Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5

Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of T-c similar or equal to 14.6 K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal x-ray and neutron powder diffraction, superconducting quantum interference device and torque magnetometry, and muon-spin rotation (mu SR). Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe1.045Se0.406Te0.594. Additionally, a minor hexagonal Fe7Se8 impurity phase was identified. The magnetic penetration depth lambda at zero temperature obtained by means of mu SR was found to be lambda(ab)(0) = 491(8) nm in the ab plane and lambda(c)(0) = 1320(14) nm along the c axis. The zero-temperature value of the superfluid density rho(s)(0) proportional to lambda(-2)(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron pnictides. The temperature dependences of both lambda(ab) and lambda(c) are well described by a two-gap s+s-wave model with the zero-temperature gap values of Delta(S)(0) = 0.51(3) meV and Delta(L)(0) = 2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter gamma(lambda)(T) = lambda(c)(T)/lambda(ab)(T) increases with decreasing temperature, in agreement with gamma(lambda)(T) observed in the iron-pnictide superconductors.