Low superfluid density and possible multigap superconductivity in theBiS2-based layered superconductorBi4O4S3

Abstract

The magnetic penetration depth $\ensuremath{\lambda}$ as a function of temperature in Bi${}_{4}$O${}_{4}$S${}_{3}$ was studied by muon-spin-spectroscopy measurements. The superfluid density of Bi${}_{4}$O${}_{4}$S${}_{3}$ is found to be very low. The dependence of ${\ensuremath{\lambda}}^{\ensuremath{-}2}$ on temperature possibly suggests the existence of two $s$-wave-type energy gaps with the zero-temperature values of 0.93 (3) and 0.09 (4) meV. The upturn in the temperature dependence of the upper critical field close to ${T}_{\mathrm{c}}$ further supports multigap superconductivity in Bi${}_{4}$O${}_{4}$S${}_{3}$. The presence of two superconducting energy gaps is consistent with theoretical and other experimental studies. However, a single-gap $s$-wave model fit with a gap of 0.88 (2) meV cannot be ruled out completely. The value of $\ensuremath{\lambda}(T)$ at $T=0$ K is estimated to be $\ensuremath{\lambda}(0)=861\phantom{\rule{0.16em}{0ex}}(17)$ nm, one of the largest of all known layered superconductors, reflecting a very low superfluid density.