30,997 research outputs found
Vertex Expansion for the Bianchi I model
A perturbative expansion of Loop Quantum Cosmological transitions amplitudes
of Bianchi I models is performed. Following the procedure outlined in [1,2] for
isotropic models, it is shown that the resulting expansion can be written in
the form of a series of amplitudes each with a fixed number of transitions
mimicking a spin foam expansion. This analogy is more complete than in the
isotropic case, since there are now the additional anisotropic degrees of
freedom which play the role of `colouring' of the spin foams. Furthermore, the
isotropic expansion is recovered by integrating out the anisotropies.Comment: 15 pages, 2 figure
Experimental Study of the Role of Atomic Interactions on Quantum Transport
We report an experimental study of quantum transport for atoms confined in a
periodic potential and compare between thermal and BEC initial conditions. We
observe ballistic transport for all values of well depth and initial
conditions, and the measured expansion velocity for thermal atoms is in
excellent agreement with a single-particle model. For weak wells, the expansion
of the BEC is also in excellent agreement with single-particle theory, using an
effective temperature. We observe a crossover to a new regime for the BEC case
as the well depth is increased, indicating the importance of interactions on
quantum transport.Comment: 4 pages, 3 figure
A foundation for multi-level modelling
Multi-level modelling allows types and instances to be mixed in the same model, however there are several proposals for how meta- models can support this. This paper proposes a meta-circular basis for meta-modelling and shows how it supports two leading approaches to multi-level modelling
Local spinfoam expansion in loop quantum cosmology
The quantum dynamics of the flat Friedmann-Lemaitre-Robertson-Walker and
Bianchi I models defined by loop quantum cosmology have recently been
translated into a spinfoam-like formalism. The construction is facilitated by
the presence of a massless scalar field which is used as an internal clock. The
implicit integration over the matter variable leads to a nonlocal spinfoam
amplitude. In this paper we consider a vacuum Bianchi I universe and show that
by choosing an appropriate regulator a spinfoam expansion can be obtained
without selecting a clock variable and that the resulting spinfoam amplitude is
local.Comment: 12 page
Stimulated Raman scattering in an optical parametric oscillator based on periodically poled MgO-doped stoichiometric LiTaO3
The evolution versus pump power of the spectrum of a singly resonant optical
parametric oscillator based on an MgO-doped periodically poled stoichiometric
lithium tantalate crystal is observed. The onset of cascade Raman lasing due to
stimulated Raman scattering in the nonlinear crystal is analyzed. Spurious
frequency doubling and sum-frequency generation phenomena are observed and
understood. A strong reduction of the intracavity Raman scattering is obtained
by a careful adjustment of the cavity losses.Comment: 6 figure
Role of Activity in Human Dynamics
The human society is a very complex system; still, there are several
non-trivial, general features. One type of them is the presence of power-law
distributed quantities in temporal statistics. In this Letter, we focus on the
origin of power-laws in rating of movies. We present a systematic empirical
exploration of the time between two consecutive ratings of movies (the
interevent time). At an aggregate level, we find a monotonous relation between
the activity of individuals and the power-law exponent of the interevent-time
distribution. At an individual level, we observe a heavy-tailed distribution
for each user, as well as a negative correlation between the activity and the
width of the distribution. We support these findings by a similar data set from
mobile phone text-message communication. Our results demonstrate a significant
role of the activity of individuals on the society-level patterns of human
behavior. We believe this is a common character in the interest-driven human
dynamics, corresponding to (but different from) the universality classes of
task-driven dynamics.Comment: 5 pages, 6 figures. Accepted by EP
Classical, quantum and total correlations
We discuss the problem of separating consistently the total correlations in a
bipartite quantum state into a quantum and a purely classical part. A measure
of classical correlations is proposed and its properties are explored.Comment: 10 pages, 3 figure
Use of high-dimensional spectral data to evaluate organic matter, reflectance relationships in soils
Recent breakthroughs in remote sensing technology have led to the development of a spaceborne high spectral resolution imaging sensor, HIRIS, to be launched in the mid-1990s for observation of earth surface features. The effects of organic carbon content on soil reflectance over the spectral range of HIRIS, and to examine the contributions of humic and fulvic acid fractions to soil reflectance was evaluated. Organic matter from four Indiana agricultural soils was extracted, fractionated, and purified, and six individual components of each soil were isolated and prepared for spectral analysis. The four soils, ranging in organic carbon content from 0.99 percent, represented various combinations of genetic parameters such as parent material, age, drainage, and native vegetation. An experimental procedure was developed to measure reflectance of very small soil and organic component samples in the laboratory, simulating the spectral coverage and resolution of the HIRIS sensor. Reflectance in 210 narrow (10 nm) bands was measured using the CARY 17D spectrophotometer over the 400 to 2500 nm wavelength range. Reflectance data were analyzed statistically to determine the regions of the reflective spectrum which provided useful information about soil organic matter content and composition. Wavebands providing significant information about soil organic carbon content were located in all three major regions of the reflective spectrum: visible, near infrared, and middle infrared. The purified humic acid fractions of the four soils were separable in six bands in the 1600 to 2400 nm range, suggesting that longwave middle infrared reflectance may be useful as a non-destructive laboratory technique for humic acid characterization
Bose-Einstein Condensate Driven by a Kicked Rotor in a Finite Box
We study the effect of different heating rates of a dilute Bose gas confined
in a quasi-1D finite, leaky box. An optical kicked-rotor is used to transfer
energy to the atoms while two repulsive optical beams are used to confine the
atoms. The average energy of the atoms is localized after a large number of
kicks and the system reaches a nonequilibrium steady state. A numerical
simulation of the experimental data suggests that the localization is due to
energetic atoms leaking over the barrier. Our data also indicates a correlation
between collisions and the destruction of the Bose-Einstein condensate
fraction.Comment: 7 pages, 8 figure
Modeling the iron oxides and oxyhydroxides for the prediction of environmentally sensitive phase transformations
Iron oxides and oxyhydroxides are challenging to model computationally as
competing phases may differ in formation energies by only several kJ/mol, they
undergo magnetization transitions with temperature, their structures may
contain partially occupied sites or long-range ordering of vacancies, and some
loose structures require proper description of weak interactions such as
hydrogen bonding and dispersive forces. If structures and transformations are
to be reliably predicted under different chemical conditions, each of these
challenges must be overcome simultaneously, while preserving a high level of
numerical accuracy and physical sophistication. Here we present comparative
studies of structure, magnetization, and elasticity properties of iron oxides
and oxyhydroxides using density functional theory calculations with plane-wave
and locally-confined-atomic-orbital basis sets, which are implemented in VASP
and SIESTA packages, respectively. We have selected hematite, maghemite,
goethite, lepidocrocite, and magnetite as model systems from a total of 13
known iron oxides and oxyhydroxides; and use same convergence criteria and
almost equivalent settings in order to make consistent comparisons. Our results
show both basis sets can reproduce the energetic stability and magnetic
ordering, and are in agreement with experimental observations. There are
advantages to choosing one basis set over the other, depending on the intended
focus. In our case, we find the method using PW basis set most appropriate, and
combine our results to construct the first phase diagram of iron oxides and
oxyhydroxides in the space of competing chemical potentials, generated entirely
from first principlesComment: 46 pages - Accepted for publication in PRB (19 journal pages),
January 201
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