Abstract

We present polarized Raman measurements from oriented single crystals of La2CuO4 and superconducting La_{2 –x}Sr_xCuO₄ La₂CuO₄ is orthorhombic at room temperature, converting to a tetragonal phase at T_ot ≃500 K. This transition has a striking effect of the Raman spectra. As T_ot, is approached from below, the α₂₂ spectrum shows a strong line at 105 cm⁻¹ that broadens and softens and a weak line at 271 cm⁻¹ that gradually disappears, both lines being unobservable in the tetragonal phase. The Sr-doped crystals, which are tetragonal at room temperature, yield an α₂₂ spectrum similar to that of the high-T phase of La₂CuO₄. These changes in the spectra are explained on the basis of a factor group analysis of the modes for the two structures. For the tetragonal phases of both doped and undoped crystals lines at 429 and 228 cm⁻¹ are assigned to the O A_1g and La A_1g modes, respectively, and a line at 230 cm⁻¹ to the O E_g mode. The spectra from La₂CuO₄ show a 2-magnon peak at ≃ 3100 cm⁻¹, with B_1g(tetragonal) symmetry; numerous lower-peaks due to normally forbidden phonon excitations; and evidence for strong second-order scattering. These last two features suggest a highly anisotropic excitonlike enhancement mechanism. The Sr-doped crystals do not show any of these features to the same extent as the undoped material. Instead, they show an interference between the phonon scattering and the background electronic scattering, leading to Fano-type line shapes for certain modes.

© 1988 Optical Society of America