Research Topics

The broad focus of Prof. Baranger's group is the interplay of electron-electron interactions and quantum interference at the nanoscale. Fundamental interest in nanophysics - the physics of small, nanometer scale, bits of solid - stems from the ability to control and probe systems on length scales larger than atoms but small enough that the averaging inherent in bulk properties has not yet occurred. Using this ability, entirely unanticipated phenomena can be uncovered on the one hand, and the microscopic basis of bulk phenomena can be probed on the other. Additional interest comes from the many links between nanophysics and nanotechnology. Within this thematic area, our work ranges from projects trying to nail down realistic behavior in well-characterized systems, to more speculative projects reaching beyond regimes investigated experimentally to date. Currently, 5 topics are being actively pursued:
1. Kondo Effect in Nanoscale Systems
The Kondo effect is a classic of many-body physics involving the correlation of an electron in an isolated level with a bulk Fermi sea. In contrast, we consider a finite size Fermi sea and so treat the non-zero level-spacing in the lead. The relevant experimental situation is two quantum dots connected by tunneling, a very small one to supply the electron in an isolated level and a large one to act as a nanoscale Fermi sea. [with R. Kaul, D. Ullmo, and Prof. S. Chandrasekharan]
2. Molecular Electronics
We have established a state-of-the-art program to calculate the electric current through single molecules. This involved substantial program development in previous years; we are now concentrating on studying various systems. For instance, we carried out an extensive study of molecules containing cobaltocene, a sandwich molecule consisting of a Co atom between two 5-member carbon rings. Cobaltocene has spin 1/2, and manipulation of this spin strongly affects the electrical conduction. Thus we have introduced the first examples of true molecular spintronics - a spin filter, spin valve, and spin switch. [with R. Liu, S.-H. Ke, and Prof. W. Yang]
3. Quantum Computing: Decoherence in Quantum Error Correction
We focus on the effects of decoherence - processes which break the quantum mechanical coherence at the basis of this type of computation. A key question is how decoherence scales as the computer becomes larger, that is, as the number of qubits increases so that the states of the computer become increasingly more complicated entangled states. Initial pessimistic estimates were circumvented by quantum error correction, a clever encoding of a single logical qubit using several physical qubits. We are studying how decoherence due to correlated noise scales in a computer using error correction. [with E. Novais and Prof. E. Mucciolo]
4. Toward Strong Interactions in Circular Quantum Dots
The “electron gas” model of electrons in solids – in which the conduction electrons interact via Coulomb forces but the ionic potential is neglected – has been a key paradigm of solid state physics. Quantum mechanically, the physical properties change dramatically depending on the balance between the strength of the Coulomb interaction and the kinetic energy. The limiting cases are well understood: for very weak interactions the particles are delocalized while for very strong interactions they localize in a Wigner crystal. The physics at intermediate densities is surprisingly rich and remains at the forefront of research. We are studying the intermediate density electron gas confined to various nanostructures by using quantum Monte Carlo techniques. [with A. Ghosal, D. Guclu, and Prof. C. Umrigar]
5. Quantum Phase Transitions
We are studying models of strongly interacting systems in which there is a quantum (zero temperature) phase transition as a function of disorder strength. The models are chosen so that there is a cooperative many-body ground state (superconductivity or ferromagnetism), and the disorder introduces inhomogeneity through quantum interference. As the inhomogeneity in the system grows, the cooperative state is eventually killed at a quantum phase transition. Through careful study using recently developed algorithms, we identify a bosonic superconductor-insulator transition which has new critical exponents which sharply disagree with previous theoretical prejudices. [with A. Priyadarshee and Prof. S. Chandrasekharan]