1,741 research outputs found
Hyperfine interaction in a quantum dot: Non-Markovian electron spin dynamics
We have performed a systematic calculation for the non-Markovian dynamics of
a localized electron spin interacting with an environment of nuclear spins via
the Fermi contact hyperfine interaction. This work applies to an electron in
the s -type orbital ground state of a quantum dot or bound to a donor impurity,
and is valid for arbitrary polarization p of the nuclear spin system, and
arbitrary nuclear spin I in high magnetic fields. In the limit of p=1 and
I=1/2, the Born approximation of our perturbative theory recovers the exact
electron spin dynamics. We have found the form of the generalized master
equation (GME) for the longitudinal and transverse components of the electron
spin to all orders in the electron spin--nuclear spin flip-flop terms. Our
perturbative expansion is regular, unlike standard time-dependent perturbation
theory, and can be carried-out to higher orders. We show this explicitly with a
fourth-order calculation of the longitudinal spin dynamics. In zero magnetic
field, the fraction of the electron spin that decays is bounded by the
smallness parameter \delta=1/p^{2}N, where N is the number of nuclear spins
within the extent of the electron wave function. However, the form of the decay
can only be determined in a high magnetic field, much larger than the maximum
Overhauser field. In general the electron spin shows rich dynamics, described
by a sum of contributions with non-exponential decay, exponential decay, and
undamped oscillations. There is an abrupt crossover in the electron spin
asymptotics at a critical dimensionality and shape of the electron envelope
wave function. We propose a scheme that could be used to measure the
non-Markovian dynamics using a standard spin-echo technique, even when the
fraction that undergoes non-Markovian dynamics is small.Comment: 22 pages, 8 figure
Microscopic Theory for the Markovian Decay of Magnetization Fluctuations in Nanomagnets
We present a microscopic theory for the phonon-driven decay of the
magnetization fluctuations in a wide class of nanomagnets where the dominant
energy is set by isotropic exchange and/or uniaxial anisotropy. Based on the
Zwanzig-Mori projection formalism, the theory reveals that the magnetization
fluctuations are governed by a single decay rate , which we further
identify with the zero-frequency portion of the associated self-energy. This
dynamical decoupling from the remaining slow degrees of freedom is attributed
to a conservation law and the discreteness of the energy spectrum, and explains
the omnipresent mono-exponential decay of the magnetization over several
decades in time, as observed experimentally. A physically transparent
analytical expression for is derived which highlights the three
specific mechanisms of the slowing down effect which are known so far in
nanomagnets.Comment: 7 page
Study protocol for the recreational stimulation for elders as a vehicle to resolve delirium superimposed on dementia (Reserve For DSD) trial
This is the final version of the article. Available from the publisher via the DOI in this record.BACKGROUND: Delirium is a state of confusion characterized by an acute and fluctuating decline in cognitive functioning. Delirium is common and deadly in older adults with dementia, and is often referred to as delirium superimposed on dementia, or DSD. Interventions that treat DSD are not well-developed because the mechanisms involved in its etiology are not completely understood. We have developed a theory-based intervention for DSD that is derived from the literature on cognitive reserve and based on our prior interdisciplinary work on delirium, recreational activities, and cognitive stimulation in people with dementia. Our preliminary work indicate that use of simple, cognitively stimulating activities may help resolve delirium by helping to focus inattention, the primary neuropsychological deficit in delirium. Our primary aim in this trial is to test the efficacy of Recreational Stimulation for Elders as a Vehicle to resolve DSD (RESERVE- DSD). METHODS/DESIGN: This randomized repeated measures clinical trial will involve participants being recruited and enrolled at the time of admission to post acute care. We will randomize 256 subjects to intervention (RESERVE-DSD) or control (usual care). Intervention subjects will receive 30-minute sessions of tailored cognitively stimulating recreational activities for up to 30 days. We hypothesize that subjects who receive RESERVE-DSD will have: decreased severity and duration of delirium; greater gains in attention, orientation, memory, abstract thinking, and executive functioning; and greater gains in physical function compared to subjects with DSD who receive usual care. We will also evaluate potential moderators of intervention efficacy (lifetime of complex mental activities and APOE status). Our secondary aim is to describe the costs associated with RESERVE-DSD. DISCUSSION: Our theory-based intervention, which uses simple, inexpensive recreational activities for delivering cognitive stimulation, is innovative because, to our knowledge it has not been tested as a treatment for DSD. This novel intervention for DSD builds on our prior delirium, recreational activity and cognitive stimulation research, and draws support from cognitive reserve theory. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT01267682This study is supported by The National Institute of Nursing Research (NINR),
the National Institutes of Health, grant number R01: NR012242.
Density-operator approaches to transport through interacting quantum dots: Simplifications in fourth-order perturbation theory
Various theoretical methods address transport effects in quantum dots beyond
single-electron tunneling while accounting for the strong interactions in such
systems. In this paper we report a detailed comparison between three prominent
approaches to quantum transport: the fourth-order Bloch-Redfield quantum master
equation (BR), the real-time diagrammatic technique (RT), and the scattering
rate approach based on the T-matrix (TM). Central to the BR and RT is the
generalized master equation for the reduced density matrix. We demonstrate the
exact equivalence of these two techniques. By accounting for coherences
(nondiagonal elements of the density matrix) between nonsecular states, we show
how contributions to the transport kernels can be grouped in a physically
meaningful way. This not only significantly reduces the numerical cost of
evaluating the kernels but also yields expressions similar to those obtained in
the TM approach, allowing for a detailed comparison. However, in the TM
approach an ad hoc regularization procedure is required to cure spurious
divergences in the expressions for the transition rates in the stationary
(zero-frequency) limit. We show that these problems derive from incomplete
cancellation of reducible contributions and do not occur in the BR and RT
techniques, resulting in well-behaved expressions in the latter two cases.
Additionally, we show that a standard regularization procedure of the TM rates
employed in the literature does not correctly reproduce the BR and RT
expressions. All the results apply to general quantum dot models and we present
explicit rules for the simplified calculation of the zero-frequency kernels.
Although we focus on fourth-order perturbation theory only, the results and
implications generalize to higher orders. We illustrate our findings for the
single impurity Anderson model with finite Coulomb interaction in a magnetic
field.Comment: 29 pages, 12 figures; revised published versio
Hall effect in quasi one-dimensional organic conductors
We study the Hall effect in a system of weakly coupled Luttinger Liquid
chains, using a Memory function approach to compute the Hall constant in the
presence of umklapp scattering along the chains. In this approximation, the
Hall constant decomposes into two terms: a high-frequency term and a Memory
function term. For the case of zero umklapp scattering, where the Memory
function vanishes, the Hall constant is simply the band value, in agreement
with former results in a similar model with no dissipation along the chains.
With umklapp scattering along the chains, we find a power-law temperature
dependance of the Hall constant. We discuss the applications to quasi 1D
organic conductors at high temperatures.Comment: Proceedings of the ISCOM conference "Sixth International Symposium on
Crystalline Organic Metals, Superconductors, and Ferromagnets", Key West,
Florida, USA (Sept. 2005), to be plublished in the Journal of Low Temperature
Physic
Progression of autosomal-dominant polycystic kidney disease in children1
Progression of autosomal-dominant polycystic kidney disease in children.BackgroundAlthough many case reports describe manifestations of autosomal-dominant polycystic kidney disease (ADPKD) in children, no longitudinal studies have examined the natural progression or risk factors for more rapid progression in a large number of children from ADPKD families.MethodsSince 1985, we have studied 312 children from 131 families with a history, a physical examination, blood and urine chemistries, an abdominal ultrasonography, and gene linkage analysis. One hundred fifteen of 185 affected children were studied multiple times for up to 15 years. Renal volumes were determined by ultrasound imaging. Graphs of mean renal volumes according to age were compared between affected and unaffected children, ADPKD children with and without early severe disease, and children with and without high blood pressure.ResultsAffected children had faster renal growth than unaffected children. ADPKD children with severe renal enlargement at a young age continued to experience faster renal growth than those with mild enlargement or normal kidney size for their age, and affected children with high blood pressure had faster renal growth than those with lower blood pressure. Glomerular filtration rate did not decrease in any children except for two with unusually severe early onset disease.ConclusionsThe progression of ADPKD clearly occurs in childhood and manifests as an increase in cyst number and renal size. This study identifies children at risk for rapid renal enlargement who may benefit the most from future therapeutic interventions
Expansion of the Gibbs potential for quantum many-body systems: General formalism with applications to the spin glass and the weakly non-ideal Bose gas
For general quantum systems the power expansion of the Gibbs potential and
consequently the power expansion of the self energy is derived in terms of the
interaction strength. Employing a generalization of the projector technique a
compact representation of the general terms of the expansion results. The
general aspects of the approach are discussed with special emphasis on the
effects characteristic for quantum systems. The expansion is systematic and
leads directly to contributions beyond mean-field of all thermodynamic
quantities. These features are explicitly demonstrated and illustrated for two
non-trivial systems, the infinite range quantum spin glass and the weakly
interacting Bose gas. The Onsager terms of both systems are calculated, which
represent the first beyond mean-field contributions. For the spin glass new
TAP-like equations are presented and discussed in the paramagnetic region. The
investigation of the Bose gas leads to a beyond mean-field thermodynamic
description. At the Bose-Einstein condensation temperature complete agreement
is found with the results presented recently by alternative techniques.Comment: 17 pages, 0 figures; revised version accepted by Phys Rev
The Central Laser Facility at the Pierre Auger Observatory
The Central Laser Facility is located near the middle of the Pierre Auger
Observatory in Argentina. It features a UV laser and optics that direct a beam
of calibrated pulsed light into the sky. Light scattered from this beam
produces tracks in the Auger optical detectors which normally record nitrogen
fluorescence tracks from cosmic ray air showers. The Central Laser Facility
provides a "test beam" to investigate properties of the atmosphere and the
fluorescence detectors. The laser can send light via optical fiber
simultaneously to the nearest surface detector tank for hybrid timing analyses.
We describe the facility and show some examples of its many uses.Comment: 4 pages, 5 figures, submitted to 29th ICRC Pune Indi
Covariance and Fisher information in quantum mechanics
Variance and Fisher information are ingredients of the Cramer-Rao inequality.
We regard Fisher information as a Riemannian metric on a quantum statistical
manifold and choose monotonicity under coarse graining as the fundamental
property of variance and Fisher information. In this approach we show that
there is a kind of dual one-to-one correspondence between the candidates of the
two concepts. We emphasis that Fisher informations are obtained from relative
entropies as contrast functions on the state space and argue that the scalar
curvature might be interpreted as an uncertainty density on a statistical
manifold.Comment: LATE
Constraints on Gamma-ray Emission from the Galactic Plane at 300 TeV
We describe a new search for diffuse ultrahigh energy gamma-ray emission
associated with molecular clouds in the galactic disk. The Chicago Air Shower
Array (CASA), operating in coincidence with the Michigan muon array (MIA), has
recorded over 2.2 x 10^{9} air showers from April 4, 1990 to October 7, 1995.
We search for gamma rays based upon the muon content of air showers arriving
from the direction of the galactic plane. We find no significant evidence for
diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma
rays to normal hadronic cosmic rays at less than 2.4 x 10^{-5} at 310 TeV (90%
confidence limit) from the galactic plane region: (50 degrees < l < 200
degrees); -5 degrees < b < 5 degrees). This limit places a strong constraint on
models for emission from molecular clouds in the galaxy. We rule out
significant spectral hardening in the outer galaxy, and conclude that emission
from the plane at these energies is likely to be dominated by the decay of
neutral pions resulting from cosmic rays interactions with passive target gas
molecules.Comment: Astrophysical Journal, submitted, 11 pages, AASTeX Latex, 3
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