Physica C 159 ( 1989 ) 689-696 North-Holland, Amsterdam 170 NMR STUDY OF Y]]a2Cn307-6 (Tc=92 K) M. HORVATIC*, Y. BERTHIER, P. BUTAUD, Y. KITAOKA +, P. SI~GRANSAN and C. BERTHIER Laboratoire de Spectrom~trie Physique *, Universit~ Joseph-Fourier Grenoble L BP 87, 38402 St Martin d'H~res Cedex, France H. KATAYAMA-YOSHIDA, Y. OKABE and T. TAKAHASHI Department of Physics, Tohoku University, Sendai 980, Japan Received 28 April 1989 Revised manuscript received 29 May 1989 We have performed ~70 NMR in YBa2Cu307 oriented powder samples in the temperature range 10-360 K for the orientation Hollcand Ho I c. The Electric Field Gradient (EFG) and Magnetic Hyperfine Shift (MHS) tensors have been determined for the four oxygen sites in the structure. The isotropic (axial) part of the MHS tensor were found to be equal to 0.21 (0.08)% for the O (2,3) site in the CuO2 plane and to 0.04 (0.05)% for the bridging oxygen O (4). Both were found temperature independent in the normal state. Below T~, they vanish to zero and the data for the MHSKcc[O(2,3)] show that this decrease is faster than expected from BCS theory. The coupling between the localized spins at the Cu (2) sites and the mobile oxygen p-holes is discussed in the light of these results. 1. Introduction Since the discovery of the high Tc superconducting oxides [ 1] the electronic structure of these com- pounds in the normal state ( T> To) and the starting Hamiltonian to describe it is still a matter of con- troversy. In the La2_xSrxCuO4 family neutron ine- lastic scattering measurements [2] have indicated that localized magnetic moments were present at the copper sites in the CuO2 plane even for concentra- tion x corresponding to superconducting com- pounds. In YBa2Cu307_6 recent NMR work [3,4] has also established that the Cu (2) ions were bear- ing magnetic moments strongly coupled by an an- tiferromagnetic exchange coupling. These results have drawn attention to the role of the oxygen p-holes in the conducting properties of these materials and raised the interest in an NMR study of ~70 in these materials [ 5-8 ]. In this paper we report on the site assignment of the various z70 NMR lines observed in YBaaCu307_6 with ~0. The electric field gradient (EFG) and magnetic hypertine shift (MHS) tensors have been determined for the four sites present in the structure [9]: 0(2) and 0(3) in the CuO2 plane, 0(4) in the BaO layer, and O ( 1 ) in the CuO chains. We also re- port the temperature dependence of MHS parallel to the c-axis (K~c) for the 0(2,3) and 0(4) oxygen nu- clei in the temperature range 5-360 K. This work is part of a more extensive investigation of 170 NMR in superconducting oxides, including also YBa2Cu306.65 [10], Bi2Sr2CaCu208 [11] and LaLssSro.lsCU04 [ 11 ], to be reported elsewhere. Perm. address: Institute of Physics of the University, POB. 304., 41001 Zagreb, Yugoslavia. + Perm. address: Department of Material Physics, Osaka Uni- versity, Toyonaka, Osaka 560, Japan. * Laboratoire associ6 au Centre National de la Recherche Scientifique. 2. Experimental Samples of composition YBa2Cu30 7 were ob- tained from a solid state reaction of powders ofY203, BaCO3 and CuO (all at least 99.99% pure) by cal- 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )