Příprava a charakterizace nanostruktur

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Zero-field spin wave turns
(AIP Publishing, 2024-03-11) Klíma, Jan; Wojewoda, Ondřej; Roučka, Václav; Molnár, Tomáš; Holobrádek, Jakub; Urbánek, Michal
Spin-wave computing, a potential successor to CMOS-based technologies, relies on the efficient manipulation of spin waves for information processing. While basic logic devices such as magnon transistors, gates, and adders have been experimentally demonstrated, the challenge for complex magnonic circuits lies in steering spin waves through sharp turns. In this study, we demonstrate with micromagnetic simulations and Brillouin light scattering microscopy experiments, that dipolar spin waves can propagate through 90 degrees turns without distortion. The key lies in carefully designed in-plane magnetization landscapes, addressing challenges posed by anisotropic dispersion. The experimental realization of the required magnetization landscape is enabled by spatial manipulation of the uniaxial anisotropy using corrugated magnonic waveguides. The findings presented in this work should be considered in any magnonic circuit design dealing with anisotropic dispersion and spin wave turns.
Next-generation photocatalytic system: Ga2O3-modified Ga Nanoislands on graphene for H2 production
(Elsevier, 2024-05-24) Bartošík, Miroslav; Edelmannová, Miroslava Filip; Mach, Jindřich; Kočí, Kamila
Study investigates Ga2O3/Ga on Graphene/SiO2/Si, prepared via temperature-controlled growth of Ga, for photocatalytic hydrogen generation, marking the first exploration of this system for such purpose. Thin surface ultra-wide band gap Ga2O3 layer effectively photoemits electron-hole pairs under UVC and the metallic Ga cores enhance electric field separating charge carriers by formation of localized surface plasmon (LSP) resonances. These effects increase hydrogen yields. Computational analysis of LSP-induced electric field enhancement quantitatively supports proposed fundamental mechanism underlying sample's photoactivity. The most active photocatalyst (Ga-M) with medium-size Ga particles (radius 14 nm) exhibited ten thousand times higher activity per gram than commercial TiO2.
Observation of Mermin-Wagner behavior in LaFeO₃/SrTiO₃ superlattices
(NATURE PORTFOLIO, 2024-06-22) Kiaba, Michal; Suter, Andreas; Salman, Zaher; Prokscha, Thomas; Chen, B.; Koster, Gertjan; Dubroka, Adam
Two-dimensional magnetic materials can exhibit new magnetic properties due to the enhanced spin fluctuations that arise in reduced dimension. However, the suppression of the long-range magnetic order in two dimensions due to long-wavelength spin fluctuations, as suggested by the Mermin-Wagner theorem, has been questioned for finite-size laboratory samples. Here we study the magnetic properties of a dimensional crossover in superlattices composed of the antiferromagnetic LaFeO3 and SrTiO3 that, thanks to their large lateral size, allowed examination using a sensitive magnetic probe - muon spin rotation spectroscopy. We show that the iron electronic moments in superlattices with 3 and 2 monolayers of LaFeO3 exhibit a static antiferromagnetic order. In contrast, in the superlattices with single LaFeO3 monolayer, the moments do not order and fluctuate to the lowest measured temperature as expected from the Mermin-Wagner theorem. Our work shows how dimensionality can be used to tune the magnetic properties of ultrathin films.
Understanding the Effect of Electron Irradiation on WS2 Nanotube Devices to Improve Prototyping Routines
(AMER CHEMICAL SOC, 2024-12-13) Kovařík, Martin; Citterberg, Daniel; Paiva de Araújo, Estácio; Šikola, Tomáš; Kolíbal, Miroslav
To satisfy the needs of the current technological world that demands high performance and efficiency, a deep understanding of the whole fabrication process of electronic devices based on low-dimensional materials is necessary for rapid prototyping of devices. The fabrication processes of such nanoscale devices often include exposure to an electron beam. A field effect transistor (FET) is a core device in current computation technology, and FET configuration is also commonly used for extraction of electronic properties of low-dimensional materials. In this experimental study, we analyze the effect of electron beam exposure on electrical properties of individual WS2 nanotubes in the FET configuration by in-operando transport measurements inside a scanning electron microscope. Upon exposure to the electron beam, we observed a significant change in the resistance of individual substrate-supported nanotubes (by a factor of 2 to 14) that was generally irreversible. The resistance of each nanotube did not return to its original state even after keeping it under ambient conditions for hours to days. Furthermore, we employed Kelvin probe force microscopy to monitor surface potential and identified that substrate charging is the primary cause of changes in nanotubes' resistance. Hence, extra care should be taken when analyzing nanostructures in contact with insulating oxides that are subject to electron exposure during or after fabrication.
Optical signatures of strain-induced ferromagnetism in a LaCoO3 thin film
(AMER PHYSICAL SOC, 2024-12-24) Abadizaman, Farzin; Munzar, Dominik; Kiaba, Michal; Dubroka, Adam
Using spectroscopic ellipsometry, we studied the optical conductivity of LaCoO3 with various degrees of strain. The optical response of the compressively strained LaCoO3 film is qualitatively similar to the one of the unstrained LaCoO3 polycrystalline sample and exhibits a redistribution of the spectral weight between '0.2 and 6 eV, which is most likely related to the thermal excitation of the high-spin (HS) states. The optical response of the ferromagnetic (FM) tensile strained film exhibits clear signatures of the FM state. Below the Curie temperature Tc = 82 K, a spectral weight transfer sets on from high energies (between 3.3 and 5.6 eV) to low energies (between 0.2 and 3.3 eV). The temperature dependence of the low-energy spectral weight can be understood in the framework of the HS biexciton model of Sotnikov et al. [SciPost Phys. 8, 082 (2020)] as corresponding to the increase of the concentration of the HS states that are stabilized below Tc. The magnitude of the redistribution of the spectral weight due to the formation of the FM state is sizable and corresponds to 0.009 e per Co ion. We discuss it in terms of the effective kinetic energy of Co 3d bands.

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