4,246 research outputs found
The one-round Voronoi game replayed
We consider the one-round Voronoi game, where player one (``White'', called
``Wilma'') places a set of n points in a rectangular area of aspect ratio r
<=1, followed by the second player (``Black'', called ``Barney''), who places
the same number of points. Each player wins the fraction of the board closest
to one of his points, and the goal is to win more than half of the total area.
This problem has been studied by Cheong et al., who showed that for large
enough and r=1, Barney has a strategy that guarantees a fraction of 1/2+a,
for some small fixed a.
We resolve a number of open problems raised by that paper. In particular, we
give a precise characterization of the outcome of the game for optimal play: We
show that Barney has a winning strategy for n>2 and r>sqrt{2}/n, and for n=2
and r>sqrt{3}/2. Wilma wins in all remaining cases, i.e., for n>=3 and
r<=sqrt{2}/n, for n=2 and r<=sqrt{3}/2, and for n=1. We also discuss complexity
aspects of the game on more general boards, by proving that for a polygon with
holes, it is NP-hard to maximize the area Barney can win against a given set of
points by Wilma.Comment: 14 pages, 6 figures, Latex; revised for journal version, to appear in
Computational Geometry: Theory and Applications. Extended abstract version
appeared in Workshop on Algorithms and Data Structures, Springer Lecture
Notes in Computer Science, vol.2748, 2003, pp. 150-16
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
Mott transition and suppression of orbital fluctuations in orthorhombic 3 perovskites
Using Wannier-functions, a low-energy Hamiltonian is derived for
orthorhombic transition-metal oxides. Electronic correlations are
treated with a new implementation of dynamical mean-field theory for non-cubic
systems. Good agreement with photoemission data is obtained. The interplay of
correlation effects and cation covalency (GdFeO-type distortions) is
found to suppress orbital fluctuations in LaTiO and even more in
YTiO, and to favor the transition to the insulating state.Comment: 4 pages, 3 figures; revised manuscrip
Trends and guidelines in online privacy policy
Online privacy policies (OPP) are important mechanisms for informing online consumers about the level of information privacy protection afforded when visiting web sites. To date, societal mechanisms and technologies have been the focus of attempts to improve the quality and effectiveness of OPPs. We present findings from a longitudinal, empirical study of online privacy policies. Our research found that although online privacy policies have improved in quality and effectiveness since 2000, they still fall well short of the level of privacy assurance desired by consumers. This study analyses trends in OPPs over the two years of the study, identifying areas of deficiency and improvements, and offering a solution in the form of a detailed set of guidelines for organisational online privacy policy. Our study adds to existing theory in this area and, more immediately, will assist businesses concerned about the effect of privacy issues on consumer web usage.<br /
Effective online privacy policies
Online privacy policies are important mechanisms for informing web site users about the level of information privacy protection afforded when visiting web sites. To date, societal mechanisms and technologies have been the focus of attempts to improve the quality and effectiveness of online privacy policies. Little attention, however, has been given to the development and use of organisational measures for this purpose. In this paper we present findings from an empirical study of online privacy policies, resulting in a set of organisational guidelines for effective online privacy policies, which extend the research base in this area and, more immediately, will assist companies concerned about the impact of privacy concerns on consumer web usage
Adding value to online privacy for consumers: remedying deficiences in online privacy policies with an holistic approach
We present findings from a longitudinal, empirical study of online privacy policies. Our research found that although online privacy policies have improved in quality and effectiveness since 2000, they still fall well short of the level of privacy assurance desired by consumers. This study has identified broad areas of deficiency in existing online privacy policies, and offers a solution in the form of an holistic framework for the development, factors and content of online privacy policies for organizations. Our study adds to existing theory in this area and, more immediately, will assist businesses concerned about the effect of privacy issues on consumer Web usage.<br /
On the mechanism for orbital-ordering in KCuF3
The Mott insulating perovskite KCuF3 is considered the archetype of an
orbitally-ordered system. By using the LDA+dynamical mean-field theory (DMFT)
method, we investigate the mechanism for orbital-ordering (OO) in this
material. We show that the purely electronic Kugel-Khomskii super-exchange
mechanism (KK) alone leads to a remarkably large transition temperature of T_KK
about 350 K. However, orbital-order is experimentally believed to persist to at
least 800 K. Thus Jahn-Teller distortions are essential for stabilizing
orbital-order at such high temperatures.Comment: 4 pages, 5 figure
Local magnetic moments in iron and nickel at ambient and Earth's core conditions
Some Bravais lattices have a particular geometry that can slow down the
motion of Bloch electrons by pre-localization due to the band-structure
properties. Another known source of electronic localization in solids is the
Coulomb repulsion in partially filled d- or f-orbitals, which leads to the
formation of local magnetic moments. The combination of these two effects is
usually considered of little relevance to strongly correlated materials. Here
we show that it represents, instead, the underlying physical mechanism in two
of the most important ferromagnets: nickel and iron. In nickel, the van Hove
singularity has an unexpected impact on the magnetism. As a result, the
electron-electron scattering rate is linear in temperature, in violation of the
conventional Landau theory of metals. This is true even at Earth's core
pressures, at which iron is instead a good Fermi liquid. The importance of
nickel in models of geomagnetism may have therefore to be reconsidered.Comment: Supplementary Information available at
https://www.nature.com/articles/ncomms16062#supplementary-informatio
Functional renormalization group study of an eight-band model for the iron arsenides
We investigate the superconducting pairing instabilities of eight-band models
for the iron arsenides. Using a functional renormalization group treatment, we
determine how the critical energy scale for superconductivity depends on the
electronic band structure. Most importantly, if we vary the parameters from
values corresponding to LaFeAsO to SmFeAsO, the pairing scale is strongly
enhanced, in accordance with the experimental observation. We analyze the
reasons for this trend and compare the results of the eight-band approach to
those found using five-band models.Comment: 11 pages, 10 figure
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