1,478 research outputs found
The plaquette phase of the square lattice quantum dimer model
The plaquette phase of the square lattice quantum dimer model is studied
using a continuous-time reptation quantum Monte Carlo method for lattices of
sizes up to 48x48 sites. We determine the location of the phase transition
between the columnar and plaquette phases to occur at V_c/J=0.60 +- 0.05 which
is significantly larger than inferred from previous exact diagonalization
studies on smaller lattices. Offdiagonal correlation functions are obtained.
They exhibit long-range order in the plaquette phase but not at the
Rokhsar-Kivelson point. We also observe significant finite-size corrections to
scaling for the transition between the plaquette phase and the critical
resonating valence bond liquid. This study demonstrates the importance of
understanding finite-size effects when considering critical properties of the
square lattice quantum dimer model.Comment: 10 pages, 18 figures, considerably revised versio
Spin of ground state baryons
We calculate the quark spin contribution to the total angular momentum of
flavor octet and flavor decuplet ground state baryons using a spin-flavor
symmetry based parametrization method of quantum chromodynamics. We find that
third order SU(6) symmetry breaking three-quark operators are necessary to
explain the experimental result Sigma_1=0.32(10). For spin 3/2 decuplet baryons
we predict that the quark spin contribution is Sigma_3=3.93(22), i.e.
considerably larger than their total angular momentum.Comment: 8 page
Stability of a hard-sphere binary quasicrystal
The stability of a quasicrystalline structure, recently obtained in a
molecular-dynamics simulation of rapid cooling of a binary melt, is analyzed
for binary hard-sphere mixtures within a density-functional approach. It is
found that this quasicrystal is metastable relative to crystalline and fluid
phases for diameter ratios above 0.83. Such trend is partially reversed for
lower diameter ratios, since the quasicrystal becomes stable with respect to
the crystal but does not reach a coexistence with the fluid.Comment: 14 pages, 6 eps figures included. Revised version to appear in Phil.
Mag.
Semiclassical ordering in the large-N pyrochlore antiferromagnet
We study the semiclassical limit of the generalization of the
pyrochlore lattice Heisenberg antiferromagnet by expanding about the saddlepoint in powers of a generalized inverse spin. To leading order,
we write down an effective Hamiltonian as a series in loops on the lattice.
Using this as a formula for calculating the energy of any classical ground
state, we perform Monte-Carlo simulations and find a unique collinear ground
state. This state is not a ground state of linear spin-wave theory, and can
therefore not be a physical (N=1) semiclassical ground state.Comment: 4 pages, 4 eps figures; published versio
The Nucleon-Mass Difference in Chiral Perturbation Theory and Nuclear Forces
A new method is developed for treating the effect of the neutron-proton mass
difference in isospin-violating nuclear forces. Previous treatments utilized an
awkward subtraction scheme to generate these forces. A field redefinition is
used to remove that mass difference from the Lagrangian (and hence from
asymptotic nucleon states) and replace its effect by effective interactions.
Previous calculations of static Class II charge-independence-breaking and Class
III charge-symmetry-breaking potentials are verified using the new scheme,
which is also used to calculate Class IV nuclear forces. Two-body forces of the
latter type are found to be identical to previously obtained results. A novel
three-body force is also found. Problems involving Galilean invariance with
Class IV one-pion-exchange forces are identified and resolved.Comment: 20 pages, 2 figures, latex - submitted to Physical Review
Predicting Transmission Suitability of Mosquito-Borne Diseases under Climate Change to Underpin Decision Making
The risk of the mosquito-borne diseases malaria, dengue fever and Zika virus is expected to shift both temporally and spatially under climate change. As climate change projections continue to improve, our ability to predict these shifts is also enhanced. This paper predicts transmission suitability for these mosquito-borne diseases, which are three of the most significant, using the most up-to-date climate change projections. Using a mechanistic methodology, areas that are newly suitable and those where people are most at risk of transmission under the best- and worst-case climate change scenarios have been identified. The results show that although transmission suitability is expected to decrease overall for malaria, some areas will become newly suitable, putting naïve populations at risk. In contrast, transmission suitability for dengue fever and Zika virus is expected to increase both in duration and geographical extent. Although transmission suitability is expected to increase in temperate zones for a few months of the year, suitability remains focused in the tropics. The highest transmission suitability in tropical regions is likely to exacerbate the intense existing vulnerability of these populations, especially children, to the multiple consequences of climate change, and their severe lack of resources and agency to cope with these impacts and pressures. As these changes in transmission suitability are amplified under the worst-case climate change scenario, this paper makes the case in support of enhanced and more urgent efforts to mitigate climate change than has been achieved to date. By presenting consistent data on the climate-driven spread of multiple mosquito-borne diseases, our work provides more holistic information to underpin prevention and control planning and decision making at national and regional levels
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
Ab initio Ti-Zr-Ni phase diagram predicts stability of icosahedral TiZrNi quasicrystals
The ab initio phase diagram determines the energetic stability of the icosahedral TiZrNi quasicrystal. The complete ab initio zero-temperature ternary phase diagram is constructed from the calculated energies of the elemental, binary and ternary Ti-Zr-Ni phases. For this, the icosahedral i-TiZrNi quasicrystal is approximated by periodic structures of up to 123 atoms/unit cell, based on a decorated-tiling model [R. G. Hennig, K. F. Kelton, A. E. Carlsson, and C. L. Henley, Phys. Rev. B 67, 134202 (2003)]. The approximant structures containing the 45-atom Bergman cluster are nearly degenerate in energy, and are all energetically stable against the competing phases. It is concluded that i-TiZrNi is a ground-state quasicrystal, as it is experimentally the low-temperature phase for its composition
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
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