16,727 research outputs found
Effects of mixed rare earth occupancy on the low temperature properties of (R, R',R''...)Ni2Ge2 single crystals
Temperature and applied magnetic field dependent magnetization measurements
on 34 single crystalline samples of (R, R',R''...)Ni2Ge2 compounds (R, R', R'',
etc. being primarily Gd-Lu, Y), were made. These measurements reveal that,
despite extremes in local moment anisotropy, the average de Gennes parameter is
a remarkably good predictor of the paramagnetic to antiferromagnetic ordering
temperature. In addition, the pronounced metamagnetic phase transitions seen in
the low temperature phase of TbNi2Ge2 are found to be remarkably robust to high
substitution levels of Gd and 25% substitutions of other heavy rare earths
Broken Symmetry as a Stabilizing Remnant
The Goldberger-Wise mechanism enables one to stabilize the length of the
warped extra dimension employed in Randall-Sundrum models. In this work we
generalize this mechanism to models with multiple warped throats sharing a
common ultraviolet brane. For independent throats this generalization is
straight forward. If the throats possess a discrete interchange symmetry like
Z_n the stabilizing dynamics may respect the symmetry, resulting in equal
throat lengths, or they may break it. In the latter case the ground state of an
initially symmetric configuration is a stabilized asymmetric configuration in
which the throat lengths differ. We focus on two- (three-) throat setups with a
Z_2 (Z_3) interchange symmetry and present stabilization dynamics suitable for
either breaking or maintaining the symmetry. Though admitting more general
application, our results are relevant for existing models in the literature,
including the two throat model with Kaluza-Klein parity and the three throat
model of flavor based on a broken Z_3 symmetry.Comment: 23 pages; v2 minor cosmetic chang
Financial optimization problems
The major objective of this thesis is to study optimization problems in finance. Most of the effort is directed towards studying the impact of transaction costs in those problems. In addition, we study dynamic meanvariance asset allocation problems. Stochastic HJB equations, Pontryagin Maximum Principle and perturbation analysis are the major mathematical techniques used.
In Chapter 1, we introduce the background literature. Following that, we use the Pontryagin Maximum Principle to tackle the problem of dynamic mean-variance asset allocation and rediscover the doubling strategy. In Chapter 2, we present one of the major results of this thesis. In this chapter, we study a financial optimization problem based on a market model without transaction costs first. Then we study the equivalent problem based on a market model with transaction costs. We find that there is a relationship between these two solutions. Using this relationship, we can obtain the solution of one when we have the solution of another.
In Chapter 3, we generalize the results of chapter 2.
In Chapter 4, we use Pontryagin Maximum Principle to study the problem limit of the no-transaction region when transaction costs tend to 0. We find that the limit is the no-transaction cost solution
Cheating and the evolutionary stability of mutualisms
Interspecific mutualisms have been playing a central role in the functioning of all ecosystems since the early history of life. Yet the theory of coevolution of mutualists is virtually nonexistent, by contrast with well-developed coevolutionary theories of competition, predator–prey and host–parasite interactions. This has prevented resolution of a basic puzzle posed by mutualisms: their persistence in spite of apparent evolutionary instability. The selective advantage of 'cheating', that is, reaping mutualistic benefits while providing fewer commodities to the partner species, is commonly believed to erode a mutualistic interaction, leading to its dissolution or reciprocal extinction. However, recent empirical findings indicate that stable associations of mutualists and cheaters have existed over long evolutionary periods. Here, we show that asymmetrical competition within species for the commodities offered by mutualistic partners provides a simple and testable ecological mechanism that can account for the long-term persistence of mutualisms. Cheating, in effect, establishes a background against which better mutualists can display any competitive superiority. This can lead to the coexistence and divergence of mutualist and cheater phenotypes, as well as to the coexistence of ecologically similar, but unrelated mutualists and cheaters
Exotic fermion multiplets as a solution to baryon asymmetry, dark matter and neutrino masses
We propose an extension to the standard model where three exotic fermion
5-plets and one scalar 6-plet are added to the particle content. By demanding
that all interactions are renormalizable and standard model gauge invariant, we
show that the lightest exotic particle in this model can be a dark matter
candidate as long as the new 6-plet scalar does not develop a nonzero vacuum
expectation value. Furthermore, light neutrino masses are generated radiatively
at one-loop while the baryon asymmetry is produced by the CP-violating decays
of the second lightest exotic particle. We have demonstrated using concrete
examples that there is a parameter space where a consistent solution to the
problems of baryon asymmetry, dark matter and neutrino masses can be obtained.Comment: 17 pages, 2 figures (REVTeX4.1), v2: some refs added, v3: typos
corrected, Sec.VI.B, C modified, this version to appear in PR
Statistical mechanics of ecosystem assembly
We introduce a toy model of ecosystem assembly for which we are able to map
out all assembly pathways generated by external invasions. The model allows to
display the whole phase space in the form of an assembly graph whose nodes are
communities of species and whose directed links are transitions between them
induced by invasions. We characterize the process as a finite Markov chain and
prove that it exhibits a unique set of recurrent states (the endstate of the
process), which is therefore resistant to invasions. This also shows that the
endstate is independent on the assembly history. The model shares all features
with standard assembly models reported in the literature, with the advantage
that all observables can be computed in an exact manner.Comment: Accepted for publication in Physical Review Letter
National scale modelling to test UK population growth and infrastructure scenarios
This paper describes an exploratory methodology used to study the national scale issues of
population growth and infrastructure implementation across the UK. The project was carried
out for the Government Office for Science in 2015, focussing on two key questions: how could
a “spatially driven” scenario provoke new thinking on accommodating forecast growth, and;
what would be the impact of transport infrastructure investments within this context.
Addressing these questions required the construction of a national scale spatial model that
also needed to integrate datasets on population and employment. Models were analysed
and profiled initially to identify existing relationships between the distribution of population
and employment against the spatial network. Based on these profiles, an experimental
methodology was used to firstly identify cities with the potential to accommodate growth,
then secondly to allocate additional population proportionally. This raises important questions
for discussion around which cities provide the benchmark for growth and why, as well as what
the optimal spatial conditions for population growth may be, and how this growth should be
accommodated locally.
Later the model was used to study the impact of High Speed Rail. As these proposed
infrastructure changes improve service (capacity, frequency, journey time), rather than
creating new topological connections, the model was adapted to be able to produce time based
catchments as an output. These catchments could then be expressed in terms of the workforce
population within an hour of every city (a potential travel to work area), as well as the number
of employment opportunities within an hour of every household
Topological Excitations in Spinor Bose-Einstein Condensates
We investigate the properties of skyrmion in the ferromagnetic state of
spin-1 Bose-Einstein condensates by means of the mean-field theory and show
that the size of skyrmion is fixed to the order of the healing length. It is
shown that the interaction between two skyrmions with oppositely rotating spin
textures is attractive when their separation is large, following a unique
power-law behavior with a power of -7/2.Comment: 4 pages, 5 figure
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