Showing papers by "Chao Zhang published in 1998"

Thermodynamic properties of a two-dimensional electron gas in the presence of a weak spatially modulated periodic potential

Abstract: A systematic investigation on the influence of an additional periodic modulation potential which is weak, either electric or magnetic in nature, and spatially modulated along one dimension, on the equilibrium thermodynamic properties of a two-dimensional electron gas in an externally applied magnetic field is presented. The application of such an additional modulation potential results in a broadening of the Landau level energy spectrum into bands whose widths oscillate as a function of the externally applied magnetic field. Such oscillations are found to reflect the commensurability of the two different length scales present in the system, namely the cyclotron diameter at the Fermi level and the period of the modulation. We show that such commensurability effects are also to be found in all thermodynamic quantities of the system. They appear at low magnetic fields as an amplitude modulation of the well-known de Haas-van Alphen-type oscillations, familiar from the homogeneous two-dimensional electron gas system in an external magnetic field, which may or may not be resolved depending on temperature and are only weakly dependent on temperature. Their origin lies in the oscillations occurring in the bandwidths and they are consequently completely different in origin from the usual de Haas-van Alphen-type oscillations. In particular, we show that commensurability oscillations are to be found in the chemical potential, Helmholtz free energy, internal energy, electronic entropy, electronic specific heat, orbital magnetization and orbital magnetic susceptibility of such weakly modulated systems. We find that the resulting commensurability oscillations in each thermodynamic function exhibit well-defined phase relations between the electric and magnetic modulations except in the case of the orbital magnetization and the orbital magnetic susceptibility. Explicit asymptotic expressions for the chemical potential, Helmholtz free energy and orbital magnetization, in the quasi-classical limit of small magnetic fields and small but finite temperatures, are also given.

Approximate Solutions for Stresses around Pin-Loaded Holes in Symmetric Composite Laminates

Abstract: The knowledge of average in-plane stress field in the vicinity of the pin-loaded holes is essential for predicting both the failure strength and edge effects of composite laminate pinned joints. In this paper, a simplified technique is proposed to approximately determine the distribution of in-plane stresses in pin-loaded composite laminates based on the values of stresses right at the pin-loaded hole edges using the Reissner variational principle. The research results show that the distribution of stresses can be represented in the forms of power formula or exponential formula, and the corresponding numerical results are in good agreement with the available experimental data.

Electronic subband structure in two-dimensional electron gases under intense laser radiations

Abstract: We present a theoretical study on the influence of intense terahertz (THz) electromagnetic (EM) radiations on the electronic structures in a modulation-doped two-dimensional (2D) semiconductor system. We find that in the presence of a THz EM radiation field polarised linearly along the 2D-plane of a 2D electron gas (2DEG), the electron density of states will be modified strongly by the intensity and the frequency of the radiation field. As a consequence, the Fermi-energy and the electron density in the system will be affected markedly by the EM radiations. The results have indicated that the photon-modified electronic subband structure in a 2DEG device can be observed by using the recently developed free-electron laser sources.

Edge effects in laminated composites with pin-loaded holes

Abstract: An analytical technique is developed for determining the three-dimensional edge effect stresses around pin-loaded holes in symmetric laminated composites Based on the superposition principle of linear elasticity theory, the stress state around pin-loaded holes is divided into two parts, one the in-plane stress state determined using the classical laminated plate theory and the other the boundary-layer stress state arising from the edge effects that only exist in the vicinity of the holes The equilibrium equations for boundary-layer stress state with zeroth-order approximation are written based on a set of power series expansions A closed-form solution for the boundary-layer stress state with zeroth-order approximation is then obtained by imposing stress functions on the variational principle of complementary energy Numerical results obtained using the present technique for cross-ply laminates are in good agreement with those obtained using finite element methods

Generation of tunable hypersonic phonons from two-dimensional electron gases under free-electron laser radiations

Abstract: A theoretical study of generation of high-frequency acoustic-phonon emission by electrons in GaAs-based two-dimensional electron gases (2DEGs), subject to terahertz (THz) free-electron laser radiations, is presented. We have studied the frequency and angular distribution of the phonon emission generated optically via deformation potential coupling. The distinctive nature for electron-photon-phonon interactions in a 2DEG results in a strong dependence of the acoustic-phonon emission on strength E0 and frequency ω of the THz laser field. Consequently, the frequency and the angle for hypersonic phonon emission can be tuned by varying E0 and/or ω.

Terahertz absorption in a double quantum well system under a magnetic field

Abstract: The absorption of electromagnetic (EM) waves in a double well structure is calculated in the terahertz frequency region. It is found that at frequencies higher than the cyclotron frequencies of electrons and holes, the EM absorption exhibits oscillatory behavior. The maxima in the absorption coefficient correspond to a situation where the photon energy is approximately the sum of the energies of the electron pair and the hole pair which are simultaneously excited by the radiation field.

Photon assisted hypersonic phonon emission from terahertz-driven two-dimensional electron gases

Abstract: A theoretical study of the generation of high-frequency acoustic phonons by electrons in GaAs-based two-dimensional (2D) electron gases (2DEGs), subject to intense terahertz (THz) electromagnetic (EM) radiations, is presented. We have studied the frequency and angular dependence of the phonon emission generated optically. The distinctive nature for electron–photon–phonon interactions in a 2DEG results in a strong dependence of the acoustic phonon emission on strength and frequency of the THz EM field. Consequently, frequency- and angle-tunable hypersonic phonon emission can be generated by varying the strength and/or frequency of the radiation field. These results are pertinent to phonon emission experiments and to the application of 2D semiconductor systems as high-frequency hypersonic sources.

Magneto-plasmon spectrum in a semiconductor under intense laser radiation

Abstract: We study how a linearly polarised intense laser irradiation affects the magneto-plasmon (MP) modes in a semiconductor. We first evaluate the electron Green's function, the density–density correlation function, and the random-phase approximation (RPA) dielectric function in the ( Q ,t ) representation in the presence of a laser field and a quantizing magnetic field. From these results, we obtain the electron density of states (DOS) and the plasmon spectrum in the ( Q ,Ω ) representation and investigate the dependence of the MP excitations on the frequency and intensity of the laser field in GaAs. The results are pertinent to the application of free-electron lasers developed recently.

Electrical Generation of Spontaneous Optical Emission in Electrically Modulated Two-Dimensional Electron Gases at Low-Temperatures

Abstract: A detailed theoretical study is presented for spontaneous optical emission from a two-dimensional electron gas (2DEG) in the presence of a unidirectional spatially periodic modulation at low temperatures. The momentum- and energy-balance equations for electron–photon interactions in the device system are solved self-consistently using the Boltzmann equation, from which we can obtain the frequency and the intensity of the electromagnetic radiation generated. The results obtained indicate that: (1) in an electrically modulated 2DEG at low temperatures, a strong electromagnetic radiation emission up to W/cm2 can be generated by applying a small d.c. electric field; (2) the radiation emission is generated indirectly in the occupied subbands through electronic transitions around the Fermi level; (3) the frequency of the radiation generated is at about 0.1 THz; (4) in the low frequency regime, spontaneous multiphoton emissions can be observed; and (5) this type of optical emission depends strongly on the sample parameters such as the electron density of the 2DEG and the modulation length.