Switching electrons from trickle to flood.

Only a relatively small number of electrons leak across the narrow gap between a surface and the tip of a sharp needle in a scanning tunneling microscope. A similar flow of tunneling electrons can occur across the microscopic junction formed by two crossed wires separated by a helium layer only one atom thick.

Now a researcher has discovered that at temperatures near absolute zero, a strong interaction between the tunneling current and a magnetic impurity -- possibly a single electron trapped somewhere in the crossed-wire junction -- can greatly increase the electron flow across this junction. Moreover, squeezing the junction ever so slightly, which would presumably shunt a trapped electron aside, readily turns the magnetic effect off and brings the current down. Releasing the pressure restores the current to its previous, high level.

"It's a switch -- if you like -- whose properties are controlled by a single electron," says Stephen Gregory of Bellcore, in Red Bank, N.J., who described his discovery at an American Physical Society meeting held this week in Indianapolis. His findings will also appear in the March 30 PHYSICAL REVIEW LETTERS.

Gregory assembles his tunnel junction from a pair of fine tungsten wires about 10 microns in diameter. A layer of helium atoms between the crossed wires acts both as a barrier and as a "glue" to hold the microscopic junction together. When cooled to 1.5 kelvins, the device carries surprisingly large currents -- as high as 1 microampere -- that cross the junction through an area barely the size of an atom. Such currents are much greater than those commonly seen in scanning tunneling microscopy.

Gregory attributes this large current to a remarkable magnetic interaction, the Kondo effect, which in some way orchestrates and facilitates the passage of electrons across the junction. "The amazing thing is that all of the electrons essentially participate in this magnetic effect together," Gregory says.

The ease with which the effect can be turned on and off suggests that a single trapped electron provides the necessary magnetic influence. Squeezing the junction shifts the trapped electron slightly so that it no longer interacts magnetically with the tunneling electrons, thereby reducing the current through the junction from a flood to a mere trickle.

"My assumption is that, despite the fact that the wires are incredibly clean, there's always a possibility there may be a residual oxygen atom or a tungsten atom that isn't that well bonded to the surface, whih is able to create a trap for an electron," Gregory says. "The gap [between the wires] is so small that the movement of an electron from one side of a trap to the other -- as little as 0.1 angstrom -- would be enough to do the job."

A tunnel junction magnetically controlled by one electron shows promise as a kind of quantum switch for novel electronic devices. It also provides a direct means of experimentally studying the Kondo effect itself, which plays a role in superconductivity and other solid-state phenomena.