8,314 research outputs found
Theory of High-Tc Superconducting Cuprates Based on Experimental Evidence
A model of superconductivity in layered high-temperature superconducting
cuprates is proposed, based on the extended saddle point singularities in the
electron spectrum, weak screening of the Coulomb interaction and
phonon-mediated interaction between electrons plus a small short -range
repulsion of Hund's, or spin-fluctuation, origin. This permits to explain the
large values of , features of the isotope effect on oxygen and copper, the
existence of two types of the order parameter, the peak in the inelastic
neutron scattering, the positive curvature of the upper critical field, as
function of temperature etc.Comment: RevTeX 3.x, 11 Postscript figures (included); send comments to
[email protected]
Habitable planets around the star Gl 581?
Radial velocity surveys are now able to detect terrestrial planets at
habitable distance from M-type stars. Recently, two planets with minimum masses
below 10 Earth masses were reported in a triple system around the M-type star
Gliese 581. Using results from atmospheric models and constraints from the
evolution of Venus and Mars, we assess the habitability of planets Gl 581c and
Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ)
boundaries determination. We provide simplified formulae to estimate the HZ
limits that may be used to evaluate the astrobiological potential of
terrestrial exoplanets that will hopefully be discovered in the near future.
Planets Gl 581c and 'd' are near, but outside, what can be considered as the
conservative HZ. Planet 'c' receives 30% more energy from its star than Venus
from the Sun, with an increased radiative forcing caused by the spectral energy
distribution of Gl 581. Its habitability cannot however be positively ruled out
by theoretical models due to uncertainties affecting cloud properties.
Irradiation conditions of planet 'd' are comparable with those of early Mars.
Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better
candidate for the first exoplanet known to be potentially habitable. A mixture
of various greenhouse gases could also maintain habitable conditions on this
planet.Comment: Astronomy and Astrophysics (2007) accepted for publicatio
Large isotope effect on in cuprates despite of a small electron-phonon coupling
We calculate the isotope coefficients and for the
superconducting critical temperature and the pseudogap temperature
in a mean-field treatment of the t-J model including phonons. The
pseudogap phase is identified with the -charge-density wave (-CDW) phase
in this model. Using the small electron-phonon coupling constant obtained previously in LDA calculations in YBaCuO,
is negative but negligible small whereas increases
from about 0.03 at optimal doping to values around 1 at small dopings in
agreement with the general trend observed in many cuprates. Using a simple
phase fluctuation model where the -CDW has only short-range correlations it
is shown that the large increase of at low dopings is rather universal
and does not depend on the existence of sharp peaks in the density of states in
the pseudogap state or on specific values of the phonon cutoff. It rather is
caused by the large depletion of spectral weight at low frequencies by the
-CDW and thus should also occur in other realizations of the pseudogap.Comment: 8 pages, 5 figures, to be publ. in PR
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