447 research outputs found
Thermal conductivity of semiconductor superlattices: Experimental study of interface scattering
We present thermal conductivity measurements performed in three short-period
(GaAs)_9(AlAs)_5 superlattices. The samples were grown at different
temperatures, leading to different small scale roughness and broadening of the
interfaces. The cross-plane conductivity is measured with a differential 3w
method, at room temperature. The order of magnitude of the overall thermal
conductivity variation is consistent with existing theoretical models, although
the actual variation is smaller than expected
Packing and Hausdorff measures of stable trees
In this paper we discuss Hausdorff and packing measures of random continuous
trees called stable trees. Stable trees form a specific class of L\'evy trees
(introduced by Le Gall and Le Jan in 1998) that contains Aldous's continuum
random tree (1991) which corresponds to the Brownian case. We provide results
for the whole stable trees and for their level sets that are the sets of points
situated at a given distance from the root. We first show that there is no
exact packing measure for levels sets. We also prove that non-Brownian stable
trees and their level sets have no exact Hausdorff measure with regularly
varying gauge function, which continues previous results from a joint work with
J-F Le Gall (2006).Comment: 40 page
Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films
Mechanical control of magnetic properties in magnetostrictive thin films
offers the unexplored opportunity to employ surface wave acoustics in such a
way that acoustic triggers dynamic magnetic effects. The strain-induced
modulation of the magnetic anisotropy can play the role of a high frequency
varying effective magnetic field leading to ultrasonic tuning of electronic and
magnetic properties of nanostructured materials, eventually integrated in
semiconductor technology. Here, we report about the opportunity to employ
surface acoustic waves to trigger magnetocaloric effect in
MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase
transition, in an interval range around room temperature (0{\deg}C -
60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase
coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of
MnAs is responsible of the observed phenomenon. By a simple anelastic model we
describe the temperature and the external magnetic field dependence of such a
huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect
could be a further interesting route for dynamic and static caloritronics and
spintronics applications in semiconductor technology
Rigidity and intermediate phases in glasses driven by speciation
The rigid to floppy transitions and the associated intermediate phase in
glasses are studied in the case where the local structure is not fully
determined from the macroscopic concentration. The approach uses size
increasing cluster approximations and constraint counting algorithms. It is
shown that the location and the width of the intermediate phase and the
corresponding structural, mechanical and energetical properties of the network
depend crucially on the way local structures are selected at a given
concentration. The broadening of the intermediate phase is obtained for
networks combining a large amount of flexible local structural units and a high
rate of medium range order.Comment: 4 pages, 4 figure
Irreversible magnetization switching using surface acoustic waves
An analytical and numerical approach is developped to pinpoint the optimal
experimental conditions to irreversibly switch magnetization using surface
acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and
two switching mechanisms are considered. In precessional switching, a small
in-plane field initially tilts the magnetization and the passage of the SAW
modifies the magnetic anisotropy parameters through inverse magneto-striction,
which triggers precession, and eventually reversal. Using the micromagnetic
parameters of a fully characterized layer of the magnetic semiconductor
(Ga,Mn)(As,P), we then show that there is a large window of accessible
experimental conditions (SAW amplitude/wave-vector, field
amplitude/orientation) allowing irreversible switching. As this is a resonant
process, the influence of the detuning of the SAW frequency to the magnetic
system's eigenfrequency is also explored. Finally, another - non-resonant -
switching mechanism is briefly contemplated, and found to be applicable to
(Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small
perpendicular field is applied opposite the initial magnetization and the
passage of the SAW lowers the domain nucleation barrier.Comment: 11 pages, 4 figure
Dual random fragmentation and coagulation and an application to the genealogy of Yule processes
The purpose of this work is to describe a duality between a fragmentation
associated to certain Dirichlet distributions and a natural random coagulation.
The dual fragmentation and coalescent chains arising in this setting appear in
the description of the genealogy of Yule processes.Comment: 14 page
Universal behavior of internal friction in glasses below T : anharmonicity vs relaxation
Comparison of the internal friction at hypersonic frequencies between a few K
and the glass transition temperature Tg for various glasses brings out general
features. At low temperature, internal friction is only weakly dependent on the
material. At high temperature but still below Tg the internal friction for
strong glasses shows a T-independent plateau in a very wide domain of
temperature; in contrast, for fragile glass, a nearly linear variation of
internal friction with T is observed. Anharmonicity appears dominant over
thermally activated relaxational processes at high temperature.Comment: accepted in Physical Review
On Exceptional Times for generalized Fleming-Viot Processes with Mutations
If is a standard Fleming-Viot process with constant mutation rate
(in the infinitely many sites model) then it is well known that for each
the measure is purely atomic with infinitely many atoms. However,
Schmuland proved that there is a critical value for the mutation rate under
which almost surely there are exceptional times at which is a
finite sum of weighted Dirac masses. In the present work we discuss the
existence of such exceptional times for the generalized Fleming-Viot processes.
In the case of Beta-Fleming-Viot processes with index we
show that - irrespectively of the mutation rate and - the number of
atoms is almost surely always infinite. The proof combines a Pitman-Yor type
representation with a disintegration formula, Lamperti's transformation for
self-similar processes and covering results for Poisson point processes
Ischiofemoral impingement: the evolutionary cost of pelvic obstetric adaptation.
Funder: Flemmish research foundationThe risk for ischiofemoral impingement has been mainly related to a reduced ischiofemoral distance and morphological variance of the femur. From an evolutionary perspective, however, there are strong arguments that the condition may also be related to sexual dimorphism of the pelvis. We, therefore, investigated the impact of gender-specific differences in anatomy of the ischiofemoral space on the ischiofemoral clearance, during static and dynamic conditions. A random sampling Monte-Carlo experiment was performed to investigate ischiofemoral clearance during stance and gait in a large (n = 40 000) virtual study population, while using gender-specific kinematics. Subsequently, a validated gender-specific geometric morphometric analysis of the hip was performed and correlations between overall hip morphology (statistical shape analysis) and standard discrete measures (conventional metric approach) with the ischiofemoral distance were evaluated. The available ischiofemoral space is indeed highly sexually dimorphic and related primarily to differences in the pelvic anatomy. The mean ischiofemoral distance was 22.2 ± 4.3 mm in the females and 29.1 ± 4.1 mm in the males and this difference was statistically significant (P < 0.001). Additionally, the ischiofemoral distance was observed to be a dynamic measure, and smallest during femoral extension, and this in turn explains the clinical sign of pain in extension during long stride walking. In conclusion, the presence of a reduced ischiofemroal distance and related risk to develop a clinical syndrome of ischiofemoral impingement is strongly dominated by evolutionary effects in sexual dimorphism of the pelvis. This should be considered when female patients present with posterior thigh/buttock pain, particularly if worsened by extension. Controlled laboratory study
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