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

Abstract

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 H c2 (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 H c2,≈c (0) and H c2,≈ab (0) derived from the WHH fitting are close to and fall within the range of the Pauli limit, for field H 0 applied parallel to the c-axis and to the ab-plane, respectively. As compared with other typical iron-based high-T c superconductors, lower values of H c2 (0) and higher superconducting anisotropy Γ(0) were observed in FeSe. © 2019 Author(s). ; The work at Yangzhou Univ. was supported by Natural Science Foundation of China (NSFC) grant # 61474096 and by NSF of Jiangsu grant # BK20180889, and at Chinese Academy of Sciences by NSFC grants # 51477167 and 41527802. D.A.C. thanks supports by the program 211 of the Russian Federation Government (RFG), agreement 02.A03.21.0006 and by the Russian Government Program of Competitive Growth of Kazan Federal Univ. A.N.V. thanks supports by Russian Foundation for Basic Research Grant No. 17-29-10007, by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISiS (Grant No. K2-2017-084), and by Act 211 of RFG, agreements 02.A03.21.0004, 02.A03.21.0006, and 02.A03.21.0011.