Kondo Effect of a Magnetic Impurity in AN Ultrasmall Metallic Grain

We study the Kondo effect generated by a single magnetic impurity embedded in an ultrasmall metallic grain within variational approach. The Kondo-singlet formation strongly depends on the parity of the number of electrons on the grain. In the odd-parity case the Kondo effect always occurs and there is a crossover from the Kondo regime to a nonmagnetic regime. In the even-parity case the singlet formation occurs only for strong coupling, and there is a crossover from mixed valence to nonmagnetic regime. We also study the single-electron tunneling conductance and the effect of the Kondo-singlet formation on it.