Jun Yamamoto

Asymmetric synthesis of pyrimidyl alkanol without adding chiral substances by the addition of diisopropylzinc to pyrimidine-5-carbaldehyde in conjunction with asymmetric autocatalysis

Enantiomerically enriched pyrimidyl alkanol with either S or R configuration was obtained stochastically from the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc without adding chiral substances in conjunction with subsequent asymmetric autocatalysis, leading to amplification of the enantiomeric excess.

Transparent nematic phase in a liquid-crystal-based microemulsion

Complex fluids are usually produced by mixing together several distinct components, the interactions between which can give rise to unusual optical and rheological properties of the system as a whole. For example, the properties of microemulsions (composed of water, oil and surfactants) are determined by the microscopic structural organization of the fluid that occurs owing to phase separation of the component elements. Here we investigate the effect of introducing an additional organizing factor into such a fluid system, by replacing the oil component of a conventional water-in-oil microemulsion with an intrinsically anisotropic fluid—a nematic liquid crystal. As with the conventional case, the fluid phase-separates into an emulsion of water microdroplets (stabilized by the surfactant as inverse micelles) dispersed in the ‘oil’ phase. But the properties are further influenced by a significant directional coupling between the liquid-crystal molecules and the surfactant tails that emerge (essentially radially) from the micelles. The result is a modified bulk-liquid crystal that is an ordered nematic at the mesoscopic level, but which does not exhibit the strong light scattering generally associated with bulk nematic order: the bulk material here is essentially isotropic and thus transparent.

One-pot asymmetric autocatalytic reaction with remarkable amplification of enantiomeric excess

Abstract A trace amount ( ca. 3 mg) of 2-methylpyrimidyl alkanol with only a slight enantiomeric excess (0.2 – 0.3% e.e.) was auto-multiplied with dramatic amplification of enantiomeric excess (up to ca. 90% e.e.) in one-pot asymmetric autocatalytic reaction using diisopropylzinc and 2-methylpyrimidine-5-carbaldehyde.

Band-Gap Energy and Effective Mass of BGaN

The band-gap energies and effective masses of boron gallium nitride (BGaN) ternaries were estimated. The band-gap energies of BxGa1-xN ternaries increase proportionally with the boron composition x. However, the effective masses of electrons and holes in BGaN ternaries are almost equal to those of GaN. We have fabricated BGaN layers on 6H–SiC substrates using metal-organic vapor phase epitaxy (MOVPE). We examined the photoluminescence spectra of these layers. The band-gap energies and effective masses estimated using the photoluminescence results are consistent with those estimated theoretically.

Dependence of Crystal Quality on Residual Strain in Strain-Controlled Thin AlN Layer Grown by Metalorganic Vapor Phase Epitaxy

Strain-controlled AlN layers were grown on (0001) 6H–SiC with a (GaN/AlN) buffer layer by metalorganic vapor phase epitaxy using an alternating-source-feeding technique (ASF). The successful strain and quality control of the thin AlN layer were experimentally demonstrated down to 0.05 µm by changing the growth conditions of the (GaN/AlN) buffer layer. The quality of the AlN layer was evaluated by not only X-ray diffraction (θ-2θ) but an X-ray rocking curve (ω-scan) from the viewpoint of c-axis tilting. The crystal quality was dependent on the residual strain in the AlN layer. The (GaN/AlN)-buffer layer is effective in improving the quality of the AlN layer.

Ultrasonic Absorption Anomaly of Brain Tissue

The internal friction of bovine brain tissue has been studied by the measurements of longitudinal ultrasonic propagation in the megahertz region and Youngs modulus in the frequency region from 10 to 1000 Hz. The ultrasonic absorption of bovine brain tissue increased monotonously as the frequency was decreased. The imaginary part of Youngs modulus appeared to have a finite value at the lower frequency limit. The results of the two kinds of mechanical measurements led to the conclusion that apparent viscosity of brain tissue increased anomalously as the frequency decreased from 5 MHz to 10 Hz. There are two possible mechanisms for the ultrasonic behaviors of brain tissue: anomalous absorption due to the existence of lamellar structures and very broad distribution of relaxation phenomena. Experimental methods to determine the contribution of the two mechanisms were discussed based upon the essential difference in the expected behaviors of the real part of moduli.

Possibility of Strain Control in AlN Layer Grown by MOVPE on (0001) 6H‐SiC with GaN/AlN Buffer

A tensile-strained AlN layer along the a-axis was grown on a (0001) 6H-SiC substrate with a (GaN/AlN) buffer layer by metalorganic vapor phase epitaxy using an alternating source feeding (ASF) technique. It was experimentally demonstrated that the strain in the AlN layer was affected by the growth conditions of the buffer layer. On the other hand, the AlN layer grown directly on a substrate without the buffer layer exhibits compressive strain along the a-axis. Strain control in the AlN layer using the ASF buffer is proposed.

Room-Temperature Photoluminescence from BAlGaN-Based Double or Single Heterostructures for UV Laser Diode

A BGaN/AlN double heterostructure or BAlGaN/AlN single was grown on 6H-SiC substrate by metalorganic vapor phase epitaxy (MOVPE). The relation between the band-gap energy and boron composition was experimentally determined for BGaN. Photoluminescence (PL) spectra were observed with 2.7% boron composition at room temperature. The PL spectrum of BAlGaN, which was near the band-edge, was observed.

Improved Optical Quality of BAlGaN/AlN MQW Structure Grown on 6H-SiC Substrate by Controlling Residual Strain Using Multi-Buffer Layer

BAlGaN and (BAlGaN/AlN) multi-quantum-wells (MQWs) structure were grown on 6H-SiC substrate by a low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). Estimated boron compositions of the BAlGaN quantum wells by an Auger electron spectroscopy (AES) analysis were 0% to 13%. Photoluminescence (PL) spectra around 260 nm were observed at room temperature. The full-width at half maximum (FWHM) of PL spectra for BAlGaN/AlN MQW structure(with 2% of boron) was narrowed from 360 meV to 179 meV, as the residual strain in the BAlGaN well layer was decreased from 1.3% to 1.0% by increasing the Al content in the quantum wells.

Layer compression modulus of the antiferroelectric liquid crystal MHPBC

Abstract The layer compression modulus B was determined near the smectic-A-smectic-Cα* phase transition in an antiferroelectric liquid crystal 4-(l-methylheptyloxycarbonyl)-phenyl 4′-octylbiphenyl-4-carboxylate (MHPBC). The temperature dependence of B clearly defined the smectic-A-smectic-Cα* phase transition temperature and supplied a new and clear evidence that the smectic-Cα* phase is a tilted phase. The pretransitional softening of B can be represented by a simple power law and explained well by the theoretical description based on renormalization-group method. These results clearly show that the transition is not of the Landau mean-field type.