1,807 research outputs found
World heritage values of Magnetic Island: the marine system
Magnetic Island is a high continental island that lies approximately 8 kilometres north of the city of Townsville on the north-western side of Cleveland Bay. It is separated from the mainland by the shallow (<15 m) West Channel. Due to its location in Cleveland Bay, the marine habitats of Magnetic Island are diverse. They are characterised by gradients ranging from very wave-protected shallow muddy environments on the leeward sides to wave-exposed windward coastlines with clearer and deeper water. Associated with the high environmental diversity is a broad range of marine communities, ranging from those that are tolerant of muddy, low light conditions to those that are typically found in less turbid environments
XXZ Bethe states as highest weight vectors of the loop algebra at roots of unity
We show that every regular Bethe ansatz eigenvector of the XXZ spin chain at
roots of unity is a highest weight vector of the loop algebra, for some
restricted sectors with respect to eigenvalues of the total spin operator
, and evaluate explicitly the highest weight in terms of the Bethe roots.
We also discuss whether a given regular Bethe state in the sectors generates an
irreducible representation or not. In fact, we present such a regular Bethe
state in the inhomogeneous case that generates a reducible Weyl module. Here,
we call a solution of the Bethe ansatz equations which is given by a set of
distinct and finite rapidities {\it regular Bethe roots}. We call a nonzero
Bethe ansatz eigenvector with regular Bethe roots a {\it regular Bethe state}.Comment: 40pages; revised versio
Spin dynamics of quantum and classical Heisenberg dimers
Analytical solutions for the time-dependent autocorrelation function of the
classical and quantum mechanical spin dimer with arbitrary spin are presented
and compared. For large spin quantum numbers or high temperature the classical
and the quantum dimer become more and more similar, yet with the major
difference that the quantum autocorrelation function is periodic in time
whereas the classical is not.Comment: 10 pages, 4 postscript figures, uses 'epsfig.sty'. Submitted to
Physica A. More information available at
http://www.physik.uni-osnabrueck.de/makrosysteme
Core Flooding Experiments and Reactive Transport Modeling of Seasonal Heat Storage in the Hot Deep Gassum Sandstone Formation
Quasiparticles governing the zero-temperature dynamics of the 1D spin-1/2 Heisenberg antiferromagnet in a magnetic field
The T=0 dynamical properties of the one-dimensional (1D)
Heisenberg antiferromagnet in a uniform magnetic field are studied via Bethe
ansatz for cyclic chains of sites. The ground state at magnetization
, which can be interpreted as a state with spinons or as a
state of magnons, is reconfigured here as the vacuum for a different
species of quasiparticles, the {\em psinons} and {\em antipsinons}. We
investigate three kinds of quantum fluctuations, namely the spin fluctuations
parallel and perpendicular to the direction of the applied magnetic field and
the dimer fluctuations. The dynamically dominant excitation spectra are found
to be sets of collective excitations composed of two quasiparticles excited
from the psinon vacuum in different configurations. The Bethe ansatz provides a
framework for (i) the characterization of the new quasiparticles in relation to
the more familiar spinons and magnons, (ii) the calculation of spectral
boundaries and densities of states for each continuum, (iii) the calculation of
transition rates between the ground state and the dynamically dominant
collective excitations, (iv) the prediction of lineshapes for dynamic structure
factors relevant for experiments performed on a variety of quasi-1D
antiferromagnetic compounds, including KCuF,
Cu(CHN, and CuGeO.Comment: 13 pages, 12 figure
Thermodynamical Properties of a Spin 1/2 Heisenberg Chain Coupled to Phonons
We performed a finite-temperature quantum Monte Carlo simulation of the
one-dimensional spin-1/2 Heisenberg model with nearest-neighbor interaction
coupled to Einstein phonons. Our method allows to treat easily up to 100
phonons per site and the results presented are practically free from truncation
errors. We studied in detail the magnetic susceptibility, the specific heat,
the phonon occupation, the dimerization, and the spin-correlation function for
various spin-phonon couplings and phonon frequencies. In particular we give
evidence for the transition from a gapless to a massive phase by studying the
finite-size behavior of the susceptibility. We also show that the dimerization
is proportional to for .Comment: 10 pages, 17 Postscript Figure
Modelling the effect of changes in vaccine effectiveness and transmission contact rates on pertussis epidemiology
AbstractThe incidence of the highly infectious respiratory disease named pertussis or whooping cough has been increasing for the past two decades in different countries, as in much of the highly vaccinated world. A decrease in vaccine effectiveness over time, especially when acellular vaccines were used for primary doses and boosters, and pathogen adaptation to the immunity conferred by vaccines have been proposed as possible causes of the resurgence. The contributions of these factors are not expected to be the same in different communities, and this could lead to different epidemiological trends. In fact, differences in the magnitude and dynamics of pertussis outbreaks as well as in the distribution of notified cases by age have been reported in various regions.Using an age-structured mathematical model designed by us, we evaluated how the changes in some of the parameters that could be related to the above proposed causes of disease resurgence – vaccine effectiveness and effective transmission rates – may impact on pertussis transmission.When a linear decrease in vaccine effectiveness (VE) was assayed, a sustained increase in pertussis incidence was detected mainly in infants and children. On the other hand, when changes in effective transmission rates (βij) were made, a dynamic effect evidenced by the presence of large peaks followed by deep valleys was detected. In this case, greater incidence in adolescents than in children was observed. These different trends in the disease dynamics due to modifications in VE or βij were verified in 18 possible scenarios that represent different epidemiological situations. Interestingly we found that both incidence trends produced by the model and their age distribution resemble the profiles obtained from data reported in several regions. The implications of these correlations are discussed
Spectrum and transition rates of the XX chain analyzed via Bethe ansatz
As part of a study that investigates the dynamics of the s=1/2 XXZ model in
the planar regime |Delta|<1, we discuss the singular nature of the Bethe ansatz
equations for the case Delta=0 (XX model). We identify the general structure of
the Bethe ansatz solutions for the entire XX spectrum, which include states
with real and complex magnon momenta. We discuss the relation between the
spinon or magnon quasiparticles (Bethe ansatz) and the lattice fermions
(Jordan-Wigner representation). We present determinantal expressions for
transition rates of spin fluctuation operators between Bethe wave functions and
reduce them to product expressions. We apply the new formulas to two-spinon
transition rates for chains with up to N=4096 sites.Comment: 11 pages, 4 figure
Lineshape predictions via Bethe ansatz for the one-dimensional spin-1/2 Heisenberg antiferromagnet in a magnetic field
The spin fluctuations parallel to the external magnetic field in the ground
state of the one-dimensional (1D) s=1/2 Heisenberg antiferromagnet are
dominated by a two-parameter set of collective excitations. In a cyclic chain
of N sites and magnetization 0<M_z<N/2, the ground state, which contains 2M_z
spinons, is reconfigured as the physical vacuum for a different species of
quasi-particles, identifiable in the framework of the coordinate Bethe ansatz
by characteristic configurations of Bethe quantum numbers. The dynamically
dominant excitations are found to be scattering states of two such
quasi-particles. For N -> \infty, these collective excitations form a continuum
in (q,\omega)-space with an incommensurate soft mode. Their matrix elements in
the dynamic spin structure factor S_{zz}(q,\omega) are calculated directly from
the Bethe wave functions for finite N. The resulting lineshape predictions for
N -> \infty complement the exact results previously derived via algebraic
analysis for the exact 2-spinon part of S_{zz}(q,\omega) in the zero-field
limit. They are directly relevant for the interpretation of neutron scattering
data measured in nonzero field on quasi-1D antiferromagnetic compounds.Comment: 10 page
Excitation Spectrum Gap and Spin-Wave Stiffness of XXZ Heisenberg Chains: Global Renormalization-Group Calculation
The anisotropic XXZ spin-1/2 Heisenberg chain is studied using
renormalization-group theory. The specific heats and nearest-neighbor spin-spin
correlations are calculated thoughout the entire temperature and anisotropy
ranges in both ferromagnetic and antiferromagnetic regions, obtaining a global
description and quantitative results. We obtain, for all anisotropies, the
antiferromagnetic spin-liquid spin-wave velocity and the Isinglike
ferromagnetic excitation spectrum gap, exhibiting the spin-wave to spinon
crossover. A number of characteristics of purely quantum nature are found: The
in-plane interaction s_i^x s_j^x + s_i^y s_j^y induces an antiferromagnetic
correlation in the out-of-plane s_i^z component, at higher temperatures in the
antiferromagnetic XXZ chain, dominantly at low temperatures in the
ferromagnetic XXZ chain, and, in-between, at all temperatures in the XY chain.
We find that the converse effect also occurs in the antiferromagnetic XXZ
chain: an antiferromagnetic s_i^z s_j^z interaction induces a correlation in
the s_i^xy component. As another purely quantum effect, (i) in the
antiferromagnet, the value of the specific heat peak is insensitive to
anisotropy and the temperature of the specific heat peak decreases from the
isotropic (Heisenberg) with introduction of either type (Ising or XY)
anisotropy; (ii) in complete contrast, in the ferromagnet, the value and
temperature of the specific heat peak increase with either type of anisotropy.Comment: New results added to text and figures. 12 pages, 18 figures, 3
tables. Published versio
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