1,242 research outputs found
Roton Interactions In Superfluid Helium
Based on a hard-sphere model for superfluid helium we derive a microscopic theory for quasiparticle interactions in this liquid. A satisfactory upper bound for the roton-roton collision frequency is obtained from this model. This theory also leads to an attractive coupling of two rotons with opposite momenta but the coupling strength is an order of magnitude larger than the phenomenological estimates based on experiments. However, as our analysis reveals, this large coupling strength is compatible with other physical considerations presented in this paper and does not contradict experimental data. © 1976 The American Physical Society.1451932194
Critical Analysis Of The Hard-sphere Model For Superfluid He4. I
The basic goal of this paper is to present the theory which stands at the basis of the calculations of the He4 excitation spectrum reported by Wong and Huang. The exact N-particle hard-sphere pseudopotential is constructed through the method of non-local-field operators developed earlier. Keeping only the two-body interaction terms, this pseudopotential replaces the exact two-particle boundary conditions which is demonstrated here and thus has been proposed as a model potential for actual helium-helium interaction in the liquid state. A pair state of the Bogoliubov type is used to calculate the ground-state energy of such a system. An expansion-parameter scheme is presented which drastically simplifies the mathematics involved and justifies previous approximations employed by one of the authors. The expansion describes the dilute case by assuming a large portion of the particles in the zero-momentum state and the dense case by assuming only a negligible portion of the particles in the Bose-Einstein state. © 1978 The American Physical Society.1731102112
Sharing the effort of the European Green Deal among countries
In implementing the European Green Deal to align with the Paris Agreement, the EU has raised its climate ambition and in 2022 is negotiating the distribution of increased mitigation effort among Member States. Such partitioning of targets among subsidiary entities is becoming a major challenge for implementation of climate policies around the globe. We contrast the 2021 European Commission proposal - an allocation based on a singular country attribute - with transparent and reproducible methods based on three ethical principles. We go beyond traditional effort-sharing literature and explore allocations representing an aggregated least regret compromise between different EU country perspectives on a fair allocation. While the 2021 proposal represents a nuanced compromise for many countries, for others a further redistribution could be considered equitable. Whereas we apply our approach within the setting of the EU negotiations, the framework can easily be adapted to inform debates worldwide on sharing mitigation effort among subsidiary entities
Effort Sharing among EU Member States: Green Deal Emission Reduction Targets for 2030 (Short Version)
After having agreed in the European Council of December 2019 on transforming the European Union to climate neutrality by 2050, the next step requires adjusting the 2030 targets on climate and energy accordingly.
This Research Brief focuses on the effort sharing (ES) sector, which covers all greenhouse gas (GHG) emissions that are neither included in the EU Emissions Trading Scheme (EU ETS) nor covered under âland use and land use changeâ1. Starting from the Effort Sharing Decision of 2018, this Brief frames the issue in the broader context of global emission budgets that are compatible with the Paris Agreement climate targets and considers criteria for allocating the respective EU budget among the EU-27.
The Research Brief (full version) is available upon request: [email protected]
High Temperature Electron Localization in dense He Gas
We report new accurate mesasurements of the mobility of excess electrons in
high density Helium gas in extended ranges of temperature and density to ascertain
the effect of temperature on the formation and dynamics of localized electron
states. The main result of the experiment is that the formation of localized
states essentially depends on the relative balance of fluid dilation energy,
repulsive electron-atom interaction energy, and thermal energy. As a
consequence, the onset of localization depends on the medium disorder through
gas temperature and density. It appears that the transition from delocalized to
localized states shifts to larger densities as the temperature is increased.
This behavior can be understood in terms of a simple model of electron
self-trapping in a spherically symmetric square well.Comment: 23 pages, 13 figure
Hopping Transport in the Presence of Site Energy Disorder: Temperature and Concentration Scaling of Conductivity Spectra
Recent measurements on ion conducting glasses have revealed that conductivity
spectra for various temperatures and ionic concentrations can be superimposed
onto a common master curve by an appropriate rescaling of the conductivity and
frequency. In order to understand the origin of the observed scaling behavior,
we investigate by Monte Carlo simulations the diffusion of particles in a
lattice with site energy disorder for a wide range of both temperatures and
concentrations. While the model can account for the changes in ionic activation
energies upon changing the concentration, it in general yields conductivity
spectra that exhibit no scaling behavior. However, for typical concentrations
and sufficiently low temperatures, a fairly good data collapse is obtained
analogous to that found in experiment.Comment: 6 pages, 4 figure
Effects of rapid prey evolution on predator-prey cycles
We study the qualitative properties of population cycles in a predator-prey
system where genetic variability allows contemporary rapid evolution of the
prey. Previous numerical studies have found that prey evolution in response to
changing predation risk can have major quantitative and qualitative effects on
predator-prey cycles, including: (i) large increases in cycle period, (ii)
changes in phase relations (so that predator and prey are cycling exactly out
of phase, rather than the classical quarter-period phase lag), and (iii)
"cryptic" cycles in which total prey density remains nearly constant while
predator density and prey traits cycle. Here we focus on a chemostat model
motivated by our experimental system [Fussmann et al. 2000,Yoshida et al. 2003]
with algae (prey) and rotifers (predators), in which the prey exhibit rapid
evolution in their level of defense against predation. We show that the effects
of rapid prey evolution are robust and general, and furthermore that they occur
in a specific but biologically relevant region of parameter space: when traits
that greatly reduce predation risk are relatively cheap (in terms of reductions
in other fitness components), when there is coexistence between the two prey
types and the predator, and when the interaction between predators and
undefended prey alone would produce cycles. Because defense has been shown to
be inexpensive, even cost-free, in a number of systems [Andersson and Levin
1999, Gagneux et al. 2006,Yoshida et al. 2004], our discoveries may well be
reproduced in other model systems, and in nature. Finally, some of our key
results are extended to a general model in which functional forms for the
predation rate and prey birth rate are not specified.Comment: 35 pages, 8 figure
An assessment of latest Cretaceous <i>Pycnodonte vesicularis</i> (Lamarck, 1806) shells as records for palaeoseasonality: a multi-proxy investigation
In order to assess the potential of the honeycomb oyster Pycnodonte vesicularis for the reconstruction of palaeoseasonality, several specimens recovered from late Maastrichtian strata in the NeuquĂ©n Basin (Argentina) were subject to a multi-proxy investigation, involving scanning techniques and trace element and isotopic analysis. Combined CT scanning and light microscopy reveals two calcite microstructures in P. vesicularis shells (vesicular and foliated calcite). Micro-XRF analysis and cathodoluminescence microscopy show that reducing pore fluids were able to migrate through the vesicular portions of the shells (aided by bore holes) and cause recrystallization of the vesicular calcite. This renders the vesicular portions not suitable for palaeoenvironmental reconstruction. In contrast, stable isotope and trace element compositions show that the original chemical composition of the foliated calcite is well-preserved and can be used for the reconstruction of palaeoenvironmental conditions. Stable oxygen and clumped isotope thermometry on carbonate from the dense hinge of the shell yield sea water temperatures of 11°C, while previous TEX86H palaeothermometry yielded much higher temperatures. The difference is ascribed to seasonal bias in the growth of P. vesicularis, causing warm seasons to be underrepresented from the record, while TEX86H palaeothermometry seems to be biased towards warmer surface water temperatures. The multi-proxy approach employed here enables us to differentiate between well-preserved and diagenetically altered portions of the shells and provides an improved methodology for reconstructing palaeoenvironmental conditions in deep time. While establishing a chronology for these shells was complicated by growth cessations and diagenesis, cyclicity in trace elements and stable isotopes allowed for a tentative interpretation of the seasonal cycle in late Maastrichtian palaeoenvironment of the NeuquĂ©n Basin. Attempts to independently verify the seasonality in sea water temperature by MgâCa ratios of shell calcite are hampered by significant uncertainty due to the lack of proper transfer functions for pycnodontein oysters. Future studies of fossil ostreid bivalves should target dense, foliated calcite rather than sampling bulk or vesicular calcite. Successful application of clumped isotope thermometry on fossil bivalve calcite in this study indicates that temperature seasonality in fossil ostreid bivalves may be constrained by the sequential analysis of well-preserved foliated calcite samples using this method
Particles-vortex interactions and flow visualization in He4
Recent experiments have demonstrated a remarkable progress in implementing
and use of the Particle Image Velocimetry (PIV) and particle tracking
techniques for the study of turbulence in He4. However, an interpretation of
the experimental data in the superfluid phase requires understanding how the
motion of tracer particles is affected by the two components, the viscous
normal fluid and the inviscid superfluid. Of a particular importance is the
problem of particle interactions with quantized vortex lines which may not only
strongly affect the particle motion, but, under certain conditions, may even
trap particles on quantized vortex cores. The article reviews recent
theoretical, numerical, and experimental results in this rapidly developing
area of research, putting critically together recent results, and solving
apparent inconsistencies. Also discussed is a closely related technique of
detection of quantized vortices negative ion bubbles in He4.Comment: To appear in the J Low Temperature Physic
Quel rÎle peut-on imputer aux banques à charte canadiennes dans la transmission des chocs monétaires des années quatre-vingt?
Cette recherche sâinscrit dans la foulĂ©e de nombreux travaux entrepris suite aux publications de Bernanke et Blinder (1988, 1992) ayant remis Ă lâavant-plan le rĂŽle jouĂ© par le systĂšme bancaire dans la transmission de la politique monĂ©taire. Nous proposons dâexaminer la dynamique inhĂ©rente Ă certains postes du bilan des banques Ă charte canadiennes suite aux mouvement des principaux taux dâintĂ©rĂȘt, habituellement jugĂ©s rĂ©vĂ©lateurs des conditions monĂ©taires du moment. Pour ce faire, nous avons recours Ă un modĂšle VAR hebdomadaire comportant Ă la fois, des Ă©lĂ©ments de lâactif et du passif des banques ainsi que les taux de rendement associĂ©s Ă divers instruments financiers. Cependant, dans le but de bien encadrer cette analyse, nous dĂ©veloppons un modĂšle formel du comportement dâune banque oĂč les seuls changements aux postes de son bilan suite aux mouvements de taux dâintĂ©rĂȘt sont dictĂ©s par des ajustements de portefeuille visant Ă tirer avantage des Ă©carts se creusant entre ceux-ci. Ce modĂšle thĂ©orique est soumis aux variations de taux dâintĂ©rĂȘt issues du modĂšle empirique VAR. Les mouvements observĂ©s aux postes du bilan de cette banque « tĂ©moin » fournissent un guide utile permettant dâinterprĂ©ter de façon Ă©clairĂ©e les rĂ©sultats empiriques obtenus. Ă cet Ă©gard, lâexercice proposĂ© montre quâil est possible dâĂ©tablir un parallĂšle assez Ă©troit entre lâĂ©volution des postes du bilan de la banque hypothĂ©tique et celle captĂ©e par le modĂšle VAR et ainsi apporte un certain support Ă lâapproche traditionnelle sur le rĂŽle jouĂ© par les banques dans la transmission des chocs monĂ©taires.This paper can be seen as a contribution to a growing literature initiated by Bernanke and Blinder (1988, 1992) which have examined the role played by the banking system in the transmission of monetary policy. We propose to study the dynamic behaviour of the balance sheet of Canadian chartered banks following a shock to some key interest rates which are good indicators of the prevailing monetary conditions. More specifically, we estimate a weekly VAR model which comprises key asset and liabilities elements as well as rates of return on major financial instruments. However, to guide this empirical inquiry, we set up a model of a representative bank which adjusts its balance sheet elements according to the interest rate spreads arising in the financial markets. This theoretical model is then subjected to the same interest rate shocks than those imposed on the VAR model: the adjustments observed in this laboratory will prove quite useful to assess the significance of the empirical results uncovered by the VAR model. Overall, we find that both approaches give rise to quite similar dynamic responses which tends to support the traditional role of the banking sector in the transmission of monetary policy
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