OPTICAL-PROPERTIES OF SINGLE-CRYSTAL SR2CUO2CL2

We report reflectance measurements of single-crystal Sr2CuO2Cl2 over t he frequency range between 30 cm(-1) and 40 000 cm(-1). Using crystals of 10 mu m to 100 mu m thickness we were also able to perform transmi ttance measurements at frequencies below the lowest phonon mode as wel l as between the highest phonon mode and the charge-transfer gap. The measurements were done on the (001) surface with ab polarization at te mperatures between 10 K to 360 K. With decreasing temperature the phon on modes shift to higher frequencies; the 340 cm(-1) stretching mode s plits below 300 K. The most notable feature in the transmittance spect rum is a weak electronic absorption, possibly magnetic in origin. This feature is not resolved in the reflectance. At the charge-transfer ga p there is an increase in the conductivity by four orders of magnitude . The charge-transfer gap energy increases with decreasing temperature s. In addition there is a temperature-dependent absorption band in the low-frequency tail of the fundamental absorption edge.