Anisotropy Reversal of the Upper Critical Field at Low Temperatures and Spin-Locked Superconductivity in K2Cr3As3

Abstract

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

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