Polaron origin for anharmonicity of the axial oxygen in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We present exact diagonalization results of an electron-phonon model Hamiltonian for the O(4)-Cu(1)-O(4) cluster in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. For large enough electron–infrared-phonon coupling the motion of holes and ions becomes strongly correlated, i.e., polaronic, leading to a double-well structure for the infrared mode, as encountered in x-ray-absorption fine structure. In contrast, the associated Raman mode shows a single-well behavior even for large electron–Raman-phonon coupling. The appearance of polarons accompanies nonadiabatic behavior yielding optical spectroscopy predictions differing from those obtained from harmonic or anharmonic lattice dynamics models.