Critical Behavior at the Metal-Nonmetal Transition in Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ)

We present the results of high-precision electrical resistivity measurements in the vicinity of the metal-nonmetal transition in TTF-TCNQ. We find a sharp negative divergence in the derivative of the resistivity, $\frac{R^{-1\mathrm{}}\mathrm{dR}}{\mathrm{dT}}$, which suggests the existence of a second-order phase transition at about 53 K. A critical-exponent analysis of this divergence implies that the onset of long-range order occurs with a classical exponent (i.e., $\beta =\frac{1}{2}$) within a three-dimensional region which extends to approximately $\frac{|T-T_c|}{T_c}=t={10}^{-1\mathrm{}}$.