1,156 research outputs found
Many-body Green's function theory for electron-phonon interactions: the Kadanoff-Baym approach to spectral properties of the Holstein dimer
We present a Kadanoff-Baym formalism to study time-dependent phenomena for
systems of interacting electrons and phonons in the framework of many-body
perturbation theory. The formalism takes correctly into account effects of the
initial preparation of an equilibrium state, and allows for an explicit
time-dependence of both the electronic and phononic degrees of freedom. The
method is applied to investigate the charge neutral and non-neutral excitation
spectra of a homogeneous, two-site, two-electron Holstein model. This is an
extension of a previous study of the ground state properties in the Hartree
(H), partially self-consistent Born (Gd) and fully self-consistent Born (GD)
approximations published in Ref. [arXiv:1403.2968]. We show that choosing a
homogeneous ground state solution leads to unstable dynamics for a sufficiently
strong interaction, and that allowing a symmetry-broken state prevents this.
The instability is caused by the bifurcation of the ground state and understood
physically to be connected with the bipolaronic crossover of the exact system.
This mean-field instability persists in the partially self-consistent Born
approximation but is not found for the fully self-consistent Born
approximation. By understanding the stability properties, we are able to study
the linear response regime by calculating the density-density response function
by time-propagation. This functions amounts to a solution of the Bethe-Salpeter
equation with a sophisticated kernel. The results indicate that none of the
approximations is able to describe the response function during or beyond the
bipolaronic crossover for the parameters investigated. Overall, we provide an
extensive discussion on when the approximations are valid, and how they fail to
describe the studied exact properties of the chosen model system.Comment: 12 figure
Fast Fourier Optimization: Sparsity Matters
Many interesting and fundamentally practical optimization problems, ranging
from optics, to signal processing, to radar and acoustics, involve constraints
on the Fourier transform of a function. It is well-known that the {\em fast
Fourier transform} (fft) is a recursive algorithm that can dramatically improve
the efficiency for computing the discrete Fourier transform. However, because
it is recursive, it is difficult to embed into a linear optimization problem.
In this paper, we explain the main idea behind the fast Fourier transform and
show how to adapt it in such a manner as to make it encodable as constraints in
an optimization problem. We demonstrate a real-world problem from the field of
high-contrast imaging. On this problem, dramatic improvements are translated to
an ability to solve problems with a much finer grid of discretized points. As
we shall show, in general, the "fast Fourier" version of the optimization
constraints produces a larger but sparser constraint matrix and therefore one
can think of the fast Fourier transform as a method of sparsifying the
constraints in an optimization problem, which is usually a good thing.Comment: 16 pages, 8 figure
A General Setting for Geometric Phase of Mixed States Under an Arbitrary Nonunitary Evolution
The problem of geometric phase for an open quantum system is reinvestigated
in a unifying approach. Two of existing methods to define geometric phase, one
by Uhlmann's approach and the other by kinematic approach, which have been
considered to be distinct, are shown to be related in this framework. The
method is based upon purification of a density matrix by its uniform
decomposition and a generalization of the parallel transport condition obtained
from this decomposition. It is shown that the generalized parallel transport
condition can be satisfied when Uhlmann's condition holds. However, it does not
mean that all solutions of the generalized parallel transport condition are
compatible with those of Uhlmann's one. It is also shown how to recover the
earlier known definitions of geometric phase as well as how to generalize them
when degeneracy exists and varies in time.Comment: 4 pages, extended result
Bat Echolocation Research: A handbook for planning and conducting acoustic studies
Echolocation provides an acoustic window on the behavior of most species of bats. Donald R. Griffin's book (Listening in the Dark: The Acoustic Orientation of Bats and Men) gave us a preview of what we might hear through this window. This Handbook illustrates how a small window has turned into a giant screen. The study of bat echolocation has progressed from a boutique curiosity and source of wonder to a discipline spanning areas of science from neurobiology through behavior, ecology, evolution, and environmental science. Many people who study bats use echolocation as a focus for their work. To some this means, for example, neurobiology, communication behavior, or environmental assessment. Whatever the goal, diversity is an underlying and overarching reality. At one level, diversity means that different species take quite different approaches to echolocation. At another level, individual differences remind us that echolocation is a conscious behavior. The same bat may, for instance, use quite different echolocation calls over the course of a night, a season, or its life. How much of echolocation behavior is inherited, and how much is learned
Thermoregulation in free-ranging Nycteris thebaica (Nycteridae) during winter : no evidence of torpor
Bats are among the most heterothermic mammals, with nearly all species investigated under free-ranging
conditions to date exhibiting some degree of daily torpor and/or hibernation. We investigated thermoregulation
during late winter by seven Nycteris thebaica in a warm, semi-arid habitat in northern South
Africa, using temperature-sensitive transmitters to measure skin temperature (Tskin). Unexpectedly, we
found no evidence for any expression of daily torpor or hibernation based on a total of 86 days of data
from 7 bats (one male and six females), despite air temperatures as low as
∼10 ◦C. Instead, daytime Tskin
was distributed unimodally with most values in the 33–35 ◦C range, and a minimum Tskin of 28.4 ◦C at a
roost temperature of 24.6 â—¦C. There are several possible reasons why N. thebaica may avoid torpor, including
predation in roosts, and the long nightly foraging periods of this species compared to many others.http://www.elsevier.com/locate /mambiohb201
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CT measurements of two-phase flow in fractured porous media
The simulation of flow in naturally fractured reservoirs commonly divides the reservoir into two continua - the matrix system and the fracture system. Flow equations are written presuming that the primary flow between grid blocks occurs through the fracture system and that the primary fluid storage is in the matrix system. The dual porosity formulation of the equations assumes that there is no flow between matrix blocks while the dual permeability formulation allows fluid movement between matrix blocks. Since most of the fluid storage is contained in the matrix, recovery is dominated by the transfer of fluid from the matrix to the high conductivity fractures. The physical mechanisms influencing this transfer have been evaluated primarily through numerical studies. Relatively few experimental studies have investigated the transfer mechanisms. Early studies focused on the prediction of reservoir recoveries from the results of scaled experiments on single reservoir blocks. Recent experiments have investigated some of the mechanisms that are dominant in gravity drainage situations and in small block imbibition displacements. The mechanisms active in multiphase flow in fractured media need to be further illuminated, since some of the experimental results appear to be contradictory. This report describes the design, construction, and preliminary results of an experiment that studies imbibition displacement in two fracture blocks. Multiphase (oil/water) displacements will be conducted at the same rate on three core configurations. The configurations are a compact core, a two-block system with a 1 mm spacer between the blocks, and a two-block system with no spacer. The blocks are sealed in epoxy so that saturation measurements can be made throughout the displacement experiments using a Computed Tomography (CT) scanner
Accurate and efficient method for the treatment of exchange in a plane-wave basis.
Published versio
Torpor in dark times : patterns of heterothermy are associated with the lunar cycle in a nocturnal bird
Many studies have shown that endotherms become more heterothermic when the costs of thermoregulation are high and /or when limited energy availability constrains thermoregulatory capacity. However, the roles of many ecological variables, including constraints on foraging opportunities and/ or success, remain largely unknown. To test the prediction that thermoregulatory patterns should be related to foraging opportunities in a heterothermic endotherm, we examined the relationship between the lunar cycle and heterothermy in Freckled Nightjars (Caprimulgus tristigma), which are visually orienting, nocturnal insectivores that are dependent on ambient light to forage. This model system provides an opportunity to assess whether variation in foraging opportunities influences the expression of heterothermy. The nightjars were active and foraged for insects when moonlight was available but become inactive and heterothermic in the absence of moonlight. Lunar illumination was a much stronger predictor of the magnitude of heterothermic responses than was air temperature (Ta). Our data suggest that heterothermy was strongly related to variation in foraging opportunities associated with the lunar cycle, even though food abundance appeared to remain relatively high throughout the study period. Patterns of thermoregulation in this population of Freckled Nightjars provide novel insights into the environmental and ecological determinants of heterothermy, with the lunar cycle, and not Ta, being the strongest predictor of torpor use.The University of Pretoria; the Natural Sciences and Engineering Research Council, Canada; and the
DST/NRF Centre of Excellence at the Percy FitzPatrick
Institute.http://www.uk.sagepub.com/journals/Journal200933ab201
Fidelity amplitude of the scattering matrix in microwave cavities
The concept of fidelity decay is discussed from the point of view of the
scattering matrix, and the scattering fidelity is introduced as the parametric
cross-correlation of a given S-matrix element, taken in the time domain,
normalized by the corresponding autocorrelation function. We show that for
chaotic systems, this quantity represents the usual fidelity amplitude, if
appropriate ensemble and/or energy averages are taken. We present a microwave
experiment where the scattering fidelity is measured for an ensemble of chaotic
systems. The results are in excellent agreement with random matrix theory for
the standard fidelity amplitude. The only parameter, namely the perturbation
strength could be determined independently from level dynamics of the system,
thus providing a parameter free agreement between theory and experiment
Overview: Will Great Power Politics Threaten Arctic Sustainability?
The eight Arctic states (Canada, Denmark/Greenland, Finland, Iceland, Norway, Sweden, the Russian Federation, and the United States) wish to maintain a position of preeminence when it comes to dealing with matters of Arctic Ocean governance. However, major non-Arctic states, while recognizing the sovereign rights of the coastal states in their economic zones and on their continental shelves, have growing interests in the maritime Arctic relating to activities such as commercial shipping, oil and gas development, fishing and ship-based tourism. They are increasingly claiming to have a legitimate interest in being consulted when it comes to addressing matters relating to the governance of such activities. Many questions arise from these new formulations: What is the appropriate mechanism for introducing the concerns of the non-Arctic North Pacific countries in forums dealing with Arctic issues? Can they achieve a significant voice in the deliberations of the Arctic Council? Can they address these issues through other mechanisms
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