Superconductivity and magnetic properties of single crystals of K0.75Fe1.66Se2 and Cs0.81Fe1.61Se2

We successfully grew the high-quality single crystals of A(x)Fe(2)Se(2) (A = K and Cs) by self-flux method. Sharp superconducting transition was observed for both types of crystals. The crystals show the onset superconducting transition temperatures (T-c) of 31 and 30 K for K and Cs compounds, respectively, with nearly 100% superconductive shielding fraction. The crystals show quite high resistivity in the normal state of more than 160 and 1300 m Omega cm maximum resistivity for KxFe2Se2 and CsxFe2Se2 single crystals, respectively. Much larger upper critical field H-c2 is inferred from low-temperature isomagnetic-field magnetoresistance in these crystals than in FeSe. The anisotropy H-c2(ab)(0)/H-c2(c)(0) is estimated to be around 3 for both of the two materials. Anisotropic peculiar magnetic behavior in normal state has been found for CsxFe2Se2