Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi4O4S3

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 fields and high temperatures is 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 2Δ/k _B T _C ～ 16.6. This value is much larger than the one predicted by the BCS theory in the weak coupling regime (2Δ/k _B T _C ～ 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 a 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.