4,213 research outputs found
Hafnium carbide formation in oxygen deficient hafnium oxide thin films
On highly oxygen deficient thin films of hafnium oxide (hafnia, HfO)
contaminated with adsorbates of carbon oxides, the formation of hafnium carbide
(HfC) at the surface during vacuum annealing at temperatures as low as 600
{\deg}C is reported. Using X-ray photoelectron spectroscopy the evolution of
the HfC surface layer related to a transformation from insulating into
metallic state is monitored in situ. In contrast, for fully stoichiometric
HfO thin films prepared and measured under identical conditions, the
formation of HfC was not detectable suggesting that the enhanced adsorption
of carbon oxides on oxygen deficient films provides a carbon source for the
carbide formation. This shows that a high concentration of oxygen vacancies in
carbon contaminated hafnia lowers considerably the formation energy of hafnium
carbide. Thus, the presence of a sufficient amount of residual carbon in
resistive random access memory devices might lead to a similar carbide
formation within the conducting filaments due to Joule heating
Monte Carlo simulations of fluid vesicles with in plane orientational ordering
We present a method for simulating fluid vesicles with in-plane orientational
ordering. The method involves computation of local curvature tensor and
parallel transport of the orientational field on a randomly triangulated
surface. It is shown that the model reproduces the known equilibrium
conformation of fluid membranes and work well for a large range of bending
rigidities. Introduction of nematic ordering leads to stiffening of the
membrane. Nematic ordering can also result in anisotropic rigidity on the
surface leading to formation of membrane tubes.Comment: 11 Pages, 12 Figures, To appear in Phys. Rev.
GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South
We present first results of our search for high-redshift galaxies in deep CCD
mosaic images. As a pilot study for a larger survey, very deep images of the
Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select
large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break
method. The data of these Lyman-break galaxies are made public as an electronic
table. These objects are good candidates for galaxies at z~3 and z~4 which is
supported by their photometric redshifts. The distributions of apparent
magnitudes and the clustering properties of the two populations are analysed,
and they show good agreement to earlier studies. We see no evolution in the
comoving clustering scale length from z~3 to z~4. The techniques presented here
will be applied to a much larger sample of U-dropouts from the whole survey in
near future.Comment: 11 pages, 11 figures, replaced with version accepted by A&A. Minor
changes and tabular appendix with LBG catalogues. Version with full
resolution figures available at
http://www.astro.uni-bonn.de/~hendrik/2544.pd
Cosmic shear analysis of archival HST/ACS data: I. Comparison of early ACS pure parallel data to the HST/GEMS Survey
This is the first paper of a series describing our measurement of weak
lensing by large-scale structure using archival observations from the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).
In this work we present results from a pilot study testing the capabilities
of the ACS for cosmic shear measurements with early parallel observations and
presenting a re-analysis of HST/ACS data from the GEMS survey and the GOODS
observations of the Chandra Deep Field South (CDFS). We describe our new
correction scheme for the time-dependent ACS PSF based on observations of
stellar fields. This is currently the only technique which takes the full time
variation of the PSF between individual ACS exposures into account. We estimate
that our PSF correction scheme reduces the systematic contribution to the shear
correlation functions due to PSF distortions to < 2*10^{-6} for galaxy fields
containing at least 10 stars. We perform a number of diagnostic tests
indicating that the remaining level of systematics is consistent with zero for
the GEMS and GOODS data confirming the success of our PSF correction scheme.
For the parallel data we detect a low level of remaining systematics which we
interpret to be caused by a lack of sufficient dithering of the data.
Combining the shear estimate of the GEMS and GOODS observations using 96
galaxies arcmin^{-2} with the photometric redshift catalogue of the GOODS-MUSIC
sample, we determine a local single field estimate for the mass power spectrum
normalisation sigma_{8,CDFS}=0.52^{+0.11}_{-0.15} (stat) +/- 0.07 (sys) (68%
confidence assuming Gaussian cosmic variance) at fixed Omega_m=0.3 for a
LambdaCDM cosmology. We interpret this exceptionally low estimate to be due to
a local under-density of the foreground structures in the CDFS.Comment: Version accepted for publication in Astronomy & Astrophysics with 28
pages, 25 figures. A version with full resolution figures can be downloaded
from http://www.astro.uni-bonn.de/~schrabba/papers/cosmic_shear_acs1_v2.pd
Spin Polarizabilities of the Nucleon from Polarized Low Energy Compton Scattering
As guideline for forthcoming experiments, we present predictions from Chiral
Effective Field Theory for polarized cross sections in low energy Compton
scattering for photon energies below 170 MeV, both on the proton and on the
neutron. Special interest is put on the role of the nucleon spin
polarizabilities which can be examined especially well in polarized Compton
scattering. We present a model-independent way to extract their energy
dependence and static values from experiment, interpreting our findings also in
terms of the low energy effective degrees of freedom inside the nucleon: The
polarizabilities are dominated by chiral dynamics from the pion cloud, except
for resonant multipoles, where contributions of the Delta(1232) resonance turn
out to be crucial. We therefore include it as an explicit degree of freedom. We
also identify some experimental settings which are particularly sensitive to
the spin polarizabilities.Comment: 30 pages, 19 figure
Explicit Delta(1232) Degrees of Freedom in Compton Scattering off the Deuteron
We examine elastic Compton scattering off the deuteron for photon energies
between 50 MeV and 100 MeV in the framework of chiral effective field theories
to next-to-leading order. We compare one theoretical scheme with only pions and
nucleons as explicit degrees of freedom to another in which the Delta(1232)
resonance is treated as an explicit degree of freedom. Whereas pion degrees of
freedom suffice to describe the experimental data measured at about 70 MeV, the
explicit Delta(1232) gives important contributions that help to reproduce the
angular dependence at higher energies. The static isoscalar dipole
polarizabilities alpha_E^s and beta_M^s are fitted to the available data,
giving results for the neutron polarizabilities
alpha_E^n=(14.2+-2.0(stat)+-1.9(syst))*10^(-4)fm^3,
beta_M^n=(1.8+-2.2(stat)+-0.3(syst))*10^(-4)fm^3. These values are in good
agreement with previous experimental analyses. Comparing them to the well-known
proton values we conclude that there is currently no evidence for significant
differences between the proton and neutron electromagnetic dipole
polarizabilities.Comment: 24 pages, 11 figure
A physical application of Kerr-Schild groups
The present work deals with the search of useful physical applications of
some generalized groups of metric transformations. We put forward different
proposals and focus our attention on the implementation of one of them.
Particularly, the results show how one can control very efficiently the kind of
spacetimes related by a Generalized Kerr-Schild (GKS) Ansatz through
Kerr-Schild groups. Finally a preliminar study regarding other generalized
groups of metric transformations is undertaken which is aimed at giving some
hints in new Ans\"atze to finding useful solutions to Einstein's equations.Comment: 18 page
- …