Number 71 (Story #1), March 13, 1992 by Phillip F. Schewe and Ben Stein THE CONDUCTANCE OF A CROSSED-WIRE TUNNEL JUNCTION can be reversibly changed by a factor of 15 through the magnetic interaction between a tunneling current and what may be a single electron trapped in the junction. Stephen Gregory of Bellcore (908-758-2847) has made measurements of conductance under low temperature conditions in a junction in which electrons tunnel between a pair of crossed tungsten 10-micron wires separated by a monolayer of helium gas (forming in effect an atomic-sized near contact). By applying a mechanical strain to the junction, Gregory found that he could switch between two distinct conductance states. The experimental data suggested to him that he was switching on and off a strong magnetic interaction, known as the Kondo effect, an interaction, studied in other areas of condensed matter physics, in which conduction electrons scatter strongly from the magnetic moments of impurity atoms. Because neither the tungsten nor the helium is magnetic, Gregory believes that the magnetic moment doing the scattering arises from an electron in an impurity atom in the junction. A tunnel junction with a single isolated magnetic dipole would not only provide optimal conditions for studying the Kondo effect but would also lead to novel quantum devices. (Physical Review Letters, 30 March 1992.)