Systematic study of the lattice dynamics of the uranium rocksalt-structure compounds

Quantum oscillations of nitrogen atoms in uranium nitride

“…The solid red curve is a fit to a superposition of Gaussian peaks at the mode positions. The measured phonon dispersion relations are consistent with earlier investigations 2,20 . The black lines in Fig.…”

“…The vibrational spectrum at energies above those of the highest optic phonon mode is generally a complicated many phonon continuum that is often weak and featureless. In sharp contrast, for the binary solid uranium nitride (UN) [2][3][4][5] , where the nitrogen atoms are very light compared with the uranium atoms, our inelastic neutron-scattering measurements reveal that the high-energy spectrum is greatly simplified and consists of a set of equally-spaced, well-defined modes that can be measured to energies at least ten times as large as that of the optic phonon modes. This data is best explained by assuming that each nitrogen atom behaves as an independent quantum harmonic oscillator (QHO), and the nitrogen motion is therefore a rare experimental realization of an exactly soluble, three-dimensional model in quantum mechanics 6 .…”

“…To perform the measurements, the same UN crystal utilized by Jackman et al 2 was aligned with the (HHL) plane horizontal and cooled to a temperature T = 5 K. Figure 2a shows representative results obtained at SEQUOIA using an incident neutron energy E i = 80 meV for a typical symmetry direction in reciprocal space. A constant Q-cut through the data, at the position corresponding to the vertical line, is illustrated in Fig.…”

“…2a are obtained from a fit of the data to a simplified model 16 that includes only nearest-neighbour U-N (65 ± 2 N m − 1 ) and U-U (50 ± 4 N m − 1 ) force constants. This model suffices for illustrative purposes, although more detailed models 2,[8][9][10]21,22 give a closer match to the observed dispersion. Some of those models also include N-N force constants, although they are found to be negligibly small compared with their U-N and U-U counterparts.…”

“…The primary magnetic and lattice excitations of UN have been investigated previously via inelastic neutron scattering 2,3,16 . Despite these efforts, several intriguing open questions remain concerning the details of the experimental spectra.…”