Spectroscopic Measurements of Discrete Electronic States in Single Metal Particles

We have made tunnel junctions containing one Al particle of diameter $<10\mathrm{}$ nm. Tunneling via discrete electronic states in the particle produces steps in the current-voltage ( $I$- $V$) curve, providing, for the first time, a spectroscopic measurement of the electronic energy levels in a metal particle. With superconducting leads, the $I$- $V$ contribution from each discrete state has the form of the BCS density of states. We can determine the parity of the electron number in the particle's ground state through the effects of an applied magnetic field on the $I$- $V$ curve.