  The magnetic penetration depth as a function of temperature in Bi4O4S3 was
studied by muon-spin-spectroscopy measurements. The superfluid density of
Bi4O4S3 is found to be very low. The dependence of penetration depth 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 Tc further supports
multi-gap superconductivity in Bi4O4S3. 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 can not be
ruled out completely. The value of penetration depth at T = 0 K is estimated to
be of 861(17) nm, one of the largest of all known layered superconductors,
reflecting a very low superfluid density.
