Mystery of Excess Low Energy States in a Disordered Superconductor in a Zeeman Field

TL;DR

This paper investigates the unexpected excess of low-energy states in disordered superconducting aluminum films under high Zeeman fields, proposing a novel disordered Larkin-Ovchinnikov phase as an explanation.

Contribution

It introduces a new disordered LO phase near the spin-paramagnetic transition, explaining subgap states through domain walls with magnetization and Andreev bound states.

Findings

Identification of subgap states not explained by BCS theory

Proposal of a disordered LO phase with sign-changing pairing amplitude

Spectral signatures linked to domain walls and Andreev bound states

Abstract

Tunneling density of states measurements of disordered superconducting (SC) Al films in high Zeeman fields reveal a significant population of subgap states which cannot be explained by standard BCS theory. We provide a natural explanation of these excess states in terms of a novel disordered Larkin-Ovchinnikov (dLO) phase that occurs near the spin-paramagnetic transition at the Chandrasekhar-Clogston critical field. The dLO superconductor is characterized by a pairing amplitude that changes sign at domain walls. These domain walls carry magnetization and support Andreev bound states, which lead to distinct spectral signatures at low energy.