10,328 research outputs found
Theory and simulation of two-dimensional nematic and tetratic phases
Recent experiments and simulations have shown that two-dimensional systems
can form tetratic phases with four-fold rotational symmetry, even if they are
composed of particles with only two-fold symmetry. To understand this effect,
we propose a model for the statistical mechanics of particles with almost
four-fold symmetry, which is weakly broken down to two-fold. We introduce a
coefficient to characterize the symmetry breaking, and find that the
tetratic phase can still exist even up to a substantial value of .
Through a Landau expansion of the free energy, we calculate the mean-field
phase diagram, which is similar to the result of a previous hard-particle
excluded-volume model. To verify our mean-field calculation, we develop a Monte
Carlo simulation of spins on a triangular lattice. The results of the
simulation agree very well with the Landau theory.Comment: 7 pages, including 12 postscript figures, uses REVTeX
Scattering lengths of Nambu-Goldstone bosons off mesons and dynamically generated heavy-light mesons
Recent lattice QCD simulations of the scattering lengths of Nambu-Goldstone
bosons off the mesons are studied using unitary chiral perturbation theory.
We show that the Lattice QCD data are better described in the covariant
formulation than in the heavy-meson formulation. The can be
dynamically generated from the coupled-channels interaction without
\textit{a priori} assumption of its existence. A new renormalization scheme is
proposed which manifestly satisfies chiral power counting rules and has
well-defined behavior in the infinite heavy-quark mass limit. Using this scheme
we predict the heavy-quark spin and flavor symmetry counterparts of the
.Comment: 22 pages, 5 figures; to appear in Physical Review
- …