12,477 research outputs found
Sound velocity and absorption measurements under high pressure using picosecond ultrasonics in diamond anvil cell. Application to the stability study of AlPdMn
We report an innovative high pressure method combining the diamond anvil cell
device with the technique of picosecond ultrasonics. Such an approach allows to
accurately measure sound velocity and attenuation of solids and liquids under
pressure of tens of GPa, overcoming all the drawbacks of traditional
techniques. The power of this new experimental technique is demonstrated in
studies of lattice dynamics, stability domain and relaxation process in a
metallic sample, a perfect single-grain AlPdMn quasicrystal, and rare gas, neon
and argon. Application to the study of defect-induced lattice stability in
AlPdMn up to 30 GPa is proposed. The present work has potential for application
in areas ranging from fundamental problems in physics of solid and liquid
state, which in turn could be beneficial for various other scientific fields as
Earth and planetary science or material research
Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)
The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of
(Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect
(MOKE). A transient MOKE signal followed by low amplitude oscillations was
evidenced, with a strong dependence on applied magnetic field, temperature and
ps-ASP amplitude. Careful interferometric measurement of the layer's thickness
variation induced by the ps-ASP allowed us to model very accurately the
resulting signal, and interpret it as the strain modulated reflectivity
(differing for probe polarizations), independently from dynamic
magnetization effects.Comment: 6 pages, 5 figure
Optimal estimates of the diffusion coefficient of a single Brownian trajectory
Modern developments in microscopy and image processing are revolutionizing
areas of physics, chemistry and biology as nanoscale objects can be tracked
with unprecedented accuracy. The goal of single particle tracking is to
determine the interaction between the particle and its environment. The price
paid for having a direct visualization of a single particle is a consequent
lack of statistics. Here we address the optimal way of extracting diffusion
constants from single trajectories for pure Brownian motion. It is shown that
the maximum likelihood estimator is much more efficient than the commonly used
least squares estimate. Furthermore we investigate the effect of disorder on
the distribution of estimated diffusion constants and show that it increases
the probability of observing estimates much smaller than the true (average)
value.Comment: 8 pages, 5 figure
A case study of spin- Heisenberg model in a triangular lattice
We study the spin- model in a triangular lattice in presence of a uniaxial
anisotropy field using a Cluster Mean-Field approach (CMF). The interplay
between antiferromagnetic exchange, lattice geometry and anisotropy forces
Gutzwiller mean-field approaches to fail in a certain region of the phase
diagram. There, the CMF yields two supersolid (SS) phases compatible with those
present in the spin- XXZ model onto which the spin- system maps.
Between these two SS phases, the three-sublattice order is broken and the
results of the CMF depend heavily on the geometry and size of the cluster. We
discuss the possible presence of a spin liquid in this region.Comment: 7 pages, 4 figures, RevTeX 4. The abstract and conclusions have been
modified and the manuscript has been extende
Determination of Fluorescence Polarization and Absorption Anisotropy in Molecular Complexes Having Threefold Rotational Symmetry
The current work concerns investigation of the polarization properties of complex molecular ensembles exhibiting threefold (C3) rotational symmetry, particularly with regard to the interplay between their structure and dynamics of internal energy transfer. We assume that the molecules or chromophores in such complexes possess strongly overlapped spectra both for absorption and fluorescence. Such trimeric structures are widely found in biological preparations, as for example the trimer of C-phycocyanin (C-PC). Higher order aggregates, e.g. hex-amers and three-hexamer rods, are also investigated and compared with the trimer case. The theory addresses both steady-state and 8-pulse excitation and establishes some links between them. Monochromophoric, bichro-mophoric and trichromophoric molecular complexes are individually examined. For steady-state excitation, analytical formulas are reported for the degree of fluorescence polarization and absorption anisotropy. It is shown that the polarization is dependent on the chromophore inclination relative to the symmetry axis, the relative efficiencies of absorption and fluorescence by chromophores of different spectral types, and the rates of energy equilibration. To assess the validity of the theory, it has been applied to C-PC aggregates. Here it was found that different C-PC aggregates provide practically identical polarization response. For S-pulse excitation we give analytical formulas for determination of the fluorescence depolarization, and also the depolarization associated with absorption recovery, both for a monochromophoric trimer and some particular cases of bichromophoric trimer. More complicated systems are analyzed by computer modeling. Thus it transpires that the initial polarization anisotropy r(t = 0) takes the value 0.4 for all considered aggregates; the long-time limit r(t →∞) has about the same value as is associated with steady-state excitation. We also show that with steady-state excitation the degree of fluorescence polarization is practically equal for various C3 aggregates of C-PC, and that the major factor determining the polarization is the chromophore orientation relative to the symmetry axis
Spatial distribution of unidentified infrared bands and extended red emission in the compact galactic HII region Sh 152
We present visible and near IR images of the compact HII region Sh 152. Some
of these images reveal the presence of Extended Red Emission (ERE) around 698
nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and 6.2
micron. Other images show the near infrared (7-12 micron) continuous emission
of the nebula. The ERE emission is found to coincide with the ionized region
and significantly differ from the UIRBs location. Also some evidence is found
in favor of grains as carriers for ERE.Comment: 3 pages, 4 figures, to be published in the proceedings of the
colloquium "The universe as seen by ISO" help in Paris, October 20-23, 1998 ;
available in html format at http://www.obs-hp.fr/preprints.htm
The molecular and dusty composition of Betelgeuse's inner circumstellar environment
The study of the atmosphere of red supergiant stars in general and of
Betelgeuse (alpha Orionis) in particular is of prime importance to understand
dust formation and how mass is lost to the interstellar medium in evolved
massive stars. A molecular shell, the MOLsphere (Tsuji, 2000a), in the
atmosphere of Betelgeuse has been proposed to account for the near- and
mid-infrared spectroscopic observations of Betelgeuse. The goal is to further
test this hypothesis and to identify some of the molecules in this MOLsphere.
We report on measurements taken with the mid-infrared two-telescope beam
combiner of the VLTI, MIDI, operated between 7.5 and 13.5 m. The data are
compared to a simple geometric model of a photosphere surrounded by a warm
absorbing and emitting shell. Physical characteristics of the shell are
derived: size, temperature and optical depth. The chemical constituents are
determined with an analysis consistent with available infrared spectra and
interferometric data. We are able to account for the measured optical depth of
the shell in the N band, the ISO-SWS spectrum and K and L band interferometric
data with a shell whose inner and outer radii are given by the above range and
with the following species: H2O, SiO and Al2O3. These results confirm the
MOLsphere model. We bring evidence for more constituents and for the presence
of species participating in the formation of dust grains in the atmosphere of
the star, i.e. well below the distance at which the dust shell is detected. We
believe these results bring key elements to the understanding of mass loss in
Betelgeuse and red supergiants in general and bring support to the dust-driven
scenario.Comment: 11 pages, 10 figures, accepted for publication in A&
Three-dimensional Gross-Pitaevskii solitary waves in optical lattices: stabilization using the artificial quartic kinetic energy induced by lattice shaking
In this Letter, we show that a three-dimensional Bose-Einstein solitary wave
can become stable if the dispersion law is changed from quadratic to quartic.
We suggest a way to realize the quartic dispersion, using shaken optical
lattices. Estimates show that the resulting solitary waves can occupy as little
as -th of the Brillouin zone in each of the three directions and
contain as many as atoms, thus representing a \textit{fully
mobile} macroscopic three-dimensional object.Comment: 8 pages, 1 figure, accepted in Phys. Lett.
Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport
Excitation transfer through interacting systems plays an important role in
many areas of physics, chemistry, and biology. The uncontrollable interaction
of the transmission network with a noisy environment is usually assumed to
deteriorate its transport capacity, especially so when the system is
fundamentally quantum mechanical. Here we identify key mechanisms through which
noise such as dephasing, perhaps counter intuitively, may actually aid
transport through a dissipative network by opening up additional pathways for
excitation transfer. We show that these are processes that lead to the
inhibition of destructive interference and exploitation of line broadening
effects. We illustrate how these mechanisms operate on a fully connected
network by developing a powerful analytical technique that identifies the
invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we
show how these principles can explain the remarkable efficiency and robustness
of excitation energy transfer from the light-harvesting chlorosomes to the
bacterial reaction center in photosynthetic complexes and present a numerical
analysis of excitation transport across the Fenna-Matthew-Olson (FMO) complex
together with a brief analysis of its entanglement properties. Our results show
that, in general, it is the careful interplay of quantum mechanical features
and the unavoidable environmental noise that will lead to an optimal system
performance.Comment: 16 pages, 9 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/09014454 . New revised version;
discussion of entanglement properties enhance
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