12,850 research outputs found
PRM4 A scoping literature review on the external validity of randomized controlled trial populations
Quantum fluctuations in quantum lattice-systems with continuous symmetry
We discuss conditions for the absence of spontaneous breakdown of continuous
symmetries in quantum lattice systems at . Our analysis is based on
Pitaevskii and Stringari's idea that the uncertainty relation can be employed
to show quantum fluctuations. For the one-dimensional systems, it is shown that
the ground state is invariant under the continuous transformation if a certain
uniform susceptibility is finite. For the two- and three-dimensional systems,
it is shown that truncated correlation functions cannot decay any more rapidly
than whenever the continuous symmetry is spontaneously broken.
Both of these phenomena occur owing to quantum fluctuations. Our theorems cover
a wide class of quantum lattice-systems having not-too-long-range interactions.Comment: 14 pages. To appear in J.Stat.Phy
A ratchet mechanism for amplification in low-frequency mammalian hearing
The sensitivity and frequency selectivity of hearing result from tuned
amplification by an active process in the mechanoreceptive hair cells. In most
vertebrates the active process stems from the active motility of hair bundles.
The mammalian cochlea exhibits an additional form of mechanical activity termed
electromotility: its outer hair cells (OHCs) change length upon electrical
stimulation. The relative contributions of these two mechanisms to the active
process in the mammalian inner ear is the subject of intense current debate.
Here we show that active hair-bundle motility and electromotility can together
implement an efficient mechanism for amplification that functions like a
ratchet: sound-evoked forces acting on the basilar membrane are transmitted to
the hair bundles whereas electromotility decouples active hair-bundle forces
from the basilar membrane. This unidirectional coupling can extend the hearing
range well below the resonant frequency of the basilar membrane. It thereby
provides a concept for low-frequency hearing that accounts for a variety of
unexplained experimental observations from the cochlear apex, including the
shape and phase behavior of apical tuning curves, their lack of significant
nonlinearities, and the shape changes of threshold tuning curves of auditory
nerve fibers along the cochlea. The ratchet mechanism constitutes a general
design principle for implementing mechanical amplification in engineering
applications.Comment: 6 pages, 4 figures, plus Supplementary Information. Animation
available on the PNAS website (http://dx.doi.org/10.1073/pnas.0914345107)
Oblique Corrections To The W Width
The lowest-order expression for the partial width to , has no oblique radiative
corrections from new physics if the measured mass is used. Here GeV/ is the muon decay constant. For
the present value of GeV/, and with
GeV, one expects MeV. The total
width is also expected to lack oblique corrections from
new physics, so that . Present data are consistent with this prediction.Comment: 15 pages (LaTeX), one PostScript figure not included (available upon
request
Consistency of the Regularization of Gauge Theories by High Covariant Derivatives
We show that regularization of gauge theories by higher covariant derivatives
and gauge invariant Pauli-Villars regulators is a consistent method if the
Pauli-Villars vector fields are considered in a covariant in the regulating
Pauli-Villars fields is pathological and the original Slavnov proposal in
covariant Landau gauge is not correct because of the appearance of massless
modes in the regulators which do not decouple when the ultraviolet regulator is
removed. In such a case the method does not correspond to the regularization of
a pure gauge theory but that of a gauge theory in interaction with massless
ghost fields. This explains the problems pointed out by Martin and Ruiz in
covariant Landau gauge. However, a minor modification of Slavnov method
provides a consistent regularization even for such a case. The regularization
that we introduce also solves the problem of overlapping divergences in a way
similar to geometric regularization and yields the standard values of the
and functions of the renormalization group equations.Comment: 20 pages, latex, 3 Postscript figures (expanded version
Structure and Magnetization of Two-Dimensional Vortex Arrays in the Presence of Periodic Pinning
Ground-state properties of a two-dimensional system of superconducting
vortices in the presence of a periodic array of strong pinning centers are
studied analytically and numerically. The ground states of the vortex system at
different filling ratios are found using a simple geometric argument under the
assumption that the penetration depth is much smaller than the spacing of the
pin lattice. The results of this calculation are confirmed by numerical studies
in which simulated annealing is used to locate the ground states of the vortex
system. The zero-temperature equilibrium magnetization as a function of the
applied field is obtained by numerically calculating the energy of the ground
state for a large number of closely spaced filling ratios. The results show
interesting commensurability effects such as plateaus in the B-H diagram at
simple fractional filling ratios.Comment: 12 pages, 19 figures, submitted for publicatio
Electrical detection of 31P spin quantum states
In recent years, a variety of solid-state qubits has been realized, including
quantum dots, superconducting tunnel junctions and point defects. Due to its
potential compatibility with existing microelectronics, the proposal by Kane
based on phosphorus donors in Si has also been pursued intensively. A key issue
of this concept is the readout of the P quantum state. While electrical
measurements of magnetic resonance have been performed on single spins, the
statistical nature of these experiments based on random telegraph noise
measurements has impeded the readout of single spin states. In this letter, we
demonstrate the measurement of the spin state of P donor electrons in silicon
and the observation of Rabi flops by purely electric means, accomplished by
coherent manipulation of spin-dependent charge carrier recombination between
the P donor and paramagnetic localized states at the Si/SiO2 interface via
pulsed electrically detected magnetic resonance. The electron spin information
is shown to be coupled through the hyperfine interaction with the P nucleus,
which demonstrates the feasibility of a recombination-based readout of nuclear
spins
Modulation of cognitive performance and mood by aromas of peppermint and ylang-ylang
This study provides further evidence for the impact of the aromas of plant essential oils on aspects of cognition and mood in healthy participants. One hundred and forty-four volunteers were randomly assigned to conditions of ylang-ylang aroma, peppermint aroma, or no aroma control. Cognitive performance was assessed using the Cognitive Drug Research computerized assessment battery, with mood scales completed before and after cognitive testing. The analysis of the data revealed significant differences between conditions on a number of the factors underpinning the tests that constitute the battery. Peppermint was found to enhance memory whereas ylang-ylang impaired it, and lengthened processing speed. In terms of subjective mood peppermint increased alertness and ylang-ylang decreased it, but significantly increased calmness. These results provide support for the contention that the aromas of essential oils can produce significant and idiosyncratic effects on both subjective and objective assessments of aspects of human behavior. They are discussed with reference to possible pharmacological and psychological modes of influence
First-principles study of orthorhombic CdTiO3 perovskite
In this work we perform an ab-initio study of CdTiO3 perovskite in its
orthorhombic phase using FLAPW method. Our calculations help to decide between
the different cristallographic structures proposed for this perovskite from
X-Ray measurements. We compute the electric field gradient tensor (EFG) at Cd
site and obtain excellent agreement with available experimental information
from a perturbed angular correlation (PAC) experiment. We study EFG under an
isotropic change of volume and show that in this case the widely used "point
charge model approximation" to determine EFG works quite well.Comment: 4 pages, 1 figure. Accepted in Physical Review
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