arXiv subjects

Hideki Yamamoto

Publications and source records attributed to Hideki Yamamoto.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2019

Evolution of charge order topology across a magnetic phase transition in cuprate superconductors

Charge order is now accepted as an integral constituent of cuprate high-temperature superconductors, one that is intimately related to other instabilities in the phase diagram including antiferromagnetism and superconductivity. Unlike nesting-induced Peierls-like density waves, the charge correlations in the CuO2 planes have been predicted to display a rich momentum space topology depending on the detailed fermiology of the system. However, to date charge order has only been observed along the high-symmetry Cu-O bond directions. Here, using resonant soft X-ray scattering, we investigate the evolution of the full momentum space topology of charge correlations in Ln2CuO4 (Ln=Nd, Pr) as a function of intrinsic electron doping. We report that, upon electron doping the parent Mott insulator, charge correlations first emerge in a hitherto-unobserved form, with full (Cinf) rotational symmetry in momentum-space. At higher doping levels, the orientation of charge correlations is sharply locked to the Cu-O bond high-symmetry directions, restoring a more conventional bidirectional charge order with enhanced correlation lengths. Through charge susceptibility calculations, we closely reproduce the drastic evolution in the topology of charge correlations across an antiferromagnetic quantum phase transition, highlighting the interplay between spin and charge degrees of freedom in electron-doped cuprates. Finally, using the established link between charge correlations and the underlying fermiology, we propose a revised phase diagram of Ln2CuO4 with a superconducting region extending toward the Mott limit.

cond-mat.str-el↗

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↗

arXiv2016

Shubnikov-de Haas quantum oscilations reveal a reconstructed Fermi surface near optimal doping in a thin film of the cuprate superconductor Pr$_{1.86}$Ce$_{0.14}$CuO$_{4\pmδ}$

We study magnetotransport properties of the electron-doped superconductor Pr$_{2-x}$Ce$_x$CuO$_{4\pmδ}$ with $x$ = 0.14 in magnetic fields up to 92~T, and observe Shubnikov de-Haas magnetic quantum oscillations. The oscillations display a single frequency $F$=255$\pm$10~T, indicating a small Fermi pocket that is $\sim$~1\% of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large hole-like cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consistent with overdoped compounds. Our study demonstrates that the exceptional chemical control afforded by high quality thin films will enable Fermi surface studies deep into the overdoped cuprate phase diagram.

cond-mat.supr-con↗

arXiv2016

Evaluation of Disorder Introduced by Electrolyte Gating through Transport Measurements in Graphene

We evaluate the degree of disorder in electrolyte gating devices through the transport measurements in graphene. By comparing the mobility in ion- and standard metal-gated devices, we show that the deposition of the ionic liquid introduces charged impurities with a density of approximately $6\times 10^{12}$ cm$^{-2}$; setting the upper limit of the mobility in graphene to 3000 cm$^2$/Vs. At higher temperature, phonons in the ionic liquid further reduce the mobility, making its upper limit 2000 cm$^2$/Vs at room temperature. Since the degree of disorder is independent of the base material, these results are valuable towards understanding disorder effects in general devices using electrolyte gating.

cond-mat.mes-hall↗

arXiv2016

Theory of intraband plasmons in doped carbon nanotubes: rolled surface-plasmons of graphene

A single-wall carbon nanotube possesses two different types of plasmons specified by the wavenumbers in the azimuthal and axial directions. The azimuthal plasmon that is caused by interband transitions has been studied, while the effect of charge doping is unknown. In this paper, we show that when nanotubes are heavily doped, intraband transitions cause the azimuthal plasmons to appear as a plasmon resonance in the near-infrared region of the absorption spectra, which is absent for light doping due to the screening effect. The axial plasmons that are inherent in the cylindrical waveguide structures of nanotubes, account for the absorption peak of the metallic nanotube observed in the terahertz region. The excitation of axial (azimuthal) plasmons requires a linearly polarized light parallel (perpendicular) to the tube's axis.

cond-mat.mes-hall↗

arXiv2016

Determination of intrinsic lifetime of edge magnetoplasmons

It is known that peculiar plasmons whose frequencies are purely imaginary exist in the interior of a two-dimensional electronic system described by the Drude model. We show that when an external magnetic field is applied to the system, these bulk plasmons are still non-oscillating and are isolated from the magnetoplasmons by the energy gap of the cyclotron frequency. These are mainly in a transverse magnetic mode and can combine with a transverse electronic mode locally at an edge of the system to form edge magnetoplasmons. With this observation, we reveal the intrinsic long lifetime of edge magnetoplasmons for the first time.

cond-mat.mes-hall↗

arXiv2015

Quantum oscillations suggest hidden quantum phase transition in the cuprate superconductor Pr$_{2}$CuO$_{4\pmδ}$

For both electron- and hole-doped cuprates, superconductivity appears in the vicinity of suppressed broken symmetry order, suggesting that quantum criticality plays a vital role in the physics of these systems. A confounding factor in identifying the role of quantum criticality in the electron-doped systems is the competing influence of chemical doping and oxygen stoichiometry. Using high quality thin films of Pr$_{2}$CuO$_{4\pmδ}$, we tune superconductivity and uncover the influence of quantum criticality without Ce substitution. We observe magnetic quantum oscillations that are consistent with the presence of small hole-like Fermi surface pockets, and a large mass enhancement near the suppression of superconductivity. Tuning these materials using only oxygen stoichiometry allows the observation of quantum oscillations and provides a new axis with which to explore the physics underlying the electron-doped side of the cuprate phase diagram.

cond-mat.supr-con↗

arXiv2008

Generic phase diagram of "electron-doped" T' cuprates

We investigated the generic phase diagram of the electron doped superconductor, Nd2-xCexCuO4, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial Oap atoms, we found that the Tc increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of Oap "impurities" on superconductivity and also magnetism in T' cuprates. Then we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd2-xCexCuO4.

cond-mat.supr-con↗

arXiv2008

Reduction dependence of superconductivity in undoped T' cuprates

We have recently achieved superconductivity in undoped T'-RE2CuO4 (RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low-PO2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high-Tc superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T'-RE2CuO4. For thin films, the amount of remnant Oap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.

cond-mat.supr-con↗

arXiv2008

Superconductivity in undoped T' cuprates with Tc over 30 K

Undoped cuprates have long been considered to be antiferromagnetic insulators. In this article, however, we report that superconductivity is achieved in undoped T'-RE2CuO4 (RE = Pr, Nd, Sm, Eu, and Gd). Our discovery was performed by using metal-organic decomposition (MOD), an inexpensive and easy-to-implement thin-film process. The keys to prepare the superconducting films are firing with low partial-pressure of oxygen and reduction at low temperatures. The highest Tc of undoped T'-RE2CuO4 is over 30 K, substantially higher than "electron-doped" analogs. Remarkably, Gd2CuO4, even the derivatives of which have not shown superconductivity so far, gets superconducting with Tconset as high as ~ 20 K. The implication of our discovery is briefly discussed.

cond-mat.supr-con↗

arXiv2007

Electronic properties of buried hetero-interfaces of LaAlO3 on SrTiO3

We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 1016 cm-2 and a mobility around 104 cm2 V-1 s-1. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.

cond-mat.mtrl-sci↗

arXiv2007

Origin of charge density at LaAlO3-on-SrTiO3 hetero-interfaces; possibility of intrinsic doping

As discovered by Ohtomo et al., a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.

cond-mat.mtrl-sci↗

arXiv2007

Dependence of electronic structure of SrRuO3 and the degree of correlation on cation off-stoichiometry

We have grown and studied high quality SrRuO3 films grown by MBE as well as PLD. By changing the oxygen activity during deposition we were able to make SrRuO3 samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). Samples with strontium vacancies were found impossible to produce since the ruthenium would precipitate out as RuO2. The volume of the unit cell of SrRuO3 becomes larger as more ruthenium vacancies are introduced. The residual resistivity ratio (RRR) and room temperature resistivity were found to systematically depend on the volume of the unit cell and therefore on the amount of ruthenium vacancies. The RRR varied from ~30 for stoichiometric samples to less than two for samples that were very ruthenium poor. The room temperature resistivity varied from 190 microOhm cm for stoichoimetric samples to over 300 microOhm cm for very ruthenium poor samples. UPS spectra show a shift of weight from the coherent peak to the incoherent peak around the Fermi level when samples have more ruthenium vacancies. Core level XPS spectra of the ruthenium 3d lines show a strong screened part in the case of stoichiometric samples. This screened part disappears when ruthenium vacancies are introduced. Both the UPS and the XPS results are consistent with the view that correlation increases as the amount of ruthenium vacancies increase.

cond-mat.mtrl-sci↗