Insulator surface, electronic states

An early success of quantum mechanics was the explanation by Wilson (1931a, b) of the reason for the sharp distinction between metals and non-metals. In crystalline materials the energies of the electron states lie in bands a non-metal is a material in which all bands are full or empty, while in a metal one or more bands are only partly full. This distinction has stood the test of time the Fermi energy of a metal, separating occupied from unoccupied states, and the Fermi surface separating them in k-space are not only features of a simple model in which electrons do not interact with one another, but have proved to be physical quantities that can be measured. Any metal-insulator transition in a crystalline material, at any rate at zero temperature, must be a transition from a situation in which bands overlap to a situation when they do not Band-crossing metal-insulator transitions, such as that of barium under pressure, are described in this book. 

The detection and measurement through photoemission spectroscopy of the single particle excitation associated with surface valence electron states is made difficult by the accompanying spectrum of the bulk material. When there is a gap in the bulk density of states at e, as in insulators and semiconductors, there is the possibility of observing surface states which lie in the gap. UPS has been used to observe such states in Si (60). It is practicable also to search for these states in metals at energies where the densities of bulk valence states are low or relatively structureless. Some suitable candidates for investigation should be the transition metals Ti, Zr, Hf, Cr, Mo, and W, which have low q(e) in the vicinity... 

The electronic properties of organic conductors are discussed by physicists in terms of band structure and Fermi surface. The shape of the band structure is defined by the dispersion energy and characterizes the electronic properties of the material (semiconductor, semimetals, metals, etc.) the Fermi surface is the limit between empty and occupied electronic states, and its shape (open, closed, nested, etc.) characterizes the dimensionality of the electron gas. From band dispersion and filling one can easily deduce whether the studied material is a metal, a semiconductor, or an insulator (occurrence of a gap at the Fermi energy). The intra- and interchain band-widths can be estimated, for example, from normal-incidence polarized reflectance, and the densities of state at the Fermi level can be used in the modeling of physical observations. The Fermi surface topology is of importance to predict or explain the existence of instabilities of the electronic gas (nesting vector concept see Chapter 2 of this book). Fermi surfaces calculated from structural data can be compared to those observed by means of the Shubnikov-de Hass method in the case of two- or three-dimensional metals [152]. 

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