4,109 research outputs found
Cross-Correlation Studies between CMB Temperature Anisotropies and 21 cm Fluctuations
During the transition from a neutral to a fully reionized universe,
scattering of cosmic microwave background (CMB) photons via free-electrons
leads to a new anisotropy contribution to the temperature distribution. If the
reionization process is inhomogeneous and patchy, the era of reionization is
also visible via brightness temperature fluctuations in the redshifted 21 cm
line emission from neutral Hydrogen. Since regions containing electrons and
neutral Hydrogen are expected to trace the same underlying density field, the
two are (anti) correlated and this is expected to be reflected in the
anisotropy maps via a correlation between arcminute-scale CMB temperature and
the 21 cm background. In terms of the angular cross-power spectrum,
unfortunately, this correlation is insignificant due to a geometric
cancellation associated with second order CMB anisotropies. The same
cross-correlation between ionized and neutral regions, however, can be studied
using a bispectrum involving large scale velocity field of ionized regions from
the Doppler effect, arcminute scale CMB anisotropies during reionization, and
the 21 cm background. While the geometric cancellation is partly avoided, the
signal-to-noise ratio related to this bispectrum is reduced due to the large
cosmic variance related to velocity fluctuations traced by the Doppler effect.
Unless the velocity field during reionization can be independently established,
it is unlikely that the correlation information related to the relative
distribution of ionized electrons and regions containing neutral Hydrogen can
be obtained with a combined study involving CMB and 21 cm fluctuations.Comment: 10 pages, 3 figure
Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor
The magnetic field dependence of the in-plane magnetic penetration depth for
optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque
magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained
from these techniques show all a pronounced decrease of the superfluid density
as the field is increased up to 1.4 T. This behavior is analysed within a
two-band model with self-consistently derived coupled gaps, where the
superfluid density related to the larger gap is field independent and the
superfluid density related to the smaller gap is strongly suppressed with
increasing field.Comment: 7 pages, 5 figure
Fluctuating Cu-O-Cu Bond model of high temperature superconductivity in cuprates
Twenty years of extensive research has yet to produce a general consensus on
the origin of high temperature superconductivity (HTS). However, several
generic characteristics of the cuprate superconductors have emerged as the
essential ingredients of and/or constraints on any viable microscopic model of
HTS. Besides a Tc of order 100K, the most prominent on the list include a
d-wave superconducting gap with Fermi liquid nodal excitations, a d-wave
pseudogap with the characteristic temperature scale T*, an anomalous
doping-dependent oxygen isotope shift, nanometer-scale gap inhomogeneity, etc..
The key role of planar oxygen vibrations implied by the isotope shift and other
evidence, in the context of CuO2 plane symmetry and charge constraints from the
strong intra-3d Coulomb repulsion U, enforces an anharmonic mechanism in which
the oxygen vibrational amplitude modulates the strength of the in-plane Cu-Cu
bond. We show, within a Fermi liquid framework, that this mechanism can lead to
strong d-wave pairing and to a natural explanation of the salient features of
HTS
The polarizability model for ferroelectricity in perovskite oxides
This article reviews the polarizability model and its applications to
ferroelectric perovskite oxides. The motivation for the introduction of the
model is discussed and nonlinear oxygen ion polarizability effects and their
lattice dynamical implementation outlined. While a large part of this work is
dedicated to results obtained within the self-consistent-phonon approximation
(SPA), also nonlinear solutions of the model are handled which are of interest
to the physics of relaxor ferroelectrics, domain wall motions, incommensurate
phase transitions. The main emphasis is to compare the results of the model
with experimental data and to predict novel phenomena.Comment: 55 pages, 35 figure
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