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The optical conductivity σ(ω) of undoped and very low doped YBa2Cu3O6.0δ is studied in detail in the midinfrared range. We collect experimental evidence for a magnetic origin of the observed absorption. The main resonance peak is well described by a bimagnon-plus-phonon absorption spectrum calculated within spin-wave theory. But spin-wave theory fails to describe further resonances at higher frequencies with significant spectral weight, which grows with both temperature and doping. This underlines the importance of quantum fluctuations for the description of short-wavelength magnetic excitations in a two-dimensional spin-1/2 system.

To explain the experimental spectra for angle-resolved photoemission we consider a modified t-J model. The modified model includes next-nearest(t') and next-next-nearest (t '') hopping as well as corrections due to ''virtual'' charge fluctuations to the spectral weights. A Dyson equation that relates the single-hole Green's functions for a given pseudospin and given spin is derived and is compared to experimental results. [S0163-1829(97)04438-X].