Anisotropy reversal of the upper critical field at low temperatures and spin-locked superconductivity inK2Cr3As3

Abstract

We report measurements of the anisotropic upper critical field ${H}_{c2}(T)$ for ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ single crystals up to 60 T and $Tg0.6\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Our results show that the upper critical field parallel to the Cr chains, ${H}_{c2}^{\ensuremath{\parallel}}(T)$, exhibits a paramagnetically limited behavior, whereas the shape of the ${H}_{c2}^{\ensuremath{\perp}}(T)$ curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves ${H}_{c2}^{\ensuremath{\perp}}(T)$ and ${H}_{c2}^{\ensuremath{\parallel}}(T)$ cross at $T\ensuremath{\approx}4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, so that the anisotropy parameter ${\ensuremath{\gamma}}_{H}(T)={H}_{c2}^{\ensuremath{\perp}}/{H}_{c2}^{\ensuremath{\parallel}}(T)$ increases from ${\ensuremath{\gamma}}_{H}({T}_{c})\ensuremath{\approx}0.35$ near ${T}_{c}$ to ${\ensuremath{\gamma}}_{H}(0)\ensuremath{\approx}1.7$ at 0.6 K. The paramagnetically limited behavior of ${H}_{c2}^{\ensuremath{\parallel}}(T)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.