Upper critical field of high-quality single crystals of KFe2As2

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 the stoichiometric iron arsenide superconductor KFe2As2. The crystals were characterized by the residual resistivity ratio rho(300 K)/rho(0) up to 3000 and the 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 an orbital-limiting mechanism of the upper critical field in clean materials. DOI: 10.1103/PhysRevB.87.134513