Flux noise in high-temperature superconductors

Spontaneously created vortex-antivortex pairs are the predominant source of flux noise in high-temperature superconductors. In principle, flux noise measurements allow to check theoretical predictions for both the distribution of vortex-pair sizes and for the vortex diffusivity. In this paper the flux-noise power spectrum is calculated for the highly anisotropic high-temperature superconductor Bi-2212, both for bulk crystals and for ultra-thin films. The spectrum is basically given by the Fourier transform of the temporal magnetic-field correlation function. We start from a Berezinskii-Kosterlitz-Thouless type theory and incorporate vortex diffusion, intra-pair vortex interaction, and annihilation of pairs by means of a Fokker-Planck equation to determine the noise spectrum below and above the superconducting transition temperature. We find white noise at low frequencies omega and a spectrum proportional to 1/omega^(3/2) at high frequencies. The cross-over frequency between these regimes strongly depends on temperature. The results are compared with earlier results of computer simulations.Comment: 9 pages, 4 PostScript figures, to be published in Phys. Rev.

NMR and NQR Fluctuation Effects in Layered Superconductors

We study the effect of thermal fluctuations of the s-wave order parameter of a quasi two dimensional superconductor on the nuclear spin relaxation rate near the transition temperature Tc. We consider both the effects of the amplitude fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations in weakly coupled layered superconductors. In the treatment of the amplitude fluctuations we employ the Gaussian approximation and evaluate the longitudinal relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair breaking effects, using the static pair fluctuation propagator D. The increase in 1/T1 due to pair breaking in D is overcompensated by the decrease arising from the single particle Green's functions. The result is a strong effect on 1/T1 for even a small amount of pair breaking. The phase fluctuations are described in terms of dynamical BKT excitations in the form of pancake vortex-antivortex (VA) pairs. We calculate the effect of the magnetic field fluctuations caused by the translational motion of VA excitations on 1/T1 and on the transverse relaxation rate 1/T2 on both sides of the BKT transitation temperature T(BKT)<Tc. The results for the NQR relaxation rates depend strongly on the diffusion constant that governs the motion of free and bound vortices as well as the annihilation of VA pairs. We discuss the relaxation rates for real multilayer systems where the diffusion constant can be small and thus increase the lifetime of a VA pair, leading to an enhancement of the rates. We also discuss in some detail the experimental feasibility of observing the effects of amplitude fluctuations in layered s-wave superconductors such as the dichalcogenides and the effects of phase fluctuations in s- or d-wave superconductors such as the layered cuprates.Comment: 38 pages, 12 figure

Surgery as a safe and effective treatment option for spheno-orbital meningioma: a systematic review and meta-analysis of surgical techniques and outcomes

Purpose The effectiveness and safety of surgery for spheno-orbital meningiomas remains subject of debate, as studies often describe different surgical approaches and reconstruction techniques with very heterogeneous outcomes. We aimed to systematically summarize and analyse the literature on spheno-orbital meningiomas regarding presenting symptoms, surgical techniques, outcomes and complications. Methods Studies were retrieved from eight databases. Original articles were included if in >= 5 patients presenting symptoms, surgical treatment and outcomes were described. Fixed- and random-effects meta-analysis was performed to estimate weighted percentages with 95%CIs of presenting symptoms, outcomes and complications. Results Thirty-eight articles were included describing 1486 patients. Proptosis was the most common presenting symptom (84%; 95%CI 76-91%), followed by unilateral visual acuity deficits (46%; 95%CI 40-51%) and visual field deficits (31%; 95%CI 20-43%). In 35/38 studies (92%), a pterional craniotomy was used. Decompression of the optic canal (82%) and the superior orbital fissure (66%) was most often performed, and usually dural (47%) and bony defects (76%) were reconstructed. In almost all patients, visual acuity (91%; 95%CI 86-96%), visual fields (87%; 95%CI 70-99%) and proptosis (96%; 95%CI 90-100%) improved. Furthermore, surgery showed improvement in 96% (95%CI 78-100%) for both diplopia and ophthalmoplegia. The most common surgical complications were hypesthesia (19%; 95%CI 10-30%), ptosis and diplopia (both 17%; 95%CI, respectively, 10-26% and 5-33%) and ophthalmoplegia (16%; 95%CI 10-24). Conclusion Patients with spheno-orbital meningioma usually present with proptosis or unilateral decreased visual acuity. Surgery shows to be effective in improving visual acuity and visual field deficits with mostly minor and well-tolerated complications.Ophthalmic researc

Frustrated two-dimensional Josephson junction array near incommensurability

To study the properties of frustrated two-dimensional Josephson junction arrays near incommensurability, we examine the current-voltage characteristics of a square proximity-coupled Josephson junction array at a sequence of frustrations f=3/8, 8/21, 0.382 $(\approx (3-\sqrt{5})/2)$, 2/5, and 5/12. Detailed scaling analyses of the current-voltage characteristics reveal approximately universal scaling behaviors for f=3/8, 8/21, 0.382, and 2/5. The approximately universal scaling behaviors and high superconducting transition temperatures indicate that both the nature of the superconducting transition and the vortex configuration near the transition at the high-order rational frustrations f=3/8, 8/21, and 0.382 are similar to those at the nearby simple frustration f=2/5. This finding suggests that the behaviors of Josephson junction arrays in the wide range of frustrations might be understood from those of a few simple rational frustrations.Comment: RevTex4, 4 pages, 4 eps figures, to appear in Phys. Rev.

An Asymmetric Cone Model for Halo Coronal Mass Ejections

Due to projection effects, coronagraphic observations cannot uniquely determine parameters relevant to the geoeffectiveness of CMEs, such as the true propagation speed, width, or source location. The Cone Model for Coronal Mass Ejections (CMEs) has been studied in this respect and it could be used to obtain these parameters. There are evidences that some CMEs initiate from a flux-rope topology. It seems that these CMEs should be elongated along the flux-rope axis and the cross section of the cone base should be rather elliptical than circular. In the present paper we applied an asymmetric cone model to get the real space parameters of frontsided halo CMEs (HCMEs) recorded by SOHO/LASCO coronagraphs in 2002. The cone model parameters are generated through a fitting procedure to the projected speeds measured at different position angles on the plane of the sky. We consider models with the apex of the cone located at the center and surface of the Sun. The results are compared to the standard symmetric cone model

Dynamic Scaling and Two-Dimensional High-Tc Superconductors

There has been ongoing debate over the critical behavior of two-dimensional superconductors; in particular for high Tc superconductors. The conventional view is that a Kosterlitz-Thouless-Berezinskii transition occurs as long as finite size effects do not obscure the transition. However, there have been recent suggestions that a different transition actually occurs which incorporates aspects of both the dynamic scaling theory of Fisher, Fisher, and Huse and the Kosterlitz-Thouless-Berezinskii transition. Of general interest is that this modified transition apparently has a universal dynamic critical exponent. Some have countered that this apparent universal behavior is rooted in a newly proposed finite-size scaling theory; one that also incorporates scaling and conventional two-dimensional theory. To investigate these issues we study DC voltage versus current data of a 12 angstrom thick YBCO film. We find that the newly proposed scaling theories have intrinsic flexibility that is relevant to the analysis of the experiments. In particular, the data scale according to the modified transition for arbitrarily defined critical temperatures between 0 K and 19.5 K, and the temperature range of a successful scaling collapse is related directly to the sensitivity of the measurement. This implies that the apparent universal exponent is due to the intrinsic flexibility rather than some real physical property. To address this intrinsic flexibility, we propose a criterion which would give conclusive evidence for phase transitions in two-dimensional superconductors. We conclude by reviewing results to see if our criterion is satisfied.Comment: 14 page

Low-energy excitations in the three-dimensional random-field Ising model

The random-field Ising model (RFIM), one of the basic models for quenched disorder, can be studied numerically with the help of efficient ground-state algorithms. In this study, we extend these algorithm by various methods in order to analyze low-energy excitations for the three-dimensional RFIM with Gaussian distributed disorder that appear in the form of clusters of connected spins. We analyze several properties of these clusters. Our results support the validity of the droplet-model description for the RFIM.Comment: 10 pages, 9 figure

Existential Communication and Leadership

The aim of this article is to introduce and explain a number of important existentialist philosophers and concepts that we believe can contribute to a critical approach to leadership theory. Emphasis is placed on understanding the nature of communication from an existentialist perspective and so Jaspers' conceptualization of existential communication is introduced along with important related concepts that may be regarded as important facets of leader communication including Being-in-the-world, the Other, intersubjectivity, dialogue and indirect communication. Particular attention is paid to Buber's ideas on communication as relationship and dialogue. Throughout, reference is made to contemporary, and what is often regarded as orthodox, thinking regarding the centrality of communication to leadership practice as a means by which to highlight the salience of an existentialist analysis

Thermodynamics of Electrolytes on Anisotropic Lattices

The phase behavior of ionic fluids on simple cubic and tetragonal (anisotropic) lattices has been studied by grand canonical Monte Carlo simulations. Systems with both the true lattice Coulombic potential and continuous-space $1/r$ electrostatic interactions have been investigated. At all degrees of anisotropy, only coexistence between a disordered low-density phase and an ordered high-density phase with the structure similar to ionic crystal was found, in contrast to recent theoretical predictions. Tricritical parameters were determined to be monotonously increasing functions of anisotropy parameters which is consistent with theoretical calculations based on the Debye-H\"uckel approach. At large anisotropies a two-dimensional-like behavior is observed, from which we estimated the dimensionless tricritical temperature and density for the two-dimensional square lattice electrolyte to be $T^*_{tri}=0.14$ and $\rho^*_{tri} = 0.70$.Comment: submitted to PR

Thermodynamic Gravity and the Schrodinger Equation

We adopt a 'thermodynamical' formulation of Mach's principle that the rest mass of a particle in the Universe is a measure of its long-range collective interactions with all other particles inside the horizon. We consider all particles in the Universe as a 'gravitationally entangled' statistical ensemble and apply the approach of classical statistical mechanics to it. It is shown that both the Schrodinger equation and the Planck constant can be derived within this Machian model of the universe. The appearance of probabilities, complex wave functions, and quantization conditions is related to the discreetness and finiteness of the Machian ensemble.Comment: Minor corrections, the version accepted by Int. J. Theor. Phy