The upper critical field and its anisotropy in RbCr${}_3$As${}_3$

Abstract

The temperature dependence of the upper critical field ($H_{c2}$) in RbCrAs${}_3$ single crystals ($T_c\approx$ 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 ($H\parallel c$, $H_{c2}^{\parallel c}$) and perpendicular ($H\perp c$, $H_{c2}^{\perp c}$) to the Cr chains. The curves $H_{c2}^{\parallel c}(T)$ and $H_{c2}^{\perp c}(T)$ cross at $\sim$ 5.5 K. As a result, the anisotropy parameter ${\hat{I}}^3(T)=H_{c2}^{\perp c}/H_{c2}^{\parallel c}(T)$ 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 $\hat{I}{¼}_0{H}_{c2}^{\parallel c}(0)\approx$ 27.2 T and $\hat{I}{¼}_0{H}_{c2}^{\perp c}(0)\approx$ 43.4 T, both exceeding the BCS weak-coupling Pauli limit $\hat{I}{¼}_0{H}_p=1.84T_c=13.4$ T. The results indicate that the paramagnetic pair breaking effect is strong for $H\parallel c$ but absent for $H\perp c$, which was further confirmed by the angle dependent magnetoresistance and $H_{c2}$ measurements.

6 pages, 5 figures