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Atoms of the mesocosm.

Atoms of the mesocosm

Mesoscopic physics is the study of a strange new world intermediate in size between the microscopic realm of atoms and molecules and the macroscopic realm of ordinary visible and touchable objects. It is suddenly popular now that fabrication techniques allow the manufacture of objects that behave according to the laws of quantum mechanics although they are much larger than the atoms and molecules to which such behavior has previously been confined.

One such possibility is the superatom, a structure made of semiconductor materials that behaves as if it were a giant atom. Recently the Japanese physicist N. Watanabe suggested that it should be possible to make such a thing, and now Takeshi Inoshita, Shuhei Ohnishi and Atsushi Oshiyama of the NEC Corporation in Kawasaki, Japan, have calculated the electronic structure of a superatom. Reporting this work in the Nov. 17 PHYSICAL REVIEW LETTERS, they stress the importance of such objects for both pure physics and technology.

A superatom would be a semiconductor heterostructure -- that is, a spherical arrangement of two kinds of semiconductor materials, a core containing a large number of electron donors (atoms from which electrons are easily detached) surrounded by a "matrix" made of a material that has a high affinity for electrons. Under the proper conditions, the donors in the core would all be ionized, and the detached electrons would reside in the matrix, where they would form an orbiting cloud bound to the core by its electric attraction, just as the electrons of an actual atom are bound to its nucleus. Of a number of possible combinations of materials, Inoshita, Ohnishi and Oshiyama chose to calculate the structure of one where the core would be an alloy of aluminum, gallium and arsenic (Al.sub.0.35.GA.sub.0.65. As) and the matrix gallium arsenide.

Using the Schrodinger equation, which describes the structures of actual atoms, they found that the electron orbits do arrange themselves into a hierarchy of quantized energy levels, although in a configuration somewhat different from that of an actual atom. The structure is well enough bounded so that one can define a "superatomic radius," in this case 355 angstroms, or hundreds of times that of an actual atom. The ground-state or lowest-energy configuration is stable at around 1[deg.]K.

All this means that superatoms should exist, and it should be possible to make "molecules," "crystals" and even "metals" out of them. These constructions should be useful for studies of collective behavior of electrons in solids, and they might have unusual magnetic properties.
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Title Annotation:research in semiconductors
Publication:Science News
Date:Nov 29, 1986
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