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_c2 in the heavy-fermion superconductor UTe₂ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure p_c ≈ 1.5 GPa, and a magnetically ordered state appears. The unusual H_c2(T) at low temperatures for H k a suggests that the multiple superconducting phases which appear under pressure have quite different H_c2. For a field applied along the hard magnetization b axis H_c2(0) is glued to the metamagnetic transition H_m, which is suppressed near pc. The suppression of H_m with pressure follows the decrease of temperature T^max, at the maximum in the susceptibility along b. The strong reinforcement of H_c2 at ambient pressure for H k b above 16 T is rapidly suppressed under pressure due to the increase of T_sc and the decrease of H_m. The change in the hierarchy of the anisotropy of H_c2(0) on approaching p_c points out that the c axis becomes the hard magnetization axis.