66,294 research outputs found
Probing magnetic order in EELS of chromite spinels using both multiple scattering (FEFF8.2) and DFT (WIEN2k)
The electron energy loss near edge structure on the O K-edge from chromite spinels contains fine structure from the hybridisation of the O p-orbitals and the Cr d-orbitals. Unlike the aluminates, a non-spin polarised calculation of this fine structure differs significantly from experimental observations. This is due to the large magnetic moment on the Cr. Calculations using simplified collinear ordering of the spins and the local spin density approximation give much improved agreement. A real space multiple scattering formalism and a reciprocal space density functional formalism give results in substantial agreement. In general, the actual spin arrangement of these chromites is not known since they are typically frustrated magnetic systems with ordering temperatures in the 10–20 K range. The calculations are based on the hypothesis that dynamic short range order persists to room temperature over the time scale of the interaction with the fast electron. However, it is possible that the observed effects are due to the strong paramagnetism present at room temperatures but which it is not possible to simulate accurately at present
A Monte-Carlo Approach to Zero Energy Quantum Scattering
Monte-Carlo methods for zero energy quantum scattering are developed.
Starting from path integral representations for scattering observables, we
present results of numerical calculations for potential scattering and
scattering off a schematic nucleus. The convergence properties of
Monte-Carlo algorithms for scattering systems are analyzed using stochastic
differential equation as a path sampling method.Comment: 30 pages, LaTeX, 8 (uuencoded, tared and gziped) postscript figure
Coherent control using adaptive learning algorithms
We have constructed an automated learning apparatus to control quantum
systems. By directing intense shaped ultrafast laser pulses into a variety of
samples and using a measurement of the system as a feedback signal, we are able
to reshape the laser pulses to direct the system into a desired state. The
feedback signal is the input to an adaptive learning algorithm. This algorithm
programs a computer-controlled, acousto-optic modulator pulse shaper. The
learning algorithm generates new shaped laser pulses based on the success of
previous pulses in achieving a predetermined goal.Comment: 19 pages (including 14 figures), REVTeX 3.1, updated conten
Computation of Light Scattering in Young Stellar Objects
A Monte Carlo light scattering code incorporating aligned non-spherical
particles is described. The major effects on the flux distribution, linear
polarisation and circular polarisation are presented, with emphasis on the
application to Young Stellar Objects (YSOs). The need for models with
non-spherical particles in order to successfully model polarisation data is
reviewed. The ability of this type of model to map magnetic field structure in
embedded YSOs is described. The possible application to the question of the
origin of biomolecular homochirality via UV circular polarisation in star
forming regions is also briefly discussed.Comment: Accepted by The Journal of Quantitative Spectroscopy and Radiative
Transfer. Replaced version corrects an error in the definition of the sense
of Cpol in the published version and other minor errors found at the proof
stag
The opposition and tilt effects of Saturn's rings from HST observations
The two major factors contributing to the opposition brightening of Saturn's
rings are i) the intrinsic brightening of particles due to coherent
backscattering and/or shadow-hiding on their surfaces, and ii) the reduced
interparticle shadowing. We utilize the Hubble Space Telescope observations for
different elevation angles B to disentangle these contributions. We assume that
the intrinsic contribution is independent of B, so that any B dependence of the
phase curves is due to interparticle shadowing, which must also act similarly
for all colors. We construct a grid of dynamical/photometric simulation models
to fit the elevation-dependent part of opposition brightening. Eliminating the
modeled interparticle component yields the intrinsic contribution to the
opposition effect: for the B and A rings it is almost entirely due to coherent
backscattering; for the C ring, an intraparticle shadow hiding contribution may
also be present.
Based on our simulations, the width of the interparticle shadowing effect is
roughly proportional to B. This follows from the observation that as B
decreases, the scattering is primarily from the rarefied low filling factor
upper ring layers, whereas at larger 's the dense inner parts are visible.
The elevation angle dependence of interparticle shadowing also explains most of
the B ring tilt effect (the increase of brightness with elevation). From
comparison of the magnitude of the tilt effect at different filters, we show
that multiple scattering can account for at most a 10% brightness increase as B
-> 26^o, whereas the remaining 20% brightening is due to a variable degree of
interparticle shadowing. The negative tilt effect of the middle A ring is well
explained by the the same self-gravity wake models that account for the
observed A ring azimuthal brightness asymmetry.Comment: Accepted to icaru
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