An exact effective Hamiltonian for a perturbed Landau level

Considers the effect of a scalar potential V (x, y) on a Landau level in two dimensions. An exact effective Hamiltonian is derived which describes the effect of the potential on a single Landau level, expressed as a power series in V/Ec, where Ec is the cyclotron energy. The effective Hamiltonian can be represented as a function H (x, p) in a one-dimensional phase space. The function H (x, p) resembles the potential V (x, y): when the area of a flux quantum is much smaller than the square of the characteristic length scale of V, then H approximately=V. Also H (x, p) retains the translational and rotational symmetries of V(x, y) exactly, but reflection symmetries are not retained beyond the lowest order of the perturbation expansion

Invariance of the correlation energy at high density and large dimension in two-electron systems

We prove that, in the large-dimension limit, the high-density correlation energy \Ec of two opposite-spin electrons confined in a $D$-dimensional space and interacting {\em via} a Coulomb potential is given by \Ec \sim -1/(8D^2) for any radial confining potential $V(r)$. This result explains the observed similarity of \Ec in a variety of two-electron systems in three-dimensional space.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let

Half-life of the electron-capture decay of 97Ru: Precision measurement shows no temperature dependence

We have measured the half-life of the electron-capture (ec) decay of 97Ru in a metallic environment, both at low temperature (19K), and also at room temperature. We find the half-lives at both temperatures to be the same within 0.1%. This demonstrates that a recent claim that the ec decay half-life for 7Be changes by $0.9% +/- 0.2% under similar circumstances certainly cannot be generalized to other ec decays. Our results for the half-life of 97Ru, 2.8370(14)d at room temperature and 2.8382(14)d at 19K, are consistent with, but much more precise than, previous room-temperature measurements. In addition, we have also measured the half-lives of the beta-emitters 103Ru and 105Rh at both temperatures, and found them also to be unchanged.Comment: 6 pages, 6 figure

Direct measurement of electrocaloric effect in lead-free Ba(SnxTi1-x)O3 ceramics

In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance

Nonstandard electroconvection in a bent-core oxadiazole material

Electroconvection (EC) phenomena have been investigated in the nematic phase of a bent-core oxadiazole material with negative dielectric anisotropy and a frequency dependent conductivity anisotropy. The formation of longitudinal roll (LR) patterns is one of the predominant features observed in the complete frequency and voltage range studied. At voltages much above the LR threshold, various complex patterns such as the "crisscrossed" pattern, bimodal varicose, and turbulence are observed. Unusually, the nonstandard EC (ns-EC) instability in this material, is observed in a regime in which we measure the dielectric and conductivity anisotropies to be negative and positive respectively. A further significant observation is that the EC displays distinct features in the high and low temperature regimes of the nematic phase, supporting an earlier report that EC patterns could distinguish between regions that have been reported as uniaxial and biaxial nematic phases

Spinning cosmic strings: a general class of solutions

In this work, we give a general class of solutions of the spinning cosmic string in Einstein's theory of gravity. After treating same problem in Einstein Cartan (EC) theory of gravity, the exact solution satisfying both exterior and interior space-times representing a spin fluid moving along the symmetry axis is presented in the EC theory. The existence of closed timelike curves in this spacetime are also examined

Temporal response to harmonic driving in electroconvection

The temporal evolution of the spatially periodic electroconvection (EC) patterns has been studied within the period of the driving ac voltage by monitoring the light intensity diffracted from the pattern. Measurements have been carried out on a variety of nematic systems, including those with negative dielectric and positive conductivity anisotropy, exhibiting "standard EC" (s-EC), those with both anisotropies negative exhibiting "non-standard EC" (ns-EC), as well as those with the two anisotropies positive. Theoretical predictions have been confirmed for stationary s-EC and ns-EC patterns. Transitions with Hopf bifurcation have also been studied. While traveling had no effect on the temporal evolution of dielectric s-EC, traveling conductive s-EC and ns-EC patterns exhibited a substantially altered temporal behavior with a dependence on the Hopf frequency. It has also been shown that in nematics with both anisotropies positive, the pattern develops and decays within an interval much shorter than the period, even at relatively large driving frequencies.Comment: 19 pages, 5 figure

EC budget support: thumbs up or down?

Budget support is a form of macro-economic financial assistance that is designed to prioritise the fight against poverty and the achievement of the Millennium Development Goals (MDGs). The European Commission (EC) is providing more and more budget support under the Cotonou Partnership Agreement (CPA), which structures the provision of assistance by the European Union (EU) to the African, Caribbean and Pacific (ACP) countries. The EC also provides macro-economic support through its cooperation agreements with a number of non-ACP countries, for which it is now also introducing budget support. During the course of its work in the ACP regions, the European Centre for Development Policy Management (ECDPM) was approached by parliamentarians, representatives of non-governmental organisations and partner government officials, all of whom had certain questions to ask about the background to and operation of budget support. These actors regard budget support as an increasingly important mechanism for development cooperation, and one that has potentially far-reaching implications for their work. At the same time, it is a relatively new mechanism, is poorly understood and is driven by a relatively small group of experts in aid agencies and partner country finance or planning ministries. We have prepared this Discussion Paper in response to these queries. Our aim is to give policy-makers and practitioners in-depth information on the nature and potential of, and the constraints associated with, budget support as a development cooperation modality. The paper explains the complex nature of budget support and its implications for the actors involved, as well as highlighting issues that have a bearing on its success. Budget support creates opportunities that are not always fully appreciated by its critics. At the same time, it creates risks which tend to be downplayed by its supporters. Against this background, this paper also aims to contribute to this intensely debated issue by providing a balanced view for those seeking to understand its complexities. As this is intended to be a relatively brief paper, it is limited to key policy issues and the challenges arising from recent experiences with budget support. For this reason, we have not devoted much space to discussing the issue of sector budget support. We advise the reader to consult the bibliography for more detailed information on the practice and technicalities of this rapidly evolving mechanism. Finally, it is important to recognise that the move towards budget support has aroused many questions among partner government officials, donor agencies, consultants and academic institutes, which tend to feed into the production of specialist documents assessing approaches and experiences. For this reason, this paper can do no more than to describe the current state of a rapidly evolving debate and practice. We hope to be able to follow up this initial study and produce further publications on specific issues, tailored to the needs of individual ACP audiences, as part of our future work