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We discuss single-electron tunneling measurements at dilutionrefrigerator temperatures on metallic grains, sufficientlysmall that the quantum levels of the conduction electrons canbe resolved. These measurements directly reveal the energyeigenvalues of the electrons in a grain that typicallycontains a few thousand conduction electrons. Suchmeasurements were first carried out a few years ago by Ralph,et al. on nanograins of Al. More recently, this workhas been extended to measurements on nanoparticles of theheavy metal Au by Davidovic and on nanoparticles ofalloys of Al and Au by Salinas, et al. This morerecent work has pointed up the need to go beyond the simplestindependent-electron model, to include the Coulomb interactionbetween electrons and also nonequilibrium electronpopulations. These interactions cause the energy levels tomerge into a continuum above the Thouless energy and can causea single quasiparticle level to show up as a cluster ofresonances. The strong spin-orbit interaction in Au cancause 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 energylevels suggestive of system spin values greater than1/2 induced by the exchangeinteraction. This paper will review the evolving progress thathas been made in interpreting these observations. |
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