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 $m$ are identified by room temperature magnetic force microscopy: (i) high-aspect ratios of $m \ge 20$ provide the favored remanent single-domain magnetization states, (ii) medium-aspect ratios $m \sim 3$ to $m \sim 20$ yield highly remanent states with closure domains and (iii) low-aspect ratios of $m \le 3$ 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 $\alpha$,\ome ga-dicyano-$\beta$,$\beta$'-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 f(R)f(R)-cosmological model coming from the request of the existence of a Noether symmetry

We present an $f(R)$-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