Temperature and angular dependence of the upper critical field in K2Cr3As3

We report measurements of the upper critical field H-c2 as functions of temperature T, polar angle. (of the field direction with respect to the crystallographic c axis), and azimuthal angle f (of the field direction relative to the a axis within the ab plane) for the Cr-based superconductor K2Cr3As3 with a quasi-one-dimensional and noncentrosymmetric crystal structure. We confirm that the anisotropy in H-c2(T) becomes inverse with decreasing temperature. At low temperatures, H-c2(theta) data are featured by two maxima at theta = 0 (H parallel to c) and p/2 (H perpendicular to c), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for H perpendicular to c are taken simultaneously into consideration. The in-plane H-c2(phi) profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristics of the H-c2(theta, phi, T) data mostly resemble those of the heavy-fermion superconductor UPt3, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in K2Cr3As3.