Optical properties of single-crystal Sr2CuO2Cl2.

We report reflectance measurements of single-crystal Sr2CuO2Cl2 over the frequency range between 30 cm and 40 000 cm. Using crystals of 10 mm to 100 mm thickness we were also able to perform transmittance measurements at frequencies below the lowest phonon mode as well as between the highest phonon mode and the charge-transfer gap. The measurements were done on the ~001! surface with ab polarization at temperatures between 10 K to 360 K. With decreasing temperature the phonon modes shift to higher frequencies; the 340 cm stretching mode splits below 300 K. The most notable feature in the transmittance spectrum is a weak electronic absorption, possibly magnetic in origin. This feature is not resolved in the reflectance. At the charge-transfer gap there is an increase in the conductivity by four orders of magnitude. The charge-transfer gap energy increases with decreasing temperatures. In addition there is a temperaturedependent absorption band in the low-frequency tail of the fundamental absorption edge.