Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces

TL;DR

This study provides direct spectroscopic evidence of odd frequency spin-triplet superconductivity at superconductor/ferromagnet interfaces, showing a transition from gapless to gapped states with magnetic order changes.

Contribution

It offers the first direct measurement of the gap structure and pairing symmetry of odd frequency superconducting states at superconductor/ferromagnet interfaces.

Findings

Detection of gapless odd frequency superconductivity via tunnelling spectroscopy.

Observation of a transition from gapless to BCS-like gapped states with magnetic order.

Confirmation of spin-triplet pairing at superconductor/ferromagnet interfaces.

Abstract

The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces.