On nonadiabatic effects in phonon spectra of metals

Experimental and theoretical research on nonadiabatic effects in metals is briefly reviewed. It is shown that these effects are most manifest in optical phonon spectra at small wave vectors $\mathbf{q}$ satisfying the condition $\mathbf{qv}_F<\omega_0(\mathbf{q})$, where $\mathbf{v}_F$ is the electron Fermi velocity, and $\omega_0(\mathbf{q})$ is the phonon frequency. Various theoretical approaches to calculating nonadiabatic effects are examined. Fröhlich's phenomenological field-theoretical model, the most widely used in such calculations, is discussed in detail, some aspects of its application are considered, and its efficiency as a tool for studying nonadiabatic effects is demonstrated.