54,808 research outputs found
The age dependence of halo clustering
We use a very large simulation of the concordance LCDM cosmogony to study the
clustering of dark matter haloes. For haloes less massive than about
1e13Msun/h, the amplitude of the two-point correlation function on large scales
depends strongly on halo formation time. Haloes that assembled at high redshift
are substantially more clustered than those that assembled more recently. The
effect is a smooth function of halo formation time and its amplitude increases
with decreasing halo mass. At 1e11 Msun/h the ``oldest'' 10% of haloes are more
than 5 times more strongly correlated than the ``youngest'' 10%. This
unexpected result is incompatible with the standard excursion set theory for
structure growth, and it contradicts a fundamental assumption of the halo
occupation distribution models often used to study galaxy clustering, namely
that the galaxy content of a halo of given mass is statistically independent of
its larger scale environment.Comment: 5 pages, 5 figures, MNRAS in press. Full resolution pdf file is
avaliable at http://www.mpa-garching.mpg.de/~gaoliang/GSW.pd
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
Halo assembly bias and its effects on galaxy clustering
The clustering of dark halos depends not only on their mass but also on their
assembly history, a dependence we term `assembly bias'. Using a galaxy
formation model grafted onto the Millennium Simulation of the LCDM cosmogony,
we study how assembly bias affects galaxy clustering. We compare the original
simulation to `shuffled' versions where the galaxy populations are randomly
swapped among halos of similar mass, thus isolating the effects of correlations
between assembly history and environment at fixed mass. Such correlations are
ignored in the halo occupation distribution models often used populate dark
matter simulations with galaxies, but they are significant in our more
realistic simulation. Assembly bias enhances 2-point correlations by 10% for
galaxies with M_bJ-5logh brighter than -17, but suppresses them by a similar
amount for galaxies brighter than -20. When such samples are split by colour,
assembly bias is 5% stronger for red galaxies and 5% weaker for blue ones. Halo
central galaxies are differently affected by assembly bias than are galaxies of
all types. It almost doubles the correlation amplitude for faint red central
galaxies. Shuffling galaxies among halos of fixed formation redshift or
concentration in addition to fixed mass produces biases which are not much
smaller than when mass alone is fixed. Assembly bias must reflect a correlation
of environment with aspects of halo assembly which are not encoded in either of
these parameters. It induces effects which could compromise precision
measurements of cosmological parameters from large galaxy surveys.Comment: 8 pages, 4 figures, accepted for publication in MNRA
An efficient mixed variational reduced order model formulation for non-linear analyses of elastic shells
The Koiter-Newton method had recently demonstrated a superior performance for non-linear analyses of structures, compared to traditional path-following strategies. The method follows a predictor-corrector scheme to trace the entire equilibrium path. During a predictor step a reduced order model is constructed based on Koiter's asymptotic post-buckling theory which is followed by a Newton iteration in the corrector phase to regain the equilibrium of forces.
In this manuscript, we introduce a robust mixed solid-shell formulation to further enhance the efficiency of stability analyses in various aspects. We show that a Hellinger-Reissner variational formulation facilitates the reduced order model construction omitting an expensive evaluation of the inherent fourth order derivatives of the strain energy. We demonstrate that extremely large step sizes with a reasonable out-of-balance residual can be obtained with substantial impact on the total number of steps needed to trace the complete equilibrium path. More importantly, the numerical effort of the corrector phase involving a Newton iteration of the full order model is drastically reduced thus revealing the true strength of the proposed formulation. We study a number of problems from engineering and compare the results to the conventional approach in order to highlight the gain in numerical efficiency for stability problems
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Non-invasive imaging of subsurface paint layers with optical coherence tomography
Optical coherence tomography (OCT) systems are fast scanning infrared Michelson interferometers designed for the non-invasive examination of the interiors of the eye and subsurface structures of biological tissues. OCT has recently been applied to the non-invasive examinations of the stratigraphy of paintings and museum artefacts. So far this is the only technique capable of imaging non-invasively the subsurface structure of paintings and painted objects. Unlike the traditional method of paint cross-section examination where sampling is required, the non-invasive and non-contact nature of the technique enables the examination of the paint cross-section anywhere on a painting, as there is no longer an issue with conservation ethics regarding the taking of samples from historical artefacts. A range of applications of the technique including the imaging of stratigraphy of paintings and painted artefacts, the imaging of underdrawings to the analysis of the optical properties of paint and varnish layers is presented. Future projects on the application of OCT to art conservation are discussed
Knowledgezoom for java: A concept-based exam study tool with a zoomable open student model
This paper presents our attempt to develop a personalized exam preparation tool for Java/OOP classes based on a fine-grained concept model of Java knowledge. Our goal was to explore two most popular student model-based approaches: open student modeling and problem sequencing. The result of our work is a Java exam preparation tool, Knowledge Zoom. The tool combines an open concept-level student model component, Knowledge Explorer and a concept-based sequencing component, Knowledge Maximizer into a single interface. This paper presents both components of Knowledge Zoom, reports results of its evaluation, and discusses lessons learned. © 2013 IEEE
Phase diagram of the two-chain Hubbard model
We have calculated the charge gap and spin gap for the two-chain Hubbard
model as a function of the on-site Coulomb interaction and the interchain
hopping amplitude. We used the density matrix renormalization group method and
developed a method to calculate separately the gaps numerically for the
symmetric and antisymmetric modes with respect to the exchange of the chain
indices. We have found very different behaviors for the weak and strong
interaction cases. Our calculated phase diagram is compared to the one obtained
by Balents and Fisher using the weak coupling renormalization group technique.Comment: 4 pages, 6 figures, to appear in PR
Scalable Quantum Networks based on Few-Qubit Registers
We describe and analyze a hybrid approach to scalable quantum computation
based on an optically connected network of few-qubit quantum registers. We show
that probabilistically connected five-qubit quantum registers suffice for
deterministic, fault-tolerant quantum computation even when state preparation,
measurement, and entanglement generation all have substantial errors. We
discuss requirements for achieving fault-tolerant operation for two specific
implementations of our approach.Comment: 4 pages, 3 figures (new figures 1 and 3
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