7,180 research outputs found
Effective calculation of LEED intensities using symmetry-adapted functions
The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions
Self-induced decoherence approach: Strong limitations on its validity in a simple spin bath model and on its general physical relevance
The "self-induced decoherence" (SID) approach suggests that (1) the
expectation value of any observable becomes diagonal in the eigenstates of the
total Hamiltonian for systems endowed with a continuous energy spectrum, and
(2), that this process can be interpreted as decoherence. We evaluate the first
claim in the context of a simple spin bath model. We find that even for large
environments, corresponding to an approximately continuous energy spectrum,
diagonalization of the expectation value of random observables does in general
not occur. We explain this result and conjecture that SID is likely to fail
also in other systems composed of discrete subsystems. Regarding the second
claim, we emphasize that SID does not describe a physically meaningful
decoherence process for individual measurements, but only involves destructive
interference that occurs collectively within an ensemble of presupposed
"values" of measurements. This leads us to question the relevance of SID for
treating observed decoherence effects.Comment: 11 pages, 4 figures. Final published versio
Contraction by Ca2+ Influx via the L-Type Ca2+ Channel Voltage Window in Mouse Aortic Segments is Modulated by Nitric Oxide
Decoherence time in self-induced decoherence
A general method for obtaining the decoherence time in self-induced
decoherence is presented. In particular, it is shown that such a time can be
computed from the poles of the resolvent or of the initial conditions in the
complex extension of the Hamiltonian's spectrum. Several decoherence times are
estimated: for microscopic systems, and
for macroscopic bodies. For the particular case of a
thermal bath, our results agree with those obtained by the einselection
(environment-induced decoherence) approach.Comment: 11 page
Self-limited oxide formation in Ni(111) oxidation
The oxidation of the Ni(111) surface is studied experimentally with low
energy electron microscopy and theoretically by calculating the electron
reflectivity for realistic models of the NiO/Ni(111) surface with an ab-initio
scattering theory. Oxygen exposure at 300 K under ultrahigh-vacuum conditions
leads to the formation of a continuous NiO(111)-like film consisting of
nanosized domains. At 750 K, we observe the formation of a nano-heterogeneous
film composed primarily of NiO(111)-like surface oxide nuclei, which exhibit
virtually the same energy-dependent reflectivity as in the case of 300 K and
which are separated by oxygen-free Ni(111) terraces. The scattering theory
explains the observed normal incidence reflectivity R(E) of both the clean and
the oxidized Ni(111) surface. At low energies R(E) of the oxidized surface is
determined by a forbidden gap in the k_parallel=0 projected energy spectrum of
the bulk NiO crystal. However, for both low and high temperature oxidation a
rapid decrease of the reflectivity in approaching zero kinetic energy is
experimentally observed. This feature is shown to characterize the thickness of
the oxide layer, suggesting an average oxide thickness of two NiO layers.Comment: 10 pages (in journal format), 9 figure
From Bloch model to the rate equations II: the case of almost degenerate energy levels
Bloch equations give a quantum description of the coupling between an atom
and a driving electric force. In this article, we address the asymptotics of
these equations for high frequency electric fields, in a weakly coupled regime.
We prove the convergence towards rate equations (i.e. linear Boltzmann
equations, describing the transitions between energy levels of the atom). We
give an explicit form for the transition rates. This has already been performed
in [BFCD03] in the case when the energy levels are fixed, and for different
classes of electric fields: quasi or almost periodic, KBM, or with continuous
spectrum. Here, we extend the study to the case when energy levels are possibly
almost degenerate. However, we need to restrict to quasiperiodic forcings. The
techniques used stem from manipulations on the density matrix and the averaging
theory for ordinary differential equations. Possibly perturbed small divisor
estimates play a key role in the analysis. In the case of a finite number of
energy levels, we also precisely analyze the initial time-layer in the rate
aquation, as well as the long-time convergence towards equilibrium. We give
hints and counterexamples in the infinite dimensional case
Indications for a Detonating Quark-Gluon Plasma
We propose a mechanism which naturally contains the relation of the hadronic gas produced in heavy-ion collisions at CERN. Our
starting assumption is the existence of a sharp front separating the
quark-gluon plasma phase from the hadronic phase. Energy-momentum conservation
across the front leads to the following consequences for an adiabatic process
a) The baryon chemical potential, , is approximately continuous across
the front. b) The temperature in the hadronic gas is higher than the phase
transition temperature due to superheating. c) In the region covered by the
experiments the velocity of the hadronic gas approximately equals the speed of
sound in the hadronic gas.Comment: Latex file 9 pages + 6 figures available as postscript file
Surface and Bulk Structural Properties of Single Crystalline Sr3Ru2O7
We report temperature and thermal-cycling dependence of surface and bulk
structures of double-layered perovskite Sr3Ru2O7 single crystals. The surface
and bulk structures were investigated using low-energy electron diffraction
(LEED) and single-crystal X-ray diffraction (XRD) techniques, respectively.
Single-crystal XRD data is in good agreement with previous reports for the bulk
structure with RuO6 octahedral rotation, which increases with decreasing
temperature (~ 6.7(6)degrees at 300 K and ~ 8.1(2) degrees at 90 K). LEED
results reveal that the octahedra at the surface are much more distorted with a
higher rotation angle (~ 12 degrees between 300 and 80 K) and a slight tilt
((4.5\pm2.5) degrees at 300 K and (2.5\pm1.7) degrees at 80 K). While XRD data
confirms temperature dependence of the unit cell height/width ratio (i.e.
lattice parameter c divided by the average of parameters a and b) found in a
prior neutron powder diffraction investigation, both bulk and surface
structures display little change with thermal cycles between 300 and 80 K.Comment: 25 pages, 5 figures, 5 tables, to appear in Physical Review
Swimming is never without risk: opening up on learning through activism and research
This article examines my own becoming as Elisabeth and as a researcher. It is about working as a support worker, coaching teams that are trying to realize inclusive education for a child, and my PhD process, which relies on these practices. My intention here is to unfold several aspects, blockages, possibilities, and tensions that can make sense of my messy struggle. The never-ending learning through working with people, listening to their stories, and taking responsibility are important ingredients of my engagement. It is necessary to provide insights and justify my multiple positions to avoid falling into a narcissistic trap. In doing so, I will seek help from Levinas and in concepts of Deleuze and Guattari to (re-)construct my own understanding
Manifestation of quantum chaos on scattering techniques: application to low-energy and photo-electron diffraction intensities
Intensities of LEED and PED are analyzed from a statistical point of view.
The probability distribution is compared with a Porter-Thomas law,
characteristic of a chaotic quantum system. The agreement obtained is
understood in terms of analogies between simple models and Berry's conjecture
for a typical wavefunction of a chaotic system. The consequences of this
behaviour on surface structural analysis are qualitatively discussed by looking
at the behaviour of standard correlation factors.Comment: 5 pages, 4 postscript figures, Latex, APS,
http://www.icmm.csic.es/Pandres/pedro.ht
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