Two-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 relatively small splitting between the highest occupied and lowest unoccupied 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 analyzed separately do not exhibit nesting properties. However these Fermi surfaces can be considered to arise from the superposition of nested 1D Fermi surfaces which are intended but hidden because of weak band hybridization. Such hidden Fermi surface nesting can lead to electronic instabilities associated with metal to metal or metal to insulator phase transitions. Some real examples of two-band systems as well as metals showing hidden Fermi surface nesting will be discussed.