Negatively-charged dust particles in plasmas can form crystal-like structures. Mitsuhiro Nambu, Sergey Vladimirov and Osamu Ishihara have suggested that the mechanism by which this occurs may be analogous to the effect which produces superconductivity.
Plasmas are essentially gases in which the constituent particles are electrically charged. Each particle is either positive or negative according to whether it has an excess of protons (+) or electrons (-). In high-temperature plasmas, such as in a fluorescent light tube or in the region around a star, most of the particles are simple, consisting of single electrons and positively charged atomic nuclei. However in recent years a great deal of interest has developed in so-called ``dusty plasmas''. These are low-temperature plasmas that contain highly charged particulates which may be as large as a micron (a thousandth of a mm) across.
Recent experiments have revealed that the particulates embedded in plasmas have an intriguing tendency to form crystalline structures - i.e. regular, repeating arrangements of particulates. The formation of these structures has been difficult to explain because the electrostatic forces between the negatively-charged dust particles themselves are repulsive, since particles with the same sense of electric charge repel one another. Prof. Mitsuhiro Nambu (Kyushu University; currently at Tokyo Metropolitan Institute of Technology), Sergey Vladimirov and Prof. Osamu Ishihara (Texas Tech University; currently at Yokohama University) have suggested that the explanation may lie in collective effects, where large numbers of the particles act in a coherent manner, between the dusty particles and the flow of positively charged ions through the static crystalline structure towards the negatively charged electrode near which the crystalline structure forms. These effects are analogous to the formation of so-called ``Cooper pairs'' of electrons that give rise to superconductivity, and this theory explains the formation of both square and hexagonal crystals, as observed experimentally.
A micrograph and sketches of an hexagonal lattice, similar to a honeycomb, which is one of the crystal structures observed in dusty plasmas. The bar below the micrograph corresponds to 200 micrometers. In the sketch the black dots represent the particulates in the top layer of the crystal, the white dots show the second layer and the shaded regions are perpendicular to the optical axis. Courtesy of Professor Lin I, National Central University, Taiwan.
A crystal-like structure can be formed out of charged dust grains in a low-temperature plasma discharge and in space plasmas such as Saturn's rings. Sergey Vladimirov and Neil Cramer have found that the grains in the crystal vibrate with definite frequencies. This will aid in the experimental diagnostics of the crystal and its background plasma.
Plasmas containing small solid particles ranging in size from nanometers to micrometers are called `dusty plasmas'. The particles acquire negative charge by a flow of fast plasma electrons onto the grains. Dusty plasmas are common in space, occurring in such diverse environments as interstellar clouds, interplanetary dust, comets, planetary rings, and the Earth's magnetosphere. In the laboratory, dusty plasmas can occur naturally in processing plasmas, such as those used for etching semiconductors. In addition, the study of the influence of dust in environmental research, such as in the Earth's ionosphere and atmosphere, is important. Dust particles can form a crystal structure known as a Coulomb solid. The crystal spacing can be large (of the order of millimeters), so the dust crystal provides a macroscopic, classical model of a solid state crystal.
Sergey Vladimirov and Neil Cramer have carried out investigations of the mechanical-electrostatic modes of vibration of a dust-plasma crystal. They have discovered transverse modes of a horizontal line of grains (where the ions flow vertically downward to a plane horizontal cathode), and of two such lines of grains, and the modes of a vertical string of grains. The last two arrangements have the unique feature that the effect of the background plasma on the mutual grain interaction is asymmetric because of the ``wake" downstream of the grains. The characteristic frequencies of the vibrations are dependent on the parameters of the plasma and the dust grains, such as the grain charge, and so measurement of the frequencies could provide diagnostics of these quantities.
A single charged dust grain is positioned at the centre of the bottom right axis. Plasma ions flow past it, creating a bow wave shown by the colored contours. Above is a 3 dimensional plot of the resulting plasma electric potential in the wake behind the dust grain. Other negatively charged dust grains may be attracted to the tops of the hills in the potential, so forming a crystal structure.
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