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.

Muon spin rotation experiments are carried out on clinoatacamite, ${\mathrm{Cu}}_2\mathrm{Cl}(\mathrm{OH}{)}_3$, which is a new geometrically frustrated system featuring a three-dimensional network of corner-sharing tetrahedral $3d$ ${\mathrm{Cu}}^{2+}$ spins. A long-range antiferromagnetic order occurs below 18.1 K with a surprisingly small entropy release of about $0.05R\ln 2/\mathrm{Cu}$. Below 6.5 K, the static long-range order transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. This work presents a novel system for studying geometric frustration.

Unconventional magnetic properties are found for a quantum spin system of mineral clinoatacamite, ${\mathrm{Cu}}_2\mathrm{Cl}{\left(\mathrm{O}\mathrm{H}\right)}_3$. Clinoatacamite is found to undergo an antiferromagnetic transition at $18.1\mathrm{K}$ with a small entropy fall $\left(0.05R\ln 2\right)$. At temperatures below $6.4\mathrm{K}$, it transits into a disordered spin-glass-like state but with large specific heat anomalies. The entire entropy fall for the spin system is about $0.31R\ln 2$, which is smaller than those for dipolar spin ice $\left(0.67R\ln 2\right)$ and for ice $I_h$ $\left(0.71R\ln 2\right)$. Geometric frustration for the quantum ${\mathrm{Cu}}^{2+}$ spins is suggested based on its crystal structure, magnetization, and specific heat studies.

Muon spin relaxation/rotation $\left(\mu \mathrm{SR}\right)$ and magnetic susceptibility measurements were carried out on antiferromagnetic nanoparticles of CuO. Nanoparticles with center size of around 5 nm were prepared by ball-milling from single crystals of CuO and investigated using $\mu \mathrm{SR}$ measurements. In the $\sim 5\mathrm{nm}$ assembly, the $T_{\mathrm{N}}$ was reduced drastically to $\sim 30\mathrm{K}$, compared with the bulk $T_{\mathrm{N}}=229\mathrm{K}$. A similar effect was observed in a system of 2 to 3 nm diameter nanorods, which was synthesized by a direct solution reaction method, where $T_{\mathrm{N}}$ was suppressed further to 13 K. The present work reports direct evidence of a dramatic finite-size effect on the magnetic transition temperature in antiferromagnetic systems.

A transparent $p$ -type semiconductor, ${\mathrm{CuAlO}}_2$ , shows selective and reversible response to ozone gas at room temperature. All existing commercial semiconductor ozone sensors are of $n$ type. This study demonstrates the feasibility of developing an inexpensive $p$ type and transparent ozone sensor. Types of transparent $p\text{–}n$ junction ozone sensors may be fabricated using the $p$ -type semiconductor ${\mathrm{CuAlO}}_2$ and existing $n$ -type transparent materials such as ${\mathrm{In}}_2{\mathrm{O}}_3$ .