Novel anisotropy of upper critical fields in Fe_1+yTe_0.6Se_0.4

Studying the upper critical field (mu H-0(c2)) 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 mu H-0(c2) and small anisotropic superconductivity, it has attracted considerable attention. However, some issues related to mu H-0(c2) are still unknown, including the effect of excess Fe content on mu H-0(c2) behavior and the origin of the crossover of the mu H-0(c2)(c) - T and mu H-0(c2)ab - T curves. In this work, the value of mu H-0(c2) 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 similar to 1.5 K, and magnetic fields up to similar to 60 T. The crossover of the mu H-0(c2)(c) - T and mu H-0(c2)ab - T curves was found to be independent of the excess Fe content. The angle dependence of mu H-0(c2) was also checked. The mu H-0(c2)(theta) symmetry at higher temperature near T-c could be fitted by anisotropic G-L model, and novel fourfold symmetry of mu H-0(c2) at lower temperature was found. Based on our spin-orientation-locking pairing model, the crossover behavior originates from the anisotropic spin-paramagnetic effect, and the novel four-fold symmetry of mu H-0(c2) could be understood by the theoretical analysis.