Browsing by Author "Shi, Xiaoyan"
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Item Anomalously Large Resistance at the Charge Neutrality Point in a Zero-Gap InAs/GaSb Bilayer(IOP Publishing Ltd) Yu, W.; Clerico, V.; Hernandez Fuentevilla, C.; Shi, Xiaoyan; Jiang, Y.; Saha, D.; Lou, W. K.; Chang, K.; Huang, D. H.; Gumbs, G.; Smirnov, D.; Stanton, C. J.; Jiang, Z.; Bellani, V.; Meziani, Y.; Diez, E.; Pan, W.; Hawkins, S. D.; Klem, J. F.; Shi, XiaoyanWe report here our recent electron transport results in spatially separated two-dimensional electron and hole gases with nominally degenerate energy subbands, realized in an InAs(10 nm)/GaSb(5 nm) coupled quantum well. We observe a narrow and intense maximum (similar to 500 k Omega) in the four-terminal resistivity in the charge neutrality region, separating the electron-like and hole-like regimes, with a strong activated temperature dependence above T = 7 Kand perfect stability against quantizing magnetic fields. We discuss several mechanisms for that unexpectedly large resistance in this zero-gap semi-metal system including the formation of an excitonic insulator state.Item Carrier Recombination in Perovskites 3D Through 0D(2022-05-01T05:00:00.000Z) Zheng, Yangzi; Campbell, Zachary; Malko, Anton V.; Gartstein, Yuri; Lv, Bing; Shi, Xiaoyan; Zheng, JieLead-halide perovskites have long been demonstrated as materials with exceptional structural and optoelectronic properties. As an important crystalline material, lead-halide perovskites have potential applications in lasers, light-emitting diodes (LEDs), photovoltaic solar cells, photon detectors and biosensors, etc. By tailoring the morphological dimensionality, low-dimensional possessed distinct properties from their bulk (3D) counterparts. Due to the strong quantum confinement and octahedral site isolation, these low dimensional metal halide hybrids at the molecular level exhibit remarkable and unique properties that are significantly different from those of ABX3 perovskites. Considering the rapid development of low dimensional metal halide perovskites, we will discuss the synthesis, characterization, application, computational studies and compare various metal halide perovskites ranging from 3D through 0D. Finally, we show that a modified atomic layer deposition technique may be successfully used to protect 0D perovskite against external environment.Item Far Infrared Edge Photoresponse and Persistent Edge Transport in an Inverted InAs/GaSb Heterostructure(American Institute of Physics Inc, 2016-01-07) Dyer, G. C.; Shi, Xiaoyan; Olson, B. V.; Hawkins, S. D.; Klem, J. F.; Shaner, E. A.; Pan, W.; Shi, XiaoyanDirect current (DC) transport and far infrared photoresponse were studied an InAs/GaSb double quantum well with an inverted band structure. The DC transport depends systematically upon the DC bias configuration and operating temperature. Surprisingly, it reveals robust edge conduction despite prevalent bulk transport in our device of macroscopic size. Under 180 GHz far infrared illumination at oblique incidence, we measured a strong photovoltaic response. We conclude that quantum spin Hall edge transport produces the observed transverse photovoltages. Overall, our experimental results support a hypothesis that the photoresponse arises from direct coupling of the incident radiation field to edge states.Item Molecular Beam Epitaxy of La2-xSrxCuO4 Films and Heterostructures(2022-08-01T05:00:00.000Z) Xu, Xiaotao; Shi, Xiaoyan; Morcos, Faruck; Lv, Bing; Lumata, Lloyd; Zakhidov, Anvar A.; Zhang, FanSince 1986, the study of high-temperature superconductivity (HTS) in cuprates has revealed a massive amount of discoveries, such as pseudogap, charge density wave, d-wave superconductivity, etc. These novel states of matter trigger even more unknowns in fundamental science and inspire enormous emergent applications. This dissertation presents our research on the archetypical La2-xSrxCuO4 (LSCO) thin films and heterostructures. Specifically, the research has been driven by several fundamental questions. For example, can we create c-axis Josephson junctions for scientific research and superconductor-based quantum computation? What controls the fundamental behaviors of interface superconductivity? To answer those questions, high-quality crystals are required. Here we utilized and improved the oxide atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique to grow atomically smooth cuprate films and heterostructures to answer the proposed research questions. The main results are presented as follows. First, we improved the ALL-MBE growth in several ways to enhance the film quality significantly. Specifically, we studied the thermal annealing of oxide substrates and developed treatment methods for LaSrAlO4(LSAO) and SrTiO3(STO) substrates. Ramp-up rate and annealing temperature are found to be the most critical parameters. We then studied the synthesis of LSCO thin films via the ALL-MBE system. A detailed recipe for the growth of LSCO thin films on LSAO substrates is presented. Unique reflection high energy electron diffraction (RHEED) pattern features are observed in LSCO films. A strategy to monitor the film growth and maintain the correct stoichiometry is developed based on the real-time RHEED feedback. We also investigated the power and stability of ozone oxidation and compiled empirical post-annealing procedures suitable for various doping levels. Substrates and LSCO films were evaluated using atomic force microscopy (AFM) and RHEED. The results indicate that they are atomically perfect with high crystallinity. Mutual inductance (MI) tests reveal that the LSCO films are uniform over the whole sample area with a sharp superconducting transition. Second, LSCO heterostructures and superlattices have been synthesized to study the HTS c-axis Josephson junction and interfacial superconductivity. The method to probe the superconducting dead layer number near the interface is introduced using a series of superlattices. At the LSCO-LSAO interface, MI and transport measurements imply that the first two LSCO layers that are near the LSAO exhibit a substantial suppression of superconductivity, resulting in a barrier that is five layers thick in total. And an overdoped LSCO protective layer is found to be effective against carrier depletion in superconducting layers. Within LSAO barriers, a thickness of 2 unit-cells of LSCO interface superconductor is synthesized. The superconducting transition of the sample is tunable with doping and demonstrates the highest transition temperature of 34 K.Item Propagation of Very Low Frequency Transmitter Signals in the Inner Magnetosphere(2022-05-01T05:00:00.000Z) Gu, Wenyao; Chen, Lunjin; Balanov, Zalman; Heelis, Roderick A.; Anderson, Phillip C.; Rodrigues, Fabiano Da Silveira; Shi, XiaoyanSignals from ground-based very low frequency (VLF) transmitters can leak through the ionosphere and propagate in the inner magnetosphere as whistler-mode waves. They interact with energetic electrons from the Earth’s radiation belts, and precipitate them into the ionosphere. The effect of wave-particle interactions is affected by signal propagations, which depend on the spatial variation of the cold plasma population (as the propagation medium). Therefore, a further understanding of transmitter signal propagation and cold plasma medium is essential for investigating wave-particle interactions. First, a case study is performed on Russian Alpha transmitter signals observed by the Van Allen Probes. The signals are in ducted propagation, experience multiple reflections, and excite triggered emissions. The ducted propagation is justified by a ray-tracing technique, and the nonlinear cyclotron resonance theory is tested by the observed triggered emissions. Second, we perform a statistical study on the distribution of the two propagation modes, ducted and nonducted, by the use of observed Russian Alpha transmitter signals. The statistics show the dominance of nonducted signals in the plasmasphere in terms of both occurrence and power. The proportion of ducted signals is enhanced at higher L-shells and during active geomagnetic conditions. Finally, we statistically analyze the spatial and temporal distributions of inner-magnetospheric cold plasma density irregularities, which are responsible for ducted propagation. The density irregularities deep inside the plasmasphere are dominant in the night and dusk sectors and show no significant variation with geomagnetic conditions. In contrast, the density irregularities in and near the plasmasphere boundary layer occur at post-midnight during quiet times and expand throughout the night sector during active times.Item Seismic Rock Physics Analysis of Organic-rich Shales Using Statistical and Machine Learning Methods(2022-08-01T05:00:00.000Z) Lee, Jaewook; Shi, Xiaoyan; Lumley, David; Zhu, Hejun; Pujana, Ignacio; Pirouz, MortazaShale is the most common sedimentary rock, which accounts for approximately 70 percent of the rocks in the crust of the Earth. In a conventional petroleum system, organic-rich shales have been regarded as source rocks generating hydrocarbon resources. Over the past decade, these shale rocks have become both the source rocks and unconventional reservoirs because of the combination of horizontal drilling and hydraulic fracturing. Although drilling and development technologies have been advancing rapidly, there is a lack of exploration involved with many shale resource operations. The current ’drilling on a grid’ strategy causes inefficient hydrocarbon production and unnecessary expense because many shale reservoirs are more heterogeneous and complex than conventional sandstone reservoirs. To identify shale reservoir production ’sweet spots’ from seismic data to help optimize recovery, we need to develop more e↵ective seismic reservoir characterization methods. Also, we have to reduce the impact of greenhouse gas emissions and associated climate change while developing unconventional oil and gas. For this reason, we need a better rock physics model (RPM) to describe shale reservoirs and understand their heterogeneity or anisotropy. However, in contrast to conventional sandstone reservoirs, many important shale reservoir rock physics properties are not well understood. Moreover, the connections between seismic, elastic, and shale rock properties are complex and need more research to improve quantitative inversion and interpretation of seismic data. There are two main geophysical problems for better shale characterization: (1) building a proper seismic RPM of organic-rich shales, and (2) estimating accurate shale-specific reservoir properties from seismic data. To help integrate these multi-disciplinary concepts, this dissertation proposes three topics as part of my PhD research: (1) Improved shale RPM methods to estimate the total organic carbon (TOC) and mineralogical brittleness index (MBI) using statistical and machine learning methods, (2) Seismic amplitude variation with o↵set (AVO) forward modeling and AVO attribute analysis to model an accurate synthetic seismic data and predict important shale properties such as TOC and clay volume, and (3) Enhanced seismic anisotropy characterization methods to estimate shale reservoir properties from anisotropic seismic data. Therefore, successful estimation of shale rock properties from seismic properties may allow us to better characterize the organic-rich shale rocks. Also, these approaches can interpret the e↵ect of variations in porosity, mineralogy, and organic carbon concentration on seismic property changes. In conclusion, these methods potentially improve the shale reservoir characterization and time- lapse monitoring for shale hydrocarbon production. They can also help to provide seismic rock physics frameworks for accurate interpretation and monitoring of the physical property changes, for example, during CO 2 enhanced oil recovery (EOR), carbon capture and storage (CCS), and geothermal energy production.