Optical phonons in copper oxides with a single CuO2 plane

Abstract

The optical phonon spectra were investigated for single crystals of various insulating copper oxides with a single CuO 2 plane, mainly for the (214)-compounds with the T-, T′- and T ∗ - structures which became superconducting by hole/electron doping. Comparing their spectra with other cuprates, (Ca, Sr)CuO 2 and Sr 2 CuO 2 Cl 2 , all the phonon modes in the ab -direction have been assigned unambiguously. From the phonon parameters, which were obtained by the oscillator fit to the experimental data, the oxygen effective charge was estimated for each material. The relatively small values of the effective charge indicate a highly covalent character in this class of materials, which should be essential for high- T c superconductivity.