X-ray studies of2kFand4kFanomalies in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)

Abstract

X-ray diffuse-scattering studies on tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) above the 54-K phase transitions reveal two types of one-dimensional scattering at the wave vector $0.295b^{*}$ (earlier assigned to the $2k_F$ wave vector) and at $0.59b^{*}$ (corresponding to twice the former wave vector) which can be attributed to two different anomalies in the phonon dispersion spectrum. Both act as precursor effects for the lower-temperature modulated phases. The two anomalies are remarkably different in their temperature dependence and the polarization of the involved phonon modes. The larger wave-vector anomaly ($4k_F$) is already visible at room temperature, sharpens slowly when the temperature is decreased, and condenses at a temperature lower than 54 K. The anomaly at $0.295b^{*}$, on the contrary, only becomes visible below 150 K, sharpens rapidly with decreasing temperature, and is the first to condense at 54 K into the earlier reported $2a\times 3.4b\times c$-modulated lattice. The $0.295b^{*}$ anomaly corresponds to a polarization with a longitudinal component along $b^{*}$ and a transverse component along $c^{*}$. The $0.59b^{*}$ anomaly on the contrary has a strictly longitudinal polarization directed along $b^{*}$. The occurrence of both polarizations ($b^{*}$ and $c^{*}$) does not affect the interpretation in terms of charge-density waves, as both of these components modify the intermolecular spacing in chain direction because of the tilt angle of the molecules in the stacks. In agreement with independent experimental data, it is suggested that the 54-K phase transition is due to the condensation of the transverse component of the $2k_F$ anomaly, while the 49-K phase transition corresponds to the condensation of both longitudinal anomalies ($2k_F$ and $4k_F$).