1,727 research outputs found
Thermodynamics of an incommensurate quantum crystal
We present a simple theory of the thermodynamics of an incommensurate quantum
solid. The ground state of the solid is assumed to be an incommensurate
crystal, with quantum zero-point vacancies and interstitials and thus a
non-integer number of atoms per unit cell. We show that the low temperature
variation of the net vacancy concentration should be as , and that the
first correction to the specific heat due to this varies as ; these are
quite consistent with experiments on solid He. We also make some
observations about the recent experimental reports of ``supersolidity'' in
solid He that motivate a renewed interest in quantum crystals.Comment: revised, new title, somewhat expande
Marine Protected Areas and commercial fisheries: the existing fishery in potential protected areas, and a modelling study of the impact of protected areas on North Sea Plaice
Dit rapport presenteert resultaten van onderzoek, dat in 2005/2006 is uitgevoerd. In het kader van de Europese Vogel en Habitat Richtlijnen dienen lidstaten te beschermen gebieden op zee aan te wijzen, wat mogelijk zou leiden tot beperkingen van visserijactiviteiten in deze gebieden. De vraag was, welke invloed dit zou hebben op de vis en visserij. In dit onderzoek is enerzijds een statische beschrijving opgesteld van de visserijinspanning en de vangsten in de voorgestelde gebieden, en is anderzijds een eerste analyse (simulatie-model) opgezet van het lange-termijn effect op migrerende vis (schol). Dit onderzoek werd eind 2006 afgerond met een concept-rapport
Generalization of Gutzwiller Approximation
We derive expressions required in generalizing the Gutzwiller approximation
to models comprising arbitrarily degenerate localized orbitals.Comment: 6 pages, 1 figure, to appear in J.Phys.Soc.Jpn. vol.6
Hubbard physics in the symmetric half-filled periodic Anderson-Hubbard model
Two very different methods -- exact diagonalization on finite chains and a
variational method -- are used to study the possibility of a metal-insulator
transition in the symmetric half-filled periodic Anderson-Hubbard model. With
this aim we calculate the density of doubly occupied sites as a function of
various parameters. In the absence of on-site Coulomb interaction ()
between electrons, the two methods yield similar results. The double
occupancy of levels remains always finite just as in the one-dimensional
Hubbard model. Exact diagonalization on finite chains gives the same result for
finite , while the Gutzwiller method leads to a Brinkman-Rice transition
at a critical value (), which depends on and .Comment: 10 pages, 5 figure
Analytical calculation of the Green's function and Drude weight for a correlated fermion-boson system
In classical Drude theory the conductivity is determined by the mass of the
propagating particles and the mean free path between two scattering events. For
a quantum particle this simple picture of diffusive transport loses relevance
if strong correlations dominate the particle motion. We study a situation where
the propagation of a fermionic particle is possible only through creation and
annihilation of local bosonic excitations. This correlated quantum transport
process is outside the Drude picture, since one cannot distinguish between free
propagation and intermittent scattering. The characterization of transport is
possible using the Drude weight obtained from the f-sum rule, although its
interpretation in terms of free mass and mean free path breaks down. For the
situation studied we calculate the Green's function and Drude weight using a
Green's functions expansion technique, and discuss their physical meaning.Comment: final version, minor correction
The Impact of Phorate on the Genetic Diversity of Wetland Aquatic Invertebraes
Impacts of the insecticide phorate on the genetic diversity of wetland invertebrates were investigated using field and laboratory studies in 1991. Electrophoretic methods were evaluated for revealing the impact of insecticides. Objectives were to determine the ability of electrophoresis to reveal the impact of phorate on invertebrates and to determine the influence of phorate on the genetic diversity in two common invertebrates. Amphipods, Hyallela azteca and mayflies, Callibaetis ferrugineus (Walsh) were placed in constructed mesocosms in wetlands and were exposed to varying amounts of phorate. Survivors and individuals from the parent population were genetically tested using cellulose acetate electrophoresis techniques. Allele frequencies were calculated for invertebrates in treatments and invertebrates from populations not exposed to phorate. Mortality of test invertebrates was significantly greater in phorate treatments than in controls (F = 5.97, P = 0.019). Chi-square analysis revealed differences in allele frequencies between the untreated populations and individuals of both species treated with phorate cx2 \u3e 8.5; df = 1,2; p \u3c 0.05). In addition, phorate appeared to eliminate, or reduce the frequency of certain genotypes in both species. Results indicate phorate selected against sensitive individuals and electrophoresis was effective at detecting differences between untreated populations and invertebrates that survived treatments. Genetic techniques should enable wetland scientists to detect the effects of pollution on invertebrate populations by monitoring genetic composition
Protestantisme en progressiviteit opnieuw bezien
The data on which this analysis is based were assembled by Dr. A. Hoogerwerf; I a random sample of 912 persons in Delft was used. The authors investigated the influence of several variables on the independent i variable of social-political-progressiveness (spp). A matrix of correlations between fifteen items of the original questionnaire was computed; eight items which were highly related to each other were found. These items were used as indicators for social-political-progressiveness by summing the answers of each respondent on every item. On these scores an analysis of variance was carried out with five independent variables; religion, age, income, level of education, and sex. The following findings were obtained; — Religion had a significant influence on spp-scores; respondents who do not have any specific religion tend to be more progressive than others. Furthermore, Catholics tend to be more progressive than Protestants and Dutch-Reformed. — It was also shown that both the level of education and income had significant influences on spp-scores; the higher the income or the educational level, the lower the progressiveness-score. — The variables age and sex did not have significant influences on spp-scores. — The five variables included in the design explained 32% of the total variance in the spp-scores. Of the remaining 68% about 27% can be explained in terms 2 of error-variance. This means that about 41% of the total variance in spp-score.I has to be explained by factors which are not included in the design
Phase diagrams of correlated electrons: systematic corrections to the mean field theory
Perturbative corrections to the mean field theory for particle-hole
instabilities of interacting electron systems are computed within a scheme
which is equivalent to the recently developed variational approach to the
Kohn-Luttinger superconductivity. This enables an unbiased comparison of
particle-particle and particle-hole instabilities within the same approximation
scheme. A spin-rotation invariant formulation for the particle-hole
instabilities in the triplet channel is developed. The method is applied to the
phase diagram of the t-t' Hubbard model on the square lattice. At the Van Hove
density, antiferromagnetic and d-wave Pomeranchuk phases are found to be stable
close to half filling. However, the latter phase is confined to an extremely
narrow interval of densities and away from the singular filling, d-wave
superconducting instability dominates
Active Microrheology of Networks Composed of Semiflexible Polymers. II. Theory and comparison with simulations
Building on the results of our computer simulation (ArXiv cond-mat/0503573)we
develop a theoretical description of the motion of a bead, embedded in a
network of semiflexible polymers, and responding to an applied force. The
theory reveals the existence of an osmotic restoring force, generated by the
piling up of filaments in front of the moving bead and first deduced through
computer simulations. The theory predicts that the bead displacement scales
like x ~ t^alfa with time, with alfa=0.5 in an intermediate- and alfa=1 in a
long-time regime. It also predicts that the compliance varies with
concentration like c^(-4/3) in agreement with experiment.Comment: 18 pages and 2 figure
Spin-independent origin of the strongly enhanced effective mass in a dilute 2D electron system
We have accurately measured the effective mass in a dilute two-dimensional
electron system in silicon by analyzing temperature dependence of the
Shubnikov-de Haas oscillations in the low-temperature limit. A sharp increase
of the effective mass with decreasing electron density has been observed. Using
tilted magnetic fields, we have found that the enhanced effective mass is
independent of the degree of spin polarization, which points to a
spin-independent origin of the mass enhancement and is in contradiction with
existing theories
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