Maki Kawai

Molecular beam epitaxy of Bi2Sr2CuOx and Bi2Sr2Ca0.85Sr0.15Cu2Ox ultra thin films at 300°C

Abstract Molecular beam epitaxy of ultra thin films of Bi2Sr2CuO6 (2201 phase) and Bi2Sr2(Ca0.85Sr0.15)Cu2O8 (2212 phase) i s realized on SrTiO3(100) at a substrate temperature of 300°C, using 10-5 Pa of NO2 as an oxidant. The structure of the ultra thin film formed is confirmed by X-ray diffraction. An in-situ RHEED (reflection high energy electron diffraction) study reveals that the film formed has the same in-plane lattice constant as that of the SrTiO3(100) substrate, namely the a and b axes are 0.39 nm. Because the a and b axes are lengthened approximately 3%, the c axis of the ultra thin 2201 film was 2.41 nm, shorter than the value for the bulk material. The crystallinity of the film strongly depended on the sequence of the oxidation process, revealing that the elementary unit of the epitaxial growth is the sub-unit of the perovskite structure. In the Bi cuprate series, Sr-Cu-Sr is the elementary unit. As a result, the sequential supply of Bi, Sr, Cu and the oxidant NO2 is necessary to realize low temperature epitaxy.

Selective and stoichiometric reaction of copper dipivaloylmethanate [Cu(DPM)2] with surface hydroxyls on SiO2

Stoichiometric reaction of copper dipivaloylmethanate [Cu(DPM)2] with the hydroxyl group (OH) on SiO2 surface was studied by infrared spectroscopy. The stoichiometric ratio of reacted OH and Cu(DPM)2 was estimated to be ∼2–3:1 from the absorbance of the OH and the CH stretching region. The ligand DPM was removed from the SiO2 surface by treatment with water at 400u2009°C. The oxidation state of Cu remaining on the surface was proven to be +1 by x‐ray photoelectron spectroscopy.

Study of successive phase transitions of the Si(001)-Bi surface by RHEED

Abstract Successive phase transitions of the 2 × n surface structures have been observed on a Si(001) surface with Bi overlayer. These structures are formed with about one monolayer of Bi. The coverage of Bi slightly decreases with increasing temperature. Since these phase transitions are accompanied by the desorption of Bi, transitions are irreversible. The dependence of n on the temperature has been measured using reflection high-energy electron diffraction (RHEED). It is revealed that n th order spots move away from fundamental spots continuously over the range from 1 13 to 1 5 as the temperature is elevated from 530 K to 820 K.

The reaction of copper and calcium dipivaloylmethanates (Cu(DPM)2 and Ca(DPM)2) with hydroxyls on oxide surface

Abstract Selective and stoichiometric reactions between surface hydroxyl groups (OH) on SiO 2 and calcium dipivaloylmethanate (Ca(DPM) 2 ) were investigated by infrared (IR) and photoelectron (XPS) spectroscopies. The stoichiometric ratio of initial surface OH and adsorbed Ca(DPM) 2 is estimated from IR absorbance to be ca. (2–3): 1. Introduction of water vapor at 673 K to this surface results in the removal of ligand DPM from the adsorbed Ca(DPM) 2 , leaving the Ca on the surface. The reactive property of Ca(DPM) 2 is identical to that of Cu(DPM) 2 , previously reported by us. A similar reaction on the surface of single crystal SrTiO 3 (100) with the Cu(DPM) 2 is also carried out.

Superconductive transition at 39.0K in (Bi, Pb)2(Sr, La)2CuOy

Abstract Bi 1.6 Pb 0.4 Sr 1.6 La 0.4 CuO y superconductor with the basic structure of Bi 2 Sr 2 CuO 6 (2201 phase) was prepared by annealing in the absence of oxygen. A high Tc value of 39.0K was observed. This result indicates that the 2201 phase in the Bi system gas an equal potential for high Tc to (La,Sr) 2 CuO 4 does.

Superconductive property of Bi2−2xPb2xSr2Can−1CunOy

Abstract Hole concentration of the system can be controlled by the substitution of Pb for Bi in Bi 2 Sr 2 Ca n −1 Cu n O y . Bi 2−2 x Pb 2 x Sr 2 Ca n −1 Cu n O y where x = 0.3 has the effect that T c decreases from 80 K to 75 K in the n = 2 phase. In the n = 3 phase T c was not affected. Difference in the effect of the hole concentration C hole on T c can be explained by the fact that Δ T c /Δ C hole for the n = 2 and 3 phases was different and that the value for the n = 3 phase was almost zero in the observed region.

Effect of S, Se and Te addition on the superconductive properties of Ba2YCu3O7−y

An addition of a small amount of sulfur or selenium in Ba2YCu3O7-y lead to the formation of the phase whose c-axis was 11.62 A, which is much shorter than that observed in sulfur-free Ba2YCu3O7-y. This phase increased the Tc(zero) value, accompanied by the sharpening of the transition width within 1 K.

Dramatic changes in the Bi and Cu valences of Bi 2201 phase annealing in absence of oxygen

Abstract The observed changes in the value of T c in the Bi 2201 phase are discussed in connection with the total valence of the system, Bi and Cu valence, and its lattice structure. Annealing the Bi 2201 phase, in the absence of oxygen (in N 2 or in Ar) leads to an increase of T c from 26.1–26.7 K to 31.2–39.0 K. It is proposed that the transformation of the position of oxygen atoms causes a change in the distribution of the total valence between the Bi-O layer and CuO 2 layer, which leads to an increase in T c .

Effect of Ba Addition on the Properties of Bi-Pb-Sr-Ca-Cu-O Superconductors

The addition of Ba to Bi-Pb-Sr-Ca-Cu-O and Bi-Sr-Ca-Cu-O superconductors exhibits the effect of raising the Tczero value to a higher temperature region, although too much Ba gives rise to semiconducting resistance-temperature behavior. The added Ba has the effect of decomposing the structure of the 80 K phase, leading to the production of BaBiO3 and BaCuO2 and enhancing the high-Tc phase formation.

Electronic states of the CuOx(x=4, 5, 6) model clusters: I. Effect of hole density and Cu-O bond lengths

Abstract As a model for the two dimensional CuO network in the high T c superconductor oxides, a DV-Xα calculation for the CuO x ( x =4, 5, 6) cluster was carried out. By varying the hole density, defined by the electron deficiency in this study, and the Cu-O bond lengths independently, the Mulliken population analysis revealed that the electron was drawn out from the O 2p at the plane site and not from Cu 3d nor from the O at the apex site for every cluster. The increase in the electron density enhances the hybridization between O 2p and Cu 3d orbitals, resulting in a rise of the overlap population between Cu and O atoms.