Raman scattering from charge-density waves and application to polyacetylene

Oscillations in an electronic gap formed by a charge-density wave are described in terms of Raman-active amplitude modes. In a multi-phonon system the authors find the Raman shifts and their intensity ratios in terms of a parameter lambda which contains all effects of electron-phonon, electron-electron and electron-disorder interactions; the single assumption in the theory is the adiabatic limit. The theory is compared with data on polyacetylene where the relation of lambda with the electronic gap yields information on the type of disorder. For an electron-phonon system an ab initio calculation of the Raman cross section is given and compared with more conventional calculations.