23,271 research outputs found
The diamagnetism above the superconducting transition in underdoped La(1.9)Sr(0.1)CuO(4) revisited: Chemical disorder or phase incoherent superconductivity?
The interplay between superconducting fluctuations and inhomogeneities
presents a renewed interest due to recent works supporting an anomalous [beyond
the conventional Gaussian-Ginzburg-Landau (GGL) scenario] diamagnetism above Tc
in underdoped cuprates. This conclusion, mainly based in the observation of new
anomalies in the low-field isothermal magnetization curves, is in contradiction
with our earlier results in the underdoped La(1.9)Sr(0.1)CuO(4) [Phys. Rev.
Lett. 84, 3157 (2000)]. These seemingly intrinsic anomalies are being presented
in various influential works as a 'thermodynamic evidence' for phase incoherent
superconductivity in the pseudogap regime, this last being at present a central
and debated issue of the cuprate superconductors' physics. Here we have
extended our magnetization measurements in La(1.9)Sr(0.1)CuO(4) to two samples
with different chemical disorder, in one of them close to the one associated
with the random distribution of Sr ions. For this sample, the corresponding
Tc-distribution may be approximated as symmetric around the average Tc, while
in the most disordered sample is strongly asymmetric. The comparison between
the magnetization measured in both samples provides a crucial check of the
chemical disorder origin of the observed diamagnetism anomalies, which are
similar to those claimed as due to phase fluctuations by other authors. This
conclusion applies also to the sample affected only by the intrinsic-like
chemical disorder, providing then a further check that the intrinsic
diamagnetism above the superconducting transition of underdoped cuprates is not
affected by the opening of a pseudogap in the normal state. It is also shown
here that once these disorder effects are overcome, the remaining precursor
diamagnetism may be accounted at a quantitative level in terms of the GGL
approach under a total energy cutoff.Comment: 13 pages, 7 figures. Minor corrections include
Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"
By using high magnetic field data to estimate the background conductivity,
Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522
(2011)] experimental evidence that the in-plane paraconductivity in cuprates is
almost independent of doping. In this Comment we also show that, in contrast
with their claims, these useful data may be explained at a quantitative level
in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors,
extended by Carballeira and coworkers to high reduced-temperatures by
introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When
combined, these two conclusions further suggest that the paraconductivity in
cuprates is conventional, i.e., associated with fluctuating superconducting
pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur
Fluctuation diamagnetism around the superconducting transition in a cuprate crystal with a reduced Meissner fraction
The magnetization around the superconducting transition was measured in a
TlPbSrCaCuO crystal affected by a considerable
reduction (55%) of its effective superconducting volume fraction but
still with a relatively sharp low-field Meissner transition, a behaviour that
may be attributed to the presence of structural inhomogeneities. By taking into
account these inhomogeneities just through the Meissner fraction, the observed
diamagnetism may still be explained, consistently above and below the
superconducting transition, in terms of the conventional Ginzburg-Landau
approach with fluctuations of Cooper pairs and vortices.Comment: 4 pages, 4 figure
Diamagnetism around the Meissner transition in a homogeneous cuprate single crystal
The in-plane diamagnetism around the Meissner transition was measured in a
TlBaCaCuO single crystal of high chemical and structural
quality, which minimizes the inhomogeneity and disorder rounding effects on the
magnetization. When analyzed quantitatively and consistently above and below
the transition in terms of the Ginzburg-Landau (GL) approach with fluctuations
of Cooper pairs and vortices, these data provide a further confirmation that
the observed Meissner transition is a conventional GL superconducting
transition in a homogeneous layered superconductor.Comment: 5 pages, including 3 figure
Optical absorption and energy-loss spectra of aligned carbon nanotubes
Optical-absorption cross-sections and energy-loss spectra of aligned
multishell carbon nanotubes are investigated, on the basis of photonic
band-structure calculations. A local graphite-like dielectric tensor is
assigned to every point of the tubules, and the effective transverse dielectric
function of the composite is computed by solving Maxwell's equations in media
with tensor-like dielectric functions. A Maxwell-Garnett-like approach
appropriate to the case of infinitely long anisotropic tubules is also
developed. Our full calculations indicate that the experimentally measured
macroscopic dielectric function of carbon nanotube materials is the result of a
strong electromagnetic coupling between the tubes. An analysis of the
electric-field pattern associated with this coupling is presented, showing that
in the close-packed regime the incident radiation excites a very localized
tangential surface plasmon.Comment: 7 pages, 12 figures, to appear in Eur. Phys. J.
Anomalous precursor diamagnetism at low reduced magnetic fields and the role of Tc inhomogeneities in the superconductors Pb55In45 and underdoped La1.9Sr0.1CuO4
The magnetic field dependence of the magnetization was measured above the
superconducting transition in a high-Tc underdoped cuprate La1.9Sr0.1CuO4 and
in a low-Tc alloy (Pb55In45). Near the superconducting transition [typically
for (T-Tc)/Tc<0.05] and under low applied magnetic field amplitudes [typically
for H/Hc2(0)<0.01, where Hc2(0) is the corresponding upper critical field
extrapolated to T=0 K] the magnetization of both samples presents a diamagnetic
contribution much larger than the one predicted by the Gaussian Ginzburg-Landau
(GGL) approach for superconducting fluctuations. These anomalies have been
already observed in cuprate compounds by various groups and attributed to
intrinsic effects associated with the own nature of these high-Tc
superconductors. However, we will see here that our results in both high and
low-Tc superconductors may be explained quantitatively, and consistently with
the GGL behavior observed at higher fields, by just taking into account the
presence in the samples of an uniform distribution of Tc inhomogeneities. These
Tc inhomogeneities, which may be in turn associated with stoichiometric
inhomogeneities, were estimated from independent measurements of the
temperature dependence of the field-cooled magnetic susceptibility under low
applied magnetic fields.Comment: 25 pages, including 6 figures and 1 table. Typos corrected. Compacte
T and CPT in B-Factories
For the Bd meson system, CP, T and CPT indirect violation can be described
using two physical parameters, epsilon and delta. The traditional observables
based on flavour tag and used in the kaon system, are not helpful in the Bd
case, and new asymmetries have to be introduced. Here such alternative
observables, based on CP tag, are presented, together with the first estimation
on the sensitivity that current asymmetric B-factories can achieve on their
measurement.Comment: 7 pages, Talk given at the International Europhysics conference on
HEP, HEP2001, July 2001, Budapest (Hungary
On the energy saved by interlayer interactions in the superconducting state of cuprates
A Ginzburg-Landau-like functional is proposed reproducing the main low-energy
features of various possible high-Tc superconducting mechanisms involving
energy savings due to interlayer interactions. The functional may be used to
relate these savings to experimental quantities. Two examples are given,
involving the mean-field specific heat jump at Tc and the superconducting
fluctuations above Tc. Comparison with existing data suggests, e.g., that the
increase of Tc due to the so-called interlayer tunneling (ILT) mechanism of
interlayer kinetic-energy savings is negligible in optimally-doped Bi-2212.Comment: 12 pages, no figures. Version history: 21-aug-2003, first version
(available on http://arxiv.org/abs/cond-mat/0308423v1); 15-jan-2004, update
to match Europhys. Lett. publication (minor grammar changes, updates in
bibliography - e.g., refs. 5 and 26
Asymptotic entanglement capacity of the Ising and anisotropic Heisenberg interactions
We compute the asymptotic entanglement capacity of the Ising interaction ZZ,
the anisotropic Heisenberg interaction XX + YY, and more generally, any
two-qubit Hamiltonian with canonical form K = a XX + b YY. We also describe an
entanglement assisted classical communication protocol using the Hamiltonian K
with rate equal to the asymptotic entanglement capacity.Comment: 5 pages, 1 figure; minor corrections, conjecture adde
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