148 research outputs found
Demonstration of an electrostatic-shielded cantilever
The fabrication and performances of cantilevered probes with reduced
parasitic capacitance starting from a commercial Si3N4 cantilever chip is
presented. Nanomachining and metal deposition induced by focused ion beam
techniques were employed in order to modify the original insulating pyramidal
tip and insert a conducting metallic tip. Two parallel metallic electrodes
deposited on the original cantilever arms are employed for tip biasing and as
ground plane in order to minimize the electrostatic force due to the capacitive
interaction between cantilever and sample surface. Excitation spectra and
force-to-distance characterization are shown with different electrode
configurations. Applications of this scheme in electrostatic force microscopy,
Kelvin probe microscopy and local anodic oxidation is discussed.Comment: 4 pages and 3 figures. Submitted to Applied Physics Letter
Coulomb-Blockade directional coupler
A tunable directional coupler based on Coulomb Blockade effect is presented.
Two electron waveguides are coupled by a quantum dot to an injector waveguide.
Electron confinement is obtained by surface Schottky gates on single
GaAs/AlGaAs heterojunction. Magneto-electrical measurements down to 350 mK are
presented and large transconductance oscillations are reported on both outputs
up to 4.2 K. Experimental results are interpreted in terms of Coulomb Blockade
effect and the relevance of the present design strategy for the implementation
of an electronic multiplexer is underlined.Comment: 4 pages, 4 figures, to be published in Applied Physics Letter
Coherent transport in Nb/delta-doped-GaAs hybrid microstructures
Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is
presented. The structures fabrication procedure is based on delta-doped layers
grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap
layer to protect the active semiconductor layers during ex situ transfer. The
superconductor is then sputter deposited in situ after thermal desorption of
the protective layer. Two types of structures in particular will be discussed,
i.e., a reference junction and the engineered one that contains an additional
insulating AlGaAs barrier inserted during the growth in the semiconductor. This
latter configuration may give rise to controlled interference effects and
realizes the model introduced by de Gennes and Saint-James in 1963. While both
structures show reflectionless tunneling-dominated transport, only the
engineered junction shows additionally a low-temperature single marked
resonance peaks superimposed to the characteristic Andreev-dominated subgap
conductance. The analysis of coherent magnetotransport in both microstructures
is successfully performed within the random matrix theory of Andreev transport
and ballistic effects are included by directly solving the Bogoliubov-de Gennes
equations. The impact of junction morphology on reflectionless tunneling and
the application of the employed fabrication technique to the realization of
complex semiconductor-superconductor systems are furthermore discussed.Comment: 9 pages, 8 figures, invited review paper, to be published in Mod.
Phys. Lett.
Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions
The specific mechanisms which leads to the formation of fractal
nanostructures by pulsed laser deposition remain elusive despite intense
research efforts, motivated mainly by the technological interest in obtaining
tailored nanostructures with simple and scalable production methods. Here we
focus on fractal nanostructures of titanium dioxide, , a strategic
material for many applications, obtained by femtosecond laser ablation at
ambient conditions. We model the fractal formation through extensive Monte
Carlo simulations based on a set of minimal assumptions: irreversible sticking
and size independent diffusion. Our model is able to reproduce the fractal
dimensions and the area distributions of the nanostructures obtained in the
experiments for different densities of the ablated material. The comparison of
theory and experiment show that such fractal aggregates are formed after
landing of the ablated material on the substrate surface by a diffusive
mechanism. Finally we discuss the role of the thermal conductivity of the
substrate and the laser fluence on the properties of the fractal
nanostructures. Our results represent an advancement towards controlling the
production of fractal nanostructures by pulsed laser deposition.Comment: 21 page
Resonant Transport in Nb/GaAs/AlGaAs/GaAs Microstructures
Resonant transport in a hybrid semiconductor-superconductor microstructure
grown by MBE on GaAs is presented. This structure experimentally realizes the
prototype system originally proposed by de Gennes and Saint-James in 1963 in
\emph{all}-metal structures. A low temperature single peak superimposed to the
characteristic Andreev-dominated subgap conductance represents the mark of such
resonant behavior. Random matrix theory of quantum transport was employed in
order to analyze the observed magnetotransport properties and ballistic effects
were included by directly solving the Bogoliubov-de Gennes equations.Comment: 7 pages REVTeX, 4 figures, to be published by World Scientific in
Proceedings of International Symposium on Mesoscopic Superconductivity and
Spintronics (NTT R&D Center Atsugi, Japan, March 2002
Low field magnetotransport in strained Si/SiGe cavities
Low field magnetotransport revealing signatures of ballistic transport
effects in strained Si/SiGe cavities is investigated. We fabricated strained
Si/SiGe cavities by confining a high mobility Si/SiGe 2DEG in a bended nanowire
geometry defined by electron-beam lithography and reactive ion etching. The
main features observed in the low temperature magnetoresistance curves are the
presence of a zero-field magnetoresistance peak and of an oscillatory structure
at low fields. By adopting a simple geometrical model we explain the
oscillatory structure in terms of electron magnetic focusing. A detailed
examination of the zero-field peak lineshape clearly shows deviations from the
predictions of ballistic weak localization theory.Comment: Submitted to Physical Review B, 25 pages, 7 figure
Ground deformation analysis at Campi Flegrei (Southern Italy) by CGPS and tide-gauge network
Campi Flegrei caldera is located 15 km west of the
city of Naples, within the central-southern sector of a
large graben called Campanian Plain. It is an active
volcanic area marked by a quasi-circular caldera
depression, formed by a huge ignimbritic eruption
occurred about 37000 years ago. This caldera was
generated by several collapses produced by strong
explosive eruptions (the last eruption, occurred in
1538, built an about 130 m spatter cone called Mt.
Nuovo). Campi Flegrei area periodically experiences
significant deformation episodes, with uplift
phenomena up to more than 3.5 m in 15 years (from
1970 to 1984), which caused during 1983-84 the
temporary evacuation of about 40000 people from the
ancient part of Pozzuoli town.
The deformation field obtainable by CGPS and tidegauge
stations plays an important role for the
modelling and interpretation of volcanic phenomena,
as well as for forecasting purposes.
The structural complexity of the Campi Flegrei area,
together with the evidence of a strong interaction
between magmatic chamber and shallow geothermal
system, calls for a detailed characterization of the
substructure and of magma-water interaction
processes.
The incoming experiment of deep drilling, down to
about 4 km, will give detailed structural and physical
constraints able to resolve the intrinsic ambiguities of
geophysical data and in particular geodetic ones.
In this poster we describe the recent ground
deformations at Campi Flegrei area by means of GPS
technique and tide gauge stations, discussing the
possible interpretations also in light of further
constraints likely coming from the next CFDDP
(Campi Flegrei Deep Drilling) deep drilling experiment
Characterization of GPS time series at the Neapolitan volcanic area by statistical analysis
The GPS time series recorded at the Neapolitan volcanic area reveals a very peculiar
behavior. When a clear deformation is observed, the amplitude distribution evolves from a
super‐Gaussian to a broader distribution. This behavior can be characterized by evaluating
the kurtosis. Spurious periodic components were evidenced by independent component
analysis and then removed by filtering the original signal. The time series for all stations was
modeled with a fifth‐order polynomial fit, which represents the deformation history at
that place. Indeed, when this polynomial is subtracted from the time series, the distributions
again become super‐Gaussian. A simulation of the deformation time evolution was
performed by superposing a Laplacian noise and a synthetic deformation history. The
kurtosis of the obtained signals decreases as the superposition increases, enlightening the
insurgence of the deformation. The presented approach represents a contribution aimed
at adding further information to the studies about the deformation at the Neapolitan volcanic
area by revealing geologically relevant data
S.A.G.NET: Rete GPS dell'Appennino meridionale.
The Matese carbonatic massive occupies the northernmost part of the campanian Apennine while Sannio mounts, located to the East of massive, consists primarily of quaternary deposits and represent the area of Apennine chain degrading to East towards the Bradanica foredeep. The area was affected in historical time by several destructive earthquakes. The first ground deformation studies in this area started from 1990-2000 with the definition of geodetic networks, covering all or part of the massive Matese, with the aim of evaluating seismogenic sources responsible for the seismicity of the area. In 2002, a careful inspection of the existing GPS benchmarks was carried out; those which had a good state of preservation and a good level of reliability were included into a new geodetic Matese network, consisting of 38 3D benchmarks. Several surveys were conducted in 2000, 2002 and 2004,with the aim of defining the strain field, defined by plano-altimetric components. In 2005, an intensive work of gathering and validating available data started, integrating data collected by previous surveys with those collected during the new survey carried out in 2006. This work describes in detail the various stages of implementing the final network S.A.G.NET, whose geometry was also bound to the distribution of the known seismogenic sources present in the area. We also show the first results obtained from data collected from2000 to 2006 and the resulting kinematic model for this area
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