161 research outputs found
Coherent-incoherent transition in the sub-Ohmic spin-boson model
We study the spin-boson model with a sub-Ohmic bath using a variational
method. The transition from coherent dynamics to incoherent tunneling is found
to be abrupt as a function of the coupling strength and to exist for
any power , where the bath coupling is described by . We find non-monotonic temperature dependence of the
two-level gap and a re-entrance regime close to the transition due
to non-adiabatic low-frequency bath modes. Differences between thermodynamic
and dynamic conditions for the transition as well as the limitations of the
simplified bath description are discussed.Comment: 12 pages, 4 figure
Environmental dependence of 8 μm luminosity functions of galaxies at z ~ 0.8: Comparison between RXJ1716.4+6708 and the AKARI NEP-deep field
Aims. We aim to reveal environmental dependence of infrared luminosity functions (IR LFs) of galaxies at z ~ 0.8 using the AKARI
satellite. AKARI’s wide field of view and unique mid-IR filters help us to construct restframe 8 μm LFs directly without relying on
SED models.
Methods. We construct restframe 8 μm IR LFs in the cluster region RXJ1716.4+6708 at z = 0.81, and compare them with a blank
field using the AKARI north ecliptic pole deep field data at the same redshift. AKARI’s wide field of view (10' × 10') is suitable to
investigate wide range of galaxy environments. AKARI’s 15 μm filter is advantageous here since it directly probes restframe 8 μm at
z ~ 0.8, without relying on a large extrapolation based on a SED fit, which was the largest uncertainty in previous work.
Results. We have found that cluster IR LFs at restframe 8 μm have a factor of 2.4 smaller L^∗ and a steeper faint-end slope than that
of the field. Confirming this trend, we also found that faint-end slopes of the cluster LFs becomes flatter and flatter with decreasing
local galaxy density. These changes in LFs cannot be explained by a simple infall of field galaxy population into a cluster. Physics
that can preferentially suppress IR luminous galaxies in high density regions is required to explain the observed results
Field-dependent dynamics of the Anderson impurity model
Single-particle dynamics of the Anderson impurity model in the presence of a
magnetic field are considered, using a recently developed local moment
approach that encompasses all energy scales, field and interaction strengths.
For strong coupling in particular, the Kondo scaling regime is recovered. Here
the frequency () and field ()
dependence of the resultant universal scaling spectrum is obtained in large
part analytically, and the field-induced destruction of the Kondo resonance
investigated. The scaling spectrum is found to exhibit the slow logarithmic
tails recently shown to dominate the zero-field scaling spectrum. At the
opposite extreme of the Fermi level, it gives asymptotically exact agreement
with results for statics known from the Bethe ansatz. Good agreement is also
found with the frequency and field-dependence of recent numerical
renormalization group calculations. Differential conductance experiments on
quantum dots in the presence of a magnetic field are likewise considered; and
appear to be well accounted for by the theory. Some new exact results for the
problem are also established
Re-weighting of somatosensory inputs from the foot and the ankle for controlling posture during quiet standing following trunk extensor muscles fatigue
The present study focused on the effects of trunk extensor muscles fatigue on
postural control during quiet standing under different somatosensory conditions
from the foot and the ankle. With this aim, 20 young healthy adults were asked
to stand as immobile as possible in two conditions of No fatigue and Fatigue of
trunk extensor muscles. In Experiment 1 (n = 10), somatosensation from the foot
and the ankle was degraded by standing on a foam surface. In Experiment 2 (n =
10), somatosensation from the foot and ankle was facilitated through the
increased cutaneous feedback at the foot and ankle provided by strips of
athletic tape applied across both ankle joints. The centre of foot pressure
displacements (CoP) were recorded using a force platform. The results showed
that (1) trunk extensor muscles fatigue increased CoP displacements under
normal somatosensatory conditions (Experiment 1 and Experiment 2), (2) this
destabilizing effect was exacerbated when somatosensation from the foot and the
ankle was degraded (Experiment 1), and (3) this destabilizing effect was
mitigated when somatosensation from the foot and the ankle was facilitated
(Experiment 2). Altogether, the present findings evidenced re-weighting of
sensory cues for controlling posture during quiet standing following trunk
extensor muscles fatigue by increasing the reliance on the somatosensory inputs
from the foot and the ankle. This could have implications in clinical and
rehabilitative areas
A Bose-Einstein condensation model for high-temperature superconductivity
I propose that a dopant charge singlet bonding state may arise from the
hybridization of molecular orbitals in a cluster containing 13 Cu atoms in the
CuO2 plane of the superconducting cuprates. This singlet state forms a
pre-formed pair with low binding energy that is spatially bounded and weakly
interacting, and that can undergo Bose-Einstein condensation. I show that this
model is able to account, in a quantitative and natural way, for many of the
thermodynamic and electronic characteristics of the superconducting cuprates,
including many of the key experimental ARPES, muSR and microwave results on the
temperature and doping dependencies of both the superfluid density and the
pairing strengths (superconducting gap, leading-edge-midpoint and psuedogap) in
these high-temperature superconductors.Comment: 28 pages, 9 figures, submitted to Phys. Rev.
Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: IV The Controlling Parameters of the Starburst SED
We combine the the stellar spectral synthesis code Starburst99, the nebular
modelling code MAPPINGSIII, and a 1-D dynamical evolution model of HII regions
around massive clusters of young stars to generate improved models of the
spectral energy distribution (SED) of starburst galaxies. We introduce a
compactness parameter, C, which characterizes the specific intensity of the
radiation field at ionization fronts in HII regions, and which controls the
shape of the far-IR dust re-emission, often referred to loosely as the dust
``temperature''. We also investigate the effect of metallicity on the overall
SED and in particular, on the strength of the PAH features. We provide
templates for the mean emission produced by the young compact HII regions, the
older (10 - 100 Myr) stars and for the wavelength-dependent attenuation
produced by a foreground screen of the dust used in our model. We demonstrate
that these components may be combined to produce a excellent fit to the
observed SEDs of star formation dominated galaxies which are often used as
templates (Arp 220 and NGC 6240). This fit extends from the Lyman Limit to
wavelengths of about one mm. The methods presented in both this paper and in
the previous papers of this series allow the extraction of the physical
parameters of the starburst region (star formation rates, star formation rate
history, mean cluster mass, metallicity, dust attenuation and pressure) from
the analysis of the pan-spectral SED.Comment: 35 pages, 21 figures, accepted for publication in ApJS full-res
available at http://www.strw.leidenuniv.nl/~brent/publications/SEDIV.pd
Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: I. The role of ISM pressure & the Molecular Cloud Dissipation Timescale
In this paper, we combine the stellar spectral synthesis code STARBURST 99,
the nebular modelling code MAPPINGS IIIq, a 1-D dynamical evolution model of
\HII regions around massive clusters of young stars and a simplified model of
synchrotron emissivity to produce purely theoretical self-consistent synthetic
spectral energy distributions (SEDs) for (solar metallicity) starbursts lasting
some years. These SEDs extend from the Lyman Limit to beyond 21 cm. We
find that two ISM parameters control the form of the SED; the pressure in the
diffuse phase of the ISM (or, equivalently, its density), and the molecular
cloud dissipation timescale. We present detailed SED fits to Arp 220 and NGC
6240, and we give the predicted colors for starburst galaxies derived from our
models for the IRAS and the Spitzer Space Observatory MIPS and IRAC
instruments. Our models reproduce the spread in observed colors of starburst
galaxies. Finally, we present absolute calibrations to convert observed fluxes
into star formation rates in the UV (GALEX), at optical wavelengths
(H), and in the IR (IRAS or the Spitzer Space Observatory). (Abstract
Truncated)Comment: 56 pages, 16 figures, accepted by The Apstrophysical Journal For
version with full, colour figures go to
http://www.mso.anu.edu.au/~bgroves/starburst
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