1,349 research outputs found
Microscopic nanomechanical dissipation in gallium arsenide resonators
We report on a systematic study of nanomechanical dissipation in
high-frequency (approximatively 300 MHz) gallium arsenide optomechanical disk
resonators, in conditions where clamping and fluidic losses are negligible.
Phonon-phonon interactions are shown to contribute with a loss background
fading away at cryogenic temperatures (3 K). Atomic layer deposition of alumina
at the surface modifies the quality factor of resonators, pointing towards the
importance of surface dissipation. The temperature evolution is accurately
fitted by two-level systems models, showing that nanomechanical dissipation in
gallium arsenide resonators directly connects to their microscopic properties.
Two-level systems, notably at surfaces, appear to rule the damping and
fluctuations of such high-quality crystalline nanomechanical devices, at all
temperatures from 3 to 300K
High Flux Helium Irradiation of Dispersion-Strengthened Tungsten Alloys and Effects of Heavy Metal Impurity Layer Deposition
Tungsten has been chosen as the plasma-facing material (PFM) for the divertor region in ITER and also a candidate PFM for future plasma-burning nuclear fusion reactors. During fusion device operation, PFMs will be exposed to low-energy He irradiation at high temperatures, resulting in sub-surface bubbles and surface morphology changes such as pores and fuzz. Carbide dispersion-strengthened W materials may enhance the ductility of W, but their behavior under high flux He irradiation remains unclear. In this work, the response of dispersion-strengthened tungsten materials to high flux, low energy He irradiation at high temperature is examined. Tungsten alloyed with 1, 5, or 10 wt. % tantalum carbide or titanium carbide exposed to these conditions result in surface pores, coral-like feature growth and sub-surface helium bubbles. Reactor-relevant helium irradiation (5x10 26_ m-2_ fluence) combined with high powered laser pulses to simulate off-normal reactor events does not significantly alter the surface morphology, as the surface nanostructures appear stable and cracks are only observed on a localized region of one sample. However, specimens show the development of an impurity layer on the surface, likely impurity deposition from the sample holder during irradiation, resulting in a mixed material layer on the surface. Helium bubbles exist in this impurity layer, and obscure conclusions about helium interactions with the carbide dispersoids. Nonetheless, it is clear that the dispersoid microstructure limits He bubble formation and subsequent surface nanostructuring, attributed to the dispersoid composition.</p
The Angular Momentum Evolution of 0.1-10 Msun Stars From the Birthline to the Main Sequence
(Abridged) Projected rotational velocities (vsini) have been measured for a
sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun)
located in the Orion star-forming complex. These measurements have been
supplemented with data from the literature for Orion stars with masses as low
as 0.1 Msun. The primary finding from analysis of these data is that the upper
envelope of the observed values of angular momentum per unit mass (J/M) varies
as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3
Msun. This power law extends smoothly into the domain of more massive stars (3
to 10 Msun), which in Orion are already on the ZAMS. This result stands in
sharp contrast to the properties of main sequence stars, which show a break in
the power law and a sharp decline in J/M with decreasing mass for stars with M
<2 Msun. A second result of our study is that this break is seen already among
the PMS stars in our Orion sample that are on radiative tracks, even though
these stars are only a few million years old. A comparison of rotation rates
seen for stars on either side of the convective-radiative boundary shows that
stars do not rotate as solid bodies during the transition from convective to
radiative tracks.Comment: to appear in Ap
Vitual kitchen : A dual-modal virtual reality platform for (re)learning of everyday life cooking activites in Alzheimer’s disease
International audienc
Tetrathiafulvalene-Based Phenanthroline Ligands: Synthesis, Crystal Structures, and Electronic Properties
Effects of structural factors on the pi-dimerization and/or disproportionation of the cation radical of extended TTF containing thiophene-based pi-conjugated spacers.
The electrochemical and chemical oxidation of extended TTF 4 and 5 are analysed by cyclic voltammetry, Visible/NIR and ESR spectroscopies, and the X-ray structures of the new salts 5·BF4(CH2Cl2) and 4·ClO4(THF)1/2 are presented. The effects of structural factors on the π-dimerization or the disproportionation reaction of the cation radical are shown. The oxidation of compound 4 presents the successive formation of stable cation radical and dication species both in dichloromethane (DCM) and in a CH3CN/THF mixture. In contrast, for compound 5, the stability of the oxidation states strongly depends on the nature of the solvent. In DCM, the oxidation of 5 proceeds by two close one-electron transfers while in CH3CN/THF the dication is directly formed via a two-electron process. The X-ray structures of the two salts reveal the formation of pi-dimers of cation radical. While the dimer (5(2))2+ is due mainly to π–π interactions between the conjugating spacer, the multiplication of the sulfur atoms in compound 4 contributes to stabilize the dimer by the combined effects of S–S and π–π interactions. Visible/NIR and ESR experiments confirm the higher tendency of 4+· to dimerize with the occurrence of dimer and monomer in solution, while for 5+· only the monomer is detected in DCM. On the other hand, by dissolution of 5·BF4(CH2Cl2) in CH3CN, only the neutral and the dicationic states of compounds 5 are observed owing to the disproportionation reaction
The [CII] 158 um Line Deficit in Ultraluminous Infrared Galaxies Revisited
We present a study of the [CII] 157.74 um fine-structure line in a sample of
15 ultraluminous infrared (IR) galaxies (L_IR>10^12 Lsun; ULIRGs) using the
Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We
confirm the observed order of magnitude deficit (compared to normal and
starburst galaxies) in the strength of the [CII] line relative to the far-IR
dust continuum emission found in our initial report (Luhman et al. 1998), but
here with a sample that is twice as large. This result suggests that the
deficit is a general phenomenon affecting 4/5 ULIRGs. We present an analysis
using observations of generally acknowledged photodissociation region (PDR)
tracers ([CII], [OI] 63 and 145 um, and FIR continuum emission), which suggests
that a high UV flux G_o incident on a moderate density n PDR could explain the
deficit. However, comparisons with other ULIRG observations, including CO
(1-0), [CI] (1-0), and 6.2 um polycyclic aromatic hydrocarbon (PAH) emission,
suggest that high G_o/n PDRs alone cannot produce a self-consistent solution
that is compatible with all of the observations. We propose that non-PDR
contributions to the FIR continuum can explain the apparent [CII] deficiency.
Here, unusually high G_o and/or n physical conditions in ULIRGs as compared to
those in normal and starburst galaxies are not required to explain the [CII]
deficit. Dust-bounded photoionization regions, which generate much of the FIR
emission but do not contribute significant [CII] emission, offer one possible
physical origin for this additional non-PDR component. Such environments may
also contribute to the observed suppression of FIR fine-structure emission from
ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The
implications for observations at higher redshifts are also revisited.Comment: to be published in The Astrophysical Journal, 58 page
The Angular Momentum Evolution of Very Low Mass Stars
We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap
Non-Equilibrium Thermodynamic Description of the Coupling between Structural and Entropic Modes in Supercooled Liquids
The density response of supercooled glycerol to an impulsive stimulated
thermal grating (q=0.63 micron^-1) has been studied in the temperature range
(T=200-340 K) where the structure rearrangement (alpha-relaxation) and thermal
diffusion occur on the same time scale. A strong interaction between the two
modes occurs giving rise to a dip in the T-dependence of the apparent thermal
conductivity and a flattening of the apparent alpha-relaxation time upon
cooling. A non-equilibrium thermodynamic (NET) model for the long time response
of relaxing fluids has been developed. The model is capable to reproduce the
experimental data and to explain the observed phenomenology.Comment: to be published in PRE Rapid Commu
Stable propagation of an ordered array of cracks during directional drying
We study the appearance and evolution of an array of parallel cracks in a
thin slab of material that is directionally dried, and show that the cracks
penetrate the material uniformly if the drying front is sufficiently sharp. We
also show that cracks have a tendency to become evenly spaced during the
penetration. The typical distance between cracks is mainly governed by the
typical distance of the pattern at the surface, and it is not modified during
the penetration. Our results agree with recent experimental work, and can be
extended to three dimensions to describe the properties of columnar polygonal
patterns observed in some geological formations.Comment: 8 pages, 4 figures, to appear in PR
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