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Structural, Electromagnetic, and Thermoelectric properties of Bi4O4S3 Superconductor

We report on the synthesis and extensive characterization of layered Bi4O4S3 superconductor. This is the optimally doped sample with Tc ~5.3 K out of a series of Bi6O4S4(SO4)_1-x samples synthesized by solid state reaction. The series was synthesized towards establishing a phase diagram of transition temperature as a function of carrier concentration. Crystal structure for Bi4O4S3 shows larger bending of Bi2-S2-Bi2 bond in the BiS2 layer compared to that for the parent phase. Scanning electron microscopy images show platelets like morphology for Bi4O4S3 signifying the layered structure. While the parent compound is found to be semiconducting, the electrical resistivity of Bi4O4S3 exhibits T^2 dependence in a small temperature range between 25 and 50 K. The typical dome structure for variation of Tc with dopant concentration is not observed. From the magneto-transport data Hc2 for Bi4O4S3 is estimated to be ~2.75T with WHH approximation and the corresponding coherence length is ~110A. Support for multiband signatures is not seen from magneto-resistance data. RF susceptibility data fits well for S-wave isotropic gap with gap value higher than BCS strong coupling limit. Hall measurements confirm dominance of electronic transport. The experimental value of Seebeck coefficient at 35K is well in accord with the calculated value deduced by using density of charge carriers from Hall experiments. The Sommerfeld constant is estimated to be ~ 1.113 mJ/K^2 mol. Evidence for thermally activated flux flow is observed.