Previous Physics Events

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 - 4:00pm Chapel Hill N.C.

Duke - UNC joint seminar at UNC, Philips 277.

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)