SuperconductingLa2CuO4+xprepared by oxygenation at high pressure: A Raman-scattering study

Superconducting crystals of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ 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. ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$, like nonsuperconducting ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.0}$, is found to have a soft phonon associated with a tetragonal-to-orthorhombic phase transition. The ${\mathit{A}}_{\mathit{g}}$ phonons of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.0}$ and ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ occur at essentially the same frequency. At room temperature, ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ has no well-defined peak from two-magnon scattering, unlike ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.0}$. However, in its phase-separated form, ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ exhibits well-defined, two-magnon scattering. This establishes that the ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.0}$ phase present in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ at low temperatures is antiferromagnetic. ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ 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 ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$ had a phonon peak at 630 ${\mathrm{cm}}^{\mathrm{-}1}$ that is absent in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4.0}$. While a definitive assignment is not possible, the frequency of this peak is consistent with a peroxidelike species in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathit{x}}$.