Magnetic-field-induced phenomena in the paramagnetic superconductor Ute₂

We present magnetoresistivity measurements on the heavy-fermion superconductor UTe₂ in pulsed magnetic fields μ₀H up to 68T and temperatures T from 1.4 to 80K. Magnetic fields applied along the three crystallographic directions a (easy magnetic axis), b, and c (hard magnetic axes), are found to induce different phenomena—depending on the field direction—beyond the low-field suppression of the superconducting state. For H ∥ a, a broad anomaly in the resistivity is observed at ₀H ’ 10T and T = 1.4K. For H ∥ c, no magnetic transition nor crossover are observed. For H ∥ b, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field ₀H_m ’ 35T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at T_CEP ’ 7K. At higher temperatures a crossover persists up to 28K, i.e., below the temperature T^max ¼ 35K where the magnetic susceptibility is maximal. A sharp maximum in the Fermi-liquid quadratic coefficient A of the low-temperature resistivity is found at H_m. It indicates an enhanced effective mass associated with critical magnetic fluctuations, possibly coupled with a Fermi surface instability.