9,814 research outputs found
Carrier-envelope phase control over pathway interference in strong-field dissociation of H
The dissociation of an H molecular-ion beam by linearly polarized,
carrier-envelope-phase-tagged 5 fs pulses at 4W/cm with a
central wavelength of 730 nm was studied using a coincidence 3D momentum
imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission
direction of H fragments relative to the laser polarization were observed.
These asymmetries are caused by interference of odd and even photon number
pathways, where net-zero photon and 1-photon interference predominantly
contributes at H+H kinetic energy releases of 0.2 -- 0.45 eV, and
net-2-photon and 1-photon interference contributes at 1.65 -- 1.9 eV. These
measurements of the benchmark H molecule offer the distinct advantage
that they can be quantitatively compared with \textit{ab initio} theory to
confirm our understanding of strong-field coherent control via the
carrier-envelope phase
Holographic Conformal Window - A Bottom Up Approach
We propose a five-dimensional framework for modeling the background geometry
associated to ordinary Yang-Mills (YM) as well as to nonsupersymmetric gauge
theories possessing an infrared fixed point with fermions in various
representations of the underlying gauge group. The model is based on the
improved holographic approach, on the string theory side, and on the
conjectured all-orders beta function for the gauge theory one. We first analyze
the YM gauge theory. We then investigate the effects of adding flavors and show
that, in the holographic description of the conformal window, the geometry
becomes AdS when approaching the ultraviolet and the infrared regimes. As the
number of flavors increases within the conformal window we observe that the
geometry becomes more and more of AdS type over the entire energy range.Comment: 20 Pages, 3 Figures. v2: references adde
Energetics and geometry of excitations in random systems
Methods for studying droplets in models with quenched disorder are critically
examined. Low energy excitations in two dimensional models are investigated by
finding minimal energy interior excitations and by computing the effect of bulk
perturbations. The numerical data support the assumptions of compact droplets
and a single exponent for droplet energy scaling. Analytic calculations show
how strong corrections to power laws can result when samples and droplets are
averaged over. Such corrections can explain apparent discrepancies in several
previous numerical results for spin glasses.Comment: 4 pages, eps files include
Absorption and wavepackets in optically excited semiconductor superlattices driven by dc-ac fields
Within the one-dimensional tight-binding minibands and on-site
Coloumbic interaction approximation, the absorption spectrum and coherent
wavepacket time evolution in an optically excited semiconductor superlattice
driven by dc-ac electric fields are investigated using the semiconductor Bloch
equations.
The dominating roles of the ratios of dc-Stark to external ac frequency, as
well as ac-Stark to external ac frequency, is emphasized. If the former is an
integer , then also harmonics are present within one Stark
frequency, while the fractional case leads to the formation of excitonic
fractional ladders. The later ratio determines the size and profile of the
wavepacket. In the absence of excitonic interaction it controls the maximum
size wavepackets reach within one cycle, while the interaction produces a
strong anisotropy and tends to palliate the dynamic wavepacket localization.Comment: 14 pages, 7 postscript figure
Power-law spin correlations in pyrochlore antiferromagnets
The ground state ensemble of the highly frustrated pyrochlore-lattice
antiferromagnet can be mapped to a coarse-grained ``polarization'' field
satisfying a zero-divergence condition From this it follows that the
correlations of this field, as well as the actual spin correlations, decay with
separation like a dipole-dipole interaction (). Furthermore, a lattice
version of the derivation gives an approximate formula for spin correlations,
with several features that agree well with simulations and neutron-diffraction
measurements of diffuse scattering, in particular the pinch-point
(pseudo-dipolar) singularities at reciprocal lattice vectors. This system is
compared to others in which constraints also imply diffraction singularities,
and other possible applications of the coarse-grained polarization are
discussed.Comment: 13 pp, revtex, two figure
Identification and Use of Frailty Indicators from Text to Examine Associations with Clinical Outcomes Among Patients with Heart Failure.
Frailty is an important health outcomes indicator and valuable for guiding healthcare decisions in older adults, but is rarely collected in a quantitative, systematic fashion in routine healthcare. Using a cohort of 12,000 Veterans with heart failure, we investigated the feasibility of topic modeling to identify frailty topics in clinical notes. Topics were generated through unsupervised learning and then manually reviewed by an expert. A total of 53 frailty topics were identified from 100,000 notes. We further examined associations of frailty with age-, sex-, and Charlson Comorbidity Index-adjusted 1-year hospitalizations and mortality (composite outcome) using logistic regression. Frailty (≤ 4 topics versu
Towards a more complete quantification of the global carbon cycle
The main
components of global carbon budget calculations are the emissions from
burning fossil fuels, cement production, and net land-use change, partly
balanced by ocean CO2 uptake and CO2 increase in the
atmosphere. The difference between these terms is referred to as the residual
sink, assumed to correspond to increasing carbon storage in the terrestrial
biosphere through physiological plant responses to changing conditions
(ΔBphys). It is often used to constrain carbon
exchange in global earth-system models. More broadly, it guides expectations
of autonomous changes in global carbon stocks in response to climatic
changes, including increasing CO2, that may add to, or subtract
from, anthropogenic CO2 emissions.
However, a budget with only these terms omits some important additional fluxes that are
needed to correctly infer ΔBphys. They are cement carbonation and
fluxes into increasing pools of plastic, bitumen, harvested-wood products, and landfill
deposition after disposal of these products, and carbon fluxes to the oceans via wind
erosion and non-CO2 fluxes of the intermediate breakdown products of methane
and other volatile organic compounds. While the global budget includes river transport of
dissolved inorganic carbon, it omits river transport of dissolved and particulate organic
carbon, and the deposition of carbon in inland water bodies.
Each one of these terms is relatively small, but together they can constitute important
additional fluxes that would significantly reduce the size of the inferred ΔBphys. We estimate here that inclusion of these fluxes would reduce ΔBphys from the currently reported 3.6 GtC yr−1 down to about 2.1 GtC yr−1
(excluding losses from land-use change). The implicit reduction in the size of
ΔBphys has important implications for the inferred magnitude of
current-day biospheric net carbon uptake and the consequent potential of future
biospheric feedbacks to amplify or negate net anthropogenic CO2 emissions.</p
Heavy ion collisions and AdS/CFT
We review some recent applications of the AdS/CFT correspondence to heavy ion
collisions including a calculation of the jet quenching parameter in N=4
super-Yang-Mills theory and quarkonium suppression from velocity scaling of the
screening length for a heavy quark-antiquark pair. We also briefly discuss
differences and similarities between QCD and N=4 Super-Yang-Mills theory.Comment: Plenary talk given at Quark Matter 2006, Shanghai, China, 14-20 Nov
2006; to appear in the conference proceedin
SrVOFeAs: A Nanolayered Bimetallic Iron Pnictide Superconductor
One of the unifying concepts in the iron-pnictide superconductors, both for
the mechanism of magnetic ordering and of unconventional order parameter
character, has been the electron and hole Fermi surfaces that are approximately
nested. Using the density functional methods that have predicted Fermi surfaces
correctly in SrFeP, we find that the recently reported superconducting
SrVOFeAs, with =37 K and no apparent competition between magnetism
and superconductivity, possesses different Fermi surface geometry and character
than previous classes of iron pnictides. The intervening layer (a V bilayer)
gives rise to bands that cross the Fermi level. Coupling to the FeAs layer is
small except for interaction along the zone boundary, however that coupling
degrades the Fermi surface nesting. SrVOFeAs, with its alternating
layers of open shell atoms, deserves further close study that should help to
understand the origin of the properties of iron pnictide compounds.Comment: 6 pages and 4 embedded figure
Reconstruction of the joint state of a two-mode Bose-Einstein condensate
We propose a scheme to reconstruct the state of a two-mode Bose-Einstein
condensate, with a given total number of atoms, using an atom interferometer
that requires beam splitter, phase shift and non-ideal atom counting
operations. The density matrix in the number-state basis can be computed
directly from the probabilities of different counts for various phase shifts
between the original modes, unless the beamsplitter is exactly balanced.
Simulated noisy data from a two-mode coherent state is produced and the state
is reconstructed, for 49 atoms. The error can be estimated from the singular
values of the transformation matrix between state and probability data.Comment: 4 pages (REVTeX), 5 figures (PostScript
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