224 research outputs found
Expectations for the Deep Impact collision from cometary nuclei modelling
Using the cometary nucleus model developed by Espinasse et al. (1991), we
calculate the thermodynamical evolution of Comet 9P/Tempel 1 over a period of
360 years. Starting from an initially amorphous cometary nucleus which
incorporates an icy mixture of H2O and CO, we show that, at the time of Deep
Impact collision, the crater is expected to form at depths where ice is in its
crystalline form. Hence, the subsurface exposed to space should not be
primordial. We also attempt an order-of-magnitude estimate of the heating and
material ablation effects on the crater activity caused by the 370 Kg
projectile released by the DI spacecraft. We thus show that heating effects
play no role in the evolution of crater activity. We calculate that the CO
production rate from the impacted region should be about 300-400 times higher
from the crater resulting from the impact with a 35 m ablation than over the
unperturbed nucleus in the immediate post-impact period. We also show that the
H2O production rate is decreased by several orders of magnitude at the crater
base just after ablation
Josephson oscillation linewidth of ion-irradiated YBaCuO junctions
We report on the noise properties of ion-irradiated YBaCuO
Josephson junctions. This work aims at investigating the linewidth of the
Josephson oscillation with a detector response experiment at 132 GHz.
Experimental results are compared with a simple analytical model based on the
Likharev-Semenov equation and the de Gennes dirty limit approximation. We show
that the main source of low-frequency fluctuations in these junctions is the
broadband Johnson noise and that the excess () noise contribution
does not prevail in the temperature range of interest, as reported in some
other types of high-T superconducting Josephson junctions. Finally, we
discuss the interest of ion-irradiated junctions to implement frequency-tunable
oscillators consisting of synchronized arrays of Josephson junctions
Direct probing of band-structure Berry phase in diluted magnetic semiconductors
We report on experimental evidence of the Berry phase accumulated by the
charge carrier wave function in single-domain nanowires made from a
(Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the
mesoscopic transport measurements is revealed as unusual patterns in the
magnetoconductance, that are clearly distinguished from the universal
conductance fluctuations. We show that these patterns appear in a magnetic
field region where the magnetization rotates coherently and are related to a
change in the band-structure Berry phase as the magnetization direction
changes. They should be thus considered as a band structure Berry phase
fingerprint of the effective magnetic monopoles in the momentum space. We argue
that this is an efficient method to vary the band structure in a controlled way
and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting
test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure
Effects of electron-phonon interactions on the electron tunneling spectrum of PbS quantum dots
We present a tunnel spectroscopy study of single PbS Quantum Dots (QDs) as
function of temperature and gate voltage. Three distinct signatures of strong
electron-phonon coupling are observed in the Electron Tunneling Spectrum (ETS)
of these QDs. In the shell-filling regime, the degeneracy of the
electronic levels is lifted by the Coulomb interactions and allows the
observation of phonon sub-bands that result from the emission of optical
phonons. At low bias, a gap is observed in the ETS that cannot be closed with
the gate voltage, which is a distinguishing feature of the Franck-Condon (FC)
blockade. From the data, a Huang-Rhys factor in the range is
obtained. Finally, in the shell tunneling regime, the optical phonons appear in
the inelastic ETS .Comment: 5 pages, 5 figure
Dynamics of two coupled vortices in a spin valve nanopillar excited by spin transfer torque
We investigate the dynamics of two coupled vortices driven by spin transfer.
We are able to independently control with current and perpendicular field, and
to detect, the respective chiralities and polarities of the two vortices. For
current densities above , a highly coherent signal
(linewidth down to 46 kHz) can be observed, with a strong dependence on the
relative polarities of the vortices. It demonstrates the interest of using
coupled dynamics in order to increase the coherence of the microwave signal.
Emissions exhibit a linear frequency evolution with perpendicular field, with
coherence conserved even at zero magnetic field
Unusual magneto-transport of YBa2Cu3O7-d films due to the interplay of anisotropy, random disorder and nanoscale periodic pinning
We study the general problem of a manifold of interacting elastic lines whose
spatial correlations are strongly affected by the competition between random
and ordered pinning. This is done through magneto-transport experiments with
YBa2Cu3O7-d thin films that contain a periodic vortex pinning array created via
masked ion irradiation, in addition to the native random pinning. The strong
field-matching effects we observe suggest the prevalence of periodic pinning,
and indicate that at the matching field each vortex line is bound to an
artificial pinning site. However, the vortex-glass transition dimensionality,
quasi-2D instead of the usual 3D, evidences reduced vortex-glass correlations
along the vortex line. This is also supported by an unusual angular dependence
of the magneto-resistance, which greatly differs from that of Bose-glass
systems. A quantitative analysis of the angular magnetoresistance allows us to
link this behaviour to the enhancement of the system anisotropy, a collateral
effect of the ion irradiation
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