Measurements of the superconducting anisotropy in FeSe with a resonance frequency technique

Utilizing a novel method with the resonance frequency of a LC circuit, we measured the superconducting anisotropy of single crystals of an Fe-based superconductor FeSe with applied magnetic field up to 16 T. We found that the temperature dependence of the upper critical field Hc2(T) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, suggesting an important role played by spin-paramagnetic effect in suppressing the superconductivity. When temperature T → 0, the values of Hc2,∥c(0) and Hc2,∥ab(0) derived from the WHH fitting are close to and fall within the range of the Pauli limit, for field H0 applied parallel to the c-axis and to the ab-plane, respectively. As compared with other typical iron-based high-Tc superconductors, lower values of Hc2(0) and higher superconducting anisotropy Γ(0) were observed in FeSe.