Bulk Superconductivity in Bismuth Oxysulfide Bi₄O₄S₃

Abstract

A very recent report on the observation of superconductivity in Bi 4 O 4 S 3 [ Mizuguchi, Y.; http://arxiv.org/abs/1207.3145 ] could potentially reignite the search for superconductivity in a broad range of layered sulfides. We report here the synthesis of Bi 4 O 4 S 3 at 500 °C by a vacuum encapsulation technique and its basic characterizations. The as-synthesized Bi 4 O 4 S 3 was contaminated with small amounts of Bi 2 S 3 and Bi impurities. The majority phase was found to be tetragonal (space group I 4/ mmm ) with lattice parameters a = 3.9697(2) Å and c = 41.3520(1) Å. Both AC and DC magnetization measurements confirmed that Bi 4 O 4 S 3 is a bulk superconductor with a superconducting transition temperature ( T c ) of 4.4 K. Isothermal magnetization ( M – H ) measurements indicated closed loops with clear signatures of flux pinning and irreversible behavior. The lower critical field ( H c1 ) at 2 K for the new superconductor was found to be ∼15 Oe. Magnetotransport measurements showed a broadening of the resistivity (ρ) and a decrease in T c (ρ = 0) with increasing magnetic field. The extrapolated upper critical field H c2 (0) was ∼31 kOe with a corresponding Ginzburg–Landau coherence length of ∼100 Å . In the normal state, the ρ ∼ T 2 dependence was not indicated. Hall resistivity data showed a nonlinear magnetic field dependence. Our magnetization and electrical transport measurements substantiate the appearance of bulk superconductivity in as-synthesized Bi 4 O 4 S 3 . On the other hand, Bi heat-treated at the same temperature is not superconducting, thus excluding the possibility of impurity-driven superconductivity in the newly discovered superconductor Bi 4 O 4 S 3 .