Improving phenomenological analysis using probabilistic neural networks : demonstration on phenomenological R-matrix

Kim, Chanhee Sungkyunkwan University 2024 국내박사

Phenomenological models are essential in modern physics research, as they can extract physical properties from measurements and predict phenomena that have never been measured. However, due to the numerous and various parameters in the model, the phenomenological analysis typically requires dedicated time and extensive effort. Additionally, some parameters may not correspond to observable features in measurements, which complicates the analysis further if there are no strict physics rules to constrain these parameters. Deep learning is currently one of the most attractive techniques in any science field because of its high capacity and vast capabilities. However, its reliability could be questionable, as it uses deep neural networks that are challenging to interpret and, therefore, often called the ``black box.'' Conventional neural networks typically give only singular predictions without uncertainty quantification. On the other hand, probabilistic neural networks, such as Bayesian neural networks, offer expressive outputs with rigorous uncertainty quantification. The calibration of uncertainty can also be explicitly tested using a subset of data samples. In this study, an analysis method using probabilistic neural networks was developed to improve the phenomenological analysis. This method can give an attractive option on top of the existing optimization process, such as χ2 minimization and Bayesian inference. Additionally, it shows how to bypass the determination of the subset of parameters that are not of primary interest and complicate the analysis. The demonstration of this method was done on the phenomenological R-matrix that has been widely used in nuclear physics. Prior to the main study, this thesis presents a fruitful discussion on phenomenological models in physics regarding their definition, conventional optimization methods, representative phenomenological models, and so on. Details of machine learning and deep learning are also introduced, including fundamentals, statistical and mathematical groundings, uncertainty quantification, examples of applications in physics research, discussion on reliability, and so on. Then, details of the proposed method, its demonstration on the phenomenological R-matrix, and a discussion on the capability and limitations of the method are presented. 현대 물리 연구에서 현상론적 모델은 실험 데이터를 이용해 측정되지 못했던 현상과 주요 물리 특성을 예측하는 역할을 하기에 필수적이다. 하지만, 모델에 있는 많고 다양한 매개 변수들로 인해 현상론적 분석은 일반적으로 많은 시간과 헌신적인 노력이 필요하다. 또한, 몇 매개 변수들은 실험 데이터에서 볼 수 있는 측정 가능한 특징과는 관련이 없는 경우도 있고, 이들의 값을 정하는 특별한 물리 규칙이 없는 경우도 있기에 분석은 더욱 힘들다. 딥러닝은 높은 수용력과 광범위한 능력 때문에 현재 어떤 과학 분야에서든 가장 매력적인 기술 중 하나이다. 하지만, 검은 상자라고도 불리는 해석하기 힘든 심층 신경망 때문에 딥러닝의 신뢰성에 의문을 품게 되는 경우가 많다. 일반적인 신경망은 불확정성 계산 없이 단수의 예측만을 한다. 반면에, 베이지안 신경망과 같은 확률적 신경망은 불확정성 계산과 함께 표현력 있는 예측이 가능하다. 또한, 데이터 표본들의 일부를 통해 불확정성의 정밀도도 분명하게 테스트할 수 있다. 이 연구에서는 확률적 신경망을 이용해 현상론적 분석을 하는 방법을 개발하였다. 이것은 기존의 방법인 χ2 최소화와 베이지안 추론에 더불어 매력적인 옵션을 제공한다. 추가로, 이 방법을 통해 일부 중요하지 않거나 분석을 방해하는 매개 변수들의 결정을 우회할 수 있다. 이 방법을 핵물리에서 널리 쓰이는 현상론적 R-매트릭스에 시연했다. 이 학위논문에서는, 주요 연구 설명에 앞서, 물리에서 현상론적 모델들의 정의, 일반적인 최적화 방법, 대표적인 모델 등에 대해 토의한다. 이후 머신러닝과 딥러닝의 기초, 통계적이고 수학적인 근거, 불확정성 계산, 응용의 예시, 신뢰성에 대한 토의 등을 구체적으로 소개한다. 마지막으로, 이 연구에서 제시하는 방법을 구체화하고 현상론적R-매트릭스에 시연, 방법의 능력 및 한계에 대해 토의한다.

Generalized quantum measurement and its applications in quantum information

이종찬 Pohang University of Science and Technology 2015 국내박사

Quantum physics, which began in the early 20th century, has revolutionized the way we understand the world. Blessing of quantum physical notions made our understanding of nature more complete in atomic electronic structure, photoelectric effect, periodic table, chemical interaction, electronic semiconductor physics, and many more. Although the quantum physics and the following semiconductor physics has driven the computer chip industry and the information age, most of the conventional modern technologies are based on or explained by classical physics. Recently, however, attempts to directly utilize quantum physical properties for information technology gave birth to a newly emerging field of quantum information science. Quantum measurement is not only at the heart of deep questions in quantum physics, but also plays an essential role in realizing quantum information science. In this thesis, many aspects of generalized quantum measurement and some other quantum resources for the application in quantum information are studied. The biggest obstacle in realizing quantum information science to a practical level is decoherence, i.e. environment induced loss of quantum coherence. Here, I experimentally study a method to suppress decoherence using weak quantum measurements. Quantum measurements are typically understood as destructive to quantum coherence. By carefully designing a protocol with a more generalized form of quantum measurement, however, the decoherence could be effectively tackled. Decoherence is also detrimental to quantum entanglement, which is an essential resource to many quantum information protocols. In some cases, the entanglement can be completely lost due to decoherence: the phenomenon called entanglement sudden death. It is experimentally demonstrated in the thesis that the entanglement loss and sudden death can be circumvented with weak measurements. Next, delayed-choice decoherence suppression scheme is proposed and experimentally demonstrated. Using entangled pair of qubits, it is possible to postpone the decision to suppress decoherence even after the decoherence itself. This protocol can be an important addition to methods for suppressing Markovian noise. Quantum measurement in the position and momentum variables are also studied. Specifically, I propose and analyze the method to probe the commutation relation of position and momentum operators with interference. One of the most important photonic quantum resource is the quantum memory. For fully utilizing spatial multimode capacity of quantum memories, spatial quantum coherence should be preserved during write-read process. In this thesis, the preservation of spatial coherence of an optical pulse in an atomic vapor quantum memory is studied. Additionally, the methods to prepare photonic quantum resources, such as quantum light source and atom-photon interface, are discussed in the text. The effect of pump focusing in spontaneous parametric down-conversion is studied in detail.

Manipulation of physical properties in oxide thin films by a local inversion symmetry breaking induced by flexoelectricity

박성민 Seoul National University 2019 국내박사

대칭의 개념은 물리학에서 놀라운 역할을 가지고 있다. 물질이 가지고 있는 대칭의 종류에 따라서 물질이 가질 수 있는 물리적 질서나 특성이 결정된다. 결정 대칭성에 따라서 고체는 잘 알려진 결정질 점군으로 분류될 수 있는데, 1830년에 32가지 유형의 형태학적 결정군으로 분류될 수 있다는 것이 증명되었다. 어떤 결정군에 속해있는지를 통해 복굴절과 같은 광학성 특성이나 압전성과 같은 전기기계적 결합이 나타날 수 있는지 여부를 알 수 있다. 전기적 특성과 기계적 특성 사이의 상관성은 매우 흥미로운 물리적 현상이며, 미시적인 크기의 전자기계부터 압전 엑츄에이터, 모터, 센서, 에너지 발전기, 전동기 단백질 및 세포막과 같은 생물 시스템에 이르기까지 다양한 분야에서 사용되고 있다. 전기 기계적 상관성의 가장 보편적인 예는 균일한 변형이 가해질 때 전기장이 물질 내부에 생성되는 압전성이다. 반면에, 불균일한 변형이 물질에 가해지는 경우에도 물질 내부에 전기장이 형성되게 되는데 이를 변전효과라고 한다. 변전효과는 전기적 분극과 불균일한 변형 사이의 상관관계로 정의될 수 있다. 이 현상의 중요성으로서 변전효과가 압전효과보다 더 보편적으로 나타난다. 즉, 더 많은 결정에서 나타날 수 있는 효과이다. 압전성은 32개중에 20개의 결정군에서 나타날 수 있는 반면, 변전성은 32개 모두의 결정군에서 나타날 수 있는 현상이다. 또한 변전효과는 물질의 크기가 작아지면 작아질수록 효과의 크기가 커진다는 재미있는 특성을 가지고 있다. 이러한 특성 때문에 압전효과가 사용되는 장치를 대체할 수 있는 효과로서 매우 큰 물리적 산업적 중요성을 가지고 있다. 본 박사논문에서는 변전효과에 의해 유도되는 반전 대칭 깨트림을 이용하여 산화물 박막의 물리적 특성을 제어하는 것이 가능하다는 것을 보여줄 것이다. 특히, 국소적인 영역에 날카로운 원자힘 현미경의 탐침을 이용하여 압력을 가하는 기술을 사용함으로써 원하는 영역에만 국소적으로 물리적 특성의 변화를 야기해낸 일들을 소개할 것이다. 나의 연구들은 산화물 박막에서 순수한 힘만을 가지고 흥미로운 물리적 현상 또는 특성을 제어하고 관찰하는 것이 가능하다는 것을 시사한다. Over the last decade, flexoelectric effect at the nanoscale in solid has shown their wide potential both scientifically and technologically. There are already some patents for commercial electronic devices to replace piezoelectric devices such as sensor, and actuator. Also, they have shown rich and intriguing physical phenomena, including the recent hot topic of domain wall and photovoltaic. Despite these extensive studies on flexoelectric effect, I believe that many interesting issues still remain untouched. In this thesis, two novel findings has been addressed: trailing flexoelectric field for manipulation of multiaxial ferroelectric, control of the electrical state in a dielectric with flexoelectric origin. Firstly, I have demonstrated that in multiaxial ferroelectric epitaxial films, the trailing flexoelectric field generated by mobile AFM tip pressing can be used as an effective tool for engineering domain structures. We have suggested several advantageous features over the electrical way. Also, we have overcome the serious drawback of mechanical switching of ferroelectric polarization with AFM tip that is switching is unidirectional, i.e. only switching of polarization up to down is possible. I believe that the finding of this mechanism of trailing flexoelectric will open a great possibility to study exotic ferroelectric domain by creating it without applying electrical bias. Also, we demonstrated flexoelectric control of the electrical state in ultrathin dielectric films. We explained this huge resistivity change in terms of tunable depolarizaton field by controlling the amount of flexoelectric polarization generated inside of the ultrathin dielectric films. Achieving breakdown using very small electric bias, we could prevent extrinsic effect such as Joule heating and permanent damage on the sample even though we applied a huge electrostatic field by means of flexoelectricity. This work overcomes a long-standing dilemma: the electrical-state switching in dielectrics requires strong fields, but when applied by strong static fields, dielectrics inevitably suffer from irreversible damage. Utilizing universal flexoelectricity, we could develop a general approach to apply non-destructive, strong electrostatic fields in various insulating systems, such as the Mott insulator. Also, our results can be extended to realize future novel devices such as flexoelectric switch and transistor. Lastly, there can be many interesting topics we can further study with this AFM tip pressing technique. Since force applied by AFM tip will generate local strain gradients, thus breaking of local inversion symmetry, any change of physical properties or intriguing phenomena induced by the inversion symmetry breaking such as band gap opening in graphene or photovoltaic in centrosymmetric materials can be studied. As those studies would be original in the aspect that mechanism is novel flexoelectricity, it would provide good chances for high-impact publications. The concept of symmetry plays an incredible role in physics. What kinds of symmetries the material possess determine the physical orders or properties the material can have. Depending on crystal symmetries, solid can be classified into well-known crystallopic point group. In solids, there exist 32 types of morphological crystalline symmetries derived in 1830 from a consideration of observed crystal forms. The point group of a crystal determines the directional variation of physical properties that arise from its structure, including optical properties such as birefringency, or electromechanical coupling such as piezoelectricity. Couplings between electrical and mechanical properties are quite intriguing physical phenomena and have been used in many applications ranging from microelectromechanical systems to biological systems such as a piezoelectric actuator, motor, sensor, energy generator, electromotor proteins, and cellular membranes. The most popular example of electromechanical couplings is piezoelectricity in polar systems (where the space inversion symmetry is broken), in which the homogeneous strain can induce electric fields and vice versa. On the other hand, there can occur a novel electromechanical coupling known as flexoelectricity in the presence of a strain gradient (i.e., inhomogeneous strain). The flexoelectricity can be defined as a coupling between polarization and strain gradient. One important aspect of this phenomena is that the flexoelectricity is more universal phenomena than the piezoelectricity. While piezoelectricity can arise in 20 point groups, flexoelectricity can arise in all 32 point groups as the strain gradients itself spontaneously break the inversion symmetry. Other than that, flexoelectricity has another important aspect. Flexoelectricity becomes larger and larger as the scale being reduced since the strain gradient is inversely proportional to the size of the sample given that the applied strain is fixed. Because of those two advantages mentioned above, flexoelectricity can play an important role in both nanoscale physics and application as it might be possible to replace the devices in which the piezoelectricity is in use. In this thesis, I will show that it is possible to tune the physical properties of oxide thin films by breaking the space inversion symmetry induced by flexoelectricity. Specifically, I exploited the technique so-called atomic force microscope (AFM) tip pressing that is applying pressure using a sharp AFM tip to induce local strain gradients, thus, break the local inversion symmetry. With this technique, local control of physical properties of oxide thin films was possible and three works related to AFM tip pressing will be addressed. Our studies on oxide thin films suggest that many interesting control of local physical properties can be possible by means of pure mechanical force.

Exploring the surface of 2D semiconductors : observation on the electronic structure near the fermi level

김민주 Graduate School, Yonsei University 2019 국내박사

2004년 그래핀의 발견 이후로, 2차원 물질에 관한 연구는 매우 급속도로 성장하고 있다. 2차원 물질이 각광받는 이유는 과학자들에게는 새로운 물리현상이 발현되는 시스템을 제공하기 때문이고 공학자들에게는 기존의 전자 및 광전소자영역의 한계를 극복할 수 있는 잠재성이 있기 때문이다. 표면 물리관점에서, 2차원 반도체의 경우 표면(z축)방향으로 dangling bond가 존재하지 않기 때문에 3차원 반도체의 표면보다는 비교적 쉽다고 생각할 수 있다. 하지만, 실제로 나타나는 전자소자/물리학적 현상을 보면 절대로 쉽다고 생각할 수 없다. 단층 2차원 반도체는 기본적으로 무극성을 갖고 층과 층 사이의 상호작용이 매우 약하다. 이것은 전계 또는 도펀트에 의한 분극이나 다른 물질과의 화학반응 (강한 상호작용)에 의해 2차원 반도체 본연의 물리적 성질이 사라질 수 있다는 것을 의미한다. 뿐만 아니라 물질 자체의 원자 결함이나 결정 대칭의 뒤틀림에 의해서도 물리적 성질이 크게 달라질 수 있다. 반대로, 전계 효과나 화학반응, 원자 결함, 결정 뒤틀림을 통해 우리가 원하는 물리적 성질을 강화시킬 수도 있다. 즉, 우리가 2차원 물질 고유의 표면과 위에서 언급한 물리적 현상들을 완전히 이해한다면, 2차원 반도체의 전자소자 응용과 새로운 물리현상을 발전시킬 수 있는 이론적 기반을 제공할 수 있다. 이러한 관점에서, 우리는 흑린 (black phosphorus, BP)의 표면의 degradation과 전이금속 디칼코제나이드 (transition metal dichalcogendies, TMDCs)에 표면산화에 의한 홀 도핑 메커니즘에 주목하였다. 먼저, BP는 매우 높은 전하이동도를 갖으며, 층 의존적 직접천이 밴드갭을 갖는다. 즉, 표면의 전자구조가 물질의 층수에 따라 크게 달라진다는 것을 의미한다. 우리는 BP의 층 의존적 전자구조가 BP의 표면 변화와 매우 밀접한 관계를 가질 것이라고 예상하였다. 우리는 원자력 간 현미경을 이용하여 48시간 동안 BP의 표면의 형상과 일함수 변화를 측정하여, 두 층의 BP보다 벌크 BP에서 산소 및 수분 결합이 빠르다는 것을 관측하였다. 제일원리계산과 마커스 이론을 결합한 이론 모델을 통해 층 의존적 산화 속도는 내재적으로 BP의 층 의존 전자구조에서 기인한다는 것을 규명한다. 우리의 이론 모델은 전도대의 전자의 밀도에 따라 표면산화속도가 매우 달라지는 것 또한 증명하였고 이는 홀 도핑을 통해 흑린의 안정성을 증가시킬 수 있다는 것을 제시한다. TMDC의 경우, 칼코겐 결함에 의한 의도하지 않는 전자도핑이 표면에 존재한다. 이러한 의도하지 않는 전자도핑은 p-type 특성을 저하시키고 메탈과의 페르미레벨 피닝을 유도하여 소자의 전기적 특성을 크게 저하시킨다고 알려져 있다. 그래서 많은 연구자들은 의도하지 않는 n-doping을 억제 시키기 위해 다양한 표면 처리방법을 사용하고 있다. 이 중에서, 우리는 TMDC의 표면산화를 이용한 홀 도핑에 주목하였다. 우리는 표면산화를 진행시키면서 TMDC의 표면의 화학적 상태와 페르미레벨 근처의 에너지레벨의 변화를 광전자/역광전자 분광을 통해 분석하였다. TMDC에서 표면산화는 칼코겐과 산소 결합과 칼코겐-산소 치환 2가지 형태로 나타나는데, 칼코겐-산소 치환의 경우 TMDC의 홀 도핑 효과가 크게 나타나는 것이 측정되었다. 이것의 원인은 칼코겐이 산소로 치환되면, transition metal의 d 오비탈의 전자를 산소가 직접 가져오기 때문에 TMDC의 홀의 농도가 크게 증가하기 때문이다. 즉 TMDC의 캐리어 농도를 효율적으로 조절하려면 전이금속의 d 오비탈의 전자량을 조절하는 것이 중요하다. Since the discovery of graphene in 2004, research on two-dimensional (2D) materials has tremendously grown. The reason why 2D materials are spotlighted is that they have the potential to overcome the limitation of conventional electronic devices while providing a new system that induced new physical phenomena, such as valleytronics, dark exciton and metal-semiconductor phase transition. From the point of view of surface physics, it can be considered that the two-dimensional semiconductor is relatively easier than the surface of the three-dimensional semiconductor because there is no dangling bond in the surface (z-axis) direction. However, 2D semiconductor-based electronic and physical phenomena are very sensitive to the surface of 2D semiconductor. A single layer of 2D semiconductors has fundamentally non-polar surface and weak interaction (vander Waals force) between layers. This means that the intrinsic properties of 2D semiconductors can be deteriorated by polarization by electric field or chemical reaction (strong interaction) other materials. Besides, their inherent properties can be greatly changed by atomic defect itself or the distortion of crystal symmetry. Conversely, it can be enhanced intrinsic properties by controlling electric field, chemical reaction, atomic defects, and crystal distortion. In other words, if we fully understand the physical phenomena raised on the surface of 2D material and on the surface abovementioned, we can provide a fundamental for developing the electronic device application of and their physical phenomena. In this regard, we have noted the degradation phenomena on the surface of black phosphorus (BP) and the hole doping mechanism by surface oxidation in transition metal dichalcogendes (TMDCs). Fast degradation remains one of the most significant challenges facing BP since the discovery of BP as a new two-dimensional material. To offer an ultimate solution for BP degradation, the complete understanding of the degradation mechanism is necessary. Despite this importance, the degradation mechanism is still lack due to the difficulties of correlating experimental measurements with the underlying physics. In this regards, we focused on the unveiling the degradation mechanism by using a scanning Kelvin probe microscopy and theoretical modeling using the Marcus-Gerischer theory and GW calculations. Our results demonstrate that there is an intrinsic correlation between the layer-dependent electronic structure and degradation. It provides not only a fundamental understanding of degradation but also the new strategy for improving the stability of BP. In the case of TMDC, the one of challenges is unintentional electron doping due to chalcogenide defects on the surface. To suppress this unintentional electron doping, various surface treatment methods are applied. Among them, we focused on surface oxidation of TMDC, which the unintentional electron doping is suppressed by hole doping by the surface oxidation. We analyzed the surface chemistry of the TMDC and the energy levels near the Fermi level by direct/inverse-photoelectron spectroscopy. Surface oxidation was observed in two forms: 1) chalcogen-oxygen bond. 2) chalcogen-oxygen substitution. In the case of chalcogen-oxygen substitution compared to chalcogen-oxygen bond, the hole doping was very effective. Its origin is that oxygen directly withdraw the electrons of the d orbitals of the transition metal when the chalcogen is replaced with oxygen.

Quantum transport in topological states of matter

이장희 Pohang University of Science and Technology 2016 국내박사

A classification of a state of matter requires a good understanding of the underlying physics of it. Whether a solid is an insulator or a conductor can be determined in terms of the band theory by describing the behavior of electrons in a reciprocal space. Spontaneous symmetry breaking also has been a powerful tool for classification of various quantum states of matter, such as liquid, solid, magnetism, or superconductivity. In 1980, Klitzing and collaborators [PRL 45, 494 (1980)] reported that the Hall conductivity in a two-dimensional electron gas is quantized as ve^2/h (v=1,2,3,...) while the longitudinal magnetoresistance becomes zero in a high magnetic field. This integer quantum Hall effect cannot be classified by the symmetry breaking or the band theory. Before long, in 1982, it was found by Thouless and collaborators [PRL 49, 405 (1982)] that the quantized Hall conductivity corresponds to a Chern number, which was the first experimental observation of a topological invariant. Since then, topology in condensed matter physics has attracted broad interest and becomes another criterion for classifying the states of matter. In 2005, Kane and Mele [PRL 95, 146801 (2005)] suggested a new topology (Z2-topology) in two-dimensions for differentiating the states of quantum spin Hall insulators from ordinary insulators. After then, the Z2-topology was extended to three dimensions, which has led to the discovery of three-dimensional (3-D) topological insulators (TIs). This thesis consists of two main parts; one consists of six chapters which focus on transport studies in the topological surface states of 3-D TIs and another consists of four chapters which are related to the emergence of an electrically confined one-dimensional (1-D) conducting channel between two dual-gated bilayer graphene (BLG) with oppositely directed electric fields. Part I is to present our recent works, which are related to topological surface states on 3-D TIs. Starting with a brief introduction of the basic concepts of the topology, we explain the state of polyacetylene and the integer quantum Hall effect as examples of topology in condensed matter physics. After describing how the topology has been developed in condensed matter physics along with the history of the discovery of 3-D TIs in chronological order, we discuss some exotic properties of the topologically protected surface states on 3-D TIs. Then, we present our results on transport properties in 3-D TIs together with a discussion of some issues related to it, such as Fermi level engineering, external gating effect, and anomalies in magnetoresistance. Finally, we present our recent work related to distributed surface currents on the 3-D TIs, which may be the cause of the inconsistency in previous results on the transport studies of 3-D TIs. In part II, we discuss another type of topological state, the emergence mechanism of which is similar to that of soliton in polyacetylene. If the band gaps in two adjacent regions in BLG are produced by opposite-directional electric fields, topological 1-D chiral modes emerge at the boundary between two gapped regions in BLG. Part II deals with our recent works to realize the state. After a brief introduction to monolayer graphene and BLG, we explain how the topological 1-D channel emerges in BLG. Then, we present the device fabrication procedure, which consists of novel stacking and transfer methods with sophisticated patterning sequence. Finally, we provide our results on transport measurements in BLG, which indicate the formation of a 1-D conducting channel between the two regions in gapped BLG. 응집 물리학에서 가장 중요한 핵심 분야 중 하나는 다양한 물질들과 양자상태 들을 특성에 따라 분류하는 것이다. 서로 다른 물질 및 양자상태 들을 어떤 명확한 기준에 근거하여 구분하기 위해서는 그 물질 및 상태에 대한 이해가 뒷받침 되어야 한다. 즉, 어느 물질의 특정 상태에 대해 명칭을 부여하고 다른 상태와 구분한다는 것은 그 특정 상태를 기술할 수 있는 충분한 지식이 축척 되었다는 것을 의미한다. 즉, 실 공간에서의 대칭성 깨짐을 통해 액체와 고체를 구분하는 것과 밴드 이론을 통해 도체와 부도체를 구분할 수 있다는 것은 고체와 액체, 그리고 도체와 부도체에 대한 이해가 이와 같은 구분을 하기에 충분하다는 것을 의미한다. 1980 년에 처음으로 발견된 양자 홀 효과는 밴드 이론과 대칭성 깨짐으로 구분할 수 없는 새로운 양자상태였다. 그러나, 1982 년에 이르러, Thouless 와 그 외 3 명의 저자들에 의해 각각의 양자 홀 상태는 Chern 숫자로 구분이 될 수 있음이 입증되면서, 물질의 상태들을 구분하는 방법론으로 위상상태가 또 하나의 기준이 될 수 있다는 것이 알려지게 되었다. 응집물리학에서의 위상상태의 역할은 그 이후 계속해서 발전되어 왔으며, 2005 년에 이르러서는, Kane 과 Mele 가 새로운 Z2-위상상태를 제안하며, 2 차원 벌집모양 격자구조에서의 양자 스핀 홀 효과를 일반적인 2 차원 부도체에서의 홀 효과와 구분 할 수 있음을 보였다. 그들은 계속해서, 이러한 Z2-위상상태를 3 차원으로 확장 할 수 있음을 제안 하였고, 이는 곧, 3 차원 위상부도체의 발견으로 연결되었다. 현재는 다양한 물질들의 양자상태 구분에 있어서 위상상태 또한 매우 중요한 역할을 할 수 있다는 것을 알게 되었고, 최근 들어서는 토폴로지역학이라는 용어의 탄생까지 이르게 되었다. 본 학위 논문은 위상상태와 관련된 양자상태에서의 전자들의 전도특성에 대한 연구 결과들이 크게 두 부분으로 나뉘어 구성되어 있다. I 부에는 3 차원 위상부도체에 관한 연구 결과들이, 그리고 II 부에는 두 층 그래핀에서 위상상태와 관련된 전도 채널에 대한 연구 결과가 정리되어 있다. I 부의 전반부는 먼저 위상상태란 개념에 대해 간단히 소개 후, 응집물리에서 위상상태가 발현되는 예로서, 폴리아세틸렌과 양자 홀 효과에 대해 설명한다. 다음, 응집물리에서의 위상상태에 대한 연구 발전 과정으로부터 3 차원 위상부도체의 탄생까지의 과정을 시대순으로 소개한 후, 위상상태로 인해 3 차원 위상부도체 표면에 나타나는 전도 채널의 독특한 특성에대해 설명한다. 그리고 이와 관련되어 진행되어 온 최근 연구 결과들을 소개하며 그 결과들에서의 문제점을 제시한다. I 부의 후반부는 앞에서 언급한 문제점과 관련하여 진행해 온 최근 우리의 연구 결과들이 기술되어 있다. II 부에서는 3 차원 위상부도체에서의 Z2-위상상태와는 다른 또 하나의 위상상태와 관련된 전도채널에 대한 내용을 다룬다. 두 층 그래핀의 밴드 갭은 게이트 전압을 이용하여 조절할 수 있는데, 이 때 두 영역의 밴드 갭을 만드는 전기장의 방향을 반대로 하게 되면, 서로 역전되어 있는 밴드 사이에 1 차원 전도 채널이 형성될 수 있음이 이론적으로 예견되었다. II 부에는 바로 이 이론적 예측을 실현하기 위한 최근의 연구 결과들이 정리되어 있다. 먼저 두 층 그래핀의 특성에 대하여 간단히 소개한 후 밴드 갭 역전 현상을 이용한 1 차원 전도 채널의 형성 원리를 설명한다. 다음 이를 실현하기 위한 시료의 제작방법을 설명한 후 그 시료에서 얻은 최근 결과들을 기술하였다. 본 학위논문 I 부에서 기술하고 있는 우리의 결과들은 앞으로 3 차원 위상부도체에 대한 전도특성 연구에 높은 신뢰성을 가진 방법론을 제시하며, 나아가 3 차원 위상부도체를 실제 사용될 전자회로에 응용하기 위한 중요한 방향을 제시할 수 있을 것으로 기대한다. 그리고 II 부에서 제시하는 두 층 그래핀에서의 1 차원 전도채널에 대한 연구 결과는 앞으로 밸리트로닉스로의 활용 방안에 대한 중요한 기초 연구 자료가 될 수 있을 것으로 기대한다.

Modulation of crystal structure in two-dimensional transition metal dichalcogenides based on strain and doping

정재훈 Graduate School, Yonsei University 2022 국내박사

Various structures in two-dimensional (2D) transition-metal dichalcogenides (TMDs) and structural phase transitions between them have garnered significant interest due to their unique physical properties of each phase. The H phase, most widely studied structure, exhibits the indirect-to-direct bandgap transition, non-hydrogenic excitons, valleytronic properties, and Ising superconductivity. Meanwhile, the T’ phase, one of the derivatives of distorted T phase, is investigated as 2D topological insulator, type-II Weyl semimetal and ferroic material. Therefore, structural phase transitions between H and T’ phases are accompanied with drastic changes in physical properties, e.g., metal–insulator transition, topological phase transition and ferroic phase transition. Since such transitions of physical properties provide a potential for application to nanoscale electronic devices and physical insight to understand quantum and classical phase transitions, the discovery of distinct physical properties in each phase and the development of method to phase engineering are significant in condensed matter physics. Especially, phase transition from high symmetry phases to low symmetry phases are very important because symmetry breaking in the process of phase transition plays a crucial role in revealing distinctive physical properties. For example, the group 7 TMDs with T’’ phase which is distorted from T phase exhibit fascinating physical properties, e.g., opening large band gap, weak interlayer interaction, and anisotropy of electrical and optical properties. Furthermore, such reduce in crystal symmetry can also manifest ferroic properties. Regarding the application to electronic devices, the metal-insulator transition via phase transition between semi-conducting H phase and semi-metallic T’ phase in group 6 TMDs opens up new opportunities for applications such as phase change memory, resistive memory and reduction of contact resistance. To facilitate practical application of phase transition, the development of methods for engineering structural phase is essential. In this dissertation, we discuss the manifestation of multiferroicity through the ferroic phase transition and the phase engineering method for the metal-insulator transition. Based on the loss of crystal symmetry during the phase transition, the intrinsic ferroelastic-ferroelectric multiferroicity in single-crystalline rhenium dichalcogenides is discussed in detail. Moreover, the change in bond configuration during the evolution of the domain wall and the preferred switching between two specific orientation states are explained based on the electron localization function and bond dissociation energy of Re-Re bonds. In addition, mechanism of phase transition in molybdenum ditelluride synthesized by molecular beam epitaxy is discussed in detail. Using X-ray and ultraviolet photoelectron spectroscopy, we revealed that the origin of phase modulation is positive charge doping by electron transfer. These results provide physical insight into the role of symmetry breaking during structural phase transition and novel method to modulate structural phases in van der Waals crystals.

Single-molecule biophysical studies of the B-Z transitions induced by biological, chemical and physical effects

김숙호 Korea University 2018 국내박사

DNA is believed to adopt non-canonical or non-B conformations such as Z-DNA, G-quadruplex, H-DNA (triplex), i-motif to name a few. These structures are implicated in numerous biological processes. Thus, any modification of its intrinsic characteristics may lead to drastic consequences. That is why various kinds of DNA alteration and damage are primary causes for cancers and enigmatic genetic diseases. Ironically, a serious damage on DNA induced intentionally can bring great benefits to us. One such example is cisplatin. Here, we studied the physical properties of cisplatin-bound DNA. Our results show that the effect of carbonates was to form monofunctional adducts and to prevent them from converting to bifunctional adducts and prove that the efficacy of cisplatin can be compromised by the presence of carbonates. And we examined the B-Z transition under chemical, biological, and physical conditions. Our results show that Z-DNA exists under high salt concentrations and the B-Z transition occurs more easily in methylated DNA. Moreover, we showed that Z-DNA can be biologically induced by binding to a Z-DNA binding protein, ADAR1, and measured thermodynamic populations of B- and Z-DNA states for ADAR1-bound DNA in both GC and TG repeats. Based on a statistical physics model, we quantitatively determined the affinities of ADAR1 to both Z-form and B-form of TG and GC repeats. We also showed that a purely physical means can induce the B-Z transition. We measured the activation energy of the transition for both TG and GC repeats and found that the energy barrier for TG is about half the barrier for GC, which explains the observed dynamics of the sequences. In summary, we gain quantitative and physical insights into various DNA-related phenomena by utilizing powerful single molecule techniques. The information newly gained as such provides physical backdrop of biological phenomena. 오른손 방향 이중 나선인 B-DNA는 표준적 DNA 구조로, 특정 서열 및 생리학적 조건에서 Z-DNA, G-quadruplex, H-DNA (triplex), i-motif 와 같은 non-B 구조로 변할 수 있다. 복제, 전사 및 재조합과 같은 다양한 유전 대사 과정에서도 유사한 조건이 조성될 수 있어 non-B 구조의 생물학적 기능이 주목받고 있다. 따라서 생명현상의 물리적 원리에 접근하기 위해서 표준 B-DNA 구조와 아울러 비표준 DNA구조의 특성을 이해하는 것이 중요하다. 한편, 표준적인 이중나선 구조라 하더라도 DNA 고유 물성에 변형 및 손상이 일어날 경우 유전 대사 과정에 문제를 일으켜 암 또는 유전병 발생의 주요 원인이 될 수 있다. 화학 요법으로 사용되는 시스플라틴은 DNA와 결합하며 국부적인 꺾임(kinking)을 일으키는 등, 의도적으로 DNA에 손상을 일으킴으로써 세포를 사멸하게 한다. 빠르게 증식하는 암세포를 효과적으로 공격하여 항암제로 널리 사용되고 있다. 일반적인 메커니즘은 잘 알려져 있지만, 생체 내 존재하는 이온에 의한 효과, 보다 구체적으로는 탄산염 이온이 존재할 때 시스플라틴과 DNA의 결합 방식 변화의 가능성에 대해서는 연구가 필요하였다. 왼손 방향의 이중 나선인 Z-DNA는 특이한 구조와 생물학적 흥미로 인해 다양한 분야에서 관심을 가져왔다. 특히, Z-DNA로 전이될 수 있는 서열(GC, TG 반복 서열)이 프로모터 부분에 많이 분포하고, 프로모터 부근에서 개방 크로마틴 구조를 안정적으로 유지하기 위해 Z-DNA가 ADAR1이나 Nrf1같은 단백질과 결합한다는 연구결과도 보고된 바 있다. 이 논문에서는 첫째, 시스플라틴과 결합한 DNA의 물리적 특성을 연구했다. 탄산염의 존재가 시스플라틴이 DNA에 단일 결합으로 붙게 하여 그 효능에 영향을 준다는 것에 대한 직접적인 증거를 얻었다. 이를 위해 단분자 생물리 기술인 자성트위저를 이용하여, 개별 DNA분자를 제어, 추적함으로써 시스플라틴의 결합도를 상황에 따라 실시간으로 추적하였다. 둘째, 이 논문에서 우리는 화학적, 생물학적 및 물리적 조건 하에서 B-Z 전이를 연구했다. Z-DNA가 높은 염 농도에서 보다 쉽게 유발되고, 메틸화된 DNA에서는 보다 낮은 염 농도에서 일어나는 결과를 명확히, 정량적으로 얻었다. 그리고 Z-DNA 결합 단백질인 ADAR1이 DNA에 결합하여 Z-DNA가 생물학적으로 유도될 수 있음을 보였고, GC 및 TG 반복 서열에서 ADAR1 결합에 의한 B 및 Z-DNA 상태의 에너지 및 열역학적 특성을 측정하였다. 통계 물리 모델을 바탕으로, TG 및 GC 반복 서열에 대해 Z 형 및 B 형 모두에 대한 ADAR1의 친화도를 정량적으로 결정하였고 B-Z 전이의 동력학적 과정을 구성했다. 마지막으로 힘과 돌림힘과 같은 역학적 조건이 B-Z 전이를 유도함을 보였을 뿐 아니라 그 양상을 정량적으로 밝혔다. 또한, 돌림힘에 의한 B-Z 전이의 활성화 에너지를 측정하고 TG 서열의 에너지 장벽이 GC 서열의 것보다 현저히 낮다는 것을 보임으로써 관련 현상의 물리적 원리를 제시할 수 있었다. 이 논문에서, 다양한 단분자 생물리 기술을 이용하여 DNA와 관련된 여러 현상에 대한 정량적, 물리적 직관을 얻고자 하였다. 이를 통해 생명현상의 물리적 원리를 밝힘과 동시에 이들 현상도 물리적 과정에 의해 영위됨을 배울 수 있었다.

Angle-resolved photoemission spectroscopy studies on the band-structure modulation of black phosphorus

김지민 Pohang University of Science and Technology 2018 국내박사

물질의 전자 구조는 그 물질이 가지고 있는 물성과 왜 그러한 물성을 가져야 하는지 그 이유를 이해하는데 큰 역할을 차지한다. 그렇기 때문에 물질의 물성을 연구할 때, 자연스럽게 물질의 전자 구조를 실험과 이론을 통해 규명하는 것으로부터 출발하게 된다. 최근 다시 각광을 받고 있는 2차원 물질인 흑린은 적당한 밴드갭과 높은 모빌리티라는 소자 응용을 위한 좋은 전자적 성질을 가지고 있지만, 이론적으로 예측되는 다양한 물리현상을 구현 및 이해하기에는 기초적인 전자 구조에 대한 연구가 부족한 상황이다. 이 연구에서는, 광전자 분광학적 방법을 바탕으로, 흑린 표면에 알칼리 금속을 도핑하는 방법을 이용하여 흑린의 저에너지 전자 구조를 탐구하였다. 도핑된 알칼리 금속은 흑린 표면에 전자를 주고 이온화되어 벌크 방향을 향하는 강한 전기장을 만든다. 이 전기장은 밴드 벤딩에 의한 2차원 표면에 갇힌 전자상태를 야기한다. 이러한 전자 밴드 구조의 변화를 각분해 광전자 분광을 이용하여 직접적으로 측정하였다. 측정한 밴드 스펙트럼은 연속적이면서 넓은 범위를 가지는 밴드갭 조절 현상을 보였다. 이 밴드갭 조절 현상은, 슈타르크 효과라고 알려져 있는 전기장에 의한 전하 밀도 변화와 동반되는 에너지 변화로 인하여 설명할 수 있다. 이러한 강한 밴드 구조 조절 현상은 단순히 밴드갭만 변화시킬 뿐만 아니라, 원자가 밴드와 전도대 밴드의 위치 관계를 뒤집음으로써, 보통의 절연체에서 디락 준금속으로의 위상적 양자 상전이를 일으킨다. 상전이가 일어나는 동안 두개의 특징적인 준금속 상태가 나타나는데, 첫번째는 브릴루앙 영역 중심에서 하나의 디락 점을 만드는 동시에 한 방향으로는 선형, 다른 방향으로는 2차함수의 밴드 분산을 가지는 이방성을 가지는 디락 준금속 상이고, 다른 하나는 한 방향으로만 한 쌍의 두 디락 점을 가지고 있는 준금속 상이다. 후자의 경우, 안정적인 한 쌍의 두 디락 점은 흑린의 특징적인 구조적 대칭성과 시간 역전 대칭성의 조합으로 나타나는 공간-시간 반전 대칭성에 의해 보호된다. 이러한 흑린의 새로운 전기장에 의한 밴드 구조 조절과 상전이는 흑린이 소자로써의 응용 가능성 및 다양한 물리 현상을 탐구하는 데 중요한 역할을 가지게 만든다. 특히, 전기장에 의해 조절되는 절연체와 금속 간 상전이는 켜고 끄는 동작에 대한 새로운 원리를 제공함으로써 이러한 원리를 이용한 새로운 소자의 제작 가능성을 제공한다. 또한, 흑린에서 나타나는 위상적 준금속 상태는 위상적 전계 효과 트랜지스터의 제작 및 안정적인 2차원 디락 혹은 바일 페르미온과 관련된 물리 현상과 위상적 양자 상전이의 상 그림을 연구하는데 유용할 것이다. Electronic band structure of matters mostly explains their characteristic physical properties and why they should have such properties. Therefore, both experimental and theoretical studies on electronic band structure become a natural starting point of condensed matter research. Black phosphorus (BP), recently revisited two-dimensional (2D) material, has attracted vast attentions owing to its promising electronic properties with moderate band gap and relatively high carrier mobility, intensive experimental studies on the electronic band structure are still needed to realize and understand various interesting physical phenomena expected theoretically, though. In this dissertation work, I have performed spectroscopic studies on the low energy electronic band structure of BP with in-situ surface doping technique of alkali metal atoms. Doped alkali metal atoms donate electrons on the surface of BP and induce a strong dipole field toward the bulk direction, giving rise to confined 2D electronic states due to surface band bending. We have directly observed strong band structure modulation of valence band (VB) and conduction band (CB) due to the induced dipole field with the help of angle-resolved photoemission spectroscopy (ARPES). Obtained ARPES spectra showed continuous and wide band gap modulation, attributed to Stark effect, due to charge density redistributions of VB and CB states under the dopant-induced electric field. This strong band structure modulation resulted in a topological quantum phase transition from direct band gap normal insulator to band-inverted topological Dirac semimetal. During the phase transition, two distinct semimetal phases are formed: the one with single Dirac node at the Brillouin zone center, and the other one with a pair of two Dirac nodes along one out of two principal axes of BP. In the band-inverted Dirac semimetal phase, a pair of two stable Dirac nodes are protected by space-time inversion symmetry, a combination of characteristic crystalline symmetry and time-reversal symmetry, of BP. This novel field-induced band structure modulation and phase transition make BP important in both of device applications and exploring rich physics. Electric field-induced phase transition between an insulator and a metal could open a possibility of new devices with novel on/off switching mechanism. Moreover, induced topological semimetal states in BP could be useful in making topological field effect transistor, studying stable 2D Dirac and Weyl fermion physics and related phase diagram of topological quantum phase transition.

Development of a new BUU Type transport model for dynamical description of nuclear reactions in heavy-ion collisions

김명국 Pusan National University 2020 국내박사

Heavy-ion collisions can produce low temperature dense nuclear matter in which densities are beyond saturation density \rho_0 = 0.16 fm^-3 and allow the studies related to nuclear structures and nuclear reactions. Although there are a lot of existing models describing the heavy-ion collisions, most of them are focusing on the final states of heavy-ion collisions. On the theoretical aspects, the transport approach is an excellent tool for studying dynamics of non-equilibrium systems from initial to final states. Understanding nuclear matter properties under the extreme conditions is one of the main goals of nuclear physics. To achieve the goal, for example, Rare isotope Accelerator complex for ON-line experiment (RAON) in Korea, is under the construction for heavy-ion collision experiments. For understanding nuclear matter properties, it is required optimized beam energy regime usually called low/intermediate energy regime. Within the beam energy regime, it is expected that the maximum densities reach up to two times saturation density \rho_max~2\rho_0. Measuring physical quantities such as n/p ratio, pion ratio, rapidity distributions, transverse flow, etc. from heavy-ion experiments, directly or indirectly, contributes importantly to understanding the nuclear dense matter. For heavy-ion collisions at low/intermediate energies, the dynamics are described through the nucleon potentials from equation of state and the nucleon-nucleon collisions. The current transport approaches have two types: Boltzmann–Uehling–Uhlenbeck (BUU) and quantum molecular dynamics (QMD). The BUU models describe time evolution of the one-body phase-space density by introducing test-particle method to solve BUU equations. Meanwhile, QMD models describe time evolution of N-body Hamiltonians which assume N-body wave function represented by Gaussian wave packets. The transport approaches can be extensively used to extract physically significant quantities such as nuclear equation of state, symmetry energy, and microscopic interactions, then they can be used to verify theoretical models by comparing them with experiments. In this study, we have developed a new Boltzmann–Uehling–Uhlenbeck (BUU) type transport called DaeJeon Boltzmann–Uehling–Uhlenbeck (DJBUU). The DJBUU is basically a transport approach including relativistic mean-field model for nucleon propagations and nucleon-nucleon collisions with quantum effects through the BUU equations. To verify the new model, we have tested numerical calculations about unphysical extreme conditions and compared ours with Transport– Code–Comparison–Project (TCCP) results. The TCCP, which began 2004 at ECT* in Trento, Italy, is the project comparing the existing transport models to understand the source of discrepancy in a given same initial condition. They have compared the results of heavy-ion collisions, box calculation of collision and blocking, mean-field dynamics, and pion productions. As a new developer of transport model, it is very helpful to compare ours with the TCCP results under the same initial conditions. Through the wide comparisons in heavy-ion collisions, we can confirm the DJBUU results are lying within the uncertainties of the TCCP results. To put it concretely, box calculations of collision and blocking, and mean-field dynamics, which are the significant ingredients of low and intermediate energy heavy-ion collisions, firmly support that the DJBUU is working properly. Also, we have applied a new equation of state called extended parity doublet model (EPDM) in the DJBUU. This model has been developed to explain the origin of nucleon mass by introducing the chiral-invariant mass. It has also been applied and studied to nuclear dense matter and nuclear structure calculations. In this study, we have estimated the possibility of significant properties related to the chiral-invariant mass in heavy-ion collisions. Based on the current version of DJBUU, it remains important tasks for the future to study on fragmentation and clustering effects, pion and \Delta productions with in-medium effects, equations of state, and etc.

Interplay among topological structure, disorders, and electron correlations : emergent phases and anomalous transport

김경민 Pohang University of Science and Technology 2018 국내박사

The understanding of topological matters lies at the heart of modern condensed matter physics. As a gapless topological matter, Weyl semimetals attract great attention because its conducting nature provides an opportunity to probe the role of topological structure in transport phenomena. In disordered systems, the interplay between topological structure and disorder casts novel physics problems: the fate of surface states, disorder-driven topological phase transitions, and an effective theory with a topological term in a strong-disorder regime. In this thesis, I investigate the interplay between topological structure and disorder on two aspects: (i) the emergence of Weyl semimetallic phases from topological insulators due to magnetic impurities and (ii) disorder-driven scaling laws in anomalous transport phenomena in Weyl semimetals. In an effective theory, topological insulator bands are represented by the Dirac theory and doped magnetic impurities are represented as local effective magnetic fields coupled to electron spins. Through the renormalization group analysis I find a fixed point at which a variance of local magnetic fields can be enormously enhanced while a band gap diminishes simultaneously due to such local fields. This fixed point is identified as a disordered Weyl metallic phase where insulating islands and Weyl metallic islands coexist inhomogeneously throughout a sample. In the presence of an external magnetic field, Weyl semimetals can be reached more easily. In an effective theory, topological insulator bands are represented by the Dirac theory and doped magnetic impurities, the O(3) vector model with a random mass term. Through the renormalization group analysis I find that a band gap gains negative feedback by spin fluctuations via a Zeeman coupling term. Due to this unusual effect, the band gap can be closed by an external magnetic field, turning a topological insulator into a Weyl semimetal. As a verification for such state, I derive a scaling formula for the negative longitudinal magneto-resistivity. Considering a Weyl metal in the presence of disorder, I investigate disorder-driven renormalization of a topological term referred an inhomogeneous theta-term responsible for the negative longitudinal magneto-resistivity. A difficulty in this study is that a naive Kubo formula calculation fails to incorporate the role of the chiral anomaly. The difficulty is circumvented by incorporating the renormalization effect of the $\theta$-term with the Boltzmann theory. As a result, I derive a scaling theory of the negative longitudinal magneto-resistivity. A breakdown of the B/T scaling behavior may be regarded as a fingerprint of the interplay between disorder scattering and topological structure in Weyl semimetal phases. Another main topic of this thesis is the interplay between electronic correlations and disorder. Strong correlations result in an electronic nematic phase where a point group symmetry of a crystal system is spontaneously broken. At the nematic quantum critical point in the absence of disorder, a non-Fermi liquid fixed point was obtained with a control parameter. However, the role of disorder should be figured out to understand anomalous transport phenomena in the metallic quantum critical point. In the presence of disorder, I construct an effective theory where intra and inter scattering channels on two momentum patches of Fermi surface are included in addition to critical order parameter fluctuations. By generalizing the co-dimension of the Fermi surface I can access to a perturbative regime for both fermion-boson interaction and disorder scattering. However, the intra scattering channel is strongly enhanced due to the extended momentum patches, inhibiting me from accessing to a weak disoder fixed point in the one loop order. This is interpreted as the interplay between electronic correlation effects and disorder scattering. Higher loop analysis is left for a future work. Lastly, I investigate the interplay between an inversion-symmetry breaking and disorder. When an inversion symmetry is broken in crystal systems, a spin-orbit coupling term originating from an effective electric field gives rise to two Fermi surfaces having different spin chiralities. In an experiment on BiTeI, the mobility of electrons in the inner Fermi surface shows a divergent behavior, increasing to order of 10^4 times that of the outer Fermi surface. This disparity is well explained within the self-consistent Born approximation on disorder scattering where the vanishing of the density of states in the inner Fermi surface is responsible for this phenomenon.