318 research outputs found
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Contrasting arbuscular mycorrhizal communities colonizing different host plants show a similar response to a soil phosphorus concentration gradient
High soil phosphorus (P) concentration is frequently shown to reduce root colonization by arbuscular mycorrhizal (AM) fungi, but the influence of P on the diversity of colonizing AM fungi is uncertain.
We used terminal restriction fragment length polymorphism (T-RFLP) of 18S rDNA and cloning to assess diversity of AM fungi colonizing maize (Zea mays), soybean (Glycene max) and field violet (Viola arvensis) at three time points in one season along a P gradient of 10–280 mg l−1 in the field.
Percentage AM colonization changed between sampling time points but was not reduced by high soil P except in maize. There was no significant difference in AM diversity between sampling time points. Diversity was reduced at concentrations of P > 25 mg l−1, particularly in maize and soybean. Both cloning and T-RFLP indicated differences between AM communities in the different host species. Host species was more important than soil P in determining the AM community, except at the highest P concentration.
Our results show that the impact of soil P on the diversity of AM fungi colonizing plants was broadly similar, despite the fact that different plants contained different communities. However, subtle differences in the response of the AM community in each host were evident
Self-similarity and novel sample-length-dependence of conductance in quasiperiodic lateral magnetic superlattices
We study the transport of electrons in a Fibonacci magnetic superlattice
produced on a two-dimensional electron gas modulated by parallel magnetic field
stripes arranged in a Fibonacci sequence. Both the transmission coefficient and
conductance exhibit self-similarity and the six-circle property. The presence
of extended states yields a finite conductivity at infinite length, that may be
detected as an abrupt change in the conductance as the Fermi energy is varied,
much as a metal-insulator transition. This is a unique feature of transport in
this new kind of structure, arising from its inherent two-dimensional nature.Comment: 9 pages, 5 figures, revtex, important revisions made. to be published
in Phys. Rev.
Unconventional localization of electrons inside of a nematic electronic phase
The magnetotransport behavior inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality, we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes, with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally dependent correlation effects, enhanced interband spin fluctuations, or a Lifshitz-like transition, which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.</p
Unconventional localization of electrons inside of a nematic electronic phase
The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals
unusual multiband effects that cannot be reconciled with a simple two-band
approximation proposed by surface-sensitive spectroscopic probes. In order to
understand the role played by the multiband electronic structure and the degree
of two-dimensionality we have investigated the electronic properties of
exfoliated flakes of FeSe by reducing their thickness. Based on
magnetotransport and Hall resistivity measurements, we assess the mobility
spectrum that suggests an unusual asymmetry between the mobilities of the
electrons and holes with the electron carriers becoming localized inside the
nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the
presence of a hole-like quasiparticle with a lighter effective mass and a
quantum scattering time three times shorter, as compared with bulk FeSe. The
observed localization of negative charge carriers by reducing dimensionality
can be driven by orbitally-dependent correlation effects, enhanced interband
spin-fluctuations or a Lifshitz-like transition which affect mainly the
electron bands. The electronic localization leads to a fragile two-dimensional
superconductivity in thin flakes of FeSe, in contrast to the two-dimensional
high-Tc induced with electron doping via dosing or using a suitable interface.Comment: 22 pages, 14 figure
Anisotropic vortex pinning in superconductors with a square array of rectangular submicron holes
We investigate vortex pinning in thin superconducting films with a square
array of rectangular submicron holes ("antidots"). Two types of antidots are
considered: antidots fully perforating the superconducting film, and "blind
antidots", holes that perforate the film only up to a certain depth. In both
systems, we observe a distinct anisotropy in the pinning properties, reflected
in the critical current Ic, depending on the direction of the applied
electrical current: parallel to the long side of the antidots or perpendicular
to it. Although the mechanism responsible for the effect is very different in
the two systems, they both show a higher critical current and a sharper
IV-transition when the current is applied along the long side of the
rectangular antidots
T cell GM-CSF expression in juvenile arthritis is contingent upon Th17 plasticity.
Objectives Granulocyte monocyte colony stimulating factor (GM-CSF) is a potent inflammatory mediator responsible for recruitment and activation of innate immune cells. Recent murine data have identified Th17 cells as a key source of GM-CSF, and suggest that T cell derived GM-CSF is instrumental in the induction of autoimmune disease. We analysed the expression of T cell derived GM-CSF in the joints of patients with Juvenile idiopathic arthritis (JIA) and investigated the development links between Th17 and GM-CSF+ T helper cells. Methods 24 patients with JIA were analysed for expression of GM-CSF and the Th17 marker CD161 in synovial and peripheral blood compartments using flow cytometry and RT-PCR. A cytokine capture assay was used to purify Th17 cells and test the plasticity of cytokine production in response to IL-12 and IL-23. Results The frequency of GM-CSF producing T helper cells were significantly enriched in JIA synovial fluid mononuclear cells (SFMC) compared to PBMC (24.1% vs 2.9% of CD4+ T cells) and closely correlated with ESR levels (r(2) =0.91, p=<0.001). Synovial GM-CSF+ T cells were predominantly CD161 positive and co-expressed interferon gamma (IFNγ) but not IL-17. Culture of Th17 cells in the presence of IL-12 led to rapid upregulation of GM-CSF and IFNγ, recapitulating the phenotype of GM-CSF expressing cells within the joint. Conclusions Our results identify a novel outcome of Th17 plasticity in humans that may account for the enrichment of GM-CSF expressing T cells found within the JIA joint. The association of GM-CSF expression with systemic inflammation highlights the potential role for Th17 related cytokines in the pathology of JIA. © 2014 American College of Rheumatology
Magnetoresistance of a 2-dimensional electron gas in a random magnetic field
We report magnetoresistance measurements on a two-dimensional electron gas
(2DEG) made from a high mobility GaAs/AlGaAs heterostructure, where the
externally applied magnetic field was expelled from regions of the
semiconductor by means of superconducting lead grains randomly distributed on
the surface of the sample. A theoretical explanation in excellent agreement
with the experiment is given within the framework of the semiclassical
Boltzmann equation.Comment: REVTEX 3.0, 11 pages, 3 Postscript figures appended. The manuscript
can also be obtained from our World Wide Web server:
http://roemer.fys.ku.dk/randmag.ht
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Atmospheric super-rotation in solar system and extra-solar planetary atmospheres
Super-rotation is a common phenomenon in solar system planetary atmospheres. Out of the four substantial atmospheres possessed by solid bodies in the solar system, the slowly rotating planet, Venus, and moon, Titan, are both well-known to have atmospheres that rotate on average substantially more quickly than does the solid surface underneath. The more rapidly rotating planets, Mars and Earth, have much weaker global super-rotation, but both can exhibit time-varying prograde jets near the equator which rotate more rapidly than the local surface. Atmospheric super-rotation is not restricted to planets with solid surfaces and shallow atmospheres. Cloud-tracking observations of the gas giants Jupiter and Saturn show that they both possess rapid prograde equatorial jets and hence exhibit local super-rotation.
Simplified global circulation models of extra-solar planets, including representations of ‘hot Jupiters’ and Earth-like planets rotating at different rates, can also show sustained super-rotating equatorial jets in different dynamical regimes. In the extra-solar planet cases in particular, the quantitative results are highly sensitive to model parameters.
In each case the detailed mechanism, or combination of mechanisms, which produces the super-rotating jets might vary, but all require longitudinally asymmetric motions, waves or eddies, to transport angular momentum up-gradient into the jets. The mechanism is not always easy to diagnose from observations and requires careful modelling. We review both observations of solar system planets and recent global circulation model results, combined in the case of Mars and Earth in the form of atmospheric reanalyses by data assimilation, together with simplified extra-solar planet simulations
Re-entrant resonant tunneling
We study the effect of electron-electron interactions on the
resonant-tunneling spectroscopy of the localized states in a barrier. Using a
simple model of three localized states, we show that, due to the Coulomb
interactions, a single state can give rise to two resonant peaks in the
conductance as a function of gate voltage, G(Vg). We also demonstrate that an
additional higher-order resonance with Vg-position in between these two peaks
becomes possibile when interactions are taken into account. The corresponding
resonant-tunneling process involves two-electron transitions. We have observed
both these effects in GaAs transistor microstructures by studying the time
evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an
isolated impurity (modulator). The heights of the two stronger peaks exibit
in-phase fluctuations. The phase of fluctuations of the smaller middle peak is
opposite. The two stronger peaks have their origin in the same localized state,
and the third one corresponds to a co-tunneling process.Comment: 9 pages, REVTeX, 4 figure
Anyons in a weakly interacting system
We describe a theoretical proposal for a system whose excitations are anyons
with the exchange phase pi/4 and charge -e/2, but, remarkably, can be built by
filling a set of single-particle states of essentially noninteracting
electrons. The system consists of an artificially structured type-II
superconducting film adjacent to a 2D electron gas in the integer quantum Hall
regime with unit filling fraction. The proposal rests on the observation that a
vacancy in an otherwise periodic vortex lattice in the superconductor creates a
bound state in the 2DEG with total charge -e/2. A composite of this
fractionally charged hole and the missing flux due to the vacancy behaves as an
anyon. The proposed setup allows for manipulation of these anyons and could
prove useful in various schemes for fault-tolerant topological quantum
computation.Comment: 7 pages with 3 figures. For related work and info visit
http://www.physics.ubc.ca/~fran
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