Reversal of the Upper Critical Field Anisotropy and Spin-Locked Superconductivity in K$_{2}$Cr$_{3}$As$_{3}$

We report the first measurements of the anisotropic upper critical field $H_{c2}(T)$ for K$_{2}$Cr$_{3}$As$_{3}$ single crystals up to 60 T and $T$\textgreater 0.6K. $H_{c2}(T)$ was determined via resistivity and proximity detector oscillator techniques. Our results show that the upper critical field parallel to the Cr chains,$ H_{c2\parallel }$, exhibits a paramagnetically-limited behavior, whereas no evidence of paramagnetic pair breaking was observed with field perpendicular to the Cr chains. As a result, the curves $H_{c2\bot }$ ($T)$ and $H_{c2\parallel }$ ($T)$ cross at $T$\textasciitilde 4 K, so that the anisotropy parameter $\gamma (T)= H_{c2\bot }$ /$H_{c2\parallel }$ increases from $\gamma $\textasciitilde 0.35 near $T_{c}$ to $\gamma $\textasciitilde 1.7 at 0.6 K. This behavior of $H_{c2}(T)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.