Principles that govern electronic transport in organic conductors and transistors

Abstract

Energy bands of organic conductors are calculated on the basis of the estimation of intermolecular overlap integrals and the tight-binding approximation. The resulting Fermi surface has been investigated by the measurements of low-temperature magnetoresistance in detail. However, we have to take electron correlation into account to explain the variation of the metal-insulator transition temperatures and the universal phase diagram. In particular, intermolecular Coulomb repulsion gives a variety of charge-order patterns, in which non-stripe charge order is important in a triangular network of organic conductors. Non-stripe charge order is an origin of flat resistivity, nonlinear conductivity, and potentially Dirac fermions. The estimation of intermolecular interaction is extended to the pdsystems, where the magnetic interactions J between the p-electrons and metal spins make a network. To discuss the charge transport in organic transistors, energy levels of the molecules are important. However, since the energy levels are considerably modified at the metal/organic interface, it is useful to use chemical doping and organic charge-transfer salts in the conducting parts of organic transistors. Temperature dependence of an organic transistor comes from the midgap trap states, but eliminating the traps in a single-crystal transistor, we can achieve band-like transport maintained down to low temperatures.