On the origin of in-gap states in homogeneously disordered ultrathin films. MoC case

TL;DR

This paper investigates the origin of in-gap states in ultrathin MoC superconducting films, revealing that interface effects significantly influence their superconducting properties and in-gap state characteristics.

Contribution

It demonstrates that interface pair breaking is a key factor in the in-gap states and superconductivity suppression in ultrathin MoC films, highlighting substrate effects.

Findings

Thinner films have lower Tc and Delta, larger Gamma.

Similar disorder levels on different substrates lead to different superconducting properties.

Interface pair breaking varies with substrate, affecting in-gap states.

Abstract

Many disordered superconducting films exhibit smeared tunneling spectra with evident in-gap states. We demonstrated that the tunneling density of states in ultrathin MoC films is gapless and can be described by the Dynes version of the BCS density of states with a strong broadening parameter Gamma accounting for the suppression of coherence peaks and increased in-gap states. The thinner the film, the lower the Tc and the superconducting energy gap Delta and the larger the Gamma. MoC films of 3 nm thickness deposited simultaneously on silicon and sapphire substrates reveal very similar scalar disorder, evidenced by the equal sheet resistance, but exhibit different superconducting characteristics of Tc, Delta and Gamma, suggesting that pair breaking responsible for the dissipation channel and the suppression of superconductivity originates on the film-substrate interface. It indicates that sapphire is a stronger pair breaker. Interface pair breaking can be operative in other cases as well.