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.

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

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.

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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.

a b s t r a c tRenewable energy is high on international and national agendas. Currently, grid-connected photovoltaic (PV) systems are a popular technology to convert solar energy into electricity. Existing PV panels have a relatively low and varying output voltage so that the converter installed between the PVs and the grid should be equipped with high step-up and versatile control capabilities. In addition, the output current of PV systems is rich in harmonics which affect the power quality of the grid. In this paper, a new multistage hysteresis control of a step-up DCeDC converter is proposed for integrating PVs into a singlephase power grid. The proposed circuitry and control method is experimentally validated by testing on a 600 W prototype converter. The developed technology has significant economic implications and could be applied to many distributed generation (DG) systems, especially for the developing countries which have a large number of small PVs connected to their single-phase distribution network.

Typically, a geared drive system is used to connect an induction motor of 1500 rpm with a Raymond Pulverizer of 105 rpm in mining applications. This system suffers from low efficiency and a heavy motor drive. This paper proposes a novel design of a 75 kW, 72/48 switched reluctance motor (SRM) for a low-speed direct-drive as for mining applications. The paper is focused on the design and comparative evaluation of the proposed machine in order to replace a geared drive system whilst providing a high torque low-speed and direct-drive solution. The machine performance is studied and the switching angle configuration of the machine is also optimised. The efficiency of the whole drive system is found to be as high as 90.19%, whereas the geared induction motor drive provides only an efficiency of 59.32% under similar operating conditions. An SRM prototype was built and experimentally tested. Simulation and experimental results show that the drive system has better performance to substitute the induction motor option in mining applications. Power [kW] 75 Weight [Kg] 678 Voltage [V] 400 The gear system outer diameter [mm] 1000 Current [A] 128 The gear system stack length [mm] 340 Frequency [Hz] 50 The output speed of a gear system [rpm] 105 Power factor at full load 0.9 The efficiency of a gear system [%] 62.98 Rate Speed [rpm] 1488 The total weight of a gear drive system [kg] 1050 Rate torque [N·m] 481 The total efficiency of a gear drive system [%] 59.32 Efficiency at full load [%] 94.2

The synthesis of chiral nonracemic materials and the polarization properties of chiral optical waveguides fabricated from them are presented. These materials are based on binaphthyl compounds designed to produce a glassy isotropic optically active material in a thin solid film form. Chiral-core optical waveguides may find use as novel elements in integrated optic devices for photonics applications.