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Molecular dynamics calculations on a model of NaNO2 in its orientationally disordered paraelectric phase are used to examine the coupling between translational phonon displacements and the anion orientations. The simulations are analyzed using the theory of Michel and coworkers. The extent of phonon-orientational coupling is measured by a wave vector dependent parameter 2, whose behaviour is found to be influenced by changes in temperature. We show that 2=1 is equivalent to the Heine-McConnell condition for the formation of an incommensurate phase. While the interionic potential that we employ leads to a disordered crystal exhibiting many of the properties of NaNO2, inadequacies of the model are clearly evident.  相似文献
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The incommensurate phase of NaNO2 is known to contain a transverse displacement modulation in addition to the modulated ferroelectric ordering, but the phase and sign of the displacement was left undetermined by the X-ray structural analysis. We discuss the phase, sign and magnitude of the displacement modulation from several points of view and build up a more detailed picture of its origin than previously. Rigorous group theoretical arguments fix the phase as zero, in contrast to the assumption of some workers, and indicate other possible aspects of the structure. The sign of the displacement is found to be positive due to the dominance of electrostatic forces and O–O repulsion over O–Na repulsion. The magnitude of the displacement seen in molecular dynamics simulations and estimated from simple atom packing agree reasonably with the X-ray value. The results confirm the displacement modulation as the essential ingredient responsible for the existence of the incommensurate phase.  相似文献
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In this communication we show that Fuliski-Kramarczyk master equation is valid for time-dependent projection operator. Thus, this equation is more general than the one proposed by Shibata and Hashitsume because no special assumption on the form of the projector is made.  相似文献
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We report molecular dynamics simulations of water confined in a cylindrical silica pore. The pore geometry and size is similar to that of typical pores in porous Vycor glass. In the present study we focus on the dependence of microscopical structural and dynamical properties on the degree of hydration of the pore. We have performed five simulations of systems between 19 and 96 % hydration. In all cases, water adsorbs strongly on the pore surface, clearly demonstrating the hydrophilic nature of the Vycor surface. Two layers of water molecules are affected strongly by the interactions with the glass surface. With decreasing degree of hydration an increasing volume in the center of the pore is devoid of water molecules. At 96 % hydration the center is a continuous and homogeneous region that has, however, a lower density than bulk water at ambient conditions. A well-pronounced mobility profile exists, where molecules in the center of the pores have substantially higher self diffusion coefficients than molecules on the pore surface. The spectral densities of center of mass and hydrogen atom motion show the signature of confinement for the molecules close to the pore surface, while the spectral densities in the center of the pore are similar to those in bulk water. The molecular dynamics results are in good agreement with recent experiments. Our data indicate that the dependence of experimental data on the level of hydration of the Vycor sample is due to the different relative contribution of molecules adsorbed on the pore surface and bulk-like molecules in the interior of the pore to the experimental averages.  相似文献
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The temporal development of incident electromagnetic plane waves across semiconductor quantum dots (QDs) is analyzed by the finite-difference time-domain method. By coating the QDs using thin metal films, surface plasmon polaritons (SPPs) can be created. As illustration, our modeling approach is applied to fluorescent multiphoton quantum dots made of cadmium sulphide of particular size (3.7 nm) and energy band gap (2.67 eV). When such a QD is coated by a metal film, a dipole-formed SPP is generated at the external surface of the coated QD by the incident electromagnetic wave with a photon energy of 1.34 eV corresponding to a two-photon process. When the thickness of the metal film is 0.37 nm, the peak intensity of the SPP oscillates through both the thin metal film and the core QD, resulting in an electromagnetic field inside the QD enhanced by a factor of 10, and thus an increased two-photon excitation that can be useful for bioimaging applications. Further increasing the metal film thickness blockades the SPP initially generated at the external surface of the coated QD from penetrating through the metal film, reducing the electromagnetic field inside the QD. PACS 73.22.-f; 78.67.Hc  相似文献
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Well-defined room-temperature photoluminescence (PL) was observed from 4H-SiC films on AlN/Si(100) complex substrates grown at temperatures below 1150 °C by the chemical vapor deposition method. The PL spectrum consists of three major emission peaks in the vicinities of 3.03, 3.17 and 3.37 eV. By the combination of experimental measurements and theoretical analysis, the origins of the PL emission peaks have been identified and associated with N donors, Al acceptors in the 4H-SiC films and the band-to-band transition between the second minimum of the conduction band and the top of valance band of the 4H-SiC. The room-temperature SiC PL can be much utilized for optoelectronic high-power, high-frequency and high-temperature applications in the ultraviolet spectral regime. PACS 81.05.Hd; 81.05.Ea; 78.55.-m  相似文献
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We study the dark current of the GaAs/AlGaAs quantum-well infrared photodetector (QWIP) by assuming a drift-diffusion carrier transport in the barriers where the electric fields are obtained by the current continuity condition and the self-consistent energy band structure. It has been shown that due to the current continuity condition, the dark currents across the QWIP devices are determined by the thermionic emission from the emitter to the multiple quantum well (MQW) region. The self-consistent calculation of the Schr?dinger and Poisson equations shows a weak electric field in the barrier region connecting to the emitter (much smaller than the average field across the QWIP at low bias) due to the accumulation of carriers in the triangle quantum well formed at the emitter-MQW interface, which results in a very small dark current at low bias. The numerical results explain well our experimental observation. PACS 73.21.Fg; 73.50.Dn; 85.60.Gz  相似文献
10.
Surface heterogeneity of activated carbons is usually characterized by adsorption energy distribution (AED) functions which can be estimated from the experimental adsorption isotherms by inverting integral equation. The experimental data of phenol adsorption from aqueous solution on activated carbons prepared from polyacrylonitrile (PAN) and polyethylene terephthalate (PET) have been taken from literature. AED functions for phenol adsorption, generated by application of regularization method have been verified. The Grand Canonical Monte Carlo (GCMC) simulation technique has been used as verification tool. The definitive stage of verification was comparison of experimental adsorption data and those obtained by utilization GCMC simulations. Necessary information for performing of simulations has been provided by parameters of AED functions calculated by regularization method.  相似文献
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