We present an experimental approach to study the ultrafast polarization switching dynamics in thin-film ferroelectrics. A semiconductor photoconductive switch with femtosecond laser illumination is used as a ''pulse generator'' to produce jitter-free, sub-100 ps rise time step-function-like electrical pulses. Quantitative measurements yield a polarization switching time, t s , of ϳ220 ps when measured with a 5 V, 68 ps rise time input electrical pulse. Modeling of the switching transients using the Merz-Ishibashi model and Merz-Shur model of switching kinetics yields a quantitative estimate of the characteristic switching time constant, t 0 , of ϳ70-90 ps.

This paper presents an accelerated particle swarm optimization (PSO)-based maximum power point tracking (MPPT) algorithm to track global maximum power point (MPP) of photovoltaic (PV) generation under partial shading conditions. Conventional PSO-based MPPT algorithms have common weaknesses of a long convergence time to reach the global MPP and oscillations during the searching. The proposed algorithm includes a standard PSO and a perturb-and-observe algorithm as the accelerator. It has been experimentally tested and compared with conventional MPPT algorithms. Experimental results show that the proposed MPPT method is effective in terms of high reliability, fast dynamic response, and high accuracy in tracking the global MPP.

Pulsed laser excitation induced conductance changes in colossal magnetoresistance material La 0.7 Ca 0.3 MnO 3 were studied on the picosecond time scale. A two-component signal was seen consisting of a fast positive transient associated with the paramagnetic insulating state and a slower negative signal associated with the ferromagnetic metallic state. The fast component corresponds to the photoionization of the Jahn-Teller small polaron. The slow component is explained in terms of the reduced carrier mobility due to photogenerated magnetic excitations. [S0031-9007(98)06798-2] PACS numbers: 75.70. -i, 78.47. + pThe observation of colossal magnetoresistance (CMR) in the doped rare earth manganites has stimulated research on these strongly coupled metallic oxide materials [1][2][3]. It is now accepted that both double exchange [5] and Jahn-Teller lattice distortions [6] play an important role in the magnetic and transport properties of the manganites. However, the phase diagram of the manganites has a number of intriguing features [4]. The ferromagnetic order in the manganites does not have a simple dependence on the Mn 31 ͞Mn 41 ratio. The ferromagnetic Curie temperature ͑T C ͒ in La 12x Ca x MnO 3 , for example, peaks close to 30% Ca 21 (corresponding to 30% Mn 41 ) concentration rather than 50% Ca 21 concentration. Further, as one enhances the Ca 21 concentration in the system, disruption of the ferromagnetic order is observed. In fact, for .50% of the Ca 21 concentration no ferromagnetic phase is seen. Also the propensity of the system to form charge ordered insulator also increases with x. These features show that spin, charge, and lattice are strongly coupled in the CMR materials [5][6][7][8].In this paper, we describe the transient transport response of La 0.7 Ca 0.3 MnO 3 upon excitation with a 100 fs laser pulse. If charge or spin excitations are introduced via pulsed photoexcitation, and the change in conductance of the material is monitored, important information about the interactions among spin, charge, and lattice, and also the relaxation processes of the nonequilibrium state may be obtained on these CMR materials. There have been some studies in the past on the picosecond spin dynamics in magnetic systems [9][10][11][12]. For example, the study by Awschalom and Halbout [9] on magnetic polaron dynamics in a dilute magnetic semiconductor Cd 12x Mn x Te showed that the spin organization and relaxation times are hundreds of picoseconds. To our knowledge, no picosecond dynamical study on CMR materials has yet been reported in the literature.The epitaxial La 0.7 Ca 0.3 MnO 3 (LCMO) films used in the study were prepared by the pulsed laser deposition process on LaAlO 3 substrates. The details of the deposition and annealing process are described elsewhere [13] and the temperature dependence of the electrical re

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This paper presents a doubly fed induction generator (DFIG) wind power system with hydrogen energy storage, with a focus on its virtual inertia adaptive control. Conventionally, a synchronous generator has a large inertia from its rotating rotor, and thus its kinetic energy can be used to damp out fluctuations from the grid. However, DFIGs do not provide such a mechanism as their rotor is disconnected with the power grid, owing to the use of back-to-back power converters between the two. In this paper, a hydrogen energy storage system is utilized to provide a virtual inertia so as to dampen the disturbances and support the grid's stability. An analytical model is developed based on experimental data and test results show that: (1) the proposed method is effective in supporting the grid frequency; (2) the maximum power point tracking is achieved by implementing this proposed system; and, (3) the DFIG efficiency is improved. The developed system is technically viable and can be applied to medium and large wind power systems. The hydrogen energy storage is a clean and environmental-friendly technology, and can increase the renewable energy penetration in the power network.