TEMPERATURE-DEPENDENCE OF NMR-SPECTRUM AT CU(2) SITE IN ANTIFERROMAGNETIC YBA2CU3O6.1

The temperature dependence of the NMR spectrum at the Cu(2) site in antiferromagnetic YBa2Cu3O6.1 was observed from 1.5 K to 77 K, well below the Neel temperature, in which the nuclear Zeeman and quadrupole frequencies, nu-L and nu-Q, were deduced. The frequency nu-L (T) follows well the relation (nu-L (0)- nu-L (T))/nu-L (0) = AT-alpha above 10 K. The parameter values are evaluated as alpha = 1.87, A = 6.83 x 10(-6) (K-alpha) and nu-L (0) = 89.80 (MHz). The fact that the exponent alpha is close to 2 suggests the presence of gapless spin wave excitations. The coefficient A is two orders of magnitude larger than the value roughly estimated from the result of the neutron scattering experiment, which suggests the presence of larger low-energy excitations in the spin wave spectrum.