Cu and O NMR studies of the magnetic properties of YBa2Cu3O6.63 (Tc=62 K).

PHYSICAL REVIEW B VOLUME 43, NUMBER Cu and O NMR studies of the magnetic properties of YBa2Cu306 M. Takigawa, * A. P. Reyes, P. C. Hammel, JANUARY 1991 (T, =62 K) J. D. Thompson, R. H. Heffner, Z. Fisk, K. C. Ott and Los Alamos Rational Laboratory, Los A/amos, Rem Mexico 87544 (Received 30 April 1990) The microscopic magnetic properties of the Cu02 planes in YBa2Cu306 63 (T, =62 K) have been investigated in Cu and NMR experiments. Unlike the fully oxygenated Y-Ba-Cu-07 (T, =90 K), the various components of the Cu and Knight-shift tensors show strong but identical temperature dependences in the normal state. This supports the picture that there is only one spin component in the Cu02 planes. The spin susceptibility deduced from Knight-shift results shows significant reduc- tion with decreasing temperature in the normal state. The temperature dependences of the nuclear-spin-relaxation rates (1/T& ) are very different for the Cu and the sites. 1/( T& T) at the O sites is nearly proportional to the spin susceptibility. 1/(T& T) at the Cu sites shows a broad peak around 150 K. We discuss these relaxation behaviors based on a model of the dynamical spin sus- ceptibility proposed by Millis, Monien, and Pines. I. INTRODUCTION The role played by magnetism, particularly the nature is one of the central issues of the high-T, copper oxide superconductors. The importance of magnetism is suggested by the proximity of the super- conducting phase to the antiferromagnetic insulating phase as the composition is changed. In the antiferro- magnetic insulators such as La2Cu04 and YBazCu306, each Cu atom in the two-dimensional (2D) Cu02 layers has 2+ valence (3d configuration). The ground state and the magnetic excitations of these compounds are well described by a 2D, s = —, ' Heisenberg model. ' The long-range antiferromagnetic order is easily destroyed by doping a small number of additional holes into the Cu02 planes. For sufficient doping the system becomes metallic and superconducting. Various high-energy spectroscopic data show that these doped holes mostly occupy the 2p states leaving the Cu valence to be 2+. ' ' and Y Knight-shift results on Yaa2Cu307 strongly support the fact that doped holes go into the 0 2p states. ' Neutron-scattering experiments performed on Sr Cu04 (Ref. 8) and YBazCu307 (Ref. 9) re- La& vealed that short-range antiferromagnetic correlations persist into the superconducting phase. However, an ap- propriate description of the magnetic correlations is still a subject of controversy. An important problem here is whether the Cu 3d and 2p holes have distinct spin de- grees of freedom and different spin dynamics' or wheth- er the hybridization between these states is so strong that the system is described by a single spin component with unique dynamics. A particular model of the latter case was presented by Zhang and Rice. They proposed that the spin of a doped hole forms a local singlet state with the neighboring Cu spin with such a large singlet binding energy that 0 2p holes make no contribution to the spin susceptibility. It may also be possible that the supercon- ducting materials are already so heavily doped that the of spin fluctuations, charge-transfer gap is not clearly defined and the states near the Fermi level are properly described by the Cu d and p hybridized band in a way similar to the heavy electron materials. ' In this case, the magnetic properties are described also by a single-spin component. Nuclear magnetic and quadrupole resonance (NMR and NQR) have been extensively used as microscopic probes of the magnetic properties of high-T, compounds. The unique power of these techniques lies in their capa- bility to distinguish behaviors at different atoms and different crystallographic sites. The most extensive reso- nance studies have been done in the Yaa2Cu307 y sys- tem, particularly on the fully oxygenated (y =0) material, using Cu, ' 0, and Y nuclei. A significant feature of this system is that one can control the concentration of doped holes and the superconducting transition tempera- ture (T, ) by changing the oxygen content y. ' In particu- lar, two homogeneous phases are known so far, one with y =0 (T, =90 K) and the other with y =0. 3 — 0. 5 (T, =60 K). It has already been reported that the magnetic proper- ties of the 60-K phase are very different from those of ful- ly oxygenated material. The magnetic susceptibility y( T) of the 60-K phase decreases with decreasing tempera- ture, ' in contrast to the T-independent Pauli-like suscep- tibility in the y =0 material, as shown in Fig. 1. This T dependence is partly associated with the spin susceptibili- ty of the Cu02 plane as revealed by the Y and Cu Knight-shift data. The Y and Cu nuclear relaxa- tion rates also show quite different behaviors from the Since the detailed magnetic proper- y =0 material. ties depend on the sample preparation method as well as the precise value and homogeneity of the oxygen content, it is important to make various NMR measurements on a single sample. This motivation led us to perform Cu and ' NMR experiments on a well-characterized YBa2Cu306 63 (y =0. 37) sample, the results of which are presented below. The American Physical Society