3,174 research outputs found
A Time-Dependent Dirichlet-Neumann Method for the Heat Equation
We present a waveform relaxation version of the Dirichlet-Neumann method for
parabolic problem. Like the Dirichlet-Neumann method for steady problems, the
method is based on a non-overlapping spatial domain decomposition, and the
iteration involves subdomain solves with Dirichlet boundary conditions followed
by subdomain solves with Neumann boundary conditions. However, each subdomain
problem is now in space and time, and the interface conditions are also
time-dependent. Using a Laplace transform argument, we show for the heat
equation that when we consider finite time intervals, the Dirichlet-Neumann
method converges, similar to the case of Schwarz waveform relaxation
algorithms. The convergence rate depends on the length of the subdomains as
well as the size of the time window. In this discussion, we only stick to the
linear bound. We illustrate our results with numerical experiments.Comment: 9 pages, 5 figures, Lecture Notes in Computational Science and
Engineering, Vol. 98, Springer-Verlag 201
Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution
An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, has
been searched for intervening MgII absorbers over the redshift range 0<z<0.15.
The total redshift path searched is 18.8, with the survey being 80% complete to
a 5-sigma rest-frame equivalent width, W_r(2796), of 0.6 Ang. Main results of
this work are: [1] Four systems were found, with a mean redshift of =0.06,
yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers with
W_r(2796)>0.6 Ang. This is consistent with the value expected if these systems
do not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6
Ang were found. It is a 2-sigma result to have a null detection of smaller
W_r(2796) systems. If this implies a turnover in the low W_r(2796) region of
the equivalent width distribution at z~0, then there is at least a 25%
reduction in the average galaxy gas cross section from z<0.2 galaxies. [3]
These systems have strong FeII absorption and are good candidates for damped
Ly-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). This
translates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs at
z~0. In tandem with the data analyzed by Rao & Turnshek, these results indicate
that the redshift number density of DLAs does not evolve from z~4 to z~0. If
the HI mass function does not evolve from z~0.5 to z~0, then the cosmological
HI mass density is also deduced to not evolve from z~4 to z~0. These z~0
results for MgII absorption-selected DLAs are at odds with those based upon
21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes
additional figures and tables and substantial modifications to the tex
Adiabatic Approximation for weakly open systems
We generalize the adiabatic approximation to the case of open quantum
systems, in the joint limit of slow change and weak open system disturbances.
We show that the approximation is ``physically reasonable'' as under wide
conditions it leads to a completely positive evolution, if the original master
equation can be written on a time-dependent Lindblad form. We demonstrate the
approximation for a non-Abelian holonomic implementation of the Hadamard gate,
disturbed by a decoherence process. We compare the resulting approximate
evolution with numerical simulations of the exact equation.Comment: New material added, references added and updated, journal reference
adde
Heat exchange mediated by a quantum system
We consider heat transfer between two thermal reservoirs mediated by a
quantum system using the generalized quantum Langevin equation. The thermal
reservoirs are treated as ensembles of oscillators within the framework of the
Drude-Ullersma model. General expressions for the heat current and thermal
conductance are obtained for arbitrary coupling strength between the reservoirs
and the mediator and for different temperature regimes. As an application of
these results we discuss the origin of Fourier's law in a chain of large, but
finite subsystems coupled to each other by the quantum mediators. We also
address a question of anomalously large heat current between the STM tip and
substrate found in a recent experiment. The question of minimum thermal
conductivity is revisited in the framework of scaling theory as a potential
application of the developed approach.Comment: 16 pages, 6 figure
Are you feeling lucky?:lottery-based scheduling for public displays
Scheduling content onto pervasive displays is a complex problem. Researchers have identified an array of potential requirements that can influence scheduling decisions, but the relative importance of these different requirements varies across deployments, with context, and over time. In this paper we describe the design and implementation of a lottery-based scheduling approach that allows for the combination of multiple scheduling policies and is easily extensible to accommodate new scheduling requirements
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
Non variability of intervening absorbers observed in the UVES spectra of the "naked-eye" GRB080319
The aim of this paper is to investigate the properties of the intervening
absorbers lying along the line of sight of Gamma-Ray Burst (GRB) 080319B
through the analysis of its optical absorption features. To this purpose, we
analyze a multi-epoch, high resolution spectroscopic observations (R=40000,
corresponding to 7.5 km/s) of the optical afterglow of GRB080319B (z=0.937),
taken with UVES at the VLT. Thanks to the rapid response mode (RRM), we
observed the afterglow just 8m:30s after the GRB onset when the magnitude was R
~ 12. This allowed us to obtain the best signal-to-noise, high resolution
spectrum of a GRB afterglow ever (S/N per resolution element ~ 50). Two further
RRM and target of opportunity observations were obtained starting 1.0 and 2.4
hours after the event, respectively. Four MgII absorption systems lying along
the line of sight to the afterglow have been detected in the redshift range 0.5
< z < 0.8, most of them showing a complex structure featuring several
components. Absorptions due to FeII, MgI and MnII are also present; they appear
in four, two and one intervening absorbers, respectively. One out of four
systems show a MgII2796 rest frame equivalent width larger than 1A. This
confirms the excess of strong MgII absorbers compared to quasars, with dn/dz =
0.9, ~ 4 times larger than the one observed along quasar lines of sight. In
addition, the analysis of multi-epoch, high-resolution spectra allowed us to
exclude a significant variability in the column density of the single
components of each absorber. Combining this result with estimates of the size
of the emitting region, we can reject the hypothesis that the difference
between GRB and QSO MgII absorbers is due to a different size of the emitting
regions.Comment: 10 pages, 15 ps figures, submitted to MNRA
Wetting and Minimal Surfaces
We study minimal surfaces which arise in wetting and capillarity phenomena.
Using conformal coordinates, we reduce the problem to a set of coupled boundary
equations for the contact line of the fluid surface, and then derive simple
diagrammatic rules to calculate the non-linear corrections to the Joanny-de
Gennes energy. We argue that perturbation theory is quasi-local, i.e. that all
geometric length scales of the fluid container decouple from the
short-wavelength deformations of the contact line. This is illustrated by a
calculation of the linearized interaction between contact lines on two opposite
parallel walls. We present a simple algorithm to compute the minimal surface
and its energy based on these ideas. We also point out the intriguing
singularities that arise in the Legendre transformation from the pure Dirichlet
to the mixed Dirichlet-Neumann problem.Comment: 22 page
Iron overload causes endolysosomal deficits modulated by NAADP-regulated 2-pore channels and RAB7A
Various neurodegenerative disorders are associated with increased brain iron content. Iron is known to cause oxidative stress, which concomitantly promotes cell death. Whereas endolysosomes are known to serve as intracellular iron storage organelles, the consequences of increased iron on endolysosomal functioning, and effects on cell viability upon modulation of endolysosomal iron release remain largely unknown. Here, we show that increasing intracellular iron causes endolysosomal alterations associated with impaired autophagic clearance of intracellular protein aggregates, increased cytosolic oxidative stress and increased cell death. These effects are subject to regulation by NAADP, a potent second messenger reported to target endolysosomal TPCNs (2-pore channels). Consistent with endolysosomal iron storage, cytosolic iron levels are modulated by NAADP, and increased cytosolic iron is detected when overexpressing active, but not inactive TPCNs, indicating that these channels can modulate endolysosomal iron release. Cell death triggered by altered intralysosomal iron handling is abrogated in the presence of an NAADP antagonist or when inhibiting RAB7A activity. Taken together, our results suggest that increased endolysosomal iron causes cell death associated with increased cytosolic oxidative stress as well as autophagic impairments, and these effects are subject to modulation by endolysosomal ion channel activity in a RAB7A-dependent manner. These data highlight alternative therapeutic strategies for neurodegenerative disorders associated with increased intracellular iron load
On the cosmological variation of the fine structure constant
A phenomenological model is proposed to explain the recent observed
cosmological variation of the fine structure constant as an effect of the
quantum vacuum, assuming a flat universe with cosmological constant
in the cases () equal to (0.3,0.7) and (1,0). Because
of the fourth Heisenberg relation, the lifetime of the virtual pairs of the
zero-point radiation must depend on the gravitational potential, so that the
quantum vacuum changes its density and acquires a relative permittivity
different from one. Since the matter was more concentrated in the past, the
gravitational potential of all the universe was stronger and the optical
density of the vacuum higher, the electron charge being then more renormalized
and smaller than now. The model is based on a first order Newtonian
approximation that is valid for the range of the observations, but not for very
high redshift, its prediction being that is
proportional to ,
being the scale factor. This agrees with the observations.Comment: 11 pages, one figure. In this new version, the effect of the
cosmological constant is considere
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