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研究生： 梁高蓁
Kao-Chen Liang
論文名稱： 反鐵磁氧化物ACu3Ti4O12、CaMnO3 及Sr2YRuO6之光譜研究
Optical studies of antiferromagnetic oxides ACu3Ti4O12, CaMnO3, and Sr2YRuO6
指導教授： 劉祥麟
Liu, Hsiang-Lin
學位類別： 碩士
Master
系所名稱： 物理學系
Department of Physics
論文出版年： 2006
畢業學年度： 94
語文別： 英文
論文頁數： 133
中文關鍵詞： 反鐵磁氧化物 、微波介電材料 、龐磁阻材料 、超導材料
英文關鍵詞： antiferromagnetic oxides, CCTO, Colossal magnetoresistance effect, CMR, superconductor
論文種類： 學術論文
相關次數： 點閱：151  下載：3 
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    • 我們研究反鐵磁氧化物CaCu3Ti4O12 (CCTO)、Na0.5La0.5Cu3Ti4O12 (NLCTO)、Na0.5Bi0.5Cu3Ti4O12 (NBCTO)、CaMnO3 及 Sr2YRuO6 之變溫光譜響應。首先，當用Na 和 La 離子或Na 和Bi 離子取代 Ca 離子時，X光繞射譜顯示NLCTO 和NBCTO 產生侷部晶格扭曲現象，同時，其靜介電常數變小，但CCTO、NLCTO 和NBCTO 樣品的反鐵磁相變溫度仍相近。有趣地是，CCTO的121 cm-1 紅外光聲子在低溫時呈現相對於高溫的紅移現象，並伴隨著線寬和強度的增加，這種異常行為在兩個摻雜樣品中效應變小。我們使用異質(extrinsic)特性的 Internal Barrier Layer Capacitance (IBLC) 效應並結合有效電荷(Born effective charge)的數據來解釋為什麼CCTO有這麼巨大的介電常數和這個介電常數隨溫度變化的行為。我們猜測由於兩個摻雜樣品扭曲的晶格結構破壞了IBLC 機制，所以，兩個摻雜樣品的介電常數比未摻雜的CCTO小很多。其次，CaMnO3 的紅外光外模振動模(～257 cm-1)在接近尼爾溫度(～125 K)，由於自旋和聲子耦合影響，產生紅移的現象，並伴隨著線寬和強度的變化。CaMnO3 的三個電子躍遷吸收峰(～2.11 eV、～3.48 eV及～6.3 eV)，在接近尼爾溫度時，其權重也呈現重新分佈的變化。最後，Sr2YRuO6的紅外光彎曲振動模(350 cm-1)和伸長振動模(545 cm-1)呈現Fano不對稱，同時，隨著溫度降低，在接近弱鐵磁相變(～135 K)時，兩者皆產生不連續藍移的現象，當溫度再降低到尼爾溫度時(～21 K)，其自旋與彎曲振動模耦合強度又再增強。藉由高頻光學電導率的分析，得到庫侖排斥能量約為～2 eV、電荷躍遷能隙約為～2.82 eV，及晶格場分裂能量約為～3.37 eV。

    We present the temperature-dependent optical reflectance studies of antiferromagnetic oxides, such as CaCu3Ti4O12 (CCTO), Na0.5La0.5Cu3Ti4O12 (NLCTO), Na0.5Bi0.5Cu3Ti4O12 (NBCTO), CaMnO3, and Sr2YRuO6. First of all, the replacement of Ca ions with Na and La, or Na and Bi ions causes NLCTO and NBCTO having the lower static dielectric constant and the more distorted crystal structure, but does not induce changes of the Néel temperature (TN). The optical conductivity spectra of CCTO reveal the phonon mode near 121cm-1 softening and increasing linewidth and intensity with decreasing temperature, but this anomalous effect becomes less significant in NLCTO and NBCTO. The internal barrier layer capacitance (IBLC) picture as an extrinsic property combining with the effective charge data can be used to interpret the giant dielectric behavior and temperature-dependent dielectric constant of CCTO. Within the framework of IBLC, the lower static dielectric constants in two doping sample are due to their distorted microsturucture. In addition, the far-infrared external phonon mode (257 cm-1) of CaMnO3 exhibits the red shifts and variation in linewidth and intensity near the Néel temperature (TN) ～125 K due to the spin-phonon coupling. Three absorptions of electronic transitions of CaMnO3 ～2.11 eV, ～3.48 eV, and ～6.3 eV show the behavior of spectral weight redistribution near TN ～125 K. Finally, the bending mode (350 cm-1) and stretching mode (545 cm-1) of Sr2YRuO6 reveal Fano-type features. With decreasing temperature, both these phonon modes reveal discontinuous blue shifts near the weak ferromagnetic phase transition (～135 K). When temperature is down to near the Néel temperature (TN) ～21 K, the strength of the spin-phonon coupling of the bending mode is getting stronger. Moreover, the Coulomb correlations U, the charge-transfer energy p-d, and the crystal-field splitting energy 10Dq are respectively ～2 eV, ～2.82 eV, and ～3.37 eV from an analysis of the optical conductivity data of Sr2YRuO6 at high frequency region.

    Chapter 1 Introduction 1 1-1 General overview 1 1-2 Motivation 2 Chapter 2 Review of previous experimental work 5 2-1 ACu3Ti4O12 5 2-2 CaMnO3 8 2-3 Sr2YRuO6 10 Chapter 3 Experimental optical techniques 22 3-1 Fourier transform infrared spectrometer 22 3-2 Grating monochrometer 23 3-3 Optical theory 24 Chapter 4 Results and discussion 31 4-1 ACu3Ti4O12 31 4-1-1 Magnetic properties 31 4-1-2 Structural properties 32 4-1-3 Optical properties 33 4-2 CaMnO3 42 4-2-1 Magnetic properties 42 4-2-2 Optical properties 43 4-3 Sr2YRuO6 47 4-3-1 Magnetic properties 47 4-3-2 Optical properties 48 Chapter 5 Summary 131

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