Anharmonic Damping of Phonon Modes in the Face-Centered Cubic Metals
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Cubic crystal system
Monotone cubic interpolation
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Soft phonon modes in strongly anharmonic crystals are often neglected in calculations of phonon-related properties. Herein, we experimentally measure the temperature effects on the band gap of cubic SrTiO3, and compare with first-principles calculations by accounting for electron-phonon coupling using harmonic and anharmonic phonon modes. The harmonic phonon modes show an increase in the band gap with temperature using either Allen-Heine-Cardona theory or finite-displacement approach, and with semilocal or hybrid exchange-correlation functionals. This finding is in contrast with experimental results that show a decrease in the band gap with temperature. We show that the disagreement can be rectified by using anharmonic phonon modes that modify the contributions not only from the significantly corrected soft modes, but also from the modes that show little correction in frequencies. Our results confirm the importance of soft-phonon modes that are often neglected in the computation of phonon-related properties and particularly in electron-phonon coupling.
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Abstract Phonon properties have been studied using reduced sound velocity of Eu 0.5 Ba 0.5 TiO 3 (EBTO). To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced sound velocity of multiferroic EBTO exhibits a kink at T N = 1.9 K. This anomalously reduced sound velocity can be interpreted as an effect of vanishing magnetic ordering above T N . What’s more, the ferroelectric subsystem cannot be influenced by the magnetic subsystem above T N for T N ≪T C in the EBTO. It was found that the reduced sound velocity decreases as T increases near ferroelectric transition TC. That is to say, the sound velocity softens near ferroelectric transition TC. It is also noteworthy that the reduced sound velocity softens when the RE (the coupling between the ferroelectric pseudo-spins and phonons), V (3) and |V (4) | (the third- and fourth-order atomic force constants of the anharmonic phonons, respectively) increase. These conclusions are all in good accordance with the experimental data and theoretical results.
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Effects of the lattice dispersion upon anharmonic decay of near-zone-boundary LA phonons in Ge are studied by making reference to the two-phonon density of states in the [100] and [111] directions. As the wave vector approaches Brillouin-zone boundaries, the two-phonon density of states increases more rapidly than ${\ensuremath{\omega}}^{2}$, implying the spontaneous decay rates of the large-wave-vector LA phonons become more frequent as compared to ${\ensuremath{\omega}}^{5}$ dependence predicted in the continuum elasticity theory.
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The prototype phonon-liquid electron-crystal β-Cu2Se has been ranked among the best thermoelectric material with its ultralow lattice thermal conductivity (κ L).
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The dynamic form factor $S(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},\ensuremath{\omega})$ and the phonon frequencies and lifetimes in Cs at $T=5,90$, and 290 K are calculated. The anharmonic properties of Cs are apparently dominated by the cubic anharmonic term. Properties such as anharmonic interference in $S(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},\ensuremath{\omega})$ which is proportional to the cubic term are found to be relatively large and should be readily observable at high temperature. The most interesting phonons are the longitudinal phonons along the [100] direction at high $Q$, which have the shortest lifetimes, and the ${T}_{1}$ phonons along [110] which are very sensitive to anharmonic contributions.
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