434 research outputs found
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
Periodic and Aperiodic Bunching in the Addition Spectra of Quantum Dot
We study electron addition spectra of quantum dots in a broad range of
electron occupancies starting from the first electron. Spectra for dots
containing <200 electrons reveal a surprising feature. Electron additions are
not evenly spaced in gate voltage. Rather, they group into bunches. With
increasing electron number the bunching evolves from occurring randomly to
periodically at about every fifth electron. The periodicity of the bunching and
features in electron tunneling rates suggest that the bunching is associated
with electron additions into spatially distinct regions within the dots.Comment: 4 pages, 2 figures. Submitted to PR
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
Quantum orientational melting of mesoscopic clusters
By path integral Monte Carlo simulations we study the phase diagram of two -
dimensional mesoscopic clusters formed by electrons in a semiconductor quantum
dot or by indirect magnetoexcitons in double quantum dots. At zero (or
sufficiently small) temperature, as quantum fluctuations of particles increase,
two types of quantum disordering phenomena take place: first, at small values
of quantum de Boer parameter q < 0.01 one can observe a transition from a
completely ordered state to that in which different shells of the cluster,
being internally ordered, are orientationally disordered relative to each
other. At much greater strengths of quantum fluctuations, at q=0.1, the
transition to a disordered (superfluid for the boson system) state takes place.Comment: 4 pages, 6 Postscript figure
Imaging Transport Resonances in the Quantum Hall Effect
We use a scanning capacitance probe to image transport in the quantum Hall
system. Applying a DC bias voltage to the tip induces a ring-shaped
incompressible strip (IS) in the 2D electron system (2DES) that moves with the
tip. At certain tip positions, short-range disorder in the 2DES creates a
quantum dot island in the IS. These islands enable resonant tunneling across
the IS, enhancing its conductance by more than four orders of magnitude. The
images provide a quantitative measure of disorder and suggest resonant
tunneling as the primary mechanism for transport across ISs.Comment: 4 pages, 4 figures, submitted to PRL. For movies and additional
infomation, see http://electron.mit.edu/scanning/; Added scale bars to
images, revised discussion of figure 3, other minor change
Two-electron state in a disordered 2D island: pairing caused by the Coulomb repulsion
We show the existence of bound two-electron states in an almost depleted
two-dimensional island. These two-electron states are carried by special
compact configurations of four single-electron levels. The existence of these
states does not require phonon mediation, and is facilitated by the
disorder-induced potential relief and by the electron-electron repulsion only.
The density of two-electron states is estimated and their evolution with the
magnetic field is discussed.Comment: 9 pages, 1 fi
Tunneling into Ferromagnetic Quantum Hall States: Observation of a Spin Bottleneck
We explore the characteristics of equilibrium tunneling of electrons from a
3D electrode into a high mobility 2D electron system. For most 2D Landau level
filling factors, we find that tunneling can be characterized by a single,
well-defined tunneling rate. However, for spin-polarized quantum Hall states
(nu = 1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to
2 orders of magnitude. The dependence of the two rates on temperature and
tunnel barrier thickness suggests that slow in-plane spin relaxation creates a
bottleneck for tunneling of electrons.Comment: 5 pages, 4 figures, submitted to PR
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~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
Anomalous structure in the single particle spectrum of the fractional quantum Hall effect
The two-dimensional electron system (2DES) is a unique laboratory for the
physics of interacting particles. Application of a large magnetic field
produces massively degenerate quantum levels known as Landau levels. Within a
Landau level the kinetic energy of the electrons is suppressed, and
electron-electron interactions set the only energy scale. Coulomb interactions
break the degeneracy of the Landau levels and can cause the electrons to order
into complex ground states. In the high energy single particle spectrum of this
system, we observe salient and unexpected structure that extends across a wide
range of Landau level filling fractions. The structure appears only when the
2DES is cooled to very low temperature, indicating that it arises from delicate
ground state correlations. We characterize this structure by its evolution with
changing electron density and applied magnetic field. We present two possible
models for understanding these observations. Some of the energies of the
features agree qualitatively with what might be expected for composite
Fermions, which have proven effective for interpreting other experiments in
this regime. At the same time, a simple model with electrons localized on
ordered lattice sites also generates structure similar to those observed in the
experiment. Neither of these models alone is sufficient to explain the
observations across the entire range of densities measured. The discovery of
this unexpected prominent structure in the single particle spectrum of an
otherwise thoroughly studied system suggests that there exist core features of
the 2DES that have yet to be understood.Comment: 15 pages, 10 figure
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