1,201 research outputs found
Method of fabricating an object with a thin wall having a precisely shaped slit
A method is described for making a structure with a cavity and a thin wall with a precisely shaped slit. An object with a cavity having two openings, one of which is to be closed by a thin wall with a slit, is placed on the surface of a fixture. The fixture surface has a slot conforming to the size and shape of the slit to be formed in the thin wall
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
Renormalized SO(5) symmetry in ladders with next-nearest-neighbor hopping
We study the occurrence of SO(5) symmetry in the low-energy sector of
two-chain Hubbard-like systems by analyzing the flow of the running couplings
(-ology) under renormalization group in the weak-interaction limit. It is
shown that SO(5) is asymptotically restored for low energies for rather general
parameters of the bare Hamiltonian. This holds also with inclusion of a
next-nearest-neighbor hopping which explicitly breaks particle-hole symmetry
provided one accounts for a different single-particle weight for the
quasiparticles of the two bands of the system. The physical significance of
this renormalized SO(5) symmetry is discussed.Comment: Final version: to appear in Phys. Rev. Lett., sched. Mar. 9
Zero-Temperature Phase Transitions of Antiferromagnetic Ising Model of General Spin on a Triangular Lattice
We map the ground-state ensemble of antiferromagnetic Ising model of spin-S
on a triangular lattice to an interface model whose entropic fluctuations are
proposed to be described by an effective Gaussian free energy, which enables us
to calculate the critical exponents of various operators in terms of the
stiffness constant of the interface. Monte Carlo simulations for the
ground-state ensemble utilizing this interfacial representation are performed
to study both the dynamical and the static properties of the model. This method
yields more accurate numerical results for the critical exponents. By varying
the spin magnitude in the model, we find that the model exhibits three phases
with a Kosterlitz-Thouless phase transition at 3/2<S_{KT}<2 and a locking phase
transition at 5/2 < S_L \leq 3. The phase diagram at finite temperatures is
also discussed.Comment: 15 pages, LaTeX; 10 figures in PostScript files; The revised version
appears in PRB (see Journal-ref). New electronic address of first author,
[email protected]
Novel spin-liquid states in the frustrated Heisenberg antiferromagnet on the honeycomb lattice
Recent experiment on a honeycomb-lattice Heisenberg antiferromagnet (AF)
BiMnO(NO) revealed a novel spin-liquid-like behavior down to
low temperature, which was ascribed to the frustration effect due to the
competition between the AF nearest- and next-nearest-neighbor interactions
and . Motivated by the experiment, we study the ordering of the
- frustrated classical Heisenberg AF on a honeycomb lattice both by
a low-temperature expansion and a Monte Carlo simulation. The model has been
known to possess a massive degeneracy of the ground state, which, however,
might be lifted due to thermal fluctuations leading to a unique ordered state,
the effect known as 'order-by-disorder'. We find that the model exhibits an
intriguing ordering behavior, particularly near the AF phase boundary. The
energy scale of the order-by-disorder is suppressed there down to extremely low
temperatures, giving rise to exotic spin-liquid states like a "ring-liquid" or
a "pancake-liquid" state accompanied by the characteristic spin structure
factor and the field-induced antiferromagnetism. We argue that the recent
experimental data are explicable if the system is in such exotic spin-liquid
states
Parity-Violating Interaction Effects in the np System
We investigate parity-violating observables in the np system, including the
longitudinal asymmetry and neutron-spin rotation in np elastic scattering, the
photon asymmetry in np radiative capture, and the asymmetries in deuteron
photo-disintegration d(gamma,n)p in the threshold region and
electro-disintegration d(e,e`)np in quasi-elastic kinematics. To have an
estimate of the model dependence for the various predictions, a number of
different, latest-generation strong-interaction potentials--Argonne v18, Bonn
2000, and Nijmegen I--are used in combination with a weak-interaction potential
consisting of pi-, rho-, and omega-meson exchanges--the model known as DDH. The
complete bound and scattering problems in the presence of parity-conserving,
including electromagnetic, and parity-violating potentials is solved in both
configuration and momentum space. The issue of electromagnetic current
conservation is examined carefully. We find large cancellations between the
asymmetries induced by the parity-violating interactions and those arising from
the associated pion-exchange currents. In the np capture, the model dependence
is nevertheless quite small, because of constraints arising through the Siegert
evaluation of the relevant E1 matrix elements. In quasi-elastic electron
scattering these processes are found to be insignificant compared to the
asymmetry produced by gamma-Z interference on individual nucleons.Comment: 65 pages, 26 figures, submitted to PR
A constrained Potts antiferromagnet model with an interface representation
We define a four-state Potts model ensemble on the square lattice, with the
constraints that neighboring spins must have different values, and that no
plaquette may contain all four states. The spin configurations may be mapped
into those of a 2-dimensional interface in a 2+5 dimensional space. If this
interface is in a Gaussian rough phase (as is the case for most other models
with such a mapping), then the spin correlations are critical and their
exponents can be related to the stiffness governing the interface fluctuations.
Results of our Monte Carlo simulations show height fluctuations with an
anomalous dependence on wavevector, intermediate between the behaviors expected
in a rough phase and in a smooth phase; we argue that the smooth phase (which
would imply long-range spin order) is the best interpretation.Comment: 61 pages, LaTeX. Submitted to J. Phys.
Order by disorder and spiral spin liquid in frustrated diamond lattice antiferromagnets
Frustration refers to competition between different interactions that cannot
be simultaneously satisfied, a familiar feature in many magnetic solids. Strong
frustration results in highly degenerate ground states, and a large suppression
of ordering by fluctuations. Key challenges in frustrated magnetism are
characterizing the fluctuating spin-liquid regime and determining the mechanism
of eventual order at lower temperature. Here, we study a model of a diamond
lattice antiferromagnet appropriate for numerous spinel materials. With
sufficiently strong frustration a massive ground state degeneracy develops
amongst spirals whose propagation wavevectors reside on a continuous
two-dimensional ``spiral surface'' in momentum space. We argue that an
important ordering mechanism is entropic splitting of the degenerate ground
states, an elusive phenomena called order-by-disorder. A broad ``spiral
spin-liquid'' regime emerges at higher temperatures, where the underlying
spiral surface can be directly revealed via spin correlations. We discuss the
agreement between these predictions and the well characterized spinel MnSc2S4
Isospin Breaking in the Pion-Nucleon Coupling from QCD Sum Rules
We use QCD sum rules for the three point function of a pseudoscalar and two
nucleonic currents in order to estimate the charge dependence of the pion
nucleon coupling constant coming from isospin violation in the
strong interaction. The effect can be attributed primarily to the difference of
the quark condensates . For the splitting
we obtain an interval of to , the uncertainties coming mainly from the input
parameters. The charged pion nucleon coupling is found to be the average of
and . Electromagnetic effects are not included.Comment: 18 pages (REVTeX) + 2 figures (as PostScript), to be published in
PRC, replaced with final version: inclusion of pi-eta mixing and N -> N*
transition
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