1,720,967 research outputs found
Evolution of electronic band reconstruction in thickness-controlled perovskite SrRuO3 thin films
No full textTransition metal perovskite oxides display a variety of emergent phenomena which are tunable by tailoring the oxygen octahedral rotation. SrRuO3, a ferromagnetic perovskite oxide, is well known to have various atomic structures and octahedral rotations when grown as thin films. However, how the electronic structure changes with the film thickness has been hardly studied. Here, using angle-resolved photoemission spectroscopy and electron diffraction techniques, we study the electronic structure of SrRuO3 thin films as a function of the film thickness. Different reconstructed electronic structures and spectral weights are observed for films with various thicknesses. We suggest that octahedral rotations on the surface can be qualitatively estimated via comparison of intensities of different bands. Our observation and methodology shed light on how structural variation and transition may be understood in terms of photoemission spectroscopy data.11Nsciescopuskc
Growth and characterization of superconductor-ferromagnet thin film heterostructure la1.85sr0.15cuo4/srruo3
No full text© 2021, Korea Institute of Applied Superconductivity and Cryogenics. All rights reserved.Superconductor-ferromagnet thin film heterostructure is an ideal system for studying the interplay between superconductivity and ferromagnetism. These two antagonistic properties combined in thin film heterostructure create interesting proximity effects such as spin-triplet superconductivity. Thin film heterostructure of optimally doped La2-xSrxCuO4(LSCO) cuprate superconductor and SrRuO3(SRO) ruthenate ferromagnet has been grown by pulsed laser deposition. Its temperature-dependent resistivity and Hall effect measurements show that our LSCO/SRO heterostructure has both superconductivity and ferromagnetism. In the Hall effect measurement results, we find additional hump-like structures appear in the anomalous Hall effect signal in the vicinity of superconducting transition. We conclude that giant magnetoresistance of the LSCO layer distorts the AHE signal, which results in a hump-like structure.11Nscopuskc
Hump-like structure in Hall signal from ultra-thin SrRuO3 films without inhomogeneous anomalous Hall effect
Full text link© 2019 Korean Physical SocietyA controversy arose over the interpretation of the recently observed hump features in Hall resistivity ρxy from ultra-thin SrRuO3 (SRO) film; it was initially interpreted to be due to topological Hall effect but was later proposed to be from existence of regions with different anomalous Hall effect (AHE). In order to settle down the issue, we performed Hall effect as well as magneto-optic Kerr-effect measurements on 4 unit cell SRO films. Clear hump features are observed in ρxy, whereas neither hump feature nor double hysteresis loop is seen in the Kerr rotation which should be proportional to the magnetization. In addition, magnetization measurement by superconducting quantum interference device shows no sign of multiple coercive fields. These results show that inhomogeneous AHE alone cannot explain the observed hump behavior in ρxy data. We suggest that emergence of the hump structure in ρxy is closely related to the growth condition11sciescopuskc
Capping and gate control of anomalous Hall effect and hump structure in ultra-thin SrRuO3 films
No full text© 2021 Author(s).Ferromagnetism and exotic topological structures in SrRuO3 (SRO) induce sign-changing anomalous Hall effect (AHE). Recently, hump structures have been reported in the Hall resistivity of SRO thin films, especially in the ultra-thin regime. We investigate the AHE and hump structure in the Hall resistivity of SRO ultra-thin films with an SrTiO3 (STO) capping layer and ionic liquid gating. STO capping results in sign changes in the AHE and modulation of the hump structure. In particular, the hump structure in the Hall resistivity is strongly modulated and even vanishes in STO-capped 4 unit cell films. In addition, the conductivity of STO-capped SRO ultra-thin films is greatly enhanced with restored ferromagnetism. We also performed ionic liquid gating to modulate the electric field at SRO/STO interface. Drastic changes in the AHE and hump structure are observed with different gate voltages. Our study shows that the hump structure as well as the AHE can be controlled by tuning inversion symmetry and the electric field at the interface.11Nsciescopu
Electric Control of 2D Van Hove Singularity in Oxide Ultra-Thin Films
No full textDivergent density of states (DOS) can induce extraordinary phenomena such as significant enhancement of superconductivity and unexpected phase transitions. Moreover, van Hove singularities (VHSs) lead to divergent DOS in 2D systems. Despite recent interest in VHSs, only a few controllable cases have been reported to date. In this work, by utilizing an atomically ultra-thin SrRuO3 film, the electronic structure of a 2D VHS is investigated with angle-resolved photoemission spectroscopy and transport properties are controlled. By applying electric fields with alkali metal deposition and ionic-liquid gating methods, the 2D VHS and the sign of the charge carrier are precisely controlled. Use of a tunable 2D VHS in an atomically flat oxide film could serve as a new strategy to realize infinite DOS near the Fermi level, thereby allowing efficient tuning of electric properties. © 2023 Wiley-VCH GmbH.11Nsciescopu
Stable humplike Hall effect and noncoplanar spin textures in SrRuO3 ultrathin films
No full text© 2021 authors. Published by the American Physical Society.We observed a humplike feature in Hall effects of SrRuO3 ultrathin films, and systematically investigated it by controlling thicknesses, temperatures and magnetic fields. The humplike feature is extremely stable, even surviving as a magnetic field is tilted by as much as 85∘. Based on the atomic-level structural analysis of a SrRuO3 ultrathin film with a theoretical calculation, we reveal that atomic rumplings at the thin-film surface enhance Dzyaloshinskii-Moriya interaction, which can generate stable chiral spin textures and a humplike Hall effect. Moreover, temperature dependent resonant x-ray measurements at the Ru L edge under a magnetic field showed that the intensity modulation of unexpected peaks was correlated with the hump region in the Hall effect. We verify that the two-dimensional property of ultrathin films generates stable noncoplanar spin textures having a magnetic order in a ferromagnetic oxide material.11Nscopu
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
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
초박막 영역에서 페로브스카이트 루테늄 산화물의 발현 현상
Full text link학위논문 (박사) -- 서울대학교 대학원 : 자연과학대학 물리·천문학부(물리학전공), 2021. 2. 김창영.본 학위 논문은 초박막 영역에서 스트론튬 페로브스카이트 루테늄 산화물 (SrRuO3) 의 발현 현상에 대해 다룬다. 초박막 형태의 응집 물질은 두께가 매우 얇아서 일반적인 덩어리 혹은 두꺼운 박막에서 나타나지 않는 새로운 물리적 특성이 나타난다. 페로브스카이트 루테늄 산화물은 수십 년 동안 잘 연구된 물질 중 하나이지만, 초박막 형태에서의 루테늄 산화물에 대한 이해는 부족한 상황이다. 본 논문에서는 초박막 스트론튬 페로브스카이트 루테늄 산화물에서 나타나는 발현 현상을 전자 수송 및 각 분해능 광전자 분광 실험을 이용해 설명한다.
전자 장치가 발전함에 따라, 전자 수송 측정을 이용하여 페로브스카이트 루테늄 산화물 박막의 저항, 홀 효과, 운반자 밀도와 같은 물리적 특성이 지난 수십 년 동안 연구되었다. 일반적으로는 페로브스카이트 루테늄 산화물의 홀 효과는 일반 홀 효과와 비정상 홀 효과의 합으로 이해되었다. 그러나, 최근 고품질의 초박막 합성이 가능하게 되면서, 초박막에서의 홀 효과 연구도 활발히 진행되기 시작하였고, 초박막의 홀 히스테리시스의 경향이 두꺼운 박막과 다르게 나타난다는 것이 보고되었다. 초박막의 홀 측정에서 혹 모양의 홀 효과가 관찰되었으며, 또한, 비정상 홀 효과의 부호가 두께에 따라 변하는 것이 보고되었다. 본 논문에서는 이러한 새로운 홀 현상의 제어 및 원인에 대해 다룬다.
본 논문은 먼저 혹 모양의 홀 효과에 대해 다룬다. 연구를 통해 혹 모양의 홀 효과는 2㎚보다 얇은 영역에서 나타난다는 것을 밝혔다. 엑스선 구조 분석을 통해 초박막의 표면에서의 반전 대칭성 붕괴는 드잘로신스키-모리야 상호 작용 및 나선형 자기 구조를 일으킬 수 있고, 따라서 혹 모양의 홀 효과를 만들 수 있다는 것을 보여준다. 또한, 본 연구에서는 스트론튬 페로브스카이트 루테늄 산화물 초박막에 다른 박막을 쌓아 헤테로 구조로 만들거나 이온 액체 게이팅 방법을 이용하여, 표면의 반전 대칭성을 제어하고 이에 따라 혹 모양의 홀 효과를 제어한다. 이러한 결과를 통해 혹 모양 홀 효과의 원리에 대해 규명하고, 혹 효과의 제어를 이용한 장치에 대해 다룬다.
이어서, 본 논문은 초박막 영역에서 나타나는 비정상 홀 효과의 부호 바뀜에 대해 다룬다. 박막의 두께, 자화, 그리고 화학 퍼텐셜의 변화가 운동량 공간에서의 베리 곡률의 합을 바꾸고, 따라서 비정상 홀 효과의 부호를 바꿀 수 있음을 보인다. 이차원 페로브스카이트 루테늄 산화물에서는 여러 부호를 가지는 베리 곡률이 나타나는데, 이는 구조에 의해 보호되는 위상 마디 구조가 존재하기 때문이다. 본 논문은 부호가 바뀌는 비정상 홀 효과의 원인 및 활용 방안에 대해서 다룬다.
초박막은 박막에 수직인 방향으로 주기성이 없고, 따라서 전자 구조 또한 덩어리 혹은 두꺼운 박막과 다르게 나타날 것으로 생각된다. 본 논문에서는 수직인 방향의 주기성이 없는 한 층의 페로브스카이트 루테늄 박막의 전자 구조에 대해 다룬다. 각 분해능 광전자 분광 장치를 통해 한 층의 페로브스카이트 루테늄 박막은 금속인 것을 관찰하였다. 또한, 박막의 구조를 제어함으로써 한 층의 페로브스카이트 루테늄 박막의 금속성을 제어할 수 있는 것을 확인하였다.This thesis comprises researches of emergent phenomena of perovskite strontium ruthenate, SrRuO (SRO), in an ultrathin regime. Due to the characteristics of the ultrathin system, a condensed material can have novel physical properties different from the bulk or thick films. Although SRO is one of the well-studied materials over the decades, SRO ultrathin films have not been studied intensively yet. Thus, we introduce the growth of high-quality SRO ultrathin films and emergent phenomena in the view of electrical transport and angle-resolved photoemission spectroscopy measurement.
As the importance of electronic devices is advanced, electrical transport measurements have been utilized to investigate many physical properties such as resistivity, Hall effect, and carrier density. In this sense, Hall effects in SRO thin films also have been widely investigated for decades. Generally, the Hall effect in SRO thin films has been explained with two Hall effects, ordinary Hall effect and anomalous Hall effect. Recently, as a high-quality SRO thin film was successfully grown, research of the Hall effects began to progress even in the ultrathin-film area. We observed that the shape of Hall hysteresis of SRO ultrathin films is different from that of an SRO thick film: hump-like structures appear and the sign of anomalous Hall effect changes. We present results and discussion on the novel Hall effects which behave differently than thick films and bulk.
We focus on the hump-like features in Hall resistivity, which appear when the thickness of thin films is thinner than 2~nm. The hump-like features are produced due to the inversion symmetry breaking on the surface of thin films, which generates Dzyaloshinskii-Moriya interaction and chiral magnetic structures. Furthermore, we control the hump-like features via capping layers on SRO and manipulating an electric field with the ionic-liquid gating method. We discuss the mechanism and possible origin of hump-like structure, explain the controversy on the emergent hump-like Hall effect, and control the hump-like structure by tuning the electric field on the surface.
Next, the sign-changing anomalous Hall effect in the ultrathin limit is continuously discussed. We show that the sign of anomalous Hall effect changes in a variation of the film thickness, magnetization, and chemical potential due to the multi-sign characteristic of Berry curvature near the Fermi level. The multi-sign Berry curvature is generated due to topological nodal structures (nodal lines and quadratic band crossing), which are protected by the four-fold-rotational and two-dimensional symmetry. Our study is the first to directly characterize the topological band structure of two-dimensional spin-polarized bands and the corresponding AHE, which could facilitate new switchable devices based on ferromagnetic ultrathin films.
The electronic band structure in ultrathin films is also expected to change due to a lack of translational symmetry along out-of-plane direction. We demonstrate SRO retains its metallicity even in a monolayer. We also introduce how the ground state changes in a monolayer SRO with changing oxygen octahedral rotation. Our systematic investigation reveals that there is a close correlation between the electronic phase and itinerant ferromagnetism in ultrathin SrRuO films. Furthermore, by exploiting the low-dimensional nature of the monolayer SrRuO, we induce the electronic transition from the correlated metal to Fermi liquid.Contents
List of Figures IV
Abstract XIX
1 Introduction 1
1.1 Transition metal oxides and metal-oxide heterostructures 2
1.2 Transition-metal-oxide ultrathin films 5
1.2.1 Strong interfacial effects 6
1.2.2 Surface-normal translational-symmetry breaking 8
1.3 Strontium ruthenate perovskite: SrRu0_3 10
1.4 Outline of Thesis 12
2 Experimental Methods 15
2.1 Pulsed laser deposition 16
2.2 Transport measurements 19
2.2.1 Hall effect 19
2.2.2 Ionic liquid gating 21
2.3 Angle-resolved photoemission spectroscopy measurement 24
3 Hump-like features in Hall effects in SrRu0_3 ultrathin films 27
3.1 Robust topological Hall effect and nontrivial magnetic order 29
3.1.1 Dzyaloshinskii-Moriya interaction and topological Hall effect 29
3.1.2 Hall effect measurements in SRO ultrathin films 31
3.1.3 Observation of in-plane magnetic order by resonant X-ray scattering 34
3.1.4 COBRA measurement and DMI calculation 37
3.1.5 Conclusion 42
3.2 Hump-like structure in Hall effect without inhomogeneous AHE 44
3.2.1 Controversy on the origin of hump structure 44
3.2.2 Interpretations for hump-like structure in Hall measurement 47
3.2.3 Magnetization measurement on SRO ultrathin films 48
3.2.4 Conclusion 54
3.3 Controlling hump structure via tuning inversion symmetry breaking 56
3.3.1 Recovering inversion symmetry with capping layer 56
3.3.2 Vanishing hump features in SrRu0_3/SrTi0_3 heterostructures 57
3.3.3 Discussion on the effect of STO capping layer 62
3.3.4 Controlling hump structures via ionic liquid gating 65
3.3.5 Conclusion 67
4 Topological structures inducing sign-varying anomalous Hall effect 68
4.1 Sign-controllable anomalous Hall effect 69
4.1.1 Two-dimensional nodal structures in ferromagnetic perovskite 69
4.1.2 Spin-resolved ARPES of two-dimensional SRO ultrathin films 73
4.1.3 Nodal structures inducing nonmonotonous AHE 76
4.1.4 Conclusion 81
5 Controlling electronic structure of SrRu0_3 extreme ultrathin films 83
5.1 Thickness-dependent phase transitions 86
5.1.1 Observation of a metallic single-atomic-layer oxide 86
5.1.2 Thickness-driven phase transitions 89
5.1.3 e^--doping-induced transition to Fermi liquid 93
5.1.4 Conclusion 94
5.2 Structure-dependent phase transitions 95
5.2.1 Controlling structure of SRO monolayer with buffer layer 96
5.2.2 Structure-dependent metal-to-insulator transition 97
5.2.3 Conclusion 98
6 References 100
7 Publication List 112
8 Korean abstract 116Docto
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