Southeast University

About: Southeast University is a education organization based out in Nanjing, China. It is known for research contribution in the topics: Computer science & MIMO. The organization has 66363 authors who have published 79434 publications receiving 1170576 citations. The organization is also known as: SEU.

The First 2D Homochiral Lead Iodide Perovskite Ferroelectrics: [R- and S-1-(4-Chlorophenyl)ethylammonium] 2 PbI 4

Abstract: 2D organic-inorganic lead iodide perovskites have recently received tremendous attention as promising light absorbers for solar cells, due to their excellent optoelectronic properties, structural tunability, and environmental stability. However, although great efforts have been made, no 2D lead iodide perovskites have been discovered as ferroelectrics, in which the ferroelectricity may improve the photovoltaic performance. Here, by incorporating homochiral cations, 2D lead iodide perovskite ferroelectrics [R-1-(4-chlorophenyl)ethylammonium]2 PbI4 and [S-1-(4-chlorophenyl)ethylammonium]2 PbI4 are successfully obtained. The vibrational circular dichroism spectra and crystal structural analysis reveal their homochirality. They both crystalize in a polar space group P1 at room temperature, and undergo a 422F1 type ferroelectric phase transition with transition temperature as high as 483 and 473.2 K, respectively, showing a multiaxial ferroelectric nature. They also possess semiconductor characteristics with a direct bandgap of 2.34 eV. Nevertheless, their racemic analogue adopts a centrosymmetric space group P21 /c at room temperature, exhibiting no high-temperature phase transition. The homochirality in 2D lead iodide perovskites facilitates crystallization in polar space groups. This finding indicates an effective way to design high-performance 2D lead iodide perovskite ferroelectrics with great application prospects.

Metamaterial-Based Thin Planar Lens Antenna for Spatial Beamforming and Multibeam Massive MIMO

Abstract: A metamaterial (MTM)-based thin planar lens antenna is proposed for spatial beamforming and multibeam massive multiple-input multiple-output systems. The antenna consists of a planar lens and a linear array of receive/transmit elements. To lower the insertion and reflection loss, the lens is formed by the two-layered ultrathin MTM-based surface separated with air and fed by substrate integrated waveguide-fed stacked-patch antennas. The effects of the focal-to-diameter ( f/D) on the power distribution of the lens are investigated to work out a design method. A planar lens antenna fed with seven elements is, for example, designed to operate at 28-GHz bands. The measured results show that the proposed antenna can achieve a scanning coverage of ±27° with a gain tolerance of 3.7 dB and a maximum gain of 24.2 dBi with an aperture efficiency of 24.5% over the operating bandwidth of 26.6-29 GHz. The lens antenna also features the advantages of compact size, low cost, lightweight, simple feeding network, and easy integration with other circuits for the next generation mobile communication and radar systems.

Finite-time stability of cascaded time-varying systems

Abstract: The uniform global finite-time stability is discussed for a cascaded time-varying system consisting of two uniformly finite-time stable subsystems. It is shown that a forward completeness condition is enough to ensure the uniform global finite-time stability of the system. For ease of reference, a particular result with a growth rate condition is also deduced. These stability results are applied to the tracking control problem of a non-holonomic wheeled mobile robot in kinematic model. Two tracking control laws are developed respectively for two different cases of the desired rotate velocity. Both control laws are continuous and can control the mobile robot to track the desired trajectory in finite time. Simulation results are provided to show the effectiveness of the method.

Space-Time-Coding Digital Metasurfaces

Abstract: We study space-time modulated digital coding metasurfaces that enable simultaneous manipulations of electromagnetic waves in both space and frequency domains, including harmonic beam steering/shaping and scattering-signature control. Our theoretical predictions are in good agreement with experimental results at microwave frequencies, and may find interesting applications to a variety of fields, including wireless communications, cognitive radars, adaptive beamforming, and holographic imaging.