Bulk Superconductivity in Bismuth Oxysulfide Bi₄O₄S₃

Abstract

A very recent report on the observation of superconductivity in Bi4O4S3 [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 Bi4O4S3 at 500 °C by a vacuum encapsulation technique and its basic characterizations. The as-synthesized Bi4O4S3 was contaminated with small amounts of Bi2S3 and Bi impurities. The majority phase was found to be tetragonal (space group I4/mmm) with lattice parameters a = 3.9697(2) Å and c = 41.3520(1) Å. Both AC and DC magnetization measurements confirmed that Bi4O4S3 is a bulk superconductor with a superconducting transition temperature (Tc) 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 (Hc1) 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 Tc (ρ = 0) with increasing magnetic field. The extrapolated upper critical field Hc2(0) was ∼31 kOe with a corresponding Ginzburg–Landau coherence length of ∼100 Å . In the normal state, the ρ ∼ T2 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 Bi4O4S3. 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 Bi4O4S3.