Comment on the double transition observed in tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ)

For a periodic system composed of layers of type A (critical temperature TcA) alternating with layers of type B (TcB<TcA), where the A and B layers are weakly coupled together, a Landau-Ginzgurg free-energy functional predicts an anomalous behavior for the transverse coherence length ξŠ¥(T) with a peak at a temperature Ts‰³TcB in addition to the usual divergence at the critical temperature Tc of the system (TcB<Tc<TcA). It is suggested that this behavior may explain the features of the double transition observed in tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) by recent conductivity and x-ray measurements.