Single electron capacitance spectroscopy of vertical quantum dots using a single electron transistor

We have incorporated an aluminum single electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single electron capacitance peaks.Comment: Submitted to Applied Physics Letters (reformatted to fit correctly on a page

A New Class of Resonances at the Edge of the Two Dimensional Electron Gas

We measure the frequency dependent capacitance of a gate covering the edge and part of a two-dimensional electron gas in the quantum Hall regime. In applying a positive gate bias, we create a metallic puddle under the gate surrounded by an insulating region. Charging of the puddle occurs via electron tunneling from a metallic edge channel. Analysis of the data allows direct extraction of this tunneling conductance. Novel conductance resonances appear as a function of gate bias. Samples with gates ranging from 1-170~$\mu$m along the edge display strikingly similar resonance spectra. The data suggest the existence of unexpected structure, homogeneous over long length scales, at the sample edge.Comment: 13 pages (revtex) including 4 figure

Disorder and interaction induced pairing in the addition spectra of quantum dots

We have investigated numerically the electron addition spectra in quantum dots containing a small number (N < 11) of interacting electrons, in presence of strong disorder. For a short-range Coulomb repulsion, we find regimes in which two successive electrons enter the dot at very close values of the chemical potential. In the strongly correlated regime these close additions, or pairing, are associated with electrons tunneling into distinct electron puddles within the dot. We discuss the tunneling rates at pairing, and we argue that our results are related to a phenomenon known as "bunching", recently observed experimentally.Comment: 4 pages, 5 figure

Dynamic nuclear polarization at the edge of a two-dimensional electron gas

We have used gated GaAs/AlGaAs heterostructures to explore nonlinear transport between spin-resolved Landau level (LL) edge states over a submicron region of two-dimensional electron gas (2DEG). The current I flowing from one edge state to the other as a function of the voltage V between them shows diode-like behavior---a rapid increase in I above a well-defined threshold V_t under forward bias, and a slower increase in I under reverse bias. In these measurements, a pronounced influence of a current-induced nuclear spin polarization on the spin splitting is observed, and supported by a series of NMR experiments. We conclude that the hyperfine interaction plays an important role in determining the electronic properties at the edge of a 2DEG.Comment: 8 pages RevTeX, 7 figures (GIF); submitted to Phys. Rev.

Localization in Artificial Disorder - Two Coupled Quantum Dots

Using Single Electron Capacitance Spectroscopy, we study electron additions in quantum dots containing two potential minima separated by a shallow barrier. Analysis of addition spectra in magnetic field allows us to distinguish whether electrons are localized in either potential minimum or delocalized over the entire dot. We demonstrate that high magnetic field abruptly splits up a low-density droplet into two smaller fragments, each residing in a potential minimum. An unexplained cancellation of electron repulsion between electrons in these fragments gives rise to paired electron additions.Comment: submitted to Phys.Rev.Let

Separately contacted edge states: A new spectroscopic tool for the investigation of the quantum Hall effect

Using an innovative combination of a quasi-Corbino sample geometry and the cross-gate technique, we have developed a method that enables us to separately contact single edge channels in the quantum Hall regime and investigate equilibration among them. Performing 4-point resistance measurements, we directly obtain information on the energetic and geometric structure of the edge region and the equilibration-length for current transport across the Landau- as well as the spin-gap. Based on an almost free choice in the number of participating edge channels and their interaction-length a systematic investigation of the parameter-space becomes possible.Comment: 8 pages, 7 figure

Random-phase Approximation Treatment Of Edge Magnetoplasmons: Edge-state Screening And Nonlocality

A random-phase approximation (RPA) treatment of edge magnetoplasmons (EMP) is presented for strong magnetic fields, low temperatures, and integer filling factors \nu. It is valid for negligible dissipation and lateral confining potentials smooth on the scale of the magnetic length \ell_{0} but sufficiently steep that the Landau-level (LL) flattening can be neglected. LL coupling, screening by edge states, and nonlocal contributions to the current density are taken into account. In addition to the fundamental mode with typical dispersion relation \omega\sim q_x \ln(q_{x}), fundamental modes with {\it acoustic} dispersion relation \omega\sim q_x are obtained for \nu>2. For \nu=1,2 a {\bf dipole} mode exists, with dispersion relation \omega\sim q_x^3, that is directly related to nonlocal responses.Comment: Text 12 pages in Latex/Revtex format, 4 Postscript figure

Repulsion of Single-well Fundamental Edge Magnetoplasmons in Double Quantum Wells

A {\it microscopic} treatment of fundamental edge magnetoplasmons (EMPs) along the edge of a double quantum well (DQW) is presented for strong magnetic fields, low temperatures, and total filling factor \nu=2. It is valid for lateral confining potentials that Landau level (LL) flattening can be neglected. The cyclotron and Zeeman energies are assumed larger than the DQW energy splitting \sqrt{\Delta^2 +4T^2}, where \Delta is the splitting of the isolated wells and T the tunneling matrix element. %hen calculated unperturbed density profile is sharp at the edge. Using a random-phase approximation (RPA), which includes local and nonlocal contributions to the current density, it is shown that for negligible tunnel coupling 2T << \Delta the inter-well Coulomb coupling leads to two DQW fundamental EMPs which are strongly renormalized in comparison with the decoupled, single-well fundamental EMP. These DQW modes can be modified further upon varying the inter-well distance d, along the z axis, and/or the separation of the wells' edges \Delta y along the y axis. The charge profile of the {\it fast} and {\it slow} DQW mode varies, respectively, in an {\it acoustic} and {\it optical} manner along the y axis and is not smooth on the \ell_{0} scale. For strong tunneling \Delta\alt 2T these DQW modes are essentially modified when \Delta is changed by applying a transverse electric field to the DQW.Comment: Text 18 pages in Latex/Revtex/Preprint format, 2 Postscript figure

Quantum and frustration effects on fluctuations of the inverse compressibility in two-dimensional Coulomb glasses

We consider interacting electrons in a two-dimensional quantum Coulomb glass and investigate by means of the Hartree-Fock approximation the combined effects of the electron-electron interaction and the transverse magnetic field on fluctuations of the inverse compressibility. Preceding systematic study of the system in the absence of the magnetic field identifies the source of the fluctuations, interplay of disorder and interaction, and effects of hopping. Revealed in sufficiently clean samples with strong interactions is an unusual right-biased distribution of the inverse compressibility, which is neither of the Gaussian nor of the Wigner-Dyson type. While in most cases weak magnetic fields tend to suppress fluctuations, in relatively clean samples with weak interactions fluctuations are found to grow with the magnetic field. This is attributed to the localization properties of the electron states, which may be measured by the participation ratio and the inverse participation number. It is also observed that at the frustration where the Fermi level is degenerate, localization or modulation of electrons is enhanced, raising fluctuations. Strong frustration in general suppresses effects of the interaction on the inverse compressibility and on the configuration of electrons.Comment: 15 pages, 18 figures, To appear in Phys. Rev.