Raman modes of the apical oxygen in mercury-based superconductors

Recent studies suggest that the apical oxygen plays an important role in high-Tc superconductivity of mercury-based superconductors HgBa2Can-1CunO2n+2+δ [Hg-12(n-1)n]. Among strong Raman peaks at ∼570 and ∼ 585 cm-1, the mode at ∼ 585 cm-1 has been determined to be the A1g mode of the apical oxygen. However, the origin of the mode near 570 cm-1 is still in question. In order to clarify the origin of the 570 cm-1 mode, we have carried out Raman measurements on the Hg1-xTlxBa2(Ca0.86Sr0.14)2Cu3O8+δ systems, where the amount of the oxygen in the Hg/Tl-O plane is controlled by the doping level of Tl ions. The results show that the 570 cm-1 peak does not arise from the modes of the interstitial oxygens in the Hg/Tl-O plane, but from the A1g-type mode of the apical oxygen. From our analysis, it is shown that the 570 cm-1 mode arises from the apical oxygen OA with the neighboring interstitial oxygen Oδ, while the 585 cm-1 mode is from the OA without the nearest Oδ neighbor. We conclude that the apical oxygens in the Hg-12(n-1)n systems can have two distinctive sets of Oδ environment.