51 research outputs found
Antiferromagnetism and singlet formation in underdoped high-Tc cuprates: Implications for superconducting pairing
The extended model is theoretically studied, in the context of hole
underdoped cuprates. Based on results obtained by recent numerical studies, we
identify the mean field state having both the antiferromagnetic and staggered
flux resonating valence bond orders. The random-phase approximation is employed
to analyze all the possible collective modes in this mean field state. In the
static (Bardeen Cooper Schrieffer) limit justified in the weak coupling regime,
we obtain the effective superconducting interaction between the doped holes at
the small pockets located around . In contrast
to the spin-bag theory, which takes into acccount only the antiferromagnetic
order, this effective force is pair breaking for the pairing without the nodes
in each of the small hole pocket, and is canceled out to be very small for the
pairing with nodes which is realized in the real cuprates.
Therefore we conclude that no superconducting instability can occur when only
the magnetic mechanism is considered. The relations of our work with other
approaches are also discussed.Comment: 20 pages, 7 figures, REVTeX; final version accepted for publicatio
Charge Fluctuations and Counterion Condensation
We predict a condensation phenomenon in an overall neutral system, consisting
of a single charged plate and its oppositely charged counterions. Based on the
``two-fluid'' model, in which the counterions are divided into a ``free'' and a
``condensed'' fraction, we argue that for high surface charge, fluctuations can
lead to a phase transition in which a large fraction of counterions is
condensed. Furthermore, we show that depending on the valence, the condensation
is either a first-order or a smooth transition.Comment: 16 pages, 1 figure, accepted to be published in PR
Counterion Condensation and Fluctuation-Induced Attraction
We consider an overall neutral system consisting of two similarly charged
plates and their oppositely charged counterions and analyze the electrostatic
interaction between the two surfaces beyond the mean-field Poisson-Boltzmann
approximation. Our physical picture is based on the fluctuation-driven
counterion condensation model, in which a fraction of the counterions is
allowed to ``condense'' onto the charged plates. In addition, an expression for
the pressure is derived, which includes fluctuation contributions of the whole
system. We find that for sufficiently high surface charges, the distance at
which the attraction, arising from charge fluctuations, starts to dominate can
be large compared to the Gouy-Chapmann length. We also demonstrate that
depending on the valency, the system may exhibit a novel first-order binding
transition at short distances.Comment: 15 pages, 8 figures, to appear in PR
Microscopic Analysis of the Non-Dissipative Force on a Line Vortex in a Superconductor: Berry's Phase, Momentum Flows and the Magnus Force
A microscopic analysis of the non-dissipative force acting on
a line vortex in a type-II superconductor at is given. We first examine
the Berry phase induced in the true superconducting ground state by movement of
the vortex and show how this induces a Wess-Zumino term in the hydrodynamic
action of the superconducting condensate. Appropriate variation of
gives and variation of the Wess-Zumino term is seen to
contribute the Magnus (lift) force of classical hydrodynamics to . This first calculation confirms and strengthens earlier work by Ao and
Thouless which was based on an ansatz for the many-body ground state. We also
determine through a microscopic derivation of the continuity
equation for the condensate linear momentum. This equation yields the
acceleration equation for the superflow and shows that the vortex acts as a
sink for the condensate linear momentum. The rate at which momentum is lost to
the vortex determines and the result obtained agrees with the
Berry phase calculation. The Magnus force contribution to is
seen to be a consequence of the vortex topology. Preliminary remarks are made
regarding finite temperature extensions, with emphasis on its relevance to the
sign anomaly occurring in Hall effect experiments done in the flux flow regime.Comment: 40 pages, RevTex, UBCTP-94-00
An individual-based model of the evolution of pesticide resistance in heterogeneous environments : Control of meligethes aeneus population in oilseed rape crops
Copyright: © 2014 Stratonovitch et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Preventing a pest population from damaging an agricultural crop and, at the same time, preventing the development of pesticide resistance is a major challenge in crop protection. Understanding how farming practices and environmental factors interact with pest characteristics to influence the spread of resistance is a difficult and complex task. It is extremely challenging to investigate such interactions experimentally at realistic spatial and temporal scales. Mathematical modelling and computer simulation have, therefore, been used to analyse resistance evolution and to evaluate potential resistance management tactics. Of the many modelling approaches available, individual-based modelling of a pest population offers most flexibility to include and analyse numerous factors and their interactions. Here, a pollen beetle (Meligethes aeneus) population was modelled as an aggregate of individual insects inhabiting a spatially heterogeneous landscape. The development of the pest and host crop (oilseed rape) was driven by climatic variables. The agricultural land of the landscape was managed by farmers applying a specific rotation and crop protection strategy. The evolution of a single resistance allele to the pyrethroid lambda cyhalothrin was analysed for different combinations of crop management practices and for a recessive, intermediate and dominant resistance allele. While the spread of a recessive resistance allele was severely constrained, intermediate or dominant resistance alleles showed a similar response to the management regime imposed. Calendar treatments applied irrespective of pest density accelerated the development of resistance compared to ones applied in response to prescribed pest density thresholds. A greater proportion of springs own oilseed rape was also found to increase the speed of resistance as it increased the period of insecticide exposure. Our study demonstrates the flexibility and power of an individual-based model to simulate how farming practices affect pest population dynamics, and the consequent impact of different control strategies on the risk and speed of resistance development.Peer reviewe
Influence of a Uniform Current on Collective Magnetization Dynamics in a Ferromagnetic Metal
We discuss the influence of a uniform current, , on the
magnetization dynamics of a ferromagnetic metal. We find that the magnon energy
has a current-induced contribution proportional to
, where is the spin-current, and
predict that collective dynamics will be more strongly damped at finite . We obtain similar results for models with and without local moment
participation in the magnetic order. For transition metal ferromagnets, we
estimate that the uniform magnetic state will be destabilized for . We discuss the relationship of this effect to
the spin-torque effects that alter magnetization dynamics in inhomogeneous
magnetic systems.Comment: 12 pages, 2 figure
Uncertainty of wheat water use: Simulated patterns and sensitivity to temperature and CO₂
Projected global warming and population growth will reduce future water availability for agriculture. Thus, it is essential to increase the efficiency in using water to ensure crop productivity. Quantifying crop water use (WU; i.e. actual evapotranspiration) is a critical step towards this goal. Here, sixteen wheat simulation models were used to quantify sources of model uncertainty and to estimate the relative changes and variability between models for simulated WU, water use efficiency (WUE, WU per unit of grain dry mass produced), transpiration efficiency (Teff, transpiration per kg of unit of grain yield dry mass produced), grain yield, crop transpiration and soil evaporation at increased temperatures and elevated atmospheric carbon dioxide concentrations ([CO2]). The greatest uncertainty in simulating water use, potential evapotranspiration, crop transpiration and soil evaporation was due to differences in how crop transpiration was modelled and accounted for 50% of the total variability among models. The simulation results for the sensitivity to temperature indicated that crop WU will decline with increasing temperature due to reduced growing seasons. The uncertainties in simulated crop WU, and in particularly due to uncertainties in simulating crop transpiration, were greater under conditions of increased temperatures and with high temperatures in combination with elevated atmospheric [CO2] concentrations. Hence the simulation of crop WU, and in particularly crop transpiration under higher temperature, needs to be improved and evaluated with field measurements before models can be used to simulate climate change impacts on future crop water demand
О перспективе извлечения йода из продукта утилизации окислителя ракетного топлива
Crop models are essential tools for assessing the threat of climate change to local and global food production. Present models used to predict wheat grain yield are highly uncertain when simulating how crops respond to temperature. Here we systematically tested 30 different wheat crop models of the Agricultural Model Intercomparison and Improvement Project against field experiments in which growing season mean temperatures ranged from 15 degrees C to 32 degrees C, including experiments with artificial heating. Many models simulated yields well, but were less accurate at higher temperatures. The model ensemble median was consistently more accurate in simulating the crop temperature response than any single model, regardless of the input information used. Extrapolating the model ensemble temperature response indicates that warming is already slowing yield gains at a majority of wheat-growing locations. Global wheat production is estimated to fall by 6% for each degree C of further temperature increase and become more variable over space and time
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