754 research outputs found
On the algorithmic construction of classifying spaces and the isomorphism problem for biautomatic groups
We show that the isomorphism problem is solvable in the class of central
extensions of word-hyperbolic groups, and that the isomorphism problem for
biautomatic groups reduces to that for biautomatic groups with finite centre.
We describe an algorithm that, given an arbitrary finite presentation of an
automatic group , will construct explicit finite models for the skeleta
of and hence compute the integral homology and cohomology of
.Comment: 21 pages, 4 figure
Observation of Quantum Effects in sub Kelvin Cold Reactions
There has been a long-standing quest to observe chemical reactions at low
temperatures where reaction rates and pathways are governed by quantum
mechanical effects. So far this field of Quantum Chemistry has been dominated
by theory. The difficulty has been to realize in the laboratory low enough
collisional velocities between neutral reactants, so that the quantum wave
nature could be observed. We report here the first realization of merged
neutral supersonic beams, and the observation of clear quantum effects in the
resulting reactions. We observe orbiting resonances in the Penning ionization
reaction of argon and molecular hydrogen with metastable helium leading to a
sharp increase in the absolute reaction rate in the energy range corresponding
to a few degrees kelvin down to 10 mK. Our method is widely applicable to many
canonical chemical reactions, and will enable a breakthrough in the
experimental study of Quantum Chemistry
Ambiguities in the partial-wave analysis of pseudoscalar-meson photoproduction
Ambiguities in pseudoscalar-meson photoproduction, arising from incomplete
experimental data, have analogs in pion-nucleon scattering. Amplitude
ambiguities have important implications for the problems of amplitude
extraction and resonance identification in partial-wave analysis. The effect of
these ambiguities on observables is described. We compare our results with
those found in earlier studies.Comment: 12 pages of text. No figure
Inversion of Randomly Corrugated Surfaces Structure from Atom Scattering Data
The Sudden Approximation is applied to invert structural data on randomly
corrugated surfaces from inert atom scattering intensities. Several expressions
relating experimental observables to surface statistical features are derived.
The results suggest that atom (and in particular He) scattering can be used
profitably to study hitherto unexplored forms of complex surface disorder.Comment: 10 pages, no figures. Related papers available at
http://neon.cchem.berkeley.edu/~dan
Abelian subgroups of Garside groups
In this paper, we show that for every abelian subgroup of a Garside
group, some conjugate consists of ultra summit elements and the
centralizer of is a finite index subgroup of the normalizer of .
Combining with the results on translation numbers in Garside groups, we obtain
an easy proof of the algebraic flat torus theorem for Garside groups and solve
several algorithmic problems concerning abelian subgroups of Garside groups.Comment: This article replaces our earlier preprint "Stable super summit sets
in Garside groups", arXiv:math.GT/060258
Electron correlation vs. stabilization: A two-electron model atom in an intense laser pulse
We study numerically stabilization against ionization of a fully correlated
two-electron model atom in an intense laser pulse. We concentrate on two
frequency regimes: very high frequency, where the photon energy exceeds both,
the ionization potential of the outer {\em and} the inner electron, and an
intermediate frequency where, from a ``single active electron''-point of view
the outer electron is expected to stabilize but the inner one is not. Our
results reveal that correlation reduces stabilization when compared to results
from single active electron-calculations. However, despite this destabilizing
effect of electron correlation we still observe a decreasing ionization
probability within a certain intensity domain in the high-frequency case. We
compare our results from the fully correlated simulations with those from
simpler, approximate models. This is useful for future work on ``real''
more-than-one electron atoms, not yet accessible to numerical {\em ab initio}
methods.Comment: 8 pages, 8 figures in an extra ps-file, submitted to Phys. Rev. A,
updated references and shortened introductio
Near-field interactions between metal nanoparticle surface plasmons and molecular excitons in thin-films: part I: absorption
In this and the following paper (parts I and II, respectively), we systematically study the interactions between surface plasmons of metal nanoparticles (NPs) with excitons in thin-films of organic media. In an effort to exclusively probe near-field interactions, we utilize spherical Ag NPs in a size-regime where far-field light scattering is negligibly small compared to absorption. In part I, we discuss the effect of the presence of these Ag NPs on the absorption of the embedding medium by means of experiment, numerical simulations, and analytical calculations, all shown to be in good agreement. We observe absorption enhancement in the embedding medium due to the Ag NPs with a strong dependence on the medium permittivity, the spectral position relative to the surface plasmon resonance frequency, and the thickness of the organic layer. By introducing a low index spacer layer between the NPs and the organic medium, this absorption enhancement is experimentally confirmed to be a near field effect In part II, we probe the impact of the Ag NPs on the emission of organic molecules by time-resolved and steady-state photoluminescence measurements
Isolated Splenic Metastasis from Rectal Carcinoma: A Rare Occurrence
The presence of isolated splenic metastasis in rectal carcinoma is uncommon and usually presents as an asymptomatic mass, noted incidentally on imaging. Splenectomy is usually performed with the goal of curing metastatic disease. It is unclear if adjuvant chemotherapy affords any benefit, and the prognosis is unknown. The case of a young woman is reported, in whom an isolated metastatic lesion in the spleen was discovered 9 months after adjuvant chemotherapy for stage III rectal adenocarcinoma. The patient has remained disease-free for nearly 5 years following splenectomy and chemotherapy. To our knowledge, this is the fourth reported case in the English literature of an isolated splenic metastatic lesion from rectal cancer. We discuss the unique presentation, the importance of post-treatment surveillance, and the implementation of multi-modality treatment strategies in this young patient
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Language support for immigrant children: a study of state schools in the UK and US
In recent decades, immigrants, refugees and asylum seekers have sought a new way of life in large numbers, often leaving their countries of origin behind in search of places that offer a better way of life. The purpose of this study was to investigate how elementary and middle school students in state schools in Reading, England (primarily speakers of Asian languages), and Richmond, Virginia (primarily speakers of Spanish), were supported academically, when most children’s first language was not English. The authors were interested in exploring whether or not there were cultural or structural differences in the way each country helped or hindered these students as they progressed through the school systems. Three UK schools in a district of approximately 100,000 and three US schools in a district of approximately 250,000 were the focus of this exploration from 2000 to 2003. Findings indicated that there were cultural and legislative differences and similarities. Teachers and administrators in both countries attempted to provide services with limited and sometimes diminishing resources. Community support varied based on resources, attitudes toward various ethnic groups, and the coping strategies adopted by these groups in their new environments. Marked differences appeared with regard to the manner in which assessments took place and how the results were made available to the public
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