Magnetic-Field-Induced Phenomena in the Paramagnetic Superconductor UTe${}_2$

We present magnetoresistivity measurements on the heavy-fermion superconductor UTe${}_2$ in pulsed magnetic fields ${\mu }_{0}H$ up to 68~T and temperatures $T$ from 1.4 to 80~K. Magnetic fields applied along the three crystallographic directions $\mathbf{a}$ (easy magnetic axis), $\mathbf{b}$, and $\mathbf{c}$ (hard magnetic axes), are found to induce different phenomena

• depending on the field direction - beyond the low-field suppression of the superconducting state. For $\mathbf{H}\parallel \mathbf{a}$, a broad anomaly in the resistivity is observed at ${\mu }_0{H}^{\ast }\simeq 10$~T and $T=1.4$~K. For $\mathbf{H}\parallel \mathbf{c}$, no magnetic transition nor crossover are observed. For $\mathbf{H}\parallel \mathbf{b}$, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field $\mu_0H_m \simeq 35$~T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at $T_{CEP}\simeq 7$~K. At higher temperatures a crossover persists up to 28~K, i.e., below the temperature $T_{\chi }^{max}=35$~K 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. Similarly to the URhGe case, we show that UTe${}_2$ is a candidate for field-induced reentrant superconductivity in the proximity of $H_m$.