619 research outputs found
Directing transport by polarized radiation in presence of chaos and dissipation
We study numerically the dynamics of particles on the Galton board of
semi-disk scatters in presence of monochromatic radiation and dissipation. It
is shown that under certain conditions the radiation leads to appearance of
directed transport linked to an underlining strange attractor. The direction of
transport can be efficiently changed by radiation polarization. The
experimental realization of this effect in asymmetric antidot superlattices is
discussed.Comment: revtex, 4 pages, 6 fig
Photocurrent in nanostructures with asymmetric antidots
The steady current induced by electromagnetic field in a 2D system with
asymmetric scatterers is studied. The scatterers are assumed to be oriented
cuts with one diffusive and another specular sides. Besides, the existence of
isotropic impurity scatterers is assumed. This simple model simulates the
lattice of half-disk which have been studied numerically recently. The model
allows the exact solution in the framework of the kinetic equation. The static
current response in the second order of electric field is obtained. The
photogalvanic tensor contains both responses to linear and circular
polarization of electromagnetic field. The model possesses non-analyticity with
regards to the rate of impurity scattering.Comment: 9 pages, 6 figure
Axiomatizing Discrete Spatial Relations
Qualitative spatial relations are used in artificial intelligence to model commonsense notions such as regions of space overlapping, touching only at their boundaries, or being separate. In this paper we extend earlier work on qualitative relations in discrete space by pre- senting a bi-intuitionistic modal logic with universal modalities, called UBiSKt. This logic has a semantics in which formulae are interpreted as subgraphs. We show how a variety of qualitative spatial relations can be defined in UBiSKt. We make essential use of a sound and complete axiomatisation of the logic and an implementation of a tableau based theorem prover to establish novel properties of these spatial relations. We also explore the role of UBiSKt in expressing spatial relations at more than one level of detail. The features of the logic allow it to rep- resent how a subgraph at a detailed level is approximated at a coarser level
Big Entropy Fluctuations in Nonequilibrium Steady State: A Simple Model with Gauss Heat Bath
Large entropy fluctuations in a nonequilibrium steady state of classical
mechanics were studied in extensive numerical experiments on a simple 2-freedom
model with the so-called Gauss time-reversible thermostat. The local
fluctuations (on a set of fixed trajectory segments) from the average heat
entropy absorbed in thermostat were found to be non-Gaussian. Approximately,
the fluctuations can be discribed by a two-Gaussian distribution with a
crossover independent of the segment length and the number of trajectories
('particles'). The distribution itself does depend on both, approaching the
single standard Gaussian distribution as any of those parameters increases. The
global time-dependent fluctuations turned out to be qualitatively different in
that they have a strict upper bound much less than the average entropy
production. Thus, unlike the equilibrium steady state, the recovery of the
initial low entropy becomes impossible, after a sufficiently long time, even in
the largest fluctuations. However, preliminary numerical experiments and the
theoretical estimates in the special case of the critical dynamics with
superdiffusion suggest the existence of infinitely many Poincar\'e recurrences
to the initial state and beyond. This is a new interesting phenomenon to be
farther studied together with some other open questions. Relation of this
particular example of nonequilibrium steady state to a long-standing persistent
controversy over statistical 'irreversibility', or the notorious 'time arrow',
is also discussed. In conclusion, an unsolved problem of the origin of the
causality 'principle' is touched upon.Comment: 21 pages, 7 figure
Survival, extinction and approximation of discrete-time branching random walks
We consider a general discrete-time branching random walk on a countable set
X. We relate local, strong local and global survival with suitable inequalities
involving the first-moment matrix M of the process. In particular we prove
that, while the local behavior is characterized by M, the global behavior
cannot be completely described in terms of properties involving M alone.
Moreover we show that locally surviving branching random walks can be
approximated by sequences of spatially confined and stochastically dominated
branching random walks which eventually survive locally if the (possibly
finite) state space is large enough. An analogous result can be achieved by
approximating a branching random walk by a sequence of multitype contact
processes and allowing a sufficiently large number of particles per site. We
compare these results with the ones obtained in the continuous-time case and we
give some examples and counterexamples.Comment: 32 pages, a few misprints have been correcte
Evaluating the effectiveness of using personal tailored risk information and taster sessions to increase the uptake of smoking cessation services: study protocol for a randomised controlled trial
Although government-funded specialist smoking cessation services in England offer advice and support to smokers motivated to quit, only a small proportion of smokers make use of this service. Evidence suggests that if smokers are proactively and personally invited to use services, use will be higher than with a standard referral made by health professionals. Computer-based systems generating personalised tailored communications also have the potential to engage with a larger proportion of the smoking population. In this study smokers are proactively invited to use the NHS Stop Smoking Service (SSS), with a personal computer-tailored letter and the offer of a no-commitment introductory session designed to give more information about the service. The primary objective is to assess the relative effectiveness on attendance at the NHS SSS, of proactive recruitment by a brief personal letter, tailored to individual characteristics, and invitation to a taster session, over a standard generic letter advertising the service
Costâeffectiveness of personal tailored risk information and taster sessions to increase the uptake of the NHS stop smoking services: the Start2quit randomized controlled trial
AIMS: To assess the costâeffectiveness of a twoâcomponent intervention designed to increase attendance at the NHS Stop Smoking Services (SSSs) in England. DESIGN: Costâeffectiveness analysis alongside a randomized controlled trial (Start2quit). SETTING: NHS SSS and general practices in England. PARTICIPANTS: The study comprised 4384 smokers aged 16 years or more identified from medical records in 99 participating practices, who were motivated to quit and had not attended the SSS in the previous 12 months. INTERVENTION AND COMPARATOR: Intervention was a personalized and tailored letter sent from the general practitioner (GP) and a personal invitation and appointment to attend a taster session providing information about SSS. Control was a standard generic letter from the GP advertising SSS and asking smokers to contact the service to make an appointment. MEASUREMENTS: Costs measured from an NHS/personal social services perspective, estimated health gains in qualityâadjusted lifeâyears (QALYs) measured with EQâ5D and incremental cost per QALY gained during both 6 months and a lifeâtime horizon. FINDINGS: During the trial period, the adjusted mean difference in costs was ÂŁ92 [95% confidence interval (CI) = âÂŁ32 to âÂŁ216) and the adjusted mean difference in QALY gains was 0.002 (95% CI = â0.001 to 0.004). This generates an incremental cost per QALY gained of ÂŁ59â401. The probability that the tailored letter and taster session is more costâeffective than the generic letter at 6 months is never above 50%. In contrast, the discounted lifeâtime healthâcare cost was lower in the intervention group, while the lifeâtime QALY gains were significantly higher. The probability that the intervention is more costâeffective is more than 83% using a ÂŁ20â000â30â000 per QALYâgained decisionâmaking threshold. CONCLUSIONS: An intervention designed to increase attendance at the NHS Stop Smoking Services (tailored letter and taster session in the services) appears less likely to be costâeffective than a generic letter in the short term, but is likely to become more costâeffective than the generic letter during the long term
Pre-service teachersâ engagement in a cross-curricular television news project: impact on professional identity
This paper focuses on the impact of pre-service teachersâ engagement in the annual BBC News School Report project on their emerging professional identity and on the evidence they provide as part of the process of becoming qualified. The research reported on is drawn from three years of enquiry. Respondents included pre-service teachers themselves, their tutors as representatives of teacher education providers and their mentors as representatives of schools in which they were placed. The methodological approach was interpretative and phenomenological with qualitative and quantitative data being analysed for emergent themes. Two years of evaluations were followed by a third year in which a set of case studies were developed. The research showed that professional identity is enhanced through being in a leading role in respect of curriculum and working with other staff. Through engagement in such projects, this paper moots that preservice teachers develop richer evidence of emerging professionalism as defined by standards of initial teacher training. Moreover, self-perception of role was modified to one in which they saw themselves, and were seen, as equals to qualified staff rather than subservient to or dependent on them. A new more equal power relationship developed as they took on responsibility for the project. Preservice teachersâ move to become full members of the professional community for which they are training was accelerated
Lâintelligence comme dispositif de pouvoir
O objetivo do presente estudo, é compreender a inscrição
individual e social de discursos usados por alunas, em diferentes
posiçÔes de classe social, para explicar os seus sucessos académicos.
Foram entrevistadas 19 alunas portuguesas do 11Âș ano de escolaridade
provenientes das classes trabalhadora e alta, com elevado rendimento
académico. A Anålise Foucaudiana do Discurso aponta para a importùncia
da posição de classe na construção dos sujeitos relativamente à sua
conceção de inteligĂȘncia e ao sucesso escolar. A inteligĂȘncia funciona
assim como um dispositivo de poder que regula as relaçÔes entre sujeitos
de diferentes classes sociais.The objective of the present study is to understand the
individual and social inscription of discourses used by students from
different social class positions to explain their academic achievement.
Nineteen portuguese students, from the 11th grade, with high academic
performance and from the working class and upper class, in northern
Portugal, were interviewed. Foucauldian Discourse Analysis points to
the importance of class position in the construction of the subjects with
respect to their concept of intelligence and academic success. Intelligence
works as a power device that regulates the relations among subjects of
different social classes.Lâobjectif de cette Ă©tude est de comprendre lâinscription
individuelle et sociale des discours utilisés par les apprenantes dans
différentes positions de classe sociale, pour expliquer leurs succÚs
académiques. Dix-neuf apprenantes portugaises de la 11Úme année de
scolaritĂ© dotĂ©es dâun bon rendement scolaire issues des classes ouvriĂšres
et moyennes supĂ©rieures ont Ă©tĂ© interviewĂ©es. LâAnalyse Foucaldienne
du Discours souligne lâimportance de la position de classe dans la
construction des sujets concernant leur conception de lâintelligence
et de la rĂ©ussite scolaire. Lâintelligence fonctionne aussi bien en tant
que dispositif de pouvoir qui régit les relations entre les individus de
différentes classes sociales.(undefined
Drawing Boundaries
In âOn Drawing Lines on a Mapâ (1995), I suggested that the different ways we have of drawing lines on maps open up a new perspective on ontology, resting on a distinction between two sorts of boundaries: fiat and bona fide. âFiatâ means, roughly: human-demarcation-induced. âBona fideâ means, again roughly: a boundary constituted by some real physical discontinuity. I presented a general typology of boundaries based on this opposition and showed how it generates a corresponding typology of the different sorts of objects which boundaries determine or demarcate. In this paper, I describe how the theory of fiat boundaries has evolved since 1995, how it has been applied in areas such as property law and political geography, and how it is being used in contemporary work in formal and applied ontology, especially within the framework of Basic Formal Ontology
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