The discovery of spin-triplet superconductivity in UTe₂ has spurred significant research due to its large, anisotropic upper critical fields exceeding the paramagnetic limit. This unconventional superconductivity is limited by a magnetic phase transition at 35 T, with a reentrant superconducting phase above 40 T. Evidence suggests topological non-triviality, including chiral in-gap surface states, a double transition in specific heat, and broken time reversal symmetry. The superconducting gap is believed to be nodal, consistent with a *p*-wave orbital symmetry. Superconductivity emerges from a renormalized electronic structure of hybridized *f*-electrons, showing heavy fermion characteristics like a large low-temperature specific heat, resistivity and susceptibility maxima, and a 4 meV Kondo hybridization gap. ARPES measurements reveal a band structure dominated by two intersecting one-dimensional sheets and a three-dimensional Fermi pocket. Magnetic interactions are crucial; while long-range magnetic order is absent in the normal state, several measurements suggest proximity to a ferromagnetic instability. Previous inelastic neutron scattering experiments have observed incommensurate wavevectors, potentially from RKKY interactions, Fermi surface nesting, or spin-ladder interactions. Changes in inelastic neutron scattering in the superconducting state near 1 meV have also been reported. This study aims to clarify the nature of these magnetic excitations through detailed inelastic neutron scattering experiments.

Prior research on UTe₂ established its unconventional spin-triplet superconductivity and associated properties, including high critical fields, topological characteristics, and a nodal superconducting gap. Studies highlighted the importance of hybridized *f*-electrons and heavy fermion behavior. The proximity to a ferromagnetic instability was suggested by various experiments, yet the precise nature of magnetic fluctuations remained unclear. Previous inelastic neutron scattering studies revealed incommensurate magnetic excitations, raising questions about their origin and relation to superconductivity. Theoretical calculations have attempted to explain the observed phenomena, proposing different scenarios for magnetic interactions and their role in driving superconductivity.

Single crystals of UTe₂ were synthesized using the chemical vapor transport method with iodine as a transport agent. Crystal orientation was determined by Laue x-ray diffraction. Inelastic neutron scattering experiments were conducted using the MACS spectrometer and preliminary measurements on the DCS spectrometer at the NIST Center for Neutron Research. Approximately 1.2 g of coaligned single crystals were used. The scattered neutron intensity, proportional to the imaginary part of the dynamic magnetic susceptibility χ″, was measured as a function of momentum transfer Q and energy transfer E. Background subtraction and symmetrization were performed during data analysis. Lorentzian lineshapes were used to fit the energy dependence of the magnetic susceptibility.

Inelastic neutron scattering measurements revealed magnetic excitations with a peak intensity at 4 meV. These excitations are strongly anisotropic, forming stripes along the crystallographic *a*-axis and modulated along the *b*-axis. They are confined to the edges of the Brillouin zones, exhibiting a clear Q-dependence. The excitations disperse asymmetrically, with a sharp minimum in the peak position at K=1.4 and 4 meV. The energy width of the excitations is comparable to their peak energy. The temperature dependence of the excitations at K=1.4 shows a decrease in intensity and a slight increase in peak position upon warming, closely following the temperature evolution of the low-field magnetic susceptibility along the *b*-axis. The 4 meV energy scale is consistent with the hybridization gap determined by scanning tunneling spectroscopy. In the superconducting state (0.2 K), the magnetic excitation spectrum is similar to that at 5 K, however a slight decrease in intensity near [0, 1.4, 0] is observed. The dispersion along [0, K, 0] is steeper for K<1.4 and gentler for K>1.4 compared to 5 K, suggesting a broader change in the magnetic excitation spectrum than the previously reported 1 meV feature.

The observed magnetic excitations in UTe₂ share similarities with those in other Kondo lattice systems, particularly URu₂Si₂, but with lower dimensionality due to the UTe₂ structure. Their presence doesn't imply a tendency toward a specific type of long-range magnetic order. The close tracking of the excitations with the bulk magnetic susceptibility suggests that these excitations are strongly tied to the underlying electronic structure of the heavy fermion state. The changes in the magnetic excitation spectrum upon entering the superconducting state indicate a significant modification of the electronic structure involving the heavy quasiparticles. This is consistent with prior observations from muon spin relaxation and nuclear magnetic resonance. The observed broad changes in the spectrum in the superconducting state at energies larger than 1 meV are difficult to reconcile with the interpretation of a single superconducting resonance feature. This suggests the possibility that the 1 meV feature might be only one manifestation of a larger-scale alteration in magnetic correlations.

Inelastic neutron scattering reveals magnetic excitations in UTe₂ consistent with Kondo lattice behavior, exhibiting paramagnetic symmetry and tracking the bulk magnetic susceptibility. These excitations are not directly linked to incipient magnetic order. Superconductivity significantly alters these excitations, indicating changes in the electronic structure involving heavy quasiparticles on energy scales much broader than previously thought. Further studies with high-energy resolution are needed to fully understand the interplay between magnetism and superconductivity in UTe₂.

The neutron scattering measurements were limited by the kinematic scattering limits of the spectrometer, preventing a complete comparison of the excitations at higher energies. The high background at certain Q-ranges made it difficult to conclusively identify the previously reported 1 meV feature in the superconducting state.