Time-Loop Formalism for Irreversible Quantum Problems: Steady State Transport in Junctions with Asymmetric Dynamics

Non-unitary quantum mechanics has been used in the past to study irreversibility, dissipation and decay in a variety of physical systems. In this letter, we propose a general scheme to deal with systems governed by non-Hermitian Hamiltonians. We argue that the Schwinger-Keldysh formalism gives a natural description for those problems. To elucidate the method, we study a simple model inspired by mesoscopic physics --an asymmetric junction. The system is governed by a non-Hermitian Hamiltonian which captures essential aspects of irreversibility.Comment: 4 pages, 4 figure

Consistent bosonization-debosonization II: The two-lead Kondo problem and the fate of its non-equilibrium Toulouse point

Following the development of a scheme to bosonize and debosonize consistently [N. Shah and C.J. Bolech, Phys. Rev B 93, 085440 (2016); arXiv:1508.03078], we present in detail the Toulouse-point analytic solution of the two-lead Kondo junction model. The existence and location of the solvable point is not modified, but the calculational methodology and the final expressions for observable quantities change markedly as compared to the existent results. This solvable point is one of the remarkably few exact results for non-equilibrium transport in correlated systems. It yields relatively simple analytical expressions for the current in the full range of temperature, magnetic field and voltage. It also shows precisely, within the limitations of the Toulouse fine-tuning, how the transport evolves depending on the relative strengths of inter-lead and intra-lead Kondo exchange couplings ranging from weak to strong. Thus its improved understanding is an important stepping stone for future research.Comment: 15 pages, 6 figure

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 tunnelling process when electrons are injected into such states. Using a non-equilibrium green function formalism, we find exact analytic expressions for the tunnelling 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.Comment: 4 pages, 1 figur

Majorana fermions in an out-of-equilibrium topological superconducting wire: an exact microscopic transport analysis of a p-wave open chain coupled to normal leads

Topological superconductors are prime candidates for the implementation of topological-quantum-computation ideas because they can support non-Abelian excitations like Majorana fermions. We go beyond the low-energy effective-model descriptions of Majorana bound states (MBSs), to derive non-equilibrium transport properties of wire geometries of these systems in the presence of arbitrarily large applied voltages. Our approach involves quantum Langevin equations and non-equilibrium Green's functions. By virtue of a full microscopic calculation we are able to model the tunnel coupling between the superconducting wire and the metallic leads realistically; study the role of high-energy non-topological excitations; predict how the behavior compares for increasing number of odd vs. even number of sites; and study the evolution across the topological quantum phase transition (QPT). We find that the normalized spectral weight in the MBSs can be remarkably large and goes to zero continuously at the topological QPT. Our results have concrete implications for the experimental search and study of MBSs.Comment: 5 pages, 4 figure

Optical response in one dimensional Mott Insulators

We study the optical response of a Mott Hubbard system in the framework of the half--filled Extended Hubbard Model using the Density Matrix Renormalization Group (DMRG) method. We discuss the appearance of excitonic features inside the spectral gap as the system goes from the Spin Density Wave (SDW) to the Charge Density Wave (CDW) phase.Comment: 4 pages, 4 figure

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.Comment: 5 pages, 2 figure

Numerical Simulation of the Nernst Effect in Extreme Type-II Superconductors: A Negative Nernst Signal and its Noise Power Spectra

Recently, different transport coefficients have been measured in High-Tc superconductors to pinpoint the nature of the pseudogap phase. In particular, the thermoelectric coefficients received a considerable attention both theoretically and experimentally. We numerically simulate the Nernst effect in extreme type-II superconductors using the time-dependent Ginzburg-Landau equations. We report the sign reversal of the thermoelectric coefficient, alpha_xy, at temperatures close to the mean-field transition temperature, Tc^{MF}(H), which qualitatively agrees with recent experiments on high-Tc materials. We also discuss the noise power spectrum of alpha_xy, which shows 1/f^beta behavior. Based on this observation, we propose an experiment to distinguish among different regimes of vortex dynamics by measuring the noise correlations of the Nernst signal.Comment: 6 pages, 6 figure