The physical mechanism for high-temperature superconductivity remains one of the most challenging issues in modern condensed matter physics. Interestingly, one of the commonalities among high-temperature superconducting cuprates and iron-based compounds is their layered structures. Even among conventional superconductors, the highest superconducting transition temperature (Tc) has been found in layered magnesium diboride MgB2. Recently, superconductivity with Tc = 4.5 K was discovered in a new superconductor Bi4O4S3. This compound has a layered structure composed of two superconducting BiS2 layers and spacer layers of Bi4O4(SO4)1x, where x indicates the deficiency of (SO4)2 ions at the interlayer sites. Since the discovery of Bi4O4S3, several other BiS2-based superconductors LnO1xF xBiS2 (Ln = La, Ce, Pr, Nd) with the highest Tc ~ 10.6 K have been reported. Both experimental and theoretical studies to date have indicated that the BiS2 layers play the role of the superconducting planes in these sulfide superconductors, similar to the CuO2 planes in the cuprate superconductors and the Fe2An2 (An = P, As, Se, Te) layers in the iron-based superconductors. A major challenge facing this new class of layered superconductors is to optimize Tc by exploring different spacer layers. Additionally, the effects of doping by either nonmagnetic or magnetic elements are important issues for investigation.
In collaboration with Professor Zhenjie Feng’s group at the Shanghai University in China, we demonstrate in this latest publication that Mn-doping in the layered sulfides Bi4O4S3 leads to stable Bi4xMnxO4S3 compounds that exhibit both long-range ferromagnetism and enhanced superconductivity for 0.075 x 0.15, with a record Tc ~ 15 K amongst all BiS2-based superconductors. Based on our experimental investigations of Bi4xMnxO4S3 and comparative studies of related compounds Bi4xCoxO4S3 and Bi4xNixO4S3, we suggest that that the coexistence of superconductivity and ferromagnetism may be attributed to Mn-doping in the spacer Bi2O2 layers away from the superconducting BiS2 layers, whereas the enhancement of Tc may be due to excess electron transfer to BiS2 from the Mn4+/Mn3+-substitutions in Bi2O2. These findings therefore shed new light on feasible pathways to enhance the Tc values of BiS2-based superconductors.