Contributions of one- and two-phonon processes to the resistivity of TTF-TCNQ

Abstract

There is a long-standing disagreement on the relative contributions of one-phonon (1p) and two-phonon (2p) scattering processes to the resistivity ϱ TTF - TCNQ in the temperature range from 300K down to 100 to 150K. The group that believes 2p processes, in particular two-libron (2λ) processes, determine ϱ has argued that their predominance is due to the fact that they cause a larger modulation of the transfer integral t than 1p processes. We have calculated 1/t times the first and second derivatives of t with respect to the displacements due to the basic external lattice modes; these are the quantities that enter into the resistivity. The calculations were done for the TCNQ chain, which contributes most of the conductivity of TTF - TCNQ. Using the resulting values, we find that the contribution of 2λ, or indeed any 2p, modes to the resistivity of the TCNQ chain is negligible. The calculated coupling constants for 1p processes involving the external modes are large enough so that, in combination with the contribution of the internal modes, they could account for the observed ϱ.