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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenSat, 13 Apr 2024 02:12:33 +0000The Hierarchical Algorithms - Theory and Applications
https://resolver.caltech.edu/CaltechETD:etd-10252007-091003
Authors: {'items': [{'id': 'Su-Zheng-Yao', 'name': {'family': 'Su', 'given': 'Zheng-Yao'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/3nfb-m047
<p>Monte Carlo simulations are one of the most important numerical techniques for investigating statistical physical systems. Among these systems, spin models are a typical example which also play an essential role in constructing the abstract mechanism for various complex systems. Unfortunately, traditional Monte Carlo algorithms are afflicted with "critical slowing down" near continuous phase transitions and the efficiency of the Monte Carlo simulation goes to zero as the size of the lattice is increased. To combat critical slowing down, a very different type of collective-mode algorithm, in contrast to the traditional single-spin-flipmode, was proposed by Swendsen and Wang in 1987 for Potts spin models. Since then, there has been an explosion of work attempting to understand, improve, or generalize it. In these so-called "cluster" algorithms, clusters of spin are regarded as one template and are updated at each step of the Monte Carlo procedure. In implementing these algorithms the cluster labeling is a major time-consuming bottleneck and is also isomorphic to the problem of computing connected components of an undirected graph seen in other application areas, such as pattern recognition.</p>
<p>A number of cluster labeling algorithms for sequential computers have long existed. However, the dynamic irregular nature of clusters complicates the task of finding good parallel algorithms and this is particularly true on SIMD (single-instruction-multiple-data machines. Our design of the Hierarchical Cluster Labeling Algorithm aims at alleviating this problem by building a hierarchical structure on the problem domain and by incorporating local and nonlocal communication schemes. We present an estimate for the computational complexity of cluster labeling and prove the key features of this algorithm (such as lower computational complexity, data locality, and easy implementation) compared with the methods formerly known. In particular, this algorithm can be viewed as a generalized scan scheme applicable to problem domains of any high dimension and of arbitrary geometry (scan is an important primitive of parallel computing). In addition, from implementation results, the hierarchical cluster labeling algorithm has proved to work equally well on MIMD machines, though originally designed for SIMD machines.</p>
<p>Based on this success, we further study the hierarchical structure hidden in the algorithm. Hierarchical structure is a conceptual framework frequently used in building models for the study of a great variety of problems. This structure serves not only to describe the complexity of the system at different levels, but also to achieve some goals targeted by the problem, i.e., an algorithm to solve the problem. In this regard, we investigate the similarities and differences between this algorithm and others, including the FFT and the Barnes-Hut method, in terms of their hierarchical structures.</p>
https://thesis.library.caltech.edu/id/eprint/4257Effects of Controlled Disorder on the Vortex Phases of High-Temperature Superconductors
https://resolver.caltech.edu/CaltechETD:etd-06212007-152240
Authors: {'items': [{'id': 'Jiang-Wen', 'name': {'family': 'Jiang', 'given': 'Wen'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/bjp0-2k03
<p>The high transition temperature, short coherence length and long penetration depth of high-temperature superconductors result in novel vortex properties associated with the large thermal and disorder fluctuations. This thesis presents systematic experimental investigations on the vortex phases of YBa2Cu3O7 single crystals with different types of controlled disorder. Measurements of dc current-voltage characteristics and ac impedance as a function of frequency are carried out on dilutely twinned YBa2Cu3O7 single crystals irradiated with 3.0 MeV protons. It is found that the moderately increased vortex pinning caused by the increasing density of controlled point defects does not change the nature of the second-order vortex-glass transitions in YBa2Cu3O7 single crystals with dilute twin boundaries, as manifested by the universal critical exponents and scaling functions for samples with different densities of point defects. In YBa2Cu3O7 single crystals with c-axis correlated columnar defects created by 0.9 GeV Pb-ion irradiation, a Bose-glass to "superfluid" transition is demonstrated by the universal critical scaling behavior of the ac impedance versus frequency isotherms. The static and dynamic critical exponents and the universal scaling functions are determined from our self-consistent critical scaling analyses. The Bose-glass transition temperature is found to decrease with the increasing angle between the applied magnetic field and the column orientation, in contrast to the angular dependence of the vortex-glass transition temperature which increases monotonically with the increasing angle due to the intrinsic sample anisotropy.</p>
<p>The interplay of vortex pinning and thermodynamic vortex phase transitions is further studied in the weak pinning limit by investigating the vortex transport properties of a nearly defect-free, untwinned YBa2Cu3O7 single crystal. Two novel phenomena are observed and studied quantitatively. The resistive hysteresis near the vortex-solid melting transition is ascribed to a current-induced non-equilibrium effect; the resistive "peak effect" below the vortex-solid transition is found to be a general phenomenon existing in extreme type-II superconductors, and is associated with the softening of the vortex-solid before the thermodynamic melting transition. It is therefore concluded that the current-induced effects are of particular importance in determining the vortex properties of extreme type-II superconductors with weak pinning.</p>
https://thesis.library.caltech.edu/id/eprint/2679Pulsed Power Discharges in Water
https://resolver.caltech.edu/CaltechTHESIS:10122016-144310923
Authors: {'items': [{'email': 'kratel@alumni.caltech.edu', 'id': 'Kratel-Axel-Wolf-Hendrik', 'name': {'family': 'Kratel', 'given': 'Axel Wolf Hendrik'}, 'show_email': 'YES'}]}
Year: 1996
DOI: 10.7907/JDPR-ZB83
<p>An Electrohydraulic Discharge Process (EHD) for the treatment of hazardous chemical wastes in water has been developed. Liquid waste in 4 L EHD reactor is directly exposed to high-energy pulsed electrical discharges between two submerged electrodes. The high-temperature (>14,000 K) plasma channel created by an EHD discharge emits ultraviolet radiation, and produces an intense shock wave as it expands against the surrounding water. A simulation of the EHD process is presented along with experimental results. The simulation assumes a uniform plasma channel with a plasma that obeys the ideal gas law and the Spitzer conductivity law. The results agree with previously published data. The simulation is used to predict the total energy efficiency, energy partitioning, maximum plasma channel temperature and pressure for the Caltech Pulsed Power Facility (CPPF). The simulation shows that capacitance, initial voltage and gap length can be used to control the efficiency of the discharge.</p>
<p>The oxidative degradation of 4-chlorophenol (4-CP), 3,4-dichloroaniline (3,4-DCA), and 2,4,6 trinitrotoluene (TNT) in an EHD reactor was explored. The initial rates of degradation for the three substrates are described by a first-order rate equation, where k<sub>0</sub> is the zero-order rate constant that accounts for direct photolysis; and k<sub>1</sub> is the first-order term that accounts for oxidation in the plasma channel region. For 4-CP in the 4.0 L reactor, the values of these two rate constants are k<sub>0</sub> = 0.73 ± 0.08 µM, and k<sub>1</sub>=(9.4 ± 1.4) x 10<sup>-4</sup>. For a 200 µM 4-CP solution this corresponds to an overall intrinsic zero-order rate constant of 0.022 M s<sup>-1</sup>, and a G-value of 4.45 x 10<sup>-3</sup>.</p>
<p>Ozone increases the rate and extent of degradation of the substrates in the EHD reactor. Combined EHD/ozone treatment of a 160 µM TNT solution resulted in the complete degradation of TNT, and a 34% reduction of the total organic carbon (TOC). The intrinsic initial rate constant of TNT degradation was 0.024 M s<sup>-1</sup>. The results of these experiments demonstrate the potential application of the EHD process for the treatment of hazardous wastes.</p>https://thesis.library.caltech.edu/id/eprint/9939Large Scale Molecular Simulations with Application to Polymers and Nano-Scale Materials
https://resolver.caltech.edu/CaltechTHESIS:10202009-133456579
Authors: {'items': [{'id': 'Gao-Guanghua', 'name': {'family': 'Gao', 'given': 'Guanghua'}, 'show_email': 'NO'}]}
Year: 1998
DOI: 10.7907/69rm-7y79
There remain practical problems to predicting structures and properties of materials from first principles, though the foundation, quantum mechanics, has been established for many years. The goals of this research are to develop methods and tools that are accurate and practical, and apply them to important problems. Two aspects of the methodology are focused.
1. The development of accurate force fields based on ab initio quantum mechanical calculations on prototype systems. Procedures were developed on polyvinyl chloride (PVC) and successfully applied on other types of polymers. They are very important to studying of amorphous polymers materials, for which current methods have not been useful in predicting important properties (e.g. moduli and glass temperature).
2. The development of Massive Parallel Simulation (MPSim) Software. MPSim is suitable for large systems (millions of atoms). It has the ability of including environmental variables (temperature, pressure, tension, and shear) and extracting physical properties (moduli and glass temperatures). The theories and algorithms implemented are summarized in the Appendix.
These methods and tools are applied to the accurate simulation of structures and properties of amorphous polymer materials and nano-materials.
Molecular dynamics (MD) simulation on polyethylene (chapter 6) was used to develop a general strategy for predicting glass transition temperatures which is expected to be very important in polymer industry. In chapter 7, these strategies were successfully applied to three important fluoro polymers.
Single-walled carbon nanotubes (SWNT), recently discovered but not very well characterized, is an interesting new class of materials. Using an accurate force field, structures and mechanical properties of these systems are studied. Chapter 2 shows that the dominating factor for deciding stable structures and mechanical properties is the tube size, not chirality. The behavior of (10, 10) nano-tube under bending are studied (chapter 3) based on energy of hypothetical toroids with different radii. Yielding curvature of 1/R_s (R_s = 183.3 (Å)) where elastic bending becomes plastic response is found. In chapter 4, closest packing of K_5C_(80) with the distribution of K atoms along tube surface similar to the stacking of stage one K_1C_8 is established as the optimum structure of K-doped SWNT crystal.
https://thesis.library.caltech.edu/id/eprint/5314The Heat Capacity of Superfluid ⁺He in the Presence of a Constant Heat Flux Near Tλ
https://resolver.caltech.edu/CaltechTHESIS:07252014-090646708
Authors: {'items': [{'id': 'Harter-Alexa-Welsh', 'name': {'family': 'Harter', 'given': 'Alexa Welsh'}, 'show_email': 'NO'}]}
Year: 2001
We present the first experimental evidence that the heat capacity of superfluid <sup>4</sup>He, at temperatures very close to the lambda transition temperature, T<sub>λ</sub>,is enhanced by a constant heat flux, Q. The heat capacity at constant Q, C<sub>Q</sub>,is predicted to diverge at a temperature T<sub>c</sub>(Q) < T<sub>λ</sub> at which superflow becomes unstable. In agreement with previous measurements, we find that dissipation enters our cell at a temperature, T<sub>DAS</sub>(Q),below the theoretical value, T<sub>c</sub>(Q). Our measurements of C<sub>Q</sub> were taken using the discrete pulse method at fourteen different heat flux values in the range 1µW/cm<sup>2</sup> ≤ Q≤ 4µW /cm<sup>2</sup>. The excess heat capacity ∆C<sub>Q</sub> we measure has the predicted scaling behavior as a function of T and Q:∆C<sub>Q</sub> • t<sup>α</sup> ∝ (Q/Q<sub>c</sub>)<sup>2</sup>, where Q<sub>c</sub>T) ~ t<sup>2ν</sup> is the critical heat current that results from the inversion of the equation for T<sub>c</sub>(Q). We find that if the theoretical value of T<sub>c</sub>( Q) is correct, then ∆C<sub>Q</sub> is considerably larger than anticipated. On the other hand,if T<sub>c</sub>(Q)≈ T<sub>DAS</sub>(Q),then ∆C<sub>Q</sub> is the same magnitude as the theoretically predicted enhancement.https://thesis.library.caltech.edu/id/eprint/8606Aspects of Non-Fermi-Liquid Metals
https://resolver.caltech.edu/CaltechETD:etd-05302002-130637
Authors: {'items': [{'id': 'Pivovarov-Evgueny-Eugene', 'name': {'family': 'Pivovarov', 'given': 'Evgueny (Eugene)'}, 'show_email': 'NO'}]}
Year: 2002
DOI: 10.7907/20WA-FT36
We consider several examples of metallic systems that exhibit non-Fermi-liquid behavior. In these examples the system is not a Fermi liquid due to the presence of a "hidden" order. The primary models are density waves with an odd-frequency-dependent order parameter and density waves with d-wave symmetry. In the first model, the same-time correlation functions vanish and there is a conventional Fermi surface. In the second model, the gap vanishes at the nodes. We derive the phase diagrams and study the thermodynamic and kinetic properties. We also consider the effects of competing orders on the phase diagram when the underlying microscopic interaction has a high symmetry.https://thesis.library.caltech.edu/id/eprint/2285New Phases of Two-Dimensional Electrons in Excited Landau Levels
https://resolver.caltech.edu/CaltechETD:etd-05272003-170504
Authors: {'items': [{'email': 'ken.b.cooper@jpl.nasa.gov', 'id': 'Cooper-Kenneth-Brian', 'name': {'family': 'Cooper', 'given': 'Kenneth Brian'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/WAE0-7Z77
The subject of this dissertation is the experimental discovery and investigation of a new class of collective phases in two-dimensional electron systems. The experiments mainly involve magnetotransport measurements in very high quality GaAs/AlGaAs semiconductor heterostructures, where a large perpendicular magnetic field serves to resolve the electrons? energy spectrum into discrete Landau levels. The most dramatic evidence of a new many-body phase is the huge and unprecedented resistance anisotropy observed only below 150 mK and around the half-filling points of the highly excited Landau levels N ≥ 2. Associated with these anisotropic states are other novel electron phases whose transport signature is a vanishing longitudinal conductivity occurring in the flanks of the same excited Landau levels. Although reminiscent of the well-understood integer quantum Hall states, the insulating phases are exceptional for being driven by electron interactions rather than single-particle localization. A persuasive theoretical picture based on "stripe" and "bubble" charge density wave formation in high Landau levels can account for many of the experimental results. For example, the broken orientational symmetry of the stripe state may underlie the observed transport anisotropy, while disorder-induced pinning of the bubble lattice could give rise to the insulating regions in high Landau levels. Further investigation of the anisotropic transport characteristics has elucidated possible symmetry-breaking mechanisms of the purported stripe phase and has provided evidence that the stripes may be more accurately described as a quantum electronic liquid crystal. In addition, experiments involving the breakdown of the insulating regions at high voltage biases may point to a depinning transition of the bubble phase. These results have spurred intense interest in the field of correlated electron systems in two dimensions and may be an indication of the variety of new phenomena in condensed matter systems still awaiting discovery.https://thesis.library.caltech.edu/id/eprint/2125Spin-Polarized Quasiparticle Transport in Cuprate Superconductors
https://resolver.caltech.edu/CaltechETD:etd-10242002-021518
Authors: {'items': [{'email': 'thept2000@yahoo.com', 'id': 'Fu-Chu-Chen', 'name': {'family': 'Fu', 'given': 'Chu-Chen'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/RYWM-VX48
The effects of spin-polarized quasiparticle transport in superconducting YBa<sub>2</sub> Cu<sub>3</sub>O<sub>7-δ</sub> (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. Transport and magnetic properties of these CMR perovskites are first investigated by inducing lattice distortions using lattice mismatching substrates. The half-metallic nature of these perovskites provides an epitaxially grown heterostructure, ideal for injection of spin-polarized current. These effects are compared with the injection of simple quasiparticles into control samples of perovskite non-magnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density (J<sub>c</sub>) as a function of the injection current density, temperature, and the thickness of the superconductor demonstrate the "self-injection effect" and reveal dramatic differences between the F-I-S and N-I-S heterostructures, with strong suppression of J<sub>c</sub> and a rapidly increasing characteristic transport length near the superconducting transition temperature Tc only in the F-I-S samples. The temperature dependence of the efficiency in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveals that a substantial chemical potential shift in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the CuO<sub>2</sub> planes.https://thesis.library.caltech.edu/id/eprint/4227Investigation of Novel Semiconductor Heterostructure Systems: I. Cerium Oxide/Silicon Heterostructures. II. 6.1 Å Semiconductor-Based Avalanche Photodiodes
https://resolver.caltech.edu/CaltechETD:etd-06022003-071834
Authors: {'items': [{'email': 'ejpcmr@yahoo.com', 'id': 'Preisler-Edward-James', 'name': {'family': 'Preisler', 'given': 'Edward James'}, 'show_email': 'YES'}]}
Year: 2003
DOI: 10.7907/59X1-WQ68
<p>The work presented in this thesis concerns the development of two different semiconductor heterostructure technologies.</p>
<p>Part I describes research in the CeO2/silicon heterostructure system. Details are presented concerning the growth of CeO2 on silicon and the reactions that take place at the CeO2/silicon interface. The evolution of this interface as a function of annealing temperature and annealing ambient are studied via in situ x-ray photoelectron spectroscopy (XPS). Studies of metal-CeO2-silicon capacitors are also presented which help to determine the usefulness of this oxide as an alternative gate dielectric for silicon-based device applications.</p>
<p>Part II involves research into the fabrication of avalanche photodiodes (APD's) utilizing the 6.1 A semiconductor system. Certain alloys of Al_xGa_(1-x)Sb are shown to greatly favor hole multiplication which is beneficial for both noise characteristics and gain-bandwidth product. Further, details are presented on the current investigation into using 6.1 A superlattices to acheive even more desirable detector performance.</p>https://thesis.library.caltech.edu/id/eprint/2373Evidence for the Josephson Effect in Quantum Hall Bilayers
https://resolver.caltech.edu/CaltechETD:etd-05192004-092920
Authors: {'items': [{'email': 'ian.spielman@nist.gov', 'id': 'Spielman-Ian-Bairstow', 'name': {'family': 'Spielman', 'given': 'Ian Bairstow'}, 'orcid': '0000-0003-1421-8652', 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/JAXZ-YV11
<p>This thesis presents tunneling measurements on bilayer two-dimensional (2D) electrons systems in GaAs/AlGaAs double quantum wells. 2D-2D tunneling is applied here as a probe of the inter-layer correlated quantum Hall state at total Landau level filling factor ν<sub>T</sub> = 1. This bilayer state is theoretically expected to be an excitonic superfluid with an associated dissipationless current and Josephson effect.</p>
<p>In addition to the conventional signatures of the quantum Hall effect ? a pronounced minimum in R<sub>xx</sub> and associated quantization of R<sub>xy</sub> - the strong inter-layer correlations lead to a step-like discontinuity in the tunneling I ? V. Although reminiscent of the DC Josephson effect, the tunneling discontinuity has a finite extent even at the lowest temperatures (the peak in conductance, dI/dV, is strongly temperature dependent even below 15 mK. The correlations develop when the inter- and intra-layer Coulomb interactions become comparable. The relative importance of which is determined by the ratio of layer separation to average electron spacing. Although this state is theoretically expected to be an excitonic superfluid, the degree to which intra-layer tunneling is Josephson-like is controversial. At a critical layer separation the zero-bias tunneling feature is lost, which we interpret as signaling the quantum phase transition to the uncorrelated state. We study the dependence of the phase transition on electron density and relative density imbalance. In the presence of a parallel magnetic field tunneling probes the response of the spectral function at finite wave vector. These tunneling spectra directly detect the expected linearly dispersing Goldstone mode; our measurement of this mode is in good agreement with theoretical expectations. There remains deep theoretical and experimental interest in this state, which represents a unprecedented convergence in the physics of quantum Hall effects and superconductivity.</p>https://thesis.library.caltech.edu/id/eprint/1869Polarizing ³He by Spin Exchange with Potassium
https://resolver.caltech.edu/CaltechETD:etd-05202004-032505
Authors: {'items': [{'email': 'guodong@caltech.edu', 'id': 'Wang-Guodong', 'name': {'family': 'Wang', 'given': 'Guodong'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/69BR-5G30
<p>We present experimental studies on the spin-exchange optical pumping of ³He using alkali metal potassium vapor. High ³He polarizations are achieved using lower laser power than traditional rubidium-³He pumping studies. In addition, spin-exchange rate coefficient measurements for potassium-³He, rubidium-³He and cesium-³He pairs are reported.</p>
<p>By spin-exchange optical pumping with potassium vapor, a high ³He polarization has been achieved in a mid-sized double-chamber glass target cell using only 5 watts of Ti:sapphire laser power. The spin-exchange rate coefficients of potassium-³He, cesium-³He and rubidium-³He pairs are measured and compared. The results are k<sub>SE</sub> = (4.0 ± 0.3) x 10⁻²⁰ cm³/sec for the potassium-³He pair, k<sub>SE</sub> = (6.5 ± 0.4) x 10⁻²⁰ cm³/sec for the rubidium-³He pair and k<sub>SE</sub> = (13.6 ± 1.3) x 10⁻²⁰ cm³/sec for the cesium-³He pair.</p>
<p>The results are consistent with theoretical predictions and confirm that the efficiency for spin-exchange polarization of ³He with potassium is significantly higher than with rubidium or cesium. The results motivate the development of high power diode lasers tuned to the appropriate potassium wavelengths. Applied to the production of polarized ³He as an atomic beam source for colliders, these developments could have significant impact on future studies of neutron spin structure. These studies may also have impact on magnetic resonance imaging using hyperpolarized ³He.</p>https://thesis.library.caltech.edu/id/eprint/1890Investigation of Spin Injection in Semiconductors: Theory and Experiment
https://resolver.caltech.edu/CaltechETD:etd-05262005-130824
Authors: {'items': [{'email': 'stephan.ichiriu@gmail.com', 'id': 'Ichiriu-Stephan-Robert', 'name': {'family': 'Ichiriu', 'given': 'Stephan Robert'}, 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/Z9CS-0V34
Spin electronics, or spintronics, is a nascent field of research whereby the spin degree of freedom in electronic devices is exercised. The electroluminescence polarization of the spin light emitting diode (spin-LED) is important in the characterization of spin injection efficiency into non-magnetic semiconductors. The validity of these measurements is questioned due to the use of large external magnetic fields during measurement and the effects of reflection and refraction within the semiconductor structure. A Monte Carlo ray-tracing simulation for the spin-LED was written to address these issues and a device-dependent polarization correction factor was calculated for the Fe/AlGaAs system to account for these effects. Spin injection into AlGaAs from Fe and Co-Cr spin aligning contacts via a Schottky barrier was measured. Fe was chosen because of the strong spin polarization of conduction electrons at the Fermi level, while Co-Cr was selected because of its properties as a perpendicular magnet for certain alloy concentrations. The contacts were epitaxially grown at room temperature by electron-beam evaporation. These samples were measured to have zero spin injection. The results were attributed to the Schottky barrier properties.https://thesis.library.caltech.edu/id/eprint/2093Evidence for Excitonic Superfluidity in a Bilayer Two-Dimensional Electron System
https://resolver.caltech.edu/CaltechETD:etd-08102004-204105
Authors: {'items': [{'id': 'Kellogg-Melinda-Jane', 'name': {'family': 'Kellogg', 'given': 'Melinda Jane'}, 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/7ZKQ-QD67
The discovery of the integer quantum Hall effect (QHE) and the fractional quantum Hall effect (FQHE) revealed that unexpected physics could be found in a seemingly very simple system: free electrons constrained to move in only two dimensions. Adding a degree of complexity to this system by bringing two of these layers of two-dimensional electrons into close proximity, multiplies the exciting physical phenomena available for study and discovery. This thesis is a report on electrical transport studies of bilayer two-dimensional electron systems (2DES) found in GaAs/AlGaAs double quantum well semiconductor heterostructures. Through studies at zero magnetic field using a fairly new transport measurement called "Coulomb drag" pure electron-electron scattering is measured with unprecedented accuracy and clarity. In large magnetic fields applied perpendicular to the electron layers, at the right combination of magnetic field strength, electron density and layer separation, a new, uniquely bilayer, many-body quantum ground state exists that can be described alternately as an itinerant pseudospin ferromagnet or as a Bose-Einstein condensate (BEC) of interlayer excitons. This bilayer quantum state was first predicted theoretically fifteen years ago, and its discovery and exploration is the basis of this thesis. In this thesis, transport measurements allow for the direct detection of the BEC of excitons by their ability to flow with vanishing resistance and vanishing influence from the large external magnetic field. Excitonic BEC has been pursued experimentally for almost 40 years, but this thesis likely represents the first detection of the elusive state. Coulomb drag is found to be an excellent probe of the phase transition out of the bilayer quantum state and is used to extend the mapping of the phase diagram into the temperature and layer density imbalance planes.
https://thesis.library.caltech.edu/id/eprint/3080Experiments in Cavity QED: Exploring the Interaction of Quantized Light with a Single Trapped Atom
https://resolver.caltech.edu/CaltechETD:etd-05272005-163452
Authors: {'items': [{'email': 'andreea.boca@jpl.nasa.gov', 'id': 'Boca-Andreea', 'name': {'family': 'Boca', 'given': 'Andreea'}, 'orcid': '0000-0001-6877-7295', 'show_email': 'YES'}]}
Year: 2005
DOI: 10.7907/BK2X-HX12
<p>The experiments discussed in this thesis focus on the interaction of a single trapped atom with the single mode of a high-finesse optical cavity, in the regime of strong coupling.</p>
<p>Chapter 1 gives a brief introduction, after which Chapter 2 describes our recent measurements of the transmission spectrum of the atom-cavity system. The spectrum exhibits a clearly resolved vacuum-Rabi splitting, in good quantitative agreement with theoretical predictions. A new Raman scheme for cooling atomic motion along the cavity axis enables a complete spectrum to be recorded for an individual atom trapped within the cavity mode, in contrast to all previous measurements of this type that have required averaging over 10^3-10^5 atoms.</p>
<p>Chapter 3 discusses our observations of photon blockade for the transmitted light in the presence of one trapped atom. Excitation of the atom-cavity system by a first photon blocks the transmission of a second one, thereby converting an incident Poissonian stream of photons into a sub-Poissonian, anti-bunched stream, as confirmed by measurements of the photon statistics of the transmitted field. The intensity correlations of the cavity transmission also reveal the energy distribution for oscillatory motion of the trapped atom.</p>
<p>Chapter 4 details a set of simple but necessary measurements of relevant experimental parameters such as cavity geometry, linewidth, mirror properties, birefringence, and detection efficiency. The thesis concludes with Appendix A, describing the efficient laser setup we use for our magneto-optical traps.</p>https://thesis.library.caltech.edu/id/eprint/2161Scanning Tunneling Spectroscopy Studies of High-Temperature Cuprate Superconductors
https://resolver.caltech.edu/CaltechETD:etd-05222006-124257
Authors: {'items': [{'id': 'Chen-Ching-Tzu', 'name': {'family': 'Chen', 'given': 'Ching-Tzu'}, 'show_email': 'NO'}]}
Year: 2006
DOI: 10.7907/11FP-3M88
<p>This thesis presents the scanning tunneling spectroscopic studies of the non-universal electronic properties among electron- and hole-doped cuprates. Tunneling spectra of the electron-doped Sr0.9La0.1CuO2 and the hole-doped YBa2Cu3O6+delta reveal distinctly different behavior in the pairing symmetries, pseudogap phenomena, satellite features, and low-energy excitations. While underdoped and optimally doped YBa2Cu3O6+delta exhibits d-wave and overdoped Ca-doped YBa2Cu3O6+delta (d+s)-wave pairing symmetry, the electron-doped Sr0.9La0.1CuO2 shows fully gapped s-wave pairing symmetry. The absence of the satellite features and pseudogap in tunneling spectra of electron-doped cuprates sharply contrasts with their general presence in hole-doped cuprates. Furthermore, the subgap low-energy spectral characteristics of Sr0.9La0.1CuO2 deviate substantially from the mean-field Bardeen-Cooper-Schrieffer theory, while those of YBa2Cu3O6+delta can be fully accounted for by the mean-field generalized Blonder-Tinkham-Klapwijk formalism.</p>
<p>Despite the aforementioned disparities, several experimental results reveal important connections between the two types of cuprates. For instance, the coexistence of the pseudogap and superconducting spectra in hole-doped cuprates and the observations of the current- and field-induced pseudogap in electron-doped cuprates suggest that competing orders, manifested as the pseudogap, coexist with superconductivity in both types of cuprates. In addition, by comparing the tunneling spectra with the high-field vortex dynamics measurements, we find that the quasiparticle spectral characteristics of Sr0.9La0.1CuO2 and YBa2Cu3O6+delta correlate with the degree of field-induced quantum phase fluctuations of the two compounds.</p>
<p>Based on these findings, we propose a simple model of coexisting density waves with superconductivity to unify the apparent non-universal phenomena among cuprate superconductors. By incorporating quantum phase fluctuations and adopting realistic band structures, numerical simulations of the quasiparticle tunneling spectra demonstrate excess subgap low-energy excitations, which is consistent with the empirical observations in Sr0.9La0.1CuO2. Furthermore, by tuning the ratio of the density waves to superconductivity, the theoretical calculations reproduce the absence of pseudogap phenomena in electron-doped cuprates and the general presence of he pseudogap in hole-doped cuprates. Thereby, we conclude that the competing orders that coexist with superconductivity in cuprate superconductors contribute to the rich cuprate phenomenology.</p>https://thesis.library.caltech.edu/id/eprint/1943Experiments on the Self-Organized Critical State of ⁴He
https://resolver.caltech.edu/CaltechETD:etd-06062006-094705
Authors: {'items': [{'id': 'Chatto-Andrew-Rosenberg', 'name': {'family': 'Chatto', 'given': 'Andrew Rosenberg'}, 'show_email': 'NO'}]}
Year: 2006
DOI: 10.7907/NN9C-1A08
<p>When a heat flux is applied downwards through a sample of ⁴He near the superfluid transition temperature T<sub>λ</sub>, the gradient in the temperature self-organizes to the gradient in T<sub>λ</sub> caused by gravity. This creates the Self-Organized Critical (SOC) state. Previous experiments have observed the state, measured the self-organization temperature T<sub>SOC</sub> vs. heat flux, and investigated a remarkable wave that only travels upwards against the flow of the heat flux.</p>
<p>We report the first results of the heat capacity of the SOC state, C<sub>∇T</sub>, for heat fluxes 60 nW/cm² < Q < 13 uW/cm² and corresponding temperatures 9 nK > T<sub>SOC</sub>-T<sub>λ</sub> > -1.1 uK. We find that C<sub>∇T</sub> tracks the static (i.e., zero heat flux) unrounded (i.e., in zero gravity) heat capacity C_0 with two exceptions. The first is that within 250 nK of T<sub>λ</sub>, C_gradT is depressed relative to C₀ and the maximum in C<sub>∇T</sub> is shifted to 50 nK below T<sub>λ</sub>. The second difference is that at high heat flux, C<sub>∇T</sub> is again depressed relative to C₀ with the departure starting at about 650 nK below T<sub>λ</sub>.</p>
<p>We present the most extensive measurements of the speed and attenuation of the SOC wave to date. We report wave speed measurements taken over our full experimental range 30 nW/cm² < Q < 13 uW/cm² and attenuation results over the limited range that produced enough attenuation to measure. We also report the first accurate calculation of the speed of the SOC wave.</p>https://thesis.library.caltech.edu/id/eprint/2479Structural Dynamics by Ultrafast Electron Crystallography
https://resolver.caltech.edu/CaltechETD:etd-01302007-102801
Authors: {'items': [{'email': 'songyech@caltech.edu', 'id': 'Chen-Songye', 'name': {'family': 'Chen', 'given': 'Songye'}, 'orcid': '0000-0001-5407-5049', 'show_email': 'YES'}]}
Year: 2007
DOI: 10.7907/PE9B-7R72
Ultrafast electron crystallography (UEC), combining the ultrafast time resolution with femtosecond lasers and the atomic spacial resolution with electron crystallography, is developed and applied to elucidate the structural dynamics in solids, surfaces and macromolecular systems. The UEC experiments for surface studies were first demonstrated on semiconductor surfaces. Coherent nonthermal motions of atoms following ultrafast laser irradiation were shown with the timescales in picoseconds, and the amplitude of the motions was determined in picometer. Using Langmuir-Blodgett films, two-dimensional crystalline monolayer, bilayers and multilayers of fatty acids and phospholipids were also studied by UEC. The atomic structures under different preparation conditions were determined. The structural dynamics following a temperature jump induced by femtosecond laser on the substrates were obtained and compared to the equilibrium temperature dependence. It was observed that a coherent anisotropic expansion solely along the aliphatic chains happens at picosecond timescale, followed by nonequilibrium contraction and restructuring at longer times. The effects of different molecules, layer thickness and substrate on the dynamics were examined. Unlike monotonic disordering in the equilibrium heating, a transient structural ordering was revealed on the picosecond timescale.https://thesis.library.caltech.edu/id/eprint/409Non-Abelian Anyons and Interferometry
https://resolver.caltech.edu/CaltechETD:etd-06042007-101617
Authors: {'items': [{'email': 'parsab@microsoft.com', 'id': 'Bonderson-Parsa-Hassan', 'name': {'family': 'Bonderson', 'given': 'Parsa Hassan'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/5NDZ-W890
This thesis is primarily a study of the measurement theory of non-Abelian anyons through interference experiments. We give an introduction to the theory of anyon models, providing all the formalism necessary to apply standard quantum measurement theory to such systems. This formalism is then applied to give a detailed analysis of a Mach-Zehnder interferometer for arbitrary anyon models. In this treatment, we find that the collapse behavior exhibited by a target anyon in a superposition of states is determined by the monodromy of the probe anyons with the target. Such measurements may also be used to gain knowledge that would help to properly identify the anyon model describing an unknown system. The techniques used and results obtained from this model interferometer have general applicability, and we use them to also describe the interferometry measurements in a two point-contact interferometer proposed for non-Abelian fractional quantum Hall states. Additionally, we give the complete description of a number of important examples of anyon models, as well as their corresponding quantities that are relevant for interferometry. Finally, we give a partial classification of anyon models with small numbers of particle types.https://thesis.library.caltech.edu/id/eprint/2447Probing Electronic Properties of Carbon Nanotubes
https://resolver.caltech.edu/CaltechETD:etd-06022008-222640
Authors: {'items': [{'email': 'jinseong@caltech.edu', 'id': 'Heo-Jinseong', 'name': {'family': 'Heo', 'given': 'Jinseong'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/A83N-TN35
Carbon nanotubes are quasi-one-dimensional objects that have many remarkable electronic properties. In Chapter I, an electrostatic force microscopy technique to probe the local density of states of single-walled carbon nanotubes (SWCNTs) under ambient conditions is described. Coupling the atomic force microscope tip motion with the quantum capacitance of nanotubes enables the van Hove singularities in the one-dimensional density of states to be resolved. We utilized this technique to identify individual semiconducting and metallic tubes, and further to estimate the chiral angle of a nanotube. Moreover, in order to realize a SWCNT interferometer, nanotube loop devices where a self-crossing geometry yields two electron paths that is a possible analog of the optical Sagnac interferometer are fabricated and explored in Chapter II. Scanning gate microscopy reveals for semiconducting devices a 0–50% transmission probability into the loop segment at the junction, which can be controlled by applying back gate voltage, hence shifting the Fermi level of the nanotube. Metallic loop devices having low contact resistance showed a large- scale conductance peak with fast oscillations superposed on it. Possible theoretical explanations including Sagnac-type interference, which takes the velocity difference between left and right movers in to account, and Fabry-Perot-type interference are compared with the experimental observations. In Chapter III, in accordance with increasing demand for developing spin-electronic devices, cobalt-filled multi-walled carbon nanotubes (Co–filled MWCNTs) are first synthesized and imaged by transmission electron microscopy, and also characterized by various spectroscopy tools like X–ray diffraction and energy dispersive X–ray spectrometry. Further, a Co–filled MWCNT device having reproducible switching in magnetoresistance was demonstrated. The last topic, in Chapter IV, covers the effects of a transverse electric field in MWCNT devices, where conductance fluctuations as a function of the transverse electric field were observed. The electric field spacing between the peaks of the fluctuations is in agreement with the theoretical predictions of band structure modulation by transverse electric fields. Future work following our experimental studies is proposed and discussed at the end of each chapter.https://thesis.library.caltech.edu/id/eprint/2410Fluid Phase Thermodynamics: I) Nucleate Pool Boiling of Oxygen under Magnetically Enhanced Gravity and II) Superconducting Cavity Resonators for High-Stability Frequency References and Precision Density Measurements of Helium-4 Gas
https://resolver.caltech.edu/CaltechETD:etd-07172007-132955
Authors: {'items': [{'email': 'corcoted@gmail.com', 'id': 'Corcovilos-Theodore-Allen', 'name': {'family': 'Corcovilos', 'given': 'Theodore Allen'}, 'orcid': '0000-0001-5716-1188', 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/YGPG-W942
<p>Although fluids are typically the first systems studied in undergraduate thermodynamics classes, we still have only a rudimentary phenomenological understanding of these systems outside of the classical and equilibrium regimes. Two experiments will be presented. First, we present progress on precise measurements of helium-4 gas at low temperatures (1 K-5 K). We study helium because at low densities it is an approximately ideal gas but at high densities the thermodynamic properties can be predicted by numerical solutions of Schroedinger's equation. By utilizing the high resolution and stability in frequency of a superconducting microwave cavity resonator we can measure the dielectric constant of helium-4 to parts in 10<sup>9</sup>, corresponding to an equivalent resolution in density. These data will be used to calculate the virial coefficients of the helium gas so that we may compare with numerical predictions from the literature. Additionally, our data may allow us to measure Boltzmann's constant to parts in 10<sup>8</sup>, a factor of 100 improvement over previous measurements. This work contains a description of the nearly-completed apparatus and the methods of operation and data analysis for this experiment. Data will be taken by future researchers.</p>
<p>The second experiment discussed is a study of nucleate pool boiling. To date, no adequate quantitative model exists of this everyday phenomenon. In our experiment, we vary one parameter inaccessible to most researchers, gravity, by applying a magnetic force to our test fluid, oxygen. Using this technique, we may apply effective gravities of 0-80 times Earth's gravitational acceleration (<i>g</i>). In this work we present heat transfer data for the boiling of oxygen at one atmosphere ambient pressure for effective gravity values between 1<i>g</i> and 16<i>g</i> . Our data describe two relationships between applied heat flux and temperature differential: at low heat flux the system obeys a power law and at high heat flux the behavior is linear. We find that the transition heat flux between these two regimes scales as the 4th root of the gravitational acceleration, which may indicate a relationship to the critical heat flux. Additionally, we find that the low heat flux power law exponent is independent of gravity and the power law scale coefficient increases linearly with gravity.</p>
https://thesis.library.caltech.edu/id/eprint/2914Studies of the Low-Energy Quasiparticle Excitations in High-Temperature Superconducting Cuprates with Scanning Tunneling Spectroscopy and Magnetization Measurements
https://resolver.caltech.edu/CaltechETD:etd-06082009-200539
Authors: {'items': [{'email': 'beyer.andy@gmail.com', 'id': 'Beyer-Andrew-David', 'name': {'family': 'Beyer', 'given': 'Andrew David'}, 'show_email': 'YES'}]}
Year: 2009
DOI: 10.7907/MM6C-AS16
<p>This thesis details the investigation of the unconventional low-energy quasiparticle excitations in both hole-and electron-type cuprate superconductors through experimental studies and theoretical modeling. The experimental studies include spatially resolved scanning tunneling spectroscopy (STS) experiments and bulk magnetization measurements, and the theoretical modeling involves developing a phenomenology that incorporates coexisting competing orders and superconductivity in the ground state of the cuprates.</p>
<p>Magnetic field and temperature dependent evolution of the spatially resolved quasiparticle excitation spectra in the electron-type cuprate La<sub>0.1</sub>Sr<sub>0.9</sub>CuO<sub>2</sub> (La-112), the simplest structured cuprate superconductor with T<sub>C</sub> = 43 K, are investigated experimentally for the first time. For temperature (T) less than the superconducting transition temperature (T<sub>C</sub>), and in zero field, the quasiparticle spectra of La-112 exhibits gapped behavior with two coherence peaks and no satellite features. For magnetic field measurements at T << T<sub>C</sub>, vortices are observed in La-112, which is the first direct observation of vortices among electron-type cuprate superconductors. Moreover, pseudogap-like spectra are revealed inside the core of vortices, where superconductivity is suppressed. The intra-vortex pseudogap-like spectra are characterized by an energy gap of V<sub>PG</sub>=(8.5±0.6)meV, while the inter-vortex quasiparticle spectra show larger peak-to-peak gap values characterized by Δ<sub>pk-pk</sub>(H) ≥ V<sub>PG</sub>, and Δ<sub>pk-pk</sub>(0)=(12.2±0.8)meV ≥Δ<sub>pk-pk</sub>(H>0). The quasiparticle spectra are found to be gapped at all locations up to the highest magnetic field examined (H = 6T) and reveal an apparent low-energy cutoff at the V<sub>PG</sub> energy scale. This finding is in stark contrast to the vortex-state quasiparticle spectra in conventional superconductors, where the intra-vortex spectra near vortex cores exhibit a sharp zero-bias conductance peak due to the complete suppression of superconductivity and the presence of continuous bound quasiparticle states. The lack of a zero-bias peak and the observation of pseudogap-like spectra in the intra-vortex quasiparticle spectra of La-112 suggest that superconductivity alone cannot describe the STS results.</p>
<p>Similar studies of the magnetic field and temperature dependent evolution of the spatially resolved quasiparticle excitation spectra in the hole-type cuprate YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> (Y-123) have also been carried out. The quasiparticle spectra for T << T<sub>C</sub>(~93 K) show satellite features at an energy higher than the superconducting gap, and the superconducting gap is found to be associated with a set of coherence peaks for H = 0. The coherence peaks are homogeneous, with a energy gap given by Δ<sub>SC</sub>=(20±1)meV, and may be attributed to superconductivity. The satellite features are less homogeneous, with a effective gap energy Δ<sub>eff</sub>=(37.8±2.0)meV. The application of magnetic fields reveal vortices in Y-123, and the intra-vortex quasiparticle spectra show two energy gaps, with one gap at the pseudogap energy scale V<sub>PG</sub>~32meV and the other gap at the subgap energy scale Δ' ~ 7-12meV < Δ<sub>SC</sub>. In contrast, the inter-vortex quasiparticle spectra reveal only one energy gap at Δ<sub>SC</sub>~20meV. A dramatic shift in the peak-to-peak gaps, Δ<sub>pk-pk</sub>(H), from Δ<sub>SC</sub> to both V<sub>PG</sub> and Δ' with increasing magnetic field is observed. In addition, higher spatial resolution STS measurements were performed in Y-123 to investigate the spatial dependence of the quasiparticle spectra in more detail. The experimental resolution allowed Fourier-transformed local density of states analysis to be performed. Energy-dependent dispersive diffraction modes attributable to quasiparticle scattering interferences (QPI) were seen, as well as three energy-independent modes not due to QPI. The energy-independent modes corresponded to periodic real-space conductance modulations along the Cu-O bonding and the nodal directions attributable to a pair-density wave, a charge-density wave, and a spin-density wave. The totality of data in Y-123 suggests that the ground state of Y-123 contains competing orders coexisting with superconductivity and not superconductivity alone.</p>
<p>In addition to the STS experiments, the effects of unconventional quasiparticle excitations on macroscopic superconductivity and vortex phase diagrams are investigated from bulk magnetization measurements on several different families of superconducting cuprate samples. Evidence for strong field-induced quantum phase fluctuations and quantum criticality are observed in the vortex phase diagrams of all samples considered. The origin of the apparent quantum criticality and strong field-induced quantum phase fluctuations due to the nearby presence of competing orders is discussed.</p>
<p>Finally, a "two-gap" phenomenological model, describing the excitations from a ground state of coexisting superconductivity and a competing order, is used to quantitatively model the unconventional quasiparticle excitations observed in the measurements of the local tunneling density of states and the angle-resolved photoemission spectroscopy (ARPES) experiments. The phenomenological model is found to provide consistent accounts for the quasiparticle tunneling data from our measurements in La-112 and Y-123, as well as experimental data by others on different cuprates.</p>
https://thesis.library.caltech.edu/id/eprint/2523Thermal Properties and Nanoelectromechanical System Based on Carbon Nanotubes
https://resolver.caltech.edu/CaltechETD:etd-03272009-033303
Authors: {'items': [{'email': 'hsinying@gmail.com', 'id': 'Chiu-Hsin-Ying', 'name': {'family': 'Chiu', 'given': 'Hsin-Ying'}, 'orcid': '0000-0002-6753-3261', 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/2T53-P806
<p>In Chapter I, the fundamental electronic properties of two-dimensional (2D) graphene and one-dimensional (1D) carbon nanotubes are discussed, along with the carbon nanotube single-electron transistors (SETs). In addition to nanotubes' extraordinary electronic properties, the phenomena of phonon transport in carbon nanotubes are also notable. In Chapter II, we discuss our experiments probing the thermal properties of multi-walled carbon nanotubes. We exploit the specific breakdown temperature under a large current, which provides an effective thermometer, in conjunction with the known power input to measure the thermal conductivity of the nanotubes. Our results reveal the exceptional micron-scale phonon mean free path at temperatures approaching 900K, and we demonstrate the first evidence for ballistic phonon propagation in nanotubes, reaching a regime where the thermal conductance of nanotubes is limited only by fundamental quantum mechanical limits imposed by their 1D nature.</p>
<p>Moreover, the combination of remarkable electrical and mechanical properties makes carbon nanotubes a highly promising candidate for nanoelectromechanical systems (NEMS). In Chapter III, we investigate using doubly clamped suspended single-walled carbon nanotubes as nanomechanical resonators at cryogenic temperatures. Their intrinsic single-electron transistor behavior provides a mixing mechanism to self-detect their motion based on their capacitance to a nearby gate electrode. We exploit our devices to attain an ultrasensitive mass sensor, realizing atomic-scale mass sensing. Finally, in Chapter IV, nanoelectromechanical switches based on using multi-walled carbon nanotubes as nanoscale linear bearings are discussed. First we demonstrate the preparation of the initial OFF state by using electrical breakdown to create gaps in a free-standing MWNT device, while subsequently the ON state is actuated with electrical forces and undergoes linear bearing motion that telescopes the inner shells to bridge the gaps. The switching cycle can be performed in double-walled nanotube devices by restoring the insulating OFF state with a controllable gate voltage. These tubular switches can potentially serve as nonvolatile memory or logic gate elements.</p>
https://thesis.library.caltech.edu/id/eprint/1185Ultrafast Electron Crystallography: Principles and Applications
https://resolver.caltech.edu/CaltechETD:etd-05082009-170032
Authors: {'items': [{'email': 'yang@uh.edu', 'id': 'Yang-Ding-Shyue-Jerry', 'name': {'family': 'Yang', 'given': 'Ding-Shyue (Jerry)'}, 'orcid': '0000-0003-2713-9128', 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/Y61P-2B24
<p>During the last 20 to 30 years, the development and application of time-resolved experimental techniques with a femtosecond temporal resolution have brought to us much knowledge about the fundamental processes in physics, chemistry and biology. Nevertheless, standard spectroscopic methods have their limitation in the determination of the transient structures during ultrafast dynamics at the atomic level, because the spatial resolution is restricted by the wavelength of the probe pulse used. In contract, with the scheme of femtosecond optical initiation and electron probing and through the diffraction phenomenon, ultrafast electron crystallography (UEC) was recently developed as a time-resolved structure-probing technique for condensed-matter studies. The short wavelength and small pulse duration of the highly accelerated electrons used provide the atomic-scale spatiotemporal resolution. In addition, the large electron–matter interaction enables the detection of small transient changes as well as the investigation of surface and interfacial phenomena.</p>
<p>This thesis describes the principles of UEC and its applications to a variety of systems, ranging from nanometer-scale structures to highly correlated materials and to interfacial assemblies. By using a prototype semiconducting material, we elucidated the fundamental processes at work in different parts of the femtosecond-to-nanosecond time range; this investigation led to a conceptual change from the consideration of laser-induced heating to the examination of nonequilibrium structural modifications as a result of the transient dynamical changes in, e.g., carriers, the crystal potential, and phonons. On the basis of such an understanding, we observed and understood the colossal unidirectional expansion induced by the photoexcitation of nanostructures to be a potential-driven result rather than a thermal one.</p>
<p>For highly correlated materials, we showed the effectiveness of UEC in resolving the transient intermediate structures during phase transformations as well as identifying new phases in the nonequilibrium state. An important breakthrough made by UEC was the confirmation of the anisotropic involvement of lattice in the electron pairing mechanism for high-temperature superconductors. In interfacial assemblies, we also found a nonequilibrium phase transformation in water and the phenomenon of ultrafast annealing for a better order in a self-assembled monolayer. With these successful experiences, we expect more condensed-matter studies by UEC to come.</p>
https://thesis.library.caltech.edu/id/eprint/1685Using Graph States for Quantum Computation and Communication
https://resolver.caltech.edu/CaltechETD:etd-05272009-130323
Authors: {'items': [{'email': 'kovid@kovidgoyal.net', 'id': 'Goyal-Kovid', 'name': {'family': 'Goyal', 'given': 'Kovid'}, 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/WR8C-1H18
<p>In this work, we describe a method to achieve fault tolerant measurement based quantum computation in two and three dimensions. The proposed scheme has an threshold of 7.8*10^-3 and poly-logarithmic overhead scaling. The overhead scaling below the threshold is also studied. The scheme uses a combination of topological error correction and magic state distillation to construct a universal quantum computer on a qubit lattice. The chapters on measurement based quantum computation are written in review form with extensive discussion and illustrative examples.</p>
<p>In addition, we describe and analyze a family of entanglement purification protocols that provide a flexible trade-off between overhead, threshold and output quality. The protocols are studied analytically, with closed form expressions for their threshold.</p>
https://thesis.library.caltech.edu/id/eprint/2177Investigations of Nanoscale Variations in Spin and Charge Transport in Manganites and Organic Semiconductors Using Spin Polarized Scanning Tunneling Spectroscopy
https://resolver.caltech.edu/CaltechTHESIS:02082010-153048901
Authors: {'items': [{'email': 'chughes@its.caltech.edu', 'id': 'Hughes-Cameron-Richard', 'name': {'family': 'Hughes', 'given': 'Cameron Richard'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/TX09-6W39
<p>Spintronics is a new class of spin-dependent electronics with great potential for nonvolatile memory and logic technology. Additionally, spintronics may be combined with optoelectronic applications to achieve higher efficiency and novel capabilities. All of these developments require growth and characterization of new materials to polarize and transport electron spin currents. In this context, spin-polarized and non spin-polarized spatially resolved conductance measurements performed by scanning tunneling microscopy (STM) are effective means to investigate the spin and charge quantum transport in magnetic and organic systems, particularly for systems that are prone to phase separations and complex magnetic properties, such as the colossal magnetoresistive (CMR) manganites La<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> (LCMO) that are known to exhibit intrinsic electronic heterogeneity due to strong electronic correlation and competing orders in the ground state. Additionally, STM measurements can provide direct information about the band structure and mobility of the organic semiconductor 8-hydroxyquinoline aluminum (Alq<sub>3</sub>) in the Alq<sub>3</sub>/LCMO heterostructures to further understand their performance in spintronic devices.</p>
<p>The manganite compound La<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> (LCMO) with a bulk doping level x = 0.3 is a ferromagnetic metal with a relatively high Curie temperature T<sub>c</sub> = 270K. This system is promising for spintronic device applications, and may be used as a spin current injector because of the gapped band structure for minority spins, a property known as half-metallicity. On the other hand, even in this bulk ferromagnetic metallic phase, inherent electronic inhomogeneity at microscopic scales is expected. To further study this effect, we have investigated x = 0.3 LCMO thin films using scanning tunneling microscopy in spectroscopic mode under varied temperature, magnetic field and spin polarization of the tunneling current. Spatially resolved maps of tunneling conductance taken with non polarized Pt/Ir tip show variations on the scale of a few hundred nanometers in size in the bulk ferromagnetic state, which are believed to be the result of intrinsic inhomogeneity of the manganites due to their tendency toward phase separation. Maps of tunneling conductance taken with spin-polarized Cr coated tips are consistent with the convolution of the LCMO and Cr density of states, and below the T<sub>c</sub> of LCMO the spin-polarized tunnel junction can be described as a spin valve configuration. The electronic homogeneity in the material increases above the magnetic ordering temperature, or with application of magnetic field in the bulk ferromagnetic state. We identified gaps in the conductance at two separate characteristic energies. The first gap of energy approximately 0.6 eV is believed to arise from a ferromagnetic insulator (FI) surface phase due to its disappearance above the Curie temperature (T<sub>c</sub>) and the dependence of gap energy on relative tip and sample magnetic orientation. The surface phase may be stabilized by Ca deficiency at the LCMO surface, corroborated by x-ray photoemission spectroscopy (XPS). Second, we observe a nearly temperature independent and spatially varying gap of approximately 0.4 eV for all zero-field tunneling spectra, which is believed to be associated with the psuedogap (PG) phenomena in the manganites. Application of a magnetic field converts the regions of PG phenomena to FI, in conjunction with an increase in the homogeneity of the lm conductance. These findings suggest that the PG phenomena arise from electronic inhomogeneity in the manganite film, in agreement with theoretical investigations, and that the vertical and lateral electronic inhomogeneity, along with its dependence on temperature and applied magnetic field, has important implications for use of these materials in high-density nanoscale spintronic devices.</p>
<p>We have also successfully deposited and investigated Alq<sub>3</sub>/LCMO heterostructures of varying thicknesses to investigate charge transport in Alq<sub>3</sub>. Bulk Alq<sub>3</sub> structural properties are preserved down to 10 nm in thickness with a -0.3 eV offset in band energies. The lack of band bending between LCMO and Alq<sub>3</sub> is suggestive of a shift in the preferred isomer from meridinial to facial at the interface. The absence of polaron states from our STM studies implies the relative unimportance of polarons in Alq<sub>3</sub> for this heterostructure. In addition, the measured mobilities on the order of 10<sup>-5</sup>cm<sup>2</sup>(Vs)<sup>-1</sup> for electrons and holes in Alq<sub>3</sub> lms deposited on heated LCMO substrates more closely resemble values of the intrinsic mobility estimated from the muon spin relaxation measurements than those from studies of the bulk LED structures, suggesting that superior film conductivity close to the fundamental limit is possible with a heated substrate during sublimation.</p>
https://thesis.library.caltech.edu/id/eprint/5546Simulation of Strongly Correlated Quantum Many-Body Systems
https://resolver.caltech.edu/CaltechTHESIS:04082011-161930834
Authors: {'items': [{'email': 'ersenbilgin@gmail.com', 'id': 'Bilgin-Ersen', 'name': {'family': 'Bilgin', 'given': 'Ersen'}, 'show_email': 'NO'}]}
Year: 2011
DOI: 10.7907/FQDK-A221
In this thesis, we address the problem of solving for the properties of interacting quantum many-body systems in thermal equilibrium. The complexity of this problem increases exponentially with system size, limiting exact numerical simulations to very small systems. To tackle more complex systems, one needs to use heuristic algorithms that approximate solutions to these systems. Belief propagation is one such algorithm that we discuss in chapters 2 and 3. Using belief propagation, we demonstrate that it is possible to solve for static properties of highly correlated quantum many-body systems for certain geometries at all temperatures. In chapter 4, we generalize the multiscale renormalization ansatz to the anyonic setting to solve for the ground state properties of anyonic quantum many-body systems. The algorithms we present in chapters 2, 3, and 4 are very successful in certain settings, but they are not applicable to the most general quantum mechanical systems. For this, we propose using quantum computers as we discuss in chapter 5. The dimension reduction algorithm we consider in chapter 5 enables us to prepare thermal states of any quantum many-body system on a quantum computer faster than any previously known algorithm. Using these thermal states as the initialization of a quantum computer, one can study both static and dynamic properties of quantum systems without any memory overhead.https://thesis.library.caltech.edu/id/eprint/6282Coherent Control of Entanglement with Atomic Ensembles
https://resolver.caltech.edu/CaltechTHESIS:05192011-130117986
Authors: {'items': [{'email': 'kyung114@gmail.com', 'id': 'Choi-Kyung-Soo', 'name': {'family': 'Choi', 'given': 'Kyung Soo'}, 'show_email': 'NO'}]}
Year: 2011
DOI: 10.7907/9T7P-2C53
<p>Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light.</p>
<p>By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems.</p>
<p>In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection.</p>
<p>Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter.</p>
<p>Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2).</p>
<p>Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.</p>https://thesis.library.caltech.edu/id/eprint/6410Variational Studies of Exotic Bose Liquid, Spin Liquid, and Magnetic Phases
https://resolver.caltech.edu/CaltechTHESIS:05282011-122022010
Authors: {'items': [{'email': 'tay@caltech.edu', 'id': 'Tay-Tiamhock', 'name': {'family': 'Tay', 'given': 'Tiamhock'}, 'show_email': 'NO'}]}
Year: 2011
DOI: 10.7907/1QT1-PN18
The strong interest in strongly correlated systems in condensed matter physics has continued unabated for the past few decades. In recent years, the number of novel, exotic quantum phases found in theoretical studies has seen a phenomenal rise. Among those interesting quantum states are bose liquids and spin liquids, where strong quantum fluctuations have prevented the systems from developing a long range order. Our work in this thesis seeks to further the understanding of frustrated systems. In the study of a hard-core boson model with ring-only exchange interactions on a square lattice, we obtain concrete numerical realization of the unconventional Exciton Bose Liquid (EBL) phase, which possesses interesting properties such as a "Bose surface" which resembles the Fermi surface in a metal, as well as unusual thermodynamic properties such as a T Log T dependence for specific heat. An equally important result from this work is the demonstration that the widely used Gutzwiller projection on slave-particle wave functions may generally fail to capture the correct long wavelength physics in the respective systems. For the Heisenberg antiferromagnet on the kagome lattice, which is a promising candidate for realizing a spin-disordered ground state, our variational study shows that the projected Schwinger boson wave function is energetically better than the Dirac spin liquid wave function when a small antiferromagnetic second-neighbor spin coupling is added to the nearest-neighbor model. We also study the anisotropic triangular Heisenberg antiferromagnetic in magnetic field, and find simple, yet accurate wave functions for various regions of the surprisingly rich phase diagram, thus providing insights into the energetics of the competing phases in this interesting model. Finally, our work also highlights permanent-type wave functions as potentially useful constructions in variational studies of systems with short-ranged correlations, e.g., a Mott insulator and a gapped spin liquid.https://thesis.library.caltech.edu/id/eprint/6470Ladder Studies of Gapless Quantum Spin Liquids: Spin Bose-metal and SU(2)-invariant Majorana Spin Liquids
https://resolver.caltech.edu/CaltechTHESIS:05142012-220503327
Authors: {'items': [{'email': 'hsinhualai@gmail.com', 'id': 'Lai-Hsin-Hua', 'name': {'family': 'Lai', 'given': 'Hsin-Hua'}, 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/YDSP-0M98
<p>The recent experimental realizations of spin-1/2 gapless quantum spin liquids in two-dimensional triangular lattice organic compounds EtMe<sub>3</sub>Sb[Pd(dmit)<sub>2</sub>]<sub>2</sub> and κ-(ET)<sub>2</sub>Cu<sub>2</sub>(CN)<sub>3</sub> have stimulated the investigation of the gapless spin liquid theories. The models in dimensions greater than one (D>1) usually involve multispin interactions, such as ring exchange interactions, that are difficult to study, while effective gauge theory descriptions are not well-controlled to give reliable physics information. Driven by the need for a systematic and controlled analysis of such phase, such models on ladders are seriously studied. This thesis first focuses on such ladder models. We propose that the gapless spin liquid phase can be accessed from a two-band interacting electron model by metal-Mott insulator phase transition. We use Bosonization analysis and weak-coupling Renormalization Group to further study the gapless spin liquid state in the presence of Zeeman magnetic fields or orbital magnetic fields. Several new exotic gapless spin liquids with dominant spin nematic correlations are predicted. In such a ladder spin liquid, we also consider the impurity effects. We conclude that the local energy textures and oscillating spin susceptibilities around the impurities are nontrivial and can be observed in the experiments. We then shift our focus to another theoretical candidate, an SU(2)-invariant spin liquid with Majorana excitations, which can also qualitatively explain the experimental phenomenology. We construct an exactly solvable Kitaev-type model realizing the long-wavelength Majorana spin liquid state and study its properties. We find that the state has equal power-law spin and spin-nematic correlations and behaves nontrivially in the presence of Zeeman magnetic fields. Finally, we realize such Majorana spin liquid states on a two-leg ladder and further explore their stability. We conclude the states can be stable against short-range interactions and gauge field fluctuations.</p>
https://thesis.library.caltech.edu/id/eprint/7029Graphene as a Platform for Novel Nanoelectronic Devices
https://resolver.caltech.edu/CaltechTHESIS:06062012-025632521
Authors: {'items': [{'email': 'brian@brianstandley.com', 'id': 'Standley-Brian-Lawrence', 'name': {'family': 'Standley', 'given': 'Brian Lawrence'}, 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/6MMB-T165
<p>Graphene's superlative electrical and mechanical properties, combined with its compatibility with existing planar silicon-based technology, make it an attractive platform for novel nanoelectronic devices. The development of two such devices is reported—a nonvolatile memory element exploiting the nanoscale graphene edge and a field-effect transistor using graphene for both the conducting channel and, in oxidized form, the gate dielectric. These experiments were enabled by custom software written to fully utilize both instrument-based and computer-based data acquisition hardware and provide a simple measurement automation system.</p>
<p>Graphene break junctions were studied and found to exhibit switching behavior in response to an electric field. This switching allows the devices to act as nonvolatile memory elements which have demonstrated thousands of writing cycles and long retention times. A model for device operation is proposed based on the formation and breaking of carbon-atom chains that bridge the junctions. Information storage was demonstrated using the concept of rank coding, in which information is stored in the relative conductance of multiple graphene switches in a memory cell.</p>
<p>The high mobility and two dimensional nature of graphene make it an attractive material for field-effect transistors. Another ultrathin layered material—graphene's insulating analogue, graphite oxide—was studied as an alternative to bulk gate dielectric materials such as Al<sub>2</sub>O<sub>3</sub> or HfO<sub>2</sub>. Transistors were fabricated comprising single or bilayer graphene channels, graphite oxide gate insulators, and metal top-gates. Electron transport measurements reveal minimal leakage through the graphite oxide at room temperature. Its breakdown electric field was found to be comparable to SiO<sub>2</sub>, typically 1–3 × 10<sup>8</sup> V/m, while its dielectric constant is slightly higher, κ ≈ 4.3.</p>
<p>As nanoelectronics experiments and their associated instrumentation continue to grow in complexity the need for powerful data acquisition software has only increased. This role has traditionally been filled by semiconductor parameter analyzers or desktop computers running <i>LabVIEW</i>. <i>Mezurit 2</i> represents a hybrid approach, providing basic <i>virtual instruments</i> which can be controlled in concert through a comprehensive scripting interface. Each virtual instrument's model of operation is described and an architectural overview is provided.</p>https://thesis.library.caltech.edu/id/eprint/7138Studies of Exciton Condensation and Transport in Quantum Hall Bilayers
https://resolver.caltech.edu/CaltechTHESIS:09262011-144749993
Authors: {'items': [{'email': 'afinck@gmail.com', 'id': 'Finck-Aaron-David-Kiyoshi', 'name': {'family': 'Finck', 'given': 'Aaron David Kiyoshi'}, 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/PQJV-SB92
This thesis is a report of the transport properties of bilayer two-dimensional electron systems found in GaAs/AlGaAs double quantum well semiconductor heterostructures. When a strong perpendicular magnetic field is applied so that the total Landau filling factor is equal to one and if the two layers are close enough together, a novel quantum Hall (QH) state with strong interlayer correlations can form. This QH state is often described as an excitonic condensate, in which electrons in one layer pair with holes in the other. As neutral particles, excitons feel no Lorentz force and are not confined to the edges of the bilayer system like charged quasiparticles are. Instead, excitons are expected to be able to move freely through the bulk and even flow without any dissipation under proper conditions (i.e.,~excitonic superfluidity). Counterflow studies that directly probe the bulk verify this exciton transport in the electrically insulating interior. We also report on studies of the phase boundary between the correlated and uncorrelated phases at total Landau filling factor one as the effective interlayer separation is tuned. When both phases are fully spin polarized at high Zeeman energy, the phase transition is much broader than when the uncorrelated phase is incompletely polarized at low Zeeman energy. This suggests a possible change in the nature of the phase transition in the regime of complete spin polarization.https://thesis.library.caltech.edu/id/eprint/6689Scanning Tunneling Spectroscopic Studies on High-Temperature Superconductors and Dirac Materials
https://resolver.caltech.edu/CaltechTHESIS:05142013-151159910
Authors: {'items': [{'email': 'mindtex@gmail.com', 'id': 'Teague-Marcus-Lawrence', 'name': {'family': 'Teague', 'given': 'Marcus Lawrence'}, 'show_email': 'YES'}]}
Year: 2013
DOI: 10.7907/M8FW-S641
<p>This thesis details the investigations of the unconventional low-energy quasiparticle excitations in electron-type cuprate superconductors and electron-type ferrous superconductors as well as the electronic properties of Dirac fermions in graphene and three-dimensional strong topological insulators through experimental studies using spatially resolved scanning tunneling spectroscopy (STS) experiments.</p>
<p>Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type cuprate La<sub>0.1</sub>Sr<sub>0.9</sub>CuO<sub>2</sub> (La-112) T<sub>C</sub> = 43 K, are investigated experimentally. For temperature (T) less than the superconducting transition temperature (TC), and in zero field, the quasiparticle spectra of La-112 exhibits gapped behavior with two coherence peaks and no satellite features. For magnetic field measurements at T < TC, first ever observation of vortices in La-112 are reported. Moreover, pseudogap-like spectra are revealed inside the core of vortices, where superconductivity is suppressed. The intra-vortex pseudogap-like spectra are characterized by an energy gap of V<sub>PG</sub> = 8.5 ± 0.6 meV, while the inter-vortex quasiparticle spectra shows larger peak-to-peak gap values characterized by Δ<sub>pk-pk</sub>(H) >V<sub>PG</sub>, and Δ<sub>pk-pk</sub> (0)=12.2 ± 0.8 meV > Δ<sub>pk-pk</sub> (H > 0). The quasiparticle spectra are found to be gapped at all locations up to the highest magnetic field examined (H = 6T) and reveal an apparent low-energy cutoff at the V<sub>PG</sub> energy scale.</p>
<p>Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type "122" iron-based Ba(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>2</sub>A<sub>s2</sub> are investigated for multiple doping levels (x = 0.06, 0.08, 0.12 with T<sub>C</sub>= 14 K, 24 K, and 20 K). For all doping levels and the T < T<sub>C</sub>, two-gap superconductivity is observed. Both superconducting gaps decrease monotonically in size with increasing temperature and disappear for temperatures above the superconducting transition temperature, T<sub>C</sub>. Magnetic resonant modes that follow the temperature dependence of the superconducting gaps have been identified in the tunneling quasiparticle spectra. Together with quasiparticle interference (QPI) analysis and magnetic field studies, this provides strong evidence for two-gap sign-changing s-wave superconductivity.</p>
<p>Additionally spatial scanning tunneling spectroscopic studies are performed on mechanically exfoliated graphene and chemical vapor deposition grown graphene. In all cases lattice strain exerts a strong influence on the electronic properties of the sample. In particular topological defects give rise to pseudomagnetic fields (B ~ 50 Tesla) and charging effects resulting in quantized conductance peaks associated with the integer and fractional Quantum Hall States.</p>
<p>Finally, spectroscopic studies on the 3D-STI, Bi<sub>2</sub>Se<sub>3</sub> found evidence of impurity resonance in the surface state. The impurities are in the unitary limit and the spectral resonances are localized spatially to within ~ 0.2 nm of the impurity. The spectral weight of the impurity resonance diverges as the Fermi energy approaches the Dirac point and the rapid recovery of the surface state suggests robust topological protection against perturbations that preserve time reversal symmetry.</p>
https://thesis.library.caltech.edu/id/eprint/7709Optimization of NEMS for Frequency Shift Sensing Applications
https://resolver.caltech.edu/CaltechTHESIS:05272016-114505090
Authors: {'items': [{'email': 'carynbullard@gmail.com', 'id': 'Bullard-Elizabeth-Caryn', 'name': {'family': 'Bullard', 'given': 'Elizabeth Caryn'}, 'orcid': '0000-0001-8579-2147', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9GH9FZ2
This thesis has two areas of focus: the application of the dynamic similarity principle in microelectromechancial systems (MEMS) and nanoelectromechanical systems (NEMS) and the study of anomalous phase noise (APN) in MEMS and NEMS. In the first portion of the thesis, we employ the dynamic similarity principle to predict the quality factor due to gas damping in MEMS and NEMS. In the second portion of the thesis, we provide a theoretical framework for sources of phase noise in MEMS and NEMS and describe the measurements that we made to quantify the temperature dependence of anomalous phase noise in silicon doubly clamped beams.https://thesis.library.caltech.edu/id/eprint/9787Advanced Applications of Nanoelectromechanical Systems
https://resolver.caltech.edu/CaltechTHESIS:05272016-154210811
Authors: {'items': [{'email': 'bearasauras@gmail.com', 'id': 'Hung-Peter-Shek-Ho', 'name': {'family': 'Hung', 'given': 'Peter Shek-Ho'}, 'orcid': '0000-0002-9034-5330', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9J38QJ3
<p>Nanoelectromechanical systems (NEMS) have advanced the technologies in a wide spectrum of fields, including nonlinear dynamics, sensors for force detection, mass spectrometry, inertial imaging, calorimetry, and charge sensing. Due to their low power consumption, fast response time, large dynamic range, high quality factor, and low mass, NEMS have achieved unprecedented measurement sensitivity. For optimized system functionalization and design, precise characterization of material properties at the nanoscale is essential. In this thesis, we will discuss three applications of NEMS: mechanical switches, using anharmonic nonlinearity to measure device and material properties, and mass spectrometry and inertial imaging.</p>
<p>The first application of NEMS we discuss is NEMS switches, switches with physical moving parts. Conventional electronics, based largely on silicon transistors, is reaching a physical limit in both size and power consumption. Mechanical switches provide a promising solution to surpass this limit by forcing a jump between the on and off states. Graphene, which is a single sheet of carbon atoms arranged in a hexagonal structure, has high mechanical strength and strong planar bonding, making it an ideal candidate for nanoelectromechanical switches. In addition, graphene is conductive, which decreases resistive heating at the contact area, therefore reducing bonding issues and subsequently reducing degradation. We demonstrate using exfoliated graphene to fabricate suspended graphene NEMS switches with successful switching.</p>
<p>The second application of NEMS we discuss in this thesis is the use of mechanical nonlinearity to measure device and material properties. While the nonlinear dynamics of NEMS have been used previously to investigate the longitudinal speed of sound of materials at nano- and micro-scales, we correct a previously attempted method that employs the anharmonicity of NEMS arising from deflection-dependent stress to interrogate the transport of RF acoustic phonons at nanometer scales. In contrast to existing approaches, this decouples intrinsic material properties, such as longitudinal speed of sound, from properties associated with linear dynamics, such as tension, of the structure. We demonstrate this approach through measurements of the longitudinal speed of sound in several NEMS devices composed of single crystal silicon along different crystal orientations. Good agreement with literature values is reported.</p>
<p>The third application of NEMS we discuss is mass spectrometry and inertial imaging. Currently, only doubly clamped beams and cantilevers have been experimentally demonstrated for mass spectrometry. We extend the one-dimension model for mass spectrometry to a novel method for inertial imaging. We further extend the theory of mass spectrometry and inertial imaging to two dimensions by using a plate geometry. We show that the mode shape is critical in performing NEMS mass spectrometry and inertial imaging, and that the mode shapes in plates deviate from the ideal scenario with isotropic stress. We experiment with various non-ideal conditions to match non-ideal mode shape observed.</p>https://thesis.library.caltech.edu/id/eprint/9800Nonlinear and Ultrafast Optical Investigations of Correlated Materials
https://resolver.caltech.edu/CaltechTHESIS:06092017-141136995
Authors: {'items': [{'email': 'cxhaochu@gmail.com', 'id': 'Chu-Hao', 'name': {'family': 'Chu', 'given': 'Hao'}, 'show_email': 'NO'}]}
Year: 2017
DOI: 10.7907/Z9VD6WHV
<p>This thesis comprises studies of 3<i>d</i>-5<i>d</i> transition metal oxides with various degrees of electronic correlation using nonlinear harmonic generation rotational anisotropy as well as time-resolved optical reflectivity methods. Specifically, we explored photo-induced phase transition in Ca<sub>2</sub>RuO<sub>4</sub> and Sr<sub>2</sub>IrO<sub>4</sub>, discovered novel electronic phases in doped Sr<sub>2</sub>IrO<sub>4</sub> and Sr<sub>3</sub>Ir<sub>2</sub>O<sub>7</sub>, and investigated different types of antiferromagnetic orders in transition metal trichalcogenides MPX<sub>3</sub>.</p>
https://thesis.library.caltech.edu/id/eprint/10334Physics and Applications of Graphene-Based Nanostructures and Nano-Meta Materials
https://resolver.caltech.edu/CaltechTHESIS:03172020-153749505
Authors: {'items': [{'email': 'chenchih.hsu@gmail.com', 'id': 'Hsu-Chen-Chih', 'name': {'family': 'Hsu', 'given': 'Chen-Chih'}, 'orcid': '0000-0003-1130-5240', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/6T02-4X35
<p>Graphene, a single layer of carbon atoms forming a honeycomb lattice structure, has been considered a wonder material for both scientific research and technological applications. Structural distortions in nano-materials can induce dramatic changes in their electronic properties. In particular, strained graphene can result in both charging effects and pseudo-magnetic fields, so that controlled strain on a perfect graphene lattice can be tailored to yield desirable electronic properties.</p>
<p>In the first part of this thesis (Chapter 2 to 5), we explore a new approach to manipulating the topological states in monolayer graphene via nanoscale strain engineering. By placing strain-free monolayer graphene on architected nanostructures to induce global inversion symmetry breaking, we demonstrate the development of giant pseudo-magnetic fields, global valley polarization, and periodic one-dimensional topological channels for protected propagation of chiral modes in strained graphene. We have also observed pseudo-magnetic field-induced quantum oscillations and valley Hall signals, including quantum valley Hall effect, by transport measurements at 1.8K.</p>
<p>The second part of this thesis focuses on the development and applications of other graphene-based nanostructures. We report PECVD techniques for the synthesis of various graphene and graphene-based nanostructures, including horizontal growth of graphene sheets, vertical growth of graphene nanostructures such as graphene nanostripes with large aspect ratios, and direct and selective deposition of multi-layer graphene on nanostructured substrates. By properly controlling the gas environment of the plasma, it is found that no active heating is necessary for the PECVD growth processes and that high-yield growth can take place in a single step on a variety of surfaces, including metallic, semiconducting, and insulating materials.</p>https://thesis.library.caltech.edu/id/eprint/13660Exciton Dynamics Studies from First-Principles Calculations: Radiative Recombination, Exciton-Phonon Interactions, and Ultrafast Exciton Relaxation
https://resolver.caltech.edu/CaltechTHESIS:05312021-191637587
Authors: {'items': [{'email': 'chenhsiaoyi1025@gmail.com', 'id': 'Chen-Hsiao-Yi', 'name': {'family': 'Chen', 'given': 'Hsiao-Yi'}, 'orcid': '000-0003-1962-5767', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/4edg-jw48
<p>Excitons are bound electron-hole pairs that dominate the optical response of semi-conductors and insulators, especially in nanoscale and wide bandgap materials where the Coulomb interaction is weakly screened. Excitons can enhance light-matter coupling at certain wavelengths, thus making their host materials candidates for optoelectronic, photovoltaic, and quantum technology devices. For instance, two-dimensional transition metal dichalcogenides have a large and tunable optical response and hold promise for next-generation ultrathin light-emitting diodes. It is remarkable that exciton properties such as the binding energy and radiative lifetime can vary by orders of magnitude in different materials and can be further tuned by material properties like defects and lattice vibrations. Therefore, quantitative studies of exciton interactions and dynamics can advance understanding of the optical response of complex materials and play a role in the design of future devices. Among theoretical studies, numerical approaches based on density functional theory (DFT) can quantitatively address the electronic structure in real materials and their response to external perturbations, enabling accurate calculations of the conductivity and dielectric properties. These first-principle methods, which employ numerical quantum mechanics and use only the atomic structure of the material as input (making no use of empirical parameters) have revolutionized studies of materials and condensed matter physics. Over the last few years, first-principles methods for studies of excitons have focused on the GW-Bathe-Salpeter equation (GW-BSE) method to compute exciton energies and optical absorption spectra. However, going beyond calculations of exciton energetics to address the exciton dynamical processes remains challenging and is an exciting new frontier of first-principles studies.</p>
<p>This thesis develops theory and novel numerical approaches to study exciton radiative and nonradiative interactions from first-principles. For the radiative processes, we demonstrate a systematic derivation of exciton radiative lifetimes in materials ranging from bulk to nanostructures and molecules. The results correctly reproduce the observed power-law temperature dependence of the radiative lifetimes. To benchmark our calculations, we study exciton radiative lifetimes in gas-phase molecules, obtaining excellent agreement between theory and experiment. Our framework is then applied in three different studies. First, we extend the radiative lifetime formula to account for the dependence on light polarization and valley occupation and investigate exciton recombination in two-dimensional transition metal dichalcogenides (2D-TMDs). We show that excitons emit light anisotropically upon recombination when they are in any quantum superposition state of the K and K' inequivalent valleys. When averaged over the emission angle and exciton momentum, our new treatment recovers the temperature-dependent radiative lifetimes derived in early literature. Second, we use the exciton energy and radiative lifetimes to identify the atomic structure of the defects in monolayer hexagonal boron nitride (h-BN). In the study, we narrow down the potential structures to nine candidates and identify the highest-likelihood structure as the V<sub>N</sub>N<sub>B</sub> defect, consisting of a nitrogen vacancy plus a carbon replacing boron in h-BN. Finally, we generalize the discussion of isotropic bulk system to accurately compute the exciton radiative lifetimes in bulk uniaxial crystals, focusing on wurtzite GaN. Our computed radiative lifetimes are in very good agreement with experiments at low temperature. We show that taking into account excitonic effect and spin-orbit coupling (to include the exciton fine structure) is essential for computing accurate radiative lifetimes. A model for exciton dissociation into free carriers allows us to compute the radiative lifetimes up to room temperature.</p>
<p>In the study of exciton non-radiative process, we focus on the exciton-phonon (ex-ph)interaction, which plays an important role to understand the dynamics of excitons in materials. We establish and implement a first-principle formalism to compute the ex-ph coupling constants by combining the electron-phonon couplings and the exciton wavefunctions from the GW-BSE approach. Using the computed ex-ph coupling matrix elements, we calculate the ex-ph relaxation times as a function of exciton energy, momentum, temperature, and phonon mode in bulk h-BN. Our calculations reveal the dominant ex-ph coupling with the longitudinal optical (LO) mode and identify the threshold for LO phonon emission with an associated ∼15 fs LO emission characteristic time. In addition, we derive the phonon-assisted photoluminescence(PL) from the ex-ph interaction and correctly reproduce the PL spectrum observed in h-BN at both 8 K and 100 K. Based on our successful study of ex-ph interactions in bulk h-BN, we extend the discussion to materials with strong spin-orbit coupling. We investigate the bright exciton linewidth broadening and PL in monolayer WSe<sub>2</sub>. The numerical results show an increase of linewidth by 20 meV from 0 K to 250 K as observed in early experiments and identify the main PL peak as a consequence of LA phonon emission while the side band is due to optical phonons. Lastly, we present results from a joint theory-experiment study of the ultrafast exciton dynamics in WSe<sub>2</sub>. We develop a Boltzmann equation for excitons and employ it to model ultrafast exciton relaxation due to ex-ph processes. The simulation and experiment both show a ~70 fs time delay for the electron intervalley scattering from the K- to the Q-valley due to exciton dynamical effects. We also develop accurate simulations of time-domain angle-resolved photoemission (ARPES) experiments, which are becoming a powerful experimental probe of exciton dynamics in condensed matter. In summary, this thesis work paves the way to quantitative studies of exciton radiative and non-radiative processes, as well as exciton ultrafast dynamics, and quantitative modeling of pump-probe experiments in materials with strongly bound excitons.</p>https://thesis.library.caltech.edu/id/eprint/14210Electron-Phonon Interactions and Charge Transport from First-Principles Calculations: Complex Crystals, Higher Order Coupling, and Steps Toward the Small Polaron Regime
https://resolver.caltech.edu/CaltechTHESIS:11252020-093720107
Authors: {'items': [{'email': 'leenienen1987@gmail.com', 'id': 'Lee-Nien-En', 'name': {'family': 'Lee', 'given': 'Nien-En'}, 'orcid': '0000-0002-3172-7750', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/b040-2y98
Electron-phonon (e-ph) interactions quantify the strength of interplay between charge carriers and lattice vibrations and critically determine the transport properties in materials near room temperature. Depending on the coupling strength, charge carriers can exhibit behaviors ranging from propagating waves extending across crystals to trapped particles localized in space. Therefore, accurately describing e-ph interactions plays a central role in quantitative transport studies on real materials. Over the last few years, first-principles methods combining density functional theory (DFT) and related techniques with the Boltzmann transport equation (BTE) have rapidly risen and reached maturity for investigating transport in various metals, semiconductors, and insulators with weak e-ph coupling. The lowest-order e-ph scattering process can be investigated starting from e-ph interactions from DFT calculations; this first-principles approach provides unambiguous quantitative prediction of transport properties such as the conductivity and mobility in common semiconductors and metals over a wide temperature range without using any empirical parameter. Encouraged by the agreement of the computed transport properties with experiment for many simple materials, this thesis aims to extend the applicability of this first-principles methodology and to further our understanding of microscopic transport mechanisms, especially in the wide temperature window near room temperature where transport is governed by e-ph scattering. We present research that expands the state of the art in three distinct ways, focusing on three research directions we pursue in this work. First, we employ the BTE to calculate the hole carrier mobility of naphthalene, an organic molecular crystal containing 36 atoms in a unit cell, the record largest system for first-principles charge transport calculations to date. The results are in excellent agreement with experiments, demonstrating that transport in some high-mobility organic semiconductors can still be explained within the band theory framework, and show that low-frequency rigid molecular motions control the electrical transport in organic molecular semiconductors in the bandlike regime. The second topic is an attempt to go beyond the lowest-order theory of e-ph interactions and quantify the importance of higher-order e-ph processes. We derive the electron-two-phonon scattering rates using many-body perturbation theory, compute them in GaAs, and quantify their impact on the electron mobility. We show that these next-to-leading order e-ph scattering rates, although smaller than the lowest-order contribution, are not negligible, and can compensate the overestimation of mobility generally made by the lowest-order BTE calculation in weakly-polar semiconductors. In the third part of the thesis, we explore the opposite extreme case in which e-ph interactions are strong and lead to the formation of localized (so-called "polaron") electronic states that become self-trapped by the interactions with the atomic vibrations. We derive a rigorous approach based on canonical transformations to compute the energetics of self-localized (small) polarons in materials with strong e-ph interactions. With the aid of \textit{ab initio} e-ph interactions, we carry out the corresponding numerical calculations to investigate the formation energy of small polaron and determine whether the charge carriers favor localized states over the Bloch waves. Due to the low computational cost of our approach, we are able to apply these calculations to various compounds, focusing on oxides, predicting the presence of small polaron in agreement with experiments in various materials. Our work paves the way to understanding small polaron formation and extending these calculations to predict transport in the polaron hopping mechanism in materials with strong e-ph coupling.https://thesis.library.caltech.edu/id/eprint/14006Synthesis of 2D Quantum Materials for Nanoelectronic and Nanophotonic Applications
https://resolver.caltech.edu/CaltechTHESIS:04262021-085536699
Authors: {'items': [{'email': 'ppo520lin@gmail.com', 'id': 'Lin-Wei-Hsiang', 'name': {'family': 'Lin', 'given': 'Wei-Hsiang'}, 'orcid': '0000-0003-0037-1277', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/vh7k-4w84
<p>2D materials have attracted tremendous attention for a variety of properties such as ultra-low body thickness, ultra-high mobility, and tunable bandgap. These unique merits of the 2D materials bring in the significant improvements and new perspectives in the digital CMOS scaling, analog performance, as well as the 3D integration of wafer stacking.</p>
<p>In this thesis, we explore van der Waals materials for future CMOS technologies. Chapter 2 introduces a compatible and a single-step method for synthesizing high-mobility monolayer graphene (MLG) in merely a few minutes by means of plasma-enhanced chemical vapor deposition (PECVD) techniques without the need of active heating. This environment enables graphene growth on different surfaces at relatively low temperatures, which paves ways to a CMOS-compatible approach to graphene synthesis. Chapter 3 describes the development of a synthesis method that controls the growth of large-area h-BN films from monolayer to 30 atomic layers, and summarizes the characterizations of the properties of these h-BN films that demonstrate the high-quality of these materials.</p>
<p>New degrees of freedom possess the immense potential and attract huge attentions as the imminent end of "Moore's Law". Compared with the traditional charge degree of freedom, spin and valley are the other two additional internal degree of freedom in solid-state electronics which enable the spintronic and valleytronic devices with high integration density, fast processing speed, low power dissipation, and non-volatility. Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are semiconductors promising for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. In chapter 4, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS₂ single crystals. By illuminating WS₂ with off-resonance circularly-polarized light and measuring the resulting spatially resolved circularly-polarized emission (P<sub>circ</sub>), we find large circular polarization increases significantly to nearly 90% at 80 K. In Chapter 5, it is reported that valley polarized PL of monolayer WS₂ can be efficiently tailored at room temperature (RT) through the surface plasmon-exciton interaction with plasmonic Archimedes spiral (PAS) nanostructures. The DVP of WS₂ using 2 turns (2T) and 4 turns (4T) of PAS can reach up to 40% and 50% at RT, respectively. Further enhancement and continuous control of excitonic valley polarization in electrostatically doped monolayer WS₂ are demonstrated. Under the circularly polarized light on WS₂-2TPAS heterostructure, 40% valley polarization of exciton without electrostatic doping is icreased to 70% by modulating the carrier doping via a backgate. This enhancement of valley polarization may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The demonstration of electrical tunability in the valley-polarized emission from WS₂-PAS heterostructures provides new strategies to harness valley excitons for application in ultrathin valleytronic devices.</p>
<p>In contrast to future optical switch applications, in Chpater 6, it is reported that Ternary tellurides based on alloying different 2D transition metal dichalcogenides can result in interesting new 2D materials with tunable optical and electrical properties. Additionally, such alloys can provide opportunities for significantly improving the electrical contact properties at the metal-semiconductor interface. In particular, realization of practical devices based on the 2D materials will require overcoming the typical Fermi-level pinning limitations of the electrical contacts at the metal-semiconductor interface and ultimately approaching the ideal Schottky-Mott limit. In this work, we develop a simple method of stacking 3D/2D electrical metal contacts onto dangling-bond-free 2D semiconductors in order to surmount the typical issue of Fermi-level pinning. Specifically, contacts of Au, graphene/Au, and WTe₂/Au are transferred onto WS<sub>1.94</sub>Te<sub>0.06</sub> alloy-based devices via a new transfer method. The WS<sub>1.94</sub>Te<sub>0.06</sub> field-effect transistors (FETs) with WTe₂/Au contacts reveal a field-effect mobility of 25 cm²V⁻¹s⁻¹, an on/off current ratio of 10⁶, and extremely low contact resistance of 8 kΩ μm. These electrical properties are far more superior to similar devices with either Au or graphene/Au contacts, which may be attributed to the fact that the work function of WTe₂ is close to the band edge of the WS<sub>1.94</sub>Te<sub>0.06</sub> alloy so that the resulting metal-semiconductor interface of the FETs are free from Fermi-level pinning. The Schottky barrier heights of the WS<sub>1.94</sub>Te<sub>0.06</sub>-FETs with WTe₂/Au contacts also follow the general trend of the Schottky-Mott limit, implying high-quality electrical contacts. Finally, in Chapter 7, several promising opportunities were proposed for future CMOS integrated circuits based on monolayer semiconductors.</p>https://thesis.library.caltech.edu/id/eprint/14128Fabrication of Pristine and Doped Graphene Nanostripes and their Application in Energy Storage
https://resolver.caltech.edu/CaltechTHESIS:02012021-171503477
Authors: {'items': [{'email': 'jacobdbagley@gmail.com', 'id': 'Bagley-Jacob-David', 'name': {'family': 'Bagley', 'given': 'Jacob David'}, 'orcid': '0000-0001-9490-1341', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/hfdw-fs13
<p>Fossil fuel usage causing rising CO<sub>2</sub> levels and leading to climate change is, perhaps, the most pressing issue of our time. However, our economic dependence on energy necessitates its usage such that reducing energy usage is not possible leaving transitioning to renewable energy technologies as the only sustainable option. Currently, the largest barrier to large scale incorporation of renewable energy sources (e.g., solar, wind) is the high cost of energy storage technologies. Electrochemical energy storage technologies (e.g., lithium-ion batteries and supercapacitors) have been identified as a key approach for enabling the transition to renewable energy technologies.</p>
<p>Graphene is a material with exceptional properties that is receiving much attention for application in various energy storage technologies and could help reduce the cost of energy storage technologies. This thesis describes a novel fabrication procedure for low-cost and efficient synthesis of high-quality graphene nanostripes (GNSPs) and their application in lithium-ion battery and supercapacitor electrodes.</p>
<p>This thesis is structured as follows. Chapter 1 outlines the motivation and technical background of this research. Chapter 2 describes the instrumentation and procedures for fabricating GNSPs. Chapter 3 describes <i>in situ</i> exfoliation of GNSPs as electrodes in supercapacitors to increase the capacitance. Chapter 4 describes synthesis and application of pyridinic-type nitrogen-doped GNSPs as a lithium-ion battery anode. Chapter 5 describes the synthesis and application of silicon-, germanium-, and tin-doped GNSPs and their application in lithium-ion battery anodes. Chapter 6 concludes and synthesizes the findings of the thesis holistically. Additionally, future outlook and potential research objectives are presented.</p>https://thesis.library.caltech.edu/id/eprint/14069Investigation of the Physical Properties of Dirac Materials
https://resolver.caltech.edu/CaltechTHESIS:07092020-003626793
Authors: {'items': [{'email': 'kyle5241@gmail.com', 'id': 'Chen-Chien-Chang', 'name': {'family': 'Chen', 'given': 'Chien-Chang'}, 'orcid': '0000-0003-0959-5584', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/bgw9-d234
<p>This 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.</p>
<p>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.</p>
<p>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 <i>vs</i>, 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.</p>
<p>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 <i>vs.</i> 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 <i>T</i> ≤ 30 mK < < <i>T<sub>c</sub><sup>bulk</sup></i> ~ 30 K) may be attributed to the finite contributions of bulk carriers to excess conduction unless T → 0.</p>
<p>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).</p>
<p>Finally, the key findings of this thesis work and the suggested future research directions are summarized.</p>
https://thesis.library.caltech.edu/id/eprint/13839Novel Light-Matter Interaction in Quasi-One-Dimensional Graphene Nanomaterials for Photonics
https://resolver.caltech.edu/CaltechTHESIS:05282021-182147719
Authors: {'items': [{'email': 'deepan.kishor@gmail.com', 'id': 'Kishore-Kumar-Deepan', 'name': {'family': 'Kishore Kumar', 'given': 'Deepan'}, 'orcid': '0000-0003-0236-8805', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/y5a2-zx57
<p>Nonlinear light-matter interaction in two-dimensional (2D) materials like graphene with unique nanostructured quasi-one-dimensionality (quasi-1D) holds the potential to address major technology opportunities in photonics from on-chip photo detection, modulation of light, and even possibly coherent light sources. In this work, we propose to use graphene, a gapless two-dimensional nanomaterial, for both nano-photonic applications and potentially energy harvesting by nano-structuring the material into nearly quasi-one-dimensional effective optical cavities with defects that act like color centers. These defects are naturally formed during its synthesis or can be engineered in the material by selective plasma radiation, is found to support a broad spectral distribution of color centers that exhibit excitation dependent photoluminescence. Through detailed investigation on the temperature and power dependence of photoluminescence from such defects, excitation dependent photoluminescence emission, we have established that these graphene nanomaterials with metastable energy states can support material excitations (e.g., excitons) that are strongly coupled to the optical modes confined within the nanostructured cavities to produce polaritonic quasiparticles, leading to many interesting nonlinear behaviors. In particular, the manifestation of blue-shifted photoluminescence, polariton lasing-like emission, multimode lasing-like emission, and distinct interference fringes, all points to the presence of novel light-matter interaction in quasi-one-dimensional graphene. Such novel light matter interactions can be exploited, among other applications, within photonic integrated circuits (PIC) by directly synthesizing graphene on silicon from a low temperature, single-step, plasma-enhanced chemical vapor deposition (PECVD) with feedstock gases of methane and hydrogen.</p>https://thesis.library.caltech.edu/id/eprint/14193Spin-Phonon Interactions and Spin Decoherence from First Principles
https://resolver.caltech.edu/CaltechTHESIS:06052022-215214933
Authors: {'items': [{'email': 'jinsoop412@gmail.com', 'id': 'Park-Jinsoo', 'name': {'family': 'Park', 'given': 'Jinsoo'}, 'orcid': '0000-0002-1763-5788', 'show_email': 'YES'}]}
Year: 2022
DOI: 10.7907/80bd-x991
<p>Developing a microscopic understanding of spin decoherence is essential to advancing quantum technologies. Electron spin decoherence due to atomic vibrations (phonons) plays a special role as it sets an intrinsic limit to the performance of spin-based quantum devices. Two main sources of phonon-induced spin decoherence, the Elliott-Yafet (EY) and Dyakonov-Perel (DP) mechanisms, have distinct physical origins and theoretical treatments. First-principles calculations of electron-phonon (<i>e</i>-ph) interactions combined with many-body perturbation theory are promising to study phonon-induced spin decoherence. However, predicting the spin response in materials remains an open challenge; methods for quantifying spin-dependent <i>e</i>-ph interactions in materials, as well as a linear response framework for spins in the presence of <i>e</i>-ph interaction is missing. In this thesis, we provide a first-principles framework for computing the relativistic spin-dependent electron-phonon interactions. We develop a formalism that unifies the modeling of EY and DP spin decoherence, and provide a rigorous many-body perturbation theory for obtaining the spin-spin correlation function including the vertex corrections due to <i>e</i>-ph interactions. We compute the phonon-dressed vertex of the spin-spin correlation function with a treatment analogous to the calculation of the anomalous electron magnetic moment in QED. We find that the vertex correction provides a giant renormalization of the electron spin dynamics in solids, greater by many orders of magnitude than the corresponding correction from photons in vacuum. We further identify the long-range quadrupole <i>e</i>-ph interaction in materials, and demonstrate its importance in the description of phonon-induced spin decoherence. We show first-principle calculations of spin-dependent <i>e</i>-ph interactions in correlated electron systems, using the framework of Hubbard-corrected density functional theory. Lastly, we provide technical details in the implementation of <i>ab-initio</i> <i>e</i>-ph interaction in PERTURBO, a software package for first-principles calculations of charge transport, spin dynamics, and ultrafast carrier dynamics in materials. In summary, the thesis demonstrates a general approach for quantitative analysis of spin decoherence in materials, advancing the quest for spin-based quantum technologies.</p>https://thesis.library.caltech.edu/id/eprint/14944Two-Dimensional Transition Metal Dichalcogenides for Ultrathin Solar Cells
https://resolver.caltech.edu/CaltechTHESIS:04082022-171550192
Authors: {'items': [{'email': 'cora.went@gmail.com', 'id': 'Went-Cora-Margaret', 'name': {'family': 'Went', 'given': 'Cora Margaret'}, 'orcid': '0000-0001-7737-3348', 'show_email': 'YES'}]}
Year: 2022
DOI: 10.7907/xrxk-3q08
<p>Ultrathin solar cells, with absorber layers less than one micron thick, have the potential to use orders of magnitude less high-quality semiconducting material than current silicon solar cells. This could be advantageous in applications that require high power output per unit weight, such as vehicle-integrated photovoltaics, or where reducing the capital cost of solar cell manufacturing is important. Transition metal dichalcogenides are a promising candidate for the semiconducting absorber layer of ultrathin solar cells due to their intrinsically passivated surfaces and their high absorption per unit thickness. </p>
<p>This thesis explores two-dimensional transition metal dichalcogenides for ultrathin photovoltaics. We start with the simplest type of solar cell, which collects carriers via a Schottky junction formed by sandwiching the absorber layer between two metal contacts with different work functions. To enable this geometry and avoid Fermi-level pinning, we develop a new process for gently transferring van der Waals metal contacts onto transition metal dichalcogenides. We measure an open-circuit voltage of 250 mV and a power conversion efficiency of 0.5% in Schottky-junction solar cells. To improve upon this efficiency, we next make carrier-selective contact solar cells, which employ wide bandgap semiconductors to selectively collect electrons on one side and holes on the other side of the absorber layer. We measure an open-circuit voltage of 520 mV and a power conversion efficiency greater than 2% in devices based on perovskite solar cell geometries, with PTAA and C60 as selective contact layers. We demonstrate that short carrier lifetimes limit the voltage in these solar cells to 750 mV, well below the detailed balance voltage limit. This motivates a more thorough understanding of the carrier dynamics at play, and we use a new pump-probe optical microscopy technique, stroboSCAT, to spatiotemporally track heat and carrier evolution in transition metal dichalcogenides. When paired with a kinetic model, we show that this technique can be used to measure lifetimes and other important material parameters even in materials with low radiative efficiencies.</p>
<p>We conclude by outlining future research directions towards achieving power conversion efficiencies greater than 10% in transition metal dichalcogenide solar cells.</p>https://thesis.library.caltech.edu/id/eprint/14543Towards Ab Initio Simulations of High-Temperature Superconductivity
https://resolver.caltech.edu/CaltechTHESIS:11022022-092201743
Authors: {'items': [{'email': 'zhcui0408@gmail.com', 'id': 'Cui-Zhi-Hao', 'name': {'family': 'Cui', 'given': 'Zhi-Hao'}, 'orcid': '0000-0002-7389-4063', 'show_email': 'YES'}]}
Year: 2023
DOI: 10.7907/y2qf-1f77
<p>High-temperature superconductors have been discovered for more than three decades. Nonetheless, the theoretical understanding of their microscopic properties remains unclear with substantial difficulties in linking the observed phenomena to material composition and structures. This thesis aims to establish a theoretical hierarchy (from lattice models to realistic materials) for faithful simulations of high temperature superconductivity.</p>
<p>We start with the lattice models of superconductors by using quantum embedding theory, whose self-consistency allows magnetic and superconducting phases to emerge. We extended the density matrix embedding theory (DMET) with improved self-consistency algorithms and determined the ground-state phase diagrams for both one-band [Chap. 3] the three-band Hubbard models [Chap. 4]. In particular, in the three-band model, we explored the atomic-scale nature of the antiferromagnetic and superconducting orders for different model parametrizations, and highlighted the role of the oxygen degrees of freedom beyond the one-band picture.</p>
<p>To go beyond the models, we therefore extended the original theory [DMET and dynamical mean-field theory (DMFT)] to ab initio realistic solids [Chap. 5]. The methods, namely the full-cell quantum embedding, are distinct from other embedding schemes in three aspects: (i) all local orbitals in a unit cell are included in the embedding problem whereas the bath orbitals are truncated according the atomic valence characters; (ii) The embedding Hamiltonian is of full quartic fermionic form rather simplified Hubbard like Hamiltonians; (iii) Many-body quantum chemistry solvers such as coupled cluster (CC) are used to generate embedding density matrix and Green’s functions. As demonstrated across a variety of semiconducting and insulating materials, the full-cell quantum embedding provides accurate energy, equation of state, spin-spin correlation functions and excited-state band structures.</p>
<p>We then applied our ab initio quantum embedding methods to the parent state of a series of cuprate superconductors [Chap. 6]. We uncovered microscopic trends in the electron correlations and revealed the link between the material composition and magnetic energy scales via a many-body picture of excitation processes involving the buffer layers. We found the competition between the in-plane superexchange and the CuO₂-buffer layer excitations, which explains the magnetic coupling difference among a series of superconducting materials.</p>
<p>Finally, we investigated the doped cuprates, where the superconducting orders enter into the phase diagram [Chap. 7]. We generalized our ab initio framework to allow the particle-number symmetry breaking states such that the superconducting orders can spontaneously emerge during the self-consistency. We showed that the d-wave superconducting magnitude increases with the pressure applied to crystals and the trend connects to the superexchange coupling J . Furthermore, we also studied the layer effect on superconductivity. Unlike the pressure effect, the layer effect between different compounds is affected by more factors - both magnetic coupling J and charge distribution matters. The work provides a promising route to study the material-specific physics in high-temperature superconductivity.</p>
<p>The aforementioned applications also relied on (i) the development and adaptation of many-body solvers, including the CC singles and doubles (CCSD) with Newton-Krylov method for better numerical convergence, and active-space quantum chemistry techniques with large-scale density matrix renormalization group. (ii) projection-based orbital localizations for metallic systems, frozen core techniques and symmetry adaptations. These contents are discussed in Chap. 2 and Appendices, including their efficient implementation and parallelization.</p>
<p>In the concluding remarks [Chap. 8], we summarized the current status and limitations of the high-temperature superconductivity studies. In addition, we proposed several possible directions to address the challenges in electronic correlation and atomic modelling of other exotic phases from an ab initio perspective.</p>https://thesis.library.caltech.edu/id/eprint/15053Strategic Advances in 2D Materials: Low-Temperature Plasma-Enhanced Chemical Vapor Deposition Growth of Graphene and Complementary Insights into MoS₂
https://resolver.caltech.edu/CaltechTHESIS:11082023-043634180
Authors: {'items': [{'email': 'stevef828@gmail.com', 'id': 'Lu-Chen-Hsuan', 'name': {'family': 'Lu', 'given': 'Chen-Hsuan'}, 'orcid': '0000-0002-4802-1332', 'show_email': 'NO'}]}
Year: 2024
DOI: 10.7907/cetf-ns02
<p>This thesis explores the intricate details of the plasma-enhanced chemical vapor deposition (PECVD) technique for growing graphene on various substrates at low temperatures. The research begins by finely optimizing the PECVD growth conditions to produce high-quality graphene on copper ink, which can potentially be used in a wide range of flexible electronics and Internet of Things (IoT) devices. The study also showcases that PECVD is an effective technique for growing graphene directly on electroplated copper over polyimide substrates, which greatly improves the resilience and environmental stability of copper circuits.</p>
<p>Furthermore, the research investigates the possibility of using PECVD to grow graphene on gold, which can be a game-changer in anti-corrosion applications and increase the longevity of gold electrode-based biosensors. The study also makes a significant breakthrough by growing nanocrystalline multilayer graphene on silver in a single step, which demonstrates exceptional oxidation resistance and opens new opportunities for hybrid graphene-silver plasmonic technologies.</p>
<p>Lastly, the thesis examines the potential and complexities of using electrodeposited (ED) copper foil as a graphene growth substrate, showing significant transformations in the properties of the ED copper foil post PECVD process. Towards the latter part of this work, attention is briefly shifted to explore the unique dipole ordering properties of monolayer molybdenum disulfide (MoS2) single crystals, which are synthesized using high-temperature chemical vapor deposition (CVD) and are van der Waals materials like graphene. Although not the main focus, this inclusion offers valuable insights into contrasting attributes and functionalities of graphene and MoS2, especially in areas like high-density data storage and non-volatile memories, and also compares the status of synthesis methods of these two types of van der Waals materials.</p>
<p>Alongside these investigations, the thesis also touches upon the prospects of both large-area PECVD graphene growth and interfacial graphene growth, identifying future paths for research and innovation. This comprehensive study highlights the versatility of low-temperature PECVD for graphene synthesis and provides insights that may reshape research and applications in flexible electronics, biosensing, and beyond. The findings of this research therefore pave ways for researchers, technology developers, and businesses to explore realistic technological applications of graphene and two-dimensional materials in various industries.</p>https://thesis.library.caltech.edu/id/eprint/16242