Spin caloritronics with superconductors: Enhanced thermoelectric effects, generalized Onsager response-matrix, and thermal spin currents

TL;DR

This paper demonstrates that tunneling between spin-split superconductors significantly enhances thermoelectric effects and thermal spin currents, surpassing bulk materials and offering tunable spin caloritronic functionalities.

Contribution

It introduces a theoretical framework showing enhanced thermoelectric response and thermal spin currents in superconductor junctions with spin-dependent particle-hole asymmetry, extending Onsager relations.

Findings

Seebeck coefficients exceeding 1 mV/K

Figures of merit around 40, surpassing bulk thermoelectrics

Thermal spin currents tunable by external parameters

Abstract

It has recently been proposed and experimentally demonstrated that it is possible to generate large thermoelectric effects in ferromagnet/superconductor structures due to a spin-dependent particle-hole asymmetry. Here, we theoretically show that quasiparticle tunneling between two spin-split superconductors enhances the thermoelectric response manyfold compared to when only one such superconductor is used, generating Seebeck coefficients ($\mathcal{S} > 1$ mV/K) and figures of merit ($ZT \simeq 40$) far exceeding the best bulk thermoelectric materials, and also becomes more resilient toward inelastic scattering processes. We present a generalized Onsager response-matrix which takes into account spin-dependent voltage and temperature gradients. Moreover, we show that thermally induced spin currents created in such junctions, even in the absence of a polarized tunneling barrier, also become largest in the case where a spin-dependent particle-hole asymmetry exists on both sides of the barrier. We determine how these thermal spin currents can be tuned both in magnitude and sign by several parameters, including the external field, temperature, and the superconducting phase-difference.