Masao Doyama

Positron lifetime spectroscopy on the high Tc superconductor Ba2YCu3O7−σ

Abstract Positron lifetime measurements were performed of the high T c superconductor Ba 2 YCu 3 O 7− σ in the temperature range 21–206 K. The second component of the lifetime decreased anomalously by undergoing the superconducting transition, while its intensity slightly increased. These results indicate that the electron density at the oxygen vacancy sites increased by the superconducting transition.

Magnetic ordering in superconducting YBa2(Cu1−xFex)3Oy oxide

Abstract Magnetic susceptibility measurement and the zero-field muon-spin-relaxation experiment have been made for the YBa 2 (Cu 1− x Fe x ) 3 O y system. The susceptibility has been measured in the field strong enough for the magnetic flux to penetrate the superconductor and the resultant temperature dependence has demonstrated the field cooled effect and a cusp at low temperatures below T c . The cusp has been observed when the high field has been applied. The dynamical depolarization rate of the muon obtained by the muon-spin-relaxation experiment has shown the maximum at the same temperature as the cusp. The results suggest that the spin glass freezing of localized magnetic moment takes place at the temperature of the cusp, which is consistent with the previous Mossbauer effect experiments. The magnetic phase diagram for the system has been obtained.

Computer Applications to Materials Science and Engineering

Engineering materials are very complicated. It is not easy to treat practical materials rigorously. The simplest materials are pure elements. Single crystals are not very common; the usual materials are polycrystalline and multiphase. Some materials are amorphous. In this chapter, an atomic and molecular approach has been taken from first principles. Concerning metals, the cohesive energies, elastic constants, and electronic structures of pure crystalline metals and of intermetallic compounds have been calculated. Random alloys present difficulties for such calculations. Using interatomic potentials, molecular dynamics have been applied to calculate the dynamic behavior of materials. Examples of calculations are presented. Kinetic equations among defects have been solved. The atomic configurations and stability of interstitial atoms and the atomic configurations of dislocation cores are shown. Using molecular dynamics, the rapid quenching of liquid, the tensile deformation of amorphous and crystalline iron, the shear plastic deformation of copper small crystals, and lattice vibrations are shown.

Structural Investigation of High Tc Bi-Sr-Ca-Cu-O

High temperature X-ray diffraction, electrical resistivity and susceptibility measurements have been made for the Bi-Sr-Ca-Cu-0 system of 1:1:1:2, 2:2:1:2 and 2:2:2:3 in the nominal composition ratio of Bi:Sr:Ca:Cu and for the system of 2:2:2:3 doped with Pb. No structural change has been observed in the temperature range from R.T. to 1123K for samples except for 1:1:1:2, where some extra peaks have appeared between 923K and 1073K. The crystal structure of a high Tc phase (Tcon=115K, Tczero=110K) in the Pb-doped sample is tetragonal with a=b=3.82 A and c=37.22 A at R.T.. The temperature dependence of lattice parameters of the low Tc phase for undoped samples and of the high Tc phase for Pb-doped samples have been obtained. The mean thermal expansion coefficient between R.T. and 1100K of c-axis of the high Tc phase is 2.9x10-5 K-1 which is larger than that of low Tc phase.

Thermal and Structural Measurements of the Mixed Crystal 1T–TaS2-xSex

The commensurate-incommensurate (C-I) charge-density wave (CDW) phase transition of the system 1T–TaS 2- x Se x (0≦ x ≦2) was studied by measuring the heat capacity and the lattice constants. From changes in the heat capacity, the intermediate phases similar to those of 1T–TaS 2 are detected only for x ≦0.85, while they are not observed for 0.90≦ x as in 1T–TaSe 2 . The lattice constants of the C-CDW phases exhibit two step-like changes around x =0.7 and 1.4. From both results, stability of the intermediate phases and the role of anion ordering are discussed. It is predicted that the C-CDW phase in 0.1≦ x ≦0.6 is strongly suppressed under pressure.

COMPUTER APPLICATIONS TO MATERIALS SCIENCE

As examples of calculations, atomic configurations of dislocation cores, using molecular dynamics, rapid quenching of liquid, tensile deformation of amorphous and crystalline iron, shear plastic deformation of copper small crystals, lattice vibrations are shown.

STRUCTURAL FLUCTUATIONS IN ARGON MICROCLUSTERS

We performed a set of extensive molecular dynamics simulations of a cluster of 79 argon atoms in order to clarify the nature of structural fluctuations in microclusters. Within a finite range of total energy, the cluster exhibited structural fluctuations between solid-like and liquid-like states and also among solid-like states. Based on our simulation, we propose that thermal expansion of the cluster initiates the structural fluctuations.

CALCULATION OF POSITRON DISTRIBUTION AND LIFETIME IN THE OXIDE SUPERCONDUCTORS

Distribution and lifetime of positrons in the oxide superconductor YBa 2 Cu 3 O 7-y (YBCO) have been calculated. It was revealed that positrons distribute between the Cu-O one-dimensional chains in YBCO. Localization of positrons at oxygen vacancy clusters was examined. The affinity of a positron to the cluster is expected to become larger with growing of the cluster size. Effects of iron or nickel substitution for copper were also investigated.

Preparation of High-Tc Phase in Bi-Sr-Ca-Cu-O System by Means of Addition of Ca2PbO4

Effect of Ca2PbO4 addition has been studied for the preparation of the high Tc superconductor in the Bi-Sr-Ca-Cu-O system. The pure high Tc phase has been obtained by the sintering of the mixture of Bi2Sr2CaCu2O8+δ, Ca2PbO4 and CuO at 1123K for 120 hours in air. It is found that the best composition ratio of Ca2PbO4 is 0.425 or 0.45 against the 1 mol low Tc phase and 1 mol CuO. The high Tc phase is more homogeneous than that sintered by the conventional solid state reaction.

Non-destructive testing of silicon single crystals by positron annihilation

A silicon single crystal of length 25 cm and diameter 10 cm grown by the Czochralski method was cut in half; one half was examined by the Lang X-ray method after heating at 1000 degrees C for 16 h and the other half was examined as-grown by positron annihilation Doppler broadening. The value of the S-parameter was at its lowest in the region of high oxygen concentration, and at its highest in the nearly perfect lattice. The results show that silicon crystals can be tested non-destructively by positron annihilation as-grown, without requiring prolonged annealing pretreatments.