Kiyoshi Mizuno

The 11B Nuclear Magnetic Resonance in CeB6 Single Crystal

NMR measurements were performed on the 11 B in CeB 6 single crystal below 4.2 K and at 77 K using the spin echo technique. The frequency was fixed mostly at 10.7 MHz and the external magnetic field was parallel to the (110) plane. The spectrum at 4.2 K was explained by taking into account the dipolar fields from ferromagnetic moments on Ce atoms. The spectrum in the [111] direction splits into two lines below 3.55 ±0.05 K, which is interpreted tentatively by the occurence of the G-type antiferromagnetic ordering with magnetic moments parallel to the external field and with the maximum component 0.1 µ B /Co around 2.4 K. The Knight shift for the [111] direction increases with decreasing temperature from +0.49 ±0.06% at 4.2 K to +2.02 ±0.25% at 1.4 K. The angular dependence of the spectrum at 2.6 K was explained to some extent by assuming the G-type and ferromagnetic components of magnetic moments. However, the spectrum at 1.8 K could not be explained by those components.

NMR Study of an Effective J value in Dilute Ag–Mn Alloys

Magnetic susceptibility and NMR measurement on dilute Ag-Mn alloys have been made at temperatures between 1.5 K and 300 K. The manganese concentrations are 25, 42, 81 and 470 ppm. The impurity contribution to the observed line widths is proportional to the impurity concentration and the averaged impurity spin which is determined from the susceptibility data. The effective J value is derived is from the proportionality constant as | J |=1.1±0.1 eV. For the derivation of the J value the local field distribution caused by the impurity due to the RKKY interaction and the direct dipolar interaction has been calculated. This magnitude of J is different by a factor of 5.0±0.5 from the value obtained from T K =40 mK.

Electronic State and Arrangement of Hydrogen Atoms in Ternary Graphite Intercalation Compounds C8KH0.61 –ESR, 1H NMR and 13C NMR Studies

We have studied ESR, 1 H NMR and 13 C NMR in ternary graphite intercalation compounds C 8 KH 0.61 prepared from the highly oriented pyrolitic graphite (HOPG). Although it has been controversial whether the hydrogen atom has a localized moment, our experiment proves non-existence of the localized moment. It is shown that the absorbed hydrogen molecules are dissolved into an atomic state and that the hydrogen atoms have a metallic character. The electronic state of hydrogen atoms and their geometrical arrangement are discussed.

NMR evidence of BCS superconductivity in Y(Ni1−xPtx)2B2C

Abstract 13 C and 11 B NMR studies have been made on the quaternary superconductor Y(Ni1−xPtx)2B2C with x=0 and 0.4. Temperature dependence of Knight shift and spin-lattice relaxation rate (SLRR) in the normal state was explained consistently among all the observed nuclei by Korringa mechanism without taking account of antiferromagnetic spin fluctuation. Bellow the superconducting transition temperature we observed a Hebel–Slichter peak in case of 13 C SLRR. These results mean that the borocarbide is a normal metal and a phonon-mediated superconductor

Pressure and Grain-Size Dependences of the Chiral-Glass Transition in Superconductive Ceramics of YBa2Cu3O7−y

Ceramic high-Tc superconductors may be viewed as a weakly coupled random Josephson network containing π-junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa2Cu3O7−y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at Tc1 and furthermore among the grains at Tc2 (<Tc1). The increase in Tc2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The Tc2 also increases with increase of pressure, however, the sharpness of the transition at Tc2 does not change. In spite of difference in Tc2s, the increasing rates of Tc2 by pressure are nearly the same.

13C NMR study of borocarbide superconductor YNi2B2C

Abstract 13 C NMR study has been made of the quaternary superconductor YNi 2 B 2 C. Temperature dependence of T 1 and K in the normal state showed normal metal behavior. Temperature dependence of T 1 in the superconducting state strongly suggests that the quaternary borocarbide is a conventional BCS superconductor.

Spin Dynamics Studied by 89Y NMRin High Tc Superconductor YBa2Cu3O6+x

89 Y NMR shifts in the high T c superconductor YBa 2 Cu 3 O 6+ x are compared with 17 O NMR new data published by Yoshinari et al . and the orbital shift 89 K orb is estimated to be 155 ppm. The nuclear spin lattice relaxation rate 89 (1/ T 1 T ) for 89 Y is found to be proportional to 17 (1/ T 1 T ) for 17 O at 2, 3 plane sites, which supports the single spin fluid model. The relation T 1 T K S =const. is realized for 89 Y nuclei in the oxygen depleted compounds YBa 2 Cu 3 O 6+ x . It is found that the relation T 1 T K S =const. means that the energy width of dynamical spin susceptibility χ ′′ ( q , ω) with the wave vector q ∼0, \(\hbar\Gamma_{0}\), is independent of temperature although the uniform susceptibility χ 0 strongly depends on temperature. It is also shown that the weight of the low q part of the spin excitation spectrum decreases with decreasing the hole concentration.

Pressure dependence of the chiral-glass transition in Y–Ba–Cu–O superconductors

Abstract Ceramic high- T c superconductors may be viewed as a weakly coupled random Josephson network containing so-called π-junctions. Kawamura and Li proposed that a novel glassy phase, a chiral-glass phase, might occur in d-wave ceramics. This prediction was supported by experimental results on YBa 2 Cu 4 O 8 ceramics. In the present work, the pressure dependence of the chiral-glass transition has been investigated by magnetic measurements on the ceramic YBa 2 Cu 4 O 8 . The transition temperature increases linearly with increase in pressure, and the singular behaviors of the susceptibilities are scaled by the transition temperature. The results suggested that the transition occurs in frustrated Josephson network and depends only on the strength of Josephson junctions between grains.

Oxygen concentration dependence of superconductivity in the Pr substituted Y1−xPrxBa2Cu3O7−y

Abstract The oxygen concentration dependence of T c is examined in Y 1−x Pr x Ba 2 Cu 3 O 7− y . The T c decreases with decreasing the oxygen concentration 7− y in not only Ortho I (6.75≦7− y ≦7.0) but also the region between Ortho II and Tetra (7− y ≦6.6) for the Pr contents of 0≦ x T c increases conversely with decrease of 7− y for 0.3≦ x y ≦6.75). The Pr content dependence of T c for Ortho II is quite different from that for Ortho I, and is preferably similar to that in the stoichiometric oxide superconductor Y 1−x Pr x Ba 2 Cu 4 O 8 .

Oxygen concentration dependence of superconductivity in Pr-substituted Ln1−xPrxBa2Cu3O7−y (Ln = Y, Nd, Gd)

The Pr content x dependence of the superconducting transition temperature Tc is examined for various oxygen concentrations by quenching the samples of the Pr-substituted Ln1−xPrxBa2Cu3O7−y, where Ln is Y, Nd and Gd. In the case of Ln = Y, the initial rate of decrease of Tc by Pr substitution (−dTcdx) decreases with decreasing oxygen concentration for 6.6 < 7 − y < 7.0, then becomes constant for 7 − y < 6.6. In contrast, the effect of Tc-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than that in the case of Ln = Y. In the case of Ln = Gd, intermediate behavior between those of Y and Nd is observed, corresponding to the ionic size of Ln.