Abstract
Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of Tc≃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 (μSR). 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 λ at zero temperature obtained by means of μSR was found to be λab(0)=491(8) nm in the ab plane and λc(0)=1320(14) nm along the c axis. The zero-temperature value of the superfluid density ρs(0)∝λ−2(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron pnictides. The temperature dependences of both λab and λc are well described by a two-gap s+s-wave model with the zero-temperature gap values of ΔS(0)=0.51(3) meV and ΔL(0)=2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter γλ(T)=λc(T)/λab(T) increases with decreasing temperature, in agreement with γλ(T) observed in the iron-pnictide superconductors.
© 2010 The American Physical Society
Bendele, M