Study of Structural and Magnetotransport Properties of Fe(Te, Se) Single Crystals

Single-crystalline samples of nominal compositions FeTe0.88 and FeTe1-xSex are synthesized using the self-flux method. The activation energy of the flux flow is estimated using the Arrhenius relation as well as by the modified thermally activated flux flow (TAFF) model. The field dependence of U-0 shows transition from single to collective vortex pinning at a magnetic field of approximate to 2 T. The analysis of field-dependent U-0 data reveals the planar and the point defect magnetic field regions for both the models. The modified TAFF model is used to study the 2D and 3D behavior of the vortex state for all the superconducting samples. The single-crystalline FeTe0.80Se0.20 and FeTe0.60Se0.40 samples show 3D behavior, whereas the FeTe0.40Se0.60 crystal shows 2D behavior. The vortex phase diagrams of all the superconducting samples are obtained by analyzing the magnetotransport data. The vortex phase diagrams reveal transitions from the unpinned to pinned vortex liquid state and the vortex liquid to vortex glass state. Below T*, the critical exponent s is in good agreement with q values for 2D/3D behavior of vortex states of all the superconducting samples.