Effect of controlled disorder on the electrical properties of TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane): High-temperature regime

The results of a study of the effects of controlled disorder on the electrical conductivity of the one-dimensional conductor, tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), are reported. We find that radiation-induced defects lead to effects which are relatively easy to establish from studies on a given set of crystals, and do not rely on subsequent synthesis and crystal growth. The temperature of the conductivity maximum increases with defect concentration [$\frac{{\mathrm{dT}}_{M}}{\mathrm{dc}}\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}{10}^{2}$ K/(percent defects)], whereas the associated disorder suppresses the phase transitions to lower temperature. The first transition, associated with the onset of long-range order in pure TTF-TCNQ shifts downward with an initial slope of 1.5 \ifmmode\times\else\texttimes\fi{} ${10}^{2}$ K/(percent defects). The 38-K transition, associated with the final locking of the charge-density waves shifts to lower temperatures with an initial slope of 2 \ifmmode\times\else\texttimes\fi{} ${10}^{2}$ K/(percent defects). The data have been analyzed in the temperature range above the conductivity maximum to obtain quantitative information on the overall question of the importance of defects on the electrical transport properties of quasi-one-dimensional conductors. Both the conductivity peak ratio and the absolute magnitude of the room-temperature conductivity are extremely sensitive to induced defects at the level of 20 to 1000 ppm. The results are discussed in the context of the Peierls instability and the structural data available for the TTF-TCNQ system.