483 research outputs found
C-axis Penetration Depth and Inter-layer Conductivity in the Thallium Based Cuprate Superconductors
The c-axis Josephson plasmon in optimally doped single-layer and bi-layer
high Tc cuprates Tl2201 and Tl2212 have been investigated using infrared
spectroscopy. We observed the plasma frequencies for these two compounds at
27.8 and 25.6 cm-1 respectively, which we interpret as a Josephson resonance
across the TlO blocking layers. No maximum in the temperature dependence of the
c-axis conductivity was observed below Tc, indicating that even in the
superconducting state a coherent quasi-particle contribution to the c-axis
conductivity is absent or very weak, in contrast to the behaviour of the
ab-plane conductivity.Comment: 4 pages, 3 figure
Improved Distances to Type Ia Supernovae with Multicolor Light Curve Shapes: MLCS2k2
We present an updated version of the Multicolor Light Curve Shape method to
measure distances to type Ia supernovae (SN Ia), incorporating new procedures
for K-correction and extinction corrections. We also develop a simple model to
disentangle intrinsic color variations and reddening by dust, and expand the
method to incorporate U-band light curves and to more easily accommodate prior
constraints on any of the model parameters. We apply this method to 133 nearby
SN Ia, including 95 objects in the Hubble flow (cz >= 2500 km/s), which give an
intrinsic dispersion of less than 7% in distance. The Hubble flow sample, which
is of critical importance to all cosmological uses of SN Ia, is the largest
ever presented with homogeneous distances. We find the Hubble flow supernovae
with Hd >= 7400 km/s yield an expansion rate that is 6.5 +/- 1.8% lower than
the rate determined from supernovae within that distance, and this can have a
large effect on measurements of the dark energy equation of state with SN Ia.
Peculiar velocities of SN Ia host galaxies in the rest frame of the Local Group
are consistent with the dipole measured in the Cosmic Microwave Background.
Direct fits of SN Ia that are significantly reddened by dust in their host
galaxies suggest their mean extinction law may be described by R_V ~= 2.7, but
optical colors alone provide weak constraints on R_V.Comment: 66 pages, 22 figures; accepted for publication in The Astrophysical
Journal. Electronic data available at
http://astro.berkeley.edu/~saurabh/mlcs2k2
Luminescent properties of Bi-doped polycrystalline KAlCl4
We observed an intensive near-infrared luminescence in Bi-doped KAlCl4
polycrystalline material. Luminescence dependence on the excitation wavelength
and temperature of the sample was studied. Our experimental results allow
asserting that the luminescence peaked near 1 um belongs solely to Bi+ ion
which isomorphically substitutes potassium in the crystal. It was also
demonstrated that Bi+ luminescence features strongly depend on the local ion
surroundings
Correlation between the Josephson coupling energy and the condensation energy in bilayer cuprate superconductors
We review some previous studies concerning the intra-bilayer Josephson
plasmons and present new ellipsometric data of the c-axis infrared response of
almost optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8}. The c-axis conductivity of
this compound exhibits the same kind of anomalies as that of underdoped
YBa_{2}Cu_{3}O_{7-delta}. We analyze these anomalies in detail and show that
they can be explained within a model involving the intra-bilayer Josephson
effect and variations of the electric field inside the unit cell. The Josephson
coupling energies of different bilayer compounds obtained from the optical data
are compared with the condensation energies and it is shown that there is a
reasonable agreement between the values of the two quantities. We argue that
the Josephson coupling energy, as determined by the frequency of the
intra-bilayer Josephson plasmon, represents a reasonable estimate of the change
of the effective c-axis kinetic energy upon entering the superconducting state.
It is further explained that this is not the case for the estimate based on the
use of the simplest ``tight-binding'' sum rule. We discuss possible
interpretations of the remarkable agreement between the Josephson coupling
energies and the condensation energies. The most plausible interpretation is
that the interlayer tunneling of the Cooper pairs provides the dominant
contribution to the condensation energy of the bilayer compounds; in other
words that the condensation energy of these compounds can be accounted for by
the interlayer tunneling theory. We suggest an extension of this theory, which
may also explain the high values of T_{c} in the single layer compounds
Tl_{2}Ba_{2}CuO_{6} and HgBa_{2}CuO_{4}, and we make several experimentally
verifiable predictions.Comment: 16 pages (including Tables) and 7 figures; accepted for publication
in Physical Review
Multiband model of high Tc superconductors
We propose an extension to other high T_{c } compounds of a model introduced
earlier for YBCO. In the ''self-doped'' compounds we assume that the doping
part (namely the BiO, HgO, TlO planes in BSCCO, HBCCO, TBCCO respectively) is
metallic, which leads to a multiband model. This assumption is supported by
band structure calculations. Taking a repulsive pairing interaction between
these doping bands and the CuO_{2} bands leads to opposite signs for the order
parameter on these bands and to nodes whenever the Fermi surfaces of these
bands cross. We show that in BSCCO the low temperature dependence of the
penetration depth is reasonably accounted for. In this case the nodes are not
located near the 45^{o} direction, which makes the experimental determination
of the node locations an important test for our model. The situation in HBCCO
and TBCCO is rather analogous to BSCCO. We consider the indications given by
NMR and find that they rather favor a metallic character for the doping bands.
Finally we discuss the cases of NCCO and LSCO which are not ''self-doped'' and
where our model does not give nodes.Comment: 11 pages, revtex, 1 figure
Kinematic Peculiarities of Gould Belt Stars
We analyzed the space velocities of Gould Belt stars younger than 125 Myr
located at heliocentric distances <650 pc. We determined the rotation and
expansion parameters of the Gould Belt by assuming the existence of a single
kinematic center whose direction was found to be the following:
and pc. The linear velocities reach their
maximum at a distance of pc from the center and are -6 km s
for the rotation (whose direction coincides with the Galactic rotation) and +4
km s for the expansion. The stellar rotation model used here is shown to
give a more faithful description of the observed velocity field than the linear
model based on the Oort constants and . We present evidence that the
young clusters Pic, Tuc/HorA, and TWA belong to the Gould Belt
structure.Comment: 17 pages, 5 figure
The Hubble Constant: A Summary of the HST Program for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids
This is the fifth and final summary paper of our 15 year program using the
Hubble Space Telescope (HST) to determine the Hubble constant using Type Ia
supernovae, calibrated with Cepheid variables in nearby galaxies that hosted
them. Several developments not contemplated at the start of the program in 1990
have made it necessary to put the summary on H_0 on a broader basis than
originally thought, making four preparatory papers necessary. The new Cepheid
distances of the subset of 10 galaxies, which were hosts of normal SNeIa, give
weighted mean luminosities in B, V, and I at maximum light of -19.49, -19.46,
and -19.22, respectively. These calibrate the adopted SNeIa Hubble diagram from
Paper III to give a global value of H_0 = 62.3 +/- 1.3 (random) +/- 5.0
(systematic). Local values of H_0 between 4.4 and 30 Mpc from Cepheids, SNeIa,
21cm-line widths, and the tip of the red-giant branch (TRGB) all agree within
5% of our global value. This agreement of H_0 on all scales from 4 - 200 Mpc
finds its most obvious explanation in the smoothing effect of vacuum energy on
the otherwise lumpy gravitational field due to the non-uniform distribution of
the local galaxies. The physical methods of time delay of gravitational lenses
and the Sunyaev-Zeldovich effect are consistent (but with large errors) with
our global value. The present result is also not in contradiction with existing
analyses of CMB data, because they either lead to wide error margins of H_0 or
depend on the choice of unwarrented priors that couple the value of H_0 with a
number of otherwise free parameters in the CMB acoustic waves. Our value of H_0
is 14% smaller than the value of H_0 found by Freedman et al. (2001) because
our independent Cepheid distances to the six SNeIa-calibrating galaxies used in
that analysis average 0.35mag larger than those used earlier.Comment: 52 pages, 9 figures, 8 tables, accepted for publication in Ap
Condensation Energy and Spectral Functions in High Temperature Superconductors
If high temperature cuprate superconductivity is due to electronic
correlations, then the energy difference between the normal and superconducting
states can be expressed in terms of the occupied part of the single particle
spectral function. The latter can, in principle, be determined from angle
resolved photoemission (ARPES) data. As a consequence, the energy gain driving
the development of the superconducting state is intimately related to the
dramatic changes in the photoemission lineshape when going below Tc. These
points are illustrated in the context of the "mode" model used to fit ARPES
data in the normal and superconducting states, where the question of kinetic
energy versus potential energy driven superconductivity is explored in detail.
We use our findings to comment on the relation of ARPES data to the
condensation energy, and to various other experimental data. In particular, our
results suggest that the nature of the superconducting transition is strongly
related to how anomalous (non Fermi liquid like) the normal state spectral
function is, and as such, is dependent upon the doping level.Comment: 10 pages, revtex, 4 encapsulated postscript figure
- …