Evidence for dominant Pauli paramagnetic effect in the upper critical field of single-crystalline FeTe0.6Se0.4

We investigated the temperature dependence of the upper critical fields H-c2(T) of a superconducting FeTe0.6Se0.4 single crystal by measuring its resistivity in static magnetic fields up to 45 T. Our observations of strong bending in the H-c2(ab)(T) curves and a nearly isotropic H-c2(ab)(0) approximate to H-c2(c)(0) approximate to 48 T support the presence of a strong Pauli paramagnetic effect. We show that the Werthamer-Helfand-Hohenberg formula that includes both the Pauli limiting and the spin-orbit scattering can effectively describe both the H-c2(ab)(T) and H-c2(c)(T) curves. An enhancement in the quasiparticle density of states or the increased scattering resulting from Te(Se) vacancies or excess Fe is discussed as a possible origin for the manifesting Pauli paramagnetic effect.