Jun Han Lee

Bio: Jun Han Lee is an academic researcher from Ulsan National Institute of Science and Technology. The author has contributed to research in topics: Physics & Condensed matter physics. The author has an hindex of 3, co-authored 10 publications receiving 228 citations.

A Room-Temperature Ferroelectric Ferromagnet in a 1D Tetrahedral Chain Network.

Abstract: Ferroelectricity occurs in crystals with broken spatial inversion symmetry. In conventional perovskite oxides, concerted ionic displacements within a 3D network of transition-metal-oxygen polyhedra (MOx ) manifest spontaneous polarization. Meanwhile, some 2D networks of MOx foster geometric ferroelectricity with magnetism, owing to the distortion of the polyhedra. Because of the fundamentally different mechanism of ferroelectricity in a 2D network, one can further challenge an uncharted mechanism of ferroelectricity in a 1D channel of MOx and estimate its feasibility. Here, ferroelectricity and coupled ferromagnetism in a 1D FeO4 tetrahedral chain network of a brownmillerite SrFeO2.5 epitaxial thin film are presented. The result provides a new paradigm for designing low-dimensional MOx networks, which is expected to benefit the realization of macroscopic ferro-ordering materials including ferroelectric ferromagnets.

Control of magnetoelectric coupling in the Co 2 Y -type hexaferrites

Abstract: We comprehensively investigated the magnetic, ferroelectric, and ME properties of ${\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Co}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Al}}_{y})}_{12}{\mathrm{O}}_{22}$ single crystals in broad doping ranges of Sr $(1.0\ensuremath{\le}x\ensuremath{\le}1.8)$ and Al $(0.00\ensuremath{\le}y\ensuremath{\le}0.08)$. Most of the investigated compounds exhibit an intriguing coexistence of two apparently competing magnetic phases: a transverse conical (TC) and alternating longitudinal conical (ALC) spin structure. The magnetic properties show that the ${\mathrm{Ba}}_{0.2}{\mathrm{Sr}}_{1.8}{\mathrm{Co}}_{2}{({\mathrm{Fe}}_{0.96}{\mathrm{Al}}_{0.04})}_{12}{\mathrm{O}}_{22}$ crystal has the highest ordering temperature and largest volume fraction of the ALC phase at zero H; further, after the application of an in-plane H, it exhibits a maximized volume fraction of the metastable TC phase, resulting in the highest ME susceptibility and electric polarization at all temperatures below 300 K. Our findings demonstrate that securing the thermal stability of the ALC phase is a crucial prerequisite to achieve optimized ME coupling in ${\mathrm{Co}}_{2}\mathrm{Y}$-type hexaferrites, pointing to a general strategy applicable to other hexaferrites as well.

An Overview of Recent Progress in the Study of Distributed Multi-agent Coordination

Abstract: This article reviews some main results and progress in distributed multi-agent coordination, focusing on papers published in major control systems and robotics journals since 2006. Distributed coordination of multiple vehicles, including unmanned aerial vehicles, unmanned ground vehicles and unmanned underwater vehicles, has been a very active research subject studied extensively by the systems and control community. The recent results in this area are categorized into several directions, such as consensus, formation control, optimization, task assignment, and estimation. After the review, a short discussion section is included to summarize the existing research and to propose several promising research directions along with some open problems that are deemed important for further investigations.

Cooperative Control for Ocean Sampling: The Glider Coordinated Control System

Abstract: The glider coordinated control system (GCCS) uses a detailed glider model for prediction and a simple particle model for planning to steer a fleet of underwater gliders to a set of coordinated trajectories. The GCCS also serves as a simulation testbed for the design and evaluation of multivehicle control laws. In this brief, we describe the GCCS and present experimental results for a virtual deployment in Monterey Bay, CA and a real deployment in Buzzards Bay, MA.

Control law design for haptic interfaces to virtual reality

Abstract: The goal of control law design for haptic displays is to provide a safe and stable user interface while maximizing the operator's sense of kinesthetic immersion in a virtual environment. This paper outlines a control design approach which stabilizes a haptic interface when coupled to a broad class of human operators and virtual environments. Two-port absolute stability criteria are used to develop explicit control law design bounds for two different haptic display implementations: the impedance display and admittance display. The strengths and weaknesses of each approach are illustrated through numerical and experimental results for a three degree-of-freedom device. The example highlights the ability of the proposed design procedure to handle some of the more difficult problems in control law synthesis for haptics, including structural flexibility and noncollocation of sensors and actuators.

An integrated flywheel energy storage system with homopolar inductor motor/generator and high-frequency drive

Abstract: The design, construction, and test of an integrated flywheel energy storage system with a homopolar inductor motor/generator and high-frequency drive is presented in this paper. The work is presented as an integrated design of flywheel system, motor, drive, and controller. The motor design features low rotor losses, a slotless stator, construction from robust and low cost materials, and a rotor that also serves as the energy storage rotor for the flywheel system. A high-frequency six-step drive scheme is used in place of pulsewidth modulation because of the high electrical frequencies. A speed-sensorless controller that works without state estimation is also described. A prototype of the flywheel system has been demonstrated at a power level of 9.4 kW, with an average system efficiency of 83% over a 30000-60000 r/min speed range.