32,763 research outputs found
Disentangling Cooper-pair formation above Tc from the pseudogap state in the cuprates
The discovery of the pseudogap in the cuprates created significant excitement
amongst physicists as it was believed to be a signature of pairing, in some
cases well above the room temperature. In this "pre-formed pairs" scenario, the
formation of pairs without quantum phase rigidity occurs below T*. These pairs
condense and develop phase coherence only below Tc. In contrast, several recent
experiments reported that the pseudogap and superconducting states are
characterized by two different energy scales, pointing to a scenario, where the
two compete. However a number of transport, magnetic, thermodynamic and
tunneling spectroscopy experiments consistently detect a signature of
phase-fluctuating superconductivity above leaving open the question of whether
the pseudogap is caused by pair formation or not. Here we report the discovery
of a spectroscopic signature of pair formation and demonstrate that in a region
of the phase diagram commonly referred to as the "pseudogap", two distinct
states coexist: one that persists to an intermediate temperature Tpair and a
second that extends up to T*. The first state is characterized by a doping
independent scaling behavior and is due to pairing above Tc, but significantly
below T*. The second state is the "proper" pseudogap - characterized by a
"checker board" pattern in STM images, the absence of pair formation, and is
likely linked to Mott physics of pristine CuO2 planes. Tpair has a universal
value around 130-150K even for materials with very different Tc, likely setting
limit on highest, attainable Tc in cuprates. The observed universal scaling
behavior with respect to Tpair indicates a breakdown of the classical picture
of phase fluctuations in the cuprates.Comment: 9 pages, 4 figure
Implications of Halo Inside-out Growth on the X-Ray Properties of Nearby Galaxy Systems within the Preheating Scenario
We present an entirely analytic model for a preheated, polytropic
intergalactic medium in hydrostatic equilibrium within a NFW dark halo
potential in which the evolution of the halo structure between major merger
events proceeds inside-out by accretion. This model is used to explain, within
a standard CDM cosmogony, the observed X-ray properties of nearby
relaxed, non-cooling flow groups and clusters of galaxies. We find that our
preferred solution to the equilibrium equations produces scaling relations in
excellent agreement with observations, while simultaneously accounting for the
typical structural characteristics of the distribution of the diffuse baryons.
In the class of preheating models, ours stands out because it offers a unified
description of the intrahalo medium for galaxy systems with total masses above
\sm 2\times 10^{13}\msun, does not produce baryonic configurations with large
isentropic cores, and reproduces faithfully the observed behavior of the gas
entropy at large radii. All this is achieved with a moderate level of energy
injection of about half a keV, which can be easily accommodated within the
limits of the total energy released by the most commonly invoked feedback
mechanisms, as well as with a polytropic index of 1.2, consistent with both
many observational determinations and predictions from high-resolution
gas-dynamical simulations of non-cooling flow clusters. More interestingly, our
scheme offers a physical motivation for the adoption of this specific value of
the polytropic index, as it is the one that best ensures the conservation after
halo virialization of the balance between the total specific energies of the
gas and dark matter components for the full range of masses investigated.Comment: 18 pages, 11 figures, accepted for publication in the Astrophysical
Journa
ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud
We present high-resolution (sub-parsec) observations of a giant molecular
cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band
6 observations trace the bulk of the molecular gas in CO(2-1) and high
column density regions in CO(2-1). Our target is a quiescent cloud (PGCC
G282.98-32.40, which we refer to as the "Planck cold cloud" or PCC) in the
southern outskirts of the galaxy where star-formation activity is very low and
largely confined to one location. We decompose the cloud into structures using
a dendrogram and apply an identical analysis to matched-resolution cubes of the
30 Doradus molecular cloud (located near intense star formation) for
comparison. Structures in the PCC exhibit roughly 10 times lower surface
density and 5 times lower velocity dispersion than comparably sized structures
in 30 Dor, underscoring the non-universality of molecular cloud properties. In
both clouds, structures with relatively higher surface density lie closer to
simple virial equilibrium, whereas lower surface density structures tend to
exhibit super-virial line widths. In the PCC, relatively high line widths are
found in the vicinity of an infrared source whose properties are consistent
with a luminous young stellar object. More generally, we find that the smallest
resolved structures ("leaves") of the dendrogram span close to the full range
of line widths observed across all scales. As a result, while the bulk of the
kinetic energy is found on the largest scales, the small-scale energetics tend
to be dominated by only a few structures, leading to substantial scatter in
observed size-linewidth relationships.Comment: Accepted by ApJ; 21 pages in AASTeX two-column styl
Recommended from our members
Scaling Effects in Laser-Based Additive Manufacturing Processes
Mechanical Engineerin
Deep high-resolution X-ray spectra from cool-core clusters
We examine deep XMM-Newton Reflection Grating Spectrometer (RGS) spectra from
the cores of three X-ray bright cool core galaxy clusters, Abell 262, Abell
3581 and HCG 62. Each of the RGS spectra show Fe XVII emission lines indicating
the presence of gas around 0.5 keV. There is no evidence for O VII emission
which would imply gas at still cooler temperatures. The range in detected gas
temperature in these objects is a factor of 3.7, 5.6 and 2 for Abell 262, Abell
3581 and HCG 62, respectively. The coolest detected gas only has a volume
filling fraction of 6 and 3 per cent for Abell 262 and Abell 3581, but is
likely to be volume filling in HCG 62. Chandra spatially resolved spectroscopy
confirms the low volume filling fractions of the cool gas in Abell 262 and
Abell 3581, indicating this cool gas exists as cold blobs. Any volume heating
mechanism aiming to prevent cooling would overheat the surroundings of the cool
gas by a factor of 4. If the gas is radiatively cooling below 0.5 keV, it is
cooling at a rate at least an order of magnitude below that at higher
temperatures in Abell 262 and Abell 3581 and two-orders of magnitude lower in
HCG 62. The gas may be cooling non-radiatively through mixing in these cool
blobs, where the energy released by cooling is emitted in the infrared. We find
very good agreement between smooth particle inference modelling of the cluster
and conventional spectral fitting. Comparing the temperature distribution from
this analysis with that expected in a cooling flow, there appears to be a even
larger break below 0.5 keV as compared with previous empirical descriptions of
the deviations of cooling flow models.Comment: 19 pages, 22 figures, accepted by MNRA
Chandra Observations of Galaxy Cluster Abell 2218
We present results from two observations (combined exposure of ~17 ks) of
galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory that were taken on October 19, 1999. Using a
Raymond-Smith single temperature plasma model corrected for galactic absorption
we find a mean cluster temperature of kT = 6.9+/-0.5 keV, metallicity of
0.20+/-0.13 (errors are 90 % CL) and rest-frame luminosity in the 2-10 keV
energy band of 6.2x10^{44} erg/s in a LambdaCDM cosmology with H_0=65 km/s/Mpc.
The brightness distribution within 4'.2 of the cluster center is well fit by a
simple spherical beta model with core radius 66".4 and beta = 0.705 . High
resolution Chandra data of the inner 2' of the cluster show the x-ray
brightness centroid displaced ~22" from the dominant cD galaxy and the presence
of azimuthally asymmetric temperature variations along the direction of the
cluster mass elongation. X-ray and weak lensing mass estimates are in good
agreement for the outer parts (r > 200h^{-1}) of the cluster; however, in the
core the observed temperature distribution cannot reconcile the x-ray and
strong lensing mass estimates in any model in which the intracluster gas is in
thermal hydrostatic equilibrium. Our x-ray data are consistent with a scenario
in which recent merger activity in A2218 has produced both significant
non-thermal pressure in the core and substructure along the line of sight; each
of these phenomena probably contributes to the difference between lensing and
x-ray core mass estimates.Comment: 33 pages, 6 figures, uses AASTeX 5.02, ApJ submitte
Dust temperature and CO-to-H2 conversion factor variations in the SFR-M* plane
Deep Herschel imaging and 12CO(2-1) line luminosities from the IRAM PdBI are
combined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sample
includes galaxies both on and above the main sequence (MS) traced by
star-forming galaxies in the SFR-M* plane. The far-infrared data are used to
derive dust masses, Mdust. Combined with an empirical prescription for the
dependence of the gas-to-dust ratio on metallicity (GDR), the CO luminosities
and Mdust values are used to derive for each galaxy the CO-to-H2 conversion
factor, alpha_co. Like in the local Universe, the value of alpha_co is a factor
of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We
also uncover a relation between alpha_co and dust temperature (Tdust; alpha_co
decreasing with increasing Tdust) as obtained from modified blackbody fits to
the far-infrared data. While the absolute normalization of the alpha_co(Tdust)
relation is uncertain, the global trend is robust against possible systematic
biases in the determination of Mdust, GDR or metallicity. Although we cannot
formally distinguish between a step and a smooth evolution of alpha_co with the
dust temperature, we can conclude that in galaxies of near-solar metallicity, a
critical value of Tdust=30K can be used to determine whether the appropriate
alpha_co is closer to the starburst value (1.0 Msun(K kms pc^2)^-1, if
Tdust>30K) or closer to the Galactic value (4.35 Msun (K kms pc^2)^-1, if
Tdust<30K). This indicator has the great advantage of being less subjective
than visual morphological classifications of mergers/SFGs, which can be
difficult at high z because of the clumpy nature of SFGs. In the absence of
far-infrared data, the offset of a galaxy from the main sequence (i.e.,
log[SSFR(galaxy)/SSFR_MS(M*,z)]) can be used to identify galaxies requiring the
use of an alpha_co conversion factor lower than the Galactic value.Comment: Accepted for publication in Astronomy and Astrophysics (A&A); 15
pages, 6 figures; V2: updated reference lis
Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductor LaFeAsO_(1-x)F_x
The nature and value of the order parameters (OPs) in the superconducting
Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of
intense debate, also connected to the pairing mechanism which is probably
unconventional. Point-contact Andreev-reflection experiments on
LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the
superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV,
which scales with the local Tc of the contact; a larger unconventional OP that
gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases
on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression
in the conductance around zero bias), that survives in the normal state up to
T* ~ 140 K (close to the Neel temperature of the undoped compound), which we
associate to antiferromagnetic spin fluctuations (AF SF) coexisting with
superconductivity. These findings point toward a complex, unconventional nature
of superconductivity in LaFeAsO_(1-x)F_x.Comment: 19 pages, 12 figures - one figure and some insets added, minor
changes to the tex
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