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We report on the lattice and band structures of complete solid-solution β-(AlxGa1–x)2O3 (x = 0–1) films that were epitaxially grown on β-Ga2O3 (100) substrates by using oxygen-radical-assisted puls...

Angle-resolved photoemission spectroscopy using a micro-focused beam spot (micro-ARPES) is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axes motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Auphotolithography pattern on Si signifies the beam spot size of 10 $\mu$m (horizontal) x 12 $\mu$m (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal., Comment: 21 pages, 8 figures

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, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO3 (SVO) sandwich a barrier layer of the band insulator SrTiO3. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons., The metal-insulator transition is typically controlled by carrier accumulation or chemical doping. Here, the authors realize an alternative method based on resonant tunnelling in a double quantum well structure of strongly correlated oxides, which offers practical advantages over conventional methods.

This study investigated the electronic structure of $\mathrm{Sr}{\mathrm{Ti}}_{1\text{\ensuremath{-}}x}{\mathrm{V}}_{x}{\mathrm{O}}_{3}$ (STVO) thin films, which are solid solutions of strongly correlated transparent conductive oxide (TCO) $\mathrm{SrV}{\mathrm{O}}_{3}$ and oxide semiconductor $\mathrm{SrTi}{\mathrm{O}}_{3}$, using in situ photoemission spectroscopy. STVO is one of the most promising candidates for correlated-metal TCO because it has the capability of optimizing the performance of transparent electrodes by varying $x$. Systematic and significant spectral changes were found near the Fermi level (${E}_{\mathrm{F}}$) as a function of $x$, while the overall electronic structure of STVO is in good agreement with the prediction of band-structure calculations. As $x$ decreases from 1.0, spectral weight transfer occurs from the coherent band near ${E}_{\mathrm{F}}$ to the incoherent states (lower Hubbard band) around 1.0--1.5 eV. Simultaneously, a pseudogap is formed at ${E}_{\mathrm{F}}$, indicating a significant reduction in quasiparticle spectral weight within close vicinity of ${E}_{\mathrm{F}}$. This pseudogap seems to evolve into an energy gap at $x=0.4$, suggesting the occurrence of a composition-driven metal-insulator transition. From angle-resolved photoemission spectroscopic results, the carrier concentration $n$ changes proportionally as a function of $x$ in the metallic range of $x=0.6--1.0$. In contrast, the mass enhancement factor, which is proportional to the effective mass (${m}^{*}$), does not change significantly with varying $x$. These results suggest that the key factor of $n/{m}^{*}$ in optimizing the performance of correlated-metal TCO is tuned by $x$, highlighting the potential of STVO to achieve the desired TCO performance in the metallic region.

The long-standing problem surrounding the metal-insulator transition of VO${}_{2}$ is the relative role of Mott and Peierls instabilities. Here, the authors investigate the change in electronic and crystal structures of nanostructured VO${}_{2}$ films, where the balance between the two instabilities is controlled as a function of thickness. $I\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ photoemission spectroscopy reveals that VO${}_{2}$ in the thin limit exhibits a Mott insulating nature without the V-V dimerization characteristic of bulk VO${}_{2}$, owing to the dominance of the Mott instability over the Peierls one.

Perovskite-type manganites, which are well-known for their intriguing physical properties such as colossal magnetoresistance (CMR) and half metalicity, have been considered as candidate materials for spintronics. However, their ferromagnetic (FM) properties are often suppressed in thin films when the thickness is reduced down to several monolayers (MLs). In order to investigate how the magnetic phases evolve near the paramagnetic (PM)-to-FM phase transition boundary, we have performed temperature-dependent x-ray magnetic circular dichroism (XMCD) experiments on a La$_{1-x}$Sr$_{x}$MnO$_3$ (LSMO, $x=0.4$) thin film, whose thickness (8 ML) is close to the boundary between the FM-metallic and the PM-insulating phases. By utilizing the element-selectiveness of XMCD, we have quantitatively estimated the fractions of the PM and superparamagnetic (SPM) phases as well as the FM one as a function of temperature. The results can be reasonably described based on a microscopic phase-separation model., Comment: 6 pages, 3 figures

CaFe 2 O 4 (CFO) is an oxide semiconductor being usable for a photocathode, but its single crystal is so far grown only by a high-pressure bulk synthesis method. Using pulsed-laser deposition, we have obtained the (010)-oriented CFO films on TiO 2 (100) substrates. The epitaxial structure included twin domains with 1.5° tilting from the surface normal and sixfold domains with in-plane rotations of ± 70° with respect to the b axis of TiO 2 , which was explained by taking a number of probable arrangements between FeO 6 and TiO 6 octahedra into account. X-ray absorption spectroscopy and ultra-violet-visible-near-infrared spectroscopy measurements revealed the electronic properties similar to those of bulk and thus the CFO films would be suitable to the investigation of photocathodic performance.

We performed systematic study on electronic structures and photoelectrochemical (PEC) properties of ilmenite-type MTiO3 (M = Mn, Fe, Co, and Ni) thin-film photoanodes for water oxidation reactions. Single-phase MTiO3 films were grown on α-Al2O3 substrates by using pulsed-laser deposition. Formation of the ilmenite-type structures and oxidation states of M2+Ti4+O3 were revealed by X-ray diffraction and X-ray absorption spectroscopy, respectively. The PEC performance in water was investigated by linear sweep voltammetry under Xe-lamp illumination and incident photon to conversion efficiency measurements under monochromatic illumination. We found that NiTiO3 acted as the most effective photoanode among MTiO3. The mechanism of the different PEC performance was discussed from the viewpoint of the electronic structures investigated by photoemission spectroscopy and band calculation based on density functional theory. We revealed that the stronger hybridization between O 2p and M 3d states was responsible for the more efficient photoanodic activity among ilmenite-type MTiO3.

We analyze the effects of microwave applied in the process of photoelectrochemical deposition of cobalt-based cocatalysts, Co–Pi, onto well-orientated flat α-Fe2O3 thin films, which were fabricated by pulsed laser deposition. As compared with conventional heating, microwave significantly affects the morphology, chemical composition, and photocatalytic activity of Co-Pi/α-Fe2O3 composite. A significant enhancement in photocurrent related to photocatalytic water oxidation is achieved by the Co–Pi catalyst prepared under microwave irradiation. This, along with its interfacial electron-transfer properties, is studied by means of electrochemical impedance spectroscopy.

Nitrogen (N) is a potential acceptor dopant making a $p$ -type semiconductor of Ga 2 O 3 . Considering highly compensative nature of metal oxides, however, high dopant concentration with low defect density is essential to the acceptor activation. In this study, we employed nitric oxide (NO) as a source of O and N during pulsed-laser deposition (PLD) of $\beta$ -Ga 2 O 3 :N films. The N concentration of the films prepared on $\beta$ -Ga 2 O 3 (100) substrates was found to be ~1021 cm−3, which is the highest ever attained. Despite of such a high concentration, the films showed high crystallinity. Moreover, both of N concentration and crystallinity of the films grown under NO plasma increased with increasing growth temperature. As a result, highly insulating properties were obtained for films grown under NO plasma. These results suggest that PLD under NO plasma is effective for high concentration N-doping into device-grade $\beta$ -Ga 2 O 3 films.

$\beta$ -Ga 2 O 3 is one of the wide-gap semiconductors investigated actively in recent years, and evaluation of its electrical properties is needed for various device applications. However, the presence of Si impurity on a $\beta$ -Ga 2 O 3 substrate has been a tantalized issue, which gives rise to parasitic conduction parallel to the interface with an overgrown epilayer. Therefore, this interface conduction must be eliminated in order to measure the electrical characteristics inherent in the epilayers. Using pulsed-laser deposition, we revealed that a semi-insulatiug $\beta$ -Ga 2 O 3 :Fe intermediate layer, as thin as 7 nm, was effective to passivate the Si impurity and the interface conduction became negligibly low. Moreover, we achieved room-temperature Hall mobility as high as 28 cm2V−1s−1 for the $\beta$ -Ga 2 O 3 :Si film grown on the intermediate layer. These results demonstrate a facile route to access fundamental electrical properties of homoepitaxial $\beta$ -Ga 2 O 3 films.

The bipolaronic insulating (BI) state can cause the emergent superconducting phase when the strength of electron–phonon interaction is deliberately reduced. In this report, we have investigated ele...

Epitaxial α-Fe2O3 thin films on conducting Ta-doped SnO2 bottom layers are prepared by pulsed-laser deposition in order to elucidate variation of the photoelectrochemical (PEC) properties for the water oxidation reaction on the α-Fe2O3 photoanodes with different orientation and thickness. Cyclic voltammetry under illumination shows that current density (photocurrent onset potential) of the m-axis oriented samples is considerably larger (lower) than that of the c-axis oriented ones. Using electrochemical impedance spectroscopy under illumination, we ascribe these behaviors to difference in surface states between c- and m-planes besides anisotropic conductivity along the individual axes. Our study demonstrates the power of utilizing well-defined epitaxial thin films to investigate the anisotropic PEC properties of visible-light-driven photoelectrodes.

We report on the epitaxial growth of titanium oxynitride (TiOxNy) films on α-Al2O3 (0001) substrates by using a pulsed-laser deposition technique and their optical properties. Using TiN as a solid source, N content (y) in the films was tuned by changing the partial pressure of O2 gas. The crystalline phase was found to evolve up to y ≈ 0.4 either from rock-salt type or from rutile-type structures. The optical absorption spectra of the films had two distinct components in regions of 0.44–2.4 eV and 2.4–6 eV, which arise from d–d and charge-transfer transitions, respectively. The former transition decreased with decreasing y and vanished at y ≈ 0.26, where a fundamental absorption edge due to the latter transition was found to be 2.4 eV. The results demonstrate a range of anion compositions in the TiOxNy being suitable for visible-light harvesting applications.

In ${\mathrm{CsW}}_{2}{\mathrm{O}}_{6}$, which undergoes a metal-insulator transition (MIT) at 213 K, the emergence of exotic properties associated with rattling motion of Cs is expected owing to its characteristic $\ensuremath{\beta}$-pyrochlore-type structure. However, a hurdle for crystal growth hampers elucidation of detailed properties and mechanisms of the MIT. Here we report on the epitaxial growth of $\ensuremath{\beta}$-pyrochlore-type ${\mathrm{CsW}}_{2}{\mathrm{O}}_{6}$ films and their electronic properties across the MIT. Using pulsed-laser deposition technique, we grew single-crystalline ${\mathrm{CsW}}_{2}{\mathrm{O}}_{6}$ films exhibiting remarkably lower resistivity compared with a polycrystalline bulk and sharp MIT around 200 K. Negative magnetoresistance and a positive Hall coefficient were found, which became pronounced below 200 K. The valence-band and core-level photoemission spectra indicated the drastic changes across the MIT. In the valence-band photoemission spectrum, the finite density of states was observed at the Fermi level in the metallic phase. In contrast, an energy gap appeared in the insulating phase. The split of $\mathrm{W}\phantom{\rule{0.28em}{0ex}}4f$ core-level spectrum suggested the charge disproportionation of ${\mathrm{W}}^{5+}$ and ${\mathrm{W}}^{6+}$ in the insulating phase. The change of spectral shape in the $\mathrm{Cs}\phantom{\rule{0.28em}{0ex}}4d$ core levels reflected the rattling motion of ${\mathrm{Cs}}^{+}$ cations. These results strongly suggest that ${\mathrm{CsW}}_{2}{\mathrm{O}}_{6}$ is an exotic material in which MIT is driven by the charge disproportionation associated with the rattling motion.

Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer., Comment: 20 pages, 4 figures

We report structural, electrical, and optical properties of Si-doped γ-Ga 2 O 3 films epitaxially grown on (100) MgAl 2 O 4 substrate by pulsed-laser deposition. The γ-Ga 2 O 3 :Si films of a metastable spinel phase had neither secondary phase nor rotation domain. A highly doped film exhibited n-type conductivity with a carrier concentration of 1.8×10 19 cm −3 and a Hall mobility of 1.6 cm 2 V −1 s −1 at 300 K. Donor activation energy was estimated to be less than 7 meV from nearly temperature-independent transport properties down to 77 K. The successful impurity doping indicates that γ-Ga 2 O 3 can be used as an n-type wide-band-gap semiconductor.

Creation and characterization of mixed valence states in transition-metal oxides are a fundamental approach to search for the unprecedented electronic and magnetic properties. In contrast to complex oxides, mixed-valence simple oxides tend to form binary or ternary phases, and turning a valence from one to next must be accompanied by structural transformations owing to a lower tolerance for oxygen non-stoichiometry. In this paper, epitaxial growth and transport properties of low-valence titanate thin films are reported to shed light on recently discovered superconducting γ-phase Ti3O5 (γ-Ti3O5). Single-phase TiO and Ti2O3 films and eutectic films including TiO, Ti2O3, and γ-Ti3O5 phases were independently grown on α-Al2O3 (0001) substrates by using pulsed-laser deposition. The X-ray diffraction measurements revealed clear epitaxial relationships with substrates and among three eutectic phases. Temperature dependence of the resistivity revealed that the γ-Ti3O5-rich films exhibited superconductivity with a maximum of transition temperature (TC) of 6.3 K. Distinct effects of the phase fraction on TC are found between TiO- and Ti2O3-enriched samples, suggesting the complex mechanisms of the superconducting proximity effect.

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 quantum phase transitions has been investigated by changing a limited number of external parameters such as pressure and magnetic field. We propose a new approach for investigating quantum criticality by changing the strength of quantum fluctuation that is controlled by the dimensional crossover in metallic quantum well (QW) structures of strongly correlated oxides. With reducing layer thickness to the critical thickness of metal-insulator transition, crossover from a Fermi liquid to a non-Fermi liquid has clearly been observed in the metallic QW of SrVO$_3$ by \textit{in situ} angle-resolved photoemission spectroscopy. Non-Fermi liquid behavior with the critical exponent ${\alpha} = 1$ is found to emerge in the two-dimensional limit of the metallic QW states, indicating that a quantum critical point exists in the neighborhood of the thickness-dependent Mott transition. These results suggest that artificial QW structures provide a unique platform for investigating novel quantum phenomena in strongly correlated oxides in a controllable fashion., Comment: 6 pages, 3 figures

Measurements of electrical resistance at high pressure were performed up to 2.5 GPa for the insulating and the superconducting epitaxial Ti4O7 films of Magneli phase. For the insulating film, the m...

We report on impacts of oxygen-radical (O ⁎ ) atmosphere for pulsed-laser deposition (PLD) of β -Ga 2 O 3 and β -(Al x Ga 1− x ) 2 O 3 films on (010) β -Ga 2 O 3 substrate in comparison with conventional PLD in O 2 atmosphere. Severe sublimation of Ga species arising from insufficient oxidation in the O 2 atmosphere resulted in substantial decrease in growth rate of the homoepitaxial films and condensation of Al content in the (Al x Ga 1− x ) 2 O 3 films. In the case of O ⁎ -assisted PLD, it was found that the growth rate of homoepitaxial films greatly recovered, and the Al content remained nearly identical to that in the target. Moreover, the use of O ⁎ allowed to reduce surface roughness of homoepitaxial films. These results indicate that O ⁎ -assisted PLD is a powerful tool for fabricating β -Ga 2 O 3 -based heterostructures.

In the perovskite-type Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO), one can control the transport and magnetic properties through varying Ce content. In the case of thin films, the properties can also be controlled by epitaxial strain from the substrate through changing it such as YAlO$_{3}$ (YAO), NdAlO$_{3}$ (NAO), and LaSrAlO$_{4}$ (LSAO). However, one cannot measure the magnetization of thin films on NAO substrates by conventional magnetization measurements because of the strong paramagnetic signals from the Nd$^{3+}$ ions. In order to eliminate the influence of Nd$^{3+}$ and to identify magnetic phases of the CCMO thin films, we have performed element-selective X-ray magnetic circular dichroism (XMCD) measurements of the Mn 2{\it p} core level. By studying the anisotropy of the XMCD intensity, we could unambiguously determine the magnetic phase diagram of the CCMO thin films., 8 pages, 3 figures, 1 table, accepted Solid State Communications

We report epitaxial structures and physical properties of double-perovskite Sr2CoRuO6 films grown using pulsed-laser deposition. Samples with a degree of Co/Ru order of 2–73% were obtained by changing growth temperature. X-ray absorption spectroscopy (XAS) on the highest ordered sample revealed that Co ions were trivalent with a high-spin configuration and Ru ions were pentavalent. We found large differences in magnetization and resistivity between the highest and lowest ordered samples as well as the absence of strong magnetism and metallicity, which are common characteristics of SrCoO3 and SrRuO3. Using resonant photoemission spectroscopy and XAS, dominant d-orbital components at the top of the occupied state (the bottom of the unoccupied state) were identified to be Ru 4d t 2g (Co 3d and Ru 4d t 2g ). These results suggest that the ground state of double-perovskite Sr2CoRuO6 is a ferrimagnetic insulator.

We have demonstrated a strain-induced metal-insulator transition for weak Jahn-Teller effects in a ${t}_{2g}$ electron system of $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$, which has metallic ground states in the bulk. A clear variation of electronic properties in $\mathrm{S}{\mathrm{m}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$ thin films was investigated as a function of epitaxial stain by changing a substrate and the film thickness. Under strong biaxial tensile strain, metallic behaviors were completely eliminated, as evidenced by the temperature dependence of resistivity. The observed metal-insulator transition was consistent with the behavior of the Ti $3d$ density of states at the Fermi level seen in valence band hard x-ray photoemission spectra.

Two-dimensional (2D) metallic states formed on the ZnO(10$\bar{1}$0) surface by hydrogen adsorption have been investigated using angle-resolved photoelectron spectroscopy (ARPES). The observed metallic state is characterized by a peak-dip-hump structure at just below the Fermi level and a long tail structure extending up to 600 meV in binding energy. The peak and hump positions are separated by about 70 meV, a value close to the excitation energy of longitudinal optical (LO) phonons. Spectral functions formulated on the basis of the 2D electron-phonon coupling well reproduce the ARPES intensity distribution of the metallic states. This spectral analysis suggests that the 2D electrons accumulated on the ZnO surface couple to the LO phonons and that this coupling is the origin of the anomalous long tail. Our results indicate that the 2D electrons at the ZnO surface are described as the electron liquid model., 5 pages, 4 figures

In order to reveal the many-body interactions in three-dimensional perovskite manganites that show colossal magnetoresistance, we performed an in situ angle-resolved photoemission spectroscopy on La_{0.6}Sr_{0.4}MnO_{3} and investigated the behavior of quasiparticles. We observed quasiparticle peaks near the Fermi momentum in both the electron and the hole bands, and clear kinks throughout the entire hole Fermi surface in the band dispersion. This isotropic behavior of quasiparticles and kinks suggests that polaronic quasiparticles produced by the coupling of electrons with Jahn-Teller phonons play an important role in the colossal magnetoresistance properties of the ferromagnetic metallic phase of three-dimensional manganites.

In this article, we review recent progress in angle-resolved photoemission (ARPES) studies of the Mott-Hubbard-type correlated electron systems SrVO3. It has the d1 electron configuration and is an ideal model compound to study electron correlation effects in normal metal. ARPES studies of bulk single-crystal SrVO3 and CaVO3 have revealed the difference in the mass renormalization of electrons between them. In-situ ARPES studies of thin films fabricated by the pulsed laser deposition method have clarified not only quasi-particle dispersions, which show a kink like high-Tc cuprates, but also finite dispersions in the incoherent part. Self-energy in a wide energy range has been deduced from the ARPES spectral weight using Kramers-Kronig transformation. The obtained self-energy has several structures which yield the incoherent feature and a pseudogap-like dip similar to the high-Tc cuprates. Quantum-well states in ultrathin films of SrVO3 have revealed sub-bands with correlated electrons. These findings of electron correlation effects outlined in the present article would provide a starting point not only for fundamental condensed-matter physics but also for the development of new devices with correlated electrons., Comment: 13 pages, 9 figures

We report epitaxial synthesis of superconducting CsxWO3 (x = 0.11, 0.20, 0.31) films on Y-stabilized ZrO2 (111) substrates. The hexagonal crystal structure was verified not only for composition within the stable region of bulk (x = 0.20, 0.31), but also for the out-of-range composition (x = 0.11). The onset of superconducting transition temperature (TC) was recorded 5.8 K for x = 0.11. We found strong correlation between TC and c-axis length, irrespective of the Cs content. The results indicate that hidden superconducting phase region of hexagonal tungsten bronze is accessible by using epitaxial synthesis of lightly doped films., Comment: 17 pages, 4 figures

Magnetic anisotropy of epitaxially grown thin films is affected by the strain from the substrates due to a combined effect of distorted electronic structure and spin-orbit interaction (SOI). As an inverse process, one expects an anisotropy of the electronic structure induced by magnetization in the presence of SOI. We have studied the charge-density anisotropy induced by magnetization in thin films of the ferromagnetic metal La$_{1-x}$Sr$_{x}$MnO$_3$ via x-ray magnetic linear dichroism (XMLD). XMLD measurements on thin films with various thicknesses have shown that the XMLD intensity is proportional to the square of the ferromagnetic moment. Using the XMLD sum rule and cluster-model calculation, it has been shown that more Mn 3$d$ electrons are distributed in orbitals elongated along the direction parallel to the spin polarization than in orbitals elongated in the direction perpendicular to it. The cluster-model calculation has shown that the effect of tensile strain from the SrTiO$_3$ substrate on the XMLD spectra is also consistent with the observed XMLD spectral line shapes., Comment: 12 pages, 4 figures

We investigated electronic properties of Ti2O3 films with film thickness of ∼150 nm. The temperature dependence of resistivity indicated characteristic phase transitions. The insulator-to-metal transition (IMT) temperature (TIMT) deviated from bulk TIMT of ∼450 K. The higher and lower TIMTs of 450–600 K and 200–300 K were found in a- and c-axes oriented films, respectively. In addition, the large anisotropy in conductivity parallel and perpendicular to the c-axis direction was observed in the insulating phase, which was attributed to large difference in Hall mobility. The higher TIMT far above room temperature and the large difference in resistivity across the IMT were promising characteristics for application of Ti2O3 films in Mottronics.

High-quality β-(AlxGa1−x)2O3 (x = 0–0.37) films were epitaxially grown on β-Ga2O3 (100) substrates by oxygen-radical-assisted pulsed-laser deposition with repeating alternate ablation of single crystals of β-Ga2O3 and α-Al2O3. The bandgap was tuned from 4.55 ± 0.01 eV (x = 0) to 5.20 ± 0.02 eV (x = 0.37), where bowing behavior was observed. The band alignment at the β-(AlxGa1−x)2O3/β-Ga2O3 interfaces was found to be type-I with conduction- and valence-band offsets of 0.52 ± 0.08 eV (0.37 ± 0.08 eV) and 0.13 ± 0.07 eV (0.02 ± 0.07 eV) for x = 0.37 (0.27), respectively. The large conduction-band offsets are ascribed to the dominant contribution of the cation-site substitution to the conduction band.

Transition metal oxides display various electronic and magnetic phases such as high-temperature superconductivity. Controlling such exotic properties by applying an external field is one of the biggest continuous challenges in condensed matter physics. Here, we demonstrate clear superconductor-insulator transition of LiTi2O4 films induced by Li-ion electrochemical reaction. A compact electrochemical cell of pseudo-Li-ion battery structure is formed with a superconducting LiTi2O4 film as an anode. Li content in the film is controlled by applying a constant redox voltage. An insulating state is achieved by Li-ion intercalation to the superconducting film by applying reduction potential. In contrast, the superconducting state is reproduced by applying oxidation potential to the Li-ion intercalated film. Moreover, superconducting transition temperature is also recovered after a number of cycles of Li-ion electrochemical reactions. This complete reversible transition originates in difference in potentials required for deintercalation of initially contained and electrochemically intercalated Li+ ions.

In situ angle-resolved photoemission spectroscopy (ARPES) has been performed on SrVO_{3} ultrathin films, which show metallic quantum well (QW) states, to unveil the origin of the anomalous mass enhancement in the QW subbands. The line-shape analysis of the ARPES spectra reveals that the strength of the electron correlation increases as the subband bottom energy approaches the Fermi level. These results indicate that the anomalous subband-dependent mass enhancement mainly arises from the quasi-one-dimensional character of confined V 3d states as a result of their orbital-selective quantization.

Titanium dioxide is one of the most popular compounds among simple oxides. Except for the fully oxidized titanate, titanium oxides have partially filled d states and their exotic properties have captured attention. Here, we report on the discovery of superconductivity in Ti4O7 and γ-Ti3O5 in a thin film form. The epitaxial Ti4O7 and γ-Ti3O5 thin films were grown using pulsed-laser deposition on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 and α-Al2O3 substrates, respectively. The highest superconducting transition temperatures are 3.0 K and 7.1 K for Ti4O7 and γ-Ti3O5, respectively. The mechanism behind the superconductivity is discussed on the basis of electrical measurements and previous theoretical predictions. We conclude that the superconductivity arises from unstabilized bipolaronic insulating states with the assistance of oxygen non-stoichiometry and epitaxial stabilization.

In situ angle-resolved photoemission spectroscopy (ARPES) has been performed on SrVO_{3} ultrathin films, which show metallic quantum well (QW) states, to unveil the origin of the anomalous mass enhancement in the QW subbands. The line-shape analysis of the ARPES spectra reveals that the strength of the electron correlation increases as the subband bottom energy approaches the Fermi level. These results indicate that the anomalous subband-dependent mass enhancement mainly arises from the quasi-one-dimensional character of confined V 3d states as a result of their orbital-selective quantization.

$\mathit{In}$ $\mathit{situ}$ photoemission spectroscopy and x-ray absorption spectroscopy (XAS) have been performed on LaNiO${}_{3}$ (LNO) ultrathin films grown on LaAlO${}_{3}$ substrates to investigate the origin of the thickness-dependent metal-insulator transition (MIT). With decreasing film thickness, the progressive weakening of a quasiparticle peak at the Fermi level (${E}_{F}$) occurs below 10 monolayer (ML), and the further depletion of spectral weight at ${E}_{F}$ leads to pseudogap behavior at 3--6 ML. The pseudogap finally evolves into a full gap, indicating that the thickness-dependent MIT takes place at a critical film thickness of 2--3 ML. The observed spectral behavior is in line with the transport properties of LNO ultrathin films. The thickness dependence of the spectral intensity is compared with realistic multiorbital dynamical mean-field theory. The experimental spectral function was found to depend on the film thickness more strongly than the theoretical one for thinner systems, indicating that the thickness-dependent MIT in LNO is caused by the crossover from three to two dimensions, during which the spatial correlations are progressively enhanced. The XAS results suggest that a charge disproportionate state is strongly suppressed in LNO ultrathin films plausibly as a result of epitaxial strain from the substrates. These results strongly suggest that a novel insulating state is realized in LNO films at a thin limit.

In situ photoemission spectroscopy and x-ray absorption spectroscopy (XAS) have been performed on LaNiO3 (LNO) ultrathin films grown on LaAlO3 substrates to investigate the origin of the thickness-dependent metal insulator transition (MIT). With decreasing film thickness, the progressive weakening of a quasiparticle peak at the Fermi level (EF ) occurs below 10 monolayer (ML), and the further depletion of spectral weight at EF leads to pseudogap behavior at 3–6 ML. The pseudogap finally evolves into a full gap, indicating that the thickness-dependent MIT takes place at a critical film thickness of 2–3 ML. The observed spectral behavior is in line with the transport properties of LNO ultrathin films. The thickness dependence of the spectral intensity is compared with realistic multiorbital dynamical mean-field theory. The experimental spectral function was found to depend on the film thickness more strongly than the theoretical one for thinner systems, indicating that the thickness-dependent MIT in LNO is caused by the crossover from three to two dimensions, during which the spatial correlations are progressively enhanced. The XAS results suggest that a charge disproportionate state is strongly suppressed in LNO ultrathin films plausibly as a result of epitaxial strain from the substrates. These results strongly suggest that a novel insulating state is realized in LNO films at a thin limit.

Polar-nonpolar interfaces between insulating LaFeO3 (LFO) and semiconducting SrTiO3 (STO) were investigated to elucidate effects of built-in potential on photocarrier dynamics during water oxidation reactions. The LFO films were grown on the TiO2- and SrO-terminated (001) Nb-doped STO substrates by using pulsed-laser deposition. The photocurrent-voltage curves varied depending on the interface termination. Potential profiles across the interface were established from these curves as well as capacitance-voltage curves. The presence of a depletion (accumulation) region near the SrO- (TiO2-) terminated interface facilitates (suppresses) the extraction of photocarriers generated in STO. On the other hand, the difference in the built-in potential in LFO barely reflects the magnitude of the photocurrent.