5,735 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
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
Technology for an intelligent, free-flying robot for crew and equipment retrieval in space
Crew rescue and equipment retrieval is a Space Station Freedom requirement. During Freedom's lifetime, there is a high probability that a number of objects will accidently become separated. Members of the crew, replacement units, and key tools are examples. Retrieval of these objects within a short time is essential. Systems engineering studies were conducted to identify system requirements and candidate approaches. One such approach, based on a voice-supervised, intelligent, free-flying robot was selected for further analysis. A ground-based technology demonstration, now in its second phase, was designed to provide an integrated robotic hardware and software testbed supporting design of a space-borne system. The ground system, known as the EVA Retriever, is examining the problem of autonomously planning and executing a target rendezvous, grapple, and return to base while avoiding stationary and moving obstacles. The current prototype is an anthropomorphic manipulator unit with dexterous arms and hands attached to a robot body and latched in a manned maneuvering unit. A precision air-bearing floor is used to simulate space. Sensor data include two vision systems and force/proximity/tactile sensors on the hands and arms. Planning for a shuttle file experiment is underway. A set of scenarios and strawman requirements were defined to support conceptual development. Initial design activities are expected to begin in late 1989 with the flight occurring in 1994. The flight hardware and software will be based on lessons learned from both the ground prototype and computer simulations
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
A Tachyonic Gluon Mass: Between Infrared and Ultraviolet
The gluon spin coupling to a Gaussian correlated background gauge field
induces an effective tachyonic gluon mass. It is momentum dependent and
vanishes in the UV only like 1/p^2. In the IR, we obtain stabilization through
a positive m^2_{conf}(p^2) related to confinement. Recently a purely
phenomenological tachyonic gluon mass was used to explain the linear rise in
the q\bar q static potential at small distances and also some long standing
discrepancies found in QCD sum rules. We show that the stochastic vacuum model
of QCD predicts a gluon mass with the desired properties.Comment: 10 pages LaTeX, 2 figures using eps
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
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
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
Gluon Condensation in Nonperturbative Flow Equations
We employ nonperturbative flow equations for an investigation of the
effective action in Yang-Mills theories. We compute the effective action
for constant color magnetic fields and examine Savvidy's
conjecture of an unstable perturbative vacuum. Our results indicate that the
absolute minimum of occurs for B=0. Gluon condensation is described
by a nonvanishing expectation value of the regularized composite operator
which agrees with phenomenological estimates.Comment: 64 pages, late
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