Y. Kubo

Structural distortion and ferroelectric properties of SrBi2(Ta1−xNbx)2O9

Ferroelectric materials of the SrBi2(Ta1−xNbx)2O9 solid-solution system were synthesized, and their structural and ferroelectric properties were investigated. Atomic displacements of the ions in the (Ta,Nb)O6 octahedron significantly increase as x increases, which leads to more structural distortion of the perovskite-type unit. The Bi2O2 layer, in contrast, is less distorted in SrBi2Nb2O9 than in SrBi2Ta2O9. The contribution of the perovskite-type unit to total ferroelectric polarization is greater in the SrBi2Nb2O9 sample, while that of the Bi2O2 layer is less; consequently, the total calculated polarization slightly increases. The ferroelectric Curie temperature also increases from 300 (SrBi2Ta2O9) to 440 °C (SrBi2Nb2O9). Three short (Ta,Nb)–O bonds in the (Ta,Nb)O6 octahedron, whose lengths are less than 2 A, have a covalent character, and the substitution of Nb for Ta makes the bonds more covalent. The strong covalent interaction of the (Ta,Nb)–O bonds increases the structural distortion, resulting in...

Spin dynamics in heavily-doped high-Tc superconductors Tl2Ba2CuO6+y with a single CuO2 layer studied by 63Cu and 205Tl NMR

Abstract Knight shift of 63 Cu and nuclear spin lattice relaxation time T 1 of 63 Cu and 205 Tl have been measured in Tl 2 Ba 2 CuO 6+ y with T c =72 K, 40 K and 0 K. In normal state, the spin part of the 63 Cu shift is T -independent and increases with decreasing T c . 1/ T 1 of 63 Cu and 205 Tl obey a T 1 T =constant law over a wide T -range from about 150 K close to T c . Above about 150 K and near T c , (1/ T 1 T ) begins to decrease. The T -linear dependence of the resistivity observed in 72 K superconductors changes into a T 2 dependence in a non-superconducting metal in which the T 1 T =constant law holds up to around 200 K. Analysis of the anisotropy of the relaxation rate, 63 R =(1/ T 1 ) ab (1/ T 1 ) c reveals that the antiferromagnetic (AF) spin fluctuations among nearest neighbor Cu sites, which survives still in the compound with T c =72 K, becomes less distinct with decreasing T c and disappears in a non-superconducting sample. These results point out that there may be an intimate interrelation between the occurrence of superconductivity and the presence of the AF spin correlation, although the magnetic coherence length, ξ M is supposed to be as small as ξ M / a ≤∼1. Experimentally, it is shown that the T -linear dependent resistivity is relevant to the presence of the AF spin correlation. In the superconducting state, the relaxation behavior is found to possess a universal feature in most of the high- T c oxides, indicating a common relaxation mechanism to work even in heavily-doped superconductors exceeding the maximum of T c .

Degradation of ferroelectric Pb(Zr, Ti)O3 under reducing conditions

The degradation mechanisms of Pb(Zr, Ti)O3 (PZT) under reducing conditions were investigated. Although no apparent changes were seen from the results of thermogravimetric and x-ray diffraction measurements after H2 annealing at typical process temperatures, the PZT material had changed: The sample appearance changed from white to black. A small amount of PZT (less than 0.3% even after 4% H2 annealing at 550 °C) decomposed into Pb in the reducing atmosphere. This, however, was too little to cause all of the change in the sample appearance. Systematic changes in the lattice constants and the black color of the sample strongly suggest that oxygen defects were introduced into the PZT during H2 annealing. Oxygen defects would produce a donor level within the PZT band gap, which would account for the change in color, and would increase leakage current in capacitors. We have also found that PbZrO3 decomposed through oxygen dissociation more easily than PbTiO3. Oxygen atoms in PbZrO3 are weakly bonded to Zr compa...

Charge dynamics in the colossal magnetoresistance pyrochlore Tl 2 Mn 2 O 7

Optical conductivity data

Effect of the pressure-temperature history on Tc and the Hall coefficient in superconducting Tl2Ba2CuO6+x

[\ensuremath{\sigma}(\ensuremath{\omega})]

Carrier density change in the colossal-magnetoresistance pyrochlore Tl 2 Mn 2 O 7

of the colossal magnetoresistance (CMR) pyrochlore

Synthesis and physical properties of Tl2Ba2CuO6+δ single crystal

{\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}

Oxygen nonstoichiometry and metal substitution in TlSr2CaCu2O7−z

are presented as functions of temperature (T) and external magnetic field

Evidence for Fermi liquid excitation in Tl2Ba2CuO6−y

(B).

Single crystal study of Tl2Ba2CuO6+δ

Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself,