We report measurements of the upper critical field $H_{\mathrm{c}2}$ as functions of temperature $T$, polar angle $\theta$ (of the field direction with respect to the crystallographic $c$ axis), and azimuthal angle $\varphi$ (of the field direction relative to the $a$ axis within the $ab$ plane) for the Cr-based superconductor ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ with a quasi-one-dimensional and noncentrosymmetric crystal structure. We confirm that the anisotropy in $H_{\mathrm{c}2}\left(T\right)$ becomes inverse with decreasing temperature. At low temperatures, $H_{\mathrm{c}2}\left(\theta \right)$ data are featured by two maxima at $\theta =0$ $\left(\mathbf{H}\parallel c\right)$ and $\pi /2$ $\left(\mathbf{H}\perp c\right)$, which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for $\mathbf{H}\perp c$ are taken simultaneously into consideration. The in-plane $H_{\mathrm{c}2}\left(\varphi \right)$ profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristics of the $H_{\mathrm{c}2}(\theta ,\varphi ,T)$ data mostly resemble those of the heavy-fermion superconductor ${\mathrm{UPt}}_3$, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$.

• Received 31 December 2015

DOI:https://doi.org/10.1103/PhysRevB.95.014502