The mid-infrared spin-wave spectrum of antiferromagnetic YBa 2 Cu 3 O 6.0 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 E op = 178.0 meV. Two further peaks at 346 meV (≈ 1.94 E op ) and 470 meV (≈ 2.6 E op ) 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 J 12 is approximately 0.55 J. 74.25.Gz, 74.25.Ha, 75.30.Ds High temperature superconductors are basically layered copper-oxide materials. It is widely accepted that the relevant electronic degrees of freedom are confined to copper-oxide planes. The number of CuO 2 planes per unit cell varies: e.g., La 2−x Sr x CuO 4 exists in a single plane form with a large spacing between planes of ≈ 13.2Å, and YBa 2 Cu 3 O 6+x has a double layer structure with intra-and interbilayer spacings of ≈ 3.3Å and 8.5Å, respectively. Electronic correlations, and hence spin dynamics [1], may depend on the type of stacking of the planes. More specifically, a sizable coupling J 12 between spins on adjacent planes of a bilayer will influence the spin excitation spectrum as well as the nature of the ground state. This may have been seen already in doped compounds: the normal state spin susceptibility of La 2−x Sr x CuO 4 extrapolates to a finite value at zero temperature, whereas it extrapolates to zero for YBa 2 Cu 3 O 6.6 [2]. This may be interpreted as a signature for the opening of a spin excitation gap in YBa 2 Cu 3 O 6.6 at low temperatures [3]-a behavior certainly not encountered in Fermi liquids. Further, a spin density wave ordering for La 2−x Sr x CuO 4 has been proposed, but for YBa 2 Cu 3 O 6.6 a singlet pairing of spins in adjacent CuO 2 planes with strong antiferromagnetic fluctuations within a plane [4,2,5]. Such a scenario seems to require an unrealistically large J 12 2.5J [6], where J is the in-plane exchange coupling of the Heisenberg Hamiltonian supposed to describe the low energy spin dynamics of a single bilayer for zero doping (x = 0). However it was argued that, for finite doping, the itinerant carriers destroy the antiferromagnetism of the insulating phase and, therefore, much smaller values of J 12 will produce a singlet interplane pairing in the conducting phase of YBa 2 Cu 3 O 6.6 .
74Up to now, no experimental evidence has been given of a sizable bilayer coupling (J 12 ∼ J). In neutron-scattering experiments on YBa 2 Cu 3 O 6+x , the in-plane coupling was determined from the dispersion of acoustic spin-waves and was found to be extremely large (J = 120 ± 20 meV [7], J = 150 meV [8], both for x = 0.15). Yet, no optical modes have been found for energies up to 60 meV [7,9], suggesting a bilayer coupling of J 12 8 meV. In Raman-1
