Anisotropy of the Upper Critical Field in the Heavy-Fermion Superconductor UTe2 under Pressure

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$intheheavy-fermionsuperconductorUTe$2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p{\rm c} \approx 1.5$GPa,andamagneticallyorderedstateappears\mathrm{.}Theunusual$H_{\rm c2}(T)$atlowtemperaturesfor$H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$\mathrm{.}Forafieldappliedalongthehardmagnetization$b$axis$H_{\rm c2} (0)$isgluedtothemetamagnetictransition$H_{\rm m}$ which is suppressed near $p_{\mathrm{c}}$. The suppression of $H_{\mathrm{m}}$ with pressure follows the decrease of temperature $T_{\chi }^{\mathrm{m}\mathrm{a}\mathrm{x}}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\mathrm{c}2}$ at ambient pressure for $H\parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\mathrm{s}\mathrm{c}}$ and the decrease of $H_{\mathrm{m}}$. The change in the hierarchy of the anisotropy of $H_{\mathrm{c}2}(0)$ on approaching $p_{\rm c}$pointsoutthatthe$c$ axis becomes the hard magnetization axis.