Nonlinear Conductivity in Dicyanoquinonediimine Complexes

Abstract

Nonlinear conductivity is observed below the metal–insulator (M–I) transitions of molecular conductors, halogen-substituted ( R 1 , R 2 -DCNQI) 2 Cu (DCNQI: dicyanoquinonediimine, R 1 , R 2 : methyl or halogen). Despite the difference of the M–I transition temperatures depending on the halogens, these compounds show nonlinear properties at similar low temperatures (<80 K), and the characteristics are regarded as “activation” type. The complex of deuterated dimethyl-DCNQI ( d 2 -DMeDCNQI) 2 Cu, which shows reentrant M–I–M transitions, exhibits irreversible switching from a low-conducting state to a high-conducting state in the intermediate I state. Since the Peierls distortion is irreversibly erased by the electric field, this phenomenon is called “Peierls memory”. In addition, “inverse” nonlinear conductivity from a high-conducting state to a low-conducting state is observed at the low-temperature M state, which is not only entirely reversible but also accompanied by a new kind of rapid current oscillation in the order of 3 kHz. These observations demonstrate metastable nature of the intermediate I state.