EXCITONIC INSTABILITIES OF DEFORMABLE LATTICE - FROM SELF-TRAPPING TOPHASE-TRANSITION

The optically created exciton will be self-trapped if its coupling to phonons is strong enough, and will moreover be self-decomposed if the electron and the hole couple to phonons in an opposite way. The bistab ility between the parity-conserved and parity-broken self-trapped exci tons was observed in alkali halides. The situation is most dramatic if the bistability between the parity-broken self-trapped exciton and th e ground state (with no exciton) comes into play since the electron-ho le pairs may then be spontaneously generated at every lattice site, re sulting in the electronic and structural phase transition. The neutral to ionic phase transition observed in a few organic charge transfer c ompounds under applied pressure or decreasing temperature can be consi dered as an example. Recent experiment revealed that TTF-chloranil, am ong others, is subject to photo-induced transient phase change over hu ndreds of unit cells per one photon. The dynamics of this process can be described in terms of self-trapping and self-multiplication of a ph oto-generated charge transfer exciton along the chain through the attr active dipolar interaction. This description of phase transition in te rms of exciton dynamics will provide a new paradigm of materiology.