74,638 research outputs found
Electron-phonon interaction for adiabatic anharmonic phonons
A model with Holstein-like electron-phonon coupling is studied in the limit
of adiabatic phonons. The phonon distribution is anharmonic with two degenerate
maxima. This model can be related to fermions in a correlated binary alloy and
describes microscopic phase separation. We discuss the weak and strong
electron-phonon coupling limit and present a qualitative phase diagram. In
terms of the phononic displacements it consists of a homogeneous, an
alternating, and a disordered phase. There is a first order phase transition
between the homogeneous and the alternating phase, and second order phase
transition between the alternating and the disordered phase. The opening of a
gap inside the disordered phase is treated by a dynamical mean-field theory.Comment: 11 pages, 3 figures, revised and published versio
Wide field weak lensing observations of A1835 and A2204
We present mass reconstructions from weak lensing for the galaxy clusters
A1835 and A2204 over 34'x34' fields using data from the ESO/MPG Wide Field
Imager. Using a background galaxy population of 22<R<25.5 we detect the
gravitational shear of A1835 at 8.8 sigma significance, and obtain best-fit
mass profiles of sigma_v=1233^{+66}_{-70} km/s for a singular isothermal sphere
model and r_{200}=1550 h^{-1} kpc, c=2.96 for a `universal' CDM profile. Using
a color-selected background galaxy population of 22<R<25.8 we detect the
gravitational shear of A2204 at 7.2 sigma significance, and obtain best-fit
mass profiles of sigma_v=1035^{+65}_{-71} km/s for a SIS model and r_{200}=1310
h^{-1} km/s, c=6.3 for a `universal' CDM profile. The gravitational shear at
distances greater than 10' is significantly detected for both clusters. The
best fit weak lensing cluster masses agree well with both X-ray and dynamical
mass measurements, although the central concentration of A1835 is much lower in
the weak lensing mass profile than that measured by recent Chandra results. We
suggest that this lower concentration is most likely a combination of
contamination of the 'background' galaxy population with cluster dwarf galaxies
and the effect of a prolate or tri-axial cluster core with the major axis lying
near the plane of the sky. We also detect a number of additional structures at
moderate significance, some of which appear to be sub-haloes associated with
the clusters.Comment: accepted to A&A, 14 pages, 13 figures, version with higher quality
images can be found at http://www.uni-bonn.de/~clow
Magnetic field induced 3D to 1D crossover in Sr0:9La0:1CuO2
The effect of the magnetic field on the critical behavior of Sr0:9La0:1CuO2
is explored in terms of reversible magnetization data. As the correlation
length transverse to the magnetic field Hi,applied along the i-axis, cannot
grow beyond the limiting magnetic length LHi, related to the average distance
between vortex lines, one expects a magnetic field induced finite size effect.
Invoking the scaling theory of critical phenomena we provide clear evidence for
this effect. It implies that in type II superconductors there is a 3D to 1D
crossover line Hpi(T). Consequently, below Tc and above Hpi(T) uperconductivity
is confined to cylinders with diameter LHi(1D). Accordingly, there is no
continuous phase transition in the (H,T)-plane along the Hc2-lines as predicted
by the mean-field treatment.Comment: 4 pages, 5 figure
Constrained probability distributions of correlation functions
Context: Two-point correlation functions are used throughout cosmology as a
measure for the statistics of random fields. When used in Bayesian parameter
estimation, their likelihood function is usually replaced by a Gaussian
approximation. However, this has been shown to be insufficient.
Aims: For the case of Gaussian random fields, we search for an exact
probability distribution of correlation functions, which could improve the
accuracy of future data analyses.
Methods: We use a fully analytic approach, first expanding the random field
in its Fourier modes, and then calculating the characteristic function.
Finally, we derive the probability distribution function using integration by
residues. We use a numerical implementation of the full analytic formula to
discuss the behaviour of this function.
Results: We derive the univariate and bivariate probability distribution
function of the correlation functions of a Gaussian random field, and outline
how higher joint distributions could be calculated. We give the results in the
form of mode expansions, but in one special case we also find a closed-form
expression. We calculate the moments of the distribution and, in the univariate
case, we discuss the Edgeworth expansion approximation. We also comment on the
difficulties in a fast and exact numerical implementation of our results, and
on possible future applications.Comment: 13 pages, 5 figures, updated to match version published in A&A
(slightly expanded Sects. 5.3 and 6
HTS quasiparticle injection devices with large current gain at 77 K
Recent progress on the development of planar QP-injection devices using YBCO and STO as an epitaxial injection barrier will be discussed. The main problem for HTS injection devices is to grow reliably a well defined, ultra-thin tunneling barrier suitable for QP tunneling. For this purpose, we used inverted cylindrical magnetron sputtering to first optimize the smoothness of our YBCO films by controlling tightly an relevant sputtering conditions. We are able to prepare smooth (001) YBCO films on (001) STO substrates on a routine basis with an average roughness varying between 1 and 2 nm. With these flat YBCO films both planar as well as grain boundary junctions were fabricated using epitaxial STO barriers between 2 and 8 nm thick and a 50 nm of Au counter electrode. Planar junctions with 6 nm STO barriers were in most cases fully insulating, in some cases, a current gain of up to 7.4 at 77 K was obtained. For 3 nm STO barriers, the highest current gain was 15 at 81 K. The injection results also show a scaling behavior with junction size. Based on the present materials development and device understanding, we consider a current gain of up to 20 at 77 K possibl
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Gesture enhancement of a virtual tutor via investigating human tutor discursive strategies: Forms and functions for proportions
We examine expert human mathematics-tutor gestures in the context of an interactive design for proportionality in order to design a virtual pedagogical agent. Early results implicate distinct gesture morphologies serving consistent contextual functionalities in guiding learners towards quantitative descriptions of proportional relations
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