Pumping electrons: Look Ma! No heat!

Heat buildup has always plagued electric circuits. As electronic components continue to shrink, even small amounts of heat become troublesome. Now, scientists have developed a way of pumping electrons through tiny circuits that may eliminate the dissipation of energy as damaging heat.

"This is a new means of making charge move," says Charles M. Marcus of Stanford University, who led the research. To make their pump, he and his team have used an existing method of making a device known as a quantum dot (SN: 4/11/98, p. 236). The dot confines electrons to a region within a thin layer of a semiconductor or metal.

To move electrons through their pump, the scientists manipulate the wave properties of electrons within the dot. By exploiting those properties, the researchers control current completely via the principles of quantum mechanics--the physics of the smallest bits of matter.

"I think this experiment is very significant," comments Qian Niu of the University of Texas at Austin. "It is really the first experiment that demonstrates that you can use a pure quantum effect to pump electrons." He and David J. Thouless of the University of Washington in Seattle formulated theories in the 1980s and early 1990s that indicated the feasibility of such quantum pumping.

In previous experiments, scientists have created electron pumps from quantum dots or other electron-confining structures. However, they have always relied to some extent on a classical-physics effect, the mutual electrostatic repulsion of electrons, to control flows in and out of the device. Pumps employing that effect can give electrons extra energy, causing undesired heating.

The new pump works by varying the probability that an electron is present at any particular location, Marcus explains. A fluctuating pattern of probabilities arises from interference between electron waves.

Marcus and his colleagues at Stanford and the University of California, Santa Barbara fabricated their micrometer-square dot in a sandwich of gallium arsenide and aluminum gallium arsenide. They describe the device, which operates at a frosty 330 millikelvins, in the March 19 Science.

Electrodes plated on the top and bottom surfaces of the semiconductor structure allow the researchers to apply voltages. Some act as confining wails to electrons. Other oscillating signals, intentionally out of sync with each other, drive the pump. When they are on, "it's as if the wails start to shake," Marcus says.

The shuddering of the walls shifts the pattern of probabilities that an electron is in a given location, making it possible for electrons to enter the dot from outside and for others to be ejected.

The research team does not make direct heat measurements on the new device but draws on other experimental indications, as well as theory, to argue that heating is minimal. "Inside the device, it's reasonable to assume there is a dissipationless current flowing," Marcus says.