Showing papers by "Masaki Azuma published in 1993"

A new family of copper oxide superconductors Srn+1CunO2n+1+δ stabilized at high pressure

Abstract: THE search for new copper oxide superconductors1 has been pursued mainly by exploring a range of chemical compositions (counter-cations and oxygen content) and reaction temperatures. We have explored the effects of an additional variable, the reaction pressure, and have thereby synthesized some new compounds, including (Ca1-ySry)1-xCuO2 which shows superconductivity at 110 K (refs 2, 3). Here we report the synthesis of a new family of high-pressure phases with the formula Srn+1CunO2n+1+δ (n = 1, 2, 3,...) extending from Sr2CuO3.1, (n = 1) to SrCuO2 (n = ∞ )4. Sr2CuO3.1 and Sr3Cu2O5+δ′ (n=2) are superconductors with transition temperatures (Tc ) of 70 and 100K, respectively. The metal sublattice of Sr2CuO3.1, is of the tetragonal K2NiF4 (or Nd2CuO4) type with lattice constants a = 3.764 A and c-12.548 A, and iodometric measurements suggest that nearly half of the oxygen atoms are missing from the counter-layers to Sr2O1.1. Homogeneous pyramidal or the oxygen atoms are missing from the counter-layers to Sr2O1.1. Homogeneous pyramidal or octahedral coordination, which has generally been believed to be indispensable to attain Tcs of ∼100 K, is thus absent. The n = 2 and 3 members seem to correspond to the high-pressure phases in the Ca–Sr–Cu–O system recently reported by Adachi et al.5.

Structure and superconductivity of the infinite-layer compound (Ca1−ySry)1−xCuO2−z

Abstract: Microstructure and superconducting properties of the infinite-layer compound A 1− x CuO 2− z (A=Ca 1− y Sr y ) have been investigated by high-resolution electron microscopy as well as magnetization and resistivity measurements. The alkaline earth deficient samples always contain defect-layers inserted almost randomly into the parent simple structure, where we suggest that the A cation vacancies are concentrated. High-resolution electron microscopy observations reveal that oxygen deficiency can simultaneously occur within the central CuO 2 sheet of the defect layer for the sake of charge compensation for the A deficiency. p-type superconductivity with a maximum T c of 110 K is realized if the oxygen defects are filled in by application of high oxygen pressure on high-pressure synthesis ( z ∼0). On the other hand, n-type superconductivity also occurs in the same structure when oxygen deficency is increased so that z > x by a heat-treatment in a reducing atmosphere under high pressure. The fundamental relationship between structure and superconductivity is discussed on the basis of the demonstrated structural uniqueness of the title compound.

Synthesis and superconducting properties of the infinite-layer compounds Sr1−xLnxCuO2(Ln=La, Nd, Sm, Gd)

Abstract: Infinite-layer-type superconductors Sr 1− x Ln x CuO 2 are synthesized under high pressure of 3 GPa for Ln=Sm, Gd as well as for Ln=La, Nd. Their chemical and superconducting properties are systematically studied as functions of doping concentration and the kind of lanthanide ion. As a result, it is demonstrated that the variation of these properties with doping concentration is similar for all the examined Ln 3+ ions. The solubility limit lies at x ≈ 0.10. CuO 2 sheets are expanded with increasing x , while their spacing decreases. The T c onset determined by magnetic measurements remains constant for any doping concentration; only the Meissner fraction increases with increasing x .

Mechanism of Carrier Doping in the Superconducting Infinite-Layer Compound A1−xCuO2−z (A=Ca1−ySry)

Abstract: Microstructure of the superconducting infinite-layer compound A 1−x CuO2−z (A = Ca1−y Sr y ) has been investigated by high-resolution electron microscopy. The superconducting samples always contain defect-layers inserted almost randomly into the parent simple structure, where we suggest that the A cation vacancies are concentrated. Structure image observations reveal that oxygen deficiency also occurs within the central CuO2 sheet of the defect layer for the sake of charge compensation for the A vacancies. P-type superconductivity with a maximum Tc of 110 K is realized when the oxygen defects are filled in by application of high oxygen pressure on high-pressure synthesis (x > z). It is suggested, moreover, that n-type superconductivity also occurs in the same structure when oxygen deficiency is increased so that x < z by a heat-treatment in a reducing atmosphere.

The Electochemical Oxidation of La2CuO4 and La2-xAxCuO4(A=Bi and Pb)

Abstract: Two kinds of methods were carried out in order to dope the oxygen in La2CuO4 crystals; the annealing under high oxygen pressure at high temperature and the electrochemical oxidation in an aqueous alkaline solution at ambient temperature. The difference in the orthorhombicity was seen in La2CuO4+z with the same z value, prepared by different routs, which indicates the different excess oxygen distribution. The electrochemical oxidation is also effective to bring the superconductivity in La2-xBixCuO4+z and La2-xPbxCuO4+z. Some samples show the Tc on set of 30K for Bi doped system and 15-20K for Pb doped one.

Superconductivity in the Infinite-Layer Phase Stabilized Under High Pressure

Abstract: Cupric-oxide superconductors are complex oxides involving counter cations like rare-earth and alkaline-earth ions. Counter cations and cupric ions compromise with each other to coexist in a crystal by adjusting their coordination numbers, bond lengths, and bond angles. Pressure and temperature evidently influence these parameters, and so, depending upon pressure and temperature, different compositions would be stabilized in a given structure and different structures would appear for a given composition. By applying 6GPa to the alkaline-earth-copper-oxygen system a new superconductor family with the highest T c of 110K has been found.