Items 1-20 of 1791

    • Journal Article

      Game of Zones: how actin-binding proteins organize muscle contraction 

      Butkevich, Eugenia; Klopfenstein, Dieter R.; Schmidt, Christoph F.
      Worm 2016; 5(2): Art. e1161880
      Locomotion of C. elegans requires coordinated, efficient transmission of forces generated on the molecular scale by myosin and actin filaments in myocytes to dense bodies and the hypodermis and cuticle enveloping body wall muscles. The complex organization of the acto-myosin scaffold with its accessory proteins provides a fine-tuned machinery regulated by effectors that guarantees that sarcomere units undergo controlled, reversible cycles of contraction and relaxation. Actin filaments in sarcomeres dynamically undergo polymerization and depolymerization. In a recent study, the actin-binding protein DBN-1, the C. elegans ortholog of human drebrin and drebrin-like proteins, was discovered to stabilize actin in muscle cells. DBN-1 reversibly changes location between actin filaments and myosin-rich regions during muscle contraction. Mutations in DBN-1 result in mislocalization of other actin-binding proteins. Here we discuss implications of this finding for the regulation of sarcomere actin stability and the organization of other actin-binding proteins.
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    • Journal Article

      Three-Dimensional Spatiotemporal Pulse-Train Solitons 

      Lahav, Oren; Kfir, Ofer; Sidorenko, Pavel; Mutzafi, Maor; Fleischer, Avner; Cohen, Oren
      Physical Review X 2017; 7(4)
      Experimental realization of three-dimensional spatiotemporal solitons, which were proposed several decades ago, is still considered a “grand challenge” in nonlinear science. Here, we present experimental observation of 3D optical spatiotemporal pulse-train solitons. A spatially bright temporally dark pulse-train beam is trapped in a bulk medium that supports two types of nonlinearities: slowly responding saturable self-focusing that collectively self-trap the beam in the transverse directions and fast self-phase modulation that self-localizes each dark notch temporally (longitudinally). This work opens the possibility for experimental investigations of various soliton phenomena, including soliton interaction in 3D, formation of multimode spatiotemporal solitons, and envisioning new entities like partially coherent spatiotemporal solitons.
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    • Journal Article

      Efficient phenotypic sex classification of zebrafish using machine learning methods 

      Hosseini, Shahrbanou; Simianer, Henner; Tetens, Jens; Brenig, Bertram; Herzog, Sebastian; Sharifi, Ahmad Reza
      Ecology and Evolution p.1-12
      Sex determination in zebrafish by manual approaches according to current guidelines relies on human observation. These guidelines for sex recognition have proven to be subjective and highly labor‐intensive. To address this problem, we present a methodology to automatically classify the phenotypic sex using two machine learning methods: Deep Convolutional Neural Networks (DCNNs) based on the whole fish appearance and Support Vector Machine (SVM) based on caudal fin coloration. Machine learning techniques in sex classification provide potential efficiency with the advantage of automatization and robustness in the prediction process. Furthermore, since developmental plasticity can be influenced by environmental conditions, we have investigated the impact of elevated water temperature during embryogenesis on sex and sex‐related differences in color intensity of adult zebrafish. The estimated color intensity based on SVM was then applied to detect the association between coloration and body weight and length. Phenotypic sex classifications using machine learning methods resulted in a high degree of association with the real sex in nontreated animals. In temperature‐induced animals, DCNNs reached a performance of 100%, whereas 20% of males were misclassified using SVM due to a lower color intensity. Furthermore, a positive association between color intensity and body weight and length was observed in males. Our study demonstrates that high ambient temperature leads to a lower color intensity in male animals and a positive association of male caudal fin coloration with body weight and length, which appears to play a significant role in sexual attraction. The software developed for sex classification in this study is readily applicable to other species with sex‐linked visible phenotypic differences.
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    • Journal Article

      Effects of small‐scale dynamo and compressibility on the Λ effect 

      Käpylä, Petri J.
      Astronomische Nachrichten 2019; 340(8) p.744-751
      The Λ effect describes a rotation‐induced nondiffusive contribution to the Reynolds stress. It is commonly held responsible for maintaining the observed differential rotation of the Sun and other late‐type stars. Here, the sensitivity of the Λ effect to small‐scale magnetic fields and compressibility is studied by means of forced turbulence simulations either with anisotropic forcing in fully periodic cubes or in density‐stratified domains with isotropic forcing. Effects of small‐scale magnetic fields are studied in cases where the magnetic fields are self‐consistently generated by a small‐scale dynamo. The results show that small‐scale magnetic fields lead to a quenching of the Λ effect which is milder than in cases where also a large‐scale field is present. The effect of compressibility on the Λ effect is negligible in the range of Mach numbers from 0.015 to 0.8. Density stratification induces a marked anisotropy in the turbulence and a vertical Λ effect if the forcing scale is roughly two times larger than the density scale height.
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    • Journal Article

      Water vapor detection in the transmission spectra of HD 209458 b with the CARMENES NIR channel 

      Sánchez-López, A.; Alonso-Floriano, F. J.; López-Puertas, M.; Snellen, I. A. G.; Funke, B.; Nagel, E.; Bauer, F. F.; Amado, P. J.; Caballero, J. A.; Czesla, S.; et al.
      Nortmann, L.Pallé, E.Salz, M.Reiners, A.Ribas, I.Quirrenbach, A.Anglada-Escudé, G.Béjar, V. J. S.Casasayas-Barris, N.Galadí-Enríquez, D.Guenther, E. W.Henning, Th.Kaminski, A.Kürster, M.Lampón, M.Lara, L. M.Montes, D.Morales, J. C.Stangret, M.Tal-Or, L.Sanz-Forcada, J.Schmitt, J. H. M. M.Zapatero Osorio, M. R.Zechmeister, M.
      Astronomy & Astrophysics 2019; 630: Art. A53
      Aims. We aim at detecting water vapor in the atmosphere of the hot Jupiter HD 209458 b and perform a multi-band study in the near infrared with CARMENES. Methods. The water vapor absorption lines from the atmosphere of the planet are Doppler-shifted due to the large change in its radial velocity during transit. This shift is of the order of tens of km s−1, whilst the Earth’s telluric and the stellar lines can be considered quasi-static. We took advantage of this shift to remove the telluric and stellar lines using SYSREM, which performs a principal component analysis including proper error propagation. The residual spectra contain the signal from thousands of planetary molecular lines well below the noise level. We retrieve the information from those lines by cross-correlating the residual spectra with models of the atmospheric absorption of the planet. Results. We find a cross-correlation signal with a signal-to-noise ratio (S/N) of 6.4, revealing H2O in HD 209458 b. We obtain a net blueshift of the signal of –5.2 −1.3+2.6 km s−1 that, despite the large error bars, is a firm indication of day- to night-side winds at the terminator of this hot Jupiter. Additionally, we performed a multi-band study for the detection of H2O individually from the three near infrared bands covered by CARMENES. We detect H2O from its 0.96–1.06 μm band with a S/N of 5.8, and also find hints of a detection from the 1.06–1.26 μm band, with a low S/N of 2.8. No clear planetary signal is found from the 1.26–1.62 μm band. Conclusions. Our significant H2O signal at 0.96–1.06 μm in HD 209458 b represents the first detection of H2O from this band individually, the bluest one to date. The unfavorable observational conditions might be the reason for the inconclusive detection from the stronger 1.15 and 1.4 μm bands. H2O is detected from the 0.96–1.06 μm band in HD 209458 b, but hardly in HD 189733 b, which supports a stronger aerosol extinction in the latter, in line with previous studies. Future data gathered at more stable conditions and with larger S/N at both optical and near-infrared wavelengths could help to characterize the presence of aerosols in HD 209458 b and other planets.
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    • Journal Article

      Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets 

      Marsh, David J. E.; Fong, Kin Chung; Lentz, Erik W.; Šmejkal, Libor; Ali, Mazhar N.
      Physical Review Letters 2019; 123(12): Art. 121601
      Antiferromagnetically doped topological insulators (ATI) are among the candidates to host dynamical axion fields and axion polaritons, weakly interacting quasiparticles that are analogous to the dark axion, a long sought after candidate dark matter particle. Here we demonstrate that using the axion quasiparticle antiferromagnetic resonance in ATIs in conjunction with low-noise methods of detecting THz photons presents a viable route to detect axion dark matter with a mass of 0.7 to 3.5 meV, a range currently inaccessible to other dark matter detection experiments and proposals. The benefits of this method at high frequency are the tunability of the resonance with applied magnetic field, and the use of ATI samples with volumes much larger than 1  mm^{3}.
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    • Journal Article

      Understanding stellar activity-induced radial velocity jitter using simultaneous K2 photometry and HARPS RV measurements 

      Oshagh, M.; Santos, N. C.; Figueira, P.; Barros, S. C. C.; Donati, J.-F.; Adibekyan, V.; Faria, J. P.; Watson, C. A.; Cegla, H. M.; Dumusque, X.; et al.
      Hébrard, E.Demangeon, O.Dreizler, S.Boisse, I.Deleuil, M.Bonfils, X.Pepe, F.Udry, S.
      Astronomy & Astrophysics 2017; 606: Art. A107
      One of the best ways to improve our understanding of the stellar activity-induced signal in radial velocity (RV) measurements is through simultaneous high-precision photometric and RV observations. This is of prime importance to mitigate the RV signal induced by stellar activity and therefore unveil the presence of low-mass exoplanets. The K2 Campaign 7 and 8 fields of view were located in the southern hemisphere, and provided a unique opportunity to gather unprecedented simultaneous high-precision photometric observation with K2 and high-precision RV measurements with the HARPS spectrograph to study the relationship between photometric variability and RV jitter. We observed nine stars with different levels of activity, from quiet to very active. We first probed the presence of any meaningful relation between measured RV jitter and the simultaneous photometric variation, and also other activity indicators (such as BIS, FWHM, log R′HK, and F8) by evaluating the strength and significance of the monotonic correlation between RVs and each indicator. We found that for the case of very active stars, strong and significant correlations exist between almost all the observables and measured RVs; however, when we move towards lower activity levels the correlations become random, and we could not reach any conclusion regarding the tendency of correlations depending on the stellar activity level. Except for the F8 whose strong correlation with RV jitter persists over a wide range of stellar activity level, and thus our result suggests that F8 might be a powerful proxy for activity-induced RV jitter over a wide range of stellar activity. Moreover, we examine the capability of two state-of-the-art modeling techniques, namely the FF′ method and SOAP2.0, to accurately predict the RV jitter amplitude using the simultaneous photometric observation. We found that for the very active stars both techniques can predict the amplitude of the RV jitter reasonably well; however, at lower activity levels the FF′ method underpredicts the RV jitter amplitude.
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    • Journal Article

      Recovery of subsurface profiles of supergranular flows via iterative inversion of synthetic travel times 

      Bhattacharya, Jishnu; Hanasoge, Shravan M.; Birch, Aaron C.; Gizon, Laurent
      Astronomy & Astrophysics 2017; 607: Art. A129
      Aims. We develop a helioseismic inversion algorithm that can be used to recover subsurface vertical profiles of two-dimensional supergranular flows from surface measurements of synthetic wave travel times. Methods. We carried out seismic wave-propagation simulations with a two-dimensional section of a flow profile that resembles an average supergranule and a starting model that only has flows at the surface. We assumed that the wave measurements are entirely without realization noise for the purpose of our test. We expanded the vertical profile of the supergranule stream function on a basis of B-splines. We iteratively updated the B-spline coefficients of the supergranule model to reduce the travel-time differences observed between the two simulations. We performed the exercise for four different vertical profiles peaking at different depths below the solar surface. Results. We are able to accurately recover depth profiles of four supergranule models at depths up to 8−10 Mm below the solar surface using f−p4 modes under the assumption that there is no realization noise. We are able to obtain the peak depth and the depth of the return flow for each model. Conclusions. A basis-resolved inversion performs significantly better than an inversion in which the flow field is inverted at each point in the radial grid. This is an encouraging result and might act as a guide in developing more realistic inversion strategies that can be applied to supergranular flows in the Sun.
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    • Journal Article

      Measuring solar active region inflows with local correlation tracking of granulation 

      Löptien, B.; Birch, A. C.; Duvall, T. L.; Gizon, L.; Proxauf, B.; Schou, J.
      Astronomy & Astrophysics 2017; 606: Art. A28
      Context. Sixteen years ago local helioseismology detected spatially extended converging surface flows into solar active regions. These flows play an important role in flux-transport models of the solar dynamo. Aims. We aim to validate the existence of the inflows by deriving horizontal flow velocities around active regions with local correlation tracking of granulation. Methods. We generate a six-year time series of full-disk maps of the horizontal velocity at the solar surface by tracking granules in continuum intensity images provided by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Results. On average, active regions are surrounded by inflows extending up to 10° from the center of the active region of magnitudes of 20–30 m/s, reaching locally up to 40 m/s, which is in agreement with results from local helioseismology. By computing an ensemble average consisting of 243 individual active regions, we show that the inflows are not azimuthally symmetric, but converge predominantly towards the trailing polarity of the active region with respect to the longitudinally and temporally averaged flow field.
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    • Journal Article

      Three planets around HD 27894 

      Trifonov, T.; Kürster, M.; Zechmeister, M.; Zakhozhay, O. V.; Reffert, S.; Lee, M. H.; Rodler, F.; Vogt, S. S.; Brems, S. S.
      Astronomy & Astrophysics 2017; 602: Art. L8
      Aims. Our new program with HARPS aims to detect mean motion resonant planetary systems around stars which were previously reported to have a single bona fide planet, often based only on sparse radial velocity data. Methods. Archival and new HARPS radial velocities for the K2V star HD 27894 were combined and fitted with a three-planet self-consistent dynamical model. The best-fit orbit was tested for long-term stability. Results. We find clear evidence that HD 27894 is hosting at least three massive planets. In addition to the already known Jovian planet with a period Pb≈ 18 days we discover a Saturn-mass planet with Pc≈ 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet (≈5.3 MJup) with a period Pd ≈ 5170 days on a moderately eccentric orbit (ed = 0.39). Conclusions. HD 27894 is hosting a massive, eccentric giant planet orbiting around a tightly packed inner pair of massive planets likely involved in an asymmetric 2:1 mean motion resonance. HD 27894 may be an important milestone for probing planetary formation and evolution scenarios.
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    • Journal Article

      Turbulent gas accretion between supermassive black-holes and star-forming rings in the circumnuclear disk 

      Chamani, Wara; Dörschner, Stephanie; Schleicher, Dominik R. G.
      Astronomy & Astrophysics 2017; 602: Art. A84
      While supermassive black-holes are known to co-evolve with their host galaxy, the precise nature and origin of this co-evolution is not clear. In this paper we explore the possible connection between star formation and black-hole growth in the circumnuclear disk (CND) to probe this connection in the vicinity close to the black-hole. We adopt here a circumnuclear disk model developed by previous works and explore both the dependence on the star formation recipe as well as the role of the gravitational field, which can be dominated by the central black-hole, the CND itself or the host galaxy. A specific emphasis is put on the turbulence regulated star formation model to explore the impact of a realistic star formation recipe. It is shown that this model helps to introduce realistic fluctuations in the black-hole and star formation rate, without overestimating them. Consistent with previous works, we show that the final black-hole masses are rather insensitive to the masses of the initial seeds, even for seed masses of up to 106M⊙. In addition, we apply our model to the formation of high-redshift quasars, as well as to the nearby system NGC 6951, where a tentative comparison is made in spite of the presence of a bar in the galaxy. We show that our model can reproduce the high black-hole masses of the high-redshift quasars within a sufficiently short time, provided a high mass supply rate from the host galaxy. In addition, it reproduces several of the properties observed in NGC 6951. With respect to the latter system, our analysis suggests that supernovae (SN) feedback may be important in creating the observed fluctuations in the star formation history as a result of negative feedback effects.
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    • Journal Article

      Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters (Corrigendum) 

      Moehler, S.; Dreizler, S.; LeBlanc, F.; Khalack, V.; Michaud, G.; Richer, J.; Sweigart, A. V.; Grundahl, F.
      Astronomy & Astrophysics 2017; 605: Art. C4
      We found that the script to determine the masses of the stars contains two errors. This script and a related one have been used to determine masses of globular cluster stars and distances to field stars in 12 papers published between 1990 and 2014. While the numerical values need to be revised none of the conclusions are affected. We provide the updated numerical values and figures for all 12 publications here. In addition we describe the effects on those refereed publications that used the distances to the field stars.
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    • Journal Article

      Remote sensing of exoplanetary atmospheres with ground-based high-resolution near-infrared spectroscopy 

      Shulyak, D.; Rengel, M.; Reiners, A.; Seemann, U.; Yan, F.
      Astronomy & Astrophysics 2019; 629: Art. A109
      Context. Thanks to the advances in modern instrumentation we have learned about many exoplanets that span a wide range of masses and composition. Studying their atmospheres provides insight into planetary origin, evolution, dynamics, and habitability. Present and future observing facilities will address these important topics in great detail by using more precise observations, high-resolution spectroscopy, and improved analysis methods. Aims. We investigate the feasibility of retrieving the vertical temperature distribution and molecular number densities from expected exoplanet spectra in the near-infrared. We use the test case of the CRIRES+ instrument at the Very Large Telescope which will operate in the near-infrared between 1 and 5 μm and resolving powers of R = 100 000 and R = 50 000. We also determine the optimal wavelength coverage and observational strategies for increasing accuracy in the retrievals. Methods. We used the optimal estimation approach to retrieve the atmospheric parameters from the simulated emission observations of the hot Jupiter HD 189733b. The radiative transfer forward model is calculated using a public version of the τ-REx software package. Results. Our simulations show that we can retrieve accurate temperature distribution in a very wide range of atmospheric pressures between 1 bar and 10−6 bar depending on the chosen spectral region. Retrieving molecular mixing ratios is very challenging, but a simultaneous observations in two separate infrared regions around 1.6 and 2.3 μm helps to obtain accurate estimates; the exoplanetary spectra must be of relatively high signal-to-noise ratio S∕N ≥ 10, while the temperature can already be derived accurately with the lowest value that we considered in this study (S∕N = 5). Conclusions. The results of our study suggest that high-resolution near-infrared spectroscopy is a powerful tool for studying exoplanet atmospheres because numerous lines of different molecules can be analyzed simultaneously. Instruments similar to CRIRES+ will provide data for detailed retrieval and will provide new important constraints on the atmospheric chemistry and physics.
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    • Journal Article

      Signature of solar g modes in first-order p-mode frequency shifts 

      Böning, Vincent G. A.; Hu, Huanchen; Gizon, Laurent
      Astronomy & Astrophysics 2019; 629: Art. A26
      Context. Solar gravity modes (g modes) are buoyancy waves that are trapped in the solar radiative zone and have been very difficult to detect at the surface. Solar g modes would complement solar pressure modes (p modes) in probing the central regions of the Sun, for example the rotation rate of the core. Aims. A detection of g modes using changes in the large frequency separation of p modes has recently been reported. However, it is unclear how p and g modes interact. The aim of this study is to evaluate to what extent g modes can perturb the frequencies of p modes. Methods. We computed the first-order perturbation to global p-mode frequencies due to a flow field and perturbations to solar structure (e.g. density and sound speed) caused by a g mode. We focused on long-period g modes and assumed that the g-mode perturbations are constant in time. The surface amplitude of g modes is assumed to be 1 mm s−1, which is close to the observational limit set by Doppler observations. Results. Gravity modes do perturb p-mode frequencies to first order if the harmonic degree of the g mode is even and if its azimuthal order is zero. The effect is extremely small. For dipole and quadrupole p modes, all frequency shifts are smaller than 0.1 nHz, or 2 × 10−8 in relative numbers. This is because the relative perturbation to solar structure quantities caused by a g mode of realistic amplitude is of the order of 10−6–10−5. Additionally, we find that structural changes dominate over advection. Surprisingly, the interaction of g and p modes takes place to a large part near the surface, where p modes spend most of their propagation times and g modes generate the largest relative changes to solar structure. This is due to the steep density stratification, which compensates the evanescent behaviour of g modes in the convection zone. Conclusions. It appears to be impossible to detect g modes solely through their signature in p-mode frequency shifts. Whether g modes leave a detectable signature in p-mode travel times under a given observational setup remains an open question.
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    • Journal Article

      Library of actions: Implementing a generic robot execution framework by using manipulation action semantics 

      Aein, Mohamad Javad; Aksoy, Eren Erdal; Wörgötter, Florentin
      The International Journal of Robotics Research 2019; 38(8) p.910-934
      When a robot has to imitate an observed action sequence, it must first understand the inherent characteristic features of the individual actions. Such features need to reflect the semantics of the action with a high degree of invariance between different demonstrations of the same action. At the same time the machine needs to be able to execute the action sequence in any appropriate situation. In this study, we introduce a new library of actions, which is a generic framework for executing manipulation actions on robotic systems by combining features that capture action semantics with a framework for execution. We focus on manipulation actions and first create a generic representation consisting of symbolic and subsymbolic components. To link these two domains we introduce a finite state machine allowing for sequential execution with error handling. The framework is developed from observing humans which provides us with a high degree of grounding. To quantitatively evaluate the scalability of the proposed approach, we conducted a large set of experiments involving different actions performed either individually or sequentially with various types of objects in different scene contexts.
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    • Journal Article

      Focus characterization of the NanoMAX Kirkpatrick–Baez mirror system 

      Osterhoff, Markus; Robisch, Anna-Lena; Soltau, Jakob; Eckermann, Marina; Kalbfleisch, Sebastian; Carbone, Dina; Johansson, Ulf; Salditt, Tim
      Journal of Synchrotron Radiation 2019; 26(4) p.1173-1180
      The focusing and coherence properties of the NanoMAX Kirkpatrick-Baez mirror system at the fourth-generation MAX IV synchrotron in Lund have been characterized. The direct measurement of nano-focused X-ray beams is possible by scanning of an X-ray waveguide, serving basically as an ultra-thin slit. In quasi-coherent operation, beam sizes of down to 56 nm (FWHM, horizontal direction) can be achieved. Comparing measured Airy-like fringe patterns with simulations, the degree of coherence |μ| has been quantified as a function of the secondary source aperture (SSA); the coherence is larger than 50% for SSA sizes below 11 µm at hard X-ray energies of 14 keV. For an SSA size of 5 µm, the degree of coherence has been determined to be 87%.
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    • Journal Article

      Average surface flows before the formation of solar active regions and their relationship to the supergranulation pattern 

      Birch, A. C.; Schunker, H.; Braun, D. C.; Gizon, L.
      Astronomy & Astrophysics 2019; 628: Art. A37
      Context. The emergence of solar active regions is an important but poorly understood aspect of the solar dynamo. Aims. Knowledge of the flows associated with the rise of active-region-forming magnetic concentrations through the near-surface layers will help determine the mechanisms of active region formation. Methods. We used helioseismic holography and granulation tracking to measure the horizontal flows at the surface that precede the emergence of active regions. We then averaged these flows over about sixty emerging active regions to reduce the noise, selecting active regions that emerge into relatively quiet Sun. To help interpret the results, we constructed a simple model flow field by generating synthetic “emergence locations” that are probabilistically related to the locations of supergranulation-scale convergence regions in the quiet Sun. Results. The flow maps obtained from helioseismology and granulation tracking are very similar (correlation coe cients for single maps around 0.96). We find that active region emergence is, on average, preceded by converging horizontal flows of amplitude about 40ms􀀀1. The convergence region extends over about 40Mm in the east-west direction and about 20Mm in the north-south direction and is centered in the retrograde direction relative to the emergence location. This flow pattern is largely reproduced by a model in which active region emergence occurs preferentially in the prograde direction relative to supergranulation inflows. Conclusions. Averaging over many active regions reveals a statistically significant pattern of near-surface flows prior to emergence. The qualitative success of our simple model suggests that rising flux concentrations and supergranule-scale flows interact during the emergence process.
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    • Journal Article

      A posteriori synchronization of scanning transmission electron microscopy signals with kilopixel per second acquisition rates 

      Meyer, Tobias; Lopatin, Sergei; Seibt, Michael; Roddatis, Vladimir
      Ultramicroscopy 2019; 200 p.62-66
      The stability and sensitivity of scanning transmission electron microscopes as well as detectors collecting e.g. electrons which su ered di erent scattering processes, or sec- ondary radiation, have increased tremendously during the last decade. In order to fully exploit capabilities of simultaneously recording various signals with up to 1000 px/s acquisition rates the central issue is their synchronization. The latter is frequently a non-trivial problem without commercially available solution especially if detectors of di erent manufacturers are involved. In this paper, we present a simple scanning pat- tern enabling a posteriori synchronization of arbitrarily many signals being recorded entirely independently. We apply the approach to the simultaneous atomic-scale ac- quisition of signals from an annular dark- eld detector and electron energy loss as well as energy-dispersive x-ray spectrometers. Errors emerging in scanning direction due to the independence of the respective processes are quanti ed and found to have a standard deviation of roughly half the pixel spacing. Since there are no intermedi- ate waiting periods to maintain synchronicity, the proposed acquisition process is, in fact, demonstrated to be 12% faster than a commercial hardware-synchronized solu- tion for identical sub-millisecond signal integration times and hence follows the trend in electron microscopy to extract more information per irradiating electron.
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    • Journal Article

      High-resolution Scanning Transmission EBIC Analysis of Misfit Dislocations at Perovskite pn-Heterojunctions 

      Meyer, T.; Kressdorf, B.; Lindner, J.; Peretzki, P.; Roddatis, V.; Jooss, C.; Seibt, M.
      Journal of Physics: Conference Series 2019; 1190: Art. 012009
      Fundamental losses of photovoltaic energy conversion are transmission of sub band gap photons and thermalisation which are the underlying physics of the Shockley-Queisser limit defining maximum conversion efficiency of single-junction solar cells. Strongly correlated materials such as perovskites are promising candidates to exceed this limit by exploiting (i) long wavelength light absorption and (ii) the existence of long-living intraband excitations indicating that harvesting hot excess carriers might be feasible in such systems. In this work, we study pn-heterojunctions produced from Pr1-xCaxMnO3 on SrTi1-yNbyO3 by means of microscopic techniques. Such systems exhibit relevant quantities such as space charge layer width, screening lengths and excess carrier diffusion lengths in the 1-10 nm range which makes the use of standard methods such as electron beam induced current a challenging task. We report scanning transmission electron beam induced current experiments of misfit dislocations at the heterojunction. The dislocation-induced reduction of the charge collection is studied with nanometer spatial resolution. Effects of surface recombination and the heterojunction electric field are discussed.
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    • Journal Article

      Branched flow and caustics in nonlinear waves 

      Green, Gerrit; Fleischmann, Ragnar
      New Journal of Physics 2019; 21(8): Art. 083020
      Rogue waves, i.e.high amplitude fluctuations in random wave fields, have been studied in several contexts, ranging from optics via acoustics to the propagation of ocean waves. Scattering by disorder, like current fields and wind fluctuations in the ocean, as well as nonlinearities in the wave equations provide widely studied mechanisms for their creation. However, the interaction of these mechanisms is largely unexplored. Hence, we study wave propagation under the concurrent influence of geometrical (disorder) and nonlinear focusing in the (current-modified) nonlinear Schrödinger equation.Weshow how nonlinearity shifts the onset distance of geometrical (disorder) focusing and alters the peak intensities of the fluctuations.Wefind an intricate interplay of both mechanisms that is reflected in the observation of optimal ratios of nonlinearity and disorder strength for the generation of rogue waves.
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