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Investigation of the Physical Properties of Dirac Materials
Get PDFThis thesis focuses on the investigation of two types of Dirac materials: topological insulators (TI) and graphene. Both materials have received much attention and stimulated intense research activities over the last decade. Although massless Dirac electron are wonderful, there will be more industrial applications if we can open the gap and make Dirac electrons massive. For topological insulators, we focus on studies of the TI/Magnetic TI (MTI) bilayer structures to induce a gap on the surface state. For graphene, the author focuses on the Moiré pattern and interlayer interaction.
For bilayer TI/MTI samples, they were investigated with scanning tunneling microscopy and spectroscopy (STM/STS), and with electrical transport measurements by means of a Physical Property Measurement System (PPMS). Details of the experimental setups for this research and their upgrades were described. For the current STM system, both the tube scanner and sample stage in the STM head had been redesigned and rebuilt, which led to better XYZ fine approach control, improved wire protection, and enhanced noise shielding. A new back gate capability was added to the sample stage. A customized commercial STM system has been commissioned, which is expected to provide a better sample holder with improved vacuum seals and easier temperature control, as well as more convenient approaches to loading samples and switching STM or AFM (atomic force microscope) tips. For PPMS, an optical probe had been designed and constructed, which enabled light-induced effects on the electrical transport properties of TIs. A new custom-made glove box has been installed, which provides a computer-controlled and self-circling gas environment to minimize the concentration of air while reduces the waste of argon. The glove box is also easy to use. This upgrade helps expand our abilities to conduct research more efficiently.
STM/STS studies of both the binary and ternary types of magnetic topological insulators (MTIs) are presented. For both binary and ternary bilayer TI/MTI systems, the majority of the density of states (DOS) spectra evolved with the temperature. At room temperature, all samples showed massless Dirac spectra. However, for temperatures below 200 K, all bilayer samples with the top pure TI layer thinner than 5QL revealed opening of a surface gap. Generally, binary TI/MTI samples exhibited smaller gapped domains, which was consistent with the finding of nearly negligible hysteretic behavior for Hall resistance vs, magnetic field sweeps at low temperatures. In contrast, ternary TI/MTI samples exhibited larger gapped domains, which implied longer range ferromagnetic order and was indeed corroborated by the apparent hysteretic behavior in the electrical transport measurements at low temperatures. Additionally, the application of c-axis magnetic fields led to slighter larger surface gaps and more uniform gap distributions, which further confirmed the physical origin of the surface gap as magnetic in nature. Besides the U or V-shaped DOS spectra, double-peak or single peak impurity resonances were also observed. These spatially localized minority spectra were found to mostly appear along the boundaries of gapped and gapless domains. Moreover, the number of impurities was founded to reach a maximum around 240 K, which corresponded to the onset temperature of localized surface gaps.
Detailed studies of the electrical transport properties of both the binary and ternary MTIs by the PPMS provided a comparison between the macroscopic information thus obtained with the microscopic information derived from STS studies. Binary TI/MTI showed an anonymous Hall effect (AHE) at 25 K while ternary TI/MTI showed AHE around 20 K. Binary TI/MTI systems exhibited weak localization (WL) behavior in the longitudinal resistance vs. magnetic field data at 2 K. The binary TI/MTI samples with a thinner top pure TI layer revealed sharper and stronger WL behavior. In contrast, for the 3QL-TI/6QL-MTI ternary sample, weak antilocalization (WAL) behavior was present for all temperatures, while WL also showed up below 13 K. The Hall resistance vs. magnetic field data for all samples of ternary TI/MTI bilayers and ternary MTI monolayer samples revealed strong hysteresis at low temperatures, in contrast to the negligible hysteretic behavior in all binary TI/MTI samples. Finally, circularly polarized light was found to enhance the AHE of the bilayer ternary TI/MTI sample while weakening that of the monolayer ternary MTI. These experimental phenomena may be mostly attributed to the different band structures and Fermi levels among the binary and ternary TI/MTI samples. In particular, we note that the observation of quantum anomalous Hall effect (QAHE) only in ternary MTI monolayers at extremely low temperatures (at T ≤ 30 mK < < Tcbulk ~ 30 K) may be attributed to the finite contributions of bulk carriers to excess conduction unless T → 0.
Simulations have been carried out to account for the Moiré patterns of graphene on Cu (111), graphene on Cu (100), twisted bilayer graphene, and Cr-doped topological insulators. The physical origin for empirically observed structural superlubricity between graphene layers has also been modeled by simulations based on the density functional theory (DFT).
Finally, the key findings of this thesis work and the suggested future research directions are summarized.</p
The Landscape of Relativistic Stellar Explosions
Get PDFFor the last half-century, relativistic outflows accompanying the final collapse of massive stars have predominantly been detected via high-energy emission (i.e., gamma-ray bursts, or GRBs). From wide-field optical and radio time-domain surveys, there have been hints of related phenomena at lower energies (e.g., X-ray flashes). For my thesis, I used the Zwicky Transient Facility to conduct the first large-scale optical survey dedicated to finding relativistic stellar explosions. I successfully detected a suite of GRB-related phenomena without relying on a GRB trigger, and followed them up with facilities across the electromagnetic spectrum including ALMA: relativistic afterglows at cosmological distances, broad-lined Ic (Ic-BL) supernovae with X-ray and radio emission, and fast-luminous transients powered by circumstellar interaction. Based on the rate of fast (intra-night) optical transients, I showed that a "clean" jet seems central to the phenomenon of collimated energetic outflows, i.e., there is no evidence for afterglow-like optical transients whose area (sky) rate greatly exceed the classical GRB rate. With a radio and millimeter-wave investigation of AT2018cow, and the discovery of a similar event in ZTF, I established a new class of engine-driven stellar explosions that arise from different progenitors to GRBs and explode embedded in dense circumstellar material. I showed that fast-luminous thermal emission can arise from late-stage eruptive mass-loss and is not necessarily linked to the presence of relativistic ejecta, complicating searches for choked jets in Ic-BL SNe. My work sets the stage for discovering and characterizing relativistic stellar explosions in large numbers during the era of ZTF Phase II, the Large Synoptic Survey Telescope (LSST), and millimeter-band facilities like ALMA and NOEMA.</p
Topological Phases of Matter: Classification, Stacking Law, and Relation to Topological Quantum Field Theory
Get PDFWe study aspects of gapped phases of matter, focusing on their classification, including the group law under stacking, and their relation to topological quantum field theories (TQFT). In one spatial dimension, it is well-known that Matrix Product States (MPS) efficiently approximate ground states of gapped systems; by showing that these states arise naturally in 1 + 1-dimensional lattice TQFT, which in turn are closely related to continuum TQFT, we provide a concrete connection between ground states of lattice systems and TQFT in 1 + 1 dimensions. We generalize this to systems with symmetries and fermions, and obtain a classification and group law for the stacking of 1 + 1-dimensional symmetry-protected topological phases. Further, we study the effect of turning on/off interactions for the classification: the phase classification of a given symmetry class of Hamiltonians can be different depending on whether we allow interactions or not, and in low dimensions we provide some concrete formulas relating the phases under the non-interacting classification and those under the interacting classification. Lastly, we study the phases of the 2 + 1-dimensional topological superconductor, and show that for all 16 phases braiding statistics of vortices, which determine the underlying TQFT, can be obtained by stacking layers of the basic p + ip superconductor.</p
Multimetallic Models of the Nitrogenase Active Site
Get PDFMotivated by the lack of an atomic-level understanding of the reduction of small molecule substrates by nitrogenase, this dissertation describes the synthesis, characterization and reactivity of well-defined model clusters of the enzyme active site.
Chapter 2 describes a series of site-differentiated, high spin iron clusters which reversibly bind carbon monoxide in redox states FeII₄ through FeIIFeIII₃. Detailed spectroscopic and thermochemical studies reveal that this remarkable reactivity can be attributed to the ability of remote metal centers to shuttle reducing equivalents to the small molecule binding site.
Chapter 3 further explores the consequences of internal electron transfer events on the thermodynamics of small molecule binding by site-differentiated, tetranuclear iron clusters. To systematically tune the electronic properties of the cluster, a Hammett series was prepared. Counterintuitively, introduction of electron-donating substituents suppresses the first CO binding event but enhances the second. Detailed spectroscopic studies revealed that the origin of this behavior can be traced to the effect of the substituents on the redox reorganization energy associated with internal electron transfer.
Chapter 4 presents the synthesis and characterization of the first open-shell diiron µ-carbyne complex, which also features a biologically relevant Fe(µ-C)(µ-H)Fe core. This electronically unusual species could be activated toward binding of N₂ upon addition of H⁺/e, which initially involves an iron-carbene intermediate.
Chapter 5 describes the synthesis and spectroscopic investigation of the first carbonbridged, bimetallic complexes featuring odd numbers of valence electrons as spectroscopic models of the critical E₄(4H) intermediate of nitrogenase. Detailed pulse EPR studies revealed the effects of electronic localization on the spectroscopic signatures of the µ-hydride motif and provide insight into the electronic distribution in a reduced state of FeMoco.
Chapter 6 describes the synthesis and characterization of terminal iron-carbene complexes, including EPR characterization of open-shell variants.
Appendix A describes unpublished efforts to prepared site-differentiated models of FeMoco featuring carbon- or sulfur-based donors.
Appendix B presents unpublished work towards modelling the cooperative activation and reduction of N₂ by diiron complexes featuring carbon-based bridging ligands.</p
Optical Light Manipulation and Imaging Through Scattering Media
Get PDFTypical optical systems are designed to be implemented in free space or clean media. However, the presence of optical scattering media scrambles light waves and becomes a problem in light field control, optical imaging, and sensing.
To address the problem caused by optical scattering media, we discuss two types of solutions in this thesis. One type of solution is active control, where active modulators are used to modulate the light wave to compensate the wave distortion caused by optical scattering. The other type of solution is computational optics, where physical and mathematical models are built to computationally reconstruct the information from the measured distorted wavefront.
In the part of active control, we first demonstrate coherent light focusing through scattering media by transmission matrix inversion. The transmission matrix inversion approach can realize coherent light control through scattering media with higher fidelity compared to conventional transmission matrix approaches. Then, by combining the pre-designed scattering metasurface with wavefront shaping, we demonstrate a beam steering system with large angular and high angular resolution. Next, we present optical-channel-based intensity streaming (OCIS), which uses only intensity information of light fields to realize light control through scattering media. This solution can be used to control spatially incoherent light propagating through scattering media. In the part of computational optics, we first demonstrate the idea of interferometric speckle visibility spectroscopy (ISVS) to measure the information cerebral blood flow. In ISVS, a camera records the speckle frames of diffused light from the human subject interferometrically, and the speckle statistics is used to calculate the speckle decorrelation time and consequently the blood flow index. Then, we compare the two methods of decorrelation time measurements - temporal sampling methods and spatial ensemble methods - and derive unified mathematical expressions for them in terms of measurement accuracy. Based on current technology of camera sensors and single detectors, our results indicate that spatial ensemble methods can have higher decorrelation time measurement accuracy compared to commonly used temporal sampling methods.</p
Probing the Inner Accretion Flow Properties Around Black Holes with X-ray Observations
Get PDFAccretion, governed by gravity, is a fundamental source of energy in the universe. Accretion is important to the growth and evolution of black holes, as well as the structure in the universe on larger scales. Accretion disks around supermassive black holes are likely to have produced most of the ionizing radiation in the universe since the epoch of reionization. Outflows launched from the accretion disk of black holes, either in the form of disk winds or jets, provide a feedback mechanism that plays an important role in the co-evolution of black holes and their host galaxies. It has been decades since the fundamental theories about black hole accretion were established. Black hole accretion has been widely studied in active galactic nuclei (AGNs) and black hole binaries ever since the early days of X-ray astronomy. However, important questions still remain regarding the fundamental physical properties of black holes, the structure and geometry of accretion disks and coronae, and the nature of disk winds and jets.
In this thesis, I report results from recent X-ray observations of an ultra-luminous infrared galaxy with a central AGN, IRAS 05189–2524, and several black hole X-ray binaries (or black hole candidates for those currently lacking dynamical mass determinations), IGR J17091–3624, MAXI J1535–571, Swift J1658.2–4242, MAXI J1631–479, and MAXI J1820+070. Most of the black hole X-ray binaries studied in this thesis were uncatalogued sources and were discovered as bright Galactic X-ray transients over the past few years, offering great opportunities for investigating black hole accretion with high quality datasets. The launch of the NuSTAR telescope in 2012 has brought the advanced capabilities of performing high sensitivity observations in the hard X-ray band and remaining free from pile-up for bright Galactic sources, providing new angles for the detailed study of observational phenomena around both supermassive and stellar-mass accreting black holes. I study the inner accretion flow properties around black holes in the above objects by conducting spectral and timing analyses of the NuSTAR observations and the simultaneous soft X-ray band data from the Swift or XMM-Newton telescope. I have searched for and analyzed various accretion related observational features in these systems (disk reflection spectra, ionized absorption caused by disk winds, quasi periodic oscillations (QPOs) in the X-ray light curves, and unusual aperiodic flux variations or accretion state changes), and interpret the results in terms of the physical properties about the inner accretion flows and the central black holes.
By modeling the relativistic disk reflection spectra, I have found that the inner edge of the optically-thick accretion disk is truncated in IGR J17091–3624 during the bright hard state, in MAXI J1631–479 during the very high state, and in MAXI J1820+070 during the faint hard state, whereas the inner accretion disk is consistent with extending down to the ISCO in MAXI J1535–571 and Swift J1658.2–4242 during their bright hard states, and in MAXI J1631–479 during its soft state. Based on all the observational evidence gathered in this thesis, the general picture about the accretion flow geometry at different accretion states seems to be more complicated than that from previously well accepted theoretical predictions. In addition, I have measured the black holes spins and inner accretion disk inclinations for IRAS 05189–2524, MAXI J1535–571, Swift J1658.2–4242, and MAXI J1631–479 using the disk reflection modeling method, where the central black holes are all found to be rapidly spinning. There are also evidence for strong disk winds detected in the X-ray spectra of some of the objects studied in this thesis with interesting implications. I summarize my studies of these individual objects at the end of this thesis in a more broader context of the characteristic behaviors of the population and their general physical implications, and discuss about the possibilities of extending my research in this direction with the upcoming new X-ray missions.</p
A Coarse Jacquet-Zagier Trace Formula for GL(n) with Applications
Get PDFIn this thesis we establish a coarse Jacquet-Zagier trace identity fo GL(n). This formula connects adjoint L-functions on GL(n) with Artin L-functions attached to certain induced Galois representations. We prove the absolute convergence when Re(s) > 1, and obtain holomorphic continuation under almost all character twists. Moreover, as an application, we obtain that holomorphy of certain adjoint L-functions for GL(n) implies Dedekind conjecture of degree n. Some nonvanishing results are also proved.</p
The Adaptive Charging Network Research Portal: Systems, Tools, and Algorithms
Get PDFMillions of electric vehicles (EVs) will enter service in the next decade, generating gigawatt-hours of additional energy demand. Charging these EVs cleanly, affordably, and without excessive stress on the grid will require advances in charging system design, hardware, monitoring, and control. Collectively, we refer to these advances as smart charging. While researchers have explored smart charging for over a decade, very few smart charging systems have been deployed in practice, leaving a sizeable gap between the research literature and the real world. In particular, we find that research is often based on simplified theoretical models. These simple models make analysis tractable but do not account for the complexities of physical systems. Moreover, researchers often lack the data needed to evaluate the performance of their algorithms on real workloads or apply techniques like machine learning. Even when promising algorithms are developed, they are rarely deployed since field tests can be costly and time-consuming.
The goal of this thesis is to develop systems, tools, and algorithms to bridge these gaps between theory and practice.
First, we describe the architecture of a first-of-its-kind smart charging system we call the Adaptive Charging Network (ACN).
Next, we use data and models from the ACN to develop a suite of tools to help researchers. These tools include ACN-Data, a public dataset of over 80,000 charging sessions; ACN-Sim, an open-source simulator based on realistic models; and ACN-Live, a platform for field testing algorithms on the ACN. Finally, we describe the algorithms we have developed using these tools. For example, we propose a practical and robust algorithm based on model predictive control, which can reduce infrastructure requirements by over 75%, increase operator profits by up to 3.4 times, and significantly reduce strain on the electric power grid. Other examples include a pricing scheme that fairly allocates costs to users considering time-of-use tariffs and demand charges and a data-driven approach to optimally size on-site solar generation with smart EV charging systems.</p
A Shock Compression Investigation of Failure Waves and Phase Transition in Soda-Lime Glass
Get PDFSoda-lime glass (SLG) and other silica glasses find use in many technological applications involving high pressures and strain rates, such as systems with laser-matter interactions, transparent armor, etc. An experimentally validated constitutive model for these glasses is required for modeling their mechanical behavior at high pressures and strain rates. Also, due to the abundance of silica in the earth's crust, understanding the behavior of these glasses at high pressures can provide significant insights into many geophysical processes. To this end, shock compression experiments are carried out on SLG to study the material's behavior under impact stresses of 5-10 GPa. These experiments are accompanied by numerical simulations and constitutive modeling of SLG to gain further insights into the reported failure-wave phenomenon and phase transitions associated with the material.
The significant findings of this study in relation to the failure-wave phenomenon were the sudden densification/compaction of SLG associated with the failure-wave and the disappearance of the failure-wave phenomenon for impact stresses above 10 GPa. When viewed in the context of the findings from past experiments, these results seem to suggest that localized densification/compaction of SLG causes nucleation of cracks and subsequent comminution in the material under shock compression. These results and observations offer a potential explanation of the mechanism underlying the failure-wave phenomenon.
Further, the shock compression and release experiments performed in this work provided significant insights into the onset of possible phase-transition in SLG under shock compression. A loading-unloading hysteresis is observed in the material’s stress-strain curve for impact stresses higher than 5.8 GPa, with the permanent/residual strain increasing with impact stress. Further analysis of these results strongly indicates that the hysteresis is more likely due to a gradual, irreversible phase transition of SLG than due to regular inelastic behavior. Thus, the results suggest that the SLG undergoes a gradual phase transition to a stiffer phase, although other properties of this phase remain unclear. It can also be noted that this phase transition is postulated to start occurring under shock compression of SLG to stresses above 5 GPa, which is also the threshold stress for the onset of the failure-wave phenomenon. It is, therefore, possible that the two phenomena are interrelated. The experimental results from this study are further used to construct a constitutive model to capture the unloading behavior of SLG.</p
Modeling Focal Ratio Degradation, Its Implications for Upcoming Fiber Spectrographs, and the Dynamics of NGC 6822
Get PDFSpectroscopy is a cornerstone of astronomical research, enabling the measurements of abundances and velocities of astronomical objects. Fiber spectroscopy, with its capability to acquire spectra from targets in densely packed fields and position fibers over targets over a large field of view, promises to greatly expand the number of astronomical objects with acquired spectra. Upcoming such projects include the Subaru Prime Focus Spectrograph, which plans to advance cosmology, galactic archaeology, and galactic evolution studies.
However, with the introduction of fibers to the spectrographic optical path also comes the introduction of the issue of focal ratio degradation (FRD). FRD is the scattering of light traveling through a fiber due to effects such as imperfections in the core-clad interface of a fiber, end face preparation, and stresses imposed on the fibers; thus, FRD is inherent to any fiber-based instrument. Focal ratio degradation scatters light to larger angles, resulting in lost light at the spectrograph and redistribution of light in the light's point spread function at the spectrograph detector. These issues are compounded by the fact that each fiber has a unique FRD 'fingerprint' and stresses during telescope operation can vary and induce dynamically changing FRD. It is also difficult to measure precisely.
This thesis focuses on characterization of FRD. It introduces a novel approach to measuring that permits extraction of the effect of angular misalignment and tests this model on fiber mounted to a Cobra fiber positioner, though this method could be utilized for any optical fiber. The effects of focal ratio degradation on the spectra are simulated and found to affect counts 3-4 pixels from the center of sky lines on the order of 1-2% for changes in FRD at the 3 milliradian level, highlighting the importance of a good understanding of FRD to model sky lines at the level desired for the Subaru prime focus spectrograph.
Multiplexed spectroscopy in NGC 6822 from KECK/DEIMOS is also presented. The ancient red giant population in NGC 6822 is prolately rotating, an unusual configuration that has been used as evidence of merger histories in other dwarf galaxies. The distribution of metallicity as a function of radius shows the oldest, most metal-poor stars are the most dispersion dominated. They also reside at larger radii, possibly pointing to a disruptive star formation history that scattered stars from the center of the galaxy over time. Future results from upcoming spectrographic surveys such as the Subaru Prime Focus Spectrograph could illuminate what is driving these peculiar features in NGC 6822 among many other scientific discoveries.</p