Midinfrared absorption in YBa2Cu3O6: Evidence for a failure of spin-wave theory for spin 1/2 in two dimensions

The optical conductivity sigma(omega) of undoped and very low doped YBa2Cu3 O6.0 delta is studied in detail in the midinfrared range. We collect experi mental evidence for a magnetic origin of the observed absorption. The main resonance peak is well described by a bimagnon-plus-phonon absorption spect rum calculated within spin-wave theory. But spin-wave theory fails to descr ibe further resonances at higher frequencies with significant spectral weig ht, which grows with both temperature and doping. This underlines the impor tance of quantum fluctuations for the description of short-wavelength magne tic excitations in a two-dimensional spin-1/2 system.