Irradiation effects in quasi-one-dimensional organic conductors: The evidence of a transverse fixed-range phonon-assisted hopping

Abstract

Several highly conducting organic quasi-one-dimensional compounds such as tetrathiafulvalene-tetracyanoquinodimethane, tetramethyltetraselenafulvalene-dimethyltetracyanoquinodimethane, hexamethylenetetraselenafulvalene-tetracyanoquinodimethane, tetraselenatetracen-iodine, and nonstoichiometric tetrathiatetracene-iodine have been irradiated at low temperature with neutrons and at room temperature with x rays. After an irradiation dose corresponding to one broken molecule in a few hundreds the resistivity-versus-dose curves become exponential over a several orders-of-magnitude increase in resistance (up to six), and the conductivity-versus-temperature curves become single-activated over a large temperature range. A model is presented which is able to explain these features qualitatively and quantitatively. Irradiation defects are considered to divide the conducting chains into segments. The transverse motion of the electrons which was originally diffusive becomes a phonon-assisted hopping from segment to segment. Previous experimental results from the Pennsylvania and the Budapest groups are considered from the point of view of this new model.