115,299 research outputs found
Multi-scale theoretical approach to X-ray absorption spectra in disordered systems: an application to the study of Zn(II) in water
We develop a multi-scale theoretical approach aimed at calculating from first
principles X-ray absorption spectra of liquid solutions and disordered systems.
We test the method by considering the paradigmatic case of Zn(II) in water
which, besides being relevant in itself, is also of interest for biology. With
the help of classical molecular dynamics simulations we start by producing
bunches of configurations differing for the Zn(II)-water coordination mode.
Different coordination modes are obtained by making use of the so-called dummy
atoms method. From the collected molecular dynamics trajectories, snapshots of
a more manageable subsystem encompassing the metal site and two solvation
layers are cut out. Density functional theory is used to optimize and relax
these reduced system configurations employing a uniform dielectric to mimic the
surrounding bulk liquid water. On the resulting structures, fully quantum
mechanical X-ray absorption spectra calculations are performed by including
core-hole effects and core-level shifts. The proposed approach does not rely on
any guessing or fitting of the force field or of the atomic positions of the
system. The comparison of the theoretically computed spectrum with the
experimental Zn K-edge XANES data unambiguously demonstrates that among the
different a priori possible geometries, Zn(II) in water lives in an octahedral
coordination mode.Comment: 8 pages, 3 figure
Semifields, relative difference sets, and bent functions
Recently, the interest in semifields has increased due to the discovery of
several new families and progress in the classification problem. Commutative
semifields play an important role since they are equivalent to certain planar
functions (in the case of odd characteristic) and to modified planar functions
in even characteristic. Similarly, commutative semifields are equivalent to
relative difference sets. The goal of this survey is to describe the connection
between these concepts. Moreover, we shall discuss power mappings that are
planar and consider component functions of planar mappings, which may be also
viewed as projections of relative difference sets. It turns out that the
component functions in the even characteristic case are related to negabent
functions as well as to -valued bent functions.Comment: Survey paper for the RICAM workshop "Emerging applications of finite
fields", 09-13 December 2013, Linz, Austria. This article will appear in the
proceedings volume for this workshop, published as part of the "Radon Series
on Computational and Applied Mathematics" by DeGruyte
Circular groups, planar groups, and the Euler class
We study groups of C^1 orientation-preserving homeomorphisms of the plane,
and pursue analogies between such groups and circularly-orderable groups. We
show that every such group with a bounded orbit is circularly-orderable, and
show that certain generalized braid groups are circularly-orderable. We also
show that the Euler class of C^infty diffeomorphisms of the plane is an
unbounded class, and that any closed surface group of genus >1 admits a C^infty
action with arbitrary Euler class. On the other hand, we show that Z oplus Z
actions satisfy a homological rigidity property: every orientation-preserving
C^1 action of Z oplus Z on the plane has trivial Euler class. This gives the
complete homological classification of surface group actions on R^2 in every
degree of smoothness.Comment: Published by Geometry and Topology Monographs at
http://www.maths.warwick.ac.uk/gt/GTMon7/paper15.abs.htm
Optimized Planar Penning Traps for Quantum Information Studies
A one-electron qubit would offer a new option for quantum information
science, including the possibility of extremely long coherence times.
One-quantum cyclotron transitions and spin flips have been observed for a
single electron in a cylindrical Penning trap. However, an electron suspended
in a planar Penning trap is a more promising building block for the array of
coupled qubits needed for quantum information studies. The optimized design
configurations identified here promise to make it possible to realize the
elusive goal of one trapped electron in a planar Penning trap for the first
time - a substantial step toward a one-electron qubit
Obstacle Numbers of Planar Graphs
Given finitely many connected polygonal obstacles in the
plane and a set of points in general position and not in any obstacle, the
{\em visibility graph} of with obstacles is the (geometric)
graph with vertex set , where two vertices are adjacent if the straight line
segment joining them intersects no obstacle. The obstacle number of a graph
is the smallest integer such that is the visibility graph of a set of
points with obstacles. If is planar, we define the planar obstacle
number of by further requiring that the visibility graph has no crossing
edges (hence that it is a planar geometric drawing of ). In this paper, we
prove that the maximum planar obstacle number of a planar graph of order is
, the maximum being attained (in particular) by maximal bipartite planar
graphs. This displays a significant difference with the standard obstacle
number, as we prove that the obstacle number of every bipartite planar graph
(and more generally in the class PURE-2-DIR of intersection graphs of straight
line segments in two directions) of order at least is .Comment: Appears in the Proceedings of the 25th International Symposium on
Graph Drawing and Network Visualization (GD 2017
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