5,393 research outputs found
The k-Point Random Matrix Kernels Obtained from One-Point Supermatrix Models
The k-point correlation functions of the Gaussian Random Matrix Ensembles are
certain determinants of functions which depend on only two arguments. They are
referred to as kernels, since they are the building blocks of all correlations.
We show that the kernels are obtained, for arbitrary level number, directly
from supermatrix models for one-point functions. More precisely, the generating
functions of the one-point functions are equivalent to the kernels. This is
surprising, because it implies that already the one-point generating function
holds essential information about the k-point correlations. This also
establishes a link to the averaged ratios of spectral determinants, i.e. of
characteristic polynomials
First principles calculation of structural and magnetic properties for Fe monolayers and bilayers on W(110)
Structure optimizations were performed for 1 and 2 monolayers (ML) of Fe on a
5 ML W(110) substrate employing the all-electron full-potential linearized
augmented plane-wave (FP-LAPW) method. The magnetic moments were also obtained
for the converged and optimized structures. We find significant contractions
( 10 %) for both the Fe-W and the neighboring Fe-Fe interlayer spacings
compared to the corresponding bulk W-W and Fe-Fe interlayer spacings. Compared
to the Fe bcc bulk moment of 2.2 , the magnetic moment for the surface
layer of Fe is enhanced (i) by 15% to 2.54 for 1 ML Fe/5 ML W(110), and
(ii) by 29% to 2.84 for 2 ML Fe/5 ML W(110). The inner Fe layer for 2
ML Fe/5 ML W(110) has a bulk-like moment of 2.3 . These results agree
well with previous experimental data
Survival Probability of a Doorway State in regular and chaotic environments
We calculate survival probability of a special state which couples randomly
to a regular or chaotic environment. The environment is modelled by a suitably
chosen random matrix ensemble. The exact results exhibit non--perturbative
features as revival of probability and non--ergodicity. The role of background
complexity and of coupling complexity is discussed as well.Comment: 19 pages 5 Figure
Coherent Optimal Control of Multiphoton Molecular Excitation
We give a framework for molecular multiphoton excitation process induced by
an optimally designed electric field. The molecule is initially prepared in a
coherent superposition state of two of its eigenfunctions. The relative phase
of the two superposed eigenfunctions has been shown to control the optimally
designed electric field which triggers the multiphoton excitation in the
molecule. This brings forth flexibility in desiging the optimal field in the
laboratory by suitably tuning the molecular phase and hence by choosing the
most favorable interfering routes that the system follows to reach the target.
We follow the quantum fluid dynamical formulation for desiging the electric
field with application to HBr molecule.Comment: 5 figure
Volterra Distortions, Spinning Strings, and Cosmic Defects
Cosmic strings, as topological spacetime defects, show striking resemblance
to defects in solid continua: distortions, which can be classified into
disclinations and dislocations, are line-like defects characterized by a delta
function-valued curvature and torsion distribution giving rise to rotational
and translational holonomy. We exploit this analogy and investigate how
distortions can be adapted in a systematic manner from solid state systems to
Einstein-Cartan gravity. As distortions are efficiently described within the
framework of a SO(3) {\rlap{\supset}\times}} T(3) gauge theory of solid
continua with line defects, we are led in a straightforward way to a Poincar\'e
gauge approach to gravity which is a natural framework for introducing the
notion of distorted spacetimes. Constructing all ten possible distorted
spacetimes, we recover, inter alia, the well-known exterior spacetime of a
spin-polarized cosmic string as a special case of such a geometry. In a second
step, we search for matter distributions which, in Einstein-Cartan gravity, act
as sources of distorted spacetimes. The resulting solutions, appropriately
matched to the distorted vacua, are cylindrically symmetric and are interpreted
as spin-polarized cosmic strings and cosmic dislocations.Comment: 24 pages, LaTeX, 9 eps figures; remarks on energy conditions added,
discussion extended, version to be published in Class. Quantum Gra
Studying Flow Close to an Interface by Total Internal Reflection Fluorescence Cross Correlation Spectroscopy: Quantitative Data Analysis
Total Internal Reflection Fluorescence Cross Correlation Spectroscopy
(TIR-FCCS) has recently (S. Yordanov et al., Optics Express 17, 21149 (2009))
been established as an experimental method to probe hydrodynamic flows near
surfaces, on length scales of tens of nanometers. Its main advantage is that
fluorescence only occurs for tracer particles close to the surface, thus
resulting in high sensitivity. However, the measured correlation functions only
provide rather indirect information about the flow parameters of interest, such
as the shear rate and the slip length. In the present paper, we show how to
combine detailed and fairly realistic theoretical modeling of the phenomena by
Brownian Dynamics simulations with accurate measurements of the correlation
functions, in order to establish a quantitative method to retrieve the flow
properties from the experiments. Firstly, Brownian Dynamics is used to sample
highly accurate correlation functions for a fixed set of model parameters.
Secondly, these parameters are varied systematically by means of an
importance-sampling Monte Carlo procedure in order to fit the experiments. This
provides the optimum parameter values together with their statistical error
bars. The approach is well suited for massively parallel computers, which
allows us to do the data analysis within moderate computing times. The method
is applied to flow near a hydrophilic surface, where the slip length is
observed to be smaller than 10nm, and, within the limitations of the
experiments and the model, indistinguishable from zero.Comment: 18 pages, 12 figure
A Multiplicity Census of Young Stars in Chamaeleon I
We present the results of a multiplicity survey of 126 stars spanning ~0.1-3
solar masses in the ~2-Myr-old Chamaeleon I star-forming region, based on
adaptive optics imaging with the ESO Very Large Telescope. Our observations
have revealed 30 binaries and 6 triples, of which 19 and 4, respectively, are
new discoveries. The overall multiplicity fraction we find for Cha I (~30%) is
similar to those reported for other dispersed young associations, but
significantly higher than seen in denser clusters and the field, for comparable
samples. Both the frequency and the maximum separation of Cha I binaries
decline with decreasing mass, while the mass ratios approach unity; conversely,
tighter pairs are more likely to be equal mass. We confirm that brown dwarf
companions to stars are rare, even at young ages at wide separations. Based on
follow-up spectroscopy of two low-mass substellar companion candidates, we
conclude that both are likely background stars. The overall multiplicity
fraction in Cha I is in rough agreement with numerical simulations of cloud
collapse and fragmentation, but its observed mass dependence is less steep than
predicted. The paucity of higher-order multiples, in particular, provides a
stringent constraint on the simulations, and seems to indicate a low level of
turbulence in the prestellar cores in Cha I.Comment: Accepted for publication in Ap
Molecular Wires Acting as Coherent Quantum Ratchets
The effect of laser fields on the electron transport through a molecular wire
being weakly coupled to two leads is investigated. The molecular wire acts as a
coherent quantum ratchet if the molecule is composed of periodically arranged,
asymmetric chemical groups. This setup presents a quantum rectifier with a
finite dc-response in the absence of a static bias. The nonlinear current is
evaluated in closed form within the Floquet basis of the isolated, driven wire.
The current response reveals multiple current reversals together with a
nonlinear dependence (reflecting avoided quasi-energy crossings) on both, the
amplitude and the frequency of the laser field. The current saturates for long
wires at a nonzero value, while it may change sign upon decreasing its length.Comment: 4 pages, 4 figures, RevTeX
Strong coupling theory for driven tunneling and vibrational relaxation
We investigate on a unified basis tunneling and vibrational relaxation in
driven dissipative multistable systems described by their N lowest lying
unperturbed levels. By use of the discrete variable representation we derive a
set of coupled non-Markovian master equations. We present analytical treatments
that describe the dynamics in the regime of strong system-bath coupling. Our
findings are corroborated by ``ab-initio'' real-time path integral
calculations.Comment: 4 LaTeX pages including 3 figure
Frustrated H-Induced Instability of Mo(110)
Using helium atom scattering Hulpke and L"udecke recently observed a giant
phonon anomaly for the hydrogen covered W(110) and Mo(110) surfaces. An
explanation which is able to account for this and other experiments is still
lacking. Below we present density-functional theory calculations of the atomic
and electronic structure of the clean and hydrogen-covered Mo(110) surfaces.
For the full adsorbate monolayer the calculations provide evidence for a strong
Fermi surface nesting instability. This explains the observed anomalies and
resolves the apparent inconsistencies of different experiments.Comment: 4 pages, 2 figures, submitted to PR
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