Supra-oscillatory critical temperature dependence of Nb-Ho bilayers

TL;DR

This study explores how the critical temperature of Nb-Ho bilayers oscillates with Ho layer thickness, revealing complex magnetic interactions and multiple superconducting transitions influenced by Ho's inhomogeneous magnetization.

Contribution

It provides a detailed analysis of Tc oscillations in Nb-Ho bilayers, incorporating singlet and triplet correlations, and explains the multiple transitions related to Ho's magnetic structure.

Findings

Multiple Tc oscillations with Ho thickness

Presence of singlet and triplet pair correlations

Two-step transition to superconductivity

Abstract

We investigate the critical temperature Tc of a thin s-wave superconductor (Nb) proximity coupled to a helical rare earth ferromagnet (Ho). As a function of the Ho layer thickness, we observe multiple oscillations of Tc superimposed on a slow decay, that we attribute to the influence of the Ho on the Nb proximity effect. Because of Ho inhomogeneous magnetization, singlet and triplet pair correlations are present in the bilayers. We take both into consideration when solving the self consistent Bogoliubov-de Gennes equations, and we observe a reasonable agreement. We also observe non-trivial transitions into the superconducting state, the zero resistance state being attained after two successive transitions which appear to be associated with the magnetic structure of Ho.