Japanese Journal of Applied Physics

We have discovered a new high-T c oxide superconductor of the Bi-Sr-Ca-Cu-O system without any rare earth element. The oxide BiSrCaCu2O x has T c of about 105 K, higher than that of YBa2Cu3O7 by more than 10 K. In this oxide, the coexistence of Sr and Ca is necessary to obtain high T c.

Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO 2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO 2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has been used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochemical reaction proceeding on a TiO 2 surface when irradiated with ultraviolet light. One includes the photo-induced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO 2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO 2 , particularly in the field of building materials. Here, we review the progress of the scientific research on TiO 2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.

InGaN multi-quantum-well (MQW) structure laser diodes (LDs) fabricated from III-V nitride materials were grown by metalorganic chemical vapor deposition on sapphire substrates. The mirror facet for a laser cavity was formed by etching of III-V nitride films without cleaving. As an active layer, the InGaN MQW structure was used. The InGaN MQW LDs produced 215 mW at a forward current of 2.3 A, with a sharp peak of light output at 417 nm that had a full width at half-maximum of 1.6 nm under the pulsed current injection at room temperature. The laser threshold current density was 4 kA/cm 2 . The emission wavelength is the shortest one ever generated by a semiconductor laser diode.

One of the (Bi 1/2 Na 1/2 )TiO 3 (BNT)-based solid solutions, Ba-modified bismuth sodium titanate, (Bi 1/2 Na 1/2 ) 1- x Ba x TiO 3 (BNBT), is studied for its dielectric and piezoelectric properties as a new group of lead-free piezoelectric ceramics. A rhombohedral (F α )-tetragonal (F β ) morphotropic phase boundary (MPB) is shown to exist at x =0.06∼0.07 by X-ray data, and dielectric and piezoelectric properties. BNBT ceramics with the MPB composition are superior as piezoelectric ceramics in high-frequency ultrasonic applications or as piezoelectric actuator materials because of a lower free permittivity, ε 33 T /ε 0 , and a high electromechanical coupling factor, k t or k 33 , along with high mechanical strength.

Distinct p-type conduction is realized with Mg-doped GaN by the low-energy electron-beam irradiation (LEEBI) treatment, and the properties of the GaN p-n junction LED are reported for the first time. It was found that the LEEBI treatment drastically lowers the resistivity and remarkably enhances the PL efficiency of MOVPE-grown Mg-doped GaN. The Hall effect measurement of this Mg-doped GaN treated with LEEBI at room temperature showed that the hole concentration is ∼2·10 16 cm -3 , the hole mobility is ∼8 cm 2 /V·s and the resistivity is ∼35 Ω·cm. The p-n junction LED using Mg-doped GaN treated with LEEBI as the p-type material showed strong near-band-edge emission due to the hole injection from the p-layer to the n-layer at room temperature.

Dye-sensitized solar cells (DSCs) using titanium dioxide (TiO 2 ) electrodes with different haze were investigated. It was found that the incident photon to current efficiency ( IPCE ) of DSCs increases with increase in the haze of the TiO 2 electrodes, especially in the near infrared wavelength region. Conversion efficiency of 11.1%, measured by a public test center, was achieved using high haze TiO 2 electrodes. This indicates that raising the haze of TiO 2 electrodes is an effective technique for improvement of conversion efficiency.

High-brightness blue, green and yellow light-emitting diodes (LEDs) with quantum well structures based on III-V nitrides were grown by metalorganic chemical vapor deposition on sapphire substrates. The typical green LEDs had a peak wavelength of 525 nm and full width at half-maximum (FWHM) of 45 nm. The output power, the external quantum efficiency and the luminous intensity of green LEDs at a forward current of 20 mA were 1 mW, 2.1% and 4 cd, respectively. The luminous intensity of green LEDs (4 cd) was about 40 times higher than that of conventional green GaP LEDs (0.1 cd). Typical yellow LEDs had a peak wavelength of 590 nm and FWHM of 90 nm. The output power of yellow LEDs was 0.5 mW at 20 mA. When the emission wavelength of III-V nitride LEDs with quantum well structures increased from the region of blue to yellow, the output power decreased dramatically.

We propose a novel material, GaInNAs, that can be formed on GaAs to drastically improve the temperature characteristics ( T 0 ) in long-wavelength-range laser diodes. The feasibility of our proposal is demonstrated experimentally.

1. Introduction - Characteristics of ferroelectrics. 2. Methods for measuring physical properties of ferroelectric materials. 3. Perovskite-type ferroelectrics (Part I). 4. Perovskite-type ferroelectrics (Part II). 5. Lithium niobate and lithium tantalate. 6. Ferroelectric tungsten - bronze type niobate crystals. 7. KDP family, TGS family and other water-soluble ferroelectric crystals. 8. Other ferroelectric crystal materials. 9. Organic ferroelectric materials and piezoelectric composites.

Field deployment of Brillouin optical correlation-domain reflectometry (BOCDR) is hindered by bulky instrumentation. We propose a remote configuration in which long-distance fibers are inserted in optical paths connected to ports 1 and 3 of the circulator, leaving only the fiber under test and circulator at the sensing site. Distributed strain measurements were demonstrated with fibers up to 4 km. Brillouin frequency shift (BFS) strain coefficients remained stable at 322–329 MHz/%, while differential-BFS standard deviations remained 2.0 MHz or less. The configuration enables remote measurement without degrading spatial resolution and can eliminate the need for a dedicated long delay fiber.

Abstract A simple 2D electron spectrometer for use in the tender and hard X-ray regime is proposed based on ray-tracing calculations. The spectrometer comprises a wide-angle deceleration lens (WDL) and a cylindrical mirror analyzer (CMA). Notably, the WDL is designed with a high deceleration ratio and a large focusing angle. This lens allows simple energy filtering by its exit aperture and projects a wide-range 2D angular distribution pattern onto a 2D detector placed at a short distance from the aperture. Here, good energy resolution can be obtained for an annular angular region, but poor energy resolution is unavoidable for a central angular region. This drawback can be compensated for by using the CMA after retracting the 2D detector and inserting an angle-limiting aperture. Moreover, high energy resolution can be achieved in the spectrum measurement mode by using an appropriate angle-limiting slit.

Abstract Optical nanofibers fabricated using a standard oxyhydrogen flame exhibit optical losses at wavelengths around 1385 nm due to absorption by embedded hydroxyl groups, posing a challenge for the realization of quantum electrodynamics systems using ytterbium atoms. Here, we establish a method for the reduction of hydroxyl groups by heating them with a laser guided within the optical nanofiber under vacuum conditions. The temperature of the optical nanofiber during heating is estimated by monitoring the phase shift of the transmitted light using an interferometer. As a result, evidence suggesting that hydroxyl groups were desorbed by laser heating was obtained.

Abstract Heavily boron-doped p -type Si layers were fabricated by thermal diffusion using B 2 O 3 . The effects of precursor concentration and annealing temperature on their electrical, morphological, and structural properties were investigated. The hole carrier concentration estimated from the sheet resistance increased systematically with increasing B 2 O 3 concentration and annealing temperature, reaching 5.85 × 10 19 cm-3 at 850℃ with 1.4% B 2 O 3 . The surface roughness of this sample was 0.143 nm and was further reduced to atomically smooth 0.076 nm by chemical mechanical polishing. X-ray diffraction and reciprocal-space mapping confirmed the preservation of high crystalline quality, while asymmetric rocking curves suggested depth-dependent strain associated with boron incorporation. Micro-Raman mapping revealed spatial variations in the local lattice state, which became more uniform after polishing. These results demonstrate a process for producing heavily doped p -type Si with high crystalline quality and an ultra-flat surface for low-resistance interfaces in Si-based spintronic devices.

Abstract In this study, Nd-Fe-B micromagnets were fabricated using a low-temperature process. The method employed Laser-Induced Forward Transfer (LIFT), utilizing an Nd-Fe-B film magnet prepared via Pulsed Laser Deposition (PLD) as the donor material. Micromagnets were formed by transferring the Nd2Fe14B phase from the donor without annealing. This study presents fundamental experimental results regarding micromagnet development using LIFT in a high-vacuum and demonstrates that coercivity can be enhanced up to approximately 500 kA/m by controlling the laser irradiation conditions. Additionally, we report that comparable coercivity values can be achieved using LIFT in air.

Abstract The energetics of triangular hexagonal boron nitride (hBN) nanoflakes with clean edges are studied using density functional theory (DFT), with particular attention to their size and edge. Formation energies of triangular hBN nanoflakes depend on the flake size, edge termination, and chemical potentials of B and N atoms: The flakes prefer N termination when borazine (B₃N₃H₆) and ammonia borane (BNH₆) are their source molecules, whereas B termination is stabilized for diborane (B₂H₆) molecule. The energetics of the flakes on the hBN sheet are also investigated to provide theoretical insight into the nucleation processes of the hBN bilayer. The flakes are bistable at or around the angles of 0˚ and 60˚, corresponding to AA’ and AB stacking, respectively. The energy barriers for rotation increase with increasing flake size, indicating that the stacking orientation of the hBN bilayer is determined at the early stage of nucleation.

Abstract Chemically cross-linked ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] films were prepared using 1,3-bis(3-aminopropyl)tetramethyldisiloxane (Bis-APTMDS), a diamine cross-linker containing a flexible siloxane segment. Fourier- transform infrared spectroscopy, X-ray photoelectron spectroscopy, and gel fraction analysis confirmed the formation of a chemically cross-linked network between P(VDF-TrFE) chains. Increasing the Bis-APTMDS content altered the surface morphology and slightly reduced the crystallinity while largely preserving the ferroelectric Form I crystalline phase. Cross-linking decreased the Young's modulus and increased the strain at break, indicating enhanced mechanical deformability. Although the positive remanent polarization decreased with increasing cross-linker content under the same applied electric field, it recovered to a value comparable to that of uncross-linked P(VDF-TrFE) under a higher maximum applied electric field. These results demonstrate that Bis-APTMDS cross-linking can enhance the mechanical deformability of P(VDF-TrFE) films while maintaining recoverable ferroelectric polarization, providing a promising strategy for the design of mechanically compliant ferroelectric polymer films for flexible piezoelectric devices.

Abstract Molecular orientation in organic semiconductors has conventionally been evaluated using macroscopic methods such as X-ray analysis. However, it has been difficult to directly evaluate the orientation of charge-carrying molecules under device operating conditions. In the present study, we determined the orientations of anti- and syn-dimethylanthradithiophene (DMADT) by applying operando electron spin resonance (ESR) method to their organic transistors. The molecular orientations were determined by combining observed anisotropic g-factors in angle-dependent ESR spectra and theoretical g-tensor calculated by a density functional theory (DFT) method. We clarified that both anti- and syn-DMADT took end-on orientation on substrate. It was also revealed that molecules were more tilted relative to the substrate in the anti-isomer than in the syn-isomer. These results are consistent with previous X-ray analyses. This work demonstrates that the operando ESR spectroscopy is an effective approach for evaluating the molecular orientation of charge carriers in organic semiconductor devices.

Abstract The leakage current of ScAlN thin films grown on n + -GaN/sapphire substrates by sputtering epitaxy at growth temperatures of 420, 550, and 600°C was investigated. As the growth temperature decreased, the device-to-device variation in leakage current was suppressed, and the average breakdown field increased from 3.3 to 4.8 MV/cm. The measured leakage current far exceeded that predicted by Fowler–Nordheim tunneling. The temperature-dependent current–voltage characteristics exhibited an exponential increase in leakage current with increasing temperature under both forward and reverse bias. Arrhenius analysis revealed that the activation energy depended on the applied electric field, with values typically ranging from approximately 0.4 to 0.6 eV. This field dependence suggests that multiple transport processes may contribute to the leakage current.

Abstract AMPA-type glutamate receptors (AMPARs) regulate synaptic function via lateral diffusion. Direct methods for manipulating their dynamics on the cell membrane remain limited. Optical trapping has emerged as a non-contact tool for manipulating biomolecules in biological systems. In this study, we investigated the diffusion dynamics of quantum dot-labeled AMPARs (QD-AMPARs) on cultured rat hippocampal neurons during optical trapping. Single-particle tracking analysis revealed that both the diffusion coefficient and step size of QD-AMPARs decreased with increasing laser power and laser irradiation time. Although the estimated optical trapping potential depth for the QD-AMPAR complex remained well below the threshold required for stable optical trapping, colocalization analysis demonstrated that QD-AMPARs were preferentially confined near postsynaptic regions enriched with the scaffold protein PSD-95. These results indicate that optical trapping, when combined with interactions involving postsynaptic scaffold proteins, provides a strategy for the non-invasive manipulation of membrane protein dynamics in living neurons.