Metal–insulator transition

A metal -insulator transition is called a phase transition, which is based on the quantum properties of matter. In this case, in particular, changes occur to the transport properties of a material, such as the electric conductivity or the reflectivity between the values ​​which are typical of metals or for insulators.

Electrical conductivity

One of the most important properties of metals is their electrical conductivity. Metals have a very high electrical conductivity, while in contrast lead to insulators electrical charges very bad. Certain materials may under specific conditions, change change in pressure, temperature, density or disorder degree from one state to the other. In the latter case one also speaks of a metal -insulator transition of the Anderson - type (see localization ( physics ) ), in the first three cases, however, of a Mott transition.

In the physical model of a metal is a material having a Fermi energy is at a temperature of 0 C, in other words at zero temperature, in a belt. As a consequence of the conduction band is not fully occupied. This results in a very high conductivity in general. With an insulator, however, the Fermi energy is located in a band gap, so that the states in the valence band are fully occupied, which completely prevents electrical conduction. At finite temperatures, this distinction is not unique in general. Metals have a lower electrical conductivity than the free electron gas. There is no perfect insulator that absolutely does not conduct current. The main reason for this are thermally excited electrons that can occupy due to their kinetic energy states well above the Fermi energy. In semiconductors, which have a very small band gap, a strong increase of the electrical conductivity due to the occupation of the conduction band can be generated by thermally excited electrons. The distinction between metal and insulator is, therefore, not always clear, and the transition between the two states is often continuous.

Mott effect

Mott effect describes the transition of an insulator into a metal due to the increase of pressure, the isolator is a simple hydrogen crystal whose atoms are arranged in hexagonal close -packing and the atomic orbitals do not overlap. This grid is an insulator, when the lattice constant is large enough. Pressed to this grid together, so we reduced the lattice constant, this material will be transferred to a metal. The critical value is a density or at a value of the lattice constant corresponding to the Mott criterion:

Wherein the Bohr radius, and n is the number density of the system. The mechanism of this transition is that the band gap is reduced, and the conduction and the valence band finally overlap. Then the Fermi energy lies within a band, and the system is a metal, even at low temperatures. The fundamental insight of this thought experiment is that turns at high enough pressure each insulator in a metal.

Electron localization

The reverse case that a metal, which is a conductor at 0 K, is an insulator, is a much more difficult to explain theoretically phenomenon Represents the breakdown of the electrical conductivity can be explained by a localization of the electrons in the metal unlokalisierten states. The theoretical interpretation is essentially based on that repel the charge carriers in the insulator state so strong that they can not propagate. This localization comes about through interaction effects. For the explanation of different models come into question. In the model of the Anderson transition ( Philip W. Anderson) localization of the electron states occurs due to statistical disorder in the system. In the Mott -Hubbard transition ( Nevill Francis Mott, John Hubbard ( physicist ) ) provide strong electron-electron correlations for a freeze on local variations of the electron concentration. With the electron -lattice coupling, there is another possibility, with a strong restriction of movement of the electrons can be described.

In this way one can understand physically why certain metal oxides, sulfides and selenides, such as NiO, insulators, etc..

Examples

An example of a temperature- driven metal -insulator transition was measured in phosphorus -doped silicon. At temperatures below 0.1 K and donor concentrations of 3.6 × 10-18 cm - 3, the highly conductive material is an insulator.

Another example is carbon, wherein the metal -insulator transition is caused by a change of the spatial structure. While graphite is metallic properties, diamond is an insulator. An interesting two-dimensional limiting case represents graph in which it is the individual isolated graphite layer.