Classical versus critical behavior at the charge-density wave instability in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane)

We present high-precision magnetic-susceptibility data in the vicinity of the 53-K transition in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane). It is argued that as T-TC→0, the temperature dependence of the magnetic susceptibility χ mirrors the electronic ordering energy and hence the temperature derivative dχdT behaves like the critical specific heat. A critical-exponent analysis of the susceptibility data yields dχdT~|t|^-12≡|(T-TC)TC|^-12 in agreement with a classical three-dimensional Ornstein-Zernike approximation for the order-parameter correlations. An anomalous contribution to the magnetic susceptibility in the vicinity of $T=48$ K was searched for but not found.