181 research outputs found
Quantum-Hall plateau-plateau transition in top-gated epitaxial graphene grown on SiC (0001)
We investigate the low-temperature magneto-transport properties of monolayer
epitaxial graphene films formed on the Si-face of semi-insulating 4H-SiC
substrates by a high temperature sublimation process. A high-k top-gate on the
epitaxial graphene is realized by inserting a fully oxidized nanometer thin
aluminum film as a seeding layer, followed by an atomic layer deposition
process. At low temperatures, the devices demonstrate a strong field effect by
the top gate with an on/off ratio of ~7 and an electron mobility up to ~3250
cm^2/Vs. After the observation of the half-integer quantum Hall effect for
monolayer epitaxial graphene films, detailed magneto-transport measurements
have been carried out including varying densities, temperatures, magnetic
fields and currents. We study the width of the distinguishable quantum-Hall
plateau to plateau transition (Landau level index n=0 to n=1) as temperature
(T) and current are varied. For both gate voltage and magnetic field sweeps and
T>10 K the transition width goes as T^{-\kappa} with exponent \kappa ~0.42.
This universal scaling exponent agrees well with those found in III-V
heterojunctions with short range alloy disorders and in exfoliated graphene.Comment: accepted by Journal of Applied Physic
Non-Markovian stochastic description of quantum transport in photosynthetic systems
We analyze several aspects of the transport dynamics in the LH1-RC core of
purple bacteria, which consists basically in a ring of antenna molecules that
transport the energy into a target molecule, the reaction center, placed in the
center of the ring. We show that the periodicity of the system plays an
important role to explain the relevance of the initial state in the transport
efficiency. This picture is modified, and the transport enhanced for any
initial state, when considering that molecules have different energies, and
when including their interaction with the environment. We study this last
situation by using stochastic Schr{\"o}dinger equations, both for Markovian and
non-Markovian type of interactions.Comment: 21 pages, 5 figure
Superradiance Transition in Photosynthetic Light-Harvesting Complexes
We investigate the role of long-lasting quantum coherence in the efficiency
of energy transport at room temperature in Fenna-Matthews-Olson photosynthetic
complexes. The excitation energy transfer due to the coupling of the light
harvesting complex to the reaction center ("sink") is analyzed using an
effective non-Hermitian Hamiltonian. We show that, as the coupling to the
reaction center is varied, maximal efficiency in energy transport is achieved
in the vicinity of the superradiance transition, characterized by a segregation
of the imaginary parts of the eigenvalues of the effective non-Hermitian
Hamiltonian. Our results demonstrate that the presence of the sink (which
provides a quasi--continuum in the energy spectrum) is the dominant effect in
the energy transfer which takes place even in absence of a thermal bath. This
approach allows one to study the effects of finite temperature and the effects
of any coupling scheme to the reaction center. Moreover, taking into account a
realistic electric dipole interaction, we show that the optimal distance from
the reaction center to the Fenna-Matthews-Olson system occurs at the
superradiance transition, and we show that this is consistent with available
experimental data.Comment: 9 page
Dynamical response of a pinned two-dimensional Wigner crystal
We re-examine a long-standing problem of a finite-frequency conductivity of a
weakly pinned two-dimensional classical Wigner crystal. In this system an
inhomogeneously broadened absorption line (pinning mode) centered at disorder
and magnetic field dependent frequency is known to appear. We show
that the relative linewidth of the pinning mode is
of the order of one in weak magnetic fields, exhibits a power-law decrease in
intermediate fields, and eventually saturates at a small value in strong
magnetic fields. The linewidth narrowing is due to a peculiar mechanism of
mixing between the stiffer longitudinal and the softer transverse components of
the collective excitations. The width of the high-field resonance proves to be
related to the density of states in the low-frequency tail of the zero-field
phonon spectrum. We find a qualitative agreement with recent experiments and
point out differences from the previous theoretical work on the subject.Comment: 19 pages, 11 figures. Supersedes cond-mat/990424
Magnetism, FeS colloids, and Origins of Life
A number of features of living systems: reversible interactions and weak
bonds underlying motor-dynamics; gel-sol transitions; cellular connected
fractal organization; asymmetry in interactions and organization; quantum
coherent phenomena; to name some, can have a natural accounting via
interactions, which we therefore seek to incorporate by expanding the horizons
of `chemistry-only' approaches to the origins of life. It is suggested that the
magnetic 'face' of the minerals from the inorganic world, recognized to have
played a pivotal role in initiating Life, may throw light on some of these
issues. A magnetic environment in the form of rocks in the Hadean Ocean could
have enabled the accretion and therefore an ordered confinement of
super-paramagnetic colloids within a structured phase. A moderate H-field can
help magnetic nano-particles to not only overcome thermal fluctuations but also
harness them. Such controlled dynamics brings in the possibility of accessing
quantum effects, which together with frustrations in magnetic ordering and
hysteresis (a natural mechanism for a primitive memory) could throw light on
the birth of biological information which, as Abel argues, requires a
combination of order and complexity. This scenario gains strength from
observations of scale-free framboidal forms of the greigite mineral, with a
magnetic basis of assembly. And greigite's metabolic potential plays a key role
in the mound scenario of Russell and coworkers-an expansion of which is
suggested for including magnetism.Comment: 42 pages, 5 figures, to be published in A.R. Memorial volume, Ed
Krishnaswami Alladi, Springer 201
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Rarity of monodominance in hyperdiverse Amazonian forests.
Tropical forests are known for their high diversity. Yet, forest patches do occur in the tropics where a single tree species is dominant. Such "monodominant" forests are known from all of the main tropical regions. For Amazonia, we sampled the occurrence of monodominance in a massive, basin-wide database of forest-inventory plots from the Amazon Tree Diversity Network (ATDN). Utilizing a simple defining metric of at least half of the trees ≥ 10 cm diameter belonging to one species, we found only a few occurrences of monodominance in Amazonia, and the phenomenon was not significantly linked to previously hypothesized life history traits such wood density, seed mass, ectomycorrhizal associations, or Rhizobium nodulation. In our analysis, coppicing (the formation of sprouts at the base of the tree or on roots) was the only trait significantly linked to monodominance. While at specific locales coppicing or ectomycorrhizal associations may confer a considerable advantage to a tree species and lead to its monodominance, very few species have these traits. Mining of the ATDN dataset suggests that monodominance is quite rare in Amazonia, and may be linked primarily to edaphic factors
WW Production Cross Section and W Branching Fractions in e+e- Collisions at 189 GeV
From a data sample of 183 pb^-1 recorded at a center-of-mass energy of roots
= 189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are
selected. Assuming Standard Model W boson decay branching fractions, the W-pair
production cross section is measured to be sigmaWW = 16.30 +- 0.34(stat.) +-
0.18(syst.) pb. When combined with previous OPAL measurements, the W boson
branching fraction to hadrons is determined to be 68.32 +- 0.61(stat.) +-
0.28(syst.) % assuming lepton universality. These results are consistent with
Standard Model expectations.Comment: 22 pages, 5 figures, submitted to Phys. Lett.
Genome-wide mapping of cystitis due to Streptococcus agalactiae and Escherichia coli in mice identifies a unique bladder transcriptome that signifies pathogen-specific antimicrobial defense against urinary tract infection
The most common causes of urinary tract infections (UTIs) are Gram-negative pathogens such as Escherichia coli; however, Gram-positive organisms, including Streptococcus agalactiae, or group B streptococcus (GBS), also cause UTI. In GBS infection, UTI progresses to cystitis once the bacteria colonize the bladder, but the host responses triggered in the bladder immediately following infection are largely unknown. Here, we used genome-wide expression profiling to map the bladder transcriptome of GBS UTI in mice infected transurethrally with uropathogenic GBS that was cultured from a 35-year-old women with cystitis. RNA from bladders was applied to Affymetrix Gene-1.0ST microarrays; quantitative reverse transcriptase PCR (qRT-PCR) was used to analyze selected gene responses identified in array data sets. A surprisingly small significant-gene list of 172 genes was identified at 24 h; this compared to 2,507 genes identified in a side-by-side comparison with uropathogenic E. coli (UPEC). No genes exhibited significantly altered expression at 2 h in GBS-infected mice according to arrays despite high bladder bacterial loads at this early time point. The absence of a marked early host response to GBS juxtaposed with broad-based bladder responses activated by UPEC at 2 h. Bioinformatics analyses, including integrative system-level network mapping, revealed multiple activated biological pathways in the GBS bladder transcriptome that regulate leukocyte activation, inflammation, apoptosis, and cytokine-chemokine biosynthesis. These findings define a novel, minimalistic type of bladder host response triggered by GBS UTI, which comprises collective antimicrobial pathways that differ dramatically from those activated by UPEC. Overall, this study emphasizes the unique nature of bladder immune activation mechanisms triggered by distinct uropathogens
Functional illness in primary care: dysfunction versus disease
<p>Abstract</p> <p>Background</p> <p>The Biopsychosocial Model aims to integrate the biological, psychological and social components of illness, but integration is difficult in practice, particularly when patients consult with medically unexplained physical symptoms or functional illness.</p> <p>Discussion</p> <p>This Biopsychosocial Model was developed from General Systems Theory, which describes nature as a dynamic order of interacting parts and processes, from molecular to societal. Despite such conceptual progress, the biological, psychological, social and spiritual components of illness are seldom managed as an integrated whole in conventional medical practice. This is because the biomedical model can be easier to use, clinicians often have difficulty relinquishing a disease-centred approach to diagnosis, and either dismiss illness when pathology has been excluded, or explain all undifferentiated illness in terms of psychosocial factors. By contrast, traditional and complementary treatment systems describe reversible functional disturbances, and appear better at integrating the different components of illness. Conventional medicine retains the advantage of scientific method and an expanding evidence base, but needs to more effectively integrate psychosocial factors into assessment and management, notably of 'functional' illness. As an aid to integration, pathology characterised by structural change in tissues and organs is contrasted with dysfunction arising from disordered physiology or psychology that may occur independent of pathological change.</p> <p>Summary</p> <p>We propose a classification of illness that includes orthogonal dimensions of pathology and dysfunction to support a broadly based clinical approach to patients; adoption of which may lead to fewer inappropriate investigations and secondary care referrals and greater use of cognitive behavioural techniques, particularly when managing functional illness.</p
Carbon uptake by mature Amazon forests has mitigated Amazon nations' carbon emissions
Background: Several independent lines of evidence suggest that Amazon forests have provided a significant carbon
sink service, and also that the Amazon carbon sink in intact, mature forests may now be threatened as a result of
different processes. There has however been no work done to quantify non-land-use-change forest carbon fluxes on
a national basis within Amazonia, or to place these national fluxes and their possible changes in the context of the
major anthropogenic carbon fluxes in the region. Here we present a first attempt to interpret results from groundbased
monitoring of mature forest carbon fluxes in a biogeographically, politically, and temporally differentiated way.
Specifically, using results from a large long-term network of forest plots, we estimate the Amazon biomass carbon balance
over the last three decades for the different regions and nine nations of Amazonia, and evaluate the magnitude
and trajectory of these differentiated balances in relation to major national anthropogenic carbon emissions.
Results: The sink of carbon into mature forests has been remarkably geographically ubiquitous across Amazonia,
being substantial and persistent in each of the five biogeographic regions within Amazonia. Between 1980 and 2010,
it has more than mitigated the fossil fuel emissions of every single national economy, except that of Venezuela. For
most nations (Bolivia, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname) the sink has probably additionally
mitigated all anthropogenic carbon emissions due to Amazon deforestation and other land use change. While the
sink has weakened in some regions since 2000, our analysis suggests that Amazon nations which are able to conserve
large areas of natural and semi-natural landscape still contribute globally-significant carbon sequestration.
Conclusions: Mature forests across all of Amazonia have contributed significantly to mitigating climate change for
decades. Yet Amazon nations have not directly benefited from providing this global scale ecosystem service. We suggest
that better monitoring and reporting of the carbon fluxes within mature forests, and understanding the drivers
of changes in their balance, must become national, as well as international, priorities
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