Upper critical field of high quality single crystals of KFe${}_2$As${}_2$

Abstract

Measurements of temperature-dependent in-plane resistivity, $\rho (T)$, were used to determine the upper critical field and its anisotropy in high quality single crystals of stoichiometric iron arsenide superconductor KFe${}_2$As${}_2$. The crystals were characterized by residual resistivity ratio, $\rho (300K)/\rho (0)$ up to 3000 and resistive transition midpoint temperature, $T_c$=3.8 K, significantly higher than in previous studies on the same material. We find increased $H_{c2}(T)$ for both directions of the magnetic field, which scale with the increased $T_c$. This unusual linear $H_{c2}(T_c)$ scaling is not expected for orbital limiting mechanism of the upper critical field in clean materials.