833 research outputs found
Hysteresis in the Mott Transition between Plasma and Insulating Gas
We show that hysteresis can occur in the transition between a neutral plasma
and the insulating gas consisting of neutral pairs bound by Coulomb attraction.
Since the transition depends sensitively on the screening length in the plasma,
regions of bistability occur in density--temperature phase space. We present
numerical results which indicate where these regions occur for systems such as
spin-polarized hydrogen, positronium gas, and excitons in a semiconductor.Comment: 9 pages (Latex/RevTex), 6 postscript figures which are in compressed
and uuencoded file, prepared using the utility "uufiles" and separately
submitted. They should be automatically included with the text when it is
downloaded. Figures also available in hard copy from the authors
([email protected]; [email protected]); paper submitted to
Phys. Rev.
Two-phonon scattering of magnetorotons in fractional quantum Hall liquids
We study the phonon-assisted process of dissociation of a magnetoroton, in a
fractional quantum Hall liquid, into an unbound pair of quasiparticles. Whilst
the dissociation is forbidden to first order in the electron-phonon
interaction, it can occur as a two-phonon process. Depending on the value of
final separation between the quasiparticles, the dissociation is either a
single event involving absorption of one phonon and emission of another phonon
of similar energy, or a two-phonon diffusion of a quasiexciton in momentum
space. The dependence of the magnetoroton dissociation time on the filling
factor of the incompressible liquid is found.Comment: 4 pages, no figure
Herd-level risk factors of bovine tuberculosis in England and Wales after the 2001 foot-and-mouth disease epidemic
We present the results of a 2005 caseâcontrol study of bovine tuberculosis (bTB) breakdowns in English and Welsh herds. The herd management, farming practices, and environmental factors of 401matched pairs of case and control herds were investigated to provide a picture of herd-level risk factors in areas of varying bTB incidence. A global conditional logistic regression model, with region-specific variants, was used to compare case herds that had experienced a confirmed bTB breakdown to contemporaneous control herds matched on region, herd type, herd size, and parish testing interval. Contacts with cattle from contiguous herds and sourcing cattle from herds with a recent history of bTB were associated with an increased risk in both the global and regional analyses. Operating a farm over several premises, providing cattle feed inside the housing, and the presence of badgers were also identified as significantly associated with an increased bTB risk. Steps taken to minimize cattle contacts with neighboring herds and altering trading practices could have the potential to reduce the size of the bTB epidemic. In principle, limiting the interactions between cattle and wildlife may also be useful; however this study did not highlight any specific measures to implement
Qualitative Properties of Magnetic Fields in Scalar Field Cosmology
We study the qualitative properties of the class of spatially homogeneous
Bianchi VI_o cosmological models containing a perfect fluid with a linear
equation of state, a scalar field with an exponential potential and a uniform
cosmic magnetic field, using dynamical systems techniques. We find that all
models evolve away from an expanding massless scalar field model in which the
matter and the magnetic field are negligible dynamically. We also find that for
a particular range of parameter values the models evolve towards the usual
power-law inflationary model (with no magnetic field) and, furthermore, we
conclude that inflation is not fundamentally affected by the presence of a
uniform primordial magnetic field. We investigate the physical properties of
the Bianchi I magnetic field models in some detail.Comment: 12 pages, 2 figures in REVTeX format. to appear in Phys. Rev.
Helical distribution of the bacterial chemoreceptor via colocalization with the Sec protein translocation machinery
In Escherichia coli, chemoreceptor clustering at a cell pole seems critical for signal amplification and adaptation. However, little is known about the mechanism of localization itself. Here we examined whether the aspartate chemoreceptor (Tar) is inserted directly into the polar membrane by using its fusion to green fluorescent protein (GFP). After induction of TarâGFP, fluorescent spots first appeared in lateral membrane regions, and later cell poles became predominantly fluorescent. Unexpectedly, TarâGFP showed a helical arrangement in lateral regions, which was more apparent when a TarâGFP derivative with two cysteine residues in the periplasmic domain was cross-linked to form higher oligomers. Moreover, similar distribution was observed even when the cytoplasmic domain of the double cysteine TarâGFP mutant was replaced by that of the kinase EnvZ, which does not localize to a pole. Observation of GFPâSecE and a translocation-defective MalEâGFP mutant, as well as indirect immunofluorescence microscopy on SecG, suggested that the general protein translocation machinery (Sec) itself is arranged into a helical array, with which Tar is transiently associated. The Sec coil appeared distinct from the MreB coil, an actin-like cytoskeleton. These findings will shed new light on the mechanisms underlying spatial organization of membrane proteins in E. coli
Quantum saturation and condensation of excitons in CuO: a theoretical study
Recent experiments on high density excitons in CuO provide evidence for
degenerate quantum statistics and Bose-Einstein condensation of this nearly
ideal gas. We model the time dependence of this bosonic system including
exciton decay mechanisms, energy exchange with phonons, and interconversion
between ortho (triplet-state) and para (singlet-state) excitons, using
parameters for the excitonic decay, the coupling to acoustic and low-lying
optical phonons, Auger recombination, and ortho-para interconversion derived
from experiment. The single adjustable parameter in our model is the
optical-phonon cooling rate for Auger and laser-produced hot excitons. We show
that the orthoexcitons move along the phase boundary without crossing it (i.e.,
exhibit a ``quantum saturation''), as a consequence of the balance of entropy
changes due to cooling of excitons by phonons and heating by the non-radiative
Auger two-exciton recombination process. The Auger annihilation rate for
para-para collisions is much smaller than that for ortho-para and ortho-ortho
collisions, explaining why, under the given experimental conditions, the
paraexcitons condense while the orthoexcitons fail to do so.Comment: Revised to improve clarity and physical content 18 pages, revtex,
figures available from G. Kavoulakis, Physics Department, University of
Illinois, Urban
A probabilistic model for gene content evolution with duplication, loss, and horizontal transfer
We introduce a Markov model for the evolution of a gene family along a
phylogeny. The model includes parameters for the rates of horizontal gene
transfer, gene duplication, and gene loss, in addition to branch lengths in the
phylogeny. The likelihood for the changes in the size of a gene family across
different organisms can be calculated in O(N+hM^2) time and O(N+M^2) space,
where N is the number of organisms, is the height of the phylogeny, and M
is the sum of family sizes. We apply the model to the evolution of gene content
in Preoteobacteria using the gene families in the COG (Clusters of Orthologous
Groups) database
AbschÀtzung der Auswirkungen der KrautfÀule auf den Bio-Kartoffelbau in verschiedenen EuropÀischen LÀndern, sowie Inventar der angewendeten Anbau- und Pflanzenschutzstrategien
Problemstellung/Ziele
Die Kraut- und KnollenfĂ€ule (Phytophthora infestans) ist die Krankheit, die im biologischen (aber auch konventionellen) Kartoffelanbau in Europa die gröĂten Probleme verursacht. Unter gĂŒnstigen klimatischen Bedingungen breitet sich die Krankheit sehr rasch aus und kann groĂe ErtragsausfĂ€lle verursachen. Innerhalb von Europa variiert der durch P. infestans verursachte wirtschaftliche Schaden stark zwischen den Regionen. Dies hĂ€ngt von verschiedenen Faktoren ab, aber in biologisch bewirtschafteten Anbausystemen immt man an, dass die klimatischen Bedingungen, die verwendeten Sorten sowie die agronomischen MaĂnahmen wie Bodenbearbeitung oder die Verwendung von Pflanzenschutzmitteln eine wichtige Rolle spielen. Die Reduktion oder das Verbot des Kupfereinsatzes im biologischen Kartoffelanbau wird deshalb auch unterschiedliche Auswirkungen in den verschiedenen EuropĂ€ischen Regionen haben.
Als Teilprojekt des EU-finanzierten Projektes Blight-MOP (QLRT 31065) wurde eine detaillierte Studie der ökonomischen und gesetzlichen Rahmenbedingungen und ein Inventar der Anbausysteme in 7 LĂ€ndern durchgefĂŒhrt, um (i) Ein Inventar der derzeitigen Anbautechniken zu erstellen, (ii) die Auswirkungen von P. infestans auf ErtrĂ€ge und Wirtschaftlichkeit zu evaluieren und die Auswirkungen eines Kupferverbotes abzuschĂ€tzen und um (iii) Pflanzenschutzstrategien von Bioproduzenten zu identifizieren, die bereits
jetzt ohne den Einsatz von Kupfer auskommen.
Fazit:
Diese Betriebsanalyse weist darauf hin, dass eine Optimierung der Einzelmassnahmen und die regionsspezfische Integration von Massnahmen zu einer substanziellen Verbesserung des Anbauerfolges fĂŒhren können. Die Betriebsanalyse zeigt auch, dass Kupfer bislang eine wichtige Rolle bei der Ertragsbildung gespielt hat.
Ein Kupferverbot ohne Angebot von praxistauglichen Alternativlösungen (wie sie innerhalb von Blight-MOP und anderen Projekten erarbeitet werden) wĂŒrde demnach zu einer starken Destabilisierung des biologischen Kartoffelanbaues und vermutlich zu einer Angebotsvernappung fĂŒhren. Andere Teilprojekte des Blight-MOP Projektes zielen auf die Verbesserung von anbautechnischen Massnahmen und die Entwicklung von Ersatzprodukten fĂŒr Kupfer ab
Dynamic nuclear polarization and spin-diffusion in non-conducting solids
There has been much renewed interest in dynamic nuclear polarization (DNP),
particularly in the context of solid state biomolecular NMR and more recently
dissolution DNP techniques for liquids. This paper reviews the role of spin
diffusion in polarizing nuclear spins and discusses the role of the spin
diffusion barrier, before going on to discuss some recent results.Comment: submitted to Applied Magnetic Resonance. The article should appear in
a special issue that is being published in connection with the DNP Symposium
help in Nottingham in August 200
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