Title Page
Abstract
Contents
Chapter 1. Introduction 21
1.1. A brief overview of superconductivity 21
1.2. Kagome metals and kagome superconductors 26
1.3. Phase transitions and the quantum phase transition 28
Chapter 2. Experimental methods 31
2.1. Single crystal growth of intermetallic materials 31
2.1.1. Cs-Sb eutectic flux growth of CsV₃Sb₅ 31
2.1.2. Chemical vapor transport growth of 2H-PdxTaSe₂[이미지참조] 35
2.2. Capillary XRD measurement 37
2.3. Elastoresistance measurement 42
2.4. Diamond anvil cell measurement 47
2.4.1. Diamond anvil cell Raman measurement 49
2.4.2. Diamond anvil cell transport measurement 51
Chapter 3. Optimized superconductivity in the vicinity of a nematic quantum critical point in the kagome superconductor Cs(V₁-xTix)₃Sb₅[이미지참조] 57
3.1. Overview 57
3.2. Introduction 59
3.3. Characterization of the Ti content in Cs(V₁-xTix)₃Sb₅[이미지참조] 63
3.4. Evolution of the CDW transition temperature 66
3.5. Evolution of the superconducting properties 69
3.6. Evolution of the nematic order: elastoresistance 72
3.7. Conclusion 81
Chapter 4. Pressure-induced quantum critical point of a strong coupling charge density wave order in 2H-Pd₀.₀₅TaSe₂ 82
4.1. Introduction to charge density waves and its origin 82
4.1.1. Weak-coupling origin of charge density wave order 82
4.1.2. Strong-coupling origin of charge density wave order 87
4.1.3. Quantum critical points of charge density wave orders 88
4.2. Crystal structure and electronic properties of 2H-Pd₀.₀₅TaSe₂ 91
4.3. Fermi liquid fits to the low-temperature resistivity 95
4.4. Raman spectra 98
4.5. Conclusion 103
Bibliography 105
Appendix A. Operation of the cubic anvil cell apparatus and the characterization of pressure-induced superconductivity in TaIrTe₄ 110
A.1. Introduction 110
A.2. Experimental details 111
A.3. Liquid Helium cool down and warmup process 114
A.4. Investigation of the high-pressure resistivity in TaIrTe₄ 116
Bibliography 123
List of publications 124
List of patents 126
국문 초록 127
Table 2.1. A comparison between RRR, ρab,₀, and TCDW between this work and other groups in the literature.[이미지참조] 34
Figure 1.1. Two unique features of superconductivity: (a) perfect electrical conductivity and (b) perfect diamagnetism. 22
Figure 1.2. (a) The calculated electronic band for a theoretical kagome lattice in the kagome-Hubbard model. (b) The calculated electron density of states for the... 27
Figure 1.3. A cartoon figure of a phenomenological temperature (T)-tuning parameter (g) phase diagram. The green color indicates the ordered phase while the... 28
Figure 2.1. (a) A photograph of a conventional glove box in Prof. Kee Hoon Kim's lab. An atmosphere of inert Ar gas with O₂ and H₂O content below 1 parts per... 32
Figure 2.2. (a) Photographs of the vanadium starting materials of the initial batch and the improved batch. The photographs were taken inside the globe box. (b)... 33
Figure 2.3. (a) A schematic picture of a CVT growth apparatus. A finite temperature gradient is set in a tube furnace, in which the hot zone temperature is... 36
Figure 2.4. (a) A photograph of the spinning capillary tube loaded in the HRXRD apparatus (PANalytical Empyrean) in Prof. Kee Hoon Kim's lab.... 39
Figure 2.5. An XRD pattern of a capillary measurement of the ground crystal of 2H-Pd₀.₀₈ TaSe₂ (black dot), the Rietveld refinement result (red line) with... 41
Figure 2.6. (a) A schematic figure of the elastoresistance measurement apparatus. (b) A schematic figure of a strain gauge and its working principle. 43
Figure 2.7. (a) Piezo voltage vs. strain plot for the xx and yy directions measured using the strain gauges at 90 K. (b) Piezo voltage vs. relative... 44
Figure 2.8. (a) N=(△R/R)xx-(△R/R)yy vs. (εxx-εyy) plot of CsV₃Sb₅ at 100 K. A black solid line represents the best fitting result for the linear dependence of N...[이미지참조] 46
Figure 2.9. (a) A photograph of a diamond anvil cell in Prof. Kee Hoon Kim's lab. The inset in red is a cartoon figure of a typical resistivity measurement... 48
Figure 2.10. (a) A photograph of a Raman spectrometer (Nanobase, XperRam200™) measuring a Raman spectrum NiCrAl-based diamond anvil...[이미지참조] 50
Figure 2.11. (a) A schematic figure of a resistivity measurement scheme in the van der Pauw configuration. The resistivity ρ can be calculated by the sheet... 52
Figure 2.12. (a) A photograph of a razor-slices Pt foil line supported by Kapton tape. (b) An image of a room-temperature curable conductive silver epoxy set... 53
Figure 2.13. (a) A photograph of a crystal with van der Pauw contacts attached to the edges. (b) A photograph of a contacted sample loaded inside a prepared... 56
Figure 3.1. (a) A schematic figure of the temperature dependent evolutions due to the known phase transitions in the vanadium kagome net of CsV₃Sb₅. (b)... 60
Figure 3.2. Schematic figures depicting the 3q charge loop currents of CsV₃Sb₅ (a) in the charge bond order state at Tnem〈T 〈TCDW and (b) in the nematic...[이미지참조] 61
Figure 3.3. (a) The X-ray intensity vs. energy graph of a Cs₁.₀₁(V₀.₉₇₃Ti₀.₀₂₇)₃Sb₅ single crystal as measured in the field emission electron probe microanalyzer... 64
Figure 3.4. (a) Capillary x-ray diffraction patterns (black dots) and the corresponding Rietveld refinement result (red line) with Rwp=28.1 and χ²...[이미지참조] 65
Figure 3.5. (a) Temperature dependence of the in-plane resistivity ρab normalized by the resistivity value of Cs(V₁-xTix)₃Sb₅ single crystals at 300 K...[이미지참조] 67
Figure 3.6. Temperature dependence of the M/H of the Cs(V₁-xTix)₃Sb₅ single crystals measured at a constant external field of μ₀H=1 T after zero-field...[이미지참조] 68
Figure 3.7. Low-temperature behaviour of the in-plane resistivity ρab in Cs(V₁-xTix)₃Sb₅ for (a) 0 ≤ x ≤ 0.015 and (b) 0.02 ≤ x ≤ 0.06. Temperature...[이미지참조] 70
Figure 3.8. A temperature(T)-doping(x) phase diagram of Cs(V₁-xTix)₃Sb₅, indicating the charge ordering temperatures TCDW and the superconducting...[이미지참조] 71
Figure 3.9. (a) Schematic illustration of the elastoresistance measurement setup. (b) N vs. (εxx-εyy) plot of CsV₃Sb₅ at several representative temperatures. (c)-(i) (top...[이미지참조] 74
Figure 3.10. (a) A temperature(T)-doping(x) phase diagram of Cs(V₁-xTix)₃Sb₅, indicating the charge ordering temperatures TCDW, the nematic transition...[이미지참조] 78
Figure 4.1. (a) (top) The atomic positions and the charge density ρ(r) of a one-dimensional atomic chain model. The atoms are equally spaced at a distance α... 84
Figure 4.2. (a) A graph of the momentum dependent phonon energy ω(q) in the theoretical free electron gas model in 1D, 2D, and 3D cases, respectively. The... 88
Figure 4.3. (a) The crystal structure of 2H-Pd₀.₀₅TaSe₂. The area in red indicates the Pd position. (b) An X-ray diffraction pattern of 2H-Pd₀.₀₅TaSe₂. A picture of a 2H-... 92
Figure 4.4. Temperature dependence of ρab (a) for 0 ≤ P ≤ 21.5 GPa and (b) for 22.8 ≤ P ≤ 34.7 GPa. The black arrow marks the charge density wave (CDW)...[이미지참조] 94
Figure 4.5. Temperature (T) dependence of ρab measured at an applied field of μ₀H=9 T in the out-of plane direction (a) for 0 ≤ P ≤ 20.4 GPa and (b) for...[이미지참조] 96
Figure 4.6. The Cp/T vs. T² curve of polycrystalline 2H-Pd₀.₀₅TaSe₂ measured at an applied field of μ₀H=9 T. The solid red line represents the linear fit to the...[이미지참조] 97
Figure 4.7. (a) The room temperature Raman spectrum of 2H-Pd₀.₀₅TaSe₂ at ambient pressure. (b) Room temperature Raman spectra of 2H-Pd₀.₀₅TaSe₂ at... 100
Figure 4.8. (a) Electronic phase diagram of 2H-Pd₀.₀₅TaSe₂ with pressure. The filled blue circles represent TCDW obtained from the ρab measurement in the 1st...[이미지참조] 102
Figure A.1. (a) a photograph is provided of a pyrophyllite plate with a thickness of 7mm. (b) Engraved pyrophyllite gaskets with dimensions of 6mm in length and... 111
Figure A.2. (a) A metallic sample of length 0.8 mm which has been prepared using the four-probe contact configuration. Various stages in the preparation of... 112
Figure A.3. (a) A photograph of a unified gasket ready for the cubic anvil cell measurement. (b) A photograph of the unified gasket loaded onto the four anvils in... 113
Figure A.4. A photograph displays the different transfer lines within the cubic anvil cell cryostat. The area enclosed by red boundaries denotes the liquid... 116
Figure A.5. In-plane resistivity vs. temperature plot of TaIrTe₄ at various pressures in the first pressure run. The applied current is set to 5 mA. (inset 1)... 118
Figure A.6. A magnefied plot of the in-plane resistivity vs. temperature plot of TaIrTe₄ at 8.5 GPa in the first pressure run. The in-plane resistivity is... 119
Figure A.7. In-plane resistivity vs. temperature plot of TaIrTe₄ at various pressures in the second pressure run. The applied current is set to 0.4 mA.... 120
Figure A.8. The temperature dependence of the in-plane resistivity divided by room temperature resistivity of TaIrTe₄ at various pressures in the initial... 121