860 research outputs found
Interaction between static holes in a quantum dimer model on the kagome lattice
A quantum dimer model (QDM) on the kagome lattice with an extensive
ground-state entropy was recently introduced [Phys. Rev. B 67, 214413 (2003)].
The ground-state energy of this QDM in presence of one and two static holes is
investigated by means of exact diagonalizations on lattices containing up to
144 kagome sites. The interaction energy between the holes (at distances up to
7 lattice spacings) is evaluated and the results show no indication of
confinement at large hole separations.Comment: 6 pages, 3 figures. IOP style files included. To appear in J. Phys.:
Condens. Matter, Proceedings of the HFM2003 conference, Grenobl
Two-flavour Schwinger model with dynamical fermions in the L\"uscher formalism
We report preliminary results for 2D massive QED with two flavours of Wilson
fermions, using the Hermitean variant of L\"uscher's bosonization technique.
The chiral condensate and meson masses are obtained. The simplicity of the
model allows for high statistics simulations close to the chiral and continuum
limit, both in the quenched approximation and with dynamical fermions.Comment: Talk presented at LATTICE96(algorithms), 3 pages, 3 Postscript
figures, uses twoside, fleqn, espcrc2, epsf, revised version (details of
approx. polynomial
Ordering monomial factors of polynomials in the product representation
The numerical construction of polynomials in the product representation (as
used for instance in variants of the multiboson technique) can become
problematic if rounding errors induce an imprecise or even unstable evaluation
of the polynomial. We give criteria to quantify the effects of these rounding
errors on the computation of polynomials approximating the function . We
consider polynomials both in a real variable and in a Hermitian matrix. By
investigating several ordering schemes for the monomials of these polynomials,
we finally demonstrate that there exist orderings of the monomials that keep
rounding errors at a tolerable level.Comment: Latex2e file, 7 figures, 32 page
Quantum spin models with exact dimer ground states
Inspired by the exact solution of the Majumdar-Ghosh model, a family of
one-dimensional, translationally invariant spin hamiltonians is constructed.
The exchange coupling in these models is antiferromagnetic, and decreases
linearly with the separation between the spins. The coupling becomes
identically zero beyond a certain distance. It is rigorously proved that the
dimer configuration is an exact, superstable ground state configuration of all
the members of the family on a periodic chain. The ground state is two-fold
degenerate, and there exists an energy gap above the ground state. The
Majumdar-Ghosh hamiltonian with two-fold degenerate dimer ground state is just
the first member of the family.
The scheme of construction is generalized to two and three dimensions, and
illustrated with the help of some concrete examples. The first member in two
dimensions is the Shastry-Sutherland model. Many of these models have
exponentially degenerate, exact dimer ground states.Comment: 10 pages, 8 figures, revtex, to appear in Phys. Rev.
The "Square Kagome" Quantum Antiferromagnet and the Eight Vertex Model
We introduce a two dimensional network of corner-sharing triangles with
square lattice symmetry. Properties of magnetic systems here should be similar
to those on the kagome lattice. Focusing on the spin half Heisenberg quantum
antiferromagnet, we generalise the spin symmetry group from SU(2) to SU(N). In
the large N limit, we map the model exactly to the eight vertex model, solved
by Baxter. We predict an exponential number of low-lying singlet states, a
triplet gap, and a two-peak specific heat. In addition, the large N limit
suggests a finite temperature phase transition into a phase with ordered
``resonance loops'' and broken translational symmetry.Comment: 5 pages, revtex, 5 eps figures include
Chemical composition and radiative properties of nascent particulate matter emitted by an aircraft turbofan burning conventional and alternative fuels
Aircraft engines are a unique source of carbonaceous
aerosols in the upper troposphere. There, these particles can more
efficiently interact with solar radiation than at ground. Due to the lack of
measurement data, the radiative forcing from aircraft exhaust aerosol
remains uncertain. To better estimate the global radiative effects of
aircraft exhaust aerosol, its optical properties need to be comprehensively
characterized. In this work we present the link between the chemical
composition and the optical properties of the particulate matter (PM)
measured at the engine exit plane of a CFM56-7B turbofan. The measurements
covered a wide range of power settings (thrust), ranging from ground idle to
take-off, using four different fuel blends of conventional Jet A-1 and
hydro-processed ester and fatty acids (HEFA) biofuel. At the two measurement
wavelengths (532 and 870 nm) and for all tested fuels, the absorption and
scattering coefficients increased with thrust, as did the PM mass. The
analysis of elemental carbon (EC) and organic carbon (OC) revealed a
significant mass fraction of OC (up to 90 %) at low thrust levels, while
EC mass dominated at medium and high thrust. The use of HEFA blends induced
a significant decrease in the PM mass and the optical coefficients at all
thrust levels. The HEFA effect was highest at low thrust levels, where the
EC mass was reduced by up to 50 %–60 %. The variability in the chemical
composition of the particles was the main reason for the strong thrust
dependency of the single scattering albedo (SSA), which followed the same
trend as the fraction of OC to total carbon (TC). Mass absorption
coefficients (MACs) were determined from the correlations between aerosol
light absorption and EC mass concentration. The obtained MAC values
(MAC532=7.5±0.3 m2 g−1 and MAC870=5.2±0.9 m2 g−1) are in excellent agreement with previous
literature values of absorption cross section for freshly generated soot.
While the MAC values were found to be independent of the thrust level and
fuel type, the mass scattering coefficients (MSCs) significantly varied with
thrust. For cruise conditions we obtained MSC532=4.5±0.4 m2 g−1 and MSC870=0.54±0.04 m2 g−1,
which fall within the higher end of MSCs measured for fresh biomass smoke.
However, the latter comparison is limited by the strong dependency of MSC on
the particles' size, morphology and chemical composition. The use of the HEFA
fuel blends significantly decreased PM emissions, but no changes were
observed in terms of EC∕OC composition and radiative properties.</p
Ischemia Induces P-Selectin-Mediated Selective Progenitor Cell Engraftment in the Isolated-Perfused Heart
Clinical trials infusing Bone Marrow Cells (BMCs) into injured hearts have produced measureable improvements in cardiac performance, but were insufficient to improve patient outcomes. Low engraftment rates are cited as probable contributor to limited improvements. To understand the mechanisms that control myocardial engraftment of BMCs following ischemia-reperfusion injury, in isolated–perfused mouse hearts, stop-flow ischemia was followed by variable-duration reperfusion (0–60 min) before addition of labeled syngenic BMCs to the perfusate. After a buffer-only wash, the heart was disaggregated. Retained BMCs (digest) and infused BMCs (aliquot) were compared by flow cytometry for c-kit and CD45 expression to determine the proportion of cell subtypes engrafted versus delivered (selectivity ratio). In these studies, a time-dependent selective retention of c-kit+ cells was apparent starting at 30 min of reperfusion, at which time c-kit+/CD45+ BMCs showed a selectivity ratio of 18 ± 2 (versus 2 ± 1 in sham-ischemic controls). To study the underlying mechanism for this selective retention, neutralizing antibodies for P-selectin or L-selectin were infused into the heart preparation and incubated with BMCs prior to BMC infusion. Blocking P-selectin in ischemic hearts ablated selectivity for c-kit+/CD45+ BMCs at 30 min reperfusion (selectivity ratio of 3 ± 1) while selectivity persisted in the presence of L-selectin neutralization (selectivity ratio of 17 ± 2). To corroborate this finding, a parallel plate flow chamber was used to study capture and rolling dynamics of purified c-kit+ versus c-kit- BMCs on various selectin molecules. C-kit+ BMCs interacted weakly with L-selectin substrates (0.03 ± 0.01% adhered) but adhered strongly to P-selectin (0.28 ± 0.04% adhered). C-kit- BMCs showed intermediate binding regardless of substrate (0.18 ± 0.04% adhered on L-selectin versus 0.17 ± 0.04% adhered on P-selectin). Myocardial ischemia–reperfusion stress induces selective engraftment of c-kit+ bone marrow progenitor cells via P-selectin activation
Atmospheric Channel Characteristics for Quantum Communication with Continuous Polarization Variables
We investigate the properties of an atmospheric channel for free space
quantum communication with continuous polarization variables. In our
prepare-and-measure setup, coherent polarization states are transmitted through
an atmospheric quantum channel of 100m length on the roof of our institute's
building. The signal states are measured by homodyne detection with the help of
a local oscillator (LO) which propagates in the same spatial mode as the
signal, orthogonally polarized to it. Thus the interference of signal and LO is
excellent and atmospheric fluctuations are autocompensated. The LO also acts as
spatial and spectral filter, which allows for unrestrained daylight operation.
Important characteristics for our system are atmospheric channel influences
that could cause polarization, intensity and position excess noise. Therefore
we study these influences in detail. Our results indicate that the channel is
suitable for our quantum communication system in most weather conditions.Comment: 6 pages, 4 figures, submitted to Applied Physics B following an
invitation for the special issue "Selected Papers Presented at the 2009
Spring Meeting of the Quantum Optics and Photonics Section of the German
Physical Society
Probing the dynamics of quasicrystal growth using synchrotron live imaging
The dynamics of quasicrystal growth remains an unsolved problem in condensed
matter. By means of synchrotron live imaging, facetted growth proceeding by the
tangential motion of ledges at the solid-melt interface is clearly evidenced
all along the solidification of icosahedral AlPdMn quasicrystals. The effect of
interface kinetics is significant so that nucleation and free growth of new
facetted grains occur in the melt when the solidification rate is increased.
The evolution of these grains is explained in details, which reveals the
crucial role of aluminum rejection, both in the poisoning of grain growth and
driving fluid flow
Green's function approach to the magnetic properties of the kagome antiferromagnet
The Heisenberg antiferromagnet is studied on the kagom\'e lattice by
using a Green's function method based on an appropriate decoupling of the
equations of motion. Thermodynamic properties as well as spin-spin correlation
functions are obtained and characterize this system as a two-dimensional
quantum spin liquid. Spin-spin correlation functions decay exponentially with
distance down to low temperature and the calculated missing entropy at T=0 is
found to be . Within the present scheme, the specific heat exhibits
a single peak structure and a dependence at low temperature.Comment: 6 (two-column revtex4) pages, 5 ps figures. Submitted to Phys. Rev.
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