Different behavior of upper critical field in Fe1−xSe single crystals*

Abstract

The temperature dependences of upper critical field (H c2) for a series of iron-deficient Fe1−x Se single crystals are obtained from the measurements of in-plane resistivity in magnetic fields up to 9 T and perpendicular to the ab plane. For the samples with lower superconducting transition temperature T c (<7.2 K), the temperature dependence of H c2 is appropriately described by an effective two-band model. For the samples with higher T c (≳ 7.2 K), the temperature dependence can also be fitted by a single-band Werthamer–Helfand–Hohenberg formula, besides the two-band model. Such a T c-dependent change in H c2(T) behavior is discussed in connection with recent related experimental results, showing an inherent link between the changes of intrinsic superconducting and normal state properties in the FeSe system.