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 YBa 2 Cu 3 O 6.1 was observed from 1.5 K to 77 K, well below the Neel temperature, in which the nuclear Zeeman and quadrupole frequencies, ν L and ν Q , were deduced. The frequency ν L ( T ) follows well the relation (ν L (0)-ν L ( T ))/ν L (0)= A T α above 10 K. The parameter values are evaluated as α=1.87, A =6.83×10 -6 ( K -α ) and ν L (0)=89.80 (MHz). The fact that the exponent α 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.