7,538 research outputs found
Effect of surface nanostructure on temperature programmed reaction spectroscopy: First-principles kinetic Monte Carlo simulations of CO oxidation at RuO2(110)
Using the catalytic CO oxidation at RuO2(110) as a showcase, we employ
first-principles kinetic Monte Carlo simulations to illustrate the intricate
effects on temperature programmed reaction spectroscopy data brought about by
the mere correlations between the locations of the active sites at a
nanostructured surface. Even in the absence of lateral interactions, this
nanostructure alone can cause inhomogeneities that cannot be grasped by
prevalent mean-field data analysis procedures, which thus lead to wrong
conclusions on the reactivity of the different surface species.Comment: 4 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
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
Magnetic field dependence of hole levels in self-assembled InAs quantum dots
Recent magneto-transport experiments of holes in InGaAs quantum dots [D.
Reuter, P. Kailuweit, A.D. Wieck, U. Zeitler, O. Wibbelhoff, C. Meier, A.
Lorke, and J.C. Maan, Phys. Rev. Lett. 94, 026808 (2005)] are interpreted by
employing a multi-band kp Hamiltonian, which considers the interaction between
heavy hole and light hole subbands explicitely. No need of invoking an
incomplete energy shell filling is required within this model. The crucial role
we ascribe to the heavy hole-light hole interaction is further supported by
one-band local-spin-density functional calculations, which show that Coulomb
interactions do not induce any incomplete hole shell filling and therefore
cannot account for the experimental magnetic field dispersion.Comment: 5 pages with 3 figures and one table. The paper has been submitted to
Phys.Rev.
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
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
First-principles kinetic Monte Carlo simulations for heterogeneous catalysis, applied to the CO oxidation at RuO2(110)
We describe a first-principles statistical mechanics approach enabling us to
simulate the steady-state situation of heterogeneous catalysis. In a first step
density-functional theory together with transition-state theory is employed to
obtain the energetics of all relevant elementary processes. Subsequently the
statistical mechanics problem is solved by the kinetic Monte Carlo method,
which fully accounts for the correlations, fluctuations, and spatial
distributions of the chemicals at the surface of the catalyst under
steady-state conditions. Applying this approach to the catalytic oxidation of
CO at RuO2(110), we determine the surface atomic structure and composition in
reactive environments ranging from ultra-high vacuum (UHV) to technologically
relevant conditions, i.e. up to pressures of several atmospheres and elevated
temperatures. We also compute the CO2 formation rates (turnover frequencies).
The results are in quantitative agreement with all existing experimental data.
We find that the high catalytic activity of this system is intimately connected
with a disordered, dynamic surface ``phase'' with significant compositional
fluctuations. In this active state the catalytic function results from a
self-regulating interplay of several elementary processes.Comment: 18 pages including 9 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
A General Effective Theory for Dense Quark Matter
A general effective action for quark matter at nonzero temperature and/or
nonzero density is derived. Irrelevant quark modes are distinguished from
relevant quark modes, and hard from soft gluon modes, by introducing two
separate cut-offs in momentum space, one for quarks, , and one for
gluons, . Irrelevant quark modes and hard gluon modes are then
exactly integrated out in the functional integral representation of the QCD
partition function. Depending on the specific choice for and
, the resulting effective action contains well-known effective
actions for hot and/or dense quark matter, for instance the ``Hard Thermal
Loop'' (HTL) or the ``Hard Dense Loop'' (HDL) action, as well as the
high-density effective theory proposed by Hong and others.Comment: 10 pages, 6 figures, contribution to proceedings of SEWM 200
CO oxidation on Pd(100) at technologically relevant pressure conditions: A first-principles kinetic Monte Carlo study
The possible importance of oxide formation for the catalytic activity of
transition metals in heterogenous oxidation catalysis has evoked a lively
discussion over the recent years. On the more noble transition metals (like Pd,
Pt or Ag) the low stability of the common bulk oxides suggests primarily
sub-nanometer thin oxide films, so-called surface oxides, as potential
candidates that may be stabilized under gas phase conditions representative of
technological oxidation catalysis. We address this issue for the Pd(100) model
catalyst surface with first-principles kinetic Monte Carlo (kMC) simulations
that assess the stability of the well-characterized (sqrt{5} x sqrt{5})R27
surface oxide during steady-state CO oxidation. Our results show that at
ambient pressure conditions the surface oxide is stabilized at the surface up
to CO:O2 partial pressure ratios just around the catalytically most relevant
stoichiometric feeds (p(CO):p(O2) = 2:1). The precise value depends sensitively
on temperature, so that both local pressure and temperature fluctuations may
induce a continuous formation and decomposition of oxidic phases during
steady-state operation under ambient stoichiometric conditions.Comment: 13 pages including 5 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Ionic conductivity and relaxation dynamics in plastic-crystals with nearly globular molecules
We have performed a dielectric investigation of the ionic charge transport
and the relaxation dynamics in plastic-crystalline 1-cyano-adamantane (CNA) and
in two mixtures of CNA with the related plastic crystals adamantane or
2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The
molecules of these compounds have nearly globular shape and, thus, the
so-called revolving-door mechanism assumed to promote ionic charge transport
via molecular reorientations in other PC electrolytes, should not be active
here. Indeed, a comparison of the dc resistivity and the reorientational
alpha-relaxation times in the investigated PCs, reveals complete decoupling of
both dynamics. Similar to other PCs, we find a significant mixing-induced
enhancement of the ionic conductivity. Finally, these solid-state electrolytes
reveal a second relaxation process, slower than the alpha-relaxation, which is
related to ionic hopping. Due to the mentioned decoupling, it can be
unequivocally detected and is not superimposed by the reorientational
contributions as found for most other ionic conductors.Comment: 9 pages, 7 figure
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