Nonlinear excitations in charge- and spin-density-wave materials — Excitons, solitons and (bi)polarons

Various nonlinear excitations in unconventional materials such as charge- and spin-density-wave (CDW and SDW) systems are reviewed. The first to be reviewed is the dynamics of photo-relaxation in a one-dimensional CDW, that is, MX chains (M=Pt or Pd). We conclude that a neutral-soliton pair is photoexcited after the relaxation from the lowest charge-transfer exciton via a self-trapped exciton (STE). Next, we introduce our results on BaBiO3 (BBO) and its doped material, Ba1−xKxBiO3 (BKBO). The former has a three-dimensional CDW as the ground state, and its absorption spectrum is reproduced with direct and indirect excitons. With very few injected holes, there occurs an almost rigid shift of the Fermi energy. When the concentration of holes exceeds some critical value, on the other hand, they are bound and localized, as bipolarons (BPʹs). Lastly, we demonstrate nonlinearity in strong electron correlation, with MX chains (M=Ni) as examples. More specifically, nonlinear couplings among excitons, and among excitons and magnon, manifest themselves in optical spectra.