In the last 30 days

• Open/close Simulating high Reynolds number flow in two-dimensional lid-driven cavity by multi-relaxation-time lattice Boltzmann method

  Chai Zhen-Hua et al 2006 Chinese Phys. 15 1855 Tag this article

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  By coupling the non-equilibrium extrapolation scheme for boundary condition with the multi-relaxation-time lattice Boltzmann method, this paper finds that the stability of the multi-relaxation-time model can be improved greatly, especially on simulating high Reynolds number ( Re) flow. As a discovery, the super-stability analysed by Lallemand and Luo is verified and the complex structure of the cavity flow is also exhibited in our numerical simulation when Re is high enough. To the best knowledge of the authors, the maximum of Re which has been investigated by direct numerical simulation is only around 50 000 in the literature; however, this paper can readily extend the maximum to 1000 000 with the above combination.

• Open/close The photoluminescence of ZnSe bulk single crystals excited by femtosecond pulse

  Li Huan-Yong et al 2006 Chinese Phys. 15 2407 Tag this article

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  This paper reports on the photoluminescence spectra of ZnSe single crystal with trace chlorine excited by the femtosecond laser pulse. Three emission bands, including second-harmonic-generation, two-photon-excited peak and a broad band at 500–700nm, were detected. The thermal strain induced by femtosecond pulse strongly influences the photoluminescence of ZnSe crystal. The corresponding strain in ZnSe crystal is estimated to be about 8.8 ×10 ⁻³ at room temperature. The zinc-vacancy, as the main point defect induced by femtosecond pulse, is successfully used to interpret the broad emission at 500–700nm. The research shows that self-activated luminescence possesses the recombination mechanism of donor–vacancy pair, and it is also influenced by a few selenium defects and the temperature. The rapid decrease in photoluminescence intensity of two-photon-excited fluorescence and second-harmonic generation emission at lower temperature is attributed to the fact that more point defects result in the thermal activation of the two-photo-absorption energy converting to the stronger recombination emission of chlorine–zinc vacancy in 500–700nm. The experimental results indicate that the femtosecond exciting photoluminescence shows a completely different emission mechanism to that of He–Cd exciting luminescence in ZnSe single crystal. The femtosecond laser exhibits a higher sensitive to the impurity in crystal materials, which can be recommended as an efficient way to estimate the trace impurity in high quality crystals.

• Open/close The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si (111) substrates

  Liu Zhe et al 2007 Chinese Phys. 16 1467 Tag this article

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  AlN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AlN/GaN superlattice buffer layer. The influence of AlN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by using AlN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.

• Open/close Spin polarization effect for Mn₂ molecule

  Yan Shi-Ying and Xu Guo-Liang 2007 Chinese Phys. 16 686 Tag this article

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  The density functional theory method (DFT) (b3p86) of Gaussian 03 has been used to optimize the structure of the Mn ₂ molecule. The result shows that the ground state of the Mn ₂ molecule is an 11-multiple state, indicating a spin polarization effect in the Mn ₂ molecule, a transition metal element molecule. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions of higher-energy states. So the ground state for Mn ₂ molecule being of an 11-multiple state is the indicative of spin polarization effect of the Mn ₂ molecule among those in the transition metal elements: that is, there are 10 parallel spin electrons in a Mn ₂ molecule. The number of non-conjugated electrons is the greatest. These electrons occupy different spacious orbitals so that the energy of the Mn ₂ molecule is minimized. It can be concluded that the effect of parallel spin in the Mn ₂ molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell–Sorbie potential functions with the parameters for the ground state and other states of the Mn ₂ molecule are derived. The dissociation energy D _e for the ground state of the Mn ₂ molecule is 1.4477  eV, equilibrium bond length R _e is 0.2506 nm, vibration frequency ω _e is 211.51 cm ⁻¹. Its force constants f ₂, f ₃, and f ₄ are 0.7240 aJ.nm ⁻², -3.35574 aJ.nm ⁻³, 11.4813 aJ.nm ⁻⁴ respectively. The other spectroscopic data for the ground state of the Mn ₂ molecule ω _eχ _e , B _e, α _e are 1.5301 cm ⁻¹, 0.0978 cm ⁻¹, 7.7825× 10 ⁻⁴ cm ⁻¹ respectively.

• Open/close Structural and electronic properties of Fe-doped BaTiO₃ and SrTiO₃

  Zhang Chao et al 2007 Chinese Phys. 16 1422 Tag this article

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  We have performed first principles calculations of Fe-doped BaTiO ₃ and SrTiO ₃. Dopant formation energy, structure distortion, band structure and density of states have been computed. The dopant formation energy is found to be 6.8 eV and 6.5 eV for Fe-doped BaTiO ₃ and SrTiO ₃ respectively. The distances between Fe impurity and its nearest O atoms and between Fe atom and Ba or Sr atoms are smaller than those of the corresponding undoped bulk systems. The Fe defect energy band is obtained, which mainly originates from Fe 3d electrons. The band gap is still an indirect one after Fe doping for both BaTiO ₃ and SrTiO ₃, but the gap changes from Γ –R point to Γ –X point.

• Open/close Non-equilibrium extrapolation method for velocity and pressure boundary conditions in the lattice Boltzmann method

  Guo Zhao-Li et al 2002 Chinese Phys. 11 366 Tag this article

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  In this paper, we propose a new approach to implementing boundary conditions in the lattice Boltzmann method (LBM). The basic idea is to decompose the distribution function at the boundary node into its equilibrium and non-equilibrium parts and then to approximate the non-equilibrium part with a first-order extrapolation of the non-equilibrium part of the distribution at the neighbouring fluid node. Schemes for velocity and pressure boundary conditions are constructed based on this method. The resulting schemes are of second-order accuracy. Numerical tests show that the numerical solutions of the LBM together with the present boundary schemes are in excellent agreement with the analytical solutions. Second-order convergence is also verified from the results. It is also found that the numerical stability of the present schemes is much better than that of the original extrapolation schemes proposed by Chen et al. (1996 Phys. Fluids 8 2527).

• Open/close Improvement in efficiency of solar cell by removing Cu_2−xSe from CIGS film surface

  Li Wei et al 2006 Chinese Phys. 15 878 Tag this article

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  CIGS thin films are deposited by sputtering and selenization. The synthesis of semiconducting polycrystalline thin films and characteristics of devices based on the CIGS absorbing layers are investigated. Their microstructures are characterized by x-ray diffraction and Raman spectroscopy. The results reveal that there exist metallic Cu _{2− x}Se compounds in CIGS film surfaces and the compounds are thought to be responsible for the degradation of the open circuit voltage of solar cells. The optimization of selenization temperature profile and copper content in the precursor surfaces is studied, concluding that the conversion efficiency may be improved by removing metallic Cu _{2− x}Se compounds from the surfaces of CIGS thin films.

• Open/close Noise-induced striped trajectories of Rossler systems

  Huang Zhi-Hua and Dong Ya-Li 2007 Chinese Phys. 16 2291 Tag this article

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  In this paper, the generation of striped trajectories in phase space by noise-injection is considered. With suitable amplitudes of noise, the steady-state system orbits appear in rectangular striped shape. The relationship between the shape (including the range and the number of stripes) and some parameters is discussed. The result shows that noise can also generate the striped shape effectively and simply, which is similar to the newly-discovered striped pattern generated by controlled Rossler systems.

• Open/close Ab initio calculations of electronic structure of anatase TiO₂

  Chen Qiang and Cao Hong-Hong 2004 Chinese Phys. 13 2121 Tag this article

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  This paper presents the results of the self-consistent calculations on the electronic structure of anatase phase of TiO ₂. The calculations were performed using the full potential-linearized augmented plane wave method (FP-LAPW) in the framework of the density functional theory (DFT) with the generalized gradient approximation (GGA). The fully optimized structure, obtained by minimizing the total energy and atomic forces, is in good agreement with experiment. We also calculated the band structure and the density of states. In particular, the calculated band structure prefers an indirect transition between valence and conduction bands of anatase TiO ₂, which may be helpful for clarifying the ambiguity in other theoretical works.

• Open/close AlGaInP thin-film LED with omni-directionally reflector and ITO transparent conducting n-type contact

  Zhang Jian-Ming et al 2007 Chinese Phys. 16 3498 Tag this article

  Article References Full text PDF (1.06 MB)

  In this paper a novel AlGaInP thin-film light-emitting diode (LED) with omni-directionally reflector (ODR) and transparent conducting indium tin oxide (ITO) n-type contact structure is proposed, and fabrication process is developed. This reflector is realized with the combination of a low-refractive-index dielectric layer and a high reflectivity metal layer. This allows the light emitted or internally reflected downwardly towards the GaAs substrate at any angle of incidence to be reflected towards the top surface of the chip. ITO n-type contact is used for anti-reflection and current spreading layers on the ODR-LED with ITO. The sheet resistance of the ITO films (95 nm) deposited on n-ohmic contact of ODR-LED is of the order 23.5Ω/□ with up to 90% transmittance (above 92% for 590–770 nm) in the visible region of the spectrum. The optical and electrical characteristics of the ODR-LED with ITO are presented and compared to conventional AS-LED and ODR-LED without ITO. It is shown that the light output from the ODR-LED with ITO at forward current 20 mA exceeds that of AS-LED and ODR-LED without ITO by about a factor of 1.63 and 0.16, respectively. A favourable luminous intensity of 218.3 mcd from the ODR-LED with ITO (peak wavelength 620 nm) could be obtained under 20 mA injection, which is 2.63 times and 1.21 times higher than that of AS-LED and ODR-LED without ITO, respectively.