arXiv subjects

M. Biehl

Publications and source records attributed to M. Biehl.

At least 19 recordsLinked to original sources

arXiv2022

A machine learning based approach to gravitational lens identification with the International LOFAR Telescope

We present a novel machine learning based approach for detecting galaxy-scale gravitational lenses from interferometric data, specifically those taken with the International LOFAR Telescope (ILT), which is observing the northern radio sky at a frequency of 150 MHz, an angular resolution of 350 mas and a sensitivity of 90 uJy beam-1 (1 sigma). We develop and test several Convolutional Neural Networks to determine the probability and uncertainty of a given sample being classified as a lensed or non-lensed event. By training and testing on a simulated interferometric imaging data set that includes realistic lensed and non-lensed radio sources, we find that it is possible to recover 95.3 per cent of the lensed samples (true positive rate), with a contamination of just 0.008 per cent from non-lensed samples (false positive rate). Taking the expected lensing probability into account results in a predicted sample purity for lensed events of 92.2 per cent. We find that the network structure is most robust when the maximum image separation between the lensed images is greater than 3 times the synthesized beam size, and the lensed images have a total flux density that is equivalent to at least a 20 sigma (point-source) detection. For the ILT, this corresponds to a lens sample with Einstein radii greater than 0.5 arcsec and a radio source population with 150 MHz flux densities more than 2 mJy. By applying these criteria and our lens detection algorithm we expect to discover the vast majority of galaxy-scale gravitational lens systems contained within the LOFAR Two Metre Sky Survey.

astro-ph.GA↗

arXiv2021

DECORAS: detection and characterization of radio-astronomical sources using deep learning

We present DECORAS, a deep learning based approach to detect both point and extended sources from Very Long Baseline Interferometry (VLBI) observations. Our approach is based on an encoder-decoder neural network architecture that uses a low number of convolutional layers to provide a scalable solution for source detection. In addition, DECORAS performs source characterization in terms of the position, effective radius and peak brightness of the detected sources. We have trained and tested the network with images that are based on realistic Very Long Baseline Array (VLBA) observations at 20 cm. Also, these images have not gone through any prior de-convolution step and are directly related to the visibility data via a Fourier transform. We find that the source catalog generated by DECORAS has a better overall completeness and purity, when compared to a traditional source detection algorithm. DECORAS is complete at the 7.5$σ$ level, and has an almost factor of two improvement in reliability at 5.5$σ$. We find that DECORAS can recover the position of the detected sources to within 0.61 $\pm$ 0.69 mas, and the effective radius and peak surface brightness are recovered to within 20 per cent for 98 and 94 per cent of the sources, respectively. Overall, we find that DECORAS provides a reliable source detection and characterization solution for future wide-field VLBI surveys.

astro-ph.IM↗

arXiv2010

A LOFAR RFI detection pipeline and its first results

Radio astronomy is entering a new era with new and future radio observatories such as the Low Frequency Array and the Square Kilometer Array. We describe in detail an automated flagging pipeline and evaluate its performance. With only a fraction of the computational cost of correlation and its use of the previously introduced SumThreshold method, it is found to be both fast and unrivalled in its high accuracy. The LOFAR radio environment is analysed with the help of this pipeline. The high time and spectral resolution of LOFAR have resulted in an observatory where only a few percent of the data is lost due to RFI.

astro-ph.IM↗

arXiv2010

Post-correlation radio frequency interference classification methods

We describe and compare several post-correlation radio frequency interference classification methods. As data sizes of observations grow with new and improved telescopes, the need for completely automated, robust methods for radio frequency interference mitigation is pressing. We investigated several classification methods and find that, for the data sets we used, the most accurate among them is the SumThreshold method. This is a new method formed from a combination of existing techniques, including a new way of thresholding. This iterative method estimates the astronomical signal by carrying out a surface fit in the time-frequency plane. With a theoretical accuracy of 95% recognition and an approximately 0.1% false probability rate in simple simulated cases, the method is in practice as good as the human eye in finding RFI. In addition it is fast, robust, does not need a data model before it can be executed and works in almost all configurations with its default parameters. The method has been compared using simulated data with several other mitigation techniques, including one based upon the singular value decomposition of the time-frequency matrix, and has shown better results than the rest.

astro-ph.IM↗

arXiv2004

Interplay of Strain Relaxation and Chemically Induced Diffusion Barriers: Nanostructure Formation in 2D Alloys

We study the formation of nanostructures with alternating stripes composed of bulk-immiscible adsorbates during submonolayer heteroepitaxy. We evaluate the influence of two mechanisms considered in the literature: (i) strain relaxation by alternating arrangement of the adsorbate species, and (ii) kinetic segregation due to chemically induced diffusion barriers. A model ternary system of two adsorbates with opposite misfit relative to the substrate, and symmetric binding is investigated by off-lattice as well as lattice kinetic Monte Carlo simulations. We find that neither of the mechanisms (i) or (ii) alone can account for known experimental observations. Rather, a combination of both is needed. We present an off-lattice model which allows for a qualitative reproduction of stripe patterns as well as island ramification in agreement with recent experimental observations for CoAg/Ru(0001) [R. Q. Hwang, Phys. Rev. Lett. 76, 4757 (1996)]. The quantitative dependencies of stripe width and degree of island ramification on the misfit and interaction strength between the two adsorbate types are presented. Attempts to capture essential features in a simplified lattice gas model show that a detailed incorporation of non-local effects is required.

cond-mat.mtrl-sci↗

arXiv2003

Kinetic model of II-VI(001) semiconductor surfaces: Growth rates in atomic layer epitaxy

We present a zinc-blende lattice gas model of II-VI(001) surfaces, which is investigated by means of Kinetic Monte Carlo (KMC) simulations. Anisotropic effective interactions between surface metal atoms allow for the description of, e.g., the sublimation of CdTe(001), including the reconstruction of Cd-terminated surfaces and its dependence on the substrate temperature T. Our model also includes Te-dimerization and the potential presence of excess Te in a reservoir of weakly bound atoms at the surface. We study the self-regulation of atomic layer epitaxy (ALE) and demonstrate how the interplay of the reservoir occupation with the surface kinetics results in two different regimes: at high T the growth rate is limited to 0.5 layers per ALE cycle, whereas at low enough T each cycle adds a complete layer of CdTe. The transition between the two regimes occurs at a characteristic temperature and its dependence on external parameters is studied. Comparing the temperature dependence of the ALE growth rate in our model with experimental results for CdTe we find qualitative agreement.

cond-mat.mtrl-sci↗

arXiv2001

Flat (001) surfaces of II-VI semiconductors: A lattice gas model

We present a two-dimensional lattice gas with anisotropic interactions which model the known properties of the surface reconstructions of CdTe and ZnSe. In constrast to an earlier publication [12], the formation of anion dimers is considered. This alters the behaviour of the model considerably. We determine the phase diagram of this model by means of transfer matrix calculations and Monte Carlo simulations. We find qualitative agreement with the results of various experimental investigations.

cond-mat.stat-mech↗

arXiv2001

Modelling (001) surfaces of II-VI semiconductors

First, we present a two-dimensional lattice gas model with anisotropic interactions which explains the experimentally observed transition from a dominant c(2x2) ordering of the CdTe(001) surface to a local (2x1) arrangement of the Cd atoms as an equilibrium phase transition. Its analysis by means of transfer-matrix and Monte Carlo techniques shows that the small energy difference of the competing reconstructions determines to a large extent the nature of the different phases. Then, this lattice gas is extended to a model of a three-dimensional crystal which qualitatively reproduces many of the characteristic features of CdTe which have been observed during sublimation and atomic layer epitaxy.

cond-mat.mtrl-sci↗

arXiv2001

Kinetic Monte Carlo Simulations of dislocations in heteroepitaxial growth

We determine the critical layer thickness for the appearance of misfit dislocations as a function of the misfit between the lattice constants of the substrate and the adsorbate from Kinetic Monte Carlo (KMC) simulations of heteroepitaxial growth. To this end, an algorithm is introduced which allows the off-lattice simulation of various phenomena observed in heteroepitaxial growth including critical layer thickness for the appearance of misfit dislocations, or self-assembled island formation. The only parameters of the model are deposition flux, temperature and a pairwise interaction potential between the particles of the system. Our results are compared with a theoretical treatment of the problem and show good agreement with a simple power law.

cond-mat.mtrl-sci↗

arXiv2001

Particle currents and the distribution of terrace sizes in unstable epitaxial growth

A solid-on-solid model of epitaxial growth in 1+1 dimensions is investigated in which slope dependent upward and downward particle currents compete on the surface. The microscopic mechanisms which give rise to these currents are the smoothening incorporation of particles upon deposition and an Ehrlich-Schwoebel barrier which hinders inter-layer transport at step edges. We calculate the distribution of terrace sizes and the resulting currents on a stepped surface with a given inclination angle. The cancellation of the competing effects leads to the selection of a stable magic slope. Simulation results are in very good agreement with the theoretical findings.

cond-mat.stat-mech↗

arXiv2001

Learning multilayer perceptrons efficiently

A learning algorithm for multilayer perceptrons is presented which is based on finding the principal components of a correlation matrix computed from the example inputs and their target outputs. For large networks our procedure needs far fewer examples to achieve good generalization than traditional on-line algorithms.

cond-mat.dis-nn↗

arXiv2000

Modelling sublimation and atomic layer epitaxy in the presence of competing surface reconstructions

We present a solid-on-solid model of a binary AB compound, where atoms of type A in the topmost layer interact via anisotropic interactions different from those inside the bulk. Depending on temperature and particle flux, this model displays surface reconstructions similar to those of (001) surfaces of II-VI semiconductors. We show, that our model qualitatively reproduces mamy of the characteristic features of these materials which have been observed during sublimation and atomic layer epitaxy. We predict some previously unknown effects which might be observed experimentally.

cond-mat.stat-mech↗

arXiv2000

The influence of the crystal lattice on coarsening in unstable epitaxial growth

We report the results of computer simulations of epitaxial growth in the presence of a large Schwoebel barrier on different crystal surfaces: simple cubic(001), bcc(001), simple hexagonal(001) and hcp(001). We find, that mounds coarse by a step edge diffusion driven process, if adatoms can diffuse relatively far along step edges without being hindered by kink-edge diffusion barriers. This yields the scaling exponents alpha = 1, beta = 1/3. These exponents are independent of the symmetry of the crystal surface. The crystal lattice, however, has strong effects on the morphology of the mounds, which are by no means restricted to trivial symmetry effects: while we observe pyramidal shapes on the simple lattices, on bcc and hcp there are two fundamentally different classes of mounds, which are encompanied by characteristic diffusion currents: a metastable one with rounded corners, and an actively coarsening configuration, which breaks the symmetry given by the crystal surface.

cond-mat.stat-mech↗

arXiv2000

Learning structured data from unspecific reinforcement

We show that a straightforward extension of a simple learning model based on the Hebb rule, the previously introduced Association-Reinforcement-Hebb-Rule, can cope with "delayed", unspecific reinforcement also in the case of structured data and lead to perfect generalization.

cond-mat↗

arXiv1999

The role of step edge diffusion in epitaxial crystal growth

The role of step edge diffusion (SED) in epitaxial growth is investigated. To this end we revisit and extend a recently introduced simple cubic solid-on-solid model, which exhibits the formation and coarsening of pyramid or mound like structures. By comparing the limiting cases of absent, very fast (significant), and slow SED we demonstrate how the details of this process control both the shape of the emerging structures as well as the scaling behavior. We find a sharp transition from significant SED to intermediate values of SED, and a continuous one for vanishing SED. We argue that one should be able to control these features of the surface in experiments by variation of the flux and substrate temperature.

cond-mat.stat-mech↗

arXiv1999

Unconventional MBE Strategies from Computer Simulations for Optimized Growth Conditions

We investigate the influence of step edge diffusion (SED) and desorption on Molecular Beam Epitaxy (MBE) using kinetic Monte-Carlo simulations of the solid-on-solid (SOS) model. Based on these investigations we propose two strategies to optimize MBE growth. The strategies are applicable in different growth regimes: During layer-by-layer growth one can exploit the presence of desorption in order to achieve smooth surfaces. By additional short high flux pulses of particles one can increase the growth rate and assist layer-by-layer growth. If, however, mounds are formed (non-layer-by-layer growth) the SED can be used to control size and shape of the three-dimensional structures. By controlled reduction of the flux with time we achieve a fast coarsening together with smooth step edges.

cond-mat.stat-mech↗

arXiv1999

Weight decay induced phase transitions in multilayer neural networks

We investigate layered neural networks with differentiable activation function and student vectors without normalization constraint by means of equilibrium statistical physics. We consider the learning of perfectly realizable rules and find that the length of student vectors becomes infinite, unless a proper weight decay term is added to the energy. Then, the system undergoes a first order phase transition between states with very long student vectors and states where the lengths are comparable to those of the teacher vectors. Additionally in both configurations there is a phase transition between a specialized and an unspecialized phase. An anti-specialized phase with long student vectors exists in networks with a small number of hidden units.

cond-mat.dis-nn↗

arXiv1998

Evaporation and Step Edge Diffusion in MBE

Using kinetic Monte-Carlo simulations of a Solid-on-Solid model we investigate the influence of step edge diffusion (SED) and evaporation on Molecular Beam Epitaxy (MBE). Based on these investigations we propose two strategies to optimize MBE-growth. The strategies are applicable in different growth regimes: during layer-by-layer growth one can reduce the desorption rate using a pulsed flux. In three-dimensional (3D) growth the SED can help to grow large, smooth structures. For this purpose the flux has to be reduced with time according to a power law.

cond-mat.mtrl-sci↗