Measurements of Superconducting Anisotropy in FeSe with Resonance Frequency Technique

Utilizing a novel method with the resonance frequency of a LC circuit, we measured the superconducting anisotropy of single crystals of Fe-based superconductor FeSe. We found that the temperature dependence of the upper critical field H_c2(T ) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, which suggests an important role played by spin-paramagnetic effect in suppressing the superconductivity. When temperature T → 0, the values of H_c2,||c(0) and H_c2,||ab(0) derived from the WHH fitting are close to and 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 H_c2(0) and higher superconducting anisotropy were found in FeSe.