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