Topological phenomena in pyrochlore iridates

오태구 서울대학교 대학원 2022 국내박사

응집물질에서의 물리는 크게 스핀 오비탈 결합과 전자 간의 상호작용으로 결정된다. 스핀 오비탈 결합과 전자 간 상호작용이 모두 밴드 폭과 비슷하게 큰 경우, 강상 위상학적 상인 바일 준금속, 액시온 부도체, 또는 위상 모트 부도체 등이 나타난다. 이들은 모두 특이한 물성을 지녔기에 주목을 점점 더 많이 받고 있다. 파이로클로르 이리듐 산화물의 화학식은 R2Ir2O7로, 상호작용과 스핀 오비탈 결합이 모두 큰 시스템이다. 따라서 이 물질군은 강상 위상학적 상을 조사할 수 있는 좋은 배경이 된다. 특히, 이 물질군은 낮은 온도에서 반강자성이 생기면서 바일 준금속이 나타날 수 있는 것으로 예측된 첫 번째 후보이다. 이 때 반강자성의 이름을 All-in-all-out (AIAO)이라고 부르는데 이는 모든 스핀이 동시에 셀 중심을 향하거나 바깥을 향하기 때문이다. 그러나 자성 바일 준금속이라는 결정적인 증거는 아직 발견되지 않았는데, 두 가지 이유가 있다. 첫번째로는 이 물질군이 대부분 부도체이므로 바일 준금속을 볼 수 있는 범위가 매우 좁다. 두번째로는 이 물질군에서 바일 준금속이 생기더라도 이에 의해서 나올 수 있는 물성이 정육면체 대칭성에 의해서 모두 사라지기 때문이다. 이 물질군에서 바일 준금속을 찾기 위해서 우리는 이 물질이 반강자성 부도체에서 상자성 도체가 되는 전이점 근처에서 섭동을 걸어주었다. 이 섭동은 바일 준금속을 볼 수 있는 범위를 넓혀 줄 뿐 아니라 대칭성도 깨줄 것으로 예상되었으므로 위의 어려움이 모두 극복될 것으로 예상되었다. 섭동으로 이용된 것은 자기장과 변형이다. 첫째로, 자기장이 반강자성 부도체와 상자성 도체 사이에 존재하는 (Ndx Pr{1-x})2Ir2O7 단결정에 걸렸을 때 매우 다양한 위상학적 준금속이 관측되었다. 군론을 이용하면 상자성 도체에 존재하는 이차 밴드 겹침이 높은 스핀 (J=3/2)을 가지기 때문에 자기장이 걸렸을 때 일반적인 제만 항 뿐 아니라 비등방성 제만 항이 추가적으로 나오는 것을 볼 수 있었다. 이 두 제만 항과 AIAO 사이의 상호관계는 4쌍 바일, 2쌍 바일, 이차 바일, 그리고 선 겹침 준금속을 만들어 내었다. 이 때 자기장이 파이로클로르 이리듐 산화물 내부의 스핀 구조를 바꾸면서 제만 항과 AIAO을 조절하는 것으로 알려졌다. 둘째로, 반강자성 부도체인 Nd2Ir2O7와 상자성 도체인 Pr2Ir2O7 박막에 각각 변형이 걸렸다. Nd2Ir2O7의 경우 변형이 부도체-도체 전이와 함께 자기장이 없을 때 이상 홀 효과를 유도하였다. 모델을 이용한 계산은 이것이 바일 준금속이 아니라 전자 및 양공 주머니가 있는 단순한 도체임을 보였다. 또한 자기장이 없을 때 자화가 없으므로, 이상 홀 효과의 원인은 바일 준금속도 자화도 아니었다. 사실 변형이 유도하는 T1-팔극자가 이상 홀 효과의 원인인데, 이는 이 팔극자가 자화랑 대칭성이 같기 때문이다. 반면 Pr2Ir2O7의 경우, 이상 홀 효과, 음의 자기저항, 평면 홀 효과 등 바일 준금속의 증거가 많이 발견되었다. 그 중에서도 평면 홀 효과는 바일 준금속의 카이랄 이상과 AIAO으로 설명되었다. 이 졸업 논문에서는 바일 준금속이 파이로클로르 준금속에서 생기는 원인과 어떻게하면 그것을 찾을 수 있는지를 제공한다. 따라서 이는 파이로클로르 이리듐 산화물에서 새로운 위상학적 상의 실험적 발견을 촉진할 것이다. Physics in condensed matter is determined by spin-orbit coupling and electronic correlation. When spin-orbit coupling and electronic correlation are comparable to the bandwidth, the correlated topological phases like Weyl semimetal, axion insulator, and topological Mott insulator emerge. Because of their unique physical phenomena, correlated topological phases are getting more attention. Pyrochlore iridates, whose chemical formula is R2Ir2O7 (R: rare-earth), have a strong correlation and spin-orbit coupling at the same time. Hence, pyrochlore iridates are playgrounds for investigating the correlated topological phases. Especially, pyrochlore iridates are the first candidate that a Weyl semimetal develops when the antiferromagnetic order is developed at a low temperature. The antiferromagnetic order is called all-in-all-out (AIAO) since all spins point from or to the unit cell center. However, the smoking gun evidence for a magnetic Weyl semimetal is missing so far for two reasons. First, the ground state is mostly an antiferromagnetic insulator, so the window for Weyl semimetal is negligible. Second, although the Weyl semimetal is present in pyrochlore iridates, the signals from the Weyl semimetal are canceled by the cubic symmetry. In order to find Weyl semimetal in pyrochlore iridates, we apply the perturbations to pyrochlore iridates near the transition point from antiferromagnetic insulator to paramagnetic metal. We expect that the perturbation widens the window for Weyl semimetal and breaks the cubic symmetry so that the difficulties above can be overcome. The perturbations used here are magnetic field and strain. Accordingly, the dissertation is divided into two parts. First, the magnetic field is applied to (NdxPr{1-x})2Ir2O7 single crystals, which is between the antiferromagnetic insulator and paramagnetic metal phase. Then, the experiment shows a variety of topological semimetals. The group theory shows that the quadratic band crossing in paramagnetic metal has a high effective spin J=3/2, so the magnetic field induces an anisotropic Zeeman term as well as a usual Zeeman term. The interplay of two Zeeman terms and AIAO order gives rise to the various topological semimetals, like the 4-pair Weyl, 2-pair Weyl, double Weyl, and nodal-line semimetals. The magnetic field controls Zeeman terms and AIAO by changing the spin configuration of pyrochlore iridates. Second, when strain is applied to Nd2Ir2O7 and Pr2Ir2O7 thin films, which are the antiferromagnetic insulator and paramagnetic metal, respectively. For Nd2Ir2O7, the strain induces the insulator-to-metal transition and anomalous Hall Effect at zero magnetic fields. The model calculation shows that strained Nd2Ir2O7 is trivial metal with electron and hole pockets. Since the magnetization is zero at zero fields, the origin of the anomalous Hall Effect is neither the magnetization nor the Weyl semimetal. In fact, the strain-induced T1-octupole is the origin of the anomalous Hall Effect, since T1-octupole has the same symmetry as magnetization. On the other hand, for Pr2Ir2O7, the experiments show numerous pieces of evidence for Weyl semimetals, such as anomalous Hall Effect, negative magnetoresistance, and planar Hall Effect. Especially, the planar Hall Effect can be explained by the chiral anomaly of Weyl semimetal and AIAO order. The results in the dissertation provide the reasons why Weyl semimetal emerges in pyrochlore iridates, and the method to find the Weyl semimetal as well. This dissertation facilitates the experimental discovery of novel topological phases in pyrochlore iridates.

Spontaneous Hall Effect and Unusual Hysteresis of Magnetic Torque in Transition Metal Dichalcogenide : 전이금속 칼코겐 화합물에서 나타나는 자발적 홀 효과와 자성 토크 특이 히스테리시스

최준영 경북대학교 대학원 2025 국내박사

The spontaneous Hall effect refers to the phenomenon where the trajectory of charge carriers bends as if under a magnetic field, even without an external magnetic field. The spontaneous Hall effect is typically observed in ferromagnetic or ferrimagnetic materials with a net magnetization but can also occur in antiferromagnetic materials that either lack net magnetization or have a net magnetization much smaller than the Hall effect. When the spontaneous Hall effect is proportional to the net magnetization, it is called the anomalous Hall effect. The anomalous Hall effect can be further divided into intrinsic and extrinsic contributions, with the intrinsic anomalous Hall effect being related to the Berry curvature in momentum space, independent of scattering mechanisms. When a fictitious magnetic field, generated by spin clusters forming a solid angle, induces the spontaneous Hall effect regardless of the net magnetization, it is called the topological Hall effect. However, spin clusters forming a solid angle are also known to act as scattering centers for the extrinsic anomalous Hall effect. Thus, accurately analyzing the origin of the spontaneous Hall effect is a crucial tool for determining magnetic structures, which in turn allows for identifying the energy-momentum dispersion relationship of the electron wave function. This paper presents magnetoresistance and Hall resistivity measurements and analysis of magnetic torque in two magnetic transition metal disulfides containing Co2+ ions. Cobalt disulfide, with a cubic pyrite structure, is an itinerant ferromagnet where the spins of conduction electrons are polarized due to the high electronic density of states and strong Coulomb interactions between electrons. In contrast, the iron and nickel disulfides of the same structure are a semiconducting ferromagnet and a Mott insulator, respectively. Therefore, substituting Co with Fe or Ni ions can significantly alter these materials’ magnetic and conductive properties. This study confirmed that at doping levels below 10%, the conductivity and ferromagnetism of cobalt disulfide are maintained, and Vegard’s law is still satisfied. Among the samples in this doping range, the sample with 5% Fe doping showed a sharp increase in the anomalous Hall effect. This result aligns well with calculations based on DFT+U , which indicate that the source of the Berry curvature in cobalt disul- fide lies at E = EF−60meV, causing the anomalous Hall effect to increase sharply as EF is tuned. While the typically reported values of anomalous Hall conductivity are in the range of 100 ∼ 1000 (Ωcm)−1, the sample with 5% Fe doping exhibited a maximum anomalous Hall conductivity of 2507 (Ωcm)−1. For reference, the expected anomalous Hall conductivity due to a single Berry curvature source in cobalt disulfide at its lattice spacing is approximately 699 (Ωcm)−1. The generally small values of anomalous Hall conductivity are due to the cancellation of multiple Berry curvature sources with opposite signs arising from symmetry. In contrast, the large anomalous Hall effect in cobalt disulfide was found to be due to four massive Dirac nodes symmetrically distributed along the Γ−K direction in momentum space, all of which have the same sign. Magnetic skyrmions with the size of the atomic level are predicted to manifest in frustrated lattice structures or in magnetic systems where interactions among more than four magnetic moments dominate. However, these structures have only been observed in a few-layer Fe or Mn adatom systems grown on Re(0001) or Cu(111) substrates. Recently, neutron scattering experiments revealed that the low- temperature magnetic structure of Co1/3TaS2, where Co2+ ions are intercalated in the van der Waals gaps of a 2D transition metal disulfide, forms a triple-Q (3Q) magnetic skyrmion structure. This structure consists of four spin sublattices arranged in a tetrahedral all-in-all-out configuration, where the spins point from the center of a 3D tetrahedron to its vertices and are distributed on the 2D intercalated hexagonal Co layer. This results in a 3Q structure, a superposition of three row-wise (1Q) states equivalent under a 120○ rotational transformation of the hexagonal lattice. The topological Hall effect in Co1/3TaS2 is induced by the fictitious magnetic field generated by the 3Q spin clusters. Despite some understanding of the magnetic structure, several phenomena in Co1/3TaS2 remain unexplained, such as (i) the nonzero net magnetization and (ii) the metamagnetic transition occurring at a critical field Hc2—different from the spin-flip transition at Hc1. In this paper, we analyze the topological Hall effect and magnetoresistance measured at various angles, revealing that the metamagnetic transition responds differently for A- and B-type domains of the 3Q structure, which we describe using a simple 2× 2 model of the magnetic transition. Additionally, analysis of magnetic torque measured along the a∗c and ac planes shows that the net magnetization of Co1/3TaS2 consists of an isotropic linear component and an anisotropic c-axis component. The unusual hysteresis in magnetic torque is explained by the hysteresis in the magnetization along the c-axis. The periodic variation of magnetic torque in the a∗a plane as a function of the applied magnetic field suggests that the magnetic ground state of Co1/3TaS2 may alternate between two 3Q states, (3Q1) and (3Q2), related by rotational symmetry. 자발적 홀 효과란 외부 자기장 없이도 마치 자기장이 있는 것처럼 전하 운반자의 궤적이 휘는 효과를 말한다. 자발적 홀 효과는 알짜 자화를 갖는 강자성체 또는 페리 자성체에서 주로 발견되지만 알짜 자화를 갖지 않거나, 홀 효과에 비해 매우 작은 알짜 자화를 갖는 반강자성체에서 관찰되기도 한다. 자발적 홀 효과가 알짜 자화에 비례하면 비정상 홀 효과라고 한다. 비정상 홀 효과는 다시 내재적 효과와 외재적 효과로 나뉘며, 산란 효과와 관계없이 모멘텀 공간의 베리 곡률과 관계된 경우를 내재적 비정상 홀 효과라고 한다. 알짜 자화와 관계 없이 고체각을 이루는 스핀 클러스터가 만드는 실공간의 가상 자기장이 자발적 홀 효과를 일으키는 경우는 위상학적 홀 효과라고 부른다. 다만, 고체각을 이루는 스핀 클러스터는 외재적 비정상 홀 효과의 스핀 산란 중심으로서 역할을 하기도 하는 것으로 알려져 있다. 따라서, 자발적 홀 효과의 원인을 정확히 분석하는 것은 자성 구조를 판가름하는 중요한 분석이고 그에 따라 전자 파동함수의 에너지-모멘텀 분산 관계를 알아낼 수 있다. 본 논문에서는 3d 전이금속 이온 Co2+이 포함된 두 가지 전이금속 이황화물 자성체의 자기저항과 홀 저항을 측정하고 자성 토크를 분석하였다. 3차원 cubic pyrite 구조를 갖는 코발트 이황화물은 충분히 큰 전자 상태 밀도와 전자 간 쿨롱 상호작용 때문에 전도 전자의 스핀이 편향되어 강자성을 띠는 itinerant ferromagnet이다. 같은 구조의 철, 니켈 이황화물은 각각 반도체 성 강자성체, 모트 절연체이기 때문에 Co site에 Fe 또는 Ni 이온의 치환으로 자성과 전도성이 크게 변화시킬 수 있는 물질이다. 본 연구에서는 10% 이하의 적당한 도핑 레벨에서는 코발트 이황화물의 전도성과 강자성이 유지되는 것과 Vegard's law를 여전히 만족하는 것을 확인하였다. 해당 도핑 범위 중 Fe가 5 % 도핑된 시료에서는 비정상 홀 효과가 급격히 커지는 결과를 보였다. 이는 DFT+U로 계산한 밴드 구조에서 코발트 이황화물의 베리 곡률 원천이 EF-60meV 에 위치하기 때문에 EF 조절에 따라 비정상 홀 효과가 급격히 커지는 계산 결과와 잘 맞다. 일반적으로 보고되는 비정상 홀 효과의 전도도 값은 100 ∼ 1000 (Ωcm)−1 범위에 불과하지만, Fe가 5 % 도핑된 코발트 이황화물의 비정상 홀 전도도는 2507 (Ωcm)−1 의 최댓값을 보였다. 참고로, 코발트 이황화물의 격자 간격에서 하나의 베리 곡률 원천이 보일 수 있는 비정상 홀 전도도는 699 (Ωcm)−1 정도다. 일반적으로 보고되는 비정상 홀 전도도가 작은 이유는 대칭성에 따라 존재하는 여러 베리 곡률 원천이 서로 부호가 다르기 때문에 상쇄되기 때문이다. 반면에 코발트 이황화물의 베리 곡률 원천은 모멘텀 공간의 Γ − K 방향에 대칭적으로 존재하는 4개의 massive Dirac node이며 서로 부호가 같은 것이 큰 비정상 홀 효과의 원인임을 보였다. 원자 수준으로 크기가 작은 자성 스커미온은 쩔쩔매는 격자 구조나, 넷 이상의 자기 모멘트가 우세하게 상호작용하는 자성체에서 발현될 것으로 예측되지만 Re(0001) 또는 Cu(111) 기판 위에 성장시킨 수 층짜리 Fe, Mn adatom 에서만 구조가 확인되었다. 2차원 전이금속 이황화물의 반데르 발스 갭에 Co2+ 이온이 intercalate 된 Co1/3TaS2 의 저온 자성 구조가 최근 중성자 산란 실험을 통해 3Q 자성 스커미온 구조로 밝혀졌다. 이 구조는 3차원 사면체의 중심에서 꼭짓점을 향하는 네 개의 스핀 부격자 구조가 2차원 intercalated Co 층에 배치된 tetrahedral all-in-all-out 구조이며, hexagonal의 120º 회전 변환에 대해 동등한 세 Row-wise (1Q) 상태가 중첩된 triple-Q (3Q) 구조이다. Co1/3TaS2 는 3Q 스핀 클러스터가 만드는 가상의 자기장에 의해 위상학적 홀 효과가 나타난다. 자성 구조가 어느 정도 밝혀졌음에도 불구하고, Co1/3TaS2 의 (i) 0이 아닌 알짜 자화와 (ii) 스핀을 뒤집는 Hc1 이 아닌, 다른 임계 자기장 Hc2 에서 발생하는 metamagnetic transition 은 아직 설명되지 않았다. 본 논문에서는 여러 각도에 따라 측정한 위상학적 홀 효과와 자기 저항을 분석하여 metamagnetic transition 이 3Q 구조의 A-, B-타입 도메인에 대해 다르게 반응하는 것을 발견하였으며 간단한 2×2 심볼을 이용해 자성 구조간 전이를 표현하였다. 다음으로 시료의 a*c, ac 평면을 따라 측정한 자성 토크를 분석하여 Co1/3TaS2 의 알짜 자화가 등방적인 선형 성분과 비등방적인 c 축 성분으로 구분된다는 것을 알아냈고 특이한 자성 토크 히스테리시스를 c 축으로만 나타나는 Co1/3TaS2 의 자화 히스테리시스로 설명하였다. 자기장에 따라 주기성이 변하는 a*a 평면상 자성 토크는 Co1/3TaS2 의 자성 바닥 상태가 3Q1 과 회전 관계에 있는 3Q2 일 가능성을 보여준다.

Topics in Condensed Matter Theory: Berry Curvature Effects in Transport and Numerical Analytic Continuation

Keyes, Lauren The Ohio State University ProQuest Dissertations & 2024 해외박사(DDOD)

소속기관이 구독 중이 아닌 경우 오후 4시부터 익일 오전 9시까지 원문보기가 가능합니다.

This thesis is composed of two unrelated pieces of work: the first develops a semiclassical theory of transport in topological magnets, and the second presents a machine learning-based method of numerical analytic continuation.In Part I, we calculate the electric and thermal currents carried by electrons in the presence of general magnetic textures in three-dimensional crystals, including three-dimensional topological spin textures. We show, within a controlled, semiclassical approach that includes all phase space Berry curvatures, that the transverse electric, thermoelectric, and thermal Hall conductivities have two contributions in addition to the usual effect proportional to a magnetic field. These are an anomalous contribution governed by the momentum-space Berry curvature arising from the average magnetization, and a topological contribution determined by the real-space Berry curvature and proportional to the topological charge density, which is non-zero in skyrmion phases. This justifies the phenomenological analysis of transport signals employed in a wide range of materials as the sum of these three parts. We prove that the Wiedemann-Franz and Mott relations hold, even in the presence of topological spin textures, and justifying their use in analyzing the transport signals in these materials. This theory also predicts the existence of the in-plane anomalous and topological Hall effects in three-dimensional, low symmetry materials. We present a symmetry analysis which predicts when the in-plane Hall effect (IPHE) is forbidden, and predict which crystal structures could harbor an IPHE which is larger than the usual out-of-plane Hall effect.In Part II, we present a method of numerical analytic continuation of determinantal Quantum Monte Carlo (DQMC) Green's functions, utilizing an unsupervised neural network (NN). Many have used supervised machine learning methods to attack this problem, but the most interesting applications of DQMC are on systems in which the physical state is totally unknown, so it is advantageous to use an unsupervised NN, which requires no prior knowledge of the system's physical state. This autoencoder-type NN trains on real DQMC data, training the NN to produce spectral functions which reproduce the DQMC data as closely as possible. Regularization is provided by 1) a pretraining step, which trains the NN on general characteristics of spectral functions, and 2) the imposition of limited physical assumptions, such as sum rules. Ultimately, this NN is shown to outperform the standard maximum entropy method on the analytic continuation of noisy data, thus saving computational effort.

Emergent phenomena of perovskite ruthenate in ultra-thin regime

손병민 서울대학교 대학원 2021 국내박사

This thesis comprises researches of emergent phenomena of perovskite strontium ruthenate, SrRuO$_3$ (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$_3$ films. Furthermore, by exploiting the low-dimensional nature of the monolayer SrRuO$_3$, we induce the electronic transition from the correlated metal to Fermi liquid. 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.

Disorder Effects in Quantum Materials

Huang, Yi University of Minnesota ProQuest Dissertations & T 2023 해외박사(DDOD)

소속기관이 구독 중이 아닌 경우 오후 4시부터 익일 오전 9시까지 원문보기가 가능합니다.

Decades of dedicated efforts in controlling disorder in conventional semiconductors have laid the foundation for our modern civilization, based on chips and all kinds of electronic devices. Nowadays, there is a growing interest in the so-called quantum materials whose properties are fundamentally altered by quantum-mechanical effects. Such quantum materials include two-dimensional heterostructures, topological insulators, graphene, superconductors, and many others. The strong interaction between electrons and topology within quantum materials gives rise to rich quantum states and phases such as quantum Hall effects and topological phases. For example, an exciting future application of quantum materials is the topological quantum computer, which is believed to be the most robust way to process quantum information. However, engineering such quantum materials must deal with ubiquitous impurities, which often ruin the delicate quantum-mechanical effects of interest and prevent the topological quantum computation from being realized. My dissertation research focuses on analyzing how the disorder affects the resistivity of different kinds of quantum materials, e.g., topological insulator thin films and wires, non-Hermitian random lasers and photonic lattices, and GaAs/AlGaAs heterostructures. Therefore, my dissertation research on improving the understanding of disorder effects in quantum materials has a broader impact on various fields from fundamental research to material engineering and technology.

Interfacial effects on magnetoresistance in nano-devices

김성종 KU-KIST Graduate School of Converging Science and 2022 국내박사

The lifestyles of people living in the high-speed and cutting-edge era represented by the 4th industrial revolution are changing from off-line to on-line. As on-line replaces off-line, people experience and need more digital technology. Due to this demand, the amount of information and data to be processed is increasing exponentially. The development of high-level storage and processing devices is required for rapid information processing. In particular, there is an urgent need to develop materials and process technologies to realize ultra-small size, ultra-low power, and high integration of the device. Among the various alternatives, spintronic device technology using the spin of electrons is receiving great attention. Spintronics technology is a technology that implements a new concept of device by using two physical properties of electrons, charge and spin, together, rather than improving or miniaturizing an existing semiconductor device using the charge of electrons. It controls electrons by distinguishing not only the charge of electrons but also spin information, that is, the spin-up and spin-down states. Compared to other types of information processing technology, it has high-speed operation, low power consumption and strong non-volatile properties. Many previous studies have made important advances in the industry with products using giant magnetoresistance, tunneling magnetoresistance, and spin transfer torque. It forms one axis of nanotechnology, which is a hot topic in recent years, and with the miniaturization of the nano scale, a new quantum mechanical phenomenon that has not been seen before is realized, and the technology is being developed in the field of nano-spintronics. This paper deals with three studies applied to spintronic devices using nanomaterials. The key keyword of these studies is the interfacial effect. The effect of each unique nanomaterial on the device surface was confirmed using various electrical measurement methods. First, the effect of the Ca-doped Bi2Se3 surface properties on the ferromagnet was confirmed through anisotropic magnetoresistance. Bi2Se3, a kind of three-dimensional topological insulator, is an insulator on the inside but a metallic material on the surface. It has a unique band structure in which the spin direction changes according to the electron motion direction. Simply put, when a current is applied in x-axis direction to the topological insulator channel, the spins are aligned in a fixed y-axis direction on the channel surface. This property is called spin-momentum locking. Anisotropic magnetoresistance determines the resistance of the channel according to the angle between the current and magnetization. When the current and magnetization are perpendicular, the scattering probability of electrons is low, which lowers the resistance, and when the current and magnetization are parallel, the scattering probability increases and the resistance increases. In the TI/FM hybrid structure, the magnetization reversal process was quantified through the effect of this surface characteristic on the magnetization of a ferromagnet having an easy-axis in the in-plane direction. Second, the intrinsic properties and local modulation of Fe3GeTe2 were confirmed through anomalous Hall effect. Two-dimensional magnetic material has the following advantages. It is possible to separate the layers in the atomic scale and maintains the magnetic properties in the monolayer. Layers can be stacked easily without considering properties such as lattice miss-match. And it reacts sensitively to various external factors that can change the magnetic properties. (i.e. magnetic/electric field, strain, spin-torque, etc.) Fe3GeTe2 has a high TC of 200 K, a metal characteristic that is easy to apply to a spin device, and a large perpendicular magnetic anisotropy. In a ferromagnetic, spin has an easy-axis that is stable and a hard-axis that is unstable depending on magnetic anisotropy. Perpendicular magnetic anisotropy refers to a state where it is comfortable for the spin to exist in the vertical direction of the channel. In general, as the thickness decreases, the in-plane state corresponds to the easy-axis. Fe3GeTe2 has a large perpendicular magnetic anisotropy despite being a very thin two-dimensional material. Anomalous Hall effect, a kind of Hall effect, refers to spin polarization by magnetization inside a ferromagnetic without an external magnetic field. When the channel has magnetization in z-axis direction, the spin moves along the y-axis of the channel, and the spin in the same direction as the magnetization becomes the majority and accumulates more. This accumulation difference is measured as Hall voltage. Through this, the magnetic anisotropy field, one of the magnetic properties, was derived from two-dimensional ferromagnet for the first time. The correlation between bulk effect and interfacial effect of Fe3GeTe2 was confirmed, and the surface was partially controlled through a heterostructure with other two-dimensional materials. Finally, the effect of the stray field of Fe3O4 nanoparticles located on the surface on the semiconductor channel was confirmed by ordinary magnetoresistance. A bio-magnetoresistive sensor for detecting liver cancer cells sensitive to magnetoresistance changes based on indium antimonide semiconductor material with a narrow bandgap and high mobility has been developed. It is a good means to give efficiency to the complicated and time-consuming liver cancer diagnosis and to detect liver cancer biomarkers present in very small amounts in the blood. Ordinary magnetoresistance increases the resistance by affecting the channel current path by Lorentz force. When an external magnetic field perpendicular to the channel is applied through the antibody-antigen-antibody-NP complex attached to the sensor surface, a stray field is strongly generated and the magnetoresistance is changed. Compared to the area where liver cancer tumors exist in the blood (about 20 ng/ml), it reacts sensitively to very low concentrations (< 1 pg/ml), and has the advantage of being selective for specific biomarker and reusable. In this research reveals several potential applications for nanomaterials and new technologies in demand today. Moving from the era of bulk to the era of interfaces, it presents the possibility as a new device, not just downsizing in thickness and size. Research on intrinsic properties of new materials is a valuable work that can help establish this new paradigm and contribute to real industrial development as well as academic research. Therefore, spintronics research using nanomaterials is expected to provide a new path for next-generation material and device research.

위상학적 절연체를 이용한 60 mV/decade 이하의 스위칭 특성을 가진 CMOS 반도체 소자 : Steep Switching Sub-60 mV/decade CMOS Devices using Topological Insulator

최현우 서울시립대학교 일반대학원 2017 국내석사

현재까지의 Complementary Metal Oxide Semiconductor (CMOS) 소자는 무어의 법칙(Moore’s Llaw)에 따라 소자의 크기를 줄이고 집적화를 높이면서 성능향상을 이뤄내는 기술 발전을 지속하였다. 하지만 소자의 구동전압을 충분히 낮추지 못하면서 그에 따른 소비 전력의 증가와 심각한 발열 문제로 기술적 한계에 봉착했다. “Boltzmann tyranny“ 라고 하는 이러한 물리적, 기술적 한계를 극복하기 위해 스팁 스위칭 소자 (Steep Switching device)의 필요성이 대두되었다. 본 연구는 이러한 스팁 스위칭 소자 중 NCFET과 유사한 방법으로 위상학적 절연체 (Toplogical insulator: TI)라는 새로운 물질을 활용, 음의 값을 갖는 양자 커패시턴스 효과를 이용하여 SS 소자를 구현하는 방법에 대해서 논의한다. TI가 기존의 다른 물질들과 구별 되면서 새로운 물질로서 큰 관심을 받은 이유는 물질의 표면은 금속과 같이 높은 전도성을 갖는 데에 반해 물질의 내부는 절연체와 같이 동작하는 것인데, 이는 물질 하나가 동시에 절연체와 전도체의 성질을 동시에 갖는 다는 것을 의미한다. 최근 연구에서는 물질 표면의 topological surface state (TSS)와 two-dimensional electron state (2DES)가 존재하는 사실이 알려지면서, 물질의 표면에 존재하는 2DES를 이용하여 양자 커패시턴스를 구현하려는 연구가 지속되고 있다. 양자 커패시턴스는 커패시터의 한 쪽 혹은 양 쪽 극판이 low density of state (DOS) system을 갖게 될 때 발생한다. 따라서 TI 물질인 Bi2Te3의 표면 상태를 이용하여 양자 커패시턴스를 관측할 가능성을 이론적으로 예측할 수 있고, 이를 실험을 통해 검증하였다. 또한, Bi2Te3를 MOSFET의 gate oxide 위에 증착하여 C-V 측정을 통해 MOS 커패시터 내에서 총 커패시턴의 증폭을 확인 할 수 있을 것이다. 커패시턴스 증폭 효과를 통해, 결과적으로 기존 보다 더 낮은 게이트 전압으로도 소자의 채널을 control하는 것이 이론적으로 가능하다는 것을 의미하며, 저전력 소자의 구현 가능성을 의미한다. In the past few decades, silicon-based CMOS technology has successfully developed by scaling its device size down. Since recently power density problem has been arisen, however, the technology faces the fundamental limit for scaling. For overcoming this obstacle, many different kinds of steep switching devices have been studied. Topological insulator (TI) is proposed to realize the steep switching devices as a promising material in this paper. The material has been reported to have exotic physical properties. The conducting two-dimensional surface state is one of the most interesting characteristic of TI. The negative quantum capacitance effect from its surface state can be used to boosts total capacitance in conventional MOSFET, and finally sub-60mV/decade devices can be accomplished. For confirming theses physical and electrical properties, Metal-TI-Oxide-Silicon capacitor (MTOSCAP) (i.e. MOSCAP which has TI as an insulator layer) is fabricated and shows positive or negative quantum capacitance in capacitance-voltage measurement at room temperature. Based on this experimental result, simulation is conducted to TI-MOSFET as a novel steep switching device structure. In this simulation, the calculated total capacitance of TI-MOSFET soars in subthreshold region due to the negative quantum capacitance effect. Furthermore, the subthreshold swing of the TI-MOSFET is also improved (i.e. SS ~ sub-60 mV/decade), and in theoretical method this result provides an opportunity that the TI-MOSFET can work as steep switching device.

Analysis of coupling effects from IEMI to multilayered PCBs in metallic enclosures

두진경 Graduate School, Yonsei University 2015 국내박사

The intentional electromagnetic interference (IEMI) penetrates into electric devices and makes malfunctions or destructions of them. To protect the electric devices against IEMI, metallic enclosures have been utilized, and the shielding effectiveness of the metallic enclosure has been studied actively. Even though many literatures report simulation and measurement techniques to probe the electromagnetic fields inside the metallic enclosure, it is not enough to predict coupling effects to a target PCB in the metallic enclosure. Moreover, the multilayered structure of the PCB should be considered because those structures have their own resonant characteristics which affect to coupling phenomena. Hence, in this dissertation, radiated coupling effects from the IEMI to a multilayered PCB in the metallic enclosure are studied. Based on the coupling mechanism in free-space, the coupling effects in the metallic enclosure are revealed, and a PCB and aperture placement guide is proposed for further reduction of the radiated coupling effects. The established coupling mechanism in the metallic enclosure is also applied to analyze coupling phenomena in several metallic waveguides which include multilayered PCBs and apertures inside. Calculated results support the coupling mechanism reasonably. The IEMI is handled as an incident plane wave, and it is assumed that the electric field intensity around the target PCB is 1 V/m. In free-space, the IEMI excites resonant modes of a multilayered PCB, and the resonant modes are differently generated in accordance with the PCB orientations. The effects of PCB resonances are appeared as coupling peaks in the frequency domain, and the maximum induced voltage is -35 dBV when a target trace includes a via-hole connecting different signal layers. The smallest coupling level is resulted when a normal vector of the PCB surface, a field polarization vector of the incident plane wave, and a wave vector indicating a direction of the propagating waves are perpendicular to each other. The smallest voltage level induced to traces is -60 dBV, regardless of the via-hole existence. Simulated and measured results considerably support the established coupling mechanism. In the metallic enclosure including a multilayered PCB, as well as an aperture, the coupling effects are also influenced from resonant modes of the entire structure, resulting peaks in the frequency domain. Below the fundamental frequency of the metallic enclosure, coupling peaks are dominantly generated by the PCB resonances, which are predictable. Whereas, above the fundamental frequency, peak frequencies are densely appeared thus they are mostly unpredictable. The maximum peak level is around -35 dBV, but it can be suppressed to -47 dBV by adjusting the placement of PCB and aperture. To reduce the coupling effects in the metallic enclosure, the placement of PCB and aperture should be handled properly, thus a useful guide is proposed in this work. The established coupling mechanism in the metallic enclosure is also utilized to analyze missile-type structures which are simplified to rectangular waveguides. The coupling effects from the IEMI to multilayered PCBs in metallic waveguide structures are investigated using the full-wave method, as well as the EMT method. For analysis using the EMT method, the modified BLT equation is proposed, and full-wave analysis data for partial structures are combined with the modified BLT equation to handle a multi-scale problem. Constructing of a topological network for waveguide structures is demonstrated considering propagating modes of the waveguide. The calculated results by the full-wave analysis, as well as the modified BLT equation, considerably support the coupling mechanism in the metallic enclosure, and especially, PCB resonant modes are obviously appeared below the fundamental frequency of the metallic enclosure. Considering the coupling mechanism investigated in this dissertation, the radiated coupling effects from the IEMI to a multilayered PCB in the metallic enclosure can be suppressed further by placing the PCB and aperture properly. It will be also helpful to predict the coupling phenomena in other metallic structures including multilayered PCBs, effectively.

Grating structure design using topology optimization at various wavelengths

임동열 Graduate School, Yonsei University 2014 국내석사

Topology optimization is regarded as great method to find optimal shape by many designers of various fields because of its design freedom. Topology optimization can be applied on many various problems such as compliant, elastic, heat and electromagnetic field problems and it can be extended wave propagation problem. The propagating of electromagnetic waves through a single sub-wavelength aperture has been studied for many years to enhance the transmittance or control the direction of radiated beam. Recently, the grating structure is regarded as one of good ways to handle this problem because of characteristic of grating structure. Grating structure positioned at the outlet of radiated light can generate the surface plasmon effect. Therefore appropriate shape of grating structure can be used as the outlet of radiated light. In this study, waveguide at infrared wavelength with asymmetric grating structures is proposed to guide the direction of radiated beam through an aperture using topology optimization. Contrary to traditional optimization, the optimization method based on the phase field method combined with reaction diffusion equation and double well potential functions calculates the sensitivity only at the boundaries not the sensitivity at all design domain. Because of this characteristic, optimization based on the phase field method is adopted in this study to obtain clear shapes. The design objective is not only maximizing the poynting vector at the measuring domain but also finding a clear and simple shape of grating structure for the manufacturing feasibility. In results, dielectric grating structures asymmetrically arranged on the metal slit surface are proposed to diffract the exit beam to a desired direction and their optimal shapes are obtained through the topology optimization process. For proving effectiveness of topology optimization in nano-scale and accuracy of results, experiment is needed. However waveguide at infrared wavelength is too small to make grating structure with optimal shape. Therefore, newly designing the grating structures at radio frequency wavelength using same optimization method is suggested to make an experiment. The simulation and optimization process has been performed by finite element analysis using the commercial package COMSOL associated with the Matlab programming

Investigating the Anomalous Thermal and Electrical Transport Phenomena in YbMnBi2 and Indium-Doped (Pb,Sn)Te Alloys

Wen, Jiamin The Ohio State University ProQuest Dissertations & 2024 해외박사(DDOD)

소속기관이 구독 중이 아닌 경우 오후 4시부터 익일 오전 9시까지 원문보기가 가능합니다.

This dissertation will center around the discussion of the investigation into the anomalous thermal and electrical transport phenomena in magnetic Weyl semimetal, YbMnBi2, as well as the characterization of its magnetization behavior. A theory-based experimental search for a new type of chiral anomaly in promising materials will also be covered.1. Thermoelectrics (TEs) are solid-state devices that can realize heat-electricity conversion. Transverse TEs require materials with a large Nernst effect, which typically requires a strong applied magnetic field. However, topological materials with magnetic order offer an alternative pathway for achieving large Nernst via the anomalous Hall effect and the accompanying anomalous Nernst effect (ANE) that arise from band topology. Here, we show that YbMnBi2 with a low Hall density and a chemical potential near the Weyl points has the highest ANE-dominated Nernst thermopower of any magnetic materials, Syx around 110 μV/K-1 (T = 254 K, 5 T ? |μ0H| ? 9 T applied along the spin canting direction), due to the synergism between classical contributions from filled electron bands, large Hall conductivity of topological origin, and large resistivity anisotropy. In addition, an appreciable thermal Hall angle of 0.02 < ∇yT/∇xT (-9 T) < 0.06 was observed (40 K < T < 310 K).2. How exactly the magnetization of YbMnBi2 changes with temperature and magnetic field remains indeterminate. Mysteries exist in the previous reports. Herein, through extensive magnetization characterization at various conditions, it was found that the magnetization behavior of YbMnBi2 showcases shared features in many aspects among multiple crystals in spite of a few sample-dependent details. The findings here hint at a more complex picture of the magnetic structure than what is currently known. This project hopefully can provide a foundation for future studies on thoroughly characterizing the magnetization behavior of YbMnBi2.3. Chiral anomaly, a signature of Weyl semimetal (WSM) phase, shows potential to efficiently modulate thermal or electrical transport in the device level, which normally requires an external magnetic field. Recently, indium-doped (Pb,Sn)Te alloys have been demonstrated to host giant Berry curvature dipoles in the WSM phase, giving rise to nonlinear Hall effect without the presence of magnetic field and magnetization. In this project, we present theory-based experimental search for a new type of chiral anomaly that is based on non-zero Berry curvature dipole. One signature of this new chiral anomaly is that in the absence of magnetic field and magnetization thermal conductivity exhibits anomalous changes with external electric field such that these variations are odd functions of E-field and proportional to it at a given temperature. In (Pb0.59Sn0.41)0.97In0.03Te single crystal, we observed a linear relationship between imposed electric field Ez along the polar axis and antisymmetric components of thermal conductivity κxx in the plane normal to z. The documented thermal conductivity behaviors with E-field in our experiments approximate theoretical predictions. This new type of chiral anomaly manifested in indium-doped (Pb,Sn)Te alloys unveils its potential for engineering a voltage-driven solid-state heat switch independent from magnetic field.