Structural, Transport, and Magnetic Properties of Bismuth Oxysulfide Superconductors

Superconducting bismuth oxy sulfide samples were synthesized with 5 and 10 % of excess Bi2O3 over and above the required quantity as per stoichiometry composition of Bi4O4S3 phase. The excess Bi2O3 acts as flux for the Bi4O4S3 phase formation and increases the superconducting volume fraction, grain size, and critical current density (J (c)) compared to the stoichiometric sample. The transmission electron microscopy (TEM) studies show the samples to be multiphase materials. The superconducting zero resistivity temperature (T (zero)) is not affected by the excess Bi2O3. However, the sample with 5 % excess Bi2O3 has the highest superconducting onset temperature (T (onset)), grain size, and J (c) value of 4.7 x 10(3) A/cm(2) among all the samples studied. The J (c) values are low due to weak links of superconducting grains in these materials. J (c) varies with applied external magnetic field as per the collective pinning model. The pinning force follows scaling law of combination of surface and point pinning. The dominating pinning mechanism changes from surface pinning to point pinning as the Bi2O3 content increases.