Publications (27)
Dynamic Walking on Highly Underactuated Point Foot Humanoids: Closing the Loop between HZD and HLIP
Adrian B. Ghansah, Jeeseop Kim, Kejun Li +1
Realizing bipedal locomotion on humanoid robots with point feet is especially challenging due to their highly underactuated nature, high degrees of freedom, and hybrid dynamics res…
Automatic quality assessment for 2D fetal sonographic standard plane based on multi-task learning
Hong Luo, Han Liu, Kejun Li +1
The quality control of fetal sonographic (FS) images is essential for the correct biometric measurements and fetal anomaly diagnosis. However, quality control requires professional…
Anomalous Hall effect from inter-superlattice scattering in a noncollinear antiferromagnet
Lilia S. Xie, Shannon S. Fender, Cameron Mollazadeh +11
Superlattice formation dictates the physical properties of many materials, including the nature of the ground state in magnetic materials. Chemical composition is commonly consider…
as a 2 eV Single-Photon Emitter Candidate in Hexagonal Boron Nitride
Kejun Li, Tyler Smart, Yuan Ping
The generation of single-photon emitters in hexagonal boron nitride around 2 eV emission is experimentally well-recognized; however the atomic nature of these emitters is unknown.…
Characteristics of solar wind rotation
Kejun Li, W. Feng
Over 54 years of hourly mean value of solar wind velocity from 27 Nov. 1963 to 31 Dec. 2017 are used to investigate characteristics of the rotation period of solar wind through aut…
Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2
Lilia S. Xie, Oscar Gonzalez, Kejun Li +16
Magnetic materials with noncollinear spin textures are promising for spintronic applications. To realize practical devices, control over the length and energy scales of such spin t…
POLAR: Preference Optimization and Learning Algorithms for Robotics
Maegan Tucker, Kejun Li, Yisong Yue +1
Parameter tuning for robotic systems is a time-consuming and challenging task that often relies on domain expertise of the human operator. Moreover, existing learning methods are n…
The Phase Shifts of the Paired Wings of Butterfly Diagrams
Kejun Li, Hongfei Liang, Wen Feng
Sunspot groups observed by Royal Greenwich Observatory/US Air Force/NOAA from May 1874 to November 2008 and the Carte Synoptique solar filaments from March 1919 to December 1989 ar…
Data-Driven Predictive Control for Robust Exoskeleton Locomotion
Kejun Li, Jeeseop Kim, Xiaobin Xiong +3
Exoskeleton locomotion must be robust while being adaptive to different users with and without payloads. To address these challenges, this work introduces a data-driven predictive…
Safety-Aware Preference-Based Learning for Safety-Critical Control
Ryan K. Cosner, Maegan Tucker, Andrew J. Taylor +7
Bringing dynamic robots into the wild requires a tenuous balance between performance and safety. Yet controllers designed to provide robust safety guarantees often result in conser…
Natural Multicontact Walking for Robotic Assistive Devices via Musculoskeletal Models and Hybrid Zero Dynamics
Kejun Li, Maegan Tucker, Rachel Gehlhar +2
Generating stable walking gaits that yield natural locomotion when executed on robotic-assistive devices is a challenging task that often requires hand-tuning by domain experts. Th…
How Spin Relaxes and Dephases in Bulk Halide Perovskites
Junqing Xu, Kejun Li, Uyen N. Huynh +4
Spintronics in halide perovskites has drawn significant attention in recent years, due to highly tunable spin-orbit fields and intriguing interplay with lattice symmetry. Spin life…
Relationship between group sunspot number and Wolf sunspot number
Kejun Li, Hongfei Liang
Continuous wavelet transform and cross-wavelet transform have been used to investigate the phase periodicity and synchrony of the monthly mean Wolf () and group () su…
Synthesizing Robust Walking Gaits via Discrete-Time Barrier Functions with Application to Multi-Contact Exoskeleton Locomotion
Maegan Tucker, Kejun Li, Aaron D. Ames
Successfully achieving bipedal locomotion remains challenging due to real-world factors such as model uncertainty, random disturbances, and imperfect state estimation. In this work…
Effect of Environmental Screening and Strain on Optoelectronic Properties of Two-Dimensional Quantum Defects
Shimin Zhang, Kejun Li, Chunhao Guo +1
Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control…
The Split Janus-faced Sun: Magnetic Rhythm and Duality in the Solar Cycle
Weiqi Chen, Kejun Li, Jingchen Xu
The solar cycle - most notably characterized by its sunspot activity patterns - serves as a cornerstone of heliospheric physics. This research uncovers a fundamental magnetic dicho…
Spin Dynamics in Hybrid Halide Perovskites -- Effect of Dynamical and Permanent Symmetry Breaking
Kejun Li, Junqing Xu, Uyen N. Huynh +7
The hybrid organic-inorganic halide perovskite (HOIP), for example MAPbBr3, exhibits extended spin lifetime and apparent spin lifetime anisotropy in experiments. The underlying mec…
Hybrid Data-Driven Predictive Control for Robust and Reactive Exoskeleton Locomotion Synthesis
Kejun Li, Jeeseop Kim, Maxime Brunet +3
Robust bipedal locomotion in exoskeletons requires the ability to dynamically react to changes in the environment in real time. This paper introduces the hybrid data-driven predict…
Single nuclear spin detection and control in a van der Waals material
Xingyu Gao, Sumukh Vaidya, Kejun Li +8
Optically active spin defects in solids are leading candidates for quantum sensing and quantum networking. Recently, single spin defects were discovered in hexagonal boron nitride…
Preferential Multi-Objective Bayesian Optimization
Raul Astudillo, Kejun Li, Maegan Tucker +3
Preferential Bayesian optimization (PBO) is a framework for optimizing a decision-maker's latent preferences over available design choices. While preferences often involve multiple…
CLF-RL: Control Lyapunov Function Guided Reinforcement Learning
Kejun Li, Zachary Olkin, Yisong Yue +1
Reinforcement learning (RL) has shown promise in generating robust locomotion policies for bipedal robots, but often suffers from tedious reward design and sensitivity to poorly sh…
Nuclear spin polarization and control in a van der Waals material
Xingyu Gao, Sumukh Vaidya, Kejun Li +11
Van der Waals layered materials are a focus of materials research as they support strong quantum effects and can easily form heterostructures. Electron spins in van der Waals mater…
ROIAL: Region of Interest Active Learning for Characterizing Exoskeleton Gait Preference Landscapes
Kejun Li, Maegan Tucker, Erdem Bıyık +6
Characterizing what types of exoskeleton gaits are comfortable for users, and understanding the science of walking more generally, require recovering a user's utility landscape. Le…
Chasing Stability: Humanoid Running via Control Lyapunov Function Guided Reinforcement Learning
Zachary Olkin, Kejun Li, William D. Compton +1
Achieving highly dynamic behaviors on humanoid robots, such as running, requires controllers that are both robust and precise, and hence difficult to design. Classical control meth…
Excited-State Dynamics and Optically Detected Magnetic Resonance of Solid-State Spin Defects from First Principles
Kejun Li, Vsevolod D. Dergachev, Ilya D. Dergachev +3
Optically detected magnetic resonance (ODMR) is an efficient and reliable method that enables initialization and readout of spin states through spin-photon interface. In general, h…
Deep Spin Defects in Zinc Oxide for High-Fidelity Single-Shot Readout
Shimin Zhang, Taejoon Park, Erik Perez +10
Wide-bandgap oxides such as ZnO are favorable hosts for spin defect qubits due to their dilute nuclear spin background and potential for ultra-high purity. Yet, a deep-level defect…
Intersystem Crossing and Exciton-Defect Coupling of Spin Defects in Hexagonal Boron Nitride
Tyler J. Smart, Kejun Li, Junqing Xu +1
Despite the recognition of two-dimensional (2D) systems as emerging and scalable host materials of single photon emitters or spin qubits, uncontrolled and undetermined chemical nat…