The dual nature of magnetism in a uranium heavy fermion system

The duality between localized and itinerant nature of magnetism in $5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single crystalline system of UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV, and surprising absence of coherent spin-waves, suggestive of itinerant magnetism. While the gap closes above the Neel temperature, strong dynamic spin correlations persist to high temperature. Nevertheless, the size and temperature dependence of the total magnetic spectral weight can be well described by local moment with $J=4$. Furthermore, polarized neutron measurements reveal that the magnetic fluctuations are mostly transverse, with little or none of the longitudinal component expected for itinerant moments. These results suggest that a dual description of local and itinerant magnetism is required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$ systems in general.