In the last 30 days

• Open/close First-principles calculations on the electronic and vibrational properties of β-V₂O₅

  Zhou Bo et al 2009 Chinese Phys. B 18 4988 Tag this article

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  Using a first-principles approach based on density functional theory, this paper studies the electronic and dynamical properties of β-V ₂O ₅. A smaller band gap and much wider split-off bands have been observed in comparison with α-V ₂O ₅. The Raman- and infrared-active modes at the Γ point of the Brillouin zone are evaluated with LO/TO splitting, where the symbol denotes the longitudinal and transverse optical model. The nonresonant Raman spectrum of a β-V ₂O ₅ powder sample is also computed, providing benchmark theoretical results for the assignment of the experimental spectrum. The computed spectrum agrees with the available experimental data very well. This calculation helps to gain a better understanding of the transition from α- to β-V ₂O ₅.

• Open/close Thickness optimization of Mo films for Cu(InGa)Se₂ solar cell applications

  Li Wei (李 微) et al 2011 Chinese Phys. B 20 068102 Tag this article

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  Mo thin films are deposited on soda lime glass (SLG) substrates using DC magnetron sputtering. The Mo film thicknesses are varied from 0.08 μm to 1.5 μm to gain a better understanding of the growth process of the film. The residual stresses and the structural properties of these films are investigated, with attention paid particularly to the film thickness dependence of these properties. Residual stress decreases and yields a typical tensile-to-compressive stress transition with the increase of film thickness at the first stages of film growth. The stress tends to be stable with the further increase of film thickness. Using the Mo film with an optimum thickness of 1 μm as the back contact, the Cu(InGa)Se ₂ solar cell can reach a conversion efficiency of 13.15%.

• Open/close Thermal stability of HfO₂/Si (001) films prepared by electron beam evaporation in ultrahigh vacuum using atomic oxygen

  Xu Run (徐 闰) et al 2010 Chinese Phys. B 19 128204 Tag this article

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  HfO ₂ films on silicon substrates have been prepared by electron beam evaporation in ultrahigh vacuum using atomic oxygen. Synchrotron radiation photon—electron spectroscopy was used to investigate the thermal stability of HfO ₂ films under an ultrahigh vacuum environment. At the temperature of 750 °C, HfO ₂ films begin to decompose. After being further annealed at 850 °C for 3 min, HfO ₂ films decomposes completely, partially to form Hf-silicide and partially to form gaseous HfO. Two chemical reactions are responsible for this decomposition process. A small amount of Hf-silicide, which is formed at the very beginning of growth, may result in the films grown subsequently to be loosened, and thereby leads to a relatively low decomposition temperature.

• Open/close Spinel lithium titanate (Li₄Ti₅O₁₂) as novel anode material for room-temperature sodium-ion battery

  Zhao Liang (赵 亮) et al 2012 Chinese Phys. B 21 028201 Tag this article

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  This is the first time that a novel anode material, spinel Li ₄Ti ₅O ₁₂ which is well known as a “zero-strain" anode material for lithium storage, has been introduced for sodium-ion battery. The Li ₄Ti ₅O ₁₂ shows an average Na storage voltage of about 1.0 V and a reversible capacity of about 145 mAh/g, thereby making it a promising anode for sodium-ion battery. Ex-situ X-ray diffraction (XRD) is used to investigate the structure change in the Na insertion/deinsertion process. Based on this, a possible Na storage mechanism is proposed.

• Open/close Bifurcations and chaotic threshold for a nonlinear system with an irrational restoring force

  Tian Rui-Lan (田瑞兰) et al 2012 Chinese Phys. B 21 020503 Tag this article

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  Nonlinear dynamical systems with an irrational restoring force often occur in both science and engineering, and always lead to a barrier for conventional nonlinear techniques. In this paper, we have investigated the global bifurcations and the chaos directly for a nonlinear system with irrational nonlinearity avoiding the conventional Taylor's expansion to retain the natural characteristics of the system. A series of transformations are proposed to convert the homoclinic orbits of the unperturbed system to the heteroclinic orbits in the new coordinate, which can be transformed back to the analytical expressions of the homoclinic orbits. Melnikov's method is employed to obtain the criteria for chaotic motion, which implies that the existence of homoclinic orbits to chaos arose from the breaking of homoclinic orbits under the perturbation of damping and external forcing. The efficiency of the criteria for chaotic motion obtained in this paper is verified via bifurcation diagrams, Lyapunov exponents, and numerical simulations. It is worthwhile noting that our study is an attempt to make a step toward the solution of the problem proposed by Cao Q J et al. (Cao Q J, Wiercigroch M, Pavlovskaia E E, Thompson J M T and Grebogi C 2008 Phil. Trans. R. Soc. A 366 635).

• Open/close Improving InGaN-LED performance by optimizing the patterned sapphire substrate shape

  Huang Xiao-Hui (黄小辉) et al 2012 Chinese Phys. B 21 037105 Tag this article

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  The epitaxial growths of GaN films and GaN-based LEDs on various patterned sapphire substrates (PSSes) with different values of fill factor ( f) and slanted angle (θ) are investigated in detail. The threading dislocation (TD) density is lower in the film grown on the PSS with a smaller fill factor, resulting in a higher internal quantum efficiency (IQE). Also the ability of the LED to withstand the electrostatic discharge (ESD) increases as the fill factor decreases. The illumination output power of the LED is affected by both θ and f. It is found that the illumination output power of the LED grown on the PSS with a lower production of tan θ and f is higher than that with a higher production of tan θ and f.

• Open/close The time-dependent Ginzburg—Landau equation for the two-velocity difference model

  Wu Shu-Zhen (吴淑贞) et al 2011 Chinese Phys. B 20 080509 Tag this article

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  A thermodynamic theory is formulated to describe the phase transition and critical phenomenon in traffic flow. Based on the two-velocity difference model, the time-dependent Ginzburg—Landau (TDGL) equation under certain condition is derived to describe the traffic flow near the critical point through the nonlinear analytical method. The corresponding two solutions, the uniform and the kink solutions, are given. The coexisting curve, spinodal line and critical point are obtained by the first and second derivatives of the thermodynamic potential. The modified Korteweg de Vries (mKdV) equation around the critical point is derived by using the reductive perturbation method and its kink—antikink solution is also obtained. The relation between the TDGL equation and the mKdV equation is shown. The simulation result is consistent with the nonlinear analytical result.

• Open/close Doctor-bladed Cu₂ZnSnS₄ light absorption layer for low-cost solar cell application

  Chen Qin-Miao (陈勤妙) et al 2012 Chinese Phys. B 21 038401 Tag this article

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  The doctor-blade method is investigated for the preparation of Cu ₂ZnSnS ₄ films for low-cost solar cell application. Cu ₂ZnSnS ₄ precursor powder, the main raw material for the doctor-blade paste, is synthesized by a simple ball-milling process. The doctor-bladed Cu ₂ZnSnS ₄ films are annealed in N ₂ ambient under various conditions and characterized by X-ray diffraction, ultraviolent/vis spectrophotometry, scanning electron microscopy, and current-voltage ( J—V) meansurement. Our experimental results indicate that (i) the X-ray diffraction peaks of the Cu ₂ZnSnS ₄ precursor powder each show a red shift of about 0.4°; (ii) the high-temperature annealing process can effectively improve the crystallinity of the doctor-bladed Cu ₂ZnSnS ₄, whereas an overlong annealing introduces defects; (iii) the band gap value of the doctor-bladed Cu ₂ZnSnS ₄ is around 1.41 eV; (iv) the short-circuit current density, the open-circuit voltage, the fill factor, and the efficiency of the best Cu ₂ZnSnS ₄ solar cell obtained with the superstrate structure of fluorine-doped tin oxide glass/TiO ₂/In ₂S ₃/Cu ₂ZnSnS ₄/Mo are 7.82 mA/cm ², 240 mV, 0.29, and 0.55%, respectively.

• Open/close All-dielectric left-handed metamaterial based on dielectric resonator: design, simulation and experiment

  Yang Yi-Ming (杨一鸣) et al 2011 Chinese Phys. B 20 014101 Tag this article

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  Dipoles with Lorentz-type resonant electromagnetic responses can realise negative effective parameters in their negative resonant region. The electric dipole and magnetic dipole can realise, respectively, negative permittivity and negative permeability, so both the field distribution forms of electric and magnetic dipoles are fundamentals in designing left-handed metamaterial. Based on this principle, this paper studies the field distribution in high-permittivity dielectric materials. The field distributions at different resonant modes are analysed based on the dielectric resonator theory. The origination and influence factors of the electric and magnetic dipoles are confirmed. Numerical simulations indicate that by combining dielectric cubes with different sizes, the electric resonance frequency and magnetic resonance frequency can be superposed. Finally, experiments are carried out to verify the feasibility of all-dielectric left-handed metamaterial composed by this means.

• Open/close Tunable broadband metamaterial absorber consisting of ferrite slabs and a copper wire

  Yang Yong-Jun (杨拥军) et al 2012 Chinese Phys. B 21 038501 Tag this article

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  A tunable broadband metamaterial absorber is demonstrated at microwave frequencies in this paper. The metamaterial absorber is composed of ferrite slabs with large resonance beamwidths and a copper wire. The theoretical analysis for the effective media parameters is presented to show the mechanism for achieving the perfect absorptivity characteristic. The numerical results of transmission, reflectance, and absorptivity indicate that the metamaterial absorber exhibits a near perfect impedance-match to free space and a high absorptivity of 98.2% for one layer and 99.97% for two layers at 9.9 GHz. The bandwidth with the absorptivity above 90% is about 2.3 GHz. Moreover, the absorption band can be shifted linearly in a wide frequency range by adjusting the magnetic bias. This metamaterial absorber opens a way to prepare perfectly matched layers for engineering applications.