Abstract
The intensity of the X-ray diffuse scattering from an electron-phonon system is calculated correct up to the second order of the electron-phonon interaction. The result consists of two terms: one is described as arising from softening of the phonon frequency due to the interaction, and the other as the shift of the center of the phonon coordinate. The former dominates when the electronic motion is faster than the phonon motion, and vice versa. It is argued that for a strongly Coulomb-correlated electron system, such as TTF–TCNQ, the electronic motion is slow enough to make the latter contribution important. A simple model for TTF–TCNQ is presented, which neglects the electron transfer entirely and for which the latter term is expressed in terms of the density correlation function of the electron system. This is calculated by applying the Monte Carlo method to a finite linear chain and also by using the hypernetted chain equation. We find a peak in the correlation function at q=4kF, which explains the 4kF anomaly in the X-ray diffuse scattering found by Kagoshima, Ishiguro and Anzai.
