97,160 research outputs found
Chemical control of orbital polarization in artificially structured transition-metal oxides: La2NiXO6 (X=B, Al, Ga, In) from first principles
The application of modern layer-by-layer growth techniques to
transition-metal oxide materials raises the possibility of creating new classes
of materials with rationally designed correlated electron properties. An
important step toward this goal is the demonstration that electronic structure
can be controlled by atomic composition. In compounds with partially occupied
transition-metal d shells, one important aspect of the electronic structure is
the relative occupancy of different d orbitals. Previous work has established
that strain and quantum confinement can be used to influence orbital occupancy.
In this paper we demonstrate a different modality for orbital control in
transition-metal oxide heterostructures, using density-functional band
calculations supplemented by a tight-binding analysis to show that the choice
of nontransition-metal counterion X in transition-metal oxide heterostructures
composed of alternating LaNiO3 and LaXO3 units strongly affects orbital
occupancy, changing the magnitude and in some cases the sign of the orbital
polarization
Prediction of combustion instability limit cycle oscillations by combining flame describing function simulations with a thermoacoustic network model
Accurate prediction of limit cycle oscillations resulting from combustion instability has been a long-standing challenge. The present work uses a coupled approach to predict the limit cycle characteristics of a combustor, developed at Cambridge University, for which experimental data are available (Balachandran, Ph.D. thesis, 2005). The combustor flame is bluff-body stabilised, turbulent and partially-premixed. The coupled approach combines Large Eddy Simulation (LES) in order to characterise the weakly non-linear response of the flame to acoustic perturbations (the Flame Describing Function (FDF)), with a low order thermoacoustic network model for capturing the acoustic wave behaviour. The LES utilises the open source Computational Fluid Dynamics (CFD) toolbox, OpenFOAM, with a low Mach number approximation for the flow-field and combustion modelled using the PaSR (Partially Stirred Reactor) model with a global one-step chemical reaction mechanism for ethylene/air. LES has not previously been applied to this partially-premixed flame, to our knowledge. Code validation against experimental data for unreacting and partially-premixed reacting flows without and with inlet velocity perturbations confirmed that both the qualitative flame dynamics and the quantitative response of the heat release rate were captured with very reasonable accuracy. The LES was then used to obtain the full FDF at conditions corresponding to combustion instability, using harmonic velocity forcing across six frequencies and four forcing amplitudes. The low order thermoacoustic network modelling tool used was the open source OSCILOS (http://www.oscilos.com). Validation of its use for limit cycle prediction was performed for a well-documented experimental configuration, for which both experimental FDF data and limit cycle data were available. The FDF data from the LES for the present test case was then imported into the OSCILOS geometry network and limit cycle oscillations of frequency 342 Hz and normalised velocity amplitude of 0.26 were predicted. These were in good agreement with the experimental values of 348 Hz and 0.21 respectively. This work thus confirms that a coupled numerical prediction of limit cycle behaviour is possible using an entirely open source numerical framework
Dynamical Mean Field Theory of Nickelate Superlattices
Dynamical mean field methods are used to calculate the phase diagram,
many-body density of states, relative orbital occupancy and Fermi surface shape
for a realistic model of -based superlattices. The model is derived
from density functional band calculations and includes oxygen orbitals. The
combination of the on-site Hunds interaction and charge-transfer between the
transition metal and the oxygen orbitals is found to reduce the orbital
polarization far below the levels predicted either by band structure
calculations or by many-body analyses of Hubbard-type models which do not
explicitly include the oxygen orbitals. The findings indicate that
heterostructuring is unlikely to produce one band model physics and demonstrate
the fundamental inadequacy of modeling the physics of late transition metal
oxides with Hubbard-like models.Comment: Values of orbitals polarizations reported in Fig. 2 corrected. We
thank E. Benckiser and M. Wu for pointing out the error
Secure Identification of Free-Floating Planets
Among the methods proposed to detect extrasolar planets, microlensing is the
only technique that can detect free-floating planets. Free-floating planets are
detected through the channel of short-duration isolated lensing events.
However, if a seemingly isolated planetary event is detected, it is difficult
to firmly conclude that the event is caused by a free-floating planet because a
wide-separation planet can also produce an isolated event. There were several
methods proposed to break the degeneracy between the isolated planetary events
produced by the free-floating and wide-separation planets, but they are
incomplete. In this paper, we show that free-floating planets can be securely
identified by conducting astrometric follow-up observations of isolated events
to be detected in future photometric lensing surveys by using high-precision
interferometers to be operated contemporarily with the photometric surveys. The
method is based on the fact that astrometric lensing effect covers much longer
range of the lens-source separation than the photometric effect. We demonstrate
that several astrometric follow-up observations of isolated planetary events
associated with source stars brighter than by using the {\it Space
Interferometry Mission} with an exposure time of for
each observation will make it possible to measure the centroid shift induced by
primaries with projected separations up to . Therefore, the
proposed method is far more complete than previously proposed methods that are
flawed by the limited applicability only to planets with projected separations
or planets accompanied by bright primaries.Comment: 5 pages including 2 figure
Mean magnetic field generation in sheared rotators
A generalized mean magnetic field induction equation for differential
rotators is derived, including a compressibility, and the anisotropy induced on
the turbulent quantities from the mean magnetic field itself and a mean
velocity shear. Derivations of the mean field equations often do not emphasize
that there must be anisotropy and inhomogeneity in the turbulence for mean
field growth. The anisotropy from shear is the source of a term involving the
product of the mean velocity gradient and the cross-helicity correlation of the
isotropic parts of the fluctuating velocity and magnetic field,
\lb{\bfv}\cdot{\bfb}\rb^{(0)}. The full mean field equations are derived to
linear order in mean fields, but it is also shown that the cross-helicity term
survives to all orders in the velocity shear. This cross-helicity term can
obviate the need for a pre-existing seed mean magnetic field for mean field
growth: though a fluctuating seed field is necessary for a non-vanishing
cross-helicity, the term can produce linear (in time) mean field growth of the
toroidal field from zero mean field. After one vertical diffusion time, the
cross-helicity term becomes sub-dominant and dynamo exponential
amplification/sustenance of the mean field can subsequently ensue. The
cross-helicity term should produce odd symmetry in the mean magnetic field, in
contrast to the usually favored even modes of the dynamo amplification in
sheared discs. This may be important for the observed mean field geometries of
spiral galaxies. The strength of the mean seed field provided by the cross-
helicity depends linearly on the magnitude of the cross-helicity.Comment: 15 pages, LaTeX, matches version accepted to ApJ, minor revision
Assessment of different urban traffic control strategy impacts on vehicle emissions
This paper investigates the influence of traffic signal control strategy on vehicle emissions, vehicle journey time and total throughput flow within a single isolated four-armed junction. Two pre-timed signal plans are considered, one with two-stages involving permissive-only opposing turns and the other with four-stages which has no conflicting traffic. Additionally, the increase in efficiency by utilising actuated signal timing where green time is re-optimised as flow values vary is investigated. A microscopic traffic simulation model is used to model flows and AIRE (Analysis of Instantaneous Road Emissions) microscopic emissions model is utilised to out- put emission levels from the flow data. A simple junction model shows that the two-stage signal plan is more efficient in both emis- sions and journey time. However, as the level of opposed turning vehicles and conflicting movement increases, the two-stage model moves to being the inferior signal plan choice and the four-stage plan outputs fewer emissions than the two-stage plan. A real-world example of a four-armed junction has been used in this study and from the traffic survey data and existing junction layout; it is rec- ommended that a two-stage plan is used as it produces lower amounts of emissions and shorter journey times compared to a four-stage plan. The results also show that nitrogen oxides (NOx) are the most sensitive to changes in flow followed by carbon dioxide (CO2), Black Carbon and then particulate matter (PM10)
Searching for sub-millisecond pulsars from highly polarized radio sources
Pulsars are among the most highly polarized sources in the universe. The NVSS
has catalogued 2 million radio sources with linear polarization measurements,
from which we have selected 253 sources, with polarization percentage greater
than 25%, as targets for pulsar searches. We believe that such a sample is not
biased by selection effects against ultra-short spin or orbit periods. Using
the Parkes 64m telescope, we conducted searches with sample intervals of 0.05
ms and 0.08 ms, sensitive to submillisecond pulsars. Unfortunately we did not
find any new pulsars.Comment: 2 pages 1 figure. To appear in "Young Neutron Stars and Their
Environments" (IAU Symposium 218, ASP Conference Proceedings), eds F. Camilo
and B. M. Gaensle
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