X-ray diffuse scattering as precursor of incommensurate Peierls transitions in one-dimensional organic charge transfer conductors

Quasi-one dimensional (ID) conductors of the TTF-TCNQ family of charge transfer salts exhibit a Peierls transition which stabilizes a periodic lattice distortion (PLD), accompanied by a charge density wave (CDW) modulation, with an incommensurate 2k(F) (and sometimes 4k(F)) wave vector related to the band filling of the ID electronic subsystem. The Peierls instability is announced on an extended temperature range by ID structural fluctuations giving rise to an X-ray diffuse scattering consisting of diffuse sheets in the reciprocal space. In this paper, we analyse quantitatively the thermal dependence of the correlation length of these ID structural fluctuations, given by the inverse half-width of these diffuse sheets, and of the peak intensity of the diffuse scattering. using the correct treatment of the thermodynamics of the Peierls chain. From this analysis we are able to extract some microscopic parameters of the electron gas under-going the Peierls instability, and. in particular, to quantify the importance of the electron-electron Coulomb repulsions. Finally, we examine the effect of the disorder on the Peierls instability and we show that defects modify the pretransitional fluctuations through the formation of Friedel oscillations whose development is controlled by the PLD-CDW response function of the Peierls chain.