Theory of measurements of electrodynamic properties in anisotropic superconductors in tilted magnetic fields. Part I: flux flow and Campbell regimes

The vortex dynamics of uniaxial anisotropic superconductors in magnetic fields applied with arbitrary orientation is theoretically studied. Focus is on the model for electrical transport experiments in the linear regime. Relevant vortex parameters, like the viscous drag, the vortex mobility and pinning constant (with point pins), together with the flux flow and Campbell resistivities, are derived in tensor form, in the very different free flux flow and pinned Campbell regimes. The applicability to the various tensor quantities of the well-known scaling laws for the angular dependence on the field orientation is commented. Moreover, it is shown that the experiments do not generally yield the intrinsic values of the anisotropic viscosity and pinning constant. Explicit expressions relating measured and intrinsic quantities are given.Comment: 13 pages, 3 figure

Theory of measurements of electrodynamic properties in anisotropic superconductors in tilted magnetic fields. Part II: high frequency regimes

The model for high frequency electrodynamics in anisotropic type-II superconductors in the vortex state is studied considering arbitrary orientations between the applied field, the applied current and the anisotropy axis. An anisotropic treatment is provided for the vortex dynamics, taking into account all the phenomena relevant at high frequency, which include flux flow, pinning and creep. The coupling between vortex motion and high frequency currents is included, providing an entirely tensor model of the electromagnetic response to high frequency fields. Examples of data analysis of angular measurements are presented, showing how to derive the angular dependence of the material properties from the measured anisotropic response. Finally, the expression of the measured angle-dependent surface impedance in the largely used thin film geometry is computed.Comment: 14 pages, 3 figure

The superconducting proposal for the CS magnet system of FAST: a preliminary analysis of the heat load due to AC losses

FAST (Fusion Advanced Studies Torus), the Italian proposal of a Satellite Facility to ITER, is a compact tokamak (R$_0$ = 1.82 m, a = 0.64 m, triangularity $\delta$ = 0.4) able to investigate non-linear dynamics effects of $\alpha$-particle behavior in burning plasmas and to test technical solutions for the first wall/divertor directly relevant for ITER and DEMO. Currently, ENEA is investigating the feasibility of a superconducting solution for the magnet system. This paper focuses on the analysis of the CS (Central Solenoid) magnet thermal behavior. In particular, considering a superconducting solution for the CS which uses the room available in the resistive design and referring to one of the most severe scenario envisaged for FAST, the heat load of the CS winding pack due to AC losses is preliminarily evaluated. The results provide a tentative baseline for the definition of the strand requirements and conductor design, that can be accepted in order to fulfil the design requirements.Comment: 15 pages, 5 figure

Analysis of the measurements of anisotropic a.c. vortex resistivity in tilted magnetic fields

Measurements of the high-frequency complex resistivity in superconductors are a tool often used to obtain the vortex parameters, such as the vortex viscosity, the pinning constant and the depinning frequency. In anisotropic superconductors, the extraction of these quantities from the measurements faces new difficulties due to the tensor nature of the electromagnetic problem. The problem is specifically intricate when the magnetic field is tilted with respect to the crystallographic axes. Partial solutions exist in the free-flux-flow (no pinning) and Campbell (pinning dominated) regimes. In this paper we develop a full tensor model for the vortex motion complex resistivity, including flux-flow, pinning, and creep. We give explicit expressions for the tensors involved. We obtain that, despite the complexity of the physics, some parameters remain scalar in nature. We show that under specific circumstances the directly measured quantities do not reflect the true vortex parameters, and we give procedures to derive the true vortex parameters from measurements taken with arbitrary field orientations. Finally, we discuss the applicability of the angular scaling properties to the measured and transformed vortex parameters and we exploit these properties as a tool to unveil the existence of directional pinning.Comment: 21 pages, 3 figures. arXiv admin note: text overlap with arXiv:1402.316

Fundamentals, International Role of Euro and 'Framing' of Expectations: What are the Determinants of the Dollar/Euro Exchange Rate?

The predictions made by economists of the value of the euro prior to its introduction were essentially based on the expected portfolio adjustment resulting from the role that it might play as an international currency. As a result, most analysts agreed that the euro would be a strong currency, appreciating against the US dollar. The first years of life of the ‘virtual’ euro contradicted such a forecast. Economists therefore abandoned predictions based on the euro as a ‘global’ money and directed their focus almost exclusively towards traditional, ‘fundamentals-based’ explanations. Among these explanations, several authors mentioned the unsatisfactory structural and institutional set up of the EMU. Nevertheless, later on, when the euro started appreciating, a different set of fundamentals had to be isolated in order to account for such behaviour. It is possible to argue, then, that the EMU economic structure and institutions are, or at least are currently perceived as, capable of supporting a strong euro, which plays the role of international money. ‘Framing’ of expectations, however, still keeps driving the behaviour of the exchange rate, so that the same structural and institutional set up may be subject to different evaluations, depending on the particular state of expectations of the international currency markets. Finally, since the available evidence suggests that the euro is starting to play an international role, I argue that the ‘international money’ and the ‘framing’ of expectations approaches explain the behaviour of the dollar/euro exchange rate better than the ‘fundamentals' one.Euro; Dollar; Fundamentals; International currency; Portfolio adjustment

Effect of nanosize BaZrO3 inclusions on vortex parameters in YBaCuO

We report on the field dependence of the microwave complex resistivity data in YBa$_2$Cu$_3$O$_{7-x}$/BaZrO$_3$ films grown by PLD at various BaZrO$_3$ content. The data, analyzed within a recently developed general framework for the mixed-state microwave response of superconductors, yield the field dependence of the fluxon parameters such as the vortex viscosity and the pinning constant. We find that pinning undergoes a change of regime when the BaZrO$_3$ content in the target increases from 2.5 mol.% to 5 mol.%. Simultaneously, the vortex viscosity becomes an increasing function of the applied magnetic field. We propose a scenario in which flux lines are pinned as bundles, and a crossover from dilute point pins to dense c-axis correlated defects takes place between 2.5 and 5 mol.% in the BZO concentration. Our data are inconsistent with vortices occupying mainly the BaZrO$_3$ sites at low fields, and suggest instead that vortices occupy both BaZrO$_3$ sites and interstitials in the YBa$_2$Cu$_3$O$_{7-x}$ matrix, even at low fields.Comment: Presented at EUCAS 2009, to be published in J. Phys.:Conf. Serie

Strategic interactions between monetary and fiscal authorities in a monetary union

In this paper we extend Nordhaus’ (1994) results to an environment which may represent the current European situation, characterised by a single monetary authority and several fiscal bodies. We show that: a) co-operation among national fiscal authorities is welfare improving only if they also co-operate with the central bank; b) when this condition is not satisfied, fiscal rules, as those envisaged in the Maastricht Treaty and in the Stability and Growth Pact, may work as co-ordination devices that improve welfare; c) the relationship between several treasuries and a single central bank makes the fiscal leadership solution collapse to the Nash one, so that, contrary to Nordhaus (1994) and Dixit and Luisa Lambertini (2001), when moving from the Nash to the Stackelberg solution, fiscal discipline no longer obtains. Also in this case we thus argue in favour of fiscal rules in a monetary union.Fiscal and monetary policy co-ordination; monetary union;international fiscal issues

Vortex state microwave response in superconducting cuprates and MgB2_2

We investigate the physics of the microwave response in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, SmBa$_{2}$Cu$_{3}$O$_{7-\delta}$ and MgB$_{2}$ in the vortex state. We first recall the theoretical basics of vortex-state microwave response in the London limit. We then present a wide set of measurements of the field, temperature, and frequency dependences of the vortex state microwave complex resistivity in superconducting thin films, measured by a resonant cavity and by swept-frequency Corbino disk. The combination of these techniques allows for a comprehensive description of the microwave response in the vortex state in these innovative superconductors. In all materials investigated we show that flux motion alone cannot take into account all the observed experimental features, neither in the frequency nor in the field dependence. The discrepancy can be resolved by considering the (usually neglected) contribution of quasiparticles to the response in the vortex state. The peculiar, albeit different, physics of the superconducting materials here considered, namely two-band superconductivity in MgB$_{2}$ and superconducting gap with lines of nodes in cuprates, give rise to a substantially increased contribution of quasiparticles to the field-dependent microwave response. With careful combined analysis of the data it is possible to extract or infer many interesting quantities related to the vortex state, such as the temperature-dependent characteristic vortex frequency and vortex viscosity, the field dependence of the quasiparticle density, the temperature dependence of the $\sigma$-band superfluid density in MgB$_{2}$Comment: 51 pages, 27 figures, to appear as a book chapter (Nova Science

Microwave vortex dynamics in Tl-2212 thin films

We present measurements of the effective surface impedance changes due to a static magnetic field, \Delta Z(H,T)=\Delta R(H,T)+\rmi \Delta X(H,T), in a Tl-2212 thin film with $T_c>$ 103 K, grown on a CeO$_2$ buffered sapphire substrate. Measurements were performed through a dielectric resonator operating at 47.7 GHz, for temperatures 60 K$\leq T<T_c$ and magnetic fields $\leq0.8$ T. We observe exceptionally large field induced variations and pronounced super-linear field dependencies in both $\Delta R(H)$ and $\Delta X(H)$ with $\Delta X(H)>\Delta R(H)$ in almost the whole $(H,T)$ range explored. A careful analysis of the data allows for an interpretation of these results as dominated by vortex dynamics. In the intermediate-high field range we extract the main vortex parameters by resorting to standard high frequency model and by taking into proper account the creep contribution. The pinning constant shows a marked decrease with the field which can be interpreted in terms of flux lines softening associated to an incipient layer decoupling. Small vortex viscosity, by an order of magnitude lower than in Y-123 are found. Some speculations about these findings are provided.Comment: pdfTeX, 4 pages, 3 figures, VORTEX 2007 proceedings, to appear in Physica