Systematic study of the transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and its selenium analogs

Abstract

We have carried out a comparative study of the dc conductivity in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and its isostructural selenium analogs TSeF-TCNQ (tetraselenafulvalene) and cis/trans-DSeDTF-TCNQ (diselenadithiafulvalene) below their metal-insulator transitions. Our study reveals the presence of a second transition in TTF-TCNQ at ∼38 K and the absence of a similar transition in TSeF-TCNQ. We have characterized the 38-K transition and shown that it involves primarily the TTF chains. From the sharpness of the two transitions in TTF-TCNQ and the single transition at ∼29 K in TSeF-TCNQ it is argued that they are related to a two- or three-dimensional ordering. The potentially disordered system DSeDTF-TCNQ does not possess any sharp transition. It is pointed out that the transition temperature defined as the sharp peak in the logarithmic derivative of the conductivity and not the conductivity itself relates more closely to the underlying thermodynamic transition. It is observed that the ratio of the zero-temperature gap to the transition temperature is the same in these systems and in many, if not all, other quasi-one-dimensional systems which undergo a metal-insulator transition. This ratio has a value of 8-10 and is considerably larger than 3.5 which is predicted by the mean-field theory. A model which is consistent with the observation of the transitions in these systems is suggested in which the coupling betwen TCNQ chains drives a two- or three-dimensional ordering of the fluctuation distortion giving rise to a metal-insulator transition.