Energy levels of an artificial atom probed with single-electron capacitance spectroscopy

Abstract

We have recently developed a spectroscopic technique which allows direct measurement of quantum energy levels. The method is based on observation of the capacitance signal resulting from single electrons tunneling into discrete quantum levels. The electrons tunnel between a metallic layer and confined states of a microscopic capacitor fabricated in GaAs. Charge transfer occurs only for bias voltages at which a quantum level resonates with the Fermi energy of the metallic layer. This creates a sequence of distinct capacitance peaks whose bias positions directly reflect the electronic spectrum of the confined structure. Using this “single-electron capacitance spectroscopy”, we map the magnetic field dependence of the ground state energies of a single quantum dot containing from 0 to 50 electrons. Along with a spectroscopic measurement of the dot's ground states, we probe tunneling rates of electrons to individual quantum states. The experimental spectra reproduce many features of a noninteracting electron model with an added fixed charging energy. However, in detailed observations deviations are apparent: exchange induces a two-electron singlet-triplet transition, self-consistency of the confinement potential causes the dot to assume a quasi two-dimensional character, and features develop which are suggestive of the fractional quantum Hall effect.