4PO5-20 Bond angle dependence of the magnetic interactions in antiferromagnetic cuprates

Tadashi Shimizu 1, Takehiko Mazmoto 1, Atsushi Goto 1, Kazuyoshi Yoshimura 2, and Koji Kosuge 2. 1 National Research Institute for Metals, Sakura, Tsukuba, Ibaraki 305-0003, Japan. 2 Department of Chemistry, Kyoto University, Kyoto 606-8502, Japan.

Presenting Author: T. Shimizu

We find experimentally a systematic dependence of the indirect magnetic (superexchange and supertransferred hyperfine) interactions on the bond angle ranging from 98 to 180° in the antiferromagnetic cuprates.

4PO5-21 Local study of antiferromagnetism in water dopped YBa₂Cu₃O_6.5

A.V. Dooglav 1, A.V. Egorov 1, P. Mendels 2, W.A. MacFarlane 2, J. Bobroff 2, N. Blanchard 2, A. Keren 3, G. Collin 4, J.F. Marucco 5, V. Guillen 5, and P.J.C. King 6. 1 Magnetic Resonance Laboratory, Kazan State University, Kazan 420008, Russia. 2 Laboratoire de Physique des Solides, UMR 8502, Bat 510, UPS Orsay, 91405 Orsay, France. 3 Dept. of Physics, Technion, Haifa 32000, Israel. 4 Laboratoire Leon Brillouin, CEA Saclay, 91191 Gif sur Yvette, France. 5 L.E.M.H.E., Bat 410, UPS Orsay, 91405 Orsay, France. 6 ISIS-RAL, Chilton-Didcot, OX110QX, England.

Presenting Author: P. Mendels

We present copper nuclear quadrupolar resonance (NQR), zero field nuclear magnetic resonance (ZFNMR) and zero field muon spin rotation experiments (ZFµSR) on a series of YBa₂Cu₃O_6.5 samples reacted at low T with water. We find that the empty CuO chains play the role of easy water insertion channels. Superconductivity is progressively destroyed and the final product obtained by insertion of 1 mole of water per formula unit is a nonsuperconducting antiferromagnetic (AF) compound which proves to be the missing AF parent of the YBa₂Cu₄O₈ superconductor. The Neel transition 395(5) K is characteristic of the undoped CuO₂ bilayer, with an AF structure similar but distinct from YBa₂Cu₃O₆. The possibility of magnetism on the Cu(1) site and the similarity of part of the ZFNMR spectrum with that of PrBa₂Cu₃O₇ will be discussed.

4PO5-22 Dynamics of the Local Magnetism Induced by a Cu Spin Vacancy in YBCO

W.A. MacFarlane 1, J. Bobroff 1, H. Alloul 1, P. Mendels 1, N. Blanchard 1, G. Collin 2, and J.-F. Marucco 3. 1 LPS Bât. 510, Universite Paris Sud, UMR8502 CNRS, Orsay 91405, France. 2 LLB, CEN Saclay, CEA-CNRS, Gif-sur-Yvette 91191, France. 3 LCNS, Université Paris Sud, Orsay 91405, France.

Presenting Author: W.A. MacFarlane

Nonmagnetic plane Cu substitutions are found to induce magnetism locally in the doped CuO₂ plane, but little is known about the spin dynamics of the defect particularly its variation with hole doping. We present the first comprehensive study of such dynamics as a function of both defect (z) and hole (n) concentration. The particular system studied is YBCO with a small fraction of the plane Cu sites substituted by nonmagnetic Li. The fluctuations of the Cu moments in the vicinity of Li are probed by both the near-neighbour ⁸⁹Y and ⁷Li NMR spin lattice relaxation. For z less than about 1 atomic % per plane, we find dilute-limit behaviour where the T dependences are z-independent at a given n. From the relaxation rates (1/T1), we extract the T dependence of the correlation time of the local magnetic moment in the normal state using a simple isolated defect model. We find that the correlation time has the same T dependence as the local static susceptibility for each n, even though it possesses significant n dependence. This results in a T- and n-independent value for the dilute-limit quantity 1/T1TK2 for ⁷Li, where K is the shift of the Li resonance (proportional to the local static susceptibility) which is remarkably reminiscent of the behaviour of Kondo impurities in conventional metals and lends strong support to the Kondo behaviour conjectured in [Ref. 1].

[1] J. Bobroff et al., submitted to PRL (cf. condmatt 9906152).

4PO5-23 Charge Ordering in NaV₆O₁₅: ⁵¹V NMR in a Single Crystal

Masayuki Itoh 1, Nobuyuki Akimoto 2, Hiroyuki Yamada 3, Masahiko Isobe 3, and Yutaka Ueda 3. 1 Department of Physics,Faculty of Science, Chiba University, Chiba 263-8522, Japan. 2 Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan. 3 Institute for Solid State Physics, University of Tokyo,Tokyo 106-8666, Japan.

Presenting Author: M. Itoh

Quantum spin systems with characteristic low-dimensional geometry have attracted much attention from both experimental and theoretical aspects. The phase transition in NaV₂O₅ first discussed as the spin-Peierls transition is considered to be a charge ordering transition in a quarter-filled spin-ladder system. In vanadium bronzes A_xV₂O₅ (A: cations) which contains NaV₂O₅ there are several phases with magnetic and electric properties different from each other. Recently we found that the charge ordering commonly takes place particularly in A_0.33V₂O₅ (AV₆O₁₅), a mixed valence oxide of V⁴⁺ and V⁵⁺, with the b structure. In this paper we have made ⁵¹V NMR experiments using a single crystal to study in detail the charge ordering in NaV₆O₁₅.

We observed at T_cO=130 K a drastic change of ⁵¹V NMR spectra due to the charge ordering. Based on the Knight shift and the electric quadrupole parameters of each V site, we will discuss the change of local magnetic and electric properties accompanied by the charge ordering. In the antiferromagnetic state below T_N=24 K we also observed ⁵¹V spectra with and without internal fields. From these NMR experimental results we will discuss a structure of the charge ordering in NaV₆O₁₅.

4PO5-24 NMR Studies of Superconducting Ba₈Ag_xSi_46-x(x = 0 ~ 6)

H. Sakamoto 1, H. Tou 1, H. Ishii 1, Y. Maniwa 1, E.A. Reny 2, and S. Yamanaka 2. 1 Department of Physics, Tokyo Metropolitan University, Minami-osawa, Hachi-oji, Tokyo, 192-0397, Japan. 2 Department of Applied Chemistry, Hiroshima University, Higashi-Hiroshima, 724, Japan.

Presenting Author: H. Sakamoto

NMR measurements of ²⁹Si and ¹³⁷Ba have been performed in Ba₈Ag_xSi_46-x (x = 0 ~ 6) to investigate their electronic properties and superconducting mechanism. Ba₈Ag_xSi_46-x belongs to silicon clathrate compounds that are attractive materials with a 3-dimensional cage structure. In Ba8Si46, which has the highest superconducting transition temperature (T_c ~ 8 K) among Ba₈Ag_xSi_46-x, the temperature dependence of the Knight shift of Si site at T > 50 K seems to be constant plus an activation type with the activation energy ~ 250 K. Below T_c the Knight shift measured at 3.9 T did not decrease with decreasing temperature, which is not expected in common superconductors. And the spin-spin relaxation rate obtained up to now exhibited a sharp peak near T_c. These results may suggest that Ba₈Si₄₆ is an unusual superconductor. The measurement of the shift of Si and Ba sites of Ba₈Ag_xSi_46-x shows that the sample with the higher T_c has the larger density of states of these sites. This fact demonstrates that the hybridization between Si 3p and Ba 5d orbitals is important for the occurrence of superconducting transition. In the meeting we will systematically discuss the obtained results for the series of Ba₈Ag_xSi_46-x (x = 0 ~ 6).

4PO5-25 Local field distribution in YNi₂B₂C superconductor

K.S. Han 1, B.J. Mean 1, K.H. Lee 1, D.H. Kim 1, S.W. Seo 1, Moohee Lee 1, B.K. Cho 2, and S.I. Lee 3. 1 Dept. of Physics, Kon-Kuk University, Seoul 143-701, Korea. 2 Department of Material Science and Technology, KJ-IST, Kwangju, 500-712, Korea. 3 National Creative Initiative Center for Superconductivity, Postech, Pohang, Kyung-Buk 790-784, Korea.

Presenting Author: M. Lee

Local field distribution in the mixed state of type II superconductors has been numerically calculated and compared with ¹¹B NMR spectra for YNi₂B₂C single crystals. We find that the just small distortion of vortex positions from the perfect lattice points is enough to wash out the low frequency shoulder. As the vortices are further distorted, the line shape changes from an asymmetric shape with a high frequency tail to a symmetric Gaussian line shape. It is found that the second moment of the field distribution has a major contribution from the high frequency tail. Consequently, a linewidth of the full width at half maximum calculated from the second moment assuming for a Gaussian line shape is overestimated.

4PO5-26 NMR studies on layered superconductor Li_0.48(THF)_yHfNCl

Hideki Tou 1, Daisuke Omata 1, Yutaka Maniwa 1, Kojiro Itoh 2, and Shoji Yamanaka 2. 1 Department of Physics, Graduate School of Science, Tokyo Metropolitan University, Hachi-oji, Tokyo 192-0397, Japan. 2 Deptertment of Applied Chemistry, Hiroshima University, Higashi-Hiroshima 739-24, Japan.

Presenting Author: H. Tou

NMR studies were performed for recently discovered layered superconductor Li_0.48(THF)_yHfNCl which shows the superconductivity below the transition temperature T_c=25 K. In the normal state, H-, Li-NMR signals are observed at around zero Knight shift. The Li-NMR shift decreases below T_c for the external magnetic field of H=3.9T, while it does not decrease below T_c for H=9.4 T. These are possible to be explained by the superconducting diamagnetic shift. From the NMR viewpoints, it is suggested that the density of states at Fermi level at Li and H sites are considerably small. By contrast, Cl-NMR signal is observed with finite shift and the NMR shift does decrease below T_c for H=9.4 T. These results strongly suggest that the two dimensional HfNCl-layer plays an important role in occurrence of the superconductivity in this system.

4PO5-27 Spin State in Perovskite Cobalt Oxides La_1-xNd_xCoO₃

Masayuki Itoh, Department of Physics, Faculty of Science, Chiba University, Chiba 263-8522, Japan, and Junichi Hashimoto, Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan

Presenting Author: J. Hashimoto

The perovskite cobalt oxide LaCoO₃ has much attention from the aspect of the anomalous spin state and the spin-state transition. The oxide is considered to undergo the transition of a trivalent cobalt ion from the low-spin (LS, t2g6) to the intermediate-spin (IS, t2g5eg1) states around 100 K and the gradual metal-insulator transition around 500 K. On the other hand, we recently showed that the spin sate in NdCoO₃ remains the low spin state up to the gradual MI transition around 580 K. Therefore in the mixed compounds La_1-xNd_xCoO₃ we can expect to control the spin state by the chemical pressure due to the substitution of La by Nd. In this paper we have made ⁵⁹Co NMR and magnetic susceptibility measurements to study the spin state in La_1-xNd_xCoO₃.

From data of magnetic susceptibility and the ⁵⁹Co Knight shift in La_1-xNd_xCoO₃, we made a phase diagram of the spin state in La_1-xNd_xCoO₃. The substitution of La by Nd was found to stabilize the LS state. We will discuss the relation between the spin sate and the MI transition, based on a model of the MI transition proposed by the band calculation in LDA+U approximation.

4PO5-28 The ²⁰⁷Pb and ¹⁷O NMR Studies Of Distribution Of The Electron Density In Metallic Phase Of BaPb_xBi_1-xO₃ Oxide

S. Verkhovskii 1, A. Gerashenko 1, K. Mikhalev 1, Yu. Piskunov 1, A. Pogudin 1, A. Yakubovskii 2, and A. Trokiner 3. 1 Institute of Metal Physics, Russian Academy of Sciences, Ekaterinburg, Russia. 2 Russian Research Center "Kurchatov Institute", Moscow, Russia. 3 Ecole Superieure de Physique et Chimie Industrielles, Paris, France.

Presenting Author: S. Verkhovskii

The results of measurements of NMR spectra, nuclear spin-lattice relaxation rate R(T) and spin echo decay time T₂ of ²⁰⁷Pb and ¹⁷O are presented for metallic phase of BaPb_xBi_1-xO₃ (0.64 £ x £ 1.0) oxides. Superconductivity in these oxides arises near the metal-semiconductor transition (x» 0.65) and the maximal critical temperature T_c,max=12K is reached at x=0.75. The main concern of this study relates to those in NMR data which give evidence of the origination and evolution of inhomogeneous spatial distribution of the carriers with E~E_F in approach to the metal-semiconductor transition. An increase of Bi-concentration leads to large distribution of the Knight shift K_S of atoms, forming sublattice of Pb(Bi)O₆ octahedra, and the largest broadened line occurs for superconducting compositions of oxide with critical temperature near T_c,max. It was established that oxygen and lead having bismuth as the neighboring atom contribute to the region of the spectra with the largest Knight shift. The gap-like behavior of Ks for compositions with x<0.8, the low-temperature features in ^207.17R(T) and the changes with x of ^207.17T₂ are discussed by taking into consideration static and dynamic effects of the charge inhomogeneity in sublattice of the octahedra Pb(Bi)O₆ on electronic motion in the conduction band of the BaPb_xBi_1-xO₃ oxide.

4PO5-29 Low-Frequency Lattice Dynamics in Ba_1-xK_xBiO₃: ³⁹K NMR Data

K. Mikhalev 1, S. Verkhovskii 1, A. Gerashenko 1, A. Yakubovskii 2, and A. Rusakov 3. 1 Institute of Metal Physics, RAS, Ural Division, Ekaterinburg, Russia. 2 Russian Research Center Kurchatov Institute, Moscow, Russia. 3 Moscow Steel and Alloys Institute, Moscow, Russia.

Presenting Author: K.N. Mikhalev

We report ³⁹K spin-lattice relaxation rate (1/T₁) measurements in Ba_1-xK_xBiO₃ with x=0.3 (semiconductor), x=0.4 (superconductor) and x=0.5 (normal metal). The main result is the presence a peak in temperature dependence of 1/T₁. The temperature of the maximum of the peak and its intensity depend from the potassium concentration. We ascribe these results as the presence of thermally activated collective rotations of BiO₆-octahedra.An activation energy and a strength of these dynamic distortions are found to increase in going from metallic to semiconducting phase of Ba_1-xK_xBiO₃.