Superconductivity and magnetic properties of single crystals ofK0.75Fe1.66Se2and Cs0.81Fe1.61Se2

Abstract

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 (${T}_{\mathrm{c}}$) 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 $\mathrm{m}\ensuremath{\Omega}$ cm maximum resistivity for ${\mathrm{K}}_{x}$Fe${}_{2}$Se${}_{2}$ and Cs${}_{x}$Fe${}_{2}$Se${}_{2}$ single crystals, respectively. Much larger upper critical field ${H}_{\mathrm{c}2}$ is inferred from low-temperature isomagnetic-field magnetoresistance in these crystals than in FeSe. The anisotropy ${H}_{\mathrm{c}2}^{\mathit{ab}}$(0)/${H}_{\mathrm{c}2}^{c}$(0) is estimated to be around 3 for both of the two materials. Anisotropic peculiar magnetic behavior in normal state has been found for Cs${}_{x}$Fe${}_{2}$Se${}_{2}$