Coexistence Of Superconductivity And Magnetism In Kxfe2-Yse2-Zsz (Z=0,0.4)

Single crystals KxFe2-ySe2-zSz (z = 0, 0.4) are successfully synthesized by self-flux method with the superconducting transition temperatures T-c(onset) of 31.3 K and 32.8 K, respectively. In contrast to external pressure effect on superconductivity, the substitution of S for Se does not suppress T-c, which suggests that chemical doping may mainly modulate the anion height from the Fe layer rather than compressing interlayer distance. In electron spin resonance (ESR) measurements, a resonance signal arising from the paramagnetic (PM) Fe ions is detected at room temperature for both KxFe2-ySe2 and KxFe2-ySe1.6S0.4 crystals. Upon cooling, the intensity of the spectrum weakens gradually and displays abrupt decrease below 140 K. The resonance signal disappears around 60 K (similar to 2 T-c), corresponding to the saturation temperature of antiferromagnetic (AFM) order revealed by recent neutron-diffraction study. The evolution of ESR spectrum is discussed by correlating with recent neutron-diffraction and Raman-scattering results, in support of the coexistence of superconductivity and AFM order in KxFe2-ySe2. Moreover, the variation of resonance field with decreasing temperature for KxFe2-ySe1.6S0.4 implies that the doping of S induces local weak ferromagnetism in the parent AFM background.