Abstract
The mid-infrared spin-wave spectrum of antiferromagnetic YBa2Cu3O6.0 was determined by infrared transmission and reflection measurements (k∥c) at T = 10 K. Excitation of single magnons of the optical branch was observed at Eop = 178.0 meV. Two further peaks at 346 meV ( ≈ 1.94Eop) and 470 meV ( ≈ 2.6Eop) both belong to the two-magnon spectrum. Linear spin-wave theory is in good agreement with the measured two-magnon spectrum, and allows to determine the exchange constant J to be about 120 meV, whereas the intrabilayer coupling J12 is approximately 0.5J.