arXiv subjects

R. Meyer

Publications and source records attributed to R. Meyer.

30 records · Page 2Linked to original sources

arXiv2008

Gravitational-Wave Astronomy with Inspiral Signals of Spinning Compact-Object Binaries

Inspiral signals from binary compact objects (black holes and neutron stars) are primary targets of the ongoing searches by ground-based gravitational-wave interferometers (LIGO, Virgo, GEO-600 and TAMA-300). We present parameter-estimation simulations for inspirals of black-hole--neutron-star binaries using Markov-chain Monte-Carlo methods. For the first time, we have both estimated the parameters of a binary inspiral source with a spinning component and determined the accuracy of the parameter estimation, for simulated observations with ground-based gravitational-wave detectors. We demonstrate that we can obtain the distance, sky position, and binary orientation at a higher accuracy than previously suggested in the literature. For an observation of an inspiral with sufficient spin and two or three detectors we find an accuracy in the determination of the sky position of typically a few tens of square degrees.

astro-ph↗

arXiv2008

Ab-initio calculation of electronic stopping power along glancing swift heavy ion tracks in perovskites

In recent experiments the irradiation of insulators of perovskite type with swift heavy ions under glancing incidence has been shown to provide a unique means to generate periodically arranged nanodots at the surface. The physical origin of these patterns has been suggested to stem from a highly anisotropic electron density distribution within the bulk. In order to show the relevance of the electron density distribution of the target we present a model calculation for the system Xe$^{+23}$ $\to$ SrTiO$_{3}$ that is known to produce the aforementioned surface modifications. On the basis of the Lindhard model of electronic stopping, we employ highly-resolved \emph{ab-initio} electron density data to describe the conversion of kinetic energy into excitation energy along the ion track. The primary particle dynamics are obtained via integration of the Newtonian equations of motion that are governed by a space- and time-dependent friction force originating from Lindhard stopping. The analysis of the local electronic stopping power along the ion track reveals a pronounced periodic structure. The periodicity length strongly varies with the particular choice of the polar angle of incidence and is directly correlated to the experimentally observed formation of periodic nanodots at insulator surfaces.

cond-mat.mtrl-sci↗

arXiv2007

Electrically probing photonic bandgap phenomena in contacted defect nanocavities

We demonstrate an electrically tunable two dimensional photonic crystal nanocavity containing InAs self assembled quantum dots. Photoluminescence and electroluminescence measurements are combined to probe the cavity mode structure and demonstrate a local electrical contact to the quantum dots. Measurements performed as a function of the electric field enable us to probe the capture, relaxation and recombination dynamics of photogenerated carriers inside the quantum dots emitting into a modified photonic environment. Furthermore, the two dimensional photonic crystal is probed by spatially dependent photocurrent spectroscopy indicating a 3.5x enhancement of the local radiative lifetime of the QDs inside the photonic crystal environment.

cond-mat.other↗

arXiv2007

Highly efficient single photon emission from single quantum dots within a two-dimensional photonic bandgap

We report highly efficient single photon generation from InGaAs self-assembled quantum dots emitting within a two-dimensional photonic bandgap. A strongly suppressed multiphoton probability is obtained for single quantum dots in bulk GaAs and those emitting into the photonic bandgap. In the latter case, photoluminescence saturation spectroscopy is employed to measure a ~17 times enhancement of the average photon extraction efficiency, when compared to quantum dots in bulk GaAs. For quantum dots in the photonic crystal we measure directly an external quantum efficiency up to 26%, much higher than for quantum dots on the same sample without a tailored photonic environment. The results show that highly efficient quantum dot single photon sources can be realized, without the need for complex nanopositioning techniques.

cond-mat.mtrl-sci↗

arXiv2007

Efficient Spatial Redistribution of Quantum Dot Spontaneous Emission from 2D Photonic Crystals

We investigate the modification of the spontaneous emission dynamics and external quantum efficiency for self-assembled InGaAs quantum dots coupled to extended and localised photonic states in GaAs 2D-photonic crystals. The 2D-photonic bandgap is shown to give rise to a 5-10 times enhancement of the external quantum efficiency whilst the spontaneous emission rate is simultaneously reduced by a comparable factor. Our findings are quantitatively explained by a modal redistribution of spontaneous emission due to the modified local density of photonic states. The results suggest that quantum dots embedded within 2D-photonic crystals are suitable for practical single photon sources with high external efficiency.

cond-mat.mtrl-sci↗

arXiv2007

Report on the first round of the Mock LISA Data Challenges

The Mock LISA Data Challenges (MLDCs) have the dual purpose of fostering the development of LISA data analysis tools and capabilities, and demonstrating the technical readiness already achieved by the gravitational-wave community in distilling a rich science payoff from the LISA data output. The first round of MLDCs has just been completed: nine data sets containing simulated gravitational wave signals produced either by galactic binaries or massive black hole binaries embedded in simulated LISA instrumental noise were released in June 2006 with deadline for submission of results at the beginning of December 2006. Ten groups have participated in this first round of challenges. Here we describe the challenges, summarise the results, and provide a first critical assessment of the entries.

gr-qc↗

arXiv2005

Manipulation of the Spontaneous Emission Dynamics of Quantum Dots in 2D Photonic Crystals

We demonstrate the ability to control the spontaneous emission dynamics of self-assembled quantum dots via the local density of optical modes in 2D-photonic crystals. We show that an incomplete 2D photonic bandgap is sufficient to significantly lengthen the spontaneous emission lifetime ($>2\times$) over a wide bandwidth ($Δλ\geq40$ nm). For dots that are both \textit{spectrally} and \textit{spatially} coupled to strongly localized ($V_{mode}\sim1.5(λ/n)^{3}$), high $Q\sim2700$ optical modes, we have directly measured a strong Purcell enhanced shortening of the emission lifetime $\geq5.6\times$, limited only by our temporal resolution. Analysis of the spectral dependence of the recombination dynamics shows a maximum lifetime shortening of $19\pm4$. From the directly measured enhancement and suppression we show that the single mode coupling efficiency for quantum dots in such structures is at least $β=92%$ and is estimated to be as large as $\sim97%$.

quant-ph↗

arXiv2003

Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials

The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.

cond-mat.mtrl-sci↗

arXiv2002

Three Emission-Line Galaxies at z~2.4

We present Keck near-infrared and WIYN optical photometry of a sample of galaxies detected by near-infrared narrowband imaging in the fields of quasar metal absorption line systems at z~2.4. Wide separations (0.6-1.6/h Mpc) from the quasars indicates that they are not directly responsible for the absorption systems. From the color excess of the galaxies we derived line fluxes, star formation rates, and equivalent widths. The data are consistent with one source having an active nucleus and two sources containing regions of star formation. The blue (R-K) colors for the sources suggest relatively lesser dust content. We discuss possible projects using current wide-field infrared instruments, which can cover an order of magnitude greater area with modest allocations of telescope time.

astro-ph↗

arXiv2002

Stacking-fault energies for Ag, Cu, and Ni from empirical tight-binding potentials

The intrinsic stacking-fault energies and free energies for Ag, Cu, and Ni are derived from molecular-dynamics simulations using the empirical tight-binding potentials of Cleri and Rosato [Phys. Rev. B 48, 22 (1993)]. While the results show significant deviations from experimental data, the general trend between the elements remains correct. This allows to use the potentials for qualitative comparisons between metals with high and low stacking-fault energies. Moreover, the effect of stacking faults on the local vibrational properties near the fault is examined. It turns out that the stacking fault has the strongest effect on modes in the center of the transverse peak and its effect is localized in a region of approximately eight monolayers around the defect.

cond-mat.mtrl-sci↗

arXiv2002

Dimensional crossover and hidden incommensurability in Josephson junction arrays of periodically repeated Sierpinski gaskets

We report a study of overdamped Josephson junction arrays with the geometry of periodically repeated Sierpinski gaskets. These model superconductors share essential geometrical features with truly random (percolative) systems. When exposed to a perpendicular magnetic field B, their euclidian or fractal behavior depends on the relation between the intervortex distance (imposed by B) and the size of a constituent gasket, and was explored with high-resolution measurements of the sample magnetoinductance L(B). In terms of the frustration parameter f expressing (in units of the superconducting flux quantum) the magnetic flux threading an elementary triangular cell of a gasket, the crossover between the two regimes occurs at f_{cN}=1/(2*4^{N}), where N is the gasket order. In the fractal regime (f>f_{cN}) a sequence of equally spaced structures corresponding to the set of states with unit cells not larger than a single gasket is observed at multiples of f_{cN}, as predicted by theory. The fine structure of L(f) radically changes in the euclidian regime (f<f_{cN}), where it is determined by the commensurability of the vortex lattice with the effective potential created by the array. Anomalies observed in both the periodicity and the symmetry of L(f) are attributed to the effect of a hidden incommensurability, which arises from the deformation of the magnetic field distribution caused by the asymmetric diamagnetic response of the superconducting islands forming the arrays.

cond-mat.supr-con↗

arXiv1997

Lattice Dynamics of Martensitic Transformations Examined by Atomistic Simulations

We have performaed molecular dynamics simulations of Fe(80)Ni(20) alloys using an inter-atomic potential of the EAM-type which allows the simulation of the martensite-austenite transition. We present results, showing the development of an inhomogeneous shear system on a nanoscale during the thermally induced austenitic transition. In addition to this we obtained the phonon dispersion relations of the martensite phase by calculating the dynamical structure factor from our simulation results. On approaching the transition temperature the phonon dispersion shows anomalies which might be connected with the formation of the microstructure during the austenitic transition.

cond-mat.mtrl-sci↗