1,720,966 research outputs found
Cu doping effects on the electronic structure of Fe1-x CuxSe
No full textUsing angle-resolved photoemission spectroscopy, we investigated the evolution of the electronic structure of Fe1-xCuxSe from x=0 to 0.10. We found that the substitution of Fe by Cu introduces extra electron carriers. The hole bands near the Γ point were observed to shift downward with increasing doping x and completely sank down below the Fermi level (EF) for x≥0.05. Meanwhile, the electron pockets near the M point became larger but lost the spectral weight near EF. Concomitantly, the effective mass of the electron bands increased with doping. Our results show how a metal-insulator transition behavior occurs upon Cu doping in view of the electronic structure and provide a platform to further investigation on the origin of emergent magnetic fluctuation in Fe1-xCuxSe.11Nsciescopu
Effect of the sample work function on alkali metal dosing induced electronic structure change
Full text linkAlkali metal dosing (AMD) has been widely used as a way to control doping without chemical substitution. This technique, in combination with angle resolved photoemission spectroscopy (ARPES), often provides an opportunity to observe unexpected phenomena. However, the amount of transferred charge and the corresponding change in the electronic structure vary significantly depending on the material. Here, we report study on the correlation between the sample work function and alkali metal induced electronic structure change for three iron-based superconductors: FeSe, Ba(Fe0.94Co0.06)(2)As-2 and NaFeAs which share a similar Fermi surface topology. Electronic structure change upon monolayer of alkali metal dosing and the sample work function were measured by ARPES. Our results show that the degree of electronic structure change is proportional to the difference between the work function of the sample and Mulliken's absolute electronegativity of the dosed alkali metal. This finding provides a possible way to estimate the AMD induced electronic structure change.11Nsciescopu
Studying local Berry curvature in 2H-WSe2 by circular dichroism photoemission utilizing crystal mirror plane
Full text linkIt was recently reported that circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) can be used to observe the Berry curvature in 2H-WSe2 (Cho et al. in Phys Rev Lett 121:186401, 2018). In that study, the mirror plane of the experiment was intentionally set to be perpendicular to the crystal mirror plane, such that the Berry curvature becomes a symmetric function about the experimental mirror plane. In the present study, we performed CD-ARPES on 2H-WSe2 with the crystal mirror plane taken as the experimental mirror plane. Within such an experimental constraint, two experimental geometries are possible for CD-ARPES. The Berry curvature distributions for the two geometries are expected to be antisymmetric about the experimental mirror plane and exactly opposite to each other. Our experimental CD intensities taken with the two geometries were found to be almost opposite near the corners of the 2D projected hexagonal Brillouin zone (BZ) and were almost identical near the center of the BZ. This observation is well explained by taking the Berry curvature or the atomic orbital angular momentum (OAM) into account. The Berry curvature (or OAM) contribution to the CD intensities can be successfully extracted through a comparison of the CD-ARPES data for the two experimental geometries. Thus, the CD-ARPES experimental procedure described provides a method for mapping Berry curvature in the momentum space of topological materials, such as Weyl semimetals.11Nsciescopu
Kondo interaction in FeTe and its potential role in the magnetic order
Full text linkFinding d-electron heavy fermion states has been an important topic as the diversity in d-electron materials can lead to many exotic Kondo effect-related phenomena or new states of matter such as correlation-driven topological Kondo insulator. Yet, obtaining direct spectroscopic evidence for a d-electron heavy fermion system has been elusive to date. Here, we report the observation of Kondo lattice behavior in an antiferromagnetic metal, FeTe, via angle-resolved photoemission spectroscopy, scanning tunneling spectroscopy and transport property measurements. The Kondo lattice behavior is represented by the emergence of a sharp quasiparticle and Fano-type tunneling spectra at low temperatures. The transport property measurements confirm the low-temperature Fermi liquid behavior and reveal successive coherent-incoherent crossover upon increasing temperature. We interpret the Kondo lattice behavior as a result of hybridization between localized Fe 3dxy and itinerant Te 5pz orbitals. Our observations strongly suggest unusual cooperation between Kondo lattice behavior and long-range magnetic order. © 2023. The Author(s).11Nsciescopu
Observation of metallic electronic structure in a single-atomic-layer oxide
No full text© 2021, The Author(s).Correlated electrons in transition metal oxides exhibit a variety of emergent phases. When transition metal oxides are confined to a single-atomic-layer thickness, experiments so far have shown that they usually lose diverse properties and become insulators. In an attempt to extend the range of electronic phases of the single-atomic-layer oxide, we search for a metallic phase in a monolayer-thick epitaxial SrRuO3 film. Combining atomic-scale epitaxy and angle-resolved photoemission measurements, we show that the monolayer SrRuO3 is a strongly correlated metal. Systematic investigation reveals that the interplay between dimensionality and electronic correlation makes the monolayer SrRuO3 an incoherent metal with orbital-selective correlation. Furthermore, the unique electronic phase of the monolayer SrRuO3 is found to be highly tunable, as charge modulation demonstrates an incoherent-to-coherent crossover of the two-dimensional metal. Our work emphasizes the potentially rich phases of single-atomic-layer oxides and provides a guide to the manipulation of their two-dimensional correlated electron systems.11Nsciescopu
철기반 초전도체 네마틱 상에 대한 전자구조 연구
Full text link학위논문(박사) -- 서울대학교대학원 : 자연과학대학 물리·천문학부(물리학전공), 2021.8. 허순상.철기반 초전도체에서는 전자구조의 회전 대칭구조가 깨져있는 네마틱 상을 가지고 있다. 이 네마틱 상은 발현 자체가 신기할 뿐만 아니라 철 기반 초전도체의 초전도 메커니즘과 연 관이 있을 것으로 예측되어 사람들의 이목을 끌었다. 그렇기에 네마틱 상이 생기는 원인을 이해하는 것이 중요한 연구과제로 여겨졌고 이를 위해 수많은 연구들이 수행되어 왔다. 하 지만 네마틱 상이 정확히 어떻게 형성되는지에 대한 정확한 이해가 여전히 되어있지 않은 상태이다. 네마틱 상을 정확히 이해하기 위해선 전자구조의 회전 대칭이 어떻게 깨져있는지 직접적으로 관측이 필요하다. 이를 위해 전자구조를 직접 관측 할 수 있는 각분해능 광전자 분광실험이 수행되었다.
FeSe는 네마틱 상을 가지고 있음에도 불구하고 다른 철기반 초전도체에서 나타나는 특성 들과 달리 특이한 특성들을 가지고 있다. 먼저, 다른 물질들이 네마틱 상과 반강자성을 동시에 가지고 있는 반면 FeSe는 반강자성이 없다는 점이다. 다른 한가지는 네마틱 상의 대표적 증 거인 저항의 비등방성이 다르게 나타나는 특성이다. 이러한 이유들로 FeSe에 대한 전자구조 연구를 진행하면 네마틱 상을 이해하는데 중요한 단서를 줄 것이라 여겨졌다. 이에 각분해능 광전자 실험 및 X-ray 흡수 실험을 수행하여 FeSe 네마틱 상의 전자구조를 규명하였다. 규 명한 전자구조의 오비탈 분포 분석을 통해 1 Fe 브릴루앙 대역에서 하나의 포켓이 사라지는 것을 확인하였다. 또한 dxz , dyz 오비탈 간의 점유도 차이가 다른 철기반 초전도체와 비교하 여 반대인 것을 확인하였다. 이 결과들은 통해 FeSe 네마틱 상에서 나오는 특이한 특성들을 이해 할 수 있었다. 더 나아가 네마틱 상의 원인으로 생각되어지던 페로 오비탈 정렬이 아니 라는 것을 밝혔다. 마지막으로 실험적으로 규명한 전자구조를 통해 네마틱 상과 초전도와의 연관성에 대해 다루었다.
앞선 결과를 통해 네마틱 상에서 dxz , dyz 밴드간의 스플리팅이 운동량 의존을 가진다는 것을 확인하였으며 이 발견이 FeSe 뿐만 아니라 다른 철기반 초전도체들에도 발견 되는 것을 확인하였다. 하지만 대표적 철기반 초전도 물질 중 하나인 LaFeAsO에 대해선 정보가 부족한 상황이였다. 이에 LaFeAsO 네마틱 상에 대해 전자구조 연구를 수행하였고 dxz , dyz 밴드간 의 스플리팅이 운동량 의존을 가진다는 것을 확인하였다. 본 연구를 통해 dxz , dyz 밴드간의 스플리팅이 철기반 초전도 네마틱 상에서 공통적으로 나타나는 것을 확인하였고 이를 만드는 것이 네마틱 상을 만드는 직접적인 원인이 될 것이라 규명하였다.Iron based superconductors have a nematic phase in which the rotational symmetry is broken in the electronic structure. This nematic phase has attracted attention because it is expected to be related to the pairing mechanism of superconductivity in iron based superconductors and is a novelty in itself. Therefore, understanding the origin of nematic phases is considered an important research task, and a number of studies have been con- ducted to find the mechanism. However, there is still no precise understanding of how exactly the nematic phase is formed. In order to understand the nematic phase accurately, it is necessary to directly observe how the rotational symmetry of the electronic structure is broken. For this reason, angle resolved photoemission spectroscopy (ARPES), which allows direct observation of the electronic structure, was conducted.
FeSe has unique properties of the nematic phase compared to other iron based super- conductors. The most peculiar aspect is the absence of long range magnetic order, which always coexists with orbital order in the iron pnictides nematic phase. Furthermore, it was reported that the resistivity anisotropy of FeSe has the opposite sign to that of iron pnictides. For this reason, the electronic structure study of FeSe was considered to provide important clue for understanding the nematic phase. Accordingly, ARPES and an X-ray absorption experiments were performed to investigate the electronic structure of the FeSe nematic phase. Through the analysis of the orbital character of the band dispersion, it was confirmed that one pocket disappeared in the 1 Fe Brillouin zone. In addition, the occupancy imbalance between the dxz and dyz orbitals was opposite compared to that of other iron based superconductors. Through these results, it was possible to understand the peculiar characteristics of the FeSe nematic phase. Furthermore, it was revealed that ferro orbital order is not the origin of the nematic phase. Finally, the relationship between the nematic phase and superconductivity has been addressed through the experimentally identified electronic structure.
Through the previous results, it was confirmed that the splitting between the dxz and dyz bands has momentum dependence in the nematic phase. Moreover, this finding was found not only in FeSe but also in other iron based superconductors. However, information about LaFeAsO, one of the representative iron based superconductor with nematic phase, was insufficient. Therefore, an electronic structure study was performed on the LaFeAsO nematic phase. It was found that the momentum dependent dxz/yz hole band splitting exist in the LaFeAsO nematic phase. Through this study, it was confirmed that the momentum dependent dxz/yz hole band splitting exist universally in the iron based superconductors. We propose that the instability for the observed universal momentum dependent band splitting should be the true origin of the nematic phase.1 Introduction 1
1.1 Overview of iron based superconductors (IBSs) 2
1.1.1 Background of IBSs 2
1.1.2 Phase diagram 3
1.1.3 Electronic structure 5
1.2 Nematic phase in IBSs 7
1.2.1 Nematic phase in IBSs 7
1.2.2 Evidence of nematic phase 9
1.2.3 Origin of nematic phase 10
1.3 Motivation 13
2 Experimental Methods 14
2.1 Angle resolved photoemission spectroscopy (ARPES) 15
2.1.1 Introduction of ARPES 15
2.1.2 Polarization dependent ARPES 18
2.2 X-ray absorption spectroscopy (XAS) 19
2.3 Detwin process 21
2.3.1 Twin domain 21
2.3.2 Detwin process 22
2.3.3 Piezo based detwin device 24
2.4 Sample growth 29
2.4.1 Chemical vapor transport (CVT) method 29
2.4.2 Growth of FeSe crystal 30
2.4.3 Sample characterization 33
3 Results I : Investigation of FeSe nematic phase 36
3.1 Background 37
3.1.1 Introduction of FeSe nematic phase 37
3.1.2 Previous electronic structure studies of FeSe nematic phase 38
3.2 Experimental detail 41
3.3 Electronic structure in the nematic phase 41
3.3.1 Fermi surface topology 41
3.3.2 Band dispersion 44
3.3.3 Orbital characterization 45
3.3.4 Temperature evolution of electronic structure 54
3.3.5 Orbital occupation measurement 57
3.4 Discussion 60
3.4.1 Orbital selective hybridization 60
3.4.2 Unique characteristic of nematic phase 62
3.4.3 Understanding mechanism of nematic phase 63
3.4.4 Relation to the superconductivity 64
4 Results II : Investigation of LaFeAsO nematic phase 67
4.1 Background 68
4.1.1 Momentum dependent band splitting in the nematic phase 68
4.1.2 Introduction of LaFeAsO nematic phase 70
4.2 Experimental detail 72
4.3 Electronic structure in the nematic phase 72
4.3.1 Electronic structure of LaFeAsO nematic phase 72
4.3.2 Temperature evolution of the electronic structure 75
4.3.3 Electronic structures of twinned and detwinned LaFeAsO 79
4.4 Discussion 80
5 Other result : Electronic structure of Cu doped FeSe 82
5.1 Background 83
5.1.1 Chemical substitution on FeSe 83
5.2 Experimental detail 86
5.3 Evolution of the electronic structure through doping 86
5.3.1 Evolution of the Fermi surface topology 86
5.3.2 Evolution of the band dispersion 88
5.4 Discussion 91
6 Summary 92
Bibliography 93
국문 초록 103박
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
