5,497 research outputs found
The Vortex Kinetics of Conserved and Non-conserved O(n) Models
We study the motion of vortices in the conserved and non-conserved
phase-ordering models. We give an analytical method for computing the speed and
position distribution functions for pairs of annihilating point vortices based
on heuristic scaling arguments. In the non-conserved case this method produces
a speed distribution function consistent with previous analytic results. As two
special examples, we simulate the conserved and non-conserved O(2) model in two
dimensional space numerically. The numerical results for the non-conserved case
are consistent with the theoretical predictions. The speed distribution of the
vortices in the conserved case is measured for the first time. Our theory
produces a distribution function with the correct large speed tail but does not
accurately describe the numerical data at small speeds. The position
distribution functions for both models are measured for the first time and we
find good agreement with our analytic results. We are also able to extend this
method to models with a scalar order parameter.Comment: 21 pages, 9 figure
Hamiltonian of a many-electron system with single-electron and electron-pair states in a two-dimensional periodic potential
Based on the metastable electron-pair energy band in a two-dimensional (2D)
periodic potential obtained previously by Hai and Castelano [J. Phys.: Condens.
Matter 26, 115502 (2014)], we present in this work a Hamiltonian of many
electrons consisting of single electrons and electron pairs in the 2D system.
The electron-pair states are metastable of energies higher than those of the
single-electron states at low electron density. We assume two different
scenarios for the single-electron band. When it is considered as the lowest
conduction band of a crystal, we compare the obtained Hamiltonian with the
phenomenological model Hamiltonian of a boson-fermion mixture proposed by
Friedberg and Lee [Phys. Rev. B 40, 6745 (1989)]. Single-electron-electron-pair
and electron-pair-electron-pair interaction terms appear in our Hamiltonian and
the interaction potentials can be determined from the electron-electron Coulomb
interactions. When we consider the single-electron band as the highest valence
band of a crystal, we show that holes in this valence band are important for
stabilization of the electron-pair states in the system
Parametric mode-mode coupling between drift waves in plasmas
Parametric mode-mode coupling between drift waves in plasma
How to interpret the growing phenomenon of private tutoring : human capital deepening, inequality increasing, or waste of resources ?
Private tutoring is now a major component of the education sector in many developing countries, yet education policy too seldom acknowledges and makes use of it. Various criticisms have been raised against private tutoring, most notably that it exacerbates social inequalities and may even fail to improve student outcomes. This paper surveys the literature for evidence on private tutoring-the extent of the tutoring phenomenon, the factors that explain its growth, and its cost-effectiveness in improving student academic performance. It also presents a framework for assessing the efficiency and equity effects of tutoring. It concludes that tutoring can raise the effectiveness of the education system under certain reasonable assumptions, even taking into account equity concerns, and it offers guidance for attacking corruption and other problems that diminish the contributions of the tutoring sector.Teaching and Learning,Tertiary Education,Education For All,Primary Education,
D^0-D^0bar mixing in \Upsilon(1S) \to D^0 D^0bar decay at Super-B
\Dz-\Dzb mixing and significant CP violation in the charm system may
indicate the signature of new physics. In this study, we suggest that the
coherent \DzDzb events from the decay of \Upsilon(1S) \to \Dz \Dzb can be
used to measure both mixing parameters and CP violation in charm decays. The
neutral mesons from decay are strongly boosted, so that it
will offer the possibility to measure the proper-time interval, ,
between the fully-reconstructed \Dz and \Dzb. Both coherent and
time-dependent information can be used to extract \Dz-\Dzb mixing parameters.
The sensitivity of the measurement should be improved at B factories or
super-B.Comment: 6 pages, 1 figure, this is the last version to appear in Phys. Rev.
Investigating the significance of coagulation kinetics on maintaining membrane permeability in an MBR following reactive coagulant dosing
In this study, the impact of kinetically controlled floc growth on sustaining membrane permeability following reactive coagulant dosing was determined using a model particle system. Floc formation was indicated to comprise of two stages following coagulant addition: (i) an initial destabilisation phase which encouraged complexation of protein and polysaccharide; and (ii) entrapment of the coarse model particles (3 µm Firefli™ microspheres) in the polymeric complex during the floc growth phase. Floc growth was characterised by an expected time lag as with conventional flocculation systems and biopolymer aggregation was kinetically favoured. When coagulant was dosed during the filtration cycle, the intermediate biopolymer aggregates (comprised of protein and polysaccharide) were preferentially transported toward the membrane increasing fouling. However, when coagulant was dosed at the onset of filtration, membrane fouling was constrained. It is asserted that by dosing at the onset of filtration: (i) early development of biopolymer aggregation is initiated which inhibits transport of the individual biopolymers to the membrane; and (ii) by dosing coagulant in the absence of a developed polarised layer, formation of biopolymer complexes local to the membrane is obviated. However, when dosing coagulant at the onset of filtration, only limited floc growth occurred which can be explained by the low applied wall shear rate and the absence of a ‘polarised’ region which ostensibly promoted floc growth when coagulant was dosed mid-filtration. Based on results from the model particle system studied, it is proposed that reactive coagulant dosing is best undertaken when: (i) filtration is stopped; (ii) modest shear is applied within the bioreactor to promote coagulant dispersion; and (iii) sufficient contact time is allowed to promote floc growth before commencement of filtration
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