Upper critical field, lower critical field and critical current density of FeTe0.60Se0.40 single crystals

Transport and magnetic studies are performed on high-quality FeTe0.60Se0.40 single crystals to determine the upper critical field (H-c2), lower critical field (H-c1) and the critical current density (J(c)). The value of the upper critical field Hc2 is very large, whereas the activation energy determined from the slope of the Arrhenius plots is found to be lower than that in the FeAs122 superconductors. The lower critical field is determined in the 'ab' and 'c' directions of the crystals, and found to have anisotropy Gamma (= H-c1 parallel to c/ H-c1 parallel to ab) similar to 4. The magnetic isotherms measured up to 12 T show the presence of fishtail behavior. The critical current density at 1.8K of the single crystals is found to be almost the same in both the 'ab' and 'c' directions in the low-field regime.