575 research outputs found
One-Dimensional Extended States in Partially Disordered Planar Systems
We obtain analytically a continuum of one-dimensional ballistic extended
states in a two-dimensional disordered system, which consists of compactly
coupled random and pure square lattices. The extended states give a marginal
metallic phase with finite conductivity in a wide energy
range, whose boundaries define the mobility edges of a first-order
metal-insulator transition. We show current-voltage duality,
scaling of the conductivity in parallel magnetic field and
non-Fermi liquid properties when long-range electron-electron interactions are
included.Comment: 4 pages, revtex file, 3 postscript file
Vortex Dynamics and Defects in Simulated Flux Flow
We present the results of molecular dynamic simulations of a two-dimensional
vortex array driven by a uniform current through random pinning centers at zero
temperature. We identify two types of flow of the driven array near the
depinning threshold. For weak disorder the flux array contains few dislocation
and moves via correlated displacements of patches of vortices in a {\it
crinkle} motion. As the disorder strength increases, we observe a crossover to
a spatially inhomogeneous regime of {\it plastic} flow, with a very defective
vortex array and a channel-like structure of the flowing regions. The two
regimes are characterized by qualitatively different spatial distribution of
vortex velocities. In the crinkle regime the distribution of vortex velocities
near threshold has a single maximum that shifts to larger velocities as the
driving force is increased. In the plastic regime the distribution of vortex
velocities near threshold has a clear bimodal structure that persists upon
time-averaging the individual velocities. The bimodal structure of the velocity
distribution reflects the coexistence of pinned and flowing regions and is
proposed as a quantitative signature of plastic flow.Comment: 12 pages, 13 embedded PostScript figure
Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2.yH2O
The Raman spectra of the parent compound NaxCoO2 (x=0.75) and the
superconducting oxyhydrates NaxCoO2.yH2O with different superconducting
temperatures (Tc) have been measured. Five Raman active phonons around 195 cm-1
(E1g), 482 cm-1, 522 cm-1, 616 cm-1 (3E2g), 663 cm-1 (A1g) appear in all
spectra. These spectra change systematically along with the intercalation of
H2O and superconducting properties. In particular, the Raman active phonons
(A1g and E1g) involving the oxygen motions within the Co-O layers show up
monotonous decrease in frequency along with superconducting temperature Tc. The
fundamental properties and alternations of other active Raman phonons in the
superconducting materials have also been discussed.Comment: 16 pages, 4 figures, 2 table
Phytoplankton Diversity and Ecology through the Lens of High Throughput Sequencing Technologies
Metabarcoding or high-throughput sequencing of a specific genetic marker is a powerful technique, widely used today, to analyze biodiversity across distinct environments and taxonomic groups. Plankton ecologists have benefited tremendously from the growing accumulation of metabarcoding studies. Novel biogeographic patterns have been established by the analysis of datasets from the Tara Oceans and Ocean Sampling Day projects. Novel lineages without cultured representatives have been uncovered. This chapter begins by going back to the steps that led Carl Woese and George Fox to define the concept of “molecular marker.” Among the multitude of exciting findings brought by high-throughput sequencing technologies, perhaps the major impacts are found in the study of picocyanobacteria and microbial eukaryotes from plankton communities. We then detail the different steps and choices that are involved in designing, performing, and analyzing a metabarcoding study. We are using a compilation of about 250 metabarcoding studies to present the major trends in terms of the gene marker used and environment probed. An alternative approach to metabarcoding developed for marine picocyanobacteria is also briefly discussed. We are then focusing on specific habitats and processes that have benefited from metabarcoding: the study of polar ecosystems, the functioning of the marine biological carbon pump, predator-prey interactions, and picoeukaryotic phytoplankton in highly urbanized lakes. Finally, we offer some perspectives on emerging trends, such as the use of metabarcodes combined with supervised machine learning for biomonitoring, the link between metabarcoding and functional diversity in trait-based studies and the massive sequencing of long DNA fragments
Topological Defects, Orientational Order, and Depinning of the Electron Solid in a Random Potential
We report on the results of molecular dynamics simulation (MD) studies of the
classical two-dimensional electron crystal in the presence disorder. Our study
is motivated by recent experiments on this system in modulation doped
semiconductor systems in very strong magnetic fields, where the magnetic length
is much smaller than the average interelectron spacing , as well as by
recent studies of electrons on the surface of helium. We investigate the low
temperature state of this system using a simulated annealing method. We find
that the low temperature state of the system always has isolated dislocations,
even at the weakest disorder levels investigated. We also find evidence for a
transition from a hexatic glass to an isotropic glass as the disorder is
increased. The former is characterized by quasi-long range orientational order,
and the absence of disclination defects in the low temperature state, and the
latter by short range orientational order and the presence of these defects.
The threshold electric field is also studied as a function of the disorder
strength, and is shown to have a characteristic signature of the transition.
Finally, the qualitative behavior of the electron flow in the depinned state is
shown to change continuously from an elastic flow to a channel-like, plastic
flow as the disorder strength is increased.Comment: 31 pages, RevTex 3.0, 15 figures upon request, accepted for
publication in Phys. Rev. B., HAF94MD
Effects of the field modulation on the Hofstadter's spectrum
We study the effect of spatially modulated magnetic fields on the energy
spectrum of a two-dimensional (2D) Bloch electron. Taking into account four
kinds of modulated fields and using the method of direct diagonalization of the
Hamiltonian matrix, we calculate energy spectra with varying system parameters
(i.e., the kind of the modulation, the relative strength of the modulated field
to the uniform background field, and the period of the modulation) to elucidate
that the energy band structure sensitively depends on such parameters:
Inclusion of spatially modulated fields into a uniform field leads occurrence
of gap opening, gap closing, band crossing, and band broadening, resulting
distinctive energy band structure from the Hofstadter's spectrum. We also
discuss the effect of the field modulation on the symmetries appeared in the
Hofstadter's spectrum in detail.Comment: 7 pages (in two-column), 10 figures (including 2 tables
An Integrated TCGA Pan-Cancer Clinical Data Resource to Drive High-Quality Survival Outcome Analytics
For a decade, The Cancer Genome Atlas (TCGA) program collected clinicopathologic annotation data along with multi-platform molecular profiles of more than 11,000 human tumors across 33 different cancer types. TCGA clinical data contain key features representing the democratized nature of the data collection process. To ensure proper use of this large clinical dataset associated with genomic features, we developed a standardized dataset named the TCGA Pan-Cancer Clinical Data Resource (TCGA-CDR), which includes four major clinical outcome endpoints. In addition to detailing major challenges and statistical limitations encountered during the effort of integrating the acquired clinical data, we present a summary that includes endpoint usage recommendations for each cancer type. These TCGA-CDR findings appear to be consistent with cancer genomics studies independent of the TCGA effort and provide opportunities for investigating cancer biology using clinical correlates at an unprecedented scale. Analysis of clinicopathologic annotations for over 11,000 cancer patients in the TCGA program leads to the generation of TCGA Clinical Data Resource, which provides recommendations of clinical outcome endpoint usage for 33 cancer types
Validation of plasma proteomic biomarkers relating to brain amyloid burden in the EMIF-Alzheimer's disease multimodal biomarker discovery cohort
We have previously investigated, discovered, and replicated plasma protein biomarkers for use to triage potential trials participants for PET or cerebrospinal fluid measures of Alzheimer's disease (AD) pathology. This study sought to undertake validation of these candidate plasma biomarkers in a large, multi-center sample collection. Targeted plasma analyses of 34 proteins with prior evidence for prediction of in vivo pathology were conducted in up to 1,000 samples from cognitively healthy elderly individuals, people with mild cognitive impairment, and in patients with AD-type dementia, selected from the EMIF-AD catalogue. Proteins were measured using Luminex xMAP, ELISA, and Meso Scale Discovery assays. Seven proteins replicated in their ability to predict in vivo amyloid pathology. These proteins form a biomarker panel that, along with age, could significantly discriminate between individuals with high and low amyloid pathology with an area under the curve of 0.74. The performance of this biomarker panel remained consistent when tested in apolipoprotein E ϵ4 non-carrier individuals only. This blood-based panel is biologically relevant, measurable using practical immunocapture arrays, and could significantly reduce the cost incurred to clinical trials through screen failure
Phase Behavior of Type-II Superconductors with Quenched Point Pinning Disorder: A Phenomenological Proposal
A general phenomenology for phase behaviour in the mixed phase of type-II
superconductors with weak point pinning disorder is outlined. We propose that
the ``Bragg glass'' phase generically transforms via two separate thermodynamic
phase transitions into a disordered liquid on increasing the temperature. The
first transition is into a glassy phase, topologically disordered at the
largest length scales; current evidence suggests that it lacks the long-ranged
phase correlations expected of a ``vortex glass''. This phase has a significant
degree of short-ranged translational order, unlike the disordered liquid, but
no quasi-long range order, in contrast to the Bragg glass. This glassy phase,
which we call a ``multi-domain glass'', is confined to a narrow sliver at
intermediate fields, but broadens out both for much larger and much smaller
field values. The multi-domain glass may be a ``hexatic glass''; alternatively,
its glassy properties may originate in the replica symmetry breaking envisaged
in recent theories of the structural glass transition. Estimates for
translational correlation lengths in the multi-domain glass indicate that they
can be far larger than the interline spacing for weak disorder, suggesting a
plausible mechanism by which signals of a two-step transition can be obscured.
Calculations of the Bragg glass-multi-domain glass and the multi-domain
glass-disordered liquid phase boundaries are presented and compared to
experimental data. We argue that these proposals provide a unified picture of
the available experimental data on both high-T and low-T materials,
simulations and current theoretical understanding.Comment: 70 pages, 9 postscript figures, modified title and minor changes in
published versio
Exploring the Bimodal Solar System via Sample Return from the Main Asteroid Belt: The Case for Revisiting Ceres
Abstract: Sample return from a main-belt asteroid has not yet been attempted, but appears technologically feasible. While the cost implications are significant, the scientific case for such a mission appears overwhelming. As suggested by the “Grand Tack” model, the structure of the main belt was likely forged during the earliest stages of Solar System evolution in response to migration of the giant planets. Returning samples from the main belt has the potential to test such planet migration models and the related geochemical and isotopic concept of a bimodal Solar System. Isotopic studies demonstrate distinct compositional differences between samples believed to be derived from the outer Solar System (CC or carbonaceous chondrite group) and those that are thought to be derived from the inner Solar System (NC or non-carbonaceous group). These two groups are separated on relevant isotopic variation diagrams by a clear compositional gap. The interface between these two regions appears to be broadly coincident with the present location of the asteroid belt, which contains material derived from both groups. The Hayabusa mission to near-Earth asteroid (NEA) (25143) Itokawa has shown what can be learned from a sample-return mission to an asteroid, even with a very small amount of sample. One scenario for main-belt sample return involves a spacecraft launching a projectile that strikes an object and flying through the debris cloud, which would potentially allow multiple bodies to be sampled if a number of projectiles are used on different asteroids. Another scenario is the more traditional method of landing on an asteroid to obtain the sample. A significant range of main-belt asteroids are available as targets for a sample-return mission and such a mission would represent a first step in mineralogically and isotopically mapping the asteroid belt. We argue that a sample-return mission to the asteroid belt does not necessarily have to return material from both the NC and CC groups to viably test the bimodal Solar System paradigm, as material from the NC group is already abundantly available for study. Instead, there is overwhelming evidence that we have a very incomplete suite of CC-related samples. Based on our analysis, we advocate a dedicated sample-return mission to the dwarf planet (1) Ceres as the best means of further exploring inherent Solar System variation. Ceres is an ice-rich world that may be a displaced trans-Neptunian object. We almost certainly do not have any meteorites that closely resemble material that would be brought back from Ceres. The rich heritage of data acquired by the Dawn mission makes a sample-return mission from Ceres logistically feasible at a realistic cost. No other potential main-belt target is capable of providing as much insight into the early Solar System as Ceres. Such a mission should be given the highest priority by the international scientific community
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