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 $T\ensuremath{-}{T}_{C}\ensuremath{\rightarrow}0$, the temperature dependence of the magnetic susceptibility $\ensuremath{\chi}$ mirrors the electronic ordering energy and hence the temperature derivative $\frac{d\ensuremath{\chi}}{\mathrm{dT}}$ behaves like the critical specific heat. A critical-exponent analysis of the susceptibility data yields $\frac{d\ensuremath{\chi}}{\mathrm{dT}}\ensuremath{\sim}{|t|}^{\frac{\ensuremath{-}1}{2}}\ensuremath{\equiv}{|\frac{(T\ensuremath{-}{T}_{C})}{{T}_{C}}|}^{\frac{\ensuremath{-}1}{2}}$ 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.