Quasi-two-dimensional conductivity

We review briefly the expressions and concepts needed to describe the electronic phenomena of surfaces and space charges. We refer the reader to the extensive literature on crystalline semiconductors (Many et al., 1965 Sze, 1969 Many, 1974 Goetzberger et al, 1976). It will be shown that the continuous distribution of gap states in amorphous semiconductors requires some changes in the expressions for the conduction and potential distribution in the space-charge region and that the quasi-two-dimensional conduction in a narrow potential well of a space-charge layer introduces some new and as yet unresolved problems. 

Quasi-two-dimensional conductivity extreme anisotropy of transport and low energy excitations. However, the least anisotropic material YBa2Cu307, has a much higher than other more 2D cuprates. 

The aim of this article is to show that the new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 [hereafter called (BEDO)CsHg] (BEDO-TTF - bis-(ethylenedioxy)tetrathiafulvalene) which contains closed and open orbits displays rather complicated oscillatory spectra associated with magnetic breakdown (MB) and quantum interference (QI) effects. Tight binding band structure calculations for this compound are proposed to characterise its Fermi surface. The aim of the article includes also an investigation of the optical conductivity anisotropy with polarized infrared reflectance spectra. 

The behaviour of the polarized reflectivity and optical conductivity spectra of new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 versus temperature for E L and E1. L are quite different. For E . L, the temperature changes of R(ro) and ct(co) are due to the decrease of the optical relaxation constant of the free carriers as expected for a metal. For E L at temperatures below 200 K, the energy gaps in the ct(co) spectra at about 4000 cm 1 and at frequencies below 700 cm 1 appear simultaneously with the two new bands of ag vibrations of the BEDO-TTF molecule activated by EMV coupling. This suggests a dimerization of the BEDO-TTF molecules in the stacks, which leads to a metal-semiconductor transition.. In the direction perpendicular to L, the studied salt shows metallic properties due to a very favourable overlap of the BEDO-TTF molecular orbitals. 

A direct verification of a quasi-two-dimensional electric conductivity, which is so unusual for TCNQ-based ARS, can be performed by studying the electric resistance anisotropy of [N-C2H5-Pz](MTCNQ)2 single crystals. 

We will see that it is the interactions between chains or sheets that is responsible for finite temperature transitions. That is the reason for labeling systems of chains or sheet as quasi-one- or two-dimensional. They exhibit one- or two-dimensional behavior until there occurs a crossover to higher dimensionality. Obviously, crystals do exist and as such must be three-dimensional. Binding forces are intrinsically three-dimensional. Let us then take for granted the existence of the crystalline backbone of organic conductors and concentrate on the electronic properties which are of interest to conduction. It is the strong anisotropy in these which is responsible for the stamp of quasi-one- or quasi-two-dimensional solids. 

---