1,389 research outputs found
Superconducting Fluctuations, Pseudogap and Phase Diagram in Cuprates
We report transport measurements using pulsed magnetic fields to suppress the
superconducting fluctuations (SCF) conductivity in a series of YBa_2Cu_3O_(6+x)
samples. These experiments allow us altogether to measure the temperature T'c
at which SCF disappear, and the pseudogap temperature T*. While the latter are
consistent with previous determinations of T*, we find that T'c is slightly
larger than similar data taken by Nernst measurements. A careful investigation
near optimal doping shows that T* becomes smaller than T'c, which is an
unambiguous evidence that the pseudogap cannot be assigned to preformed pairs.
Studies of the incidence of disorder on both T'c and T* allow us to propose a
phase diagram including disorder which explains most observations done in other
cuprate families, and to discuss the available knowledge on the pseudogap line
in the phase diagram.Comment: New version with minor correction
Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling
The recently discovered exoplanet Gl581d is extremely close to the outer edge
of its system's habitable zone, which has led to much speculation on its
possible climate. We have performed a range of simulations to assess whether,
given simple combinations of chemically stable greenhouse gases, the planet
could sustain liquid water on its surface. For best estimates of the surface
gravity, surface albedo and cloud coverage, we find that less than 10 bars of
CO2 is sufficient to maintain a global mean temperature above the melting point
of water. Furthermore, even with the most conservative choices of these
parameters, we calculate temperatures above the water melting point for CO2
partial pressures greater than about 40 bar. However, we note that as Gl581d is
probably in a tidally resonant orbit, further simulations in 3D are required to
test whether such atmospheric conditions are stable against the collapse of CO2
on the surface.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere
High-resolution spectra of Pluto in the 1.66 um region, recorded with the
VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to
constrain the spatial and vertical distribution of methane in Pluto's
atmosphere and to search for mid-term (4 year) variability. A sensitivity study
to model assumptions (temperature structure, surface pressure, Pluto's radius)
is performed. Results indicate that (i) no variation of the CH4 atmospheric
content (column-density or mixing ratio) with Pluto rotational phase is present
in excess of 20 % (ii) CH4 column densities show at most marginal variations
between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this
time frame. As stellar occultations indicate that Pluto's surface pressure has
continued to increase over this period, this implies a concomitant decrease of
the methane mixing ratio (iii) the data do not show evidence for an
altitude-varying methane distribution; in particular, they imply a roughly
uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high
concentrations of low-temperature methane near the surface can be ruled out.
Our results are also best consistent with a relatively large (> 1180 km) Pluto
radius. Comparison with predictions from a recently developed global climate
model GCM indicates that these features are best explained if the source of
methane occurs in regional-scale CH4 ice deposits, including both low latitudes
and high Northern latitudes, evidence for which is present from the rotational
and secular evolution of the near-IR features due to CH4 ice. Our "best guess"
predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17
ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible
longitudinal variations.Comment: 21 pages, 6 figure
3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability
The inner edge of the classical habitable zone is often defined by the
critical flux needed to trigger the runaway greenhouse instability. This 1D
notion of a critical flux, however, may not be so relevant for inhomogeneously
irradiated planets, or when the water content is limited (land planets).
Here, based on results from our 3D global climate model, we find that the
circulation pattern can shift from super-rotation to stellar/anti stellar
circulation when the equatorial Rossby deformation radius significantly exceeds
the planetary radius. Using analytical and numerical arguments, we also
demonstrate the presence of systematic biases between mean surface temperatures
or temperature profiles predicted from either 1D or 3D simulations.
Including a complete modeling of the water cycle, we further demonstrate that
for land planets closer than the inner edge of the classical habitable zone,
two stable climate regimes can exist. One is the classical runaway state, and
the other is a collapsed state where water is captured in permanent cold traps.
We identify this "moist" bistability as the result of a competition between the
greenhouse effect of water vapor and its condensation. We also present
synthetic spectra showing the observable signature of these two states.
Taking the example of two prototype planets in this regime, namely Gl581c and
HD85512b, we argue that they could accumulate a significant amount of water ice
at their surface. If such a thick ice cap is present, gravity driven ice flows
and geothermal flux should come into play to produce long-lived liquid water at
the edge and/or bottom of the ice cap. Consequently, the habitability of
planets at smaller orbital distance than the inner edge of the classical
habitable zone cannot be ruled out. Transiting planets in this regime represent
promising targets for upcoming observatories like EChO and JWST.Comment: Accepted for publication in Astronomy and Astrophysics, complete
abstract in the pdf, 18 pages, 18 figure
BaCu3O4: High Temperature Magnetic Order in One-Dimensional S=1/2 Diamond-Chains
The magnetic properties of the alkaline earth oxocuprate BaCu3O4 are
investigated. We show that the characteristic Cu3O4 layers of this material can
be described with diamond chains of antiferromagnetically coupled Cu 1/2 spins
with only a weak coupling between two adjacent chains. These Cu3O4 layers seem
to represent a so far unique system of weakly coupled one-dimensional magnetic
objects where the local AF ordering of the Cu2+ ions leads to an actual net
magnetic moment of an isolated diamond chain. We demonstrate a magnetic
transition at a high N\'eel temperature T_{N}=336 K
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Martian meso/micro-scale winds and surface energy budget
Regional, diurnal and seasonal variations of surface
temperature are particularly large on Mars. This is mostly due to the Martian surface remaining close to radiative equilibrium. Contrary to most terrestrial locations, contributions of sensible heat flux (i.e. conduction/convection exchanges between atmosphere and surface) to the surface energy budget [hereinafter SEB] are negligible on Mars owing to lowatmospheric density and heat capacity (e.g. Figure 2 in Savijärvi and Kauhanen, 2008). This radiative control of surface temperature is a key characteristic of the Martian environment and has crucial consequences on the the Martian geology, meteorology, exobiology, etc.
In order to identify the impact of this Martian peculiarity to near-surface regional-to-local atmospheric circulations,
we employ our recently-built Martian limited-area meteorological model (Spiga and Forget, 2009). We use horizontal resolutions adapted to the dynamical phenomena we aim to resolve: from several tens of kilometers to compute regional winds (mesoscale simulations) to several tens of meters to compute atmospheric boundary-layer winds (microscale or turbulent-resolving simulations, also called Large-Eddy Simulations, LES)
Unconventional high-energy-state contribution to the Cooper pairing in under-doped copper-oxide superconductor HgBaCaCuO
We study the temperature-dependent electronic B1g Raman response of a
slightly under-doped single crystal HgBaCaCuO with a
superconducting critical temperature Tc=122 K. Our main finding is that the
superconducting pair-breaking peak is associated with a dip on its
higher-energy side, disappearing together at Tc. This result hints at an
unconventional pairing mechanism, whereas spectral weight lost in the dip is
transferred to the pair-breaking peak at lower energies. This conclusion is
supported by cellular dynamical mean-field theory on the Hubbard model, which
is able to reproduce all the main features of the B1g Raman response and
explain the peak-dip behavior in terms of a nontrivial relationship between the
superconducting and the pseudo gaps.Comment: 7 pages 4 figure
Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs
The resistivity, Hall effect and transverse magnetoresistance (MR) have been
measured in low residual resistivity single crystals of LiFeAs. A comparison
with angle resolved photoemission spectroscopy and quantum oscillation data
implies that four carrier bands unevenly contribute to the transport. However
the scattering rates of the carriers all display the T^2 behavior expected for
a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2
variation permit us to extract the superconducting fluctuation contribution to
the conductivity. Though below Tc the anisotropy of superconductivity is rather
small, the superconducting fluctuations display a quasi ideal two-dimensional
behavior which persists up to 1.4 Tc. These results call for a refined
theoretical understanding of the multiband behavior of superconductivity in
this pnictide.Comment: 8pages with supplementary material, 6 figure
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