Mid-infrared Spectrum of Dilute Holes in Antiferromagnetic CuO2 layers.

The optical absorption spectrum from 0.1 to 1.5 eV photon energy is presented for extremely lightly oxygen doped crystals of La 2 CuO 4 . Crystals grown by two different methods have different low temperature spectra and different temperature dependencies. However, at room temperature the spectra are nearly identical, suggesting that they result from the optical absorption of free isolated holes in the two dimensional antiferromagnetic CuO 2 square lattice.