VTechWorks staff will be away for the Thanksgiving holiday beginning at noon on Wednesday, November 27, through Friday, November 29. We will resume normal operations on Monday, December 2. Thank you for your patience.

Browsing by Author "Soghomonian, Victoria G."

Now showing 1 - 16 of 16
  • Thumbnail Image
    Applications of Layer-by-Layer Films in Electrochromic Devices and Bending Actuators
    Jain, Vaibhav (Virginia Tech, 2009-09-02)
    This thesis presents work done to improve the switching speed and contrast performance of electrochromic devices. Layer-by-Layer (LbL) assembly was used to deposit thin electrochromic films of materials ranging from organic, inorganic, conducting polymers, etc. The focus was on developing new materials with high contrast and long lifecycles. A detailed switching-speed study of solid-state EC devices of already-developed (PEDOT (Poly(3,4-ethylenedioxythiophene)), polyviologen, inorganic) materials and some new materials (Prodot-Sultone) was performed. Work was done to achieve the optimum thickness and number of bilayers in LbL films resulting in high-contrast and fast switching. Device sizes were varied for comparison of the performance of the lab-made prototype device with the commercially available "small pixel" size displays. Symmetrical EC devices were fabricated and tested whenever conducting polymers are used as an EC material. This symmetrical configuration utilizes conducting polymers as an electroactive layer on each of two ITO-coated substrates; potential is applied to the two layers of similar conducting polymers and the device changes color from one redox state to another. This method, along with LbL film assembly, are the main factors in the improvement of switching speed results over already-published work in the literature. PEDOT results show that EC devices fabricated by LbL assembly with a switching speed of less than 30 ms make EC flat-panel displays possible by adjusting film thickness, device size, and type of material. The high contrast value (84%) for RuP suggests that its LbL films can be used for low-power consumption displays where contrast, not fastest switching, is the prime importance. In addition to the electrochromic work, this thesis also includes a section on the application of LbL assembly in fabricating electromechanical bending actuators. For bending actuators based on ionic polymer metal composites (IPMCs), a new class of conductive composite network (CNC) electrode was investigated, based on LbL self-assembled multilayers of conductive gold (Au) nanoparticles. The CNC of an electromechanical actuator fabricated with 100 bilayers of polyallylamine hydrochloride (PAH)/Au NPs exhibits high strain value of 6.8% with an actuation speed of 0.18 seconds for a 26 µm thick IPMC with 0.4 µm thick LbL CNCs under 4 volts.
  • Thumbnail Image
    Applications of Neutrino Physics
    Christensen, Eric Kurt (Virginia Tech, 2014-09-02)
    Neutrino physics has entered a precision era in which understanding backgrounds and systematic uncertainties is particularly important. With a precise understanding of neutrino physics, we can better understand neutrino sources. In this work, we demonstrate dependency of single detector oscillation experiments on reactor neutrino flux model. We fit the largest reactor neutrino flux model error, weak magnetism, using data from experiments. We use reactor burn-up simulations in combination with a reactor neutrino flux model to demonstrate the capability of a neutrino detector to measure the power, burn-up, and plutonium content of a nuclear reactor. In particular, North Korean reactors are examined prior to the 1994 nuclear crisis and waste removal detection is examined at the Iranian reactor. The strength of a neutrino detector is that it can acquire data without the need to shut the reactor down. We also simulate tau neutrino interactions to determine backgrounds to muon neutrino and electron neutrino measurements in neutrino factory experiments.
  • Thumbnail Image
    Characterization of electrical conductivity in a zeolitelike material
    Soghomonian, Victoria G.; Heremans, Jean J. (AIP Publishing, 2009-10)
    We present the electrical characterization of a zeolitelike oxo-vanadium arsenate framework. The experimentally obtained electronic and ionic conductivities and their interactions are discussed. Further, we investigate the potential use of electrically conducting zeolitelike materials in electrical energy storage applications, in light of the material's structural and electronic characteristics. (C) 2009 American Institute of Physics. [doi:10.1063/1.3251070]
  • Thumbnail Image
    The Daya Bay Reactor Neutrino Experiment
    Meng, Yue (Virginia Tech, 2014-09-22)
    The Daya Bay reactor neutrino experiment is a high sensitivity experiment designed to determine the last unknown neutrino mixing angle $theta_{13}$ by measuring disappearance of reactor antineutrinos emitted from six 2.9 $GW_{th}$ reactors at the Daya Bay Nuclear Power Station. There are eight identical Gd-loaded liquid scintillator detectors deployed in two near (flux-weighted baseline 512 $m$ and 561 $m$) and one far (1579 $m$) underground experimental halls to detect the inverse beta decay interaction. This dissertation describes the Daya Bay Experiment and individual contributions to this experiment. Chapter 1 reviews the history of the neutrino and the neutrino oscillation phenomena. The reactor based neutrino experiments in different times are described in this chapter in detail. It presents the motivation of the Daya Bay Experiment. In Chapter 2, the neutrino detection method and the $theta_{13}$ relative measurement method are introduced. This chapter focuses on the design of the Daya Bay Experiment, including antineutrino detector, calibration system, muon veto system and muon tagging system. Chapter 3 shows the design, development, construction, and assembly of Muon Pool PMT calibration system, and presents an algorithm of calculating the muon pool PMT timing offset values. Chapter 4 focuses on the manufacture, installation and commissioning of RPC HV system. Chapter 5 presents the analyses of the radioactive isotopes induced by comic muons. The Daya Bay detector energy response model is also described in detail. The relative rate analysis results exclude a zero value from $sin^22theta_{13}$ with a significance of 7.7 standard deviation using 139 days of data, 28909 (205308) antineutrino candidates which were recorded at the far hall (near halls) and shows $sin^22theta_{13} = 0.089pm0.011$ in a three-neutrino framework. A combined analysis of the $overline nu_e$ rates and energy spectra based on the detector energy response model improved measurement of the mixing angle $sin^22theta_{13} = 0.090^{+0.008}_{-0.009}$ by using 217 days of data, 41589 (203809 and 92912) antineutrino candidates were detected in the far hall (near halls). Also the first direct measurement of the $overline nu_e$ mass-squared difference $|Delta m^2_{ee}|= (2.59^{+0.19}_{-0.20})times10^{-3}$ $eV^2$. It is consistent with $|Delta m^2_{mumu}|$ measured by muon neutrino disappearance, supporting the three-flavor oscillation model.
  • Thumbnail Image
    Electrical injection and detection of spin polarization in InSb/ferromagnet nanostructures
    Kim, Yong-Jae (Virginia Tech, 2012-07-30)
    We present studies of the electical detection of spin injection and transport in InSb/CoFe heterostructures. As a narrow gap semiconductor, InSb has a high mobility and strong spin-orbit interaction. Using ferromagnetic CoFe, lateral InSb/CoFe devices are fabricated by semiconductor processing techniques. The saturation magnetizations of various CoFe electrodes with different widths are calculated from Hall measurements in which the fringing fields of the CoFe electrodes are detected. A magnetic model provides reasonable estimation of the saturation magnetization for micrometer scale geometries. The interface magnetoresistance measurements of InSb/CoFe thin film layered structures present a unique peak at low field, having a symmetric behavior in magnetic field with a critical field Hc and a strong temperature dependence. We attribute our signal to a ferromagnetic phase in the InSb induced by spin injection. In a non-local lateral spin valve measurement, we observed the following. Firstly, Hc of the lateral spin valve signals is identical to Hc of interface magnetoresistance signals. Secondly, the non-local lateral spin valve signals are strongly dependent on temperature, which is also a unique characteristic magnetoresistance. Thirdly, the signals are tunable in response to an applied injector bias. Lastly, the signals are dependent on the exact interfaces. Based on these observations, the detected signals may be considered as spin current signals. The Hall and magnetoresistance signals are measured locally and non-locally in InSb/CoFe Hall devices. The non-local magnetoresistance signals exhibit asymmetric behavior in applied magnetic field which are considered as signatures of spin phenomena. The non-local Hall signals present switching behavior with the CoFe magnetization switching at the coercive field. The non-local Hall signals in a perpendicular field show Hc, similarly seen in non-local lateral spin valves. Inverse spin Hall effect measurements with tilted magnetic fields show an in-plane magnetic field dependence in non-local type Hall signal and a perpendicular magnetic field dependence in the local Hall measurement. We have found that the signal can have its origin in a spin current from our observation of Hc and hysteresis in the magnetization traces. As yet, the spin current transport mechanism is unknown.
  • Thumbnail Image
    Electrically conducting microporous frameworks
    (United States Patent and Trademark Office, 2014-10-07)
    Electrically conducting vanadium arsenate or vanadium phosphate materials are described. The materials include a vanadium arsenate or vanadium phosphate framework structure about organic template and water molecules which may be removed to leave a microporous structure. The three-dimensional vanadium framework may provide electronic conductivity, while the extra-framework constituents may provide ionic conductivity.
  • Thumbnail Image
    Experimental Measurements by Antilocalization of the Interactions between Two-Dimensional Electron Systems and Magnetic Surface Species
    Zhang, Yao (Virginia Tech, 2014-06-18)
    Low-temperature weak-localization (WL) and antilocalization (AL) magnetotransport measurements are sensitive to electron interference, and thus can be used as a probe of quantum states. The spin-dependent interactions between controllable surface magnetism and itinerant electrons in a non-magnetic host provide insight for spin-based technologies, magnetic data storage and quantum information processing. This dissertation studies two different host systems, an In$_{0.53}$Ga$_{0.47}$As quantum well at a distance from the surface of a heterostructure, and an accumulation layer on an InAs surface. Both the systems are two-dimensional electron systems (2DESs), and possess prominent Rashba spin-orbit interaction caused by structural inversion asymmetry, which meets the prerequisites for AL. The surface local moments influence the surrounding electrons in two ways, increasing their spin-orbit scattering, and inducing magnetic spin-flip scattering, which carries information about magnetic interactions. The two effects modify the AL signals in opposing directions: the spin-flip scattering of electrons shrinks the signal, and requires a close proximity to the species, whereas the increase of spin-orbit scattering broadens and increases the signal. Accordingly, we only observe an increase in spin-orbit scattering in the study of the interactions between ferromagnetic Co$_{0.6}$Fe$_{0.4}$ nanopillars and the relatively distant InGaAs quantum well. With these CoFe nanopillars, a decrease in spin decoherence time is observed, attributed to the spatially varying magnetic field from the local moments. A good agreement between the data and a theoretical calculation suggests that the CoFe nanopillars also generate an appreciable average magnetic field normal to the surface, of value $\sim$ 35 G. We also performed a series of comparative AL measurements to experimentally investigate the interactions and spin-exchange between InAs surface accumulation electrons and local magnetic moments of rare earth ions Sm$^{3+}$, Gd$^{3+}$, Ho$^{3+}$, of transition metal ions Ni$^{2+}$, Co$^{2+}$, and Fe$^{3+}$, and of Ni$^{2+}$-, Co$^{2+}$-, and Fe$^{3+}$-phthalocyanines deposited on the surface. The deposited species generate magnetic scattering with magnitude dependent on their electron configurations and effective moments. Particularly for Fe$^{3+}$, the significant spin-flip scattering due to the outermost 3d shell and the fairly high magnetic moments modifies the AL signal into a WL signal. Experiments indicate a temperature-independent magnetic spin-flip scattering for most of the species except for Ho$^{3+}$ and Co$^{2+}$. Ho$^{3+}$ yields electron spin-flip rates proportional to the square root of temperature, resulting from transitions between closely spaced energy levels of spin-orbit multiplets. In the case of Co$^{2+}$, either a spin crossover or a spin-glass system forms, and hence spin-flip rates transit between two saturation regions as temperature varies. Concerning the spin-orbit scattering rate, we observe an increase for all the species, and the increase is correlated with the effective electric fields produced by the species. In both 2DESs, the inelastic time is inversely proportional to temperature, consistent with phase decoherence via the Nyquist mechanism. Our method provides a controlled way to probe the quantum spin interactions of 2DESs, either in a quantum well, or on the surface of InAs.
  • Thumbnail Image
    Improvement of the Optical and Mechanical Properties of Silica Nanoparticle Ionic Self-Assembled Multilayer Anti-Reflection Coatings on Glass and Polycarbonate Substrates
    Ridley, Jason Ian (Virginia Tech, 2010-02-05)
    This thesis presents the characterization of the optical and mechanical properties of silica nanoparticle films fabricated by ionic self-assembly, also known as layer-by-layer (LbL) deposition. Utilizing electrostatic attraction of oppositely-charged materials permits uniform and rapid growth of the constituents onto planar and curved surfaces. In this work, silica nanoparticles are adsorbed onto glass and polycarbonate substrates, as well as micron-scale glass fibers, with the purpose of improving the optical quality of the respective media. Several methods are presented to improve the adhesion and cohesion of silica nanoparticle films on glass substrates. In the first method, the substrate and nanoparticle surfaces are coated with materials containing sulfonate end groups. Next, a photo-reactive polycation known as diazo-resin (DAR) is used in ISAM deposition with the modified silica nanoparticles. Subsequent exposure to UV converts the ionic bonds between the DAR and sulfonate groups into covalent ones. The second method to improve the mechanical strength is to heat the ISAM silica nanoparticle film at a high enough temperature (500 °C) to remove the polymer and partially fuse the nanoparticles. This technique is known as calcination and is shown to significantly improve the mechanical robustness of the film without compromising the optical properties. The final method involves the deposition of precursor and capping polymer layers around bulk silica nanoparticle films with both bilayer and quadlayer designs. The addition of these polymer layers improves the surface contact between adjacent nanoparticles but reduces the film porosity and consequently the optical transparency. Currently the calcination technique is the only one that significantly improves the film adhesion and cohesion, but suggestions are offered to potentially improve the performance of films made by the other two methods. An alternative way to functionalize polycarbonate substrates for silica nanoparticle ISAM deposition is also presented. The molecular structure of polycarbonate at the surface can be modified by exposing it to deep UV (λ = 185, 254 nm). By doing so, the surface becomes populated with carboxylate species, and thus permits ISAM deposition of poly(allylamine hydrochloride) (PAH) and silica nanoparticles. A variety of spectroscopic methods show that the molecular structure is changed by this procedure, and SEM shows that UV treatment improves the uniformity of ISAM films on polycarbonate. Finally, PAH/silica nanoparticle ISAM films are deposited onto glass fibers. The fibers are used for mechanical reinforcement of polymer composite optical media. The role of the nanoparticle film on the fibers is to reduce light scattering at the interfaces of materials with different thermo-optic coefficients, in other words, transmittance losses associated with changes in temperature. Fiber bundles coated with silica nanoparticles suffer from unacceptable levels of aggregation, and hence do not currently improve the transmittance over the temperature spectrum. Some evidence is presented, however, to suggest that the transparency can be improved if fiber aggregation during ISAM deposition can be avoided.
  • Thumbnail Image
    Measurement by antilocalization of interactions between InAs surface electrons and magnetic surface species
    Zhang, Y.; Kallaher, R. L.; Soghomonian, Victoria G.; Heremans, Jean J. (American Physical Society, 2013-02-25)
    We show that antilocalization measurements can be used to experimentally study the interactions between InAs surface electrons and local moments of the rare earth ions Sm3+, Gd3+, and Ho3+ on the surface. Magnetic spin-flip scattering and spin-orbit scattering of the accumulation layer electrons are affected by the proximity of the rare earth ions. The spin-flip rate carries information about magnetic interactions. Within the temperature range studied, Sm3+ and Gd3+ yield temperature-independent electron spin-flip rates in proportion to their magnetic moments. In proximity to Ho3+ the InAs electrons however show a spin-flip rate increasing with temperature. We interpret the spin-flip rate due to Ho3+ as resulting from transitions between closely spaced energy levels of the ion on the surface. The experiments also show that the strength of spin-orbit interaction can be modified by the surface species. DOI: 10.1103/PhysRevB.87.054430
  • Thumbnail Image
    Mesoscopic quantum interference experiments in InGaAs and GaAs two-dimensional systems
    Ren, Shaola (Virginia Tech, 2015-06-16)
    The study of quantum interference in solid-state systems yields insight in fundamental properties of mesoscopic systems. Electron quantum interference constitutes an important method to explore mesoscopic physics and quantum decoherence. This dissertation focuses on two-dimensional (2D) electron systems in $delta-$Si doped n-type In$_{0.64}$Ga$_{0.36}$As/In$_{0.45}$Al$_{0.55}$As, 2D hole systems in Si-doped p-type GaAs/Al$_{0.35}$Ga$_{0.65}$As and C-doped p-type GaAs/\Al$_{0.24}$Ga$_{0.76}$As heterostructures. The low temperature experiments study the magnetotransport of nano- and micro-scale lithographically defined devices fabricated on the heterostructures. These devices include a single ring interferometer and a ring interferometer array in 2D electron system, Hall bar geometries and narrow wires in 2D hole systems. The single ring interferometer yields pronounced Aharonov-Bohm (AB) oscillations with magnetic flux periodicity of h/e over a wide range of magnetic field. The periodicity was confirmed by Fourier transformation of the oscillations. The AB oscillation amplitude shows a quasi-periodic modulation over applied magnetic field due to local magnetic flux threading through the interferometer arms. Further study of current and temperature dependence of the amplitude of the oscillations indicates that the Thouless energy forms the measure of excitation energies giving quantum decoherence. An in-plane magnetic field was applied to the single ring interferometer to study the Berry's phase and the Aharonov-Casher effect. The ring interferometer array yields both AB oscillations and Altshuler-Aronov-Spivak (AAS) oscillations, the latter with magnetic flux periodicity of h/2e. The AAS oscillations require time-reversal symmetry and hence can be used to qualify time-reversal symmetry breaking. More importantly, the fundamental mesoscopic dephasing length associated with time-reversal symmetry breaking under applied magnetic field, an effective magnetic length, can be obtained by the analysis of the AAS oscillations over magnetic field. A theoretical model for confined ballistic system is confirmed by experimental data fitting. The AAS oscillations are barely resolved above 0.16 T and their amplitude decays with increasing magnetic field. The AB oscillations exist till above 2 T and their amplitude doesn't show the monotonic decay with increasing magnetic field. The different behavior of the AAS and AB oscillations originates in the different symmetries, respectively temporal and spatial, that they are sensitive to. The p-type 2D GaAs system has strong spin-orbit interaction (SOI). Antilocalization in a Hall bar geometry was analyzed by the 2D Hikami-Larkin-Nagaoka (HLN) theory to obtain the spin coherence time and phase coherence time. The 2D hole systems we studied have low density and high mobility, quite different from the 2D electron systems. These high-quality 2D hole systems demonstrate semi-classical ballistic phenomena in mesoscopic structures preferentially to quantum-coherence phenomena.
  • Thumbnail Image
    Physical, electrical and electrochemical characterizations of transition metal compounds for electrochemical energy storage
    Yuan, Qifan (Virginia Tech, 2015-02-03)
    Electrochemical energy storage has been widely used in various areas, including new energy sources, auto industry, and information technology. However, the performance of current electrochemical energy storage devices does not meet the requirements of these areas that include both high energy and power density, fast recharge time, and long lifetime. One solution to meet consumer demands is to discover new materials that can substantially enhance the performance of electrochemical energy storage devices. In this dissertation we report four transition metal materials systems with potential applications in electrochemical energy storage. Nanoscale and nanostructured materials are expected to play important roles in energy storage devices because of their enhanced and sometimes unique physical and chemical properties. Studied here is the comparative electrochemical cation insertion into a nanostructured vanadium oxide, a promising electrode material candidate, for the alkali metal ions Li+, Na+ and K+ and the organic ammonium ion, in aqueous electrolyte solutions. Observed are the distinctive insertion processes of the different ions, which yield a correlation between physical degradation of the material and a reduction of the calculated specific charge. The results reveal the potential of this nanostructured vanadium oxide material for energy storage. Vanadium based electrochemical systems are of general interest, and as models for vanadium based solid-state electrochemical processes, the solution state and the solid-state electrochemical properties of two cryolite-type compounds, (NH4)3VxGa1-xF6, and Na3VF6, are studied. The electrochemical behavior of (NH4)3VxGa1-xF6 explored the possibility of using this material as an electrolyte for solid state energy storage systems. Zeolite-like materials have large surface to volume ratios, with ions and neutral species located in the nanometer sized pores of the 3-dimensional framework, potentially yielding high energy density storage capabilities. Yet the insulating nature of known zeolite-like materials has limited their use for electrical energy storage. Studied here are two vanadium based zeolite-like structures, the oxo-vanadium arsenate [(As6V15O51)-9]∞, and the oxo-vanadium phosphate [(P6V15O51)-9]∞, where the former shows electronic conduction in the 3-dimensional framework. Mixed electronic and ionic conductivity, from the framework and from the cations located within the framework, respectively, is measured in the oxo-vanadium arsenate, and allows the use of this material in electrochemical double-layer capacitor configuration for energy storage. By contrast, the oxo-vanadium phosphate shows ionic conduction only. Lastly, a new strontium manganese vanadate with a layered structure exhibiting mixed protonic and electronic conductivity is studied. The various transition metal compounds and materials systems experimentally studied in this thesis showcase the importance of novel materials in future energy storage schemes.
  • Thumbnail Image
    Quantum interference measurement of spin interactions in a bio-organic/semiconductor device structure
    Deo, Vincent; Zhang, Yao; Soghomonian, Victoria G.; Heremans, Jean J. (Springer Nature, 2015-03-30)
    Quantum interference is used to measure the spin interactions between an InAs surface electron system and the iron center in the biomolecule hemin in nanometer proximity in a bio-organic/semiconductor device structure. The interference quantifies the influence of hemin on the spin decoherence properties of the surface electrons. The decoherence times of the electrons serve to characterize the biomolecule, in an electronic complement to the use of spin decoherence times in magnetic resonance. Hemin, prototypical for the heme group in hemoglobin, is used to demonstrate the method, as a representative biomolecule where the spin state of a metal ion affects biological functions. The electronic determination of spin decoherence properties relies on the quantum correction of antilocalization, a result of quantum interference in the electron system. Spin-flip scattering is found to increase with temperature due to hemin, signifying a spin exchange between the iron center and the electrons, thus implying interactions between a biomolecule and a solid-state system in the hemin/InAs hybrid structure. The results also indicate the feasibility of artificial bioinspired materials using tunable carrier systems to mediate interactions between biological entities.
  • Thumbnail Image
    Quantum transport in mesoscopic systems of Bi and other strongly spin-orbit coupled materials
    Rudolph, Martin (Virginia Tech, 2013-05-03)
    Systems with strong spin-orbit coupling are of particular interest in solid state physics as an avenue for observing and manipulating spin physics using standard electrical techniques. This dissertation focuses on the characteristics of elemental bismuth (Bi), which exhibits some of the strongest intrinsic spin-orbit coupling of all elements, and InSb, which exhibits some of the strongest intrinsic spin-orbit coupling of all compound semiconductors. The experiments performed study the quantum transport signatures of nano- and micron-scale lithographically defined devices as well as spin-orbit coupled material/ferromagnet interfaces. All Bi structures are fabricated from Bi thin "films, and hence a detailed analysis of
    the characteristics of Bi "film growth by thermal evaporation is provided. Morphologically and electrically high quality "films are grown using a two stage deposition procedure. The phase and spin coherence of Bi geometries constrained in one, two, and three dimensions are systematically studied by analysis of the weak antilocalization transport signature, a quantum interference phenomenon sensitive to spin-orbit coupling. The "findings indicate that the phase coherence scales proportionally to the limiting dimension of the structure for sizes less than 500 nm. Specifically, in Bi wires, the phase coherence length is approximately as long as the wire width. Dephasing due to quantum confinement e"ffects limit the phase coherence in small Bi structures, impairing the observation of controlled interference phenomena in nano-scale Bi rings. The spin coherence length is independent of dimensional constraint by the film thickness, but increases significantly as the lateral dimensions, such as wire width, are constrained. This is a consequence of the quantum transport contribution from the strongly spin-orbit coupled Bi(001) surface state. To probe the Bi surface state further, Bi/CoFe junctions are fabricated. The anisotropic magnetoresistance of the CoFe is modifi"ed when carriers tunnel into the CoFe from Bi, possibly due to a spin dependent tunneling process or an interaction between the spin polarized density of states in CoFe and the anisotropic spin-orbit coupled density of states in Bi. InSb/CoFe junctions are studied as InSb "films are a simpler spin-orbit coupled system compared to Bi "films. For temperatures below 3.5 K, a large, symmetric, and abrupt negative magnetoresistance is observed. The low-"field high resistance state has similar temperature and magnetic "field dependences as the superconducting phase, but a superconducting component in the device measurements seems absent. A differential conductance measurement of the InSb/CoFe interface during spin injection indicates a quasiparticle gap present at the Fermi energy, coinciding with the large magnetoresistance.
  • Thumbnail Image
    Quantum-coherent transport in low-dimensional mesoscopic structures and thin films
    Xie, Yuantao (Virginia Tech, 2018-01-10)
    This thesis experimentally studies quantum interference phenomena and quantum coherence in mesoscopic systems, and quantum transport as well as magnetotransport in various materials system. One overarching aim is exploring the different mechanisms that give rise to quantum phase decoherence in lithographically patterned mesoscopic structures, of importance in the field of quantum technologies and spintronics. Various mesoscopic structures, namely quantum stadia, quantum wires, and side-gated rings, were fabricated to function as quantum interference devices and platforms to study quantum coherence on two-dimensional electron systems in InGaAs/InAlAs heterostructures. The mesoscopic structures were fabricated by photolithography and electron-beam lithography. The dependence of quantum coherence on geometry or temperature is investigated for each of the quantum interference devices. In the case of quantum stadia, phase coherence lengths were extracted by universal conductance fluctuations, and the extracted phase coherence lengths show a dependence on both temperature and geometry. Phase coherence lengths decreased with increasing temperature, as expected. Moreover, phase coherence lengths also varied with the width-to-length ratio and length of the side wires connected to the stadia, where competition between Nyquist decoherence and environmental coupling decoherence mechanisms coexists. For the quantum wires studied, the phase coherence lengths were extracted from antilocalization signals. Antilocalization measurements provide a sensitive mean of probing the quantum mechanical correction to electronic transport. The phase coherence lengths increased as the wire length increased, due to reduction of the environmental coupling that induces decoherence at the ends of a wire; longer wires tend to have longer phase coherence lengths. In related work, the thesis shows that the spin coherence length, as limited by spin-orbit interaction, increases as the wire width decreases. Decoherence in side-gated rings was measured from the amplitudes of the quantum-mechanical Aharonov-Bohm oscillations. The side gates allow for an in-plane controllable electric field. Asymmetrically biased side-gate voltages allow for the breaking of the two-dimensional parity symmetry of the ring device, effectively resulting in reduced amplitude of the Aharonov-Bohm oscillations. The mechanism that contributes to decoherence in these rings appears to be related to the breaking of the spatial symmetry. Measurements of antilocalization and weak-localization as well as magnetotransport were used to probe interesting or unique quantum mechanical phenomena in the following two, quite different, materials system: bismuth iridate thin films, and Ge/AlAs heterostructures on GaAs or Si substrates. Both materials are of interest for future quantum technologies and devices. Measurements in bismuth iridate thin films reveal interesting transport characteristics such as logarithmic temperature dependence of the resistivity, multiple charge carriers, and antilocalization due to spin-orbit interaction in the system. Weak localization measurements in the Ge/AlAs heterostructure on GaAs or Si substrates show that single carrier transport is essentially located in the Ge layer only. Further, the weak localization results indicate the near-absence of spin-orbit interaction for carriers in the electronically active Ge layer, suggesting the potential use of this materials system as a promising candidate for future electronic device applications. In short, quantum transport and interference measurements probe the quantum-mechanical behavior of materials system for future quantum, spin and electronic technologies. Mesoscopic patterned geometries in InGaAs/InAlAs heterostructures offer a wide range of interesting and unique platforms to study quantum-mechanical phenomena, specifically quantum decoherence, in the solid state. The decoherence phenomena observed and the investigations to the underlying mechanisms studied and modeled in this thesis may be transferred to similar materials system, enriching the knowledge in the field of quantum technologies. Magnetotransport and quantum transport were also applied to Ge/AlAs heterostructures and bismuth iridate thin films, to study the properties of their carrier systems.
  • Thumbnail Image
    Spin States in Bismuth and Its Surfaces: Hyperfine Interaction
    Jiang, Zijian (Virginia Tech, 2021-01-07)
    The hyperfine interaction between carrier spins and nuclear spins is an important component in exploring spin-dependent properties in materials with strong spin orbit interaction.However hyperfine interaction has been less studied in bismuth (Bi), a heavy element exhibiting a strong Rashba-like spin-orbit interaction in its two-dimensional surface states due to the broken spatial inversion symmetry. In this dissertation we experimentally explore the carrier spin polarization due to transport under strong spin-orbit interaction and the nuclear polarization resulting from the relatively unexplored hyperfine interaction on Bi(111) films.The carrier and nuclear spin polarizations are expected to dynamically interact, a topic with ramifications to other materials where surface states with noteworthy properties play a role.To achieve this goal, an optimized van der Waals epitaxy growth technique for Bi(111) on mica substrates was developed and used, resulting in flat Bi surfaces with large grain sizes and a layered step height of 0.39±0.015 nm, corresponding to one Bi(111) bilayer height. A comparison between Bi(111) films grown on three different substrates (mica, InSb(111)B, and Si(111)) is discussed, for which scanning electron microscopy and atomic force microscopy are applied to obtain the structural and morphological characteristics on the film surface. Magnetotransport measurements are carried out to extract the transport properties of theBi(111) films. Using the high quality Bi(111) film deposited on mica, we develop quantum magnetotransport techniques as delicate tools to study hyperfine interaction. The approach is based on measuring quantum corrections to the conductivity due to weak antilocalization, which depend on the coherence of the spin state of the carriers. The carrier spin polarization is generated by a strong DC current in the Bi(111) surface states (here called the Edelstein effect), which then induces dynamic nuclear polarization by hyperfine interaction. Quantum transport antilocalization measurements in the Bi(111) thin-films grown on mica indicate a suppression of antilocalization by the in-plane Overhauser field from the nuclear polarization, and allow for the quantification of the Overhauser field, which is shown to depend on both polarization duration and the DC current magnitude. Various delay times between the polarization and the measurement result in an exponential decay of the Overhauser field, driven by relaxation time T1. We observe that in the Bi surface states, the appreciable electron density and strong spin-orbit interaction allow for dynamic nuclear polarization in the absence of an external magnetic field.
  • Thumbnail Image
    Spin-orbit or Aharonov-Casher edge states in semiconductor systems
    Xu, Lingling (Virginia Tech, 2015-08-21)
    We present studies of edge states induced by the Aharonov-Casher vector potential or Rashba-type spin-orbit interaction using quantum transport in InGaAs/InAlAs herterostructures. The Aharonov-Casher effect is electromagnetically dual to the Aharonov-Bohm effect and is predicted to lead to edge states in a parabolic confinement at two-dimensional sample edges. As a narrow gap material, InGaAs has a low effective mass, high mobility, and strong spin-orbit interaction, which indicate that it can be used as a good material to detect the Aharonov-Casher effect or SOI interaction. Using InGaAs, we measured the magnetoresistance in a quantum antidot in narrow short channels in a tilted magnetic field. The fine structure (mT spacing) observed in the magnetoresistance indicate a probable energy spacing between AC edge states. We also fabricated side-gate channel structures in InGaAs/InAlAs quantum wells and investigated the values of the Rashba spin-orbit coupling constant α using the weak antilocalization analysis as a function of the side-gate voltage. We take the effect of the finite width into account and find the corrected values. With the simulation of electric fields in the wide channel and narrow channel, we found that the electric field components can be changed using side-gate voltages. While our results do not indicate which electric field component is responsible, the data indicate that the deduced spin-orbit strength values in a narrow channel are tunable by the side-gate voltage.