Anomalous double peak structure in Nb/Ni superconductor/ferromagnet tunneling DOS

TL;DR

This study investigates the density of states in Nb/Ni superconductor/ferromagnet bilayers, revealing an anomalous double-peak structure and an inverted DOS at larger Ni thicknesses, indicating new physics beyond existing models.

Contribution

It reports the observation of an anomalous double-peak and inverted DOS in Nb/Ni bilayers, challenging current theoretical understanding and suggesting new physics.

Findings

Intermediate Ni thickness shows double-peak DOS structure.

Thicker samples exhibit inverted DOS previously unreported in similar structures.

Standard models cannot reproduce the sub-gap features observed.

Abstract

We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers as a function of Ni thickness, $d_F$. Our thinnest samples show the usual DOS peak at $\pm\Delta_0$, whereas intermediate-thickness samples have an anomalous ``double-peak'' structure. For thicker samples ($d_F \geq 3.5$ nm), we see an ``inverted'' DOS which has previously only been reported in superconductor/weak-ferromagnet structures. We analyze the data using the self-consistent non-linear Usadel equation and find that we are able to quantitatively fit the features at $\pm\Delta_0$ if we include a large amount of spin-orbit scattering in the model. Interestingly, we are unable to reproduce the sub-gap structure through the addition of any parameter(s). Therefore, the observed anomalous sub-gap structure represents new physics beyond that contained in the present Usadel theory.