855 research outputs found
Pressure-induced structural transitions in MgH
The stability of MgH has been studied up to 20~GPa using
density-functional total-energy calculations. At ambient pressure
-MgH takes a TiO-rutile-type structure. -MgH is
predicted to transform into -MgH at 0.39~GPa. The calculated
structural data for - and -MgH are in very good agreement
with experimental values. At equilibrium the energy difference between these
modifications is very small, and as a result both phases coexist in a certain
volume and pressure field. Above 3.84~GPa -MgH transforms into
-MgH; consistent with experimental findings. Two further
transformations have been identified at still higher pressure: i) - to
-MgH at 6.73 GPa and (ii) - to -MgH at
10.26~GPa.Comment: 4 pages, 4 figure
Binary continuous random networks
Many properties of disordered materials can be understood by looking at
idealized structural models, in which the strain is as small as is possible in
the absence of long-range order. For covalent amorphous semiconductors and
glasses, such an idealized structural model, the continuous-random network, was
introduced 70 years ago by Zachariasen. In this model, each atom is placed in a
crystal-like local environment, with perfect coordination and chemical
ordering, yet longer-range order is nonexistent. Defects, such as missing or
added bonds, or chemical mismatches, however, are not accounted for. In this
paper we explore under which conditions the idealized CRN model without defects
captures the properties of the material, and under which conditions defects are
an inherent part of the idealized model. We find that the density of defects in
tetrahedral networks does not vary smoothly with variations in the interaction
strengths, but jumps from close-to-zero to a finite density. Consequently, in
certain materials, defects do not play a role except for being thermodynamical
excitations, whereas in others they are a fundamental ingredient of the ideal
structure.Comment: Article in honor of Mike Thorpe's 60th birthday (to appear in J.
Phys: Cond Matt.
Simulations of Time-Resolved X-Ray Diffraction in Laue Geometry
A method of computer simulation of Time-Resolved X-ray Diffraction (TRXD) in
asymmetric Laue (transmission) geometry with an arbitrary propagating strain
perpendicular to the crystal surface is presented. We present two case studies
for possible strain generation by short-pulse laser irradiation: (i) a
thermoelastic-like analytic model; (ii) a numerical model including effects of
electron-hole diffusion, Auger recombination, deformation potential and thermal
diffusion. A comparison with recent experimental results is also presented.Comment: 9 pages, 11 figure
Time dependence of Bragg forward scattering and self-seeding of hard x-ray free-electron lasers
Free-electron lasers (FELs) can now generate temporally short, high power
x-ray pulses of unprecedented brightness, even though their longitudinal
coherence is relatively poor. The longitudinal coherence can be potentially
improved by employing narrow bandwidth x-ray crystal optics, in which case one
must also understand how the crystal affects the field profile in time and
space. We frame the dynamical theory of x-ray diffraction as a set of coupled
waves in order to derive analytic expressions for the spatiotemporal response
of Bragg scattering from temporally short incident pulses. We compute the
profiles of both the reflected and forward scattered x-ray pulses, showing that
the time delay of the wave is linked to its transverse spatial shift
through the simple relationship , where
is the grazing angle of incidence to the diffracting planes. Finally,
we apply our findings to obtain an analytic description of Bragg forward
scattering relevant to monochromatically seed hard x-ray FELs.Comment: 11 pages, 6 figure
Spatiotemporal Response of Crystals in X-ray Bragg Diffraction
The spatiotemporal response of crystals in x-ray Bragg diffraction resulting
from excitation by an ultra-short, laterally confined x-ray pulse is studied
theoretically. The theory presents an extension of the analysis in symmetric
reflection geometry [1] to the generic case, which includes Bragg diffraction
both in reflection (Bragg) and transmission (Laue) asymmetric scattering
geometries. The spatiotemporal response is presented as a product of a
crystal-intrinsic plane wave spatiotemporal response function and an envelope
function defined by the crystal-independent transverse profile of the incident
beam and the scattering geometry. The diffracted wavefields exhibit amplitude
modulation perpendicular to the propagation direction due to both angular
dispersion and the dispersion due to Bragg's law. The characteristic measure of
the spatiotemporal response is expressed in terms of a few parameters: the
extinction length, crystal thickness, Bragg angle, asymmetry angle, and the
speed of light. Applications to self-seeding of hard x-ray free electron lasers
are discussed, with particular emphasis on the relative advantages of using
either the Bragg or Laue scattering geometries. Intensity front inclination in
asymmetric diffraction can be used to make snapshots of ultra-fast processes
with femtosecond resolution
Supersonic strain front driven by a dense electron-hole plasma
We study coherent strain in (001) Ge generated by an ultrafast
laser-initiated high density electron-hole plasma. The resultant coherent pulse
is probed by time-resolved x-ray diffraction through changes in the anomalous
transmission. The acoustic pulse front is driven by ambipolar diffusion of the
electron-hole plasma and propagates into the crystal at supersonic speeds.
Simulations of the strain including electron-phonon coupling, modified by
carrier diffusion and Auger recombination, are in good agreement with the
observed dynamics.Comment: 4 pages, 6 figure
On the Early History of Current Algebra
The history of Current Algebra is reviewed up to the appearance of the
Adler-Weisberger sum rule. Particular emphasis is given to the role current
algebra played for the historical struggle in strong interaction physics of
elementary particles between the S-matrix approach based on dispersion
relations and field theory. The question whether there are fundamental
particles or all hadrons are bound or resonant states of one another played an
important role in this struggle and is thus also regarded.Comment: 17 page
Topology of amorphous tetrahedral semiconductors on intermediate lengthscales
Using the recently-proposed ``activation-relaxation technique'' for
optimizing complex structures, we develop a structural model appropriate to
a-GaAs which is almost free of odd-membered rings, i.e., wrong bonds, and
possesses an almost perfect coordination of four. The model is found to be
superior to structures obtained from much more computer-intensive tight-binding
or quantum molecular-dynamics simulations. For the elemental system a-Si, where
wrong bonds do not exist, the cost in elastic energy for removing odd-membered
rings is such that the traditional continuous-random network is appropriate.
Our study thus provides, for the first time, direct information on the nature
of intermediate-range topology in amorphous tetrahedral semiconductors.Comment: 4 pages, Latex and 2 postscript figure
Field-Induced Quasiparticle Excitation in Ca(AlSi): Evidence for unconventional Superconductivity
The temperature () and magnetic field () dependence of the magnetic
penetration depth, , in Ca(AlSi) exhibits
significant deviation from that expected for conventional BCS superconductors.
In particular, it is inferred from a field dependence of () at 2.0 K that the quasiparticle excitation is strongly enhanced by the
Doppler shift. This suggests that the superconducting order parameter in
Ca(AlSi) is characterized by a small energy scale
K originating either from anisotropy or multi-gap
structure.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
- …