71,413 research outputs found
Coral Reefs, Fisheries, and Food Security: Integrated Approaches to Addressing Multiple Challenges in the Coral Triangle
The Coral Triangle is the most biologically and economically valuable marine ecosystem on the planet. Covering just three percent of the globe, the region represents more than half of the world's reefs and boasts 76 percent of its known coral species. Sustaining more than 130 million people who rely directly on the marine ecosystems for their livelihoods and food, the marine habitats of the Coral Triangle contribute billions of dollars each year toward the economies of the region.Although the environmental imperative for preserving this area of incredible value and biodiversity is obvious, the growing pressures and threats from widespread poverty, rapid development, and global demands continue to place enormous strain on the natural marine resources of the Coral Triangle
Spin Gaps in a Frustrated Heisenberg model for CaVO
I report results of a density matrix renormalization group (DMRG) study of a
model for the two dimensional spin-gapped system CaVO. This study
represents the first time that DMRG has been used to study a two dimensional
system on large lattices, in this case as large as , allowing
extrapolation to the thermodynamic limit. I present a substantial improvement
to the DMRG algorithms which makes these calculations feasible.Comment: 10 pages, with 4 Postscript figure
On Deriving Space-Time From Quantum Observables and States
We prove that, under suitable assumptions, operationally motivated data
completely determine a space-time in which the quantum systems can be
interpreted as evolving. At the same time, the dynamics of the quantum system
is also determined. To minimize technical complications, this is done in the
example of three-dimensional Minkowski space.Comment: 19 pages, to appear in Communications in Mathematical Physics; minor
corrections mad
Workshop on web information seeking and interaction
The World Wide Web has provided access to a diverse range of information sources and systems. People engaging with this rich network of information may need to interact with different technologies, interfaces, and information providers in the course of a single search task. These systems may offer different interaction affordances and require users to adapt their informationseeking strategies. Not only is this challenging for users, but it also presents challenges for the designers of interactive systems, who need to make their own system useful and usable to broad user groups. The popularity of Web browsing and Web search engines has given rise to distinct forms of information-seeking behaviour, and new interaction styles, but we do not yet fully understand these or their implications for the development of new systems
Energetics of Domain Walls in the 2D t-J model
Using the density matrix renormalization group, we calculate the energy of a
domain wall in the 2D t-J model as a function of the linear hole density
\rho_\ell, as well as the interaction energy between walls, for J/t=0.35. Based
on these results, we conclude that the ground state always has domain walls for
dopings 0 < x < 0.3. For x < 0.125, the system has (1,0) domain walls with
\rho_\ell ~ 0.5, while for 0.125 < x < 0.17, the system has a possibly
phase-separated mixture of walls with \rho_\ell ~ 0.5 and \rho_\ell =1. For x >
0.17, there are only walls with \rho_\ell =1. For \rho_\ell = 1, diagonal (1,1)
domain walls have very nearly the same energy as (1,0) domain walls.Comment: Several minor changes. Four pages, four encapsulated figure
Comment on "Kagome Lattice Antiferromagnet Stripped to Its Basics"
Density matrix renormalization group (DMRG) calculations on large systems (up
to 3096 spins) indicate that the ground state of the Heisenberg model on a
3-chain Kagome strip is spontaneously dimerized. This system has degenerate
ground states and a gap to triplet and singlet excitations. These results are
in direct contradiction with recent results of Azaria et al (Phys. Rev. Lett.
81, 1694 (1998)) and suggest a need for a reexamination of the underlying field
theory.Comment: 1 page, submitted to PR
Phase diagram of the half-filled Hubbard chain with next-nearest-neighbor hopping
We investigate the ground-state phase diagram of the half-filled
one-dimensional Hubbard model with next-nearest-neighbor hopping using the
Density-Matrix Renormalization Group technique as well as an unrestricted
Hartree-Fock approximation. We find commensurate and incommensurate disordered
magnetic insulating phases and a spin-gapped metallic phase in addition to the
one-dimensional Heisenberg phase. At large on-site Coulomb repulsion , we
make contact with the phase diagram of the frustrated Heisenberg chain, which
has spin-gapped phases for sufficiently large frustration. For weak ,
sufficiently large next-nearest-neighbor hopping leads to a band
structure with four Fermi points rather than two, producing a spin-gapped
metallic phase. As is increased in this regime, the system undergoes a
Mott-Hubbard transition to a frustrated antiferromagnetic insulator
A 212-nt long RNA structure in the Tobacco necrosis virus-D RNA genome is resistant to Xrn degradation
Plus-strand RNA viruses can accumulate viral RNA degradation products during infections. Some of these decay intermediates are generated by the cytosolic 5′-to-3′ exoribonuclease Xrn1 (mammals and yeast) or Xrn4 (plants) and are formed when the enzyme stalls on substrate RNAs upon encountering inhibitory RNA structures. Many Xrn-generated RNAs correspond to 3′-terminal segments within the 3′-UTR of viral genomes and perform important functions during infections. Here we have investigated a 3′-terminal small viral RNA (svRNA) generated by Xrn during infections with Tobacco necrosis virus-D (family Tombusviridae). Our results indicate that (i) unlike known stalling RNA structures that are compact and modular, the TNV-D structure encompasses the entire 212 nt of the svRNA and is not functionally transposable, (ii) at least two tertiary interactions within the RNA structure are required for effective Xrn blocking and (iii) most of the svRNA generated in infections is derived from viral polymerase-generated subgenomic mRNA1. In vitro and in vivo analyses allowed for inferences on roles for the svRNA. Our findings provide a new and distinct addition to the growing list of Xrn-resistant viral RNAs and stalling structures found associated with different plant and animal RNA viruses.York University Librarie
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
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