2,276 research outputs found
Temperature-dependent "phason" elasticity in a random tiling quasicrystal
Both ``phason'' elastic constants have been measured from Monte Carlo
simulations of a random-tiling icosahedral quasicrystal model with a
Hamiltonian. The low-temperature limit approximates the ``canonical-cell''
tiling used to describe several real quasicrystals. The elastic constant K2
changes sign from positive to negative with decreasing temperature; in the
``canonical-cell'' limit, K2/K1 appears to approach -0.7, about the critical
value for a phason-mode modulation instability. We compare to the experiments
on i-AlPdMn and i-AlCuFe.Comment: 5 pages, 2 Postscript figures, LaTeX, uses revtex4, submitted to PR
First-principles prediction of a decagonal quasicrystal containing boron
We interpret experimentally known B-Mg-Ru crystals as quasicrystal
approximants. These approximant structures imply a deterministic decoration of
tiles by atoms that can be extended quasiperiodically. Experimentally observed
structural disorder corresponds to phason (tile flip) fluctuations.
First-principles total energy calculations reveal that many distinct tilings
lie close to stability at low temperatures. Transfer matrix calculations based
on these energies suggest a phase transition from a crystalline state at low
temperatures to a high temperature state characterized by tile fluctuations. We
predict BMgRu forms a decagonal quasicrystal that is
metastable at low temperatures and may be thermodynamically stable at high
temperatures.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Depleted pyrochlore antiferromagnets
I consider the class of "depleted pyrochlore" lattices of corner-sharing
triangles, made by removing spins from a pyrochlore lattice such that every
tetrahedron loses exactly one. Previously known examples are the "hyperkagome"
and "kagome staircase". I give criteria in terms of loops for whether a given
depleted lattice can order analogous to the kagome \sqrt{3} \times \sqrt{three}
state, and also show how the pseudo-dipolar correlations (due to local
constraints) generalize to even the random depleted case.Comment: 6pp IOP latex, 1 figure; Proc. "Highly Frustrated Magnetism 2008",
Sept 2008, Braunschwei
Effective Hamiltonian and low-lying energy clustering patterns of four-sublattice antiferromagnets
We study the low-lying energy clustering patterns of quantum antiferromagnets
with p sublattices (in particular p=4). We treat each sublattice as a large
spin, and using second-order degenerate perturbation theory, we derive the
effective (biquadratic) Hamiltonian coupling the p large spins. In order to
compare with exact diagonalizations, the Hamiltonian is explicitly written for
a finite-size lattice, and it contains information on energies of excited
states as well as the ground state. The result is applied to the
face-centered-cubic Type I antiferromagnet of spin 1/2, including
second-neighbor interactions. A 32-site system is exactly diagonalized, and the
energy spectrum of the low-lying singlets follows the analytically predicted
clustering pattern.Comment: 17 pages, 4 table
Spontaneous Currents in Spinless Fermion Lattice Models at the Strong-Coupling Limit
What kind of lattice Hamiltonian manifestly has an ordered state with
spontaneous orbital currents? We consider interacting spinless fermions on an
array of square plaquettes, connected by weak hopping; the array geometry may
be a 2 x 2L ladder, a 2 x 2 x 2L "tube", or a 2L x 2L square grid. At half
filling, we derive an effective Hamiltonian in terms of pseudospins, of which
one component represents orbital currents, and find the conditions sufficient
for orbital current long-range order. We consider spinfull variants of the
aforesaid spinless models and make contact with other spinfull models in the
literature purported to possess spontaneous currents.Comment: added two new references following recent communicatio
Exact ground states and correlation functions of chain and ladder models of interacting hardcore bosons or spinless fermions
By removing one empty site between two occupied sites, we map the ground
states of chains of hardcore bosons and spinless fermions with infinite
nearest-neighbor repulsion to ground states of chains of hardcore bosons and
spinless fermions without nearest-neighbor repulsion respectively, and
ultimately in terms of the one-dimensional Fermi sea. We then introduce the
intervening-particle expansion, where we write correlation functions in such
ground states as a systematic sum over conditional expectations, each of which
can be ultimately mapped to a one-dimensional Fermi-sea expectation. Various
ground-state correlation functions are calculated for the bosonic and fermionic
chains with infinite nearest-neighbor repulsion, as well as for a ladder model
of spinless fermions with infinite nearest-neighbor repulsion and correlated
hopping in three limiting cases. We find that the decay of these correlation
functions are governed by surprising power-law exponents.Comment: 20 pages, 18 figures, RevTeX4 clas
Intercept Surveys: An Overlooked Method for Data Collection
Intercept surveys are a tool Extension educators can use to capture local data quickly and with minimal cost. We used intercept surveys at city farmers\u27 markets to test the efficacy of food safety signage. From our experience with the intercept survey process, we identified a set of best practices that can benefit other Extension educators interested in developing and implementing this type of research
Phason elasticity of a three-dimensional quasicrystal: transfer-matrix method
We introduce a new transfer matrix method for calculating the thermodynamic
properties of random-tiling models of quasicrystals in any number of
dimensions, and describe how it may be used to calculate the phason elastic
properties of these models, which are related to experimental measurables such
as phason Debye-Waller factors, and diffuse scattering wings near Bragg peaks.
We apply our method to the canonical-cell model of the icosahedral phase,
making use of results from a previously-presented calculation in which the
possible structures for this model under specific periodic boundary conditions
were cataloged using a computational technique. We give results for the
configurational entropy density and the two fundamental elastic constants for a
range of system sizes. The method is general enough allow a similar calculation
to be performed for any other random tiling model.Comment: 38 pages, 3 PostScript figures, self-expanding uuencoded compressed
tar file, LaTeX using RevTeX macros and epsfig.st
Effect of Quantum Fluctuations on Magnetic Ordering in CaVO
We present a theoretical model for CaVO: the -depleted square
spin- Heisenberg model which includes both the nearest-neighbor coupling
() and the next-nearest-neighbor coupling (), where and are
antiferromagnetic. Recent experiments of the neutron diffraction by Harashina
et.al. report the magnetic ordering at low temperatures, which may be called as
a stripe phase. It is shown that the observed spin structure is not stable in
the classical theory. By employing the modified spin wave theory, we show that
the stripe phase is stabilized by the quantum fluctuations for .
In CaVO, the coupling constants are estimated as by
comparing the theoretical and experimental results.Comment: submitted to J. Phys. Soc. Jp
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