Magazine article Science News

Electrons in Boxes: Probing Artificial Atoms to Stretch Quantum Physics

Magazine article Science News

Electrons in Boxes: Probing Artificial Atoms to Stretch Quantum Physics

Article excerpt

Weird things happen when particles are confined to tiny boxes.

An atom, for example, can be pictured as a spherical container for electrons. The attractive electrical force between the negatively charged electrons and the atom's positively charged nucleus serves as the container's walls.

According to quantum theory, electrons trapped in such minuscule packages follow only certain orbits, each representing a specific energy. Researchers can determine those energies by measuring the wavelengths of light absorbed or emitted by atoms as electrons jump abruptly from one energy level to another.

Nowadays, they can also probe the shenanigans of electrons inside microscopic semiconductor structures called quantum dots. In these artificial atoms an electric field traps an electron in much the same way that a bowl confines a rolling marble.

"One can consider the artificial atom as a tiny laboratory in which quantum mechanics and the effects of electron-electron interactions can be studied," says Raymond C. Ashoori of the Massachusetts Institute of Technology.

Scientists can construct a wide variety of quantum dots to explore how electrons behave in boxes many times the size of a typical atom or in containers shaped like rods, pancakes, misshapen disks, or distorted spheres.

"It's fun to imagine and study how quantum mechanics plays itself out in all sorts of geometries--geometries that atoms can't have," says Paul L. McEuen of the University of California, Berkeley.

Surprises abound. No computer yet can handle the calculations necessary to determine the detailed behavior of a bunch of charged particles in a box of arbitrary shape. "So we don't know what's going to happen in our experiments," McEuen remarks.

There's also a practical aspect. The characteristics of solids typically reflect properties of their microscopic building blocks. "If we could engineer new types of artificial atoms, we could then assemble them into new kinds of solids--ones that could not be realized with real atoms," McEuen says.

Several research groups reported results of quantum dot experiments at an American Physical Society meeting held last month in Los Angeles.

Leo P. Kouwenhoven and his coworkers at the Delft University of Technology in the Netherlands studied the ground state--in which electrons have the lowest possible energy--as well as excited states of a pancake-shaped quantum dot, 0.1 micrometer [micro] m across, containing 1 to 12 electrons.

By incorporating this quantum dot into a device resembling a transistor and measuring current-voltage relationships, the researchers determined the dot's ground state as they added electrons one by one.

In real atoms, the order in which electrons fill up different energy levels follows a set of rules devised many decades ago by German spectroscopist Friedrich Hund. The Dutch team discovered that electrons obey the same sort of rules in filling energy levels in what is essentially an oversized, two-dimensional atom. For example, the energy levels of pancake helium, with two electrons in the quantum dot, displayed the same sorts of complexities exhibited by real helium.

"Theorists had not predicted that one would see Hund's rules [applied] in quantum dots," Ashoori explains. The observation have since led to theoretical calculations that explain many features of a quantum dot's energy spectrum.

"These experiments beautifully illustrate that for a high-symmetry quantum dot of a few electrons, the ideas of atomic physics coupled with many-body quantum calculations can give a relatively complete qualitative and quantitative description of the observed behavior," McEuen commented in the Dec. 5, 1997 Science.

Charles M. Marcus and his colleagues at Stanford University have taken a somewhat different tack, focusing on irregularly shaped quantum dots containing about 200 electrons chilled to millikelvin temperatures. …

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