1,510 research outputs found
Erupted magma volume estimates at Santiaguito and Pacaya Volcanoes, Guatemala using digital elevation models
High resolution digital elevation models (DEMs) of Santiaguito and Pacaya volcanoes, Guatemala, were used to estimate volume changes and eruption rates between 1954 and 2001. The DEMs were generated from contour maps and aerial photography, which were analyzed in ArcGIS 9.0®. Because both volcanoes were growing substantially over the five decade period, they provide a good data set for exploring effective methodology for estimating volume changes. The analysis shows that the Santiaguito dome complex grew by 0.78 ± 0.07 km3 (0.52 ± 0.05 m3 s-1) over the 1954-2001 period with nearly all the growth occurring on the El Brujo (1958-75) and Caliente domes (1971-2001). Adding information from field data prior to 1954, the total volume extruded from Santiaguito since 1922 is estimated at 1.48 ± 0.19 km3. Santiaguito’s growth rate is lower than most other volcanic domes, but it has been sustained over a much longer period and has undergone a change toward more exogenous and progressively slower extrusion with time. At Santiaguito some of the material being added at the dome is subsequently transported downstream by block and ash flows, mudflows and floods, creating channel shifting and areas of aggradation and erosion. At Pacaya volcano a total volume of 0.21 ± 0.05 km3 was erupted between 1961 and 2001 for an average extrusion rate of 0.17 ± 0.04 m3 s-1. Both the Santiaguito and Pacaya eruption rate estimates reported here are minima, because they do not include estimates of materials which are transported downslope after eruption and data on ashfall which may result in significant volumes of material spread over broad areas. Regular analysis of high resolution DEMs using the methods outlined here, would help quantify the effects of fluvial changes to downstream populated areas, as well as assist in tracking hazards related to dome collapse and eruption
Effect of external magnetic field on electron spin dephasing induced by hyperfine interaction in quantum dots
We investigate the influence of an external magnetic field on spin phase
relaxation of single electrons in semiconductor quantum dots induced by the
hyperfine interaction. The basic decay mechanism is attributed to the
dispersion of local effective nuclear fields over the ensemble of quantum dots.
The characteristics of electron spin dephasing is analyzed by taking an average
over the nuclear spin distribution. We find that the dephasing rate can be
estimated as a spin precession frequency caused primarily by the mean value of
the local nuclear magnetic field. Furthermore, it is shown that the hyperfine
interaction does not fully depolarize electron spin. The loss of initial spin
polarization during the dephasing process depends strongly on the external
magnetic field, leading to the possibility of effective suppression of this
mechanism.Comment: 10 pages, 2 figure
Field-induced thermal metal-to-insulator transition in underdoped LSCO
The transport of heat and charge in cuprates was measured in undoped and
heavily-underdoped single crystal La_{2-x}Sr_xCuO_{4+delta} (LSCO). In
underdoped LSCO, the thermal conductivity is found to decrease with increasing
magnetic field in the T --> 0 limit, in striking contrast to the increase
observed in all superconductors, including cuprates at higher doping. The
suppression of superconductivity with magnetic field shows that a novel thermal
metal-to-insulator transition occurs upon going from the superconducting state
to the field-induced normal state.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas
Recent measurements of a 2D electron gas subjected to microwave radiation
reveal a magnetoresistance with an oscillatory dependence on the ratio of
radiation frequency to cyclotron frequency. We perform a diagrammatic
calculation and find radiation-induced resistivity oscillations with the
correct period and phase. Results are explained via a simple picture of current
induced by photo-excited disorder-scattered electrons. The oscillations
increase with radiation intensity, easily exceeding the dark resistivity and
resulting in negative-resistivity minima. At high intensity, we identify
additional features, likely due to multi-photon processes, which have yet to be
observed experimentally.Comment: 5 pages, 3 figures; final version as published in Phys Rev Let
Electrochemical Biosensors: Recommended Definitions and Classification
Two Divisions of the International Union of Pure and Applied Chemistry (IUPAC),
namely Physical Chemistry (Commission I.7 on Biophysical Chemistry, formerly Steering
Committee on Biophysical Chemistry) and Analytical Chemistry (Commission V.5 on
Electroanalytical Chemistry), have prepared recommendations on the definition, classification
and nomenclature related to electrochemical biosensors; these recommendations could, in the
future, be extended to other types of biosensors.
An electrochemical biosensor is a self-contained integrated device, which is capable of
providing specific quantitative or semi-quantitative analytical information using a biological
recognition element (biochemical receptor) which is retained in direct spatial contact
with an electrochemical transduction element. Because of their ability to be repeatedly
calibrated, we recommend that a biosensor should be clearly distinguished from a bioanalytical system, which requires additional processing steps, such as reagent addition. A device which is
both disposable after one measurement, i.e. single use, and unable to monitor the analyte
concentration continuously or after rapid and reproducible regeneration should be designated a
single-use biosensor.
Biosensors may be classified according to the biological specificity-conferring mechanism
or, alternatively, the mode of physicochemical signal transduction. The biological recognition
element may be based on a chemical reaction catalysed by, or on an equilibrium reaction with,
macromolecules that have been isolated, engineered or present in their original biological
environment. In the latter case, equilibrium is generally reached and there is no further, if any,
net consumption of analyte(s) by the immobilized biocomplexing agent incorporated into the
sensor. Biosensors may be further classified according to the analytes or reactions that they
monitor: direct monitoring of analyte concentration or of reactions producing or consuming
such analytes; alternatively, an indirect monitoring of inhibitor or activator of the biological
recognition element (biochemical receptor) may be achieved.
A rapid proliferation of biosensors and their diversity has led to a lack of rigour in defining
their performance criteria. Although each biosensor can only truly be evaluated for a
particular application, it is still useful to examine how standard protocols for performance
criteria may be defined in accordance with standard IUPAC protocols or definitions. These
criteria are recommended for authors, referees and educators and include calibration
characteristics (sensitivity, operational and linear concentration range, detection and quantitative
determination limits), selectivity, steady-state and transient response times, sample
throughput, reproducibility, stability and lifetime
Transport properties in the d-density wave state: Wiedemann-Franz law
We study the Wiedemann-Franz (WF) law in the d-density wave (DDW) model. Even
though the opening of the DDW gap profoundly modifies the electronic
density of states and makes it dependent on energy, the value of the WF ratio
at zero temperature (T=0) remains unchanged. However, neither electrical nor
thermal conductivity display universal behavior. For finite temperature, with T
greater than the value of the impurity scattering rate at zero frequency
i.e. , the usual WF ratio is obtained only in
the weak scattering limit. For strong scattering there are large violations of
the WF law.Comment: 1 figur
Observation of Apparently Zero-Conductance States in Corbino Samples
Using Corbino samples we have observed oscillatory conductance in a
high-mobility two-dimensional electron system subjected to crossed microwave
and magnetic fields. On the strongest of the oscillation minima the conductance
is found to be vanishingly small, possibly indicating an insulating state
associated with these minima.Comment: 4 pages, 3 figures, RevTex
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