Anharmoniset

Anharmoniset refers to a class of higher-order effects in solid-state physics where the relationship between the elastic constants of a material and its vibrational modes deviates from a harmonic description. In other words, the vibrations of a material's atoms or molecules do not behave in a mathematically predictable way, leading to non-linear interactions.

At low temperatures, the vibrational modes of a material can be described using harmonic oscillator theory, which assumes that the restoring force is proportional to the displacement of the atoms or molecules. However, as the temperature increases, or when the material's energy levels become more dense, anharmonic effects become significant. These effects can be both intrinsically related to the material's internal structure and their interactions or arise extrinsically from external influences.

There are two main types of anharmonic effects that occur within a solid: third-order and second-order effects. Third-order effects are typically smaller and are only relevant at higher temperatures or in materials with weak couplings between atoms or molecules. Second-order effects are more significant, as they result from the pairwise interactions between atoms or molecules and can lead to acoustic phonon-phonon interactions.

The anharmonic effects can lead to changes in the material's thermal expansion coefficient, the temperature dependence of the elastic constants, and can affect the thermal conductivity and specific heat capacity measurements. This understanding of anharmonic effects is essential in a wide range of scientific and technological applications, from developing new materials for thermal and energy management to understanding the behavior of high-temperature materials in extreme environments.