COLLECTIVE EXCITATIONS IN THE ANHARMONIC CRYSTAL

We consider a lattice model of an anharmonic crystal and compute the i sothermal (chi(T)) and adiabatic (chi(S)) susceptibilities for the lat tice displacement below T-c. A detailed analysis is given in terms of the dimension of the lattice and a decay parameter of the harmonic for ce. It turns out that there is a phase-space region for which 0 < chi( T)/chi(S) < 1, indicating breaking of ergodicity, and a region for whi ch chi(T) is finite, indicating the presence of a plasmon frequency du e to the anharmonicity.