Seiki Takebayashi

Processing and properties of QMG materials

Abstract Recent progress of Quench and Melt Growth (QMG) process is reviewed. QMG process is one of the melt processes for Y based superconductor. A pseudo single crystal (QMG crystal) which includes finely dispersed 211 phase in the matrix of 123 phase was produced by the original QMG process. The critical current density of the QMG crystal that has no high angle grain boundaries has been exceeding 104A/cm2 at 77K and 1T. The original QMG process was improved to make larger QMG crystals of which crystallographic orientation was controlled. Modified QMG process can be characterized by Pt addition in precursor preparation and seeding technique in growth process. Crystallinity of the QMG crystal made by this modified process was investigated. It appeared that there were two kinds of domain structures that depend on the growth direction. QMG crystals enlarged by this modified process are applicable to various fields. For instance, QMG crystal can operate as a bulk magnet. The maximum value of flux density was 4.5T at 40K on the surface. It was found that QMG crystal is thermally stable above 40K at least.

High Magnetic Flux Trapping by Melt-Grown YBaCuO Superconductors

Trapped magnetic flux density of melt-grown YBaCuO bulk superconductors was evaluated. A single-grained disc-shaped bulk sample with a size of 45 mm×15 mm trapped very high magnetic flux density after field cooling. One disc trapped a maximum surface flux density of 0.72 T, while the maximum of double-piled discs was 1.35 T after 100 seconds. A polygrained sample showed a much lower maximum. The pinning potential of this single-grained disc was 0.17 eV, and the overall critical current density estimated from the trapped flux density was about 7×103 A/cm2. Since these values are lower than those of a small-sized sample, weak links are still expected to exist even in the single-grained sample. Domain structure within a grain is a strong candidate for this weak link.

Effect of Pt Addition on Melt-Processed YBaCuO Superconductors

Platinum addition in precursors of melt processing could produce finely dispersed Y2BaCuO5 (211) inclusions of about 1 µm in YBa2Cu3Ox matrix. This fine dispersion of 211 resembles that of quench-and-melt-grown materials. The platinum-added sample exhibited a critical current density exceeding 2×104A/cm2 at 77 K and 1 T. Platinum is considered to enhance the nucleation of 211 which forms by the pertitectic reaction between Y2O3 and liquid phase.

The optical study of the doping effect in single-crystal Bi2Sr2(Ca, A)Cu2O8+x (A=YorNd)

Abstract The reflectivity of single-crystal Bi 2 Sr 2 (Ca, A)Cu 2 O 8+ x (A=Y or Nd) was measured and transformed to the optical conduc tivity through the Kramers-Kronig relation. A broad peak is observed in the mid-infrared region in the conductivity of the semiconducting samples, and the position of the peak shifts to O eV with increasing doped-carrier concentration. For the superconducting sample, the peak is located near O eV where a Drude model is approximately valid in the mid-infrared region, while a Lorentz model is appropriate for the mid-infrared spectra of the other samples. The conductivity was fitted by two Lorentz oscillators and one Drude term, and the electronic states of the high- T c cuprates were discussed in terms of the obtained fitting parameters.

1/f conduction noise in the high-temperature superconductor Bi-Sr-Ca-Cu-O system

Abstract Conduction noise was investigated in the normal state of the 2212 phase of the Bi-Sr-Ca-Cu-O system for polycrystals, highly oriented thin films, and single crystals. It was found that large noise with 1/ f spectrum in this material is generated as a bulk effect. The magnitude of the noise power is 7–10 orders of magnitude larger than in normal metals even in single crystals. Several possible origins are discussed.

Effect of substitution of 3d metals for Cu in Bi2(Sr0.6Ca0.4)3Cu2Oy

Abstract The effect of the substitution of 3d metals (Fe, Co, Ni and Zn) for Cu was investigated. Lattice constants, DC resistivity, Hall effect, magnetic susceptibilities in normal and superconducting phases were measured. The change of the Hall coefficient with the substitution was much smaller than those in YBa 2 Cu 3 O y or in Y or lanthanide substituted Bi 2 (Sr, Ca) 3 Cu 2 O y . The change of T c with the substitution of Fe, Co, Ni with localized magnetic moment did not differ much from that of Zn with no magnetic moment.

Optical study of Bi2Sr2Can−1CunO4+2n+δ—Doping effects, n-dependence of the spectra and characteristics of the CuO2 plane

Abstract Optical reflectivity of single crystals of the Bi-based Cu and Co oxides was measured and analyzed. The doping effects in the heavily doped region, the n -dependence, and the comparison between Cu and Co oxides are discussed. The optical spectra for various cuprates are compared in order to find a common feature of the superconducting CuO 2 plane.

Study of YBa2Cu3O7 Superconductor by Low-Temperature Scanning Tunneling Microscopy and Spectroscopy

The electronic structure of YBa2Cu3O7 (YBCO) superconductor was studied by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS), since the electronic states of CuO2 planes in the superconducting state may shed light on the mechanism of high-Tc superconductivity. Experiments were performed on the cleaved surface of a bulk single-grain YBCO specimen at 4.2 K. Two different STS conductance spectra were obtained, a BCS-like one and a metalliclike one. These spectra were observed alternately with a periodicity of 0.8±0.2 nm, suggesting that YBCO has a two-dimensional structure in which CuO2 planes are strongly superconducting and CuO chains are metallic-like.

Conductance fluctuation of BiSrCaCuO system

Abstract Conduction noise was investigated in normal state of 2212 phase of Bi Sr Ca Cu O system for polycrystals, highly oriented thin films, and single crystals. The magnitude of the noise power is 7–8 orders of magnitude larger than in normal metals even in single crystals. By the investigation of the size effect, this noise was found to be the bulk effect.

The optical properties of the doped holes in Bi2Sr2(Ca,Nd)Cu2O8+δ and (Nd,Sr)CoO3

The optical reflectivity spectra of Bi 2 Sr 2 (Ca,Nd)Cu 2 O 8+δ and (Nd,Sr)CoO 3 were measured in the range from 0.03 to 3 eV, and transformed to the optical conductivity through the Kramers-Kronig relation. The obtained conductivity spectra for doped samples have a broad absorption in the mid-infrared region for both systems. The broad absorption shifts to 0 eV with doping in Bi-based cuprates, while it stays constant near 0.3 eV in (Nd,Sr)CoO 3 . The electronic states of high-T c cuprates and Co oxides are discussed by comparing the obtained spectra of the two materials.