6,675 research outputs found
Towards a semantic modeling of learners for social networks
The Friend of a Friend (FOAF) ontology is a vocabulary for mapping social networks. In this paper we propose an extension to FOAF in order to allow it to model learners and their social networks. We analyse FOAF alongside different learner modeling standards and specifications, and based on this analysis we introduce a taxonomy of the different features found in those models. We then compare the learner models and FOAF against the taxonomy to see how their characteristics have been shaped by their purpose. Based on this we propose extensions to FOAF in order to produce a learner model that is capable of forming the basis of a semantic social network.<br/
Mott-Peierls Transition in the extended Peierls-Hubbard model
The one-dimensional extended Peierls-Hubbard model is studied at several band
fillings using the density matrix renormalization group method. Results show
that the ground state evolves from a Mott-Peierls insulator with a correlation
gap at half-filling to a soliton lattice with a small band gap away from
half-filling. It is also confirmed that the ground state of the Peierls-Hubbard
model undergoes a transition to a metallic state at finite doping. These
results show that electronic correlations effects should be taken into account
in theoretical studies of doped polyacetylene. They also show that a
Mott-Peierls theory could explain the insulator-metal transition observed in
this material.Comment: 4 pages with 3 embedded eps figure
Supporting the 'Sharing Institution' - Practical Steps Towards a More Open Teaching and Learning Culture
4th International Conference on Open RepositoriesThis presentation was part of the session : Conference PresentationsDate: 2009-05-19 03:00 PM – 04:30 PMInstitutional repositories for research output have developed progressively over the last few years. Although a primary motivation is Open Access both institutional and academic needs must also be met in order to foster this spirit effectively. There is now a greater emphasis on creating a more open culture for teaching and learning and institutions are again beginning to play their part more readily. On a larger scale, there are pioneering global examples of courses being preserved and complex learning materials being deposited in national and international databases. But what does fostering a more open culture in the practice of teaching mean for the institution itself and its academics? The virtual learning environment has given greater opportunities for effective administration of courses but in other ways it has had the opposite effect on sharing and re-use. There is potential for institutional solutions which are complementary to the global landscape. In this paper we report on the practical experiences and issues met, in setting up a institutional resource, EdShare, as a vehicle for sharing educational materials more easily in a multi-disciplinary institution. With constructive feedback from faculty, within the EdSpace project, EdShare has migrated into a more visual, web 2.0 style, resource with a flexible deposit process promoting 'micro-sharing'. It now offers a range of sharing options to support the teaching workflow in an encouraging atmosphere. Other features will support the growing emphasis on multidisciplinary teaching and collaboration - both internally and externally.JIS
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
Field-induced gap in the spin-1/2 antiferromagnetic Heisenberg chain: A density matrix renormalization group study
We study the spin-1/2 antiferromagnetic Heisenberg chain in both uniform and
(perpendicular) staggered magnetic fields using the density-matrix
renormalization-group method. This model has been shown earlier to describe the
physics of the copper benzoate materials in magnetic field. In the present
work, we extend the study to more general case for a systematic investigation
of the field-induced gap and related properties of the spin-1/2
antiferromagnetic Heisenberg chain. In particular, we explore the high magnetic
field regime where interesting behaviors in the field-induced gap,
magnetization, and spin correlation functions are found. Careful examination of
the low energy properties and magnetization reveals interesting competing
effects of the staggered and uniform fields. The incommensurate behavior in the
spin correlation functions is demonstrated and discussed in detail. The present
work reproduces earlier results in good agreement with experimental data on
copper benzoate and predicts new interesting field-induced features at very
high magnetic field.Comment: 8 pages, 6 figure
Effects of substituting rare-earth ion R by non-magnetic impurities in - theory and numerical DMRG results
In this paper we study the effect of substituting R (rare-earth ion) by
non-magnetic ions in the spin-1 chain material . Using a
strong-coupling expansion and numerical density matrix renormalization group
calculations, we show that spin-wave bound states are formed at the impurity
site. Experimental consequences of the bound states are pointed out.Comment: 5 pages, 4 postscript figure
Incremental expansions for Hubbard-Peierls systems
The ground state energies of infinite half-filled Hubbard-Peierls chains are
investigated combining incremental expansion with exact diagonalization of
finite chain segments. The ground state energy of equidistant infinite Hubbard
(Heisenberg) chains is calculated with a relative error of less than for all values of using diagonalizations of 12-site (20-site)
chain segm ents. For dimerized chains the dimerization order parameter as a
function of the onsite repulsion interaction has a maximum at nonzero
values of , if the electron-phonon coupling is lower than a critical
value . The critical value is found with high accuracy to be
. For smaller values of the position of the maximum of is
approximately , and rapidly tends to zero as approaches from
below. We show how our method can be applied to calculate breathers for the
problem of phonon dynamics in Hubbard-Peierls systems.Comment: 4 Pages, 3 Figures, REVTE
Density Matrix Renormalization Group Study of One-Dimensional Acoustic Phonons
We study the application of the density matrix renormalization group (DMRG)
to systems with one-dimensional acoustic phonons. We show how the use of a
local oscillator basis circumvents the difficulties with the long-range
interactions generated in real space using the normal phonon basis. When
applied to a harmonic atomic chain, we find excellent agreement with the exact
solution even when using a modest number of oscillator and block states (a few
times ten). We discuss the use of this algorithm in more complex cases and
point out its value when other techniques are deficient.Comment: 12 pages. To be published in PRB rapid co
Bond-versus-site doping models for off-chain-doped Haldane-gap system Y Ba Ni O
Using the density matrix renormalization-group technique, we calculate the
impurity energy levels for two different effective models of off-chain doping
for quasi-one-dimensional Heisenberg chain compound Y Ba Ni O:
ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping.
Thresholds of the impurity strength for the appearance of localized states are
found for both models. However, the ground-state and low-energy excitations for
weak impurity strength are different for these two models and the difference
can be detected by experiments.Comment: 5 pages, 5 eps figures included, to be published in Phys. Rev.
Multiple Histogram Method for Quantum Monte Carlo
An extension to the multiple-histogram method (sometimes referred to as the
Ferrenberg-Swendsen method) for use in quantum Monte Carlo simulations is
presented. This method is shown to work well for the 2D repulsive Hubbard
model, allowing measurements to be taken over a continuous region of
parameters. The method also reduces the error bars over the range of parameter
values due the overlapping of multiple histograms. A continuous sweep of
parameters and reduced error bars allow one to make more difficult
measurements, such as Maxwell constructions used to study phase separation.
Possibilities also exist for this method to be used for other quantum systems.Comment: 4 pages, 5 figures, RevTeX, submitted to Phys. Rev. B Rapid Com
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