Spontaneous symmetry breaking Jahn-Teller effect

Non-adiabatic coupling is also termed vibronic coupling as the resulting breakdown of the adiabatic picture is due to coupling between the nuclear and electi onic motion. A well-known special case of vibronic coupling is the Jahn-Teller effect [14,164-168], in which a symmetrical molecule in a doubly degenerate electronic state will spontaneously distort so as to break the symmetry and remove the degeneracy. 

At semiconductor surfaces, the spontaneous symmetry breaking due to the Jahn-Teller effect is often accompanied by a rehybridi2ation of the surface atoms as sketched in Figure 9.11. Bulk atoms exhibit an sp hybridization of their valence orbitals. At the surface the atoms may change their energy by slight inward or outward displacements. This is accompanied by rehybridization and a variation of the bond between the first and second atomic layers. 

The most important consequence of the breakdown of the B-O approximation is indisputably the Jahn-Teller (J-T) effect [7], where the structure defined on the basis of this approximation does not in fact hold. The important role of the J-T effect is emphasized in Bersuker s book [8] Moreover, since the J-T effect has been shown to be the only source of spontaneous distortion of high-symmetry configurations, we come to the conclusion that the J-T effect is a unique mechanism of all the symmetry breakings in condensed matter. It is of course well known that problems related to the definition of crystallic structure in solid-state physics are mostly ignored assuming only BO structures. Nevertheless it is often criticised by scientists dedicated to studies of the J-T effect. For instance the proper understanding of superconductors should be evidently based on a solution of the non-adiabatic problem but the impact on the crystallic structure is neither reflected in the Frohlich Hamiltonian [9,10] nor in the BCS theory [11]. 

---