  Electric transport and scanning tunneling spectrum (STS) have been
investigated on polycrystalline samples of the new superconductor Bi4O4S3. A
weak insulating behavior in the resistive curve has been induced in the normal
state when the superconductivity is suppressed by applying a magnetic field.
Interestingly, a kink appears on the temperature dependence of resistivity near
4 K at all high magnetic fields above 1 T when the bulk superconductivity is
completely suppressed. This kink associated with the upper critical field as
well as the wide range of excess conductance at low field and high temperature
are explained as the possible evidence of strong superconducting fluctuation.
From the tunneling spectra, a superconducting gap of about 3 meV is frequently
observed yielding a ratio of 2\Delta/(kB*Tc) ~ 16.6. This value is much larger
than the one predicted by the BCS theory in the weak coupling regime
(2\Delta/(kB*Tc) ~ 3.53), which suggests the strong coupling superconductivity
in the present system. Furthermore, the gapped feature persists on the spectra
until 14 K in the STS measurement, which suggests a prominent fluctuation
region of superconductivity. Such superconducting fluctuation can survive at
very high magnetic fields, which are far beyond the critical fields for bulk
superconductivity as inferred both from electric transport and tunneling
measurements.
