The Transverse Optic Mode Anharmonic Self-Energy of NaCℓ from Measurements by Dispersive Fourier Transform Spectroscopy

摘要

The interatomic forces in a crystal are usually strongly dependent on the interatomic spacing and consequently the lattice potential energy for a crystal can generally be written as a power series in the displacements of the atoms from their equilibrium positions. If such an expansion is terminated at the quadratic terms, then this constitutes the so-called harmonic approximation. In the harmonic approximation the lattice vibrations are true normal modes such that if energy is channeled uniquely into any one lattice mode, then it will remain undissipated in that mode. The harmonic approximation would therefore predict, for instance, that the spectral profiles of lattice vibrations would be a set of undamped, temperature-independent resonances. These and other such predictions are, of course, in marked discord with experimental evidence. As a result, the anharmonic terms in a lattice potential must be considered if a full understanding of many of the physical properties of solids is to be achieved.