  High-quality single crystals of K0.8Fe2Se1.4S0.4 are successfully synthesized
by self-flux method with the superconducting transition temperatures Tconset =
32.8 K and Tczero = 31.2 K. In contrast to external pressure effect on
superconductivity, the substitution of S for Se does not suppress Tc, which
suggests that chemical doping may mainly modulate the anion height from
Fe-layer rather than compressing interlayer distance. The investigation of the
micromagnetism by electron spin resonance shows clear evidence for strong spin
fluctuation at temperatures above Tc. Accompanied by the superconducting
feature spectra, a novel resonance signal develops gradually upon cooling below
Tc, indicating the coexistence of superconductivity and magnetism in
K0.8Fe2Se1.4S0.4 crystal.
