일차원 위상학적 시스템과 1UC-FeSe/SrTiO3 초전도체에 대한 주사터널링현미경 연구

학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 물리·천문학부, 2018. 2. 국양.The discovery of the integer quantum hall effect has led to a lot of research on the system topology. Studies on 2D or 3D topological systems, which have been studied extensively in recent years, have improved the understanding of new physical properties that the system topology has not been known so far. Studies on 1D topological systems have also been initiated by Su-Schrieffer-Heeger(SSH), and many exotic phenomena such as the Majorana zero mode have been found so far. There are not many examples of 1D systems that have been isolated yet. Through this thesis I report that the Se atom is self-assemble on Ge (111) and is arranged in a staggered hopping structure, and its unique structure makes a system similar to polyacetylene. It has been shown that topological edge states are possibly created at both ends of the wire. In the same way, the self-assembled island as well as the one-dimensional wire were found to have a state at the edge. Besides the topological insulator, there is another high-Tc superconductor (HTSC) as another strongly correlated system. The paring mechanism in HTSC is not well known yet. Many HTSCs have been studied with STMs, but in this paper we have studied FeSe / SrTiO3(100), an interface induced thin film superconductor first reported in Xue group in 2012. Two-dimensional high-temperature superconductors and the unusually high superconducting phase transition temperatures only appear on SrTiO3 or Titanate substrates have led to a number of studies on the mechanism of this exotic phenomenon. I observed at the dopant state while adjusting the doping level in this article, and I studied at how its ordering contributes to the formation of high Tc. We also studied the bosonic mode which makes pairing based on the Migdal-Eliashberg model.1. Introduction 1 2. Scanning Tunneling Microscopy 3 2.1. Quantum Tunneling of Electrons 3 2.1.1. Introduction 3 2.1.2. Bardeen Tunneling 5 2.1.3. Inelastic Electron Tunneling Spectroscopy 9 2.2. Scanning Tunneling Microscopy 11 3. Experimental Setup and Method 13 3.1. STM System Setup 13 3.1.1. STM Head 13 3.1.2. Ultra High Vacuum System 14 3.1.3. Micro Four Point Probes 16 3.2. System for in situ Sample Preparation 19 3.2.1. Sputter and annealing 19 3.2.2. Source preparation 20 4. One-dimensional Topological System -Se on Ge(111) 22 4.1. 1D Topological system 22 4.1.1. Introduction 22 4.1.2. Su-Schrieffer-Heeger Model 22 4.2. Sample Preparation and Characterization 24 4.3. One-dimensional Se wire on Ge(111) 25 4.4. Se island on Ge(111) 31 4.5. DFT calculation of the system 35 4.5.1. Geometry Optimization 35 4.6. Modeling and Simulation 37 4.6.1. One-dimensional wire calculation 37 4.6.2. Se island calculation and analysis 40 4.7. Discussion 44 5. Thin Film High-Tc Superconductor: FeSe on SrTiO3(100) 46 5.1. Introduction 46 5.1.1. Phonon mediated Superconductivity (BCS theory) 46 5.1.2. Unconventional superconductivity. 49 5.2. Electron-Boson interaction 51 5.2.1. Eliashberg Theory 51 5.2.2. Superconducting mechanism in FeSe/STO 57 5.3. Sample Preparation and Characterization 58 5.3.1. STO Cleaning 59 5.3.2. FeSe Growth and doping 60 5.4. Oxygen vacancy dopant ordered state on 1UC-FeSe/STO 62 5.4.1. Oxygen vacancy ordering in High-Tc 62 5.4.2. Dopant state in FeSe/STO Insulating Phase 63 5.4.3. Dopant ordering in FeSe/STO 66 5.5. Tunneling spectroscopy of FeSe/STO 68 5.5.1. Eliashberg Feature 68 5.5.2. Magnetic Field dependence 70 5.6. Discussion 73 Bibliography 75 Appendix 81 A. STM2 Instrumentation 81 A.1 STM Design 81 A.2 Multifunctional Tip Holder Stage 83 B. Cryogenic preamplifier 85 B.1 Bias voltage stability measurement 85 B.2 Circuit analysis 87 B.3 Cabling 90 C. Sn/InSb(111) 92 D. SnSe on Ge(111) 93Docto