The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCo_0.8Ru_0.2Al, reaching 23 microvolts per kelvin. Uranium’s 5f electrons provide strong electronic correlations that lead to narrow bands, a known route to producing a large thermoelectric response. In addition, uranium’s strong spin-orbit coupling produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that in UCo_0.8Ru_0.2Al at least 148 Weyl nodes, and two nodal lines, exist within 60 millielectron volt of the Fermi level. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Multicomponent superconductivity in intermediate-valence UTe ₂ emerges from an antiferromagnetic quantum critical point.

UTe₂ is a newly-discovered unconventional superconductor wherein multicomponent topological superconductivity is anticipated based on the presence of two superconducting transitions and time-reversal symmetry breaking in the superconducting state. The observation of two superconducting transitions, however, remains controversial. Here we demonstrate that UTe₂ single crystals displaying an optimal superconducting transition temperature at 2 K exhibit a single transition and remarkably high quality supported by their large residual resistance ratio and small residual heat capacity in the superconducting state. Our results shed light on the intrinsic superconducting properties of UTe₂ and bring into question whether UTe₂ is a multicomponent superconductor at ambient pressure.

Topological superconductivity is a long-sought state of matter in bulk materials, and the odd-parity superconductor ${\mathrm{UTe}}_2$ is a prime candidate. The recent observation of a field-trainable spontaneous Kerr signal in ${\mathrm{UTe}}_2$ at the onset of superconductivity provides strong evidence that the superconducting order parameter is multicomponent and breaks time-reversal symmetry. Here, we perform Kerr effect measurements on a number of ${\mathrm{UTe}}_2$ samples—grown via both chemical vapor transport and the molten-salt-flux methods—that show a single superconducting transition between 1.6 K and 2.1 K. Our results show no evidence for a spontaneous Kerr signal in zero-field measurements. This implies that the superconducting state of ${\mathrm{UTe}}_2$ does not intrinsically break time-reversal symmetry. Instead, we observe a field-trainable signal that varies in magnitude between samples and between different locations on a single sample, which is a sign of inhomogeneous magnetic regions. Our results provide an examination of representative ${\mathrm{UTe}}_2$ samples and place strong constraints on the superconducting order parameter of ${\mathrm{UTe}}_2$.

The newly discovered superconductor $\mathrm{U}{\mathrm{Te}}_2$ is a strong contender for a topological spin-triplet state wherein a multicomponent order parameter arises from two nearly degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that $\mathrm{U}{\mathrm{Te}}_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature ($T_c$). Notably, samples with higher $T_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line (constant pressure) in the pressure-temperature phase diagram but also provide a consistent scenario for the sample dependence of $\mathrm{U}{\mathrm{Te}}_2$.

We report anisotropic resistivity measurements of the heavy-fermion superconductor ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ in magnetic fields up to 16 T and temperatures down to 0.35 K. The measured ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ resistivity shows a signature corresponding to the suggested quadrupole-density-wave order state at $T_0\sim 0.5$ K for both measured directions. For a magnetic field applied along the tetragonal $a$ axis, $T_0$ is enhanced with magnetic field reaching $\sim 1.75$ K at 16 T. Further, a magnetic field-induced transition occurs at ${\mu }_0{H}_m\sim 8.1$ T corresponding to a change to a new broken symmetry state. For a magnetic field applied along the $c$ axis, $T_0$ is suppressed below our base temperature $\sim 0.35$ K by ${\mu }_{0}H\sim 4.5$ T, a field close to the previously reported field-induced transition within the superconducting state suggested to be from an even-parity to an odd-parity state. Our results indicate that the multiple superconducting phases in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ are intimately tied to the suppression of the proposed quadrupole-density-wave phase at $T_0$.