Crystal structure, physical properties and superconductivity inAxFe2Se2single crystals

We studied the correlation among structure, transport properties and superconductivity in different A x Fe 2 Se 2 single crystals ( A =K, Rb and Cs). Two sets of (00 l ) reflections are observed in the x-ray single-crystal diffraction patterns, and they arise from the intrinsic inhomogeneous distribution of the intercalated alkali atoms. The occurrence of superconductivity is closely related to the c -axis lattice constant, and the A content is crucial to superconductivity. The hump observed in resistivity seems to be irrelevant to the superconductivity. There exist many deficiencies within the FeSe layers in A x Fe 2 Se 2 , although their T c does not change so much. In this sense, superconductivity is robust to the vacancies within the FeSe layers. Very high resistivity in the normal state should be ascribed to such defects in the conducting FeSe layers. A x Fe 2 Se 2 ( A =K, Rb and Cs) single crystals show the same susceptibility behavior in the normal state, and no anomaly is observed in susceptibility at the hump temperature in resistivity. The clear jump in specific heat for Rb x Fe 2 Se 2 and K x Fe 2 Se 2 single crystals indicates the good bulk superconductivity of these crystals.