1,629 research outputs found
Probing the Electrostatics of Integer Quantum Hall Edges with Momentum-Resolved Tunnel Spectroscopy
We present measurements of momentum-resolved magneto-tunneling from a
perpendicular two-dimensional (2D) contact into integer quantum Hall (QH) edges
at a sharp edge potential created by cleaved edge overgrowth. Resonances in the
tunnel conductance correspond to coincidences of electronic states of the QH
edge and the 2D contact in energy-momentum space. With this dispersion relation
reflecting the potential distribution at the edge we can directly measure the
band bending at our cleaved edge under the influence of an external voltage
bias. At finite bias we observe significant deviations from the flat-band
condition in agreement with self-consistent calculations of the edge potential
The structure of a single sharp quantum Hall edge probed by momentum-resolved tunneling
Momentum resolved magneto-tunnelling spectroscopy is performed at a single
sharp quantum Hall edge. We directly probe the structure of individual integer
quantum Hall (QH) edge modes, and find that an epitaxially overgrown cleaved
edge realizes the sharp edge limit, where the Chklovskii picture relevant for
soft etched or gated edges is no longer valid. The Fermi wavevector in the
probe quantum well probes the real-space position of the QH edge modes, and
reveals inter-channel distances smaller than both the magnetic length and the
Bohr radius. We quantitatively describe the lineshape of principal conductance
peaks and deduce an edge filling factor from their position consistent with the
bulk value. We observe features in the dispersion which are attributed to
fluctuations in the ground energy of the quantum Hall system.Comment: 4 pages, 3 figure
Dynamics of Nucleation in the Ising Model
Reactive pathways to nucleation in a three-dimensional Ising model at 60% of
the critical temperature are studied using transition path sampling of single
spin flip Monte Carlo dynamics. Analysis of the transition state ensemble (TSE)
indicates that the critical nuclei are rough and anisotropic. The TSE,
projected onto the free energy surface characterized by cluster size, N, and
surface area, S, indicates the significance of other variables in addition to
these two traditional reaction coordinates for nucleation. The transmission
coefficient along N is ~ 0.35, and this reduction of the transmission
coefficient from unity is explained in terms of the stochastic nature of the
dynamic model.Comment: In press at the Journal of Physical Chemistry B, 7 pages, 8 figure
Acute idiopathic heart failure following laparoscopic myotomy for achalasia of the esophagus
Background: Stress-induced cardiomyopathy, also known as takotsubo cardiomyopathy, is not fully understood. It is thought to occur in patients who have signs and symptoms consistent with acute myocardial infarction but display no obstructive coronary lesions during heart catheterization. Characteristics include transient left ventricular dysfunction, wall motion abnormalities on echocardiogram, new electrocardiographic ST-segment changes, and the occurrence of a precipitating stressor. Case Report: We present a patient who underwent Heller myotomy and suffered acute heart failure in the immediate postoperative period. Left heart catheterization revealed clean coronary arteries, and the patient fully recovered days later. While difficult to fully exclude drug-related causes, we believe this case to be consistent with takotsubo cardiomyopathy. Conclusion: This unusual postoperative complication following uneventful laparoscopic surgery should be kept in mind when unsuspected cardiovascular compromise is seen in the early perioperative recovery period. In addition to the rare occurrence of acute coronary ischemia syndromes and possible perioperative pulmonary embolic events, cardiovascular decompensation related to acute stress syndromes or idiopathic pharmacologic responses must be considered. Even patients who seem most healthy can have complications that warrant immediate attention and treatment
Dynamics of Dissipative Quantum Hall Edges
We examine the influence of the edge electronic density profile and of
dissipation on edge magnetoplasmons in the quantum Hall regime, in a
semiclassical calculation. The equilibrium electron density on the edge,
obtained using a Thomas-Fermi approach, has incompressible stripes produced by
energy gaps responsible for the quantum Hall effect. We find that these stripes
have an unobservably small effect on the edge magnetoplasmons. But dissipation,
included phenomenologically in the local conductivity, proves to produce
significant oscillations in the strength and speed of edge magnetoplasmons in
the quantum Hall regime.Comment: 23 pages including 10 figure
Fermi liquid to Luttinger liquid transition at the edge of a two-dimensional electron gas
We present experimental results on the tunneling into the edge of a two
dimensional electron gas (2DEG) obtained with a GaAs/AlGaAs cleaved edge
overgrown structure in a strong perpendicular magnetic field. While the 2DEG
exhibits typical fractional quantum Hall features of a very high mobility
sample, we observe the onset of a non-linear current-voltage characteristic in
the vicinity of nu=1. For filling factor nu<1 the system is consistent with a
non-Fermi liquid behavior, such as a Luttinger liquid, whereas for nu>1 we
observe an Ohmic tunneling resistance between the edge and a three dimensional
contact, typical for a Fermi liquid. Hence, at the edge, there is a transition
from a Luttinger liquid to a Fermi liquid. Finally, we show that the Luttinger
liquid exponent at a given filling factor is not universal but depends on
sample parameters.Comment: 4 pages, 4 figure
The visibility of IQHE at sharp edges: Experimental proposals based on interactions and edge electrostatics
The influence of the incompressible strips on the integer quantized Hall
effect (IQHE) is investigated, considering a cleaved-edge overgrown (CEO)
sample as an experimentally realizable sharp edge system. We propose a set of
experiments to clarify the distinction between the large-sample limit when bulk
disorder defines the IQHE plateau width and the small-sample limit smaller than
the disorder correlation length, when self-consistent edge electrostatics
define the IQHE plateau width. The large-sample or bulk QH regime is described
by the usual localization picture, whereas the small-sample or edge regime is
discussed within the compressible/incompressible strips picture, known as the
screening theory of QH edges. Utilizing the unusually sharp edge profiles of
the CEO samples, a Hall bar design is proposed to manipulate the edge potential
profile from smooth to extremely sharp. By making use of a side-gate
perpendicular to the two dimensional electron system, it is shown that the
plateau widths can be changed or even eliminated altogether. Hence, the
visibility of IQHE is strongly influenced when adjusting the edge potential
profile and/or changing the dc current direction under high currents in the
non-linear transport regime. As a second investigation, we consider two
different types of ohmic contacts, namely highly transmitting (ideal) and
highly reflecting (non-ideal) contacts. We show that if the injection contacts
are non-ideal, however still ohmic, it is possible to measure directly the
non-quantized transport taking place at the bulk of the CEO samples. The
results of the experiments we propose will clarify the influence of the edge
potential profile and the quality of the contacts, under quantized Hall
conditions.Comment: Substantially revised version of manuscript arXiv:0906.3796v1,
including new figures et
Infrared Hall effect in high Tc superconductors: Evidence for non-Fermi liquid Hall scattering
Infrared (20-120 cm-1 and 900-1100 cm-1) Faraday rotation and circular
dichroism are measured in high Tc superconductors using sensitive polarization
modulation techniques. Optimally doped YBCO thin films are studied at
temperatures down to 15 K and magnetic fields up to 8 T. At 1000 cm-1 the Hall
conductivity varies strongly with temperature in contrast to the longitudinal
conductivity which is nearly independent of temperature. The Hall scattering
rate has a T^2 temperature dependence but, unlike a Fermi liquid, depends only
weakly on frequency. The experiment puts severe constraints on theories of
transport in the normal state of high Tc superconductors.Comment: 8 pages, 3 figure
Assessing small area estimates via artificial populations from KBAABB: a kNN-based approximation to ABB
Comparing and evaluating small area estimation (SAE) models for a given
application is inherently difficult. Typically, we do not have enough data in
many areas to check unit-level modeling assumptions or to assess unit-level
predictions empirically; and there is no ground truth available for checking
area-level estimates. Design-based simulation from artificial populations can
help with each of these issues, but only if the artificial populations (a)
realistically represent the application at hand and (b) are not built using
assumptions that could inherently favor one SAE model over another. In this
paper, we borrow ideas from random hot deck, approximate Bayesian bootstrap
(ABB), and k nearest neighbor (kNN) imputation methods, which are often used
for multiple imputation of missing data. We propose a kNN-based approximation
to ABB (KBAABB) for a different purpose: generating an artificial population
when rich unit-level auxiliary data is available. We introduce diagnostic
checks on the process of building the artificial population itself, and we
demonstrate how to use such an artificial population for design-based
simulation studies to compare and evaluate SAE models, using real data from the
Forest Inventory and Analysis (FIA) program of the US Forest Service. We
illustrate how such simulation studies may be disseminated and explored
interactively through an online R Shiny application
Mid-infrared Hall effect in thin-film metals: Probing the Fermi surface anisotropy in Au and Cu
A sensitive mid-infrared (MIR, 900-1100 cm-1, 112-136 meV) photo-elastic
polarization modulation technique is used to measure simultaneously Faraday
rotation and circular dichroism in thin metal films. These two quantities
determine the complex AC Hall conductivity. This novel technique is applied to
study Au and Cu thin films at temperatures down to 20 K and magnetic fields up
to 8 T. The Hall frequency is consistent with band theory predictions. We
report the first measurement of the MIR Hall scattering rate, which is
significantly lower than that derived from Drude analysis of zero magnetic
field MIR transmission measurements. This difference is qualitatively explained
in terms of the anisotropy of the Fermi surface in Au and Cu.Comment: 14 pages of text, 5 figure
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