The ferroelectricity of (Al_1−xSc_x)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al_1−xSc_x)N films were prepared at 400 °C on (111)Pt/TiO_x/SiO₂/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N₂ gas or a mixture of N₂ and Ar gases. The film deposited under N₂ gas showed larger remanent polarization than those under N₂+ Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N₂ gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study.

One of the present intensive concerns about the high-temperature superconductors is whether charge stripes are a key to superconductivity. Here we report observation of charge stripes in the simplest copper oxide, CuO, by real-space images obtained by electron microscopy. Charge-ordered domains and normal-lattice domains exist alternatively in the vapor-grown single crystal of CuO. Since CuO consists of the Cu-O bonding, which is a basic material feature for high- $T_c$ cuprates, the discovery of charge stripes in this basic compound has important implications for discussing the mechanism of superconductivity in complex cuprates.

The elastico-mechanoluminescence (EML) properties of CaZnOS:Mn²⁺ are investigated. The CaZnOS:Mn²⁺/epoxy resin composite can simultaneously “feel” (sense) and “see” (image) various types of mechanical stress over a wide energy and frequency range (ultrasonic vibration, impact, friction and compression) as an intense red emission (610 nm) from Mn²⁺ ions. Further, the accurate linear relation between emission intensity and different stress parameters (intensity, energy and deformation rate) are confirmed. The EML mechanism is explained using a piezoelectrically induced trapped carrier excitation mode. All the results imply that CaZnOS:Mn²⁺ has potential as a stress probe to sense and image multiple mechanical stresses and decipher the stress intensity distribution.

Elastico-deformation luminescence in strontium aluminates was investigated systematically using precisely controlled pure-phase Eu-doped strontium aluminates of ${\mathrm{SrAl}}_{12}{\mathrm{O}}_{19},$ ${\mathrm{Sr}}_4{\mathrm{Al}}_{14}{\mathrm{O}}_{25},$ ${\mathrm{SrAl}}_4{\mathrm{O}}_7,$ $\mathrm{\alpha -}{\mathrm{SrAl}}_2{\mathrm{O}}_4,$ $\mathrm{\beta -}{\mathrm{SrAl}}_2{\mathrm{O}}_4,$ ${\mathrm{Sr}}_3{\mathrm{Al}}_2{\mathrm{O}}_6$ and their mixed phases. This study revealed that only the $\mathrm{\alpha -}{\mathrm{SrAl}}_2{\mathrm{O}}_4$ phase produces strong elastico-deformation luminescence; other strontium aluminates show no deformation luminescence. Correlation of deformation luminescence and crystal structure was found. The $\mathrm{\alpha -}{\mathrm{SrAl}}_2{\mathrm{O}}_4$ has the lowest symmetry, crystallizing in a monoclinic structure. This finding can be applied in designing strong elastico-deformation-luminescent materials.