The ground-state and first-excited-state energy of the bound surface polaron

Abstract

The authors consider the problem of an exterior surface polaron bound to the surface of a polar crystal both in the case where it is free to move on the surface and in the case where it is bound to a hydrogenic defect. In both cases the ground state and the first relaxed excited state are calculated. The non-diagonal Fock approximation is used to obtain the energies using a variational technique valid for any value of the electron-phonon coupling constant alpha or of the Rydberg constant R. Qualitatively, the authors obtain the same type of behaviour for the surface case as is found for the free or bound bulk polaron. Quantitatively, for equal values of alpha and R, the self-energy and the binding energy are larger in the surface case than in the bulk case. The polaron is thus more stable in the two-dimensional case. Finally, for a given crystal, the values of alpha and R are found to be larger for the surface polaron than for the bulk case.