Superconductivity in doped FeTe1−xSx (x = 0.00 to 0.25) single crystals

Abstract

We report self flux growth and characterization of FeTe1-xSx (x = 0.00 to 0.25) single crystal series. Surface x-ray diffraction (XRD) exhibited crystalline nature with growth in (00l) plane. Micro-structural (electron microscopy) images of representative crystals showed the slab-like morphology and near stoichiometric composition. Powder XRD analysis (Rietveld) of single crystals exhibited tetragonal structure with P4/nmm space group and decreasing a and c lattice parameters with increase in x. Electrical resistivity measurements (ρ-T) showed superconductivity with at 9.5 K and 8.5 K for x = 0.10 and x = 0.25 respectively. The un-doped crystal, i.e., x = 0.00, exhibited known step like anomaly at around 70 K. Upper critical field Hc2(0), as calculated from magneto transport [ρ(T)H], for x = 0.25 crystal is around 60Tesla and 45Tesla in H//ab and H//c directions. Thermal activation energy [U0(H)] calculated for x = 0.10 and 0.25 crystals followed weak power law, indicating single vortex pinning at low fields. Mossbauer spectra for FeTe1-xSx crystals at 300 K and 5 K are compared with non superconducting FeTe. Both quadrupole splitting (QS) and isomer shift (IS) for S doped crystals were found to decrease. Also at 5 K the hyperfine field for x = 0.10 superconducting crystal is decreased substantially from 10.6Tesla (FeTe) to 7.2Tesla. For x = 0.25 crystal, though small quantity of un-reacted Fe is visible at room temperature, but unlike x = 0.10, the low temperature (5 K) ordered FeTe hyperfine field is nearly zero.