682 research outputs found
A stratified transect approach captures reef complexity with canopy-forming organisms
On the Great Barrier Reef (GBR), persistent changes to reef communities have begun to be documented, and on inshore reefs these shifts may favour the proliferation of macroalgae. Critical to understanding changes to reef community structure in response to anthropogenic impacts is developing effective methods to accurately document the abundance of different reef organisms. Effective monitoring must be time and cost efficient, replicable, and able to sufficiently and accurately detect disturbances to allow development of strategies to mitigate their impacts. Traditional techniques to document coral reef communities (i.e. photo-quadrats, benthic intercept transects) rely on planar views, which tend to either over- or under-represent canopy-forming organisms. As canopy-forming organisms are likely to be affected by anthropogenic influences (corals negatively, algae positively), it is essential for monitoring programs to implement methods sufficient to document changes to the vertical dimension of coral reefs. Here we build on previous work to document the canopy effect in coral-dominated ecosystems and propose a new survey approach suitable for implementation in algal-dominated systems. A vertically stratified transect, modified from a traditional point intercept transect, captures benthic and canopy-forming members of reef communities and provides information on three-dimensional complexity. To test the capability of the new method to detect changes in vertical reef structure, seaweed was removed from experimental quadrats and monitoring techniques were applied before and after four months of regrowth. A stratified method more accurately captured the three-dimensional change resulting from algal canopy growth, while resolving the over- and under-representation of algal biomass in two traditional techniques. We propose that a stratified transect method improves abundance estimates of canopy-forming organisms whilst maintaining data compatibility with traditional methods
Individual risk assessment and information technology to optimise screening frequency for diabetic retinopathy.
To access publisher full text version of this article. Please click on the hyperlink in Additional Links field.AIMS/HYPOTHESIS: The aim of this study was to reduce the frequency of diabetic eye-screening visits, while maintaining safety, by using information technology and individualised risk assessment to determine screening intervals. METHODS: A mathematical algorithm was created based on epidemiological data on risk factors for diabetic retinopathy. Through a website, www.risk.is , the algorithm receives clinical data, including type and duration of diabetes, HbA(1c) or mean blood glucose, blood pressure and the presence and grade of retinopathy. These data are used to calculate risk for sight-threatening retinopathy for each individual's worse eye over time. A risk margin is defined and the algorithm recommends the screening interval for each patient with standardised risk of developing sight-threatening retinopathy (STR) within the screening interval. We set the risk margin so that the same number of patients develop STR within the screening interval with either fixed annual screening or our individualised screening system. The database for diabetic retinopathy at the Department of Ophthalmology, Aarhus University Hospital, Denmark, was used to empirically test the efficacy of the algorithm. Clinical data exist for 5,199 patients for 20 years and this allows testing of the algorithm in a prospective manner. RESULTS: In the Danish diabetes database, the algorithm recommends screening intervals ranging from 6 to 60 months with a mean of 29 months. This is 59% fewer visits than with fixed annual screening. This amounts to 41 annual visits per 100 patients. CONCLUSION: Information technology based on epidemiological data may facilitate individualised determination of screening intervals for diabetic eye disease. Empirical testing suggests that this approach may be less expensive than conventional annual screening, while not compromising safety. The algorithm determines individual risk and the screening interval is individually determined based on each person's risk profile. The algorithm has potential to save on healthcare resources and patients' working hours by reducing the number of screening visits for an ever increasing number of diabetic patients in the world
Weakly frustrated two-dimensional Heisenberg antiferromagnets: thermodynamic properties from a non-perturbative approach
We analyze the thermodynamic properties of the spin-S two-dimensional quantum
Heisenberg antiferromagnet on a square lattice with nearest and next-nearest
neighbor couplings in the Neel phase (J_2/J_1<0.4) employing the quantum
hierarchical reference theory (QHRT), a non-perturbative implementation of the
renormalization group method to quantum systems. We investigate the staggered
susceptibility, the structure factors and the correlation length at finite
temperature and for different values of the frustration ratio. From the finite
temperature results, we also extrapolate ground state properties, such as spin
stiffness and spontaneous staggered magnetization, providing an estimate of the
extent of quantum corrections. The behavior of these quantities as a function
of frustration may provide some hint on the breakdown of the Neel phase at zero
temperature for larger values of J_2
Operator Algebra in Chern-Simons Theory on a Torus
We consider Chern-Simons gauge theory on a torus with both nonrelativistic
and relativistic matter. It is shown that the Hamiltonian and two total momenta
commute among themselves only in the physical Hilbert space. We also discuss
relations among degenerate physical states, degenerate vacua, and the existence
of multicomponent Schrodinger wavefunctions.Comment: 12 pages, TPI-Minn-92/41-T, UMN-TH-1105/9
Topological quantum memory
We analyze surface codes, the topological quantum error-correcting codes
introduced by Kitaev. In these codes, qubits are arranged in a two-dimensional
array on a surface of nontrivial topology, and encoded quantum operations are
associated with nontrivial homology cycles of the surface. We formulate
protocols for error recovery, and study the efficacy of these protocols. An
order-disorder phase transition occurs in this system at a nonzero critical
value of the error rate; if the error rate is below the critical value (the
accuracy threshold), encoded information can be protected arbitrarily well in
the limit of a large code block. This phase transition can be accurately
modeled by a three-dimensional Z_2 lattice gauge theory with quenched disorder.
We estimate the accuracy threshold, assuming that all quantum gates are local,
that qubits can be measured rapidly, and that polynomial-size classical
computations can be executed instantaneously. We also devise a robust recovery
procedure that does not require measurement or fast classical processing;
however for this procedure the quantum gates are local only if the qubits are
arranged in four or more spatial dimensions. We discuss procedures for
encoding, measurement, and performing fault-tolerant universal quantum
computation with surface codes, and argue that these codes provide a promising
framework for quantum computing architectures.Comment: 39 pages, 21 figures, REVTe
Modified Spin Wave Thoery of the Bilayer Square Lattice Frustrated Quantum Heisenberg Antiferromagnet
The ground state of the square lattice bilayer quantum antiferromagnet with
nearest and next-nearest neighbour intralayer interaction is studied by means
of the modified spin wave method. For weak interlayer coupling, the ground
state is found to be always magnetically ordered while the quantum disordered
phase appear for large enough interlayer coupling. The properties of the
disordered phase vary according to the strength of the frustration. In the
regime of weak frustration, the disordered ground state is an almost
uncorrelated assembly of interlayer dimers, while in the strongly frustrated
regime the quantum spin liquid phase which has considerable N\'eel type short
range order appears. The behavior of the sublattice magnetization and spin-spin
correlation length in each phase is discussed.Comment: 15 pages, revtex, figures upon reques
Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene
We have measured a strictly linear pi-plasmon dispersion along the axis of
individualized single wall carbon nanotubes, which is completely different from
plasmon dispersions of graphite or bundled single wall carbon nanotubes.
Comparative ab initio studies on graphene based systems allow us to reproduce
the different dispersions. This suggests that individualized nanotubes provide
viable experimental access to collective electronic excitations of graphene,
and it validates the use of graphene to understand electronic excitations of
carbon nanotubes. In particular, the calculations reveal that local field
effects (LFE) cause a mixing of electronic transitions, including the 'Dirac
cone', resulting in the observed linear dispersion
Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting
Along mid-ocean ridges the extending crust is segmented1 on length scales of 10–1,000 km. Where rift segments are offset from one another, motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis. Where segments overlap, non-transform offsets with a variety of geometries2 accommodate shear motions. Here we use micro-seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north Iceland. Between the rift segments, we identify a series of faults that are aligned sub-parallel to the orientation of the main rift. These faults slip through left-lateral strike-slip motion. Yet, movement between the overlapping rift segments is through right-lateral motion. Together, these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf-faulting mechanism, named after its resemblance to a tilting row of books on a shelf3. The faults probably reactivated existing crustal weaknesses, such as dyke intrusions, that were originally oriented parallel to the main rift and have since rotated about 15° clockwise. Reactivation of pre-existing, rift-parallel weaknesses contrasts with typical mid-ocean ridge transform faults and is an important illustration of a non-transform offset accommodating shear motion between overlapping rift segments
Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions in the "natural" gauge
We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.Comment: 4 pages, Latex, 3 figures included as eps files,minor correction
A Monte Carlo study of O(3) antiferromagnetic models in three dimensions
We study three antiferromagnetic formulations of the O(3) spin model in three
dimensions by means of Monte Carlo simulations: 1. a two parameter
model with nearest and next to nearest neighbors couplings in a cubic lattice;
2. a face centered cubic lattice with nearest neighbors interaction; 3. a cubic
lattice with a set of fully frustrating couplings. We discuss in all cases the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first order.Comment: 24 pages, uuencoded gzipped postscript file. 13 figures include
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