臺灣博碩士論文加值系統

本論文探討過摻雜之高溫超導薄膜Y1-xCaxBa2Cu3O7-隨著不同濃度的鈣摻雜、激發光源及溫度之雷射拉曼散射光譜的變化。首先，我們發現117 cm-1聲子-鋇原子振動模，會隨著鈣原子的摻雜，出現聲子硬化及譜線加寬的現象，此現象暗示著鈣原子的加入除了取代釔原子之外，也部份地取代鋇原子;而339 cm-1聲子-銅氧平面上的氧做反向的振動，其不對稱形狀之改變是由於鈣的不等價取代釔，使得銅氧面上的電動濃度增加所導致。其次，我們發現由銅氧鏈上的缺陷所引起的230 及590 cm-1振動模的強度在激發光源為紅色( 632.8 nm )時最強，而此現象可由共振效應來解釋。隨著溫度的下降，超導能隙打開的影響，鋇原子、銅氧面上的銅及氧原子，明顯地受到self-energy效應。鋇原子的振動模在超導溫度時，出現一不連續聲子硬化的現象，而譜線也急遽地變窄；銅原子的振動模，在超導臨界溫度以下出現聲子軟化及譜線變窄的現象；銅氧平面上的氧做反向的振動模，在超導臨界溫度以下出現聲子軟化、譜線加寬及強度增強的現象。這些聲子的頻率及半高寬之變化深受self-energy效應的影響。然而聲子受到強耦合self-energy效應的影響會隨著鈣摻雜進釔鋇銅氧薄膜的濃度越高而減弱。最後，我們在Y1-xCaxBa2Cu3O7- 薄膜中觀察到雙磁振子激發的拉曼散射訊號。我們以二維正方晶格的海森堡反鐵磁性模型進行擬合以分析雙磁振子。從擬合的結果我們得知純的釔鋇銅氧薄膜的雙磁振子激發位置位於2900 cm-1附近，且隨著鈣原子的摻雜，出現雙磁振子軟化、譜線加寬及強度減弱的現象。在更多鈣摻雜的Y1-xCaxBa2Cu3O7- 薄膜中，雙磁振子的激發已不可見，暗示著其反鐵磁有序排列小於兩倍的晶格常數。

We report an inelastic light scattering study of overdoped Y1-xCaxBa2Cu3O7- films as a function of excitation wavelength, temperature and doping. At room temperature, the 117 cm-1 phonon, involving c-axis vibrations of the Ba atoms, becomes hardening and broadening by the Ca doping, suggesting Ca ions also partially substitute the Ba sites besides the replacement of Y ions. The generation of additional holes in the CuO2 planes due to the partial replacement of Y3+ by Ca2+ induces asymmetric lineshape changes of planar oxygen vibrational mode at ~ 339 cm-1. Additionally, the appearance of two phonon modes at ~ 230 and 590 cm-1 which are due to defect-induced Cu-O chain related modes gains significant intensity when the excitation approaches the red (632.8 nm) region, suggesting that light couples to these two modes via some intermediate excitation states with energy around 2 eV. As the temperature is lowered, the Fano-coupled Ba, and planar oxygen as well as planar copper modes in YBa2Cu3O7- show remarkable self-energy effects when the superconducting gap opens: (i) the Ba mode exhibits a discontinuity of the hardening below Tc. The linewidth narrows dramatically with a cusp occurring at Tc; (ii) the Cu mode exhibits a softening below Tc in all films. The narrowing of the linewidth is also observed below Tc; and (iii) the planar oxygen mode exhibits a large softening and an enhancement of the linewidth and its intensity below Tc. Notably, these strong self-energy effects are vanishing with increasing Ca concentration into the overdoped regime. Furthermore, the observed B1g two-magnon excitation peak near 2900 cm-1 in YBa2Cu3O7- is significantly broadened, weakened, and shifts to the lower frequency with increasing Ca content. For more overdoped samples, no two-magnon excitation is visible, indicating an antiferromagnetic correlation below twice the lattice parameter.

Table of Contents
Abstract (in Chinese) …………………………………………………...i
Abstract (in English) …………………………………………………..ii
Acknowledgements ..………………………………………………….iii
List of Tables ……………….....………………………………………vi
List of Figures ……………………………….……………………….vii
Chapter 1 Introduction …………………………..…………………1
§ 1-1 General introduction ……………………………………………..1
§ 1-2 Motivation ……………………………………………………….2
§ 1-3 Scope …………………………………………………………….4
Chapter 2 Theory of Raman Scattering ……..…...………………9
§ 2-1 Classical theory ………………………………………………...10
§ 2-2 Quantum theory ………………………………………………..13
§ 2-3 Resonant Raman scattering …………………………………..14
§ 2-4 Group theory and excitation symmetry ………………………..15
Chapter 3 Experimental Details …………………………………20
§ 3-1 Sample preparation and characterization ………………………20
3-1-1 Sample preparation ……………………………..……………20
3-1-2 Structure ………………………….…………………………..20
3-1-3 Transport properties …………………..……………………...21
3-1-4 Surface morphology ……………….………………………...22
§ 3-2 Raman experimental instrument …………..……………………23
3-2-1 Light source system …………………….……...…………….24
3-2-2 Raman spectrometer system …………………………………24
3-2-3 Detection system ……………..……………………………...26
3-2-4 Data processing and corrections ……………...……………...26
Chapter 4 Review of Previous Experimental Work ………......35
Chapter 5 Results and Discussion ………...……………………..39
§ 5-1 Phononic Raman scattering …………………………………….39
5-1-1 Polarization dependence …………………………………….39
5-1-2 Doping effects ……………...………………………………..41
5-1-3 Resonant Raman scattering ………………………………….44
5-1-4 Temperature effects ……………..…………………………...44
§ 5-2 Magnetic Raman scattering …………………………………….48
5-2-1 A model for the two-magnon excitations ………….………...49
5-2-2 Evolution of magnetic fluctuations with doping …………….51
Chapter 6 Summary ………………………………..………...……96
References ……………………………………………………………98
Appendix ……………………………………………………………102

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