1,499 research outputs found
First-principles study of the ferroelastic phase transition in CaCl_2
First-principles density-functional calculations within the local-density
approximation and the pseudopotential approach are used to study and
characterize the ferroelastic phase transition in calcium chloride (CaCl_2). In
accord with experiment, the energy map of CaCl_2 has the typical features of a
pseudoproper ferroelastic with an optical instability as ultimate origin of the
phase transition. This unstable optic mode is close to a pure rigid unit mode
of the framework of chlorine atoms and has a negative Gruneisen parameter. The
ab-initio ground state agrees fairly well with the experimental low temperature
structure extrapolated at 0K. The calculated energy map around the ground state
is interpreted as an extrapolated Landau free-energy and is successfully used
to explain some of the observed thermal properties. Higher-order anharmonic
couplings between the strain and the unstable optic mode, proposed in previous
literature as important terms to explain the soft-phonon temperature behavior,
are shown to be irrelevant for this purpose. The LAPW method is shown to
reproduce the plane-wave results in CaCl_2 within the precision of the
calculations, and is used to analyze the relative stability of different phases
in CaCl_2 and the chemically similar compound SrCl_2.Comment: 9 pages, 6 figures, uses RevTeX
High-temperature phase transitions in SrBi_2Ta_2O_9 film: a study by THz spectroscopy
Time-domain THz transmission experiment was performed on a film deposited on sapphire substrate. Temperatures between 300
and 923 K were investigated and complex permittivity spectra of the film were
determined. The lowest frequency optic phonon near 28 cm reveals a slow
monotonic decrease in frequency on heating with no significant anomaly near the
phase transitions. We show that the dielectric anomaly near the ferroelectric
phase transition can be explained by slowing down of a relaxational mode,
observed in the THz spectra. A second harmonic generation signal observed in a
single crystal confirms a loss of center of symmetry in the ferroelectric phase
and a presence of polar clusters in the intermediate ferroelastic phase.Comment: subm. to J. Phys.: Condens. Matte
New Symmetries in Crystals and Handed Structures
For over a century, the structure of materials has been described by a
combination of rotations, rotation-inversions and translational symmetries. By
recognizing the reversal of static structural rotations between clockwise and
counterclockwise directions as a distinct symmetry operation, here we show that
there are many more structural symmetries than are currently recognized in
right- or left-handed handed helices, spirals, and in antidistorted structures
composed equally of rotations of both handedness. For example, though a helix
or spiral cannot possess conventional mirror or inversion symmetries, they can
possess them in combination with the rotation reversal symmetry. Similarly, we
show that many antidistorted perovskites possess twice the number of symmetry
elements as conventionally identified. These new symmetries predict new forms
for "roto" properties that relate to static rotations, such as rotoelectricity,
piezorotation, and rotomagnetism. They also enable symmetry-based search for
new phenomena, such as multiferroicity involving a coupling of spins, electric
polarization and static rotations. This work is relevant to structure-property
relationships in all material structures with static rotations such as
minerals, polymers, proteins, and engineered structures.Comment: 15 Pages, 4 figures, 3 Tables; Fig. 2b has error
Qualitative characterization of healthcare wastes
The biological hazard inherent in the clinical wastes should be considered during the management and treatment process as well as the disposal into the environment. In this chapter, the risks associated with the clinical wastes as well as the management of these wastes are discussed. The chapter focused on reviewing the types of healthcare wastes generated from hospitals and clinics as well as the regulations and management practices used for these wastes. Moreover, the health risk associated with the infectious agents which have the potential to be transmitted into the environment. It has appeared that the clinical wastes represent real hazards for the human health and the environment if they were not managed properly
Amorphous AlN films grown by ALD from trimethylaluminum and monomethylhydrazine
The great interest in aluminium nitride thin films has been attributed to their excellent dielectric, thermal and mechanical properties. Here we present the results of amorphous AlN films obtained by atomic layer deposition. We used trimethylaluminum and monomethylhydrazine as the precursors at a deposition temperature of 375-475 °C. The structural and mechanical properties and chemical composition of the synthesized films were investigated in detail by X-ray diffraction, X-ray photoelectron spectroscopy, electron and probe microscopy and nanoindentation. The obtained films were compact and continuous, exhibiting amorphous nature with homogeneous in-depth composition, at an oxygen content of as low as 4 at%. The mechanical properties were comparable to those of AlN films produced by other techniques
Magnetic superspace groups and symmetry constraints in incommensurate magnetic phases
Although superspace formalism has become the standard approach for the
analysis of structurally modulated crystals, it has remained during the last
thirty years almost unexplored as a practical tool to deal with magnetic
incommensurate structures. This situation has recently changed with the
development of new computer tools for magnetic phases based on this formalism.
In this context we show here that, as in the case of nonmagnetic incommensurate
systems, the concept of superspace symmetry provides a simple, efficient and
systematic way to characterize the symmetry and rationalize the structural and
physical properties of incommensurate magnetic materials. The method introduces
significant advantages over the most commonly employed method of representation
analysis for the description of the magnetic structure of a crystal. But, more
importantly, in contrast with that method, it consistently yields and
classifies all degrees of freedom of the system. The knowledge of the
superspace group of an incommensurate magnetic material allows to predict its
crystal tensor properties and to rationalize its phase diagram, previous to any
appeal to microscopic models or mechanisms. This is especially relevant when
the properties of incommensurate multiferroics are being studied. We present
first a summary of the superspace method under a very practical viewpoint
particularized to magnetic modulations. Its relation with the usual
representation analysis is then analyzed in detail, with the derivation of
important general rules for magnetic modulations with a single propagation
vector. The power and efficiency of the formalism is illustrated with various
selected examples, including some multiferroic materials
GeantV: Results from the prototype of concurrent vector particle transport simulation in HEP
Full detector simulation was among the largest CPU consumer in all CERN
experiment software stacks for the first two runs of the Large Hadron Collider
(LHC). In the early 2010's, the projections were that simulation demands would
scale linearly with luminosity increase, compensated only partially by an
increase of computing resources. The extension of fast simulation approaches to
more use cases, covering a larger fraction of the simulation budget, is only
part of the solution due to intrinsic precision limitations. The remainder
corresponds to speeding-up the simulation software by several factors, which is
out of reach using simple optimizations on the current code base. In this
context, the GeantV R&D project was launched, aiming to redesign the legacy
particle transport codes in order to make them benefit from fine-grained
parallelism features such as vectorization, but also from increased code and
data locality. This paper presents extensively the results and achievements of
this R&D, as well as the conclusions and lessons learnt from the beta
prototype.Comment: 34 pages, 26 figures, 24 table
Experimental evidence of stochastic resonance without tuning due to non Gaussian noises
In order to test theoretical predictions, we have studied the phenomenon of
stochastic resonance in an electronic experimental system driven by white non
Gaussian noise. In agreement with the theoretical predictions our main findings
are: an enhancement of the sensibility of the system together with a remarkable
widening of the response (robustness). This implies that even a single resonant
unit can reach a marked reduction in the need of noise tuning.Comment: 4 pages, 3 figure
Stochastic Resonance of Ensemble Neurons for Transient Spike Trains: A Wavelet Analysis
By using the wavelet transformation (WT), we have analyzed the response of an
ensemble of (=1, 10, 100 and 500) Hodgkin-Huxley (HH) neurons to {\it
transient} -pulse spike trains () with independent Gaussian noises.
The cross-correlation between the input and output signals is expressed in
terms of the WT expansion coefficients. The signal-to-noise ratio (SNR) is
evaluated by using the {\it denoising} method within the WT, by which the noise
contribution is extracted from output signals. Although the response of a
single (N=1) neuron to sub-threshold transient signals with noises is quite
unreliable, the transmission fidelity assessed by the cross-correlation and SNR
is shown to be much improved by increasing the value of : a population of
neurons play an indispensable role in the stochastic resonance (SR) for
transient spike inputs. It is also shown that in a large-scale ensemble, the
transmission fidelity for supra-threshold transient spikes is not significantly
degraded by a weak noise which is responsible to SR for sub-threshold inputs.Comment: 20 pages, 4 figure
Phonons from neutron powder diffraction
The spherically averaged structure function \soq obtained from pulsed
neutron powder diffraction contains both elastic and inelastic scattering via
an integral over energy. The Fourier transformation of \soq to real space, as
is done in the pair density function (PDF) analysis, regularizes the data, i.e.
it accentuates the diffuse scattering. We present a technique which enables the
extraction of off-center phonon information from powder diffraction experiments
by comparing the experimental PDF with theoretical calculations based on
standard interatomic potentials and the crystal symmetry. This procedure
(dynamics from powder diffraction(DPD)) has been successfully implemented for
two systems, a simple metal, fcc Ni, and an ionic crystal, CaF. Although
computationally intensive, this data analysis allows for a phonon based
modeling of the PDF, and additionally provides off-center phonon information
from powder neutron diffraction
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