Structural, electromagnetic and thermoelectric properties of Bi_4O_4S_3 superconductor

摘要

We report on the synthesis and extensive characterization of the layered Bi_4O_4S_3 superconductor. This is the optimally doped sample with T_c ~ 5.3 K out of a series of Bi_6O_4S_4(SO_4)_(1-x) samples synthesized by solid state reaction. The series was prepared towards establishing a phase diagram of the transition temperature as a function of carrier concentration. The crystal structure for Bi_4O_4S_3 shows a different Bi-S-Bi bond angle as compared to that for the parent phase. Scanning electron microscopy images show a platelet-like morphology for Bi_4O_4S_3, signifying the layered structure. While the parent compound is found to be semiconducting, the electrical resistivity of Bi_4O_4S_3 exhibits a T~2 dependence in a small temperature range between 25 and 50 K. The typical dome structure for variation of T_c with dopant concentration is not observed. From the magneto-transport data H_(c2) for Bi_4O_4S_3 is estimated to be ~2.75 T using the WHH approximation and the corresponding coherence length is ~110 ?. Support for multi-band signatures is not seen from the magneto-resistance data. The rf susceptibility data fits well for an S-wave isotropic gap with a gap value higher than the BCS strong coupling limit. Hall measurements confirm the dominance of electronic transport with a charge carrier density of 4.405 × 10~(19) cm~(-3) at 10 K. The experimental value of the Seebeck coefficient at 35 K is well in accord with the calculated value deduced by using the density of charge carriers from Hall experiments. The Sommerfeld constant γ is estimated to be 1.113 mJ K~(-2) mol~(-1). Evidence for thermally activated flux flow is observed and the pinning potential is found to scale as B~(-0.3) for B < 0.1 T and B~(-199) for B > 0.1 T.