Critical behavior at electronically driven structural transitions in anisotropic metals

Abstract

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 $\left(\frac{1}{\rho }\right)\left(\frac{d\rho }{\mathrm{dT}}\right)$, 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 $T_c\simeq 53 \mathrm{and} 29$ 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 $T-T_c\to 0^{+}$, the resistivity is most probably dominated by an enhancement in the electron-phonon scattering associated with the critical fluctuations in the $q=2\mathrm{}{k}_F$ phonons. (ii) For $T-T_c\to 0^{-}$, 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 $\rho$ in the high-temperature phase suggests strongly that the fluctuation growth in these materials is rather isotropic and that for $T$ above $T_c$ there does not exist a large one-dimensional correlation length.