Masaki Kobayashi

The University of Tokyo | Todai · Faculty & Graduate School of Engineering

The Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC>300K). To clarify the mechanism of the high−TC ferromagnetism, we have investigated the electronic structure and magnetic properties...

Polymer nanocomposites (PNCs) exhibit excellent electrical properties owing to charge trapping provided by nanofillers. However, the role of nanofillers in trap formation at the microscopic level is poorly understood. In this study, we propose a method to determine the charge trap depth of nanofillers in PNCs using X-ray photoelectron spectroscopy...

Developing technology to realize oxide-based nanoscale planar integrated circuits is in high demand for next-generation multifunctional electronics. Oxide circuits can have a variety of unique functions, including ferromagnetism, ferroelectricity, multiferroicity, superconductivity, and mechanical flexibility. In particular, for spin-transistor app...

As charge injection from metal into dielectric polymer causes degradation and breakdown of electronic and power devices, the band alignment at the metal/polymer interface should be revealed. To investigate the dominant factors of the band alignment at the metal/polymer interface, we carried out a combined experimental and first-principles computati...

Lithium-ion deintercalation/intercalation during charge/discharge processes is one of the essential reactions that occur in the layered cathodes of lithium-ion batteries, and the performance of the cathode can be expressed as the sum of the reactions that occur in the local area of the individual cathode particles. In this study, the spatial distri...

The stable performance of organic photodiodes (OPDs) is crucial for realizing reliable photosensing and their facile integration into larger systems. However, OPDs with the commonly used ZnO electron transport layer (ETL) suffer from photoinstability, where the dark current increases by several orders of magnitude after light irradiation, thereby a...

The two-dimensional electron gas (2DEG) formed at interfaces between SrTiO3 (STO) and other oxide insulating layers is promising for use in efficient spin-charge conversion due to the large Rashba spin-orbit interaction (RSOI). However, these insulating layers on STO prevent the propagation of a spin current injected from an adjacent ferromagnetic...

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 mol...

The structural similarity between NbNx and AlN facilitates the integration of superconductors into semiconductor optoelectronic devices. The abrupt interface between wide‐bandgap AlN and superconducting NbNx is essential for future nitride quantum devices, such as Josephson junction‐based quantum bits. In this study, the fundamental properties of N...

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both n- and p-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-dop...

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 mol...

Intervalence charge transfer (IVCT) refers to the transfer of electrons between two metal (M) sites with different oxidation states, through a bridging ligand: Mn+1+M′m→Mn+M′m+1. It is considered that the IVCT is related to the hopping probability of electrons (or electron mobility) in solids. Controlling the conductivity of ferromagnetic semicondu...

Since charge injection from metal into dielectric leads to electrical degradation and failure of various electrical and power devices, it is essential to reveal the band alignment of the metal/dielectric interface. However, the band alignment of the metal/dielectric interface remains theoretically unexplored and has not been experimentally obtained...

Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC > 300 K). To clarify the mechanism of the high-TC ferromagnetism, we have investigated the electronic structure and magnetic properties...

The morphology of active material particles has a significant impact on the charge-discharge cycle performances of lithium-ion batteries because each crystal surface constructed by different elemental arrangement indicates various surface...

We investigated the Ru 4d and O 2p electronic structure and magnetic properties of an ultrahigh-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-scre...

Flexoelectricity is a universal property associated with dielectric materials, wherein they exhibit remanent polarization induced by strain gradient. Rare-earth iron garnets, R3Fe5O12, are ferrimagnetic insulators with useful magnetic properties. However, they are unlikely to show remanent dielectric polarization because of their centrosymmetric st...

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both n- and p-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-dop...

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-scr...

Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds embedded with a regular interval in a semiconduc...

A fundamental understanding of the interfacial magnetic properties in ferromagnetic heterostructures is essential for utilizing ferromagnetic materials for spintronic device applications. Here, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO3(LAO)/La0.6Sr0.4MnO3(LSMO)/Nb:SrTiO3(Nb:STO) heteros...

Investigating heterointerfaces with test structures is crucial for fabricating electronic and photonic devices. Recently, we proposed and developed a PbS colloidal quantum dot/ZnO/Si hybrid infrared detector for infrared-sensitive optoelectronic devices on silicon-based large-scale integrated circuits. The conventional circular transfer length meth...

Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream p-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in n-type FMSs is theoretically nontrivial. Soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) is a powerful approach to examine the mec...

Inter-valence charge transfer (IVCT) is electron transfer between two metal $M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} + M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related to the hopping probability of electron (or the electron mobility) in solids. Since controlling the conductivity of ferro...

The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, whic...

(Ga1−x,Fex)Sb is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature (TC) is above 300 K when the Fe concentration x is equal to or higher than ∼0.20. However, the origin of the high TC in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use resonant photoemission spectrosc...

In order to understand the mechanism of the ferromagnetism in the ferromagnetic semiconductor Ga 1 − xMn xAs [(Ga,Mn)As], we have investigated the magnetic behavior on a microscopic level through systematic temperature ( T)- and magnetic field ( H)-dependent soft x-ray magnetic circular dichroism (XMCD) experiments at the Mn L 2 , 3 absorption edge...

Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTi...

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We perform hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm 3d core-level spectra, we estimate the valence of Sm to be ∼2.96, pro...

Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream p-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in n-type FMSs is theoretically non-trivial. Soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) is a powerful approach to examine the me...

(Ga$_{1-x}$,Fe$_x$)Sb is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature ($T_{\rm C}$) is above 300 K when the Fe concentration $x$ is equal to or higher than ~0.20. However, the origin of the high $T_{\rm C}$ in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use reso...

(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature (TC) is above room temperature. However, the origin of ferromagnetism with high TC remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB)...

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We have performed hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm 3$d$ core-level spectra, we have estimated the valence of Sm t...

We have studied the magnetization process of the new insulating ferromagnetic semiconductor (Al,Fe)Sb by means of x-ray magnetic circular dichroism. For an optimally doped sample with 10% Fe, a magnetization was found to rapidly increase at low magnetic fields and to saturate at high magnetic fields at room temperature, well above the Curie tempera...

(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature ($T_{\rm C}$) is above room temperature. However, the origin of ferromagnetism with high $T_{\rm C}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the...

To investigate the relationship between the charge redistribution and ferromagnetism at the interface between perovskite transition-metal oxides LaNiO3 (LNO) and LaMnO3 (LMO), we performed x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements on the sandwiched layer in the trilayer structures. In the LNO/LMO...

To investigate the relationship between the charge redistribution and ferromagnetism at the heterointerface between perovskite transition-metal oxides LaNiO$_3$ (LNO) and LaMnO$_3$ (LMO), we performed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) measurements. In the LNO/LMO heterostructures with asymmetric charge redis...

We have studied the magnetization process of the new insulating ferromagnetic semiconductor (Al,Fe)Sb by means of x-ray magnetic circular dichroism. For an optimally doped sample with 10% Fe, a magnetization was found to rapidly increase at low magnetic fields and to saturate at high magnetic fields at room temperature, well above the Curie tempera...

Controlling quantum critical phenomena in strongly correlated electron systems, which emerge in the neighborhood of a quantum phase transition, is a major challenge in modern condensed matter physics. Quantum critical phenomena are generated from the delicate balance between long-range order and its quantum fluctuation. So far, the nature of quantu...

We have firstly studied the electrical conductivity and the electronic structure of the Nd0.6Sr0.4FeO3−δ (NSFO) thin film on Al2O3(0001) substrate deposited by RF magnetron sputtering. The prepared thin film has larger lattice constant than the bulk crystal due to the stress from the substrate and the oxygen vacancies. The Fe 2p photoemission spect...

We have studied the multi-band electronic structure of BaNi2(As1−xPx)2 (x = 0.00 and 0.092) in which the P substitution suppresses unusual Ni–Ni zigzag bond order and induces strong coupling superconductivity. At x = 0.092, all the Fermi surfaces predicted by the ab-initio band calculations are successfully identified including the small electron p...

Yttria-stabilized zirconia (Zr0.92Y0.08O2-δ:YSZ) thin films have been prepared on Al2O3 (0001) substrates by RF magnetron sputtering. The as-deposited YSZ thin films prepared at 500 ºC exhibited the (111) and (200) peak at 2θ values of 30.0º and 34.8º, respectively, which close to the peak positions of the YSZ single crystal. The Zr⁴⁺, Y³⁺ valence...

The typical Kondo insulator, YbB12, is a candidate of topological Kondo insulators. To clarify the surface electronic structure of not only bulk but also surface by photoelectron measurements, we developed a method to obtain a clean surface of YbB12(001) by annealing at 1650 K in an ultra-high vacuum. By surface-sensitive photoelectron measurements...

Resonant photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) were used to investigate the effect of lithiation on the electronic structure of Fe3O4 thin film relevant to the operation mechanism of nanoionic devices to enable magnetic property tuning. Comparison of the Fe 2p XAS spectrum for lithiated Fe3O4 (Li–Fe3O4) with that...

The in-plane-oriented BaPrO3-δ thin film with mixed valence states has been prepared on an Al2O3 (0001) substrate by RF magnetron sputtering. The lattice constant decreases with increasing film thickness. The thin film has the mixed valence states of Pr⁴⁺ and Pr³⁺. The ～168 nm thickness film with small lattice constant exhibits high electrical cond...

We use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce 4f electronic states of a new BiS2 layered superconductor CeO1−xFxBiS2, for polycrystalline and single-crystal samples. The Ce 3d spectrum of the single crystal of nominal composition x=0.7 has no f0 component and the spectral shape closely resembles the ones observed for Ce tri...

Nd0.6Sr0.4FeO3 (NSFO)thin films with various thicknesses have been deposited on Al2O3 (0001) substrates by RF magnetron sputtering. The lattice constant decreases with increasing film thickness. The electrical conductivity is higher than that of the bulk crystal. The conductivity at 500˚C does not depend on the oxygen gas partial pressure. The valu...

Yttria-stabilized zirconia (Zr0.92Y0.08O2-δ :YSZ) thin films have prepared on Al2O3 substrates by RF magnetron sputtering. The as-deposited YSZ thin film prepared at 500 ºC exhibits the (020) peak at 2θ ~34.8°, which corresponds to the peak position of the YSZ single crystal. The Zr⁴⁺, Y³⁺ and valence states were confirmed by photoemission spectros...

The in-plane-oriented BaPrO3-δ thin film with mixed valence states has been prepared on an Al2O3 (0001) substrate by RF magnetron sputtering. The lattice constant decreases with increasing film thickness. The thin film has the mixed valence states of Pr⁴⁺ and Pr³⁺. The ~168 nm thickness film with small lattice constant exhibits high electrical cond...

The BaPrO 3 (BPO) is expected as a candidate for anode materials in solid oxide fuel cell (SOFC) using oxide protonics material as a solid electrolyte, due to a possible presence of mixed conductivity or holes (electrons)and protons. BPO ceramics has the possibility of electron-ion mixed conduction because of auto-ionization of Pr ⁺⁴ /Pr ⁺³ . There...

Ln 1- x Sr x FeO 3 (Ln=La, Nd) bulk crystal exhibits various electrical and magnetic properties by Ln substitution. In particular, Ln 1-x Sr x FeO 3 has been extensively studied in the bulk and thin film forms due to electrochemical application of cathode electrode for Solid Oxide Fuel Cell (SOFC). However, La 1- x Sr x FeO 3 thin film as electrode...

Yttria stabilized zirconia (YSZ) is known as solid electrolyte materials with oxide ion conduction and is used for sensor and solid oxide fuel cells (SOFCs). Recently, Wagner et al. has reported proton conductivity in YSZ single crystals at high temperature. Furthermore, Scherrer et al. has also reported proton conduction in YSZ thin films prepared...

Supplementary Figures 1-3

Clarification of the position of the Fermi level ($E_\mathrm{F}$) is important in understanding the origin of ferromagnetism in the prototypical ferromagnetic semiconductor Ga$_{1-x}$Mn$_x$As (GaMnAs). In a recent publication, Souma $et$ $al$. [Sci. Rep. $\mathbf{6}$, 27266 (2016)], have investigated the band structure and the $E_\mathrm{F}$ positi...

The a- and c-axes-oriented BaCe0.85Ru0.05Y0.10O3−δ (BCRY) thin films have been deposited on Nb-SrTiO3(100) substrates by radio frequency (RF) magnetron sputtering. Such BCRY thin films have mixed valence states of Ce4+ and Ce3+. The activation energies (E A) for the conductivity of films thicker than 200 nm are 0.23–0.26 eV, which corresponds to ha...

We have prepared c-axis controlled α-Fe2O3 thin films on Al2O3 substrates by RF magnetron sputtering and studied their electronic structure by soft-X-ray spectroscopy. The lattice constant of c-axis increases with increasing film thickness due to the relaxation of lattice mismatch between α-Fe2O3 and Al2O3 and formation of oxygen vacancies. The ele...

Anatase TiO2−δ thin film was prepared by RF magnetron sputtering using oxygen radical and Ti-metal target. Degrees of the TiO2−δ crystal orientation in the thin film depends of the oxygen gas pressure () in the radical gun. The (004)- and (112)-oriented TiO2−δ thin films crystallized without postannealing have the mixed valence Ti4+/Ti3+ state. The...

We have prepared b-axis-oriented VO2 thin films by RF magnetron sputtering using oxygen radicals as the reactive gas. The VO2 thin films consist of a mixed-valence V3+/V4+ state formed by oxygen vacancies. The V3+ ratio strongly depends on the film thickness and the oxygen partial pressure of the radical gun during deposition. The lattice constant...

The structural and electrical properties of a c-axis-oriented BaCe0.85Ru0.05Y0.10O3-delta (BCRY) thin film on an Al2O3(0001) substrate depending on film thickness have been studied. The lattice constant of the c-axis decreases with increasing film thickness. The electrical conductivity is higher in the thin film with a small lattice constant. The a...

To investigate the interfacialcharge-transfer phenomena between perovskite transition metal oxides LaNiO3 (LNO) and LaMnO3 (LMO), we have performed in situx-ray absorption spectroscopy(XAS) measurements on LNO/LMO multilayers. The Ni-L2,3 and Mn-L2,3XAS spectra clearly show the occurrence of electron transfer from Mn to Ni ions in the interface reg...

A synergistic effect between strong electron correlation and spin-orbit interaction (SOI) has been theoretically predicted to result in a new topological state of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surf...

The Ru5+ and Y3+-doped BaCeO3 (BaCe0.85Ru0.05Y0.10O3-δ: BCRY) thin film has been prepared on (0001) Al2O3 substrate by RF magnetron sputtering. The BCRY thin film crystallized at 600°C exhibits a- and c-axes orientations. The electrical conductivity of BCRY thin film is higher than that of BaCe0.90Y0.10O3-δ (BCY) thin film. The activation energy (E...

Carbon-based cathode catalysts derived from a hyperbranched iron phthalocyanine polymer (HB-FePc) were characterized, and their active-site formation mechanism was studied by synchrotron-based spectroscopy. The properties of the HB-FePc catalyst are compared with those of a catalyst with high oxygen reduction reaction (ORR) activity synthesized fro...

An in-plane-oriented BaPrO3-δ thin film with mixed valence states has been prepared on an Al2O3(0001) substrate by RF magnetron sputtering. With increasing crystallization temperature (Tsub), the lattice constant decreases and the orientation changes from the a-axis to the b-axis. The thin film prepared above Tsub = 800 °C exhibits a higher proton...

We prepared c-axis-controlled V2O3 thin films by RF magnetron sputtering and proved their metal-insulator transition (MIT) in terms of electronic structure. The lattice constant of the c-axis depends on the film thickness and the lattice mismatch of the substrate and V2O3. MIT is observed at a temperature of ∼150 K in the V2O3 thin films with the l...

The electronic structure of b-axis oriented VO2 thin films in monoclinic phase with various thicknesses has been studied by soft-X-ray spectroscopy. The VO2 thin films show the mixed-valence V3+/V4+ state, which is created by oxygen vacancies. The V3+ state and oxygen vacancies increase with increasing film thickness. The thinnest film of 56 nm exh...

The physical properties and electronic structure of Sm-doped CeO2 (Ce0.90Sm0.10O2−δ) in the thin-film form have been studied. The as-deposited thin film exhibits (111) orientation on an Al2O3(0001) substrate. The lattice constant of the thin film is larger than that of the bulk crystal. The Ce0.90Sm0.10O2−δ thin film has the mixed valence states of...

The physical properties and electronic structure of c-axis-oriented Ti1−xFexO2−δ thin films have been studied by soft X-ray spectroscopy. The c-axis lattice constant increases with increasing Fe concentration. Fe ions have mixed valence states of Fe2+ and Fe3+ with a high-spin configuration. The intensity of the unoccupied Ti 3d state decreases and...

In order to investigate the mechanism of ferromagnetic ordering in the new n-type magnetic semiconductor (In,Fe)As co-doped with Be, we have performed X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) studies of ferromagnetic and paramagnetic samples. The spectral line shapes suggest that the ferromagnetism is intrinsic ori...

In order to reveal many-body interactions in the three-dimensional (3D) perovskite manganite, we have performed an $in$ $situ$ angle-resolved photoemission spectroscopy (ARPES) on La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO) and investigated the behaviors of quasiparticles. We observe quasiparticle peaks around the Fermi momentum, both in the electron and th...

The electronic and magnetic properties of Fe atoms in the ferromagnetic semiconductor (In,Fe)As codoped with Be have been studied by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe $L_{2,3}$ edge. The XAS and XMCD spectra showed simple spectral line shapes similar to Fe metal, but the ratio of the orbital...

The electronic structure of doped Mn in (Ga,Mn)As is studied by resonant inelastic X-ray scattering (RIXS). From configuration-interaction cluster-model calculations, the line shapes of the Mn $L_3$ RIXS spectra can be explained by $d$-$d$ excitations from the Mn$^{3+}$ ground state, dominated by charge-transferred states, rather than a Mn$^{2+}$ g...

The fundamental advantage of soft-X-ray ARPES (SX-ARPES) with photon energies around 1 keV compared to the conventional vacuum-ultraviolet ARPES (VUV-ARPES) is an increase of the photoelectron escape depth. This enhances the bulk sensitivity of the ARPES experiment as well as enables access to buried systems such as interfaces, heterostructures, an...