1,721,464 research outputs found
Seung-Hun Lee
No full text학위논문(석사)--아주대학교 일반대학원 :재료공학과,2011. 2In graphite and graphitized carbonaceous materials, it is known that they have different Li storage capacity and electrochemical properties depending on the structures. In particular, graphene which have outstanding physical, chemical and electrochemical properties was the most attention for alternative anode material of Li-ion batteries.Abstract 1
List of Tables 3
List of Figures 4
1. 서론 7
2. 연구 배경 11
2.1 흑연계 물질의 나노구조 18
2.2 그래핀 연구동향 23
3. 선행기술 조사 24
3.1 그래핀 제조방법 24
3.2 흑연막 전해박리에 의한 그래핀 제조 26
4. 실험 방법 27
4.1 그래핀 제조 27
4.1.1 전해박리에 의한 흑연막 표면 개질 27
4.1.2 전해박리에 의한 그래핀 나노시트 제조 31
4.1.3 전기화학적 물성 측정 방법 34
4.1.4 그래핀 나노시트의 물성 평가 35
5. 실험 결과 및 고찰 36
5.1 그래핀 나노시트에 의한 흑연막 표면 개질 36
5.2 그래핀 나노시트 물성 측정 46
6. 결론 61
Reference 62Maste
Boundary-obstructed topological superconductor in buckled honeycomb lattice under perpendicular electric field
No full textIn this work, we show that a buckled honeycomb lattice can host a boundary-obstructed topological superconductor (BOTS) in the presence of f-wave spin-triplet pairing (fSTP). The underlying buckled structure allows for the manipulation of both chemical potential and sublattice potential using a double gate setup. Although a finite sublattice potential can stabilize the fSTP with a possible higher-order band topology, because it also breaks the relevant symmetry, the stability of the corner modes is not guaranteed. Here we show that the fSTP on the honeycomb lattice gives a BOTS under nonzero sublattice potential, thus the corner modes can survive as long as the boundary is gapped. Also, by examining the large sublattice potential limit where the honeycomb lattice can be decomposed into two triangular lattices, we show that the boundary modes in the normal state are the quintessential ingredient leading to the BOTS. Thus the effective boundary Hamiltonian becomes nothing but the Hamiltonian for Kitaev chains, which eventually gives the corner modes of the BOTS. © 2023 American Physical Society.11Nsciescopu
Odd-Parity Spin-Triplet Superconductivity in Centrosymmetric Antiferromagnetic Metals
No full textWe propose a route to achieve odd-parity spin-triplet (OPST) superconductivity in metallic collinear antiferromagnets with inversion symmetry. Owing to the existence of hidden antiunitary symmetry, which we call the effective time-reversal symmetry (eTRS), the Fermi surfaces of ordinary antiferromagnetic metals are generally spin degenerate, and spin-singlet pairing is favored. However, by introducing a local inversion symmetry breaking perturbation that also breaks the eTRS, we can lift the degeneracy to obtain spin-polarized Fermi surfaces. In the weak-coupling limit, the spin-polarized Fermi surfaces constrain the electrons to form spin-triplet Cooper pairs with odd parity. Interestingly, all the odd-parity superconducting ground states we obtained host nontrivial band topologies manifested as chiral topological superconductors, second-order topological superconductors, and nodal superconductors. We propose that double perovskite oxides with collinear antiferromagnetic or ferrimagnetic ordering, such as SrLaVMoO6, are promising candidate systems where our theoretical ideas can be applied to.11Nsciescopu
홀-반전성스핀-삼중항위상초전도체에대한 자성의영향
No full text학위논문(박사) -- 서울대학교대학원 : 자연과학대학 물리·천문학부(물리학전공), 2024. 2. 양범정.Effect of magnetism on topological odd-parity spin-triplet superconductivity Seung Hun Lee Department of Physics and Astronomy The Graduate School Seoul National University A Topological superconductor is a symmetry-protected topological phase that has particle-hole symmetry. In centrosymmetric systems, topological phases can be obtained from trivial phases via the transition so-called the band inversion process. Since the odd-parity spin-triplet pairing interaction describes a natural band inversion process that occurs during the superconduct- ing phase transition in the tight-binding theory of superconductivity written in the Bogoliubov-de Gennes formalism, the odd-parity spin-triplet super-conductors have high possibility to possess non-trivial band topology. However, unlike conventional spin-singlet superconductors, most of the spin-triplet superconductor candidates are thought to be related to magnetism. This dissertation aims to further our understanding of the interplay of magnetism and odd-parity spin-triplet superconductivity, and the band topology of the resulting superconducting phases. The main body of the dissertation consists of two parts. In the first part, we discuss the odd-parity spin-triplet pairing constrained by magnetism. We have uncovered a new route to achieve spin-triplet superconductivity in centrosymmetric collinear anti- ferromagnets, instead of conventional approaches based on ferromagnets. The central idea is to break effective time-reversal symmetry, the ubiquitous symmetry of collinear antiferromagnets, by breaking local inversion symmetry. Surprisingly, all superconducting ground states we obtained are topological superconductors including chiral, spin-chiral, nodal, and second-order topological superconductors. This means that the superconductivity coexisting with stable antiferromagnetism, which has been largely unexplored yet, is a promising avenue to search for novel topological superconductivity. We then investigate how more general real space magnetic ordering of a metallic system induces spin-momentum locking texture on the Fermi surface, and thereby determines the gap structures and topology of its superconducting states. We demonstrate our theory in the all-in-all-out-ordered kagome antiferromagnet, a representative case that satisfies the conditions to have non-trivial winding Fermi surface spin texture. Our study generalizes the idea of altermagnetic spin texture in the momentum space to non-collinear magnetic ordered cases, and suggests the applicability of non-collinear antiferromagnetic metals in realized various topological superconducting phases. In the second part, we discuss the odd-parity spin-triplet superconductivity mediated by spin fluctuation. Based on DFT+DMFT calculation about UTe2, we find the emergence of correlated isotropic three-dimensional Fermi surface at low temperatures, arising from Kondo hybridization-driven Lifshitz transition. We confirmed that this cor-related Fermi surface is not only consistent with low-temperature behaviors reported by various recent experiments but also responsible for time-reversal symmetry broken Weyl superconductivity, which was evidenced by the Kerr rotation measurements.위상 초전도체는 입자-양공 대칭성을 가지는 대칭성-보호 위상 상태이다. 반전 대칭성이 있는 계에서, 위상 상태는 일반 상태로부터 소위 띠역전이라는 과정을 거쳐 나타날 수 있다. 홀-반전성 스핀-삼중항 쿠퍼 쌍에 의해 일어나는 초전도 상전이는 자연스럽게 띠 역전 과정을 수반하기 때문에, 홀-반전성 스핀-삼중항 초전도체체 평범하지 않은 띠 위상 성질을 가질 가능성이 높다. 그러나 일반적인 스핀-단일항 초전도체들과는 다르게, 대부분의 스핀-삼중항 초전도체 후보들은 자성과 밀접한 관련이 있는 것으로 여겨진다. 본 학위논문의 목표는 자성과 스핀-삼중항 초전도 현상의 상호작용과, 그 결과로 나타나는 초전도 상태의 띠 위상에 대해 더 깊이 이해하는 것이다.
본 학위논문의 본문은 크게 두 부분으로 나뉘어 있다. 첫 번째 장은 자성에 의해 제약되는 홀-반전성 스핀-삼중항 쿠퍼 쌍을 다룬다. 우리는 강자성체에 기반한 기존 방법 대신, 반전대칭성이 있는 공선적인 반강자성체에서 스핀-삼중항 초전도 현상이 나타날 수 있는 조건에 대해 탐구한다. 핵심은 국소적으로 반전대칭성을 깸으로써 공선적인 반강자성체에 널리 존재하는 유효 시간역전대칭성을 함께 깨는 것이다. 유효 시간역전대칭성이 없는 공선 반강자성체를 기술하기 위해 우리가 도입한 모형으로부터 나타날 수 있는 초전도 상태는 모두 일차, 이차 위상 초전도체를 포함한 다양한 위상 성질을 가지고 있다. 이는 아직 반강자성과 공존하는 초전도 현상에 대한 연구가 부족한 이 시점에, 반강자성 초전도체가 새로운 위상 초전도 현상을 발견할 가능성이 높은 영역 임을 의미한다. 다음으로, 우리는 어떻게 스핀-궤도 상호작용 없이도 실공간의 자성이 운동량 공간의 페르미 면 위에서 스핀-운동량 잠김을 만들 수 있는지, 그리고 그 페르미 면이 쿠퍼 쌍의 형성과 초전도 상태의 위상 성질에 어떤 영향을 끼치는지에 대한 이론을 전개한다. 우리는 카고메 격자 위의 비공선적 반강자성체 모형에 우리의 이론을 적용시켜 비공선적 반강자성 금속을 다양한 위상 초전도 상태를 만드는 것에 활용할 수 있음을 보였다.
두 번째 장은 스핀 요동에 의해 매개되는 홀-반전성 스핀-삼중항 초전도 현상을 다룬다. 밀도범함수 이론과 동적 평균장 이론에 기반한 계산을 통해, 우리는 낮은 온도의 UTe2에서 곤도 효과에 의한 립시츠 전이가 일어나 등방적인 삼차원 페르미 면이 나타나는 것을 발견했다. 우리는 이 페르미 면을 이용해 저온 영역에서 UTe2가 보이는 성질과 관련된 여러 최신 실험 결과들을 뿐 아니라, 커 효과 측정 결과 등이 시사하는 시간 역전대칭성이 깨진 바일 초전도 상태까지 성공적으로 설명할 수 있음을 확인했다.Abstract i
List of Contents vii
Chapter 1 Introduction 1
Chapter 2 Preliminaries 7
2.1 Symmetries of superconducting states 7
2.2 Wilson loop spectrum 8
2.3 Atomic limit and symmetry indicators for band topologies of superconducting states 9
Chapter 3 Part I: Odd-parity spin-triplet pairing constrained by Fermi surface spin texture 11
3.1 Collinear antiferromagnetism and superconductivity 12
3.1.1 Effective time-reversal symmetry of collinear antiferromagnetic 12
3.1.2 Modeling two-dimensional collinear antiferromagnetic 14
3.1.3 Group theoretical classification of pairing functions 16
3.1.4 Mean-field theoretical analysis 18
3.1.5 Topological Superconductivity in two-dimensional
eTRS-broken collinear antiferromagnetic 22
3.1.6 Application to double perovskites 23
3.2 Non-collinear antiferromagnetism and superconductivity 26
3.2.1 FS spin texture and pairing symmetry 26
3.2.2 Magnetic ordering and FS spin texture 28
3.2.3 Kagome non-collinear antiferromagnetic 31
3.2.4 Fermi surface spin texture and superconducting gap structure 32
3.2.5 Topological superconductivity in kagome non-collinear antiferromagnetic 35
Chapter 4 Part II: Odd-parity spin-triplet superconductivity mediated by spin fluctuation 41
4.1 Physical properties of UTe2 42
4.2 Band structure calculations 43
4.2.1 DFT+DMFT calculation at T = 11 K 43
4.2.2 Emergence of correlated 3D FS at lower temperatures 45
4.2.3 Computational details 47
4.3 Superconductivity of UTe2 50
4.3.1 Superconducting instability 50
4.3.2 Topological superconductivity 52
4.4 Supporting details 56
4.4.1 Validation of parameters in the DFT+DMFT calculations 56
4.4.2 Temperature-dependent behavior of self-energy and coherent temperature 56
4.5 Comparison with experiments 60
4.5.1 Temperature-dependent behavior of resistivity 60
4.5.2 Sign reversal of Hall coefficient 60
4.5.3 Zp point spectral weight 61
4.6 Analytic calculation details 63
4.6.1 Fermiology 63
4.6.2 Tight-binding Hamiltonian 64
4.6.3 Linearized Eliashberg equation 67
4.6.4 Topological invariant 69
4.6.5 Spontaneous TRS breaking 69
Chapter 5 Conclusion 73
Bibliography 77
Abstract in Korean 89박
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Appropriate Similarity Measures for Author Cocitation Analysis
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Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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