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85 results on '"Kopecek, Jaromir"'

1. Elastic constants of single-crystalline NiTi studied by resonant ultrasound spectroscopy

Author

Bodnárová, Lucie, Janovská, Michaela, Ševčík, Martin, Frost, Miroslav, Kadeřávek, Lukáš, Kopeček, Jaromír, Seiner, Hanuš, and Sedlák, Petr

Subjects
Condensed Matter - Materials Science
Abstract

Contactless, laser-based resonant ultrasound spectroscopy was utilized to monitor changes in elastic properties in single-crystalline NiTi shape memory alloy. It was observed that the elastic behavior of the temperature-induced B19$^\prime$ martensite adopts the symmetry elements of the parent austenite phase, and thus, the changes over the transformation temperature can be represented by the temperature evolution of three cubic elastic coefficients. The experiments confirm that the transition during the cooling run is preceded by pronounced softening of the $c_{44}$ elastic coefficient, which leads to nearly complete vanishing of elastic anisotropy prior to the transition. Below the transition, this coefficient remains soft, and the character of anisotropy switches from $c_{44}/c^\prime>1$ to $c_{44}/c^\prime<1$. We rationalize this behavior from the mechanical instability of the B19$^\prime$ lattice with respect to shears along the (001)$_{B19^\prime}$ plane, which is known from first-principles calculations., Comment: Manuscript submitted to Shape Memory and Superelasticity

Published
2025

2. Laue Indexing with Optimal Transport

Author

Kacprzak, Tomasz, Samothrakitis, Stavros, Larsen, Camilla Buhl, Kopeček, Jaromír, Strobl, Markus, Polatidis, Efthymios, and Obozinski, Guillaume

Subjects
Condensed Matter - Materials Science, Electrical Engineering and Systems Science - Image and Video Processing
Abstract

Laue tomography experiments retrieve the positions and orientations of crystal grains in a polycrystalline samples from diffraction patterns recorded at multiple viewing angles. The use of a broad wavelength spectrum beam can greatly reduce the experimental time, but poses a difficult challenge for the indexing of diffraction peaks in polycrystalline samples; the information about the wavelength of these Bragg peaks is absent and the diffraction patterns from multiple grains are superimposed. To date, no algorithms exist capable of indexing samples with more than about 500 grains efficiently. To address this need we present a novel method: Laue indexing with Optimal Transport (LaueOT). We create a probabilistic description of the multi-grain indexing problem and propose a solution based on Sinkhorn Expectation-Maximization method, which allows to efficiently find the maximum of the likelihood thanks to the assignments being calculated using Optimal Transport. This is a non-convex optimization problem, where the orientations and positions of grains are optimized simultaneously with grain-to-spot assignments, while robustly handling the outliers. The selection of initial prototype grains to consider in the optimization problem are also calculated within the Optimal Transport framework. LaueOT can rapidly and effectively index up to 1000 grains on a single large memory GPU within less than 30 minutes. We demonstrate the performance of LaueOT on simulations with variable numbers of grains, spot position measurement noise levels, and outlier fractions. The algorithm recovers the correct number of grains even for high noise levels and up to 70% outliers in our experiments. We compare the results of indexing with LaueOT to existing algorithms both on synthetic and real neutron diffraction data from well-characterized samples., Comment: Data and code available at https://github.com/LaueOT/laueotx. 19 Pages, 8 figures

Published
2024

36. Optically transparent composite diamond/Ti electrodes

Author

Ashcheulov, Petr, Taylor, Andrew, More-Chevalier, Joris, Kovalenko, Alexander, Remeš, Zdeněk, Drahokoupil, Jan, Hubík, Pavel, Fekete, Ladislav, Klimša, Ladislav, Kopeček, Jaromír, Remiášová, Jarmila, Kohout, Michal, Frank, Otakar, Kavan, Ladislav, and Mortet, Vincent

Published
2017
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45. Synthesis and Magnetic Properties of Carbon Doped and Reduced SrTiO3 Nanoparticles

Author

Makarova, Marina V, Prokhorov, Andrey, Stupakov, Alexander, Kopecek, Jaromir, Drahokoupil, Jan, Trepakov, Vladimir, Dejneka, Alexander, Makarova, Marina V, Prokhorov, Andrey, Stupakov, Alexander, Kopecek, Jaromir, Drahokoupil, Jan, Trepakov, Vladimir, and Dejneka, Alexander

Abstract

We report on the studies of the synthesis, structural, and magnetic properties of undoped SrTiO3 (STO), carbon-doped STO:C, and reduced STO STO:R nanoparticles. Fine (similar to 20-30 nm) and coarse (similar to 100 nm) nanoparticles with a single phase of cubic perovskite-type structure were sintered by thermal decomposition of SrTiO(C2O4)(2). Magnetization loops of fine STO:C and STO:R nanoparticles at low temperatures and an almost linear decrease in magnetization with temperature indicate the realization of a soft, ferromagnetic state in them, with a pronounced disorder effect characteristic of doped dilute magnetic semiconductors. Oxidation and particle size increase suppress the magnetic manifestations, demonstrating the importance of surface-related defects and oxygen deficiency in the emergence of magnetism. It was found that oxygen vacancies and doping with carbon make similar contributions to the magnetization, while complementary electron paramagnetic resonance, together with magnetization measurement studies, show that the most probable state of oxygen vacancies, which determine the appearance of magnetic properties, are charged F+ oxygen vacancies and C-impurity centers, which tend to segregate on the surface of nanoparticles.

Published
2023

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