6,523 research outputs found
Black hole formation from point-like particles in three-dimensional anti-de Sitter space
We study collisions of many point-like particles in three-dimensional anti-de
Sitter space, generalizing the known result with two particles. We show how to
construct exact solutions corresponding to the formation of either a black hole
or a conical singularity from the collision of an arbitrary number of massless
particles falling in radially from the boundary. We find that when going away
from the case of equal energies and discrete rotational symmetry, this is not a
trivial generalization of the two-particle case, but requires that the excised
wedges corresponding to the particles must be chosen in a very precise way for
a consistent solution. We also explicitly take the limit when the number of
particles goes to infinity and obtain thin shell solutions that in general
break rotational invariance, corresponding to an instantaneous and
inhomogeneous perturbation at the boundary. We also compute the stress-energy
tensor of the shell using the junction formalism for null shells and obtain
agreement with the point particle picture.Comment: 42 pages, 9 figures; v2: fixed some typo
Performance of the coupled cluster singles and doubles method on two-dimensional quantum dots
An implementation of the coupled-cluster single- and double excitations
(CCSD) method on two-dimensional quantum dots is presented. Advantages and
limitations are studied through comparison with other high accuracy approaches
for two to eight confined electrons. The possibility to effectively use a very
large basis set is found to be an important advantage compared to full
configuration interaction implementations. For the two to eight electron ground
states, with a confinement strength close to what is used in experiments, the
error in the energy introduced by truncating triple excitations and beyond is
shown to be on the same level or less than the differences in energy given by
two different Quantum Monte Carlo methods. Convergence of the iterative
solution of the coupled cluster equations is, for some cases, found for
surprisingly weak confinement strengths even when starting from a
non-interacting basis. The limit where the missing triple and higher
excitations become relevant is investigated through comparison with full
Configuration Interaction results.Comment: 11 pages, 1 figure, 5 table
Fermionization of two-component few-fermion systems in a one-dimensional harmonic trap
The nature of strongly interacting Fermi gases and magnetism is one of the
most important and studied topics in condensed-matter physics. Still, there are
many open questions. A central issue is under what circumstances strong
short-range repulsive interactions are enough to drive magnetic correlations.
Recent progress in the field of cold atomic gases allows to address this
question in very clean systems where both particle numbers, interactions and
dimensionality can be tuned. Here we study fermionic few-body systems in a one
dimensional harmonic trap using a new rapidly converging effective-interaction
technique, plus a novel analytical approach. This allows us to calculate the
properties of a single spin-down atom interacting with a number of spin-up
particles, a case of much recent experimental interest. Our findings indicate
that, in the strongly interacting limit, spin-up and spin-down particles want
to separate in the trap, which we interpret as a microscopic precursor of
one-dimensional ferromagnetism in imbalanced systems. Our predictions are
directly addressable in current experiments on ultracold atomic few-body
systems.Comment: 12 pages, 6 figures, published version including two appendices on
our new numerical and analytical approac
A streamwise-constant model of turbulent pipe flow
A streamwise-constant model is presented to investigate the basic mechanisms
responsible for the change in mean flow occuring during pipe flow transition.
Using a single forced momentum balance equation, we show that the shape of the
velocity profile is robust to changes in the forcing profile and that both
linear non-normal and nonlinear effects are required to capture the change in
mean flow associated with transition to turbulence. The particularly simple
form of the model allows for the study of the momentum transfer directly by
inspection of the equations. The distribution of the high- and low-speed
streaks over the cross-section of the pipe produced by our model is remarkably
similar to one observed in the velocity field near the trailing edge of the
puff structures present in pipe flow transition. Under stochastic forcing, the
model exhibits a quasi-periodic self-sustaining cycle characterized by the
creation and subsequent decay of "streamwise-constant puffs", so-called due to
the good agreement between the temporal evolution of their velocity field and
the projection of the velocity field associated with three-dimensional puffs in
a frame of reference moving at the bulk velocity. We establish that the flow
dynamics are relatively insensitive to the regeneration mechanisms invoked to
produce near-wall streamwise vortices and that using small, unstructured
background disturbances to regenerate the streamwise vortices is sufficient to
capture the formation of the high- and low-speed streaks and their segregation
leading to the blunting of the velocity profile characteristic of turbulent
pipe flow
An interpolatory ansatz captures the physics of one-dimensional confined Fermi systems
Interacting one-dimensional quantum systems play a pivotal role in physics.
Exact solutions can be obtained for the homogeneous case using the Bethe ansatz
and bosonisation techniques. However, these approaches are not applicable when
external confinement is present. Recent theoretical advances beyond the Bethe
ansatz and bosonisation allow us to predict the behaviour of one-dimensional
confined systems with strong short-range interactions, and new experiments with
cold atomic Fermi gases have already confirmed these theories. Here we
demonstrate that a simple linear combination of the strongly interacting
solution with the well-known solution in the limit of vanishing interactions
provides a simple and accurate description of the system for all values of the
interaction strength. This indicates that one can indeed capture the physics of
confined one-dimensional systems by knowledge of the limits using wave
functions that are much easier to handle than the output of typical numerical
approaches. We demonstrate our scheme for experimentally relevant systems with
up to six particles. Moreover, we show that our method works also in the case
of mixed systems of particles with different masses. This is an important
feature because these systems are known to be non-integrable and thus not
solvable by the Bethe ansatz technique.Comment: 22 pages including methods and supplementary materials, 11 figures,
title slightly change
Resolving all-order method convergence problems for atomic physics applications
The development of the relativistic all-order method where all single,
double, and partial triple excitations of the Dirac-Hartree-Fock wave function
are included to all orders of perturbation theory led to many important results
for study of fundamental symmetries, development of atomic clocks, ultracold
atom physics, and others, as well as provided recommended values of many atomic
properties critically evaluated for their accuracy for large number of
monovalent systems. This approach requires iterative solutions of the
linearized coupled-cluster equations leading to convergence issues in some
cases where correlation corrections are particularly large or lead to an
oscillating pattern. Moreover, these issues also lead to similar problems in
the CI+all-order method for many-particle systems. In this work, we have
resolved most of the known convergence problems by applying two different
convergence stabilizer methods, reduced linear equation (RLE) and direct
inversion of iterative subspace (DIIS). Examples are presented for B, Al,
Zn, and Yb. Solving these convergence problems greatly expands the
number of atomic species that can be treated with the all-order methods and is
anticipated to facilitate many interesting future applications
Aerosol exposure versus aerosol cooling of climate: what is the optimal emission reduction strategy for human health?
Particles, climate change, and health have thought-provoking interactions. Air pollution is one of the largest environmental problems concerning human health. On the other hand, aerosol particles can have a cooling effect on climate and a reduction of those emissions may result in an increased temperature globally, which in turn may have negative health effects. The objective of this work was to investigate the "total health effects" of aerosol emissions, which include both exposure to particles and consequences for climate change initiated by particles. As a case study the "total health effect" from ship emissions was derived by subtracting the number of deaths caused by exposure with the estimated number of lives saved from the cooling effect of the emissions. The analysis showed that, with current level of scientific understanding, it could not be determined whether ship emissions are negative or positive for human health on a short time scale. This first attempt to approximate the combined effect of particle emissions on health shows that reductions of particulate air pollution will in some cases (black carbon) have win-win effects on health and climate, but sometimes also cause a shift from particle exposure-related health effects towards an increasing risk of health consequences from climate change. Thus, measures to reduce aerosol emissions have to be coupled with climate change mitigation actions to achieve a full health benefit on a global level
Gravitational infall in the hard wall model
An infalling shell in the hard wall model provides a simple holographic model
for energy injection in a confining gauge theory. Depending on its parameters,
a scalar shell either collapses into a large black brane, or scatters between
the hard wall and the anti-de Sitter boundary. In the scattering regime, we
find numerical solutions that keep oscillating for as long as we have followed
their evolution, and we provide an analytic argument that shows that a black
brane can never be formed. This provides examples of states in infinite-volume
field theory that never thermalize. We find that the field theory expectation
value of a scalar operator keeps oscillating, with an amplitude that undergoes
modulation.Comment: 7 pages, 4 figure
Predictable Disruption Tolerant Networks and Delivery Guarantees
This article studies disruption tolerant networks (DTNs) where each node
knows the probabilistic distribution of contacts with other nodes. It proposes
a framework that allows one to formalize the behaviour of such a network. It
generalizes extreme cases that have been studied before where (a) either nodes
only know their contact frequency with each other or (b) they have a perfect
knowledge of who meets who and when. This paper then gives an example of how
this framework can be used; it shows how one can find a packet forwarding
algorithm optimized to meet the 'delay/bandwidth consumption' trade-off:
packets are duplicated so as to (statistically) guarantee a given delay or
delivery probability, but not too much so as to reduce the bandwidth, energy,
and memory consumption.Comment: 9 page
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