Nonlinear thermoelectricity with electron-hole symmetric systems

TL;DR

This paper demonstrates that nonlinear thermoelectric effects, including negative conductance, can occur in electron-hole symmetric systems like superconducting tunnel junctions, beyond the linear regime.

Contribution

It reveals that electron-hole symmetry breaking is not necessary for thermoelectric effects outside the linear regime, using superconducting junctions as a model.

Findings

Nonlinear thermoelectric effects can occur without explicit symmetry breaking.

A superconducting tunnel junction can exhibit negative conductance under thermal bias.

The effect is linked to spontaneous electron-hole symmetry breaking.

Abstract

In the linear regime, thermo-electric effects between two conductors are possible only in the presence of an explicit breaking of the electron-hole symmetry. We consider a tunnel junction between two electrodes and show that this condition is no longer required outside the linear regime. In particular, we demonstrate that a thermally-biased junction can display an absolute negative conductance (ANC), and hence thermo-electric power, at a small but finite voltage bias, provided that the density of states of one of the electrodes is gapped and the other is monotonically decreasing. We consider a prototype system that fulfills these requirements, namely a tunnel junction between two different superconductors where the Josephson contribution is suppressed. We discuss this nonlinear thermo-electric effect based on the spontaneous breaking of electron-hole symmetry in the system, characterize its main figures of merit and discuss some possible applications.