11,801 research outputs found
Systematic Inclusion of High-Order Multi-Spin Correlations for the Spin- Models
We apply the microscopic coupled-cluster method (CCM) to the spin-
models on both the one-dimensional chain and the two-dimensional square
lattice. Based on a systematic approximation scheme of the CCM developed by us
previously, we carry out high-order {\it ab initio} calculations using
computer-algebraic techniques. The ground-state properties of the models are
obtained with high accuracy as functions of the anisotropy parameter.
Furthermore, our CCM analysis enables us to study their quantum critical
behavior in a systematic and unbiased manner.Comment: (to appear in PRL). 4 pages, ReVTeX, two figures available upon
request. UMIST Preprint MA-000-000
Phase Transitions in the Spin-Half J_1--J_2 Model
The coupled cluster method (CCM) is a well-known method of quantum many-body
theory, and here we present an application of the CCM to the spin-half J_1--J_2
quantum spin model with nearest- and next-nearest-neighbour interactions on the
linear chain and the square lattice. We present new results for ground-state
expectation values of such quantities as the energy and the sublattice
magnetisation. The presence of critical points in the solution of the CCM
equations, which are associated with phase transitions in the real system, is
investigated. Completely distinct from the investigation of the critical
points, we also make a link between the expansion coefficients of the
ground-state wave function in terms of an Ising basis and the CCM ket-state
correlation coefficients. We are thus able to present evidence of the
breakdown, at a given value of J_2/J_1, of the Marshall-Peierls sign rule which
is known to be satisfied at the pure Heisenberg point (J_2 = 0) on any
bipartite lattice. For the square lattice, our best estimates of the points at
which the sign rule breaks down and at which the phase transition from the
antiferromagnetic phase to the frustrated phase occurs are, respectively, given
(to two decimal places) by J_2/J_1 = 0.26 and J_2/J_1 = 0.61.Comment: 28 pages, Latex, 2 postscript figure
Optical alignment system Patent
Electro-optical/computer system for aligning large structural members and maintaining correct positio
Profile blunting and flow blockage in a yield stress fluid: A molecular dynamics study
The flow of a simple glass forming system (a 80:20 binary Lennard-Jones
mixture) through a planar channel is studied via molecular dynamics
simulations. The flow is driven by an external body force similar to gravity.
Previous studies show that the model exhibits both a static [Varnik et al. J.
Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich Phys.
Rev. B 73, 174209 (2006)] yield stress in the glassy phase. \blue{These
observations are corroborated by the present work, where we investigate how the
presence of a yield stress may affect the system behavior in a Poiseuille-type
flow geometry.} In particular, we observe a blunted velocity profile across the
channel: A relatively wide region in the channel center flows with a constant
velocity (zero shear rate) followed by a non linear change of the shear rate as
the walls are approached. The observed velocity gradients are compared to those
obtained from the knowledge of the shear stress across the channel and the
flow-curves (stress versus shear rate), the latter being determined in our
previous simulations of homogeneous shear flow. Furthermore, using the value of
the (dynamic) yield stress known from previous simulations, we estimate the
threshold body force for a complete arrest of the flow. Indeed, a blockage is
observed as the imposed force falls below this threshold value. Small but
finite shear rates are observed at stresses above the dynamic but below the
static yield stress. We discuss the possible role of the \blue{stick-slip like
motion} for this observation.Comment: 22 pages, 8 figure
Continuum coupled cluster expansion
We review the basics of the coupled-cluster expansion formalism for numerical
solutions of the many-body problem, and we outline the principles of an
approach directed towards an adequate inclusion of continuum effects in the
associated single-energy spectrum. We illustrate our findings by considering
the simple case of a single-particle quantum mechanics problem.Comment: 16 pages, 1 figur
Correlation-induced metal insulator transition in a two-channel fermion-boson model
We investigate charge transport within some background medium by means of an
effective lattice model with a novel form of fermion-boson coupling. The bosons
describe fluctuations of a correlated background. By analyzing groundstate and
spectral properties of this transport model, we show how a metal-insulator
quantum phase transition can occur for the half-filled band case. We discuss
the evolution of a mass-asymmetric band structure in the insulating phase and
establish connections to the Mott and Peierls transition scenarios.Comment: 4 pages, 4 figures, 1 table, revised version accepted for publication
in Phys. Rev. Let
Cosmic Electromagnetic Fields due to Perturbations in the Gravitational Field
We use non-linear gauge-invariant perturbation theory to study the
interaction of an inflation produced seed magnetic field with density and
gravitational wave perturbations in an almost
Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetime. We compare the effects
of this coupling under the assumptions of poor conductivity, infinite
conductivity and the case where the electric field is sourced via the coupling
of velocity perturbations to the seed field in the ideal magnetohydrodynamic
(MHD) regime, thus generalizing, improving on and correcting previous results.
We solve our equations for long wavelength limits and numerically integrate the
resulting equations to generate power spectra for the electromagnetic field
variables, showing where the modes cross the horizon. We find that the rotation
of the electric field dominates the power spectrum on small scales, in
agreement with previous arguments.Comment: 16 pages, 3 figures, published in PR
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