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.

Interface depinning versus absorbing-state phase transitions

According to recent numerical results from lattice models, the critical exponents of systems with many absorbing states and an order parameter coupled to a non-diffusive conserved field coincide with those of the linear interface depinning model within computational accuracy. In this paper the connection between absorbing state phase transitions and interface pinning in quenched disordered media is investigated. For that, we present a mapping of the interface dynamics in a disordered medium into a Langevin equation for the active-site density and show that a Reggeon-field-theory like description, coupled to an additional non-diffusive conserved field, appears rather naturally. Reciprocally, we construct a mapping from a discrete model belonging in the absorbing state with-a-conserved-field class to a discrete interface equation, and show how a quenched disorder is originated. We discuss the character of the possible noise terms in both representations, and overview the critical exponent relations. Evidence is provided that, at least for dimensions larger that one, both universality classes are just two different representations of the same underlying physics.Comment: 8 page

Implementing an On-the-fly Garbage Collector for Java

Java uses garbage collection (GC) for the automatic reclamation of computer memory no longer required by a running application. GC implementations for Java Virtual Machines (JVM) are typically designed for single processor machines, and do not necessarily perform well for a server program with many threads running on a multiprocessor. We designed and implemented an on-the-fly GC, based on the algorithm of Doligez, Leroy and Gonthier [13, 12] (DLG), for Java in this environment. An on-the-fly collector, a collector that does not stop the program threads, allows all processors to be utilized during collection and provides uniform response times. We extended and adapted DLG for Java (e.g., adding support for weak references) and for modern multiprocessors without sequential consistency, and added performance improvements (e.g., to keep track of the objects remaining to be traced). We compared the performance of our implementation with stop-the-world mark-sweep GC. Our measurements show th..

Imaging of gynecological disease: clinical and ultrasound characteristics of ovarian dysgerminoma.

OBJECTIVES: To describe the clinical history and ultrasound findings in patients with ovarian dysgerminoma. METHODS: This was a retrospective study of patients with a histological diagnosis of ovarian dysgerminoma who had undergone preoperative ultrasound examination. The patients were identified from the databases of 11 ultrasound centers. The tumors were described by the principal investigator at each contributing center on the basis of ultrasound images, ultrasound reports and research protocols (when applicable) using the terms and definitions of the International Ovarian Tumor Analysis (IOTA) group. In addition, three authors reviewed all available electronic ultrasound images (gray-scale images and color/power Doppler images were available for 18 patients and 14 patients, respectively) and described them using subjective evaluation of gray-scale and color Doppler ultrasound findings (here called pattern recognition). RESULTS: Twenty-one patients with ovarian dysgerminoma were identified (including one woman with bilateral masses). Twenty patients had a primary ovarian dysgerminoma (including the one with bilateral masses) and one patient had a recurrence of dysgerminoma in her retained ovary. One of the 21 patients was pregnant. All tumors except one were pure dysgerminomas, one being a mixed germinal cell tumor with 30% dysgerminoma component. Median age was 20 (range, 16-31) years. Information on clinical symptoms was available for 18 patients. In four patients, the tumor was detected incidentally, whereas 14 patients presented with one or more of the following symptoms: acute pain (n = 4), chronic pain (n = 8), bloating (n = 8), menstrual disorders (n = 5) and infertility problems (n = 1). One (5%) patient had ascites. Using the IOTA terms and definitions, all but one dysgerminoma were moderately (43%) or very well (50%) vascularized solid tumors. One tumor was multilocular-solid. According to pattern recognition, most dysgerminomas were highly vascularized, purely solid tumors with heterogeneous internal echogenicity divided into several lobules, had a smooth and sometimes lobulated contour and were well-defined relative to the surrounding organs. CONCLUSION: The ultrasound finding of a highly vascularized, large, solid, lobulated adnexal mass with irregular internal echogenicity in a woman 20-30 years old should raise the suspicion of ovarian dysgerminom