Anisotropy reversal of the upper critical field at low temperatures and spin-locked superconductivity in K2Cr3As3

We report measurements of the anisotropic upper critical field Hc2(T) for K2Cr3As3 single crystals up to 60 T and T>0.6K. Our results show that the upper critical field parallel to the Cr chains, Hc2¥(T), exhibits a paramagnetically limited behavior, whereas the shape of the Hc2¥(T) curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves Hc2¥(T) and Hc2¥(T) cross at T≈4K, so that the anisotropy parameter γH(T)=Hc2¥/Hc2¥(T) increases from γH(Tc)≈0.35 near Tc to γH(0)≈1.7 at 0.6 K. The paramagnetically limited behavior of Hc2¥(T) is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.