Transport properties of La 2Ϫx Sr x Cu 1Ϫy Zn y O 4 in the overdoped region have been investigated in detail. In a narrow region around xϭ0.22 in yϭ0, we have found anomalously less-metallic behaviors of the electrical resistivity and the thermoelectric power than usual. The value of x around which the anomalies occur decreases through the substitution of Zn; namely, with increasing y. In the anomalously less-metallic samples, superconductivity has been found to be a little suppressed. These anomalies are not connected with the structural phase transition between the tetragonal high-temperature phase (I4/mmm) and the orthorhombic midtemperature one (Bmab). There is a possibility that an ordering of holes and/or spins is formed or fluctuates in the samples exhibiting the anomalous properties in the overdoped region. ͓S0163-1829͑99͒03202-6͔

Nickel fine particles with diameters under 600 nm were synthesized rapidly and continuously in water at temperatures ranging from 300 to 380 °C and at 30 MPa. Nickel particles formed through the following two steps: formation of Fe 3 O 4 nuclei by hydrothermal synthesis from FeSO 4 aqueous solution and precipitation of nickel on the surfaces of the nuclei (i.e., Ni coating) by hydrogen reduction from Ni(CH 3 COO) 2 aqueous solution. The substance 1,10-phenanthroline was used as a complex for preventing hydrolysis of Ni 2+ because hydrolysis promotes formation of nickel oxide particles at higher temperatures. Thickness of Ni shell on the Fe 3 O 4 nuclei increased with increasing reaction temperature and Ni/Fe molar ratio in the feed solution. The 1,10-phenanthroline was thermally stable up to 380 °C for residence times of ca. 19 s, which means that recycle after product formation is probably possible.

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Recent experimental work on the 1/8 problem by our group is reviewed not only in the La-based cuprate but also in the Bi-and Y-based ones. In the partially Zn-substituted Bi2Sr2Cai_ a: Ya:(Cui_ 2/ Zn 2/ )208+5, we have found anomalous suppression of superconductivity at x = 0.30 -0.35, where the hole concentration per Cu p ~ 1/8. Moreover, it has been revealed from the //SR measurements that the Cu-spin fluctuations exhibit slowing-down behavior at low temperatures in these samples. These results are suggestive of the stripe correlations of holes and spins tending to be pinned by Zn at p ~ 1/8 in the Bi-based cuprate as well as in the La-based cuprate. In Yi_ a3 Caa;Ba2Cu307_ < $, the 60 K plateau of T c has been found not to be correlated with the oxygen content but to be interpreted as being due to the suppression of superconductivity at p ~ 1/8. Accordingly, we conclude that the 1/8 problem is common to all high-T c cuprates with the CuC>2 plane. Moreover, we have found transport and T c anomalies around x = 0.22 in La2-a;Sr a ;Cui_ 2/ M z/ 04 (M = Zn, Ga), suggesting a possibility that an order of holes and/or spins is formed or fluctuates in these samples in the overdoped region.