10,384 research outputs found
Quantum information analysis of electronic states at different molecular structures
We have studied transition metal clusters from a quantum information theory
perspective using the density-matrix renormalization group (DMRG) method. We
demonstrate the competition between entanglement and interaction localization.
We also discuss the application of the configuration interaction based
dynamically extended active space procedure which significantly reduces the
effective system size and accelerates the speed of convergence for complicated
molecular electronic structures to a great extent. Our results indicate the
importance of taking entanglement among molecular orbitals into account in
order to devise an optimal orbital ordering and carry out efficient
calculations on transition metal clusters. We propose a recipe to perform DMRG
calculations in a black-box fashion and we point out the connections of our
work to other tensor network state approaches
State estimation in quantum homodyne tomography with noisy data
In the framework of noisy quantum homodyne tomography with efficiency
parameter , we propose two estimators of a quantum state whose
density matrix elements decrease like , for
fixed known and . The first procedure estimates the matrix
coefficients by a projection method on the pattern functions (that we introduce
here for ), the second procedure is a kernel estimator of the
associated Wigner function. We compute the convergence rates of these
estimators, in risk
Nazi Punks Folk Off: Leisure, Nationalism, Cultural Identity and the Consumption of Metal and Folk Music
Far-right activists have attempted to infiltrate and use popular music scenes to propagate their racialised ideologies. This paper explores attempts by the far right to co-opt two particular music scenes: black metal and English folk. Discourse tracing is used to explore online debates about boundaries, belonging and exclusion in the two scenes, and to compare such online debates with ethnographic work and previous research. It is argued that both scenes have differently resisted the far right through the policing of boundaries and communicative choices, but both scenes are compromised by their relationship to myths of whiteness and the instrumentality of the pop music industry
Action semantics in retrospect
This paper is a themed account of the action semantics project, which Peter Mosses has led since the 1980s. It explains his motivations for developing action semantics, the inspirations behind its design, and the foundations of action semantics based on unified algebras. It goes on to outline some applications of action semantics to describe real programming languages, and some efforts to implement programming languages using action semantics directed compiler generation. It concludes by outlining more recent developments and reflecting on the success of the action semantics project
Interpretation of AIRS Data in Thin Cirrus Atmospheres Based on a Fast Radiative Transfer Model
A thin cirrus cloud thermal infrared radiative transfer model has been developed for application to cloudy
satellite data assimilation. This radiation model was constructed by combining the Optical Path Transmittance
(OPTRAN) model, developed for the speedy calculation of transmittances in clear atmospheres, and
a thin cirrus cloud parameterization using a number of observed ice crystal size and shape distributions.
Numerical simulations show that cirrus cloudy radiances in the 800–1130-cm^(-1) thermal infrared window are
sufficiently sensitive to variations in cirrus optical depth and ice crystal size as well as in ice crystal shape
if appropriate habit distribution models are selected a priori for analysis. The parameterization model has
been applied to the Atmospheric Infrared Sounder (AIRS) on board the Aqua satellite to interpret clear
and thin cirrus spectra observed in the thermal infrared window. Five clear and 29 thin cirrus cases at
nighttime over and near the Atmospheric Radiation Measurement program (ARM) tropical western Pacific
(TWP) Manus Island and Nauru Island sites have been chosen for this study. A X^2-minimization program
was employed to infer the cirrus optical depth and ice crystal size and shape from the observed AIRS
spectra. Independent validation shows that the AIRS-inferred cloud parameters are consistent with those
determined from collocated ground-based millimeter-wave cloud radar measurements. The coupled thin
cirrus radiative transfer parameterization and OPTRAN, if combined with a reliable thin cirrus detection
scheme, can be effectively used to enhance the AIRS data volume for data assimilation in numerical
weather prediction models
Anomalous Radio-Wave Scattering from Interstellar Plasma Structures
This paper considers scattering screens that have arbitrary spatial
variations of scattering strength transverse to the line of sight, including
screens that are spatially well confined, such as disks and filaments. We
calculate the scattered image of a point source and the observed pulse shape of
a scattered impulse. The consequences of screen confinement include: (1) Source
image shapes that are determined by the physical extent of the screen rather
than by the shapes of much-smaller diffracting microirregularities. These
include image elongations and orientations that are frequency dependent. (2)
Variation with frequency of angular broadening that is much weaker than the
trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including
frequency-independent cases; and (3) Similar departure of the pulse broadening
time from the usually expected \nu^{-4} scaling law. We briefly discuss
applications that include scattering of pulses from the Crab pulsar by
filaments in the Crab Nebula; image asymmetries from Galactic scattering of the
sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active
galactic nuclei by intervening galaxies. We also address the consequences for
inferences about the shape of the wavenumber spectrum of electron density
irregularities, which depend on scaling laws for the image size and the pulse
broadening. Future low-frequency (< 100 MHz) array observations will also be
strongly affected by the Galactic structure of scattering material. Our
formalism is derived in the context of radio scattering by plasma density
fluctuations. It is also applicable to optical, UV and X-ray scattering by
grains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by
ApJ, revised version has minor changes to respond to referee comments and
suggestion
Low-Lying Excited States and Low-Temperature Properties of an Alternating Spin-1 / Spin-1/2 Chain : A DMRG study
We report spin wave and DMRG studies of the ground and low-lying excited
states of uniform and dimerized alternating spin chains. The DMRG procedure is
also employed to obtain low-temperature thermodynamic properties of the system.
The ground state of a 2N spin system with spin-1 and spin-1/2 alternating from
site to site and interacting via an antiferromagnetic exchange is found to be
ferrimagnetic with total spin from both DMRG and spin wave analysis.
Both the studies also show that there is a gapless excitation to a state with
spin and a gapped excitation to a state with spin .
Surprisingly, the correlation length in the ground state is found to be very
small from both the studies for this gapless system. For this very reason, we
show that the ground state can be described by a variational ``ansatz'' of the
product type. DMRG analysis shows that the chain is susceptible to a
conditional spin-Peierls' instability. The DMRG studies of magnetization,
magnetic susceptibility () and specific heat show strong magnetic-field
dependence. The product shows a minimum as a function of
temperature() at low-magnetic fields and the minimum vanishes at
high-magnetic fields. This low-field behaviour is in agreement with earlier
experimental observations. The specific heat shows a maximum as a function of
temperature and the height of the maximum increases sharply at high magnetic
fields. It is hoped that these studies will motivate experimental studies at
high-magnetic fields.Comment: 22 pages in latex; 16 eps figures available upon reques
Magnetic properties of a new molecular-based spin-ladder system: (5IAP)2CuBr4*2H2O
We have synthesized and characterized a new spin-1/2 Heisenberg
antiferromagnetic ladder: bis 5-iodo-2-aminopyridinium tetrabromocuprate(II)
dihydrate. X-ray diffraction studies show the structure of the compound to
consist of well isolated stacked ladders and the interaction between the Cu(2+)
atoms to be due to direct Br...Br contacts. Magnetic susceptibility and
magnetization studies show the compound to be in the strong-coupling limit,
with the interaction along the rungs (J' ~ 13 K) much greater than the
interaction along the rails (J ~ 1 K). Magnetic critical fields are observed
near 8.3 T and 10.4 T, respectively, establishing the existence of the energy
gap.Comment: 10 pages, 4 figures, submitted to Phys. Rev. B Figure 4 did not
print. *.eps files replaced with figures.ps fil
Finding Radio Pulsars in and Beyond the Galactic Center
Radio-wave scattering is enhanced dramatically for Galactic center sources in
a region with radius >~ 15 arc min. Using scattering from Sgr A* and other
sources, we show that pulse broadening for pulsars in the Galactic center is
{\em at least} 6.3 \nu^{-4} seconds (\nu = radio frequency in GHz) and is most
likely 50--200 times larger because the relevant scattering screen appears to
be within the Galactic center region itself. Pulsars beyond---but viewed
through---the Galactic center suffer even greater pulse broadening and are
angularly broadened by <~ 2 {\em arc min}. Periodicity searches at radio
frequencies are likely to find only long period pulsars and, then, only if
optimized by using frequencies >~ 7 GHz and by testing for small numbers of
harmonics in the power spectrum. The optimal frequency is where \Delta_{0.1} is the distance of the
scattering region from Sgr A* in units of 0.1 kpc, P is the period (seconds),
and \alpha is the spectral index. A search for compact sources using aperture
synthesis may be far more successful than searches for periodicities because
the angular broadening is not so large as to desensitize the survey. We
estimate that the number of {\em detectable} pulsars in the Galactic center may
range from <= 1 to 100, with the larger values resulting from recent, vigorous
starbursts. Such pulsars provide unique opportunities for probing the ionized
gas, gravitational potential, and stellar population near Sgr A*.Comment: 13 pages, 4 PS figures, LaTeX and requires AASTeX macro aas2pp4,
accepted by ApJ, also available as
http://astrosun.tn.cornell.edu/SPIGOT/papers/pulsar/gc_psr.web
Improved method of porcine renal allografting for transplantation Research
This study presents a refined, reproducible, and clinically appropriate animal model of renal transplantation. A pair of kidneys are harvested from a donor pig and preserved in Euro-Collins' solution (4°C). After a set period of preservation, the allografts are transplanted to two recipient pigs. The abdomen is entered through a midline incision. The right common iliac artery and vein are dissected and bilateral native nephrectomy is performed. Each allograft is then randomly assigned and transplanted to the recipients. Three minutes before un- clamping, 100 mg offurosemide and 10 g of mannitol are given IV. Immediately after reperfusion, urine output is measured for 1 h. The allograft is biopsied and ureteroneocyslostomy is created. Cystostomy is then placed using a 16F Foley catheter. The bladder neck is ligated to secure complete diversion of urine, and the abdomen is closed in layers. This kidney transplant model allows an absolutely paired study of the kidney allograft function from the same donor and also collection of pure urine at any time postoperatively, obviating the need for metabolic cages or sedation for urinary collection. This model and its unique modifications allow various transplant studies, including organ preservation, immunosuppressive protocol, and the prevention of reperfusion injury from oxygen free radicals © 1991 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
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