Akihiko Hayashi’s research while affiliated with The University of Tokyo and other places

We studied the vortex state of an oriented polycrystalline high-T-c superconductor Tl2Ba2CuO6+delta (T-c similar to 85 K) by Tl-205 NMR spin-echo technique. Although more than 10 years have passed since the sample was synthesized, no aging effects were found in the low-field magnetic susceptibility and the zero-field Cu-63,Cu-65 nuclear quadrupole resonance spectrum. From (TI)-T-205 NMR spectra at 2T, we observed vortex melting, vortex partial melting, imperfect vortex lattices. and the absence of static antiferromagnetic ordering at vortex cores. (c) 2006 Elsevier B.V. All rights reserved.

We report a 205Tl NMR study of vortex states in an aligned polycrystalline sample of an overdoped high-Tc superconductor, Tl2Ba2CuO6+δ (Tc˜ 85 K), at a magnetic field of 2 T along the c axis. We observed an imperfect vortex lattice, so-called Bragg glass, at T=5 K, the coexistence of vortex solid and vortex liquid between 10 and 60 K, and vortex melting between 65 and 85 K. No evidence for local antiferromagnetic ordering at vortex cores was found for our sample.

We report a (205)T1 NMR study of vortex state for an oriented polycrystalline sample of an overdoped high-T-c superconductor T1(2)Ba(2)CuO(6+) (delta)(T(c)similar to 85 K) with magnetic field 2 T along the c-axis. An obvious Redfield pattern of the (205)T1 NMR spectrum for our sample at T= 5 K indicates the vortex lattice state without local antiferromagnetic order at vortex cores. We emphasize that nonexponential nuclear spin-lattice relaxation curves indicate an imperfect vortex lattice or Bragg glass phase. (c) 2005 Elsevier Ltd. All rights reserved.

The high Tc superconductor YBa2Cu3Oy (YBCO) is an oxygen non-stoichiometric compound and its oxygen content y changes from 6.0 to 7.0 under various conditions (temperature and oxygen partial pressure). At around room temperature, the crystal structure of YBCO is tetragonal in 6.0<y<6.4 and orthorhombic in 6.4<y<7.0. The relation between oxygen content and ortho-tetra transition at y>6.6 has been unknown because of the restriction of experimental methods. In this experiments, using the newly developed TG-DTA making it possible to measure under high pressures, we have investigated oxygen non-stoichiometry of YBCO under high oxygen pressure (>1atm). In results, we obtained new knowledge of the dependence of O-T transition temperature on oxygen content y (>6.6).

Interaction between intercellular adhesion molecule 1 (ICAM-1) and lymphocyte function-associated antigen 1 (LFA-1) might be involved in the pathogenesis of liver diseases. We investigated whether monoclonal antibodies (mAbs) against these two adhesion molecules could inhibit the formation of primary biliary cirrhosis (PBC)-like lesions in an animal model using graft-versus-host reaction (GVHR) with major histocompatibility complex class II disparity. PBC-like hepatic lesions such as cellular infiltration of portal area and nonsupprative destructive cholangitis (NSDC) were generated by injecting spleen T cells of C57BL/6 (B6) mice into (B6. C-H-2bm12 X B6) F1 mice. In the liver of these mice, increased number of LFA-1-positive cells and enhanced expression of ICAM-1 on sinusoidal endothelial cells and bile duct epithelial cells were observed immunohistochemically, when compared with F1 mice without GVHR. Hepatic lesions of these mAb-treated mice were almost completely inhibited in these mice compared with GVHR mice. Furthermore, we studied to determine which anti-LFA-1 mAb or anti-ICAM-1 mAb was essential to inhibit the hepatic lesions. Mice solely treated with anti-LFA-1 mAb showed significant inhibition of hepatic lesions, whereas treatment with anti-ICAM-1 mAb could not inhibit the lesions. Despite the inhibition of hepatic lesions, induction of GVHR and production of antimitochondrial antibodies were not impaired in mAb-treated mice. We conclude that LFA-1 mediates cell infiltration into the liver in this murine model of GVHR and suggest a possible therapeutic role of mAbs to this adhesion molecule in selective autoimmune liver diseases.

We report Cu NMR and NQR studies of the cupric sulfide CuS from 4.2 K to room temperature. We have assigned the site for two distinct 63Cu NMR lines based on an abrupt change of the Cu NMR spectrum at 55 K due to a structural phase transition for both the powder and the oriented CuS. We found that the Cu nuclear spin-lattice relaxation rate 1/T1 is magnetic in origin and highly anisotropic, and that the temperature dependence of 1/T1T is similar to that of the uniform magnetic susceptibility chi0(T). The orbital contribution to the relaxation rate and the nature of the electron spin correlation at the Cu sites are discussed.

The anomalous metallic state below the charge-density-wave (CDW) transition temperature of Rb3Cu8S6 has been studied by NMR. This compound shows metal-CDW-metal transitions at 110 K(Tc1) and 50 K(Tc2), respectively. In the Cu spectra below Tc2, a broad hump appears just between two peaks of the double-horned spectrum characteristic for CDW states, indicating the existence of domain walls on the CDW chains. On the other hand, Rb spectra suggest that the electronic states of the Rb sites slightly change below Tc2. This may reflect a small change in the electronic states of the Cu-S planes where CDW's develop, and suggests a possibility that the domain walls might act as spinless solitons, as is observed in polyacetylene chains.

The fact that the magnetic susceptibilities in the paramagnetic regime of the parent insulating materials La2CuO4 and YBa2Cu3O6 increase monotonically with temperature is well explainable in terms the square-lattice 2D Heisenberg model. It is predicted by the model that the position of chi(max) occurs at a temperature close to the intraplanar exchange parameter J. We show that the Ga-based cuprate YSr2Cu2GaO7 is an orthorhombic antiferromagnet with T-N = 387 K. Above T-N, two broad maxima of magnetic susceptibility are observed. The first broad maximum is located at 520 K. The position of the second maximum lies beyond the measurable range (> 800 K). Thus, the observed susceptibility behavior disagrees with that predicted by the isotropic S=1/2 Heisenberg model. Such a discrepancy is tentatively explained by a loss of the symmetry in the CuO2 network.

Oxygen nonstoichiometry and structural phase transitions in La2NiO4+δ were studied by thermogravimetry (TG) and in-situ X-ray powder diffraction. Doubly charged O2− ions are proposed as the species of the excess oxygen from thermodynamical analysis of TG data. A transition from tetragonal 14/mmm to orthorhombic Fmmm phase was found in the wide range of 0.06 ≤ δ < 0.11, which showed a significant time dependence. The phase segregation of pseudo-tetragonal phase from tetragonal 14/mmm was observed for 0.03 ≤ δ < 0.06. An equilibrium phase diagram of La2NiO4+δ is proposed for 0 ≤ δ ≤ 0.18. The character of the phase transitions is discussed in the light of the temperature dependence of the orthorhombicity in each orthorhombic phase.

We report the temperature(T)-dependent anisotropy r(≡63Wab/63Wcc) of the planar 63Cu nuclear spin-latticerelaxation rates 63Walpha (alpha=ab,cc) in the high-TcCu oxides, YBa2Cu4O8, YBa2Cu3O6.96 andTl2Ba2CuO6+delta, from around Tc up to about 300 K. Wediscuss within the spin-fluctuation model that the anisotropy r isdependent only on the antiferromagnetic correlation length xi andthat its T-dependence for the lightly-doped material is different fromthat of the over-doped ones. We provide the T-dependent anisotropy ras direct evidence of the T-dependent xi .