Abstract Scope |
Harmonic models are convenient for free energy, F, and entropy, S. It is tempting to obtain the temperature dependence of F and S by correcting harmonic frequencies, ω, for effects of volume and temperature, V, T. This can be done using perturbation theory, which is often successful for S(ω(V,T)) [1]. When the anharmonicity is large, however, new behaviors arise. In a highly nonlinear medium, the input vibrational amplitude, Ψ, becomes Ψ' = Ψ+ηΨ2. When this medium is driven by two frequencies, e.g., Ψ = exp(-iω1t)+exp(-iω2t), Ψ' exhibits spectral components at new frequencies ω1+ω2 and ω1-ω2, called "intermodulation phonon sidebands". They were recently observed in NaBr at 300 K [2]. At higher T, NaBr showed an additional nonlinear phenomenon, where phonon spectral weight moved between different regions of momentum and energy space. This effect is under study, as are its effects on F and S. [1] DOI: 10.1103/PhysRevLett.125.085504. [2] DOI: 10.1103/PhysRevB.103.134302. |