Tunneling Through Metallic Quantum Dots

Abstract

We discuss single-electron tunneling measurements at dilution refrigerator temperatures on metallic grains, sufficiently small that the quantum levels of the conduction electrons can be resolved. These measurements directly reveal the energy eigenvalues of the electrons in a grain that typically contains a few thousand conduction electrons. Such measurements were first carried out a few years ago by Ralph, et al. on nanograins of Al. More recently, this work has been extended to measurements on nanoparticles of the heavy metal Au by Davidović and on nanoparticles of alloys of Al and Au by Salinas, et al. This more recent work has pointed up the need to go beyond the simplest independent-electron model, to include the Coulomb interaction between electrons and also nonequilibrium electron populations. These interactions cause the energy levels to merge into a continuum above the Thouless energy and can cause a single quasiparticle level to show up as a cluster of resonances. The strong spin-orbit interaction in Au can cause levels to split in magnetic fields with a g-factor of ∼ 0.3, instead of the free electron g = 2. In addition, there is evidence for a proliferation of low-lying energy levels suggestive of system spin values greater than 1/2 induced by the exchange interaction. This paper will review the evolving progress that has been made in interpreting these observations.