3,549 research outputs found
Mn-doped II-VI quantum dots: artificial molecular magnets
The notion of artifical atom relies on the capability to change the number of
carriers one by one in semiconductor quantum dots, and the resulting changes in
their electronic structure. Organic molecules with transition metal atoms that
have a net magnetic moment and display hysteretic behaviour are known as single
molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped
with a controlled number of Mn atoms and with a number of carriers controlled
either electrically or optically paves the way towards a new concept in
nanomagnetism: the artificial single molecule magnet. Here we study the
magnetic properties of a Mn-doped CdTe quantum dot for different charge states
and show to what extent they behave like a single molecule magnet.Comment: Conference article presented at QD2006, Chamonix, May 200
Some new results on an old controversy: is perturbation theory the correct asymptotic expansion in nonabelian models?
Several years ago it was found that perturbation theory for two-dimensional
O(N) models depends on boundary conditions even after the infinite volume limit
has been taken termwise, provided . There ensued a discussion whether the
boundary conditions introduced to show this phenomenon were somehow anomalous
and there was a class of `reasonable' boundary conditions not suffering from
this ambiguity. Here we present the results of some computations that may be
interpreted as giving some support for the correctness of perturbation theory
with conventional boundary conditions; however the fundamental underlying
question of the correctness of perturbation theory in these models and in
particular the perturbative function remain challenging problems of
mathematical physics.Comment: 4 pages, 3 figure
Vacuum Nodes and Anomalies in Quantum Theories
We show that nodal points of ground states of some quantum systems with
magnetic interactions can be identified in simple geometric terms. We analyse
in detail two different archetypical systems: i) a planar rotor with a
non-trivial magnetic flux , ii) Hall effect on a torus. In the case of
the planar rotor we show that the level repulsion generated by any reflection
invariant potential is encoded in the nodal structure of the unique vacuum
for . In the second case we prove that the nodes of the first
Landau level for unit magnetic charge appear at the crossing of the two
non-contractible circles , with holonomies
for any reflection invariant potential
. This property illustrates the geometric origin of the quantum translation
anomaly.Comment: 14 pages, 2 ps-figures, to appear in Commun. Math. Phy
Ab initio calculations of structures and stabilities of (NaI)_nNa+ and (CsI)_nCs+ cluster ions
Ab initio calculations using the Perturbed Ion model, with correlation
contributions included, are presented for nonstoichiometric (NaI)_nNa+ and
(CsI)_nCs+ (n=1-14) cluster ions. The ground state and several low-lying
isomers are identified and described. Rocksalt ground states are common and
appear at cluster sizes lower than in the corresponding neutral systems. The
most salient features of the measured mobilities seem to be explained by
arguments related to the changes of the compactness of the clusters as a
function of size. The stability of the cluster ions against evaporation of a
single alkali halide molecule shows variations that explain the enhanced
stabilities found experimentally for cluster sizes n=4, 6, 9, and 13. Finally,
the ionization energies and the orbital eigenvalue spectrum of two (NaI)_13Na+
isomers are calculated and shown to be a fingerprint of the structure.Comment: 8 pages plus 13 postscript figures, LaTeX. Accepted for publication
in Phys, Rev. B; minor changes including a more complete comparison to pair
potential result
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Modelling the developmental patterning of finiteness marking in English, Dutch, German and Spanish using MOSAIC
In this paper we apply MOSAIC (Model of Syntax Acquisition in Children) to the simulation of the developmental patterning of children’s Optional Infinitive (OI) errors in four languages: English, Dutch, German and Spanish. MOSAIC, which has already simulated this phenomenon in Dutch and English, now implements a learning mechanism that better reflects the theoretical assumptions underlying it, as well as a chunking mechanism which results in frequent phrases being treated as one unit. Using one, identical model that learns from child-directed speech, we obtain a close quantitative fit to the data from all four languages, despite there being considerable cross-linguistic and developmental variation in the OI phenomenon. MOSAIC successfully simulates the difference between Spanish (a pro-drop language where OI errors are virtually absent), and Obligatory Subject languages that do display the OI phenomenon. It also highlights differences in the OI phenomenon across German and Dutch, two closely related languages whose grammar is virtually identical with respect to the relation between finiteness and verb placement. Taken together, these results suggest that (a) cross-linguistic differences in the rates at which children produce Optional Infinitives are graded, quantitative differences that closely reflect the statistical properties of the input they are exposed to and (b) theories of syntax acquisition need to consider more closely the role of input characteristics as determinants of quantitative differences in the cross-linguistic patterning of phenomena in language acquisition
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Using Ontology Research in Semantic Web Applications
In the light of improving the World Wide Web, researchers are working towards the Semantic Web. Ontologies and ontology-based applications are its basic ingredients. Several ontological environments, categorizations and methodologies can be found in the literature. This paper shows how we have investigated the state of the art in these areas in an ontology building process that is the basis for an application developed at the later stage in an events organisation domain
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A Generic Communications Module for Cooperative 3D Visualization and Modelling over the Internet: the Collaborative API
Cooperative three-dimensional visualization and modeling applications allow a distributed group of users to work together with a model they share. To implement this kind of applications the underlying communications system must provide reliable and ordered multicast of users interactions. Due to the high complexity that characterizes the models, network bandwidth requirements have limited their use to intranets or in a few cases to very high-speed Internet connections.
In this paper we present a communications module that solves this problem. The library exposed, which is called Collaborative API, supports the creation of very efficient cooperative 3D visualization and modeling applications by optimizing the use of the network resources.
The Collaborative API, implements a new communications architecture: the dynamic client/server. The communications module presented in this paper is illustrated by two examples of applications that use it to provide cooperative 3D visualization over the Internet
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