arXiv subjects

Yoshitaka Taniyasu

Publications and source records attributed to Yoshitaka Taniyasu.

35 records · Page 2Linked to original sources

arXiv2023

Reduction of Interlayer Interaction in Multilayer Stacking Graphene with Carbon Nanotube Insertion: Insights from Experiment and Simulation

The creation of multilayer graphene (Gr), while preserving the brilliant properties of monolayer Gr derived from its unique band structure, can expand the application field of Gr to the macroscale. However, the energy-favorable AB stacking structure in the multilayer Gr induces a strong interlayer interaction and alters the band structure. Consequently, the intrinsic properties of each monolayer are degraded. In this work, we insert carbon nanotubes (CNTs) as nanospacers to modulate the microstructure of multilayer stacking Gr. Nanospacers can increase the interlayer distance and reduce the interlayer interaction. The Gr/CNT stacking structure is experimentally fabricated using a dry transfer method in a layer-by-layer manner. Raman spectroscopy verifies the reduction in the interlayer interaction within the stacking structure. Atomic force microscopy shows an increase in the interlayer distance, which can explain the weakening of the interlayer interactions. The microstructure of the stacked Gr and CNTs is studied by molecular dynamics simulation to systematically investigate the effect of CNT insertion. We found that the distribution distance, size, and arrangement of the CNT can modulate the interlayer distance. These results will help us to understand and improve the properties of the composite systems consisting of Gr and CNTs.

cond-mat.mes-hall↗

arXiv2023

Magnetic anisotropy driven by ligand in 4d transition metal oxide SrRuO3

The origin of magnetic anisotropy in magnetic compounds is a longstanding issue in solid state physics and nonmagnetic ligand ions are considered to contribute little to magnetic anisotropy. Here, we introduce the concept of ligand driven magnetic anisotropy in a complex transition-metal oxide. We conducted X ray absorption and X ray magnetic circular dichroism spectroscopies at the Ru and O edges in the 4d ferromagnetic metal SrRuO3. Systematic variation of the sample thickness in the range below 10 nm allowed us to control the localization of Ru 4d t2g states, which affects the magnetic coupling between the Ru and O ions. We found that the orbital magnetization of the ligand induced via hybridization with the Ru 4d orbital determines the magnetic anisotropy in SrRuO3.

cond-mat.mtrl-sci↗

arXiv2023

Impedance-matched High-overtone Bulk Acoustic Resonator

A high-overtone bulk acoustic resonator (HBAR), in which a piezoelectric transducer is set on an acoustic cavity, has been attracting attention in both fundamental research and RF applications due to its scalability, high frequency, and high quality factor. The acoustic impedance matching in HBARs is crucial for efficient acoustic power transfer from the piezoelectric transducer to the cavity. However, impedance mismatch remains in most HBARs due to the metal layer insertion between the piezoelectric layer and cavity substrate. In this study, we fabricated a nearly impedance-matched high-quality HBAR using an epitaxial AlN piezoelectric layer directly grown on a conductive SiC cavity substrate with no metal layer insertion. The small impedance mismatch was verified from the variation in the free spectral range (FSR), which is comparable to the best value in previously reported HBARs. The experimentally obtained FSR spectra was greatly reproduced by using the Mason model. Broadband phonon cavity modes up to the K-band (26.5 GHz) were achieved by reducing the thickness of the AlN layer from 800 to 200 nm. The high figure of merit of $f\times\text{Q} \sim 1.3\times 10^{13}\ \textrm{Hz}$ at 10 GHz was also obtained. Our nearly impedance-matched high-quality HBAR will enable the development of RF applications, such as low-phase noise oscillators and acoustic filters, as well as research on high-frequency acoustic systems hybridized with electric, optical, and magnetic systems.

cond-mat.mes-hall↗

arXiv2023

Stoichiometric growth of SrTiO3 films via Bayesian optimization with adaptive prior mean

Perovskite insulator SrTiO3 is expected to be applied to the next generation of electronic and photonic devices as high-k capacitors and photocatalysts. However, reproducible growth of highly insulating stoichiometric SrTiO3 films remains challenging due to the difficulty of the precise stoichiometry control in perovskite oxide films. Here, to grow stoichiometric SrTiO3 thin films by fine-tuning multiple growth conditions, we developed a new Bayesian optimization (BO)-based machine learning method that encourages the exploration of the search space by varying the prior mean to get out of suboptimal growth condition parameters. Using simulated data, we demonstrate the efficacy of the new BO method, which reproducibly reaches the global best conditions. With the BO method implemented in machine-learning-assisted molecular beam epitaxy (ML-MBE), highly insulating stoichiometric SrTiO3 film with no absorption in the band gap was developed in only 44 MBE growth runs. The proposed algorithm provides an efficient experimental design platform that is not as dependent on the experience of individual researchers and will accelerate not only oxide electronics but also various material syntheses.

cond-mat.mtrl-sci↗

arXiv2023

Scattering-dependent transport of SrRuO3 films: From Weyl fermion transport to hump-like Hall effect anomaly

Recent observation of quantum transport phenomena of Weyl fermions has brought much attention to 4d ferromagnetic perovskite SrRuO3 as a magnetic Weyl semimetal. Besides, the hump-like Hall effect anomaly, which might have a topological origin, has also been reported for this material. Here, we show that the emergence of such phenomena is governed by the degree of scattering determined by the defect density (Ru-deficiency- and/or interface-driven-defect scattering) and measurement temperature (phonon scattering), where the former is controlled by varying the growth conditions of the SrRuO3 films in molecular beam epitaxy as well as the film thickness. The resulting electronic transport properties can be classified into three categories: clean, intermediate, and dirty regimes. The transport of Weyl fermions emerges in the clean regime, whereas that of topologically trivial conduction electrons in the ferromagnetic metal state prevail in the intermediate and dirty regimes. In the clean and intermediate regimes, anomalous Hall resistivity obeys a scaling law incorporating the intrinsic Karplus-Luttinger (Berry phase) and extrinsic side-jump mechanisms. The hump-like Hall effect anomaly is observed only in the dirty regime, which is contrary to the scaling law between anomalous Hall resistivity and longitudinal resistivity. Hence, we conclude that this anomaly is not inherent to the material and does not have a topological origin. We also provide defect- and temperature-dependent transport phase diagrams of stoichiometric SrRuO3 and Ru-deficient SrRu0.7O3 where the appearance of Weyl fermions and hump-like Hall effect anomaly are mapped. These diagrams may serve as a guideline for designing SrRu1-xO3-based spintronic and topological electronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Isotropic orbital magnetic moments in magnetically anisotropic SrRuO3 films

Epitaxially strained SrRuO3 films have been a model system for understanding the magnetic anisotropy in metallic oxides. In this paper, we investigate the anisotropy of the Ru 4d and O 2p electronic structure and magnetic properties using high-quality epitaxially strained (compressive and tensile) SrRuO3 films grown by machine-learning-assisted molecular beam epitaxy. The element-specific magnetic properties and the hybridization between the Ru 4d and O 2p orbitals were characterized by Ru M2,3-edge and O K-edge soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements. The magnetization curves for the Ru 4d and O 2p magnetic moments are identical, irrespective of the strain type, indicating the strong magnetic coupling between the Ru and O ions. The electronic structure and the orbital magnetic moment relative to the spin magnetic moment are isotropic despite the perpendicular and in-plane magnetic anisotropy in the compressive-strained and tensile-strained SrRuO3 films; i.e., the orbital magnetic moments have a negligibly small contribution to the magnetic anisotropy. This result contradicts Bruno model, where magnetic anisotropy arises from the difference in the orbital magnetic moment between the perpendicular and in-plane directions. Contributions of strain-induced electric quadrupole moments to the magnetic anisotropy are discussed, too.

cond-mat.str-el↗

arXiv2022

Bayesian optimization with experimental failure for high-throughput materials growth

A crucial problem in achieving innovative high-throughput materials growth with machine learning and automation techniques, such as Bayesian optimization (BO) and robotic experimentation, has been a lack of an appropriate way to handle missing data due to experimental failures. Here, we propose a new BO algorithm that complements the missing data in the optimization of materials growth parameters. The proposed method provides a flexible optimization algorithm capable of searching a wide multi-dimensional parameter space. We demonstrate the effectiveness of the method with simulated data as well as in its implementation for actual materials growth, namely machine-learning-assisted molecular beam epitaxy (ML-MBE) of SrRuO3, which is widely used as a metallic electrode in oxide electronics. Through the exploitation and exploration in a wide three-dimensional parameter space, while complementing the missing data, we attained tensile-strained SrRuO3 film with a high residual resistivity ratio of 80.1, the highest among tensile-strained SrRuO3 films ever reported, in only 35 MBE growth runs.

cond-mat.mtrl-sci↗

arXiv2021

High-mobility two-dimensional carriers from surface Fermi arcs in magnetic Weyl semimetal films

High-mobility two-dimensional carriers originating from surface Fermi arcs in magnetic Weyl semimetals are highly desired for accessing exotic quantum transport phenomena and for topological electronics applications. Here, we demonstrate high-mobility two-dimensional carriers that show quantum oscillations in magnetic Weyl semimetal SrRuO3 epitaxial films by systematic angle-dependent, high-magnetic field magnetotransport experiments. The exceptionally high-quality SrRuO3 films were grown by state-of-the-art oxide thin film growth technologies driven by machine learning algorithm. The quantum oscillations for the 10-nm SrRuO3 film show a high quantum mobility of 3500 cm2/Vs, a light cyclotron mass, and two-dimensional angular dependence, which can be attributed to the surface Fermi arcs. The linear thickness dependence of the phase shift of the quantum oscillations provides evidence for the non-trivial nature of the quantum oscillations mediated by the surface Fermi arcs. In addition, at low temperatures and under magnetic fields of up to 52 T, the quantum limit of SrRuO3 manifests the chiral anomaly of the Weyl nodes. Emergence of the hitherto hidden two-dimensional Weyl states in a ferromagnetic oxide pave the way to explore novel quantum transport phenomena for topological oxide electronics.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity in infinite-layer CaCuO2-brownmillerite Ca2Fe2O5 superlattices

High-temperature cuprate superconductors have naturally a superlattice structure. Infinite-layer CaCuO2 is the common ingredient of cuprates with superconducting transition temperatures above 100 K. However, infinite-layer CaCuO2 by itself does not superconduct. Here we show that superconductivity emerges in artificial superlattices built from infinite-layer CaCuO2 and brownmillerite Ca2Fe2O5 grown by molecular beam epitaxy. X-ray diffraction and electron microscopy characterizations showed that the crystal quality of the infinite-layer CaCuO2 in the superlattices significantly improved compared to bare thin-films of CaCuO2. We found that the induction of superconductivity in [(CaCuO2)n(Ca2Fe2O5)m]N superlattices is also subject to the oxidizing environment used during the cool-down procedure and therefore to a minimization of oxygen vacancies within the CuO2 planes. The inserted Ca2Fe2O5 layers buffer charge imbalances triggered by point defect formation during growth, minimizing cationic defects in the infinite-layer CaCuO2 layers thus stabilizing monolithic infinite-layer CaCuO2 slabs; embedding CaCuO2 within a superlattice enables extended two-dimensional CuO2 planes and therefore superconductivity while Ca2Fe2O5 serves similar to the charge-reservoir layers in the cuprate superconductors synthesized from incongruent melts.

cond-mat.supr-con↗

arXiv2021

Single-domain perpendicular magnetization induced by the coherent O 2p-Ru 4d hybridized state in an ultra-high-quality SrRuO3 film

We investigated the Ru 4d and O 2p electronic structure and magnetic properties of an ultra-high-quality SrRuO3 film on SrTiO3 grown by machine-learning-assisted molecular beam epitaxy. The high itinerancy and long quantum lifetimes of the quasiparticles in the Ru 4d t2g-O 2p hybridized valence band are confirmed by observing the prominent well-screened peak in the Ru 3d core-level photoemission spectrum, the coherent peak near the Fermi energy in the valence band spectrum, and quantum oscillations in the resistivity. The element-specific magnetic properties and the hybridization between the Ru 4d and O 2p orbitals were characterized by Ru M2,3-edge and O K-edge soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements. The ultra-high-quality SrRuO3 film with the residual resistivity ratio of 86 shows the large orbital magnetic moment of oxygen ions induced by the strong orbital hybridization of the O 2p states with the spin-polarized Ru 4d t2g states. The film also shows single-domain perpendicular magnetization with an almost ideal remanent magnetization ratio of 0.97. These results provide detailed insights into the relevance between orbital hybridization and the perpendicular magnetic anisotropy in SrRuO3/SrTiO3 systems.

cond-mat.mtrl-sci↗

arXiv2021

Wide-range epitaxial strain control of electrical and magnetic properties in high-quality SrRuO3 films

Epitaxial strain in 4d ferromagnet SrRuO3 films is directly linked to the physical properties through the strong coupling between lattices, electrons, and spins. It provides an excellent opportunity to tune the functionalities of SrRuO3 in electronic and spintronic devices. However, a thorough understanding of the epitaxial strain effect in SrRuO3 has remained elusive due to the lack of systematic studies. This study demonstrates wide-range epitaxial strain control of electrical and magnetic properties in high-quality SrRuO3 films. The epitaxial strain was imposed by cubic or pseudocubic perovskite substrates having a lattice mismatch of -1.6 to 2.3% with reference to bulk SrRuO3. The Poisson ratio, which describes the two orthogonal distortions due to the substrate clamping effect, is estimated to be 0.33. The Curie temperature (TC) and residual resistivity ratios of the series of films are higher than or comparable to the highest reported values for SrRuO3 on each substrate, confirming the high crystalline quality of the films. A TC of 169 K is achieved in a tensile-strained SrRuO3 film on the DyScO3 (110) substrate, which is the highest value ever reported for SrRuO3. The TC (146-169 K), magnetic anisotropy (perpendicular or in-plane magnetic easy axis), and metallic conduction (residual resistivity at 2 K of 2.10 - 373 μΩcm) of SrRuO3 are widely controlled by epitaxial strain. These results provide guidelines to design SrRuO3-based heterostructures for device applications.

cond-mat.mtrl-sci↗

arXiv2021

Structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy: comparison with stoichiometric SrRuO3

We investigate structural and transport properties of highly Ru-deficient SrRu0.7O3 thin films prepared by molecular beam epitaxy on (001) SrTiO3 substrates. To distinguish the influence of the two types of disorders in the films, Ru vacancies within lattices and disorders near the interface, SrRu0.7O3 thin films with various thicknesses (t = 1-60 nm) were prepared. It was found that the influence of the former dominates the electrical and magnetic properties when t > 5-10 nm, while that of the latter does when t < 5-10 nm. Structural characterizations revealed that the crystallinity, in terms of the Sr and O sublattices, of SrRu0.7O3 thin films, is as high as that of the ultrahigh-quality SrRuO3 ones. The Curie temperature (TC) analysis elucidated that SrRu0.7O3 (TC = 140 K) is a material distinct from SrRuO3 (TC = 150 K). Despite the large Ru deficiency (30%), the SrRu0.7O3 films showed metallic conduction when t > 5 nm. In high-field magnetoresistance measurements, the fascinating phenomenon of Weyl fermion transport was not observed for the SrRu0.7O3 thin films irrespective of thickness, which is in contrast to the stoichiometric SrRuO3 films. The (magneto)transport properties suggest that a picture of carrier scattering due to the Ru vacancies is appropriate for SrRu0.7O3, and also that proper stoichiometry control is a prerequisite to utilizing the full potential of SrRuO3 as a magnetic Weyl semimetal and two-dimensional spin-polarized system. Nevertheless, the large tolerance in Ru composition (30 %) to metallic conduction is advantageous for some practical applications where SrRu1-xO3 is exploited as an epitaxial conducting layer.

cond-mat.mtrl-sci↗

arXiv2020

Thickness-dependent quantum transport of Weyl fermions in ultra-high-quality SrRuO3 films

The recent observation of Weyl fermions in the itinerant 4d ferromagnetic perovskite SrRuO3 points to this material being a good platform for exploring novel physics related to a pair of Weyl nodes in epitaxial heterostructures. In this letter, we report the thickness-dependent magnetotransport properties of ultra-high-quality epitaxial SrRuO3 films grown under optimized conditions on SrTiO3 substrates. Signatures of Weyl fermion transport, i.e., unsaturated linear positive magnetoresistance accompanied by a quantum oscillation having a π Berry phase, were observed in films with thicknesses as small as 10 nm. Residual resistivity increased with decreasing film thickness, indicating disorder near the interface between SrRuO3 and the SrTiO3 substrate. Since this disorder affects the magnetic and electrical properties of the films, the Curie temperature decreases and the coercive field increases with decreasing thickness. Thickness-dependent magnetotransport measurements revealed that the threshold residual resistivity ratio (RRR) to observe Weyl fermion transport is 21. These results provide guidelines for realizing quantum transport of Weyl fermions in SrRuO3 near heterointerfaces.

cond-mat.mtrl-sci↗

arXiv2020

Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide

Magnetic Weyl fermions, which occur in magnets, have novel transport phenomena related to pairs of Weyl nodes, and they are, of both, scientific and technological interest, with the potential for use in high-performance electronics, spintronics and quantum computing. Although magnetic Weyl fermions have been predicted to exist in various oxides, evidence for their existence in oxide materials remains elusive. SrRuO3, a 4d ferromagnetic metal often used as an epitaxial conducting layer in oxide heterostructures, provides a promising opportunity to seek for the existence of magnetic Weyl fermions. Advanced oxide thin film preparation techniques, driven by machine learning technologies, may allow access to such topological matter. Here we show direct quantum transport evidence of magnetic Weyl fermions in an epitaxial ferromagnetic oxide SrRuO3: unsaturated linear positive magnetoresistance (MR), chiral-anomaly-induced negative MR, Pi Berry phase accumulated along cyclotron orbits, light cyclotron masses and high quantum mobility of about 10000 cm2/Vs. We employed machine-learning-assisted molecular beam epitaxy (MBE) to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic quantum transport properties. We also clarified the disorder dependence of the transport of the magnetic Weyl fermions, and provided a brand-new diagram for the Weyl transport, which gives a clear guideline for accessing the topologically nontrivial transport phenomena. Our results establish SrRuO3 as a magnetic Weyl semimetal and topological oxide electronics as a new research field.

cond-mat.mtrl-sci↗

arXiv2019

Machine-learning-assisted thin-film growth: Bayesian optimization in molecular beam epitaxy of SrRuO3 thin films

Materials informatics exploiting machine learning techniques, e.g., Bayesian optimization (BO), has the potential to offer high-throughput optimization of thin-film growth conditions through incremental updates of machine learning models in accordance with newly measured data. Here, we demonstrated BO-based molecular beam epitaxy (MBE) of SrRuO3, one of the most-intensively studied materials in the research field of oxide electronics, mainly owing to its unique nature as a ferromagnetic metal. To simplify the intricate search space of entangled growth conditions, we ran the BO for a single condition while keeping the other conditions fixed. As a result, high-crystalline-quality SrRuO3 film exhibiting a high residual resistivity ratio (RRR) of over 50 as well as strong perpendicular magnetic anisotropy was developed in only 24 MBE growth runs in which the Ru flux rate, growth temperature, and O3-nozzle-to-substrate distance were optimized. Our BO-based search method provides an efficient experimental design that is not as dependent on the experience and skills of individual researchers, and it reduces experimental time and cost, which will accelerate materials research.

cond-mat.mtrl-sci↗

arXiv2018

Surface structures of graphene covered Cu (103)

We studied the surface structures of chemical vapor deposited (CVD) graphene on Cu(103). The graphene covered Cu surface had (103) facets parallel to the Cu[010] direction, on which triangular patterns were formed. In contrast, the bare Cu surface showed no facets. Post-growth thermal annealing in an ultra-high vacuum induced surface changes on the Cu(103) facets. The reorganization of the Cu surface by the post-growth thermal annealing led to a change in the lattice strain and hole doping level of the CVD-grown graphene.

cond-mat.mtrl-sci↗

arXiv2018

Ferromagnetism above 1000 K in highly cation-ordered double-perovskite insulator Sr3OsO6

Magnetic insulators have been intensively studied for over 100 years, and they, in particular ferrites, are considered to be the cradle of magnetic exchange interactions in solids. Their wide range of applications include microwave devices and permanent magnets . They are also suitable for spintronic devices owing to their high resistivity, low magnetic damping, and spin-dependent tunneling probabilities. The Curie temperature is the crucial factor determining the temperature range in which any ferri/ferromagnetic system remains stable. However, the record Curie temperature has stood for over eight decades in insulators and oxides (943 K for spinel ferrite LiFe5O8). Here we show that a highly B-site ordered double-perovskite, Sr2(SrOs)O6 (Sr3OsO6), surpasses this long standing Curie temperature record by more than 100 K. We revealed this B-site ordering by atomic-resolution scanning transmission electron microscopy. The density functional theory (DFT) calculations suggest that the large spin-orbit coupling (SOC) of Os6+ 5d2 orbitals drives the system toward a Jeff = 3/2 ferromagnetic (FM) insulating state. Moreover, the Sr3OsO6 is the first epitaxially grown osmate, which means it is highly compatible with device fabrication processes and thus promising for spintronic applications.

cond-mat.mtrl-sci↗