Superconductivity and magnetic properties of single crystals of ${\mathrm{K}}_{0.75}$Fe${}_{1.66}$Se${}_2$ and Cs${}_{0.81}$Fe${}_{1.61}$Se${}_2$

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 $\mathrm{m}\Omega$ 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${}_2$

Figure 1

(a) The single crystal x-ray diffraction pattern of ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ and Cs${}_x$Fe${}_2$Se${}_2$ (offset vertically for clarity); (b) x-ray diffraction pattern of the powdered ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ and Cs${}_x$Fe${}_2$Se${}_2$ (offset vertically for clarity), all the peaks can be well indexed. The hump in XRD pattern for Cs${}_x$Fe${}_2$Se${}_2$ comes from the Mylar film, which has been used to protect the sample from air.

Figure 2

(a) The resistivity of ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ around $T_{\mathrm{c}}$ (solid line), and its ZFC and FC susceptibility taken at 10 Oe with the magnetic field parallel to the $\mathit{ab}$ plane. (b) The resistivity of Cs${}_x$Fe${}_2$Se${}_2$ around $T_{\mathrm{c}}$ (solid line), and its ZFC and FC susceptibility taken at 10 Oe with the magnetic field parallel to the $\mathit{ab}$ plane.

Figure 3

The temperature dependence of resistivity for ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ and Cs${}_x$Fe${}_2$Se${}_2$.

Figure 4

(a and c) The temperature dependence of resistivity for ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ crystal with the magnetic field parallel and perpendicular to the $c$ axis, respectively. (b and d) The temperature dependence of resistivity for Cs${}_x$Fe${}_2$Se${}_2$ crystal with the magnetic field parallel and perpendicular to the $c$ axis, respectively. (e and f) The temperature dependence of $H_{\mathrm{c}2}$($T$) for ${\mathrm{K}}_x$Fe${}_2$Se${}_2$ and Cs${}_x$Fe${}_2$Se${}_2$, respectively.

Figure 5

The magnetic susceptibility at 5 T for Cs${}_x$Fe${}_2$Se${}_2$ crystal with the magnetic field along and perpendicular to $c$ axis.