Upper critical field and its anisotropy in RbCr3As3

Abstract

The temperature dependence of the upper critical field $({H}_{c2})$ in ${\mathrm{RbCr}}_{3}{\mathrm{As}}_{3}$ single crystals $({T}_{c}\ensuremath{\approx}7.3$ K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel $(H\ensuremath{\parallel}c,{H}_{c2}^{\ensuremath{\parallel}c})$ and perpendicular $(H\ensuremath{\perp}c,{H}_{c2}^{\ensuremath{\perp}c})$ to the Cr chains. The curves ${H}_{c2}^{\ensuremath{\parallel}c}(T)$ and ${H}_{c2}^{\ensuremath{\perp}c}(T)$ cross at $\ensuremath{\sim}5.5$ K. As a result, the anisotropy parameter $\ensuremath{\gamma}(T)={H}_{c2}^{\ensuremath{\perp}c}/{H}_{c2}^{\ensuremath{\parallel}c}(T)$ increases from 0.5 near ${T}_{c}$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of ${\ensuremath{\mu}}_{0}{H}_{c2}^{\ensuremath{\parallel}c}(0)\ensuremath{\approx}27.2$ T and ${\ensuremath{\mu}}_{0}{H}_{c2}^{\ensuremath{\perp}c}(0)\ensuremath{\approx}43.4$ T, both exceeding the BCS weak-coupling Pauli limit ${\ensuremath{\mu}}_{0}{H}_{p}=1.84{T}_{c}=13.4$ T. The results indicate that the paramagnetic pair-breaking effect is strong for $H\ensuremath{\parallel}c$ but absent for $H\ensuremath{\perp}c$, which was further confirmed by the angle dependent magnetoresistance and ${H}_{c2}$ measurements.