525 research outputs found
Mid-infrared emission and absorption in strained and relaxed direct bandgap GeSn semiconductors
By independently engineering strain and composition, this work demonstrates
and investigates direct band gap emission in the mid-infrared range from GeSn
layers grown on silicon. We extend the room-temperature emission wavelength
above ~4.0 {\mu}m upon post-growth strain relaxation in layers with uniform Sn
content of 17 at.%. The fundamental mechanisms governing the optical emission
are discussed based on temperature-dependent photoluminescence, absorption
measurements, and theoretical simulations. Regardless of strain and
composition, these analyses confirm that single-peak emission is always
observed in the probed temperature range of 4-300 K, ruling out defect- and
impurity-related emission. Moreover, carrier losses into thermally-activated
non-radiative recombination channels are found to be greatly minimized as a
result of strain relaxation. Absorption measurements validate the direct band
gap absorption in strained and relaxed samples at energies closely matching
photoluminescence data. These results highlight the strong potential of GeSn
semiconductors as versatile building blocks for scalable, compact, and
silicon-compatible mid-infrared photonics and quantum opto-electronics
Discovery of the brightest T dwarf in the northern hemisphere
We report the discovery of a bright (H=12.77) brown dwarf designated SIMP
J013656.5+093347. The discovery was made as part of a near-infrared proper
motion survey, SIMP (Sondage Infrarouge de Mouvement Propre), which uses proper
motion and near-infrared/optical photometry to identify brown dwarf candidates.
A low resolution (lambda/dlambda~40) spectrum of this brown dwarf covering the
0.88-2.35 microns wavelength interval is presented. Analysis of the spectrum
indicates a spectral type of T2.5+/-0.5. A photometric distance of 6.4+/-0.3 pc
is estimated assuming it is a single object. Current observations rule out a
binary of mass ratio ~1 and separation >5 AU. SIMP 0136 is the brightest T
dwarf in the northern hemisphere and is surpassed only by Eps Indi Bab over the
whole sky. It is thus an excellent candidate for detailed studies and should
become a benchmark object for the early-T spectral class.Comment: 4 pages, 3 figures, To be published in November 1, 2006 issue of
ApJL. Following IAU recommendation, the survey acronym (IBIS) was changed to
SIM
Recoil velocity at 2PN order for spinning black hole binaries
We compute the flux of linear momentum carried by gravitational waves emitted
from spinning binary black holes at 2PN order for generic orbits. In particular
we provide explicit expressions of three new types of terms, namely
next-to-leading order spin-orbit terms at 1.5 PN order, spin-orbit tail terms
at 2PN order, and spin-spin terms at 2PN order. Restricting ourselves to
quasi-circular orbits, we integrate the linear momentum flux over time to
obtain the recoil velocity as function of orbital frequency. We find that in
the so-called superkick configuration the higher-order spin corrections can
increase the recoil velocity up to about a factor 3 with respect to the
leading-order PN prediction. Furthermore, we provide expressions valid for
generic orbits, and accurate at 2PN order, for the energy and angular momentum
carried by gravitational waves emitted from spinning binary black holes.
Specializing to quasi-circular orbits we compute the spin-spin terms at 2PN
order in the expression for the evolution of the orbital frequency and found
agreement with Mik\'oczi, Vas\'uth and Gergely. We also verified that in the
limit of extreme mass ratio our expressions for the energy and angular momentum
fluxes match the ones of Tagoshi, Shibata, Tanaka and Sasaki obtained in the
context of black hole perturbation theory.Comment: 28 pages (PRD format), 1 figure, reference added, version published
in PRD, except that the PRD version contains a sign error: the sign of the
RHS of Eqs.(4.26) and (4.27) is wrong; it has been corrected in this
replacemen
Nanoparticules d'or de l'imagerie par résonance magnétique à la radiosensibilisation
Cette thèse approfondit l'étude de nanoparticules d'or de 5 nm de diamètre recouvertes de diamideéthanethioldiethylènetriaminepentacétate de gadolinium (DTDTPA:Gd), un agent de contraste pour l'imagerie par résonance magnétique (IRM). En guise de ciblage passif, la taille des nanoparticules a été contrôlée afin d'utiliser le réseau de néovaisseaux poreux et perméable des tumeurs. De plus les tumeurs ont un drainage lymphatique déficient qui permet aux nanoparticules de demeurer plus longtemps dans le milieu interstitiel de la tumeur. Les expériences ont été effectuées sur des souris Balb/c femelles portant des tumeurs MC7-L1. La concentration de nanoparticules a pu être mesurée à l'IRM in vivo . La concentration maximale se retrouvait à la fin de l'infusion de 10 min. La concentration s'élevait à 0.3 mM dans la tumeur et de 0.12 mM dans le muscle environnant. Les nanoparticules étaient éliminées avec une demi-vie de 22 min pour les tumeurs et de 20 min pour le muscle environnant. Les nanoparticules ont été fonctionnalisées avec le peptide Tat afin de leur conférer des propriétés de ciblage actif La rétention de ces nanoparticules a ainsi été augmentée de 1600 %, passant d'une demi-vie d'élimination de 22 min à 350 min. La survie des souris a été mesurée à l'aide de courbes Kaplan-Meier et d'un modèle mathématique évalue l'efficacité de traitements. Le modèle nous permet, à l'aide de la vitesse de croissance des tumeurs et de l'efficacité des traitements, de calculer la courbe de survie des spécimens. Un effet antagoniste a été observé au lieu de l'effet synergétique attendu entre une infusion de Au@DTDTPA:Gd et l'irradiation aux rayons X. L'absence d'effet synergétique a été attribuée à l'épaisseur du recouvrement de DTDTPA:Gd qui fait écran aux électrons produits par l'or. De plus, le moyen d'ancrage du recouvrement utilise des thiols qui peuvent s'avérer être des capteurs de radicaux. De plus, contrairement a ce qui était escompté, un effet chimiothérapeutique de ces nanoparticules a été observé in vitro et in vivo . Par contre, le mécanisme précis de cet effet est encore à être expliquer, mais on sait déjà que les nanoparticules d'or affectent les fonctions des macrophages ainsi que l'angiogenèse
Hydraulic Conductivity and Water Retention Curve of Highly Compressible Materials- From a Mechanistic Approach through Phenomenological Models
Intrinsic universality in tile self-assembly requires cooperation
We prove a negative result on the power of a model of algorithmic
self-assembly for which it has been notoriously difficult to find general
techniques and results. Specifically, we prove that Winfree's abstract Tile
Assembly Model, when restricted to use noncooperative tile binding, is not
intrinsically universal. This stands in stark contrast to the recent result
that, via cooperative binding, the abstract Tile Assembly Model is indeed
intrinsically universal. Noncooperative self-assembly, also known as
"temperature 1", is where tiles bind to each other if they match on one or more
sides, whereas cooperative binding requires binding on multiple sides. Our
result shows that the change from single- to multi-sided binding qualitatively
improves the kinds of dynamics and behavior that these models of nanoscale
self-assembly are capable of. Our lower bound on simulation power holds in both
two and three dimensions; the latter being quite surprising given that
three-dimensional noncooperative tile assembly systems simulate Turing
machines. On the positive side, we exhibit a three-dimensional noncooperative
self-assembly tile set capable of simulating any two-dimensional noncooperative
self-assembly system.
Our negative result can be interpreted to mean that Turing universal
algorithmic behavior in self-assembly does not imply the ability to simulate
arbitrary algorithmic self-assembly processes.Comment: Added references. Improved presentation of definitions and proofs.
This article uses definitions from arXiv:1212.4756. arXiv admin note: text
overlap with arXiv:1006.2897 by other author
Interference of Clocks: A Quantum Twin Paradox
The phase of matter waves depends on proper time and is therefore susceptible
to special-relativistic (kinematic) and gravitational time dilation (redshift).
Hence, it is conceivable that atom interferometers measure general-relativistic
time-dilation effects. In contrast to this intuition, we show that light-pulse
interferometers without internal transitions are not sensitive to gravitational
time dilation, whereas they can constitute a quantum version of the
special-relativistic twin paradox. We propose an interferometer geometry
isolating the effect that can be used for quantum-clock interferometry.Comment: 9 Pages, 2 Figure
Planar parallel phonon Hall effect and local symmetry breaking
Y-kapellasite [Y3Cu9(OH)19Cl8] is a frustrated antiferromagnetic insulator
which remains paramagnetic down to a remarkably low N\'eel temperature of about
2 K. Having studied this material in the paramagnetic regime, in which phonons
are the only possible heat carriers, we report the observation of a planar
parallel thermal Hall effect coming unambiguously from phonons. This is an
advantage over the Kitaev quantum spin liquid candidates {\alpha}-RuCl3 and
Na2Co2TeO6 where in principle other heat carriers can be involved [1-4]. As it
happens, Y-kapellasite undergoes a structural transition attributed to the
positional freezing of a hydrogen atom below about 33 K. Above this transition,
the global crystal symmetry forbids the existence of a planar parallel signal -
the same situation as in Na2Co2TeO6 and cuprates [3-5]. This points to the
notion of a local symmetry breaking at the root of the phonon Hall effect. In
this context, the advantage of Y-kapellasite over Na2Co2TeO6 (with high levels
of Na disorder and stacking faults) and cuprates (with high levels of disorder
coming from dopants and oxygen vacancies) is its clean structure, where the
only degree of freedom available for local symmetry breaking is this hydrogen
atom randomly distributed over six equivalent positions above 33 K. This
provides a specific and concrete case for the general idea of local symmetry
breaking leading to the phonon Hall effect in a wide range of insulators.Comment: 16 pages, 4 figures, 3 table
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