Signatures of superconducting triplet pairing in Ni--Ga-bilayer junctions

TL;DR

This paper investigates how superconducting triplet pairing manifests in Ni-Ga bilayer junctions through experimental conductance measurements and neutron reflectometry, revealing signatures of triplet superconductivity at the interface.

Contribution

It introduces the concept of conductance shoulders as a transport signature of triplet pairing and combines experimental and theoretical analysis to support this finding.

Findings

Identification of conductance shoulders as a signature of triplet pairing

Observation of nonuniform magnetization near the Ni-Ga interface

Evidence of induced triplet superconductivity from Meissner response

Abstract

Ni-Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al2O3(/EuS)/Ni-Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)-Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)-Ga (30 nm) bilayers entail completely different physics, and give rise to novel large-bias (when compared to the superconducting gap of the thin Al film as a reference) conductance-peak subseries that we term conductance shoulders. These conductance shoulders might attract considerable attention also in similar magnetic superconducting bilayer junctions, as we predict them to offer an experimentally well-accessible transport signature of superconducting triplet pairings that are induced around the interface of the Ni-Ga bilayer. We further substantiate this claim performing complementary polarized neutron reflectometry measurements on the bilayers, from which we deduce (1) a nonuniform magnetization structure in Ga in a several nanometer-thick area around the Ni-Ga boundary and can simultaneously (2) satisfactorily fit the obtained data only considering the paramagnetic Meissner response scenario. While the latter provides independent experimental evidence of induced triplet superconductivity inside the Ni-Ga bilayer, the former might serve as the first experimental hint of its potential microscopic physical origin.