It is indicated that the overall features of the phase diagram of electron and hole doped high T_C cuprates can be understood based on the mean field theory of the t-J model.

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The phenomenological Ginzburg-Landau theory is developed for the high-T_c superconductors assuming the spin and charge degrees of freedom are separated. The superconductivity is realized when both the fermion pairing and the bose condensation occurs. We propose the free energy functional including these two order parameters, which are coupled with the gauge field. The global phase diagram as well as the vortex structure is discussed.

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The electronic structure of high-Tc copper oxides is investigated by the optical reflectivity spectra focusing on the evolution of the low-energy spectrum with doping and on the nature of the metallic state realized by the doping. The experimental results indicate that rapid rearrangement is occuring on doping from the conduction and valence band states of the original charge-transfer insulator and thus reconstructed state is not ordinary Fermi liquid in the high-Tc regime.

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High performance of the critical current density J_c in Y₁Ba₂Cu₃O₇ films prepared by chemical vapor deposition has been demonstrated. The pinning mechanism in high-T_c superconducting oxides was explored from a viewpoint of the anisotropic behavior of J_c, and the global pinning force F_p was investigated in terms of the flux creep. It was found that the global pinning force on the basis of the flux creep theory well describes the J_c properties in CVD-Y₁Ba₂Cu₃O₇ films. The scaling in the form F_p∝(B/B_c2)^1/2(1-B/B_c2)^3/2 of the unrelaxed state which is not limited by the flux creep was presented.

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Defects such as cracks and weak link regions are considered to be the causes for the low critical current densities in melt-processed Y-Ba-Cu-O in comparison with those in single-crystalline thin films. In order to understand the influence of those factors on the critical current density, the measurement of the imaginary ac susceptibility was carried out for two melt-processed Y-Ba-Cu-O specimens; one specimen contained fairly large number density of cracks and the other did not. The imaginary ac susceptibility was found to deviate from the prediction from the critical state model near the critical temperature due to the reversible motion of fluxoids only for the specimen with cracks. From the comparison of the experimental result with the theoretical calculation based on Campbell's model for the reversible fluxoid motion, the effective width of the channels of current paths was found to be comparable to the mean spacing of cracks. This shows that the cracks are the dominant factors which limit the critical current density. Such an effect of the reversible fluxoid motion was not observed in the specimen without the cracks. That is, the influence of weak link region was not appreciable in the present experiments at the do magnetic field of 1.0 T. This is attributed to a large size of the domains in comparison with Campbell's ac penetration depth.

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The results of experimental investigation of the electrical resistivity and magnetic permeability of ceramic rod-shaped sample in temperature range from 300 to 970K in flowing oxygen and nitrogen are presented. The data were obtained by SRPM method, vectorially measuring the difference in the impedance of gold circular solenoid coil with and without a sample under investigation. The measurements were performed at frequency 1MHz with the help of computer controlled experimental set-up at heating and cooling rate of 3K/min. The results of investigation of electrical resistivity performed by this method in different ambient are compared with the data obtained by the ac four-probe method with press contacts.

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The electrical character (flux-pinning, Josephson junction, or resistive) of the grain boundaries in approximately twenty flux-grown YBa₂Cu₃O₇-δ bicrystals was determined in previous studies. A selection of these same bicrystals now have been thinned for study by transmission and scanning transmission electron microscopy. High-spatial resolution imaging and analytical techniques reveal microstructural differences among these boundaries that are consistent with their diverse electrical characteristics. The observations offer preliminary insight into some of the features that control the grain boundary superconducting properties and re-emphasize the very fine scale on which the grain boundary electrical character is determined.

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I discuss a few of what I believe to be open questions in the solid state chemistry of high T_c superconductors. Topics include: standards for the announcement of the discovery of a “new” superconductor, two-phase vs. solid solution behavior in (La, Sr)₂CuO₄, (Nd, Ce)₂CuO₄, and Ba₂YCu₃O_7-δ, universal curves of T_c vs. x, and, briefly, the doping mechanism in “Bi₂Sr₂CaCu₂O_8+δ”, and very high T_c superconductors and metastable or difficult to synthesize phases.

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To investigate the interaction of a crystalline lattice with high-temperature superconductivity as well as with magnetic flux lines, we measured temperature dependence of elastic constants and ultrasonic attenuation in single-crystalline La_1.86Sr_0.14CuO₄ in magnetic fields of 0-14 T. In zero field, the elastic response to the superconducting transition was found to be remarkably anisotropic. The elastic constant C₃₃ shows abrupt softening just below T_c. In contrast, C₁₁ revealed no abrupt softening but change in the slope of temperature dependence. In magnetic field, elastic constants exhibit either an additional hardening due to the stiffness of the flux line lattice or an apparent softening probably due to the depression of superconductivity.

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The AC hysteretic loss at 77K in three samples of melt-processed YBa₂Cu₃O_x (YBCO) has been measured using vibrating sample magnetometry. A magnetic method of determining the effective radius of imperfect cylinders is demonstrated. The hysteretic losses of various size samples are intercompared by normalizing the data to some arbitrarily chosen reference radius, R_ref. Normalized loss then depends primarily on J_c. It is demonstrated that for field-sweep amplitudes, H_m, up to a few times H_c1 the Meissner surface current makes an important contribution to the total hysteretic loss, but that at higher fields, although still below the penetration value, H*, the loss can be closely approximated by the H_c1-ignoring Bean value, Q_hB, which follows the well-known law Q_hB∝H_m³. A hysteretic method of determining the lower critical field, valid for thick, high-J_c samples that defy the conventional M versus H approach, is described and demonstrated. Using this method, the 77-K H_c1s of YBCO prepared by two variants of the melt-processing method were found to be 225 and 163 Oe. The H_c1s of two samples prepared independently by the same melt process were found to be 163 and 162 Oe.

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A thick 2212 layer with an uniform composition and a strong (100) preferred orientation has been synthesized by the reaction diffusion between a Sr-base oxide substrate and a Bi-base oxide coating layer. A Jc(4.2K) in the order of 10⁶A/cm² has been obtained. The Ag addition to the coating layer enhances the diffusion. The Ag is pushed out from the diffusion layer to the surface of the specimen forming a laminated structure composed of (100) oriented 2212 phase and Ag. The multilayered 2212 tape has been also prepared using the reaction diffusion in this study.

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The ablation process of high Tc YBa₂Cu₃O_7-x superconducting thin films flashed by a pulsed KrF excimer laser(λ=248nm) or a pulsed Nd-YAG laser (λ=1.06μm) is studied numerically. The dynamics of pulsed nanosecond laser ablation is simulated by the solution of the one-dimensional heat flow equation. The finite element method is applied to solve the equation including the temperature dependence of the thermal conductivity of YBaCuO thin films, the movement of a vapor-liquid interface and the energy of laser-generated plume. The ablation threshold energy density for YBaCuO thin film flashed by a pulsed KrF excimer laser is 0.29J/cm² and that for a pulsed Nd-YAG laser is estimated to be 0.89J/cm², and the effect of laser-generated plume energy is discussed.

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Studies on the thin-film multilayer structures of tunnel junction (S/I/S′) and proximity tunnel junction (S/N/I/S′) geometries are presented (S: HTSC, S′: LTSC, N: normal material, I: insulator). The observed tunnel and Josephson characteristics are reported and discussed. A new tunneling model taking the special features of high T_c superconductors into account is proposed to interret the observed data. Some data on the proximity tunneling effect are also presented.

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Low temperature molecular beam epitaxial (MBE) growth of Bi cuprates are realized on various substrates. Ultra thin films of Bi2201 and Bi2212 could be formed under low temperature (<673K) using 10^-5Pa of NO₂ as an oxidant. The in-plane lattice constants of the ultra-thin films, with thickness less than 6nm, formed under the low temperature epitaxial growth condition on the substrates with less than 3% lattice mismatch were similar to those of the substrates, with the Poisson's ratio of ca. 0.1, in the case of Bi2201 phase. Relaxation of the lattice strain caused by the epitaxial growth was examined by varying the thickness of the epitaxially grown films on SrTiO₃(100). It was revealed that the averaged lattice constant observed by the X-ray diffraction became to show the value of the bulk material around the film thickness of 10 half c unit cells. In this paper, details of the lattice relaxation are discussed.

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The oxidation ability of various oxidants for Bi₂Sr₂CaCuO_x and related cuprates were evaluated both by experiments and by a thermodynamical calculation. When Bi₂Sr₂CaCu₂O_x sputtered films were treated at 400°C with such oxidants as photo-activated O₂, NO₂, O₃, and N₂O, significant Tc change was observed. This Tc change was found in parallel with the variation of c-lattice parameter and verified to originate from oxygen nonstoichiometry in the films by the Hall coefficient measurement. The highly oxidative property of NO₂, ozone, and atomic oxygen derived from this experiment was supported by the thermodynamical calculation for the oxidation of Cu₂O to CuO. The results were further utilized to grow alkaline earth cuprate films by pulsed laser deposition in ultrahigh vacuum (laser MBE). In situ RHEED and XPS analyses clearly indicated that alkaline earth cuprates could be grown epitaxially and laterally under such conditions as 700°C and 1×10^-7 NO₂pressure.

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The nonlinear I-V characteristics in magnetic fields at 0.12 and 0.4T of 100nm-thick Bi₂Sr₂CaCu₂O_x films on MgO(100) substrates were measured. In addition to the published results on YBa₂Cu₃O_x and Tl₂Ba₂CaCu₂O_x, the present measurement suggests that nonlinear I-V characteristics of high-T_c superconductors are universal and in agreement with a theoretical prediction based on a three-dimensional superconducting “vortex-glass” phase.

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The critical current density and resistivity of Bi-Sr-Ca-Cu-O films of thickness 5 to 300nm prepared by MOCVD have been investigated. Magnetic Fields up to 6T were applied parallel and perpendicular to the film surface. Zero resistance temperatures (T_c0) for all samples were about 70K. Critical current densities (J_c) for all samples (4.2K, zero applied field) were about 1×10⁶A/cm², but the form of J_c(T) varied with film thickness. This suggested the existence of isolated layers in the film containing different junctions. The films displayed strong anisotropy for magnetic fields applied parallel to, and perpendicular to, the film surface. We suspect this result to originate in the non-penetration of flux quanta (due to the large effective H_c1) and in the potential barrier (surface barrier) to flux penetration at the surface of the thin films in addition to the intrinsic anisotropy of the Bi-based superconductors.

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A photo-response of High-T_c superconducting films (Bi-Sr-Ca-Cu-O) was investigated to know the possibilities of high temperature operating superconducting photodetectors.
Bi-Sr-Ca-Cu-O films were deposited on MgO (100) substrates with AC sputtering. The photo-response was measured for a chopped He-Ne laser irradiation at the temperature from 10 to 300K. The signal was detected using a Lock-in Amp. at the chopping frequency of the laser radiation.
The photo-response of the films were observed at the temperature region of 10-80K. The signal showed a sharp maximum near T_c (about 70K) and linear current dependencies at whole temperatures, which implied the bolometric behaviour.
The detector performance including direct interactions with superconducting electrons in such as Vortex-flow detectors are discussed.

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The diffusion coefficient of oxygen in superconducting BiSrCaCuO thin films was determined in oxygen ambient in temperature range from 670K to 820K. The measurements of a diffusion coefficient were performed by potentiostatic method, employing a YSZ as a solid electrolyte. The calculations were carried out using the data, obtained by simultaneousely monitoring the time dependences of both the current flowing along the sample and solid electrolyte as well as the electrical resistivity of thin film measured by ac four-probe method. The changes of a diffusion coefficient in the temperature region investigated are discussed.

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