Anisotropic superconductivity of high quality FeSe1-x Single crystal

Abstract

We investigate the upper critical field anisotropy ΓH and the magnetic penetration depth anisotropy Γλ of a high-quality FeSe1-x single crystal using angular dependent resistivity and torque magnetometry up to 14 T. High quality single crystals of FeSe1-x were successfully grown using KCl-AlCl3 flux method, which shows a sharp superconducting transition at TC ~ 9 K and a high residual resistivity ratio of ~ 25. We found that the anisotropy ΓH near TC is a factor of two larger than found in the poor-quality crystals, indicating anisotropic 3D superconductivity of FeSe1-x. Similar to the 1111-type Fe pnictides, the anisotropies Γλ and ΓH show distinct temperature dependence; ΓH decreases but Γλ increases with lowering temperature. These behaviors can be attributed to multi-band superconductivity, but different from the case of MgB2. Our findings suggest that the opposite temperature dependence of Γλ and ΓH is the common properties of Fe-based superconductors.