2 research outputs found

    Breakdown of Migdal's theorem and intensity of electron-phonon coupling in high-Tc superconductors

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    In this article we quantify the possible effects of the breakdown of Migdal's theorem on the electron-phonon (e-p) coupling constant, on the critical temperature and on the superconducting gap by examining different kinds of superconducting materials either with low and high critical temperature. We use the theoretical approach developed by Grimaldi, Pietronero and Strassler [PRB 52, 10516 & 10530 (1995)] on experimental data taken both from literature and from our recent break-junction tunneling experiments in Bi_2Sr_2CaCu_2O_{8+x} (BSCCO) [Physica C 275, 162 (1997) and cond-mat/9612220]. The results show that a large violation of the Migdal's theorem (as in BSCCO) yields to a large increase of the observed e-p coupling constant and, in a first approximation and by direct solution of the Eliashberg equations, to large but different increases of Tc and gap. The same theory gives no modifications when applied to low-Tc conventional superconductors. In BSCCO, the renormalized values of the e-p coupling constant and of the ratio 2Delta/kTc (1.85 and 4.98, respectively) are compatible with a strong-coupling electron-phonon origin of superconductivity.Comment: 11 pages REVTeX file, 3 PostScript figures, to be published in Phys. Rev. B (Rapid Commun.

    The Cosmic Evolution Survey (COSMOS) -- Overview

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    The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) with large-scale structure (LSS) over the redshift range z >0.5> 0.5 to 6. The survey includes multi-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 \sq\deg area, including HST imaging. Given the very high sensitivity and resolution of these datasets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS datasets, and summarize the science goals.Comment: 22 pages, 3 figure
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