466 research outputs found
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
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
Topographic Mapping of the Quantum Hall Liquid using a Few-Electron Bubble
A scanning probe technique was used to obtain a high-resolution map of the
random electrostatic potential inside the quantum Hall liquid. A sharp metal
tip, scanned above a semiconductor surface, sensed charges in an embedded
two-dimensional electron gas. Under quantum Hall effect conditions, applying a
positive voltage to the tip locally enhanced the 2D electron density and
created a ``bubble'' of electrons in an otherwise unoccupied Landau level. As
the tip scanned along the sample surface, the bubble followed underneath. The
tip sensed the motions of single electrons entering or leaving the bubble in
response to changes in the local 2D electrostatic potential.Comment: 4 pages, 3 JPG figures, Revtex. For additional info and AVI movies,
visit http://electron.mit.edu/st
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
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~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
Effects of Auditory Rhythm and Music on Gait Disturbances in Parkinson’s Disease
Gait abnormalities such as shuffling steps, start hesitation, and freezing are common and often incapacitating symptoms of Parkinson’s disease (PD) and other parkinsonian disorders. Pharmacological and surgical approaches have only limited efficacy in treating these gait disorders. Rhythmic auditory stimulation (RAS), such as playing marching music or dance therapy, has been shown to be a safe, inexpensive, and an effective method in improving gait in PD patients. However, RAS that adapts to patients’ movements may be more effective than rigid, fixed-tempo RAS used in most studies. In addition to auditory cueing, immersive virtual reality technologies that utilize interactive computer-generated systems through wearable devices are increasingly used for improving brain-body interaction and sensory-motor integration. Using multisensory cues, these therapies may be particularly suitable for the treatment of parkinsonian freezing and other gait disorders. In this review, we examine the affected neurological circuits underlying gait and temporal processing in PD patients and summarize the current studies demonstrating the effects of RAS on improving these gait deficits
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