Composition and annealing effects on superconductivity in sintered and arc-melted Fe_1+εTe_0.50Se_0.50

We present the results of x-ray diffraction (XRD), resistivity, and ac magnetic susceptibility measurements to study the effects of the iron concentration on the “11”- structure superconductor Fe_1+εTe_0.50Se_0.50 (0 ≤ ε ≤ 0.15). Samples were initially sintered in sealed quartz tubes or melted in an arc furnace and later annealed at a variety of temperatures: 425°C, 600°C, 675°C for sintered samples and 650°C for arc-melted specimens. The XRD data show a predominant tetragonal phase with a secondary hexagonal phase. The resistivity measurements show that the superconducting transition temperature, T_C ≈ 14 K, is independent of iron concentration 1+ε, suggesting a single superconducting line compound. The magnetic screening fraction varies with Fe concentration and annealing temperature, with a maximum value at ε = 0.07. Larger superconducting fractions are associated with larger a-axis lattice constants, hinting at a possible chalcogen-vacancy electron doping. Thus we suggest that such behavior originates from an electron-doped, chalcogen deficient stoichiometry.