希土類と鉛で同時置換した変調構造のない(Bi,Pb)2Sr2(Ca,R)Cu2Oz+a(R:rare earth=Y)のフォノンラマン散乱 (酸化物超伝導体の合成と特性<特集>)

概要

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Phonon Raman spectra of the modulation-free Bi1.2 Pb0.8 Sr2 Ca1-y, YyCu2Oz(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 O8+d (Bi2212), most notably in high-frequency three prominent oxygen modes observed at 290, 460 and 630cm-1, which we assigned to AgOBi+B1gOCu, Ag OBi and Ag Oap _??_ x, respectively. The energies of the three oxygen modes show drastic change with a decrease of the hole concentration: (1)Ag OBi+B1g OCu. mode is shifted towards higher frequencies, (2)AgOBi mode hardly varies for y<0.6, while softens for y>0.6, and Ag Oap _??_ x mode is shifted towards lower frequencies. The observed changes in these oxygen modes are similar to those observed for the modulated Bi2 Sr2 Ca1-y Yy Cu2Oz. 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.