2-BAND SYSTEMS AND HIDDEN NESTING - NOVEL ASPECTS OF THE ELECTRONIC-STRUCTURE OF LOW-DIMENSIONAL METALS

Some molecules leading to conducting charge transfer salts exhibit a r elatively small splitting between the highest occupied and lowest unoc cupied molecular orbitals. As a result, the partially filled bands of their charge transfer salts can only be adequately described when both orbitals of the same molecule (and thus, the bands they generate) are explicitly taken into account. These systems are thus called two-band systems and lead to interesting band structures. Some low-dimensional oxides and bronzes (but also charge transfer molecular salts) having several partially filled bands exhibit Fermi surfaces that when analyz ed separately do not exhibit nesting properties. However these Fermi s urfaces can be considered to arise from the superposition of nested 1D Fermi surfaces which are intended but hidden because of weak band hyb ridization. Such hidden Fermi surface nesting can lead to electronic i nstabilities associated with metal to metal or metal to insulator phas e transitions. Some real examples of two-band systems as well as metal s showing hidden Fermi surface nesting will be discussed.