Reentrant superconductivity in conical-ferromagnet/superconductor nanostructures

TL;DR

This paper investigates reentrant superconductivity in a bilayer of superconductor and spiral magnetic material, revealing temperature-induced transitions between superconducting and normal states with unique thermodynamic properties.

Contribution

It provides a self-consistent analysis of reentrant superconductivity in superconductor/spiral-magnet bilayers using Bogoliubov-de Gennes equations, highlighting temperature-driven phase transitions.

Findings

Superconductivity reenters as temperature decreases within certain magnetic layer thicknesses.

The system exhibits a strictly reentrant transition with identical low and high temperature phases.

Entropy analysis shows superconducting state can be less ordered than the normal state.

Abstract

We study a bilayer consisting of an ordinary superconductor and a magnet with a spiral magnetic structure of the {\rm Ho} type. We use a self consistent solution of the Bogolioubov-de Gennes equations to evaluate the pair amplitude, the transition temperature, and the thermodynamic functions, namely, the free energy and entropy. We find that for a range of thicknesses of the magnetic layer the superconductivity is reentrant with {\it temperature} $T$: as one lowers $T$ the system turns superconducting, and when $T$ is further lowered it turns normal again. This behavior is reflected in the condensation free energy and the pair potential, which vanish both above the upper transition and below the lower one. The transition is strictly reentrant: the low and high temperature phases are the same. The entropy further reveals a range of temperatures where the superconducting state is {\it less ordered} than the normal one.