32,048 research outputs found
Comet dust as a mixture of aggregates and solid particles: model consistent with ground-based and space-mission results
The most successful model of comet dust presents comet particles as
aggregates of submicron grains. It qualitatively explains the spectral and
angular change in the comet brightness and polarization and is consistent with
the thermal infrared data and composition of the comet dust obtained {\it in
situ} for comet 1P/Halley. However, it experiences some difficulties in
providing a quantitative fit to the observational data. Here we present a model
that considers comet dust as a mixture of aggregates and compact particles. The
model is based on the Giotto and Stardust mission findings that both aggregates
(made mainly of organics, silicates, and carbon) and solid silicate particles
are present in the comet dust. We simulate aggregates as {\bf Ballistic
Cluster-Cluster Aggregates (BCCA)} and compact particles as polydisperse
spheroids with some distribution of the aspect ratio. The particles follow a
power-law size distribution with the power -3 that is close to the one obtained
for comet dust {\it in situ}, at studies of the Stardust returned samples, and
the results of ground-based observations of comets. The model provides a good
fit to the angular polarization curve. It also reproduces the positive spectral
gradient of polarization, red color of the dust, and {\bf low albedo. It also
has the ratio of compact to fluffy particles close to the one found {\it in
situ} for comet 1P/Halley} and the mass ratio of silicate to carbonaceous
materials equal to unity that is in accordance with the elemental abundances of
Halley's dust found by Giotto mission.Comment: "Earth and Planetary Science" (Japan), in pres
Phase separation in the vicinity of the surface of -(BEDT-TTF)Cu[N(CN)]Br by fast cooling
Partial suppression of superconductivity by fast cooling has been observed in
the organic superconductor -(BEDT-TTF)Cu[N(CN)]Br by two means:
a marked sample size effect on the magnetic susceptibility and direct imaging
of insulating regions by scanning microregion infrared reflectance
spectroscopy. Macroscopic insulating regions are found in the vicinity of the
crystalline surface after fast cooling, with diameters of 50--100 m and
depths of a few m. The very large in-plane penetration depth reported to
date ( 24--100 m) can be explained by the existence of the
insulating regions.Comment: Several rhetoric alternations to avoid misleadings. 6 pages, 3
figures. to be publihsed in Phys. Rev.
Zn and Ni doping effects on the low-energy spin excitations in LaSrCuO
Impurity effects of Zn and Ni on the low-energy spin excitations were
systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn, Ni) by
neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a spin gap of 4meV
below Tc in the antiferromagnetic(AF) incommensurate spin excitation. In
Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In
Zn:y=0.008 and Zn:y=0.011, however, the magnetic signals at 3meV decrease below
Tc and increase again at lower temperature, indicating an in-gap state. In
Zn:y=0.017, the low-energy spin state remains unchanged with decreasing
temperature, and elastic magnetic peaks appear below 20K then exponentially
increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below
3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however,
shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the
absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also
at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega
increases, the magnetic peak width broadens and the peak position, i.e.
incommensurability, shifts toward the magnetic zone center (pi pi). We
interpret the impurity effects as follows: Zn locally makes a
non-superconducting island exhibiting the in-gap state in the superconducting
sea with the spin gap. Zn reduces the superconducting volume fraction, thus
suppressing Tc. On the other hand, Ni primarily affects the superconducting
sea, and the spin excitations become more dispersive and broaden with
increasing energy, which is recognized as a consequence of the reduction of
energy scale of spin excitations. We believe that the reduction of energy scale
is relevant to the suppression of Tc.Comment: 13pages, 14figures; submitted to Phys. Rev.
Regularization dependence on phase diagram in Nambu-Jona-Lasinio model
We study the regularization dependence on meson properties and the phase
diagram of quark matter by using the two flavor Nambu-Jona-Lasinio model. We
find that the meson properties and the phase structure do not show drastically
difference depending the regularization procedures. We also find that the
location or the existence of the critical end point highly depends on the
regularization methods and the model parameters. Then we think that
regularization and parameters are carefully considered when one investigates
the QCD critical end point in the effective model studies.Comment: 28 page
High purith low defect FZ silicon
The most common intrinsic defects in dislocation-free float zone (FZ) silicon crystals are the A- and B-type swirl defects. The mechanisms of their formation and annihilation have been extensively studied. Another type of defect in dislocation-free FZ crystals is referred to as a D-type defect. Concentrations of these defects can be minimized by optimizing the growth conditions, and the residual swirls can be reduced by the post-growth extrinsic gettering process. Czochralski (Cz) silicon wafers are known to exhibit higher resistance to slip and warpage due to thermal stress than do FZ wafers. The Cz crystals containing dislocations are more resistant to dislocation movement than dislocated FZ crystals because of the locking of dislocations by oxygen atoms present in the Cz crystals. Recently a transverse magnetic field was applied during the FZ growth of extrinsic silicon. Resultant flow patterns, as revealed by striation etching and spreading resistance in Ga-doped silicon crystals, indicate strong effects of the transverse magnetic field on the circulation within the melt. At fields of 5500 gauss, the fluid flow in the melt volume is so altered as to affect the morphology of the growing crystal
Transcranial magnetic stimulation over sensorimotor cortex disrupts anticipatory reflex gain modulation for skilled action
Skilled interactions with new environments require flexible changes to the transformation from somatosensory signals to motor outputs. Transcortical reflex gains are known to be modulated according to task and environmental dynamics, but the mechanism of this modulation remains unclear. We examined reflex organization in the sensorimotor cortex. Subjects performed point- to- point arm movements into predictable force fields. When a small perturbation was applied just before the arm encountered the force field, reflex responses in the shoulder muscles changed according to the upcoming force field direction, indicating anticipatory reflex gain modulation. However, when a transcranial magnetic stimulation (TMS) was applied before the reflex response to such perturbations so that the silent period caused by TMS overlapped the reflex processing period, this modulation was abolished, while the reflex itself remained. Loss of reflex gain modulation could not be explained by reduced reflex amplitudes nor by peripheral effects of TMS on the muscles themselves. Instead, we suggest that TMS disrupted interneuronal networks in the sensorimotor cortex, which contribute to reflex gain modulation rather than reflex generation. We suggest that these networks normally provide the adaptability of rapid sensorimotor reflex responses by regulating reflex gains according to the current dynamical environment
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