64,773 research outputs found
Localized Electron States Near a Metal-Semiconductor Nanocontact
The electronic structure of nanowires in contact with metallic electrodes of
experimentally relevant sizes is calculated by incorporating the electrostatic
polarization potential into the atomistic single particle Schr\"odinger
equation. We show that the presence of an electrode produces localized
electron/hole states near the electrode, a phenomenon only exhibited in
nanostructures and overlooked in the past. This phenomenon will have profound
implications on electron transport in such nanosystems. We calculate several
electrode/nanowire geometries, with varying contact depths and nanowire radii.
We demonstrate the change in the band gap of up to 0.5 eV in 3 nm diameter CdSe
nanowires and calculate the magnitude of the applied electric field necessary
to overcome the localization.Comment: 11 pages 4 figure
A Public Health Framework for Screening Mammography: Evidence-Based Versus Politically Mandated Care
This Viewpoint highlights the societal risks of politically motivated mandates relating to public health guidelines. Although the Affordable Care Act mandated insurance coverage for U.S. Preventive Services Task Force (USPSTF)-recommended preventive services, it went further for mammography screening. Instead of relying on the most recent USPSTF guidelines, Congress amended the ACA to require the Department of Health and Human Services (DHHS) to use its 2002 guidelines, which recommended screening every 1-2 years starting at age 40. The FY 2016 Consolidated Appropriations Act instructs DHHS to interpret any reference to “current” USPSTF breast cancer screening recommendations to mean those issued “before 2009” — in other words, its 2002 recommendations. Essentially, Congress is requiring health insurers to ignore modern scientific assessments, and instead use 14-year-old guidance.
The ACA improved the public’s health by guaranteeing that insurers provide uniform, cost-free access to preventive services based on modern evidence of effectiveness. The public’s health is best served when women’s personal decisions about screening are informed by evidence rather than political considerations. The Congress’s paternalistic response to USPSTF mammography-screening recommendations vividly illuminate the social costs of politically mandated care. Rather than benefiting women, political interference with science can discourage shared decision-making, increase harms from screening, and sow public doubt about the value and integrity of science
Adiabatic self-tuning in a silicon microdisk optical resonator
We demonstrate a method for adiabatically self-tuning a silicon microdisk resonator. This mechanism is not only able to sensitively probe the fast nonlinear cavity dynamics, but also provides various optical functionalities like pulse compression, shaping, and tunable time delay
Electrical impedance imaging in two-phase, gas-liquid flows: 1. Initial investigation
The determination of interfacial area density in two-phase, gas-liquid flows is one of the major elements impeding significant development of predictive tools based on the two-fluid model. Currently, these models require coupling of liquid and vapor at interfaces using constitutive equations which do not exist in any but the most rudimentary form. Work described herein represents the first step towards the development of Electrical Impedance Computed Tomography (EICT) for nonintrusive determination of interfacial structure and evolution in such flows
P-band in a rotating optical lattice
We investigate the effects of rotation on the excited bands of a tight
binding lattice, focusing particulary on the first excited (p-) band. Both the
on-site energies and the hopping between lattice sites are modified by the
effective magnetic field created by rotation, causing a non-trivial splitting
and magnetic fine structure of the p-band. We show that Peierls substitution
can be modified to describe p-band under rotation, and use this method to
derive an effective Hamiltonian. We compare the spectrum of the effective
Hamiltonian with a first principles calculation of the magnetic band structure
and find excellent agreement, confirming the validity of our approach. We also
discuss the on-site interaction terms for bosons and argue that many-particle
phenomena in a rotating p-band can be investigated starting from this effective
Hamiltonian.Comment: 7 pages, 4 figures, new discussion of effective Hamiltonian,
references adde
Interaction of Close-in Planets with the Magnetosphere of their Host Stars I: Diffusion, Ohmic Dissipation of Time Dependent Field, Planetary Inflation, and Mass Loss
The unanticipated discovery of the first close-in planet around 51 Peg has
rekindled the notion that shortly after their formation outside the snow line,
some planets may have migrated to the proximity of their host stars because of
their tidal interaction with their nascent disks. If these planets indeed
migrated to their present-day location, their survival would require a halting
mechanism in the proximity of their host stars. Most T Tauri stars have strong
magnetic fields which can clear out a cavity in the innermost regions of their
circumstellar disks and impose magnetic induction on the nearby young planets.
Here we consider the possibility that a magnetic coupling between young stars
and planets could quench the planet's orbital evolution. After a brief
discussion of the complexity of the full problem, we focus our discussion on
evaluating the permeation and ohmic dissipation of the time dependent component
of the stellar magnetic field in the planet's interior. Adopting a model first
introduced by C. G. Campbell for interacting binary stars, we determine the
modulation of the planetary response to the tilted magnetic field of a
non-synchronously spinning star. We first compute the conductivity in the young
planets, which indicates that the stellar field can penetrate well into the
planet's envelope in a synodic period. For various orbital configurations, we
show that the energy dissipation rate inside the planet is sufficient to induce
short-period planets to inflate. This process results in mass loss via Roche
lobe overflow and in the halting of the planet's orbital migration.Comment: 47 pages, 12 figure
Sliding of Electron Crystal of Finite Size on the Surface of Superfluid He-4 Confined in a Microchannel
We present a new study of the nonlinear transport of a two-dimensional
electron crystal on the surface of liquid helium confined in a 10
micrometer-wide channel in which the effective length of the crystal can be
varied from 10 to 215 micrometers. At low driving voltages, the moving electron
crystal is strongly coupled to deformation of the liquid surface arising from
resonant excitation of surface capillary waves, ripplons, while at higher
driving voltages the crystal decouples from the deformation. We find strong
dependence of the decoupling threshold of the driving electric field acting on
the electrons, on the size of the crystal. In particular, the threshold
electric field significantly decreases when the length of the crystal becomes
shorter than 25 micrometers. We explain this effect as arising from weakening
of surface deformations due to radiative loss of resonantly-excited ripplons
from an electron crystal of finite size, and we account for the observed effect
using an instructive analytical model.Comment: 5 figure
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