電荷密度波材料BaRuO3、BaIrO3、Sr2IrO4之光譜研究

Abstract

我們研究具有4d電子軌域的（9R） 、和具有5d電子軌域的 及 單晶樣品之光譜特性，並探討電荷密度波或磁性相變對這些系統之晶格與內部電子結構所造成的影響。 首先，我們分析（9R） 的拉曼散射光譜，與之前的論文結果[Phys. Rev. B 65, 235113 (2002)]完全吻合。其次， 的變溫拉曼散射光譜顯示其中Ag（Ir）及Bg（O）振動模在電荷密度波相變溫度以下，產生強烈的藍位移及分裂成兩個峰的現象，顯示聲子與電子的耦合強度增強。此外，相似的聲子異常情形也發生在Sr2IrO4的鐵磁有序及電荷密度波相變溫度之下。 最後，我們分析 單晶的全頻光譜，我們估計Sr2IrO4的庫侖排斥能量U大約為1.09 eV，晶格場分裂能量10Dq大約為2.94 eV，而電荷躍遷能隙Δpd大約為3.20 eV。由於 的5d電子軌域比3d或4d系統更加地延伸，所以庫侖排斥能量U減小，且與O 2p軌域有更多的交錯重疊，導致其電荷躍遷能隙Δpd及晶格場分裂能量10Dq增加。

We report the Raman-scattering and optical reflectance measurements of single-crystalline BaRuO3, BaIrO3 and Sr2IrO4 as a function of temperature. These materials are interesting on account of their large spatial extent of 4d- and 5d-electron orbitals. The room-temperature Raman sprctra of BaRuO3 are consistent with those of the previous reports published in [Phys. Rev. B 65, 235113 (2002).]. By lowering the temperature and crossing the charge-density-wave (CDW) transitions, the lattice parameters of some certain phonon modes in BaIrO3 and Sr2IrO4 show abnormal behaviors, reflecting the strong electron-phonon coupling effects play an important role in the CDW state of these materials. Moreover, the on-set Coulomb repulsion energy U, the crystal-field splitting 10Dq,and the charge-transfer energy Δpd of Sr2IrO4 are estimated to be ~ 1.09, 2.94, and 3.20 eV from analyzing the optical conductivity data. Compared with the case of 3d and 4d, the parameter of U is decreasing, in agreement with the extended nature of the orbitals in the 5d oxides; 10Dq and Δpd are increasing, indicating the enhanced d-p hybridization.