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1.
Using QHD model the parametric interaction of a laser radiation in an unmagnetised piezoelectric semiconductor has been studied. It is found that the Bohm potential in the electron dynamics enhances the gain coefficient of parametrically generated modes whereas reduces the threshold pump intensity. 相似文献
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Stimulated Brillouin scattering of a plane polarised electromagnetic wave propagating perpendicular to a static magnetic field
has been investigated analytically in ann-type piezoelectric semiconductor-plasma. Using coupled mode theory the dispersion relation is obtained and the threshold
value of the amplitude of electromagnetic wave for the onset of instability is studied for both the forward and back-scattered
modes. The role of the magnetostatic field on the threshold conditions for the unstable mode has been discussed. 相似文献
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S. Ghosh M.P. Rishi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,19(2):223-230
The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely
magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated
wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the
induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric
interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation
is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the
acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation
in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers
the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic
field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily
doped regime.
Received 5 November 2001 相似文献
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本文所论述的轴对称环状静电模是指环形磁约束等离子体(如托卡马克)中环向模数为零的近理想静电流体模,它包含有测地声模和基频率与之较低的声模;也含有所谓的‘近零频带状流’.本文根据冷离子流体模型在圆形磁面构成的准环坐标系中的表示,对涉及以上三种模式的漂移波湍流参量激发理论,在一级环形效应近似下,进行了系统讨论,并证明了带状流的四个新命题.利用对漂移波能谱的参数化描写,注意到由漂移波能谱径向有限宽度所引发的特性,如波能传播量的双Landau奇点,揭示了有限宽度对径向δ谱所得结果的重要修正:如,对近零频带状流和测地声模的参量激发条件带来的严格限制.此外,还讨论了密度带状流在高q条件下被激发的可能性.本文选用合理的物理参数.采用图示方法详细地讨论了有关的数值结果.分析表明,测地声模和近零频带状流的参量激发不可能发生在同一小半径处;如果测地声模被参量激发,也应能观察到密度带状流. 相似文献
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The possible lattice formation of grains of chosen material in a magnetized current carrying n-type piezoelectric semiconductor plasma has been examined. In addition to the repulsive Coulomb potential, there appears
a non-Coulombic oscillatory potential among the highly charged grains due to the strong resonant collective interaction of
the grains and the electron-acoustic mode of the host semiconductor giving rise to the possibility of the lattice formation
of grains of new materials. 相似文献
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G. Sharma S. Ghosh 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,11(2):301-307
Based on hydrodynamic model of plasmas an analytical investigation of frequency modulational interaction between copropagating
high frequency pump and acoustic mode and consequent amplification (steady-state and transient) of the modulated waves is
carried out in a magnetised piezoelectric semiconductor medium. The phenomenon of modulational amplification is treated as
four wave interaction process involving cubic nonlinearity of the medium. Gain constants, threshold-pump intensities and optimum-pulse
duration for the onset of modulational instabilities are estimated. The analysis has been performed in non-dispersive regime
of the acoustic mode, which is one of the preconditions for achieving an appreciable initial steady-state growth of the modulated
signal wave. It is found that the transient gain constant diminishes very rapidly if one chooses the pump pulse duration beyond
the maximum gain point. Moreover, the desired value of the gain can be obtained by adjusting intensity and pulse duration
of the pump and doping concentration of the medium concerned.
Received 12 August 1999 and Received in final form 17 December 1999 相似文献
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The parametric excitation of acoustohelicon waves has been studied in a piezoelectric semiconductor in the presence of a strong high frequency oscillatory electric field. The threshold electric field amplitude and the growth rate of the unstable mode have been obtained analytically and for n-InSb at 77 K the unstable mode is found to be propagating with a growth rate ~103 s?1 when the crystal is irradiated with a 10.6 μm CO2 laser. 相似文献
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由压电驱动器驱动的快速反射镜(FSM)广泛应用于各种精密稳定跟踪系统,FSM的控制精度决定了系统的跟踪精度。但压电驱动器具有严重的迟滞非线性干扰,针对这一缺点,应用自适应径向基RBF神经网络对迟滞干扰进行非线性逼近,并在此基础上结合滑模控制和反演法,设计了自适应反演滑模(ABSM)控制器。仿真实验表明,相对于滑模控制器,ABSM控制器的最大跟踪误差和均方根误差为分别降低了57.26%和52.53%,提高了FSM的控制精度。 相似文献
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S. Ghosh M.P. Rishi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(2):275-279
Using hydrodynamical model of semiconductor plasma analytical investigations are made for the amplitude modulation as well
as demodulation of an electromagnetic wave in a transversely magnetised acousto-optic semiconducting plasma. The inclusion
of carriers diffusion adds new dimension in the analysis presented here. Analysis are made under different wave number regions
over a wide range of cyclotron frequency. It has been seen that diffusion of charge carriers modifies amplitude modulation
and demodulation processes effectively. Numerical estimations are made for n-InSb crystal irradiated by pump wave of frequency 1.6 T s-1. Complete absorption of the waves takes place in all the possible wave lengths regimes when the cyclotron frequency becomes exactly equal to (v
2 + )1/2.
Received 28 February 2002 Published online 19 July 2002 相似文献
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Vinit SharmaPriyanka Manchanda Pankaj K. SahotaRalph Skomski Arti Kashyap 《Journal of magnetism and magnetic materials》2012,324(5):786-791
Density-functional calculations are used to determine the electronic structure and magnetic properties of dilute magnetic semiconductors with the composition X1−xMnxN (X=Al, Ga, In, x=6.25% and 12.5%). Emphasis is on the interatomic exchange as a function of the Mn-Mn distance. Our superlattice calculations show that the Mn dopants are spin-polarized with a half-metallic band gap and a magnetic moment of 4 μB per Mn atom at x=6.25 and 12.5%. The Mn (3d) bands lie in the band gap but partially hybridize with valence band or N 2p electrons, depending on the group-III element and on the spin direction. To calculate the exchange interaction parameters Jij, we have used a Green-function approach. The interaction between Mn atoms extends over several interatomic interactions and is mediated by nitrogen (2p) electrons. The exchange is always ferromagnetic and largest for the first nearest neighbors, but substantial ferromagnetic interactions persist over Mn-Mn distances up to sixth nearest neighbors in the considered supercell. 相似文献
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The parametric excitation of a helicon and an acoustic wave in a piezoelectric semiconductor-plasma in the presence of a strong magnetic field has been investigated using the coupled mode theory. The expressions for the threshold value of the electric field required for the onset of instability and for the growth rate well above the threshold have been obtained. It is observed that an acoustic wave of higher frequency and higher phase velocity than that of the pump wave cannot be excited. The analysis has been applied to the case of n type InSb sample where the threshold value of the electric field is found to be of the order 5.2 × 103 Vm?1 and the growth rate at an electric field 5.2 × 104 Vm?1 is of the order of 8.7 × 1010 sec?1. 相似文献
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Absolute instability of the acoustic wave (excited parametrically) has been explored on the basis of the Bers and Briggs criterion in the presence of a large transverse static magnetic field. In the magnetoplasma threshold the electric field decreases to about 0.4 times the value at zero magnetic field. 相似文献
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The transition to turbulence (spatio-temporal chaos) in a wide class of spatially extended dynamical system is due to the loss of transversal stability of a chaotic attractor lying on a homogeneous manifold (in the Fourier phase space of the system), causing spatial mode excitation. Since the latter manifests as intermittent spikes this has been called a bubbling transition. We present numerical evidences that this transition occurs due to the so-called blowout bifurcation, whereby the attractor as a whole loses transversal stability and becomes a chaotic saddle. We used a nonlinear three-wave interacting model with spatial diffusion as an example of this transition. 相似文献
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Investigations of thickness-shear mode elastic constant and damping of shunted piezoelectric materials with a coupling resonator 下载免费PDF全文
Shear-mode piezoelectric materials have been widely used to shunt the damping of vibrations where utilizing surface or interface shear stresses. The thick-shear mode(TSM) elastic constant and the mechanical loss factor can change correspondingly when piezoelectric materials are shunted to different electrical circuits. This phenomenon makes it possible to control the performance of a shear-mode piezoelectric damping system through designing the shunt circuit. However, due to the difficulties in directly measuring the TSM elastic constant and the mechanical loss factor of piezoelectric materials, the relationships between those parameters and the shunt circuits have rarely been investigated. In this paper, a coupling TSM electro–mechanical resonant system is proposed to indirectly measure the variations of the TSM elastic constant and the mechanical loss factor of piezoelectric materials. The main idea is to transform the variations of the TSM elastic constant and the mechanical loss factor into the changes of the easily observed resonant frequency and electrical quality factor of the coupling electro–mechanical resonator. Based on this model, the formular relationships are set up theoretically with Mason equivalent circuit method and they are validated with finite element(FE) analyses. Finally, a prototype of the coupling electro–mechanical resonator is fabricated with two shear-mode PZT5 A plates to investigate the TSM elastic constants and the mechanical loss factors of different circuit-shunted cases of the piezoelectric plate. Both the resonant frequency shifts and the bandwidth changes observed in experiments are in good consistence with the theoretical and FE analyses under the same shunt conditions. The proposed coupling resonator and the obtained relationships are validated with but not limited to PZT5 A. 相似文献
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Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense
circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma.
The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically
unstable mode have been obtained for two cases (i)B
0=0 and (ii) B0 ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth
rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also
found to be insensitive to the electron thermal velocity. 相似文献