Low superfluid density and possible multigap superconductivity in the BiS_2-based layered superconductor Bi_4O_4S_3

摘要

The magnetic penetration depth X as a function of temperature in Bi_4O_4S_3 was studied by muon-spin-spectroscopy measurements. The superfluid density of Bi_4O_4S_3 is found to be very low. The dependence of λ~(-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_c further supports multigap superconductivity in Bi_4O_4S_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 λ(T) at T = 0 K is estimated to be λ(0) = 861 (17) nm, one of the largest of all known layered superconductors, reflecting a very low superfluid density.