Temperature and angular dependence of the upper critical field in K2Cr3As3

We report measurements of the upper critical field Hc2 as functions of temperature T, polar angle θ (of the field direction with respect to the crystallographic c axis), and azimuthal angle φ (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 Hc2(T) becomes inverse with decreasing temperature. At low temperatures, Hc2(θ) data are featured by two maxima at θ=0 (Hc) and π/2 (Hc), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for Hc are taken simultaneously into consideration. The in-plane Hc2(φ) profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristics of the Hc2(θ,φ,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.