2,803 research outputs found
Attosecond counter-rotating-wave effect in xenon driven by strong fields
No full textWe investigate the subfemtosecond dynamics of a highly excited xenon atom coherently driven by a strong control field at which the Rabi frequency of the system is comparable to the frequency of a driving laser. The widely used rotating-wave approximation breaks down at such fields, resulting in features such as the counter-rotating-wave (CRW) effect. We present a time-resolved observation of the CRW effect in the highly excited 4d-1np xenon using attosecond transient absorption spectroscopy. Time-dependent many-body theory confirms the observation and explains the various features of the absorption spectrum seen in experiment. ?2017 American Physical Society.111Ysciescopu
Dynamical imaginary-time correlations from auxiliary fields quantum Monte Carlo
No full textQuantum Monte Carlo (QMC) simulations of many body fermionic systems are considerably complicated by the well known sign
problem. Although very accurate approximation schemes have been developed for the calculation of static properties, like
structure functions and energy, the possibility of extending such methodologies to the investigation of dynamical properties is still
largely unexplored. Recently, a number of innovative QMC methods have been conceived which map the imaginary time
evolution into a random walk in the abstract manifold of Slater determinants. In such approaches the sign problem is not
circumvented and still requires approximations, but emerges in a different - and hopefully easier to handle - way.
We have focused on the phaseless auxiliary Fields QMC method (AFQMC), developed by Shiwei Zhang. Generalizing the formal
manipulations suggested by Assaad et al., we propose a practical scheme to evaluate dynamic correlation functions in imaginary
time, giving access to the study of excitations and response functions of interacting fermionic systems.
We have explored systematically the effects of the phaseless approximation, underlying the AFQMC technique and its dynamical
generalization, via the study of exactly solvable simple models, comparing AFQMC predictions with exact solutions. We will present
also results about a two-dimensional electron liquid, providing comparisons with other QMC techniques
Formulation of the twisted-light-matter interaction at the phase singularity: Beams with strong magnetic fields
No full textThe formulation of the interaction of matter with singular light fields needs special care. In a recent article [G. F. Quinteiro, Phys. Rev. A 91, 033808 (2015)PLRAAN1050-294710.1103/PhysRevA.91.033808] we have shown that the Hamiltonian describing the interaction of a twisted-light beam having parallel orbital and spin angular momenta with a small object located close to the phase singularity can be expressed only in terms of the electric field of the beam. Here we complement our study by providing an interaction Hamiltonian for beams having antiparallel orbital and spin angular momenta. Such beams may exhibit unusually strong magnetic effects. We further extend our formulation to radially and azimuthally polarized beams. The advantages of our formulation are that for all beams the Hamiltonian is written solely in terms of the electric and magnetic fields of the beam and as such it is manifestly gauge invariant. Furthermore, it is intuitive by resembling the well-known expressions in the dipole-electric and dipole-magnetic moment approximations.Fil: Quinteiro, Guillermo Federico. Universidad de Buenos Aires. Facultad de Ciencias Exactas. Departamento de Ecología, Genética y Evolución. Buenos Aires; Argentina. Westfalische Wilhelms Universitat; AlemaniaFil: Reiter, D.E.. Westfalische Wilhelms Universitat; AlemaniaFil: Kuhn, T.. Westfalische Wilhelms Universitat; Alemani
Review of the thesis: “The activities of the Soviet police to combat crime and public protection in Western Siberia in 1925-1937” by D.E. Kuznetsov
No full textThe article analyses the thesis “The activities of the Soviet police to combat crime and protect public order in Western Siberia in 1925-1937” by D.E. Kuznetsov. The structure and logic of the construction of the work, the validity of the conclusions, the merits of the dissertation research and its controversial points are considered. Special attention is paid to the source of the dissertation. In conclusion, the author of the article summaries that the contents of D.E. Kuznetsov's facts, assessments and conclusions can be used to develop textbooks on the history of crime, the history of law enforcement bodies, the history of Russia
The Sun's global photospheric and coronal magnetic fields : observations and models
No full text2012LRSP....9....6M Funding: STFC, the Leverhulme Trust and European Commission’s Seventh Framework Programme (FP7/2007-2013) under the grant agreement SWIFF (project no. 263340, http://www.swiff.eu).In this review, our present day understanding of the Sun's global photospheric and coronal magnetic fields is discussed from both observational and theoretical viewpoints. Firstly, the large-scale properties of photospheric magnetic fields are described, along with recent advances in photospheric magnetic flux transport models. Following this, the wide variety of theoretical models used to simulate global coronal magnetic fields are described. From this, the combined application of both magnetic flux transport simulations and coronal modeling techniques to describe the phenomena of coronal holes, the Sun's open magnetic flux and the hemispheric pattern of solar filaments is discussed. Finally, recent advances in non-eruptive global MHD models are described. While the review focuses mainly on solar magnetic fields, recent advances in measuring and modeling stellar magnetic fields are described where appropriate. In the final section key areas of future research are identified.Peer reviewe
Using and evaluating CASE tools : from software engineering to phenomenology
No full textCASE (Computer-Aided Systems Engineering) is a recent addition to the long line of
"silver bullets" that promise to transform information systems development, delivering
new levels of quality and productivity. CASE is particularly intriguing because
information systems (IS) practitioners spend their working lives applying information
technology (IT) to other people's work, and now they are applying it to themselves.
CASE research to date has been dominated by accounts of tool development,
normative writings (for example practitioner success stories) and surveys recording
IT specialists' perceptions. There have been very few in-depth studies of tool use,
and very few attempts to quantify benefits, therefore the essence of the CASE process
remains largely unexplored, and the views of stakeholders other than the IT specialists
have yet to be heard.
The research presented here addresses these concerns by adopting a hybrid research
approach combining action research, grounded theory and phenoinenology and using
both qualitative and quantitative data in order to tell the story of a system developer's
experience in using CASE tools in three information systems projects for a major UK
car manufacturer over a four year period. The author was the lead developer on all
three projects. Action research is a learning process, the researcher is an explorer.
At the start of this project it was assumed that the tools would be the focus of the
work. As the research progressed it became evident that the tools were but part of
a richer organisational context in which culture, politics, history, external initiatives
and cognitive limitations played important roles. The author continued to record
experiences and impressions of tool use in the project diary together with quality and
productivity metrics. But the diary also became home to a story of organisational
developments that had not originally been foreseen.
The principal contribution made by the work is to identity the narrow positivistic
nature of CASE knowledge, and to show via the research stories the overwhelming
importance of organisational context to systems development success and how the
exploration of context is poorly supported by the tools. Sixteen further contributions
are listed in the Conclusions to the thesis, including a major extension to Wynekoop
and Conger's CASE research taxonomy, an identification of the potentially
misleading nature of quantitative IS assessment and further evidence of the limitations
of the "scientific" approach to systems development.
The thesis is completed by two proposals for further work. The first seeks to
advance IS theory by developing further a number of emerging process models of IS
development. The second seeks to advance IS practice by asking the question "How
can CASE tools be used to stimulate awareness and debate about the effects of
organisational context?", and outlines a programme of research in this area
Learning generative texture models with extended Fields-of-Experts
No full textWe evaluate the ability of the popular Field-of-Experts (FoE) to model structure in images. As a test case we focus on modeling synthetic and natural textures. We find that even for modeling single textures, the FoE provides insufficient flexibility to learn good generative models – it does not perform any better than the much simpler Gaussian FoE. We propose an extended version of the FoE (allowing for bimodal potentials) and demonstrate that this novel formulation, when trained with a better approximation of the likelihood gradient, gives rise to a more powerful generative model of specific visualstructure that produces significantly better results for the texture task
Monitoring the fluvial palynomorph load in a lowland temperate catchment and its relationship to suspended sediment and discharge
No full textDespite it being a component of the seston we know very little about fluvial (waterborne) pollen and spore (palynomorph) transport. This paper presents the results of a monitoring programme conducted over two years and at a catchment scale in South West England. A hierarchical monitoring network was established with flood peak samples taken at 9 sub-catchments, intra-hydrograph samples taken in two sub-catchments and time-integrated sampling undertaken at one location. In addition sampling was undertaken of probable palynomorph sources such as channel bed and bank sediments, and the airborne pollen flux was monitored using modified Tauber traps. The results support previous research in illustrating how the vast majority of fluvial pollen and spores are transported during floods (91%) and that the main control on waterborne palynomorph assemblages is the catchment vegetation and its spatial distribution but with a long-distance (extra-catchment) component. However, strong seasonal effects are also shown, and the importance of distinctive sources such as the riparian input, bed re-suspension and overland flow into drains and tributaries is revealed. Fine sediment in river pools appears to act as a selective store of damaged cereal type pollen grains derived from arable fields. Although pollen does form part of composite particles the data presented here suggest that the majority of the pollen is transported as single grains. Fluvial palynomorph loading is strongly dependant upon discharge and so concentrations in laminated or varved sediments could be regarded as a proxy for flood magnitude
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Exposure and risk calculations for disposal of wastes having minimal radioactivity
No full textThe US Nuclear Regulatory Commission is currently considering revision of rules 10 CFR 20 and 10 CFR 61, which cover disposal of solid wastes containing minimal activity radioactivity. In support of these revised rules, we have evaluated the consequences of disposing of four waste streams at four types of disposal areas located in three different geographic regions. Consequences are expressed in terms of human exposures and associated health effects. Each geographic region has its own climate and geology. Example waste streams, waste disposal methods, and geographic regions chosen for this study are clearly specified. The PRESTO-II methodology was used to evaluate radionuclide transport and health effects. This methodology was developed to assess radiological impacts to a static local population for a 1000-year period following disposal. The modeling of pathways and processes of migration from the trench to exposed populations included the following considerations: groundwater transport, overland flow, erosion, surface water dilution, resuspension, atmospheric transport, deposition, inhalation, and ingestion of contaminated beef, milk, crops, and water. 9 references, 2 figures, 3 tables
Snake based Unsupervised Texture Segmentation using Gaussian Markov Random Field Models
No full textA functional for unsupervised texture segmentation is investigated in this paper. An auto-normal model based on Markov Random Fields is employed to model textures. The functional investigated here is optimized with respect to the model parameters and the evolving contour to simultaneously estimate model parameters and find the boundaries between textures. Experimental results applied on the textures of the Brodatz album demonstrate the superior performance and the higher speed of convergence of this algorithm in comparison with a traditional stochastic algorithm in the literature
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