We analysed associations between exposure to nightlife businesses and severe acute respiratory syndrome coronavirus 2 PCR test results at a tertiary hospital in Tokyo between March and April 2020. A nightlife group was defined as those who had worked at or visited the businesses. We included 1517 individuals; 196 (12.9%) were categorised as the nightlife group. After propensity score matching, the proportion of positive PCR tests in the nightlife group was significantly higher than that in the non-nightlife group (nightlife, 63.8%; non-nightlife, 23.0%; P < 0.001). An inclusive approach to mitigate risks related to the businesses needs to be identified.

The magnetohydrodynamic equilibrium of a helical-axis heliotron with large-vacuum magnetic islands is investigated. Since the helical-axis heliotron is an $L\,{=}\,1/M\,{=}\,4$ low-magnetic-shear configuration, there is the sensitivity to the perturbed field. By assuming the external trim coil, low-order magnetic islands with $n/m\,{=}\,$1/2 can be generated. The effect and change of low-order ($n/m\,{=}\,1/2$) vacuum islands to the finite $\beta$ equilibrium are investigated. For a finite β equilibrium, $n/m\,{=}\,$1/2 islands disappear but $n/m\,{=}\, 4/8$ islands appear by the pressure-induced field. For another case, $n/m\,{=}\,$1/2 islands disappear and other resonances are suppressed. This property depends on the phase of islands.

Adhesion between swollen hydrogels was studied using a simple tack-evaluation technique by a point contact in air at room temperature. The hydrogel used here was poly(sodium acrylate) (PSA) gel physically cross-linked by aluminum ions. The adhesion force and the separation energy showed a power-law increase with separation velocity, normal force, and waiting period　prior to separate. Effects of the degrees of protonation in PSA as well as of dehydration on the adhesive properties are also presented. The results are discussed in terms of the surface molecular interaction and the bulk elastic and viscous properties.

Growth and characterization of p-type 4H-SiC epitaxial layers grown on (11-20) substrates are reported. P-type 4H-SiC epilayers with smooth surface morphology have been grown on (11-20) substrates by low-pressure, hot-wall type CVD with SiH4–C3H8–H2–TMA system. The doping concentration can be controlled in the range from about 1×1016cm−3 to 1×1019cm−3. Anisotropy of the crystalline quality is observed by x-ray diffraction measurement. P-type epilayers, in which near band-gap emissions are dominated and D-A pair peak is not observed, are obtained. Hole mobility of (11-20) epilayers is smaller than that of (0001) epilayers probably due to the lack of crystalline quality compared to (0001) epilayers. The results of both low-temperature photoluminescence and the temperature dependence of Hall effect measurements indicate that the boron concentration as undoped impurity in (11-20) epilayer is lower than that of (0001) epilayer. This may be caused by the smaller incorporation efficiency of boron into (11-20) epilayer than that of (0001) epilayer.

The influence of growth conditions and susceptor purity on the residual contamination in undoped 4H-SiC epitaxial layer grown at higher temperature such as 1600 °C is investigated. Residual N concentration is found to increase with growth temperature. Growth temperature dependence of residual N concentration is stronger than that of Al or B. The effect of degradation of SiC coating layer on the purity of the epitaxial layer is studied. SiC coating layer is degraded after very few repetition of growth-run. SIMS measurement reveals that the concentration of N, Al, B, Ti and V in epitaxial layer increases with the deterioration of SiC coating layer. Therefore, serious consideration on the effect of contamination from the susceptor graphite is required. By using various grades of graphite as susceptor, the dependence of the purity of epitaxial layer on the susceptor purity is studied. High concentration (1017 to 1018 cm−3) of N is found in graphite. Contamination of all types of impurities such as p-type impurities, transition metal impurities and N in graphite is found to affect the purity of epitaxial layers.