Study of the modulated structures in 3d transition metal oxides

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Title:	Study of the modulated structures in 3d transition metal oxides
Other Titles:	3d過渡金屬氧化物中調制結構的研究
Authors:	李書翰;Lee, Shu-Han
Contributors:	淡江大學物理學系博士班杜昭宏;Du, Chao-Hung
Keywords:	X光散射;共振散射;第一原理計算;La2-xSrxNiO4;SrFeO3-δ;X-ray scattering;Resonant Scattering;First-Principle Calculation
Date:	2017
Issue Date:	2018-08-03 14:45:51 (UTC+8)
Abstract:	本論文主要針對兩種不同的3d過渡金屬氧化物，La1.66Sr0.33NiO4(LSNO)及SrFeO2.81(SFO)，在低溫下所產生的電荷及電子自旋有序調制結構的相變化進行研究，其中利用硬X光繞射及共振軟X光繞射技術來量測電荷及電子自旋有序調制結構的相變化。在La2-xSrxNiO4系統中，當Sr摻雜量為x = 0.33時，在低溫下所產生的電荷有序波向量及電子自旋有序波向量為QCO(4.66 0 3)及QSO(0.66 0 0)，而其相變溫度分別為TCO = 230 (K)和TSO = 190 (K)，特別的是，電荷有序調制結構的相變化過程中會產生"反向有序到無序"(Inverse Order-Disorder Transition)的相變化，而本論文中亦針對此現象探討。而在另一個3d過渡金屬氧化物系統，SrFeO2.81，在低溫下所產生的電荷有序波向量及電子自旋有序波向量為QCO(0 0 3.37)及QSO(0 0 0.5)，而其相變溫度分別為TCO = 110 (K)和TSO = 70 (K)，在此系統中除了有豐富的調制結構的相變化，亦產生四方體轉單斜晶的結構變化，而此結構變化亦伴隨著遲滯現象的發生。針對SFO系統，本論文中利用光譜計算來模擬低溫下Fe L3,2-edge的螢光吸收光譜，並利用低溫下單斜晶的原子結構來做第一原理(LDA+U)的計算，透過態密度(Density of State)及能帶結構(Band Structure)來分析在低溫下SFO系統的電子結構，並確認了反鐵磁相以及電荷轉移的傳導機制。 We report the phase transition of charge and spin modulations in two different kinds of 3d transition metal oxides, La1.66Sr0.33NiO4(LSNO) and SrFeO2.81(SFO). Hard X-Ray and resonant soft X-Ray diffractions were used to measure charge and spin ordering reflections as a function of temperature. In La2-xSrxNiO4, hole doped by strontium, x = 0.33, the wave vector of charge ordering and spin ordering were located at QCO(4.66 0 3) and QSO(0.66 0 0) and their phase transition temperatures were observed to be TCO = 230 (K) and TSO = 190 (K), respectively. Interestingly, the charge ordering was observed to display an inverse order-disorder transition at about 230 K where the spin ordering forms. We show that this inverse transition is due to the interlayer coupling between the charge and spin ordering. This discovery points out the importance of the interlayer correlation in strongly correlated electron systems. The oxygen-defective ferrite, SrFeO2.81, also has both of charge and spin modulated structures at low temperatures, which accompanies with the unusual transport behavior. Using X-Ray scattering, the wave vector changes of reflection were located at QCO(0 0 3.37) and QSO(0 0 0.5) with the transition temperatures of TCO ~ 110 (K) and TSO ~ 70 (K), respectively. In addition, SFO also shows a lattice distortion at around 60 K where a hysteresis transition occurs. For further understanding the electronic structures, we used the first-principles LDA+U method to simulate the absorption spectrum at the Fe2,3-edge.
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