NONLINEAR LATTICE RELAXATIONS AND PROLIFERATION OF EXCITONS IN ONE-DIMENSIONAL AND 2-DIMENSIONAL CHARGE-DENSITY WAVES

One- and two-dimensional extended Peierls-Hubbard models are investigated, so as to clarify nonlinear natures of lattice relaxation processes of photogenerated excitons in charge-density wave (CDW) states. This theory is mainly based on the adiabatic approximation for phonons and the mean-field theory for interelectron interactions, but is also reinforced by taking the electron-hole correlation into account, so that we can obtain the exciton. Potential energy surfaces relevant to the relaxation of the exciton are calculated by this theoretical method, and the possible relaxation paths are clarified. In the one-dimensional CDW, the exciton relaxes down to a macroscopic excited state, in which the phase of the Peierls distortion is completely inverted from that of the starting ground state. In the two-dimensional case, on the other hand, the exciton relaxes down to form a local excited domain, wherein the spin-density-wave-type order appears over several metallic sites.