TY - JOUR A1 - Xu, Pengbo A1 - Deng, Weihua A1 - Sandev, Trifce T1 - Levy walk with parameter dependent velocity BT - hermite polynomial approach and numerical simulation JF - Journal of physics : A, Mathematical and theoretical N2 - To analyze stochastic processes, one often uses integral transform (Fourier and Laplace) methods. However, for the time-space coupled cases, e.g. the Levy walk, sometimes the integral transform method may fail. Here we provide a Hermite polynomial expansion approach, being complementary to the integral transform method, to the Levy walk. Two approaches are compared for some already known results. We also consider the generalized Levy walk with parameter dependent velocity. Namely, we consider the Levy walk with velocity which depends on the walking length or on the duration of each step. Some interesting features of the generalized Levy walk are observed, including the special shapes of the probability density function, the first passage time distributions, and various diffusive behaviors of the mean squared displacement. KW - Hermite polynomial expansion KW - Levy walk KW - anomalous diffusion KW - parameter KW - dependent velocity Y1 - 2020 U6 - https://doi.org/10.1088/1751-8121/ab7420 SN - 1751-8113 SN - 1751-8121 VL - 53 IS - 11 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Schneider, Sebastian A1 - Hoffmann-Vogel, Regina T1 - Electrostatic forces above graphene nanoribbons and edges interpreted as partly hydrogen-free JF - Nanoscale N2 - Graphene nanoribbons' electronic transport properties strongly depend on the type of edge, armchair, zigzag or other, and on edge functionalization that can be used for band-gap engineering. For only partly hydrogenated edges interesting magnetic properties are predicted. Electric charge accumulates at edges and corners. Scanning force microscopy has so far shown the centre of graphene nanoribbons with atomic resolution using a quartz crystal tuning fork sensor of high stiffness. Weak long-range electrostatic forces related to the charge accumulation on the edges of graphene nanoribbons could not be imaged so far. Here, we show the electrostatic forces at the corners and edges of graphene nanoribbons are amenable to measurement. We use soft cantilevers and a bimodal imaging technique to combine enhanced sensitivity to weak long-range electrostatic forces with the high resolution of the second-frequency shift. Additionally, in our work the edges of the nanoribbons are mainly hydrogen-free, opening to the route to investigations of partly hydrogenated magnetic nanoribbons. Y1 - 2020 U6 - https://doi.org/10.1039/d0nr03348k SN - 2040-3364 SN - 2040-3372 VL - 12 IS - 34 SP - 17895 EP - 17901 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Schulz, Christian A1 - Lieutenant, Klaus A1 - Xiao, Jie A1 - Hofmann, Tommy A1 - Wong, Deniz A1 - Habicht, Klaus T1 - Characterization of the soft X-ray spectrometer PEAXIS at BESSY II JF - Journal of synchrotron radiation N2 - The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV to 1600 eV allowing for RIXS measurements in the range 200-1200 eV. The monochromator optics can be operated in different configurations to provide either high flux with up to 10(12) photons s(-1) within the focal spot at the sample or high energy resolution with a full width at half maximum of <40 meV at an incident photon energy of similar to 400 eV. The measured total energy resolution of the RIXS spectrometer is in very good agreement with theoretically predicted values obtained by ray-tracing simulations. PEAXIS features a 5 m-long RIXS spectrometer arm that can be continuously rotated about the sample position by 106 degrees within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. Selected scientific examples are presented to demonstrate the instrument capabilities, including measurements of excitations in single-crystalline NiO and in liquid acetone employing a fluid cell sample manipulator. Planned upgrades of the beamline and the RIXS spectrometer to further increase the energy resolution to similar to 100 meV at 1000 eV incident photon energy are discussed. KW - resonant inelastic X-ray scattering KW - X-ray photoelectron spectroscopy KW - soft X-ray spectroscopy KW - soft X-ray beamline KW - X-ray emission KW - X-ray KW - absorption KW - BESSY II Y1 - 2020 U6 - https://doi.org/10.1107/S1600577519014887 SN - 1600-5775 VL - 27 SP - 238 EP - 249 PB - International Union of Crystallography CY - Chester ER - TY - JOUR A1 - Yeste, Maria Pilar A1 - Primus, Philipp-Alexander A1 - Alcantara, Rodrigo A1 - Cauqui, Miguel Angel A1 - Calvino, Juan Jose A1 - Pintado, José María A1 - Blanco, Ginesa T1 - Surface characterization of two Ce0.62Zr0.38O2 mixed oxides with different reducibility JF - Applied surface science : a journal devoted to applied physics and chemistry of surfaces and interfaces N2 - This paper presents a study of the surface properties of two Ce/Zr mixed oxides with different reducibility, obtained by applying distinct thermal ageing treatments to an oxide with the composition Ce0.62Zr0.38O2. The surface composition was investigated by XPS. Chemical reactivity of the surface was studied by adsorption of the probe molecules CO2, D-2 and methanol. Nanostructural characterization was carried out by XRD, Raman and high-resolution Eu3+ spectroscopy (FLNS). The characterization showed only slight variations in surface composition and bulk Ce-Zr distribution, but hardy differences concerning the type and strength of acidic surface centres, as well as strong differences in the ability to dissociate hydrogen. Structural variations between both samples were identified by comparing the optical spectra of Eu3+ in surface doped samples. KW - Ce/Zr KW - Surface properties KW - Reactive adsorption KW - Hydrogen activation Y1 - 2020 U6 - https://doi.org/10.1016/j.apsusc.2019.144255 SN - 0169-4332 SN - 1873-5584 VL - 503 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Sandev, Trifce A1 - Iomin, Alexander A1 - Kocarev, Ljupco T1 - Hitting times in turbulent diffusion due to multiplicative noise JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We study a distribution of times of the first arrivals to absorbing targets in turbulent diffusion, which is due to a multiplicative noise. Two examples of dynamical systems with a multiplicative noise are studied. The first one is a random process according to inhomogeneous diffusion, which is also known as a geometric Brownian motion in the Black-Scholes model. The second model is due to a random processes on a two-dimensional comb, where inhomogeneous advection is possible only along the backbone, while Brownian diffusion takes place inside the branches. It is shown that in both cases turbulent diffusion takes place as the one-dimensional random process with the log-normal distribution in the presence of absorbing targets, which are characterized by the Levy-Smirnov distribution for the first hitting times. Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevE.102.042109 SN - 2470-0045 SN - 2470-0053 VL - 102 IS - 4 PB - American Institute of Physics CY - Woodbury, NY ER - TY - JOUR A1 - Massolt, Joost Willem A1 - Borowski, Andreas T1 - Perceived relevance of university physics problems by pre-service physics teachers BT - personal constructs JF - International journal of science education N2 - Pre-service physics teachers often do not recognise the relevance for their future career in their university content knowledge courses. A lower perceived relevance can, however, have a negative effect on their motivation and on their academic success. Several intervention studies have been undertaken with the goal to increase this perceived relevance. A previous study shows that conceptual physics problems used in university physics courses are perceived by pre-service physics teachers as more relevant for their future career than regular, quantitative problems. It is however not clear, what the students' meaning of the construct 'relevance' is: what makes a problem more relevant to them than another problem? To answer this question, N = 7 pre-service teachers were interviewed using the repertory grid technique, based on the personal construct theory. Nine physics problems were discussed with regards to their perceived relevance and with regards to problem properties that distinguish these problems from each other. We are able to identify six problem properties that have a positive influence on the perceived relevance. Physics problems that are based on these properties should therefore potentially have a higher perceived relevance, which can have a positive effect on the motivation of the pre-service teachers who solve these problems. KW - Motivation KW - physics education KW - pre-service teachers KW - repertory grid Y1 - 2020 U6 - https://doi.org/10.1080/09500693.2019.1705424 SN - 0950-0693 SN - 1464-5289 VL - 42 IS - 2 SP - 167 EP - 189 PB - Routledge, Taylor & Francis Group CY - Abingdon ER - TY - JOUR A1 - Evsevleev, Sergei A1 - Paciornik, Sidnei A1 - Bruno, Giovanni T1 - Advanced deep learning-based 3D microstructural characterization of multiphase metal matrix composites JF - Advanced engineering materials N2 - The quantitative analysis of microstructural features is a key to understanding the micromechanical behavior of metal matrix composites (MMCs), which is a premise for their use in practice. Herein, a 3D microstructural characterization of a five-phase MMC is performed by synchrotron X-ray computed tomography (SXCT). A workflow for advanced deep learning-based segmentation of all individual phases in SXCT data is shown using a fully convolutional neural network with U-net architecture. High segmentation accuracy is achieved with a small amount of training data. This enables extracting unprecedently precise microstructural parameters (e.g., volume fractions and particle shapes) to be input, e.g., in micromechanical models. KW - computed tomography KW - convolutional neural networks KW - deep learning KW - metal KW - matrix composites KW - segmentations Y1 - 2020 U6 - https://doi.org/10.1002/adem.201901197 SN - 1438-1656 SN - 1527-2648 VL - 22 IS - 4 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Mishurova, Tatiana A1 - Bruno, Giovanni A1 - Evsevleev, Sergei A1 - Sevostianov, Igor T1 - Determination of macroscopic stress from diffraction experiments BT - a critical discussion JF - Journal of applied physics N2 - The paper is motivated by some inconsistencies and contradictions present in the literature on the calculation of the so-called diffraction elastic constants. In an attempt at unifying the views that the two communities of Materials Science and Mechanics of Materials have on the subject, we revisit and define the terminology used in the field. We also clarify the limitations of the commonly used approaches and show that a unified methodology is also applicable to textured materials with a nearly arbitrary grain shape. We finally compare the predictions based on this methodology with experimental data obtained by in situ synchrotron radiation diffraction on additively manufactured Ti-6Al-4V alloy. We show that (a) the transverse isotropy of the material yields good agreement between the best-fit isotropy approximation (equivalent to the classic Kroner's model) and the experimental data and (b) the use of a general framework allows the calculation of all components of the tensor of diffraction elastic constants, which are not easily measurable by diffraction methods. This allows us to extend the current state-of-the-art with a predictive tool. Y1 - 2020 U6 - https://doi.org/10.1063/5.0009101 SN - 0021-8979 SN - 1089-7550 VL - 128 IS - 2 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Makwana, Kirit D. A1 - Yan, Huirong T1 - Properties of magnetohydrodynamic modes in compressively driven plasma turbulence JF - Physical Review X N2 - We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes-namely, the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving while varying the Alfven Mach number (M-A), plasma beta, and the sonic Mach number from subsonic to transsonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfven-mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high M-A. The slow-mode properties are similar to the Alfven mode. On the other hand, the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast-mode behavior does not show any transition in going from low to high M-A. We find indications that there is some interaction between the different modes, and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic-ray transport and magnetic reconnection are discussed. KW - Astrophysics KW - Plasma Physics Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevX.10.031021 SN - 2160-3308 VL - 10 IS - 3 PB - American Physical Society (APS) CY - College Park ER - TY - GEN A1 - Makwana, Kirit D. A1 - Yan, Huirong T1 - Properties of magnetohydrodynamic modes in compressively driven plasma turbulence T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes-namely, the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving while varying the Alfven Mach number (M-A), plasma beta, and the sonic Mach number from subsonic to transsonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfven-mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high M-A. The slow-mode properties are similar to the Alfven mode. On the other hand, the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast-mode behavior does not show any transition in going from low to high M-A. We find indications that there is some interaction between the different modes, and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic-ray transport and magnetic reconnection are discussed. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1225 KW - mhd turbulence KW - star formation KW - simulations KW - Anisotropy KW - diffusion Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-531607 SN - 1866-8372 VL - 10 IS - 3 PB - American Physical Society (APS) CY - College Park ER -