NMR RELAXATION RATE OF O-17 IN SR2CUO2CL2 - PROBING 2-DIMENSIONAL MAGNONS AT SHORT DISTANCES

We calculate the nuclear relaxation rate 1/T-1(O) of oxygen in the und oped quasi-two-dimensional quantum Heisenberg antiferromagnet Sr2CuO2C l2 above the Neel temperature. The calculation is performed at two-loo p order with the help of the Dyson-Maleev formulation of the spin-wave expansion, taking all scattering processes involving two and three ma gnons into account. At low temperatures T we find 1/T-1(O)=c(1)T(3)+c( 2)T(4)+O(T-5), and give explicit expressions for the coefficients c(1) (two-magnon scattering) and c(2) (three-magnon scattering). We compar e our result with a recent experiment by Thurber et al. and show that this experiment directly probes the existence of short-wavelength magn ons in a two-dimensional antiferromagnet.