Thermoelectric detection of Andreev states in unconventional superconductors

TL;DR

This paper theoretically demonstrates a thermoelectric effect in superconductor-ferromagnet junctions that is sensitive to surface states, especially low-energy Andreev bound states, revealing complex pairing phenomena.

Contribution

It introduces a new thermoelectric effect driven by Andreev processes that can detect surface states in unconventional superconductors, highlighting coexistence of pairing symmetries.

Findings

Thermoelectric current reverses with low-energy Andreev bound states.

Effect arises only with coexistence of even and odd-frequency Cooper pairs.

Surface states significantly influence thermoelectric response.

Abstract

We theoretically describe a thermoelectric effect that is entirely due to Andreev processes involving the formation of Cooper pairs through the coupling of electrons and holes. The Andreev thermoelectric effect can occur in ballistic ferromagnet-superconductor junctions with a dominant superconducting proximity effect on the ferromagnet, and it is very sensitive to surface states emerging in unconventional superconductors. We consider hybrid junctions in two and three dimensions to demonstrate that the thermoelectric current is always reversed in the presence of low-energy Andreev bound states at the superconductor surface. A microscopic analysis of the proximity-induced pairing reveals that the thermoelectric effect only arises if even and odd-frequency Cooper pairs coexist in mixed singlet and triplet states. Our results are an example of the richness of emergent phenomena in systems that combine magnetism and superconductivity, and they open a pathway for exploring exotic surface states in unconventional superconductors.