We report measurements of the anisotropic upper critical field $H_{c2}\left(T\right)$ for ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ single crystals up to 60 T and $T>0.6\mathrm{K}$. Our results show that the upper critical field parallel to the Cr chains, $H_{c2}^{\parallel }\left(T\right)$, exhibits a paramagnetically limited behavior, whereas the shape of the $H_{c2}^{\perp }\left(T\right)$ curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves $H_{c2}^{\perp }\left(T\right)$ and $H_{c2}^{\parallel }\left(T\right)$ cross at $T\approx 4\mathrm{K}$, so that the anisotropy parameter ${\gamma }_H\left(T\right)=H_{c2}^{\perp }/H_{c2}^{\parallel }\left(T\right)$ increases from ${\gamma }_H\left(T_c\right)\approx 0.35$ near $T_c$ to ${\gamma }_H\left(0\right)\approx 1.7$ at 0.6 K. The paramagnetically limited behavior of $H_{c2}^{\parallel }\left(T\right)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.

• Received 20 May 2015
• Revised 9 June 2015

DOI:https://doi.org/10.1103/PhysRevB.91.220505