Transport in spinless superconducting wires

We consider electron transport in a model of a spinless superconductor described by a Kitaev-type lattice Hamiltonian where the electron interactions are modeled through a superconducting pairing term. The superconductor is sandwiched between two normal metals that are kept at different temperatures and chemical potentials and that are themselves modeled as noninteracting spinless fermions. For this setup we compute the exact steady-state properties of the system using the quantum-Langevin-equation approach. Closed-form exact expressions for particle current, energy current, and other two-point correlations are obtained in the Landauer-type forms and involve two nonequilibrium Green's functions. The current expressions are found to be the sum of three terms having simple physical interpretations. We then discuss a numerical approach where we construct the time evolution of the two-point correlators of the system from the eigenspectrum of the complete quadratic Hamiltonian describing the system and leads. By starting from an initial state corresponding to the leads in thermal equilibrium and the system in an arbitrary state, the long-time solution for the correlations, before recurrence times, gives us steady-state properties. We use this independent numerical method for verifying the results of the exact solution. We also investigate analytically the presence of high-energy bound states and obtain expressions for their contributions to two-point correlators. As applications of our general formalism we present results on thermal conductance and on the conductance of a Kitaev chain with next-nearest-neighbor interactions which allows topological phases with different winding numbers.