中国物理B

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Analysis of robustness of urban bus network

任涛, 王一帆, 刘苗苗, 徐艳杰

2016 (2): 20101-020101. doi: 10.1088/1674-1056/25/2/020101

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In this paper, the invulnerability and cascade failures are discussed for the urban bus network. Firstly, three static models(bus stop network, bus transfer network, and bus line network) are used to analyse the structure and invulnerability of urban bus network in order to understand the features of bus network comprehensively. Secondly, a new way is proposed to study the invulnerability of urban bus network by modelling two layered networks, i.e., the bus stop-line network and the bus line-transfer network and then the interactions between different models are analysed. Finally, by modelling a new layered network which can reflect the dynamic passenger flows, the cascade failures are discussed. Then a new load redistribution method is proposed to study the robustness of dynamic traffic. In this paper, the bus network of Shenyang City which is one of the biggest cities in China, is taken as a simulation example. In addition, some suggestions are given to improve the urban bus network and provide emergency strategies when traffic congestion occurs according to the numerical simulation results.

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Stochastic response of van der Pol oscillator with two kinds of fractional derivatives under Gaussian white noise excitation

杨勇歌, 徐伟, 孙亚辉, 谷旭东

2016 (2): 20201-020201. doi: 10.1088/1674-1056/25/2/020201

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This paper aims to investigate the stochastic response of the van der Pol (VDP) oscillator with two kinds of fractional derivatives under Gaussian white noise excitation. First, the fractional VDP oscillator is replaced by an equivalent VDP oscillator without fractional derivative terms by using the generalized harmonic balance technique. Then, the stochastic averaging method is applied to the equivalent VDP oscillator to obtain the analytical solution. Finally, the analytical solutions are validated by numerical results from the Monte Carlo simulation of the original fractional VDP oscillator. The numerical results not only demonstrate the accuracy of the proposed approach but also show that the fractional order, the fractional coefficient and the intensity of Gaussian white noise play important roles in the responses of the fractional VDP oscillator. An interesting phenomenon we found is that the effects of the fractional order of two kinds of fractional derivative items on the fractional stochastic systems are totally contrary.

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Improved kernel gradient free-smoothed particle hydrodynamics and its applications to heat transfer problems

雷娟棉, 彭雪莹

2016 (2): 20202-020202. doi: 10.1088/1674-1056/25/2/020202

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Kernel gradient free-smoothed particle hydrodynamics (KGF-SPH) is a modified smoothed particle hydrodynamics (SPH) method which has higher precision than the conventional SPH. However, the Laplacian in KGF-SPH is approximated by the two-pass model which increases computational cost. A new kind of discretization scheme for the Laplacian is proposed in this paper, then a method with higher precision and better stability, called Improved KGF-SPH, is developed by modifying KGF-SPH with this new Laplacian model. One-dimensional (1D) and two-dimensional (2D) heat conduction problems are used to test the precision and stability of the Improved KGF-SPH. The numerical results demonstrate that the Improved KGF-SPH is more accurate than SPH, and stabler than KGF-SPH. Natural convections in a closed square cavity at different Rayleigh numbers are modeled by the Improved KGF-SPH with shifting particle positions, and the Improved KGF-SPH results are presented in comparison with those of SPH and finite volume method (FVM). The numerical results demonstrate that the Improved KGF-SPH is a more accurate method to study and model the heat transfer problems.

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Solving unsteady Schrödinger equation using the improved element-free Galerkin method

程荣军, 程玉民

2016 (2): 20203-020203. doi: 10.1088/1674-1056/25/2/020203

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By employing the improved moving least-square (IMLS) approximation, the improved element-free Galerkin (IEFG) method is presented for the unsteady Schrödinger equation. In the IEFG method, the two-dimensional (2D) trial function is approximated by the IMLS approximation, the variation method is used to obtain the discrete equations, and the essential boundary conditions are imposed by the penalty method. Because the number of coefficients in the IMLS approximation is less than in the moving least-square (MLS) approximation, fewer nodes are needed in the entire domain when the IMLS approximation is used than when the MLS approximation is adopted. Then the IEFG method has high computational efficiency and accuracy. Several numerical examples are given to verify the accuracy and efficiency of the IEFG method in this paper.

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Simulation of the 3D viscoelastic free surface flow by a parallel corrected particle scheme

任金莲, 蒋涛

2016 (2): 20204-020204. doi: 10.1088/1674-1056/25/2/020204

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In this work, the behavior of the three-dimensional (3D) jet coiling based on the viscoelastic Oldroyd-B model is investigated by a corrected particle scheme, which is named the smoothed particle hydrodynamics with corrected symmetric kernel gradient and shifting particle technique (SPH_CS_SP) method. The accuracy and stability of SPH_CS_SP method is first tested by solving Poiseuille flow and Taylor-Green flow. Then the capacity for the SPH_CS_SP method to solve the viscoelastic fluid is verified by the polymer flow through a periodic array of cylinders. Moreover, the convergence of the SPH_CS_SP method is also investigated. Finally, the proposed method is further applied to the 3D viscoelastic jet coiling problem, and the influences of macroscopic parameters on the jet coiling are discussed. The numerical results show that the SPH_CS_SP method has higher accuracy and better stability than the traditional SPH method and other corrected SPH method, and can improve the tensile instability.

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Weak value amplification via second-order correlated technique

崔挺, 黄靖正, 刘翔, 曾贵华

2016 (2): 20301-020301. doi: 10.1088/1674-1056/25/2/020301

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We propose a new framework combining weak measurement and second-order correlated technique. The theoretical analysis shows that weak value amplification (WVA) experiment can also be implemented by a second-order correlated system. We then build two-dimensional second-order correlated function patterns for achieving higher amplification factor and discuss the signal-to-noise ratio influence. Several advantages can be obtained by our proposal. For instance, detectors with high resolution are not necessary. Moreover, detectors with low saturation intensity are available in WVA setup. Finally, type-one technical noise can be effectively suppressed.

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Quantum frequency doubling based on tripartite entanglement with cavities

郭娟, 魏志峰, 张素英

2016 (2): 20302-020302. doi: 10.1088/1674-1056/25/2/020302

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We analyze the entanglement characteristics of three harmonic modes, which are the output fields from three cavities with an input tripartite entangled state at fundamental frequency. The entanglement properties of the input beams can be maintained after their frequencies have been up-converted by the process of second harmonic generation. We have calculated the parametric dependences of the correlation spectrum on the initial squeezing factor, the pump power, the transmission coefficient, and the normalized analysis frequency of cavity. The numerical results provide references to choose proper experimental parameters for designing the experiment. The frequency conversion of the multipartite entangled state can also be applied to a quantum communication network.

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Enhancing parameter precision of optimal quantum estimation by quantum screening

黄江, 郭有能, 谢钦

2016 (2): 20303-020303. doi: 10.1088/1674-1056/25/2/020303

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We propose a scheme of quantum screening to enhance the parameter-estimation precision in open quantum systems by means of the dynamics of quantum Fisher information. The principle of quantum screening is based on an auxiliary system to inhibit the decoherence processes and erase the excited state to the ground state. By comparing the case without quantum screening, the results show that the dynamics of quantum Fisher information with quantum screening has a larger value during the evolution processes.

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Mach-Zehnder interferometer with squeezed and EPR entangled optical fields

于旭东, 李卫, 朱诗尧, 张靖

2016 (2): 20304-020304. doi: 10.1088/1674-1056/25/2/020304

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We study a scheme for Mach-Zehnder (MZ) interferometer as a quantum linear device by injecting two-mode squeezed input states into two ports of interferometer. Two-mode squeezed states can be changed into two types of inputs for MZ interferometer: two squeezed states and Einstein-Podolsky-Rosen (EPR) entangled states. The interference patterns of the MZ interferometer vary periodically as the relative phase of the two arms of the interferometer is scanned, and are measured by the balanced homodyne detection system. Our experiments show that there are different interference patterns and periodicity of the output quantum states for two cases which depend on the relative phase of input optical fields. Since MZ interferometer can be used to realize some quantum operations, this work will have the important applications in quantum information and metrology.

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Hong-Ou-Mandel interference with two independent weak coherent states

陈华, 安雪碧, 伍娟, 银振强, 王双, 陈巍, 韩正甫

2016 (2): 20305-020305. doi: 10.1088/1674-1056/25/2/020305

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Recently, the Hong-Ou-Mandel (HOM) interference between two independent weak coherent pulses (WCPs) has been paid much attention due to the measurement-device-independent (MDI) quantum key distribution (QKD). Using classical wave theory, articles reported before show that the visibility of this kind of HOM-type interference is ≤50% . In this work, we analyze this kind of interference using quantum optics, which reveals more details compared to the wave theory. Analyses confirm the maximum visibility of 50%. And we conclude that the maximum visibility of 50% comes from the two single-photon states in WCPs, without considering the noise. In the experiment, we successfully approach the visibility of 50% by using WCPs splitting from the single pico-second laser source and phase scanning. Since this kind of HOM interference is immune to slow phase fluctuations, both the realized and proposed experiment designs can provide stable ways of high-resolution optical distance detection.

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Bidirectional transfer of quantum information for unknown photons via cross-Kerr nonlinearity and photon-number-resolving measurement

Jino Heo, Chang-Ho Hong, Dong-Hoon Lee, Hyung-Jin Yang

2016 (2): 20306-020306. doi: 10.1088/1674-1056/25/2/020306

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We propose an arbitrary controlled-unitary (CU) gate and a bidirectional transfer scheme of quantum information (BTQI) for unknown photons. The proposed CU gate utilizes quantum non-demolition photon-number-resolving measurement based on the weak cross-Kerr nonlinearities (XKNLs) and two quantum bus beams; the proposed CU gate consists of consecutive operations of a controlled-path gate and a gathering-path gate. It is almost deterministic and is feasible with current technology when a strong amplitude of the coherent state and weak XKNLs are employed. Compared with the existing optical multi-qubit or controlled gates, which utilize XKNLs and homodyne detectors, the proposed CU gate can increase experimental realization feasibility and enhance robustness against decoherence. According to the CU gate, we present a BTQI scheme in which the two unknown states of photons between two parties (Alice and Bob) are mutually swapped by transferring only a single photon. Consequently, by using the proposed CU gate, it is possible to experimentally implement the BTQI scheme with a certain probability of success.

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A quantum walk in phase space with resonator-assisted double quantum dots

边志浩, 秦豪, 詹翔, 李剑, 薛鹏

2016 (2): 20307-020307. doi: 10.1088/1674-1056/25/2/020307

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We implement a quantum walk in phase space with a new mechanism based on the superconducting resonator-assisted double quantum dots. By analyzing the hybrid system, we obtain the necessary factors implementing a quantum walk in phase space: the walker, coin, coin flipping and conditional phase shift. The coin flipping is implemented by adding a driving field to the resonator. The interaction between the quantum dots and resonator is used to implement conditional phase shift. Furthermore, we show that with different driving fields the quantum walk in phase space exhibits a ballistic behavior over 25 steps and numerically analyze the factors influencing the spreading of the walker in phase space.

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Efficient entanglement concentration for arbitrary less-entangled NOON state assisted by single photons

周澜, 盛宇波

2016 (2): 20308-020308. doi: 10.1088/1674-1056/25/2/020308

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We put forward two efficient entanglement concentration protocols (ECPs) for arbitrary less-entangled NOON state. Both ECPs only require one pair of less-entangled NOON state and an auxiliary photon. In the first ECP, the auxiliary photon is shared by two parties, while in the second ECP, the auxiliary photon is only possessed by one party, which can increase the practical success probability by avoiding the transmission loss and simplify the operations. Moreover, both ECPs can be used repeatedly to get a high success probability. Based on the above features, our two ECPs, especially the second one, may be useful in the quantum information processing.

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A-site ordered perovskiteCaCu₃Cu₂Ir₂O_12-δ with square-planar and octahedral coordinated Cu ions

赵庆, 殷云宇, 戴建洪, 沈希, 胡志伟, 杨俊叶, 王清涛, 禹日成, 李晓东, 龙有文

2016 (2): 20701-020701. doi: 10.1088/1674-1056/25/2/020701

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A novel CaCu₃Cu₂Ir₂O_12-δ polycrystalline sample was synthesized at 8 GPa and 1373 K. Rietveld structural analysis shows that this compound crystallizes in an AA'₃B₄O₁₂-type A-site ordered perovskite structure with space group Im-3. X-ray absorption spectra reveal a +2-charge state for both the square-planar and octahedral coordinated Cu ions, and the valence state of Ir is found to be about +5. Although the A-site Ca and the A' -site Cu²⁺ are 1:3 ordered at fixed atomic positions, the distribution of B-site Cu²⁺ and Ir⁵⁺ is disorderly. As a result, no long-range magnetic ordering is observed at temperatures down to 2 K. Electrical transport and heat capacity measurements demonstrate itinerant electronic behavior. The crystal structure is stable with pressure up to 35.7 GPa at room temperature.

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Tune-out wavelengths for the alkaline-metal atoms

于伟威, 于荣梅, 程勇军, 周雅君

2016 (2): 23101-023101. doi: 10.1088/1674-1056/25/2/023101

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An approximation formula is developed to determine the tune-out wavelengths for the ground states of the alkaline-metal atoms lithium, sodium and cesium from the existing relativistic reduced matrix elements and experimental energies. The first longest tune-out wavelengths for Li, Na, and Cs are 670.971 nm, 589.557 nm, and 880.237 nm, respectively. This is in good agreement with the previous high precise results of 670.971626 nm, 589.5565 nm, and 880.25 nm from the relativistic all-order many-body perturbation theory (RMBPT) calculation [Phys. Rev. A 84 043401 (2011)].

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Tuning the energy gap of bilayer α -graphyne by applying strain and electric field

杭阳, 吴文志, 于进, 郭万林

2016 (2): 23102-023102. doi: 10.1088/1674-1056/25/2/023102

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Our density functional theory calculations show that the energy gap of bilayer α -graphyne can be modulated by a vertically applied electric field and interlayer strain. Like bilayer graphene, the bilayer α -graphyne has electronic properties that are hardly changed under purely mechanical strain, while an external electric field can open the gap up to 120 meV. It is of special interest that compressive strain can further enlarge the field induced gap up to 160 meV, while tensile strain reduces the gap. We attribute the gap variation to the novel interlayer charge redistribution between bilayer α -graphynes. These findings shed light on the modulation of Dirac cone structures and potential applications of graphyne in mechanical-electric devices.

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Up-conversion luminescence polarization control in Er³⁺-doped NaYF₄ nanocrystals

张晖, 姚云华, 张诗按, 卢晨晖, 孙真荣

2016 (2): 23201-023201. doi: 10.1088/1674-1056/25/2/023201

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We propose a femtosecond laser polarization modulation scheme to control the up-conversion (UC) luminescence in Er³⁺-doped NaYF₄ nanocrystals dispersed in the silicate glass. We show that the UC luminescence can be suppressed when the laser polarization is changed from linear through elliptical to circular, and the higher repetition rate will yield the lower control efficiency. We theoretically analyze the physical control mechanism of the UC luminescence polarization modulation by considering on- and near-resonant two-photon absorption, energy transfer up-conversion, and excited state absorption, and show that the polarization control mainly comes from the contribution of near-resonant two-photon absorption. Furthermore, we propose a method to improve the polarization control efficiency of UC luminescence in rare-earth ions by applying a two-color femtosecond laser field.

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Frequency dependence of quantum path interference in non-collinear high-order harmonic generation

钟诗阳, 贺新奎, 滕浩, 叶蓬, 汪礼锋, 何鹏, 魏志义

2016 (2): 23301-023301. doi: 10.1088/1674-1056/25/2/023301

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High-order harmonic generation (HHG) driven by two non-collinear beams including a fundamental and its weak second harmonic is numerically studied. The interference of harmonics from adjacent electron quantum paths is found to be dependent on the relative delay of the driving pulse, and the dependences are different for different harmonic orders. This frequency dependence of the interference is attributed to the spatial frequency chirp in the HHG beam resulting from the harmonic dipole phase, which in turn provides a potential way to gain an insight into the generation of high-order harmonics. As an example, the intensity dependent dipole phase coefficient α is retrieved from the interference fringe.

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Energy and rotation-dependent stereodynamics of H(²S) + NH(a¹Δ)→H₂(X¹Σ_g⁺) + N(²D) reaction

李永庆, 杨云帆, 于洋, 张永嘉, 马凤才

2016 (2): 23401-023401. doi: 10.1088/1674-1056/25/2/023401

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Quasi-classical trajectory calculations are performed to study the stereodynamics of the H(²S) + NH(a¹Δ)→H₂(X¹Σ_g⁺) + N(²D) reaction based on the first excited state NH₂(1²A') potential energy surface reported by Li et al. [Li Y Q and Varandas A J C 2010 J. Phys. Chem. A 9644] for the first time. We observe the changes of differential cross-sections at different collision energies and different initial reagent rotational excitations. The influence of collision energy on the k-k' distribution can be attributed to a purely impulsive effect. Initial reagent rotational excitation transforms the reaction mechanism from insertion to abstraction. The effect of initial reagent rotational excitations on k-k' distribution can be explained by the rotational excitation enlarging the rotational rate of reagent NH in the entrance channel to reduce the probability of collision between incidence H atom and H atom of target molecular. We also investigate the changes of vector correlations and find that the rotational angular momentum vector j' of the product H₂ is not only aligned, but also oriented along the y axis. The alignment parameter, the disposal of total angular momentum and the reaction mechanism are all analyzed carefully to explain the polarization behavior of the product rotational angular moment.

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X-ray emission from 424-MeV/u C ions impacting on selected target

周贤明, 程锐, 雷瑜, 孙渊博, 王瑜玉, 王兴, 徐戈, 梅策香, 张小安, 陈熙萌, 肖国青, 赵永涛

2016 (2): 23402-023402. doi: 10.1088/1674-1056/25/2/023402

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The K-shell x-rays of Ti, V, Fe, Co, Ni, Cu, and Zn induced by 424-MeV/u C⁶⁺ ion impact are measured. It is found that the K x-ray shifts to the high energy side and the intensity ratio of K/K is larger than the atomic data, owing to the L-shell multiple-ionization. The x-ray production cross sections are deduced from the experimental counts and compared with the binary encounter approximation (BEA), plane wave approximation (PWBA) and energy-loss Coulomb-repulsion perturbed-stationary-state relativistic (ECPSSR) theoretical predictions. The BEA model with considering the multiple-ionization fluorescence yield is in better consistence with the experimental results. In addition, the cross section as a function of target atomic K-shell binding energy is presented.

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Anomalous propagation conditions of electromagnetic wave observed over Bosten Lake, China in July and August, 2014

孙正, 宁辉, 唐敬, 谢永杰, 石鹏飞, 王建华, 王柯

2016 (2): 24101-024101. doi: 10.1088/1674-1056/25/2/024101

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Atmospheric duct is a common phenomenon over large bodies of water, and it can significantly affect the performance of many radio systems. In this paper, a two-month (in July and August, 2014) sounding experiment in ducting conditions over Bosten Lake was carried out at a littoral station (41.89° N, 87.22° E) with high resolution GPS radiosondes, and atmospheric ducts were observed for the first time in this area. During the two months, surface and surface-based ducts occurred frequently over the Lake. Strong diurnal variations in ducting characteristics were noticed in clear days. Ducting occurrence was found at its lowest in the early morning and at its highest (nearly 100%) in the afternoon. Duct strength was found increasing from early morning to forenoon, and reaching its maximum in the afternoon. But contrarily, duct altitude experienced a decrease in a clear day. Then the meteorological reasons for the variations were discussed in detail, turbulent bursting was a possible reason for the duct formation in the early morning and the prevailing lake-breeze front was the main reason in the afternoon. The propagation of electromagnetic wave in a ducting environment was also investigated. A ray-tracing framework based on Runge-Kutta method was proposed to assess the performance of radio systems, and the precise critical angle and grazing angle derived from the ray-tracing equations were provided. Finally, numerical investigations on the radar performance in the observed ducting environments have been carried out with high accuracy, which demonstrated that atmospheric ducts had made great impacts on the performance of radio systems. The range/height errors for radar measurement induced by refraction have also been presented, too, which shows that the height errors were very large for trapped rays when the total range was long enough.

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Property of slice square polycapillary x-ray optics

彭诗棋, 刘志国, 孙天希, 王锴, 易龙涛, 杨魁, 陈曼, 王金榜

2016 (2): 24102-024102. doi: 10.1088/1674-1056/25/2/024102

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A geometrical description of square polycapillary x-ray optics and the basic theory of the transmission of x-rays are presented. A method of numerical calculation is developed based on ray-tracing theory. The method simulates the intensity distribution of x-rays propagating through slice square polycapillary x-ray optics. The simulation results are compared with the experimental results.

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New pattern recognition system in the e-nose for Chinese spirit identification

曾慧, 李强, 谷宇

2016 (2): 24201-024201. doi: 10.1088/1674-1056/25/2/024201

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This paper presents a new pattern recognition system for Chinese spirit identification by using the polymer quartz piezoelectric crystal sensor based e-nose. The sensors are designed based on quartz crystal microbalance (QCM) principle, and they could capture different vibration frequency signal values for Chinese spirit identification. For each sensor in an 8-channel sensor array, seven characteristic values of the original vibration frequency signal values, i.e., average value (A), root-mean-square value (RMS), shape factor value (S_f), crest factor value (C_f), impulse factor value (I_f), clearance factor value (CL_f), kurtosis factor value (K_v) are first extracted. Then the dimension of the characteristic values is reduced by the principle components analysis (PCA) method. Finally the back propagation (BP) neutral network algorithm is used to recognize Chinese spirits. The experimental results show that the recognition rate of six kinds of Chinese spirits is 93.33% and our proposed new pattern recognition system can identify Chinese spirits effectively.

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Lensless ghost imaging through the strongly scattering medium

杨哲, 赵连洁, 赵学亮, 秦伟, 李俊林

2016 (2): 24202-024202. doi: 10.1088/1674-1056/25/2/024202

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Lensless ghost imaging has attracted much interest in recent years due to its profound physics and potential applications. In this paper we report studies of the robust properties of the lensless ghost imaging system with a pseudo-thermal light source in a strongly scattering medium. The effects of the positions of the strong medium on the ghost imaging are investigated. In the lensless ghost imaging system, a pseudo-thermal light is split into two correlated beams by a beam splitter. One beam goes to a charge-coupled detector camera, labeled as CCD2. The other beam goes to an object and then is collected in another charge-coupled detector camera, labeled as CCD1, which serves as a bucket detector. When the strong medium, a pane of ground glass disk, is placed between the object and CCD1, the bucket detector, the quality of ghost imaging is barely affected and a good image could still be obtained. The quality of the ghost imaging can also be maintained, even when the ground glass is rotating, which is the strongest scattering medium so far. However, when the strongly scattering medium is present in the optical path from the light source to CCD2 or the object, the lensless ghost imaging system hardly retrieves the image of the object. A theoretical analysis in terms of the second-order correlation function is also provided.

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Experimental study of the dependences of retrieval efficiencies on time delay between magneto-optical-trap being turned off and optical storage

陈力荣, 徐忠孝, 李萍, 温亚飞, 曾炜卿, 武跃龙, 田龙, 李淑静, 王海

2016 (2): 24203-024203. doi: 10.1088/1674-1056/25/2/024203

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We report an experimental study of electromagnetically induced transparency (EIT)-based light storage in a cloud of cold atoms loaded into a magneto-optical-trap (MOT). After the MOT is turned off, the retrieval efficiencies of right- and left-circularly polarized signal light fields each as a function of storage time are measured for different time delays between MOT off and the storage event, respectively. The results show that in the delay ranging from 0.015 ms to 3.5 ms, the retrieval efficiency for a zero-storage time (0.2 us) and the storage lifetime can exceed 15% and 1.4 ms, respectively. The measured results will provide important help for optimizing the storage of the polarized entanglement photons in cold atomic ensembles.

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2-μm single longitudinal mode GaSb-based laterally coupled distributed feedback laser with regrowth-free shallow-etched gratings by interference lithography

杨成奥, 张宇, 廖永平, 邢军亮, 魏思航, 张立春, 徐应强, 倪海桥, 牛智川

2016 (2): 24204-024204. doi: 10.1088/1674-1056/25/2/024204

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We report a type-I GaSb-based laterally coupled distributed-feedback (LC-DFB) laser with shallow-etched gratings operating a continuous wave at room temperature without re-growth process. Second-order Bragg gratings are fabricated alongside the ridge waveguide by interference lithography. Index-coupled LC-DFB laser with a cavity of 1500 μm achieves single longitudinal mode continuous-wave operation at 20 ℃ with side mode suppression ratio (SMSR) as high as 24 dB. The maximum single mode continuous-wave output power is about 10 mW at room temperature (uncoated facet). A low threshold current density of 230 A/cm² is achieved with differential quantum efficiency estimated to be 93 mW/A. The laser shows a good wavelength stability against drive current and working temperature.

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Diode-pumped Kerr-lens mode-locked femtosecond Yb:YAG ceramic laser

高子叶, 朱江峰, 汪珂, 王军利, 王兆华, 魏志义

2016 (2): 24205-024205. doi: 10.1088/1674-1056/25/2/024205

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We experimentally demonstrated a diode-pumped Kerr-lens mode-locked femtosecond laser based on an Yb:YAG ceramic. Stable laser pulses with 97-fs duration, 2.8-nJ pulse energy, and 320-mW average power were obtained. The femtosecond oscillator operated at a central wavelength of 1049 nm and a repetition rate of 115 MHz. To the best of our knowledge, this is the first demonstration of a Kerr-lens mode-locked operation in a diode-pumped Yb:YAG ceramic laser with sub-100 fs pulse duration.

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Passively mode-locked erbium-doped fiber laser via a D-shape-fiber-based MoS₂ saturable absorber with a very low nonsaturable loss

段利娜, 苏玉龙, 王勇刚, 李璐, 王茜, 王屹山

2016 (2): 24206-024206. doi: 10.1088/1674-1056/25/2/024206

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We report on the generation of conventional and dissipative solitons in erbium-doped fiber lasers by the evanescent field interaction between the propagating light and a multilayer molybdenum disulfide (MoS₂) thin film. The MoS₂ film is fabricated by depositing the MoS₂ water-ethanol mixture on a D-shape-fiber (DF) repetitively. The measured nonsaturable loss, saturable optical intensity, and the modulation depth of this device are 13.3%, 110 MW/cm², and 3.4% respectively. Owing to the very low nonsaturable loss, the laser threshold of conventional soliton is as low as 4.8 mW. The further increase of net cavity dispersion to normal regime, stable dissipation soliton pulse trains with a spectral bandwidth of 11.7 nm and pulse duration of 116 ps are successfully generated. Our experiment demonstrates that the MoS₂-DF device can indeed be used as a high performance saturable absorber for further applications in ultrafast photonics.

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Design of LD in-band direct-pumping side surface polished micro-rod Nd:YVO₄ laser

张文启, 王飞, 柳强, 巩马理

2016 (2): 24207-024207. doi: 10.1088/1674-1056/25/2/024207

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To diminish the thermal load, two ways, that is, in-band direct pumping and micro-rod crystal, could be adopted at the same time. The efficiency of LD in-band direct-pumping side surface polished micro-rod Nd:YVO₄ laser is numerically analyzed. By optimizing parameters such as crystal length, laser mode radius, pump beam radius, doping concentration and crystal cross-section size, the overall efficiency can reach over 50%. It is found that with micro-rod crystal implemented in the laser oscillator, high overall efficiency LD in-band direct-pumping Nd:YVO₄ laser could be realized. High efficiency combined with low thermal load makes this laser an outstanding scheme for building high-power Nd:YVO₄ lasers.

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Wavelength modulation spectroscopy at 1530.32 nm for measurements of acetylene based on Fabry-Perot tunable filter

李运龙, 杨兵初, 许雪梅

2016 (2): 24208-024208. doi: 10.1088/1674-1056/25/2/024208

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Sensitive detection of acetylene (C₂H₂) is performed by absorption spectroscopy and wavelength modulation spectroscopy (WMS) based on Fiber Fabry-Perot tunable filter (FFP-TF) at 1530.32 nm. After being calibrated by Fiber Bragg Grating (FBG), FFP-TF is frequency-multiplexed and modulated at 20 Hz and 2.5 kHz respectively to achieve wavelength modulation. The linearity with 0.9907 fitting coefficient is obtained by measuring different concentrations in a 100 ppmv-400 ppmv range. Furthermore, the stability of the system is analyzed by detecting 50 ppmv and 100 ppmv standard gases for 2 h under room temperature and ambient pressure conditions respectively. The precision of 11 ppmv is achieved by calculating the standard deviation. Therefore, the measuring system of C₂H₂ detection can be applied in practical applications.

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19-fs pulse generated by supercontinuum compression

张华强, 王鹏, 刘文军, 姚翳蕾, 徐志敬, 李健

2016 (2): 24209-024209. doi: 10.1088/1674-1056/25/2/024209

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Supercontinuum generation and compression in a length of 50-mm photonic crystal fiber with pulse of nanojoule energy are investigated theoretically and experimentally. Chirped mirror pair is used for dispersion compensation and pulse compression. Pulse characteristics are measured by frequency-resolved optical gating. And 19-fs pulse is generated.

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Measurements of atmospheric NO₃ radicals in Hefei using LED-based long path differential optical absorption spectroscopy

卢雪, 秦敏, 谢品华, 段俊, 方武, 凌六一, 沈兰兰, 刘建国, 刘文清

2016 (2): 24210-024210. doi: 10.1088/1674-1056/25/2/024210

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NO₃ radicals accumulate during the night, thereby being the most critical night oxidant. Owing to the low concentration and dramatic variation, the detection of atmospheric NO₃ radicals is still challenging. In this paper, an LED-based Long Path Differential Optical Absorption Spectroscopy (LPDOAS) instrument is developed for measuring the atmospheric NO₃ radicals. This instrument is composed of a Schmidt-Cassegrain telescope, a combined emitting and receiving fiber, and a red LED equipped with a thermostat, and has a center wavelength of 660 nm, covering the NO₃ strongest absorption peak (662 nm). The influence of LED temperature fluctuations is discussed. The temperature of the LED lamp with a home-made thermostat is tested, showing a stability of ± 0.1 ℃. The principle and fitting analyses of LED-LPDOAS are presented. A retrieval example and a time series of NO₃ radical concentrations with good continuity for one night are shown. The detection limit of NO₃ for 2.6-km optical path is about 10 ppt.

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Tunable acoustic radiation pattern assisted by effective impedance boundary

钱枫, 全力, 王力维, 刘晓宙, 龚秀芬

2016 (2): 24301-024301. doi: 10.1088/1674-1056/25/2/024301

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The acoustic wave propagation from a two-dimensional subwavelength slit surrounded by metal plates decorated with Helmholtz resonators (HRs) is investigated both numerically and experimentally in this work. Owing to the presence of HRs, the effective impedance of metal surface boundary can be manipulated. By optimizing the distribution of HRs, the asymmetric effective impedance boundary will be obtained, which contributes to generating tunable acoustic radiation pattern such as directional acoustic beaming. These dipole-like radiation patterns have high radiation efficiency, no fingerprint of sidelobes, and a wide tunable range of the radiation pattern directivity angle which can be steered by the spatial displacements of HRs.

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Analysis of underwater decoupling properties of a locally resonant acoustic metamaterial coating

黄凌志, 肖勇, 温激鸿, 杨海滨, 温熙森

2016 (2): 24302-024302. doi: 10.1088/1674-1056/25/2/024302

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This paper presents a semi-analytical solution for the vibration and sound radiation of a semi-infinite plate covered by a decoupling layer consisting of locally resonant acoustic metamaterial. Formulations are derived based on a combination use of effective medium theory and the theory of elasticity for the decoupling material. Theoretical results show good agreements between the method developed in this paper and the conventional finite element method (FEM), but the method of this paper is more efficient than FEM. Numerical results also show that system with acoustic metamaterial decoupling layer exhibits significant noise reduction performance at the local resonance frequency of the acoustic metamaterial, and such performance can be ascribed to the vibration suppression of the base plate. It is demonstrated that the effective density of acoustic metamaterial decoupling layer has a great influence on the mechanical impedance of the system. Furthermore, the resonance frequency of locally resonant structure can be effectively predicted by a simple model, and it can be significantly affected by the material properties of the locally resonant structure.

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Experimental and numerical studies of nonlinear ultrasonic responses on plastic deformation in weld joints

项延训, 朱武军, 邓明晰, 轩福贞

2016 (2): 24303-024303. doi: 10.1088/1674-1056/25/2/024303

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The experimental measurements and numerical simulations are performed to study ultrasonic nonlinear responses from the plastic deformation in weld joints. The ultrasonic nonlinear signals are measured in the plastic deformed 30Cr2Ni4MoV specimens, and the results show that the nonlinear parameter monotonically increases with the plastic strain, and that the variation of nonlinear parameter in the weld region is maximal compared with those in the heat-affected zone and base regions. Microscopic images relating to the microstructure evolution of the weld region are studied to reveal that the change of nonlinear parameter is mainly attributed to dislocation evolutions in the process of plastic deformation loading. Meanwhile, the finite element model is developed to investigate nonlinear behaviors of ultrasonic waves propagating in a plastic deformed material based on the nonlinear stress-strain constitutive relationship in a medium. Moreover, a pinned string model is adopted to simulate dislocation evolution during plastic damages. The simulation and experimental results show that they are in good consistency with each other, and reveal a rising acoustic nonlinearity due to the variations of dislocation length and density and the resulting stress concentration.

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Bubble nonlinear dynamics and stimulated scattering process

时洁, 杨德森, 时胜国, 胡博, 张昊阳, 胡诗涌

2016 (2): 24304-024304. doi: 10.1088/1674-1056/25/2/024304

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A complete understanding of the bubble dynamics is deemed necessary in order to achieve their full potential applications in industry and medicine. For this purpose it is first needed to expand our knowledge of a single bubble behavior under different possible conditions including the frequency and pressure variations of the sound field. In addition, stimulated scattering of sound on a bubble is a special effect in sound field, and its characteristics are associated with bubble oscillation mode. A bubble in liquid can be considered as a representative example of nonlinear dynamical system theory with its resonance, and its dynamics characteristics can be described by the Keller-Miksis equation. The nonlinear dynamics of an acoustically excited gas bubble in water is investigated by using theoretical and numerical analysis methods. Our results show its strongly nonlinear behavior with respect to the pressure amplitude and excitation frequency as the control parameters, and give an intuitive insight into stimulated sound scattering on a bubble. It is seen that the stimulated sound scattering is different from common dynamical behaviors, such as bifurcation and chaos, which is the result of the nonlinear resonance of a bubble under the excitation of a high amplitude acoustic sound wave essentially. The numerical analysis results show that the threshold of stimulated sound scattering is smaller than those of bifurcation and chaos in the common condition.

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Underwater asymmetric acoustic transmission structure using the medium with gradient change of impedance

胡博, 时洁, 时胜国, 孙玉, 朱中锐

2016 (2): 24305-024305. doi: 10.1088/1674-1056/25/2/024305

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We propose an underwater asymmetric acoustic transmission structure comprised of two media each with a gradient change of acoustic impedance. By gradually increasing the acoustic impedances of the media, the propagating direction of the acoustic wave can be continuously bent, resulting in allowing the acoustic wave to pass through along the positive direction and blocking acoustic waves from the negative one. The main advantages of this structure are that the asymmetric transmission effect of this structure can be realized and enhanced more easily in water. We investigate both numerically and experimentally the asymmetric transmission effect. The experimental results show that a highly efficient asymmetric acoustic transmission can be yielded within a remarkable broadband frequency range, which agrees well with the numerical prediction. It is of potential practical significance for various underwater applications such as reducing vibration and noise.

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Calculation of multi-layer plate damper under one-axial load

闫辉, 张露, 姜洪源, Alexander M. Ulanov

2016 (2): 24306-024306. doi: 10.1088/1674-1056/25/2/024306

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A multi-layer damper with waved plates under one-axial load is considered. A method of theoretical calculation of its energy dissipation coefficient is proposed. An experimental research of own frequencies and vibration transfer ratios for different parameters of damper structure, harmonic vibration load and random load is performed. Results of this research are approximated by functions; it is possible to use these functions for the calculation of the damper too.

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Numerical investigation of a coupled moving boundary model of radial flow in low-permeable stress-sensitive reservoir with threshold pressure gradient

刘文超, 刘曰武, 牛丛丛, 韩国锋, 万义钊

2016 (2): 24701-024701. doi: 10.1088/1674-1056/25/2/024701

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The threshold pressure gradient and formation stress-sensitive effect as the two prominent physical phenomena in the development of a low-permeable reservoir are both considered here for building a new coupled moving boundary model of radial flow in porous medium. Moreover, the wellbore storage and skin effect are both incorporated into the inner boundary conditions in the model. It is known that the new coupled moving boundary model has strong nonlinearity. A coordinate transformation based fully implicit finite difference method is adopted to obtain its numerical solutions. The involved coordinate transformation can equivalently transform the dynamic flow region for the moving boundary model into a fixed region as a unit circle, which is very convenient for the model computation by the finite difference method on fixed spatial grids. By comparing the numerical solution obtained from other different numerical method in the existing literature, its validity can be verified. Eventually, the effects of permeability modulus, threshold pressure gradient, wellbore storage coefficient, and skin factor on the transient wellbore pressure, the derivative, and the formation pressure distribution are analyzed respectively.

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Development of a new correlation to calculate permeability for flows with high Knudsen number

Esmaeil Dehdashti

2016 (2): 24702-024702. doi: 10.1088/1674-1056/25/2/024702

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Flows with high Knudsen number play a prominent role in many engineering applications. The present study is an effort toward the simulation of flow with high Knudsen number using modified lattice Boltzmann method (LBM) through a porous medium in a channel. The effect of collision between molecules and solid walls, which is required to accurately simulate transition flow regime, is taken into account using a modified relaxation time. Slip velocity on the wall, which is another significant difficulty in simulating transition flow regime, is captured using the slip reflection boundary condition (SRBC). The geometry of porous medium is considered as in-line and staggered. The results are in good agreement with previous works. A new correlation is obtained between permeability, Knudsen number and porosity for flows in transition flow regimes.

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Wind tunnel experiments on flow separation control of an Unmanned Air Vehicle by nanosecond discharge plasma aerodynamic actuation

陈康, 梁华

2016 (2): 24703-024703. doi: 10.1088/1674-1056/25/2/024703

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Plasma flow control (PFC) is a new kind of active flow control technology, which can improve the aerodynamic performances of aircrafts remarkably. The flow separation control of an unmanned air vehicle (UAV) by nanosecond discharge plasma aerodynamic actuation (NDPAA) is investigated experimentally in this paper. Experimental results show that the applied voltages for both the nanosecond discharge and the millisecond discharge are nearly the same, but the current for nanosecond discharge (30 A) is much bigger than that for millisecond discharge (0.1 A). The flow field induced by the NDPAA is similar to a shock wave upward, and has a maximal velocity of less than 0.5 m/s. Fast heating effect for nanosecond discharge induces shock waves in the quiescent air. The lasting time of the shock waves is about 80 us and its spread velocity is nearly 380 m/s. By using the NDPAA, the flow separation on the suction side of the UAV can be totally suppressed and the critical stall angle of attack increases from 20° to 27° with a maximal lift coefficient increment of 11.24%. The flow separation can be suppressed when the discharge voltage is larger than the threshold value, and the optimum operation frequency for the NDPAA is the one which makes the Strouhal number equal one. The NDPAA is more effective than the millisecond discharge plasma aerodynamic actuation (MDPAA) in boundary layer flow control. The main mechanism for nanosecond discharge is shock effect. Shock effect is more effective in flow control than momentum effect in high speed flow control.

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Cycloid motions of grains in a dust plasma

张永亮, 冯帆, 刘富成, 董丽芳, 贺亚峰

2016 (2): 25201-025201. doi: 10.1088/1674-1056/25/2/025201

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Hypocycloid and epicycloid motions of irregular grains (pine pollen) are observed for the first time in a dust plasma in a two-dimensional (2D) horizontal plane. These cycloid motions can be regarded as a combination of a primary circle and a secondary circle. An inverse Magnus force originating from the spin of the irregular grain gives rise to the primary circle. Radial confinement resulting from the electrostatic force and the ion drag force, together with inverse Magnus force, plays an important role in the formation of the secondary circle. In addition, the cyclotron radius is seen to change periodically during the cycloid motion. Force analysis and comparison experiments have shown that the cycloid motions are distinctive features of an irregular grain immersed in a plasma.

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Sheath structure in plasma with two species of positive ions and secondary electrons

赵晓云, 项农, 欧靖, 李德徽, 林滨滨

2016 (2): 25202-025202. doi: 10.1088/1674-1056/25/2/025202

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The properties of a collisionless plasma sheath are investigated by using a fluid model in which two species of positive ions and secondary electrons are taken into account. It is shown that the positive ion speeds at the sheath edge increase with secondary electron emission (SEE) coefficient, and the sheath structure is affected by the interplay between the two species of positive ions and secondary electrons. The critical SEE coefficients and the sheath widths depend strongly on the positive ion charge number, mass and concentration in the cases with and without SEE. In addition, ion kinetic energy flux to the wall and the impact of positive ion species on secondary electron density at the sheath edge are also discussed.

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A hybrid mode of one- and two-surface multipactor on grooved dielectric surface

蔡利兵, 王建国, 程国新, 朱湘琴

2016 (2): 25203-025203. doi: 10.1088/1674-1056/25/2/025203

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A hybrid mode of one- and two-surface multipactor on the grooved dielectric surface is studied in detail using both an analytical approach and two-dimensional particle-in-cell (2D PIC) simulations. When the groove width L< eE₀/(4π m_ef²), there are one-surface multipactor and one-order two-surface multipactor on the grooved dielectric surface, and only one slope of the groove has the multipactor anytime. When L< eE₀/(4π m_ef²), both slopes may have the multipactors. The electron surface density of the multipactor discharge has a sharp increase at the length L< eE₀/(4π m_ef²).

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Bandgap narrowing in the layered oxysulfide semiconductor Ba₃Fe₂O₅Cu₂S₂: Role of FeO₂ layer

张韩, 金士锋, 郭丽伟, 申士杰, 林志萍, 陈小龙

2016 (2): 26101-026101. doi: 10.1088/1674-1056/25/2/026101

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A new layered Cu-based oxychalcogenide Ba₃Fe₂O₅Cu₂S₂ has been synthesized and its magnetic and electronic properties were revealed. Ba₃Fe₂O₅Cu₂S₂ is built up by alternatively stacking [Cu₂S₂]^2- layers and iron perovskite oxide [(FeO₂)(BaO)(FeO₂)]^2- layers along the c axis that are separated by barium ions with Fe³⁺ fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuCh-based (Ch = S, Se, Te) compounds the +3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba₃Fe₂O₅Cu₂S₂ is an antiferromagnetic semiconductor with a Néel temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3d states of Fe ions that antiferromagnetically arranged in FeO₂ layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr₃Sc₂O₅Cu₂S₂.

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Phase transition and chemical decomposition of liquid carbon dioxide and nitrogen mixture under extreme conditions

姜晓旭, 陈冠宇, 李玉同, 程新路, 唐翠明

2016 (2): 26102-026102. doi: 10.1088/1674-1056/25/2/026102

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Thermodynamic and chemical properties of liquid carbon dioxide and nitrogen (CO₂-N₂) mixture under the conditions of extremely high densities and temperatures are studied by using quantum molecular dynamic (QMD) simulations based on density functional theory including dispersion corrections (DFT-D). We present equilibrium properties of liquid mixture for 112 separate density and temperature points, by selecting densities ranging from ρ =1.80 g/cm³to 3.40 g/cm³ and temperatures from T=500 K to 8000 K. In the range of our study, the liquid CO₂-N₂ mixture undergoes a continuous transition from molecular to atomic fluid state and liquid polymerization inferred from pair correlation functions (PCFs) and the distribution of various molecular components. The insulator-metal transition is demonstrated by means of the electronic density of states (DOS).

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Innovative technologies for powder metallurgy-based disk superalloys: Progress and proposal

贾崇林, 葛昌纯, 燕青芝

2016 (2): 26103-026103. doi: 10.1088/1674-1056/25/2/026103

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Powder metallurgy (PM) superalloys are an important class of high temperature structural materials, key to the rotating components of aero engines. In the purview of the present challenges associated with PM superalloys, two novel approaches namely, powder preparation and the innovative spray-forming technique (for making turbine disk) are proposed and studied. Subsequently, advanced technologies like electrode-induction-melting gas atomization (EIGA), and spark-plasma discharge spheroidization (SPDS) are introduced, for ceramic-free superalloy powders. Presently, new processing routes are sought after for preparing finer and cleaner raw powders for disk superalloys. The progress of research in spray-formed PM superalloys is first summarized in detail. The spray-formed superalloy disks specifically exhibit excellent mechanical properties. This paper reviews the recent progress in innovative technologies for PM superalloys, with an emphasis on new ideas and approaches, central to the innovation driving techniques like powder processing and spray forming.

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High-pressure structural properties of tetramethylsilane

秦振兴, 陈晓嘉

2016 (2): 26104-026104. doi: 10.1088/1674-1056/25/2/026104

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High-pressure structural properties of tetramethylsilane are investigated by synchrotron powder x-ray diffraction at pressures up to 31.1 GPa and room temperature. A phase with the space group of Pnma is found to appear at 4.2 GPa. Upon compression, the compound transforms to two following phases: the phase with space groups of P2₁/c at 9.9 GPa and the phase with P2/m at 18.2 GPa successively via a transitional phase. The unique structural character of P2₁/c supports the phase stability of tetramethylsilane without possible decomposition upon heavy compression. The appearance of the P2/m phase suggests the possible realization of metallization for this material at higher pressure.

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Fabrication of CoFe₂O₄ ferrite nanowire arrays in porous silicon template and their local magnetic properties

郑辉, 韩满贵, 邓龙江

2016 (2): 26201-026201. doi: 10.1088/1674-1056/25/2/026201

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CoFe₂O₄ ferrite nanowire arrays are fabricated in porous silicon templates. The porous silicon templates are prepared via metal-assisted chemical etching with gold (Au) nanoparticles as the catalyst. Subsequently, CoFe₂O₄ ferrite nanowires are successfully synthesized into porous silicon templates by the sol-gel method. The magnetic hysteresis loop of nanowire array shows an isotropic feature of magnetic properties. The coercivity and squareness ratio (M_r/M_s) of ensemble nanowires are found to be 630 Oe (1 Oe=79.5775 A·m^-1 and 0.4 respectively. However, the first-order reversal curve (FORC) is adopted to reveal the probability density function of local magnetostatic properties (i.e., interwire interaction field and coercivity). The FORC diagram shows an obvious distribution feature for interaction field and coercivity. The local coercivity with a value of about 1000 Oe is found to have the highest probability.

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Lattice stabilities, mechanical and thermodynamic properties of Al₃Tm and Al₃Lu intermetallics under high pressure from first-principles calculations

张旭东, 姜伟

2016 (2): 26301-026301. doi: 10.1088/1674-1056/25/2/026301

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The effects of high pressure on lattice stability, mechanical and thermodynamic properties of L1₂ structure Al₃Tm and Al₃Lu are studied by first-principles calculations within the VASP code. The phonon dispersion curves and density of phonon states are calculated by using the PHONONPY code. Our results agree well with the available experimental and theoretical values. The vibrational properties indicate that Al₃Tm and Al₃Lu keep their dynamical stabilities in L1₂ structure up to 100 GPa. The elastic properties and Debye temperatures for Al₃Tm and Al₃Lu increase with the increase of pressure. The mechanical anisotropic properties are discussed by using anisotropic indices A^{m G}, A^U, A^Z, and the three-dimensional (3D) curved surface of Young's modulus. The calculated results show that Al₃Tm and Al₃Lu are both isotropic at 0 GPa and anisotropic under high pressure. In the present work, the sound velocities in different directions for Al₃Tm and Al₃Lu are also predicted under high pressure. We also calculate the thermodynamic properties and provide the relationships between thermal parameters and temperature/pressure. These results can provide theoretical support for further experimental work and industrial applications.

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Phase transition, elastic and electronic properties of topological insulator Sb₂Te₃ under pressure: First principle study

卢清, 张怀勇, 程艳, 陈向荣, 姬广富

2016 (2): 26401-026401. doi: 10.1088/1674-1056/25/2/026401

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The phase transition, elastic and electronic properties of three phases (phaseI, II, and III) of Sb₂Te₃ are investigated by using the generalized gradient approximation (GGA) with the PBESOL exchange-correlation functional in the framework of density-functional theory. Some basic physical parameters, such as lattice constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, acoustic velocity, and Debye temperature Θ are calculated. The obtained lattice parameters under various pressures are consistent with experimental data. Phase transition pressures are 9.4GPa (I→II) and 14.1GPa (II→III), which are in agreement with the experimental results. According to calculated elastic constants, we also discuss the ductile or brittle characters and elastic anisotropies of three phases. PhasesI and III are brittle, while phaseII is ductile. Of the three phases, phaseII has the most serious degree of elastic anisotropy and phaseIII has the slightest one. Finally, we investigate the partial densities of states (PDOSs) of three phases and find that the three phases possess some covalent features.

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First principle investigation of the electronic and thermoelectric properties of Mg₂C

Kulwinder Kaur, Ranjan Kumar

2016 (2): 26402-026402. doi: 10.1088/1674-1056/25/2/026402

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In this paper, electronic and thermoelectric properties of Mg₂C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters, bulk modulus, band gap and thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg₂C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor (PF) increases with temperature. The thermoelectric properties of Mg₂C have been calculated in a temperature range of 100 K-1200 K.

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First-principle investigation on the thermodynamics of X₂N₂O (X= C, Si, Ge) compounds

熊青云, 沈启霞, 李蕊子, 申江, 田付阳

2016 (2): 26501-026501. doi: 10.1088/1674-1056/25/2/026501

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The structures under different pressures, elastic properties, electronic structures and lattice vibrations of the X₂N₂O (X= C, Si, Ge) compounds are investigated by using the first-principle method. Based on the phonon density of state, the thermodynamic properties of the present compounds are studied under different pressures and at different temperatures. The structural parameters including the bond lengths and bond angles are in agreement with available experimental measurements and theoretical calculations. We employ the elastic theory to calculate the nine independent elastic constants (C_ij) and the derived elastic moduli (B, G, E, v). Results indicate that these X₂N₂O (X= C, Si, Ge) compounds are mechanically stable and show the brittle behaviors. The electronic properties of the present compounds are analyzed by using the band structure and density of states. The phonon dispersion calculations imply that the present compounds are dynamically stable. Based on the quasi-harmonic approximation, the calculations of the specific heat indicate that the temperature in a range of 0 K-1500 K and pressure in a range of 0 GPa-40 GPa have a large effect on the thermal quantities of Ge₂N₂O, compared with on those of the C₂N₂O and Si₂N₂O compounds.

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Vibrational features of confined water in nanoporous TiO₂ by Raman spectra

高欣, 王强, 孙刚, 李晨曦, 胡林

2016 (2): 26801-026801. doi: 10.1088/1674-1056/25/2/026801

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Raman spectra of confined water adsorbed in nanoporous TiO₂ are obtained in experiment. TiO₂ samples with different pore diameters under different humidity conditions are investigated. The results indicate that the symmetric vibrational mode of water molecule is destroyed when relative humidity decreases. This indicates that the interaction between water molecules and surface of TiO₂ becomes stronger when the distance between water molecules and surface turns smaller, and the interaction plays a major role in depressing the symmetric vibrational peak. The spectra of confined water in TiO₂ and Vycor are compared. When filling fractions are the same, their spectra show distinctions no matter whether they are in partial filling condition or in full filling condition. The spectra of HDO confined in TiO₂ with different filling fractions are compared with each other. There is no clear distinction among their vibrational peaks, and the peaks mainly relate to asymmetric vibration. Therefore, the interaction between water molecules and the wall of pore decouples the symmetric vibrational mode only, and the influences on asymmetric vibrational mode show little differences among different filling fractions.

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Effect of amorphous lamella on the crack propagation behavior of crystalline Mg/amorphous Mg-Al nanocomposites

宋海洋, 李玉龙

2016 (2): 26802-026802. doi: 10.1088/1674-1056/25/2/026802

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The effects of amorphous lamella on the crack propagation behavior in crystalline/amorphous (C/A) Mg/Mg-Al nanocomposites under tensile loading are investigated using the molecular dynamics simulation method. The sample with an initial crack of orientation (1210) [0001] is considered here. For the nano-monocrystal Mg, the crack growth exhibits brittle cleavage. However, for the C/A Mg/Mg-Al nanocomposites, the 'double hump' behavior can be observed in all the stress-strain curves regardless of the amorphous lamella thickness. The results indicate that the amorphous lamella plays a critical role in the crack deformation, and it can effectively resist the crack propagation. The above mentioned crack deformation behaviors are also disclosed and analyzed in the present work. The results here provide a strategy for designing the high-performance hexagonal-close-packed metal and alloy materials.

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First-principles calculations of structural and electronic properties of Tl_xGa_1-xAs alloys

G. Bilgeç Akyüz, A. Y. Tunali, S. E. Gulebaglan, N. B. Yurdasan

2016 (2): 27101-027101. doi: 10.1088/1674-1056/25/2/027101

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The zincblende ternary alloys Tl_xGa_1-xAs (0<x<1) are studied by numerical analysis based on the plane wave pseudopotential method within the density functional theory and the local density approximation. To model the alloys, 16-atom supercells with the 2× 2× 2 dimensions are used and the dependency of the lattice parameter, bulk modulus, electronic structure, energy band gap, and optical bowing on the concentration x are analyzed. The results indicate that the ternary Tl_xGa_1-xAs alloys have an average band gap bowing parameter of 4.48 eV for semiconductor alloys and 2.412 eV for semimetals. It is found that the band gap bowing strongly depends on composition and alloying a small Tl content with GaAs produces important modifications in the band structures of the alloys.

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Different variation behaviors of resistivity for high-temperature-grown and low-temperature-grown p-GaN films

杨静, 赵德刚, 江德生, 陈平, 刘宗顺, 朱建军, 乐伶聪, 李晓静, 何晓光, 张立群, 杨辉

2016 (2): 27102-027102. doi: 10.1088/1674-1056/25/2/027102

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Two series of p-GaN films grown at different temperatures are obtained by metal organic chemical vapor deposition (MOCVD). And the different variation behaviors of resistivity with growth condition for high- temperature(HT)-grown and low-temperature(LT)-grown p-GaN films are investigated. It is found that the resistivity of HT-grown p-GaN film is nearly unchanged when the NH₃ flow rate or reactor pressure increases. However, it decreases largely for LT-grown p-GaN film. These different variations may be attributed to the fact that carbon impurities are easy to incorporate into p-GaN film when the growth temperature is low. It results in a relatively high carbon concentration in LT-grown p-GaN film compared with HT-grown one. Therefore, carbon concentration is more sensitive to the growth condition in these samples, ultimately, leading to the different variation behaviors of resistivity for HT- and LT-grown ones.

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Evaluation of electrical and optical characteristics of ZnO/CdS/CIS thin film solar cell

Hadi Zarei, Rasoul Malekfar

2016 (2): 27103-027103. doi: 10.1088/1674-1056/25/2/027103

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In this study, device modeling and simulation are conducted to explain the effects of each layer thickness and temperature on the performance of ZnO/CdS/CIS thin film solar cells. Also, the thicknesses of the CIS and CdS absorber layers are considered in this work theoretically and experimentally. The calculations of solar cell performances are based on the solutions of the well-known three coupling equations: the continuity equation for holes and electrons and the Poisson equation. Our simulated results show that the efficiency increases by reducing the CdS thickness. Increasing the CIS thickness can increase the efficiency but it needs more materials. The efficiency is more than 19% for a CIS layer with a thickness of 2 μm. CIS nanoparticles are prepared via the polyol route and purified through centrifugation and precipitation processes. Then nanoparticles are dispersed to obtain stable inks that could be directly used for thin-film deposition via spin coating. We also obtain x-ray diffraction (XRD) peak intensities and absorption spectra for CIS experimentally. Finally, absorption spectra for the CdS window layer in several deposition times are investigated experimentally.

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Electronegativity explanation on the efficiency-enhancing mechanism of the hybrid inorganic-organic perovskite ABX₃ from first-principles study

陈清源, 黄杨, 黄鹏儒, 马泰, 曹超, 何垚

2016 (2): 27104-027104. doi: 10.1088/1674-1056/25/2/027104

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Organic-inorganic hybrid perovskites play an important role in improving the efficiency of solid-state dye-sensitized solar cells. In this paper, we systematically explore the efficiency-enhancing mechanism of ABX₃ (A =CH₃NH₃; B = Sn, Pb; X = Cl, Br, I) and provide the best absorber among ABX₃ when the organic framework A is CH₃NH₃ by first-principles calculations. The results reveal that the valence band maximum (VBM) of the ABX₃ is mainly composed of anion X p states and that conduction band minimum (CBM) of the ABX₃ is primarily composed of cation B p states. The bandgap of the ABX₃ decreases and the absorptive capacities of different wavelengths of light expand when reducing the size of the organic framework A, changing the B atom from Pb to Sn, and changing the X atom from Cl to Br to I. Finally, based on our calculations, it is discovered that CH₃NH₃SnI₃ has the best optical properties and its light-adsorption range is the widest among all the ABX₃ compounds when A is CH₃NH₃. All these results indicate that the electronegativity difference between X and B plays a fundamental role in changing the energy gap and optical properties among ABX₃ compounds when A remains the same and that CH₃NH₃SnI₃ is a promising perovskite absorber in the high efficiency solar batteries among all the CH₃NH₃BX₃ compounds.

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Current induced nonequilibrium spin polarization in semiconductor-nanowire/s-wave superconductor junctions with strong spin-orbit coupling

刘乃清, 黄立捷, 王瑞强, 胡梁宾

2016 (2): 27201-027201. doi: 10.1088/1674-1056/25/2/027201

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We have studied the characteristics of current induced nonequilibrium spin polarization in semiconductor-nanowire/s-wave superconductor junctions with strong spin-orbit coupling. It was found that within some parameter regions the magnitude of the current induced nonequilibrium spin polarization density in such structures will increase (or decrease) with the decrease (or increase) of the charge current density, in contrast to that found in normal spin-orbit coupled semiconductor structures. It was also found that the unusual characteristics of the current induced nonequilibrium spin polarization in such structures can be well explained by the effect of the Andreev reflection.

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Infrared laser-induced fast photovoltaic effect observed in orthorhombic tin oxide film

赵嵩卿, 张际蕊, 施宏杰, 闫坤坤, 黄春, 杨立敏, 杨睿, 赵昆

2016 (2): 27202-027202. doi: 10.1088/1674-1056/25/2/027202

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The SnO₂/SnO with an orthorhombic structure is a material known to be stable at high pressures and temperatures and expected to have new optical and electrical properties. The authors report a new finding of the infrared laser induced a fast photovoltaic effect arising from orthorhombic tin oxide film with an indirect band gap (～ 2.4 eV) which is deposited by pulsed laser deposition. The rising time of the photovoltaic signal is about 3 ns with a peak value of 4.48 mV under the pulsed laser beam with energy density 0.015 mJ/mm². The relation between the photovoltages and laser positions along the line between two electrodes of the film is also exhibited. A possible mechanism is put forward to explain this phenomenon. All data and analyses demonstrate that the orthorhombic tin oxide with an indirect band gap could be used as a candidate for an infrared photodetector which can be operated at high pressures and temperatures.

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Polarization-independent terahertz wave modulator based on graphene-silicon hybrid structure

杜亮亮, 李泉, 李绍限, 胡放荣, 熊显名, 栗岩锋, 张文涛, 韩家广

2016 (2): 27301-027301. doi: 10.1088/1674-1056/25/2/027301

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In this study, we propose and demonstrate a broadband polarization-independent terahertz modulator based on graphene/silicon hybrid structure through a combination of continuous wave optical illumination and electrical gating. Under a pump power of 400 mW and the voltages ranging from -1.8 V to 1.4 V, modulation depths in a range of -23%-62% are achieved in a frequency range from 0.25 THz to 0.65 THz. The modulator is also found to have a transition from unidirectional modulation to bidirectional modulation with the increase of pump power. Combining the Raman spectra and Schottky current-voltage characteristics of the device, it is found that the large amplitude modulation is ascribed to the electric-field controlled carrier concentration in silicon with assistance of the graphene electrode and Schottky junction.

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Quantum information transfer between topological and conventional charge qubits

栗军, 邹艳

2016 (2): 27302-027302. doi: 10.1088/1674-1056/25/2/027302

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We propose a scheme to realize coherent quantum information transfer between topological and conventional charge qubits. We first consider a hybrid system where a quantum dot (QD) is tunnel-coupled to a semiconductor Majorana-hosted nanowire (MNW) via using gated control as a switch, the information encoded in the superposition state of electron empty and occupied state can be transferred to each other through choosing the proper interaction time to make measurements. Then we consider another system including a double QDs and a pair of parallel MNWs, it is shown that the entanglement information transfer can be realized between the two kinds of systems. We also realize long distance quantum information transfer between two quantum dots separated by an MNW, by making use of the nonlocal fermionic level formed with the pared Majorana feimions (MFs) emerging at the two ends of the MNW. Furthermore, we analyze the teleportationlike electron transfer phenomenon predicted by Tewari et al. [Phys. Rev. Lett. 100, 027001 (2008)] in our considered system. Interestingly, we find that this phenomenon exactly corresponds to the case that the information encoded in one QD just returns back to its original place during the dynamical evolution of the combined system from the perspective of quantum state transfer.

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Effect of gate length on breakdown voltage in AlGaN/GaN high-electron-mobility transistor

罗俊, 赵胜雷, 宓珉瀚, 陈伟伟, 侯斌, 张进成, 马晓华, 郝跃

2016 (2): 27303-027303. doi: 10.1088/1674-1056/25/2/027303

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The effects of gate length L_G on breakdown voltage V_BR are investigated in AlGaN/GaN high-electron-mobility transistors (HEMTs) with L_G = 1 μm～ 20 μm. With the increase of L_G, V_BR is first increased, and then saturated at L_G=3 μm. For the HEMT with L_G=1 μm, breakdown voltage V_BR is 117 V, and it can be enhanced to 148 V for the HEMT with L_G = 3 μm. The gate length of 3 μm can alleviate the buffer-leakage-induced impact ionization compared with the gate length of 1 μm, and the suppression of the impact ionization is the reason for improving the breakdown voltage. A similar suppression of the impact ionization exists in the HEMTs with L_G>3 μm. As a result, there is no obvious difference in breakdown voltage among the HEMTs with L_G = 3 μm～ 20 μm, and their breakdown voltages are in a range of 140 V-156 V.

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Effect of graphene tunnel barrier on Schottky barrier height of Heusler alloy Co₂MnSi/graphene/n-Ge junction

李桂芳, 胡晶, 吕辉, 崔智军, 候晓伟, 刘诗斌, 杜永乾

2016 (2): 27304-027304. doi: 10.1088/1674-1056/25/2/027304

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We demonstrate that the insertion of a graphene tunnel barrier between Heusler alloy Co₂MnSi and the germanium (Ge) channel modulates the Schottky barrier height and the resistance-area product of the spin diode. We confirm that the Fermi level is depinned and a reduction in the electron Schottky barrier height (SBH) occurs following the insertion of the graphene layer between Co₂MnSi and Ge. The electron SBH is modulated in the 0.34 eV-0.61 eV range. Furthermore, the transport mechanism changes from rectifying to symmetric tunneling following the insertion. This behavior provides a pathway for highly efficient spin injection from a Heusler alloy into a Ge channel with high electron and hole mobility.

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Modeling of a triple reduced surface field silicon-on-insulator lateral double-diffused metal-oxide-semiconductor field-effect transistor with low on-state resistance

王裕如, 刘祎鹤, 林兆江, 方冬, 李成州, 乔明, 张波

2016 (2): 27305-027305. doi: 10.1088/1674-1056/25/2/027305

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An analytical model for a novel triple reduced surface field (RESURF) silicon-on-insulator (SOI) lateral double-diffused metal-oxide-semiconductor (LDMOS) field effect transistor with n-type top (N-top) layer, which can obtain a low on-state resistance, is proposed in this paper. The analytical model for surface potential and electric field distributions of the novel triple RESURF SOI LDMOS is presented by solving the two-dimensional (2D) Poisson's equation, which can also be applied to single, double and conventional triple RESURF SOI structures. The breakdown voltage (BV) is formulized to quantify the breakdown characteristic. Besides, the optimal integrated charge of N-top layer (Q_ntop) is derived, which can give guidance for doping the N-top layer. All the analytical results are well verified by numerical simulation results, showing the validity of the presented model. Hence, the proposed model can be a good tool for the device designers to provide accurate first-order design schemes and physical insights into the high voltage triple RESURF SOI device with N-top layer.

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A uniform doping ultra-thin SOI LDMOS with accumulation-mode extended gate and back-side etching technology

张彦辉, 魏杰, 尹超, 谭桥, 刘建平, 李鹏程, 罗小蓉

2016 (2): 27306-027306. doi: 10.1088/1674-1056/25/2/027306

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A uniform doping ultra-thin silicon-on-insulator (SOI) lateral-double-diffused metal-oxide-semiconductor (LDMOS) with low specific on-resistance (R_on,sp) and high breakdown voltage (BV) is proposed and its mechanism is investigated. The proposed LDMOS features an accumulation-mode extended gate (AG) and back-side etching (BE). The extended gate consists of a P- region and two diodes in series. In the on-state with V_GD>0, an electron accumulation layer is formed along the drift region surface under the AG. It provides an ultra-low resistance current path along the whole drift region surface and thus the novel device obtains a low temperature distribution. The R_on,sp is nearly independent of the doping concentration of the drift region. In the off-state, the AG not only modulates the surface electric field distribution and improves the BV, but also brings in a charge compensation effect to further reduce the R_on,sp. Moreover, the BE avoids vertical premature breakdown to obtain high BV and allows a uniform doping in the drift region, which avoids the variable lateral doping (VLD) and the “hot-spot” caused by the VLD. Compared with the VLD SOI LDMOS, the proposed device simultaneously reduces the R_on,sp by 70.2% and increases the BV from 776 V to 818 V.

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Controllable synthesis of ultrathin vanadium oxide nanobelts via an EDTA-mediated hydrothermal process

秦玉香, 刘成, 谢威威, 崔梦阳

2016 (2): 27307-027307. doi: 10.1088/1674-1056/25/2/027307

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Ultrathin VO₂ nanobelts with rough alignment features are prepared on the induction layer-coated substrates by an ethylenediaminetetraacetic acid (EDTA)-mediated hydrothermal process. EDTA acts as a chelating reagent and capping agent to facilitate the one-dimensional (1D) preferential growth of ultrathin VO₂ nanobelts with high crystallinities and good uniformities. The annealed induction layer and concentration of EDTA are found to play crucial roles in the formation of aligned and ultrathin nanobelts. Variation in EDTA concentration can change the VO₂ morphology of ultrathin nanobelts into that of thick nanoplates. Mild annealing of ultrathin VO₂ nanobelts at 350 ℃ in air results in the formation of V₂O₅ nanobelts with a nearly unchanged ultrathin structure. The nucleation and growth mechanism involved in the formations of nanobelts and nanoplates are proposed. The ethanol gas sensing properties of the V₂O₅ nanobelt networks-based sensor are investigated in a temperature range from 100 ℃ to 300 ℃ over ethanol concentrations ranging from 3 ppm to 500 ppm. The results indicate that the V₂O₅ nanobelt network sensor exhibits high sensitivity, good reversibility, and fast response-recovery characteristics with an optimal working temperature of 250 ℃.

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Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

杨雷雷, 刘大勇, 陈东猛, 邹良剑

2016 (2): 27401-027401. doi: 10.1088/1674-1056/25/2/027401

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The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO (R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe₂As₂ (R = Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magnetic-field-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.

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Superconductivity of bilayer phosphorene under interlayer compression

黄桂芹, 邢钟文

2016 (2): 27402-027402. doi: 10.1088/1674-1056/25/2/027402

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According to first-principles calculations, it is our prediction that bilayer phosphorene (BLP) will become a quasi-two-dimensional superconductor under a certain degree of interlayer compression. A decreasing interlayer distance may realize the transition in the BLP from a semiconducting phase to a metallic phase. On the other hand, a severe vertical compression may make the BLP lattice become dynamically unstable. It is found that in the stable metallic phase of the BLP, interlayer phonon modes dominate the electron-phonon coupling λ. The obtained λ can be greater than 1 and the superconducting temperature T_c can be higher than 10 K.

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Magnetic entropy change and magnetic properties of LaFe_11.5Si_1.5 after controlling the Curie temperature by partial substitution of Mn and hydrogenation

傅斌, 韩洁

2016 (2): 27501-027501. doi: 10.1088/1674-1056/25/2/027501

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Magnetic properties and magnetic entropy changes of La(Fe_1-xMn_x)_11.5Si_1.5H_y compounds are investigated. Their Curie temperatures are adjusted to room temperature by partial Mn substitution for Fe and hydrogen absorption in 1-atm (1 atm=1.01325×10⁵ Pa) hydrogen gas. Under a field change from 0 T to 2 T, the maximum magnetic entropy change for La(Fe_0.99Mn_0.01)_11.5Si_1.5H_1.61 is -11.5 J/kg. The suitable Curie temperature and large value of Δ S_m make it an attractive potential candidate for the room temperature magnetic refrigeration application.

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Influence of trap-assisted tunneling on trap-assisted tunneling current in double gate tunnel field-effect transistor

蒋智, 庄奕琪, 李聪, 王萍, 刘予琪

2016 (2): 27701-027701. doi: 10.1088/1674-1056/25/2/027701

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Trap-assisted tunneling (TAT) has attracted more and more attention, because it seriously affects the sub-threshold characteristic of tunnel field-effect transistor (TFET). In this paper, we assess subthreshold performance of double gate TFET (DG-TFET) through a band-to-band tunneling (BTBT) model, including phonon-assisted scattering and acoustic surface phonons scattering. Interface state density profile (D_it) and the trap level are included in the simulation to analyze their effects on TAT current and the mechanism of gate leakage current.

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Improvement in electrical properties of high-κfilm on Ge substrate by an improved stress relieved pre-oxide method

樊继斌, 丁晓甫, 刘红侠, 谢鹏飞, 张袁涛, 廖清良

2016 (2): 27702-027702. doi: 10.1088/1674-1056/25/2/027702

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High-κ/Ge gate stack has recently attracted a great deal of attention as a potential candidate to replace planar silicon transistors for sub-22 generation. However, the desorption and volatilization of GeO hamper the development of Ge-based devices. To cope with this challenge, various techniques have been proposed to improve the high-κ/Ge interface. However, these techniques have not been developed perfectly yet to control the interface. Therefore, in this paper, we propose an improved stress relieved pre-oxide (SRPO) method to improve the thermodynamic stability of the high-κ/Ge interface. The x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) results indicate that the GeO volatilization of the high-κ/Ge gate stack is efficiently suppressed after 500 ℃ annealing, and the electrical characteristics are greatly improved.

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Realization of a flux-driven memtranstor at room temperature

申世鹏, 尚大山, 柴一晟, 孙阳

2016 (2): 27703-027703. doi: 10.1088/1674-1056/25/2/027703

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The memtranstor has been proposed to be the fourth fundamental circuit memelement in addition to the memristor, memcapacitor, and meminductor. Here, we demonstrate the memtranstor behavior at room temperature in a device made of the magnetoelectric hexaferrite (Ba_0.5Sr_1.5Co₂Fe₁₁AlO₂₂) where the electric polarization is tunable by external magnetic field. This device shows a nonlinear q-φ relationship with a butterfly-shaped hysteresis loop, in agreement with the anticipated memtranstor behavior. The memtranstor, like other memelements, has a great potential in developing more advanced circuit functionalities.

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Superluminal light attenuated by strong dispersion of complex refractive index

Abdurahman Ahmed Yonis, Vadim Nickolaevich Mal'nev, Belayneh Mesfin Ali

2016 (2): 27801-027801. doi: 10.1088/1674-1056/25/2/027801

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The propagation of narrow packets of electromagnetic waves (EMWs) in frequency dispersive medium with the consideration of the complex refractive index is studied. It is shown that counting in the dispersion of the complex refractive index within the context of the conventional expression of the group velocity of narrow wave packets of EMWs propagating in a dispersive medium results in the appearance of additional constraints on the group velocity, which dictates that the physically acceptable group velocity can only be realized in the case of a negligible imaginary part of the group index. In this paper, the conditions that allow one to realize the physically acceptable group velocity are formulated and analyzed numerically for the relevant model of the refractive index of a system of two-level atoms in the optical frequency range. It is shown that in the frequency band where superluminal light propagation is expected, there is a strong dispersion of the refractive index that is accompanied with strong absorption, resulting in a strongly attenuated superluminal light.

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Molten-salt synthesis and composition-dependent luminescent properties of barium tungsto-molybdate-based solid solution phosphors

贺香红, 叶招莲, 关明云, 连宁, 孙建华

2016 (2): 27802-027802. doi: 10.1088/1674-1056/25/2/027802

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Pr³⁺-activated barium tungsto-molybdate solid solution phosphor Ba(Mo_1-zW_z)O₄:Pr³⁺ is successfully fabricated via a facile molten-salt approach. The as-synthesized microcrystal is of truncated octahedron and exhibits deep-red-emitting upon blue light excitation. Powder x-ray diffraction and Raman spectroscopy techniques are utilized to investigate the formation of solid solution phosphor. The luminescence behaviors depend on the resulting composition of the microcrystals with fixed Pr³⁺-doping concentration, while the host lattices remain in a scheelite structure. The forming solid solution via the substitution of [WO₄] for [MoO₄] can significantly enhance its luminescence, which may be due to the fact that Ba(Mo_1-zW_z)O₄:Pr³⁺ owns well-defined facets and uniform morphologies. Owing to its properties of high phase purity, well-defined facets, highly uniform morphologies, exceptional chemical and thermal stabilities, and stronger emission intensity, the resulting solid solution phosphor is expected to find potential applications in phosphor-converted white light-emitting diodes (LEDs).

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Molecular dynamics simulation of Cu_n clusters scattering from a single-crystal Cu (111) surface: The influence of surface structure

罗先文, 王勐, 胡碧涛

2016 (2): 27901-027901. doi: 10.1088/1674-1056/25/2/027901

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By performing a molecular dynamics simulation, fragmentation of Cu_n clusters scattering from a single-crystal Cu (111) surface is studied. The interactions among copper atoms are modeled by tight-binding potential, and the positions of the copper clusters at each time step are calculated by integrating the Newton equations of motion. The percentage of unfragmented clusters depends on the incident velocities, angles of incidence, and surface structure. The influence of surface structure on the fragment distribution is discussed, and the clusters appear to be more stable under an axial channeling condition. The fragment distribution shifting toward the small fragment range for cluster scattering along a random direction is confirmed, indicating that the cluster undergoes more intensive fragmentation.

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Effects of Ni doping on the structural properties and collapse of magnetic ordering in NdFe₁-xNi_xO₃ (0.1≤x≤ 0.7) orthoferrites

I. Ahmad, M. J. Akhtar, M. Siddique

2016 (2): 28101-028101. doi: 10.1088/1674-1056/25/2/028101

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NdFe₁-xNi_xO₃ (0.1≤x≤ 0.7) orthoferrites are synthesized by solid state reaction method, and the structural properties of these materials are investigated by employing x-ray diffraction (XRD), scanning electron microscopy (SEM) and Mössbauer spectroscopy. The orthorhombic structure is observed in all systems; however, with the increase in Ni doping, the increase in tolerance factor and the decrease in the cell volume are observed. Orthorhombic distortion decreases with Ni content increasing up to 50%, while above 50% Ni doping it increases. SEM examination indicates the increases in grain size and intermixing of grains with increase in Ni concentration. Comparison between bulk and theoretical densities shows that in each of all samples porosity is less than 2%. Mössbauer spectroscopic investigations are performed to explain local structure, Fe oxidation states and collapse of the magnetic ordering. In these samples the Fe oxidation state remains +3 and there is no considerable increase in hole states observed; however due to mismatch of the ionic radii between Fe³⁺ and Ni³⁺, octahedral distortions, sagging and distribution of hyperfine parameters increase with increase in Ni concentration. The major factors behind the collapse of magnetic ordering in the Ni-doped systems are the weakening of the super-exchange interactions, decrease in the Neel temperature, increase in spin-spin relaxation frequency and high spin to low spin transition.

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Room temperature NO₂ gas sensing of Au-loaded tungsten oxide nanowires/porous silicon hybrid structure

王登峰, 梁继然, 李昌青, 闫文君, 胡明

2016 (2): 28102-028102. doi: 10.1088/1674-1056/25/2/028102

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In this work, we report an enhanced nitrogen dioxide (NO₂) gas sensor based on tungsten oxide (WO₃) nanowires/porous silicon (PS) decorated with gold (Au) nanoparticles. Au-loaded WO₃ nanowires with diameters of 10 nm-25 nm and lengths of 300 nm-500 nm are fabricated by the sputtering method on a porous silicon substrate. The high-resolution transmission electron microscopy (HRTEM) micrographs show that Au nanoparticles are uniformly distributed on the surfaces of WO₃ nanowires. The effect of the Au nanoparticles on the NO₂-sensing performance of WO₃ nanowires/porous silicon is investigated over a low concentration range of 0.2 ppm-5 ppm of NO₂ at room temperature (25 ℃). It is found that the 10-Å Au-loaded WO₃ nanowires/porous silicon-based sensor possesses the highest gas response characteristic. The underlying mechanism of the enhanced sensing properties of the Au-loaded WO₃ nanowires/porous silicon is also discussed.

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Nanodots and microwires of ZrO₂ grown on LaAlO₃ by photo-assisted metal-organic chemical vapor deposition

郭峰, 汪薪生, 庄仕伟, 李国兴, 张宝林, 周本初

2016 (2): 28103-028103. doi: 10.1088/1674-1056/25/2/028103

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ZrO₂ nanodots are successfully prepared on LaAlO₃ (LAO) (100) substrates by photo-assisted metal-organic chemical vapor deposition (MOCVD). It is indicated that the sizes and densities of ZrO₂ nanodots are controllable by modulating the growth temperature, oxygen partial pressure, and growth time. Meanwhile, the microwires are observed on the surfaces of substrates. It is found that there is an obvious competitive relationship between the nanodots and the microwires. In a growth temperature range from 500 ℃ to 660 ℃, the microwires turn longest and widest at 600 ℃, but in contrast, the nanodots grow into the smallest diameter at 600 ℃. This phenomenon could be illustrated by the energy barrier, decomposition rate of Zr(tmhd)₄, and mobility of atoms. In addition, growth time or oxygen partial pressure also affects the competitive relationship between the nanodots and the microwires. With increasing oxygen partial pressure from 451 Pa to 752 Pa, the microwires gradually grow larger while the nanodots become smaller. To further achieve the controllable growth, the coarsening effect of ZrO₂ is modified by varying the growth time, and the experimental results show that the coarsening effect of microwires is higher than that of nanodots by increasing the growth time to quickly minimize ZrO₂ energy density.

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In-situ characterization of electrochromism based on ITO/PEDOT:PSS towards preparation of high performance device

王学进, 郭正飞, 曲婧毓, 潘坤, 祁铮, 李泓

2016 (2): 28201-028201. doi: 10.1088/1674-1056/25/2/028201

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Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is usually sandwiched between indium tin oxide (ITO) and a functional polymer in order to improve the performance of the device. However, because of the strong acidic nature of PEDOT:PSS, the instability of ITO/PEDOT:PSS interface is also observed. The mechanism of degradation of the device remains is unclear and needs to be further studied. In this article, we investigate the in-situ electrochromism of PEDOT:PSS to disclose the cause of the degradation. X-ray photoelectron spectroscopy (XPS) was used to characterize the PEDOT:PSS films, as well as the PEDOT:PSS plus polyethylene glycol (PEG) films with and without indium ions. The electrochromic devices (ECD) based on PEDOT:PSS and PEG with and without indium ions are carried out by in-situ micro-Raman and laser reflective measurement (LRM). For comparison, ECD based on PEDOT:PSS and PEG films with LiCl, KCl, NaCl or InCl₃ are also investigated by LRM. The results show that PEDOT:PSS is further reduced when negatively biased, and oxidized when positively biased. This could identify that PEDOT:PSS with indium ions from PEDOT:PSS etching ITO will lose dopants when negatively biased. The LRM shows that the device with indium ions has a stronger effect on the reduction property of PEDOT:PSS-PEG film than the device without indium ions. The contrast of the former device is 44%, that of the latter device is about 3%. The LRM also shows that the contrasts of the device based on PEDOT:PSS+PEG with LiCl, KCl, NaCl, InCl₃ are 30%, 27%, 15%, and 18%, respectively.

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First principle study of LiXS₂ (X = Ga, In) as cathode materials for Li ion batteries

饶凤雅, 宁芳华, 蒋礼威, 曾祥明, 吴木生, 徐波, 欧阳楚英

2016 (2): 28202-028202. doi: 10.1088/1674-1056/25/2/028202

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From first principle calculations, we demonstrate that LiXS₂ (X = Ga, In) compounds have potential applications as cathode materials for Li ion batteries. It is shown that Li can be extracted from the LiXS₂ lattice with relatively small volume change and the XS₄ tetrahedron structure framework remains stable upon delithiation. The theoretical capacity and average intercalation potential of the LiGaS₂ (LiInS₂) cathode are 190.4 (144.2) mAh/g and 3.50 V (3.53 V). The electronic structures of the LiXS₂ are insulating with band gaps of 2.88 eV and 1.99 eV for X= Ga and In, respectively. However, Li vacancies, which are formed through delithiation, change the electronic structure substantially from insulating to metallic structure, indicating that the electrical conductivities of the LiXS₂ compounds should be good during cycling. Li ion migration energy barriers are also calculated, and the results show that Li ion diffusions in the LiXS₂ compounds can be as good as those in the currently widely used electrode materials.

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Investigations of the optical properties of Si surface with microwires for solar cell applications

郦莉, 吴仕良, 虞栋, 王伟, 刘文超, 吴小山, 张凤鸣

2016 (2): 28401-028401. doi: 10.1088/1674-1056/25/2/028401

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Combined with the rough textures which are usually used for fabricating the planar solar cells, textured-microwire (MW) structures (there are MW textures on the tops of MWs) are fabricated. Both simulative and experimental investigations of the optical properties of Si surface with microwires are carried out and the results show that surfaces of textured-MW structures exhibit lower reflectance in the short wavelength range, but higher reflectance in the long wavelength range than the conventional textured surface. It is also shown, consequently, that the textured-MW structures could absorb more photons in the short wavelength range, which could help improve the performance of the solar cells.

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Improving the performance of perovskite solar cells with glycerol-doped PEDOT:PSS buffer layer

李建丰, 赵创, 张恒, 同军锋, 张鹏, 杨春燕, 夏养君, 范多旺

2016 (2): 28402-028402. doi: 10.1088/1674-1056/25/2/028402

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In this paper, we investigate the effects of glycerol doping on transmittance, conductivity and surface morphology of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)) (PEDOT:PSS) and its influence on the performance of perovskite solar cells．The conductivity of PEDOT:PSS is improved obviously by doping glycerol. The maximum of the conductivity is 0.89 S/cm when the doping concentration reaches 6 wt%, which increases about 127 times compared with undoped. The perovskite solar cells are fabricated with a configuration of indium tin oxide (ITO)/PEDOT:PSS/CH₃NH₃PbI₃/PC₆₁BM/Al, where PEDOT:PSS and PC₆₁BM are used as hole and electron transport layers, respectively. The results show an improvement of hole charge transport as well as an increase of short-circuit current density and a reduction of series resistance, owing to the higher conductivity of the doped PEDOT:PSS. Consequently, it improves the whole performance of perovskite solar cell. The power conversion efficiency (PCE) of the device is improved from 8.57% to 11.03% under AM 1.5 G (100 mW/cm² illumination) after the buffer layer has been modified.

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Performance improvement of AlGaN-based deep ultraviolet light-emitting diodes with double electron blocking layers

张诚, 孙慧卿, 李旭娜, 孙浩, 范宣聪, 张柱定, 郭志友

2016 (2): 28501-028501. doi: 10.1088/1674-1056/25/2/028501

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The AlGaN-based deep ultraviolet light-emitting diodes (LED) with double electron blocking layers (d-EBLs) on both sides of the active region are investigated theoretically. They possess many excellent performances compared with the conventional structure with only a single electron blocking layer, such as a higher recombination rate, improved light output power and internal quantum efficiency (IQE). The reasons can be concluded as follows. On the one hand, the weakened electrostatic field within the quantum wells (QWs) enhances the electron-hole spatial overlap in QWs, and therefore increases the probability of radioactive recombination. On the other hand, the added n-AlGaN layer can not only prevent holes from overflowing into the n-side region but also act as another electron source, providing more electrons.

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Reverse-feeding effect of epidemic by propagators in two-layered networks

吴大宇, 赵艳萍, 郑木华, 周杰, 刘宗华

2016 (2): 28701-028701. doi: 10.1088/1674-1056/25/2/028701

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Epidemic spreading has been studied for a long time and is currently focused on the spreading of multiple pathogens, especially in multiplex networks. However, little attention has been paid to the case where the mutual influence between different pathogens comes from a fraction of epidemic propagators, such as bisexual people in two separated groups of heterosexual and homosexual people. We here study this topic by presenting a network model of two layers connected by impulsive links, in contrast to the persistent links in each layer. We let each layer have a distinct pathogen and their interactive infection is implemented by a fraction of propagators jumping between the corresponding pairs of nodes in the two layers. By this model we show that (i) the propagators take the key role to transmit pathogens from one layer to the other, which significantly influences the stabilized epidemics; (ii) the epidemic thresholds will be changed by the propagators; and (iii) a reverse-feeding effect can be expected when the infective rate is smaller than its threshold of isolated spreading. A theoretical analysis is presented to explain the numerical results.

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Performance analysis of LDPC codes on OOK terahertz wireless channels

刘纯, 王长, 曹俊诚

2016 (2): 28702-028702. doi: 10.1088/1674-1056/25/2/028702

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Atmospheric absorption, scattering, and scintillation are the major causes to deteriorate the transmission quality of terahertz (THz) wireless communications. An error control coding scheme based on low density parity check (LDPC) codes with soft decision decoding algorithm is proposed to improve the bit-error-rate (BER) performance of an on-off keying (OOK) modulated THz signal through atmospheric channel. The THz wave propagation characteristics and channel model in atmosphere is set up. Numerical simulations validate the great performance of LDPC codes against the atmospheric fading and demonstrate the huge potential in future ultra-high speed beyond Gbps THz communications.

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Terahertz multi-metal-wire hybrid-cladding hollow waveguide for refractive index sensing

于莹莹, 李绪友, 何昆鹏, 孙波

2016 (2): 28703-028703. doi: 10.1088/1674-1056/25/2/028703

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We propose a design of terahertz refractive index sensing based on the multi-metal-wire (MMW) hybrid-cladding hollow waveguide. The proposed terahertz hybrid-cladding hollow waveguide comprises one air core in the center surrounding MMW surrounded dielectric. The central air core is used for filling lossless measurands and transmitting terahertz light. In particular, the refractive index sensing is realized by measuring the mode field area (MFA) variation of radially polarized mode. The modal effective refractive index, mode field intensity distribution, and mode field area properties responding to the measurand refractive indexes for different operating frequencies and structure dimensions are investigated, respectively. Simulations show that the proposed terahertz refractive index sensor can realize easily the measurement of the measurand refractive index. Meanwhile, the effects of operating frequency and structure parameters on sensitivity and measurement accuracy are also studied. In view of the trade-off between sensitivity and measurement accuracy, the reasonable choice of the operating frequency and structure parameters can optimize appropriately the sensitivity and measurement accuracy, and the sensitivity can reach approximately 0.585 mm²/RIU (RIU is short for refraction index units) with the proper frequency and structure parameter.

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Simple phase extraction in x-ray differential phase contrast imaging

刘鑫, 郭金川, 雷耀虎, 李冀, 牛憨笨

2016 (2): 28704-028704. doi: 10.1088/1674-1056/25/2/028704

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A fast and simple method to extract phase-contrast images from interferograms is proposed, and its effectiveness is demonstrated through simulation and experiment. For x-ray differential phase contrast imaging, a strong attenuation signal acts as an overwhelming background intensity that obscures the weak phase signal so that no obvious phase-gradient information is detectable in the raw image. By subtracting one interferogram from another, chosen at particular intervals, the phase signal can be isolated and magnified.

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Flexible impedance and capacitive tensile load Sensor based on CNT composite

Zubair Ahmad, Kh S Karimov, Farid Touati

2016 (2): 28801-028801. doi: 10.1088/1674-1056/25/2/028801

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In this paper, the fabrication and investigation of flexible impedance and capacitive tensile load sensors based on carbon nanotube (CNT) composite are reported. On thin rubber substrates, CNTs are deposited from suspension in water and pressed at elevated temperature. It is found that the fabricated load cells are highly sensitive to the applied mechanical force with good repeatability. The increase in impedance of the cells is observed to be 2.0 times while the decrease in the capacitance is found to be 2.1 times as applied force increases up to 0.3 N. The average impedance and capacitive sensitivity of the cell are equal to 3.4 N^-1 and 1.8 N^-1, respectively. Experimental results are compared with the simulated values, and they show that they are in reasonable agreement with each other.

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Dynamic feature analysis in bidirectional pedestrian flows

杨晓霞, Winnie Daamen, Serge Paul Hoogendoorn, 董海荣, 姚秀明

2016 (2): 28901-028901. doi: 10.1088/1674-1056/25/2/028901

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Analysis of dynamic features of pedestrian flows is one of the most exciting topics in pedestrian dynamics. This paper focuses on the effect of homogeneity and heterogeneity in three parameters of the social force model, namely desired velocity, reaction time, and body size, on the moving dynamics of bidirectional pedestrian flows in the corridors. The speed and its deviation in free flows are investigated. Simulation results show that the homogeneous higher desired speed which is less than a critical threshold, shorter reaction time or smaller body size results in higher speed of flows. The free dynamics is more sensitive to the heterogeneity in desired speed than that in reaction time or in body size. In particular, an inner lane formation is observed in normal lanes. Furthermore, the breakdown probability and the start time of breakdown are focused on. This study reveals that the sizes of homogeneous desired speed, reaction time or body size play more important roles in affecting the breakdown than the heterogeneities in these three parameters do.

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A 0.33-THz second-harmonic frequency-tunable gyrotron

李铮迪, 杜朝海, 戚向波, 罗里, 刘濮鲲

2016 (2): 29401-029401. doi: 10.1088/1674-1056/25/2/029401

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Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interaction mechanism about propagating waves, featured by wave competition and wave cooperation, is presented and provides a new insight into the beam-wave interaction. The two different features revealed in the two different operation regions of low-order axial modes (LOAMs) and high-order axial modes (HOAMs) respectively determine the characteristic of the overall performance of the device essentially. The device performance is obtained by the simulation based on the time-domain nonlinear theory and shows that using a 12-kV/150-mA electron beam and TE_-3,4 mode, the second harmonic gyrotron can generate terahertz radiations with frequency-tuning ranges of about 0.85 GHz and 0.60 GHz via magnetic field and beam voltage tuning, respectively. Additionally, some non-stationary phenomena in the mode startup process are also analyzed. The investigation in this paper presents guidance for future developing high-performance frequency-tunable gyrotrons toward terahertz applications.

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