鐵硒碲與金的C軸方向穿隧元件中電導能譜與超導能隙結構之研究

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研究生：	沈育佃 Shen, Yu-Tien
論文名稱：	鐵硒碲與金的C軸方向穿隧元件中電導能譜與超導能隙結構之研究 Conductance spectra and superconducting gap structures observed in c-axis FeSe0.3Te0.7 / Au junctions
指導教授：	齊正中 Chi, Cheng-Chung
口試委員:	王明杰 Wang, Ming-Jye 林大欽 Ling, Dah-Chin 吳茂昆 Wu, Mow-Kuen 陳正中 Chen, Jeng-chung
學位類別：	博士 Doctor
系所名稱：	理學院 - 物理學系 Department of Physics
論文出版年：	2013
畢業學年度：	101
語文別：	英文
論文頁數：	83
中文關鍵詞：	鐵基超導、超導-金屬元件、電導能譜、超導能隙
外文關鍵詞：	iron based superconductors, c-axis superconductor-normal junction, tunneling spectra, gap structures
相關次數：	點閱：3 下載：0
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我們將雷射濺鍍法成長出來的鐵硒碲薄膜製作成C方向的鐵硒碲-金穿遂元件，並研究分析該元件的電導能譜。實驗中，當量測的溫度低於鐵硒碲的超導溫度時，除了明顯的ZBCP現象產生外其它的類似超導能隙的結構也隨著鐵硒碲的超導特性而產生。由於這些結構隨著磁性以及溫度的變化，有著相應的反應，該結構可以被認定為與超導特性有關連。透過extended BTK理論的計算，我們的實驗結果可以被模擬出來，鐵硒碲的兩個超導能隙大小在兩個樣品中分別為4、6meV以及7、10.5meV。雖然兩個樣品上看到的數值有差異，不過兩個超導能隙的比例均為1.5。在更高偏壓的區域，我們還發現了另一個大結構的類超導能隙結構。除此之外，我們還發現了通氧可讓鐵硒碲薄膜的超導特性變好的結果。

The electric transport properties of superconducting c-axis FeSe0.3Te0.7/Au (S/N) junctions, fabricated using pulsed laser deposition, have been investigated in the temperature range of 2 K to the superconducting transition temperature Tc, and in the presence of applied magnetic fields from 0 to 9 T. A large zero-bias conductance peak has always been observed in every conductance spectrum of the junctions. In addition, we have found several gap-like features. Using the extended BTK theory with the currently favored nodeless s±-wave symmetry, our conductance spectrum can be reproduced qualitatively with Δ1 = 4 meV, and Δ2 = 6 meV at 2 K in sample #1 and Δ1 = 7 meV, and Δ2 = 10.5 meV at 2 K in sample #2. Although there is an inconsistency of the gap values in FeSeTe, the relative ratio between two gaps are both 1.5, which is the same as the published ARPES data for FeSeTe samples of similar compositions. We note that the experimental conductance spectra are substantially below the one calculated in the high bias range. Furthermore, there are unaccounted conductance minima at approximately 15.4 meV (20 meV) in junction #1 (#2) whatever the cause for these conductance minima may also be the reason for the discrepancy. In addition, we have accidentally encountered an interesting Tc enhancement effect due to oxygen-annealing effect for the pulse-deposited FeSeTe thin films. The Tc of the as-grown thin film can be increased up to 3 K after an oxygen-annealing process.

Abstract                                                 i
Acknowledgement                                          ii
Table of Contents                                        iv
List of Figures                                          vi
List of Tables                                           xi

Chapter 1: Introduction                                  1
1-1. History of Superconductors                          1
1-2. Introduction to Iron-Based Superconductors          3
1-3. Motivation of This Thesis                           6

Chapter 2: Theory                                        7
2-1. Conventional BTK Theory                             7
2-1.1. Andreev Reflection                                8
2-1.2. Derivation of the BTK Theory                      8
2-2. Generalized BTK Theory with d-Wave Superconductor   16
2-3. Generalized BTK Theory with s±-Wave Superconductor  20

Chapter 3: Thin Film and Junction Fabrication            23
3-1. Introduction to Thin Film Fabrication System        23
3-1.1. Pulse Laser Deposition System                     24
3-1.2. Thermal Evaporation System                        27
3-2. Quality of the FeSeTe Thin Films                    29
3-2.1. Transport Properties                              29
3-2.2. X-ray Diffraction Patterns                        33
3-2.3. Rutherford Backscattering Spectrometry            35
3-2.4. Other Measurements                                37
3-3. Fabrication of the c-axis FeSeTe / Au Junctions     39
3-3.1. Introduction to Photolithography Process          40
3-3.2. Fabrication Process of the S/N Junctions          43

Chapter 4: c-axis FeSeTe / Au Junction Measurement       46
4-1. Experimental Setup of the Differential Conductance 
Measurement System                                       46
4-2. Measurements of R-T Behavior of the Junctions       47
4-3. Differential Conductance Spectra of the Junctions   50
4-3.1.Temperature Dependence of Conductance Spectra      50
4-3.2. Magnetic Field Dependence of Conductance Spectra  57

Chapter 5: Simulation Results and Discussion             67
5-1. Zero Bias Conductance Peak                          67
5-2. Simulation Based on Nodeless s±-wave Symmetry       69
5-3. Discussion                                          72

Chapter 6: Conclusion                                    77

Appendix: Fourier Analysis                              78
References                                             80

                                

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