The vibrational excitations of crystalline solids corresponding to acoustic or optic one-phononmodes 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 uraniumnitride, showing well-defined, equally spaced, high-energy vibrational modes in addition to theusual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillatorand characterizes independent motions of light nitrogen atoms, each found in an octahedralcage of heavy uranium atoms. This is an unexpected and beautiful experimental realizationof one of the fundamental, exactly solvable problems in quantum mechanics. There are alsopractical implications, as the oscillator modes must be accounted for in the design of generationIV nuclear reactors that plan to use uranium nitride as a fuel.
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