171 research outputs found
On the roughness of the paths of RBM in a wedge
Reflected Brownian motion (RBM) in a wedge is a 2-dimensional stochastic process Z whose state space in ℝ2 is given in polar coordinates by S = {(r, θ) : r ≥ 0, 0 ≤ θ ≤ ξ } for some 0 α, the strong p-variation of the sample paths of Y is finite on compact intervals, and, for 0 < p ≤ α, the strong p-variation of Y is infinite on [0,T ] whenever Z has been started from the origin. We also show that on excursion intervals of Z away from the origin, (Z,Y ) satisfies the standard Skorokhod problem for X. However, on the entire time horizon (Z,Y ) does not satisfy the standard Skorokhod problem for X, but nevertheless we show that it satisfies the extended Skorkohod problem
Anderson-Mott transition as a quantum glass problem
We combine a recent mapping of the Anderson-Mott metal-insulator transition
on a random-field problem with scaling concepts for random-field magnets to
argue that disordered electrons near an Anderson-Mott transition show
glass-like behavior. We first discuss attempts to interpret experimental
results in terms of a conventional scaling picture, and argue that some of the
difficulties encountered point towards a glassy nature of the electrons. We
then develop a general scaling theory for a quantum glass, and discuss critical
properties of both thermodynamic and transport variables in terms of it. Our
most important conclusions are that for a correct interpretation of experiments
one must distinguish between self-averaging and non-self averaging observables,
and that dynamical or temperature scaling is not of power-law type but rather
activated, i.e. given by a generalized Vogel-Fulcher law. Recent mutually
contradicting experimental results on Si:P are discussed in the light of this,
and new experiments are proposed to test the predictions of our quantum glass
scaling theory.Comment: 25pp, REVTeX, 5 ps figs, final version as publishe
Doping a semiconductor to create an unconventional metal
Landau Fermi liquid theory, with its pivotal assertion that electrons in
metals can be simply understood as independent particles with effective masses
replacing the free electron mass, has been astonishingly successful. This is
true despite the Coulomb interactions an electron experiences from the host
crystal lattice, its defects, and the other ~1022/cm3 electrons. An important
extension to the theory accounts for the behaviour of doped semiconductors1,2.
Because little in the vast literature on materials contradicts Fermi liquid
theory and its extensions, exceptions have attracted great attention, and they
include the high temperature superconductors3, silicon-based field effect
transistors which host two-dimensional metals4, and certain rare earth
compounds at the threshold of magnetism5-8. The origin of the non-Fermi liquid
behaviour in all of these systems remains controversial. Here we report that an
entirely different and exceedingly simple class of materials - doped small gap
semiconductors near a metal-insulator transition - can also display a non-Fermi
liquid state. Remarkably, a modest magnetic field functions as a switch which
restores the ordinary disordered Fermi liquid. Our data suggest that we have
finally found a physical realization of the only mathematically rigourous route
to a non-Fermi liquid, namely the 'undercompensated Kondo effect', where there
are too few mobile electrons to compensate for the spins of unpaired electrons
localized on impurity atoms9-12.Comment: 17 pages 4 figures supplemental information included with 2 figure
Mean field theory of the Mott-Anderson transition
We present a theory for disordered interacting electrons that can describe
both the Mott and the Anderson transition in the respective limits of zero
disorder and zero interaction. We use it to investigate the T=0 Mott-Anderson
transition at a fixed electron density, as a the disorder strength is
increased. Surprisingly, we find two critical values of disorder W_{nfl} and
W_c. For W > W_{nfl}, the system enters a ``Griffiths'' phase, displaying
metallic non-Fermi liquid behavior. At even stronger disorder, W=W_c > W_{nfl}
the system undergoes a metal insulator transition, characterized by the linear
vanishing of both the typical density of states and the typical quasiparticle
weight.Comment: 4 pages, 2 figures, REVTEX, eps
Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment
In this paper, we review theoretical and experimental research on rare region
effects at quantum phase transitions in disordered itinerant electron systems.
After summarizing a few basic concepts about phase transitions in the presence
of quenched randomness, we introduce the idea of rare regions and discuss their
importance. We then analyze in detail the different phenomena that can arise at
magnetic quantum phase transitions in disordered metals, including quantum
Griffiths singularities, smeared phase transitions, and cluster-glass
formation. For each scenario, we discuss the resulting phase diagram and
summarize the behavior of various observables. We then review several recent
experiments that provide examples of these rare region phenomena. We conclude
by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative
Ising chain fixed, references added, v3: final version as publishe
MrBayes 3.2: Efficient Bayesian Phylogenetic Inference and Model Choice Across a Large Model Space
Since its introduction in 2001, MrBayes has grown in popularity as a software package for Bayesian phylogenetic inference using Markov chain Monte Carlo (MCMC) methods. With this note, we announce the release of version 3.2, a major upgrade to the latest official release presented in 2003. The new version provides convergence diagnostics and allows multiple analyses to be run in parallel with convergence progress monitored on the fly. The introduction of new proposals and automatic optimization of tuning parameters has improved convergence for many problems. The new version also sports significantly faster likelihood calculations through streaming single-instruction-multiple-data extensions (SSE) and support of the BEAGLE library, allowing likelihood calculations to be delegated to graphics processing units (GPUs) on compatible hardware. Speedup factors range from around 2 with SSE code to more than 50 with BEAGLE for codon problems. Checkpointing across all models allows long runs to be completed even when an analysis is prematurely terminated. New models include relaxed clocks, dating, model averaging across time-reversible substitution models, and support for hard, negative, and partial (backbone) tree constraints. Inference of species trees from gene trees is supported by full incorporation of the Bayesian estimation of species trees (BEST) algorithms. Marginal model likelihoods for Bayes factor tests can be estimated accurately across the entire model space using the stepping stone method. The new version provides more output options than previously, including samples of ancestral states, site rates, site dN/dS rations, branch rates, and node dates. A wide range of statistics on tree parameters can also be output for visualization in FigTree and compatible software
Complexity of the International Agro-Food Trade Network and Its Impact on Food Safety
With the world’s population now in excess of 7 billion, it is vital to ensure the chemical and microbiological safety of our food, while maintaining the sustainability of its production, distribution and trade. Using UN databases, here we show that the international agro-food trade network (IFTN), with nodes and edges representing countries and import-export fluxes, respectively, has evolved into a highly heterogeneous, complex supply-chain network. Seven countries form the core of the IFTN, with high values of betweenness centrality and each trading with over 77% of all the countries in the world. Graph theoretical analysis and a dynamic food flux model show that the IFTN provides a vehicle suitable for the fast distribution of potential contaminants but unsuitable for tracing their origin. In particular, we show that high values of node betweenness and vulnerability correlate well with recorded large food poisoning outbreaks
Circulating Hepatitis B Surface Antigen Particles Carry Hepatocellular microRNAs
Hepatitis B virus (HBV) produces high quantities of subviral surface antigen particles (HBsAg) which circulate in the blood outnumbering virions of about 1\103–6 times. In individuals coinfected with the defective hepatitis Delta virus (HDV) the small HDV-RNA-genome and Delta antigen circulate as ribonucleoprotein complexes within HBsAg subviral particles. We addressed the question whether subviral HBsAg particles may carry in the same way cellular microRNAs (miRNAs) which are released into the bloodstream within different subcellular forms such as exosomes and microvescicles. Circulating HBsAg particles were isolated from sera of 11 HBsAg carriers by selective immunoprecipitation with monoclonal anti-HBs-IgG, total RNA was extracted and human miRNAs were screened by TaqMan real-time quantitative PCR Arrays. Thirty-nine human miRNAs were found to be significantly associated with the immunoprecipitated HBsAg, as determined by both comparative DDCT analysis and non-parametric tests (Mann-Whitney, p<0.05) with respect to controls. Moreover immunoprecipitated HBsAg particles contained Ago2 protein that could be revealed in ELISA only after 0.5% NP40. HBsAg associated miRNAs were liver-specific (most frequent = miR-27a, miR-30b, miR-122, miR-126 and miR-145) as well as immune regulatory (most frequent = miR-106b and miR-223). Computationally predicted target genes of HBsAg-associated miRNAs highlighted molecular pathways dealing with host-pathoge
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