3,894 research outputs found
CMB anisotropies in the presence of a stochastic magnetic field
Primordial magnetic fields present since before the epoch of matter-radiation
equality have an effect on the anisotropies of the cosmic microwave background.
The CMB anisotropies due to scalar perturbations are calculated in the gauge
invariant formalism for magnetized adiabatic initial conditions. Furthermore
the linear matter power spectrum is calculated. Numerical solutions are
complemented by a qualitative analysis.Comment: 26 pages, 21 figures; sections 2 and 4 expanded; matches version
published in PR
Structure and composition dynamics of single-crystal copper interfaces and Fe-Ag and Fe-Cu nanoparticles
For heterogeneous (electro)catalysis, the catalytic properties of a material are foremost determined by its structure and composition at the interface between the catalyst and the reaction environment. However, both the structure and composition are not static factors, but parameters that respond dynamically to their environment. A deeper understanding of these dynamic processes is therefore required to be able to, in turn, make targeted improvements to electrocatalytic materials. Here, three projects are presented that each address the surface alterations that result from specific catalyst functionalization and activation treatments. First, we show the oxidation of Cu(100) and Cu(111) surfaces via a low-pressure O2-plasma treatment, investigated by scanning tunneling microscopy, x-ray spectroscopy and low energy electron diffraction. We traced the evolution of Cu, Cu2O and CuO and the surface structure over time and found that the surface orientation plays a critical role in the time-dependent oxidation process. The results show that initial surface reconstruction is accompanied by island growth, coalescencing into a thin layer for longer exposure times. The composition trace reveals an initial Cu2O overlayer until eventually a thicker CuO is grown on top of it. The second study investigates the effects of a pulsed potential on the surface morphology of a Cu(100) single crystal. The stepped surface of a UHV-prepared crystal experiences large alterations in the form of cubic protrusions evolving during the pulsed potential in a certain potential window, resulting in a reconstructed surface and highly reactive structural motifs. We also identified key potentials and pulse durations and their corresponding structural changes to allow for separation of morphological from oxidation state effects in future studies. In the final project, nuclear resonant inelastic X-ray scattering (NRIXS) and X-ray absorption fine-structure spectroscopy (XAFS) measurements were employed to gain insight into the dynamic structure and surface composition of FeCu and FeAg nanoparticles under CO2RR conditions. We also extracted their correlations with the catalytic activity and selectivity. The formation of a core-shell structure during CO2RR for FeAg NP was inferred from the phonon density of states (PDOS), extracted from NRIXS data, and XAFS measurements
Optimization of nanostructured permalloy electrodes for a lateral hybrid spin-valve structure
Ferromagnetic electrodes of a lateral semiconductor-based spin-valve
structure are designed to provide a maximum of spin-polarized injection
current. A single-domain state in remanence is a prerequisite obtained by
nanostructuring Permalloy thin film electrodes. Three regimes of aspect ratios
are identified by room temperature magnetic force microscopy: (i)
high-aspect ratios of provide the favored remanent single-domain
magnetization states, (ii) medium-aspect ratios to yield
highly remanent states with closure domains and (iii) low-aspect ratios of lead to multi-domain structures. Lateral kinks, introduced to bridge the
gap between micro- and macroscale, disturb the uniform magnetization of
electrodes with high- and medium-aspect ratios. However, vertical flanks help
to maintain a uniformly magnetized state at the ferromagnet-semiconcuctor
contact by domain wall pinning.Comment: revised version, major structural changes, figures reorganized,6
pages, 8 figures, revte
Electronic transport properties through thiophenes on switchable domains
The electronic transport of electrons and holes through stacks of
,\ome ga-dicyano-,'-dibutyl- quaterthiophene (DCNDBQT)
as part of a nov el organic ferroic field-effect transistor (OFFET) is
investigated. The novel ap plication of a ferroelectric instead of a dielectric
substrate provides the poss ibility to switch bit-wise the ferroelectric
domains and to employ the polarizat ion of these domains as a gate field in an
organic semiconductor. A device conta ining very thin DCNDBQT films of around
20 nm thickness is intended to be suitab le for logical as well as optical
applications. We investigate the device proper ties with the help of a
phenomenological model called multilayer organic light-e mitting diodes
(MOLED), which was extended to transverse fields. The results sho wed, that
space charge and image charge effects play a crucial role in these org anic
devices
Atomic position localization via dual measurement
We study localization of atomic position when a three-level atom interacts
with a quantized standing-wave field in the Ramsey interferometer setup. Both
the field quadrature amplitude and the atomic internal state are measured to
obtain the atomic position information. It is found that this dual measurement
scheme produces an interference pattern superimposed on a diffraction-like
pattern in the atomic position distribution, where the former pattern
originates from the state-selective measurement and the latter from the field
measurement. The present scheme results in a better resolution in the position
localization than the field-alone measurement schemes. We also discuss the
measurement-correlated mechanical action of the standing-wave field on the atom
in the light of Popper's test.Comment: 6.5 pages and 5 figure
Exact -cosmological model coming from the request of the existence of a Noether symmetry
We present an -cosmological model with an exact analytic solution,
coming from the request of the existence of a Noether symmetry, which is able
to describe a dust-dominated decelerated phase before the current accelerated
phase of the universe.Comment: 4 pages, 2 figures, Contribution to the proceedings of Spanish
Relativity Meeting 2008, Salamanca, Sapin, 15-19 September 200
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