Direct Observation of Vortices and Antivortices Generation in Phase-Separated Superconductor Sn-Pb Solder

Quantized vortices in type-II superconductors provide insights into the mechanisms of superconductivity. However, the generation of antivortices, characterized by magnetization antiparallel to the external magnetic field, remains less understood. In this study, we investigate Sn-Pb solder, a superconductor with phase-separated Sn and Pb phases, and report the observation of both vortices and antivortices. Scanning SQUID (superconducting quantum interference device) microscopy revealed the presence of both vortices and antivortices, while magneto-optical imaging demonstrated flux avalanches. Our results demonstrate that Sn in Sn-Pb solder behaves as a type-II superconductor when magnetic fluxes are trapped, despite bulk Sn being a type-I superconductor with a transition temperature (TcSn) of 3.7 K. Our findings suggest that the size effect and proximity effect with Pb contribute synergistically to induce type-II superconductivity in Sn Notably, vortices were observed at temperatures as high as 5 K, exceeding the bulk TcSn. Furthermore, the interplay between the type-I superconducting Pb phase and the type-II superconducting Sn phase results in the generation of antivortices, providing a mechanism to accommodate excess magnetic flux. This study shed light on new research on composites combining type-I and type-II superconductors.