CU-63 NQR NMR-STUDY OF ZN-SUBSTITUTED YBA2CU4O8 - EFFECT OF IMPURITY ON SPIN-GAP-LIKE BEHAVIOR

The nuclear-spin-lattice relaxation rate, 1/T1, and the Knight shift, K(ab), of Cu-63 in the plane have been measured for YBa2(Cu1-xZnx)4O8 (x=0.01 and 0.02) in order to unravel the underlying problem of spin dynamics in YBa2Cu4O8. It Was found that the substitution of Zn into the CuO2 plane causes a continuous increase of 1/T1T down to T(c) in contrast to the marked reduction of 1/T1T at far above T(c) for the undoped sample, while the Knight shift is invariant. These findings show that the suppression of the spin fluctuation spectral weight around Q=(pi, pi) at low energies is removed by substituting Zn into the CuO2 plane, while that around q=(0, 0) is not affected appreciably. The q-dependent change of spin-fluctuations with Zn-doping may be crucial in elucidating the cause of the decrease of 63(1/T1T) above T(c).