Transport properties and the anisotropy of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals in normal and superconducting states

The transport and superconducting properties of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with $T_c\approx31$ K were studied. Both in-plane and out-of-plane resistivity was measured by a modified Montgomery method. The in-plane resistivity is almost the same for all studied samples, unlike the out-of-plane resistivity, which differs considerably. We have found that the resistivity anisotropy $\gamma=\rho_c/\rho_{ab}$ is almost independent of temperature and lies in the range 10–30 for the studied samples. This indicates the extrinsic nature of high out-of-plane resistivity, which may be due to the presence of flat defects along Fe-As layers in the samples. This statement is supported by comparatively small effective mass anisotropy, obtained from the upper critical field measurements, and from the observation of the so-called “Friedel transition”, which indicates the existence of some disorder in the samples in the c-direction.