SUPERCONDUCTING LA2CUO4+X PREPARED BY OXYGENATION AT HIGH-PRESSURE - A RAMAN-SCATTERING STUDY

Superconducting crystals of La2CuO4+x prepared by high-pressure oxygenation have been analyzed by Raman spectroscopy. A direct comparison of the role of excess oxygen was made by examining the same crystals with and without excess oxygen. La2CuO4+x, like nonsuperconducting La2CuO4.0, is found to have a soft phonon associated with a tetragonal-to-orthorhombic phase transition. The A(g) phonons of La2CuO4.0 and La2CuO4+x occur at essentially the same frequency. At room temperature, La2CuO4+x has no well-defined peak from two-magnon scattering, unlike La2CuO4.0. However, in its phase-separated from, La2CuO4+x exhibits well-defined, two-magnon scattering. This establishes that the La2CuO4.0 phase present in La2CuO4+x at low temperatures is antiferromagnetic. La2CuO4+x samples prepared by slightly different methods are found to have differing amounts of excess oxygen, as indicated by variations in the intensity of the phonon and magnetic scattering. Certain samples of La2CuO4+x had a phonon peak at 630 cm-1 that is absent in La2CuO4.0. While a definitive assignment is not possible, the frequency of this peak is consistent with a peroxidelike species in La2CuO4+x.