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    Intrinsically Localized Lattice Vibrations in Crystalline Lattices

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    Genre
    Thesis/Dissertation
    Date
    2019
    Author
    Agyare, Benjamin Adu
    Advisor
    Metz, Andreas
    Committee member
    Perdew, John P.
    Wu, Xifan
    Matsika, Spiridoula
    Department
    Physics
    Subject
    Physics
    Physics, Condensed Matter
    Ilms
    Intrinsic
    Lattice
    Modes
    Phonons
    Vibrations
    Permanent link to this record
    http://hdl.handle.net/20.500.12613/644
    
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    DOI
    http://dx.doi.org/10.34944/dspace/626
    Abstract
    We examined the formation of Intrinsically Localized Modes (ILMs) for a pair of harmonic phonons along the direction [111] of the Sodium Iodide (NaI) crystalline lattice. The tendency for ILMs to form at a certain center-of-mass momentum ▁q and corresponding relative momentum vector ▁k is attributed to the van-Hove singularities condition in the non-interaction two-phonon density of states continuum. We observed that, as ▁q converges to the high-symmetry point L=▁q (π/a,π/a,π/a) of the Brillouin zone, the relative momentum vector ▁k remains invariant at ▁k (π/2,π/2,π/2) for a certain threshold value of ▁q, and coalesces at the upper-edge of the two-phonon density of states spectrum with high degeneracy in the two-phonon critical energy. We conclude that the excitation spectra of the pairs of harmonic phonon excitations become energetically degenerate past the threshold ▁q value towards L at the invariant vector ▁k, announcing the strong presence of ILMs. The calculated ILMs were observed at critical energies of 20.0 meV and 25.0 meV for the spring coupling constants ratios K_2/K_1 ≈0.598 and K_2/K_1 ≈0.202 respectively. Reports of Inelastic Neutron Scattering experiments have identified one-phonon breather excitations energy of 10.2 meV at elevated temperatures of 555 K. The formation of ILMs, or multi-phonon bound states, is expected to arise as a result of the anharmonic interactions that lift these degeneracies to enhance the formation of ILMs.
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