Previous Physics Events

Thursday, September 22, 2016 - 11:30am Physics 298

Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [1]. Additionally, we carried out the modification of hyperbolic response in meta-structures comprised of a mononlayer graphene deposited on hBN [2]. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the "hyperlens" for subdiffractional focusing and imaging using a slab of hBN [3]. [1] S. Dai et al., Science 343, 1125 (2014) [2] S. Dai et al., Nature Nanotechnology 10, 682 (2015) [3] S. Dai et al., Nature Communications 6, 6963 (2015)

Wednesday, September 21, 2016 - 3:30pm Gross Hall 330

"Machine Learning at The New York Times
The Data Science group at The New York Times develops and deploys machine learning solutions to newsroom and business problems.
Re-framing real-world questions as machine learning tasks requires not only adapting and extending models and algorithms to new or special cases but also sufficient breadth to know the right method for the right challenge. I'll first outline how unsupervised, supervised, and reinforcement learning methods are increasingly used in human applications for description, prediction, and prescription, respectively. I'll then focus on the 'prescriptive' cases, showing how methods from the reinforcement learning and causal inference literatures can be of direct impact in engineering, business, and decision-making more generally.

Informal lunch with undergraduates at 11:45

Seminar begins at 3:30 p.m., with reception to follow.

Wednesday, September 21, 2016 - 3:30pm Physics 128

"Testing String Theory? Scale Invariance in Expanding Strongly Interacting Fermi Gases"

Optically-trapped, ultra-cold gases of spin ½-up and spin ½-down 6Li atoms model high temperature superconductors, neutron matter, and even the quark-gluon plasma that existed microseconds after the Big Bang. A bias magnetic field tunes the gas to a collisional (Feshbach) resonance, where the dilute atomic cloud becomes the most strongly interacting, non-relativistic fluid known: Shock waves are produced when two clouds collide. I will describe our recent observations of scale-invariant expansion and new measurements of quantum viscosity η and entropy s in such clouds. The η/s ratio obtained in the experiments is comparable to that of a quark-gluon plasma, close to the minimum conjectured for a "perfect fluid" using scale-invariant string theory methods.

Coffee and cookies will be served before the event in room 128.

Tuesday, September 6, 2016 - 3:30pm Physics 298

We calculate the medium photon production due to Compton and annihilation processes by taking into account the (3+1)-dimensional anisotropic hydrodynamics of the quark gluon plasma (QGP) expected to be formed in relativistic heavy ion collisions. We have taken the momentum-space anisotropy into account in the computation of the photon production rate finally. We present the predictions for high-energy photon yields as a function of transverse momentum and rapidity. We conclude that high energy photon production is extremely sensitive to the assumed level of initial momentum-space anisotropy of the quark-gluon plasma.

Wednesday, August 31, 2016 - 2:00pm Physics 278

Since the earliest measurements of neutrinos, neutron interactions
have been an important part of the detection process. This talk will
focus on measurements of neutrinoless double beta decay, which have
the potential to determine the Dirac/Majorana nature of the neutrino.
In particular, the EXO-200 and nEXO experiments will be highlighted.
After the experimental status and main results are discussed, two
techniques involving neutron interactions will be described. First,
the backgrounds from cosmogenic neutrons in EXO-200 have been studied
in detail, leading to a technique to use neutron capture signals to
reject these backgrounds from future data, improving sensitivity.
Second, neutron inelastic scattering may be used to calibrate a future
large xenon detector such as nEXO. This novel technique offers
advantages over traditional external gamma source calibration.

Monday, July 18, 2016 - 11:30am Physics 298

We propose a circuit-QED-based single-microwave-photon detector for dark matter axion searches. The strong coupling between a superconducting qubit and microwave photons makes it possible to detect the extremely weak axion signal with a high sensitivity. With realistic experimental parameters, the single-photon detector has a detection efficiency 99.5% and a very low dark count of 0.5% of the thermal noise. At low temperatures, the single-photon detector can outperform linear amplifiers by several orders of magnitude in the signal-to-noise ratio (SNR). This improved SNR could significantly speed up the search for axion particles. Beyond axion searches, the proposed single-photon detector can become an important component of circuit-QED toolbox and find direct applications in experiments such as remote entanglement.

Friday, May 13, 2016 - 2:00pm Physics 278

The search for supersymmetry (SUSY) is a long-standing important task
in particle physics. One prime motivation for weak-scale SUSY is that
it protects the Higgs vacuum expectation value without unnatural
fine-tuning of the theory parameters. Light stops one of the key
features of natural SUSY. In this talk, I will review searches for
direct stop pair production with the CMS detector in the LHC. The
searches are performed using 2 fb-1 proton-proton collision data at a
center of mass energy of 13 TeV.

Monday, May 9, 2016 - 2:00pm Physics 278

Dark Matter in the form of Weakly Interacting Massive Particles (WIMPs) is typically expected to induce nuclear recoils in a terrestrial detector target with an annually modulated rate due to the motion of the Earth around the Sun. Although such a modulation has been observed by the DAMA/LIBRA collaboration it is difficult to interpret it as a dark matter signal, given the null results from other experiments. Experimental anomalies like the annual modulation signal observed in the DAMA/LIBRA project cannot be explained by traditional dark matter scattering on atomic nuclei, but could be accommodated if dark matter scatters predominantly off electrons or if most of the energy is release in the form of photons.
We will present the results of the XENON experiment on leptophilic dark matter searches and we will contrast them with the DAMA signal.

Wednesday, April 13, 2016 - 4:30pm Gross Hall 107 - Ahmadieh Family Auditorium

Kate Scholberg and Chris Walter, Duke Physics professors, will tell the story of neutrinos, focusing discovery of oscillations of neutrinos from the atmosphere by Super-Kamiokande, a 50-kton water Cherenkov detector inside a mountain in Japan.

Reception will follow

Wednesday, April 13, 2016 - 3:30pm Physics 130

Title and abstract forthcoming. Coffee and cookies will be served before the event in Physics room 130.

Friday, April 8, 2016 - 9:00pm None

Join us at the Duke Teaching Observatory for the Statewide Star Party, the kickoff event of the 2016 North Carolina Science Festival.

You will observe a variety of celestial objects including Jupiter and its moons, multiple star systems, nebulae, star clusters, and galaxies. In addition, we will have a few educational activities for adults, teens and children that will show you many exciting aspects of how to use a telescope beyond just looking through the eyepiece. You will make a star clock to tell time with the night sky, learn how to use stars for navigation with just your hands and eyes, and more!

In case of rain or poor viewing conditions, the event will be held Saturday, April 9. Check our webpage before coming.

Thursday, April 7, 2016 - 11:30am Physics 298

Ultracold atomic gases are a pristine platform for studying the physics of strongly-correlated quantum systems. Motivated by the search for exotic forms of superfluidity, we have studied a two-component, strongly-interacting two-dimensional Fermi gas with spin imbalance. The low-temperature phase diagram of such a gas may include several interesting phases, including conventional superfluids, Sarma or FFLO states as well as Fermi liquid phases. We have observed phase separation in the trapped gas between a spin-balanced condensed phase, a partially polarized phase and fully polarized normal gas. We have mapped out the behavior of the gas at different polarizations and interaction strengths across the BEC-BCS crossover. The lower dimensionality enhances the stability of exotic superfluid phases like the FFLO phase, a superfluid with a spatially modulated gap, whose direct signatures we are well-equipped to search for using a microscope capable of detecting single atoms in the gas. In the BEC limit, the FFLO phase can be thought of as a soliton train in the superfluid. Connecting to this idea, I will describe experiments where we have imprinted solitons onto a fermionic superfluid and observed their motion in the trap and their decay into other topological excitations.

Wednesday, April 6, 2016 - 4:30pm Gross Hall 107 - Ahmadieh Family Auditorium

Because fluids flow and readily change their shape in response to small forces, liquid drops have frequently been used to model phenomena as diverse as the dynamics of star formation or the statics of nuclear shape. The exhilarating spray from waves crashing onto the shore, the distressing nighttime sound of a leaking faucet, or the indispensable role of bubbles dissolving gas into the oceans attest to their ubiquitous presence and profound importance in our everyday world. A liquid drop, as it breaks into pieces, is also an excellent starting point for investigating topological or shape transitions. Although part of our common experience, these changes are far from understood and upon careful investigation reveal delightful and profound surprises. In this lecture, I will give the life history of a liquid drop - from its birth to its eventual demise as it fades into memory - illustrating the passage of its existence with the scientific surprises that determine its fate. The commonplace is extraordinary!

Bio: Sidney Nagel is Stein-Freiler Distinguished Service Professor of Experimental Condensed Matter Physicist in the Department of Physics, James Franck Institute, Enrico Fermi Institute, and the University of Chicago. He is a member of the National Academy of Sciences, and among numerous honors, he received the Oliver E. Buckley Condensed Matter Prize in 1999.He recieved his PhD from Princeton Univeristy in 1974.

Faculty Host: Robert Behringer

Wednesday, April 6, 2016 - 3:30pm Gross Hall 330

Estimating the support size of a distribution is a classical problem in statistics, dating back to the early work of Fisher, Good-Turing, and the influential work by Efron-Thisted on "how many words did Shakespeare know." This problem has also been investigated by the CS theory community under the umbrella of property testing. In the sublinear regime where the alphabet cardinality far exceeds the sample size, the challenge lies in how to extrapolate the number of unseen symbols based on what have been observed so far.

Tuesday, April 5, 2016 - 3:00pm Physics 119

"Exploiting Disorder for Global Response: Independence of Bond-Level Contributions" - We are customarily taught to understand ordinary solids by considering perturbations about a perfect crystal. This approach becomes increasingly untenable as the amount of disorder in the solid increases; for a glass with no well-defined long-range order, a crystal is a terrible starting point for understanding the glass's rigidity or its excitations. Is there an alternative - the opposite of a crystal - where order, rather than disorder is the perturbation? Jamming is an alternate way of creating rigid solids that are qualitatively different from crystals.

In a crystal with only one atom per unit cell, all atoms play the same role in producing the solid's global response to external perturbations. Jammed disordered materials are not similarly constrained and a new principle emerges: independence of bond-level response. Using networks where individual bonds can be successively removed, one can drive the overall system to different regimes of behavior. Consequently one can exploit disorder to achieve unique, varied, textured and tunable global response.

Wednesday, March 23, 2016 - 3:30pm Physics 130

"Active Matter Builds Life" - Thermodynamic non-equilibrium is a defining feature of living systems on all levels of organization. Cells and tissues are built of "active matter", dynamic materials with built-in force generators. Such materials self-organize in biological systems into well-ordered dynamic steady states, sustained by the dissipation of metabolic energy. The materials show striking collective phenomena on a mesoscopic scale, reminiscent of second order phase transitions and criticality. We use advanced light microscopy as well as microscopic motion and force-sensing techniques to characterize the complex mechanical properties of and the motion and stress patterns in biological active matter, in particular the actin cortex, all the way from reconstituted model systems to cells and tissues. We quantitate thermodynamic non-equilibrium using fundamental concepts of statistical physics such as the fluctuation-dissipation theorem and the principle of detailed balance. Faculty Host: Robert P. Behringer. Coffee and cookies will be served before the event in Physics room 130.

Wednesday, March 23, 2016 - 5:00am None

A penumbral lunar eclipse is a subtle phenomenon occurs when the Moon moves through the outer part of Earth's shadow. It is not as spectacular as a total or partial lunar eclipses and could be recognized as a slight shadow across the Moon.

A couple of telescopes will be set on the deck in front of the French Family Science Center. The eclipse starts at 5:40 am. Come earlier to see Saturn, Jupiter, Mars, Pluto and different deep sky objects.

Check Duke Teaching Observatory webpage for a possible cancellation message before coming to the observation.

Tuesday, March 22, 2016 - 3:30pm Raleigh, NC 

Currently there does not exist an accepted way to nonperturbatively regulate chiral gauge theories, such as the Standard Model. A lattice formulation leads to mirror fermions that we do not see, and decoupling via a big mass would break the gauge symmetry. Past approaches for eliminating the mirror fermions include (i) breaking the gauge symmetry, (ii) endowing them with an exotic strong interaction in hopes that it induces a mass gap. I explore an alternative where the mirror fermions decouple from the visible world via soft form factors. An interesting feature of the mechanism is that the gauge field topology remains sensitive to their existence. Could these exotic fermions exist in Nature?

Monday, March 21, 2016 - 11:30am Physics 299

Note the unusual weekday! -- Majorana modes in hybrid superconductor-semiconductor nanowire devices can be probed via tunneling spectroscopy which shows a zero bias peak (ZBP) in differential conductance. However, alternative mechanisms such as disorder or formation of quantum dots can also give rise to ZBPs, and obscure experimental studies of Majoranas. Further, a soft induced superconducting gap presents an outstanding challenge for the demonstration of their topological protection. In this talk we show that with device improvements, we reach low-disorder transport regime with clear quantized conductance plateaus and Andreev enhancement, approaching the theoretical limit. Tunneling spectroscopy shows a hard induced superconducting gap without any formation of quantum dots. Together with extremely stable ZBPs observed in large gate voltage and magnetic field ranges, we exclude various alternative theories besides the formation of localized Majorana modes. Majoranas are formed when the Zeeman energy E_Z and the chemical potential mu satisfy the condition E_Z>(Delta^2+mu^2)^(1/2), with Delta the superconducting gap. This Majorana condition outlines the topologically non-trivial phase and predicts a particular dependence of ZBPs on the gate voltage (chemical potential) and the external magnetic field (Zeeman). Our gate voltage and magnetic field dependence of ZBPs map out a ZBP phase diagram which is consistent with the Majorana topological phase diagram.

Thursday, March 10, 2016 - 11:30am Physics 298

A fundamental challenge to quantum information processing is to protect information from the detrimental effects of the environment. A milestone in addressing this problem was the development of the theory of quantum error correction (QEC). In this work, we build upon previous studies in order to evaluate the fidelity of a quantum state protected by one of the best QEC codes: the surface code. We discuss the protection of a quantum state for different spectral functions, bath temperatures and QEC cycles times. Analytical results are supported by finite size scaling analyses of Monte Carlo and exact diagonalization of finite lattices. Our results demonstrate a finite threshold that explicitly depends on the bath mediated qubit-qubit interaction range and temperature of the bath.

Tuesday, March 8, 2016 - 3:30pm Chapel Hill, N.C.

The reach of ab initio many-body techniques has increased tremendously in recent years, owing to new developments in many-body theory as well as advances in their numerical implementation. Coupled Cluster, Self-Consistent Green's Function, and In-Medium Similarity Renormalization Group (IM-SRG) calculations are routinely performed for isotopes in the A ~ 100 region. Moreover, these techniques have been extended to tackle open-shell nuclei, either directly or through the auxiliary step of deriving valence-space interactions for use with existing Shell Model technology.

One of the most powerful aspects of \emph{ab initio} methods is their capability to provide results for energies and other observables with systematic uncertainties. Together with new accurate nuclear forces (and operators) derived from Chiral Effective Field Theory, they provide a consistent framework --- and a road map --- for a predictive description of nuclei. This will have a critical impact on the search for the limits of nuclear existence, tests of fundamental symmetries (e.g., the search for neutrinoless double beta decay), our understanding of quenching and effective charges in phenomenological Shell Model calculations etc.

Wednesday, March 2, 2016 - 3:30pm Physics 130

"Revisiting Decades-Old Physics to Improve Modern Molecular Imaging" Molecular imaging-the use of chemical signatures to image function instead of merely structure-promises to enable a new generation of clinical modalities that can revolutionize both diagnosis and treatment. I will focus on two specific modalities-magnetic resonance and optical imaging-and discuss how a close coupling between basic physics on the one hand, and focused clinical questions on the other hand, enable new and important applications. In magnetic resonance, one of the most startling results of the last few decades was our work that showed coherences survive at room temperature between pairs of nuclear spins separated by many microns or millimeters. This gives insight into fundamental questions in quantum mechanics, such as the differences between coherence, correlation and entanglement; it also lets us image temperature in vivo, measure local anisotropy, and detect tissue activation. We have used fundamental symmetries in spin physics to populate states which are immune to most relaxation mechanisms (and thus persist for minutes to hours). Coupled with a new catalytic approach to enhance molecular magnetization by about a factor of 100,000 over thermal values, this can improve the molecular information in clinical imaging... Please see the Physics' website for the rest of the abstract. Faculty Host: Mark Kruse. Coffee and cookies will be served before the event in Physics Room 130.

Tuesday, March 1, 2016 - 2:00pm Physics 278

Two-phase liquid noble element-based detectors are used in a variety
of fundamental physics experiment applications. This seminar will
focus on xenon-based detectors, specifically the Large Underground
Xenon (LUX) experiment and the Particle Identification in Xenon at
Yale (PIXeY) and Compton-imaging Detector in Xenon (CoDeX)
detectors. The LUX dark matter detector is a 350 kg two-phase
liquid/gas xenon time projection chamber located at the 4850 ft level
of the Sanford Underground Research Facility in Lead, SD. Gamma
radiation from detector components produces a significant number of
the background events seen by the LUX detector. The gamma ray
background model implemented in a re-analysis of the first science run
builds on the model employed in the original results
announcement. PIXeY and CoDeX are two relatively small (approximately
30 kg) two-phase time projection chambers that were constructed at
Yale University. The focus of efforts with these detectors is to
demonstrate the efficacy of liquid xenon as a medium for Compton
gamma-ray imaging for homeland security applications.

Thursday, February 25, 2016 - 11:30am Physics 298

The phase diagram of electron-doped pnictides as a function of temperature, electronic density, and isotropic quenched disorder strength obtained by means of computational techniques applied to a three-orbital (xz, yz, xy) spin-fermion model with lattice degrees of freedom will be presented. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. The use of computational techniques allows to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, Monte Carlo simulations reveal that the fast reduction with doping of the N\'{e}el T_N and the structural T_S transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range considered, changes in the Fermi Surface produced by electron doping affect only slightly both critical temperatures. Comparisons with STM and neutron scattering experiments will be presented.

Ref: S. Liang, C. Bishop, A. Moreo, and E. Dagotto, Phys. Rev. B 92, 104512 (2015)

Tuesday, February 23, 2016 - 3:30pm Raleigh, NC 

Massive stars end their lives in a violent core collapse. While the expectation for the majority of them has been the formation of a protoneutron star and an optical supernova explosion, an alternate channel is the collapse to a black hole with only a limited optical transient. I will discuss recent supernova simulation results and on-going observational studies which are starting to shed light on the question of how massive stars end their lives. I will also discuss the unique insights that neutrinos can provide in future supernova neutrino datasets, including both Galactic events as well as the diffuse supernova neutrino background.

Wednesday, February 17, 2016 - 3:30pm Physics 130

"Swimming Strokes of Microorganisms: Collective Motion by Molecular Motors" The beating patterns of sperm flagella and the breast-stroke like swimming strokes of ciliates are driven by the molecular motor dynein. This motor generates sliding forces between adjacent microtubule doublets within the axoneme, the motile cytoskeletal structure. To create regular, oscillatory beating patterns, the activities of the dyneins must be coordinated both spatially and temporally. It is thought that coordination is mediated by stresses or strains that build up within the moving axoneme, but it is not known which components of stress or strain are involved, nor how they feed back on the dyneins. To answer this question, we measured the beating patterns of isolated, reactivate axonemes of the unicellular alga Chlamydomonas. We compared the measurements in wildtype and mutant cells with models derived from different feedback mechanisms. We found that regulation by changes in axonemal curvature was the only mechanism that accords with the measurements. Faculty Host: Henry Greenside. Coffee and cookies will be served before the event in Physics room 130.

Tuesday, February 16, 2016 - 3:30pm Raleigh, NC 

I will describe how to arrive at an effective action for dissipative hydrodynamics starting from the microscopic Schwinger-Keldysh construction. The essential ingredients will the doubling of degrees of freedom, an emergent abelian symmetry that is associated with entropy, and a BRST supersymmetry that ensures fluctuation-dissipation relation. I will show how these elements combine to lead to a non-linear viscous fluid effective action and outline some interesting applications of this line of thought.

Thursday, February 11, 2016 - 11:30am Physics 298

In 2006, Alexei Kitaev proposed that certain 2D honeycomb magnets with a strong spin-orbit coupling in the low-spin (S=1/2) ground state and a high degree of bond anisotropy will form a quantum-spin-liquid (QSL). This QSL is special since its excitations can be solved exactly to predict the presence of 2D Majorana Fermions. If realized in a solid-state material, these hold the promise for the technology of lossless topological qubits that survive to high temperatures. In this talk, I will present the search of these elusive quasi-particles in a two-dimensional (2D) honeycomb magnet a-RuCl3. Using a combination of thermodynamic, diffraction and neutron-scattering measurements in powder and single-crystal samples, we delve in detail the ground-state in the material, and show that this material has the ingredients necessary for satisfying the Heisenberg-Kitaev Hamiltonian. A spin-wave analysis yields that anisotropic Kitaev interactions are indeed the leading-order coupling in this material. Although the ground state is antiferromagnetic at lowest temperatures, this material shows a broad excitation spectrum independent of the long-range ordering, - which could not be explained with standard classical theories. Excitingly, when compared with the exact quantum calculations of the pure Kitaev QSL ground state, the data matches the broad fractionalized excitations expected from itinerant Majorana Fermions.

Tuesday, February 9, 2016 - 3:30pm Physics 298

In high energy collisions of heavy-ions, experimental findings of collective flow are customarily associated with the presence of a thermalized medium expanding according to the laws of hydrodynamics, which has been dubbed a 'quark-gluon plasma'. Just a few years ago, there was near consensus in the field that systems created in proton-nucleus and proton-proton collisions were too small, too short-lived, and contained too few particles to form a quark-gluon plasma. Then the experimental results started to come in. In all of these small systems, the experimental signals turned out to be similar in type and magnitude to those found in heavy-ion collisions, severely challenging the 'consensus'. In this talk, I will discuss under which conditions hydrodynamics can reasonably be applied to small systems in proton-nucleus and proton-proton collisions, whether it can be used to quantitatively describe experimental data, and what we may learn about QCD transport coefficients and non-equililibrium QCD dynamics along the way.

TBA
Tuesday, February 2, 2016 - 3:30pm Raleigh, NC 

TBA

Wednesday, January 27, 2016 - 3:30pm Physics 130

"Initial Conditions of the Universe" - I will describe my proposal for the wavefunction of the universe on the landscape multiverse which led to a theory of the selection of the initial conditions of the universe and a series of testable predictions. The series of anomalies derived from this theory before observations were available, such as: the Cold Spot; the low multipole alignment; the temperature low multipole suppression, the power asymmetry and, the preferred direction in the CMB, were first seen by WMAP and confirmed by Planck 2013. I will discuss the implications and the status of the theory in the light of Planck 2015 data. The only remaining test is the prediction of a dark flow. Faculty Host: Mark Kruse. Coffee and cookies will be served before the event in Physics room 130.

Tuesday, January 26, 2016 - 3:30pm Physics 298

One of the primary aims of the ongoing nuclear collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies is to create a Quark Gluon Plasma (QGP). The heavy quarks (HQ), charm and bottom constitutes a unique probe of the QGP properties. Both at RHIC and LHC energies a puzzling relation between the nuclear modification factor R_AA(p_T) and the elliptic flow v_2(p_T) related to heavy quark has been observed which challenged all the existing models. We discuss, within a Langevin and a Boltzamann approach, how the temperature dependence of the heavy quark drag coefficient is responsible to address for a large part of such a puzzle. We point out that for the same R_AA(p_T) one can generate 2-3 times more v_2 depending on the temperature dependence of the heavy quark drag coefficient. A drag coefficient which increases as the temperature approaches to the critical value (T_c) is a major ingredient for a simultaneous description of R_AA(p_T) and v_2(p_T). The impact of differen realization of the fluctuation-dissipation theorem on the HQ observables including the heavy-flavor pair correlations will be also addressed.

Thursday, January 21, 2016 - 11:30am Physics 298

We show that placing a quantum system in contact with an environment can enhance non-Fermi-liquid correlations, rather than destroying quantum effects as is typical. The system consists of two quantum dots in series with two leads; the highly resistive leads couple charge flow through the dots to the electromagnetic environment (noise). The similarity to the two impurity Kondo model suggests that there will be a quantum phase transition between a Kondo phase and a local singlet phase. However, this transition is destabilized by charge tunneling between the two leads. Our main result is that sufficiently strong quantum noise suppresses this charge transfer and leads to stabilization of the quantum phase transition. We present the phase diagram, the ground state degeneracy at the four fixed points, and the leading temperature dependence of the conductance near these points.

Tuesday, January 19, 2016 - 3:30pm Physics 298

TBA

Friday, December 11, 2015 - 7:30pm Duke Teaching Observatory

Observe the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Tuesday, December 8, 2015 - 3:30pm Physics 298

The problem of constructing supersymmetric lattice theories is an old one dating back to the earliest days of lattice gauge theory and has resisted solution until quite recently. I will show how new ideas borrowed from topological field theory have afforded a solution to this problem and describe one of the most interesting applications; the construction of a supersymmetric lattice regularization of N=4 super Yang-Mills theory. Large scale Monte Carlo simulations of this theory are now possible and have the potential to yield new insights into the structure of this theory at strong coupling and away from the planar limit.

Wednesday, December 2, 2015 - 2:30pm Physics 278

AGILE is a small mission of the Italian Space Agency launched on April
27, 2007, and primarily devoted to high-energy astrophysics. Although
its small dimensions and the overall very stringent constrains, in
more than eight years of operation AGILE was able to deliver
fundamental results on the physics of cosmic accelerators, including
supermassive black holes at the center of active galactic nuclei,
gamma-ray bursts, microquasars, pulsars and supernova remnants. This
large record of achievements culminated with the awarding of the Bruno
Rossi Prize by the American Astronomical Society for the discovery of
the variability of the Crab pulsar, and the establishment of supernova
remnants as the sites for galactic cosmic-ray acceleration. Moreover,
AGILE is observing our own Earth, being one of the few operative
satellites capable of detecting Terrestrial Gamma-ray Flashes,
powerful bursts of radiation associated with thunderstorms and
lightning activity. I will review the main scientific achievements of
the AGILE mission, from the extreme distances of the universe to our
restless atmosphere, with particular emphasis on the key factors that
make a small satellite become a highly successful mission.

TBA
Tuesday, December 1, 2015 - 3:30pm Raleigh N.C.

TBA

Friday, November 20, 2015 - 7:30pm Duke Teaching Observatory

Observe the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Thursday, November 19, 2015 - 1:30pm Duke North, Room 2003

"Looking for Life in the Outer Solar System: Enceladus, Titan, Europa" - Beyond Mars, three moons in the outer solar system-Jupiter's Europa, Saturn's Titan and Enceladus- appear to host environments that are provisionally suitable for sustaining life. All three contain liquid water oceans beneath their icy crusts, while one (Titan) has surface seas of methane and ethane. The oceans of Europa and Enceladus are salty, and the latter's is charged with organic molecules. I will describe the data from Galileo (for Europa) and Cassini (for Titan and Enceladus) that lead to the assertion of habitability, and discuss future exploration plans including the Europa Multiple Flyby mission, now under development for launch in the early 2020's. Jonathan Lunine's Bio: http://astro.cornell.edu/members/jonathan-lunine.html

Thursday, November 19, 2015 - 11:30am Physics 298

Changing the ground state of a system via a non-thermal parameter such as doping or pressure has been a topic of great interest in condensed matter physics for decades, most notably in the study of quantum phase transitions and the associated quantum critical regime. The rare-earth fluoride LiHoF_4 and the related dilution series LiHo_{1-x}Y_xF4, long known to be a realization of the S=1/2 Transverse Ising Model, can be used to study state tuning as a function of several tuning parameters. In addition to the well-understood field-induced ferromagnet to paramagnet quantum phase transition in pure LiHoF_4, there are a number of subtler transitions which are seen in the diluted materials. For x~=0.5, application of a magnetic field transverse to the Ising axis transitions the system into a realization of the Random-Field Ising Model, an important generic model for studying the effects of disorder. In the more highly dilution limit, at x~0.05, ordered ferromagnetic states are no longer stable due to frustration effects. Instead, the system shows behavior consistent with either a spin glass or a non-freezing spin liquid, and can be tuned between these two regimes by varying the degree of thermal connectivity with an external heat bath or by tuning the rate of quantum tunneling via an external magnetic field.

Tuesday, November 17, 2015 - 3:30pm Raleigh, NC 

The interior of a massive star in the final years prior to core-collapse supernova is in a state of violent unrest. Vigorous convective shell burning of silicon, oxygen and carbon drives large scale plume motions that can interact with each other and significantly alter the structure of the progenitor star. Under such conditions the star departs from perfect hydrostatic equilibrium and spherical symmetry and may experience dynamical pulses that, in some cases, can lead to episodic mass-loss. I will present preliminary results from multidimensional simulations of convection in massive stars in the few seconds to hours preceding the supernova explosion. The proper, dynamical multidimensional treatment of convection will be bench-marked against parametrized one-dimensional mixing length theory predictions and the implications for realistic core-collapse supernova progenitor models will be discussed. I will also present a new method to characterize pre-supernova convection, the method of Vector Spherical Harmonics decomposition, and discuss how it can be used to make realizations of core-collapse models in future simulations.

Thursday, November 12, 2015 - 11:30am Physics 298

Recent advances in techniques for synthesizing multivalent micron-sized particles have generated interest in the types of structures that can be formed through self-assembly. Simulations have shown that relatively simple building blocks can create complex structures, including crystals and quasicrystals. We are interested in the creation of a limit-periodic structure, which is ordered but nonperiodic and consists of a union of periodic lattices with ever-increasing lattice constants.
We consider the possibility of forming a limit-periodic structure out of a collection of particles based on the Taylor-Socolar tile. We find through Monte Carlo simulations that identical hard disks with magnetic dipoles embedded around the perimeter spontaneously form the limit-periodic structure through a hierarchy of phase transitions even when the ground state is periodic. The initial phase transitions drive the system into a deep free energy well, preventing the formation of competing periodic phases.
The possibility of the realization of the limit-periodic structure raises the question about the nature of its phonon modes. We calculate the phonon modes of ball and spring models of periodic structures that contain elements of the limit-periodic pattern. We identify interesting sets of extended low-participation-ratio modes of the limit-periodic structure. We show that each set of modes forms a hierarchy and present a heuristic argument for the existence of the set

Wednesday, November 11, 2015 - 3:30pm Physics 128

"Do WIMPs Rule? The LUX & LZ Experiments and the Search for Cosmic Dark Matter"

Dark Matter remains a profound mystery at the intersection of particle physics, astrophysics, and cosmology. One of the leading candidates, the Weakly Interacting Massive Particle, or WIMP, may be detectable using terrestrial particle detectors. Recent technological advances are enabling very rapid increases in sensitivity in the search for these particles. I will talk about the LUX experiment, a liquid xenon time projection chamber, which currently holds the best upper limit over much of the WIMP mass range. I will also developments toward a larger follow up experiment, LZ, which will just begin to measure a background neutrino signal that will set a fundamental limit our ability to search for WIMP dark matter.

Faculty Host: Haiyan Gao

Refreshments will be served before and after the event in Physics room 128.

Friday, November 6, 2015 - 7:30pm Duke Teaching Observatory

Observe the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Friday, November 6, 2015 - 1:30pm French Family Science Center Atrium

Triangle Universities Nuclear Lab is celebrating 50 years of research and education with a weekend program at Duke Nov. 6-8. More info at http://today.duke.edu/2015/10/tunl50preview

Wednesday, November 4, 2015 - 3:30pm Physics 128

"Observing quantum effects in the motion of a millimeter-sized object"

A major challenge in physics is to understand how the classical behavior of macroscopic objects emerges from laws that are fundamentally quantum mechanical. The field of optomechanics seeks to address this issue by studying quantum effects in the motion of macroscopic objects that are coupled to individual photons. In the past few years, experiments have demonstrated a number of quantum effects in these devices, including ground-state cooling, entanglement, and the quantum back-action of displacement measurements. I will give an overview of our group's work on these topics. I will also describe our recent work on optomechanical effects in superfluid helium devices.

Faculty Host: Gleb Finkelstein

Refreshments will be served before and after the event in Physics room 128.

Tuesday, October 27, 2015 - 3:30pm Raleigh N.C.

The nature of dark matter and the explosion mechanism of core-collapse supernovae remain two of the biggest questions in astrophysics. A keV mass sterile neutrino species may provide a solution to both of these problems. In core-collapse supernovae, sterile neutrinos can efficiently transport energy from the protoneutron star core to the stalled shock via oscillations between electron neutrinos and sterile neutrinos. We have performed simulations of core-collapse supernovae including a sterile neutrino with mass and mixing angle of a dark matter candidate. We have found that some choices of mass and mixing angle result in enhanced neutrino reheating and result in successful explosions, even in models that would not otherwise explode.

Friday, October 23, 2015 - 8:00pm Duke Teaching Observatory

Observe the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Thursday, October 22, 2015 - 11:30am Physics 298

Understanding magnetism is a complex undertaking: it relies on our knowledge of the exact position of magnetic ions in a crystal and their interactions. More important, at its core, this is fundamentally a quantum problem. In general, knowledge of the magnetic properties of a single atom will not tell much about the magnetic properties of a material, and requires understanding the cooperative effects of many degrees of freedom, particularly the spin. Starting from the chemistry, "cooking" a single crystal with enough purity and without defects is already an enormous challenge. In addition, being able to "design" a material with the desired geometry and interactions is only possible in a few cases, usually using organic molecules as a fundamental building block. In the past decade we have witnessed enormous progress in experiments that consist of placing magnetic atoms at predetermined positions on substrates, and building magnetic nanostructures, one atom at a time. The electrons in the substrate mediate the interactions between the spins, and scanning tunneling microscopy allows one to study their properties. In order to understand these interactions, we rely on a theory developed decades ago by Ruderman, Kittel, Kasuya, and Yosida, dubbed "RKKY theory", which applies when the spins are classical. The quantum nature of the electronic spin introduces another degree of complexity, and competition with other quantum phenomena: the Kondo effect. This competition is quite subtle

Wednesday, October 21, 2015 - 9:45am Physics 278

Jacob Daughhetee (Georgia Tech)

Monday, October 19, 2015 - 3:00pm Physics 278

Vincent Fischer (CEA)

Friday, October 9, 2015 - 8:00pm Duke Teaching Observatory

Observe the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Thursday, October 8, 2015 - 11:30am Physics 298

We have studied the strong coupling of a single artificial atom, formed from a superconducting qubit, to an open transmission line. This produces an almost ideal 1D quantum electrodynamic system. In a series of experiments, we have already demonstrated a number of interesting physical effects. For instance, we demonstrated that the coherent scattering of microwaves from the atom leads to the strong extinction (>99%) of the transmitted light, an effect long predicted but never observed in conventional quantum optics. In the same work, we demonstrated a rudimentary single-photon router in the microwave regime. In the next experiment, we proved that the microwaves scattered from the artificial atom are definitively nonclassical. We did this by demonstrating photon antibunching in the reflected signal, using a microwave photon statistics analyzer developed in our lab. In a third experiment, we demonstrated the giant cross-Kerr effect, an effective interaction between two photons mediated by the artificial atom. There has been great interest in the cross-Kerr effect for a number of potential applications such as photonic quantum gates and quantum nondemolition measurements (QND) of photons. In more recent experiments, we have studied the interaction of the artificial atom with its own image in a superconducting "mirror." In this talk, I will review these experiments and discuss prospects for future work.

Wednesday, October 7, 2015 - 3:30pm Physics 128

"Liquid Sodium Models of the Earth's Core" -- During the current solar maximum, we have seen a host of x-class flares and coronal mass ejections from the sun. The fact that we saw little danger from them on Earth is due to our planetary magnetic field, which shields us from the Sun's charged particle radiation. That field has weakened throughout recorded human history. The origin and dynamics of the magnetic fields of the Earth, Sun, gas giants, and nearly every massive astrophysical object raise numerous questions not resolved by existing theoretical, computational and experimental work. By using liquid sodium models of the Earth's core, we hope to better understand what determines the Earth's magnetic field strength, pattern and dynamics by probing the effects of turbulence, Lorentz forces and rotation on core dynamics. While it is not possible to match every aspect of core dynamics in the lab, the experiments we perform have a comparable force balance among rotation, magnetic fields and advection. It is possible using these experiments to match many parameters thought to occur in the Earth's outer core. Faculty Host: Mark Kruse. Refreshments will be served before and after the event in Physics room 128.

Tuesday, October 6, 2015 - 11:45am French Family Science Center 2231

"Scalable Quantum Networks of Trapped Atomic Ions"

Laser-cooled and trapped atomic ions are standards for quantum information science, acting as qubits with unsurpassed levels of quantum coherence while also allowing near-perfect measurement. When qubit state-dependent laser forces are applied to a collection of atomic ions, their Coulomb interaction is modulated in a way that allows the entanglement of the qubits through quantum gates that can form the basis of a quantum computer. Using a similar laser configuration, we have implemented tunable long-range interacting spin models with up to 20 trapped ions, the largest collection of interacting qubits yet demonstrated. Scaling to even larger numbers can be accomplished by coupling trapped ion qubits to optical photons, where entanglement can be formed over remote distances for applications in quantum communication and distributed quantum computing. By employing such a modular and reconfigurable architecture, ion trap quantum networks can be scaled to useful dimensions, for future applications that are impossible using classical processing.

Coffee and cookies will be served before the event.

Thursday, October 1, 2015 - 11:30am Physics 298

Combining superconductivity and the quantum Hall (QH) effect is a promising route for creating new types of topological excitations. Despite this potential, signatures of superconductivity in the quantum Hall regime remain scarce, and a superconducting current through a quantum Hall weak link has so far eluded experimental observation. Here we demonstrate the existence of a novel type of Josephson coupling through a QH region at magnetic fields as high as 2 Tesla. The supercurrent is mediated by states encompassing QH edge channels, which are flowing on the opposite sides of the sample. The edges are coupled together by the hybrid electron-hole modes at the interfaces between the QH region and the superconducting contacts. These chiral modes, which share some features with Majorana modes, are formed when electron and hole edge states are mixed by the superconductor.

Wednesday, September 30, 2015 - 3:30pm Physics 128

"Is Statistical Physics Relevant to Climate Science?"

The science of climate emerged out of a spate of basic physics questions, ranging from the "dark heat of Joseph Fourier to John Tyndall's measurements of the infrared absorption of gases. Today, no one can deny that the Earth's climate system is a complex nonlinear dynamical system. Three main research approaches are common: (1) Observation of the past and present state of the system and extrapolation to the future. (2) Numerical simulations using Global Circulation Models, which treat the system with the deterministic approach of weather forecasting by modeling the processes on a coarse-grained scale. (3) Constructing a low-order description of the system/subsystem of the climate, in the vein of theoretical physics. There are substantial cultural and technical differences between these approaches. All have value and all have limitations. However, it appears that the ready availability of computing power has made approach (3) less favorable. Here I show that there are a number of climate scale phenomena that can be fruitfully examined quantitatively with the methods of statistical physics. in so doing I hope to try and stimulate broader participation of fellow physicists in these compelling and societally relevant problems.

Faculty Host: Bob Behringer

Refreshments will be served before and after the event in Physics room 128.

Friday, September 25, 2015 - 8:30pm Duke Teaching Observatory

Cancelled due to poor weather conditionsObserve the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Wednesday, September 23, 2015 - 3:30pm Physics 128

"The Path to Magnetic Fusion Energy"

When the possibility of fusion as an energy source for electricity generation was realized in the 1950s, understanding of the plasma state was primitive. The fusion goal has been paced by, and has stimulated, the development of plasma physics. Our understanding of complex, nonlinear processes in plasmas is now mature. We can routinely produce and manipulate 100 million degree plasmas with remarkable finesse, and we can identify a path to commercial fusion power. The international experiment, ITER, will create a burning (self-sustained) plasma and produce 500 MW of thermal fusion power. This talk will summarize the physics progress in fusion research to date, and the remaining steps to fusion power.

Faculty Host: Henry Greenside

Refreshments will be served before and after the event in Physics room 128.

Friday, September 18, 2015 - 8:00am Bostock Library - The Edge Workshop Room

Neutron scattering, neutron diffraction, and neutron imaging provide information that is highly complementary to that from other microscopic scattering techniques, and more recent technical developments have fueled the expansion of the field, originally focused on condensed matter physics, into materials science, soft matter and biomolecular systems, engineering, and the arts. Exciting opportunities exist, for specialists as well as non-specialists, to do neutron research at ORNL¿s state-of-the-art facilities, either through collaborations with ORNL researchers or as external users. The NScD operates the High Flux Isotope Reactor (HFIR), the United States' highest flux reactor-based neutron source and the Spallation Neutron Source (SNS), the world's most intense pulsed accelerator based neutron source. Together these facilities operate 30 instruments for neutron scattering, neutron diffraction, and neutron imaging research, each year carrying out in excess of 1,000 experiments in the physical, chemical, materials, biological and medical sciences. HFIR also provides unique facilities for isotope production and neutron irradiation.

Tuesday, September 15, 2015 - 3:30pm Raleigh N.C.

I will present some applications of effective field theory in hadron structure and spectrum. Finite range regularization will be discussed in the extrapolation of lattice data on magnetic form factors, strange form factors, charge radii, first moments, etc. The unitary chiral perturbation theory is applied to study the open charm and hidden charm states. The recent work on the asymmetry of sea quark parton distribution function in proton will also be discussed.

Friday, September 11, 2015 - 9:00pm Duke Teaching Observatory

Event canceled due to cloud cover

Observe the solar system objects, binary stars, star clusters, galaxies and nebulas through the university's five Meade LX200 GPS Schmidt-Cassegrain telescopes. See website for directions.

Tuesday, September 8, 2015 - 12:00pm Sanford 223, Rhodes Conference Room

Nasser Hadian, a prominent professor of political science at the faculty of law and political science at the University of Tehran will headline a lunchtime talk "The Iran Nuclear Deal: What's at Stake." He will offer the Iranian perspective on the Joint Comprehensive Plan of Action (JCPOA), the evolving Iran-US relationship, and Iran¿s geopolitical role in the region in the aftermath of the agreement.

Mark Emamian, a senior research engineer in the Duke Physics Department at the Free Electron Laser Lab, will give a brief history of the Iran nuclear program from a technical perspective. The talk will be moderated by Peter Feaver, director of the American Grand Strategy Program (AGS) at the Sanford School of Public Policy, and Professor of Political Science, Duke University. Hadian's visit presents the Duke and Durham community with a rare opportunity to hear about the inner dynamics of this deal from an insider point of view. Free and open to the public. A light lunch will be served, first come first served.

Thursday, September 3, 2015 - 5:00pm Fitzpatrick Center Schiciano Auditorium Side A

Join us as we define the future of technology and engineering. At Northrop Grumman, we¿ve developed the Future Technical Leaders (FTL) Program - a professional development opportunity aimed at identifying and investing in Northrop Grumman¿s next generation of technologists, engineers, architects and leaders.

Our FTL Program is seeking talented MS and PhD 2013-2014 graduates
interested in learning more about our exclusive three year rotational program
working side-by-side with senior technical management. It¿s the chance of a
lifetime¿one that only a few receive.

Majors of interest are:
Analytics Aerospace Engineering (systems)
Mathematics Systems Engineering (technical)
Physics Information Systems/Information Technology
Computer Science Operations Research
Computer Engineering Cyber Security & Information Assurance
Electrical Engineering Electrical & Computer Engineering
Health/Bio IS/IT Bio Informatics/Neuroscience

Across our career areas and around the globe, we see the value of our
performance every day. We are Northrop Grumman. And forward thinking
is at the heart of what we do.

Discover more about our careers in the FTL Program at
www.northropgrumman.com/ftl

Wednesday, September 2, 2015 - 3:30pm Physics 128

"Self-Organization and Mechanics of Active Matter"

Bird flocks, bacterial suspensions, and colloids propelled by self-catalytic reactions are examples of active matter collectives of individually driven, dissipative units with fascinating emergent behavior at large scales. Active systems self-organize in complex patterns, with transitions between ordered and disordered states, and spontaneously generate forces and sustained motion. In this talk I will highlight common properties of these diverse systems and describe recent progress in understanding and classifying their complex behavior using theory and simulations.

Faculty Host: Bob Behringer

Refreshments will be served before and after the event in Physics room 128.

Thursday, August 13, 2015 - 10:00am Gross Hall 330

Abstract
In this work, we develop a class of parallel domain decomposition method for
simulating blood flow in three-dimensional compliant arteries. We model the
fluid-structure interaction problem by using a monolithically coupled system
of linear elasticity equation and incompressible Navier-Stokes equations in
an arbitrary Lagrangian-Eulerian framework. The monolithic fluid-structure
system is discretized with a fully-implicit finite element method on unstructured
moving meshes, and solved by a Newton-Krylov algorithm preconditioned
with restricted additive Schwarz methods. The investigations focuses
on the performance of the one-level and two-level overlapping Schwarz preconditioner
for Newton-Krylov methods as well as the efficiency and parallel
scalability of the algorithm for solving the complicated coupled system.
Simulations based on the patient-specific pulmonary artery geometries are
performed on a supercomputer. Our algorithm is shown to be scalable with
thousands processors and for problems with millions of unknowns.

Yuqi Wu is an Acting Assistant Professor in Applied Mathematics at the
University of Washington. He received his Ph.D in Applied Mathematics
from the University of Colorado. Dr. Wu¿s research focuses on numerical
algorithms for partial di¿erential equations, computational fluid dynamics,
parallel computing and reduced-order models.

Wednesday, August 12, 2015 - 9:00pm Duke Teaching Observatory

The Fall 2015 observation season at Duke Teaching Observatory opens this Wednesday, August 12 at 9pm. The main event of the evening is Perseids Meteor Shower. Few telescopes will be set for Solar system and deep sky objects observation. See directions at http://www.phy.duke.edu/duke-teaching-observatory#Directions.

Thursday, July 16, 2015 - 10:00pm Duke Teaching Observatory

Testing Events Block for Teaking Observatory

Friday, July 10, 2015 - 2:00pm Physics 278

It has been unknown whether most massive stars end their lives as supernova explosions, or whether a significant fraction collapse to black holes with only a limited optical transient. I will discuss recent supernova simulation results and on-going observational studies which is starting to paint a consistent picture for how massive stars end their lives. I will discuss the unique neutrino signatures this scenario predicts, and how they can be tested by future supernova neutrino datasets, including both Galactic supernovae as well as the diffuse supernova neutrino background.

Monday, June 29, 2015 - 11:00am Physics 298

The spectral density (SD) function has a central role in the study of open quantum systems (OQSs). We show a spectroscopic method for measuring the SD, requiring neither the OQS to be initially excited nor its time evolution to be tracked in time, which is not limited to the weak-coupling regime. This is achieved through one-dimensional photon scattering along a waveguide for a zero-temperature reservoir coupled to a two-level OQS via the rotating wave approximation. We find that the SD profile associated with the OQSs own reservoir can be exactly mapped into a universal simple function of the photon's reflectance and transmittance. As such, it can be straightforwardly inferred from the photon reflection and transmission spectra. (host: H. Baranger)

Thursday, June 18, 2015 - 11:00am Physics 298

Using first principles quantum Monte Carlo simulations and the methods developed in arXiv:1502.02980, we study the real-time dynamics of large open quantum spin systems in two-spatial dimensions. In particular, the dynamics of the considered systems is entirely driven by dissipative processes. Several different dissipative processes as well as various initial ordered states, such as Heisenberg antiferromagnetism, are studied in our investigation. We find that the time scales governing the approach towards a new equilibrium phase at late times are determined by the symmetry of the dissipative processes. Specifically, a slow equilibration is observed for the processes conserving any of the magnetization Fourier modes. With the same technology, the non-equilibrium transport of magnetization in large open quantum spin systems is investigated as well.
(Host: H. Baranger)

Wednesday, May 13, 2015 - 8:00am Physics 128

Term I classes begin

Monday, May 11, 2015 - 3:30pm Physics 128

"Phase transitions and non-equilibrium behavior in biological systems"

The mechanics of cells and tissues is largely governed by scaffolds of filamentous proteins such as collagen. Evidence is emerging that such structures can exhibit rich mechanical phase behavior. A classic example of a mechanical phase transition was identified by Maxwell for macroscopic engineering structures: networks of struts or springs exhibit a continuous, second-order phase transition at the isostatic point, where the number of constraints imposed by connectivity just equals the number of mechanical degrees of freedom. We will present recent theoretical predictions and experimental evidence for mechanical phase transitions in in both synthetic and biological soft materials. Living systems, of course, typically operate far from thermodynamic equilibrium, which affects both their dynamics and mechanical response. As a result of enzymatic activity at the molecular scale, living systems characteristically violate detailed balance, a fundamental principle of equilibrium statistical mechanics. We discuss violations of detailed balance at the meso-scale in living systems.

Wednesday, April 29, 2015 - 3:30pm Physics 128

"Femtosecond Hard X-ray Lasers for Imaging Atomic Structure and Dynamics"

The recent invention of the hard X-ray laser has created a host of new opportunities for the study of atomic processes and dynamics in many fields. Since the Linac Coherent Light Source (LCLS) started operation in late 2009 at SLAC we have collected femtosecond pulsed coherent X-ray scattering from many molecular systems. It has been found that sufficiently brief X-ray pulses terminate before radiation damage commences, opening up many opportunities for new experiments in time- resolved imaging with atomic spatial resolution at room temperature, in condensed matter physics, materials science and biology. I will review some of these, including pump-probe experiments on the large molecular complexes involved in photosynthesis, and on a drug target molecule for sleeping sickness. A new approach to disentangling orientational disorder will also be demonstrated, aimed at reconstructing the image of one molecule, using the scattering from many in random orientations in solution, without modeling, based on angular correlation functions. The first "molecular movies", which track chemical reactions in time will be shown, which may be synchonized with snapshot spectroscopy. I'll also describe the new approaches to the phase problem which these experiments suggest. A review of all this work can be found in Rev Mod Phys. 75, 102601 (2012).

Faculty Host: Glenn Edwards

Refreshments will be served before and after the event in room 128.

Monday, April 27, 2015 - 8:30am Environment Hall Field Auditorium

Please register to participate in the Duke-Oak Ridge Collaboration Building event, designed (in collaboration with Larry Carin and Robert Calderbank) to provide information about how to connect with Oak Ridge National Labs which can possibly benefit you in multiple ways through opportunities to develop research collaborations, access funding for your graduate students through these research collaborations, and access ORNL's user facilities. Oak Ridge desires to connect with Duke around research and graduate students, which can take place through research collaboration, the Go! program for funding graduate students (see attached description), and joint faculty appointments. We have thus developed a collaboration event to take place April 27th that will provide additional background about these collaborative opportunities and introduce ORNL programs, institutes, and centers to Duke faculty. The schedule for the collaboration event is attached above. Register to participate at https://duke.qualtrics.com/SE/?SID=SV_0JPEEZbkYkhbVKl . Registration closes April 17. There is no cost for the program, but we need a good estimate of attendees and an understanding of the research interests represented (as we plan to provide information about your research interests and desires for connection to Oak Ridge ahead of the meeting).

Friday, April 24, 2015 - 8:30pm Duke Teaching Observatory

Observe the sky through modern 10" telescopes, guided
by Duke physicists. Everyone welcome. Weather dependent

Thursday, April 16, 2015 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

Saturday, April 11, 2015 - 8:30am Physics 130

Spring session of a series of regional meetings on mathematical string theory. See event web page for details

Tuesday, April 7, 2015 - 4:30pm Gross Hall 107

First in this Natural Sciences Colloquium Series sponsored by Deans Kiehart and Patton.

TBA
Tuesday, April 7, 2015 - 3:30pm Raleigh N.C.

TBA

Wednesday, April 1, 2015 - 3:30pm Physics 128

"Additive Manufacturing and Architected Materials"

Material properties are governed by the chemical composition and spatial arrangement of constituent elements at multiple length scales. This fundamentally limits material properties with respect to each other creating trade-offs when selecting materials for specific applications. For example, strength and density are inherently linked so that, in general, the more dense the material, the stronger it is in bulk form. We are combining advanced microstructural design, using flexure and screw theory as well as topology optimization, with advanced additive micro- and nanomanufacturing techniques to create new material systems with previously unachievable property combinations. We have demonstrated designer properties resulting from architected materials in polymers, metals, ceramics and combinations thereof. Our manufacturing techniques include Projection Microstereolithography (P¿SL), Direct Ink Writing (DIW), and Electrophoretic Deposition (EPD). These tools are capable of generating the designed structures which are highly three-dimensional micro- and nano-scale architectures with multiple constituent materials in the same structure. Design of materials, fabrication, and characterization will all be discussed.

Faculty Host: Glenn Edwards

Refreshments served before and after the event in room 128.

Tuesday, March 31, 2015 - 4:30pm Physics 128

Professor Ratner is hosted by the Duke University Chemistry and Physics Departments and the Duke University Chapter of Sigma Xi.

Friday, March 27, 2015 - 11:30am Physics 298

Abstract: Angle-resolved photoemission spectroscopy (ARPES) has recently emerged as one of the most powerful probes of the electronic structure of a solid, directly giving the detailed energy-momentum dispersion relations (band structures), Fermi surfaces, etc. ¿ properties that are the starting point for almost all analyses of the physical properties of a solid. Going more deeply, ARPES also has unique abilities to uncover the dynamical interaction effects or ¿self energies¿ that dominate the physical properties of correlated electron materials.
Perhaps the most famous and exotic of correlated electron materials are high-temperature cuprate superconductors, which have exotic ¿normal¿ and superconducting states, neither of which are understood. Here I show our latest results on this problem, focusing not just on the pairing energy scale (the gap) but also the pair-breaking energy scale. In contrast to conventional superconductors in which the superconducting transition temperature Tc is set by the pairing energy alone, I show that Tc in the cuprates is set by a crossover between the pairing and pair-breaking energy scales, each of which is strongly temperature-dependent. Uncovering the nature of this pair-breaking is an important future direction for the field.
Time permitting, I will briefly discuss how ultrafast pump-probe ARPES experiments are becoming a powerful tool for studies of out-of-equilibrium physics, including correlation effects.

TBA
Thursday, March 26, 2015 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke.

Thursday, March 26, 2015 - 11:30am Physics 298

Abstract: Forces may be induced by fluctuations of any thermodynamic parameter. The most well known fluctuation induced force is the Van-der-Waals/Casimir force, which is due to the dipolar fluctuations. A less studied force is the force due to monopolar charge fluctuations, which are related to the capacitance of the system. This talk will focus on monopolar charge fluctuation forces in solid state nanodevices. In particular the force is calculated for a parallel plate system[1] involving both a graphitic nanostructure and a metal plate. Both the classical and quantum mechanical effects will be discussed. This force will be shown to be comparable or greater than the Van-der Waals/Casimir force in systems with reduced dimensionality, such as ones involving graphitic nanostructures.
Host: H. Greenside

Wednesday, March 25, 2015 - 3:30pm Physics 128

"From Chaos to Cures; controlling the complex spatiotemporal dynamics of cardiac arrhythmias using a theoretical, numerical and experimental integrative approach"

The heart is an electro-mechanical system in which, under normal conditions, electrical waves propagate in a coordinated manner to initiate an efficient contraction. In pathologic states, single and multiple rapidly rotating spiral and scroll waves of electrical activity can appear and generate complex spatiotemporal patterns of activation that inhibit contraction and can be lethal if untreated. Despite much study, many questions remain regarding the mechanisms that initiate, perpetuate, and terminate reentrant waves in cardiac tissue. In this talk, we will discuss how we use a combined experimental, numerical and theoretical approach to better understand the dynamics of cardiac arrhythmias. We will show how large scale GPU simulations and state-of-the-art optical mapping with voltage-sensitive fluorescent dyes can be used to image the electrical waves present in cardiac tissue, leading to new insights about their underlying dynamics. We will present experimental data from hearts ranging from zebra fish to rabbits, cats, dogs, pigs and horses and discuss how period-doubling bifurcations that arise at fast heart rates can lead to complex spatiotemporal patterns and multistability between single and multiple spiral waves in two and three dimensions. Then we will show how control algorithms tested in computer simulations can be used in experiments to continuously guide the system toward an unstable fixed point in order to prevent and terminate complex electrical patterns characteristic of arrhythmias. We will proceed to establish a relationship between the response of cardiac tissue to an electric field and the spatial distribution of heterogeneities due to the coronary vascular structure, and discuss how in response to a pulsed electric field E, these heterogeneities serve as nucleation sites for the generation of intramural electrical waves with a source density ρ(E) and a characteristic time constant τ for tissue excitation that obeys a power law. We will finish by showing how these results can be applied in vitro and in vivo to develop a novel low energy control algorithm that could be use clinically that requires only 10% of the energy currently used by standard methods to defibrillate the heart.

Faculty Host: Henry Greenside

Refreshments will be served before and after the event in room 128

Thursday, March 19, 2015 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

Wednesday, February 25, 2015 - 3:30pm Physics 128

"Light and life: How biological species respond to UV radiation?" - Sunlight is essential to life on earth both as an energy source to fuel photosynthesis for chemical energy and as a biological signal to control circadian rhythm in the life cycle. However, the UV radiation in sunlight, especially after recent ozone depletion, is detrimental to biosphere by causing DNA damage and cell dysfunction. Here, we present our recent studies on two important biological systems: One is to reverse damaged DNA to prevent potential skin cancer and the other is to trigger signal transduction for UV protection. With femtosecond spectroscopy and molecular biology methods, we mapped out the entire functional evolution in real time by following the dynamics from the beginning to the end, including a series of elementary processes of ultrafast energy transfer, electron transfer and chemical bond breaking and making. These biological dynamics occur ultrafast on the picosecond time scales and are in synergy to maximize biological quantum efficiency. These results reveal the molecular mechanism and functional photocycle at the most fundamental level and provide the molecular basis for future biomedical applications. Faculty Host: Glenn Edwards. Refreshments served before and after the event in room 128.

Thursday, February 19, 2015 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke.

Wednesday, February 18, 2015 - 3:30pm Physics 128

CANCELLED, WILL RESCHEDULE SOON "The Path to Magnetic Fusion Energy" - When the possibility of fusion as an energy source for electricity generation was realized in the 1950s, understanding of the plasma state was primitive. The fusion goal has been paced by, and has stimulated, the development of plasma physics. Our understanding of complex, nonlinear processes in plasmas is now mature. We can routinely produce and manipulate 100 million degree plasmas with remarkable finesse, and we can identify a path to commercial fusion power. The international experiment, ITER, will create a burning (self-sustained) plasma and produce 500 MW of thermal fusion power. This talk will summarize the physics progress in fusion research to date, and the remaining steps to fusion power. Faculty Host: Glenn Edwards. Refreshments served before and after the event in room 128.

TBA
Thursday, February 12, 2015 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

Thursday, February 5, 2015 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke. A detection of primordial gravity waves has been heralded not only as a smoking gun for the existence of inflation, but also as a way to establish the scale at which inflation took place. In this talk I will review the connection between a confirmed detection of primordial gravity waves and the scale of inflation. In particular, I will discuss whether other primordial sources of gravity waves could lead to a misinterpretation of the data. I will also discuss challenges for inflation model building and the implications for the post-inflationary, "Big Bang" universe.

Wednesday, January 21, 2015 - 3:30pm Physics 128

"Ten Unsolved Problems in Polymer Physics" Polymer physics is one of the youngest areas of physics, and one of the very few ones that were born outside physics and then later, far beyond the age of infancy, evolved to become an accepted member of physics family. Eighty years ago, scientists were still arguing whether polymers are giant macromolecules or colloidal association of smaller molecules. By the end of last century polymer physics grew into a mature field and evolved to include a much broader class of materials to become the soft matter physics. Nevertheless a number of major problems remains unsolved. Two groups of these unsolved problems will be briefly reviewed in this presentation. The first group is related to the grand challenge of the first principle quantitative description of topological entanglements between overlapping polymers. Recent advances in this area include models of non-linear elasticity of entangled networks and fractal structure of non-concatenated overlapping rings. The second group is related to the reversible bonds between polymers that allow design of self-healing materials. Grand challenges in this group include self-replication with exponential growth, active polymers, and the minimal model of life. Faculty Host: Joshua Socolar. Refreshments will be served before and after the event in Physics room 128.

Friday, January 16, 2015 - 5:00pm East Campus

Conference for undergraduate women in physics, co-sponsored by the APS, Duke, NC State, UNC and NCCU. Registered participants only.

Thursday, January 8, 2015 - 3:30pm Raleigh N.C.

I review physical scenarios in different vacua of N=2 supersymmetric QCD deformed by the mass term for the adjoint matter. I focus on dynamical scenarios which can serve as a prototypes of what we observe in the real world QCD. The above mentioned deformation breaks supersymmetry down to N=1 and at large deformations the theory flows to a ''more realistic'' theory: N=1 supersymmetric QCD. It turns out that the standard Seiberg-Witten scenario of quark confinement does not survive the N=1 deformation. However, the different phase which we call "instead-of-confinement" phase appears to be more promising.

Tuesday, December 9, 2014 - 3:30pm Physics 298

In ultra-relativistic heavy ion collisions a fireball of hot and dense matter is created. When the energy density inside the fireball is very high, liberation of partons is expected to occur and a new phase of matter, the Quark Gluon Plasma (QGP) is formed. Short lived hadronic resonances are sensitive to the medium properties of a heavy-ion collision, in particular to the temperature, density and expansion velocity. Resonances decaying into hadrons are used to estimate the time span and hadronic interaction cross section in the hadronic phase between chemical and kinetic freeze-out. The detection of early decoupled resonances aims at studying chiral symmetry restoration via their mass shift and width broadening. This talk will summarize the results from LHC and RHIC energies.

Thursday, December 4, 2014 - 3:30pm Raleigh N.C.

Starting from a brief review of early exact results in super-Yang-Mills (such as beta functions in N=1) I discuss recent developments of these ideas in newly emerged models (e.g. particular heterotic 2D sigma models) and in newly emerged applications in 4D super Yang-Mills.

Tuesday, December 2, 2014 - 3:30pm Chapel Hill N.C.

TBA

Tuesday, November 25, 2014 - 10:00am Physics 128

After a brief introduction to iron based high temperature superconductivity, this presentation will focus on two recent results: (1) the unexpected complexity of magnetic states found in real-space Hartree-Fock studies of Se-based compounds when in the presence of regularly spaced iron vacancies and also in the case of two-leg ladders, and (2) the much studied spin nematic state and its diverging susceptibility above the critical temperature T_S analyzed from the perspective of Monte Carlo simulations of the spin fermion model. The presentation is based on the following publications:

P. Dai, JP Hu, and E.D., Nat. Phys. 8, 709 (2012); E.D., RMP 85, 849 (2013);
Q. Luo et al., PRB 84, 140506(R) (2011); Q. Luo et al., PRB 87, 024404 (2013);
S. Liang et al., PRL 111, 047004 (2013); and S. Liang et al., arXiv:1405.6395

Thursday, November 20, 2014 - 4:30pm Physics 278

Halloween is only a few weeks behind us... In this talk we will discuss how Project Poltergeist shaped neutrino physics for decades to come and made nuclear reactors the workhorse of early oscillation searches. A renaissance of reactor neutrino experiments around 2010 lead to the very precise measurement of one of the mixing parameters, theta13 -- in the run-up to this measurement flux calculations from the 1980's were scrutinized and surprisingly the flux was found to be higher than previously expected leading to the so-called reactor anomaly. The reactor anomaly points a type of neutrino even more elusive than regular neutrinos, the ghostly sterile neutrino. We will review the calculations performed so far and highlight some of the open questions. In the final part of the talk we will point out how current attempts to settle the question of the sterile neutrino will impact our future ability to use neutrinos to peer into the cores of nuclear reactors to safeguards against the diversion of plutonium, which may play an important role in banishing the specter of nuclear terrorism.

Thursday, November 20, 2014 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

Thursday, November 20, 2014 - 3:30pm Raleigh N.C.

During the merger of two neutron stars, matter can be ejected in the interstellar medium through different channels. The ejection mechanism, as well as the expansion timescale, can influence deeply the matter properties and, eventually, the subsequent nucleosynthesis. In this talk, I will present results regarding the formation and the properties of a neutrino-driven wind in the aftermath of a binary neutron star merger, in presence of a long living hyper massive neutron star. Implications in terms of nucleosynthesis and electromagnetic counterparts will be also discussed.

Wednesday, November 19, 2014 - 3:30pm Physics 128

"Searching for Vibrations from the Big Bang with BICEP2"

Moments after the Big Bang, our observable universe underwent a violent growth spurt called inflation. The inflationary expansion flung apart the observable universe from a causally-connected sub-atomic volume, and established a primordial spectrum of scalar perturbations that led to the temperature anisotropies observed in the cosmic microwave background. Our team has been making precise degree-scale polarization measurements of the CMB from the south pole with the BICEP/Keck series of experiments in search of a distinctive ¿B-mode¿ pattern, a hallmark of tensor perturbations associated with a background of gravitational waves generated by inflation. I will discuss our experiments and measurements, recent results, and the prospects for improved measurements coming in the near future.

Faculty host: Prof. Emeritus Horst Meyer.

Refreshments will be served before and after the event in room 128.

Tuesday, November 18, 2014 - 3:30pm Raleigh N.C.

TBA

TBA
Thursday, November 13, 2014 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke.

Tuesday, November 11, 2014 - 3:30pm Raleigh N.C.

Time crystals are a class of systems which exhibit motion in their lowest-energy states. Their Lagrangians have unconventional kinetic terms and their Hamiltonians are multivalued functions of momentum, yet they are amenable to quantization. Field theoretic generalizations may have applications in condensed matter physics and cosmology.
Refs: arXiv:1202.2537, arXiv:1207.2677, arXiv:1210.3545.

Thursday, November 6, 2014 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

Thursday, November 6, 2014 - 11:30am Physics 298

The surface code is a promising alternative for implementing fault-tolerant, large-scale quantum information processing. Its high threshold for single-qubit errors under stochastic noise is one of its most attractive features. We develop an exact formulation for the fidelity of the surface code that allows us to probe much further on this promise of strong protection. This formulation goes beyond the stochastic single-qubit error model approximation and can take into account both correlated errors and inhomogeneities in the coupling between physical qubits and the environment. Exact results for the fidelity threshold of the surface code are obtained for several relevant types of noise. Analytical predictions for a representative case are confirmed by Monte Carlo simulations.

Wednesday, November 5, 2014 - 3:30pm Physics 128

"Quantum Entanglement for Fun and Profit: 101 Uses for Schroedinger's Cat"

Nearly 80 years after Schroedinger described entanglement as the quintessential nonclassical phenomenon, and 50 years after Bell showed the inconsistency of quantum correlations with local realism, the quantum information revolution seeks to use its almost magical properties to enable new feats in information processing. As we shall see, entanglement can now be produced at high rates with exquisite precision, enabling unprecedented tests of nonlocality and such feats as quantum cryptography and teleportation. I will describe some of these miracles, and our investigations into how the usual benefits can be further extended, by using more complex quantum states (e.g., hyper-entanglement). Time and appetites permitting, I may give a brief lesson in quantum cooking.

Faculty Host: Daniel Gauthier.

Refreshments will be served before and after the event in room 128.

Thursday, October 30, 2014 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke.

Wednesday, October 29, 2014 - 3:30pm Physics 128

We live in a universe whose properties are remarkably well described by a very early epoch of accelerating expansion, termed inflation. One generic prediction of inflation is a relic background of stochastic gravitational radiation. These tensor metric perturbations leave a distinctive imprint in the polarization of the microwave background. In March 2014, the BICEP experiment at the South Pole announced the detection of this signal for the first time, although subsequent analysis cast doubt on whether polarized emission from dust in the Milky Way galaxy might be contaminating the signal.

Wednesday, October 22, 2014 - 3:30pm Physics 128

"Recent Discoveries of Cosmic Ray Anomalies"

One of the most exciting possibilities in cosmic ray research is the potential to discover new phenomena. A number of elementary particles were discovered in cosmic rays before modern-day accelerators became available to study their detailed properties. In spite of the rapid progress in physics over the last century, the possible presence of cosmological antimatter and the nature of dark matter in the universe remain mysteries. Balloon-borne and space based instruments configured with particle detectors have been flown to search for these exotic sources, and to explore possible limits to particle acceleration in supernova. They have also been used to study cosmic-ray origin, acceleration and propagation. Recent results, their implications, and the outlook for future experiments will be presented.

Faculty Host: Kate Scholberg.

Refreshments will be served before and after the event in room 128.

TBA
Tuesday, October 21, 2014 - 3:30pm Raleigh N.C.

TBA

Thursday, October 9, 2014 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke.

TBA
Thursday, October 2, 2014 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

Wednesday, October 1, 2014 - 3:30pm Physics 128

"Superconducting Quantum Circuits"

The principles of quantum mechanics were introduced about a hundred years ago to explain the properties of microscopic systems like atoms and molecules. Recently, macroscopic systems in the form of electrical circuits containing billions of atoms have attained sufficient perfection that radiofrequency currents circulating in their wires can constitute single photons controllably exchanged with a measurement apparatus. These quantum circuits exploit both the dissipation-less character of superconductivity and the non-linearity of the Josephson effect. It is even now possible to design such macroscopic artificial atoms to perform functions unattainable with natural ones. Superconducting integrated circuits serving as quantum bits illustrate the problem of engineering a quantum electrodynamic system from top to bottom. A simple Lego-like set of three basic elements - linear capacitances, linear inductances and non-linear Josephson inductances - can be combined with almost no limitations. Can circuit architecture mitigate or even eliminate decoherence due to unavoidable defects of basic electrical components? This key question addressed to superconducting quantum bits will be discussed starting from the present entries of their "Mendeleev table".

Faculty Host: Horst Meyer.

Refreshments will be served before and after the event in room 128.

TBA
Tuesday, September 30, 2014 - 3:30pm Chapel Hill N.C.

TBA

Wednesday, September 24, 2014 - 3:30pm Physics 128

"Building with Crystals of Light and Quantum Matter: From Clocks to Computers"

Understanding the behavior of interacting electrons in solids or liquids is at the heart of modern quantum science and necessary for technological advances. However, the complexity of their interactions generally prevents us from coming up with an exact mathematical description of their behavior. Precisely engineered ultracold gases are emerging as a powerful tool for unraveling these challenging physical problems. In this talk, I will present recent developments at JILA on using atoms in crystals of light for the investigation of complex many-body phenomena and magnetism. I will also discuss a new research direction of using atomic clocks not only as precise time keepers but also as unique quantum laboratories for the investigation of new forms of matter with no known counterpart in nature.

Faculty Hosts: Profs. Haiyan Gao and Daniel Gauthier

Refreshments will be served before and after the event in room 128.

TBA
Tuesday, September 23, 2014 - 3:30pm Raleigh N.C.

TBA

Thursday, September 18, 2014 - 3:00pm Physics 119

Duke - UNC joint seminar at Duke.

Thursday, September 11, 2014 - 11:30am Physics 298

Gleb M. Akselrod (Duke University): Control of the radiative properties of emitters such as molecules, quantum dots, and color centers is central to nanophotonic and quantum optical devices, including lasers and single photon sources. Plasmonic cavities and nanoantennas can strongly modify the excitation and decay rates of nearby emitters by altering the local density of states. I will describe our work in the groups of Profs. M. Mikkelsen and D. Smith on large enhancements of fluorescence and spontaneous emission rates of molecules and quantum dots embedded in plasmonic nanoantennas with sub-10-nm gap sizes. The nanoantennas consist of colloidally synthesized silver nanocubes coupled to a metallic film, separated by a ~10 nm spacer layer with embedded molecules and quantum dots. Each nanocube resembles a nanoscale patch antenna whose plasmon resonance can be changed independent of its local field enhancement. We directly probe and control the nanoscale photonic environment of the embedded emitters including the local field enhancement, dipole orientation and spatial distribution of emitters. This enables the design and experimental demonstration of Purcell factors ~1,000 while maintaining high quantum efficiency and directional emission. Finally, I will discuss progress on coupling colloidal quantum dots to the plasmonic nanopatch antennas in order to realize a fast (<GHz) single photon source.

Thursday, September 4, 2014 - 4:00pm Chapel Hill N.C.

Location: UNC Chapel Hill, Phillips 277

The calculation of the entanglement properties of strongly
coupled many-body systems, in particular Renyi and von Neumann entropies, continues to be an active research area with many open questions. In this talk, I will outline the challenges and describe some of the advances,by my group and others, towards the characterization of entanglement innon-relativistic many-fermion systems using novel lattice Monte Carlo strategies.

Thursday, September 4, 2014 - 11:30am Physics 298

The problems of solid fractures and granular media have occupied physicists and engineers for centuries. These phenomenons are classically addressed within the framework of continuum and discrete mechanics. Still, in both cases, stress concentration at the local scale along with intrinsic microstructural heterogeneities make the observation at the global scale dramatically dependent of the very small scale. This yields peculiar statistical behaviour known as earthquake dynamics. In the case of fracture, we designed an experimental setup that allows growing well-controlled tensile cracks in brittle heterogeneous solids of tunable microstructure. This allowed us to characterize quantitatively the crackling dynamics of cracks, also to evidence intriguing statistical similarities between the seismicity associated with this simple situation (single crack under tension) and the much more complex situation of multicracking in compressive fracture and in earthquakes. In the case of granular materials, we experimentally study the deformation of a three-dimensional sphere packings subjected to macroscopic deformation. We address the non-linear force response of a disordered packing under compression, force network dynamics and explore the statistics of plastic rearrangements inside cyclically loaded packings.

Tuesday, August 26, 2014 - 3:30pm Physics Faculty Lounge

Among the discovered new hadron XYZ states, the X(3872) might be the most elusive one. In this talk, I will briefly review the possible explanations of X(3872), and put stress on its production at hadron colliders. Based on the calculation in nonrelativistic QCD, we argue that a substantial $\chi_{c1}(2P)$ component is needed to explain the large production rates of X(3872) at the Tevatron and LHC. And this is also supported by the observed large decay branching fraction of X(3872) to $\psi(2S)\gamma$, as well as the large production rate of X(3872) in B meson decays.

Monday, July 14, 2014 - 1:30pm Physics 298

The resonant-level model is a paradigmatic quantum system which serves as a basis for many other quantum impurity models. We provide a comprehensive analysis of the non-equilibrium transport near a quantum phase transition in a spinless dissipative resonant-level model [1-4]. A detailed derivation of a rigorous mapping of our system onto an effective Kondo model is presented. A controlled energy-dependent renormalization group approach [5] is applied to compute the non-equilibrium current in the presence of a finite bias voltage V. In the linear response regime V ->0, the system exhibits as a function of the dissipative strength a localized-delocalized quantum transition of the Kosterlitz-Thouless (KT) type. We address fundamental issues of the non-equilibrium transport near the quantum phase transition. We furthermore provide new signatures of the transition in the finite-frequency current noise and AC conductance via the recently developed Functional Renormalization Group (FRG) approach. Our work on dissipative resonant level has direct relevance to the experiments in a quantum dot coupled to resistive environment done at Duke, namely H. Mebrahtu et al., Nature 488, 61, (2012).

Tuesday, May 6, 2014 - 3:30pm Physics 298

We will first recall why the nucleon spin sructure is such a difficult and subtle
problem, not yet fully understood in QCD. We will then describe the quantum statistical approach to parton distributions and some recent results, in particular related to the nucleon spin structure. Future measurements are challenging to check the validity of this novel framework.

Thursday, May 1, 2014 - 3:30pm Raleigh N.C.

Primordial nucleosynthesis (or BBN) is one of the three observational evidences for the Big¿Bang model. It is very special as it involves only a dozen main nuclear reactions and because, contrary to stellar models, within the standard BBN model, the thermodynamic con- ditions can be calculated from first principles, that can be tested in that way. Hence, it is possible to accurately calculate the abundances of the produced ¿light elements¿: 4He, D, 3He and 7Li, using the baryonic density of the Universe deduced from the analysis of the Cosmic Microwave Background anisotropies (WMAP and Planck satellites). Even though they span a range of five orders of magnitude, there is indeed a good overall agreement between 4He, D and 3He primordial abundances, either deduced from observation or from BBN calculations. However, there is a tantalizing discrepancy of a factor of ¿3 between the primordial 7Li abun- dance deduced from observations of halo stars, and the BBN calculations. Solutions to this problem have been proposed, involving stellar physics (observational bias, surface depletion), non standard BBN models (variation of constants, relic particles,....), or nuclear physics (extra reactions, resonances or neutron sources). In spite of this lithium problem, BBN remains a valuable tool to probe the physics of the early Universe as it is, when we look back in time, the last milestone of known laboratory physics. It can hence be used to test deviations from standard theories.

Thursday, April 24, 2014 - 11:30am Physics 298

The electronic properties of graphene are well described by a non-interacting Dirac Hamiltonian with a fourfold symmetry associated with spin and valley, an additional degree of freedom due to the hexagonal crystal lattice of graphene. As a result, graphene exhibits a variety of peculiar phenomena such as an anomalous quantum Hall effect. At high magnetic fields, the electron kinetic energy is quenched by the Landau quantization, and Coulomb interactions become the dominant energy scale of the system. This results in a variety of new electronic phases, whose ground states depend on the competition between symmetry breaking interactions.
Observing these so-called fractional quantum Hall (FQH) phases is challenging in graphene because potential fluctuations induced by disorder blur out transport signatures of FQH states. I will describe fabrication techniques to overcome these difficulties and obtain devices with carrier mobility exceeding one million cm2/Vs in FET or bipolar geometries. This quality allows for the observation of a plethora of fractional quantum Hall phases at filling factors following the composite fermion theory. The sequence of fractions, as well as the magnetic field dependence of their activation gaps, informs us about the spin and valley polarization of the ground state in each phase.

Wednesday, April 23, 2014 - 3:30pm Physics 128

"Dripping, Jetting, Drops and Wetting: The Magic of Microfluidics"

This talk will discuss the use of microfluidic devices to precisely control the flow and mixing of fluids to make drops, and will explore a variety of uses of these drops. These drops can be used to create new materials that are difficult to synthesize with any other method. These materials have great potential for use for encapsulation and release and for drug delivery and for cosmetics. I will also show how the exquisite control afforded by microfluidic devices provides enabling technology to use droplets as microreactors to perform reactions at remarkably high rates using very small quantities of fluids. I will demonstrate how this can be used for new fundamental and technological applications.

Hosted by the Duke University Physics and Chemistry Departments The Duke University Chapter of Sigma Xi

Faculty Host: Bob Behringer

Refreshments will be served after the event in room 128

Tuesday, April 22, 2014 - 2:50pm Physics 119

The stiffness of cells is commonly assumed to depend on the stiffness of their surrounding: bone cells are much stiffer than neurons, and each exists in surrounding tissue that matches the cell stiffness. In this talk, I will discuss new measurements of cell stiffness, and show that that cell stiffness is strongly correlated to cell volume. This affects both the mechanics and the gene expression in the cell, and even impacts on the differentiation of stem cells.

Tuesday, April 22, 2014 - 9:15am Physics 298

Ultimately, every quantum system of interest is coupled to some form of environment which leads to decoherence. Until our recent study, it was assumed that, as long as the environment is memory-less (i.e. Markovian), the temporal coherence decay is always exponential-- to such a degree that this behavior was synonymously associated with decoherence. However, the situation can change if the system itself is a many-body system. In this case, the interplay between dissipation and internal interactions gives rise to a wealth of novel phenomena. In particular, we have discovered recently that the coherence decay can change to a power law.
After recapitulating the mathematical framework and basic notions of decoherence, I will discuss an open XXZ chain for which the decoherence time diverges in the thermodynamic limit. The coherence decay is then algebraic instead of exponential. In contrast, decoherence in the open transverse-field Ising model is found to be always exponential. In this case, the internal interactions can both facilitate and impede the environment-induced decoherence. The results are based on quasi-exact simulations using a matrix product representation of the density operator (time-dependent density matrix renormalization group) and explained on the basis of perturbative treatments.
Reference: Z. Cai and T. Barthel, PRL 111, 150403 (2013)

Monday, April 21, 2014 - 3:00pm Physics 128

** Please note this event is on MONDAY not Wednesday. ** The non-locality of quantum many-body systems, and hence their information content, can be quantified by entanglement measures. For ground states of condensed matter systems, I will discuss how the entanglement scales with the subsystem size, and how it behaves under time-evolution after a sudden change of system parameters (quench). The available number of degrees of freedom in a quantum many-body system grows exponentially with the system size. However, the scaling behavior of the entanglement indicates that the quantum states of interest exhaust only a much smaller number of effective degrees of freedom. This is exploited in non-perturbative simulation techniques based on so-called tensor network states, which are a way to parametrize relevant effective degrees of freedom and are particularly valuable for strongly-correlated regimes. I will describe how this approach can be employed to simulate systems of all particle statistics in order to study ground states, thermal states, and non-equilibrium phenomena. Besides explaining the main ideas, I will highlight applications of the techniques to quantum magnets and ultra-cold atomic gases in some of my projects and outline further plans and ideas. Faculty Host: Harold Baranger. Refreshments will be served after the event in room 128.

Friday, April 18, 2014 - 4:00pm Bostock Library 023

This course, which has as a prerequisite "Introduction to Unix" offered March 24&26 (or equivalent experience), provides an introduction to scientific computing using the Python programming language. The course covers basic data types, data structures, control flow statements, and commonly used functions from the Python standard library. We will also touch on popular third party libraries that provide facilities for efficient mathematical and statistical function, data visualization and plotting, and domain specific tasks (e.g. bioinformatics, image processing). In addition to serving as an introduction to scientific programming, this course will discuss guidelines and tools that help investigators to adhere to principles of reproducible research when carrying out computational and statistical analyses. Registration required. Please use this link: http://sites.duke.edu/researchcomputing/2014/02/25/introduction-to-scien... to access the registration form and get more information.

Thursday, April 17, 2014 - 11:00am Physics 278

We will discuss a correlation seen between the dark matter content and the ellipticity of elliptical galaxies. The analysis method for this investigation will be described and the origin of the correlation -whether it is physical or an observational/methodological bias- will be discussed. If of physical origin, the correlation found would imply that at equal luminosities, rounder medium-size elliptical galaxies appear to contain less dark matter than flatter elliptical galaxies. This would be puzzling in the context of the conventional model of cosmological structure formation. We will conclude by discussing a possible scenario explaining such correlation.

Wednesday, April 16, 2014 - 3:30pm Physics 128

For important biological functions such as wound healing, embryonic development, and cancer tumorogenesis, cells must initially rearrange and move over relatively large distances, like a liquid. Subsequently, these same tissues must undergo buckling and support shear stresses, like a solid. Our work suggests that biological tissues can accommodate these disparate requirements because the tissues are close to glass or jamming transition. This is important because most existing studies of disease focus on single-cell motility, but in glassy tissues the dominant contributions to cell migration come from collective effects and constraints imposed by neighbors. I will discuss a new theoretical framework for predicting rates of cell migration in epithelial (skin) cell layers, and explain how similar models predict surface tension in tissues and cell shape changes that generate left-right asymmetry in embryos. I will also discuss our current work to predict how cancer cells migrate through dense tissues and understand how active cell processes (such as cell polarization) alter the physics of glasses.

Faculty Host: Robert Behringer.

Refreshments will be served after the event in room 128.

Tuesday, April 15, 2014 - 3:30pm Physics 298

Discovering a new state of matter is always a major event for the scientific community. As the science moves forward, those new states are always more difficult to discover andand need increasing effort to be understood. The field of nuclear physics does not make exception to this rule: more then 30 years ago, it was predicted that a so-called quark-gluon plasma could be achieved in ultra-relativistic heavy ion collisions, the prominent feature of this statebeing that the fundamental constituents of hadrons would to be deconfined. Since then, its quest has generated a considerable interest both from the experimental and theoretical viewpoints.Nevertheless, many of its properties are still only partly understood. One of the main difficulties encountered when aiming at characterizing such QGP state is its very short lifetime, of the order of 10 fm/c. This obviously forbids the use of external probes. One has to rely on particles produced within the QGP itself to characterize its features and quantify its properties.  In this talk, we will concentrate on what we could learn from heavy quarks observables, like the production of D and B mesons or J/psi and upsilons. In particular, we will focus on the question of the quenching of open heavy flavours.

Tuesday, April 15, 2014 - 11:30am Physics 298

SrCu2(BO3)2 (SCBO) has corner-sharing Cu2+ spin-1/2 dimers lying on a square lattice, corresponding to the two-dimensional Shastry-Sutherland model. I will present the results of the first single-crystal neutron scattering measurements at high pressure at the Spallation Neutron Source, which combined with computer modeling, show how true antiferromagnetic order can emerge with a crossover into the third dimension. We find a subtle symmetry change as a function of temperature for pressures above 4.5 GPa, linked to antiferromagnetism and the tilting of the dimers out of the plane. The inclusion of Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland Hamiltonian helps explain the observations. Additionally, interest has focused on finite magnetization of SCBO at low temperatures, wherein the lowest energy of the three triplet states is driven to zero energy. This is related to Bose-Einstein condensation of the triplet excitations, which occurs for fields near 20 T and higher. At higher magnetic fields, plateaus have been observed in the magnetization, which have been interpreted in terms of preferred filling of the singlet ground state with increasing densities of triplet excitations. I will present how pressure can be used as tuning parameter to drive the system across the quantum phase diagram and study the evolution of the quantized magnetic plateaus. Host: Harold Baranger

Monday, April 14, 2014 - 3:00pm Physics 128

** Please note this event is on MONDAY not Wednesday. **

The interactions that define how spins arrange themselves in a material play a fundamental role in a wide variety of physical phenomena from quantum magnetism to quantum critical phenomena to exotic superconductivity. My talk will build on one of the first problems encountered in elementary quantum mechanics - the description of a system containing two spin 1/2 identical particles - asking how a collection of these spins forms an ordered state. The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a two-dimensional square lattice, has played an influential role in developing this general field because it is sufficiently simple to be exactly soluble, but sufficiently rich to capture the interesting physics. In this talk, I will present high-resolution x-ray and neutron scattering studies of the physical realization of the Shastry-Sutherland model, SrCu2(BO3)2, as it is tuned with pressure . The ratio of the intra and inter-dimer exchange interactions in this compound is close to a quantum critical point, where the ground state is predicted to transform from a non-magnetic singlet state to magnetic entities with only short-range correlations to a full antiferromagnet as a function of the ratio of the strength of the dimer interactions. I will demonstrate how the combination of high quality single crystals, high magnetic fields, GigaPascals of pressure, high resolution spallation neutron and synchrotron x-ray measurements, as well as liquid helium temperatures, permits new insights into quantum magnets with competing ground states.

Refreshments will be served after the event in Physics 128.

Thursday, April 10, 2014 - 4:30pm Physics 278

There is a broad effort in the U.S. neutrino physics community to develop the technologies necessary to build a kiloton scale liquid argon time projection chamber (LArTPC) detector for the Long Baseline Neutrino Experiment (LBNE). Liquid argon scintillation light collection is an essential component of this R&D effort, as it can be used to determine the absolute drift time of an event, reject cosmic backgrounds, and complement TPC-based particle reconstructions. The challenge for liquid argon scintillation light collection systems is that the scintillation light is emitted with a peak wavelength in the far UV (128 nm) and cannot be directly detected by a photomultiplier tube. Instead, the photons must first be wavelength-shifted to the sensitive range of the light detection elements. In this talk we briefly review LArTPC technology using the MicroBooNE detector as our example. We then focus on recent efforts to understand and improve liquid argon scintillation light collection efficiencies, and their implications for both the neutrino and dark matter communities.

TBA
Thursday, April 10, 2014 - 3:30pm Physics 298

TBA

Wednesday, April 9, 2014 - 3:30pm Physics 128

Complex systems are characterized by an abundance of meta-stable states. To describe such systems statistically, one must understand how states are sampled, a difficult task in general when thermal equilibrium does not apply. This problem arises in various fields of science, and here I will focus on a simple example, sand. Sand can flow until one jammed configuration (among the exponentially many possible ones) is reached. I will argue that these dynamically-accessible configurations are atypical, implying that in its solid phase sand "remembers" that it was flowing just before it jammed. As a consequence, it is stable, but barely so. I will argue that this marginal stability answers long-standing questions both on the solid and liquid phase of granular materials, and will discuss tentatively the applicability of this idea to other systems.

Faculty Host: Bob Behringer.

Refreshments will be provided after the event in room 128.

Wednesday, April 9, 2014 - 3:30pm Raleigh N.C.

The apparent finding of a 125-GeV light Higgs boson closes unitarity of the minimal Standard Model (SM), that is weakly interacting: this is an exceptional feature not generally true if new physics exists beyond the mass gap found at the LHC up to 700 GeV. Such new physics induces departures of the low-energy dynamics for the minimal electroweak symmetry-breaking sector with three Goldstone bosons (equivalent to longitudinal W bosons) and one light scalar from the SM couplings. We calculate the scattering amplitudes among these four particles and their partial-wave projections in effective theory. For this we employ the Electroweak Chiral Lagrangian extended by one light scalar and carry out the complete one-loop computation at high energy including the counterterms needed for perturbative renormalization, of dimension eight. For most of parameter space, the scattering is strongly interacting (with the SM a remarkable exception). We therefore explore various unitarization methods, find and study a natural second sigma-like scalar pole of the W_L W_L amplitude, and also map out how additional new resonances in these scattering amplitudes correlate with the parameters of the low-energy Lagrangian density. Based on arXiv:1308.1629 (JPG, in press) and arXiv:1311.5993 in collaboration with Rafael L. Delgado and Antonio Dobado.

Monday, April 7, 2014 - 3:00pm Physics 128

** Please note this event is on MONDAY not Wednesday and will be held at 3:00pm not 3:30pm. **

Many complex mesoscopic systems, ranging from synthetic colloids to active biological cells, exhibit a rich variety of pattern-forming behavior. In this talk, I will show you how anisotropy in two model systems, anisotropic shaped colloids and bacterial communities, affect complex pattern formation. During the directed self-assembly of colloidal systems, shape anisotropy can greatly influence resulting structures. We have developed a technique called roughness controlled depletion attraction which allows us to prove the phase space of 2D Brownian systems for a variety of anisotropic shapes such as triangles, squares, and other polygons. We have discovered several unanticipated effects, such as local chiral symmetry breaking in a triatic liquid crustal phase of uniform triangles. Anisotropy also plays a large role in the formation of bacterial communities called biofilms. Biofilms are a major human health hazard as well as being an impediment in many industrial and medical settings. By using soft condensed matter techniques, we present for the first time the dynamics of colony formation at early stages of biofilm development for Pseudomonas aeruginosa. We found that Pseudomonas aeruginosa does not follow an isotropic random walk as commonly assumed, but instead obeys a new form of polysaccharide-guided dynamics such that the distribution of surface visitation follows a power law. This power law behavior may benefit bacteria social organization during biofilm formation.

Refreshments will be served after the event in room 128.

Wednesday, April 2, 2014 - 3:30pm Physics 128

Whether it's for applications that exploit the ultra-low energy scales, sensitivity, or complexity of quantum systems, quantum mechanics will play an ever increasing role in engineering. In the past decade, the nascent field of quantum engineering has produced quite good devices and clearer proposals for high level operations. What's less clear is what happens in between, in the realm of several interacting, modular quantum devices. In my opinion, tackling this regime will require finding quantum generalizations to electrical engineering concepts and techniques. For example, just as one often uses Kirchhoff's laws rather than Maxwell's equations to analyze electrical circuits, what approximations to quantum electrodynamics are needed to understand networks of quantum devices and fields? I will summarize my efforts to further this engineering perspective on quantum optical, superconducting microwave and mechanical systems. As broad overview of my work, this talk will touch on the quantum switching of a single-atom optical nonlinearity, the design of all-quantum feedback circuits, and fully coherent and lossless superconducting microwave networks for sequential logic and state squeezing. Refreshments will be served after the Colloquium in room 128.

Tuesday, April 1, 2014 - 3:30pm Raleigh N.C.

I will report a complete lattice calculation of the quark and glue components of the proton momentum and angular momenta. Preliminary results on the quark spin contribution from the anomalous Ward identity will also be reported. Hadron mass can be decomposed in terms of the quark kinetic energy, quark condensate, glue component and the trace anomaly. Results of such a division for the pseudoscalar and vector mesons from light to charm quarks will be presented.

Tuesday, April 1, 2014 - 11:30am Physics 298

Spin liquid states are ground states of quantum spin systems that do not spontaneously break any global symmetry. In the last decade tremendous progresses have been made in searching for spin liquid states in real materials. So far three different kinds of spin liquid materials all show very similar yet very exotic phenomena in experiments: these systems have metallic specific heat and spin susceptibility despite the fact that they are all insulators! We propose a universal spin liquid state that can explain all the major universal experimental facts of these materials, and we demonstrate that this spin liquid state has a very competitive energy with a realistic spin Hamiltonian. Predictions are made based on our theory that can be checked by future experiments.
Host: Albert Chang

Thursday, March 27, 2014 - 11:30am Physics 298

Some of the simplest systems accessible to experiments with ultracold gases in optical lattices are dimers: atoms in a double-well optical lattice, or atoms in a single optical trap, but with two interacting spin states. These systems are very accurately represented by the Bose-Hubbard dimer. A quantum model with many degrees of freedom, the Bose-Hubbard dimer can be approximated by classical equations of motion for just two variables, z, the imbalance in the two wells' atomic populations, and phi, the wells' relative phase. We study how much of the quantum system's behavior is captured by this simple classical picture. Surprisingly, the classical model not only predicts the dynamics of z and phi, but also contains information about the entanglement of the modes. It can therefore be used to shed light on the counterintuitive technique of enhancing entanglement though controlled dissipation. Further features of the quantum model can be recovered through semiclassical quantization of the equations of motion. This approach allows us to obtain closed-form, nonperturbative estimates of the tunneling rate between the modes.
Hosts: Harold Baranger and Josh Socolar

Wednesday, March 26, 2014 - 1:30pm Teer 106

This two-day / six-unit course will provide students with a basic introduction to Linux and Unix systems in use in many of the biological and computational research departments around campus. Attendees will have access to a Linux computational server to practice various tasks and perform labs in order to familiarize themselves with the environment. The class materials will cover a variety of tasks from those often considered simple, such as logging in, through more advanced tasks like building an application. The course includes lectures, informal Q & A, and hands-on activities/labs. Registration required. Please use this link: http://sites.duke.edu/researchcomputing/2014/02/25/introduction-to-unix-... to access the registration form and get more information.

Tuesday, March 25, 2014 - 3:30pm Physics 298

It is shown that the acoustic scaling patterns of anisotropic flow for different event shapes at a fixed collision centrality (shape-engineered events), provide robust constraints for the event-by-event fluctuations in the initial-state density distribution from ultrarelativistic heavy ion collisions. The empirical scaling parameters also provide a dual-path method for extracting the specific shear viscosity eta/s of the quark-gluon plasma (QGP) produced in these collisions. A calibration of these scaling parameters via detailed viscous hydrodynamical model calculations, gives eta/s estimates for the plasma produced in collisions of Au+Au at RHIC and Pb+Pb at LHC. The estimates are insensitive to the initial-state geometry models considered.

Tuesday, March 25, 2014 - 11:30am Physics 298

The discovery of topological band insulators has created a revolution in condensed matter physics. We generalize the essential idea of band inversion and symmetry protection to experimentally feasible superconducting systems with time-reversal symmetry. When such a one-dimensional system becomes topological nontrivial, a Majorana Kramers pair appears on the boundary, producing quantized tunneling conductance plateaus and unprecedented fractional Josephson effects. The latter effects have two significant implications: (i) the existence of a "periodic building" unifying all the free-fermion topological systems and (ii) the possibility of fractionalization in superconductors.

Monday, March 24, 2014 - 3:00pm Physics 128

** Please note this event is on MONDAY not Wednesday and will be held at 3:00pm not 3:30pm. ** "From Topological Insulators to Majorana Fermions" - The discovery of topological insulators has created a revolution in condensed matter science that has far ranging implications over coming decades. I will introduce a simple way to understand the essential ideas of band inversion and symmetry protection. I will then apply these ideas to insulators, semimetals, and superconductors. In the superconductor case, Majorana fermion(s) may appear on the boundary and induces fractional Josephson effects. All these topological aspects in solid-state systems can be fit into an elegant "periodic building", with the Kitaev table being its ground floor. Experimental signatures, potential applications, and future directions will be discussed. Refreshments will be served after the event in room 128.

Tuesday, March 18, 2014 - 3:30pm Physics 298

Hadronic many-body theory predicts a strong broadening of the rho-meson spectral
function in hot and dense matter, leading to a melting of its resonance structure
as the pseudo-critical temperature is approached from below. Pertinent calculations
of thermal dilepton spectra in heavy-ion collisions, which additionally include
radiation from the quark-gluon plasma phase, are largely consistent with experimental
measurements which now cover a rather large range of collision energies, from SPS to
RHIC. The main part of this talk is devoted to analyzing the implications of this
scenario for the long-standing question of chiral symmetry restoration. Toward this
end, a combination of QCD and chiral Weinberg sum rules is utilized with inputs from
lattice-QCD for in-medium condensates and order parameters. Rather stringent
constraints on the a1(1260) spectral function - the chiral partner of the rho - are
deduced. Solutions are found which satisfy the temperature-dependent sum rules
accurately, thus suggesting that the rho melting scenario is compatible with chiral
restoration.

Tuesday, March 11, 2014 - 3:30pm Physics 298

Perturbative QCD is a powerful tool for calculating the properties of jets at the LHC. However, there are many jet observables for which non-perturbative input from QCD is needed. In this talk, I present three case studies at the boundary between perturbative and non-perturbative QCD---ratio observables, track-based measurements, and hadronization effects---all of which are relevant for new physics searches at the LHC.

Thursday, February 27, 2014 - 11:45am Physics 128

In nature real systems are coupled to a large number of macroscopic degrees of freedom which play an important role in determining their phase coherence. To understand the role of the environment it is customary to begin with a simple model of a qubit (two level system) coupled with an infinite number of quantum oscillators (bosons). While the weak coupling limit of this model is well understood by using perturbative approaches, a complete analytical theory beyond the perturbation theory still needs to be addressed. In this work we present a generalized variational coherent state ansatz for the ground state of the qubit-photon system, which is supported by constructing quantum tomography of the states using Numerical Renormalization Group calculations. We show that at strong coupling the ground state wave-function of the joint spin-boson system is highly entangled with emerging non-adiabatic features (Schrodinger cat like states of the environment). The Wigner distributions of the bosonic wave-function projected in different spin sectors support this strongly non-adiabatic nature of the wave-function. Furthermore, we calculate the entanglement entropy of the spin and a single bosonic mode subsystem. The joint entropy shows a peak structure around the Kondo scale, which further confirms the non-polaronic effect in the ground state wave-function. Host: Harold Baranger

Tuesday, February 25, 2014 - 3:30pm Physics 298

In the past ten years, the study of scattering amplitudes in quantum field theory has led to a revolutionary reformulation of the subject. This revolution began with the discovery, in 2005, of a recursive expansion for scattering amplitudes (to leading order) in terms of planar, trivalent, two-colored graphs---called "on-shell diagrams." Around the same time that these diagrams were first drawn by physicists, they also started to appear in the mathematical literature (for entirely independent reasons) in the context of what is known as the "positroid stratification" of Grassmannian manifolds. Recently, these two independent lines of research came together, leading to many valuable insights on both sides. In my talk, I will outline the physical ideas behind these developments, and explain the many deep connections which have been found between scattering amplitudes and the geometry and combinatorics of the positroid stratification of the Grassmannian.

Thursday, February 20, 2014 - 11:30am Physics 128

To circumvent the limitations of conventional computers in tackling complex physical processes, Richard Feynman proposed nearly thirty years ago a means of using well-understood quantum systems called quantum simulators (or quantum emulators) to emulate similar, but otherwise poorly understood, quantum systems. Among the various physical systems that could be used to build a quantum simulator, one possibility is the use of regular arrays of atoms or ions that are held in place by laser fields. In this talk, we describe how a quantum simulator is also possible through photons propagating through a nonlinear optical waveguide and interacting with cold atomic ensemble placed inside the fiber. Host: Albert Chang

Wednesday, February 19, 2014 - 3:30pm Physics 128

*CANCELLED** "Giant Impact Models of Lunar Origin" - Nearly all recent work on lunar origin has focused on the giant impact theory, which proposes that the collision of a planet-sized body with the forming Earth produced a disk of debris that later accumulated into the Moon. The impact theory is strongly favored because it provides a natural explanation for the Moon's lack of a large iron core and the Earth's rapid initial rotation rate. However impacts capable of producing a lunar-sized Moon typically produce a disk of material derived from the impactor rather than from the Earth. This would most naturally produce a Moon whose composition differed from that of the Earth's mantle. Instead, the silicate Earth and the Moon are compositionally indistinguishable in multiple respects. I will describe current giant impact models, which are studied through 3D hydrodynamical simulations of planet-planet collisions. "Canonical" impacts involving a Mars-size impactor can explain the current angular momentum of the Earth-Moon system, but require post-impact mixing between the disk and the Earth to explain the similar compositions of the Earth and Moon. "High angular momentum" impacts can produce a disk with the same composition as the Earth's mantle, but require a gravitational resonance with the Sun to subsequently alter the spin rate of the Earth. Faculty Host: Horst Meyer. Dr. Canup will also give the Hertha Sponer Lecture on Thursday, February 20, sponsored by the President's Office

Tuesday, February 18, 2014 - 3:30pm Physics 298

I introduce light-cone physics as a large momentum effective field theory (LMEF). This notion allows formulating Euclidean lattice calculations of parton physics which is otherwise considered impossible.

Monday, February 17, 2014 - 3:30pm Physics 128

** Please note this event is for Monday not Wednesday. **

"Collective Dynamics of Laboratory Insect Swarms"

Self-organized collective animal behavior--in swarms, flocks, schools, herds, or crowds--is ubiquitous throughout the animal kingdom. In part because it is so generic, it has engaged and fascinated scientists from many disciplines, from biology to physics to engineering. But despite this broad interest, little empirical data exists for real animals; modelers have therefore been forced to settle for only qualitative large-scale information or to make ad hoc assumptions about the low-level inter-individual interactions. To address this dearth of data, we have conducted a laboratory study of swarms of the non-biting midge Chironomus riparius. Using multicamera stereoimaging and three-dimensional particle tracking, we measure the trajectories and kinematics of each individual insect in the swarm, and study their statistics and interactions. I will give an overview of our measurements, including the statistical mechanics of the swarm as a whole and the behavior of individual insects, and will discuss some of the implications of our results for modeling.

Faculty Host: Bob Behringer

Refreshments will be served after the Colloquium in room 128.

Thursday, February 13, 2014 - 11:30am Physics 128

This event has been postponed due to the winter weather. It will be rescheduled later this semester. Spin liquid states are ground states of quantum spin systems that do not spontaneously break any global symmetry. In the last decade tremendous progresses have been made in searching for spin liquid states in real materials. So far three different kinds of spin liquid materials all show very similar yet very exotic phenomena in experiments: these systems have metallic specific heat and spin susceptibility despite the fact that they are all insulators! We propose a universal spin liquid state that can explain all the major universal experimental facts of these materials, and we demonstrate that this spin liquid state has a very competitive energy with a realistic spin Hamiltonian. Predictions are made based on our theory that can be checked by future experiments. Host: Harold Baranger

Tuesday, February 11, 2014 - 3:30pm Physics 298

We discuss results on the Polyakov loop susceptibilities in SU(3)
lattice gauge theory. The longitudinal and transverse fluctuations of the
Polyakov loop, as well as, that of its absolute value will be introduced
in the context of the confinement-deconfinement phase transition.
We will indicate the influences of fermions
on the Polyakov loop fluctuations, based on lattice calculations in 2- and
(2+1)-flavors QCD. We show, that ratios of different susceptibilities of
the Polyakov loop are excellent probes of critical behavior. We will
formulate an effective model for the
Polyakov loop coupled to fermions and discus its applications to QCD
thermodynamics. We emphasize the role of fluctuations to fully explore
properties of QCD in the limit of heavy flavours.

Friday, February 7, 2014 - 2:30pm Physics 278

In March 2012, the Double Chooz reactor neutrino experiment published its most precise result so far: sin22¿13 = 0.109 ± 0.030(stat.) ± 0.025(syst.). The statistical significance is 99.9% away from the no-oscillation hypothesis. The systematic uncertainties from background and detection efficiency are smaller than the first publication of the Double Chooz experiment. The neutron detection efficiency, one of the biggest contributions in detection systematic uncertainties, is the first part of my talk. 252Cf is used to determine the neutron detection efficiency in this study. The neutron detection efficiency from the 252Cf result is confirmed by the electron antineutrino data and Monte Carlo simulations. The seasonal variation in detector performance and the seasonal variations of the muon intensity are described in the second part of my talk. The detector stability is confirmed by observation of two phenomena: 1) the electron antineutrino rate, which is seen to be uncorrelated with the liquid scintillator temperature, and 2) the daily muon rate, which has the expected correspondence with the effective atmospheric temperature. The correlation between the muon rate and effective atmospheric temperature is further analyzed to determine the ratio of kaon to pion in the local atmosphere. Finally, the talk concludes with the potential instabilities from neutron detection efficiency and seasonal variation and estimation of how these potential instabilities affect the result of sin22¿

Wednesday, February 5, 2014 - 3:30pm Physics 128

"Tradition to Enlightenment The Evolution of Intro Physics at the University of Illinois"

About 17 years ago we significantly changed the way we teach intro physics at UIUC. The innovation, which in hindsight seems almost trivial, was to define these courses in terms of their content and infrastructure rather than in terms of the faculty assigned to teach them. Having our courses rest on a solid departmental foundation rather than on the shoulders of faculty means that faculty have the time and freedom to innovate, making incremental yet significant improvements to these courses over time. In this talk I will discuss this evolution as well as several of the resulting innovations, including prelectures, just in time teaching, peer instruction, and a new approach to labs.

Faculty Host: Glenn Edwards

Refreshments will be served after the Colloquium in room 128.

Monday, February 3, 2014 - 4:00pm Physics 298

Particle physics, as it is known today, is a union of electroweak theory and quantum chronodynamics, collectively called the standard model (SM). Despite the tremendous success of the SM, it falls short of answering some of the fundamental questions of the nature, hence, cannot be considered the final theory of particles and their interactions. More and more experiments are being designed to probe the SM at higher energies and intensities, and address its shortcomings. The Large Hadron Collider was built at CERN to provide proton-proton collisions at 14 TeV center of mass energies. It provides access to the physics that takes place on the smaller scales and higher energies than has ever been achieved in the laboratories. The high energy and intensity provided by the LHC enables us to perform studies that may not have been feasible before. I will present the first study of the helicity distributions for a Z¿ di-boson production process at hadron colliders, from the experiment that gave you the Higgs boson and put supersymmetry in coma - CMS. It is a multidimensional angular analysis of two leptons (muons or electrons) and a photon, where leptons originate from a Z boson decay, aiming to measure the helicity amplitudes that govern the process. Angular analyses, in general, are a good way to study the properties of the particles or processes, and this particular analysis may in addition provide the sensitivity to the anomalous couplings that are prohibited by the standard mode

Tuesday, January 21, 2014 - 3:30pm Physics 298

I discuss the structure of longitudinal chromomagnetic fields which
develop in heavy-ion collisions. Rather than being homogeneous, Bz is
found to exhibit domain-like structures in the transverse plane. The
expectation values of spatial Wilson loops exhibit area law scaling
for radii larger than the inverse saturation momentum, indicating
uncorrelated magnetic flux fluctuations at such scales. The
corresponding spatial string tension is approximately invariant
under a Z(2) rotation of the SU(2) Wilson loops. I discuss the
failure of a naive perturbative expansion to reproduce area law
scaling and the role of magnetic screening.

Friday, January 17, 2014 - 2:30pm Physics 278

One of the top priorities of the first run of the Large Hadron Collider has been to understand the Higgs mechanism and how it gives mass to particles. The Higgs decay to photons has been crucial for both the discovery of the new boson at 125 GeV and the measurement of its properties. The reconstruction of Higgs events relies on an excellent energy measurement of the two photons and the angle between them, which makes it deceptively simple. However, the messy environment of rapid pp-collisions and effects in the CMS detector invite significant improvements to optimize the Higgs sensitivity. This talk will focus on the key creative ideas that led to improvements in sensitivity like the use of a multivariate energy correction, using a multivariate classifier to remove background, and improved photon reconstruction using the CMS Particle Flow technique.

Thursday, January 16, 2014 - 2:00pm Physics 298

LUX (Large Underground Xenon) is a dark matter direct detection experiment deployed at the 4850' level of the Sanford Underground Research Facility (SURF) in Lead, SD, operating a 370 kg dual-phase xenon TPC. We have recently reported the results of the first WIMP search dataset, taken during the period of April to August 2013, presenting the analysis of 85.3 live-days with a fiducial volume of 118 kg. The experiment exhibited a sensitivity to spin-independent WIMP-nucleon elastic scattering with a minimum upper limit on the cross section of 7.6 x 10^-46 cm^2 at a WIMP mass of 33 GeV/c^2, establishing the best limits in the literature. This sensitivity is inconsistent with the low-mass WIMP signal interpretations of the results from several recent direct detection experiments. This talk will provide an overview of the experiment, focusing in the recent science results.

Thursday, January 16, 2014 - 11:00am Physics 278

SNO+ is a scintillator-based neutrino experiment that will be housed at the SNOLAB facility, located two km underground in Sudbury, Ontario, Canada. The SNO+ detector will be capable of exploring many new areas of neutrino physics including neutrinoless double beta decay and low energy solar neutrinos. This talk will give an overview of the SNO+ detector, will discuss the construction status of the experiment, and will outline the main physics goals that SNO+ hopes to achieve. Two areas which have been the main focus of my own research will be emphasized, namely the tagging of alpha backgrounds using pulse shape discrimination and using pep solar neutrinos as a probe for light sterile neutrinos.

Tuesday, January 14, 2014 - 3:30pm Raleigh N.C.

In this talk I will discuss my research on computational simulations of the central engines of core-collapse supernovae, the endgame of massive star evolution. The main message I hope to instill is that not all massive stars are destroyed equally, and that there is, in fact, a great diversity in the products of core collapse. In addition to a brief introduction to core-collapse supernovae, I will focus on three main areas of my research. First, I will shed some light on the question of which core collapse events are more likely to successfully explode as a supernova and which events are more likely to fail and lead the formation of a stellar mass black hole---the so-called un-novae. Second, I will present results where we extended the methods used to study core-collapse supernovae in spherical symmetry to 2D and 3D simulations with the goal of exploring the core-collapse supernova explosion mechanism. I will also present some early results of a systematic study of core collapse in two dimensions. Finally, with neutrino radiation transport simulations in spherical symmetry, I will quantitatively show how the expected neutrino signal at Earth from a Galactic or near-Galactic core-collapse supernova varies with the interior structure of massive stars. This provides a potential way for neutrino astrophysics to help constrain the poorly understood advanced burning stages of stellar evolution. I will elaborate on what is needed to elevate these neutrino predictions to a level where

Monday, December 9, 2013 - 7:00pm Raleigh N.C.

At NCSU
It has recently been realized that some studies of supersymmetric gauge theories, when properly interpreted, lead to insights whose importance transcends supersymmetry. I will illustrate the insightful nature of supersymmetry by two examples having to do with the microscopic description of the thermal deconfinement transition, in non-supersymmetric pure Yang-Mills theory and in QCD with adjoint fermions. A host of strange ``topological" molecules will be seen to be the major players in the confinement-deconfinement dynamics. Interesting connections between topology, ``condensed-matter" gases of electric and magnetic charges, and attempts to interpret the divergent perturbation series will emerge.

Monday, November 25, 2013 - 7:00pm Raleigh N.C.

Wally Melnitchouk (JLAB) [at NCSU]

Wednesday, November 20, 2013 - 7:00pm Bryan Research 103

Eric Betzig (Janelia Farm Research Campus, HHMI)

Tuesday, November 19, 2013 - 7:00pm Physics 128

Eric Betzig (Janelia Farms)

Monday, November 18, 2013 - 7:00pm Raleigh N.C.

Brad Meyer (Clemson U.) [at NCSU]

Wednesday, March 20, 2013 - 8:00pm Raleigh N.C.

Marcus Bluhm (SUBATECH, Nantes) [at NCSU]

Monday, March 18, 2013 - 8:00pm Physics 298

Debasish Banerjee (Bern U.)

Sunday, March 17, 2013 - 8:00pm Physics 298

Lance Labun (National Taiwan U.)

Tuesday, March 5, 2013 - 7:00pm Hudson Hall 125

Kang-Kuen Ni (JILA)

Monday, March 4, 2013 - 7:00pm Physics 298

Eugenio Bianchi (Perimeter Institute)