Kazuhiro Sakuyama

New superconducting cuprates (Pb, Cu)(Eu, Ce)2(Sr, Eu)2Cu2Oz

Abstract New superconducting Pb-based layered copper oxides, (Pb, Cu)(Eu, Ce)2(Sr, Eu)2Cu2Oz, have been synthesized. This new superconducting phase or the “1222” phase characteristically contains a (Pb, Cu)-O monolayer and a “fluorite-type” block layer which is sandwiched between the basal planes of double Cu-O5 pyramids. Crystallographycally, this structure agrees with that recently proposed for nonsuperconducting PbPr2.4Sr1.6Cu2.6Oz by Adachi et al. A superconductivity transition with the onset temperature of 25 K is measured for the nearly single-phase sample of the nominal composition of ( Pb 1 2 Cu 1 2 ) ( Eu 3 4 Ce 1 4 )2( Sr 7 8 Eu 1 8 )2Cu2Oz. This is the first superconductor of the “1222” structure.

Superconductivity in Pb-based layered copper oxides, (Pb0.5Cu0.5)(Sr,Ba)2(Y,Ca)Cu2O7

Abstract The effects of Ba-substitution for Sr in Pb-based superconducting layered copper oxides. (Pb0.5Cu0.5)Sr2(Y,Ca)Cu2Oz (z∼7), are investigated. Ba-free samples are not superconducting at temperatures above 4.2 K, probably because the solubility of Ca is too low to exhibit superconductivity. However, for the samples in which Ba partially substitutes for Sr, superconductivity transitions with an onset temperature of 70 K have been observed. The Tc value of 70 K is as high as that previously reported for Pb2Sr2(Y,Ca)Cu3O8. The superconducting phase is identified to have the crystallographic structure of the “1212” type.

Effects of Sb-doping on the formation of superconducting phases in Bi-Pb-Sr-Ca-Cu oxides

Abstract The effects of doped Sbon the formation of superconducting phases in [Bi0.8(Pb1-xSbx)0.2]2Sr2 Ca2Cu2.8Oy are investigated for x in the range of 0 to 0.5. Electrical resistivity measurements, powder X-ray diffraction and differential thermal analyses are performed. The coexistence of Pb and Sb is likely to cause a partially-melted state at a relatively low temperature of ≈840°C. The superconducting phase with 110 K transition temperature may rapidly grow in the partially-melted state. For x=0.2, samples showing zero resistivity at temperatures above 100 K can be obtained by sintering at 860°C for a period much shorter than that required for sintering samples without Sb.

Transport properties of the Pb-based superconducting cuprate (Pb1+x/2Cu1-x)Sr2(Y1-xCax)Cu2Oy

Abstract The resistivity, Hall effect and Seebeck effect of (Pb (1+ x )/2)Sr 2 (Y 1- x Ca x )Cu 2 O y ( x ⩾0.35) were determined. Superconductivity appeared only for x =0.30 and 0.35 and only when the sample was quenched from high temperature. Transport data showed that hole carriers increase with x and with decreasing oxygen content, y , from 7.1 to 7.0 by quenching. This indicates that, unlike the case of “Tl-1212”, the “Pb-1212” system cannot be explained by over-doping arguments. Positive magnetoconductance was observed for the slowly cooled sample with x =0.35. Substitution and quenching effects on the transport properties are discussed from the viewpoints of self-doping, charge transfer and localization.

Superconductivity in Nd2CuO4−xFx

Abstract We report the effects of fluorine substitution for oxygen in Nd 2 CuO 4 . For the samples of Nd 2 CuO 4− x F x , X-ray powder diffraction analysis shows that the fluorine ions have successfully been doped into the oxygen sites in the tetragonal Nd 2 CuO 4 lattice and the maximum fluorine solubility limit is found to be X cr = 0.3. As x increases, the lattice constant a increases linearly, while the lattice constant c decreases linearly. In particular, for the samples with x ≥0.2 which are sintered at 1050 and 1100°C, the superconducting transition is observed around 27 K in both resistivity and magnetic susceptibility measurements. On the contrary, for the samples with x ≥0.2 which are sintered at 950 and 1000°C, a strong diamagnetic signal is observed in the temperature region below 27 K, though the zero-resistance state is not observed down to 4.2 K. This result suggests that the electrical conduction pass does not form throughout the sample when sintered at 950 and 1000°C because of the small grains and high density of pores.

HRTEM study of new superconducting lead cuprate (Pb, Cu) (Eu, Ce)2(Eu, Sr)2Cu2Oz

Abstract Crystallographic properties of a new lead cuprate superconducting material (Pb, Cu) (Eu, Ce) 2 (Eu, Sr) 2 Cu 2 O z are examined by electron probe micro-analysis and transmission electron microscopy. It is confirmed that although the sample examined here is not of single-phase, the composition of the superconducting phase is close to the nominal composition. From TEM observations, it follows that the crystal structure of the new phase is similar to that of Tl-1222 which consists of a fluorite type block, a rock salt type layer and a perovskite block.

Substitution effects of Ca for Sr in Pb-based layered copper oxides, (Pb(1+x)/2Cu(1−x)/2Sr2(Y1−xCax)Cu2O7+δ

Abstract (Pb/Cu)-1212 cuprates in which Sr sites are partially substituted by Ca, (Pb (1+x)/2 Cu (1−x)/2) (Sr 1−y Ca y ) 2 (Y 1−x Ca x )Cu 2 O 7+δ , are synthesized and characterized. While single-phase samples are obtained only in the range of 0≦ ×≦ 0.35 for y=0, partial substitution of Ca for Sr stabilizes the (Pb/Cu)-1212 compounds which contain Ca in the Y sites with x>0.35. For x = 0.5, nearly single-phase samples are obtained for y = 0.05 and 0.1. Superconductivity transition temperature as high as 80 K is measured for the sample with x = 0.5 and y = 0.1.

High-Pressure transport phenomena in (Pb(1+x)2Cu(1−x)2)Sr2(Y1−xCax)Cu2O7+δ

Abstract Pressure effect on Tc for “Pb-1212” phase is investigated up to 2GPa using a piston- cylinder apparatus. Unusually large pressure coefficient of Tc is observed ( dln T c d p =0.65 GPa −1 , Tc=16.8K at latm.). Pressure induced-delocalization of holes is discussed.

Synthesis and 57Fe Mössbauer Spectroscopy of (Pb0.5Fe0.5)Sr2(Y0.5Ca0.5)Cu2Oz (z ≈ 7)

A nonsuperconducting lead cuprate having the “1:2:1:2” structure and a nominal composition of (Pb0.5Fe0.5)Sr2(Y0.5Ca0.5)Cu2Oz is synthesized as a single-phase polycrystalline form. The temperature dependence of electrical resistivity is slightly metallic at temperatures around 300 K, but it increases after an annealing at 800°C in air and subsequent quenching into liquid N2. The 57Fe Mossbauer spectroscopy is performed in order to discuss the site distribution of Fe atoms.

(Pb,Cu)Sr/sub 2/(Y,Ca)Cu/sub 2/O/sub z/, new superconducting layered copper oxides

Single-phase ceramics of Pb-based layered copper oxides, (Pb/sub (1+x)/2/Cu/sub (1-x)/2/)Sr/sub 2/(Y/sub 1-x/Ca/sub x/)Cu/sub 2/O/sub x/ (x=0 approximately 0.35), were successfully synthesized in an oxidizing atmosphere in contrast to the case of Pb/sub 2/Sr/sub 2/(Y,Ca)Cu/sub 3/O/sub 8/. Superconductivity is observed for the samples containing appropriate amounts of Ca such that x>0.25 when annealed in air and then quenched into liquid nitrogen. As the Ca content, x, increases, the superconductivity transition temperature increases. The superconducting onset temperature of 65 K is measured for the multiphase samples with x=0.5.