Browsing by Author "Zhao, Yang"
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Item A novel and efficient engine for P-/S-wave-mode vector decomposition for vertical transverse isotropic elastic reverse time migration(Society of Exploration Geophysicists, 2022) Zhang, Lele; Liu, Lu; Niu, Fenglin; Zuo, Jiahui; Shuai, Da; Jia, Wanli; Zhao, YangWave-mode decomposition plays a very important role in elastic reverse time migration (ERTM). Improved imaging quality can be achieved due to reduced wave-mode crosstalk artifacts. The current state-of-the-art methods for anisotropic wavefield separation are based on either splitting model strategy, low-rank approximation, or lower-upper (LU) factorization. Most of these involve expensive matrix computation and Fourier transforms with strong model assumptions. Based on the anisotropic-Helmholtz (ani-Helmholtz) decomposition operator and decoupled formulations, we develop a novel and efficient P-/S-wave-mode vector decomposition method in vertical transverse isotropic (VTI) media with application in ERTM. We first review the basics of classical Helmholtz decomposition and isotropic decoupled formulations. In addition, the ani-Helmholtz decomposition operator is built from the P- and S-wave polarizations of the Christoffel equation in VTI media. We then derive novel decoupled formulations of anisotropic P-/S-waves based on the obtained ani-Helmholtz operator. Moreover, we use the first-order Taylor expansion to approximate the normalization term from the derived decoupled formulations and obtain an efficient ani-Helmholtz decomposition approach, which produces vector P- and S-wavefields with correct units, phases, and amplitudes. Compared with the previous studies, our approach mitigates model assumptions, avoids intricate calculations, such as LU factorization or low-rank approximation, and only needs three fast Fourier transforms at each time step. In addition, the graphic processing unit technique is used to dramatically accelerate various functions of ERTM, such as wavefields extrapolation, decomposition, and imaging. Three synthetic examples demonstrate the effectiveness and feasibility of our proposed approach.Item In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe2As2(Springer Nature, 2020) Liu, Panpan; Klemm, Mason L.; Tian, Long; Lu, Xingye; Song, Yu; Tam, David W.; Schmalzl, Karin; Park, J. T.; Li, Yu; Tan, Guotai; Su, Yixi; Bourdarot, Frédéric; Zhao, Yang; Lynn, Jeffery W.; Birgeneau, Robert J.; Dai, PengchengA small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe2As2, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature Ts. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature TN ( < Ts) and create in-plane resistivity anisotropy above Ts. Here we use neutron polarization analysis to show that such a strain on BaFe2As2 also induces a static or quasi-static out-of-plane (c-axis) AF order and its associated critical spin fluctuations near TN/Ts. Therefore, uniaxial pressure necessary to detwin single crystals of BaFe2As2 actually rotates the easy axis of the collinear AF order near TN/Ts, and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.Item Local events-based fast RTM surface-offset gathers via dip-guided interpolation(Springer Nature, 2021) Zhao, Yang; Niu, Feng-Lin; Fu, Lei; Cheng, Cheng; Chen, Jin-Hong; Huo, Shou-DongReverse Time Migration (RTM) Surface Offset Gathers (SOGs) are demonstrated to deliver more superior residual dip information than ray-based approaches. It appears more powerful in complex geological settings, such as salt areas. Still, the computational cost of constructing RTM SOGs is a big challenge in applying it to 3D field data. To tackle this challenge, we propose a novel method using dips of local events as a guide for RTM gather interpolation. The residual-dip information of the SOGs is created by connecting local events from depth-domain to time-domain via ray tracing. The proposed method is validated by a synthetic experiment and a field example. It mitigates the computational cost by an order of magnitude while producing comparable results as fully computed RTM SOGs.Item Marrying Application-Level Opportunities with Algorithm-Hardware Co-Design towards Ubiquitous Edge Intelligence(2023-04-21) Zhao, Yang; Lin, YingyanArtificial Intelligence (AI) algorithms, especially Deep Neural Networks (DNNs), recently achieved record-breaking performance (i.e., task accuracy) in a wide range of applications. This has motivated a growing demand for bringing powerful AI-powered functionalities into edge devices, such as Virtual Reality/Augmented Reality (VR/AR) and medical devices, towards ubiquitous edge intelligence. On the other hand, the powerful performance of AI algorithms comes with much increased computational complexity and memory storage requirements, which stand at odd with the limited compute/storage resources on edge devices. To close the aforementioned gap for enabling more extensive AI-powered edge intelligence, we advocate harmonizing AI algorithms and dedicated accelerators via algorithm-accelerator co-design and leveraging application-level opportunities to minimize redundant computations in the processing pipeline. First, to tackle the efficiency bottleneck caused by the required massive random-access memory (DRAM) accesses when accelerating DNNs, we propose an algorithm-accelerator co-design technique called SmartExchange to trade higher-cost memory storage/accesses for lower-cost computations, for boosting the acceleration efficiency of both DNN inference and training. In particular, on the algorithm level, we enforce a hardware-friendly DNN weight structure, where only a small basis matrix and a sparse and readily-quantized coefficient matrix are needed to be stored for each layer and the remaining majority of weights can be recovered from lower-cost computations. On the hardware level, we further design a dedicated accelerator to leverage the SmartExchange-enforced algorithm structure for improving both the energy efficiency and processing latency of acceleration. Second, motivated by the promising results achieved by the above algorithm-accelerator co-design technique SmartExchange, we explore and develop dedicated algorithm-accelerator co-design techniques for two real-world applications, which are further empowered with application-level opportunities to maximize the achievable efficiency. In particular, we consider two representative and increasingly demanded AI-powered intelligent applications, one is eye tracking on VR/AR devices which is to estimate the gaze directions of human eyes and the other is cardiac detection on medical implants which is to perform intracardiac electrogram (EGM) to electrocardiogram (ECG) conversion (i.e., EGM-to-ECG conversion) on pacemakers. Among these applications, we find that there consistently exist application-level opportunities to be leveraged for largely reducing the computation/data movement redundancy within the processing pipeline. Therefore, we develop a tailored processing pipeline for each application and then pair it with dedicated algorithm-accelerator co-design techniques to further boost the overall system efficiency while maintaining the task performance, as elaborated below: For the eye tracking application, we propose a predict-then-focus pipeline that first extracts region-of-interests (ROIs), which is only 24% (average) of the original eye images for gaze estimation, to reduce computational redundancy. Additionally, the temporal correlation of eyes across frames is leveraged so that only 5% of the frames require ROIs adjustment over time. On top of those, we develop a dedicated accelerator and integrate both the algorithm and accelerator into a real hardware prototype system, dubbed i-FlatCam, consisting of a lensless camera and a chip prototype fabricated in a 28nm CMOS technology for validation. Real-hardware measurements show that the i-FlatCam system is the first to simultaneously meets all three requirements of eye tracking required by next-generation AR/VR devices. After that, we take another big leap towards accelerating an eye segmentation-involved pipeline for eye tracking towards more general eye tracking in AR/VR, where the segmentation result can enable more downstream tasks in addition to eye tracking. The resulting system is called EyeCoD and is validated with a multi-chip hardware prototype setting. For the EGM-to-ECG conversion application, we propose an application-aware processing pipeline where a precise and more complex conversion algorithm is only incurred in instant response to the detected anomaly (i.e., arrhythmia) and a coarse conversion is activated otherwise to avoid unnecessary computations. Furthermore, we develop a dedicated accelerator called e-G2C which is tailored for the above processing pipeline to further boost energy efficiency. For evaluation, the e-G2C processor is fabricated in a 28nm CMOS technology and achieves 0.14-8.31 μJ/inference energy efficiency outperforming prior arts under similar complexity, enabling real-time detection/conversion, and promising possibly life-critical interventions.Item Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$(American Physical Society, 2015) Zhang, Chenglin; Park, J.T.; Lu, Xingye; Yu, Rong; Li, Yu; Zhang, Wenliang; Zhao, Yang; Lynn, J.W.; Si, Qimiao; Dai, PengchengWe use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe0.985Co0.015As which has coexisting static antiferromagnetic (AF) order and superconductivity (Tc=15 K, TN=30 K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er1=6 meV at the AF ordering wave vector QAF=Q1=(1,0) and its twinned domain Q2=(0,1). For partially detwinned NaFe0.985Co0.015As with the static AF order mostly occurring at QAF=(1,0), we still find a double resonance at both wave vectors with similar intensity. Since Q1=(1,0) characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.Item Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe0.985Co0.015As(American Physical Society, 2015) Zhang, Chenglin; Park, J.T.; Lu, Xingye; Yu, Rong; Li, Yu; Zhang, Wenliang; Zhao, Yang; Lynn, J.W.; Si, Qimiao; Dai, PengchengWe use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe0.985Co0.015As which has coexisting static antiferromagnetic (AF) order and superconductivity (Tc=15 K, TN=30 K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near Er1=3.25ﾠmeV and a broad dispersionless mode at Er1=6 meV at the AF ordering wave vector QAF=Q1=(1,0) and its twinned domain Q2=(0,1). For partially detwinned NaFe0.985Co0.015As with the static AF order mostly occurring at QAF=(1,0), we still find a double resonance at both wave vectors with similar intensity. Since Q1=(1,0) characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.Item Spin waves and spatially anisotropic exchange interactions in the $S=2$ stripe antiferromagnet ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_{2}$(American Physical Society, 2015) Wang, Meng; Valdivia, P.; Yi, Ming; Chen, J.X.; Zhang, W.L.; Ewings, R.A.; Perring, T.G.; Zhao, Yang; Harriger, L.W.; Lynn, J.W.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Yao, D. X.; Birgeneau, R.J.An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb0.8Fe1.5S2 throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of 27.6±4.2μ2B/Fe, consistent with S≈2. Unlike XFe2As2 (X=Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb0.8Fe1.5S2 is a Mott-like insulator with fully localized 3d electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.Item Spin waves and spatially anisotropic exchange interactions in the S=2 stripe antiferromagnet Rb0.8Fe1.5S2(American Physical Society, 2015) Wang, Meng; Valdivia, P.; Yi, Ming; Chen, J.X.; Zhang, W.L.; Ewings, R.A.; Perring, T.G.; Zhao, Yang; Harriger, L.W.; Lynn, J.W.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Yao, D.X.; Birgeneau, R.J.An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb0.8Fe1.5S2 throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of 27.6±4.2μ2B/Fe, consistent with S≈2. Unlike XFe2As2 (X=Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb0.8Fe1.5S2 is a Mott-like insulator with fully localized 3d electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.Item Temperature-dependent optical properties of titanium nitride(AIP Publishing LLC, 2017) Briggs, Justin A.; Naik, Gururaj V.; Zhao, Yang; Petach, Trevor A.; Sahasrabuddhe, Kunal; Goldhaber-Gordon, David; Melosh, Nicholas A.; Dionne, Jennifer A.The refractory metal titanium nitride is promising for high-temperature nanophotonic and plasmonic applications, but its optical properties have not been studied at temperatures exceeding 400 °C. Here, we perform in-situ high-temperature ellipsometry to quantify the permittivity of TiN films from room temperature to 1258 °C. We find that the material becomes more absorptive at higher temperatures but maintains its metallic character throughout visible and near infrared frequencies. X-ray diffraction, atomic force microscopy, and mass spectrometry confirm that TiN retains its bulk crystal quality and that thermal cycling increases the surface roughness, reduces the lattice constant, and reduces the carbon and oxygen contaminant concentrations. The changes in the optical properties of the material are highly reproducible upon repeated heating and cooling, and the room-temperature properties are fully recoverable after cooling. Using the measured high-temperature permittivity, we compute the emissivity, surface plasmon polariton propagation length, and two localized surface plasmon resonance figures of merit as functions of temperature. Our results indicate that titanium nitride is a viable plasmonic material throughout the full temperature range explored.