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With the immense increase in the computational power available for the material science community in recent years, a range of new discoveries were made possible. Accurate investigations of large scale atomic systems, however, still come with an extremely high computational demand. While the recent development of Graphics Processing Unit (GPU) accelerated supercomputing might offer a solution to some extent, most well known electronic structure codes have yet to be fully ported to utilize this new power. With a soaring demand for new and better materials from both science and industry, a more efficient approach for the investigation of material properties needs to be implemented. The use of Machine Learning (ML) to obtain Interatomic Potentials (IP) which far outperform the classical potentials has increased greatly in recent years. With successful implementation of ML methods utilizing neural networks or Gaussian basis functions, the accuracy of ab-initio methods can be achieved at the demand of simulations with empirical potentials. Most ML approaches, however, require high accuracy data sets to be trained sufficiently. If no such data is available for the system of interest, the immense cost of creating a viable data set from scratch can quickly negate the benefit of using ML. In this diploma project, the elastic and thermodynamic properties of the Ti0.5Al0.5N random alloy at elevated temperature are therefore investigated using an Active Learning (AL) approach with the Machine Learning Interatomic Potentials (MLIP) package. The obtained material properties are found to be in good agreement with results from computationally demanding ab-initio studies of Ti0.5Al0.5N, at a mere fraction of the demand. The AL approach requires no high accuracy data sets or previous knowledge about the system, as the model is initially trained on low accuracy data which is removed from the training set (TS) at a later stage. This allows for an iterative process of improving and expanding the data set used to train the IP, without the need for large amounts of data.

<p>Topological materials have been under intense investigation for more than 30 years and have experienced astonishing growth  over the last decade. The two-dimensional electron gas has long served as a model system for the exploration of topological physics, supporting a diverse array of strongly correlated emergent phenomena. Indeed, some of the most stunning topological phases in condensed matter systems are the integer and fractional quantum Hall states forming in two-dimensional electron gases.</p> <p>It was realized early on that electron localization in the bulk has an important role in attaining topological phases, where the sample bulk is well described by randomly localized electrons, known as the Anderson insulator. However, a different type of topological phases forms when charge carriers order in the bulk. Such a charge ordering can only occur in the presence of strong electron-electron interactions and low disorder. Localization of this kind is of a collective nature and differs fundamentally from the single particle physics of the Anderson insulator. The nature of charge ordering, however, is more nuanced than first thought. Indeed, in high Landau levels, Hartree-Fock theories predict the proliferation of numerous exotic bulk insulators, where in the limit of no disorder electrons cluster together and form a hexagonal lattice. Initial observations of these highly correlated insulating phases were limited to low disorder two-dimensional electron gases confined to GaAs/AlGaAs heterostructures. However, recent discoveries of charge ordering in two-dimensional electron gases confined to graphene highlight the universality of this phenomena, irrespective of host material. While progress has been made in understanding the collective insulators residing within integer quantum Hall plateaus, many aspects remain unresolved. In this Dissertation, I discuss the transport properties and energetics of collective insulators forming near integer quantum Hall plateaus in the latest generation of very low disorder two-dimensional electron gases.</p> <p>In chapter  1  I briefly introduce recent developments in our current understanding of the integer quantum Hall effect, where the topological phase is described by both a topological invariant as well as a local order parameter related to the Landau symmetry breaking paradigm. Next, I introduce the basic principles of two-dimensional electron gases confined to semiconductor heterostructures and provide a short summary of recent technological breakthroughs in molecular beam epitaxial growth protocols. The chapter concludes with an introduction to the essential physics of both the integer and the fractional quantum Hall effect.</p> <p>Chapter  2  contains a brief review of the existing literature on the collective insulators forming in sufficiently low disorder two-dimensional electron gases. The primary focus of chapter  2  is on the unique magnetotransport patterns seen at various Landau level filling factors, which support the collective insulator  interpretation. Throughout this chapter I tend to lean on theoretical models that describe these collective phases through the lens of the Hartree-Fock theory; however, it is important to note that both density matrix renormalization group theories and direct diagonalization of small electron systems reach similar conclusions.</p> <p>In chapter  3  I present our data displaying the hallmark transport signatures of a collective insulator residing within the flanks of the nu = 1 integer quantum Hall plateau. Our sample belongs to the latest generation of low disorder 2DEGs confined to GaAs/AlGaAs. I provide a detailed analysis of its development in both temperature and filling factor. The distinct transport signatures we observe strongly overlap in filling factor with prior microwave resonance, surface acoustic wave, compressibility, and tunneling measurements, all of which point to the formation of a collective insulator known as the integer quantum Hall Wigner solid. One puzzling aspect, however, is that while the latter measurements exhibit the integer quantum Hall Wigner solid in older generation samples, transport signatures of this phase appear to be present only in the newest and highest mobility samples. By using distinct features in the magnetoresistance, I propose a stability diagram of the integer quantum Hall Wigner solid in nu −T phase space. Analysis of magnetoresistance profiles at fixed filling factors display sharp peaks within the region of integer quantum Hall Wigner solid phase. It is believed that these sharp peaks are a shared property of collective insulators forming in low disorder two-dimensional electron gases and signal the onset of the electron solid formation. Additional analysis of the magnetoresistance profiles suggests activated transport behavior with a gap energy comparable to that of the plateau center. Lastly, I present large signal measurements of the nu = 1 integer quantum Hall Wigner solid. The data displays strong non-linear behavior in the current-voltage characteristics consistent with the depinning and sliding conduction. However, similar threshold conduction is also seen in the current-voltage characteristics near the center of the integer quantum Hall plateau, where the bulk is an Anderson insulator. Much to our surprise, trends in the threshold current are monotonic in filling factor.</p> <p>In chapter  4  I report on the recent emergence of a newly observed collective insulator residing within the nu = 2 integer quantum Hall plateau and centered at filling factor nu = 1.79. Based on the range of filling factors which stabilizes this collective insulator, we find it distinct from the aforementioned integer quantum Hall Wigner solid. Indeed, the transport behavior is eerily reminiscent to the reentrant insulating phase seen at low filling factors between 1/5 < nu < 2/9. Hence, we term this collective insulator the reentrant integer quantum Hall Wigner solid. Evoking concepts of particle-hole symmetry, we find the reentrant integer quantum Hall Wigner solid to be one member of the larger family of Wigner solids, which is intimately linked through this fundamental symmetry of the system.</p> <p>Lastly in chapter  5 , I explore the energetics of the collective insulators which develop in the N = 2 and N = 3 Landau level, specifically the two- and three-electron bubble phase. We extract the onset temperatures of these exotic bulk insulators from the sharp peaks in the magnetoresistance at fixed filling factor. We compare our measured onset energies with the cohesive energies found from numerical calculations. We find the onset temperatures for the both two- and three-electron bubble phase show an approximately linear trend in filling factor within a single Landau level. In addition, we observe that the three-electron bubble phase has a larger onset temperature than the two-electron bubble phase, a result which is inconsistent with some numerical energy calculations. Thus, our measurements of bubble phase energetics call attention to the importance of the short-range Coulomb interaction in the formation of multi-electron bubble phases and is expected to serve as guide towards the refinement of existing theoretical models.</p>

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23696号 / 理博第4786号 / 新制||理||1685(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 柳瀬 陽一, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Long-wavelength (1.3-μm) vertical-cavity surface-emitting lasers (VCSELs) are of great interest as low-cost, high performance light sources for fiber-optic metro and access networks. During recent years the main development effort in this field has been directed towards all epitaxial GaAs-based structures by employing novel active materials. Different active region candidates for GaAs-based 1.3-μm VCSELs such as GaInNAs/GaAs QWs, GaAsSb QWs or InAs/InGaAs QDs have been investigated. However, the difficult growth and materials properties of these systems have so far hampered any real deployment of the technology. More recently, a new variety of VCSELs have been developed at KTH as based on highly strained InGaAs QWs and negative gain cavity detuning to reach the 1.3-μm wavelength window. The great benefit of this approach is that it is fully compatible with standard materials and processing methods. The aim of this thesis is to investigate long-wavelength (1.3-μm) VCSELs using ~1.2-μm In0.4GaAs/GaAs Multiple Quantum Wells (MQWs). A series of QW structures, DBR structures and laser structures, including VCSELs and Broad Area lasers (BALs) were grown by metal-organic vapor phase epitaxy (MOVPE) and characterized by various techniques: Photoluminescence (PL), high-resolution x-ray diffraction (XRD), atomic force microscopy (AFM), high accuracy reflectance measurements as well as static and dynamic device characterization. The work can be divided into three parts. The first part is dedicated to the optimization and characterization of InGaAs/GaAs QWs growth for long wavelength and strong luminescence. A strong sensitivity to the detailed growth conditions, such as V/III ratio and substrate misorientation is noted. Dislocations in highly strained InGaAs QW structure and Sb as surfactant assisted in InGaAs QW growth are also discussed here. The second part is related to the AlGaAs/GaAs DBR structures. It is shown that the InGaAs VCSELs with doped bottom DBRs have significantly lower slope efficiency, output power and higher threshold current. By a direct study of buried AlGaAs/GaAs interfaces, this is suggested to be due to doping-enhanced Al-Ga hetero-interdiffusion. In the third part, singlemode, high-performance 1.3-μm VCSELs based on highly strained InGaAs QWs are demonstrated. Temperature stable singlemode performance, including mW-range output power and 10 Gbps data transmission, is obtained by an inverted surface relief technique. / QC 20101126

<p>Soft condensed matter systems are a very diverse and challenging subject to study. To understand the complex macro-properties of such systems one approach is to characterize the microscopic structure and dynamics. A powerful technique for determining micro and nanoscale properties is scattering of radiation sources. Light, electron and neutron scattering techniques provide insight into the complicated molecular structures and the processes happening on these small scales.</p> <p>We have used neutron and x-ray scattering techniques to determine structural and dynamical information from two different types of soft condensed matter systems. The microscopic nature of a cobalt magnetic fluid was studied using neutron scattering, and the structure and dynamics of molecules within lipid bilayers was studied with the use of both neutron and x-ray scattering.</p> <p>Under strong magnetic fields, our cobalt fluid's small magnetic particles formed short chains, which we observed using neutron scattering.</p> <p>In the lipid bilayer systems which were studied we determined the positional orientation of cholesterol, Aspirin, and ethanol molecules, observed the effect of temperature on some of these systems, characterized domains and dynamics, and recreated the molecular structures of Alzheimer's protein in a brain-like membrane.</p> / Master of Science (MSc)

The first-principles calculation based on density functional theory (DFT) was used to study the oxidation resistance of atomically flat and atomic-step edges of Cu(111), diffusion of Cu atoms in different surfaces of alumina and interface properties of alumina and Cu(111), and magnetic properties of Mn-substituted strontium hexaferrite. The dissociation of oxygen molecules is the primary reason for the corrosion of metals, which deteriorates their application. Cu(111) flat surface, mono-atomic, and multi-atomic step edges were used to study oxygen diffusion. Penetration of oxygen on a Cu(111) flat surface requires high energy, indicating oxidation resistance. Our DFT result of oxygen diffusion into a mono-atomic step edge is an endothermic reaction. But the penetration of the O atom at the multi-atomic step edge is an exothermic reaction. We find mono-atomic step is as imperious as the flat surface; on the other hand, multi-atomic step edges are vulnerable to oxidation. This finding is consistent with the experimental results. In the second project, we find Al-terminated surface of alumina is more stable than Oterminated. The result of the ideal work of adhesion shows that the O-terminated surface has a better interface with Cu(111). Single Cu and clusters of Cu atoms were diffused through the Al and O terminating surfaces. The energy barrier for the diffusion of Cu atoms on the O-terminated surface is much higher than on the Al-terminating surface. Furthermore, the activation energies for clusters of multiple atoms migration are higher than for a single Cu atom. The results validate experimentally observed growth modes in the early stage of thin film growth. Next, we perform a DFT calculation to investigate the site preference and magnetic properties of Mn-substituted strontium hexaferrite. The site occupancies of substituted atoms were estimated by calculating the substitution energy. The magnetic properties of substituted hexaferrite were calculated by using formation probabilities. A decrease in saturation magnetization was observed with increasing Mn concentration.

Periodic driving is a ubiquitous tool for controlling experimental quantum systems. When the drive fields are of comparable, incommensurate frequencies, new theoretical tools are required to treat the resulting quasiperiodic time dependence. Similarly, new and surprising phenomena of topological origin may emerge in this regime, including the quantized pumping of energy from one drive field to another. This dissertation will describe how to exploit this energy pumping to coherently translate––or boost––quantum states of a cavity in the Fock basis. This protocol enables the preparation of highly excited Fock states for use in quantum metrology––one need only boost low occupation Fock states. Energy pumping, and hence boosting, may be achieved nonadiabatically as a robust edge effect associated to an anomalous localized topological phase (ALTP) of fermions on a wire, called the quasiperiodic Floquet-Thouless energy pump (QP pump). We present a simple coupled-layer model for the QP pump, and describe the broader topological classification which characterizes its robust properties. Finally, we argue that energy pumping by the edge modes is robust to the introduction of weak interactions between fermions, making the QP pump a stable, interacting, non-equilibrium phase of matter.

<p dir="ltr">Topological phase of matter is a special quantum phase of gapped Hamiltonian that is beyond the Landau's symmetry breaking paradigm. Topological phase of matter exhibit extraordinary topology-depedent properties and provide significant resources for quantum computation, as it can support anyons as lowest excitations, which contribute to fault-tolerant topological quantum computation, while its ground state are naturally quantum error correcting codes. </p><p dir="ltr">This thesis focused on studying topological phase of matter and its potential contribution to quantum computation. The author first works on the Ribbon operators in the Kitaev Quantum Double model with semisimple Hopf algebra $H$, which captures the anyonic excitations of $\mathbf{D}(H)$, Second, twist defects are studied in the Kitaev spin liquid context and shows the potential contribution to quantum computations by manipulating defects. Third, two classes of topological floquet code are introduced, to overcome the high cost of many-body syndrome operators and also gives new construction of topological orders.</p><p dir="ltr"><br></p><p dir="ltr"><br></p>

Imposing dimensional restrictions on graphene sheets and adding impurities can give rise to carbon nanostructures with magnetic properties. In this work, zigzag graphene nanoribbons, zGNRs, with nitrogen and fluorine adatoms are investigated for magnetic properties of interest for spin devices. Geometry optimizations were done determining which position along a zGNR electrode that N and F would favorably attach to. Edge positions were determined as the most stable attachment site. M-cell zGNR electrodes (M = 1-3) edge-functionalized by N and F adatoms were investigated with respect to their band structures and spin densities in antiferromagnetic and ferromagnetic, AFM and FM, configurations. Focus was placed on band structures showing spin gaps, indicating potential for magnetoresistive devices. Devices were modeled for 2-cell and 3-cell electrodes with nitrogen adatoms, and the respective transmission spectra were compared. Attaching N adatoms to zGNRs turned out to be a mode of controlled manipulation of their spin configurations. Spin gaps were identified in units based on 3-cell-zGNR electrodes.