5,904 research outputs found
Effective Average Action of Chern-Simons Field Theory
The renormalization of the Chern-Simons parameter is investigated by using an
exact and manifestly gauge invariant evolution equation for the scale-dependent
effective average action.Comment: 14 pages, late
Renormalization of the Topological Charge in Yang-Mills Theory
The conditions leading to a nontrivial renormalization of the topological
charge in four--dimensional Yang--Mills theory are discussed. It is shown that
if the topological term is regarded as the limit of a certain nontopological
interaction, quantum effects due to the gauge bosons lead to a finite
multiplicative renormalization of the theta--parameter while fermions give rise
to an additional shift of theta. A truncated form of an exact renormalization
group equation is used to study the scale dependence of the theta--parameter.
Possible implications for the strong CP--problem of QCD are discussed.Comment: 31 pages, late
CO oxidation at Pd(100): A first-principles constrained thermodynamics study
The possible formation of oxides or thin oxide films (surface oxides) on late
transition metal surfaces is recently being recognized as an essential
ingredient when aiming to understand catalytic oxidation reactions under
technologically relevant gas phase conditions. Using the CO oxidation at
Pd(100) as example, we investigate the composition and structure of this model
catalyst surface over a wide range of (T,p)-conditions within a multiscale
modeling approach where density-functional theory is linked to thermodynamics.
The results show that under the catalytically most relevant gas phase
conditions a thin surface oxide is the most stable "phase" and that the system
is actually very close to a transition between this oxidic state and a reduced
state in form of a CO covered Pd(100) surface.Comment: 13 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Cosmological Perturbations in Renormalization Group Derived Cosmologies
A linear cosmological perturbation theory of an almost homogeneous and
isotropic perfect fluid Universe with dynamically evolving Newton constant
and cosmological constant is presented. A gauge-invariant formalism
is developed by means of the covariant approach, and the acoustic propagation
equations governing the evolution of the comoving fractional spatial gradients
of the matter density, , and are thus obtained. Explicit solutions
are discussed in cosmologies where both and vary according to
renormalization group equations in the vicinity of a fixed point.Comment: 22 pages, revtex, subeqn.sty, to appear on IJMP
The role of Background Independence for Asymptotic Safety in Quantum Einstein Gravity
We discuss various basic conceptual issues related to coarse graining flows
in quantum gravity. In particular the requirement of background independence is
shown to lead to renormalization group (RG) flows which are significantly
different from their analogs on a rigid background spacetime. The importance of
these findings for the asymptotic safety approach to Quantum Einstein Gravity
(QEG) is demonstrated in a simplified setting where only the conformal factor
is quantized. We identify background independence as a (the ?) key prerequisite
for the existence of a non-Gaussian RG fixed point and the renormalizability of
QEG.Comment: 2 figures. Talk given by M.R. at the WE-Heraeus-Seminar "Quantum
Gravity: Challenges and Perspectives", Bad Honnef, April 14-16, 2008; to
appear in General Relativity and Gravitatio
Renormalization group improved gravitational actions: a Brans-Dicke approach
A new framework for exploiting information about the renormalization group
(RG) behavior of gravity in a dynamical context is discussed. The
Einstein-Hilbert action is RG-improved by replacing Newton's constant and the
cosmological constant by scalar functions in the corresponding Lagrangian
density. The position dependence of and is governed by a RG
equation together with an appropriate identification of RG scales with points
in spacetime. The dynamics of the fields and does not admit a
Lagrangian description in general. Within the Lagrangian formalism for the
gravitational field they have the status of externally prescribed
``background'' fields. The metric satisfies an effective Einstein equation
similar to that of Brans-Dicke theory. Its consistency imposes severe
constraints on allowed backgrounds. In the new RG-framework, and
carry energy and momentum. It is tested in the setting of homogeneous-isotropic
cosmology and is compared to alternative approaches where the fields and
do not carry gravitating 4-momentum. The fixed point regime of the
underlying RG flow is studied in detail.Comment: LaTeX, 72 pages, no figure
Fluorescent nanodiamonds for FRET-based monitoring of a single biological nanomotor FoF1-ATP synthase
Color centers in diamond nanocrystals are a new class of fluorescence markers
that attract significant interest due to matchless brightness, photostability
and biochemical inertness. Fluorescing diamond nanocrystals containing defects
can be used as markers replacing conventional organic dye molecules, quantum
dots or autofluorescent proteins. They can be applied for tracking and
ultrahigh-resolution localization of the single markers. In addition the spin
properties of diamond defects can be utilized for novel magneto-optical imaging
(MOI) with nanometer resolution. We develop this technique to unravel the
details of the rotary motions and the elastic energy storage mechanism of a
single biological nanomotor FoF1-ATP synthase. FoF1-ATP synthase is the enzyme
that provides the 'chemical energy currency' adenosine triphosphate, ATP, for
living cells. The formation of ATP is accomplished by a stepwise internal
rotation of subunits within the enzyme. Previously subunit rotation has been
monitored by single-molecule fluorescence resonance energy transfer (FRET) and
was limited by the photostability of the fluorophores. Fluorescent nanodiamonds
advance these FRET measurements to long time scales.Comment: 10 pages, 4 figure
Interplay between nanometer-scale strain variations and externally applied strain in graphene
We present a molecular modeling study analyzing nanometer-scale strain
variations in graphene as a function of externally applied tensile strain. We
consider two different mechanisms that could underlie nanometer-scale strain
variations: static perturbations from lattice imperfections of an underlying
substrate and thermal fluctuations. For both cases we observe a decrease in the
out-of-plane atomic displacements with increasing strain, which is accompanied
by an increase in the in-plane displacements. Reflecting the non-linear elastic
properties of graphene, both trends together yield a non-monotonic variation of
the total displacements with increasing tensile strain. This variation allows
to test the role of nanometer-scale strain variations in limiting the carrier
mobility of high-quality graphene samples
The Complex Gap in Color Superconductivity
We solve the gap equation for color-superconducting quark matter in the 2SC
phase, including both the energy and the momentum dependence of the gap,
\phi=\phi(k_0,\vk). For that purpose a complex Ansatz for \phi is made. The
calculations are performed within an effective theory for cold and dense quark
matter. The solution of the complex gap equation is valid to subleading order
in the strong coupling constant g and in the limit of zero temperature. We find
that, for momenta sufficiently close to the Fermi surface and for small
energies, the dominant contribution to the imaginary part of arises from
Landau-damped magnetic gluons. Further away from the Fermi surface and for
larger energies the other gluon sectors have to be included into Im\phi. We
confirm that Im contributes a correction of order g to the prefactor of
\phi for on-shell quasiquarks sufficiently close to the Fermi surface, whereas
further away from the Fermi surface Im\phi and Re\phi are of the same order.
Finally, we discuss the relevance of Im\phi for the damping of quasiquark
excitations.Comment: 23 pages, 3 figures, 8 tables. Typos corrected, minor corrections to
the text. Accepted for publication in PR
Fractal space-times under the microscope: A Renormalization Group view on Monte Carlo data
The emergence of fractal features in the microscopic structure of space-time
is a common theme in many approaches to quantum gravity. In this work we carry
out a detailed renormalization group study of the spectral dimension and
walk dimension associated with the effective space-times of
asymptotically safe Quantum Einstein Gravity (QEG). We discover three scaling
regimes where these generalized dimensions are approximately constant for an
extended range of length scales: a classical regime where , a
semi-classical regime where , and the UV-fixed point
regime where . On the length scales covered by
three-dimensional Monte Carlo simulations, the resulting spectral dimension is
shown to be in very good agreement with the data. This comparison also provides
a natural explanation for the apparent puzzle between the short distance
behavior of the spectral dimension reported from Causal Dynamical
Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic
Safety.Comment: 26 pages, 6 figure
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