Kei-ichi Koga

Metal-Insulator Transition in the Bi2Sr2Ca1-xYxCu2O8+y System

Metal-insulator transition driven by the change of the carrier concentration was investigated in the 80 K superconductor Bi2Sr2CaCu2O8+y. The carrier concentration was controlled by the substitution of yttrium for calcium. Experimental results from structural, transport, and magnetic studies are discussed in comparison with other high-temperature superconductor systems, La2-xSrxCuO4 and YBa2Cu3O7-y.

NMR Studies of 17O in the Normal State of YBa2Cu3O6+x

The temperature dependence of both the Knight shift ( K ) tensor and the nuclear spin-lattice relaxation rate (1/ T 1 ) in the normal state have been studied, by using the magnetically oriented powered samples of YBa 2 Cu 3 O 6+ x with x =0.96, 0.8 and 0.6. From the K -χ plot for the planer oxygen sites (O(2, 3)), we have determined the principal values of the hyperfine field tensor and found that those hardly depend on the hole concentration. The remarkable feature has emerged from the T -dependence of the 1/( T 1 T ) at the O(2, 3) sites; i.e. it reveals almost the same T -dependence of the spin parts of the Knight shift. This observation suggests that the energy width of the spin excitations around q ∼0 is T -independent in the normal state. The Knight shift tensor at the apical oxygen sites has also been measured and is discussed.

Characterization of Metamorphic Phases of Ba2YCu3O9-δ

We have recently found several metamorphic phases of Ba2YCu3O9-δ. In addition to the high Tc orthorhombic phase, two tetragonal and another orthorhombic phases of them were studied with X-ray diffraction, susceptibility and resistivity measurements. We found that the high Tc phase is a weak Pauli paramagnet and one of the tetragonal phases is a magnetic semiconductor with a phase transition around 8 K. The results are briefly discussed in relevance to the superconductivity in this series of oxides.

Characterization of a Non-Superconducting Cuprate, Bi2Sr2YCu2O8.5

A new Bi-containing cuprate, Bi2Sr2YCu2O8.5, was found in the course of substitution study for high Tc compounds Bi2Sr2CaCu2O8+y. Bi2Sr2YCu2O8.5 is a semiconductor and undergoes an antiferromagnetic transition at about 13 K which is considered to originate from the ordering of Cu in the CuO2 planes. An electron diffraction study revealed the existence of a superstructure. The unit cell taking account of the superstructure is 1×8×1 of the original unit cell (5.47×5.43×30.18 A3). Analogy of three kinds of oxide systems, (La1-xSrx)2CuO4, YBa2Cu3O6+x, and Bi2Sr2CaxY1-xCu2O8+y, is discussed.

Proton NMR in degraded powder of YBa2Cu3O7−δ

Pulsed NMR of 1H has been applied to characterize aged powders of YBa2Cu3O7-δ and its family. A proton site with an internal field of 150 Oe, which is from antiferromagnetically ordered copper moments, has been observed in antiferromagnetic, tetragonal Yba2Cu3O7-δ. A proton signal inside the superconducting region of orthorhombic YBa2Cu3O7-δ has been observed together with signals from regions of nonmagnetic, nonsuperconducting degraded products for both cases. Overall features are interpreted along with a recently proposed topotactic mechanism of hydration reaction.

Local magnetic moment in Al-Mn based quasicrystals

Mossbauer spectra and magnetic susceptibility have been measured for Al-Mn based quasicrystals containing Fe. The separations of outer peaks of the Mossbauer spectra at 4.2 K with external magnetic fields of 3.0 and 4.5 T correspond to the ones due only to the applied field, indicating that the Fe atoms have substantially no local magnetic moment. The magnetic susceptibilities for quasicrystalline Al 72 Si 6 (Mn 1- x Fe x ) 22 with x =0, 0.01 and 0.25 obey the Curie-Weiss law, giving approximately the same effective moment per atom of transition elements, int spite of the substitution of non-magnetic Fe for Mn. The above facts is, in turn, interpreted by either the fact that (1) there are magnetic and non-magnetic Mn sites and Fe atoms occupy preferentially the latter sites, or that (2) the itinerant electrorns are responsible for the temperature-dependent susceptibility.

Transition from valence-fluctuating state with a gap to magnetic Kondo state in CeNi1−xCuxSn

Abstract Magnetic, thermal and transport measurements on the pseudo-ternary system CeNi1−xCuxSn (0 ≤ x ≤ 0.5) demonstrate that a valence-fluctuating state with a pseudo-gap for x = 0 evolves to an antiferromagnetic Kondo state for x ≥ 0.13 through a heavy-fermion region. The energy gap for x = 0 was found to be smeared out not only by the partial substitution of about 10% of Cu but also by magnetic fields higher than 20 T.

Superconducting and magnetic properties of the oxygen-vacancy ordered ortho-II phase of Ba2YCu3O6.5

Abstract Superconducting oxygen-vacancy ordered ortho-II phase samples of Ba 2 YCu 3 O 6.5 were prepared with a new method of low temperature annealing. Electron diffraction analyses confirmed the ordered structure with a doubled cell along the a -axis of the orthorhombic triple perovskite. Contrary to previously reported results for quenched samples of the same phase, the annealed samples exhibited a metallic temperature dependence of resistivity and an almost linearly increasing magnetic susceptibility with temperature above its T c (∼53 K). Significance of this temperature dependence of susceptibility without Curie-Weiss contribution is discussed in comparison with the recently proposed model for the La 2- x (Sr, Ba) x CuO 4 system. Resistivity measurements in magnetic fields up to 8 T revealed an anomaly around 35 K, implying existence of another superconducting phase. From low temperature specific heat measurements we found 4.0 mJ/mol-K 2 for the residual γ value in the superconducting state which is about the same found for good quality ortho-I samples.

Crystal structure and superconductivity of La2−xBaxCuO4 (0.03≤x≤0.24)

Abstract High-resolution neutron powder diffraction has been performed to investigate the crystal structure of La 2− x Ba x CuO 4 as a function of x (0.03≤ x ≤0.24). The crystal parameters at 115 K refined by profile analysis show that the Cu-planar O(1) bond lengths decrease by 0.008 A when x is increased by 0.1. This result indicates that the overlap of Cu and planar O orbitals, and thus the charge transfer, increases with x . The length of the Cu-apical O(2) bonds decreases at a rate of about one half of that for Cu-O(1), suggesting that the apical oxygens O(2) also play an important role in the charge transfer process. The changes of the Cu-O(1) bond lengths of the compound with x =0.125 at 15 K are further studied in connection with the structural phase transition to the low-temperature tetragonal (LTT) phase. These results are discussed in terms of their implication for the electronic structure and superconductivity of this system.

Superconductivity and magnetism of Bi2Sr2CaCu2Ox Effect of quenching and pulverization

Abstract The effect of quenching on Bi 2 Sr 2 CaCu 2 O x (BSCCO) has been investigated. Quenching enhances the critical temperature T c . The hole concentration n h (∼ 5 × 10 21 cm −3 ) decreases as the quenching temperature T q increases. The normal-state static susceptibility χ DC (∼ 5 × 10 −7 emu/g) of the BSCCO pellet is almost independent of temperature but its magnitude decreases with the increase of T q . Thermogravimetry (TG) analysis shows that the change in the oxygen content is very small when the sample is heated. The Cu nuclear quadrupole resonance (NQR) spectra are very broad. In addition, we have observed a large Curie-Weiss term in χ DC and χ ESR of powdered samples. From the Curie constant, the number of localized Cu 2+ spins is estimated to reach 30% of total Cu atoms. This component almost disappears by re-annealing. In all cases, χ ESR has almost the same magnitude as χ DC , in marked contrast with YBa 2 Cu 3 O 7−δ . The origin of the normal-state susceptibility is discussed.