Mott isolator

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As Mott insulators materials are referred to by the band model actually electrically conductive should be, but in the experiment as insulators prove. The name goes back to the British physicist Sir Nevill F. Mott , who dealt with this phenomenon. An example of a Mott insulator is nickel (II) oxide .

history

In 1974 Mott predicted states of matter in which the central assumption of the ribbon model - each electron moves independently and only senses the presence of the other electrons based on an averaged potential - no longer applies. The reason for this is the slight interactions between the electrons, which result from the classic Coulomb repulsion between particles with the same charge. They lead to a change in the periodic potentials of the atoms: the individual potential walls increase. This is particularly important in solids in which the Coulomb threshold and delocalized electrons are in a sensitive balance. Even small changes in the form or height of the potential cause metal- insulator transitions in them . The conduction electrons can no longer move freely through the crystal . They behave like spatially localized particles bound to individual atoms and are placed in the potential wells in the changed energy landscape like eggs in an egg carton. The material becomes an insulator.

Mott insulators are ideally suited to investigate fundamental questions of solid-state physics as well as atomic physics , including the individual interaction of electrons with one another under given conditions. They also play a role in some theories explaining high temperature superconductors .

Manufacturing

Metal-insulator junctions of the Mott type can be brought about , for example, by changing the pressure or by doping with foreign atoms. If the pressure in a non-conductive sample is increased, the distance between the (ionized) atoms becomes smaller. The number of free electrons and their mutual influence increases to the same extent. In the event of a material-dependent, critical pressure, the conduction electrons are suddenly released and the solid becomes a metallic conductor. If semiconductors such as silicon carbide or gallium arsenide are doped with suitable materials, the number of electrons also increases. According to the ribbon model, a metal should have been created, but interactions between the electrons according to the Mott model turn the material into an insulator.

A completely different class of Mott insulators can be realized with the help of thin surface layers. In large solids, the tendency of electrons to delocalize generally wins, making them electrical conductors. However, if the electrons are spatially restricted, for example to the extremely thin surface layer of a semiconductor, a Mott insulator should be created. A systematic search for such materials began in the 1990s. A classic experiment is the manufacture of a Mott insulator on a silicon surface. Implanted boron initially depletes electrons, then potassium is absorbed until enough electrons are available to achieve metallic conductivity according to the Bloch-Wilson model. To investigate the electronic configuration, the potassium-covered silicon surface is irradiated with (short-wave) light, which knocks electrons out of the material. The energy distribution of the emitted photoelectrons shows a clear band gap between filled and empty energy bands, the characteristic of an insulator.

Conversion of a Bose-Einstein condensate into a Mott isolator

In 2002, Munich scientists reported for the first time about the conversion of a Bose-Einstein condensate (BEC) into a Mott isolator near absolute temperature zero. The researchers captured the superfluid quantum collective in a three-dimensional network of laser beams and pushed its members to precisely defined grid locations. The community disintegrated into individual atoms, which were locked in individual wave troughs. With the laser grid, the depth and periodicity of the energy landscape could also be changed as desired and thus switched back and forth between the Mott isolator and a Bose-Einstein condensate. Such Mott isolators, as switching and computing elements, open up new perspectives on the way to a quantum computer .

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