Surface and interface effects in epitaxially grown niobium and iron pnictide superconductors

다국어 초록 (Multilingual Abstract)

Superconductivity has been one of the most actively studied field in physics since it was discovered. In BCS superconductor, it is well explained microscopic phenomena by BCS theory, yet some BCS superconductor studies such as low dimensional superconductor or superconductor junction behaviors are still performed. For high TC superconductor such as iron pnictide compound, more phenological studies are needed since theory that fully describe the mechanism is not established. During my Ph.D course, I have performed the scanning tunneling microscopy and spectroscopy (STM/STS) studies for niobium and iron pnictide superconductors to extend the understanding of superconductivity. For this study, I constructed a cryogenic STM system and an STM combined molecular beam epitaxy (MBE) / pulsed deposition (PLD) growth system. I observed that the superconducting gap of niobium appears when film thickness is over 40 Å, and investigated the suppression of superconductivity in disordered niobium film correspond to Finkel’stein’s model. In epitaxially grown niobium film on W(110) substrate, I observed the oscillation of electron density around superconducting gap, which is analyzed to be an interface effect of superconducting and normal junction ? Tomasch effect. I synthesized Co doped BaFe2As2 film by PLD, and measured with STM/STS. I observed surface state effect by electronic screening of barium atoms on the surface, which made the superconducting gap be distorted. And I found some features which has not been reported with cleaved surface. For another iron pnictide superconductor, LiFeAs, I performed the STM measurement of epitaxially grown film by PLD.

목차 (Table of Contents)

• Abstract i
• List of Figures vii
• Chapter 1 Superconductivity 1
• 1.1 Superconductivity Overview 1
• 1.2 BCS Superconductor 3
• 1.2.1 BCS Theory 3
• 1.2.2 Strong Coupling Superconductivity 6
• 1.3 Iron Based Superconductor 8
• 1.3.1 Discovery of Iron Based Superconductor 8
• 1.3.2 Origin of Superconductivity 10
• Bibliography 12
• Chapter 2 Instrumentation 15
• 2.1 Scanning Tunneling Microscopy 15
• 2.1.1 STM Theory 15
• 2.1.2 Quasiparticle Interference 20
• 2.2 Construction of 4K-STM 21
• 2.2.1 STM Design Overview 21
• 2.2.2 STM Head 22
• 2.2.3 Vibration Isolation Stage 26
• 2.2.4 4 K Cryostat 27
• 2.2.5 System Performance 29
• 2.3 STM combined PLD/MBE system 30
• 2.3.1 In-situ Growth of Iron Based Superconductor 30
• 2.3.2 Thin Film Growth by Pulsed Laser Deposition 31
• 2.3.3 Construction of STM combined PLD/MBE system 34
• 2.3.4 Vacuum Transfer Chamber 36
• Bibliography 38
• Chapter 3 STM Study of Niobium 41
• 3.1 Nb on SrTiO3(001) 41
• 3.1.1. Low Dimensional Superconductor 41
• 3.1.2 SrTiO3(001) Substrate 43
• 3.1.3 Niobium Deposition 45
• 3.1.4 Nb on SrTiO3(001) 45
• 3.1.5 Suppression of Superconductivity Due to Disorder 47
• 3.2 Nb on W(110): Interface Effect 49
• 3.2.1 W(110) Substrate 49
• 3.2.2 Nb on W(110) 51
• 3.2.3 Proximity Effect 54
• 3.2.4 Tomasch Oscillation 56
• Bibliography 58
• Chapter 4 STM Study of Iron-based Superconductor 62
• 4.1 Co doped BaFe2As2 62
• 4.1.1 Target Preparation 65
• 4.1.2 Epitaxial growth by PLD 67
• 4.1.3 STM Study of Epitaxially Grown Co-doped BaFe2As2 Film on SrTiO3(001) 69
• 4.1.4 Surface State Effect of BaFe2As2 on SrTiO3(001) 72
• 4.1.5 Spatial Distribution of Co Dopant Site 77
• 4.1.6 Quasiparticle Interference 78
• 4.1.7 Surface Contamination and Restoration of Co-doped BaFe2As2 80
• 4.2 LiFeAs on SrTiO3(001) 83
• 4.2.1 Sample Preparation 84
• 4.2.2 STM Study of Epitaxially Grown LiFeAs on SrTiO3(001) 86
• Bibliography 88
• Chapter 5 Conclusion 93
• APPENDIX 96
• Appendix A: Superconducting Magnet Fabrication 97
• Appendix B Oxygen Atoms and Molecules on Cu(110) 100
• B.1 Dynamics of O2 on Cu(110) 100
• B.2 Experimental Setup 100
• B.3 O2 on Cu(110) substrate 101
• B.4 CO pickup by STM tip 103
• B.5 Oxygen Dissociation 105
• Bibliography 106