5,090 research outputs found
Continuum model study of optical absorption by hybridized moir\'e excitons in transition metal dichalcogenide heterobilayers
We propose a continuum model for the theoretical study of hybridized moir\'e
excitons in transition metal dichalcogenides heterobilayers, and we use a
variational method to solve the exciton wavefunction and calculate the optical
absorption spectrum. The exciton continuum model is built by the charge
continuum model for electrons and holes in moir\'e superlattices, thereby
preserving the moir\'e periodicity and lattice symmetry from the charge
continuum model. The momentum-space shift of interlayer electron-hole
distribution is included, and thus the indirect nature of interlayer excitons
is described. Spin and valley degrees of freedom and related interactions are
omitted in this model, except for the spin-orbit energy splitting of A and B
excitons. In long moir\'e-wavelength and zero charge-transfer-coupling limits,
the exciton model and the optical absorption formula can be reduced to the
counterparts of an isolated exciton. This continuum model is applied to the
simulation of optical absorption by hybridized moir\'e excitons in
/ and / heterobilayers.
Twist-angle and electric-field dependences of absorption spectra are studied.
Calculated spectra are compared with experimental observations in the
literature, and correspondences of signatures are found. The deficiency and the
potential of the present model are discussed.Comment: 13 pages, 6 figure
Low-complexity face-assisted video coding
[[abstract]]This paper presents a novel face-assisted video coding scheme to enhance the visual quality of the face regions in video telephony applications. A skin-color based face detection and tracking scheme is proposed to locate the face regions in real-time. After classifying the macroblocks into the face and non-face regions, we present a dynamic distortion weighting adjustment (DDWA) scheme to drop the static non-face macroblocks, and the saved bits are used to compensate the face region by adjusting the distortion weighting of the face macroblocks. The quality of face regions will thus be enhanced. Moreover, the computation originally required for the skipped macroblocks can also be saved. The experimental results show that the proposed method can significantly improve the PSNR and the subjective quality of face regions, while the degradation introduced on the non-face areas is relatively insensitive to human perception. The proposed algorithm is fully compatible with the H.263 standard, and the low complexity feature makes it well suited to implement for real-time applications[[fileno]]2030144030041[[department]]電機工程學
Experimental analysis of power harvesting on vehicle vibration using smart piezoelectric materials
In this paper the experimental analysis for power harvesting from mechanical vibration on a vehicle has been studied by using QuickPack smart materials with piezoelectric effect. The finite element ANSYS method (ANSYS FEM) was applied to explore the required mechanical structure, modal and harmonic analysis, and electrical feature, i.e., output voltage, admittance. The experimental platform consists of a shocker and a lever, which simulated a periodical oscillation on vehicle vibration, for evaluating conversion efficiency from mechanical energy to electrical energy. During loading experiments of power generation, the electromechanical coupling characteristics of smart materials were investigated via a proposed testing circuit. Also, various electrical output loadings were specified within resistance of 5~3000 kΩ. Through the experiment analysis, the power harvesting test with a buck converter at the output terminal was processed to obtain the spectrum analysis of output voltage within the vibrating frequencies below 200 Hz, controlled by the electromagnetic shaker. Based on the comparison between ANSYS FEM and spectrum analysis, the optimal results of mechanical oscillating quantities have been verified by the maximum output voltage for the QuickPack NQ45N material. Hence, the optimum power harvesting of the smart material has the maximum output power of 0.18 mW at 26-Hz-vibration on a vehicle
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