823 research outputs found
Onset voltage shift due to non-zero Landau ground state level in coherent magnetotransport
Coherent electron transport in double-barrier heterostructures with parallel
electric and magnetic fields is analyzed theoretically and with the aid of a
quantum simulator accounting for 3-dimensional transport effects. The
onset-voltage shift induced by the magnetic field in resonant tunneling diodes,
which was previously attributed to the cyclotron frequency inside the
well is found to arise from an upward shift of the non-zero ground (lowest)
Landau state energy in the entire quantum region where coherent transport takes
place. The spatial dependence of the cyclotron frequency is accounted for and
verified to have a negligible impact on resonant tunneling for the device and
magnetic field strength considered. A correction term for the onset-voltage
shift arising from the magnetic field dependence of the chemical potential is
also derived. The Landau ground state with its nonvanishing finite harmonic
oscillator energy is verified however to be the principal
contributor to the onset voltage shift at low temperatures.Comment: 13 pages, and 3 figures. Accepted for publication in Phys. Rev.
A Variable Density Sampling Scheme for Compressive Fourier Transform Interferometry
Fourier Transform Interferometry (FTI) is an appealing Hyperspectral (HS)
imaging modality for many applications demanding high spectral resolution,
e.g., in fluorescence microscopy. However, the effective resolution of FTI is
limited by the durability of biological elements when exposed to illuminating
light. Overexposed elements are subject to photo-bleaching and become unable to
fluoresce. In this context, the acquisition of biological HS volumes based on
sampling the Optical Path Difference (OPD) axis at Nyquist rate leads to
unpleasant trade-offs between spectral resolution, quality of the HS volume,
and light exposure intensity. We propose two variants of the FTI imager, i.e.,
Coded Illumination-FTI (CI-FTI) and Structured Illumination FTI (SI-FTI), based
on the theory of compressive sensing (CS). These schemes efficiently modulate
light exposure temporally (in CI-FTI) or spatiotemporally (in SI-FTI).
Leveraging a variable density sampling strategy recently introduced in CS, we
provide near-optimal illumination strategies, so that the light exposure
imposed on a biological specimen is minimized while the spectral resolution is
preserved. Our analysis focuses on two criteria: (i) a trade-off between
exposure intensity and the quality of the reconstructed HS volume for a given
spectral resolution; (ii) maximizing HS volume quality for a fixed spectral
resolution and constrained exposure budget. Our contributions can be adapted to
an FTI imager without hardware modifications. The reconstruction of HS volumes
from CS-FTI measurements relies on an -norm minimization problem promoting
a spatiospectral sparsity prior. Numerically, we support the proposed methods
on synthetic data and simulated CS measurements (from actual FTI measurements)
under various scenarios. In particular, the biological HS volumes can be
reconstructed with a three-to-ten-fold reduction in the light exposure.Comment: 45 pages, 11 figure
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GPU-optimized Code for Long-term Simulations of Beam-beam Effects in Colliders
We report on the development of the new code for long-term simulation of beam-beam effects in particle colliders. The underlying physical model relies on a matrix-based arbitrary-order symplectic particle tracking for beam transport and the Bassetti-Erskine approximation for beam-beam interaction. The computations are accelerated through a parallel implementation on a hybrid GPU/CPU platform. With the new code, a previously computationally prohibitive long-term simulations become tractable. We use the new code to model the proposed medium-energy electron-ion collider (MEIC) at Jefferson Lab
GPU Accelerated Long-Term Simulations of Beam-Beam Effects in Colliders
We present an update on the development of the new code for long-term simulation of beam-beam effects in particle colliders. The underlying physical model relies on a matrix-based arbitrary-order particle tracking (including a symplectic option) for beam transport and the generalized Bassetti-Erskine approximation for beam-beam interaction. The computations are accelerated through a parallel implementation on a hybrid GPU/CPU platform. With the new code, previously computationally prohibitive long-term simulations become tractable. The new code will be used to model the proposed Medium-energy Electron-Ion Collider (MEIC) at Jefferson Lab
Nonequilibrium Green's function theory for transport and gain properties of quantum cascade structures
The transport and gain properties of quantum cascade (QC) structures are
investigated using a nonequilibrium Green's function (NGF) theory which
includes quantum effects beyond a Boltzmann transport description. In the NGF
theory, we include interface roughness, impurity, and electron-phonon
scattering processes within a self-consistent Born approximation, and
electron-electron scattering in a mean-field approximation. With this theory we
obtain a description of the nonequilibrium stationary state of QC structures
under an applied bias, and hence we determine transport properties, such as the
current-voltage characteristic of these structures. We define two contributions
to the current, one contribution driven by the scattering-free part of the
Hamiltonian, and the other driven by the scattering Hamiltonian. We find that
the dominant part of the current in these structures, in contrast to simple
superlattice structures, is governed mainly by the scattering Hamiltonian. In
addition, by considering the linear response of the stationary state of the
structure to an applied optical field, we determine the linear susceptibility,
and hence the gain or absorption spectra of the structure. A comparison of the
spectra obtained from the more rigorous NGF theory with simpler models shows
that the spectra tend to be offset to higher values in the simpler theories.Comment: 44 pages, 16 figures, appearing in Physical Review B Dec 200
High-Fidelity Simulations of Long-Term Beam-Beam Dynamics on GPUs
Future machines such as the Electron Ion Collider (MEIC), linac-ring machines (eRHIC) or LHeC are particularly sensitive to beam-beam effects. This is the limiting factor for long-term stability and high luminosity reach. The complexity of the non-linear dynamics makes it challenging to perform such simulations typically requiring millions of turns. Until recently, most of the methods have involved using linear approximations and/or tracking for a limited number of turns. We have developed a framework which exploits a massively parallel Graphical Processing Units (GPU) architecture to allow for tracking millions of turns in a sympletic way up to an arbitrary order. The code is called GHOST for GPU-accelerated High-Order Symplectic Tracking. As of now, there is no other code in existence that can accurately model the single-particle non-linear dynamics and the beam-beam effect at the same time for a large enough number of turns necessary to verify the long-term stability of a collider. Our approach relies on a matrix-based arbitrary-order symplectic particle tracking for beam transport and the Bassetti-Erskine approximation for the beam-beam interaction
Health Links: Who Acts as a Source of Health Information in a Social Network?
Background: Members of a social network can influence the preventive health choices and cancer screening behaviors of other network members.
Study Design: We conducted in-person interviews with 438 insured adults ages 40-70 in Massachusetts, Hawaii, and Georgia. We gathered information on social network communication regarding routine health topics and cancer screening. Participants reported whether family members and friends ask them for information or advice on health topics. Characteristics of each respondentâs social network were explored, including number of people with whom the participant has discussed colorectal cancer (CRC) screening. Participants were asked whether communication with social network members had ever led them to seek cancer screening.
Principal Findings:80% of respondents in our group described themselves as a source of health information or advice for others in their social network (89% of women vs. 68% of men, p
Conclusions: People who identified themselves as a source of health information within their social network were more likely to have discussed CRC screening with others and communicated with more people. Further study is necessary to understand the roles played by these individuals. As âhealth information ambassadors,â they may be effective targets for interventions that promote preventive screening
Pass This Message Along: Self-edited Email Messages Promoting Colon Cancer Screening among Friends and Family
Encouraging communication within a social network may promote uptake of desired medical services or health behaviors. Little is known about the use of this approach to promote colorectal cancer (CRC) screening. We conducted in-person interviews with 438 insured adults ages 42-73 in Massachusetts, Hawaii, and Georgia.
Participants were shown a sample message in which the sender shares that he has completed a colonoscopy and urges the recipient to discuss CRC screening with a doctor. We asked participants to edit the message to create one they would be willing to send to friends and family via email or postcard. Changes to the message were recorded. Edited text was analyzed for content and concordance with original message.
The majority of participants (61.6%) modified the message; 14.2% added to or reframed the existing personalizing words (e.g. adding âbecause I love youâ), 10.3% added urgency to the message (e.g. âplease donât delayâ) and 8% added reassurance (e.g. âItâs really not that bad.â) Almost one in five (18.3%) deleted a negatively framed sentence on colon cancer risks. In 5.7% of cases, the meaning of at least one sentence was changed but only 2.7% created messages with factual inaccuracies.
Modifiable messages transmitted within a social network offer a way for screened individuals to promote CRC screening. Further study is needed to identify the optimal combination of user-generated content and pre-written text, allowing for creation of messages that are acceptable to senders, persuasive and factually accurate
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