66 research outputs found
Diffusion in a Random Velocity Field: Spectral Properties of a Non-Hermitian Fokker-Planck Operator
We study spectral properties of the Fokker-Planck operator that describes
particles diffusing in a quenched random velocity field. This random operator
is non-Hermitian and has eigenvalues occupying a finite area in the complex
plane. We calculate the eigenvalue density and averaged one-particle Green's
function, for weak disorder and dimension d>2. We relate our results to the
time-evolution of particle density, and compare them with numerical
simulations.Comment: 4 pages, 2 figure
Transition from localized to extended eigenstates in the ensemble of power-law random banded matrices
We study statistical properties of the ensemble of large random
matrices whose entries decrease in a power-law fashion
. Mapping the problem onto a nonlinear
model with non-local interaction, we find a transition from localized
to extended states at . At this critical value of the system
exhibits multifractality and spectral statistics intermediate between the
Wigner-Dyson and Poisson one. These features are reminiscent of those typical
for the mobility edge of disordered conductors. We find a continuous set of
critical theories at , parametrized by the value of the coupling
constant of the model. At all states are expected to be
localized with integrable power-law tails. At the same time, for
the wave packet spreading at short time scale is superdiffusive: , which leads to a modification of the
Altshuler-Shklovskii behavior of the spectral correlation function. At
the statistical properties of eigenstates are similar to those
in a metallic sample in dimensions. Finally, the region
is equivalent to the corresponding Gaussian ensemble of random
matrices . The theoretical predictions are compared with results of
numerical simulations.Comment: 19 pages REVTEX, 4 figure
Statistics of delay times in mesoscopic systems as a manifestation of eigenfunction fluctuations
We reveal a general explicit relation between the statistics of delay times
in one-channel reflection from a mesoscopic sample of any spatial dimension and
the statistics of the eigenfunction intensities in its closed counterpart. This
opens a possibility to use experimentally measurable delay times as a sensitive
probe of eigenfunction fluctuations. For the particular case of quasi-one
dimensional geometry of the sample we use an alternative technique to derive
the probability density of partial delay times for any number of open channels.Comment: 12 pages; published version with updated reference
Freezing of dynamical exponents in low dimensional random media
A particle in a random potential with logarithmic correlations in dimensions
is shown to undergo a dynamical transition at . In
exact results demonstrate that , the static glass transition
temperature, and that the dynamical exponent changes from at high temperature to in the glass phase. The same
formulae are argued to hold in . Dynamical freezing is also predicted in
the 2D random gauge XY model and related systems. In a mapping between
dynamics and statics is unveiled and freezing involves barriers as well as
valleys. Anomalous scaling occurs in the creep dynamics.Comment: 5 pages, 2 figures, RevTe
Random Dirac Fermions and Non-Hermitian Quantum Mechanics
We study the influence of a strong imaginary vector potential on the quantum
mechanics of particles confined to a two-dimensional plane and propagating in a
random impurity potential. We show that the wavefunctions of the non-Hermitian
operator can be obtained as the solution to a two-dimensional Dirac equation in
the presence of a random gauge field. Consequences for the localization
properties and the critical nature of the states are discussed.Comment: 5 pages, Latex, 1 figure, version published in PR
Anderson Transitions
The physics of Anderson transitions between localized and metallic phases in
disordered systems is reviewed. The term ``Anderson transition'' is understood
in a broad sense, including both metal-insulator transitions and
quantum-Hall-type transitions between phases with localized states. The
emphasis is put on recent developments, which include: multifractality of
critical wave functions, criticality in the power-law random banded matrix
model, symmetry classification of disordered electronic systems, mechanisms of
criticality in quasi-one-dimensional and two-dimensional systems and survey of
corresponding critical theories, network models, and random Dirac Hamiltonians.
Analytical approaches are complemented by advanced numerical simulations.Comment: 63 pages, 39 figures, submitted to Rev. Mod. Phy
Haloes gone MAD: The Halo-Finder Comparison Project
[abridged] We present a detailed comparison of fundamental dark matter halo
properties retrieved by a substantial number of different halo finders. These
codes span a wide range of techniques including friends-of-friends (FOF),
spherical-overdensity (SO) and phase-space based algorithms. We further
introduce a robust (and publicly available) suite of test scenarios that allows
halo finder developers to compare the performance of their codes against those
presented here. This set includes mock haloes containing various levels and
distributions of substructure at a range of resolutions as well as a
cosmological simulation of the large-scale structure of the universe. All the
halo finding codes tested could successfully recover the spatial location of
our mock haloes. They further returned lists of particles (potentially)
belonging to the object that led to coinciding values for the maximum of the
circular velocity profile and the radius where it is reached. All the finders
based in configuration space struggled to recover substructure that was located
close to the centre of the host halo and the radial dependence of the mass
recovered varies from finder to finder. Those finders based in phase space
could resolve central substructure although they found difficulties in
accurately recovering its properties. Via a resolution study we found that most
of the finders could not reliably recover substructure containing fewer than
30-40 particles. However, also here the phase space finders excelled by
resolving substructure down to 10-20 particles. By comparing the halo finders
using a high resolution cosmological volume we found that they agree remarkably
well on fundamental properties of astrophysical significance (e.g. mass,
position, velocity, and peak of the rotation curve).Comment: 27 interesting pages, 20 beautiful figures, and 4 informative tables
accepted for publication in MNRAS. The high-resolution version of the paper
as well as all the test cases and analysis can be found at the web site
http://popia.ft.uam.es/HaloesGoingMA
Observational evidence for gravitationally trapped massive axion(-like) particles
Unexpected astrophysical observations can be explained by gravitationally
captured massive particles, which are produced inside the Sun or other Stars
and are accumulated over cosmic times. Their radiative decay in solar outer
space would give rise to a `self-irradiation' of the whole star, providing the
time-independent component of the corona heating source. In analogy with the
Sun-irradiated Earth atmosphere, the temperature and density gradient in the
corona - chromosphere transition region is suggestive for an omnipresent
irradiation of the Sun. The same scenario fits other astrophysical X-ray
observations. The radiative decay of a population of such elusive particles
mimics a hot gas. X-ray observatories, with an unrivalled sensitivity below ~10
keV, can search for such particles. The elongation angle relative to the Sun is
the relevant new parameter.Comment: 35 pages, LaTeX, 9 figures. Accepted by Astroparticle Physic
The Circumgalactic Medium in Massive Halos
This chapter presents a review of the current state of knowledge on the cool
(T ~ 1e4 K) halo gas content around massive galaxies at z ~ 0.2-2. Over the
last decade, significant progress has been made in characterizing the cool
circumgalactic gas in massive halos of Mh ~ 1e12-1e14 Msun at intermediate
redshifts using absorption spectroscopy. Systematic studies of halo gas around
massive galaxies beyond the nearby universe are made possible by large
spectroscopic samples of galaxies and quasars in public archives. In addition
to accurate and precise constraints for the incidence of cool gas in massive
halos, detailed characterizations of gas kinematics and chemical compositions
around massive quiescent galaxies at z ~ 0.5 have also been obtained. Combining
all available measurements shows that infalling clouds from external sources
are likely the primary source of cool gas detected at d >~ 100 kpc from massive
quiescent galaxies. The origin of the gas closer in is currently less certain,
but SNe Ia driven winds appear to contribute significantly to cool gas found at
d < 100 kpc. In contrast, cool gas observed at d <~ 200 kpc from luminous
quasars appears to be intimately connected to quasar activities on parsec
scales. The observed strong correlation between cool gas covering fraction in
quasar host halos and quasar bolometric luminosity remains a puzzle. Combining
absorption-line studies with spatially-resolved emission measurements of both
gas and galaxies is the necessary next step to address remaining questions.Comment: 29 pages, 7 figures, invited review to appear in "Gas Accretion onto
Galaxies", Astrophysics and Space Science Library, eds. A. Fox & R. Dave, to
be published by Springe
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