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 $T-T_C\to 0$, the temperature dependence of the magnetic susceptibility $\chi$ mirrors the electronic ordering energy and hence the temperature derivative $\frac{d\chi }{\mathrm{dT}}$ behaves like the critical specific heat. A critical-exponent analysis of the susceptibility data yields $\frac{d\chi }{\mathrm{dT}}\sim {\mathrm{}\left|t\right|\mathrm{}}^{\frac{-1\mathrm{}}{2}}\equiv {\left|\frac{(T-T_C)}{T_C}\right|}^{\frac{-1\mathrm{}}{2}}$ 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 $T=48\mathrm{}$ K was searched for but not found.