  The question of how localized electrons interact with delocalized electrons
is central to many problems at the forefront of solid state physics. The
simplest example is the Kondo phenomenon, which occurs when an impurity atom
with an unpaired electron is placed in a metal, and the energy of the unpaired
electron is far below the Fermi energy. At low temperatures a spin singlet
state is formed between the unpaired localized electron and delocalized
electrons at the Fermi energy.
  The confined droplet of electrons interacting with the leads of a single
electron transistor (SET) is closely analogous to an impurity atom interacting
with the delocalized electrons in a metal. (Meir, Wingreen and Lee, 1993) We
report here measurements on a new generation of SETs that display all the
aspects of the Kondo phenomenon: the spin singlet forms and causes an
enhancement of the zero-bias conductance when the number of electrons on the
artificial atom is odd but not when it is even. The singlet is altered by
applying a voltage or magnetic field or by increasing the temperature, all in
ways that agree with predictions. (Wingreen and Meir 1994)
