学位論文 Study of Critical Behaviour on Layered Organic Conductors

上原, 友敬

内容記述
指導教員 : 谷口弘三
Contents1 Introduction 71.1 Organic Conductor System κ-(BEDT-TTF)2X . . . . . . . . . . . 71.2 Mott Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.3 Antiferromagnetic Transition . . . . . . . . . . . . . . . . . . . . 151.4 Superconducting fluctuation . . . . . . . . . . . . . . . . . . . . 172 Experiments 192.1 Magnetisation Measurement . . . . . . . . . . . . . . . . . . . . 192.2 Helium-gas Pressurise Technique . . . . . . . . . . . . . . . . . . 222.3 Muon Spin Relaxation/Rotation Measurement . . . . . . . . . . . 243 Results for Mott transition 303.1 Measurement under ambient pressure . . . . . . . . . . . . . . . 303.2 Measurement under pressure I――Phase Diagram . . . . . . . . . . 323.3 Measurement under pressure II――Criticality of Mott transition . . 374 Results for Antiferromagnetic Transition 404.1 κ-(BEDT-TTF)2Cu[N(CN)2]Cl . . . . . . . . . . . . . . . . . . . 404.2 κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br . . . . . . . . . . . . . . . . . 464.3 κ-(d8-BEDT-TTF)2Cu[N(CN)2]Cl . . . . . . . . . . . . . . . . . 535 Results for superconducting fluctuation 645.1 κ-(BEDT-TTF)2Cu[N(CN)2]Br . . . . . . . . . . . . . . . . . . . 645.2 Pressurised κ-(BEDT-TTF)2Cu[N(CN)2]Cl . . . . . . . . . . . . 696 Remarks 74A Appendix iA.1 Muon site estimation . . . . . . . . . . . . . . . . . . . . . . . . iReferences xiii
Over several decades, the strongly correlated system is one of the most attractive target systems for solid state physicist because of numerous physical properties they show. Since the strongly correlated system shows various states, the electrons with strong correlation are performing the most important role in such system. One of the best stages to investigate the effect of electronic correlation is the organic system because of its simpleness. In addition to that, the most part of organic material with strong correlation can be facilitated by chemical reaction, which leads to low-impurity samples which helps us to perform precise measurements. The purpose of this research is to put a detailed investigation on the criticality of transitions on molecule-based strong correlation system,κ-(BEDT-TTF)2X. This system is known to show superconducting, magnetic and Mott transition. The criticalities of these three transitions are strongly related to fundamental questions in the field of solid state physics. However, the investigation of the criticality is not reported with a few exceptions. In this dissertation, we report experimental results for these criticalities by μSR and magnetisation measurements. We succeed in clarifying several points such as i) Critical exponent of Mott transition in quasi-two-dimensional system probed by thermodynamic quantity, ii) Criticality of true (zero-field) magnetic ordering inκ-(BEDT-TTF)2X, and iii) Criticality of superconducting fluctuation which can be regarded as a precursor of superconductivity. Among them, we found that the Mott transition in this system has critical exponents which never observed by thermodynamic quantity, and that agrees to the theoretical prediction of quantum-Mott transition. This behaviour considered as a result of the low-temperature critical endpoint. In addition to that, we discovered that superconducting fluctuation (SCF) in this system has a rapid-growth nature which is an important clue to clarify its origin. We also find that magnetic ordering temperature in κ-(BEDT-TTF)2X shows intriguing behaviour while the system approaches to Mott transition, which may suggest the dipolar interaction as an origin of magnetic ordering.
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