STRONG ELECTRON-PHONON COUPLING AND THE ORTHORHOMBIC TO TETRAGONAL TRANSITION IN La2CuO4

Abstract

It is proposed that the main contribution to the electron-photon coupling in ionic metals arises from the deformation induced variation of the crystal field on the ionic sites which are involved in conduction. The latter are assumed here to be the oxygen sites in the CuO 2 planes of the layered metal oxides. The coupling of holes on those sites to the tilting mode of the La 2 CuO 4 lattice is investigated in detail. Although the coupling is quadratic in small tilting displacement the large value of the corresponding coupling constant explains the destabilization of the tilted (orthorhombic) La 2 CuO 4 lattice on increasing the hole concentration. It is shown that the holes are suppressing the tilt locally, creating the regions of the tetragonal please, as observed recently in the photogeneration experiments. The stability of the corresponding small polarons (tiltons) is discussed in detail.