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

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

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

Strong Reduction of the Field-Dependent Microwave Surface Resistance in YBCO with BaZrO_3 Inclusions

We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa$_2$Cu$_3$O$_{7-\delta}$ films on SrTiO$_3$ substrates. BaZrO$_3$ crystallites were included in the films using composite targets containing BaZrO$_3$ inclusions with mean grain size smaller than 1 $\mu$m. X-ray diffraction showed single epitaxial relationship between BaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-\delta}$. The effective surface resistance was measured at 47.7 GHz for 60$< T <$90 K and 0$< \mu_0H <$0.8 T. The magnetic field had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-\delta}$ and YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$, while for $\mu_0H=$0 only a reduction of $T_c$ in the YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$ film was observed, consistent with dc measurements. At low enough $T$, in moderate fields YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$ exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO$_3$ inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO$_3$ inclusions determine very steep pinning potentials.Comment: LaTeX, 6 pages, 4 figures, uses jpconf.cls and jpconf11.clo class files, talk given at EUCAS 2007, submitted to J. Phys.: Conf. Serie

When Your “Problem” Becomes Mine: Adult Female Siblings’ Perspectives of Having a Brother with a Disability

This study is a retrospective look at adult female siblings’ perspectives of their childhoods and present identities based on having a brother with a disability. This paper focuses on siblings’ experiences within educational and public domains, and how such experiences have shaped their personalities and career choices. Qualitative findings were organized into four central themes, each with subcate-gories: (a) Siblings’ Fear and Worries, in-school bullying, school friends coming to the home; (b) Anger and Resentment, towards the perceived advantages of hav-ing a disability, towards teachers; (c) Positive Impacts, acceptance of others in varied public contexts, advocacy; and (d) Career Choices, entering the teaching profession. Findings are discussed in relation to both the emotional and educa-tional significance for siblings

New aspects of microwave properties of Nb in the mixed state

We present a study of the frequency dependence of the vortex dynamics in a conventional superconductor. We have employed a swept-frequency, Corbino-disk technique to investigate the temperature (3.6K-Tc) and high-field (from Hc2/2 to Hc2) microwave complex resistivity in Nb thin (20-40 nm) films as a function of the frequency (1-20 GHz). We have found several previously unnoticed features: (i) a field-dependent depinning frequency in the GHz range; (ii) deviations from the accepted frequency dependence, that can be ascribed to some kind of vortex creep; (iii) the presence of switching phenomena, reminiscent of vortex instabilities. We discuss the possible origin of the features here reported.Comment: 5 pages, 3 figures, presented at VORTEX VI Conference, to appear on Physica

Vortex state microwave resistivity in Tl-2212 thin films

We present measurements of the field induced changes in the 47 GHz complex resistivity, $\Delta \tilde \rho(H,T)$, in Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+x}$ (TBCCO) thin films with $T_{c}\simeq$ 105 K, prepared on CeO$_{2}$ buffered sapphire substrates. At low fields ($\mu_{0}H<$10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level $\Delta \tilde \rho(H,T)$ exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property $\Delta \tilde \rho(H,T)=\Delta \tilde \rho[H/H^{*}(T)]$, where the scaling field $H^{*}(T)$ maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.Comment: 8 pages, 3 figures, proceedings of 9th HTSHFF, accepted for publication on J. Supercon