48 results on '"Michael Hanke"'
Search Results
2. Towards a reliable implementation of least-squares collocation for higher index differential-algebraic equations—Part 1: basics and ansatz function choices
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Roswitha März and Michael Hanke
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Collocation ,Applied Mathematics ,Numerical analysis ,Basis function ,010103 numerical & computational mathematics ,Function (mathematics) ,01 natural sciences ,010101 applied mathematics ,Overdetermined system ,Theory of computation ,Applied mathematics ,0101 mathematics ,Differential algebraic equation ,Ansatz ,Mathematics - Abstract
In the two parts of the present note we discuss several questions concerning the implementation of overdetermined least-squares collocation methods for higher index differential-algebraic equations (DAEs). Since higher index DAEs lead to ill-posed problems in natural settings, the discrete counterparts are expected to be very sensitive, which attaches particular importance to their implementation. In the present Part 1, we provide a robust selection of basis functions and collocation points to design the discrete problem. We substantiate a procedure for its numerical solution later in Part 2. Additionally, in Part 1, a number of new error estimates are proven that support some of the design decisions.
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- 2021
3. Influence of Proximity to Supporting Substrate on van der Waals Epitaxy of Atomically Thin Graphene/Hexagonal Boron Nitride Heterostructures
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Martin Heilmann, J. Marcelo J. Lopes, Alexander S. Prikhodko, Nikolai I. Borgardt, Michael Hanke, and Alexander Sabelfeld
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Materials science ,business.industry ,Graphene ,Heterojunction ,Hexagonal boron nitride ,02 engineering and technology ,Substrate (electronics) ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,chemistry.chemical_compound ,chemistry ,Transmission electron microscopy ,Boron nitride ,law ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business ,Layer (electronics) ,Molecular beam epitaxy - Abstract
Combining graphene and the insulating hexagonal boron nitride (h-BN) into two-dimensional heterostructures is promising for novel, atomically thin electronic nanodevices. A heteroepitaxial growth, in which these materials are grown on top of each other, will be crucial for their scalable device integration. However, during this so-called van der Waals epitaxy, not only the atomically thin substrate itself must be considered but also the influences from the supporting substrate below it. Here, we report not only a substantial difference between the formation of h-BN on single- (SLG) and on bi-layer epitaxial graphene (BLG) on SiC, but also vice versa, that the van der Waals epitaxy of h-BN at growth temperatures well below 1000 °C affects the varying number of graphene layers differently. Our results clearly demonstrate that the additional graphene layer in BLG enhances the distance to the corrugated, carbon-rich interface of the supporting SiC substrate and thereby diminishes its influence on the van der Waals epitaxy, leading to a homogeneous formation of a smooth, atomically thin heterostructure, which will be required for a scalable device integration of 2D heterostructures.
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- 2020
4. Least-squares collocation for higher-index linear differential-algebraic equations: Estimating the instability threshold
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Roswitha März, Caren Tischendorf, and Michael Hanke
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Algebra and Number Theory ,Collocation ,Index (economics) ,Applied Mathematics ,Computational mathematics ,010103 numerical & computational mathematics ,Computer Science::Numerical Analysis ,01 natural sciences ,Instability ,Mathematics::Numerical Analysis ,010101 applied mathematics ,Overdetermined system ,Computational Mathematics ,Initial value problem ,Applied mathematics ,Boundary value problem ,0101 mathematics ,Differential algebraic equation ,Mathematics - Abstract
Differential-algebraic equations with higher-index give rise to essentially ill-posed problems. The overdetermined least-squares collocation for differential-algebraic equations which has been prop ...
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- 2018
5. Neutron-capture elements in dwarf galaxies III. A homogenized analysis of 13 dwarf spheroidal and ultra-faint galaxies
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Michael Hanke, Camilla Juul Hansen, Á. Skúladóttir, Almudena Arcones, Eva K. Grebel, and Moritz Reichert
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MILKY-WAY SATELLITE ,Ursa Major ,Stellar mass ,dwarf [galaxies] ,Metallicity ,fundamental parameters [stars] ,FOS: Physical sciences ,Ursa Minor ,CHEMICAL EVOLUTION ,Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,01 natural sciences ,LTE LINE FORMATION ,CORE-COLLAPSE SUPERNOVAE ,0103 physical sciences ,abundances [galaxies] ,Astrophysics::Solar and Stellar Astrophysics ,GIANT BRANCH STARS ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics ,evolution [galaxies] ,Dwarf galaxy ,Physics ,EXTREMELY METAL-POOR ,010308 nuclear & particles physics ,R-PROCESS ,Astronomy and Astrophysics ,BOOTES ,Astrophysics - Astrophysics of Galaxies ,Galaxy ,abundances [stars] ,Supernova ,Space and Planetary Science ,Astrophysics of Galaxies (astro-ph.GA) ,TRANSITION-PROBABILITIES ,S-PROCESS ,catalogs ,MULTIELEMENT ABUNDANCE MEASUREMENTS - Abstract
Context. We present a large homogeneous set of stellar parameters and abundances across a broad range of metallicities, involving 13 classical dwarf spheroidal (dSph) and ultra-faint dSph (UFD) galaxies. In total, this study includes 380 stars in Fornax, Sagittarius, Sculptor, Sextans, Carina, Ursa Minor, Draco, Reticulum II, Bootes I, Ursa Major II, Leo I, Segue I, and Triangulum II. This sample represents the largest, homogeneous, high-resolution study of dSph galaxies to date. Aims. With our homogeneously derived catalog, we are able to search for similar and deviating trends across different galaxies. We investigate the mass dependence of the individual systems on the production of α-elements, but also try to shed light on the long-standing puzzle of the dominant production site of r-process elements. Methods. We used data from the Keck observatory archive and the ESO reduced archive to reanalyze stars from these 13 classical dSph and UFD galaxies. We automatized the step of obtaining stellar parameters, but ran a full spectrum synthesis (1D, local thermal equilibrium) to derive all abundances except for iron to which we applied nonlocal thermodynamic equilibrium corrections where possible. Results. The homogenized set of abundances yielded the unique possibility of deriving a relation between the onset of type Ia supernovae and the stellar mass of the galaxy. Furthermore, we derived a formula to estimate the evolution of α-elements. This reveals a universal relation of these systems across a large range in mass. Finally, we show that between stellar masses of 2.1 × 107 M⊙ and 2.9 × 105 M⊙, there is no dependence of the production of heavy r-process elements on the stellar mass of the galaxy. Conclusions. Placing all abundances consistently on the same scale is crucial to answering questions about the chemical history of galaxies. By homogeneously analyzing Ba and Eu in the 13 systems, we have traced the onset of the s-process and found it to increase with metallicity as a function of the galaxy’s stellar mass. Moreover, the r-process material correlates with the α-elements indicating some coproduction of these, which in turn would point toward rare core-collapse supernovae rather than binary neutron star mergers as a host for the r-process at low [Fe/H] in the investigated dSph systems.
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- 2020
6. Purveyors of fine halos. II. Chemodynamical association of halo stars with Milky Way globular clusters
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Ulrich Bastian, Michael Hanke, Z. Prudil, Eva K. Grebel, and Andreas Koch
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Physics ,education.field_of_study ,010308 nuclear & particles physics ,Milky Way ,Population ,FOS: Physical sciences ,Astronomy and Astrophysics ,Sagittarius Stream ,Astrophysics ,01 natural sciences ,Astrophysics - Astrophysics of Galaxies ,Galactic halo ,Stars ,Space and Planetary Science ,Globular cluster ,Astrophysics of Galaxies (astro-ph.GA) ,0103 physical sciences ,Galaxy formation and evolution ,Halo ,education ,010303 astronomy & astrophysics - Abstract
We present chemodynamical links between the present-day Milky Way halo field star population and Galactic globular clusters (GCs) using a dataset that combines information from the $\rm{\it Gaia}$ space mission and the Sloan Digital Sky Survey (SDSS-IV, DR14). Moreover, we incorporated a sample of halo giant stars with a distinct chemical signature (strong CN bandheads) that resembles the light-elements anomaly otherwise only seen in the second generation of GC stellar populations. Using three different tagging techniques, we could successfully establish unique associations between 151 extratidal stars in the neighborhood of eight GCs. In addition, we traced the possible origin of about $62\%$ of the sample of CN-strong giants to their potential host clusters. Several of the involved GCs have been brought into connection with the Gaia-Enceladus and Sequoia merger events. By establishing kinematic and chemical connections between 17 CN-strong stars and their surrounding fields, we could identify co-moving groups of stars at the same [Fe/H] with a possible cluster origin. We found strong evidence that four CN-strong stars and their associates are connected to the Sagittarius stream whilst their tightly confined [Fe/H] may hint to a birth site in M 54. Finally, we provide tentative estimates for the fraction of first-generation cluster stars among all stars lost to the halo. In the immediate cluster vicinity, this value amounts to $50.0\pm16.7\%$ while the associations in the halo field rather imply $80.2_{-5.2}^{+4.9}\%$. We speculate that -- if proven real by spectroscopic follow-up -- the disparity between these numbers could indicate a major contribution of low-mass clusters to the overall number of stars escaped to the halo or, alternatively, point toward a strong mass loss from the first generation during early cluster dissolution. [abridged], Comment: 20 pages (+9 pages of appendices), 39 figures, 4 tables, accepted for publication in A&A
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- 2020
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7. Atmospheric parameters of Cepheids from flux ratios with ATHOS: I. The temperature scale
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Bertrand Lemasle, Michael Hanke, Jesper Storm, Eva K. Grebel, and Giuseppe Bono
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Physics ,Settore FIS/05 ,010308 nuclear & particles physics ,Cepheid variable ,Metallicity ,Scale of temperature ,Flux ,FOS: Physical sciences ,Astronomy and Astrophysics ,Context (language use) ,Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Effective temperature ,01 natural sciences ,Astrophysics - Solar and Stellar Astrophysics ,Space and Planetary Science ,0103 physical sciences ,Astrophysics::Solar and Stellar Astrophysics ,Surface brightness ,Small Magellanic Cloud ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics ,Solar and Stellar Astrophysics (astro-ph.SR) - Abstract
Context: The effective temperature is a key parameter governing the properties of a star. For stellar chemistry, it has the strongest impact on the accuracy of the abundances derived. Since Cepheids are pulsating stars, determining their effective temperature is more complicated that in the case of non-variable stars. Aims: We want to provide a new temperature scale for classical Cepheids, with a high precision and full control of the systematics. Methods: Using a data-driven machine learning technique employing observed spectra, and taking great care to accurately phase single-epoch observations, we have tied flux ratios to (label) temperatures derived using the infrared surface brightness method. Results: We identified 143 flux ratios that allow us to determine the effective temperature with a precision of a few K and an accuracy better than 150 K, which is in line with the most accurate temperature measures available to date. The method does not require a normalization of the input spectra and provides homogeneous temperatures for low- and high-resolution spectra, even at the lowest signal-to-noise ratios. Due to the lack of a dataset of sufficient sample size for Small Magellanic Cloud Cepheids, the temperature scale does not extend to Cepheids with [Fe/H] < -0.6 dex but nevertheless provides an exquisite, homogeneous means of characterizing Galactic and Large Magellanic Cloud Cepheids. Conclusions: The temperature scale will be extremely useful in the context of spectroscopic surveys for Milky Way archaeology with the WEAVE and 4MOST spectrographs. It paves the way for highly accurate and precise metallicity estimates, which will allow us to assess the possible metallicity dependence of Cepheids' period-luminosity relations and, in turn, to improve our measurement of the Hubble constant H0., 16 pages, 13 figures, accepted in A&A
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- 2020
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8. A high-precision abundance analysis of the nuclear benchmark star HD 20
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Hans-Günter Ludwig, Michael Hanke, Camilla Juul Hansen, Luciano Piersanti, Sergio Cristallo, Eva K. Grebel, Andrew McWilliam, ITA, USA, and DEU
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Nuclear reaction ,Physics ,Metallicity ,FOS: Physical sciences ,Astronomy and Astrophysics ,Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Light curve ,Astrophysics - Astrophysics of Galaxies ,01 natural sciences ,Galactic halo ,Stars ,Supernova ,Astrophysics - Solar and Stellar Astrophysics ,Space and Planetary Science ,Nucleosynthesis ,Astrophysics of Galaxies (astro-ph.GA) ,0103 physical sciences ,Nuclear astrophysics ,Astrophysics::Solar and Stellar Astrophysics ,010306 general physics ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics ,Solar and Stellar Astrophysics (astro-ph.SR) - Abstract
We present our chemical abundance investigation of the metal-poor ([Fe/H]=-1.60 dex), r-process-enriched ([Eu/Fe]=0.73 dex) halo star HD 20 using novel and archival high-resolution spectra at outstanding signal-to-noise ratios. By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with non-LTE analyses of iron lines, we derive a set of highly accurate and precise stellar parameters. These allow us to delineate a chemical pattern comprised of solid detections of 48 elements, including 28 neutron-capture elements, which establishes HD 20 among the few benchmark stars that have almost complete patterns with low systematic dependencies on the stellar parameters. Our light-element (Z, Comment: 23 pages (+10 pages appendix), 27 figures, 8 tables, revised version resubmitted to A&A
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- 2020
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9. Random orthogonal matrix simulation with exact means, covariances, and multivariate skewness
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Wolfgang K. Schief, Alex Weissensteiner, Spiridon Penev, and Michael Hanke
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Multivariate statistics ,021103 operations research ,Information Systems and Management ,Multivariate analysis ,General Computer Science ,business.industry ,0211 other engineering and technologies ,Context (language use) ,02 engineering and technology ,Management Science and Operations Research ,01 natural sciences ,Measure (mathematics) ,Industrial and Manufacturing Engineering ,010104 statistics & probability ,Skewness ,Modeling and Simulation ,Statistics ,Econometrics ,Arbitrage ,Orthogonal matrix ,0101 mathematics ,business ,Risk management ,Mathematics - Abstract
We develop a simulation algorithm that generates multivariate samples with exact means, covariances, and multivariate skewness. If required for financial applications, absence of arbitrage can be ensured. Potential applications include the simulation of risk factors for the risk management of financial institutions. We use the Kollo measure of multivariate skewness, which is more informative for these applications than the Mardia skewness previously used in this context.
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- 2017
10. A reliable direct numerical treatment of differential–algebraic equations by overdetermined collocation: An operator approach
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Michael Hanke and Roswitha März
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Polynomial ,Collocation ,Conjecture ,Applied Mathematics ,010103 numerical & computational mathematics ,Computer Science::Numerical Analysis ,01 natural sciences ,Mathematics::Numerical Analysis ,010101 applied mathematics ,Overdetermined system ,Computational Mathematics ,Operator (computer programming) ,Applied mathematics ,0101 mathematics ,Differential algebraic equation ,Mathematics - Abstract
Recently reported experiments and theoretical contributions concerning overdetermined polynomial collocation applied to higher-index differential–algebraic equations give rise to the conjecture tha ...
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- 2021
11. Least-squares collocation for linear higher-index differential–algebraic equations
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Stefan Wurm, Roswitha Mrz, Caren Tischendorf, Michael Hanke, and Ewa Weinmller
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Backward differentiation formula ,Independent equation ,Applied Mathematics ,Mathematical analysis ,Numerical methods for ordinary differential equations ,010103 numerical & computational mathematics ,Exponential integrator ,01 natural sciences ,010101 applied mathematics ,Computational Mathematics ,Collocation method ,Orthogonal collocation ,0101 mathematics ,Numerical partial differential equations ,Numerical stability ,Mathematics - Abstract
Differentialalgebraic equations with higher index give rise to essentially ill-posed problems. Therefore, their numerical approximation requires special care. In the present paper, we state the notion of ill-posedness for linear differentialalgebraic equations more precisely. Based on this property, we construct a regularization procedure using a least-squares collocation approach by discretizing the pre-image space. Numerical experiments show that the resulting method has excellent convergence properties and is not much more computationally expensive than standard collocation methods used in the numerical solution of ordinary differential equations or index-1 differentialalgebraic equations. Convergence is shown for a limited class of linear higher-index differentialalgebraic equations.
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- 2017
12. Huge impact of compressive strain on phase transition temperatures in epitaxial ferroelectric K x Na 1−x NbO 3 thin films
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Pierre-Eymeric Janolin, Martin Schmidbauer, Jutta Schwarzkopf, Tobias Breuer, Michael Hanke, S. Ganschow, L. von Helden, Laura Bogula, Leibniz-Institut für Kristallzüchtung, Laboratoire Structures, Propriétés et Modélisation des solides (SPMS), Institut de Chimie du CNRS (INC)-CentraleSupélec-Centre National de la Recherche Scientifique (CNRS), Paul Drude Institute for solid state electronics, Fachbereich Physik [Marburg], and Philipps Universität Marburg
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010302 applied physics ,Phase transition ,Materials science ,Piezoelectric coefficient ,Physics and Astronomy (miscellaneous) ,Condensed matter physics ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Ferroelectricity ,Reciprocal lattice ,Piezoresponse force microscopy ,Phase (matter) ,0103 physical sciences ,[PHYS.COND.CM-MS]Physics [physics]/Condensed Matter [cond-mat]/Materials Science [cond-mat.mtrl-sci] ,Orthorhombic crystal system ,0210 nano-technology ,Monoclinic crystal system - Abstract
International audience; We present a study in which ferroelectric phase transition temperatures in epitaxial KxNa1−xNbO3 films are altered systematically by choosing different (110)-oriented rare-earth scandate substrates and by variation of the potassium to sodium ratio. Our results prove the capability to continuously shift the ferroelectric-to-ferroelectric transition from the monoclinic MC to orthorhombic c-phase by about 400 °C via the application of anisotropic compressive strain. The phase transition was investigated in detail by monitoring the temperature dependence of ferroelectric domain patterns using piezoresponse force microscopy and upon analyzing structural changes by means of high resolution X-ray diffraction including X-ray reciprocal space mapping. Moreover, the temperature evolution of the effective piezoelectric coefficient d33,f was determined using double beam laser interferometry, which exhibits a significant dependence on the particular ferroelectric phase.
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- 2019
13. Milky Way archaeology using RR Lyrae and type II Cepheids
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I. Ferraro, Noriyuki Matsunaga, Mario Nonino, Giuliana Fiorentino, Michael Hanke, R. da Silva, Brian Chaboyer, C. E. Martínez-Vázquez, Bertrand Lemasle, Z. Prudil, J. Crestani, Frédéric Thévenin, C. K. Gilligan, M. Fabrizio, H. Lala, Massimo Dall'Ora, G. Altavilla, A. J. Koch-Hansen, Giuseppe Bono, Massimo Marengo, Vittorio F. Braga, and Eva K. Grebel
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Cepheid variable ,Metallicity ,Milky Way ,FOS: Physical sciences ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,RR Lyrae variable ,01 natural sciences ,0103 physical sciences ,Astrophysics::Solar and Stellar Astrophysics ,010303 astronomy & astrophysics ,Solar and Stellar Astrophysics (astro-ph.SR) ,Astrophysics::Galaxy Astrophysics ,Physics ,Settore FIS/05 ,010308 nuclear & particles physics ,Astronomy and Astrophysics ,Horizontal branch ,Astrophysics - Astrophysics of Galaxies ,Red-giant branch ,Stars ,Astrophysics - Solar and Stellar Astrophysics ,13. Climate action ,Space and Planetary Science ,Astrophysics of Galaxies (astro-ph.GA) ,Astrophysics::Earth and Planetary Astrophysics ,Variable star - Abstract
We present a chemo-dynamical study of the Orphan stellar stream using a catalog of RR~Lyrae pulsating variable stars for which photometric, astrometric, and spectroscopic data are available. Employing low-resolution spectra from the Sloan Digital Sky Survey (SDSS), we determined line-of-sight velocities for individual exposures and derived the systemic velocities of the RR~Lyrae stars. In combination with the stars' spectroscopic metallicities and \textit{Gaia} EDR3 astrometry, we investigated the northern part of the Orphan stream. In our probabilistic approach, we found 20 single mode RR~Lyrae variables likely associated with the Orphan stream based on their positions, proper motions, and distances. The acquired sample permitted us to expand our search to nonvariable stars in the SDSS dataset, utilizing line-of-sight velocities determined by the SDSS. We found 54 additional nonvariable stars linked to the Orphan stream. The metallicity distribution for the identified red giant branch stars and blue horizontal branch stars is, on average, $-2.13\pm0.05$ dex and $-1.87\pm0.14$ dex, with dispersions of 0.23 and 0.43dex, respectively. The metallicity distribution of the RR~Lyrae variables peaks at $-1.80\pm0.06$ dex and a dispersion of 0.25dex. Using the collected stellar sample, we investigated a possible link between the ultra-faint dwarf galaxy Grus II and the Orphan stream. Based on their kinematics, we found that both the stream RR~Lyrae and Grus II are on a prograde orbit with similar orbital properties, although the large uncertainties on the dynamical properties render an unambiguous claim of connection difficult. At the same time, the chemical analysis strongly weakens the connection between both. We argue that Grus II in combination with the Orphan stream would have to exhibit a strong inverse metallicity gradient, which to date has not been detected in any Local Group system., 22 pages, 12 figures, Accepted for publication in A&A, final version
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- 2021
14. The role of substrate on the growth of 2D heterostructures by CVD
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Taís Orestes Feijó, Cláudio Radtke, Gabriel Vieira Soares, Joao Marcelo J. Lopes, Gabriela Copetti, Ester Riedner Figini Gerling, and Michael Hanke
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Materials science ,General Physics and Astronomy ,Heterojunction ,02 engineering and technology ,Surfaces and Interfaces ,General Chemistry ,Substrate (electronics) ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Epitaxy ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Amorphous solid ,symbols.namesake ,X-ray photoelectron spectroscopy ,Chemical engineering ,symbols ,Thermal stability ,0210 nano-technology ,Raman spectroscopy - Abstract
Van der Waals heterostructures have captured scientific attention due to their unique properties obtained through the stacking of 2D crystals. Large-area synthesis of such structures is crucial to implement them in future applications. In this work, we investigated the effect of the substrate on the structural properties of MoS2 grown by chemical vapor deposition on epitaxial graphene (EG)/SiC(0 0 0 1) and SiO2/Si(1 0 0). We also investigated the effect of each substrate on the thermal stability of the resulting MoS2 in face of hexagonal boron nitride (h-BN) growth by MBE on its surface. Structural characterization of samples was performed by atomic force microscopy and Raman spectroscopy. MoS2 islands grown on EG offered improved morphology in comparison to those formed on amorphous SiO2. Synchrotron-based grazing-incidence X-ray diffraction has confirmed epitaxial growth of MoS2 on EG at a relatively low temperature (of 600 °C). X-ray photoelectron spectroscopy data have revealed that MoS2/EG remains stable during h-BN growth, while the use of SiO2 as substrate led to its complete degradation. These results elucidate the effect of different substrates on the growth behavior and thermal stability of MoS2 and illustrate the great potential of EG as a template for scalable fabrication of high-quality van der Waals heterostructures.
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- 2021
15. Large-area van der Waals epitaxy and magnetic characterization of Fe3GeTe2 films on graphene
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Adriana I. Figueroa, Eugenio Zallo, Dietmar Czubak, Michael Hanke, Manfred Ramsteiner, Charles Guillemard, Sergio O. Valenzuela, Juan Rubio-Zuazo, Jesús López-Sánchez, Manuel Valvidares, J. Marcelo J. Lopes, and European Commission
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Magnetic 2D materials ,Van der waals heterostructures ,Materials science ,Van der Waals heterostructures ,FOS: Physical sciences ,02 engineering and technology ,01 natural sciences ,law.invention ,Condensed Matter::Materials Science ,law ,0103 physical sciences ,Van der waals epitaxy ,General Materials Science ,010306 general physics ,Condensed Matter - Materials Science ,Condensed matter physics ,Graphene ,Mechanical Engineering ,Van der Waals epitaxy ,Materials Science (cond-mat.mtrl-sci) ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Characterization (materials science) ,Ferromagnetism ,Mechanics of Materials ,0210 nano-technology ,Molecular beam epitaxy - Abstract
Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT) - a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM) - directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices., They also acknowledge the provision of beamtime under the project HC-4068 at the European Synchrotron Radiation Facility (ESRF), located in Grenoble (France). ICN2 researchers acknowledge support from the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 881603 (Graphene Flagship).
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- 2021
16. Temperature dependence of three-dimensional domain wall arrangement in ferroelectric K0.9Na0.1NbO3 epitaxial thin films
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Adriana Ladera, Leonard von Helden, Jutta Schwarzkopf, Jianjun Wang, Long Qing Chen, Martin Schmidbauer, Laura Bogula, Michael Hanke, and Bo Wang
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010302 applied physics ,Diffraction ,Phase transition ,Materials science ,Condensed matter physics ,General Physics and Astronomy ,02 engineering and technology ,Atmospheric temperature range ,021001 nanoscience & nanotechnology ,01 natural sciences ,Ferroelectricity ,Condensed Matter::Materials Science ,Piezoresponse force microscopy ,Domain wall (magnetism) ,0103 physical sciences ,ddc:530 ,Orthorhombic crystal system ,0210 nano-technology ,Monoclinic crystal system - Abstract
Journal of applied physics 128(18), 184101 (2020). doi:10.1063/5.0029167 special issue: "Domains and Domain Walls in Ferroic Materials", The three-dimensional arrangement and orientation of domain walls in ferroelectric K$_{0.9}$Na$_{0.1}$NbO$_{3}$/(110)NdScO$_{3}$ epitaxial thin films wereinvestigated at different temperatures both experimentally by means of piezoresponse force microscopy and three-dimensional x-ray diffractionand theoretically by three-dimensional phase-field simulations. At room temperature, a well-ordered herringbone-like domain patternappears in which there is a periodic arrangement of a$_1$a$_2$/M$_C$ monoclinic phases. Four different types of domain walls are observed, whichcan be characterized by out-of-plane tilt angles of ±45° and in-plane twist angles of ±21°. For the orthorhombic high-temperature phase, aperiodic $_1$a$_2$ stripe domain pattern with exclusive in-plane polarization is formed. Here, two different types of domain walls are observed,both of them having a fixed out-of-plane domain wall angle of 90° but distinguished by different in-plane twist angles of ±45°. The experimentalresults are fully consistent with three-dimensional phase-field simulations using anisotropic misfit strains. The qualitative agreementbetween the experiment and the theory applies, in particular, to the wide phase transition range between about 180 °C and 260 °C. In thistemperature range, a complex interplay of coexisting monoclinic $_1$a$_2$/M$_C$ and orthorhombic $_1$a$_2$ phases takes place., Published by American Inst. of Physics, Melville, NY
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- 2020
17. Monoclinic MA domains in anisotropically strained ferroelectric K0.75Na0.25NbO3 films on (110) TbScO3 grown by MOCVD
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Jan Sellmann, Albert Kwasniewski, D. Braun, Jutta Schwarzkopf, Michael Hanke, and Martin Schmidbauer
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010302 applied physics ,Materials science ,Condensed matter physics ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Ferroelectricity ,General Biochemistry, Genetics and Molecular Biology ,Polarization density ,Piezoresponse force microscopy ,0103 physical sciences ,X-ray crystallography ,Metalorganic vapour phase epitaxy ,Thin film ,0210 nano-technology ,Anisotropy ,Monoclinic crystal system - Abstract
A highly regular one-dimensional domain pattern is formed in ferroelectric K0.75Na0.25NbO3 thin films grown on (110) TbScO3 substrates using metal–organic chemical vapour deposition (MOCVD). The domain pattern exhibits a lateral periodicity of about 50 nm and extends over several micrometres. The monoclinic symmetry of the domains is controlled by the elastic anisotropy of K0.75Na0.25NbO3 and the anisotropic lattice strain, which is highly compressive in one in-plane direction and weakly tensile in the corresponding orthogonal direction. Using piezoresponse force microscopy and X-ray diffraction, the monoclinic MA phase is identified, which is associated with both a strong vertical and a lateral electric polarization component. The lateral component of the polarization vector is collinear with the ±[{\overline 1}10]pc shear direction of the pseudocubic unit cell of the film and changes periodically by 180° in adjacent domains. A structural variant of a 90° rotated MA domain pattern, where the monoclinic distortion of the pseudocubic unit cells occurs along ±[110]pc, is also observed. However, this variant appears with significantly lower probability, in agreement with energy considerations based on linear elasticity theory. Thus, the incorporation of highly anisotropic lattice strain provides the opportunity to grow one-dimensional nanostructures with high ferroelectric properties.
- Published
- 2016
18. The Orbit of the New Milky Way Globular Cluster FSR1716 = VVV-GC05
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Gergely Hajdu, Felipe Gran, Maren Hempel, Angeles Pérez-Villegas, Márcio Catelan, A. Rojas-Arriagada, Javier Alonso-García, Dante Minniti, José G. Fernández-Trincado, Michael Hanke, Edmundo Moreno Díaz, Manuela Zoccali, Rodrigo Contreras Ramos, Univers, Transport, Interfaces, Nanostructures, Atmosphère et environnement, Molécules (UMR 6213) (UTINAM), Université de Franche-Comté (UFC), Université Bourgogne Franche-Comté [COMUE] (UBFC)-Université Bourgogne Franche-Comté [COMUE] (UBFC)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS), Institute of Neuroscience and Medicine [Jülich] (INM-1), Pontificia Universidad Católica de Chile (UC), Department of Astronomy (DA PUC CHILE), Pontificia Universidad Catolica, Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université de Franche-Comté (UFC), and Université Bourgogne Franche-Comté [COMUE] (UBFC)-Université Bourgogne Franche-Comté [COMUE] (UBFC)
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Vista Variables in the Via Lactea ,Milky Way ,FOS: Physical sciences ,Variables: RR Lyrae [Stars] ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,RR Lyrae variable ,01 natural sciences ,0103 physical sciences ,Astrophysics::Solar and Stellar Astrophysics ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics ,ComputingMilieux_MISCELLANEOUS ,Physics ,individual (FSR1716 = VVV-GC-5) [Globular clusters] ,010308 nuclear & particles physics ,Astronomy and Astrophysics ,kinematics and dynamics [Galaxy] ,Horizontal branch ,Astrophysics - Astrophysics of Galaxies ,Galaxy ,Stars ,Space and Planetary Science ,Astrophysics of Galaxies (astro-ph.GA) ,Globular cluster ,Proper motions ,Astrophysics::Earth and Planetary Astrophysics ,Instability strip ,[PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph] ,disk [Galaxy] - Abstract
We use deep multi-epoch near-IR images of the VISTA Variables in the Via Lactea (VVV) Survey to measure proper motions (PMs) of stars in the Milky Way globular cluster FSR1716 = VVV-GC05. The color-magnitude diagram of this object, made using PM selected members, shows an extended horizontal branch, nine confirmed RR Lyrae members in the instability strip, and possibly several hotter stars extending to the blue. Based on the fundamental-mode (ab-type) RR Lyrae stars that move coherently with the cluster, we confirmed that FSR1716 is an Oosterhoff I globular cluster with a mean period Pab = 0.574 days. Intriguingly, we detect tidal extensions to both sides of this cluster in the spatial distribution of PM selected member stars. Also, one of the confirmed RRabs is located 11 arcmin in projection from the cluster center, suggesting that FSR1716 may be losing stars due to the gravitational interaction with the Galaxy. We also measure radial velocities (RVs) for five cluster red giants selected using the PMs. The combination of RVs and PMs allow us to compute for the first time the orbit of this globular cluster, using an updated Galactic potential. The orbit results to be confined within |Zmax | < 2.0 kpc, and has eccentricity 0.4 < e < 0.6, with perigalactic distance 1.5 < Rperi (kpc) < 2.3, and apogalactic distance 5.3 < Rapo (kpc) < 6.4. We conclude that, in agreement with its relatively low metallicity ([Fe/H]= -1.4 dex), this is an inner halo globular cluster plunging into the disk of the Galaxy. As such, this is a unique object to test the dynamical processes that contribute to the disruption of Galactic globular clusters., 6 figures, 4 tables, accepted in APJ
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- 2018
19. Ordered structure of FeGe2 formed during solid-phase epitaxy
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S Gaucher, Achim Trampert, Bernd Jenichen, Xing Huang, Holm Kirmse, Benedikt Haas, Jens Herfort, Michael Hanke, Elena Willinger, and Steven C. Erwin
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Diffraction ,Materials science ,Physics and Astronomy (miscellaneous) ,Elastic energy ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,Synchrotron ,law.invention ,Condensed Matter::Materials Science ,Crystallography ,Tetragonal crystal system ,Electron diffraction ,law ,Condensed Matter::Superconductivity ,Phase (matter) ,0103 physical sciences ,General Materials Science ,010306 general physics ,0210 nano-technology ,Molecular beam epitaxy - Abstract
${\mathrm{Fe}}_{3}\mathrm{Si}/\mathrm{Ge}(\mathrm{Fe},\mathrm{Si})/{\mathrm{Fe}}_{3}\mathrm{Si}$ thin-film stacks were grown by a combination of molecular beam epitaxy and solid-phase epitaxy (Ge on ${\mathrm{Fe}}_{3}\mathrm{Si}$). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron x-ray diffraction. The Ge(Fe,Si) films crystallize in the well-oriented, layered tetragonal structure ${\mathrm{FeGe}}_{2}$ with space group $P4mm$. This kind of structure does not exist as a bulk material and is stabilized by the solid-phase epitaxy of Ge on ${\mathrm{Fe}}_{3}\mathrm{Si}$. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.
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- 2018
20. Kinematics of outer halo globular clusters: M 75 and NGC 6426
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Michael Hanke, Nikolay Kacharov, and Andreas Koch
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Physics ,010308 nuclear & particles physics ,Metallicity ,Milky Way ,FOS: Physical sciences ,Astronomy and Astrophysics ,Orbital eccentricity ,Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,01 natural sciences ,Astrophysics - Astrophysics of Galaxies ,Luminosity ,Galactic halo ,Stars ,Space and Planetary Science ,Globular cluster ,Astrophysics of Galaxies (astro-ph.GA) ,0103 physical sciences ,Astrophysics::Solar and Stellar Astrophysics ,Halo ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics - Abstract
Globular clusters (GCs) and their dynamic interactions with the Galactic components provide an important insight into the structure and formation of the early Milky Way. Here, we present a kinematic study of two outer halo GCs based on a combination of VLT/FORS2, VLT/FLAMES, and Magellan/MIKE low- and high-resolution spectroscopy of 32 and 27 member stars, respectively. Although both clusters are located at Galactocentric distances of 15 kpc, they have otherwise very different properties. M 75 is a luminous and metal-rich system at [Fe/H] = $-1.2$ dex, a value that we confirm from the calcium triplet region. This GC shows mild evidence for rotation with an amplitude of A$_{\rm rot}\sim$5 km s$^{-1}$. One of the most metal-poor GCs in the Milky Way (at [FeII/H] = $-2.3$ dex), NGC 6426 exhibits marginal evidence of internal rotation at the 2 km s$^{-1}$ level. Both objects have velocity dispersions that are consistent with their luminosity. Although limited by small-number statistics, the resulting limits on their $A_{\rm rot}/\sigma_0$ ratios suggest that M 75 is a slow rotator driven by internal dynamics rather than being effected by the weak Galactic tides at its large distances. Here, M 75 ($A_{\rm rot}/\sigma=0.31$) is fully consistent with the properties of other, younger halo clusters. At $A_{\rm rot}/\sigma_0=0.8\pm0.4$, NGC 6426 appears to have a remarkably ordered internal motion for its low metallicity, but the large uncertainty does not allow for an unambiguous categorization as a fast rotator. An accretion origin of M 75 cannot be excluded, based on the eccentric orbit, which we derived from the recent data release 2 of Gaia, and considering its younger age., Comment: 9 pages, 9 figures, accepted for publication in Astronomy & Astrophysics
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- 2018
21. 2D or Not 2D: Strain Tuning in Weakly Coupled Heterostructures
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Felix R. L. Lange, Rui Ning Wang, Stefano Cecchi, Michael Hanke, Matthias Wuttig, Raffaella Calarco, Wang, R, Lange, F, Cecchi, S, Hanke, M, Wuttig, M, and Calarco, R
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010302 applied physics ,Materials science ,Condensed matter physics ,van der Waals epitaxy ,Superlattice ,superlattice ,Heterojunction ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Biomaterials ,Strain engineering ,molecular beam epitaxy ,0103 physical sciences ,Electrochemistry ,strain engineering ,Van der waals epitaxy ,0210 nano-technology ,phase change material ,2d strain ,Molecular beam epitaxy - Abstract
A route to realize strain engineering in weakly bonded heterostructures is presented. Such heterostructures, consisting of layered materials with a pronounced bond hierarchy of strong and weak bonds within and across their building blocks respectively, are anticipated to grow decoupled from each other. Hence, they are expected to be unsuitable for strain engineering as utilized for conventional materials which are strongly bonded isotropically. Here, it is shown for the first time that superlattices of layered chalcogenides (Sb2Te3/GeTe) behave neither as fully decoupled two-dimensional (2D) materials nor as covalently bonded three-dimensional (3D) materials. Instead, they form a novel class of 3D solids with an unparalleled atomic arrangement, featuring a distribution of lattice constants, which is tunable. A map to identify further material combinations with similar characteristic is given. It opens the way for the design of a novel class of artificial solids with unexplored properties.
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- 2018
22. Strain Engineering of Ferroelectric Domains in KxNa1−xNbO3 Epitaxial Layers
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Martin Schmidbauer, Jutta Schwarzkopf, Michael Hanke, Reinhard Uecker, and D. Braun
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Materials science ,ferroelectric domains ,Materials Science (miscellaneous) ,02 engineering and technology ,01 natural sciences ,lcsh:Technology ,Strain engineering ,0103 physical sciences ,Thin film ,Materials ,metal-organic chemical vapor deposition ,010302 applied physics ,Condensed matter physics ,lcsh:T ,epitaxial growth ,Linear elasticity ,021001 nanoscience & nanotechnology ,Polarization (waves) ,Piezoelectricity ,Ferroelectricity ,X-ray diffraction ,Crystallography ,Piezoresponse force microscopy ,thin films ,strain engineering ,ddc:620 ,KxNa1−xNbO3 ,0210 nano-technology ,piezoresponse force microscopy ,Monoclinic crystal system - Abstract
Frontiers in Materials 4, 26 (2017). doi:10.3389/fmats.2017.00026, The application of lattice strain through epitaxial growth of oxide films on lattice mismatched perovskite-like substrates strongly influences the structural properties of ferroelectric domains and their corresponding piezoelectric behavior. The formation of different ferroelectric phases can be understood by a strain-phase diagram, which is calculated within the framework of the Landau–Ginzburg–Devonshire theory. In this paper, we illustrate the opportunity of ferroelectric domain engineering in the KxNa1−xNbO3 lead-free material system. In particular, the following examples are discussed in detail: (i) Different substrates (NdGaO3, SrTiO3, DyScO3, TbScO3, and GdScO3) are used to systematically tune the incorporated epitaxial strain from compressive to tensile. This can be exploited to adjust the NaNbO3 thin film surface orientation and, concomitantly, the vector of electrical polarization, which rotates from mainly vertical to exclusive in-plane orientation. (ii) In ferroelectric NaNbO3, thin films grown on rare-earth scandate substrates, highly regular stripe domain patterns are observed. By using different film thicknesses, these can be tailored with regard to domain periodicity and vertical polarization component. (iii) A featured potassium concentration of x = 0.9 of KxNa1−xNbO3 thin films grown on (110) NdScO3 substrates favors the coexistence of two equivalent, monoclinic, but differently oriented ferroelectric phases. A complicated herringbone domain pattern is experimentally observed which consists of alternating MC and a1a2 domains. The coexistence of different types of ferroelectric domains leads to polarization discontinuities at the domain walls, potentially enabling high piezoelectric responses. In each of these examples, the experimental results are in excellent agreement with predictions based on the linear elasticity theory., Published by Frontiers Media, Lausanne
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- 2017
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23. Strain Driven Shape Evolution of Stacked (In,Ga)N Quantum Disks Embedded in GaN Nanowires
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David van Treeck, Achim Trampert, Michael Hanke, and Javier Bartolomé
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Materials science ,Fabrication ,business.industry ,Mechanical Engineering ,Nanowire ,Stacking ,Bioengineering ,Crystal growth ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Molecular physics ,Optics ,Transmission electron microscopy ,0103 physical sciences ,General Materials Science ,Light emission ,010306 general physics ,0210 nano-technology ,business ,Quantum ,Quantum well - Abstract
The fabrication of nanowires with axial multiquantum wells or disks presenting a homogeneous size and shape distribution along the whole stack is still an unresolved challenge, despite being essential for narrowing their light emission bandwidth. In this work we demonstrate that the commonly observed change in the shape of the disks along the stacking direction proceeds in a systematic, predictable way. High- resolution transmission electron microscopy of stacked (In,Ga)N quantum discs embedded in GaN nanowires with diameters of ∼40 nm and lengths of ∼700 nm and finite element method calculations show that, contrary to what is normally assumed, this change is not related to the radial growth of the nanowires, which is shown to be negligible, but to the strain relaxation of the whole active region. A simple model is proposed to account for the experimental observations. The model assumes that each disk reaches an equilibrium shape that minimizes the overall energy of the system, given by the sum of the surface and strain energies of the disk itself and the barrier below. The strain state of the barrier is affected by the presence of the disk buried directly below in a way that depends on its shape. This gives rise to a cumulative process, which makes the aspect ratio of each quantum disk to be smaller compared to the disk grown just before, in qualitative agreement with the experimental observations. The obtained results imply that strain relaxation is an important factor to bear in mind for the design of multiquantum disks with controlled shape along the stacking direction in any lattice mismatched nanowire system.
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- 2017
24. Structure and Composition of Isolated Core-Shell(In,Ga)N/GaNRods Based on Nanofocus X-Ray Diffraction and Scanning Transmission Electron Microscopy
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Achim Trampert, Michael Hanke, Zongzhe Cheng, Maik Kahnt, Christian G. Schroer, Andreas Waag, Lars Nicolai, Hergo-Heinrich Wehmann, Gerald Falkenberg, Hao Zhou, Michael Niehle, Jana Hartmann, and Thilo Krause
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010302 applied physics ,Diffraction ,Materials science ,Scattering ,Shell (structure) ,General Physics and Astronomy ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,Rod ,0103 physical sciences ,X-ray crystallography ,Scanning transmission electron microscopy ,Relaxation (physics) ,Deformation (engineering) ,0210 nano-technology - Abstract
Nanofocus x-ray diffraction is used to investigate the structure and local strain field of an isolated ðIn; GaÞN=GaN core-shell microrod. Because the high spatial resolution of the x-ray beam is only 80 × 90 nm2, we are able to investigate several distinct volumes on one individual side facet. Here, we find a drastic increase in thickness of the outer GaN shell along the rod height. Additionally, we performed highangle annular dark-field scanning-transmission-electron-microscopy measurements on several rods from the same sample showing that (In,Ga)N double-quantum-well and GaN barrier thicknesses also increase strongly along the height. Moreover, plastic relaxation is observed in the top part of the rod. Based on the experimentally obtained structural parameters, we simulate the strain-induced deformation using the finiteelement method, which serves as the input for subsequent kinematic scattering simulations. The simulations reveal a significant increase of elastic in-plane relaxation along the rod height. However, at a certain height, the occurrence of plastic relaxation yields a decrease of the elastic strain. Because of the experimentally obtained structural input for the finite-element simulations, we can exclude unknown structural influences on the strain distribution, and we are able to translate the elastic relaxation into an indium concentration which increases by a factor of 4 from the bottom to the height where plastic relaxation occurs.
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- 2017
25. A hybrid MBE-based growth method for large-area synthesis of stacked hexagonal boron nitride/graphene heterostructures
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Joseph M. Wofford, Xianjie Liu, Thilo Krause, Manfred Ramsteiner, Henning Riechert, Michael Hanke, J. Marcelo J. Lopes, and S. Nakhaie
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GRAPHENE ,Solid-state chemistry ,Materials science ,Photoemission spectroscopy ,Materialkemi ,02 engineering and technology ,Substrate (electronics) ,01 natural sciences ,Article ,law.invention ,Lattice constant ,law ,0103 physical sciences ,GRAZING INCIDENCE DIFFRACTION ,Materials Chemistry ,010306 general physics ,Multidisciplinary ,Grazing incidence diffraction ,business.industry ,Graphene ,BORON NITRIDE CRYSTALLIZATION ,Heterojunction ,021001 nanoscience & nanotechnology ,Optoelectronics ,0210 nano-technology ,business ,Molecular beam epitaxy - Abstract
Van der Waals heterostructures combining hexagonal boron nitride (h-BN) and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. In particular, the growth of h-BN on graphene has proven to be challenging due to the inertness of the graphene surface. Here we exploit a scalable molecular beam epitaxy based method to allow both the h-BN and graphene to form in a stacked heterostructure in the favorable growth environment provided by a Ni(111) substrate. This involves first saturating a Ni film on MgO(111) with C, growing h-BN on the exposed metal surface, and precipitating the C back to the h-BN/Ni interface to form graphene. The resulting laterally continuous heterostructure is composed of a top layer of few-layer thick h-BN on an intermediate few-layer thick graphene, lying on top of Ni/MgO(111). Examinations by synchrotronbased grazing incidence diffraction, X-ray photoemission spectroscopy, and UV-Raman spectroscopy reveal that while the h-BN is relaxed, the lattice constant of graphene is significantly reduced, likely due to nitrogen doping. These results illustrate a different pathway for the production of h-BN/graphene heterostructures, and open a new perspective for the large-area preparation of heterosystems combining graphene and other 2D or 3D materials. Funding Agencies|Leibniz Association; Alexander von Humboldt Foundation
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- 2017
26. Multirate method for co-simulation of electrical-chemical systems in multiscale modeling
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Jeanette Hellgren Kotaleski, Mikael Djurfeldt, Michael Hanke, Upinder S. Bhalla, and Ekaterina Brocke
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Backward differentiation formula ,Mathematical optimization ,Computer and Information Sciences ,Coupled integration ,Discretization ,Computer science ,Cognitive Neuroscience ,Models, Neurological ,Stability (learning theory) ,010103 numerical & computational mathematics ,Co-simulation ,01 natural sciences ,Article ,03 medical and health sciences ,Cellular and Molecular Neuroscience ,0302 clinical medicine ,Electricity ,Component (UML) ,Coupled system ,Electrochemistry ,Humans ,Multiscale modeling ,0101 mathematics ,Adaptive time step integration ,Neurosciences ,Multiscale simulation ,Data- och informationsvetenskap ,Sensory Systems ,Multirate integration ,Integrator ,Parallel numerical integration ,Theory of computation ,030217 neurology & neurosurgery ,Algorithms - Abstract
Multiscale modeling by means of co-simulation is a powerful tool to address many vital questions in neuroscience. It can for example be applied in the study of the process of learning and memory formation in the brain. At the same time the co-simulation technique makes it possible to take advantage of interoperability between existing tools and multi-physics models as well as distributed computing. However, the theoretical basis for multiscale modeling is not sufficiently understood. There is, for example, a need of efficient and accurate numerical methods for time integration. When time constants of model components are different by several orders of magnitude, individual dynamics and mathematical definitions of each component all together impose stability, accuracy and efficiency challenges for the time integrator. Following our numerical investigations in Brocke et al. (Frontiers in Computational Neuroscience, 10, 97, 2016), we present a new multirate algorithm that allows us to handle each component of a large system with a step size appropriate to its time scale. We take care of error estimates in a recursive manner allowing individual components to follow their discretization time course while keeping numerical error within acceptable bounds. The method is developed with an ultimate goal of minimizing the communication between the components. Thus it is especially suitable for co-simulations. Our preliminary results support our confidence that the multirate approach can be used in the class of problems we are interested in. We show that the dynamics ofa communication signal as well as an appropriate choice of the discretization order between system components may have a significant impact on the accuracy of the coupled simulation. Although, the ideas presented in the paper have only been tested on a single model, it is likely that they can be applied to other problems without loss of generality. We believe that this work may significantly contribute to the establishment of a firm theoretical basis and to the development of an efficient computational framework for multiscale modeling and simulations. Electronic Supplementary Material The online version of this article (doi:10.1007/s10827-017-0639-7) contains supplementary material, which is available to authorized users.
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- 2017
27. Scanning X-ray nanodiffraction from ferroelectric domains in strained K0.75Na0.25NbO3 epitaxial films grown on (110) TbScO3
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Michael Hanke, Albert Kwasniewski, D. Braun, Jutta Schwarzkopf, S.J. Leake, C. Feldt, L. von Helden, Martin Schmidbauer, Leibniz-Institut für Kristallzüchtung, Leibniz Inst Forschungsverbund Berlin, Paul Drude Inst Festkorperelekt, Hausvogteipl 5-7, D-10117 Berlin, Germany, and European Synchrotron Radiation Facility (ESRF)
- Subjects
[PHYS]Physics [physics] ,KxNa1-xNbO3 ,Materials science ,ferroelectric domains ,Condensed matter physics ,Linear elasticity ,Elastic energy ,02 engineering and technology ,Substrate (electronics) ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,Ferroelectricity ,General Biochemistry, Genetics and Molecular Biology ,0104 chemical sciences ,Crystallography ,Lattice constant ,Kx Na1-x NbO3 ,strained epitaxial films ,Domain (ring theory) ,Orthorhombic crystal system ,0210 nano-technology ,X-ray nanodiffraction - Abstract
Scanning X-ray nanodiffraction on a highly periodic ferroelectric domain pattern of a strained K0.75Na0.25NbO3 epitaxial layer has been performed by using a focused X-ray beam of about 100 nm probe size. A 90°-rotated domain variant which is aligned along [1{\overline 1}2]TSO has been found in addition to the predominant domain variant where the domains are aligned along the [{\overline 1}12]TSO direction of the underlying (110) TbScO3 (TSO) orthorhombic substrate. Owing to the larger elastic strain energy density, the 90°-rotated domains appear with significantly reduced probability. Furthermore, the 90°-rotated variant shows a larger vertical lattice spacing than the 0°-rotated domain variant. Calculations based on linear elasticity theory substantiate that this difference is caused by the elastic anisotropy of the K0.75Na0.25NbO3 epitaxial layer.
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- 2017
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28. Nucleation and growth of atomically thin hexagonal boron nitride on Ni/MgO(111) by molecular beam epitaxy
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Martin Heilmann, Michael Hanke, J. Marcelo J. Lopes, S. Nakhaie, and Thilo Krause
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010302 applied physics ,Materials science ,Grazing incidence diffraction ,Scanning electron microscope ,business.industry ,Wide-bandgap semiconductor ,Nucleation ,General Physics and Astronomy ,02 engineering and technology ,Dielectric ,021001 nanoscience & nanotechnology ,01 natural sciences ,Crystal ,symbols.namesake ,0103 physical sciences ,symbols ,Optoelectronics ,0210 nano-technology ,Raman spectroscopy ,business ,Molecular beam epitaxy - Abstract
Scalable fabrication of atomically thin hexagonal boron nitride (h-BN) films is highly important for the future implementation of this two-dimensional dielectric in various applications. In this contribution, we report on systematical growth experiments of few-layer thick h-BN, synthesized by molecular beam epitaxy (MBE), on crystalline Ni films deposited on MgO(111). The samples are studied using scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and synchrotron-based grazing incidence diffraction. Growth parameters for the realization of continuous h-BN films with high structural quality are presented and discussed. Additionally, our study also aims at gaining insight into the nucleation and growth behavior of h-BN on the Ni surface, which is crucial for achieving further improvement in terms of crystal quality and thickness homogeneity of h-BN layers grown not only by MBE but also by other methods.
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- 2019
29. Spontaneous formation of three-dimensionally ordered Bi-rich nanostructures within GaAs1-x Bi x /GaAs quantum wells
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Esperanza Luna, Mircea Guina, Mingjian Wu, Michael Hanke, Achim Trampert, and Janne Puustinen
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010302 applied physics ,Nanostructure ,Grazing incidence diffraction ,Materials science ,Condensed matter physics ,Spinodal decomposition ,Mechanical Engineering ,Bioengineering ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Microstructure ,01 natural sciences ,Mechanics of Materials ,Transmission electron microscopy ,Quantum dot ,0103 physical sciences ,General Materials Science ,Electrical and Electronic Engineering ,0210 nano-technology ,Quantum well ,Molecular beam epitaxy - Abstract
In this work, we report on the spontaneous formation of ordered arrays of nanometer-sized Bi-rich structures due to lateral composition modulations in Ga(As,Bi)/GaAs quantum wells grown by molecular beam epitaxy. The overall microstructure and chemical distribution is investigated using transmission electron microscopy. The information is complemented by synchrotron x-ray grazing incidence diffraction, which provides insight into the in-plane arrangement. Due to the vertical inheritance of the lateral modulation, the Bi-rich nanostructures eventually shape into a three-dimensional assembly. Whereas the Bi-rich nanostructures are created via two-dimensional phase separation at the growing surface, our results suggest that the process is assisted by Bi segregation which is demonstrated to be strong and more complex than expected, implying both lateral and vertical (surface segregation) mass transport. As demonstrated here, the inherent thermodynamic miscibility gap of Ga(As,Bi) alloys can be exploited to create highly uniform Bi-rich units embedded in a quantum confinement structure.
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- 2016
30. Thermal expansion of single-crystalline β-Ga2O3 from RT to 1200 K studied by synchrotron-based high resolution x-ray diffraction
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Zbigniew Galazka, Michael Hanke, Achim Trampert, and Zongzhe Cheng
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010302 applied physics ,Diffraction ,Materials science ,Physics and Astronomy (miscellaneous) ,Phonon ,02 engineering and technology ,Atmospheric temperature range ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,Thermal expansion ,Synchrotron ,law.invention ,law ,0103 physical sciences ,X-ray crystallography ,0210 nano-technology ,Anisotropy ,Monoclinic crystal system - Abstract
The anisotropic coefficient of thermal expansion for single-crystalline monoclinic β-Ga2O3 has been precisely measured by synchrotron-based high resolution x-ray diffraction in the temperature range from 298 to 1200 K. We derived values along the three main crystallographic directions, based on spacings for (600), (020), and ( 2 ¯ 04 ) lattice planes. αa changes non-linearly between 0.10 × 10–6 K–1 and 2.78 × 10–6 K–1 in the temperature range of 298 to 1200 K, while the values of αb and αc along the further two directions are nearly double. Within the Einstein model, we have numerically modeled the functional dependencies applying a single phonon mode.
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- 2018
31. Ferroelectric monoclinic phases in strained K0.70Na0.30NbO3 thin films promoting selective surface acoustic wave propagation
- Author
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Michael Hanke, Martin Schmidbauer, Jutta Schwarzkopf, Leonard von Helden, Roger Wördenweber, and Sijia Liang
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010302 applied physics ,Diffraction ,Materials science ,Condensed matter physics ,Mechanical Engineering ,Surface acoustic wave ,Bioengineering ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Ferroelectricity ,Surface coating ,Piezoresponse force microscopy ,Mechanics of Materials ,0103 physical sciences ,X-ray crystallography ,General Materials Science ,Electrical and Electronic Engineering ,Thin film ,0210 nano-technology ,Monoclinic crystal system - Abstract
We present a detailed analysis of the ferroelectric domain structure of K0.70Na0.30NbO3 thin films on (110) TbScO3 grown by metal-organic chemical vapor deposition. Upon piezoresponse force microscopy and nanofocus x-ray diffraction measurements we derive a domain model revealing monoclinic MC domains. The complex domain pattern is formed out of four co-existing in-plane orientations of the shearing direction of the monoclinic unit cell resulting in four types of superdomains each being composed of well-ordered stripe domains. Finally, we present surface acoustic wave (SAW) experiments that exhibit extraordinary signal intensities given the low thickness of the tested film. Moreover, the SAW propagation is found to occur selectively along the identified shearing directions.
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- 2018
32. Growth mode evolution during (100)-oriented β-Ga2O3 homoepitaxy
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Achim Trampert, Michael Hanke, Zongzhe Cheng, and Zbigniew Galazka
- Subjects
010302 applied physics ,Diffraction ,Materials science ,Reflection high-energy electron diffraction ,business.industry ,Mechanical Engineering ,Bioengineering ,Crystal growth ,02 engineering and technology ,General Chemistry ,Substrate (electronics) ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,Electron diffraction ,Mechanics of Materials ,0103 physical sciences ,Optoelectronics ,General Materials Science ,Electrical and Electronic Engineering ,0210 nano-technology ,business ,Surface reconstruction ,Molecular beam epitaxy - Abstract
This work focuses on homoepitaxial growth of β-Ga2O3 on (100)-oriented substrates during molecular beam epitaxy. It provides a comprehensive study on the growth mode by combining in situ with ex situ tools. In situ reflection high-energy electron diffraction (RHEED) indicates 2D layer-by-layer mode accompanied by (1 × 1) surface reconstruction. The homoepitaxial layers are grown pseudomorphic with the substrate without in-plane strain as probed by in-plane azimuthal RHEED and out-of-plane synchrotron-based high resolution x-ray diffraction. In contrast to the substrate, stacking faults and twin domains are present in the layer.
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- 2018
33. Hierarchy and scaling behavior of multi-rank domain patterns in ferroelectric K0.9Na0.1NbO3 strained films
- Author
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Martin Schmidbauer, Michael Hanke, D. Braun, and Jutta Schwarzkopf
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010302 applied physics ,Materials science ,Condensed matter physics ,Mechanical Engineering ,Elastic energy ,Bioengineering ,02 engineering and technology ,General Chemistry ,Dielectric ,Plasticity ,021001 nanoscience & nanotechnology ,01 natural sciences ,Ferroelectricity ,Mechanics of Materials ,0103 physical sciences ,General Materials Science ,Herringbone pattern ,Electrical and Electronic Engineering ,Thin film ,0210 nano-technology ,Anisotropy ,Monoclinic crystal system - Abstract
The formation process of a ferroelectric multi-rank domain pattern in the thickness range of 7-52 nm is investigated for monoclinic K0.9Na0.1NbO3 strained epitaxial films on (110) NdScO3 substrates. Although the elastic strain energy density is degenerated for two pseudocubic orientations, a distinctive hierarchy of domain evolution is observed with exclusive in-plane a1a2 domains for very thin films and the retarded onset of a ferroelectric MC phase at larger film thickness. This is accompanied by a thickness dependent transformation from stripe domains to a herringbone pattern and, eventually, for the thickest film, to a checkerboard-like structure. These transformations in the domain arrangement and width are correlated to energetic aspects as depolarization field and anisotropic strain relaxation in the film. While for the MC domains plastic strain relaxation is throughout observed, the a1a2 domains show a two-step strain relaxation mechanism starting with an in-plane elastic shearing, which is followed by plastic lattice relaxation. Our results highlight a pathway for engineering and patterning of periodic ferroelectric domain structures.
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- 2017
34. Phase formation and strain relaxation of Ga2O3 on c-plane and a-plane sapphire substrates as studied by synchrotron-based x-ray diffraction
- Author
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Michael Hanke, Zongzhe Cheng, Oliver Bierwagen, Patrick Vogt, and Achim Trampert
- Subjects
010302 applied physics ,Diffraction ,Materials science ,Physics and Astronomy (miscellaneous) ,Plane (geometry) ,Relaxation (NMR) ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,Synchrotron ,law.invention ,Crystallography ,law ,0103 physical sciences ,X-ray crystallography ,Sapphire ,0210 nano-technology ,Monoclinic crystal system ,Molecular beam epitaxy - Abstract
Heteroepitaxial Ga2O3 was deposited on c-plane and a-plane oriented sapphire by plasma-assisted molecular beam epitaxy and probed by ex-situ and in-situ synchrotron-based x-ray diffraction. The investigation on c-plane sapphire determined a critical thickness of around 33 A, at which the monoclinic β-phase forms on top of the hexagonal α-phase. A 143 A thick single phase α-Ga2O3 was observed on a-plane sapphire, much thicker than the α-Ga2O3 on c-plane sapphire. The α-Ga2O3 relaxed very fast in the first 30 A in both out-of-plane and in-plane directions as measured by the in-situ study.
- Published
- 2017
35. Tunable ferroelectric domain wall alignment in strained monoclinic KxNa1−xNbO3 epitaxial films
- Author
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Martin Schmidbauer, Michael Hanke, D. Braun, Jutta Schwarzkopf, and A. Kwasniewski
- Subjects
010302 applied physics ,Materials science ,Physics and Astronomy (miscellaneous) ,Condensed matter physics ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,Ferroelectricity ,Crystallography ,Domain wall (magnetism) ,0103 physical sciences ,Domain (ring theory) ,Domain engineering ,Herringbone pattern ,Thin film ,0210 nano-technology ,Monoclinic crystal system - Abstract
Epitaxial growth of ferroelectric KxNa1−xNbO3 thin films on (110) NdScO3 substrates results in the formation of domain walls which significantly differ in their arrangement from commonly observed 45° or 90° inclinations. The deviation is attributed to the monoclinic symmetry of the evolving a1a2/MC herringbone pattern and can be controlled by the epitaxial strain. In this work, tuning of the in-plane domain angle is systematically performed by the variation of the potassium content in KxNa1−xNbO3. The experimental data are in full agreement with a theoretical model. The observed behavior yields a promising pathway for domain engineering and patterning of periodic structures.
- Published
- 2017
36. Strain engineering of monoclinic domains in K x Na1−x NbO3 epitaxial layers: a pathway to enhanced piezoelectric properties
- Author
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Toni Markurt, Michael Hanke, Jutta Schwarzkopf, D. Braun, and Martin Schmidbauer
- Subjects
010302 applied physics ,Materials science ,Condensed matter physics ,Mechanical Engineering ,Bioengineering ,02 engineering and technology ,General Chemistry ,Crystal structure ,Dielectric ,021001 nanoscience & nanotechnology ,Polarization (waves) ,01 natural sciences ,Ferroelectricity ,Piezoelectricity ,Crystallography ,Strain engineering ,Mechanics of Materials ,0103 physical sciences ,General Materials Science ,Electrical and Electronic Engineering ,0210 nano-technology ,Anisotropy ,Monoclinic crystal system - Abstract
A novel concept to obtain a ferroelectric material with enhanced piezoelectric properties is proposed. This approach is based on the combination of two pathways: (i) the evolution of a ferroelectric monoclinic phase and, (ii) the coexistence of different types of ferroelectric domains leading to polarization discontinuities at the domain walls. Each of these pathways enables polarization rotation in the material which is responsible for giant piezoelectricity. Targeted incorporation of anisotropic epitaxial lattice strain is used to implement this approach. The feasibility of our concept is demonstrated for K0.9Na0.1NbO3 epitaxial layers grown on NdScO3 substrates where the coexistence of (100)pc and (001)pc pseudocubic oriented monoclinic domains is experimentally verified. This coexistence results in a complex periodic domain pattern with alternating emergence of ferroelectric in-plane a 1 a 2 and inclined M C monoclinic phases, which differ in the direction of the electrical polarization vector. Our approach opens the possibility to exploit ferroelectric properties in both vertical and lateral directions and to achieve enhanced piezoelectric properties in lead-free material caused by singularities at the domains walls.
- Published
- 2017
37. Influence of strain relaxation in axial ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1-x}{\rm{N}}/\mathrm{GaN}$ nanowire heterostructures on their electronic properties
- Author
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Oliver Marquardt, Oliver Brandt, Vladimir M. Kaganer, Michael Hanke, Thilo Krause, and Javier Martín-Sánchez
- Subjects
Materials science ,Condensed matter physics ,Mechanical Engineering ,Nanowire ,Bioengineering ,Heterojunction ,02 engineering and technology ,General Chemistry ,Electron ,021001 nanoscience & nanotechnology ,Polarization (waves) ,Electrostatics ,01 natural sciences ,Mean field theory ,Mechanics of Materials ,0103 physical sciences ,General Materials Science ,Electrical and Electronic Engineering ,010306 general physics ,0210 nano-technology ,Ground state ,Quantum well - Abstract
We present a systematic theoretical study of the influence of elastic strain relaxation on the built-in electrostatic potentials and the electronic properties of axial [Formula: see text] nanowire (NW) heterostructures. Our simulations reveal that for a sufficiently large ratio between the thickness of the [Formula: see text] disk and the diameter of the NW, the elastic relaxation leads to a significant reduction of the built-in electrostatic potential in comparison to a planar system of similar layer thickness and In content. In this case, the ground state transition energies approach constant values with increasing thickness of the disk and only depend on the In content, a behavior usually associated to that of a quantum well free of built-in electrostatic potentials. We show that the structures under consideration are by no means field-free, and the built-in potentials continue to play an important role even for ultrathin NWs. In particular, strain and the resulting polarization potentials induce complex confinement features of electrons and holes, which depend on the In content, shape, and dimensions of the heterostructure.
- Published
- 2017
38. Supernormal hardness increase of dilute Ga(As, N) thin films
- Author
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Michael Hanke, Achim Trampert, Jonas Berggren, and Esperanza Luna
- Subjects
Materials science ,Relaxation (NMR) ,General Physics and Astronomy ,chemistry.chemical_element ,Mineralogy ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,Nitrogen ,Electronegativity ,Cracking ,Solid solution strengthening ,chemistry ,0103 physical sciences ,Composite material ,Thin film ,Dislocation ,010306 general physics ,0210 nano-technology - Abstract
Hardness of epitaxial GaAs1−xNx films on GaAs(001) with different film thicknesses, varying from 80 to 700 nm, and nitrogen compositions x between zero (pure GaAs) and 0.031, were studied by means of nano-indentation. As a result, a disproportionate and monotonic increase by 17% in hardness was proved in the dilute range from GaAs to GaAs0.969N0.031. We are tracing this observation to solid solution strengthening, an extrinsic effect based on dislocation pinning due to interstitial nitrogen. On the other hand, intrinsic effects related to different electronegativities of As and N (i.e., altered bonding conditions) could be ruled out. Furthermore, in tensilely strained GaAs1−xNx layers, the appearance of cracks acts as the main strain relieving mechanism. A correlation between cracking and hardness reduction is investigated and discussed as a further relaxation pathway.
- Published
- 2017
39. Insights into the chemical composition of the metal-poor Milky Way halo globular cluster NGC 6426
- Author
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Andreas Koch, Michael Hanke, Camilla Juul Hansen, and Andrew McWilliam
- Subjects
Physics ,010308 nuclear & particles physics ,Red giant ,Milky Way ,FOS: Physical sciences ,Astronomy and Astrophysics ,Astrophysics ,Astrophysics - Astrophysics of Galaxies ,01 natural sciences ,Spectral line ,Stars ,Space and Planetary Science ,Astrophysics of Galaxies (astro-ph.GA) ,Globular cluster ,0103 physical sciences ,Cluster (physics) ,Halo ,Hypernova ,010303 astronomy & astrophysics - Abstract
We present our detailed spectroscopic analysis of the chemical composition of four red giant stars in the halo globular cluster NGC 6426. We obtained high-resolution spectra using the Magellan2/MIKE spectrograph, from which we derived equivalent widths and subsequently computed abundances of 24 species of 22 chemical elements. For the purpose of measuring equivalent widths, we developed a new semi-automated tool, called EWCODE. We report a mean Fe content of [Fe/H] = -2.34$\pm$0.05 dex (stat.) in accordance with previous studies. At a mean $\alpha$-abundance of [(Mg,Si,Ca)/3 Fe] = 0.39$\pm$0.03 dex, NGC 6426 falls on the trend drawn by the Milky Way halo and other globular clusters at comparably low metallicities. The distribution of the lighter $\alpha$-elements as well as the enhanced ratio [Zn/Fe] = 0.39 dex could originate from hypernova enrichment of the pre-cluster medium. We find tentative evidence for a spread in the elements Mg, Si, and Zn, indicating an enrichment scenario, where ejecta of evolved massive stars of a slightly older population polluted a newly born younger one. The heavy element abundances in this cluster fit well into the picture of metal-poor globular clusters, which in that respect appear to be remarkably homogeneous. The pattern of the neutron-capture elements heavier than Zn point towards an enrichment history governed by the r-process with only little -if any- sign of s-process contributions. This finding is supported by the striking similarity of our program stars to the metal-poor field star HD 108317., Comment: 14 pages, 12 figures, accepted for publication in Astronomy & Astrophysics
- Published
- 2017
40. Structural properties of Co2TiSi films on GaAs(001)
- Author
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Xiang Kong, Bernd Jenichen, Michael Hanke, Jens Herfort, Holm Kirmse, U. Jahn, Steven C. Erwin, M. T. Dau, and Achim Trampert
- Subjects
Diffraction ,Condensed Matter - Materials Science ,Materials science ,Condensed matter physics ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,General Physics and Astronomy ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,Magnetization ,Magnetic anisotropy ,Lattice constant ,Electron diffraction ,Transmission electron microscopy ,Condensed Matter::Superconductivity ,0103 physical sciences ,X-ray crystallography ,010306 general physics ,0210 nano-technology ,Molecular beam epitaxy - Abstract
Co$_{2}$TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the \textit{L$2_1$} and \textit{B}2 structures and considering the influence of non--stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13~nm these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found \textit{L$2_1$} and \textit{B}2 ordered regions, however the [Co]/[Ti]--ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic \textit{B}2--ordering with the best order parallel to the axes of the columns.
- Published
- 2016
41. Nanofocus x-ray diffraction and cathodoluminescence investigations into individual core–shell (In,Ga)N/GaN rod light-emitting diodes
- Author
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Manfred Burghammer, Martin Rosenthal, Hao Zhou, Hergo-Heinrich Wehmann, Michael Niehle, Achim Trampert, Michael Hanke, Zongzhe Cheng, Johannes Ledig, Andreas Waag, Thilo Krause, and Jana Hartmann
- Subjects
Diffraction ,Materials science ,chemistry.chemical_element ,Bioengineering ,Cathodoluminescence ,02 engineering and technology ,01 natural sciences ,law.invention ,Optics ,law ,0103 physical sciences ,General Materials Science ,Electrical and Electronic Engineering ,010302 applied physics ,Beam diameter ,business.industry ,Scattering ,Mechanical Engineering ,General Chemistry ,021001 nanoscience & nanotechnology ,Wavelength ,chemistry ,Mechanics of Materials ,X-ray crystallography ,0210 nano-technology ,business ,Indium ,Light-emitting diode - Abstract
Employing nanofocus x-ray diffraction, we investigate the local strain field induced by a five-fold (In,Ga)N multi-quantum well embedded into a GaN micro-rod in core-shell geometry. Due to an x-ray beam width of only 150 nm in diameter, we are able to distinguish between individual m-facets and to detect a significant in-plane strain gradient along the rod height. This gradient translates to a red-shift in the emitted wavelength revealed by spatially resolved cathodoluminescence measurements. We interpret the result in terms of numerically derived in-plane strain using the finite element method and subsequent kinematic scattering simulations which show that the driving parameter for this effect is an increasing indium content towards the rod tip.
- Published
- 2016
42. Strain dynamics during La2O3/Lu2O3 superlattice and alloy formation
- Author
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Peter Rodenbach, Michael Niehle, Frank Grosse, Achim Trampert, Michael Hanke, and André Proessdorf
- Subjects
010302 applied physics ,Materials science ,Condensed matter physics ,Superlattice ,General Physics and Astronomy ,02 engineering and technology ,Crystal structure ,021001 nanoscience & nanotechnology ,01 natural sciences ,Crystallographic defect ,Dark field microscopy ,Condensed Matter::Materials Science ,Crystallography ,Transmission electron microscopy ,0103 physical sciences ,Scanning transmission electron microscopy ,0210 nano-technology ,High-resolution transmission electron microscopy ,Molecular beam epitaxy - Abstract
The dynamics of strain relaxation and intermixing during molecular beam epitaxy of La2O3 and Lu2O3 superlattices and alloys consisting of both binaries on Si(111) have been studied by real-time in situ grazing incidence x-ray diffraction and high resolution transmission electron microscopy. The presence of both hexagonal and cubic polymorphs of La2O3 influences the epitaxial formation within the superlattice. The process of strain relaxation is closely related to the presence of a (La,Lu)2O3 alloy adopting a cubic symmetry. It is formed by interdiffusion of La and Lu atoms reducing internal lattice mismatch within the superlattice. An interface thickness dominated by interdiffusion regions of about 3 monolayers is determined by high-angle annular dark field scanning transmission electron microscopy.
- Published
- 2016
43. Bowing effect in elastic constants of dilute Ga(As,N) alloys
- Author
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Michael Hanke, Jonas Berggren, and Achim Trampert
- Subjects
010302 applied physics ,Materials science ,Physics and Astronomy (miscellaneous) ,Bowing ,Thermodynamics ,Modulus ,chemistry.chemical_element ,02 engineering and technology ,Nanoindentation ,021001 nanoscience & nanotechnology ,01 natural sciences ,Nitrogen ,Finite element method ,Condensed Matter::Materials Science ,Crystallography ,chemistry ,Indentation ,0103 physical sciences ,Thin film ,0210 nano-technology ,Elastic modulus - Abstract
We study the elastic properties of dilute Ga(As,N) thin films grown on GaAs(001) by means of nano-indentation and complementary dynamic finite element calculations. The experimental results of indentation modulus are compared with simulations in order to extract the cubic elastic constants cij as a function of nitrogen content of the Ga(As,N) alloys. Both, indentation modulus and elastic constants decrease with increasing nitrogen content, which proves a strong negative bowing effect in this system in contrast to Vegard's law.
- Published
- 2016
44. Counterintuitive strain distribution in axial (In,Ga)N/GaN nanowires
- Author
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Thilo Krause, Achim Trampert, Oliver Brandt, and Michael Hanke
- Subjects
010302 applied physics ,Diffraction ,Condensed Matter - Materials Science ,Materials science ,Physics and Astronomy (miscellaneous) ,Condensed matter physics ,Linear elasticity ,Nanowire ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Physics::Optics ,Infinitesimal strain theory ,02 engineering and technology ,Radius ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,Reciprocal lattice ,0103 physical sciences ,Shear stress ,Deformation (engineering) ,0210 nano-technology - Abstract
We study the three-dimensional deformation field induced by an axial (In,Ga)N segment in a GaN nanowire. Using the finite element method within the framework of linear elasticity theory, we study the dependence of the strain field on the ratio of segment length and nanowire radius. Contrary to intuition, the out-of-plane-component of the elastic strain tensor is found to assume large negative values for a length-to-radius ratio close to one. We show that this unexpected effect is a direct consequence of the deformation of the nanowire at the free sidewalls and the associated large shear strain components. Simulated reciprocal space maps of a single (In,Ga)N/GaN nanowire demonstrate that nanofocus x-ray diffraction is a suitable technique to assess this peculiar strain state experimentally.
- Published
- 2016
45. Diffraction at GaAs/Fe3Si core/shell nanowires: The formation of nanofacets
- Author
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Bernd Jenichen, Michael Hanke, Achim Trampert, Jens Herfort, Maria Hilse, and Steven C. Erwin
- Subjects
Diffraction ,Materials science ,III-V semiconductors ,Nanowire ,General Physics and Astronomy ,FOS: Physical sciences ,02 engineering and technology ,Substrate (electronics) ,Epitaxy ,01 natural sciences ,Molecular physics ,Condensed Matter::Materials Science ,0103 physical sciences ,Epitaxy Density functional theory ,010306 general physics ,Condensed Matter - Materials Science ,Nanowires ,Materials Science (cond-mat.mtrl-sci) ,021001 nanoscience & nanotechnology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,lcsh:QC1-999 ,X-ray diffraction ,Reciprocal lattice ,Nanolithography ,Transmission electron microscopy ,X-ray crystallography ,0210 nano-technology ,lcsh:Physics - Abstract
GaAs/Fe$_{3}$Si core/shell nanowire structures were fabricated by molecular-beam epitaxy on oxidized Si(111) substrates and investigated by synchrotron x-ray diffraction. The surfaces of the Fe$_3$Si shells exhibit nanofacets. These facets consist of well pronounced Fe$_3$Si{111} planes. Density functional theory reveals that the Si-terminated Fe$_3$Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the x-ray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and poly-crystalline films located between the wires. Fe$_3$Si nanofacets cause streaks in the x-ray reciprocal space map rotated by an azimuthal angle of 30{\deg} compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1$\overline{1}$0] in accordance with the x-ray results. Additional maxima in the x-ray scans indicate the onset of chemical reactions between Fe$_{3}$Si shells and GaAs cores occurring at increased growth temperatures., Comment: 15 pages, 5 figures
- Published
- 2016
46. Nucleation mechanisms of self-induced GaN nanowires grown on an amorphous interlayer
- Author
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Henning Riechert, Michael Hanke, M. Knelangen, Achim Trampert, Vincent Consonni, Lutz Geelhaar, Laboratoire des matériaux et du génie physique (LMGP ), Institut National Polytechnique de Grenoble (INPG)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), and Paul-Drude-Institut für Festkörperelektronik (PDI)
- Subjects
010302 applied physics ,Materials science ,Reflection high-energy electron diffraction ,Condensed matter physics ,Surface stress ,Nanowire ,Nucleation ,02 engineering and technology ,[CHIM.MATE]Chemical Sciences/Material chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Surface energy ,Electronic, Optical and Magnetic Materials ,Amorphous solid ,Condensed Matter::Materials Science ,0103 physical sciences ,Critical radius ,Laplace pressure ,0210 nano-technology - Abstract
The formation mechanisms of GaN nanowires grown on a Si${}_{x}$N${}_{y}$ amorphous interlayer within a self-induced approach by molecular beam epitaxy have been investigated by combining in situ reflection high-energy electron-diffraction measurements with ex situ high-resolution transmission electron microscopy imaging. It is found that GaN initially nucleates as spherical cap-shaped islands with a wetting angle of $42\ifmmode\pm\else\textpm\fi{}{7}^{\ifmmode^\circ\else\textdegree\fi{}}$. Subsequently, these islands coarsen and undergo a shape transition toward the nanowire morphology at an experimental critical radius of 5 nm. As the epitaxial constraint is very weak on an amorphous interlayer, the equivalent Laplace pressure due to the effects of surface stress has been taken into account. Analytical and finite-element method calculations show that the Laplace pressure results at the nanoscale dimensions in significant volume elastic strain in both spherical caps and nanowires. From thermodynamic considerations, it is revealed that the related strain energy density is slightly in favor of the shape transition toward the nanowire geometry owing to its higher ability to relieve the strain. Nevertheless, the anisotropy of surface energy is an even stronger driving force, since the nanowires are composed of $c$- and $m$-planes with very low surface energies. It is deduced that an energy barrier does exit for the shape transition and may be related to edge effects, resulting in a growth condition-dependent critical radius.
- Published
- 2011
47. Monodisperse (In, Ga)N insertions in catalyst-free-grown GaN(0001) nanowires
- Author
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Oliver Brandt, Michael Hanke, Achim Trampert, M. Knelangen, Lutz Schrottke, and Esperanza Luna
- Subjects
Diffraction ,Photoluminescence ,Materials science ,Dispersity ,Nanowire ,Bioengineering ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,0103 physical sciences ,General Materials Science ,Electrical and Electronic Engineering ,010302 applied physics ,business.industry ,Mechanical Engineering ,Relaxation (NMR) ,Heterojunction ,General Chemistry ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Crystallography ,Transmission electron microscopy ,Mechanics of Materials ,Optoelectronics ,business ,0210 nano-technology ,Molecular beam epitaxy - Abstract
Vertical stacks of (In, Ga)N insertions in GaN nanowires are grown by molecular beam epitaxy. The chemical composition and strain within the structure are probed by a combination of high-resolution x-ray diffraction, transmission electron microscopy, and geometrical phase analysis. The (In, Ga)N insertions are coherently strained. Finite-element simulations strongly support an effective strain relaxation mechanism of the surrounding GaN matrix due to the nanowire geometry, leading to high-quality (In, Ga)N/GaN nanowire heterostructures. An intense green photoluminescence emission is observed and attributed to an inter-well transition between the stacked (In, Ga)N insertions.
- Published
- 2011
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48. Efficient Integration of Coupled Electrical-Chemical Systems in Multiscale Neuronal Simulations
- Author
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Upinder S. Bhalla, Michael Hanke, Mikael Djurfeldt, Ekaterina Brocke, and Jeanette Hellgren Kotaleski
- Subjects
Backward differentiation formula ,Theoretical computer science ,parallel numerical integration ,Computer science ,Beräkningsmatematik ,Computation ,Neuroscience (miscellaneous) ,010103 numerical & computational mathematics ,Co-simulation ,01 natural sciences ,Field (computer science) ,coupled system ,lcsh:RC321-571 ,03 medical and health sciences ,Cellular and Molecular Neuroscience ,0302 clinical medicine ,Methods ,backward differentiation formula ,0101 mathematics ,lcsh:Neurosciences. Biological psychiatry. Neuropsychiatry ,Computational mathematics ,coupled systems ,Multiscale modeling ,multiscale modeling ,multiscale simulation ,Mathematical theory ,Computational Mathematics ,Coupling (computer programming) ,adaptive time step integration ,coupled integration ,co-simulation ,Algorithm ,030217 neurology & neurosurgery ,Neuroscience - Abstract
Multiscale modeling and simulations in neuroscience is gaining scientific attention due to its growing importance and unexplored capabilities. For instance, it can help to acquire better understanding of biological phenomena that have important features at multiple scales of time and space. This includes synaptic plasticity, memory formation and modulation, homeostasis. There are several ways to organize multiscale simulations depending on the scientific problem and the system to be modeled. One of the possibilities is to simulate different components of a multiscale system simultaneously and exchange data when required. The latter may become a challenging task for several reasons. First, the components of a multiscale system usually span different spatial and temporal scales, such that rigorous analysis of possible coupling solutions is required. Then, the components can be defined by different mathematical formalisms. For certain classes of problems a number of coupling mechanisms have been proposed and successfully used. However, a strict mathematical theory is missing in many cases. Recent work in the field has not so far investigated artifacts that may arise during coupled integration of different approximation methods. Moreover, in neuroscience, the coupling of widely used numerical fixed step size solvers may lead to unexpected inefficiency. In this paper we address the question of possible numerical artifacts that can arise during the integration of a coupled system. We develop an efficient strategy to couple the components comprising a multiscale test problem in neuroscience. We introduce an efficient coupling method based on the second-order backward differentiation formula (BDF2) numerical approximation. The method uses an adaptive step size integration with an error estimation proposed by Skelboe (2000). The method shows a significant advantage over conventional fixed step size solvers used in neuroscience for similar problems. We explore different coupling strategies that define the organization of computations between system components. We study the importance of an appropriate approximation of exchanged variables during the simulation. The analysis shows a substantial impact of these aspects on the solution accuracy in the application to our multiscale neuroscientific test problem. We believe that the ideas presented in the paper may essentially contribute to the development of a robust and efficient framework for multiscale brain modeling and simulations in neuroscience. QC 20161024
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