Hisao Yamauchi

Overdoped cuprates with high-temperature superconducting transitions

Evidence for high-Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both x-ray absorption near-edge spectroscopy and bond valence sum measurements are >2.3, and are in close agreement with each other for structures in the homologous series (Cu0.75Mo0.25)Sr2(Y,Ce)sCu2O5+2s+δ with s = 1, 2, 3, and 4. The record short apical oxygen distance, at odds with current theory, suggests the possibility of a new pairing mechanism. The possibility that the charge reservoir layers are able to screen long range coulomb interactions and thus enhance Tc is discussed.

Effect of Post-Annealing on Tc in Pb-Doped Bi-2223 Superconductors

(Bi,Pb)-2223 superconductors with nominal composition of Bi1.85Pb0.35Sr2−xCa2+xCu3.1Oy (x=0 and 0.1) were synthesized. They are virtually single-phase and show Tc (mag.) around 109K~110K. By applying an encapsulation and low temperature post-annealing process, Tc increased to 115 K by SQUID measurement. From the resistivity, thermoelectric power measurements and the evolution of the lattice constants, it is suggested that the increases in Tc may due to the decreases in hole concentration.

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.

Synthesis and Characterization of Bi-based 1212-type Materials

Virtually single phase of Bi-based 1212-type materials was prepared with the nominal composition of (Bi0.3Cu0.7)Sr2YCu2Oz, which was insulator after sintering in air. Following annealing in different oxygen pressure, the oxygen content z was greatly increased, and superconductivity was induced at T c onset =18K, and T c R=0 =15K for z=7.125. The superconducting volume fraction was found to be small, and the conductivity of its non-metallic regions in the annealed samples follows the variable range hopping law, suggesting that carrier localization is responsible for the non-metallic conduction behavior in this system.

Nature of electron-lattice interaction in superconducting oxides

Various experimental results show that in high temperature superconducting oxides the lattice responds rather anomalously to the onset of superconductivity. This suggests that some unconventional electron-lattice interaction may be at work in these solids which could be important to the mechanism of high temperature superconductivity. We describe our recent experimental observations by neutron and X-ray scattering as well as some theoretical results which support this view, and discuss their implications.

Pressure Enhancement of Tc up to 150K in HgBa2Ca2Cu3Oy

The onset temperature(T c ) of Meissner signal for HgBa2Ca2Cu3Oy(which is 134K at ambient pressure) has been measured under high pressure up to 11GPa by using a cubic anvil device. The T c increases, without any change in both transition width and volume fraction, at the rate of dT c /dP = 2.0K/GPa up to 6GPa and at higher pressure dT c /dP decreases gradually with increasing pressure. The highest value observed in this experiment reaches 150K at 11GPa. This is higher than those reported for any other high-temperature superconductors. This result is discussed from the structural and electronic view points in comparison with other cuprate superconductors. This work was supported by NEDO.

Oxygen-content dependence of the flux pinning in Pb-substituted Bi-2212 superconductors

The Pb-substituted Bi-2212 superconductors with different oxygen contents were synthesized by post-annealing the samples in air or varying the sintering temperature in N2. With decreasing oxygen content, Tc increased and the structure changed in the following series: tetragonal → orthorhombic → “monoclinic”. The same tendency was observed ever when the Pb content increased. It was confirmed that there existed two kinds of pinning mechanisms in this system. One that was effective at low magnetic fields was enhanced by the inhomogeneity in the oxygen distribution. The other that was effective at high magnetic fields increased near the boundary between orthorhombic and “monoclinic” phases.

Superconductivity in Pb-Based Copper Oxides with 1212 Structure

Superconducting Pb-based copper oxides with the 1212 structure have been synthesized in an oxidizing atmosphere in contrast to those with the 2213 structure usually synthesized in a reducing atmosphere. Superconductivity of the Pb-based 1212 copper oxides are strongly dependent on the synthetic conditions as well as the chemical compositions. Although the samples of nearly single- phase of the 1212 phase are obtained in a starting composition of Pb0.5 (Sr1.5Ca1.33Y0.67)Cu2Oz, they are not superconductive at temperatures above 4.2 K. On the other hand, the samples with a starting composition of PbSr2Ca0.5 Cu2Oz are of multi-phase but show a superconductivity transition with an onset temperature of 27 K and a zero resistivity temperature of 16 K.