2,864 research outputs found
Spin Gap in the Single Spin-1/2 Chain Cuprate SrCaCuO
We report Cu nuclear magnetic resonance and muon spin rotation
measurements on the S=1/2 antiferromagnetic Heisenberg spin chain compound
SrCaCuO. An exponentially decreasing spin-lattice
relaxation rate 1/T indicates the opening of a spin gap. This behavior is
very similar to what has been observed for the cognate zigzag spin chain
compound SrCaCuO, and confirms that the occurrence of a
spin gap upon Ca doping is independent of the interchain exchange coupling
. Our results therefore generally prove the appearance of a spin gap in an
antiferromagnetic Heisenberg spin chain induced by a local bond disorder of the
intrachain exchange coupling . A low temperature upturn of 1/T evidences
growing magnetic correlations. However, zero field muon spin rotation
measurements down to 1.5 K confirm the absence of magnetic order in this
compound which is most likely suppressed by the opening of the spin gap.Comment: 5 pages, 4 figure
Quantum Maxwell-Bloch equations for spatially inhomogeneous semiconductor lasers
We present quantum Maxwell-Bloch equations (QMBE) for spatially inhomogeneous
semiconductor laser devices. The QMBE are derived from fully quantum mechanical
operator dynamics describing the interaction of the light field with the
quantum states of the electrons and the holes near the band gap. By taking into
account field-field correlations and field-dipole correlations, the QMBE
include quantum noise effects which cause spontaneous emission and amplified
spontaneous emission. In particular, the source of spontaneous emission is
obtained by factorizing the dipole-dipole correlations into a product of
electron and hole densities. The QMBE are formulated for general devices, for
edge emitting lasers and for vertical cavity surface emitting lasers, providing
a starting point for the detailed analysis of spatial coherence in the near
field and far field patterns of such laser diodes. Analytical expressions are
given for the spectra of gain and spontaneous emission described by the QMBE.
These results are applied to the case of a broad area laser, for which the
frequency and carrier density dependent spontaneous emission factor beta and
the evolution of the far field pattern near threshold are derived.Comment: 22 pages RevTex and 7 figures, submitted to Phys.Rev.A, revisions in
abstract and in the discussion of temporal coherenc
High Adenylyl Cyclase Activity and \u3cem\u3eIn Vivo\u3c/em\u3e cAMP Fluctuations in Corals Suggest Central Physiological Role
Corals are an ecologically and evolutionarily significant group, providing the framework for coral reef biodiversity while representing one of the most basal of metazoan phyla. However, little is known about fundamental signaling pathways in corals. Here we investigate the dynamics of cAMP, a conserved signaling molecule that can regulate virtually every physiological process. Bioinformatics revealed corals have both transmembrane and soluble adenylyl cyclases (AC). Endogenous cAMP levels in live corals followed a potential diel cycle, as they were higher during the day compared to the middle of the night. Coral homogenates exhibited some of the highest cAMP production rates ever to be recorded in any organism; this activity was inhibited by calcium ions and stimulated by bicarbonate. In contrast, zooxanthellae or mucus had \u3e1000-fold lower AC activity. These results suggest that cAMP is an important regulator of coral physiology, especially in response to light, acid/base disturbances and inorganic carbon levels
Boltzmann equation and hydrodynamic fluctuations
We apply the method of invariant manifolds to derive equations of generalized
hydrodynamics from the linearized Boltzmann equation and determine exact
transport coefficients, obeying Green-Kubo formulas. Numerical calculations are
performed in the special case of Maxwell molecules. We investigate, through the
comparison with experimental data and former approaches, the spectrum of
density fluctuations and address the regime of finite Knudsen numbers and
finite frequencies hydrodynamics.Comment: This is a more detailed version of a related paper: I.V. Karlin, M.
Colangeli, M. Kroger, PRL 100 (2008) 214503, arXiv:0801.2932. It contains
comparison between predictions and experiment, in particular. 11 pages, 6
figures, 2 table
Impact of different leaf velocities and dose rates on the number of monitor units and the dose-volume-histograms using intensity modulated radiotherapy with sliding-window technique
<p>Abstract</p> <p>Background</p> <p>Intensity modulated radiotherapy (IMRT) using sliding window technique utilises a leaf sequencing algorithm, which takes some control system limitations like dose rates (DR) and velocity of the leafs (LV) into account. The effect of altering these limitations on the number of monitor units and radiation dose to the organs at risk (OAR) were analysed.</p> <p>Methods</p> <p>IMRT plans for different LVs from 1.0 cm/sec to 10.0 cm/sec and different DRs from 100 MU/min to 600 MU/min for two patients with prostate cancer and two patients with squamous cell cancer of the scalp (SCCscalp) were calculated using the same "optimal fluence map". For each field the number of monitor units, the dose volume histograms and the differences in the "actual fluence maps" of the fields were analysed.</p> <p>Results</p> <p>With increase of the DR and decrease of the LV the number of monitor units increased and consequentially the radiation dose given to the OAR. In particular the serial OARs of patients with SCCscalp, which are located outside the end position of the leafs and inside the open field, received an additional dose of a higher DR and lower LV is used.</p> <p>Conclusion</p> <p>For best protection of organs at risk, a low DR and high LV should be applied. But the consequence of a low DR is both a long treatment time and also that a LV of higher than 3.0 cm/sec is mechanically not applicable. Our recommendation for an optimisation of the discussed parameters is a leaf velocity of 2.5 cm/sec and a dose rate of 300–400 MU/min (prostate cancer) and 100–200 MU/min (SCCscalp) for best protection of organs at risk, short treatment time and number of monitor units.</p
Magnetic frustration, phase competition and the magneto-electric effect in NdFe3(BO3)4
We present an element selective resonant magnetic x-ray scattering study of
NdFe3(BO3)4 as a function of temperature and applied magnetic field. Our
measurements show that the magnetic order of the Nd sublattice is induced by
the Fe spin order. When a magnetic field is applied parallel to the hexagonal
basal plane, the helicoidal spin order is suppressed and a collinear ordering,
where the moments are forced to align in a direction perpendicular to the
applied magnetic field, is stabilized. This result excludes a non-collinear
spin order as the origin of the magnetically induced electric polarization in
this compound. Instead our data imply that magnetic frustration results in a
phase competition, which is the origin of the magneto-electric response.Comment: 5 pages, 3 figure
Probing liquid surface waves, liquid properties and liquid films with light diffraction
Surface waves on liquids act as a dynamical phase grating for incident light.
In this article, we revisit the classical method of probing such waves
(wavelengths of the order of mm) as well as inherent properties of liquids and
liquid films on liquids, using optical diffraction. A combination of simulation
and experiment is proposed to trace out the surface wave profiles in various
situations (\emph{eg.} for one or more vertical, slightly immersed,
electrically driven exciters). Subsequently, the surface tension and the
spatial damping coefficient (related to viscosity) of a variety of liquids are
measured carefully in order to gauge the efficiency of measuring liquid
properties using this optical probe. The final set of results deal with liquid
films where dispersion relations, surface and interface modes, interfacial
tension and related issues are investigated in some detail, both theoretically
and experimentally. On the whole, our observations and analyses seem to support
the claim that this simple, low--cost apparatus is capable of providing a
wealth of information on liquids and liquid surface waves in a non--destructive
way.Comment: 25 pages, 12 figures, to appear in Measurement Science and Technology
(IOP
The OPERA experiment Target Tracker
The main task of the Target Tracker detector of the long baseline neutrino
oscillation OPERA experiment is to locate in which of the target elementary
constituents, the lead/emulsion bricks, the neutrino interactions have occurred
and also to give calorimetric information about each event. The technology used
consists in walls of two planes of plastic scintillator strips, one per
transverse direction. Wavelength shifting fibres collect the light signal
emitted by the scintillator strips and guide it to both ends where it is read
by multi-anode photomultiplier tubes. All the elements used in the construction
of this detector and its main characteristics are described.Comment: 25 pages, submitted to Nuclear Instrument and Method
Marine harmful algal blooms, human health and wellbeing : challenges and opportunities in the 21st century
Author Posting. © Marine Biological Association of the United Kingdom, 2015. This is the author's version of the work. It is posted here by permission of Marine Biological Association of the United Kingdom for personal use, not for redistribution. The definitive version was published in Journal of the Marine Biological Association of the United Kingdom 96 (2016): 61-91, doi:10.1017/S0025315415001733.Microalgal blooms are a natural part of the seasonal cycle of photosynthetic organisms in marine ecosystems. They are key
components of the structure and dynamics of the oceans and thus sustain the benefits that humans obtain from these aquatic
environments. However, some microalgal blooms can cause harm to humans and other organisms. These harmful algal
blooms (HABs) have direct impacts on human health and negative influences on human wellbeing, mainly through their consequences
to coastal ecosystem services (valued fisheries, tourism and recreation) and other marine organisms and environments.
HABs are natural phenomena, but these events can be favoured by anthropogenic pressures in coastal areas. Global
warming and associated changes in the oceans could affect HAB occurrences and toxicity as well, although forecasting the
possible trends is still speculative and requires intensive multidisciplinary research. At the beginning of the 21st century,
with expanding human populations, particularly in coastal and developing countries, there is an urgent need to prevent
and mitigate HABs’ impacts on human health and wellbeing. The available tools to address this global challenge include
maintaining intensive, multidisciplinary and collaborative scientific research, and strengthening the coordination with stakeholders,
policymakers and the general public. Here we provide an overview of different aspects to understand the relevance
of the HABs phenomena, an important element of the intrinsic links between oceans and human health and wellbeing.The research was funded in part by the UK Medical Research
Council (MRC) and UK Natural Environment Research
Council (NERC) for the MEDMI Project; the National
Institute for Health Research Health Protection Research
Unit (NIHR HPRU) in Environmental Change and Health
at the London School of Hygiene and Tropical Medicine in
partnership with Public Health England (PHE), and in collaboration
with the University of Exeter, University College
London and the Met Office; and the European Regional
Development Fund Programme and European Social Fund
Convergence Programme for Cornwall and the Isles of Scilly
(University of Exeter Medical School).
EB was supported by the CTM2014-53818-R project, from
the Spanish Government (MINECO). KDA was in receipt of
funding from the BBSRC-NERC research programme for
multidisciplinary studies in sustainable aquaculture: health,
disease and the environment. P. Hess was supported by
Ifremer (RISALTOX) and the Regional Council of the Pays
de la Loire (COSELMAR). Porter Hoagland was supported
by the US National Science Foundation under NSF/CNH
grant no. 1009106.2016-05-2
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