Fishtail effect and the vortex phase diagram of high-entropy alloy superconductor

High-entropy alloy (HEA) is an attracting topic raising in materials science and condensed matter physics. Although several types of superconductors have been discovered in HEAs, the critical currents (Jc) of HEA superconductors remain uncharacterized up to now. Here, we systematically study the current-carrying ability of (TaNb)0.7(HfZrTi)0.5 HEA at various heat treatment conditions. We obtained the high upper critical field and large current carrying ability, which point to optimistic applications. Interestingly, the fishtail or second peak effect was found for the first time in HEA superconductors, and the position of the vortex pinning force shows a maximum at 0.72 of the reduced field, which is quite different from the cuprates and iron-based high-Tc superconductors. Together with the resistive measurements, the vortex phase diagram is obtained for HEA superconductor.