When conduction electrons are confined within a metal particle smaller than about 10 nm in diameter, it is not appropriate to use the usual assumption that the electronic energy levels inside the metal form a continuum of states. We have recently developed a technique by which the spectrum of levels inside a single metal particle can be measured for the first time. This is useful because it provides a powerful, fundamentally new probe into the physics of the interacting electrons inside a metal. The effects of all the forces which determine the electronic properties can now be examined directly at the level of individual eigenstates, as in atomic physics. We are currently investigating the eigenstates within superconducting particles.
Dramatic changes can then result in the resistance, magnetization, and many other properties.
One example of this general class of problems is the Kondo effect for a magnetic impurity in a metal. Cooperative behavior among many quantum defects in a metal can produce bizarre and fascinating electronic states that are just beginning to be understood. We are studying the physics of quantum defects by measuring their properties in metal devices small enough to provide the sensitivity to probe individual impurities. In both magnets and superconductors, interactions between the conduction electrons lead to properties very different from a non-interacting electron gas. Research in our group is driven by the development of new techniques for fabricating small devices with interesting physical properties.