Phonon Raman Scattering of the Modulation-free (Bi, Pb)₂Sr₂(Ca, R)Cu₂O_z+aby Simultaneous Substitution of Bi by Pb and Ca by R(R:rare earth=Y)

Abstract

Phonon Raman spectra of the modulation-free Bi_1.2 Pb_0.8 Sr₂ Ca_1-y, Y_yCu₂O_z(0.2≤y≤0.8) have been investigated both in polycrystalline and single crystal forms. Polarized Raman spectroscopic measurements of single crystals made it possible to clarify some controversies on the Raman mode assignments for Bi₂ Sr₂ CaCu₂ O_8+d (Bi2212), most notably in high-frequency three prominent oxygen modes observed at 290, 460 and 630cm_-1, which we assigned to A_gO_Bi+B_1gO_Cu, A_g O_Bi and A_g O_{ap _??_ x}, respectively. The energies of the three oxygen modes show drastic change with a decrease of the hole concentration: (1)A_g O_Bi+B_1g O_Cu. mode is shifted towards higher frequencies, (2)A_gO_Bi mode hardly varies for y<0.6, while softens for y>0.6, and A_g O_{ap _??_ x} mode is shifted towards lower frequencies. The observed changes in these oxygen modes are similar to those observed for the modulated Bi₂ Sr₂ Ca_1-y Y_y Cu₂O_z. We propose that the Y/Ca substitution both in the modulated and modulation-free Bi2212 systems induced a hole filling on the CuO₂ plane, which can explain the behaviour of the oxygen modes observed in their Raman spectra.