31,757 research outputs found
Poisoning of Hydrogen Dissociation at Pd (100) by Adsorbed Sulfur Studied by ab initio Quantum Dynamics and ab initio Molecular Dynamics
We report calculations of the dissociative adsorption of H_2 at Pd (100)
covered with 1/4 monolayer of sulfur using quantum dynamics as well as
molecular dynamics and taking all six degrees of freedom of the two H atoms
fully into account. The ab initio potential-energy surface (PES) is found to be
very strongly corrugated. In particular we discuss the influence of tunneling,
zero-point vibrations, localization of the nuclei's wave function when narrow
valleys of the PES are passed, steering of the approaching H_2 molecules
towards low energy barrier configurations, and the time scales of the center of
mass motion and the other degrees of freedom. Several ``established'' concepts,
which were derived from low-dimensional dynamical studies, are shown to be not
valid.Comment: 4 pages, 3 figures, submitted to Surf. Sci. Lett. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Stability of networks of delay-coupled delay oscillators
Dynamical networks with time delays can pose a considerable challenge for
mathematical analysis. Here, we extend the approach of generalized modeling to
investigate the stability of large networks of delay-coupled delay oscillators.
When the local dynamical stability of the network is plotted as a function of
the two delays then a pattern of tongues is revealed. Exploiting a link between
structure and dynamics, we identify conditions under which perturbations of the
topology have a strong impact on the stability. If these critical regions are
avoided the local stability of large random networks can be well approximated
analytically
Gauge Fields and Space-Time
In this article I attempt to collect some ideas,opinions and formulae which
may be useful in solving the problem of gauge/ string / space-time
correspondence This includes the validity of D-brane representation, counting
of gauge-invariant words, relations between the null states and the Yang-Mills
equations and the discussion of the strong coupling limit of the string sigma
model. The article is based on the talk given at the "Odyssey 2001" conference.Comment: 20 page
Measuring and engineering entropy and spin squeezing in weakly linked Bose-Einstein condensates
We propose a method to infer the single-particle entropy of bosonic atoms in
an optical lattice and to study the local evolution of entropy, spin squeezing,
and entropic inequalities for entanglement detection in such systems. This
method is based on experimentally feasible measurements of
non-nearest-neighbour coherences. We study a specific example of dynamically
controlling atom tunneling between selected sites and show that this could
potentially also improve the metrologically relevant spin squeezing
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Emergent bipartiteness in a society of knights and knaves
We propose a simple model of a social network based on so-called
knights-and-knaves puzzles. The model describes the formation of networks
between two classes of agents where links are formed by agents introducing
their neighbours to others of their own class. We show that if the proportion
of knights and knaves is within a certain range, the network self-organizes to
a perfectly bipartite state. However, if the excess of one of the two classes
is greater than a threshold value, bipartiteness is not observed. We offer a
detailed theoretical analysis for the behaviour of the model, investigate its
behaviou r in the thermodynamic limit, and argue that it provides a simple
example of a topology-driven model whose behaviour is strongly reminiscent of a
first-order phase transitions far from equilibrium.Comment: 12 pages, 5 figure
Spin squeezing, entanglement and quantum metrology with Bose-Einstein condensates
Squeezed states, a special kind of entangled states, are known as a useful
resource for quantum metrology. In interferometric sensors they allow to
overcome the "classical" projection noise limit stemming from the independent
nature of the individual photons or atoms within the interferometer. Motivated
by the potential impact on metrology as wells as by fundamental questions in
the context of entanglement, a lot of theoretical and experimental effort has
been made to study squeezed states. The first squeezed states useful for
quantum enhanced metrology have been proposed and generated in quantum optics,
where the squeezed variables are the coherences of the light field. In this
tutorial we focus on spin squeezing in atomic systems. We give an introduction
to its concepts and discuss its generation in Bose-Einstein condensates. We
discuss in detail the experimental requirements necessary for the generation
and direct detection of coherent spin squeezing. Two exemplary experiments
demonstrating adiabatically prepared spin squeezing based on motional degrees
of freedom and diabatically realized spin squeezing based on internal hyperfine
degrees of freedom are discussed.Comment: Phd tutorial, 23 pages, 17 figure
Classical bifurcation at the transition from Rabi to Josephson dynamics
We report on the experimental realization of an internal bosonic Josephson
junction in a Rubidium spinor Bose-Einstein condensate. The measurement of the
full time dynamics in phase space allows the characterization of the
theoretically predicted -phase modes and quantitatively confirms
analytical predictions, revealing a classical bifurcation. Our results suggest
that this system is a model system which can be tuned from classical to the
quantum regime and thus is an important step towards the experimental
investigation of entanglement generation close to critical points
String Spectrum of 1+1-Dimensional Large N QCD with Adjoint Matter
We propose gauging matrix models of string theory to eliminate unwanted
non-singlet states. To this end we perform a discretised light-cone
quantisation of large N gauge theory in 1+1 dimensions, with scalar or
fermionic matter fields transforming in the adjoint representation of SU(N).
The entire spectrum consists of bosonic and fermionic closed-string
excitations, which are free as N tends to infinity. We analyze the general
features of such bound states as a function of the cut-off and the gauge
coupling, obtaining good convergence for the case of adjoint fermions. We
discuss possible extensions of the model and the search for new non-critical
string theories.Comment: 20 pages (7 figures available from authors as postscipt files),
PUPT-134
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