C. J. Bolech

Observing Majorana Bound States in p-wave Superconductors Using Noise Measurements in Tunneling Experiments

The zero-energy bound states at the edges or vortex cores of chiral p-wave superconductors should behave like Majorana fermions. We introduce a model Hamiltonian that describes the tunneling process when electrons are injected into such states. Using a nonequilibrium Green function formalism, we find exact analytic expressions for the tunneling current and noise and identify experimental signatures of the Majorana nature of the bound states to be found in the shot noise. We discuss the results in the context of different candidate materials that support triplet superconductivity. Experimental verification of the Majorana character of midgap states would have important implications for the prospects of topological quantum computation.

Universal Statistics of the Critical Depinning Force of Elastic Systems in Random Media

We study the rescaled probability distribution of the critical depinning force of an elastic system in a random medium. We put in evidence the underlying connection between the critical properties of the depinning transition and the extreme value statistics of correlated variables. The distribution is Gaussian for all periodic systems, while in the case of random manifolds there exists a family of universal functions ranging from the Gaussian to the Gumbel distribution. Both of these scenarios are a priori experimentally accessible in finite, macroscopic, disordered elastic systems.

Solution of the two-channel Anderson impurity model: implications for the heavy fermion UBe13.

We solve the two-channel Anderson impurity model using the Bethe-ansatz. We determine the ground state and derive the thermodynamics, obtaining the impurity entropy and specific heat over the full range of temperature. We show that the low-temperature physics is given by a line of fixed points describing a two-channel non-Fermi-liquid behavior in the integral valence regime associated with moment formation as well as in the mixed valence regime where no moment forms. We discuss the relevance for the theory of UBe13.

Universal out-of-equilibrium transport in Kondo-correlated quantum dots: renormalized dual fermions on the Keldysh contour.

The nonlinear conductance of semiconductor heterostructures and single molecule devices exhibiting Kondo physics has recently attracted attention. We address the observed sample dependence of the measured steady state transport coefficients by considering additional electronic contributions in the effective low-energy model underlying these experiments that are absent in particle-hole symmetric setups. A novel version of the superperturbation theory of Hafermann et al. in terms of dual fermions is developed, which correctly captures the low-temperature behavior. We compare our results with the measured transport coefficients.

Expansion of 1D polarized superfluids: the Fulde-Ferrell-Larkin-Ovchinnikov state reveals itself.

We study the expansion dynamics of a one-dimensional polarized Fermi gas after its sudden release from confinement using both the mean-field Bogoliubov-dexa0Gennes and the numerically unbiased time-evolving block decimation methods. Our results show that experimentally observable spin density modulations, directly related to the presence of a Fulde-Ferrell-Larkin-Ovchinnikov state, develop during the expansion of the cloud. Our work therefore provides a robust theoretical proposal for the detection of this long-sought state.

Point-contact tunneling involving low-dimensional spin-triplet superconductors.

We modify and extend previous microscopic calculations of tunneling in superconducting junctions based on a nonequilibrium Green function formalism to include the case of spin-triplet pairing. We show that distinctive features are present in the I-V characteristics of different kinds of junctions, in particular, when the effects of magnetic fields are taken into account, that permit to identify the type of pairing. We discuss the relevance of these results in the context of quasi-one-dimensional organic superconductors such as (TMTSF)2PF6 and layered compounds like Sr2RuO(4).

Long-time behavior of the momentum distribution during the sudden expansion of a spin-imbalanced Fermi gas in one dimension.

We study the sudden expansion of spin-imbalanced ultracold lattice fermions with attractive interactions in one dimension after turning off the longitudinal confining potential. We show that the momentum distribution functions of majority and minority fermions quickly approach stationary values due to a quantum distillation mechanism that results in a spatial separation of pairs and majority fermions. As a consequence, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) correlations are lost during the expansion. Furthermore, we argue that the shape of the stationary momentum distribution functions can be understood by relating them to the integrals of motion in this integrable quantum system. We discuss our results in the context of proposals to observe FFLO correlations, related to recent experiments by Liao et al., Nature (London) 467, 567 (2010).

Prediction of the capacitance line shape in two-channel quantum dots.

We propose a setup to realize two-channel Kondo physics using quantum dots. We discuss how the charge fluctuations on a small dot can be accessed by using a system of two single-electron transistors arranged in parallel. We derive a microscopic Hamiltonian description of the setup that allows us to make the connection with the two-channel Anderson model (of extended use in the context of heavy-fermion systems) and in turn make detailed predictions for the differential capacitance of the dot. Its line shape, which we determined precisely, shows a robust behavior that should be experimentally verifiable.

Breakdown of the local density approximation in interacting systems of cold fermions in strongly anisotropic traps

We consider spin-polarized mixtures of cold fermionic atoms on the BEC side of the Feshbach resonance. We demonstrate that a strongly anisotropic confining potential can give rise to a double-peak structure in the axial distribution of the density difference and a polarization-dependent aspect ratio of the minority species. Both phenomena appear as a result of the breakdown of the local density approximation for the phase-separated regime. We speculate on the implications of our findings for the unitary regime.

Mapping of the anisotropic two-channel Anderson model onto a Fermi-Majorana biresonant level model.

We establish the correspondence between an extended version of the two-channel Anderson model and a particular type of biresonant level model. For certain values of the parameters the new model becomes quadratic. We calculate in closed form the entropy and impurity occupation as functions of temperature and identify the different physical energy scales of the problem. We show how, as the temperature goes to zero, the model approaches a universal line of fixed points non-Fermi liquid in nature.