831 research outputs found
Microfluidic and Nanofluidic Cavities for Quantum Fluids Experiments
The union of quantum fluids research with nanoscience is rich with
opportunities for new physics. The relevant length scales in quantum fluids,
3He in particular, are comparable to those possible using microfluidic and
nanofluidic devices. In this article, we will briefly review how the physics of
quantum fluids depends strongly on confinement on the microscale and nanoscale.
Then we present devices fabricated specifically for quantum fluids research,
with cavity sizes ranging from 30 nm to 11 microns deep, and the
characterization of these devices for low temperature quantum fluids
experiments.Comment: 12 pages, 3 figures, Accepted to Journal of Low Temperature Physic
Stochastic volatility and leverage effect
We prove that a wide class of correlated stochastic volatility models exactly
measure an empirical fact in which past returns are anticorrelated with future
volatilities: the so-called ``leverage effect''. This quantitative measure
allows us to fully estimate all parameters involved and it will entail a deeper
study on correlated stochastic volatility models with practical applications on
option pricing and risk management.Comment: 4 pages, 2 figure
Nonlinear porous medium flow with fractional potential pressure
We study a porous medium equation, with nonlocal diffusion effects given by
an inverse fractional Laplacian operator. We pose the problem in n-dimensional
space for all t>0 with bounded and compactly supported initial data, and prove
existence of a weak and bounded solution that propagates with finite speed, a
property that is nor shared by other fractional diffusion models.Comment: 32 pages, Late
Scanning Fourier Spectroscopy: A microwave analog study to image transmission paths in quantum dots
We use a microwave cavity to investigate the influence of a movable absorbing
center on the wave function of an open quantum dot. Our study shows that the
absorber acts as a position-selective probe, which may be used to suppress
those wave function states that exhibit an enhancement of their probability
density near the region where the impurity is located. For an experimental
probe of this wave function selection, we develop a technique that we refer to
as scanning Fourier spectroscopy, which allows us to identify, and map out, the
structure of the classical trajectories that are important for transmission
through the cavity.Comment: 4 pages, 5 figure
Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24
Supergranulation is a component of solar convection that manifests itself on
the photosphere as a cellular network of around 35 Mm across, with a turnover
lifetime of 1-2 days. It is strongly linked to the structure of the magnetic
field. The horizontal, divergent flows within supergranule cells carry local
field lines to the cell boundaries, while the rotational properties of
supergranule upflows may contribute to the restoration of the poloidal field as
part of the dynamo mechanism that controls the solar cycle. The solar minimum
at the transition from cycle 23 to 24 was notable for its low level of activity
and its extended length. It is of interest to study whether the convective
phenomena that influences the solar magnetic field during this time differed in
character to periods of previous minima. This study investigates three
characteristics (velocity components, sizes and lifetimes) of solar
supergranulation. Comparisons of these characteristics are made between the
minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008,
respectively. It is found that whereas the lifetimes are equal during both
epochs (around 18 h), the sizes are larger in 1996 (35.9 +/- 0.3 Mm) than in
2008 (35.0 +/- 0.3 Mm), while the dominant horizontal velocity flows are weaker
(139 +/- 1 m/s in 1996; 141 +/- 1 m/s in 2008). Although numerical differences
are seen, they are not conclusive proof of the most recent minimum being
inherently unusual.Comment: 22 pages, 5 figures. Solar Physics, in pres
Small BGK waves and nonlinear Landau damping
Consider 1D Vlasov-poisson system with a fixed ion background and periodic
condition on the space variable. First, we show that for general homogeneous
equilibria, within any small neighborhood in the Sobolev space W^{s,p}
(p>1,s<1+(1/p)) of the steady distribution function, there exist nontrivial
travelling wave solutions (BGK waves) with arbitrary minimal period and
traveling speed. This implies that nonlinear Landau damping is not true in
W^{s,p}(s<1+(1/p)) space for any homogeneous equilibria and any spatial period.
Indeed, in W^{s,p} (s<1+(1/p)) neighborhood of any homogeneous state, the long
time dynamics is very rich, including travelling BGK waves, unstable
homogeneous states and their possible invariant manifolds. Second, it is shown
that for homogeneous equilibria satisfying Penrose's linear stability
condition, there exist no nontrivial travelling BGK waves and unstable
homogeneous states in some W^{s,p} (p>1,s>1+(1/p)) neighborhood. Furthermore,
when p=2,we prove that there exist no nontrivial invariant structures in the
H^{s} (s>(3/2)) neighborhood of stable homogeneous states. These results
suggest the long time dynamics in the W^{s,p} (s>1+(1/p)) and particularly, in
the H^{s} (s>(3/2)) neighborhoods of a stable homogeneous state might be
relatively simple. We also demonstrate that linear damping holds for initial
perturbations in very rough spaces, for linearly stable homogeneous state. This
suggests that the contrasting dynamics in W^{s,p} spaces with the critical
power s=1+(1/p) is a trully nonlinear phenomena which can not be traced back to
the linear level
Sequential design of computer experiments for the estimation of a probability of failure
This paper deals with the problem of estimating the volume of the excursion
set of a function above a given threshold,
under a probability measure on that is assumed to be known. In
the industrial world, this corresponds to the problem of estimating a
probability of failure of a system. When only an expensive-to-simulate model of
the system is available, the budget for simulations is usually severely limited
and therefore classical Monte Carlo methods ought to be avoided. One of the
main contributions of this article is to derive SUR (stepwise uncertainty
reduction) strategies from a Bayesian-theoretic formulation of the problem of
estimating a probability of failure. These sequential strategies use a Gaussian
process model of and aim at performing evaluations of as efficiently as
possible to infer the value of the probability of failure. We compare these
strategies to other strategies also based on a Gaussian process model for
estimating a probability of failure.Comment: This is an author-generated postprint version. The published version
is available at http://www.springerlink.co
The Nucleon Spectral Function at Finite Temperature and the Onset of Superfluidity in Nuclear Matter
Nucleon selfenergies and spectral functions are calculated at the saturation
density of symmetric nuclear matter at finite temperatures. In particular, the
behaviour of these quantities at temperatures above and close to the critical
temperature for the superfluid phase transition in nuclear matter is discussed.
It is shown how the singularity in the thermodynamic T-matrix at the critical
temperature for superfluidity (Thouless criterion) reflects in the selfenergy
and correspondingly in the spectral function. The real part of the on-shell
selfenergy (optical potential) shows an anomalous behaviour for momenta near
the Fermi momentum and temperatures close to the critical temperature related
to the pairing singularity in the imaginary part. For comparison the selfenergy
derived from the K-matrix of Brueckner theory is also calculated. It is found,
that there is no pairing singularity in the imaginary part of the selfenergy in
this case, which is due to the neglect of hole-hole scattering in the K-matrix.
From the selfenergy the spectral function and the occupation numbers for finite
temperatures are calculated.Comment: LaTex, 23 pages, 21 PostScript figures included (uuencoded), uses
prc.sty, aps.sty, revtex.sty, psfig.sty (last included
YREC: The Yale Rotating Stellar Evolution Code
The stellar evolution code YREC is outlined with emphasis on its applications
to helio- and asteroseismology. The procedure for calculating calibrated solar
and stellar models is described. Other features of the code such as a non-local
treatment of convective core overshoot, and the implementation of a
parametrized description of turbulence in stellar models, are considered in
some detail. The code has been extensively used for other astrophysical
applications, some of which are briefly mentioned at the end of the paper.Comment: 10 pages, 2 figures, ApSS accepte
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