2,487 research outputs found
A vast 4000-year-old spatial pattern of termite mounds
The origins of many large-scale âbiogenicâ earthen structures are controversial, because often the species that built them have vanished. This is especially true when they form regular (over-dispersed), self-organized vegetation patterns [1]. Here, we describe a vast array of soil mounds constructed by termites (Syntermes dirus) that has persisted for up to 4000 years and covers an estimated 230,000 km2 of seasonally dry tropical forest in a relatively undisturbed and climatically stable region of Northeast Brazil. The mounds are not nests, but rather they are generated by the excavation of vast inter-connecting tunnel networks, resulting in approximately 10 km3 of soil being deposited in 200 million conical mounds that are 2.5 m tall and approximately 9 m in diameter. S. dirus termites are still present in the soil surrounding the mounds and we found that intra-specific aggression occurred at a scale much larger than an individual mound. We suggest that the complex network of tunnels built to access episodic leaf-fall has allowed for the optimization of waste soil removal, which over thousands of years has formed an over-dispersed spatial pattern of mounds
Edge effects in 3D dosimetry: characterisation and correction of the non-uniform dose response of PRESAGEÂŽ.
Previous work has shown that PRESAGEÂŽ can be used successfully to perform 3D dosimetric measurements of complex radiotherapy treatments. However, measurements near the sample edges are known to be difficult to achieve. This is an issue when the doses at air-material interfaces are of interest, for example when investigating the electron return effect (ERE) present in treatments delivered by magnetic resonance (MR)-linac systems. To study this effect, a set of 3.5 cm-diameter cylindrical PRESAGEÂŽ samples was uniformly irradiated with multiple dose fractions, using either a conventional linac or an MR-linac. The samples were imaged between fractions using an optical-CT, to read out the corresponding accumulated doses. A calibration between TPS-predicted dose and optical-CT pixel value was determined for individual dosimeters as a function of radial distance from the axis of rotation. This data was used to develop a correction that was applied to four additional samples of PRESAGEÂŽ of the same formulation, irradiated with 3D-CRT and IMRT treatment plans, to recover significantly improved 3D measurements of dose. An alternative strategy was also tested, in which the outer surface of the sample was physically removed prior to irradiation. Results show that for the formulation studied here, PRESAGEÂŽ samples have a central region that responds uniformly and an edge region of 6-7 mm where there is gradual increase in dosimeter response, rising to an over-response of 24%-36% at the outer boundary. This non-uniform dose response increases in both extent and magnitude over time. Both mitigation strategies investigated were successful. In our four exemplar studies, we show how discrepancies at edges are reduced from 13%-37% of the maximum dose to between 2 and 8%. Quantitative analysis shows that the 3D gamma passing rates rise from 90.4, 69.3, 63.7 and 43.6% to 97.3, 99.9, 96.7 and 98.9% respectively
Chandra observations of Cygnus OB2
Cygnus OB2 is the nearest example of a massive star forming region,
containing over 50 O-type stars and hundreds of B-type stars. We have analyzed
two Chandra pointings in Cyg OB2, detecting ~1700 X-ray sources, of which ~1450
are thought to be members of the association. Optical and near-IR photometry
has been obtained for ~90% of these sources from recent deep Galactic plane
surveys. We have performed isochrone fits to the near-IR color-magnitude
diagram, deriving ages of 3.5(+0.75,-1.0) and 5.25(+1.5,-1.0) Myrs for sources
in the two fields, both with considerable spreads around the pre-MS isochrones.
The presence of a second population in the region, somewhat older than the
present-day O-type stars, has been suggested by other authors and fits with the
ages derived here. The fraction of sources with inner circumstellar disks (as
traced by the K-band excess) is found to be very low, but appropriate for a
population of age ~5 Myrs. We measure the stellar mass functions and find a
power-law slope of Gamma = -1.09 +/- 0.13, in good agreement with the global
mean value estimated by Kroupa. A steepening of the mass function at high
masses is observed and we suggest this is due to the presence of the previous
generation of stars that have lost their most massive members. Finally,
combining our mass function and an estimate of the radial density profile of
the association suggests a total mass of Cyg OB2 of ~30,000 Msun, similar to
that of many of our Galaxy's most massive star forming regions.Comment: 6 pages, 4 figures, conference proceedings for JENAM 2010: Star
Clusters in the Era of Large Surveys, Editors: A.Moitinho and J. Alve
Tonic Activation of GluN2C/GluN2D-Containing NMDA Receptors by Ambient Glutamate Facilitates Cortical Interneuron Maturation
Developing cortical GABAergic interneurons rely on genetic programs, neuronal activity, and environmental cuesto construct inhibitory
circuits during early postnatal development. Disruption of these events can cause long-term changes in cortical inhibition and may be
involved in neurological disorders associated with inhibitory circuit dysfunction. We hypothesized that tonic glutamate signaling in the
neonatal cortex contributesto, and is necessary for,the maturation of cortical interneurons. Totestthis hypothesis, we used mice of both
sexes to quantify extracellular glutamate concentrations in the cortex during development, measure ambient glutamate-mediated activation of developing cortical interneurons, and manipulatetonic glutamate signaling using subtype-specific NMDA receptor antagonists
in vitro and in vivo. We report that ambient glutamate levels are high (100 nM) in the neonatal cortex and decrease (to 50 nM) during
the first weeks of life, coincident with increases in astrocytic glutamate uptake. Consistent with elevated ambient glutamate, putative
parvalbumin-positive interneurons in the cortex (identified using G42:GAD1-eGFP reporter mice) exhibit a transient, tonic NMDA
current at the end of the first postnatal week. GluN2C/GluN2D-containing NMDA receptors mediate the majority of this current and
contribute to the resting membrane potential and intrinsic properties of developing putative parvalbumin interneurons. Pharmacological blockade of GluN2C/GluN2D-containing NMDA receptors in vivo during the period of tonic interneuron activation, but not later,
leads to lasting decreases in interneuron morphological complexity and causes deficits in cortical inhibition later in life. These results
demonstrate that dynamic ambient glutamate signaling contributes to cortical interneuron maturation via tonic activation of GluN2C/
GluN2D-containing NMDA receptor
High-sensitivity diamond magnetometer with nanoscale resolution
We present a novel approach to the detection of weak magnetic fields that
takes advantage of recently developed techniques for the coherent control of
solid-state electron spin quantum bits. Specifically, we investigate a magnetic
sensor based on Nitrogen-Vacancy centers in room-temperature diamond. We
discuss two important applications of this technique: a nanoscale magnetometer
that could potentially detect precession of single nuclear spins and an optical
magnetic field imager combining spatial resolution ranging from micrometers to
millimeters with a sensitivity approaching few femtotesla/Hz.Comment: 29 pages, 4 figure
Valley-spin blockade and spin resonance in carbon nanotubes
Manipulation and readout of spin qubits in quantum dots made in III-V
materials successfully rely on Pauli blockade that forbids transitions between
spin-triplet and spin-singlet states. Quantum dots in group IV materials have
the advantage of avoiding decoherence from the hyperfine interaction by
purifying them with only zero-spin nuclei. Complications of group IV materials
arise from the valley degeneracies in the electronic bandstructure. These lead
to complicated multiplet states even for two-electron quantum dots thereby
significantly weakening the selection rules for Pauli blockade. Only recently
have spin qubits been realized in silicon devices where the valley degeneracy
is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade
can be observed by lifting valley degeneracy through disorder. In clean
nanotubes, quantum dots have to be made ultra-small to obtain a large energy
difference between the relevant multiplet states. Here we report on
low-disorder nanotubes and demonstrate Pauli blockade based on both valley and
spin selection rules. We exploit the bandgap of the nanotube to obtain a large
level spacing and thereby a robust blockade. Single-electron spin resonance is
detected using the blockade.Comment: 31 pages including supplementary informatio
Micro-pharmacokinetics: quantifying local drug concentration at live cell membranes
Fundamental equations for determining pharmacological parameters, such as the binding afnity of a ligand for its target receptor, assume a homogeneous distribution of ligand, with concentrations in the immediate vicinity of the receptor being the same as those in the bulk aqueous phase. It is, however, known that drugs are able to interact directly with the plasma membrane, potentially increasing local ligand concentrations around the receptor. We have previously reported an infuence of ligand-phospholipid interactions on ligand binding kinetics at the β2-adrenoceptor, which resulted in distinct âmicro-pharmacokineticâ ligand profles. Here, we directly quantifed the local concentration of BODIPY630/650-PEG8-S-propranolol (BY-propranolol), a fuorescent derivative of the classical β-blocker propranolol, at various distances above membranes of single living cells using fuorescence correlation spectroscopy. We show for the frst time a signifcantly increased ligand concentration immediatel adjacent to the cell membrane compared to the bulk aqueous phase. We further show a clear role of both the cell membrane and the β2-adrenoceptor in determining high local BY-propranolol concentrations at the cell surface. These data suggest that the true binding afnity of BY-propranolol for the β2-adrenoceptor is likely far lower than previously reported and highlights the critical importance of understanding the âmicro-pharmacokineticâ profles of ligands for membrane-associated proteins
Nanoscale spin rectifiers controlled by the Stark effect
The control of orbital and spin state of single electrons is a key ingredient
for quantum information processing, novel detection schemes, and, more
generally, is of much relevance for spintronics. Coulomb and spin blockade (SB)
in double quantum dots (DQDs) enable advanced single-spin operations that would
be available even for room-temperature applications for sufficiently small
devices. To date, however, spin operations in DQDs were observed at sub-Kelvin
temperatures, a key reason being that scaling a DQD system while retaining an
independent field-effect control on the individual dots is very challenging.
Here we show that quantum-confined Stark effect allows an independent
addressing of two dots only 5 nm apart with no need for aligned nanometer-size
local gating. We thus demonstrate a scalable method to fully control a DQD
device, regardless of its physical size. In the present implementation we show
InAs/InP nanowire (NW) DQDs that display an experimentally detectable SB up to
10 K. We also report and discuss an unexpected re-entrant SB lifting as a
function magnetic-field intensity
Proximity to Sports Facilities and Sports Participation for Adolescents in Germany
Objectives - To assess the relationship between proximity to specific sports facilities and participation in the corresponding sports activities for adolescents in Germany.
Methods - A sample of 1,768 adolescents aged 11â17 years old and living in 161 German communities was examined. Distances to the nearest sports facilities were calculated as an indicator of proximity to sports facilities using Geographic Information Systems (GIS). Participation in specific leisure-time sports activities in sports clubs was assessed using a self-report questionnaire and individual-level socio-demographic variables were derived from a parent questionnaire. Community-level socio-demographics as covariates were selected from the INKAR database, in particular from indicators and maps on land development. Logistic regression analyses were conducted to examine associations between proximity to the nearest sports facilities and participation in the corresponding sports activities.
Results - The logisitic regression analyses showed that girls residing longer distances from the nearest gym were less likely to engage in indoor sports activities; a significant interaction between distances to gyms and level of urbanization was identified. Decomposition of the interaction term showed that for adolescent girls living in rural areas participation in indoor sports activities was positively associated with gym proximity. Proximity to tennis courts and indoor pools was not associated with participation in tennis or water sports, respectively.
Conclusions - Improved proximity to gyms is likely to be more important for female adolescents living in rural areas
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
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