3,335 research outputs found
Spatially anisotropic Heisenberg Kagome antiferromagnet
In the search for spin-1/2 kagome antiferromagnets, the mineral volborthite
has recently been the subject of experimental studies [Hiroi et al.,2001]. It
has been suggested that the magnetic properties of this material are described
by a spin-1/2 Heisenberg model on the kagome lattice with spatially anisotropic
exchange couplings. We report on investigations of the Sp(N) symmetric
generalisation of this model in the large N limit. We obtain a detailed
description of the dependence of possible ground states on the anisotropy and
on the spin length S. A fairly rich phase diagram with a ferrimagnetic phase,
incommensurate phases with and without long range order and a decoupled chain
phase emerges.Comment: 6 pages, 6 figures, proceedings of the HFM2006 conference, to appear
in a special issue of J. Phys.: Condens. Matte
Monte Carlo Simulation of the Heisenberg Antiferromagnet on a Triangular Lattice: Topological Excitations
We have simulated the classical Heisenberg antiferromagnet on a triangular
lattice using a local Monte Carlo algorithm. The behavior of the correlation
length , the susceptibility at the ordering wavevector , and
the spin stiffness clearly reflects the existence of two temperature
regimes -- a high temperature regime , in which the disordering
effect of vortices is dominant, and a low temperature regime ,
where correlations are controlled by small amplitude spin fluctuations. As has
previously been shown, in the last regime, the behavior of the above quantities
agrees well with the predictions of a renormalization group treatment of the
appropriate nonlinear sigma model. For , a satisfactory fit of the
data is achieved, if the temperature dependence of and is
assumed to be of the form predicted by the Kosterlitz--Thouless theory.
Surprisingly, the crossover between the two regimes appears to happen in a very
narrow temperature interval around .Comment: 13 pages, 8 Postscript figure
A Factorization Algorithm for G-Algebras and Applications
It has been recently discovered by Bell, Heinle and Levandovskyy that a large
class of algebras, including the ubiquitous -algebras, are finite
factorization domains (FFD for short).
Utilizing this result, we contribute an algorithm to find all distinct
factorizations of a given element , where is
any -algebra, with minor assumptions on the underlying field.
Moreover, the property of being an FFD, in combination with the factorization
algorithm, enables us to propose an analogous description of the factorized
Gr\"obner basis algorithm for -algebras. This algorithm is useful for
various applications, e.g. in analysis of solution spaces of systems of linear
partial functional equations with polynomial coefficients, coming from
. Additionally, it is possible to include inequality constraints
for ideals in the input
The effectiveness of polder systems on peak discharge capping of floods along the middle reaches of the Elbe River in Germany
International audienceIn flood modelling, many one-dimensional (1-D) hydrodynamic models are too restricted in capturing the spatial differentiation of processes within a polder or system of polders and two-dimensional (2-D) models are very demanding in data requirements and computational resources. The latter is an important consideration when uncertainty analyses using the Monte Carlo techniques are to complement the modelling exercises. This paper describes the development of a quasi-2-D modeling approach, which still calculates the dynamic wave in 1-D but the discretisation of the computational units are in 2-D, allowing a better spatial representation of the flow in polders and avoiding large additional expenditure on data pre-processing and computational time. The model DYNHYD (1-D hydrodynamics) from the WASP5 modeling package was used as a basis for the simulations and extended to incorporate the quasi-2-D approach. A local sensitivity analysis shows the sensitivity of parameters and boundary conditions on the filling volume of polders and capping of the peak discharge in the main river system. Two flood events on the Elbe River, Germany were used to calibrate and test the model. The results show a good capping effect on the flood peak by the proposed systems. The effect of capping reduces as the flood wave propagates down stream from the polders (up to 0.5 cm of capping is decreased for each additional kilometer from the polder)
Nonequilibrium Current in the One Dimensional Hubbard Model at Half-Filling
Nonlinear transport in the one dimensional Hubbard model at half-filling
under a finite bias voltage is investigated by the adaptive time-dependent
density matrix renormalization group method. For repulsive on-site interaction,
dielectric breakdown of the Mott insulating ground state to a current-carrying
nonequilibrium steady state is clearly observed when the voltage exceeds the
charge gap. It is found that by increasing the voltage further the
current-voltage characteristics are scaled only by the charge gap and the
scaling curve exhibits almost linear dependence on the voltage whose slope is
suppressed by the electron correlation. In the case of attractive interaction
the linear conductance is the perfect one which agrees with the
prediction by the Luttinger liquid theory.Comment: 4 pages, 7 figure
The LOPES experiment - recent results, status and perspectives
The LOPES experiment at the Karlsruhe Institute of Technology has been taking
radio data in the frequency range from 40 to 80 MHz in coincidence with the
KASCADE-Grande air shower detector since 2003. Various experimental
configurations have been employed to study aspects such as the energy scaling,
geomagnetic dependence, lateral distribution, and polarization of the radio
emission from cosmic rays. The high quality per-event air shower information
provided by KASCADE-Grande has been the key to many of these studies and has
even allowed us to perform detailed per-event comparisons with simulations of
the radio emission. In this article, we give an overview of results obtained by
LOPES, and present the status and perspectives of the ever-evolving experiment.Comment: Proceedings of the ARENA2010 conference, Nantes, Franc
Absolute calibration of the LOPES antenna system
Radio emission in extensive air showers arises from an interaction with the
geomagnetic field and is subject of theoretical studies. This radio emission
has advantages for the detection of high energy cosmic rays compared to
secondary particle or fluorescence measurement methods. Radio antennas like the
LOPES30 antenna system are suited to investigate this emission process by
detecting the radio pulses. The characteristic observable parameters like
electric field strength and pulse length require a calibration which was done
with a reference radio source resulting in an amplification factor representing
the system behavior in the environment of the KASCADE-Grande experiment.
Knowing the amplification factor and the gain of the LOPES antennas LOPES30 is
calibrated absolutely for systematic analyses of the radio emission.Comment: 5 pages, Proceedings of International Workshop on Acoustic and Radio
EeV Neutrino detection Activities: ARENA, May 17-19, 2005, DESY Zeuthe
Risk assessment and mapping of extreme floods in non-dyked communities along the Elbe and Mulde Rivers
International audienceAssessing and mapping damage risk of floods for large river basins is still in its infancy. Damage risk is understood to be the combination of flood hazard and the vulnerability of communities to a flood of a particular return period. Risk is calculated and mapped for two communities in which dykes are not located for flood protection: Meissen on the Elbe River and Döbeln in the Mulde catchment. Different methodologies for the computation of flood depth and inundation extent of varying flood return periods (hazard) are compared. Exposure and relative damage to the flooding (vulnerability) based on land-use coverages of different scale are also compared and discussed. A property asset coverage completes the data requirements for the construction of the risk maps. Recommendations for continued research on risk assessments of large river basins conclude the study
The spectrum of high-energy cosmic rays measured with KASCADE-Grande
The energy spectrum of cosmic rays between 10**16 eV and 10**18 eV, derived
from measurements of the shower size (total number of charged particles) and
the total muon number of extensive air showers by the KASCADE-Grande
experiment, is described. The resulting all-particle energy spectrum exhibits
strong hints for a hardening of the spectrum at approximately 2x10**16 eV and a
significant steepening at c. 8x10**16 eV. These observations challenge the view
that the spectrum is a single power law between knee and ankle. Possible
scenarios generating such features are discussed in terms of astrophysical
processes that may explain the transition region from galactic to extragalactic
origin of cosmic rays.Comment: accepted by Astroparticle Physics June 201
Bag Formation in Quantum Hall Ferromagnets
Charged skyrmions or spin-textures in the quantum Hall ferromagnet at filling
factor nu=1 are reinvestigated using the Hartree-Fock method in the lowest
Landau level approximation. It is shown that the single Slater determinant with
the minimum energy in the unit charge sector is always of the hedgehog form. It
is observed that the magnetization vector's length deviates locally from unity,
i.e. a bag is formed which accommodates the excess charge. In terms of a
gradient expansion for extended spin-textures a novel O(3) type of effective
action is presented, which takes bag formation into account.Comment: 13 pages, 3 figure
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