28,237 research outputs found
The hexagonal versus the square lattice
We establish Schmutz Schaller's conjecture that the hexagonal lattice is
`better' than the square lattice.
Schmutz Schaller (Bulletin of the AMS 35 (1998), p. 201), motivated by
considerations from hyperbolic geometry, conjectured that in dimensions 2 to 8
the best known lattice sphere packings have `maximal lengths' and goes on to
write: "In dimension 2 the conjecture means in particular that the hexagonal
lattice is `better' than the square lattice. More precisely, let 0<h_1<h_2<...
be the positive integers, listed in ascending order, which can be written as
h_i=x^2+3y^2 for integers x and y. Let 0<q_1<q_2<... be the positive integers,
listed in ascending order, which can be written as q_i=x^2+y^2 for integers x
and y. Then the conjecture is that q_i<=h_i for i=1,2,3,..."
Our proof requires computational prime number theory in combination with
methods from a preprint of the first author (to appear in Math. Comp.),
arXiv:math.NT/0112100.Comment: 24 pages, 6 figures, 2 table
The 5f localization/delocalization in square and hexagonal americium monolayers: A FP-LAPW electronic structure study
The electronic and geometrical properties of bulk americium and square and
hexagonal americium monolayers have been studied with the full-potential
linearized augmented plane wave (FP-LAPW) method. The effects of several common
approximations are examined: (1) non-spin polarization (NSP) vs. spin
polarization (SP); (2) scalar-relativity (no spin-orbit coupling (NSO)) vs.
full-relativity (i.e., with spin-orbit (SO) coupling included); (3)
local-density approximation (LDA) vs. generalized-gradient approximation (GGA).
Our results indicate that both spin polarization and spin orbit coupling play
important roles in determining the geometrical and electronic properties of
americium bulk and monolayers. A compression of both americium square and
hexagonal monolayers compared to the americium bulk is also observed. In
general, the LDA is found to underestimate the equilibrium lattice constant and
give a larger total energy compared to the GGA calculations. While spin orbit
coupling shows a similar effect on both square and hexagonal monolayer
calculations regardless of the model, GGA versus LDA, an unusual spin
polarization effect on both square and hexagonal monolayers is found in the LDA
results as compared with the GGA results. The 5f delocalization transition of
americium is employed to explain our observed unusual spin polarization effect.
In addition, our results at the LDA level of theory indicate a possible 5f
delocalization could happen in the americium surface within the same Am II (fcc
crystal structure) phase, unlike the usually reported americium 5f
delocalization which is associated with crystal structure change. The
similarities and dissimilarities between the properties of an Am monolayer and
a Pu monolayer are discussed in detail.Comment: 22 pages, 8 figure
Scaling prediction for self-avoiding polygons revisited
We analyse new exact enumeration data for self-avoiding polygons, counted by
perimeter and area on the square, triangular and hexagonal lattices. In
extending earlier analyses, we focus on the perimeter moments in the vicinity
of the bicritical point. We also consider the shape of the critical curve near
the bicritical point, which describes the crossover to the branched polymer
phase. Our recently conjectured expression for the scaling function of rooted
self-avoiding polygons is further supported. For (unrooted) self-avoiding
polygons, the analysis reveals the presence of an additional additive term with
a new universal amplitude. We conjecture the exact value of this amplitude.Comment: 17 pages, 3 figure
Piezoelectric mechanism of orientation of a bilayer Wigner crystal in a GaAs matrix
A mechanism for orientation of bilayer classical Wigner crystals in a
piezoelectric medium is considered. For the GaAs system the piezoelectric
correction to the electrostatic interaction between electrons is calculated. It
is shown that taking into account the correction due to the piezoelectric
effect leads to a dependence of the total energy of the electron crystal on its
orientation with respect to the crystallographic axes of the surrounding
matrix. A generalization of Ewald's method is obtained for calculating the
anisotropic interaction between electrons in a Wigner crystal. The method is
used to calculate the energy of bilayer Wigner crystals in electron layers
parallel to the crystallographic planes (001), (0-11), and (111) as a function
of their orientation and the distance between layers, and the energetically
most favorable orientation for all types of electron lattices in a bilayer
system is found. It is shown that phase transitions between structures with
different lattice symmetry in a Wigner crystal can be accompanied by a change
of its orientation.Comment: 11 pages, 4 eps figures include
Design of two-dimensional particle assemblies using isotropic pair interactions with an attractive well
Using ground-state and relative-entropy based inverse design strategies,
isotropic interactions with an attractive well are determined to stabilize and
promote as- sembly of particles into two-dimensional square, honeycomb, and
kagome lattices. The design rules inferred from these results are discussed and
validated in the dis- covery of interactions that favor assembly of the highly
open truncated-square and truncated-hexagonal lattices.Comment: 11 pages, 5 figures and supplemental materia
Lyapunov spectral analysis of a nonequilibrium Ising-like transition
By simulating a nonequilibrium coupled map lattice that undergoes an
Ising-like phase transition, we show that the Lyapunov spectrum and related
dynamical quantities such as the dimension correlation length~ are
insensitive to the onset of long-range ferromagnetic order. As a function of
lattice coupling constant~ and for certain lattice maps, the Lyapunov
dimension density and other dynamical order parameters go through a minimum.
The occurrence of this minimum as a function of~ depends on the number of
nearest neighbors of a lattice point but not on the lattice symmetry, on the
lattice dimensionality or on the position of the Ising-like transition. In
one-space dimension, the spatial correlation length associated with magnitude
fluctuations and the length~ are approximately equal, with both
varying linearly with the radius of the lattice coupling.Comment: 29 pages of text plus 15 figures, uses REVTeX macros. Submitted to
Phys. Rev. E
Via Hexagons to Squares in Ferrofluids: Experiments on Hysteretic Surface Transformations under Variation of the Normal Magnetic Field
We report on different surface patterns on magnetic liquids following the
Rosensweig instability. We compare the bifurcation from the flat surface to a
hexagonal array of spikes with the transition to squares at higher fields. From
a radioscopic mapping of the surface topography we extract amplitudes and
wavelengths. For the hexagon--square transition, which is complex because of
coexisting domains, we tailor a set of order parameters like peak--to--peak
distance, circularity, angular correlation function and pattern specific
amplitudes from Fourier space. These measures enable us to quantify the smooth
hysteretic transition. Voronoi diagrams indicate a pinning of the domains. Thus
the smoothness of the transition is roughness on a small scale.Comment: 17 pages, 14 figure
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