Upper critical field, anisotropy, and superconducting properties of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ single crystals

Abstract

The temperature dependent resistivity of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ (x = 0.23, 0.25, 0.28 and 0.4) single crystals and the angle dependent resistivity of superconducting Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ single crystals were measured in magnetic fields up to 9 T. The measurements of temperature dependent resistivity for samples with different doping levels revealed very high upper critical fields which increase with the transition temperature monotonously, and a very low superconducting anisotropy ratio $\hat{I}“=H_{c2}^{ab}/H_{c2}^c\approx$ 2. By scaling the resistivity in the frame of the anisotropic Ginzburg-Landau theory, the angle dependent resistivity of the Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy $\hat{I}“$ was determined alternatively for each temperature and was found to be between two and three.

5 pages, 5 figures