Different behavior of upper critical field in Fe_1-xSe single crystals

The temperature dependences of upper critical field (H_c2) for a series of iron-deficient Fe_1-xSe 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.