Vortex Phase Diagram and Pinning Mechanisms of Air‐Annealed FeTe_0.8S_0.2 Single Crystals

The vortex phase diagram and superconducting properties of air‐annealed FeTe 0.8 S 0.2 single crystals are investigated. The annealed samples exhibit a superconducting transition, as confirmed by temperature‐dependent resistivity measurements. The thermally activated energy is calculated from the magnetotransport measurements, and the analysis shows that as the magnetic field increases, there is a crossover from single to collective vortex pinning. The vortex phase diagrams have been determined by analyzing magnetic field‐dependent resistivity, revealing the transitions from an unpinned vortex liquid region to a pinned vortex liquid state and further transition from this pinned state to a vortex glass state. Temperature‐dependent magnetization measurement determines the superconducting volume fraction. The critical current densities, as a function of the magnetic field ( J C ( H )), have been estimated from the magnetization versus magnetic field loops measured at various temperatures. Bean's critical state model estimates the J C values, and for an optimized annealed crystal, the self‐field J C at 2 K is . The normalized pinning force density in the samples is determined using the Dew Hughes model to identify the pinning mechanisms. The temperature‐dependent J C data analysis indicates the presence of ‐pinning in the samples. Theoretical models analyze experimental observations to understand the vortex pinning mechanisms.