Isotope scattering of dispersive phonons in Ge

Abstract

With the use of a lattice-dynamical model which reproduces observed phonon dispersion curves very accurately, the isotope scattering of near-zone-boundary acoustic phonons in Ge is investigated theoretically. We show that the lattice dispersion which becomes appreciable in the THz-frequency region leads to the phonon relaxation time that decreases more rapidly than w-4 • This deviation from the w-4 dependence (the w-4 behavior is predicted by the continuum elasticity theory) amounts to about 20% at I THz and is enhanced heavily at the zone boundaries. Calculations are based on the lowest-order perturbation ~heory. Higherorder corrections are estimated as well and shown to be negligibly small.

Heavy-impurity resonance, hybridization, and phonon spectral functions inFe1−xMxSi (M=Ir, Delaire1, Al-Qasir2, May3 et al. 2015Phys. Rev. B402190View full textAdd to dashboardCite

“…From both INS experiments and simulations, we identified a clear resonance mode associated with the vibration of heavy Os and Ir atoms, which are dynamically decoupled from the phonon dispersions of the host lattice. The widely used expressions for mass-mismatch scattering of phonons by point-defects, are typically derived using perturbation theory 11,12 , assuming that the dispersion frequencies in the crystal are unchanged. For large mass contrast, this assumption is expected to break down, since a new resonance mode emerges, and it can hybridize with the acoustic phonon branches as shown here.…”

“…Massmismatch scattering of phonons in thermoelectric alloys is a broadly adopted strategy to hinder phonon propagation, for example in Si-Ge alloys 2,3 , half-heusler compounds [4][5][6][7] , skutterudites [8][9][10] , and other compounds. Isotopic-impurity scattering, where the mass contrast is relatively low, and the force-constants unperturbed, has been theoretically investigated using perturbation theory 11,12 , resulting in formulas that are widely-used. However, in the case of large mass-contrast, some of the approximations used in these derivations become questionable.…”

Rethinking phonons: The issue of disorder

“…Instead, the VCA accounts for the deviation from such a simplified model by superimposing an additional mechanism for phonon scattering, namely that of compositional impurities/ defects. In this sense, the VCA treats the presence of dissimilar elements in an alloy lattice as though they act as scattering centers for the phonon gas, and the expressions used to model this effect were derived by Tamura 26 in the context of modeling isotope scattering. This additional scattering mechanism then ultimately yields much more frequent scattering, which reduces the TC in much better agreement with experimental data, 13,22,23,25 as compared to a simple rule of mixtures.…”

Phonon scattering rates and atomic ordering inAg1−xSb1+xTe2+x(Ma1, Delaire2, Specht3 et al. 2014Phys. Rev. B130120View full textAdd to dashboardCite

“…It is possible that point-defect scattering has a stronger contribution to the scattering of optical modes, however, since it is expected to scale like g(ω) ω 2 (Refs. 47,48). Also, we emphasize that our model only considered single-defects.…”