358 research outputs found
The Antiferromagnetic Heisenberg Model on Fullerene-Type Symmetry Clusters
The nearest neighbor antiferromagnetic Heisenberg model is
considered for spins sitting on the vertices of clusters with the connectivity
of fullerene molecules and a number of sites ranging from 24 to 32. Using
the permutational and spin inversion symmetries of the Hamiltonian the low
energy spectrum is calculated for all the irreducible representations of the
symmetry group of each cluster. Frustration and connectivity result in
non-trivial low energy properties, with the lowest excited states being
singlets except for . Same hexagon and same pentagon correlations are the
most effective in the minimization of the energy, with the
symmetry cluster having an unusually strong singlet intra-pentagon correlation.
The magnetization in a field shows no discontinuities unlike the icosahedral
fullerene clusters, but only plateaux with the most pronounced for
. The spatial symmetry as well as the connectivity of the clusters appear
to be important for the determination of their magnetic properties.Comment: Extended to include low energy spectra, correlation functions and
magnetization data of clusters up to 32 site
Effects of spin vacancies on magnetic properties of the Kitaev-Heisenberg model
We study the ground state properties of the Kitaev-Heisenberg model in a
magnetic field and explore the evolution of spin correlations in the presence
of non-magnetic vacancies. By means of exact diagonalizations, the phase
diagram without vacancies is determined as a function of the magnetic field and
the ratio between Kitaev and Heisenberg interactions. We show that in the
(antiferromagnetic) stripe ordered phase the static susceptibility and its
anisotropy can be described by a spin canting mechanism. This accounts as well
for the transition to the polarized phase when including quantum fluctuations
perturbatively. Effects of spin vacancies depend sensitively on the type of the
ground state. In the liquid phase, the magnetization pattern around a single
vacancy in a small field is determined, and its spatial anisotropy is related
to that of non-zero further neighbor correlations induced by the field and/or
Heisenberg interactions. In the stripe phase, the joint effect of a vacancy and
a small field breaks the six-fold symmetry of the model and stabilizes a
particular stripe pattern. Similar symmetry-breaking effects occur even at zero
field due to effective interactions between vacancies. This selection mechanism
and intrinsic randomness of vacancy positions may lead to spin-glass behavior.Comment: 13 pages, 10 figure
An Approach to Agent-Based Service Composition and Its Application to Mobile
This paper describes an architecture model for multiagent systems that was developed in the European project LEAP (Lightweight Extensible Agent Platform). Its main feature is a set of generic services that are implemented independently of the agents and can be installed into the agents by the application developer in a flexible way. Moreover, two applications using this architecture model are described that were also developed within the LEAP project. The application domain is the support of mobile, virtual teams for the German automobile club ADAC and for British Telecommunications
Internal state conversion in ultracold gases
We consider an ultracold gas of (non-condensed) bosons or fermions with two
internal states, and study the effect of a gradient of the transition frequency
between these states. When a RF pulse is applied to the sample,
exchange effects during collisions transfer the atoms into internal states
which depend on the direction of their velocity. This results, after a short
time, in a spatial separation between the two states. A kinetic equation is
solved analytically and numerically; the results agree well with the recent
observations of Lewandowski et al.Comment: Accepted version, to appear in PR
Longitudinal spin waves in a dilute Bose gas
We present a kinetic theory for a dilute noncondensed Bose gas of two-level
atoms that predicts the transient spin segregation observed in a recent
experiment. The underlying mechanism driving spin currents in the gas is due to
a mean field effect arising from the quantum interference between the direct
and exchange scattering of atoms in different spin states. We numerically solve
the spin Boltzmann equation, using a one dimensional model, and find excellent
agreement with experimental data.Comment: 4.5 pages, 3 embedded color figure
Observation of anomalous spin-state segregation in a trapped ultra-cold vapor
We observe counter-intuitive spin segregation in an inhomogeneous sample of
ultra-cold, non-condensed Rubidium atoms in a magnetic trap. We use spatially
selective microwave spectroscopy to verify a model that accounts for the
differential forces on two internal spin states. In any simple understanding of
the cloud dynamics, the forces are far too small to account for the dramatic
transient spin polarizations observed. The underlying mechanism remains to be
elucidated.Comment: 5 pages, 3 figure
Intrinsic susceptibility and bond defects in the novel 2D frustrated antiferromagnet BaSnZnCrGaO
We present microscopic and macroscopic magnetic properties of the highly
frustrated antiferromagnet BaSnZnCrGaO,
respectively probed with NMR and SQUID experiments. The -variation of the
intrinsic susceptibility of the Cr frustrated kagom\'{e} bilayer,
, displays a maximum around 45 K. The dilution of the magnetic
lattice has been studied in detail for . Novel dilution
independent defects, likely related with magnetic bond disorder, are evidenced
and discussed. We compare our results to SrCrGaO. Both
bond defects and spin vacancies do not affect the average susceptibility of the
kagom\'{e} bilayers.Comment: Published in Phys. Rev. Lett. 92, 217202 (2004). Only minor changes
as compared to previous version. 4 pages, 4 figure
The Antiferromagnetic Heisenberg Model on Clusters with Icosahedral Symmetry
The antiferromagnetic Heisenberg model is considered for spins
located on the vertices of the dodecahedron and the icosahedron, which belong
to the point symmetry group . Taking into account the permutational and
spin inversion symmetries of the Hamiltonian results in a drastic reduction of
the dimensionality of the problem, leading to full diagonalization for both
clusters. There is a strong signature of the frustration present in the systems
in the low energy spectrum, where the first excited states are singlets.
Frustration also results in a doubly-peaked specific heat as a function of
temperature for the dodecahedron. Furthermore, there is a discontinuity in the
magnetization as a function of magnetic field for the dodecahedron, where a
specific total spin sector never becomes the ground state in a field. This
discontinuity is accompanied by a magnetization plateau. The calculation is
also extended for where both systems again have singlet excitations.
The magnetization of the dodecahedron has now two discontinuities in an
external field and also magnetization plateaux, and the specific heat of the
icosahedron a two-peak structure as a function of temperature. The similarities
between the two systems suggest that the antiferromagnetic Heisenberg model on
a larger cluster with the same symmetry, the 60-site cluster, will have similar
properties
Emergent Ising degrees of freedom in frustrated two-leg ladder and bilayer Heisenberg antiferromagnets
Based on exact diagonalization data for finite quantum Heisenberg
antiferromagnets on two frustrated lattices (two-leg ladder and bilayer) and
analytical arguments we map low-energy degrees of freedom of the spin models in
a magnetic field on classical lattice-gas models. Further we use
transfer-matrix calculations and classical Monte Carlo simulations to give a
quantitative description of low-temperature thermodynamics of the quantum spin
models. The classical lattice-gas model yields an excellent description of the
quantum spin models up to quite large temperatures. The main peculiarity of the
considered frustrated bilayer is a phase transition which occurs at low
temperatures for a wide range of magnetic fields below the saturation magnetic
field and belongs to the two-dimensional Ising model universality class.Comment: 17 pages, 8 figure
Normal-superfluid interaction dynamics in a spinor Bose gas
Coherent behavior of spinor Bose-Einstein condensates is studied in the
presence of a significant uncondensed (normal) component. Normal-superfluid
exchange scattering leads to a near-perfect local alignment between the spin
fields of the two components. Through this spin locking, spin-domain formation
in the condensate is vastly accelerated as the spin populations in the
condensate are entrained by large-amplitude spin waves in the normal component.
We present data evincing the normal-superfluid spin dynamics in this regime of
complicated interdependent behavior.Comment: 5 pages, 4 fig
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