Compact Central WENO Schemes for Multidimensional Conservation Laws

We present a new third-order central scheme for approximating solutions of systems of conservation laws in one and two space dimensions. In the spirit of Godunov-type schemes,our method is based on reconstructing a piecewise-polynomial interpolant from cell-averages which is then advanced exactly in time. In the reconstruction step, we introduce a new third-order as a convex combination of interpolants based on different stencils. The heart of the matter is that one of these interpolants is taken as an arbitrary quadratic polynomial and the weights of the convex combination are set as to obtain third-order accuracy in smooth regions. The embedded mechanism in the WENO-like schemes guarantees that in regions with discontinuities or large gradients, there is an automatic switch to a one-sided second-order reconstruction, which prevents the creation of spurious oscillations. In the one-dimensional case, our new third order scheme is based on an extremely compact point stencil. Analogous compactness is retained in more space dimensions. The accuracy, robustness and high-resolution properties of our scheme are demonstrated in a variety of one and two dimensional problems.Comment: 24 pages, 5 figure

Financial stability issues arising from current risk premia configurations and dynamics.

One of the major financial stability issues, we currently need to focus upon, relates to risk premia dynamics. Recent market turbulences have indeed, once more, shown that such dynamics can change quickly. True, the well known long term interest rates conundrum, highlighted by M. Greenspan, might appear less crucial an issue in 2006 than in 2004/2005. But several other market configurations appear puzzling. First, long term interest rates remain relatively low at this stage of the cycle. However, the jury is still out regarding the question, whether low term premia will be permanent. Second, credit markets remain buoyant, with very aggressive pricing. Low credit spreads often only match average historical rates of default without providing additional risk premium. A benign scenario can obviously account for such a configuration, but this is only one scenario among many other more risky ones. Third, the contrast with high equity risk premia is puzzling. In the past, at the specific time when excesses from previous bubbles had to be wiped out without too much damage to economic growth, the decrease in credit spreads in the face of high equity risk premia might have served financial stability. But, currently and for the longer term, these advantages seem less clear. Higher credit risk premia premia might indeed bode better for future financial stability. Fourth, recent market tensions have highlighted some vulnerabilities. This should lead us i) to remain cautious in our assessment of potential future short term market dynamics and ii) put on top of our agenda an enhancement of the resilience in face of longer term vulnerabilities. Risk premia move constantly because of changes in fundamental factors as well as in risk appetite. This often smoothly contributes to financial stability. However some characteristics of the present environment could make such adjustments challenging in the future. First, less accommodative global liquidity makes financial market dynamics more complex to predict. Second, bank lending standards might be looser than in a steady state. If credit standards are finally tightened more strongly, this would benefit financial stability in the medium term. But the transition includes challenges for borrowers which have relied on cheap credit in recent years. Third, cyclical lows on financial market volatility might be behind us. Fourth, financial institutions risk management often relies on fragile diversification related assumptions. This makes sense in a stable environment, when correlations do not change quickly. However, diversification might not protect as expected in time of stress. And a quick unwinding of positions is not always possible. Fifth, model risk remains a field where improvements are needed in order to strengthen financial stability over a full financial cycle. The main market related risks, in a context of less ample global liquidity, include changes in risk aversion and risk premia, with the potential for a breakdown in traditional correlations. If significant portfolio reallocation flows take place, market liquidity issues have the potential to transform vulnerabilities into financial instability. These risks might not be perfectly captured by some risk models currently used by financial institutions (including stress-tests). The main financial system related risks include the scenario in which financial institutions would not decrease sufficiently their search for yield in the future, in an environment where higher expected returns could be subject to increased risks. We can certainly envisage an ideal exit scenario in term of financial stability. However, risk premia are not controlled by any single institution and are subject to complex and uncertain dynamics. In fact, financial market dynamics might be more challenging in the future than in recent years. Financial institutions should therefore continuously enhance their resiliency in face of the variety of potential scenarii. The various conditions which can prevail during a full economic and financial cycle need to be seriously taken into account. This means that financial institutions should continue to ensure to avail of sharp risk management practices, emphasising the cases when risks (unfortunately) materialise over the short term, even for those risks which are often relegated to long term scenarii. They should also increase their sensitivity to the potential difficulty of a quick unwinding of some positions in difficult market conditions. The risks, that we highlighted, certainly warrant i), at the micro level, a monitoring of financial institutions risk management and ii), at the macro level, of the dynamics behind potential crowded trades. It is the role of central banks and all public authorities, individually and in co-operation, to foster a risk cycle management oriented attitude, which allows combining the advantages of real and financial innovation with financial stability and sustainable growth.

Low temperature irreversibility induced by thermal cycles on two prototypical phase separated manganites

We have studied the effect of irreversibility induced by repeated thermal cycles on the electric transport and magnetization of polycrystalline samples of La0.5Ca0.5MnO3 and La0.325Pr0.3Ca0.375MnO3. An increase of the resistivity and a decrease of the magnetization at different temperature ranges after cycling is obtained in the temperature range between 300 K and 30 K. Both compounds are known to exhibit intrinsic submicrometric coexistence of phases and undergo a sequence of phase transitions related to structural changes. Changes induced by thermal cycling can be partially inhibited by applying magnetic field and hydrostatic pressure. Our results suggest that the growth and coexistence of phases with different structures gives rise to microstructural tracks and strain accommodation, producing the observed irreversibility. Irrespective of the actual ground state of each compound, the effect of thermal cycling is towards an increase of the amount of the insulating phase in both compounds.Comment: to appear in Journal of Alloys and Compounds (2003

Simultaneous electric and magnetic field induced nonvolatile memory

We investigate the electric field induced resistive switching effect and magnetic field induced fraction enlargement on a polycrystalline sample of a colossal magnetoresistive compound displaying intrinsic phase coexistence. Our data show that the electric effect (presumably related to the presence of inhomogeinities) is present in a broad temperature range(300 to 20 K), being observable even in a mostly homogeneous ferromagnetic state. In the temperature range in which low magnetic field determines the phase coexistence fraction, both effects, though related to different mechanisms, are found to determine multilevel nonvolatile memory capabilities simultaneously.Comment: Submited to AP

The electrical current effect in phase separated La5/8-yPryCa3/8MnO3: Charge order melting vs. Joule heating

We have studied the effect of electric field on transport properties of the prototypical phase separated manganite La5/8-yPryCa3/8MnO3 with y=0.34. Our results show that the suggested image in which the charge ordered state is melted by the appliance of an electric current and/or voltage has to be revised. We were able to explain the observed resistivity drop in terms of an artifact related to Joule heating and the particular hysteresis that the system under study display, common to many other phase separated manganites.Comment: 2 figures. Accepted in J. Appl. Phy

Gain, phase and frequency stability of DSS-42 and DSS-43 vor Voyage Uranus encounter

Theoretically rigorous definitions are derived of such parameters as RF signal path length, phase delay, and phase/frequency stability in a Cassegrainian antenna applicable to a narrow bandwidth channel, as well as algorithms for evaluating these parameters. This work was performed in support of the Voyager spacecraft encounter with Uranus in January 1986. The information was needed to provide Voyager/Uranus radio science researchers with a rotational basis for deciding the best strategy to operate the three antennas involved during the crucial 5-hour occultation period of the encounter. Such recommendations are made at the end of the article

Antiperiodic dynamical 6-vertex model by separation of variables II: Functional equations and form factors

We pursue our study of the antiperiodic dynamical 6-vertex model using Sklyanin's separation of variables approach, allowing in the model new possible global shifts of the dynamical parameter. We show in particular that the spectrum and eigenstates of the antiperiodic transfer matrix are completely characterized by a system of discrete equations. We prove the existence of different reformulations of this characterization in terms of functional equations of Baxter's type. We notably consider the homogeneous functional $T$-$Q$ equation which is the continuous analog of the aforementioned discrete system and show, in the case of a model with an even number of sites, that the complete spectrum and eigenstates of the antiperiodic transfer matrix can equivalently be described in terms of a particular class of its $Q$-solutions, hence leading to a complete system of Bethe equations. Finally, we compute the form factors of local operators for which we obtain determinant representations in finite volume.Comment: 52 page

Correlation between magnetic and transport properties of phase separated La0.5_{0.5}Ca0.5_{0.5}MnO3_{3}

The effect of low magnetic fields on the magnetic and electrical transport properties of polycrystalline samples of the phase separated compound La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is studied. The results are interpreted in the framework of the field induced ferromagnetic fraction enlargement mechanism. A fraction expansion coefficient af, which relates the ferromagnetic fraction f with the applied field H, was obtained. A phenomenological model to understand the enlargement mechanism is worked out.Comment: 3 pages, 3 figures, presented at the Fifth LAW-MMM, to appear in Physica B, Minor change

The electron plasma - Experiment theory and applications

Production, stability, and control of pure electron and electron rich plasmas - crossed field electron bea

Measurement of electric fields in the ionosphere Final report, Aug. 1966 - Sep. 1969

Measurement of electric fields in environmen