Evolution of correlation strength in KxFe2-ySe2 superconductor doped with S

We report the evolution of thermal transport properties of KxFe2-ySe2 with sulfur substitution at Se sites. Sulfur doping suppresses the superconducting T-c. The Seebeck coefficient of all crystals in the low-temperature range can be described very well by a diffusive thermoelectric response model. The zero-temperature extrapolated value of the Seebeck coefficient divided by temperature, S/T, gradually decreases from -0.48 mu V/K-2 to a very small value, similar to 0.03 mu V/K-2, where T-c is completely suppressed. The normal-state electron Sommerfeld term (gamma n) of specific heat also decreases with the increase in sulfur content. The decrease of S/T and gamma(n) reflects a suppression of the density of states at the Fermi energy, or a change in the Fermi surface that would induce the suppression of correlation strength. Our results imply little relevance of strong electron correlations to superconductivity.