Nature Commun. 3, 1124 (2012)

Collective excitations such as phonons, the quantized lattice vibrations that occur in crystalline solids, are a central concept in condensed-matter physics. On the one hand they determine the physical properties of a crystal, such as the heat capacity, and on the other hand, they provide a framework for reducing the many-body phenomena that occur in complex systems into tractable theoretical models. Adam Aczel and colleagues find a beautiful example of this correspondence in uranium nitride, a rock salt-like structure that has recently been put forward as a possible fuel for next-generation nuclear reactors. Using a time-of-flight neutron scattering approach they observe, in addition to the usual phonon modes, a set of clear and equally spaced high-energy vibrational modes. This spectrum is best explained by assuming that each nitrogen atom behaves as an independent harmonic oscillator trapped within an octahedral cage of heavy uranium atoms. The researchers therefore conclude that uranium nitride is an ideal experimental realization of a quantum harmonic oscillator, one of the few quantum mechanical models that can be solved analytically.