Nonequilibrium electrical, thermal and spin transport in open quantum systems of topological superconductors, semiconductors and metals

TL;DR

This paper provides exact theoretical expressions and numerical validation for electrical, thermal, and spin transport in open quantum systems involving topological superconductors, semiconductors, and metals, highlighting quantized conductance phenomena near topological transitions.

Contribution

It introduces exact formulas for steady-state currents in open quantum systems with topological materials and explores their transport properties, including quantized conductance and thermoelectric effects.

Findings

Thermal conductance peaks sharply at topological phase transition.

Quantized zero-bias spin conductance in topological superconductor wires.

Large thermoelectric currents near topological transition.

Abstract

We study nonequilibrium transport in various open quantum systems whose systems and leads/baths are made of topological superconductors (TSs), semiconductors, and metals. Using quantum Langevin equations and Green's function method, we derive exact expressions for steady-state electrical, thermal, and spin current at the junctions between a system and leads. We validate these current expressions by comparing them with the results from direct time-evolution simulations. We then show how an electrical current injected in TS wires divides into two parts carried by single electronic excitations and Cooper pairs. We further show ballistic thermal transport in an open TS wire in the topological phase under temperature or voltage bias. The thermal current values grow significantly near the topological phase transition, where thermal conductance displays a sharp quantized peak as predicted earlier. We relate the quantized thermal conductance to the zero-frequency thermoelectric transmission coefficient of the open TS wire. We also observe a large thermoelectric current near the topological transition of the TS wires. We introduce a differential spin conductance which displays a quantized zero-bias peak at zero temperature for a spinful TS wire in the topological phase. The role of superconducting baths in transport is demonstrated by thoroughly examining the features of zero-temperature differential electrical conductance and thermal conductance in open systems with TS baths. Our new thermoelectric and spin transport findings in various two-terminal geometries are beneficial to the present challenges in probing the emergence of Majorana quasi-particles in experiments.