335 research outputs found
Anharmonic Self-Energy of Phonons: Ab Initio Calculations and Neutron Spin Echo Measurements
We have calculated (ab initio) and measured (by spin-echo techniques) the
anharmonic self-energy of phonons at the X-point of the Brillouin zone for
isotopically pure germanium. The real part agrees with former, less accurate,
high temperature data obtained by inelastic neutron scattering on natural
germanium. For the imaginary part our results provide evidence that transverse
acoustic phonons at the X-point are very long lived at low temperatures, i.e.
their probability of decay approaches zero, as a consequence of an unusual
decay mechanism allowed by energy conservation.Comment: 8 pages, 2 figures, pdf fil
A purely geometric distance to the binary star Atlas, a member of the Pleiades
We present radial velocity and new interferometric measurements of the double
star Atlas, which permit, with the addition of published interferometric data,
to precisely derive the orbital parameters of the binary system and the masses
of the components. The derived semi-major axis, compared with its measured
angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely
geometrical way. Under the assumption that the location of Atlas is
representative of the average distance of the cluster, we confirm the distance
value generally obtained through main sequence fitting, in contradiction with
the early Hipparcos result (118.3+-3.5 pc).Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Reduced dimensionality multiphysics model for efficient VCSEL optimization
The ICT scene is dominated by short-range intra-datacenter interconnects and networking, requiring high speed and stable operations at high temperatures. GaAs/AlGaAs vertical-cavity surface-emitting lasers (VCSELs) emitting at 850–980 nm have arisen as the main actors in this framework. Starting from our in-house 3D fully comprehensive VCSEL solver VENUS, in this work we present the possibility of downscaling the dimensionality of the simulation, ending up with a multiphysics 1D solver (D1ANA), which is shown to be capable of reproducing the experimental data very well. D1ANA is then extensively applied to optimize high-temperature operation, by modifying cavity detuning and distributed Bragg’s reflector lengths
A search for solar-like oscillations in the Am star HD 209625
The goal is to test the structure of hot metallic stars, and in particular
the structure of a near-surface convection zone using asteroseismic
measurements. Indeed, stellar models including a detailed treatement of the
radiative diffusion predict the existence of a near-surface convection zone in
order to correctly reproduce the anomalies in surface abundances that are
observed in Am stars. The Am star HD 209625 was observed with the Harps
spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory
(Chile) during 9 nights in August 2005. This observing run allowed us to
collect 1243 radial velocity (RV) measurements, with a standard deviation of
1.35 m/s. The power spectrum associated with these RV measurements does not
present any excess. Therefore, either the structure of the external layers of
this star does not allow excitation of solar-like oscillations, or the
amplitudes of the oscillations remain below 20-30 cm/s (depending on their
frequency range).Comment: 5 pages, 4 figures, A&A accepte
Non-linear optical susceptibilities, Raman efficiencies and electrooptic tensors from first-principles density functional perturbation theory
The non-linear response of infinite periodic solids to homogenous electric
fields and collective atomic displacements is discussed in the framework of
density functional perturbation theory. The approach is based on the 2n + 1
theorem applied to an electric-field-dependent energy functional. We report the
expressions for the calculation of the non-linear optical susceptibilities,
Raman scattering efficiencies and electrooptic coefficients. Different
formulations of third-order energy derivatives are examined and their
convergence with respect to the k-point sampling is discussed. We apply our
method to a few simple cases and compare our results to those obtained with
distinct techniques. Finally, we discuss the effect of a scissors correction on
the EO coefficients and non-linear optical susceptibilities
Robust design of Si/Si3N4 high contrast grating mirror for mid-infrared VCSEL application
A Si/Si3N4 high contrast grating mirror has been designed for a VCSEL
integration in mid-infrared ({\lambda} = 2.65 m). The use of an
optimization algorithm which maximizes a VCSEL mirror quality factor allowed
the adjustment of the grating parameters while keeping large and shallow
grating pattern. The robustness with respect to fabrication error has been
enhanced thanks to a precise study of the grating dimension tolerances. The
final mirror exhibits large high reflectivity bandwidth with a polarization
selectivity and several percent of tolerance on the grating dimensions.Comment: The final publication is available at http://www.springerlink.com,
Optical and Quantum Electronics (2012) Online Firs
Anharmonic Decay of Vibrational States in Amorphous Silicon
Anharmonic decay rates are calculated for a realistic atomic model of
amorphous silicon. The results show that the vibrational states decay on
picosecond timescales and follow the two-mode density of states, similar to
crystalline silicon, but somewhat faster. Surprisingly little change occurs for
localized states. These results disagree with a recent experiment.Comment: 10 pages, 4 Postscript figure
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