Charging of small two-dimensional electron puddles

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.Includes bibliographical references (p. 138-147).We study electron additions in 2D quantum dots of varying sizes and over a wide range of electron densities using Single Electron Capacitance Spectroscopy For high electron densities in dots of any size, we observe a conventional pattern of nearly periodic Coulomb blockade However, the addition spectra of electron droplets larger than 0.2 [mu]m diameter and below a critical electron density (no = 1 x 1011cm-2 in all of our dots) are highly nonperiodic and contain pairs and bunches two or more successive electrons can enter the dot at nearly the same energy; they show almost no sign of repelling each other Application of high perpendicular magnetic field increases n0 , creating a sharp boundary between periodic and "paired" parts of the addition spectrum Previously, we hypothesized that disorder and electron interactions within the low-density dot split it into two spatially separate droplets, and pairing arises once this localization occurs. We have produced experiments to study this transition in a controlled fashion. One probes the spatial extent of electronic wave functions by investing the dependence of these energies on changes in the dot confirming potential We find that for low electron densities, electrons occupy distinct spatial sites localized within the dot At higher densities, the electrons become delocalized, and all wavefuncions are spread over the full dot area The transition occurs around the critical electron density n0 = 1 x 1011cm-2 For densities Just below the critical density our data establish the existence of electronic states localized at the dot's periphery We also create a dot with a potential profile containing two minimal separated by a barrier. Our studies conclusively demonstrate that under precisely the same conditions for observation of the paired electron additions, a low-density electron droplet inside the dot indeed splits up into smaller fragments, each resigning a disorder minimum We find that the two electrons added as a pair actually enter into spatially-distinct regions within a dot and we measure the remnant residual interact between the fragments Surprisingly, it displays nearly complete independence on the strength of the applied field for fields larger than required for the localization transition.by Misha Brodsky.Ph.D