235 research outputs found
Modeling of the source mechanism of the April 5, 2003 paroxysmal eruption at Stromboli volcano (Italy) by the inversion of broadband seismic data
Abstract:
On April 5, 2003, one of the largest eruptions in the last decades was observed at Stromboli
volcano, Italy. The eruption occurred in a period of anomalous volcanic activity, after a previous
explosion in December 2002 interrupted the typical moderate "Strombolian" behaviour. An
exhaustive analysis of the available broadband seismic data is here presented and related to the
observed eruption phases. Prominent features of the seismic signals include a very long
period signal a few tens of seconds prior to the explosive eruption, as well as a strong energetic
signal a few seconds after the onset of the eruption
Influence of topograhy on the seismic waveforms associated to eruptive events at Stromboli volcano
The steep topography, which characterizes certain volcanic areas, may strongly influence
the displacement field associated to seismic signals. As a consequence, the
interpretation of seismic data for the inversion of the seismic source and the crustal
structure properties should carefully take into account these effects. We propose a set
of numerical simulations for seismic wave propagation in a 3-D homogeneous model
of Stromboli volcano, Italy, based on the application of a pseudospectral technique.
The model assumes a topography with a discretization of 100 m on the horizontal directions,
and 1 m on the vertical direction, while bathymetry is not yet included. We
estimate the surface displacement field for different seismic sources, reproducing possible
phenomena occurring during an eruptive process. These include purely isotropic
sources, the realistic inclusion of an additional deviatoric CLVD component, which
may take into account the effects of explosive events in presence of conduits and the
final fall-out of material at the volcanic surface. Different durations of the source time
function are tested in order to compare the effects of topography for seismic radiations
with a variable range of frequency content. The comparison of results highlights the
effects, which are strictly related to the presence of a steep topography
Direct observation of micron-scale ordered structure in a two-dimensional electron system
We have applied a novel scanned probe method to directly resolve the interior
structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling
geometry. We find that the application of a perpendicular magnetic field can
induce surprising density modulations that are not static as a function of the
field. Near six and four filled Landau levels, stripe-like structures emerge
with a characteristic wave length ~2 microns. Present theories do not account
for ordered density modulations on this length scale.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Scanning-probe Single-electron Capacitance Spectroscopy
The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface. As in other electric-field-sensitive scanned-probe techniques, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz[superscript ½] at 0.3 K[superscript 5].National Science Foundation (U.S.) (DMR-0305461)National Science Foundation (U.S.) (DMR-0906939)National Science Foundation (U.S.) (DMR-0605801)Michigan State University. Institute for Quantum Science
Modifying the surface electronic properties of YBa2Cu3O7-delta with cryogenic scanning probe microscopy
We report the results of a cryogenic study of the modification of
YBa2Cu3O7-delta surface electronic properties with the probe of a scanning
tunneling microscope (STM). A negative voltage applied to the sample during STM
tunneling is found to modify locally the conductance of the native degraded
surface layer. When the degraded layer is removed by etching, the effect
disappears. An additional surface effect is identified using Scanning Kelvin
Probe Microscopy in combination with STM. We observe reversible surface
charging for both etched and unetched samples, indicating the presence of a
defect layer even on a surface never exposed to air.Comment: 6 pages, 4 figures. To appear in Superconductor Science and
Technolog
Anisotropy and periodicity in the density distribution of electrons in a quantum-well
We use low temperature near-field optical spectroscopy to image the electron
density distribution in the plane of a high mobility GaAs quantum well. We find
that the electrons are not randomly distributed in the plane, but rather form
narrow stripes (width smaller than 150 nm) of higher electron density. The
stripes are oriented along the [1-10 ] crystal direction, and are arranged in a
quasi-periodic structure. We show that elongated structural mounds, which are
intrinsic to molecular beam epitaxy, are responsible for the creation of this
electron density texture.Comment: 10 pages, 3 figure
Electrostatics of Inhomogeneous Quantum Hall Liquid
The distribution of electron density in the quantum Hall liquid is considered
in the presence of macroscopic density gradient caused by side electrodes or
inhomogeneous doping. In this case different Landau levels are occupied in
different regions of a sample. These regions are separated by incompressible
liquid. It is shown that the applicability of the approach by Chklovskii et al.
is substantially restricted if the density gradient is not very large and
disorder is important. Due to the fluctuations of the remote donor's density
the liquid in the transition region can not be considered as completely
incompressible. In the typical situation, when the gap between Landau levels is
not much larger than the energy of disorder, the transition region is a wide
band where electron density, averaged over the fluctuations, is independent of
magnetic field. The band is a random mixture of regions occupied by electrons
of upper level, by holes of lower level and by incompressible liquid. The width
of this band is calculated and an analytical expression for the fraction of
incompressible liquid in different parts of this band is given.Comment: 12 pages, RevTe
Random Matrix Model for Superconductors in a Magnetic Field
We introduce a random matrix ensemble for bulk type-II superconductors in the
mixed state and determine the single-particle excitation spectrum using random
matrix theory. The results are compared with planar tunnel junction experiments
in PbBi thin films. More low energy states appear than in the
Abrikosov-Gor'kov-Maki or Ginzburg-Landau descriptions, consistent with
observations.Comment: 4 pages, 1 postscript figure, to appear in Phys. Rev. Let
Observation of the screening signature in the lateral photovoltage of electrons in the Quantum Hall regime
The lateral photovoltage generated in the plane of a two-dimensional electron
system (2DES) by a focused light spot, exhibits a fine-structure in the quantum
oscillations in a magnetic field near the Quantum Hall conductivity minima. A
double peak structure occurs near the minima of the longitudinal conductivity
oscillations. This is the characteristic signature of the interplay between
screening and Landau quantization.Comment: 4 pages, 4 figures, to be published in Phys. Rev.
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