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.

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.

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.

Disorder Effects in Quantum Materials

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

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.

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

Manipulation and study of emergent phenomena in oxide ultrathin films

김동한 서울대학교 대학원 2024 국내박사

본 논문에서는 ABO3 페로브스카이트 구조를 갖는 루테네이트와 이리데이트의 초박막 시스템에서 나타나는 새로운 현상들을 연구한다. 여러 흥미로운 현상이 활발히 보고되고 있는 루테네이트의 경우, 다양한 접근법을 통해 발현 현상을 조절하는 데 중점을 두었다. 한편, 이리데이트에 대한 연구는 전자 구조적 관점에서 초박막에서 나타나는 새로운 현상에 초점을 맞추었다. 이 논문의 결과 섹션의 전반부에서는 SRO 초박막의 수송 현상 조작에 대해 다룬다. SRO의 순회 강자성은 초박막 두께 조건에서 부호가 변하는 변칙 홀 효과 (AHE), 홀 저항의 혹 구조, 부호가 변하는 일반 홀 효과 등 다양한 수송 현상을 유도한다. 본 연구에서는 초박막 SRO 필름의 자기 수송 현상을 이해하기 위해 초박막의 반전 대칭성 깨짐 (ISB) 정도를 조절하여 홀 저항의 AHE와 혹 구조를 제어한다. ISB를 제어하기 위한 방법으로 캡핑층 증착과 전기장 게이팅을 사용했다. 특히 초박막 금속 박막의 게이팅 실험에 효과적인 이온성 액체 게이팅 방법을 사용하여 SRO 박막에 전기장을 인가하였다. 다음으로 SRO 초박막의 또 다른 수송 특성인 일반 홀 효과의 부호 변화를 탐구하였다. 이 효과는 SRO에서 나타나는 2차원 (2D) 반 호브 특이점 (VHS)의 전기적 제어를 통해 연구하였다. 2D VHS는 SRO를 포함한 스트론튬 루테네이트의 전자 구조의 핵심 특성으로, SRO의 순회 강자성 및 Sr2RuO4의 초전도성과 관련하여 발산 상태 밀도 (DOS)로 작용하는 것으로 이해되고 있다. 우리는 수송 측정에서 이온성 액체 게이팅 실험과 함께 알칼리 금속 도징에 의한 표면 전자 도핑으로 초박막 SRO 필름의 2D VHS를 조작하여 ARPES 측정을 수행하였다. 결과 섹션 후반부의 초박막 SIO 필름 연구에서는 주로 전자 구조의 변화와 숨겨진 현상들을 연구하는 데 중점을 두었다. 페로브스카이트 이리데이트는 큰 스핀-궤도 결합 (SOC)으로 인해 페르미 준위 근처에서 단일 Jeff = 1/2 상태의 비교적 복잡하지 않은 전자 구조를 보이기 때문에 초박막 SIO 필름에서 치수 감소가 효과적이다. 본 논문에서는 상자성 준금속 SIO가 두 단위셀 (uc)과 1 uc의 두께 사이에서 금속에서 절연체로 전이되는 것을 ARPES의 직접 전자구조 측정을 통해 보였다. 금속-절연체 전이의 이론적 타당성을 동적 평균장 이론 (DMFT) 계산을 통해 논의하고, 압축 변형에 의해 금속 단층을 얻을 수 있음을 본 논문에서 ARPES 측정을 통해 실험적으로도 증명하였다. 또한, 온도 의존적 실험과 함께 금속 SIO 단층에 대한 정밀한 ARPES 측정은 단층 SIO를 포함한 초박막 SIO 필름이 전자 구조 관점에서 평면 내 반자성 질서를 나타낸다는 것을 보여준다. 단층 SIO 필름에서 관찰된 전자 구조는 DFT+SOC+U 계산을 통해 조사되었으며, 평면 내 반자기 질서가 온도에 따른 ARPES 측정 결과를 가장 잘 설명할 수 있음을 입증했다. 평면 내 반자성 금속 단층 SIO 박막에 대한 이 연구는 스트론튬 이리듐 연구의 오랜 목표인 Jeff = 1/2 초전도 실현에 돌파구를 마련하는 데 도움이 될 수 있다. The field of ultrathin film is an emerging area of oxide materials research. In the works in this thesis, we study emergent phenomena within ultrathin film systems of ruthenate and iridate with ABO3 perovskite structure. For ruthenate, where a number of interesting phenomena have been actively reported, our emphasis is on manipulating emergent phenomena through various approaches. Meanwhile, our investigation of iridates focuses on emergent phenomena in ultrathin films from an electronic structure perspective. The first half of the results section in this thesis is dedicated to manipulating transport phenomena in ultrathin SRO films. The itinerant ferromagnetism of SRO gives rise to a variety of transport phenomena, including the sign-changing anomalous Hall effect (AHE), the presence of a hump structure in Hall resistivity, and the occurrence of a sign-changing ordinary Hall effect under ultrathin thickness conditions. To understand these magnetic transport phenomena observed in ultrathin SRO films, we control the AHE and the hump structure in Hall resistivity by manipulating the degree of inversion symmetry breaking (ISB) in the ultrathin film. Two methods, namely capping layer deposition and electric field gating, are employed to control ISB. Specifically, we apply an electric field to the SRO film using the ionic liquid gating method, known for its effectiveness in gating experiments on ultrathin metallic films. Next, we study another transport property of ultrathin SRO films - the sign-changing ordinary Hall effect. This is investigated through the electric control of the two-dimensional (2D) Van Hove singularity (VHS) in oxide ultrathin films. The 2D VHS stands out as a crucial characteristic of the electronic structure of strontium ruthenate, including SRO. It is recognized as a divergent density of states (DOS) associated with the itinerant ferromagnetism of SRO and the superconductivity of Sr2RuO4. We manipulate the 2D VHS of ultrathin SRO films through surface electron doping via alkali metal dosing in angle-resolved photoemission spectroscopy (ARPES) measurements and conduct ionic liquid gating experiments in transport measurements. In the second half of the results section, we focus on ultrathin SIO films to explore the evolution of the electronic structure and hidden emergent phenomena. Thickness reduction proves to be particularly effective in ultrathin SIO films, as perovskite iridates exhibit a relatively straightforward electronic structure characterized by a single Jeff = 1/2 state near the Fermi level due to substantial spin-orbit coupling (SOC). Through direct electronic structure measurements via angle-resolved photoemission spectroscopy (ARPES), we show that the paramagnetic semi-metal SIO undergoes a metal-insulator transition between a thickness of 2 unit-cells (uc) and 1 uc. The theoretical feasibility of this transition is discussed using dynamical mean field theory (DMFT) calculations, demonstrating that a metallic monolayer can be achieved under compressive strain. This is also experimentally validated through ARPES measurements in this thesis. Furthermore, precise ARPES measurements on metallic SIO monolayer, along with temperature-dependent experiments, reveal that ultrathin SIO films, including monolayer SIO, exhibit in-plane antiferromagnetic order from an electronic structure perspective. The observed electronic structure in the monolayer SIO film is scrutinized using density functional theory (DFT)+SOC+U calcula- tions, demonstrating that in-plane antiferromagnetic ordering best explains the temperature-dependent ARPES measurement results. This investigation of in-plane antiferromagnetic metal monolayer SIO films holds the potential to contribute significantly to realizing Jeff = 1/2 superconductivity, a longstanding goal in strontium iridate research.

Shape optimization of the silver nanoparticle for improving efficiency of thin film solar cells

변수환 Graduate School, Yonsei University 2014 국내석사

Low transfer efficiency of thin film solar cells is one of the reasons why the use of thin film solar cells is restricted in practical area. Therefore, there are many ideas to solve low transfer efficiency problem. For example, Nano-scale structure added on incident layer can increase time which incident light stay in absorbing layer and modified structure of reflecting plate reduced interference with incident light and reflected light. One of these various ways is using surface plasmon effect. Surface plasmon effect is enhancement phenomena caused by interaction between metal nanoparticle and dielectric materials and makes strong electromagnetic field near metal nanoparticle. Therefore the strong electromagnetic field caused by surface plasmon effect can increase energy which thin film solar cell can absorb so that the transfer efficiency of thin film solar cells can be improved. The important factor of incrementing surface plasmon effect caused by metal nanoparticle is shape of metal nanoparticle since optical properties of metal nanoparticles are determined by its shape. If shape of metal nanoparticles is designed by using optimization method, thin film solar cells can absorb more light than before. In this case, the appropriate optimization method is level-set method. Level-set method is one of topology optimization methods which defined boundaries of structure as design domain so that the optimal design can avoid traditional weakness of topology optimization method such as intermediated density problem and checkerboard pattern problem. Specifically, level-set method based on phase field function updated by reaction and diffusion equation is suitable level-set method for this case since it can control complexity of optimal shape and have a less computational load. In this study, the optimal shape of silver nanoparticles for improving transfer efficiency of thin film solar cells was proposed by using level-set method based on phase field function.

Satellite Network Design with High Frequency and Optical Communication

Yonghwa Lee DGIST 2022 국내박사

종래의 위성 통신은 지상 통신시스템을 보조하는 역할을 주로 수행해 오며 매우 작은 전송 용량의 서비스를 제공해왔다. 그러나 최근 5G 상용화와 B5G/6G 기술 개발이 복격화되며 위성 통신이 새롭게 주목 받고 있으나 아직 지상 통신 시스템과 같이 원활한 광대역 통신 서비스 제공에는 어려움이 있어 많은 도전과제들을 해결해야하는 상황이다. 따라서 본 논문에서는 정지궤도 위성 및 저궤도 위성 통신 시스템에서 신뢰성 있고 원활한 서비스 제공을 위한 네트워크 시스템 성능 분석을 수행하고 앞으로 새로운 위성 통신 네트워크 설계를 위한 초석을 마련하고 나아가야할 방향을 제시한다. 첫 번째는 고주파 대역을 사용하는 정지궤도 위성통신에서의 이동통신서비스에 관한 분석이다. 지상 시스템 기술의 발전에 따라 위성통신에서도 이동통신 수요가 증가하고 있으며, 기존의 저주파 대역이 포화상태에 근접함에 따라 고주파 혹은 초고주파대역을 사용해야하는 상황에 놓여 있다. 고주파 대역에서의 이동통신 서비스는 매우 큰 성능 저하를 초래하므로 이를 타개하고 이동위성서비스의 제공 가능성을 확인하기 위해 지상으로부터의 피드백 채널 정보를 활용하여 전송 전력을 제어하는 시스템을 제안하였다. 두 번째는 새로운 저궤도 위성 토폴로지에서 위성간 연결성에 대한 분석이다. 최근 수 백 혹은 수천 이상의 위성이 배치된 콘스텔레이션 모델의 등장으로 위성 네트워크의 시스템 모델을 재정의해야 한다. 그에 따라 증가된 위성, 저궤도 위성의 시변-위성 토폴로지 특성과 함게 광통신 시스템의 정확한 링크 연결에 대한 요구사항을 고려하여 위성 간 가시성 분석을 수행하였고, 메가 콘스텔레이션에 적합한 위성 가시성 매트릭스 모델 및 설게 방식을 제안하였다. 세 번째는 위성 간 통신에서 광 통신 시스템을 활용하는 경우 종단간 라우팅에 대한 분석이다. 증가된 메가 콘스텔레이션 모델에서 더 효율적이고 빠른 신호 전송을 위해 광통신 시스템을 활용할 때 발생할 수 있는 지연 모델을 설계하였다. 실제 시스템에서 데이터 전송 모델에 고려되어 정확한 성능이 평가될 필요가 있음을 확인하였다. 또한, 위성 간 광통신 시스템의 장점 분석을 위해 패킷 정송, 핸드오버 전략 및 통신 방식(라디오/광학)에 따른 종단간 라우팅에서의 지연 성능을 라디오 주파수 대역 시스템과 비교/평가하였다. 본 연구는 그 동안 정지된 시스템에 서비스를 제공하던 위성통신에서 이동통신 시스템의 가능성을 확인하였고 저고도 시스템의 잠재역도 함께 확인할 수 있었다. 또한 새로운 저궤도 위성통신 네트워크에서의 효율적인 테이터 전송을 위한 시스템 설계와 더불어 그로 인해 발생할 수 있는 문제점에 대한 고려를 통해 더욱 현실적인 환경에서의 시스템 성능 평가로 제안한 방식들이 실제 시스템 개발 및 사용화에 적용 및 실현될 수 있을 것 이다. Satellite communications are a type of wireless communication system that use geostationary orbit (GEO), medium-Earth orbit (MEO), or low-Earth orbit (LEO) satellites to provide an inter- continental data transmission and maritime or aerial communication services. In particular, because satellite communications are not affected by network damage from natural disasters (tolerance capability), these systems can provide temporary networks in emergency regions where ground infrastructure is paralyzed. However, GEO satellite-based networks, which are mainly employed in conventional satellite communications, have chronic problems such as extremely long delays and high costs. To overcome these difficulties, high throughput satellite (HTS) systems have been developed that can support a performance generally 20-times higher or more than those of traditional systems. In addition, GEO-based mobile communications are being studied along with the development of terrestrial mobile technologies. Despite the disadvantages of GEO satellite communications, satellite communications are currently receiving significant attention and will become a core technology for a three- dimensional communication design according to the 5G 3GPP non-terrestrial network (NTN) and the 6G standards in the near future. Therefore, in this study, 1) GEO satellite mobile services with high throughput satellite (HTS) technology and 2) a satellite network design with optical inter-satellite communication were researched. In a study of GEO satellite-based mobile services, there is an influential issue of a carrier frequency shift above 20 GHz owing to saturation of the lower frequency bands. Previously, the frequency bands from the L (1–2 GHz) to C (4–8 GHz) band have mainly been adopted; however, it is now necessary to provide services with high-frequency bands above the Ka band (20 GHz). Since satellites are located at very high altitudes, they are sensitive to weather conditions as the carrier frequency increases. Accordingly, we propose a satellite transmission power control method that can assist stable mobile satellite communication using the channel state information (CSI) of a ground-satellite link with HTS system. Two channel models are then applied to scrutinize the system performance according to channel variations, which are a Lutz’s land mobile satellite (LMS) channel as a representative satellite channel model and Nakagami-m fading as one of the statistical models. The proposed transmission power control approach is then evaluated for various user speeds at various altitudes of the MEO, LEO satellites and a high-altitude platform (HAP) as well as GEO satellite. As mentioned above, satellite systems are attracting attention as one of the key technologies in 5G and 6G communications. LEO satellites have recently emerged as an alternative system that can complement the chronic problems of GEO satellites. For improved satellite communications, companies such as SpaceX, Amazon, and Blue Origin have developed a novel LEO satellite topology for broadband Internet services, which is a mega-constellation. The main purpose of a mega-constellation is to provide stable telecommunication services that deploy an enormous number of satellites. Owing to the shortened link distances between satellites resulting from their increased number, there have been many attempts to arrange the optical inter-satellite links (ISLs). We therefore propose a novel satellite visibility matrix and routing algorithm with optical ISLs when considering the time- varying characteristics of the LEO satellite topology. The visibility matrix was developed by analyzing the relative position changes of the neighboring satellites. In addition, the antenna movement angle between visible satellites was also investigated for strict optical antenna constraints owing to the very narrow beam divergence. The system performance was then evaluated through a comparison with the visibility matrix in prior studies in terms of the average end-to-end link distance and hop counts. As a result, we can verify that the proposed visibility matrix outperformed the previous study by 75.45 % in terms of the end-to-end link distance and 52.85% in terms of the hop counts in the mega-constellation architecture. Dynamic routing path discovery is required depending on the satellite topology variation caused by the movement of the LEO satellites. However, optical communications have delay overheads in the link establishment procedure due to the narrow beam divergence, and we propose a routing algorithm applied in a changing topology environment with a packet trans- mission and optical ISL. The algorithm utilizes an iterative path searching method to respond to a time-varying satellite topology. We evaluated the system performance by comparing the satellite net- works with RF and optical ISL environments in terms of the average hop count, end-to-end delay, and number of iterations in a route discovery.