1,563 research outputs found
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Functional Renormalization of Noncommutative Scalar Field Theory
In this paper we apply the Functional Renormalization Group Equation (FRGE)
to the non-commutative scalar field theory proposed by Grosse and Wulkenhaar.
We derive the flow equation in the matrix representation and discuss the theory
space for the self-dual model. The features introduced by the external
dimensionful scale provided by the non-commutativity parameter, originally
pointed out in \cite{Gurau:2009ni}, are discussed in the FRGE context. Using a
technical assumption, but without resorting to any truncation, it is then shown
that the theory is asymptotically safe for suitably small values of the
coupling, recovering the result of \cite{disertori:2007}. Finally, we
show how the FRGE can be easily used to compute the one loop beta-functions of
the duality covariant model.Comment: 38 pages, no figures, LaTe
Experimental and theoretical modeling of Fe , Co , Cu , Mn based electrocatalysts for oxygen reduction
Abstract Experience gained during efforts towards optimization of noble metal free electrocatalysts for oxygen reduction is simultaneously used to understand the chemical and morphological necessities for inducing efficient multielectron transfer catalysis. The analysis of many preparative experimental steps between the moderately performing metal porphyrines and the highly efficient transition metal and sulfur containing pyrolised catalyst material contributes to the following model of the catalyst 1. The metals function enclosed in nitrogen or graphitic environment where they are shielded against oxidation. 2. The metals can be exchanged but are not identical in their efficiency. 3. Higher efficiency is only achieved, when the function of a binary reaction center is warranted. 4. The carbonization of the environment is critical and provides intercalated metal centers and attached metal complexes in graphite environment for interaction with the nitrogen chelated partner center in the simultaneously obtained graphene layers. Experimental support for these models from EXAFS, RAMAN, Mössbauer and X ray spectroscopy is given and a parallel is drawn with the cytochrome oxidase oxygen reduction catalysis, which is proposed to proceed according to roughly the same mechanism. A special effort is made to discuss strategies for shielding and protecting catalytically active abundant transition metals against chemical reaction with oxygen or hydrogen peroxide
Preferred foliation effects in Quantum General Relativity
We investigate the infrared (IR) effects of Lorentz violating terms in the
gravitational sector using functional renormalization group methods similar to
Reuter and collaborators. The model we consider consists of pure quantum
gravity coupled to a preferred foliation, described effectively via a scalar
field with non-standard dynamics. We find that vanishing Lorentz violation is a
UV attractive fixed-point of this model in the local potential approximation.
Since larger truncations may lead to differing results, we study as a first
example effects of additional matter fields on the RG running of the Lorentz
violating term and provide a general argument why they are small.Comment: 12 pages, no figures, compatible with published versio
The Link between General Relativity and Shape Dynamics
We show that one can construct two equivalent gauge theories from a linking
theory and give a general construction principle for linking theories which we
use to construct a linking theory that proves the equivalence of General
Relativity and Shape Dynamics, a theory with fixed foliation but spatial
conformal invariance. This streamlines the rather complicated construction of
this equivalence performed previously. We use this streamlined argument to
extend the result to General Relativity with asymptotically flat boundary
conditions. The improved understanding of linking theories naturally leads to
the Lagrangian formulation of Shape Dynamics, which allows us to partially
relate the degrees of freedom.Comment: 19 pages, LaTeX, no figure
Electron Impact Ionization Close to the Threshold: Classical Calculations
In this paper we present Classical Trajectory Monte Carlo (CTMC) calculations
for single and multiple electron ionization of Argon atoms and ions in the
threshold region. We are able to recover the Wannier exponents a for the
power-law behavior of the cross section s versus excess energy: the exact value
of the exponent as well as the existence of its saturation for multiple
ionization appear to be related to how the total binding energy is shared
between target electrons.Comment: 9 pages. To be published in Journal of Physics
Towards new background independent representations for Loop Quantum Gravity
Recently, uniqueness theorems were constructed for the representation used in
Loop Quantum Gravity. We explore the existence of alternate representations by
weakening the assumptions of the so called LOST uniqueness theorem. The
weakened assumptions seem physically reasonable and retain the key requirement
of explicit background independence. For simplicity, we restrict attention to
the case of gauge group U(1).Comment: 22 pages, minor change
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