Strange attractions in quantum dots.

Because all electrons have a negative electric charge, they repel each other. Even in an atom, where they are attracted to a positively charged nucleus, the orbiting electrons tend to stay as far apart as possible.

Paradoxically, under certain circumstances, this strong, intrinsic repulsion can make it seem that electrons attract one another.

"When you put together [electron] repulsion and quantum mechanics, you get a short-range attraction," says physicist Raymond C. Ashoori of the Massachusetts Institute of Technology.

This effect, described in the Feb. 1 Nature, can't be observed in a real atom. It occurs inside a microscopic box-a quantum dot-fabricated out of semiconductors to hold a specified number of mobile electrons (SN: 2/20/93, p. 118).

As in an ordinary atom, the confined electrons can have only certain well-defined energies. They also steer clear of each other, partly because they exert a repulsive force and partly because the rules of quantum mechanics specify that individual electrons must occupy different energy levels. Because a typical quantum dot is much roomier than an atom, researchers can study subtle quantum effects not evident on atomic scales. In such a setting, a magnetic field can induce quantum effects that push the electrons into bunches, as if they were attracting each other, leaving gaps elsewhere.