過渡金屬氧化物之近緣吸收光譜研究

Abstract

在本論文中將利用同步輻中心高度極化光源進行變溫的O K-edge和Cu L-edge X光吸收光譜(XANES)實驗，探討YBa2(Cu1-xZnx)3Oy ( x= 0、0.05，6.35≤y≤6.9)薄膜的銅氧平面費米能階(Fermi level；EF)附近能帶之光譜權重(spectral weight)，於偽能隙(Pseudogap)上下之溫區是否有所差異，進而檢證Philip Phillips et al. (Phys. Rev. Lett. 99, 046404 (2007))所提出高溫超導體中Hidden charge 2e boson model之新理論。實驗結果為光譜權重有所變化，與理論定性相符。 本論文亦對CuCrO4和ReCo0.5Mn0.5O3(其中Re=Eu、Nd)等樣品進行XANES實驗研究。在CuCrO4樣品中進行Cr K-edge及Cu L和K-edge之XANES實驗研究，實驗結果此樣品的Cu存在著Cu1.4+與Cu3+ 兩種不同價數。 本論文亦在ReCo0.5Mn0.5O3(其中Re=Eu、Nd)樣品進行Co和Mn的L、K-edge及O K-edge XANES光譜實驗，探討不同稀土族化合物其過渡金屬離子之電子組態。實驗結果當Re 不同時其過渡金屬離子價數仍為Mn4+/Co2+電荷有序狀態，此現象不符合簡單的游離能估計。本論文結果引出在其它不同離子半徑大小的ReCo0.5Mn0.5O3中，此Mn4+/Co2+電荷有序狀態是否繼續維持的問題。我們也發現當在Re=Eu與Nd的情形下，Mn的價數稍有不同，此價數的變化與Re離子大小是否有相關，值得持續探討。

In this thesis, we measure the x-ray absorption spectroscopy (XAS) at O K- and Cu L-edge of YBa2(Cu1-xZnx)3Oy( x= 0、0.05，6.35≤y≤6.9) above and below the pseudogap temperature T*. We compare the spectral weight above and below the pseudogap temperature to testify the theory based on the Hidden charge 2e boson model in the high-temperature superconductors proposed by Robert G. Leigh, Philip Phillips, and Ting-Pong Choy (Phys. Rev. Lett. 99, 046404 (2007)). The experimental results support the theory with evidence of the change of spectral weight. We also measure XAS on CuCrO4 and ReCo0.5Mn0.5O3 (Re=Eu、Nd). By both the Cr K- and Cu L K-edge spectroscopy on CuCrO4, we have discovered that the Cu ions have two different valence states, i.e. Cu1.4+ and Cu3+. We measure XAS at L、K-edge for ReCo0.5Mn0.5O3, and use XAS to clarify the valence states of Co and Mn ions in the samples and discuss the electron configurations of transition metal ions in different rare earth compounds. For both Re= Eu and Nd, the valence state of the transition-metal ions is Mn4+/Co2+. The phenomenon can’t be understood by the simple estimation of ionization energy. The results of this thesis raise an issue on the robustness of the Mn4+/Co2+ state with different Re ionic sizes. Furthermore, the valence state of Mn is slight different between the cases of Re= Eu and Nd. Whether the change of the valence state is associated with the ionic size of Re, it is another interesting question to investigate.