Temperature Dependence of NMR Spectrum at Cu(2) Site in Antiferromagnetic YBa₂Cu₃O_6.1

Abstract

The temperature dependence of the NMR spectrum at the Cu(2) site in antiferromagnetic YBa₂Cu₃O_6.1 was observed from 1.5 K to 77 K, well below the Néel 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)=AT^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.