Structural, electromagnetic and thermoelectric properties of Bi4O4S3 superconductor

Abstract

We report on the synthesis and extensive characterization of the 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 prepared towards establishing a phase diagram of the transition temperature as a function of carrier concentration. The crystal structure for Bi4O4S3 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 Bi4O4S3, signifying the layered structure. While the parent compound is found to be semiconducting, the electrical resistivity of Bi4O4S3 exhibits a T2 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 ∼2.75 T using the WHH approximation and the corresponding coherence length is ∼110 Å. 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 × 1019 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−1.99 for B > 0.1 T.