Lifetimes of impurity states in crossed magnetic and electric fields

We study the quantum dynamics of localized impurity states created by a point interaction for an electron moving in two dimensions under the influence of a perpendicular magnetic field and an in-plane weak electric field. All impurity states are unstable in presence of the electric field. Their lifetimes are computed and shown to grow in a Gaussian way as the electric field tends to zero.Comment: 13 pages, no figures, submitted to J. Math. Phy

Formulas and flexibility in trade negotiations : sensitive agricultural products in the WTO's Doha agenda

Many trade negotiations involve large cuts in high tariffs, with flexibilities allowing much smaller cuts for an agreed number of politically-sensitive products. The effects of these flexibilities on market access opportunities are difficult to predict, creating particular problems for developing countries in assessing whether to support a proposed agreement. Some widely-used ad hoc approaches to identifying likely sensitive products -- such as the highest-bound-tariff rule -- suggest that the impacts of a limited number of such exceptions on average tariffs and on market access are likely to be minor. This paper uses a rigorous specification based on the apparent objectives of policy makers in setting the pre-negotiation tariff. Applying this approach with detailed data allows the authors to assess the implications of sensitive-product provisions for average agricultural tariffs, economic welfare, and market access under the Doha negotiations. The authors conclude that highest-tariff rules are likely to seriously underestimate the impacts on average tariffs, and that treating even 2 percent of tariff lines as sensitive is likely to have a sharply adverse impact on economic welfare. The impacts on market access are also adverse, but much smaller, perhaps reflecting the mercantilist focus of the negotiating process.Free Trade,International Trade and Trade Rules,Markets and Market Access,Debt Markets,Trade Policy

Domino tilings and related models: space of configurations of domains with holes

We first prove that the set of domino tilings of a fixed finite figure is a distributive lattice, even in the case when the figure has holes. We then give a geometrical interpretation of the order given by this lattice, using (not necessarily local) transformations called {\em flips}. This study allows us to formulate an exhaustive generation algorithm and a uniform random sampling algorithm. We finally extend these results to other types of tilings (calisson tilings, tilings with bicolored Wang tiles).Comment: 17 pages, 11 figure

Irreversibility and Chaos in Active Particle Suspensions

Active matter has been the object of huge amount of research in recent years for its important fundamental and applicative properties. In this paper we investigate active suspensions of micro-swimmers through direct numerical simulation, so that no approximation is made at the continuous level other than the numerical one. We consider both pusher and puller organisms, with a spherical or ellipsoidal shape. We analyse the velocity and the characteristic scales for an homogeneous two-dimensional suspension and the effective viscosity under shear. We bring evidences that the complex features displayed are related to a spontaneous breaking of the time-reversal symmetry. We show that chaos is not a key ingredient, whereas a large enough number of interacting particles and a non-spherical shape are needed to break the symmetry and are therefore at the basis of the phenomenology. Our numerical study also shows that pullers display some collective motion, though with different characteristics from pushers

Experiments on transformation thermodynamics: Molding the flow of heat

It has recently been shown theoretically that the time-dependent heat conduction equation is form-invariant under curvilinear coordinate transformations. Thus, in analogy to transformation optics, fictitious transformed space can be mapped onto (meta-)materials with spatially inhomogeneous and anisotropic heat-conductivity tensors in the laboratory space. On this basis, we design, fabricate, and characterize a micro-structured thermal cloak that molds the flow of heat around an object in a metal plate. This allows for transient protection of the object from heating, while maintaining the same downstream heat flow as without object and cloak.Comment: 10 pages, 4 figure

Scattering problems in elastodynamics

In electromagnetism, acoustics, and quantum mechanics, scattering problems can routinely be solved numerically by virtue of perfectly matched layers (PMLs) at simulation domain boundaries. Unfortunately, the same has not been possible for general elastodynamic wave problems in continuum mechanics. In this paper, we introduce a corresponding scattered-field formulation for the Navier equation. We derive PMLs based on complex-valued coordinate transformations leading to Cosserat elasticity-tensor distributions not obeying the minor symmetries. These layers are shown to work in two dimensions, for all polarizations, and all directions. By adaptative choice of the decay length, the deep subwavelength PMLs can be used all the way to the quasi-static regime. As demanding examples, we study the effectiveness of cylindrical elastodynamic cloaks of the Cosserat type and approximations thereof

Consequences of short interruptions of bouts walking on estimates of compliance to physical activity guidelines

Current guidelines on physical activity suggest that 30 min of moderate intensity physical activity can be accumulated in continuous bouts of at least 10 min. It has been shown by use of activity monitoring that it is difficult to achieve 10 min of completely uninterrupted walking in the free-living urban environment where we have obstacles such as roads to cross. The aim of this study was to examine the effect of short interruptions in walking on the rate of oxygen uptake (ml . kg . min(-1)) to determine if walking with short interruptions can still be considered continuous. This leads to a more meaningful understanding as to what is a physiological break in activity. This is an important consideration for measurement of physical activity especially when exploring measurement by accelerometry. In a laboratory setting a repeated measure design was used to replicate interrupted walking in urban setting. Healthy volunteers (N = 10) walked on a treadmill with walking interruptions of 10 s, 50 s and 100 s. Oxygen uptake was measured using a gas analysis system. 10 s interruptions in walking had no significant effect on the VO2 . kg . min(-1). However two breaks of 50 s or 100 s introduced into a 5 min brisk walking bout showed a significant reduction in oxygen uptake requirements and metabolic equivalent of task (MET) (p < 0.001) compared to continuous walking for the same amount of effective walking, but only the 100 s walking period could not be considered greater than 3 MET during the interval. Short periods of brisk walking interrupted by 10 s breaks can be considered continuous physical activity, but when walking is interrupted by longer breaks e.g. 50 s, there is a significant reduction in oxygen uptake requirement suggesting that it is not continuous anymore and should be considered as fragmented

The role of short-time work schemes during the global financial crisis and early recovery: A cross-country analysis

There has been a strong interest in short-time work (STW) schemes during the global financial crisis. Using data for 23 OECD countries for the period 2004 Q1 to 2010 Q4, this paper analyses the quantitative effects of STW programmes on labour market outcomes. Special attention is given to the dynamic aspects of the relationship between output shocks and labour market outcomes. The results indicate the STW raises hours flexibility by increasing the output elasticity of working time and helps to preserve jobs in the context of a recession by making employment and unemployment less elastic with respect to output. A key finding is that the timing of STW is crucial. While STW helped preserving a significant number of jobs during the crisis, its continued use during the recovery may have slowed the job-content of the recovery. By the end of 2010, the net effect of STW on employment was negligible or may even have become negative. However, the gross impact of STW on the number of jobs saved per quarter remains large and positive in the majority of countries

Vibrotactile sensitivity in active touch: effect of pressing force

An experiment was conducted to study the effects of force produced by active touch on vibrotactile perceptual thresholds. The task consisted in pressing the fingertip against a flat rigid surface that provided either sinusoidal or broadband vibration. Three force levels were considered, ranging from light touch to hard press. Finger contact areas were measured during the experiment, showing positive correlation with the respective applied forces. Significant effects on thresholds were found for vibration type and force level. Moreover, possibly due to the concurrent effect of large (unconstrained) finger contact areas, active pressing forces, and long duration stimuli, the measured perceptual thresholds are considerably lower than what previously reported in the literature