Raman spectra of high temperature superconductors

The results of Raman scattering from phonons and superconducting gap excitations in the high-T_c compounds YBa₂ Cu₃ O_7-δ and Bi₂ Sr₂ CaCu₂ O_8+Δ are presented and compared. In the normal state, both materials exhibit strong electronic interband scattering, which manifests itself as a broad background continuum having both A_1g and B_1g symmetry components. At low temperature there occurs a redistribution of this electronic scattering with the formation of a broad pronounced peak. This may indicate the formation of a superconducting gap in these compounds. The two symmetries exhibit a distinctly different redistribution, however, implying strong anisotropy. At temperatures well below T_c, both compounds exhibit residual low-energy scattering, a feature suggestive of the coexistence of normal electrons and superconducting quasiparticles. Phonon modes at 115 and 330 cm^-1 in YBa₂ Cu₃ O_7-δ show antiresonances with the continuum corroborating the redistribution of the continuum from low to higher frequencies as the gap forms.