Vibration of the apical oxygens in mercury-based superconductors 1

Abstract

Mercury-based superconductors HgBa2Can-1CunO2n+2+δ, Hg-12(n-1)n, show strong Raman peaks at ∼ 570 cm-1 and ∼ 585 cm-1. The mode at ∼ 585 cm-1 is unambiguously the A1g mode of the apical oxygen OA. However, the origin of the mode at ∼ 570 cm-1 is still in question. In order to clarify the origin of the 570 cm-1 mode, we have done Raman measurements on the Tl-doped Hg-1223 samples, where the amount of the oxygen Oδ in the Hg/Tl-O plane is controlled by the doping level of Tl ions. The results show that the 570 cm-1 mode is not from the motion of Oδ, but from the A1g-type motion of OA- This is in contradiction with the factor group analysis of the ideal Hg-12(n-1)n structure with fully-empty or fully-occupied Oδ position, which shows that there is only one A1g mode for OA- The result indicates that, in the Hg12(n-1)n systems with partially-occupied Oδ, there are two possible environments for OA; one with Oδ(neighbors, the other without any nearest Oδ neighbor. © 1996 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA.