Title Page
Abstract
Contents
Chapter 1. Introduction 10
1.1. Charge density wave and topological semimetal 10
1.1.1. Charge density wave 11
1.1.2. Topological semimetal 13
1.2. Cobalt selenides 15
Chapter 2. Experimental 17
2.1. Molecular Beam Epitaxy (MBE) 17
2.2. Reflection High Energy Electron Diffraction (RHEED) 19
2.3. Angle-resolved photoemission spectroscopy (ARPES) 20
2.4. Thin film growth method 22
2.5. Density functional theory (DFT) 23
Chapter 3. Results and discussion 23
3.1. Room temperature charge density wave in orthorhombic cobalt selenide 24
3.1.1. Orthorhombic cobalt diselenide (o-CoSe₂) 24
3.1.2. Quasi-1D electronic structure of o-CoSe₂ 26
3.1.3. Temperature dependent pseudo gap evolution 29
3.1.4. Electron-phonon coupling in Orthorhombic cobalt diselenide 31
3.2. Topological state in hexagonal cobalt selenide - graphene 34
3.2.1. Hexagonal cobalt selenide - graphene (h-CoSe/G) heterostructure 34
3.2.2. computational investigation in h-CoSe on graphene 37
3.2.3. Electronic bands dispersion measured with different photon energies and polarizations 41
3.2.4. Weyl nodal line in hexagonal cobalt selenide 43
Chapter 4. Conclusion 45
Bibliography 48
Abstract (in Korean) 52
Figure 1. Three primary models of CDW phase transitions in transition metal chalcogenide. a Peierls instability in metallic 1D wire system, showing pseudo... 11
Figure 2. Topological semimetal. a schematic of unique band structure of Topological Dirac Fermion and Topological Weyl Fermion. b schematic of... 13
Figure 3. Thin film growth mechanism of Molecular beam epitaxy. a and b conventional epitaxial thin film layer which lattice constant is larger and... 17
Figure 4. In-situ Reflection high energy electron diffraction. a schematic image describing in-situ RHEED monitoring during sample growth. 6 The thin... 19
Figure 5. Schematics of ARPES measurement and each experimental factors. b and c Energy and momentum conservation rules that can analyze arpes... 20
Figure 6. Structure of o-CoSe₂ film. a Atomic structure of o-CoSe₂. b In-situ RHEED image of o-CoSe2 / G grown by MBE. c atomic resolution STM... 24
Figure 7. Quasi-1D electronic structure and Q vector. Arpes spectra aquired with s-polarized a,c,e,g(upper panels), p-polarized b,d,f,h(lower panels) and... 26
Figure 8. Temperature dependence of CDW gap. a and c show intensity plot along G-Y direction at 10 K and 360 K. b and d is symmetrized image of a... 29
Figure 9. electron-phonon coupling near nested point of α band. a Intensity plot at 10K along G-X direction. b and c curvature image along momentum... 31
Figure 10. Hexagonal cobalt selenide graphene hybrid (CoSe-G) superstructure. a Superstructure calculated by structural optimization. b In-... 34
Figure 11. Theoretical calculations of different atomic configurations. 37
Figure 12. Experimental and Computational Electronic Band Dispersion. Calculated spin-polarized electronic structure and ARPES intensity plot... 41
Figure 13. Constant energy contour with Weyl Nodal lines. a-f Constant energy ARPES intensity maps taken at several different EB from 0 meV to -...[이미지참조] 43