614 research outputs found
Nonlinear Transport in a Quantum Point Contact due to Soft Disorder Induced Coherent Mode Mixing
We show that the coherent mixing of different transverse modes, due to
forward scattering of carriers by soft impurity- or boundary potentials leads
to a nonlinear, asymmetric current response of quantum point contacts (QPC).
The oscillating contribution to the current is sensitive both to driving
voltage and to gate voltage in direct analogy to the electrostatic
Aharonov-Bohm effect.
Our calculations are in a good agreement with recent experimental data
showing small-scale conductivity nonlinearities and asymmetry in QPC.Comment: 4 pages, 2 figures (availiable upon request), REVTEX, Applied Physics
Report 93-4
Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts
The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched
GaAs/GaAlAs quantum point contacts, by measuring the differential conductance
as a function of source-drain and gate bias as well as a function of
temperature. We investigate in detail how, for a given gate voltage, the
differential conductance depends on the finite bias voltage and find a
so-called self-gating effect, which we correct for. The 0.7 anomaly at zero
bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at
finite bias. Varying the gate voltage the transition between the 1.0 and the
0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate
voltage dependent energy difference . This energy difference is
compared with the activation temperature T_a extracted from the experimentally
observed activated behavior of the 0.7 anomaly at low bias. We find \Delta =
k_B T_a which lends support to the idea that the conductance anomaly is due to
transmission through two conduction channels, of which the one with its subband
edge \Delta below the chemical potential becomes thermally depopulated as the
temperature is increased.Comment: 9 pages (RevTex) with 9 figures (some in low resolution
Spin-dependent thermoelectric transport coefficients in near-perfect quantum wires
Thermoelectric transport coefficients are determined for semiconductor
quantum wires with weak thickness fluctuations. Such systems exhibit anomalies
in conductance near 1/4 and 3/4 of 2e^2/h on the rising edge to the first
conductance plateau, explained by singlet and triplet resonances of conducting
electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak,
and J.H. Jefferson, Phys. Rev. B 62, 12985 (2000)]. We extend this work to
study the Seebeck thermopower coefficient and linear thermal conductance within
the framework of the Landauer-Buettiker formalism, which also exhibit anomalous
structures. These features are generic and robust, surviving to temperatures of
a few degrees. It is shown quantitatively how at elevated temperatures thermal
conductance progressively deviates from the Wiedemann-Franz law.Comment: To appear in Phys. Rev. B 2002; 3 figure
Resonant transmission through an open quantum dot
We have measured the low-temperature transport properties of a quantum dot
formed in a one-dimensional channel. In zero magnetic field this device shows
quantized ballistic conductance plateaus with resonant tunneling peaks in each
transition region between plateaus. Studies of this structure as a function of
applied perpendicular magnetic field and source-drain bias indicate that
resonant structure deriving from tightly bound states is split by Coulomb
charging at zero magnetic field.Comment: To be published in Phys. Rev. B (1997). 8 LaTex pages with 5 figure
Conductance anomalies and the extended Anderson model for nearly perfect quantum wires
Anomalies near the conductance threshold of nearly perfect semiconductor
quantum wires are explained in terms of singlet and triplet resonances of
conduction electrons with a single weakly-bound electron in the wire. This is
shown to be a universal effect for a wide range of situations in which the
effective single-electron confinement is weak. The robustness of this generic
behavior is investigated numerically for a wide range of shapes and sizes of
cylindrical wires with a bulge. The dependence on gate voltage, source-drain
voltage and magnetic field is discussed within the framework of an extended
Hubbard model. This model is mapped onto an extended Anderson model, which in
the limit of low temperatures is expected to lead to Kondo resonance physics
and pronounced many-body effects
Emergency department utilization after hospitalization discharge for acute stroke: The COMprehensive Post-Acute Stroke Services (COMPASS) study
Each year nearly 800,000 people in the United States experience a stroke. Those that survive are at high risk for complications after hospital discharge. Providing appropriate care during the recovery from this complex condition is a challenge for patients, caregivers, and health care providers. Understanding emergency department (ED) utilization after a stroke may provide insights into long-term management of stroke, inform interventions, improve patient outcomes, and reduce medical costs. A comprehensive transitional care model for post-acute stroke care may influence the need to seek ED care for downstream events after a stroke. To date, most transitional care trials exploring post-stroke healthcare utilization were conducted outside of the U.S. health-care system. We examined data from the Comprehensive Post-Acute Stroke Services (COMPASS) study, a cluster-randomized pragmatic trial of a post-discharge transitional care model for stroke survivors and their caregivers compared with usual care
Parental occupational exposure to pesticides, animals and organic dust and risk of childhood leukemia and central nervous system tumors:Findings from the International Childhood Cancer Cohort Consortium (I4C)
Measurement of the open-charm contribution to the diffractive proton structure function
Production of D*+/-(2010) mesons in diffractive deep inelastic scattering has
been measured with the ZEUS detector at HERA using an integrated luminosity of
82 pb^{-1}. Diffractive events were identified by the presence of a large
rapidity gap in the final state. Differential cross sections have been measured
in the kinematic region 1.5 < Q^2 < 200 GeV^2, 0.02 < y < 0.7, x_{IP} < 0.035,
beta 1.5 GeV and |\eta(D*+/-)| < 1.5. The measured cross
sections are compared to theoretical predictions. The results are presented in
terms of the open-charm contribution to the diffractive proton structure
function. The data demonstrate a strong sensitivity to the diffractive parton
densities.Comment: 35 pages, 11 figures, 6 table
Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter
Data collected by the Pierre Auger Observatory through 31 August 2007 showed
evidence for anisotropy in the arrival directions of cosmic rays above the
Greisen-Zatsepin-Kuz'min energy threshold, \nobreak{eV}. The
anisotropy was measured by the fraction of arrival directions that are less
than from the position of an active galactic nucleus within 75 Mpc
(using the V\'eron-Cetty and V\'eron catalog). An updated
measurement of this fraction is reported here using the arrival directions of
cosmic rays recorded above the same energy threshold through 31 December 2009.
The number of arrival directions has increased from 27 to 69, allowing a more
precise measurement. The correlating fraction is , compared
with expected for isotropic cosmic rays. This is down from the early
estimate of . The enlarged set of arrival directions is
examined also in relation to other populations of nearby extragalactic objects:
galaxies in the 2 Microns All Sky Survey and active galactic nuclei detected in
hard X-rays by the Swift Burst Alert Telescope. A celestial region around the
position of the radiogalaxy Cen A has the largest excess of arrival directions
relative to isotropic expectations. The 2-point autocorrelation function is
shown for the enlarged set of arrival directions and compared to the isotropic
expectation.Comment: Accepted for publication in Astroparticle Physics on 31 August 201
Atmospheric effects on extensive air showers observed with the Surface Detector of the Pierre Auger Observatory
Atmospheric parameters, such as pressure (P), temperature (T) and density,
affect the development of extensive air showers initiated by energetic cosmic
rays. We have studied the impact of atmospheric variations on extensive air
showers by means of the surface detector of the Pierre Auger Observatory. The
rate of events shows a ~10% seasonal modulation and ~2% diurnal one. We find
that the observed behaviour is explained by a model including the effects
associated with the variations of pressure and density. The former affects the
longitudinal development of air showers while the latter influences the Moliere
radius and hence the lateral distribution of the shower particles. The model is
validated with full simulations of extensive air showers using atmospheric
profiles measured at the site of the Pierre Auger Observatory.Comment: 24 pages, 9 figures, accepted for publication in Astroparticle
Physic
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