Rebounding electrons in quantum arenas.

In the early days of atomic theory, physicists often pictured an atom as a miniature solar system with electrons orbiting at any distance from a central nucleus. The advent of quantum mechanics altered that image.

Quantum theory specified that an electron's position is determined by a kind of map, called a wave function, that mathematically describes a quantum particle's probability of being at any particular location as it travels in a stable, periodic orbit.

By measuring the energy of electrons trapped in a microscopic box threaded by a strong magnetic field, researchers have now observed the quantum equivalent of the motion of electrons following irregular or chaotic orbits.

Physicists Laurence Eaves, T. M. Fromhold, and their coworkers at the University of Nottingham in England, along with collaborators at the University of Tokyo, report their findings in the April 18 Nature.

Theorists had predicted that this chaotic motion would correspond to wave functions having features called scars, which represent concentrations of probability associated with periodic, but unstable, electron orbits (SN: 11/2/91, p. 282). Under appropriate conditions, such quantum scars can manifest themselves as abrupt increases in an electric current.

Eaves and his colleagues obtained their results using a tunnel diode, in which electrons leak into a so-called quantum well consisting of a thin layer of gallium arsenide sandwiched between walls of aluminum gallium arsenide.

Confined to the well, an electron bounces from wall to wall like a billiard ball.

In a strong magnetic field at right angles to the walls, the electron spirals around a magnetic field line as it bounces back and forth. If the magnetic field is tilted, the electron's corkscrew motion becomes more complicated-even chaotic.

As the researchers changed the voltage across the tunnel diode, they observed large fluctuations in current due to the appearance and disappearance of wave function scars. 'We've shown that for quantum particles, scars can be rather important experimentally,' Eaves says. Now, the researchers are looking into what happens to this chaotic behavior when two quantum wells are coupled together.

Researchers had previously seen traces of scars in the patterns created by microwaves trapped inside thin, metallic boxes (SN: 4/29/95, p. 264) and in the spectrum of light emitted by hydrogen atoms in a magnetic field but not among electrons in a semiconductor device.

'Interesting as the hydrogen atom and the microwave experiments are, a novel tunnel diode and devices like it are more likely to show up in practical applications,' notes Eric J. Heller of Harvard University.