Superconductivity, magnetism, and stoichiometry of single crystals of Fe1+y(Te1-xSx)z

We report synthesis of high-quality Fe1+y(Te1-xSx)(z) single crystals and a comprehensive study of structural, magnetic, and transport properties. We demonstrate the very small upper critical field anisotropy of Fe1+y(Te1-xSx)(z), gamma(H)=H-c2(c)/H-c2(perpendicular to c). The value of gamma(H) reaches 1.05 at T=0.65T(C) for Fe1.12Te0.83S0.11 while still maintaining large values of upper critical field. There is high sensitivity to material stoichiometry which includes vacancies on the Te(S) site. Our results reveal competition and coexistence of magnetic order and percolative superconductivity for x >= 0.03 while zero resistivity is achieved for x >= 0.1.