Spin susceptibility of tetrathiafulvalene tetracyanoquinodimethane, TTF-TCNQ, in the semiconducting regime: Comparison with conductivity

New spin-susceptibility data, determined by electron-spin resonance on single crystals, are reported below the 53-K metal-semiconductor transition in tetrathiafulvalene-tetracyanoquinodimethane, TTF-TCNQ. The measured susceptibility is decomposed into the TTF and TCNQ contributions. Each of these contributions behave differently through the 38- and 53-K phase transitions, allowing us to determine the respective roles of the two stacks in these transitions. We conclude that the TCNQ stack drives the transition at 53 K and that the TTF stack susceptibility is most affected by the 38-K transition. In addition, the temperature dependence of the TTF spin susceptibility is compared to that of the dc conductivity on samples from the same batch. We find that for $T<\mathrm{}38\mathrm{}$ K their activation energies are quite different, ∼ 100-125 K for the susceptibility and ∼ 220 K for the conductivity, which is believed to be primarily on the TTF stack. Three explanations are offered to account for this difference in activation energies: (a) intramolecular Coulomb energies are significant for the TTF stack, (b) the mobility of electrons on the TTF stack is activated, and (c) existence of microdomains and amplitude solitons.