Control of Hole Density in Russellite Bi2WO6 via Intentional Chemical Doping.

Based on the fundamental design concept of modulating the valence band maximum of oxides and subsequent predictions through computational approaches, several lone-pair ns2-based p-type oxide semiconductors, such as Sn2+- or Bi3+-based complex oxides, have been developed. Thus far, the bandgap can be modified via tuning of the chemical composition, whereas the hole density cannot be intentionally controlled because of the poor chemical stability of Sn2+ and/or the formation of oxygen vacancies. The inability to control hole density prohibits the design and realization of emergent electronic devices based on p- and n-type oxide semiconductors. Herein, we report the control of hole density via intentional chemical doping in polycrystalline Bi2WO6. While the holes of polycrystalline Nb- or Ta-doped Bi2WO6 are strongly trapped by grain boundaries, the hole density obtained at high temperatures monotonically increases with the increase in the doping concentration. This study provides important insights into the development of practical p-type oxide semiconductors.

Destabilization of Sn2+ 5s2 Lone-Pair States of SnO through Dimensional Crossover.

Materials exhibiting unique electronic properties arising from a characteristic crystal structure have physical properties that are sensitive to structural dimensionality. This study involves the destabilization of Sn 5s2 lone-pair states of SnO films by decreasing their structural dimensionality in the out-of-plane direction. The inherent dispersive band structure of the SnO films remained unchanged between 80 and 11 nm. Below 11 nm, their dispersive band structure disappeared, the O/Sn ratio increased, and the carrier type changed from the p type to the n type, whereas the Sn valency remained constant at +2. These unconventional changes arose from the electronic separation corresponding to the Debye length, which is proportional to permittivity, and were attributed to weakened interactions between Sn 5s2 lone-pair electrons. Therefore, designing low-permittivity materials is beneficial for reducing the crystallite size required for stabilizing lone-pair states. These results are essential for designing emergent p-type oxides and improving their semiconducting properties and performance in transparent or high-power electronics.

Beamline commissioning for microscopic measurements with ultraviolet and soft X-ray beam at the upgraded beamline BL-13B of the Photon Factory.

Beamline 13 of the Photon Factory has been in operation since 2010 as a vacuum ultraviolet and soft X-ray undulator beamline for X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and angle-resolved photoelectron spectroscopy (ARPES) experiments. The beamline and the end-station at branch B have been recently upgraded, enabling microscopic XPS, XAS, and ARPES measurements to be performed. In 2015, a planar undulator insertion device was replaced with an APPLE-II (advanced planar polarized light emitter II) undulator. This replacement allows use of linear, circular, and elliptical polarized light between 48 and 2000 eV with photon intensities of 109-1013 photons s-1. For microscopic measurements, a toroidal post-mirror was renewed to have more focused beam with profile sizes of 78 µm (horizontal) × 15 µm (vertical) and 84 µm × 11 µm at photon energies of 100 and 400 eV, respectively. A high-precision sample manipulator composed of an XYZ translator, a rotary feedthrough, and a newly developed goniometer, which is essential for microscopic measurements, has been used to control a sample specimen in six degrees of freedom, i.e. translation in the X, Y, and Z directions and rotation in the polar, azimuthal, and tilt directions. To demonstrate the performance of the focused beams, one- and two-dimensional XPS and XAS scan measurements of a copper grid have been performed. It was indicated from analysis of XPS and XAS intensity maps that the actual spatial resolution can be determined by the beam size.

Characteristic Electronic Structure of SnO Film Showing High Hole Mobility.

Actual knowledge of the intrinsic electronic characteristics of p-type oxide semiconductors should help guide the design of innovative electronic devices. The electronic characteristics of oxide semiconductors in thin-film form potentially differ from those in the bulk form owing to lattice strain. In this Letter, we report on the empirical band structure of stannous oxide (SnO) film, which has been shown to have a higher hole mobility than the theoretically expected values for SnO in the bulk form. In vacuo angle-resolved photoemission spectroscopy measurements reveal that the uppermost valence band is anisotropic between the out-of-plane and in-plane directions, and more dispersive than the theoretical predictions. Our findings unveil the underlying mechanism of the semiconductor properties of SnO films and suggest a suitable device structure based on the electronic characteristics.

Bipolar Semiconducting Properties in α-SnWO4 Based on the Characteristic Defect Structure.

Diodes, memories, logic circuits, and most other current information technologies rely on the combined use of p- and n-type semiconductors. Although oxide semiconductors have many technologically attractive functionalities, such as transparency and high dopability to enable their use as conducting films, they typically lack bipolar conductivity. In particular, the absence of p-type semiconducting properties owing to the innate electronic structures of oxides represents a bottleneck for the development of practical devices. Here, bipolar semiconducting properties are demonstrated in α-SnWO4 within a 100 °C temperature window after appropriate thermal treatment. Comprehensive spectroscopic observations reveal that Sn4+ is present in p-type α-SnWO4 in a notably greater quantity than in n-type. This result strongly suggests that the Sn4+ substitutional defects on the W6+ sites contribute to hole-carrier generation in α-SnWO4. We also find that oxygen vacancies are initially formed in Sn-O-W bonds and migrate to W-O-W bonds with changes in semiconducting properties from p-type to n-type. These findings suggest useful strategies for exploring p-type oxide semiconductors and controlling their carrier type by utilizing the octahedral structure.

Development of a high-precision XYZ translator and estimation of beam profile of the vacuum ultraviolet and soft X-ray undulator beamline BL-13B at the Photon Factory.

A high-precision XYZ translator was developed for the microanalysis of electronic structures and chemical compositions on material surfaces by electron spectroscopy techniques, such as photoelectron spectroscopy and absorption spectroscopy, utilizing the vacuum ultraviolet and soft X-ray synchrotron radiation at an undulator beamline BL-13B at the Photon Factory. Using the high-precision translator, the profile and size of the undulator beam were estimated. They were found to strongly depend on the photon energy but were less affected by the polarization direction. To demonstrate the microscopic measurement capability of an experimental apparatus incorporating a high-precision XYZ translator, the homogeneities of an SnO film and a naturally grown anatase TiO2 single crystal were investigated using X-ray absorption and photoemission spectroscopies. The upgraded system can be used for elemental analyses and electronic structure studies at a spatial resolution in the order of the beam size.

In-gap state generated by La-on-Sr substitutional defects within the bulk of SrTiO3.

Local distortion in the conduction pathway has a significant influence on the conducting properties of oxides. The electronic states induced in the band gap of SrTiO3 by La doping were investigated using photoemission spectroscopy (PES) and soft X-ray emission spectroscopy (SXES); moreover, the local distortion in the conduction pathway was examined using extended X-ray absorption fine structure (EXAFS). An itinerant state and a localized state were observed as a metallic state and an in-gap state, respectively, in the PES spectra and as inelastic peaks in the SXES spectra. This implied that the itinerant state and the in-gap state coexisted within the bulk. From EXAFS results, it was observed that La doped into SrTiO3 substituted Sr and locally distorted the conduction pathway. The results showed that some electrons doped by La-on-Sr substitution are trapped/localized by the local distortion in the conduction pathway, whereas the remaining doped electrons itinerate in the pristine conduction pathway with no distortion.

Accurate and efficient data acquisition methods for high-resolution angle-resolved photoemission microscopy.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental technique in materials science, as it can directly probe electronic states inside solids in energy (E) and momentum (k) space. As an advanced technique, spatially-resolved ARPES using a well-focused light source (high-resolution ARPES microscopy) has recently attracted growing interests because of its capability to obtain local electronic information at micro- or nano-metric length scales. However, there exist several technical challenges to guarantee high precision in determining translational and rotational positions in reasonable measurement time. Here we present two methods of obtaining k-space mapping and real-space imaging in high-resolution ARPES microscopy. One method is for k-space mapping measurements that enables us to keep a target position on a sample surface during sample rotation by compensating rotation-induced displacements (tracing acquisition method). Another method is for real-space imaging measurements that significantly reduces total acquisition time (scanning acquisition method). We provide several examples of these methods that clearly indicate higher accuracy in k-space mapping as well as higher efficiency in real-space imaging, and thus improved throughput of high-resolution APRES microscopy.

Development of laser-based scanning µ-ARPES system with ultimate energy and momentum resolutions.

We have developed a laser-based scanning angle-resolved photoemission spectroscopy system (µ-ARPES) equipped with a high precision 6-axis control system, realizing not only high-resolution photoemission spectroscopy in energy and momentum, but also spatial resolution of a µm scale. This enables our µ-ARPES system to probe fine details of intrinsic electronic states near the Fermi level such as the superconducting gaps and lifetime broadening.

Enhancement of ferromagnetism by oxygen isotope substitution in strontium ruthenate SrRuO3.

The oxygen isotope effect of the ferromagnetic transition in itinerant ferromagnet strontium ruthenate SrRuO3 with a Curie temperature Tc of 160 K is studied. We observed for the first time a shift of ∆Tc ~ 1 K by oxygen isotope substitution of 16O → 18O in SrRuO3 by precise measurements of DC and AC magnetizations. The results surprisingly lead to the noteworthy inverse isotope effect with negative coefficient α = -∂ lnTc/∂ lnM. The Raman spectra indicate that the main vibration frequency of 16O at 363 cm-1 shifts to 341 cm-1 following oxygen isotope substitution 18O. This shift is remarkably consistent with the Debye frequency being proportional to ∝ 1√M where M is the mass of an oxygen atom. The positive isotope shift of ∆Tc can be understood by taking account of the electron-phonon interaction.

'True' bosonic coupling strength in strongly correlated superconductors.

Clarifying the coupling between electrons and bosonic excitations (phonons or magnetic fluctuations) that mediate the formation of Cooper pairs is pivotal to understand superconductivity. Such coupling effects are contained in the electron self-energy, which is experimentally accessible via angle-resolved photoemission spectroscopy (ARPES). However, in unconventional superconductors, identifying the nature of the electron-boson coupling remains elusive partly because of the significant band renormalization due to electron correlation. Until now, to quantify the electron-boson coupling, the self-energy is most often determined by assuming a phenomenological 'bare' band. Here, we demonstrate that the conventional procedure underestimates the electron-boson coupling depending on the electron-electron coupling, even if the self-energy appears to be self-consistent via the Kramers-Kronig relation. Our refined method explains well the electron-boson and electron-electron coupling strength in ruthenate superconductor Sr2RuO4, calling for a critical revision of the bosonic coupling strength from ARPES self-energy in strongly correlated electron systems.

Te concentration dependent photoemission and inverse-photoemission study of FeSe1-xTex.

We have characterized the electronic structure of FeSe1-x Te x for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

Superconductors: unusual oxygen isotope effects in cuprates?

The possibility that a pairing boson might act as the 'glue' to bind electrons into a Cooper pair in superconductors with a high critical temperature (T(c)) is being actively pursued in condensed-matter physics. Gweon et al. claim that there is a large and unusual oxygen-isotope effect on the electronic structure, indicating that phonons have a special importance in high-temperature superconductors. However, we are unable to detect this unusual oxygen-isotope effect in new data collected under almost identical material and experimental conditions. Our findings point towards a more conventional influence of phonons in these materials.