臺灣博碩士論文加值系統

層狀鈷氧化物Bi2Sr2CoO6+δ引起科學家廣泛興趣的主因是由於它的結構與Bi2Sr2CuO6+δ相似。不同含氧量的Bi2Sr2CoO6+δ展現出令人驚奇的電子與磁的特性。在本論文中，我們以在鈷K吸收邊緣不同偏極方向之X光吸收光譜與共振非彈性X光散射光譜等技術來探討Bi2Sr2CoO6+δ單晶中鈷氧鍵結能態、電子軌域分布情形以及費米能階附近的電荷躍遷行為。從X光吸收光譜研究中顯示，由於Bi2Sr2CoO6+δ晶體內CoO6所形成的八面體具有很強的形變使其破壞了晶格結構D4h的對稱性，此八面體的形變造成了鈷原子本身4p軌域的混成或是4p軌域與3d軌域的混成。我們推測造成此高度軌域混成之原因可能是來自於氧原子的參雜，當氧含量δ越大CoO6所形成的八面體形變量就越大，實驗結果證實δ > 0.25的形變量比δ ~ 0.25大。從共振非彈性X光散射光譜可以發現，樣品Tpaek ~ 280 K在2.8電子伏特, 4.6電子伏特, 7.0電子伏特等三處具有能量損失的激發態，類似的情形也發生在樣品Tpaek ~ 150 K上， 能量損失激發態出現在3.0電子伏特, 4.6電子伏特及7.7電子伏特處。這些能量損失激發態的位置可以對應到鈷原子3d軌域以及氧原子2p軌域間的電荷躍遷機制。其中我們可以清楚的辨認出4.5電子伏特的能量損失激發態出現在吸收光譜中鈷原子的電荷躍遷1s→4px’, 4py’伴隨著氧之共價電子轉移至鈷上O 2p→Co 3dx’2- y’2的過程。

Layered cobalt oxide Bi2Sr2CoO6+δ have attracted much interest because it is isostructural to the high-Tc superconductor Bi2Sr2CuO6+δ. Furthermore, Bi2Sr2CoO6+δ exhibit a series of surprising electronic and magnetic properties with various oxygen contents. We have carried out polarization dependent X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) measurements to study the orbital symmetry and the energies of ligand to metal charge transfer excitations. The XAS measurement reveals that because the centrosymmetry is broken from D4h symmetry due to strong local distortion, there exists a large mixing among 4p states or hybridization between 4p and 3d states. This distortion of CoO6 octahedron with δ > 0.25 is more severe than δ ~ 0.25 due to extra oxygen insertion
In the RIXS measurement, spectra of the Tpeak~280 K sample exhibit three clear inelastic features around 2.8 eV, 4.6 eV and 7.0 eV. Similarly in the Tpeak~150 K sample three peaks appear around 3.0 eV, 4.6 eV and 7.7 eV. The lack of dispersion suggests the highly localized characteristics of these excitations. These features may be referred to charge transfer mechanism from a 2p electron of O to a 3d hole of Co in the CoO2 plane. Particularly the energy loss feature at 4.5 eV can be clearly identified as a charge transfer excitation because it resonates at the absorption transition of 1s→4px’, 4py’ with ligand to metal charge transfer (LMCT) from O 2p→Co 3dx’2- y’2.

Chapter 1 Introduction 1
Chapter 2 Backgrounds 3
2.1 Sample Information 3
2.1.1 Sample Introduction 3
2.1.2 Sample Preparation 3
2.1.3 Sample Physical Properties 5
2.1.3.1 Mott-Hubbard Insulator and Charge Transfer Insulator 11
2.1.3.2 Crystal Field Theory and Jahn-Teller Distortion 14
2.2 Beamline Setup 18
Chapter 3 X-ray Absorption Spectroscopy 21
3.1 Introduction 21
3.2 Dipole and Quadrupole Transition 23
3.3 Experiment Setup and Result 25
3.3.1 Pre-Edge Part 32
3.3.2 Edge Part 39
Chapter 4 Resonant Inelastic X-ray Scattering 55
4.1 Introduction 55
4.2 Principle and Theory 57
4.2.1 Principle 57
4.2.2 Theory 59
4.2.3 RIXS Spectrometer Setup 64
4.3 Incident Photon Energy Dependence 68
4.4 Momentum Transfer Dependence 73
4.5 Discussion 80
4.5.1 Co Kβ2,5 and Kβ” Fluorescence of Bi2Sr2CoO6+δ 80
4.5.2 Energy Loss Feature 83
Chapter 5 Conclusions 92
Chapter 6 Reference 94

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