CU-63 NMR AND NQR STUDIES OF HIGH-T-C CUPRATES UNDER PRESSURE

The spin-lattice relaxation rate 1/T-1 and the Knight shift were measu red for La1.85Sr0.15CuO4 and YBa2Cu4O8 under pressure to 1.4 GPa. The results together with the previously reported pressure dependence of t he nuclear quadrupole frequency, nu(Q), give evidence that the local h ole distribution changes under pressure. In La1.85Sr0.15CuO4, the hole density in the Cu 3d orbit decreases, accompanying a suppression of t he antiferromagnetic spin fluctuation of the low-energy component. In YBa2Cu4O8, by contrast, the spin fluctuation changes little while the total hole number in the CuO2 plane increases. We argue that such chan ges in the hole distribution are responsible for the T-c increase in t he respective compounds. We also found that the so-called ''spin-gap'' behavior in YBa2Cu4O8 becomes more pronounced under pressure.