Large polaron in an anharmonic crystal lattice

Extending the large Frohlich polaron problem to an anharmonic lattice the author studies a polaronic state with a large radius of the wave function. The appropriate anharmonic part of the e-ph interaction Hamiltinian is derived, based on methods of quantun field theory. Then, with the help of perturbation theory, the anharmonic contribution to the ground-state energy of a polaron is calculated. To estimate shows that the anharmonicity can considerably increase the large polaron binding energy in a situation of a moderately strong e-ph coupling. A comparison of a developed theory with other anharmonic models of a polaronic state is drawn.