5,885 research outputs found
Parametric instability of the helical dynamo
We study the dynamo threshold of a helical flow made of a mean (stationary)
plus a fluctuating part. Two flow geometries are studied, either (i) solid body
or (ii) smooth. Two well-known resonant dynamo conditions, elaborated for
stationary helical flows in the limit of large magnetic Reynolds numbers, are
tested against lower magnetic Reynolds numbers and for fluctuating flows (zero
mean). For a flow made of a mean plus a fluctuating part the dynamo threshold
depends on the frequency and the strength of the fluctuation. The resonant
dynamo conditions applied on the fluctuating (resp. mean) part seems to be a
good diagnostic to predict the existence of a dynamo threshold when the
fluctuation level is high (resp. low).Comment: 37 pages, 8 figure
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]
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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