臺灣博碩士論文加值系統

高溫超導銅氧化物的超導性是由於電子或電洞摻雜到莫特絕緣體中產生而成。這些化合物具有豐富且複雜的相圖，對應於他們不同的電子特性。當溫度低於相變溫度Tc時，超導能隙將打開。自從數十年前的發現以來，銅氧化物的高溫超導機制仍然是個謎，特別是偽能隙相的潛在物理仍不清楚。共振非彈性X光散射(RIXS)是一種強大的雙粒子能譜方法，可用於研究從超導態轉變到偽能隙態的能譜特徵。

此論文研究鉍鍶銅氧高溫超導體(Bi2212)中“電子電洞對”激發的現象，量測入射光能量在氧K邊緣和銅L邊緣附近的RIXS能譜。我們在銅L邊緣RIXS能譜的溫度變化結果中，意想不到地發現，超導態所貢獻的“電子電洞對” 激發比偽能隙態強。此一發現，可被一種基於簡單的雙分量費米子模型解釋，此新方法體現了電子的“細分化”。我們的結果指出了一種理解超導銅氧化物電子結構的新途徑。

The superconductivity of high-temperature superconductor cuprates is created by doping holes or electrons into the Mott insulator. Such compounds have a rich and complex phase diagram corresponding to their diverse electronic properties. The superconducting gap opens as the temperature goes below the transition temperature Tc. Ever since its discovery decades ago, the mechanism of high-temperature superconductivity in cuprates remains a mystery. In particular, the underlying physics of the pseudogap phase remains unclear. Resonant inelastic X-ray scattering (RIXS) is a powerful two-particle spectroscopic method for studying the spectral feature under the transition from the superconducting state to the pseudogap state.

This thesis uses RIXS to investigate the electron-hole excitations in Bi2212 with the incident photon energy tuned to the O K-edge and the Cu L3-edge. Our temperature­dependent RIXS spectra at the Cu L3-edge show that there exists an unexpected enhancement of the exciton intensity in the superconducting state compared to that in the pseudogap state. A new scheme based on a simple two-component fermion model, which embodies electron fractionalization, explains such enhancement well. Our results point toward a novel route for understanding the electronic structure of superconducting cuprates.

Table of Contents
摘要 . i
Abstract. ii
Table of Contents . iii
List of Figures. v
1 Introduction. 1
1.1 Introduction to superconductivity . 1
1.2 Gap physics of cuprates4
1.2.1 Superconducting gap. 4
1.2.2 Pseudogap 6
1.3 X­ray spectroscopy 9
2 Hubbard model and electron fractionalization 11
2.1 Hubbard model11
2.2 Green’s function12
2.3 Two­component fermion model . 17
3 X­ray spectroscopy. 21
3.1 X­ray absorption spectroscopy21
3.2 Resonant inelastic soft X­ray scattering . 24
3.2.1 RIXS process. 26
3.2.2 Elementary excitations by RIXS . 28
3.2.3 RIXS intensity of excitations. 30
3.2.4 RIXS experimental setups 32
4 Excitations of Bi2Sr2CaCu2O8+δ 37
4.1 Sample condition 37
4.2 Paramagnon excitations39
4.3 Exciton excitations 39
4.4 Dynamical spin and charge structure factor. 40
4.5 Cu L­edge RIXS data of Bi2212 41
4.6 Cu L­edge RIXS temperature dependent data of Bi2212 45
4.7 O K­edge RIXS data of Bi2212 49
5 Conclusion and future work 53
References 55

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