Critical behavior at electronically driven structural transitions in anisotropic metals

We present the results cf high-precision electrical-resistivity measurements in the vicinity of the three-dimensional structural transitions in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) and TSeF-TCNQ (tetraselenafulvalene-tetracyanoquinodimethane). We find a sharp negative divergence in the derivative of the resistivity (1ρ)(dρdT), which is similar in form to the resistive anomalies observed in metallic antiferromagnets and suggests the existence of a second-order phase transition at about Tc53 and 29 K in TTF-TCNQ and TSeF-TCNQ, respectively. A systematic comparison between theory and experiment including a critical-exponent analysis of the resistive anomaly has led to the following conclusions: (i) For T-Tc→0⁺, the resistivity is most probably dominated by an enhancement in the electron-phonon scattering associated with the critical fluctuations in the q=2kF phonons. (ii) For T-Tc→0^-, the resistivity appears to be dominated by the opening of an electronic gap at the Fermi surface. (iii) The overall structure of the resistive anomaly including the temperature dependence of ρ in the high-temperature phase suggests strongly that the fluctuation growth in these materials is rather isotropic and that for T above Tc there does not exist a large one-dimensional correlation length.