Mott insulators, electronic

Since the discovery of the first organic conductors based on TTF, [TTF]C1 in 1972 [38] and TTF - TCNQ in 1973 [39], TTF has been the elementary building block of hundreds of conducting salts [40] (1) charge-transfer salts if an electron acceptor such as TCNQ is used, and (2) cation radical salts when an innocent anion is introduced by electrocrystallization [41]. In both cases, a mixed-valence state of the TTF is required to allow for a metallic conductivity (Scheme 5), as the fully oxidized salts of TTF+ cation radicals most often either behave as Mott insulators (weakly interacting spins) or associate into... 

The above simple picture of solids is not universally true because we have a class of crystalline solids, known as Mott insulators, whose electronic properties radically contradict the elementary band theory. Typical examples of Mott insulators are MnO, CoO and NiO, possessing the rocksalt structure. Here the only states in the vicinity of the Fermi level would be the 3d states. The cation d orbitals in the rocksalt structure would be split into t g and eg sets by the octahedral crystal field of the anions. In the transition-metal monoxides, TiO-NiO (3d -3d% the d levels would be partly filled and hence the simple band theory predicts them to be metallic. The prediction is true in TiO... 

Antiferromagnetic insulators of the kind discussed here are sometimes called Mott insulators . It was thought until recently that a crystalline array of one-electron atoms at a sufficient distance apart would necessarily be of this type. In fact, however, many crystals in which each metal atom has a spin and that are... 

In this section we discuss the properties of an electron in the conduction band of an antiferromagnetic insulator. This may be a simple Mott insulator, but, since the experimental evidence is related to them, we first discuss materials like EuSe, where the europium ion has seven 4f-electrons and electrons can be introduced into the conduction band by doping with GdSe the ion Gd2 + has the same number of f-electrons but one more electron in an outer shell, so a Gd ion acts as a donor. 

In this book we treat the discontinuous nature of the transition using an analysis introduced by Brinkman and Rice (1970a, b). This applies to bandcrossing transitions and transitions in an array of one-electron centres. We term the latter Mott transitions when the centres have a moment we do not limit the term to cases when the moment is that of a single spin, and indeed such cases are rare (Chapter 3). The insulating antiferromagnetic state is sometimes called a Mott insulator . A Mott transition can be accompanied by a change of structure (see Section 3 below). 

The band structures of the transition metal monoxides including NiO have been a topic of considerable interest for many years, and study of spectra and transport properties continues in an effort to determine band widths, separations and electrostatic correlation energies. NiO is a Mott insulator (96) and the localized electron description assumed here is probably appropriate. Augmented plane wave band structure calculations have recently been made for NiO and other monoxides (97) and a localized electron multiple scattering Xa calculation for NiO (98). Neither type of calculation includes electron-electron correlation effects. 

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