145 research outputs found
A theoretical study on the statics and dynamics of magnetic domain walls and skyrmions
No full textThis thesis conducts a comprehensive theoretical study and numerical simulations on the statics and dynamics of magnetic domain walls (DWs) and skyrmions, two prominent examples of topological solitons in magnetic materials. For DWs, the focus is on their motion in ferrimagnetic nanowires, driven by external magnetic fields or spin-polarized currents. The research addresses challenges like the Walker breakdown, which limits DW speed. A significant finding is that high-speed DW motion occurs near the angular momentum compensation point (AMCP). The study uses energy conservation principles to explain DW dynamics, proving that static DWs cannot exist under uniform external fields and deriving a velocity formula consistent with experimental results. In the skyrmion section, the thesis investigates ferromagnetic skyrmion pinning by disk-shaped defects and the dynamics of current-driven antiferromagnetic skyrmions in disordered systems. It reveals how skyrmion type and disk size affect pinning and explores the impact of disorder on skyrmion trajectories. Overall, this work enhances the understanding of DWs and skyrmions, offering insights for future spintronic device development based on topological magnetic solitons.</p
Operation of BGO with SiPM readout at dry ice and liquid nitrogen temperatures
No full textThe light yield and decay constant of BGO were measured at both dry ice and
liquid nitrogen temperatures using two SiPMs directly coupled to a
cm cubic BGO crystal. With the measured light yield
(5.20.3 PE/keV at dry ice temperature and 10.50.4 PE/keV at liquid
nitrogen temperature) and decay constants, potential applications of BGO in
ToF-PET and SPECT were discussed
Author response image 1. Author response
No full textAnkyrin adaptors together with their spectrin partners coordinate diverse ion channels and cell adhesion molecules within plasma membrane domains and thereby promote physiological activities including fast signaling in the heart and nervous system. Ankyrins specifically bind to numerous membrane targets through their 24 ankyrin repeats (ANK repeats), although the mechanism for the facile and independent evolution of these interactions has not been resolved. Here we report the structures of ANK repeats in complex with an inhibitory segment from the C-terminal regulatory domain and with a sodium channel Nav1.2 peptide, respectively, showing that the extended, extremely conserved inner groove spanning the entire ANK repeat solenoid contains multiple target binding sites capable of accommodating target proteins with very diverse sequences via combinatorial usage of these sites. These structures establish a framework for understanding the evolution of ankyrins' membrane targets, with implications for other proteins containing extended ANK repeat domains.</p
Ji yu jin chang sheng xue xuan fu de zi yun xing he zi fu dong er wei zhi dong qi
No full textM.Phil.Acoustic radiation force in the near-field of a vibrating source can be utilized to lift and transport objects, which provides a non-contact driving technology in addition to maglev. This research presents a novel self-running planar motion stage utilizing near-field acoustic transportation phenomenon and coupled resonant vibration of the device.Finite element analysis models have been established to find the needed resonant modes and optimize the design of the structure. The first longitudinal and the second bending modes are designed to have an identical frequency to create elliptical vibration of the structure, which can provide vertical levitation force as well as the propulsion force. A dynamic model based on the Reynolds Equation has been established to study the levitation and driving mechanism of the proposed device. The levitation and driving forces are analyzed; and the relationship between the driving force and the input phase difference is investigated to optimize the following position control. A capacitance surface encoder is proposed to provide the levitation height and two dimensional position feedback for the stage. The encoder design is based on the measurement of capacitance change between the driving stage and the metal sensing plates embedded in the ground. The lateral position (in the XY plane) of the circular stage can be uniquely determined based on the capacitance values.A prototype including the stage, encoder, and controller is implemented and characterized to demonstrate the performance of the proposed design. Two motion characteristics of the stage are first tested: one is the influence of the excitation voltage amplitude on the levitation height, and the other is the influence of the amplitude and phase angles of the excitation signals on the velocity and thrust force. In the calibration of the encoder, the calculated coordinates are firstly compensated for the scaling and the misalignment of the encoder axis and the precision stage. And the systematic errors from the nonlinear features are identified and fitted to a third-order polynomial equation in each axis. In the control scheme, the capacitance encoder works as the feedback module, and a LabVIEW program is used for date acquisition and processing. The stage is tested and demonstrated to follow a circular trajectory with a radius of 3.5mm.This work investigates a novel self-floating and self-running two-dimensional actuator using near-field acoustic levitation and coupled resonant vibration of the device. Compared with the conventional designs based on near-field acoustic levitation, the proposed design eliminates any external component such as guide rail, which contributes to bigger working space. This design also has two-dimensional motion capability with higher load capacity and energy efficiency. A dynamic model based on the Reynolds Equation is established to study the driving mechanism. A capacitance surface encoder is proposed to provide the two dimensional position feedback for the stage. A prototype including the stage, encoder, and controller is implemented and characterized to demonstrate the capability of the proposed design.振動源在近場中產生的輻射力可以用於提升和運輸物體，這現象可用於非接觸驅動技術，並有別於磁懸浮技術。本研究提出了一種新型的自運行平面運動機構，該機構利用了近場聲學傳播現象和裝置的耦合諧振。通過有限元分析可以找到機構的共振模態，並且用於指導結構的優化設計。通過結構設計，使得一階縱向模態和二階橫向模態具有相同的振動頻率，以產生橢圓振動，橢圓振動同時提供懸浮立和前進驅動力。建立了基於雷諾方程的運動模型，來分析機構的懸浮和運動機制。通過分析懸浮力和驅動力，獲得驅動力與輸入信號相位差之間的關係，用於之後定位平臺的位置控制。基於表面電容的編碼器被用於定位平臺的位置反饋，這個反饋包括了平臺的懸浮高度以及它的二維平面位置。編碼器可以測量懸浮平臺和固定在地上的金屬感應板間的電容。基於測得的電容值，通過相應的計算，可以得出平臺在XY平面內的確定位置。建立了包括運動器，編碼器和控制器在內的實驗模型，用於驗證設計的性能。首先是關於驅動器運動性能的測定，第一個是電壓大小對懸浮高度的影響，第二個是電壓大小以及電壓相位角對於運動速度和驅動力的影響。在編碼器的校準過程中，首先補償了由於編碼器和精密平臺錯位以及縮放而引起的誤差， 然後運用三階多項式來擬合每個軸的非線性系統誤差。在控制方案中，電容式編碼器起反饋作用，一個LabVIEW程序用於對電信號的處理。在運動測試中，平臺行走了一個半徑為3.5mm的圓形軌跡。這項研究成功研發了一種新型的自懸浮和自運動的二維運動機構，這個機構運用了近場超聲懸浮和耦合共振效應。與傳統的近場超聲懸浮裝置相比，新的設計不需要導軌之類的任何外部部件，這有助於提供更大的工作空間。該設計具有二維運動能力，並有更高的負載能力和能量效率。建立了基於雷諾方程的運動模型來研究其運動機制。用電容編碼器來當作二維定位平臺的位置反饋。包括運動機構，編碼器和控制器在內的實驗模型被建立，用於驗證設計的性能。Chen, Keyu.Thesis M.Phil. Chinese University of Hong Kong 2017.Includes bibliographical references (leaves 56-60).Abstracts also in Chinese.Title from PDF title page (viewed on 06, February, 2020).Chen, Keyu
A Mean Field Game Approach to Relative Investment-Consumption Games with Habit Formation
No full textThis paper studies an optimal investment-consumption problem for competitive
agents with exponential or power utilities and a common finite time horizon.
Each agent regards the average of habit formation and wealth from all peers as
benchmarks to evaluate the performance of her decision. We formulate the
n-agent game problems and the corresponding mean field game problems under the
two utilities. One mean field equilibrium is derived in a closed form in each
problem. In each problem with n agents, an approximate Nash equilibrium is then
constructed using the obtained mean field equilibrium when n is sufficiently
large. The explicit convergence order in each problem can also be obtained. In
addition, we provide some numerical illustrations of our results.Comment: arXiv admin note: substantial text overlap with arXiv:2206.13341 by
other author
Light yield of cold undoped CsI crystal down to 13 keV and the application of such crystals in neutrino detection
No full textThe light yield of an undoped CsI crystal at about 77 Kelvin was measured to be photo-electrons (PE) per keV electron-equivalent (keVee) in the energy range of [13, 60] keVee using X and -rays from an Am radioactive source. Based on this experimental result, the performance of 10 kg cryogenic inorganic scintillating crystals coupled to SiPM arrays to probe non-standard neutrino interactions through the detection of coherent elastic neutrino-nucleus scatterings at the spallation neutron source, Oak Ridge National Laboratory, was examined in detail
Trade-Off and Synergy among Ecosystem Services in the Guanzhong-Tianshui Economic Region of China
No full textNatural ecosystems provide society with important goods and services. With rapidly increasing populations and excessive utilization of natural resources, humans have been enhancing the production of some services at the expense of others. Although the need for certain trade-offs between conservation and development is urgent, having only a small number of efficient methods to assess such trade-offs has impeded progress. This study focuses on the evaluation of ecosystem services under different land use schemes. It reveals the spatial and temporal distributions of and changes in ecosystem services. Based on a correlation rate model and distribution mapping, the trade-offs and synergies of these ecosystem services can be found. Here, we also describe a new simple approach to quantify the relationships of every trade-off and synergy. The results show that all ecosystem services possess trade-offs and synergies in the study area. The trend of improving carbon sequestration and water interception indicate that these key ecosystem services have the strongest synergy. And the decrease in regional agricultural production and other services, except water yield, may be considered as trade-offs. The synergy between water yield and agricultural production was the most significant, while the trade-off between water interception and carbon sequestration was the most apparent, according to our interaction quantification model. The results of this study have implications for planning and monitoring the future management of natural capital and ecosystem services, and can be integrated into land use decision-making
Optimal wideband sequential sensing in cognitive radios via deep reinforcement learning
No full textAbstract In cognitive radios, wideband sequential sensing plays an important role, which is able to quickly identify temporary available transmission opportunities by adaptively allocating sensing resources. This paper proposes a Markov decision process for modelling the optimal control of sequential sensing, which provides a general formulation capturing various practical features, including sampling cost, sensing requirement, sensing budget etc. For solving the optimal sensing policy, a model‐augmented deep reinforcement learning algorithm is proposed, which enjoys high learning stability and efficiency, compared to conventional reinforcement learning algorithms
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