Recent Submissions

  • Journal Article

    Transit least-squares survey. II. Discovery and validation of 17 new sub- to super-Earth-sized planets in multi-planet systems from K2 

    Heller, René; Hippke, Michael; Rodenbeck, Kai
    Astronomy & Astrophysics 2019; 627: Art. A66
    The extended Kepler mission (K2) has revealed more than 500 transiting planets in roughly 500 000 stellar light curves. All of these were found either with the box least-squares algorithm or by visual inspection. Here we use our new transit least-squares (TLS) algorithm to search for additional planets around all K2 stars that are currently known to host at least one planet. We discover and statistically validate 17 new planets with radii ranging from about 0.7 Earth radii (R ) to roughly 2:2 R and a median radius of 1:18 R . EPIC 201497682.03, with a radius of 0:692+0:059 􀀀0:048 R , is the second smallest planet ever discovered with K2. The transit signatures of these 17 planets are typically 200 ppm deep (ranging from 100 ppm to 2000 ppm), and their orbital periods extend from about 0.7 d to 34 d with a median value of about 4 d. Fourteen of these 17 systems only had one known planet before, and they now join the growing number of multi-planet systems. Most stars in our sample have subsolar masses and radii. The small planetary radii in our sample are a direct result of the higher signal detection e ciency that TLS has compared to box-fitting algorithms in the shallow-transit regime. Our findings help in populating the period-radius diagram with small planets. Our discovery rate of about 3.7% within the group of previously known K2 systems suggests that TLS can find over 100 additional Earth-sized planets in the data of the Kepler primary mission.
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  • Journal Article

    Magnetic fields in M dwarfs from the CARMENES survey 

    Shulyak, D.; Reiners, A.; Nagel, E.; Tal-Or, L.; Caballero, J. A.; Zechmeister, M.; Béjar, V. J. S.; Cortés-Contreras, M.; Martin, E. L.; Kaminski, A.; et al.
    Ribas, I.Quirrenbach, A.Amado, P. J.Anglada-Escudé, G.Bauer, F. F.Dreizler, S.Guenther, E. W.Henning, T.Jeffers, S. V.Kürster, M.Lafarga, M.Montes, D.Morales, J. C.Pedraz, S.
    Astronomy & Astrophysics 2019; 626(A86)
    Context. M dwarfs are known to generate the strongest magnetic fields among main-sequence stars with convective envelopes, but we are still lacking a consistent picture of the link between the magnetic fields and underlying dynamo mechanisms, rotation, and activity. Aims. In this work we aim to measure magnetic fields from the high-resolution near-infrared spectra taken with the CARMENES radial-velocity planet survey in a sample of 29 active M dwarfs and compare our results against stellar parameters. Methods. We used the state-of-the-art radiative transfer code to measure total magnetic flux densities from the Zeeman broadening of spectral lines and filling factors. Results. We detect strong kG magnetic fields in all our targets. In 16 stars the magnetic fields were measured for the first time. Our measurements are consistent with the magnetic field saturation in stars with rotation periods P < 4 d. The analysis of the magnetic filling factors reveal two different patterns of either very smooth distribution or a more patchy one, which can be connected to the dynamo state of the stars and/or stellar mass. Conclusions. Our measurements extend the list ofMdwarfs with strong surface magnetic fields. They also allow us to better constrain the interplay between the magnetic energy, stellar rotation, and underlying dynamo action. The high spectral resolution and observations at near-infrared wavelengths are the beneficial capabilities of the CARMENES instrument that allow us to address important questions about the stellar magnetism
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  • Journal Article

    A zone-plate-based two-color spectrometer for indirect X-ray absorption spectroscopy 

    Döring, Florian; Risch, Marcel; Rösner, Benedikt; Beye, Martin; Busse, Philipp; Kubiček, Katharina; Glaser, Leif; Miedema, Piter S.; Soltau, Jakob; Raiser, Dirk; et al.
    Guzenko, Vitaliy A.Szabadics, LukasKochanneck, LeifBaumung, MaxBuck, JensJooss, ChristianTechert, SimoneDavid, ChristianDöring, FlorianRisch, MarcelRösner, BenediktBeye, MartinBusse, PhilippKubiček, KatharinaGlaser, LeifMiedema, Piter S.Soltau, JakobRaiser, DirkGuzenko, Vitaliy A.Szabadics, LukasKochanneck, LeifBaumung, MaxBuck, JensJooss, ChristianTechert, SimoneDavid, Christian
    Journal of Synchrotron Radiation 2019; 26(4) p.1266-1271
    X-ray absorption spectroscopy (XAS) is a powerful element-specific technique that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La0.6Sr0.4MnO3 by analysis of three emission lines simultaneously at the detector, namely the O 2p-1s, Mn 3s-2p and Mn 3d-2p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3s-2p and Mn 3d-2p transitions when scanning the Mn L-edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to-noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.
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  • Journal Article

    Direct characterization of cytoskeletal reorganization during blood platelet spreading 

    Paknikar, Aishwarya K.; Eltzner, Benjamin; Köster, Sarah
    Progress in Biophysics and Molecular Biology 2019; 144 p.166-176
    Blood platelets are the key cellular players in blood clotting and thus of great biomedical importance. While spreading at the site of injury, they reorganize their cytoskeleton within minutes and assume a flat appearance. As platelets possess no nucleus, many standard methods for visualizing cytoskeletal components by means of fluorescence tags fail. Here we employ silicon-rhodamine actin and tubulin probes for imaging these important proteins in a time-resolved manner. We find two distinct timescales for platelet spread area development and for cytoskeletal reorganization, indicating that although cell spreading is most likely associated with actin polymerization at the cell edges, distinct, stress-fiber-like actin structures within the cell, which may be involved in the generation of contractile forces, form on their own timescale. Following microtubule dynamics allows us to distinguish the role of myosin, microtubules and actin during early spreading.
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  • Journal Article

    Electrification in granular gases leads to constrained fractal growth 

    Singh, Chamkor; Mazza, Marco G.
    Scientific Reports 2019; 9(1): Art. 9049
    The empirical observation of aggregation of dielectric particles under the influence of electrostatic forces lies at the origin of the theory of electricity. The growth of clusters formed of small grains underpins a range of phenomena from the early stages of planetesimal formation to aerosols. However, the collective effects of Coulomb forces on the nonequilibrium dynamics and aggregation process in a granular gas – a model representative of the above physical processes – have so far evaded theoretical scrutiny. Here, we establish a hydrodynamic description of aggregating granular gases that exchange charges upon collisions and interact via the long-ranged Coulomb forces. We analytically derive the governing equations for the evolution of granular temperature, charge variance, and number density for homogeneous and quasi-monodisperse aggregation. We find that, once the aggregates are formed, the granular temperature of the cluster population, the charge variance of the cluster population and the number density of the cluster population evolve in such a way that their non-dimensional combination obeys a physical constraint of nearly constant dimensionless ratio of characteristic electrostatic to kinetic energy. This constraint on the collective evolution of charged clusters is confirmed both by our theory and our detailed molecular dynamics simulations. The inhomogeneous aggregation of monomers and clusters in their mutual electrostatic field proceeds in a fractal manner. Our theoretical framework is extendable to more precise charge exchange mechanisms, a current focus of extensive experimentation. Furthermore, it illustrates the collective role of long-ranged interactions in dissipative gases and can lead to novel designing principles in particulate systems
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  • Journal Article

    Distributional semantics of objects in visual scenes in comparison to text 

    Lüddecke, Timo; Agostini, Alejandro; Fauth, Michael; Tamosiunaite, Minija; Wörgötter, Florentin
    Artificial Intelligence 2019; 274 p.44-65
    The distributional hypothesis states that the meaning of a concept is defined through the contexts it occurs in. In practice, often word co-occurrence and proximity are analyzed in text corpora for a given word to obtain a real-valued semantic word vector, which is taken to (at least partially) encode the meaning of this word. Here we transfer this idea from text to images, where pre-assigned labels of other objects or activations of convolutional neural networks serve as context. We propose a simple algorithm that extracts and processes object contexts from an image database and yields semantic vectors for objects. We show empirically that these representations exhibit on par performance with state-of-the-art distributional models over a set of conventional objects. For this we employ well-known word benchmarks in addition to a newly proposed object-centric benchmark.
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  • Journal Article

    Doubly-charged Higgs boson at a future electron-proton collider 

    Dev, P. S. Bhupal; Khan, Sarif; Mitra, Manimala; Rai, Santosh Kumar
    Physical Review D 2019; 99(11)
    We explore the discovery prospect of the doubly-charged component of an SU(2)L-triplet scalar at the future e−p collider FCC-eh, proposed to operate with an electron beam energy of 60 GeV and a proton beam energy of 50 TeV. We consider the associated production of the doubly-charged Higgs boson along with leptons and jet(s), and its subsequent prompt decay to same-sign lepton pair. This occurs for O(1) Yukawa coupling of the scalar triplet with charged leptons, which is expected for reasonably small vacuum expectation values of the neutral component of the triplet field that governs the neutrino mass generation in the type-II seesaw. We present our analysis for two different final states, 3l+≥1j and an inclusive ≥2l+≥1j channel. Considering its decay to electrons only, we find that the doubly-charged Higgs boson with a mass around a TeV could be observed at the 3σ confidence level with O(200)  fb−1 of integrated luminosity, while masses up to 2 TeV could be probed within a few years of data accumulation. The signal proposed here becomes essentially background free, if it is triggered in the μμ mode and a 5σ discovery is achievable in this channel for a TeV-scale doubly-charged Higgs boson with an integrated luminosity as low as O(50)  fb−1. We also highlight the sensitivity of FCC-eh to the Yukawa coupling responsible for the production of the doubly-charged Higgs boson as a function of its mass in both the ee and μμ channels.
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  • Journal Article

    FIMP dark matter candidate(s) in a B − L model with inverse seesaw mechanism 

    Abdallah, Waleed; Choubey, Sandhya; Khan, Sarif
    Journal of High Energy Physics 2019; 2019(6)
    Abstract: The non-thermal dark matter (DM) production via the so-called freeze-in mechanism provides a simple alternative to the standard thermal WIMP scenario. In this work, we consider a popular U(1)B�����L extension of the standard model (SM) in the context of inverse seesaw mechanism which has at least one (fermionic) FIMP DM candidate. Due to the added Z2 symmetry, a SM gauge singlet fermion, with mass of order keV, is stable and can be a warm DM candidate. Also, the same Z2 symmetry helps the lightest righthanded neutrino, with mass of order GeV, to be a stable or long-lived particle by making a corresponding Yukawa coupling very small. This provides a possibility of a two component DM scenario as well. Firstly, in the absence of a GeV DM component (i.e., without tuning its corresponding Yukawa coupling to be very small), we consider only a keV DM as a single component DM, which is produced by the freeze-in mechanism via the decay of the extra Z0 gauge boson associated to U(1)B�����L and can consistently explain the DM relic density measurements. In contrast with most of the existing literature, we have found a reasonable DM production from the annihilation processes. After numerically studying the DM production, we show the dependence of the DM relic density as a function of its relevant free parameters. We use these results to obtain the parameter space regions that are compatible with the DM relic density bound. Secondly, we study a two component DM scenario and emphasize that the current DM relic density bound can be satis ed for a wide range of parameter space.
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  • Journal Article

    CuO-CeO2 nanocomposite catalysts produced by mechanochemical synthesis 

    Borchers, Ch.; Martin, M. L.; Vorobjeva, G. A.; Morozova, O. S.; Firsova, A. A.; Leonov, A. V.; Kurmaev, E. Z.; Kukharenko, A. I.; Zhidkov, I. S.; Cholakh, S. O.
    AIP Advances 2019; 9(6): Art. 065115
    Mechanochemical synthesis based on ball-milling of individual oxides was applied as a one-step preparation technique for CuO-CeO2 catalyst for preferential CO oxidation in H2 excess. The mechanical energy dose transferred to the original powder mixture determines both the catalyst composition and activity. It is found that after 90 min of milling (corresponding to a dose of 372 kJ mol–1), a mixture of 10 wt.% CuO-CeO2 powder exhibits a CO conversion of 97% at 423 K. Four active oxygen states, which are not observed in case of pure CeO2, were detected in the nanocomposite lattice and attributed to the presence of Cu in surface sites as well as in subsurface bulk sites of CeO2, in nearest neighbor and next nearest neighbor positions. Correspondingly, oxidation of CO to CO2 was found to occur in a two-stage process with Tmax = 395/460 K, and oxidation of H2 to H2O likewise in a four-stage process with Tmax = 426/448/468/516 K. The milled powder consists of CeO2 crystallites sized 8–10 nm agglomerated to somewhat larger aggregates, with CuO dispersed on the surface of the CeO2 crystallites, and to a lesser extent present as Cu2O.
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  • Journal Article

    Common origin of baryon asymmetry, Dark Matter and neutrino mass 

    Biswas, Anirban; Choubey, Sandhya; Covi, Laura; Khan, Sarif
    Journal of High Energy Physics 2019; 2019(5): Art. 193
    In this work, we explain three beyond standard model (BSM) phenomena, namely neutrino masses, the baryon asymmetry of the Universe and Dark Matter, within a single model and in each explanation the right handed (RH) neutrinos play the prime role. Indeed by just introducing two RH neutrinos we can generate the neutrino masses by the Type-I seesaw mechanism. The baryon asymmetry of the Universe can arise from thermal leptogenesis from the decay of lightest RH neutrino before the decoupling of the electroweak sphaleron transitions, which redistribute the B − L number into a baryon number. At the same time, the decay of the RH neutrino can produce the Dark Matter (DM) as an asymmetric Dark Matter component. The source of CP violation in the two sectors is exactly the same, related to the complex couplings of the neutrinos. By determining the comoving number density for different values of the CP violation in the DM sector, we obtain a particular value of the DM mass after satisfying the relic density bound. We also give prediction for the DM direct detection (DD) in the near future by different ongoing DD experiments.
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  • Journal Article

    Flagella-like Beating of a Single Microtubule 

    Vilfan, Andrej; Subramani, Smrithika; Bodenschatz, Eberhard; Golestanian, Ramin; Guido, Isabella
    Nano Letters 2019; 19(5) p.3359-3363
    Kinesin motors can induce a buckling instability in a microtubule with a fixed minus end. Here we show that by modifying the surface with a protein-repellent functionalization and using clusters of kinesin motors, the microtubule can exhibit persistent oscillatory motion resembling the beating of sperm flagella. The observed period is of the order of 1 min. From the experimental images we theoretically determine a distribution of motor forces that explains the observed shapes using a maximum likelihood approach. A good agreement is achieved with a small number of motor clusters acting simultaneously on a microtubule. The tangential forces exerted by a cluster are mostly in the range 0-8 pN toward the microtubule minus end, indicating the action of 1 or 2 kinesin motors. The lateral forces are distributed symmetrically and mainly below 10 pN, while the lateral velocity has a strong peak around zero. Unlike well-known models for flapping filaments, kinesins are found to have a strong "pinning" effect on the beating filaments. Our results suggest new strategies to utilize molecular motors in dynamic roles that depend sensitively on the stress built-up in the system.
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  • Journal Article

    Formation and development of the male copulatory organ in the spider Parasteatoda tepidariorum involves a metamorphosis-like process 

    Quade, Felix Simon Christian; Holtzheimer, Jana; Frohn, Jasper; Töpperwien, Mareike; Salditt, Tim; Prpic, Nikola-Michael
    Scientific Reports 2019; 9(1): Art. 6945
    Spiders have evolved a unique male copulatory organ, the pedipalp bulb. The morphology of the bulb is species specific and plays an important role in species recognition and prezygotic reproductive isolation. Despite its importance for spider biodiversity, the mechanisms that control bulb development are virtually unknown. We have used confocal laser scanning microscopy (CLSM) and diffusible iodine-based contrast-enhanced micro computed tomography (dice-µCT) to study bulb development in the spider Parasteatoda tepidariorum. These imaging technologies enabled us to study bulb development in situ, without the use of destructive procedures for the first time. We show here that the inflated pedipalp tip in the subadult stage is filled with haemolymph that rapidly coagulates. Coagulation indicates histolytic processes that disintegrate tibia and tarsus, similar to histolytic processes during metamorphosis in holometabolous insects. The coagulated material contains cell inclusions that likely represent the cell source for the re-establishment of tarsus and tibia after histolysis, comparable to the histoblasts in insect metamorphosis. The shape of the coagulated mass prefigures the shape of the adult tarsus (cymbium) like a blueprint for the histoblasts. This suggests a unique role for controlled coagulation after histolysis in the metamorphosis-like morphogenesis of the male pedipalp.
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  • Journal Article

    CMEs in the Heliosphere: II. A Statistical Analysis of the Kinematic Properties Derived from Single-Spacecraft Geometrical Modelling Techniques Applied to CMEs Detected in the Heliosphere from 2007 to 2017 by STEREO/HI-1 

    Barnes, D.; Davies, J. A.; Harrison, R. A.; Byrne, J. P.; Perry, C. H.; Bothmer, V.; Eastwood, J. P.; Gallagher, P. T.; Kilpua, E. K. J.; Möstl, C.; et al.
    Rodriguez, L.Rouillard, A. P.Odstrčil, D.
    Solar Physics 2019; 294(5)
    Recent observations with the Heliospheric Imagers (HIs) onboard the twin NASA Solar Terrestrial Relations Observatory (STEREO) spacecraft have provided unprecedented observations of a large number of coronal mass ejections (CMEs) in the inner heliosphere. In this article we discuss the generation of the HIGeoCAT CME catalogue and perform a statistical analysis of its events. The catalogue was generated as part of the EU FP7 HELCATS (Heliospheric Cataloguing, Analysis and Techniques Service) project ( www.helcats-fp7.eu/ ). It is created by generating time/elongation maps for CMEs using observations from the inner (HI-1) and outer (HI-2) cameras along a position angle close to the CME apex. Next, we apply single-spacecraft geometric-fitting techniques to determine the kinematic properties of these CMEs, including their speeds, propagation directions, and launch times. The catalogue contains a total of 1455 events (801 from STEREO-A and 654 from STEREO-B) from April 2007 to the end of August 2017. We perform a statistical analysis of the properties of CMEs in HIGeoCAT and compare the results with those from the Large Angle Spectrometric Coronagraph (LASCO) CDAW catalogues (Yashiro et al.J. Geophys. Res. Space Phys.109, A07105, 2004) and the COR-2 catalogue of Vourlidas et al. (Astrophys. J.838, 141, 2004) during the same period. We find that the distributions of both speeds and latitudes for the HIGeoCAT CMEs correlate with the sunspot number over the solar cycle. We also find that the HI-derived CME speed distributions are generally consistent with coronagraph catalogues over the solar cycle, albeit with greater absolute speeds due to the differing methods with which each is derived.
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  • Journal Article

    Leidenfrost Pattern Formation and Boiling 

    Prabhakaran, Prasanth; Krekhov, Alexei; Bodenschatz, Eberhard; Weiss, Stephan
    Journal of Statistical Physics 2019; 175(3-4) p.598-616
    We report on Leidenfrost patterns and boiling with compressed sulfur hexafluoride (SF6). The experiments were carried out in a large aspect ratio Rayleigh–Bénard convection cell, where the distance between the horizontal plates is comparable with the capillary length of the working fluid. Pressures and temperatures were chosen such that the bottom plate was above and the top plate was below the liquid–vapor transition temperature of SF6. As a result, SF6 vapor condenses at the top plate and forms drops that grow in size. Leidenfrost patterns are formed as the drops do not fall but levitate by the vapor released in the gap between the hot bottom plate and the colder drops. When the size of these drops became too large, one or more vapor bubbles—chimneys—form inside them. We determine the critical size for the formation of a chimney as a function of the capillary length. For even larger drops and extended puddles many disconnected chimneys occur that can grow to sizes large enough for the formation of new drops inside them. By varying the temperatures and the pressure in the system, we observe various such patterns. When the area covered by a puddle becomes large it touches the hot bottom plate locally and boils off rapidly. This can be attributed to a local reduction of the bottom plate surface temperature below the Leidenfrost temperature.
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  • Journal Article

    Erratum to: Soft-drop thrust 

    Baron, Jeremy; Marzani, Simone; Theeuwes, Vincent
    Journal of High Energy Physics 2019; 2019(5): Art. 056
    This note corrects the de nition of soft-drop thrust presented in ref. [1] so that it is collinear safe for all values of the soft-drop angular parameter 0.
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  • Journal Article

    Full NLO corrections to 3-jet production and R32 at the LHC 

    Reyer, Max; Schönherr, Marek; Schumann, Steffen
    The European Physical Journal C 2019; 79(4): Art. 321
    We present the evaluation of the complete set of NLO corrections to three-jet production at the LHC. To this end we consider all contributions of O(αnsαm) with n+m=3 and n+m=4. This includes in particular also subleading Born contributions of electroweak origin, as well as electroweak virtual and QED real-radiative corrections. As an application we present results for the three- over two-jet ratio R32. While the impact of non-QCD corrections on the total cross section is rather small, they can exceed −10% for high jet transverse momenta. The R32 observable turns out to be very stable against electroweak corrections, receiving absolute corrections below 5% even in the high-pT region.
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  • Journal Article

    Extreme velocity gradients in turbulent flows 

    Buaria, Dhawal; Pumir, Alain; Bodenschatz, Eberhard; Yeung, P. K.
    New Journal of Physics 2019; 21(4): Art. 043004
    Fully turbulent flows are characterized by intermittent formation of very localized and intense velocity gradients. These gradients can be orders of magnitude larger than their typical value and lead to many unique properties of turbulence. Using direct numerical simulations of the Navier–Stokes equations with unprecedented small-scale resolution, we characterize such extreme events over a significant range of turbulence intensities, parameterized by the Taylor-scale Reynolds number (Rl). Remarkably, we find the strongest velocity gradients to empirically scale as t l - Rb K 1 , with b »  0.775 0.025,where tK is theKolmogorov time scale (with its inverse, t-K1, being the rms of velocity gradient fluctuations). Additionally, we observe velocity increments across very small distances r  h,where η is theKolmogorov length scale, to be as large as the rms of the velocity fluctuations. Both observations suggest that the smallest length scale in the flow behaves as h l R-a,with a = b - 1 2 , which is at odds with predictions from existing phenomenological theories.Wefind that extreme gradients are arranged in vortex tubes, such that strain conditioned on vorticity grows on average slower than vorticity, approximately as a power law with an exponent g < 1, which weakly increaseswith Rl.Using scaling arguments,we get b = (2 - g)-1,which suggests that βwould also slowly increasewith Rl.We conjecture that approaching themathematical limit of infinite Rl, strain and vorticity would scale similarly resulting in g = 1and hence extreme events occurring at a scale h l R-1/2 corresponding to b = 1.
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  • Journal Article

    Eigenstate thermalization and quantum chaos in the Holstein polaron model 

    Jansen, David; Stolpp, Jan; Vidmar, Lev; Heidrich-Meisner, Fabian
    Physical Review B 2019; 99(15): Art. 155130
    The eigenstate thermalization hypothesis (ETH) is a successful theory that provides sufficient criteria for ergodicity in quantum many-body systems. Most studies were carried out for Hamiltonians relevant for ultracold quantum gases and single-component systems of spins, fermions, or bosons. The paradigmatic example for thermalization in solid-state physics are phonons serving as a bath for electrons. This situation is often viewed from an open-quantum-system perspective. Here, we ask whether a minimal microscopic model for electron-phonon coupling is quantum chaotic and whether it obeys ETH, if viewed as a closed quantum system. Using exact diagonalization, we address this question in the framework of the Holstein polaron model. Even though the model describes only a single itinerant electron, whose coupling to dispersionless phonons is the only integrability-breaking term, we find that the spectral statistics and the structure of Hamiltonian eigenstates exhibit essential properties of the corresponding random-matrix ensemble. Moreover, we verify the ETH ansatz both for diagonal and off-diagonal matrix elements of typical phonon and electron observables, and show that the ratio of their variances equals the value predicted from random-matrix theory.
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  • Journal Article

    A Theoretical Framework to Derive Simple, Firing-Rate-Dependent Mathematical Models of Synaptic Plasticity 

    Lappalainen, Janne; Herpich, Juliane; Tetzlaff, Christian
    Frontiers in Computational Neuroscience 2019; 13: Art. 26
    Synaptic plasticity serves as an essential mechanism underlying cognitive processes as learning and memory. For a better understanding detailed theoretical models combine experimental underpinnings of synaptic plasticity and match experimental results. However, these models are mathematically complex impeding the comprehensive investigation of their link to cognitive processes generally executed on the neuronal network level. Here, we derive a mathematical framework enabling the simplification of such detailed models of synaptic plasticity facilitating further mathematical analyses. By this framework we obtain a compact, firing-rate-dependent mathematical formulation, which includes the essential dynamics of the detailed model and, thus, of experimentally verified properties of synaptic plasticity. Amongst others, by testing our framework by abstracting the dynamics of two well-established calcium-dependent synaptic plasticity models, we derived that the synaptic changes depend on the square of the presynaptic firing rate, which is in contrast to previous assumptions. Thus, the here-presented framework enables the derivation of biologically plausible but simple mathematical models of synaptic plasticity allowing to analyze the underlying dependencies of synaptic dynamics from neuronal properties such as the firing rate and to investigate their implications in complex neuronal networks.
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  • Journal Article

    Time-resolved x-ray phase-contrast tomography of sedimenting micro-spheres 

    Ruhlandt, A; Salditt, T
    New Journal of Physics 2019; 21(4): Art. 043017
    We have implemented a time-dependent (dynamic) x-ray tomography of sedimenting micro-spheres suspended in water. To achieve phase contrast at high magnification we use the divergent and highly coherent beam emitted from an x-ray waveguide. Holograms are recorded with 5 ms acquisition time while the sample is rotated at 1 Hz, over a run of 40 s. We show that under these conditions, more than 20 000 individual particle trajectories can be tracked. The analysis of the trajectories shows apparent super-diffusive behavior due to collective flow patterns, as also further evidenced by plotting the temporal averaged spatial distribution of particle densities and velocities.
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