Structure, magnetization, specific heat, and microwave properties of KxFe2-ySe2

Temperature-dependent magnetization, specific heat, and magnetic field modulated microwave spectroscopy (MFMMS) measurements were performed on single crystals of KxFe2-ySe2. Magnetization measurements yield a superconducting transition temperature (T-c) of similar to 30 K, with a diamagnetic shielding fraction of nearly 90%. Specific heat measurements revealed a 'jump' at T-c, Delta C/T vertical bar T-c, of about 6.8 +/- 1 mJ mol(-1) K-2, consistent with bulk superconductivity in KxFe2-ySe2. Moreover, MFMMS measurements detect the superconductivity of KxFe2-ySe2. with a peak with an onset at T-c(mu) similar to 28 K, close to the values of T-c determined from the magnetization and specific heat measurements. The presence and the shape of the low temperature MFMMS signal could be ascribed to a complex dissipation mechanism and percolative superconductivity.