2,269 research outputs found
Deep mantle structure and the postperovskite phase transition
Seismologists have known for many years that the lowermost mantle of the Earth is complex. Models based on observed seismic phases sampling this region include relatively sharp horizontal discontinuities with strong zones of anisotropy, nearly vertical contrasts in structure, and small pockets of ultralow velocity zones (ULVZs). This diversity of structures is beginning to be understood in terms of geodynamics and mineral physics, with dense partial melts causing the ULVZs and a postperovskite solid–solid phase transition producing regional layering, with the possibility of large-scale variations in chemistry. This strong heterogeneity has significant implications on heat transport out of core, the evolution of the magnetic field, and magnetic field polarity reversals
Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy
The process of natural aging in pure ternary Al-Mg-Si alloys was studied by
positron annihilation lifetime spectroscopy in real time in order to clarify
the sequence and kinetics of clustering and precipitation. It was found that
natural aging takes place in at least five stages in these alloys, four of
which were directly observed. This is interpreted as the result of complex
interactions between vacancies and solute atoms or clusters. One of the early
stages of positron lifetime evolution coincides with a clustering process
observed by differential scanning calorimetry (DSC) and involves the formation
of a positron trap with \sim 0.200 ns lifetime. In later stages, a positron
trap with a higher lifetime develops in coincidence with the DSC signal of a
second clustering reaction. Mg governs both the kinetics and the lifetime
change in this stage. Within the first 10 min after quenching, a period of
nearly constant positron lifetime was found for those Mg-rich alloys that later
show an insufficient hardness response to artificial aging, the so-called
"negative effect." The various processes observed could be described by two
effective activation energies that were found by varying the aging temperature
from 10 to 37\degree C.Comment: arXiv admin note: same as v2, to correct mistaken v
Finite Temperature Behavior of the Quantum Hall Effect in Bilayer Electron Systems
An effective field theoretic description of bilayer electron systems
stabilized by Coulomb repulsion in a single wide quantum well is examined using
renormalization group techniques. The system is found to undergo a crossover
from a low temperature strongly correlated quantum Hall state to a high
temperature compressible state. This picture is used to account for the recent
experimental observation of an anomalous transition in bilayer electron systems
(T. S. Lay, {\em et al.} Phys. Rev. B {\bf 50}, 17725 (1994)). An estimate for
the crossover temperature is provided, and it is shown that its dependence on
electron density is in reasonable agreement with i the experiment.Comment: Corrected typos, and changed content, 5 pages and 2 figures, accepted
in Phys. Rev.
Density of states and electron concentration of double heterojunctions subjected to an in-plane magnetic field
We calculate the electronic states of
AlGaAs/GaAs/AlGaAs double heterojunctions subjected to
a magnetic field parallel to the quasi two-dimensional electron gas. We study
the energy dispersion curves, the density of states, the electron concentration
and the distribution of the electrons in the subbands. The parallel magnetic
field induces severe changes in the density of states, which are of crucial
importance for the explanation of the magnetoconductivity in these structures.
However, to our knowledge, there is no systematic study of the density of
states under these circumstances. We attempt a contribution in this direction.
For symmetric heterostructures, the depopulation of the higher subbands, the
transition from a single to a bilayer electron system and the domination of the
bulk Landau levels in the centre the wide quantum well, as the magnetic field
is continuously increased, are presented in the ``energy dispersion picture''
as well as in the ``electron concentration picture'' and in the ``density of
states picture''.Comment: J. Phys.: Condens. Matter 11 No 26 (5 July 1999) 5131-5141 Figures
(three) embedde
Observations of teleseismic P wave coda for underground explosions
The early P wave coda (5–15 sec after the first arrival) of underground explosions at the Nevada Test Site is studied in the time domain using 2082 teleseismic short-period recordings, with the intent of identifying near-source contributions to the signals in the frequency range 0.2–2.0 Hz. Smaller magnitude events tend to have relatively high coda levels in the 0.4–0.8 Hz frequency band for both Yucca Flat and Pahute Mesa explosions. Coda complexity in this low-frequency passband is negatively correlated with burial depth for Pahute Mesa events but is only weakly correlated with depth for Yucca Flat events. Enhanced excitation of relatively long-period scattered waves for smaller, less deeply buried events is required to explain this behavior. Coda complexity in the 0.8–1.1 Hz band is positively correlated with magnitude and depth for Pahute Mesa events, but has no such dependence for Yucca Flat events. This may result from systematic variations between the spectra of direct signals and coda arrivals caused by pP interference for the largest events, all of which were detonated at Pahute Mesa. Another possible explanation is a frequency-dependent propagation effect on the direct signals of the larger events, most of which were located in the center of the mesa overlying strong lateral velocity gradients in the crust and upper mantle. Event average complexity varies spatially for both test sites, particularly in the 0.8–1.1 Hz band, providing evidence for frequency-dependent focussing or scattering by near-source structure. Both the direct arrivals and the early coda have strong azimuthal amplitude patterns that are produced by defocussing by mantle heterogeneity. The direct arrivals have stronger coherent azimuthal patterns than the early coda for Pahute Mesa events, indicating more pronounced deep crustal and shallow mantle defocussing for the direct signals. However, for Yucca Flat events the direct arrivals have less coherent azimuthal patterns than the coda, suggesting that a highly variable component of near-source scattering preferentially affecting the downgoing energy is superimposed on a pattern produced by mantle heterogeneity that affects the entire signal. This complicated behavior of the direct arrivals may be the result of triplications and caustics produced by the complex basement structure known to underlie the Yucca Flat test site. The presence of strong azimuthal patterns in the early coda indicates that source strength estimates based on early coda are subject to biases similar to those affecting estimates based on direct arrivals.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43106/1/24_2005_Article_BF01772598.pd
Magnetic Anisotropy in Quantum Hall Ferromagnets
We show that the sign of magnetic anisotropy energy in quantum Hall
ferromagnets is determined by a competition between electrostatic and exchange
energies. Easy-axis ferromagnets tend to occur when Landau levels whose states
have similar spatial profiles cross. We report measurements of integer QHE
evolution with magnetic-field tilt. Reentrant behavior observed for the QHE at high tilt angles is attributed to easy-axis anisotropy. This
interpretation is supported by a detailed calculation of the magnetic
anisotropy energy.Comment: 12 pages, 3 figures, submitted to Phys. Rev. Let
Calculating the transfer function of noise removal by principal component analysis and application to AzTEC observations
Instruments using arrays of many bolometers have become increasingly common
in the past decade. The maps produced by such instruments typically include the
filtering effects of the instrument as well as those from subsequent steps
performed in the reduction of the data. Therefore interpretation of the maps is
dependent upon accurately calculating the transfer function of the chosen
reduction technique on the signal of interest. Many of these instruments use
non-linear and iterative techniques to reduce their data because such methods
can offer improved signal-to-noise over those that are purely linear,
particularly for signals at scales comparable to that subtended by the array.
We discuss a general approach for measuring the transfer function of principal
component analysis (PCA) on point sources that are small compared to the
spatial extent seen by any single bolometer within the array. The results are
applied to previously released AzTEC catalogues of the COSMOS, Lockman Hole,
Subaru XMM-Newton Deep Field, GOODS-North and GOODS-South fields. Source flux
density and noise estimates increase by roughly +10 per cent for fields
observed while AzTEC was installed at the Atacama Submillimeter Telescope
Experiment and +15-25 per cent while AzTEC was installed at the James Clerk
Maxwell Telescope. Detection significance is, on average, unaffected by the
revised technique. The revised photometry technique will be used in subsequent
AzTEC releases.Comment: 14 pages, 4 figure
Wigner crystallization and metal-insulator transition of two-dimensional holes in GaAs/AlGaAs at B=0
We report the transport properties of a low disorder two-dimensional hole
system (2DHS) in the GaAs/AlGaAs heterostructure, which has an unprecedentedly
high peak mobility of , with hole density of in the temperature range of
. From their T, p, and electric field dependences, we find that
the metal-insulator transition in zero magnetic field in this exceptionally
clean 2DHS occurs at , which is in good agreement with the
critical for Wigner crystallization , predicted by
Tanatar and Ceperley for an ideally clean 2D system.Comment: 4 pages, 4 Postscript figure
Anomalous spin-splitting of two-dimensional electrons in an AlAs Quantum Well
We measure the effective Lande g-factor of high-mobility two-dimensional
electrons in a modulation-doped AlAs quantum well by tilting the sample in a
magnetic field and monitoring the evolution of the magnetoresistance
oscillations. The data reveal that |g| = 9.0, which is much enhanced with
respect to the reported bulk value of 1.9. Surprisingly, in a large range of
magnetic field and Landau level fillings, the value of the enhanced g-factor
appears to be constant.Comment: 4 pages, 3 figure
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