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

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  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  and the larger the  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,  and  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.I.

Zeeman-driven superconductor-insulator transition in strongly disordered MoC films: Scanning tunneling microscopy and transport studies in a transverse magnetic field

“…= 3.8 ⁄ in agreement with our previously published results [6,7]. The gapless superconducting DOS described by the Dynes modification of the BCS DOS [18] has been recently microscopically explained by the presence of local pairbreaking fields at arbitrary potential disorder [19].…”

“…As shown in our previous paper [7] the superconducting properties of MoC films are sensitive to the surface morphology. In slightly thinner areas of the sample the tunneling spectra reveal higher spectral broadening D, smaller value of the superconducting gap Δ and higher in-gap conductance.…”

“…The gapless superconducting DOS described by the Dynes modification of the BCS DOS [18] has been recently microscopically explained by the presence of local pairbreaking fields at arbitrary potential disorder [19]. In MoC such pair breaking fields are formed at the interface between the substrate and the film [7]. Figure 2b) shows an effect of perpendicular magnetic field on the tunneling spectra at = 0.5 K. When B is increased, the superconducting gap is gradually filled, and the coherence peaks are smeared.…”

“…In fermionic scenario [5] the amplitude of the superconducting order parameter is fully suppressed driving the superconducting transition temperature to zero. Our previous papers demonstrated that increased disorder in ultrathin MoC films leads to the fermionic scenario of the suppression of superconductivity [6,7].…”

Microscopic origin of the Dynes parameterΓof theLaAlO3–SrTiO3interface superconductor

“…For example, the T c of MoC films depends strongly on the film thickness and increases from 0 to 0.85 with decreasing film thickness. Nevertheless, T c is solely governed by 0 for all these MoC films [8,25].…”