A neutron scattering study of the quasi-one-dimensional conductor

Abstract

The Peierls phase transition in the quasi-one-dimensional conductor is investigated by means of elastic and inelastic neutron scattering. The effective critical exponent , extracted from the temperature dependence of the integrated intensity from the CDW satellite reflections, is anomalously low, suggesting that the phase transition may be of first order. The intensity distribution among symmetry-related satellite reflections indicates a domain structure with slowly fluctuating domain populations. Correlation lengths associated with the diverging `central peak' are determined and are found to be nearly isotropic, at variance with results obtained on other quasi-one-dimensional compounds, such as platinum chains (KCP) or blue bronze, . Doping with 1.2% Nb has a severe effect on the modulated state. The low-temperature satellites are replaced by a diffuse scattering distribution elongated along . The absence of a phonon soft mode and the presence of a diverging central peak at the phase transition is interpreted within the framework of strong electron-phonon coupling. Finally, we propose a Ginzburg-Landau phenomenological model, where the interplay between the electronically coupled optical-like order parameter (Ta-atom tetramerization along the chain axis) and the elastic deformations lies at the origin of the phase transition in .