2023/05/08 by Pan, Yongqiang, Sun, Yue, Zhou, Nan +6
#FOS: Physical sciences #Superconductivity (cond-mat.supr-con)
paper · doi:10.48550/arxiv.2305.04515
Studying the upper critical field (μ0H_\rmc2) and its anisotropy of superconductors is of great importance because it can provide an unusual insight into the pair-breaking mechanism. Since Fe1+yTe1-xSex exhibits the high μ0H_\rmc2 and small anisotropic superconductivity, it has attracted considerable attention. However, some issues related to μ0H_\rmc2 are still unknown, including the effect of excess Fe content on μ0H_\rmc2 behavior and the origin of the crossover of the μ0H_\rmc2c -- T and μ0H_\rmc2ab -- T curves. In this work, the value of μ0H_\rmc2 of Fe1+yTe0.6Se0.4 single crystals with controlled amounts of excess Fe was obtained by resistivity measurements over a wide range of temperatures down to ∼ 1.5 K, and magnetic fields up to ∼ 60 T. The crossover of the μ0H_\rmc2c -- T and μ0H_\rmc2ab -- T curves was found to be independent of the excess Fe content. The angle dependence of μ0H_\rmc2 was also checked. The μ0H_\rmc2(θ) symmetry at higher temperature near Tc could be fitted by anisotropic G-L model, and novel fourfold symmetry of μ0H_\rmc2 at lower temperature was found. Based on our spin-locking pairing model, the crossover behavior originates from the anisotropic spin-paramagnetic effect, and the novel fourfold symmetry of μ0H_\rmc2 could be understood by our extended anisotropic G-L model.