Quantum oscillations of nitrogen atoms in uranium nitride

The vibrational excitations of crystalline solids corresponding to acoustic or optic one phonon modes appear as sharp features in measurements such as neutron spectroscopy. In contrast, many-phonon excitations generally produce a complicated, weak, and featureless response. Here we present time-of-flight neutron scattering measurements for the binary solid uranium nitride (UN), showing well-defined, equally-spaced, high energy vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the fundamental, exactly-solvable problems in quantum mechanics. There are also practical implications, as the oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use UN as a fuel.Comment: 25 pages, 10 figures, submitted to Nature Communications, supplementary information adde

Actinide 5f Systems: Experimental Determination of the Magnetic Response Function

Understanding of metallic actinide systems is in confusion. Searches for crystal-field levels with neutron spectroscopy have, for the most part, been unsuccessful, despite the acknowledged importance of the 5f electrons in determining the magnetic behavior. In systems such as UAl/sub 2/, USn/sub 3/ and UN a broad response function, S(Q vector,..omega..), reminiscent of that found in intermediate valent compounds, exists. Neutron inelastic scattering experiments on single crystals have shown the small influence of the crystal field. Instead we find an unusual response function dominated by the longitudinal susceptibility chi/sup zz/(Q vector,,..omega..) such that transverse excitations - conventional spin waves - do not exist at low energies. As yet a detailed theoretical interpretation of the measurements does not exist. Indeed, the small, although not necessarily negligible, role of the crystal field presents conceptual difficulties if we anticipate behavior analogous to that found in many lanthanide 4f systems. Some alternate approaches are discussed