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Topological Oscillators: Good Vibrations

Next generation of time-of-flight inelastic neutron scattering instruments are now enabling the exploration of new realms of dynamics in solids.  In this talk I will briefly describe spectrometers now operating at the Spallation Neutron Source, and then focus on one recent experiment where startling and beautiful results have been obtained.  The particular example is a measurement of vibrational excitations in a single crystal of uranium nitride (UN), which has a simple cubic rock-salt structure.  In addition to the usual acoustic and optic phonon excitations one observes a ladder of equally spaced well-defined modes extending to energies at least as high as 500 meV.  Analysis of the momentum and energy dependence of the scattering and additional insights obtained from ab-initio calculations show that each individual nitrogen atom is a nearly ideal realization of the isotropic three dimensional quantum harmonic oscillator, one of the few exactly solvable problems in quantum mechanics.  This behavior arises in part because of the large mass difference between the nitrogen and uranium atoms. Knowledge of the oscillator modes is important for at least one potential application of UN.  The phenomenon should be present in other materials and it has now been seen in uranium carbide.   For more information see Nature Communications 3, 1124 (2012); also research highlights comment in Nature Materials 11, 1002 (2012).

*Work in collaboration with A. A.  Aczel, G. E. Granroth, G. J. MacDougall, W. J. L. Buyers, D. L. Abernathy, G. D. Samolyuk and G. M. Stocks, and recently Yuen Yiu.  Research at the SNS, ORNL is supported by the U. S. Department of Energy, Office of Basic Energy Sciences, and Scientific User Facilities Division.