Non-Ohmic current–voltage characteristics in the positively charged tetrathiafulvalene (TTF) molecular crystals

Abstract

The mechanism of an extraordinary increase in conductivity at larger applied voltage than the critical voltage generally observed in the one-dimensional electronic systems such as tetrathiafulvalenium–tetracyanoquinodimethanide (TTF–TCNQ) is quantitatively investigated. The tunneling effects of the nondissipative diamagnetic currents between the two neighboring localized TTF unit cells and the current–voltage characteristics at low temperature process (0 K < T < 14 K) are investigated. The transition probability ( P TC) of electrons between two neighboring unit cells is estimated to be in the order of 10 −3–10 −4. The estimated density of states near the Fermi level in the Peierls distorted structures ( N PD( ɛ F)) on the basis of the experimental results of the current–voltage characteristics is always larger than that ( N NM( ɛ F)) in the normal metallic states in TTF–TCNQ molecular crystals. This result can be rationalized from the fact that the density of states near the Fermi level in the Peierls distorted states is enhanced as a result of the congestion of the energy levels. That is, the N PD( ɛ F) values estimated on the basis of the tunneling effect theory suggested in this paper are well rationalized. The tunneling effects theory of the nondissipative diamagnetic currents is compared with the incommensurate charge-density-wave (ICDW) sliding theory. According to our quantitative calculated results, the phenomena described above can be understood if we consider that such an extraordinary increase in conductivity originates from the tunneling effects of the nondissipative localized diamagnetic currents between the two neighboring unit cells.