820 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
Stability of the hard-sphere icosahedral quasilattice
The stability of the hard-sphere icosahedral quasilattice is analyzed using
the differential formulation of the generalized effective liquid approximation.
We find that the icosahedral quasilattice is metastable with respect to the
hard-sphere crystal structures. Our results agree with recent findings by
McCarley and Ashcroft [Phys. Rev. B {\bf 49}, 15600 (1994)] carried out using
the modified weighted density approximation.Comment: 15 pages, 2 figures available from authors upon request, (revtex),
submitted to Phys. Rev.
Magnetic Properties of Undoped
The Heisenberg antiferromagnet, which arises from the large Hubbard
model, is investigated on the molecule and other fullerenes. The
connectivity of leads to an exotic classical ground state with
nontrivial topology. We argue that there is no phase transition in the Hubbard
model as a function of , and thus the large solution is relevant for
the physical case of intermediate coupling. The system undergoes a first order
metamagnetic phase transition. We also consider the S=1/2 case using
perturbation theory. Experimental tests are suggested.Comment: 12 pages, 3 figures (included
Recursion and Path-Integral Approaches to the Analytic Study of the Electronic Properties of
The recursion and path-integral methods are applied to analytically study the
electronic structure of a neutral molecule. We employ a tight-binding
Hamiltonian which considers both the and valence electrons of carbon.
From the recursion method, we obtain closed-form {\it analytic} expressions for
the and eigenvalues and eigenfunctions, including the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) states, and the Green's functions. We also present the local densities
of states around several ring clusters, which can be probed experimentally by
using, for instance, a scanning tunneling microscope. {}From a path-integral
method, identical results for the energy spectrum are also derived. In
addition, the local density of states on one carbon atom is obtained; from this
we can derive the degree of degeneracy of the energy levels.Comment: 19 pages, RevTex, 6 figures upon reques
A Menu of Activities in Different Intelligence Areas to Differentiate Instruction for Upper Elementary Students Related to the Book Because of Winn-Dixie
Todayâs elementary classrooms are becoming more diverse, requiring teachers to provide effective instruction to children with a wide range of academic performance, ability, background, and interest. This work focuses on the development of a menu of differentiated instructional activities for teaching literacy to upper elementary students. The author, a former elementary teacher, worked with a faculty member and her literacy methods class of undergraduate preservice elementary teachers to develop this teaching unit based on a popular, award-winning childrenâs novel (DiCamillo, 2000). The menu of various activities organized in ten different intelligence areas (logical-mathematical, linguistic, bodilykinesthetic, spatial, musical, interpersonal, intrapersonal, naturalist, moral, and spiritual) and arranged at different levels of Bloomâs taxonomy (knowledge, comprehension, application, analysis, synthesis and evaluation) allows a teacher to easily select assignments designed to deepen comprehension or appreciation of this literary work to meet the individual needs of students. Each activity is accompanied by an example correct response so that teachers may gauge its demands. The reactions of preservice teachers to this project from beginning to end were examined. Before participation in the project, preservice teachers emphasized fame and recognition as being motivators and effects of publication. After participation in the project, they focused on the effects of the work on their employment opportunities, emphasizing a sense of accomplishment and downplaying fame/recognition. [5 tables, 11 references, 2 appendices
Rules for Computing Symmetry, Density and Stoichiometry in a Quasi-Unit-Cell Model of Quasicrystals
The quasi-unit cell picture describes the atomic structure of quasicrystals
in terms of a single, repeating cluster which overlaps neighbors according to
specific overlap rules. In this paper, we discuss the precise relationship
between a general atomic decoration in the quasi-unit cell picture atomic
decorations in the Penrose tiling and in related tiling pictures. Using these
relations, we obtain a simple, practical method for determining the density,
stoichiometry and symmetry of a quasicrystal based on the atomic decoration of
the quasi-unit cell taking proper account of the sharing of atoms between
clusters.Comment: 14 pages, 8 figure
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
Nonequilibrium dynamics of fully frustrated Ising models at T=0
We consider two fully frustrated Ising models: the antiferromagnetic
triangular model in a field of strength, , as well as the Villain
model on the square lattice. After a quench from a disordered initial state to
T=0 we study the nonequilibrium dynamics of both models by Monte Carlo
simulations. In a finite system of linear size, , we define and measure
sample dependent "first passage time", , which is the number of Monte
Carlo steps until the energy is relaxed to the ground-state value. The
distribution of , in particular its mean value, , is shown to
obey the scaling relation, , for both models.
Scaling of the autocorrelation function of the antiferromagnetic triangular
model is shown to involve logarithmic corrections, both at H=0 and at the
field-induced Kosterlitz-Thouless transition, however the autocorrelation
exponent is found to be dependent.Comment: 7 pages, 8 figure
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 %â60 %. 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 gâ1 and MAC870=5.2±0.9 m2 gâ1) 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 gâ1 and MSC870=0.54±0.04 m2 gâ1,
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 ECâOC composition and radiative properties.</p
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