20,492 research outputs found
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
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
Modeling magnesium escape from HD209458b atmosphere
Transit observations in the MgI line of HD209458b revealed signatures of
neutral magnesium escaping the upper atmosphere of the planet, while no
atmospheric absorption was found in the MgII doublet. Here we present a 3D
particle model of the dynamics of neutral and ionized magnesium populations,
coupled with an analytical modeling of the atmosphere below the exobase.
Theoretical MgI absorption line profiles are directly compared with the
absorption observed in the blue wing of the line during the planet transit.
Observations are well-fitted with an escape rate of neutral magnesium in the
range 2x10^7-3.4x10^7 g/s, an exobase close to the Roche lobe (Rexo in the
range 2.1-4.3 Rp, where Rp is the planet radius) and a planetary wind velocity
at the exobase vpl=25km/s. The observed velocities of the planet-escaping
magnesium up to -60km/s are well explained by radiation pressure acceleration,
provided that UV-photoionization is compensated for by electron recombination
up to about 13Rp. If the exobase properties are constrained to values given by
theoretical models of the deeper atmosphere (Rexo=2Rp and vpl=10km/s), the best
fit to the observations is found at a similar electron density and escape rate
within 2 sigma. In all cases, the mean temperature of the atmosphere below the
exobase must be higher than about 6100 K. Simulations predict a redward
expansion of the absorption profile from the beginning to the end of the
transit. The spatial and spectral structure of the extended atmosphere is the
result of complex interactions between radiation pressure, planetary gravity,
and self-shielding, and can be probed through the analysis of transit
absorption profiles in the MgI line.Comment: 16 pages, 24 figure
The MgI line: a new probe of the atmospheres of evaporating exoplanets
Transit observations of HD209458b in the UV revealed signatures of neutral
magnesium escaping the planet's upper atmosphere. The absorption detected in
the MgI line provides unprecedented information on the physical conditions at
the altitude where the atmospheric blow-off takes place. Here we use a 3D model
of atmospheric escape to estimate the transit absorption signatures in the MgI
line of their host stars. The detectability of these signatures depends on the
brightness of the star and the escape rate of neutral magnesium. We identify a
sample of potentially evaporating exoplanets that covers a wide range of
stellar and planetary properties, and whose extended exospheres might be
detected through MgI line observations with current UV facilities, allowing
further steps in comparative exoplanetology.Comment: 4 pages, 2 figure
Three qubits can be entangled in two inequivalent ways
Invertible local transformations of a multipartite system are used to define
equivalence classes in the set of entangled states. This classification
concerns the entanglement properties of a single copy of the state.
Accordingly, we say that two states have the same kind of entanglement if both
of them can be obtained from the other by means of local operations and
classical communcication (LOCC) with nonzero probability. When applied to pure
states of a three-qubit system, this approach reveals the existence of two
inequivalent kinds of genuine tripartite entanglement, for which the GHZ state
and a W state appear as remarkable representatives. In particular, we show that
the W state retains maximally bipartite entanglement when any one of the three
qubits is traced out. We generalize our results both to the case of higher
dimensional subsystems and also to more than three subsystems, for all of which
we show that, typically, two randomly chosen pure states cannot be converted
into each other by means of LOCC, not even with a small probability of success.Comment: 12 pages, 1 figure; replaced with revised version; terminology
adapted to earlier work; reference added; results unchange
Electromagnetic dipole moments of charged baryons with bent crystals at the LHC
We propose a unique program of measurements of electric and magnetic dipole
moments of charm, beauty and strange charged baryons at the LHC, based on the
phenomenon of spin precession of channeled particles in bent crystals. Studies
of crystal channeling and spin precession of positively- and negatively-charged
particles are presented, along with feasibility studies and expected
sensitivities for the proposed experiment using a layout based on the LHCb
detector.Comment: 19 pages, 13 figure
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