2,644 research outputs found
Aqueous alteration on main belt primitive asteroids: results from visible spectroscopy
This work focuses on the study of the aqueous alteration process which acted
in the main belt and produced hydrated minerals on the altered asteroids. The
aqueous alteration is particularly important for unraveling the processes
occurring during the earliest times of the Solar System history, as it can give
information both on the asteroids thermal evolution and on the localization of
water sources in the asteroid belt. We present new spectral observations in the
visible region of 80 asteroids belonging to the primitive classes C, G, F, B
and P. We combine the present observations with the visible spectra of
asteroids available in the literature for a total of 600 primitive main belt
asteroids. Our analysis shows that the aqueous alteration sequence starts from
the P-type objects, practically unaltered, and increases through the F, B, C,
and G asteroids. Around 50% of the observed C-type asteroids show absorption
features in the vis. range due to hydrated silicates, implying that more than
70% of them will have a 3 m absorption band and thus hydrated minerals on
their surfaces. The process dominates in primitive asteroids located between
2.3 and 3.1 AU, that is at smaller heliocentric distances than previously
suggested. The aqueous alteration process dominates in the 50--240 km sized
primitive asteroids, while it is less effective for bodies smaller than 50 km.
No correlation is found between the aqueous alteration process and the
asteroids albedo or orbital elements. Comparing the 0.7 m band
parameters of hydrated silicates and CM2 carbonaceous chondrites, we see that
the band center of meteorites is at longer wavelengths than that of asteroids.
This difference on center positions may be attributed to different minerals
abundances, and to the fact that CM2 available on Earth might not be
representative of the whole aqueous altered asteroids population.Comment: Icarus, accepted for publication on 28 January 2014 Manuscript pages:
38; Figures: 13 ; Tables:
Andreev Level Qubit
We investigate the dynamics of a two-level Andreev bound state system in a
transmissive quantum point contact embedded in an rf-SQUID. Coherent coupling
of the Andreev levels to the circulating supercurrent allows manipulation and
read out of the level states. The two-level Hamiltonian for the Andreev levels
is derived, and the effect of interaction with the quantum fluctuations of the
induced flux is studied. We also consider an inductive coupling of qubits, and
discuss the relevant SQUID parameters for qubit operation and read out.Comment: 4 pages, 1 figur
Spectral variability on primitive asteroids of the Themis and Beagle families: space weathering effects or parent body heterogeneity?
Themis is an old and statistically robust asteroid family populating the
outer main belt, and resulting from a catastrophic collision that took place
2.51.0 Gyr ago. Within the old Themis family a young sub-family, Beagle,
formed less than 10 Myr ago, has been identified. We present the results of a
spectroscopic survey in the visible and near infrared range of 22 Themis and 8
Beagle families members. The Themis members investigated exhibit a wide range
of spectral behaviors, while the younger Beagle family members look spectrally
bluer with a smaller spectral slope variability. The best meteorite spectral
analogues found for both Themis and Beagle families members are carbonaceous
chondrites having experienced different degrees of aqueous alteration,
prevalently CM2 but also CV3 and CI, and some of them are chondrite samples
being unusual or heated. We extended the spectral analysis including the data
available in the literature on Themis and Beagle families members, and we
looked for correlations between spectral behavior and physical parameters using
the albedo and size values derived from the WISE data. The analysis of this
larger sample confirm the spectral diversity within the Themis family and that
Beagle members tend to be bluer and to have an higher albedo. The differences
between the two family may be partially explained by space weathering
processes, which act on these primitive surfaces in a similar way than on
S-type asteroids, i.e. producing reddening and darkening. However we see
several Themis members having albedos and spectral slopes similar to the young
Beagle members. Alternative scenarios are proposed including heterogeneity in
the parent body having a compositional gradient with depth, and/or the survival
of projectile fragments having a different composition than the parent body.Comment: Manuscript pages: 40; Figures: 15 ; Tables: 4 Icarus (2016),in pres
Josephson junction qubit network with current-controlled interaction
We design and evaluate a scalable charge qubit chain network with
controllable current-current coupling of neighbouring qubit loops via local
dc-current gates. The network allows construction of general N-qubit gates. The
proposed design is in line with current main stream experiments.Comment: 4 pages, 4 figure
Ion counting efficiencies at the IGISOL facility
At the IGISOL-JYFLTRAP facility, fission mass yields can be studied at high
precision. Fission fragments from a U target are passing through a Ni foil and
entering a gas filled chamber. The collected fragments are guided through a
mass separator to a Penning trap where their masses are identified. This
simulation work focuses on how different fission fragment properties (mass,
charge and energy) affect the stopping efficiency in the gas cell. In addition,
different experimental parameters are varied (e. g. U and Ni thickness and He
gas pressure) to study their impact on the stopping efficiency. The simulations
were performed using the Geant4 package and the SRIM code. The main results
suggest a small variation in the stopping efficiency as a function of mass,
charge and kinetic energy. It is predicted that heavy fragments are stopped
about 9% less efficiently than the light fragments. However it was found that
the properties of the U, Ni and the He gas influences this behavior. Hence it
could be possible to optimize the efficiency.Comment: 52 pages, 44 figure
Characterization of a Be(p,xn) neutron source for fission yields measurements
We report on measurements performed at The Svedberg Laboratory (TSL) to
characterize a proton-neutron converter for independent fission yield studies
at the IGISOL-JYFLTRAP facility (Jyv\"askyl\"a, Finland). A 30 MeV proton beam
impinged on a 5 mm water-cooled Beryllium target. Two independent experimental
techniques have been used to measure the neutron spectrum: a Time of Flight
(TOF) system used to estimate the high-energy contribution, and a Bonner Sphere
Spectrometer able to provide precise results from thermal energies up to 20
MeV. An overlap between the energy regions covered by the two systems will
permit a cross-check of the results from the different techniques. In this
paper, the measurement and analysis techniques will be presented together with
some preliminary results.Comment: 3 pages, 3 figures, also submitted as proceedings of the
International Conference on Nuclear Data for Science and Technology 201
Domain size effects on the dynamics of a charge density wave in 1T-TaS2
Recent experiments have shown that the high temperature incommensurate (I)
charge density wave (CDW) phase of 1T-TaS2 can be photoinduced from the lower
temperature, nearly commensurate (NC) CDW state. Here we report a time-resolved
x-ray diffraction study of the growth process of the photoinduced I-CDW
domains. The layered nature of the material results in a marked anisotropy in
the size of the photoinduced domains of the I-phase. These are found to grow
self-similarly, their shape remaining unchanged throughout the growth process.
The photoinduced dynamics of the newly formed I-CDW phase was probed at various
stages of the growth process using a double pump scheme, where a first pump
creates I-CDW domains and a second pump excites the newly formed I-CDW state.
We observe larger magnitudes of the coherently excited I-CDW amplitude mode in
smaller domains, which suggests that the incommensurate lattice distortion is
less stable for smaller domain sizes.Comment: 8 pages, 8 figure
Surface effects on the Mott-Hubbard transition in archetypal VO
We present an experimental and theoretical study exploring surface effects on
the evolution of the metal-insulator transition in the model Mott-Hubbard
compound Cr-doped VO. We find a microscopic domain formation that is
clearly affected by the surface crystallographic orientation. Using scanning
photoelectron microscopy and X-ray diffraction, we find that surface defects
act as nucleation centers for the formation of domains at the
temperature-induced isostructural transition and favor the formation of
microscopic metallic regions. A density functional theory plus dynamical mean
field theory study of different surface terminations shows that the surface
reconstruction with excess vanadyl cations leads to doped, and hence more
metallic surface states, explaining our experimental observations.Comment: 5 pages, 4 figure
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