Metamagnetic transition in heavy fermion superconductor Ute₂

We studied the magnetization of the recently discovered heavy fermion superconductor UTe₂ up to 56 T in pulsed-magnetic fields. A first-order metamagnetic transition was clearly observed at H_m = 34.9 T, when a magnetic field H was applied along the orthorhombic hard-magnetization b-axis. The transition has a critical end point at 11 K and 34.7 T, where the first order transition terminates and changes into a crossover regime. Using the thermodynamic Maxwell relation, we evaluated the field dependence of the Sommerfeld coefficient of specific heat, which is directly related to the superconducting pairing. From the analysis, we found a significant enhancement of the effective mass centered at H_m, which is reminiscent of the field-reentrant superconductivity of the ferromagnet URhGe in transverse fields. We discuss the origin of their field-robust superconductivity.