One of the most remarkable achievements of modern condensed matter physics is the discovery of topological phases of matter. Materials in a non-trivial topological phase or the topological insulators can be distinguished by their unique electronic and transport properties which are indifferent to different types of perturbations and thus open new routes towards the dissipationless transport. Explaining their properties requires proper involvement of relativistic approach as well as topological analysis. Among different classes of topological insulators, the Z2 topological insulators have drawn special attention due to their strong spin-orbit coupling which makes them a promising candidate for spintronics application, especially for magnetic memory devices. Due to their inherent strong spin-orbit coupling, they provide an efficient way to manipulate electronic spin with an applied electric field via spin orbit torque. The topological insulators have been found to be far more superior in manipulating the magnetic order parameter of a ferromagnet compared to the conventional heavy metals like platinum or tantalum. Another milestone in magnetic memory devices is marked by the introduction of antiferromagnetic memory devices which has not drawn any attention for long time as they cannot be controlled by an applied magnetic field. Recently it has been found that in case of a non-centrosymmetric antiferromagnet, the magnetic order parameter can be manipulated by with spin-orbit torque which also have been verified experimentally. The advantages of antiferromagnetic devices over ferromagnetic devices are that they allow faster switching speed and they are immune to an external magneticfield which are two highly solicited properties for next generation spintronic devices. This thesis is focused on understanding the transport properties in topologically nontrivial materials and their interface with different magnetic material. We use simplified continuum model as well as tight binding models to capture the salient features of these systems. Using non-equilibrium Green's function we explore their transport properties as well as spin-charge conversion mechanism. Our finding would provide a better understanding of these new class of materials and thus would be instrumental to discover new mechanisms to manipulate their properties.

Presented here are electron-induced electron yield measurements from high-resistivity, high-yield materials to support a model for the yield of uncharged insulators. These measurements are made using a low-fluence, pulsed electron beam and charge neutralization to minimize charge accumulation. They show charging induced changes in the total yield, as much as 75%, even for incident electron fluences of <3 fC/mm2, when compared to an uncharged yield. The evolution of the yield as charge accumulates in the material is described in terms of electron recapture, based on the extended Chung and Everhart model of the electron emission spectrum and the dual dynamic layer model for internal charge distribution. This model is used to explain charge-induced total yield modification measured in high-yield ceramics, and to provide a method for determining electron yield of uncharged, highly insulating, high-yield materials. A sequence of materials with progressively greater charge susceptibility is presented. This series starts with low-yield Kapton derivative called CP1, then considers a moderate-yield material, Kapton HN, and ends with a high-yield ceramic, polycrystalline aluminum oxide. Applicability of conductivity (both radiation induced conductivity (RIC) and dark current conductivity) to the yield is addressed. Relevance of these results to spacecraft charging is also discussed.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18772号 / 理博第4030号 / 新制||理||1581(附属図書館) / 31723 / 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 川上 則雄, 教授 田中 耕一郎, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Phase transitions are critical in understanding the properties of different phases of matter, and their identification is an essential research focus in condensed matter physics. However, defining phase transitions for topological systems is more complex than for common mesoscale materials. This complexity is further compounded when disorders are present in the system. In this thesis work, we provide a comprehensive review of machine learning, topological insulators, and the conventional approach to classifying different topological phases. We focus on the Benalcazar, Bernevig, and Hughes (BBH) model, a higher-order topological insulator model, and investigate the challenges of identifying phase transitions in topological systems, particularly in the presence of disorders. To overcome these challenges, we implement the diffusion maps method, which accurately predicts the same transition points as traditional numerical calculations for both clean and disordered systems. Moreover, we demonstrate the efficacy of the diffusion maps method in predicting the transition point for the topological Anderson insulator. Our findings suggest that this approach has the potential to be generalized and applied to a broader range of disordered systems. Overall, this thesis work provides a novel method for identifying phase transition points in topological systems, which could have significant implications for the design and development of future topological materials.

Pages viii, xii, xiv, 32, 110, 182, 188, 194 were blank and therefore omitted. / <p> The functional density of photonic integrated circuits can be significantly increased by stacking multiple waveguide layers. These vertically-integrated devices require optical couplers to switch light signals between their layers. In this thesis, optical coupling between two stacked silicon-on-insulator slab waveguides has been demonstrated with a coupling efficiency of 68±4%, obtained with a coupler length of 3535 μm. The main advantage of using a silicon-based material system for photonic integrated circuits is its compatability with existing electronics manufacturing processes, facilitating cost-effective fabrication and the monolithic integration of both photonics and electronics on a single device. </p> <p> Coupling between more complex silicon-on-insulator waveguide structures with lateral confinement was then demonstrated. The coupling ratio between stacked silicon rib wavelengths was measured to be 54±4%, while ratios of 71±4% and 93±4% were obtained for stacked channel waveguide and multimode interferometer-based couplers respectively. The corresponding coupler lengths for these three designs were 572 μm, 690 μm and 241 μm respectively. The sensitivity of these couplers to the input wavelength and polarization state has also been evaluated. These vertical-integrated couplers, along with other structures, have been thoroughly simulated, including their tolerance to fabrication errors. Novel fabrication processes used to demonstrate coupling in proof-of-concept devices have been developed, including an in-house wafer bonding procedure. </p> / Thesis / Doctor of Philosophy (PhD)

<p> The analysis, construction and performance of a new type of evaporated thin-film filter is described. It has the capability of frequency tuning by means of a single bias voltage.</p> <p> The device is basically a uniformly distributed resistance-capacitance network (URC) with a thin semiconductor film incorporated in the structure. The resistive film, insulator and semiconductor films constitute a metal-insulator-semiconductor system (MIS) whose capacitance is bias dependent. The device differs from other proposed tunable thin-film filters in the fact that the capacitance can be altered independently of the distributed resistance.</p> <p> A theory is developed to account for the effects of MIS-losses and parasitic inductance on the performance of URC-null networks. These parasitic effects manifest themselves in four different ways: (i) A shift of the null from the position predicted by the ideal theory. (ii) A change in the notch parameter α for an optimum null. (iii) A detuning effect in the case of tunable notch filters. (iv) A reduction in the available tuning range; the so-called "tuning range compression".</p> <p> Experimental units were made by vacuum evaporation of Nichrome, Y2O3, CdS and Al onto alumina substrates. An attempt is made to relate the device characteristics to the material properties. In this respect, the relative dielectric permittivity and thickness of the insulator, the doping density of the semiconductor, the semiconductor-insulator interface states and the bulk states in the semiconductor are found to be the most important parameters.</p> <p> Filters were built which operated in the frequency range 600 kHz to 6 MHz. The best notch filter has a tuning capability of 30% with a ±15V bias voltage. The notch depth is over 60 dB in most cases. A tunable bandpass amplifier which was constructed, has a centre frequency which can be shifted between 6 MHz and 7 MHz with a ±3.5V bias voltage. The Q of this particular device was measured to be 742.</p> / Thesis / Doctor of Philosophy (PhD)

<p> This work presents theoretical calculations of some of the most important electrical characteristics of thin film, insulator-dominated semiconductor devices. Solutions of prespecified accuracy for an a.c. transmission line model of the semiconductor in such configurations as the MIS (Metal-Insulator-Semiconductor) structure and the SIS (Semiconductor-Insulator-Semiconductor) structure are given. Together with an accurate solution of the d.c. bias problem, exact C-V (Capacitance-Voltage) and G-V (Conductance-Voltage) characteristics can be found at any frequency. SRH (Shockley-Reed-Hall) impurity centres and surface states have been included in both the d.c. and a.c. solutions. In addition, accurate studies of the low temperature dopant impurity response can be made since the d.c. solution uses full Fermi integrals over arbitrary densities of states with the impurity dopant band treated like an SRH centre for the a.c. solution.</p> <p> In non-equilibrium situations, such as those which occur with the application of light or carrier injection by tunnelling, the a.c. solution requires active elements in the transmission line model but the transmission line can still be solved to a prespecified accuracy provided an accurate solution of the d.c. bias problem can be found. In this thesis the d.c. solution for the case of light-induced pair production is considered under the assumption of bulk controlled d.c. quasi Fermi level shifts. Thus the accuracy of the related a.c. conductance and capacitance solutions is dependent on the reliability of this assumption.</p> <p> The detailed treatment of the a.c. admittance of the MIS structure is justified by its technological importance and fundamental insight gained on impurity centre and surface state effects.</p> <p> The other work presented in this thesis is devoted to a study of a new thin film device structure, the SIS diode. First, the a.c. admittance characteristics for the thick insulator case are predicted to a prespecified accuracy. Then a simple treatment for part of the SIS d.c. tunnelling problem is done to qualitatively predict the effects of such parameters as doping density, temperature and insulator thickness on current-voltage characteristics. Finally, a simple generalization of the a.c. transmission line to include the effects of tunnelling is given which allows accurate solution for the a.c. admittance of tunnelling SIS diodes if the complete d.c. non-equilibrium problem can be accurately solved.</p> <p> Practical application of the thick-insulator SIS diode will undoubtedly stem from its wide range of interesting low and high frequency response characteristics. Among the most interesting of these characteristics is a bell-shaped high frequency C-V response previously unobserved in other two terminal devices.</p> <p> An interesting negative resistance feature, which partly resembles the negative resistance region of the p-n junction tunnel diode, is analyzed for the degenerate SIS p-i-n thin insulator structure. While the thick insulator SIS device has recently been fabricated and the bell-shaped high frequency response experimentally observed, experimental verification of the tunneling characteristics of the SIS diode has as yet not been fully realized.</p> / Thesis / Doctor of Philosophy (PhD)

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No description available.

<p> An optical temperature sensor and Fourier spectrometer, working in the 1550nm telecommunications wavelength range, were fabricated in silicon-on-insulator. Both devices were based on asymmetric Mach-Zehnder Interferometer waveguide geometries. The temperature sensor underwent a two phase design. The various asymmetry factors, due to different path length differences, of the Mach-Zehnder arms resulted in different levels of temperature sensitivity, which in turn was the driving mechanism behind the Fourier spectrometer. Due to the asymmetry of the Mach-Zehnder arms, there exists an inherent optical path length difference which is further changed with temperature variation due to the thermo-optic effect. The phase I temperature sensor showed an accuracy of 1-2°C and a sensitivity of 0.5°C for ΔL of 37.23μm and 23.46μm, respectively. The phase II temperature sensor design, which allowed for self normalization, resulted in a 1°C temperature accuracy and a 0.5°C sensitivity for a ΔL of 27.85μm. Both the phase I and II temperature sensors showed repeatable and stable results for the temperature range of 20-100°C, and agreed well with the theoretical design performance. Upon analysis of the highly asymmetric Mach-Zehnder designs it was found that both the 1.05cm and 3.05cm path length differences resulted in a temperature accuracy of 0.1°C, with a 0.05°C sensitivity over a small temperature range.</p> <p> The Fourier spectrometer exhibited decent agreeability with theoretical design performance and demonstrated proof of concept. A 1.05cm path length difference was insufficient to resolve two wavelengths at 1546.12nm and 1564.68nm, which agreed with the theoretical model. However, the 3.05cm ΔL was sufficient to resolve the two wavelengths in a repeatable manner.</p> / Thesis / Master of Applied Science (MASc)