Study of TAFF and vortex phase of Fe_xTe_0.60Se_0.40 (0.970 ≤ x ≤ 1.030) single crystals

Single crystalline samples with starting compositions Fe_xTe_0.60Se_0.40 (x = 0.970, 0.985, 1, 1.015 and 1.030) have been grown using the self-flux method to study the effect of Fe variation (x) on the structural and superconducting properties of the grown single crystalline samples. All the grown samples are found superconducting and among all, the FeTe_0.60Se_0.40 (x = 1) sample shows the highest superconducting transition temperature (T_C). For the samples with x other than 1, a decrease in the superconducting properties, such as T_C, activation energy (U₀) and upper critical field (H_C2(0)), has been observed. The thermal activation energy (TAE) of the flux flow has been calculated using the conventional method and modified thermally activated flux flow (TAFF) model. The values of U₀(H) show transition from the single vortex pinning regime to the collective vortex pinning regime at 2T. Also, the presence of planer defects has been estimated via both models. Furthermore, the modified TAFF model suggests three dimentional (3D) behaviour for all the grown single crystals. The vortex phase diagrams have been established by analysing the magnetoresistance data. The analysis reveals the transitions from unpinned to pinned vortex liquid region and also from the pinned vortex liquid state to vortex glass state. In the vortex phase diagram, below the critical temperature (T*), the critical exponent (s) is in a good agreement with the 'q' values for the 3D behaviour of vortex states for all the grown samples. (paper)