Pauli-limited upper critical field of Fe1+yTe1-xSex

In this work, we investigated the temperature dependence of the upper critical field mu H-0(c2)(T) of Fe1.02(3)Te0.61(4)Se0.39(4) and Fe1.05(3)Te0.89(2)Se0.11(2) single crystals by measuring the magnetotransport properties in stable dc magnetic fields up to 35 T. Both crystals show that mu H-0(c2)(T) in the ab plane and along the c-axis exhibit saturation at low temperatures. The anisotropy of mu H-0(c2)(T) decreases with decreasing temperature, becoming nearly isotropic when the temperature T --> 0. Furthermore, mu H-0(c2)(0) deviates from the conventional Werthamer-Helfand-Hohenberg theoretical prediction values for both field directions. Our analysis indicates that the spin-paramagnetic pair-breaking effect is responsible for the temperature-dependent behavior of mu H-0(c2)(T) in both field directions.