2,076 research outputs found
Intrinsic peculiarities of real material realizations of a spin-1/2 kagome lattice
Spin-1/2 magnets with kagome geometry, being for years a generic object of
theoretical investigations, have few real material realizations. Recently, a
DFT-based microscopic model for two such materials, kapellasite Cu3Zn(OH)6Cl2
and haydeeite Cu3Mg(OH)6Cl2, was presented [O. Janson, J. Richter and H.
Rosner, arXiv:0806.1592]. Here, we focus on the intrinsic properties of real
spin-1/2 kagome materials having influence on the magnetic ground state and the
low-temperature excitations. We find that the values of exchange integrals are
strongly dependent on O--H distance inside the hydroxyl groups, present in most
spin-1/2 kagome compounds up to date. Besides the original kagome model,
considering only the nearest neighbour exchange, we emphasize the crucial role
of the exchange along the diagonals of the kagome lattice.Comment: 4 pages, 4 figures. A paper for the proceedings of the HFM 2008
conferenc
Microscopic magnetic modeling for the =1/2 alternating chain compounds NaCuSbO and NaCuTeO
The spin-1/2 alternating Heisenberg chain system NaCuSbO features
two relevant exchange couplings: within the structural CuO
dimers and between the dimers. Motivated by the controversially
discussed nature of , we perform extensive density-functional-theory
(DFT) calculations, including DFT+ and hybrid functionals. Fits to the
experimental magnetic susceptibility using high-temperature series expansions
and quantum Monte Carlo simulations yield the optimal parameters =
217 K and = 174 K with the alternation ratio 1.25. For the closely related system
NaCuTeO, DFT yields substantially enhanced , but weaker
. The comparative analysis renders the buckling of the chains as the
key parameter altering the magnetic coupling regime. Numerical simulation of
the dispersion relations of the alternating chain model clarify why both
antiferromagnetic and ferrromagnetic can reproduce the experimental
magnetic susceptibility data.Comment: published version: 11 pages, 8 figures, 5 tables + Supplemental
materia
Crystal water induced switching of magnetically active orbitals in CuCl2
The dehydration of CuCl2*2(H2O) to CuCl2 leads to a dramatic change in
magnetic behavior and ground state. Combining density functional electronic
structure and model calculations with thermodynamical measurements we reveal
the microscopic origin of this unexpected incident -- a crystal water driven
switching of the magnetically active orbitals. This switching results in a
fundamental change of the coupling regime from a three-dimensional
antiferromagnet to a quasi one-dimensional behavior. CuCl2 can be well
described as a frustrated J1-J2 Heisenberg chain with ferromagnetic exchange J1
and J2/J1 ~ -1.5 for which a helical ground state is predicted.Comment: 6 pages, 5 figures, 1 table (PRB, accepted
Unicyclic Components in Random Graphs
The distribution of unicyclic components in a random graph is obtained
analytically. The number of unicyclic components of a given size approaches a
self-similar form in the vicinity of the gelation transition. At the gelation
point, this distribution decays algebraically, U_k ~ 1/(4k) for k>>1. As a
result, the total number of unicyclic components grows logarithmically with the
system size.Comment: 4 pages, 2 figure
Reconstruction of Random Colourings
Reconstruction problems have been studied in a number of contexts including
biology, information theory and and statistical physics. We consider the
reconstruction problem for random -colourings on the -ary tree for
large . Bhatnagar et. al. showed non-reconstruction when and reconstruction when . We tighten this result and show non-reconstruction when and reconstruction when .Comment: Added references, updated notatio
Electronic structure and magnetic properties of the spin-1/2 Heisenberg system CuSe2O5
A microscopic magnetic model for the spin-1/2 Heisenberg chain compound
CuSe2O5 is developed based on the results of a joint experimental and
theoretical study. Magnetic susceptibility and specific heat data give evidence
for quasi-1D magnetism with leading antiferromagnetic (AFM) couplings and an
AFM ordering temperature of 17 K. For microscopic insight, full-potential DFT
calculations within the local density approximation (LDA) were performed. Using
the resulting band structure, a consistent set of transfer integrals for an
effective one-band tight-binding model was obtained. Electronic correlations
were treated on a mean-field level starting from LDA (LSDA+U method) and on a
model level (Hubbard model). In excellent agreement of experiment and theory,
we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour
intra-chain interaction of 165 K and a non-frustrated inter-chain coupling of
20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2,
Bi2CuO4), general implications for a magnetic ordering in presence of
inter-chain frustration are made.Comment: 20 pages, 8 figures, 3 table
Leadership Statistics in Random Structures
The largest component (``the leader'') in evolving random structures often
exhibits universal statistical properties. This phenomenon is demonstrated
analytically for two ubiquitous structures: random trees and random graphs. In
both cases, lead changes are rare as the average number of lead changes
increases quadratically with logarithm of the system size. As a function of
time, the number of lead changes is self-similar. Additionally, the probability
that no lead change ever occurs decays exponentially with the average number of
lead changes.Comment: 5 pages, 3 figure
A particle swarm optimization based memetic algorithm for dynamic optimization problems
Copyright @ Springer Science + Business Media B.V. 2010.Recently, there has been an increasing concern from the evolutionary computation community on dynamic optimization problems since many real-world optimization problems are dynamic. This paper investigates a particle swarm optimization (PSO) based memetic algorithm that hybridizes PSO with a local search technique for dynamic optimization problems. Within the framework of the proposed algorithm, a local version of PSO with a ring-shape topology structure is used as the global search operator and a fuzzy cognition local search method is proposed as the local search technique. In addition, a self-organized random immigrants scheme is extended into our proposed algorithm in order to further enhance its exploration capacity for new peaks in the search space. Experimental study over the moving peaks benchmark problem shows that the proposed PSO-based memetic algorithm is robust and adaptable in dynamic environments.This work was supported by the National Nature Science Foundation of China (NSFC) under Grant No. 70431003 and Grant No. 70671020, the National Innovation Research Community Science Foundation of China under
Grant No. 60521003, the National Support Plan of China under Grant No. 2006BAH02A09 and the Ministry of Education, science, and Technology in Korea through the Second-Phase of Brain Korea 21 Project in 2009, the Engineering and Physical Sciences Research
Council (EPSRC) of UK under Grant EP/E060722/01 and the Hong Kong Polytechnic University Research Grants under Grant G-YH60
Increased prevalence of sleep disturbances and daytime sleepiness in subjects with bronchial asthma: a population study of young adults in three European countries
To access publisher full text version of this article. Please click on the hyperlink in Additional Links fieldThe aim of this study was to investigate whether asthma is associated with decreased quality of sleep and increased daytime sleepiness. The study involved a random population of 2,202 subjects supplemented by 459 subjects with suspected asthma, aged 20-45 yrs. The subjects were from Reykjavik (Iceland), Uppsala and Göteborg (Sweden) and Antwerp (Belgium), and participated in the European Community Respiratory Health Survey. The investigation included a structured interview, methacholine challenge, skinprick tests and a questionnaire on sleep disturbances. Participants in Iceland and Sweden also estimated their sleep times and made peak expiratory flow (PEF) recordings during a period of 1 week. Asthma was defined as self-reported physician-diagnosed asthma with current asthma-related symptoms (n = 267). Difficulties inducing sleep (DIS) and early morning awakenings (EMA) were about twice as common, and daytime sleepiness 50% more common, in asthmatics compared with subjects without asthma. After adjusting for possible confounders, a positive association was found between asthma and: DIS (odds ratio (OR) = 1.8); EMA (OR = 2.0); daytime sleepiness (OR = 1.6); snoring (OR = 1.7); and self reported apnoeas (OR = 3.7). Allergic rhinitis, which was reported by 71% of subjects with asthma, was independently related to DIS (OR = 2.0) and daytime sleepiness (OR = 1.3). A significant correlation was found between the number of asthma-related symptoms and sleep disturbances (p < 0.001). Asthma is associated with decreased subjective quality of sleep and increased daytime sleepiness. Concurrent allergic rhinitis may be an important underlying cause of sleep impairment in asthmatic patients
Early ComeOn+ Adaptive Optics Observation of GQ Lup and its Substellar Companion
An analysis of adaptive optics K-band imaging data of GQ Lup acquired in 1994
by the first generation adaptive optics system ComeOn+ at the ESO 3.6m optical
telescope in La Silla is presented. The data reveal a likely candidate for the
low-mass companion recently reported in the literature. An a posteriori
detection in the 11 year old data would provide a useful astrometric data point
for the very long period (~1000 yr) orbit of the GQ Lup system. However, the
data is severely contaminated by speckle noise at the given projected
separation, which decreases the confidence of the detection. Still, from the
data we can conclude that GQ Lup B is not an unrelated background source, but
instead a physical companion to GQ Lup A. We present here the reduction and
analysis of the ComeOn+ images, as well as the results. We also discuss the
nature of the companion based on data and models available in the scientific
literature and examine claims made regarding the classification of the object
as a planet.Comment: 8 pages, 4 figures, accepted for publication in A&
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