中国物理B

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Robust H_∞ control for uncertain Markovian jump systems with mixed delays

R Saravanakumar, M Syed Ali

2016 (7): 70201-070201. doi: 10.1088/1674-1056/25/7/070201

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We scrutinize the problem of robust H_∞ control for a class of Markovian jump uncertain systems with interval time-varying and distributed delays. The Markovian jumping parameters are modeled as a continuous-time finite-state Markov chain. The main aim is to design a delay-dependent robust H_∞ control synthesis which ensures the mean-square asymptotic stability of the equilibrium point. By constructing a suitable Lyapunov-Krasovskii functional (LKF), sufficient conditions for delay-dependent robust H_∞ control criteria are obtained in terms of linear matrix inequalities (LMIs). The advantage of the proposed method is illustrated by numerical examples. The results are also compared with the existing results to show the less conservativeness.

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Gazeau-Klauder coherent states examined from the viewpoint of diagonal ordering operation technique

Dušan Popov, Romeo Negrea, Miodrag Popov

2016 (7): 70301-070301. doi: 10.1088/1674-1056/25/7/070301

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In this paper we investigate the Gazeau-Klauder coherent states using a newly introduced diagonal ordering operation technique, in order to examine some of the properties of these coherent states. The results coincide with those obtained from other purely algebraic methods, but the calculations are greatly simplified. We apply the general theory to two cases of Gazeau-Klauder coherent states: pseudoharmonic as well as the Morse oscillators.

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Improving the intensity and efficiency of compressed echo in rare-earth-ion-doped crystal

马秀荣, 梁裕卿, 王松, 张双根, 单云龙

2016 (7): 70302-070302. doi: 10.1088/1674-1056/25/7/070302

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We investigate the intensity and efficiency of a compressed echo, which is important in arbitrary waveform generation (AWG). A new model of compressed echo is proposed based on the optical Bloch equations, which exposes much more detailed parameters than the conventional model, such as the time delay of the chirp lasers, the nature of the rare-earth-ion-doped crystal, etc. According to the novel model of compressed echo, we find that reducing the time delay of the chirp lasers and scanning the lasers around the center frequency of the inhomogeneously broadened spectrum, while utilizing a crystal with larger coherence time and excitation lifetime can improve the compressed echo's intensity and efficiency. The theoretical analysis is validated by numerical simulations.

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Intercept-resend attack on six-state quantum key distribution over collective-rotation noise channels

Kevin Garapo, Mhlambululi Mafu, Francesco Petruccione

2016 (7): 70303-070303. doi: 10.1088/1674-1056/25/7/070303

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We investigate the effect of collective-rotation noise on the security of the six-state quantum key distribution. We study the case where the eavesdropper, Eve, performs an intercept-resend attack on the quantum communication between Alice, the sender, and Bob, the receiver. We first derive the collective-rotation noise model for the six-state protocol and then parameterize the mutual information between Alice and Eve. We then derive quantum bit error rate for three intercept-resend attack scenarios. We observe that the six-state protocol is robust against intercept-resend attacks on collective rotation noise channels when the rotation angle is kept within certain bounds.

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Entanglement properties between two atoms in the binomial optical field interacting with two entangled atoms

刘堂昆, 张康隆, 陶宇, 单传家, 刘继兵

2016 (7): 70304-070304. doi: 10.1088/1674-1056/25/7/070304

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The temporal evolution of the degree of entanglement between two atoms in a system of the binomial optical field interacting with two arbitrary entangled atoms is investigated. The influence of the strength of the dipole-dipole interaction between two atoms, probabilities of the Bernoulli trial, and particle number of the binomial optical field on the temporal evolution of the atomic entanglement are discussed. The result shows that the two atoms are always in the entanglement state. Moreover, if and only if the two atoms are initially in the maximally entangled state, the entanglement evolution is not affected by the parameters, and the degree of entanglement is always kept as 1.

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Collapse-revival of squeezing of two atoms in dissipative cavities

邹红梅, 方卯发

2016 (7): 70305-070305. doi: 10.1088/1674-1056/25/7/070305

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Based on the time-convolutionless master-equation approach, we investigate the squeezing dynamics of two atoms in dissipative cavities. We find that the atomic squeezing is related to initial atomic states, atom-cavity couplings, non-Markovian effects and resonant frequencies of an atom and its cavity. The results show that a collapse-revival phenomenon will occur in the atomic squeezing and this process is accompanied by the buildup and decay of entanglement between two atoms. Enhancing the atom-cavity coupling can increase the frequency of the collapse-revival of the atomic squeezing. The stronger the non-Markovian effect is, the more obvious the collapse-revival phenomenon is. In particular, if the atom-cavity coupling or the non-Markovian effect is very strong, the atomic squeezing will tend to a stably periodic oscillation in a long time. The oscillatory frequency of the atomic squeezing is dependent on the resonant frequency of the atom and its cavity.

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Bursting phenomena as well as the bifurcation mechanism in a coupled BVP oscillator with periodic excitation

张晓芳, 吴磊, 毕勤胜

2016 (7): 70501-070501. doi: 10.1088/1674-1056/25/7/070501

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We explore the complicated bursting oscillations as well as the mechanism in a high-dimensional dynamical system. By introducing a periodically changed electrical power source in a coupled BVP oscillator, a fifth-order vector field with two scales in frequency domain is established when an order gap exists between the natural frequency and the exciting frequency. Upon the analysis of the generalized autonomous system, bifurcation sets are derived, which divide the parameter space into several regions associated with different types of dynamical behaviors. Two typical cases are focused on as examples, in which different types of bursting oscillations such as subHopf/subHopf burster, subHopf/fold-cycle burster, and double-fold/fold burster can be observed. By employing the transformed phase portraits, the bifurcation mechanism of the bursting oscillations is presented, which reveals that different bifurcations occurring at the transition between the quiescent states (QSs) and the repetitive spiking states (SPs) may result in different forms of bursting oscillations. Furthermore, because of the inertia of the movement, delay may exist between the locations of the bifurcation points on the trajectory and the bifurcation points obtained theoretically.

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Multifractal modeling of the production of concentrated sugar syrup crystal

闭胜, 高剑波

2016 (7): 70502-070502. doi: 10.1088/1674-1056/25/7/070502

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High quality, concentrated sugar syrup crystal is produced in a critical step in cane sugar production: the clarification process. It is characterized by two variables: the color of the produced sugar and its clarity degree. We show that the temporal variations of these variables follow power-law distributions and can be well modeled by multiplicative cascade multifractal processes. These interesting properties suggest that the degradation in color and clarity degree has a system-wide cause. In particular, the cascade multifractal model suggests that the degradation in color and clarity degree can be equivalently accounted for by the initial “impurities” in the sugarcane. Hence, more effective cleaning of the sugarcane before the clarification stage may lead to substantial improvement in the effect of clarification.

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Modeling random telegraph signal noise in CMOS image sensor under low light based on binomial distribution

张钰, 逯鑫淼, 王光义, 胡永才, 徐江涛

2016 (7): 70503-070503. doi: 10.1088/1674-1056/25/7/070503

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The random telegraph signal noise in the pixel source follower MOSFET is the principle component of the noise in the CMOS image sensor under low light. In this paper, the physical and statistical model of the random telegraph signal noise in the pixel source follower based on the binomial distribution is set up. The number of electrons captured or released by the oxide traps in the unit time is described as the random variables which obey the binomial distribution. As a result, the output states and the corresponding probabilities of the first and the second samples of the correlated double sampling circuit are acquired. The standard deviation of the output states after the correlated double sampling circuit can be obtained accordingly. In the simulation section, one hundred thousand samples of the source follower MOSFET have been simulated, and the simulation results show that the proposed model has the similar statistical characteristics with the existing models under the effect of the channel length and the density of the oxide trap. Moreover, the noise histogram of the proposed model has been evaluated at different environmental temperatures.

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Bifurcation and chaos in high-frequency peak current mode Buck converter

常昌远, 赵欣, 杨帆, 吴承恩

2016 (7): 70504-070504. doi: 10.1088/1674-1056/25/7/070504

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Bifurcation and chaos in high-frequency peak current mode Buck converter working in continuous conduction mode (CCM) are studied in this paper. First of all, the two-dimensional discrete mapping model is established. Next, reference current at the period-doubling point and the border of inductor current are derived. Then, the bifurcation diagrams are drawn with the aid of MATLAB. Meanwhile, circuit simulations are executed with PSIM, and time domain waveforms as well as phase portraits in i_L-v_C plane are plotted with MATLAB on the basis of simulation data. After that, we construct the Jacobian matrix and analyze the stability of the system based on the roots of characteristic equations. Finally, the validity of theoretical analysis has been verified by circuit testing. The simulation and experimental results show that, with the increase of reference current I_ref, the corresponding switching frequency f is approaching to low-frequency stage continuously when the period-doubling bifurcation happens, leading to the converter tending to be unstable. With the increase of f, the corresponding I_ref decreases when the period-doubling bifurcation occurs, indicating the stable working range of the system becomes smaller.

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Wavefront sensing based on phase contrast theory and coherent optical processing

黄磊, 边琪, 周晨露, 李腾浩, 巩马理

2016 (7): 70701-070701. doi: 10.1088/1674-1056/25/7/070701

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A novel wavefront sensing method based on phase contrast theory and coherent optical processing is proposed. The wavefront gradient field in the object plane is modulated into intensity distribution in a gang of patterns, making high-density detection available. By applying the method, we have also designed a wavefront sensor. It consists of a classical coherent optical processing system, a CCD detector array, two pieces of orthogonal composite sinusoidal gratings, and a mechanical structure that can perform real-time linear positioning. The simulation results prove and demonstrate the validity of the method and the sensor in high-precision measurement of the wavefront gradient field.

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Configuration interaction studies on the spectroscopic properties of PbO including spin—orbit coupling

罗旺, 李瑞, 盖志强, 艾瑞波, 张宏民, 张晓美, 闫冰

2016 (7): 73101-073101. doi: 10.1088/1674-1056/25/7/073101

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Lead oxide (PbO), which plays the key roles in a range of research fields, has received a great deal of attention. Owing to the large density of electronic states and heavy atom Pb including in PbO, the excited states of the molecule have not been well studied. In this work, high level multireference configuration interaction calculations on the low-lying states of PbO have been carried out by utilizing the relativistic effective core potential. The effects of the core-valence correlation correction, the Davidson modification, and the spin-orbital coupling on the electronic structure of the PbO molecule are estimated. The potential energy curves of 18 Λ-S states correlated to the lowest dissociation limit (Pb (³P_g) + O(³P_g)) are reported. The calculated spectroscopic parameters of the electronic states below 30000 cm^-1, for instance, X¹Σ⁺, 1³⁺, and 1³Σ^-, and their spin-orbit coupling interaction, are compared with the experimental results, and good agreements are derived. The dipole moments of the 18 Λ-S states are computed with the configuration interaction method, and the calculated dipole moments of X¹Σ⁺ and 1³Σ⁺ are consistent with the previous experimental results. The transition dipole moments from 1¹Π, 2¹Π, and 2¹Σ⁺ to X¹Σ⁺ and other singlet excited states are estimated. The radiative lifetime of several low-lying vibrational levels of 1¹Π, 2¹Π, and 2¹Σ⁺ states are evaluated.

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Automatic compensation of magnetic field for a rubidium space cold atom clock

李琳, 吉经纬, 任伟, 赵鑫, 彭向凯, 项静峰, 吕德胜, 刘亮

2016 (7): 73201-073201. doi: 10.1088/1674-1056/25/7/073201

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When the cold atom clock operates in microgravity around the near-earth orbit, its performance will be affected by the fluctuation of magnetic field. A strategy is proposed to suppress the fluctuation of magnetic field by additional coils, whose current is changed accordingly to compensate the magnetic fluctuation by the linear and incremental compensation. The flight model of the cold atom clock is tested in a simulated orbital magnetic environment and the magnetic field fluctuation in the Ramsey cavity is reduced from 17 nT to 2 nT, which implied the uncertainty due to the second order Zeeman shift is reduced to be less than 2×10^-16. In addition, utilizing the compensation, the magnetic field in the trapping zone can be suppressed from 7.5 μT to less than 0.3 μT to meet the magnetic field requirement of polarization gradients cooling of atoms.

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Two-photon double ionization of helium by chirped few-cycle attosecond pulses: From nonsequential to sequential regime

童瑶, 姜维超, 伍攀, 彭良友

2016 (7): 73202-073202. doi: 10.1088/1674-1056/25/7/073202

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The two-photon double ionization (TPDI) dynamics of helium by chirped attosecond pulses are theoretically studied by solving the two-electron time-dependent Schrödinger equation in its full dimensions. We show that both the differential and the total double ionization probability can be significantly controlled by adjusting the chirp. The dependence of the TPDI on the chirp can be quite different for different photon energies, relying on the central photon energy being in the sequential region, nonsequential region, or translation region. The physics which lead to the chirp dependence for different photon energies are addressed. Present findings are well reproduced by a model based on the second-order time-dependent perturbation theory.

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Differential cross sections of positron—hydrogen collisions

于荣梅, 濮春英, 黄晓玉, 殷复荣, 刘旭焱, 焦利光, 周雅君

2016 (7): 73401-073401. doi: 10.1088/1674-1056/25/7/073401

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We make a detailed study on the angular differential cross sections of positron-hydrogen collisions by using the momentum-space coupled-channels optical (CCO) method for incident energies below the H ionization threshold. The target continuum and the positronium (Ps) formation channels are included in the coupled-channels calculations via a complex equivalent-local optical potential. The critical points, which show minima in the differential cross sections, as a function of the scattering angle and the incident energy are investigated. The resonances in the angular differential cross sections are reported for the first time in this energy range. The effects of the target continuum and the Ps formation channels on the different cross sections are discussed.

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Optimization of loss and gain multilayers for reducing the scattering of a perfect conducting cylinder

余振中, 赵国树, 孙罡, 司海飞, 杨忠

2016 (7): 74101-074101. doi: 10.1088/1674-1056/25/7/074101

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Reduction of electromagnetic scattering from a conducting cylinder could be achieved by covering it with optimized multilayers of normal dielectric and plasmonic material. The plasmonic material with intrinsic losses could degrade the cloaking effect. Using a genetic algorithm, we present the optimized design of loss and gain multilayers for reduction of the scattering from a perfect conducting cylinder. This multilayered structure is theoretically and numerically analyzed when the plasmonic material with low loss and high loss respectively is considered. We demonstrate by full-wave simulation that the optimized nonmagnetic gain-loss design can greatly compensate the decreased cloaking effect caused by loss material, which facilitates the realization of practical electromagnetic cloaking, especially in the optical range.

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Electromagnetic backscattering from one-dimensional drifting fractal sea surface II:Electromagnetic backscattering model

谢涛, William Perrie, 赵尚卓, 方贺, 于文金, 何宜军

2016 (7): 74102-074102. doi: 10.1088/1674-1056/25/7/074102

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Sea surface current has a significant influence on electromagnetic (EM) backscattering signals and may constitute a dominant synthetic aperture radar (SAR) imaging mechanism. An effective EM backscattering model for a one-dimensional drifting fractal sea surface is presented in this paper. This model is used to simulate EM backscattering signals from the drifting sea surface. Numerical results show that ocean currents have a significant influence on EM backscattering signals from the sea surface. The normalized radar cross section (NRCS) discrepancies between the model for a coupled wave-current fractal sea surface and the model for an uncoupled fractal sea surface increase with the increase of incidence angle, as well as with increasing ocean currents. Ocean currents that are parallel to the direction of the wave can weaken the EM backscattering signal intensity, while the EM backscattering signal is intensified by ocean currents propagating oppositely to the wave direction. The model presented in this paper can be used to study the SAR imaging mechanism for a drifting sea surface.

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A method for generating double-ring-shaped vector beams

陈欢, 凌晓辉, 陈知红, 李钱光, 吕昊, 余华清, 易煦农

2016 (7): 74201-074201. doi: 10.1088/1674-1056/25/7/074201

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We propose a method for generating double-ring-shaped vector beams. A step phase introduced by a spatial light modulator (SLM) first makes the incident laser beam have a nodal cycle. This phase is dynamic in nature because it depends on the optical length. Then a Pancharatnam-Berry phase (PBP) optical element is used to manipulate the local polarization of the optical field by modulating the geometric phase. The experimental results show that this scheme can effectively create double-ring-shaped vector beams. It provides much greater flexibility to manipulate the phase and polarization by simultaneously modulating the dynamic and the geometric phases.

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Entanglement of movable mirror and cavity field enhanced by an optical parametric amplifier

张彩云, 李虎, 潘桂侠, 圣宗强

2016 (7): 74202-074202. doi: 10.1088/1674-1056/25/7/074202

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A scheme to generate entanglement in a cavity optomechanical system filled with an optical parametric amplifier is proposed. With the help of the optical parametric amplifier, the stationary macroscopic entanglement between the movable mirror and the cavity field can be notably enhanced, and the entanglement increases when the parametric gain increases. Moreover, for a given parametric gain, the degree of entanglement of the cavity optomechanical system increases with increasing input laser power.

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Photon statistics of pulse-pumped four-wave mixing in fiber with weak signal injection

刘楠楠, 刘宇宏, 李嘉敏, 李小英

2016 (7): 74203-074203. doi: 10.1088/1674-1056/25/7/074203

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We study the photon statistics of pulse-pumped four-wave mixing in fibers with weak coherent signal injection by measuring the intensity correlation functions of individual signal and idler fields. The experimental results show that the intensity correlation function of individual signal (idler) field g_s(i)⁽²⁾ decreases with the intensity of signal injection. After applying narrow band filter in signal (idler) band, the value of g_s(i)⁽²⁾ decreases from 1.9±0.02 (1.9±0.02) to 1.03±0.02 (1.05±0.02) when the intensity of signal injection varies from 0 to 120 photons/pulse. The results indicate that the photon statistics changes from Bose-Einstein distribution to Poisson distribution. We calculate the intensity correlation functions by using the multi-mode theory of four-wave mixing in fibers. The theoretical curves well fit the experimental results. Our investigation will be useful for mitigating the crosstalk between quantum and classical channels in a dense wavelength division multiplexing network.

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Enhanced Kerr nonlinearity in a quantized four-level graphene nanostructure

Ghahraman Solookinejad, M Panahi, E Ahmadi, Seyyed Hossein Asadpour

2016 (7): 74204-074204. doi: 10.1088/1674-1056/25/7/074204

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In this paper, a new model is proposed for manipulating the Kerr nonlinearity of right-hand circular probe light in a monolayer of graphene nanostructure. By using the density matrix equations and quantum optical approach, the third-order susceptibility of probe light is explored numerically. It is realized that the enhanced Kerr nonlinearity with zero linear absorption can be provided by selecting the appropriate quantities of controllable parameters, such as Rabi frequency and elliptical parameter of elliptical polarized coupling field. Our results may be useful applications in future all-optical system devices in nanostructures.

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Generation of few-cycle laser pulses: Comparison between atomic and molecular gases in a hollow-core fiber

黄志远, 戴晔, 赵睿睿, 王丁, 冷雨欣

2016 (7): 74205-074205. doi: 10.1088/1674-1056/25/7/074205

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We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber. From the perspective of self-phase modulation (SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation (MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.

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Broadband tunable Raman soliton self-frequency shift to mid-infrared band in a highly birefringent microstructure fiber

王伟, 毕新英, 王珺琪, 屈玉玮, 韩颖, 周桂耀, 齐跃峰

2016 (7): 74206-074206. doi: 10.1088/1674-1056/25/7/074206

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Raman soliton self-frequency shifted to mid-infrared band (λ> 2 μ) has been achieved in an air-silica microstructure fiber (MF). The MF used in our experiment has an elliptical core with diameters of 1.08 and 2.48 μ for fast and slow axis. Numerical simulation shows that each fundamental orthogonal polarization mode has two wide-spaced λ_ZDW and the λ_ZDW pairs located at 701/2110 nm and 755/2498 nm along the fast and slow axis, respectively. Using 810-nm Ti:sapphire femtosecond laser as pump, when the output power varies from 0.3 to 0.5 W, the furthest red-shift Raman solitons in both fast and slow axis shift from near-infrared band to mid-infrared band, reaching as far as 2030 and 2261 nm. Also, mid-infrared Raman solitons can always be generated for pump wavelength longer than 790 nm if output pump power reaches 0.5 W. Specifically, with pump power at 0.5 W, the mid-infrared soliton in slow axis shifts from 2001 to 2261 nm when the pump changes from 790 nm to 810 nm. This means only a 20 nm change of pump results in 260 nm tunability of a mid-infrared soliton.

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Ultra-low power anti-crosstalk collision avoidance light detection and ranging using chaotic pulse position modulation approach

郝杰, 巩马理, 杜鹏飞, 卢宝杰, 张帆, 张海涛, 付星

2016 (7): 74207-074207. doi: 10.1088/1674-1056/25/7/074207

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A novel concept of collision avoidance single-photon light detection and ranging (LIDAR) for vehicles has been demonstrated, in which chaotic pulse position modulation is applied on the transmitted laser pulses for robust anti-crosstalk purposes. Besides, single-photon detectors (SPD) and time correlated single photon counting techniques are adapted, to sense the ultra-low power used for the consideration of compact structure and eye safety. Parameters including pulse rate, discrimination threshold, and number of accumulated pulses have been thoroughly analyzed based on the detection requirements, resulting in specified receiver operating characteristics curves. Both simulation and indoor experiments were performed to verify the excellent anti-crosstalk capability of the presented collision avoidance LIDAR despite ultra-low transmitting power.

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Generation of entangled TEM₀₁ modes withperiodically poled KTiOPO₄ crystal

杨荣国, 王晶静, 张静, 孙恒信

2016 (7): 74208-074208. doi: 10.1088/1674-1056/25/7/074208

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Spatial quantum optics based on the high-order transverse mode is important for the super-resolution measurement and quantum image beyond the shot noise level. Quantum entanglement of the transverse plane Hermite-Gauss TEM₀₁ mode has been demonstrated experimentally in this paper. Two squeezed TEM₀₁ modes, which are generated by a pair of degenerate optical parametric amplifiers (DOPA) with the nonlinear crystals of periodically poled KTiOPO₄, have been combined to produce TEM₀₁ mode entanglement using a beam splitter. The 1.5 dB for the sum of amplitude and 1.2 dB for the difference of phase below shot-noise level is achieved with the measurement system of a Bell state detection.

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Three-dimensional flow of Powell-Eyring nanofluid with heat and mass flux boundary conditions

Tasawar Hayat, Ikram Ullah, Taseer Muhammad, Ahmed Alsaedi, Sabir Ali Shehzad

2016 (7): 74701-074701. doi: 10.1088/1674-1056/25/7/074701

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This article investigates the three-dimensional flow of Powell-Eyring nanofluid with thermophoresis and Brownian motion effects. The energy equation is considered in the presence of thermal radiation. The heat and mass flux conditions are taken into account. Mathematical formulation is carried out through the boundary layer approach. The governing partial differential equations are transformed into the nonlinear ordinary differential equations through suitable variables. The resulting nonlinear ordinary differential equations have been solved for the series solutions. Effects of emerging physical parameters on the temperature and nanoparticles concentration are plotted and discussed. Numerical values of local Nusselt and Sherwood numbers are computed and examined.

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Structure and switching of single-stranded DNA tethered to a charged nanoparticle surface

赵新军, 高志福

2016 (7): 74702-074702. doi: 10.1088/1674-1056/25/7/074702

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Using a molecular theory, we investigate the temperature-dependent self-assembly of single-stranded DNA (ssDNA) tethered to a charged nanoparticle surface. Here the size, conformations, and charge properties of ssDNA are taken into account. The main results are as follows: i) when the temperature is lower than the critical switching temperature, the ssDNA will collapse due to the existence of electrostatic interaction between ssDNA and charged nanoparticle surface; ii) for the short ssDNA chains with the number of bases less than 10, the switching of ssDNA cannot happen, and the critical temperature does not exist; iii) when the temperature increases, the electrostatic attractive interaction between ssDNA and charged nanoparticle surface becomes weak dramatically, and ssDNA chains will stretch if the electrostatic attractive interaction is insufficient to overcome the elastic energy of ssDNA and the electrostatic repulsion energy. These findings accord well with the experimental observations. It is predicted that the switching of ssDNA will not happen if the grafting densities are too high.

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Role of hydrogen bonding in solubility of poly(N-isopropylacrylamide) brushes in sodium halide solutions

赵新军, 高志福

2016 (7): 74703-074703. doi: 10.1088/1674-1056/25/7/074703

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By employing molecular theory, we systematically investigate the shift of solubility of poly(N-isopropylacrylamide) (PNIPAM) brushes in sodium halide solutions. After considering PNIPAM-water hydrogen bonds, water-anion hydrogen bonds, and PNIPAM-anion bonds and their explicit coupling to the PNIPAM conformations, we find that increasing temperature lowers the solubility of PNIPAM, and results in a collapse of the layer at high enough temperatures. The combination of the three types of bonds would yield a decrease in the solubility of PNIPAM following the Hofmeister series: NaCl >NaBr >NaI. PNIPAM-water hydrogen bonds are affected by water-anion hydrogen bonds and PNIPAM-anion bonds. The coupling of polymer conformations and the competition among the three types of bonds are essential for describing correctly a decrease in the solubility of PNIPAM brushes, which is determined by the free energy associated with the formation of the three types of bonds. Our results agree well with the experimental observations, and would be very important for understanding the shift of the lower critical solution temperature of PNIPAM brushes following the Hofmeister series.

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Segregation behavior of magnetic ions in continuous flowing solution under gradient magnetic field

冀冰, 吴平, 任菡, 张师平, Abdul Rehman, 王立

2016 (7): 74704-074704. doi: 10.1088/1674-1056/25/7/074704

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The research of magnetic separation starts from magnetic solid particles to nanoparticles, and in the research progress, particles become smaller gradually with the development of application of magnetic separation technology. Nevertheless, little experimental study of magnetic separation of molecules and ions under continuous flowing conditions has been reported. In this work, we designed a magnetic device and a “layered” flow channel to study the magnetic separation at the ionic level in continuous flowing solution. A segregation model was built to discuss the segregation behavior as well as the factors that may affect the separation. The magnetic force was proved to be the driving force which plays an indispensable role leading to the segregation and separation. The flow velocity has an effect on the segregation behavior of magnetic ions, which determines the separation result. On the other hand, the optimum flow velocity which makes maximum separation is related to the initial concentration of solution.

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Recrystallization of freezable bound water in aqueous solutions of medium concentrations

赵立山, 潘礼庆, 纪爱玲, 曹则贤, 王强

2016 (7): 75101-075101. doi: 10.1088/1674-1056/25/7/075101

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For aqueous solutions with freezable bound water, vitrification and recrystallization are mingled, which brings difficulty to application and misleads the interpretation of relevant experiments. Here, we report a quantification scheme for the freezable bound water based on the water-content dependence of glass transition temperature, by which also the concentration range for the solutions that may undergo recrystallization finds a clear definition. Furthermore, we find that depending on the amount of the freezable bound water, different temperature protocols should be devised to achieve a complete recrystallization. Our results may be helpful for understanding the dynamics of supercooled aqueous solutions and for improving their manipulation in various industries.

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Structural and mechanical properties of Al-C-N films deposited at room temperature by plasma focus device

Z A Umar, R Ahmad, R S Rawat, M A Baig, J Siddiqui, T Hussain

2016 (7): 75201-075201. doi: 10.1088/1674-1056/25/7/075201

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The Al-C-N films are deposited on Si substrates by using a dense plasma focus (DPF) device with aluminum fitted central electrode (anode) and by operating the device with CH₄/N₂ gas admixture ratio of 1:1. XRD results verify the crystalline AlN (111) and Al₃CON (110) phase formation of the films deposited using multiple shots. The elemental compositions as well as chemical states of the deposited Al-C-N films are studied using XPS analysis, which affirm Al-N, C-C, and C-N bonding. The FESEM analysis reveals that the deposited films are composed of nanoparticles and nanoparticle agglomerates. The size of the agglomerates increases at a higher number of focus deposition shots for multiple shot depositions. Nanoindentation results reveal the variation in mechanical properties (nanohardness and elastic modulus) of Al-C-N films deposited with multiple shots. The highest values of nanohardness and elastic modulus are found to be about 11 and 185 GPa, respectively, for the film deposited with 30 focus deposition shots. The mechanical properties of the films deposited using multiple shots are related to the Al content and C-N bonding.

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Preparation and structural properties of thin carbon films by very-high-frequency magnetron sputtering

高明伟, 叶超, 王响英, 何一松, 郭佳敏, 杨培芳

2016 (7): 75202-075202. doi: 10.1088/1674-1056/25/7/075202

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Growth and structural properties of thin a-C films prepared by the 60 MHz very-high-frequency (VHF) magnetron sputtering were investigated. The energy and flux of ions impinging the substrate were also analyzed. It is found that the thin a-C films prepared by the 60 MHz sputtering have a lower growth rate, a smooth surface, and more sp³ contents. These features are related to the higher ion energy and the lower ions flux onto the substrate. Therefore, the 60 MHz VHF sputtering is more suitable for the preparation of thin a-C film with more sp³ contents.

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Behaviors of Zn₂GeO₄ under high pressure and high temperature

杨淑雯, 彭放, 李文涛, 胡启威, 晏小智, 雷力, 李晓东, 贺端威

2016 (7): 76101-076101. doi: 10.1088/1674-1056/25/7/076101

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The structural stability of Zn₂GeO₄ was investigated by in-situ synchrotron radiation angle dispersive x-ray diffraction. The pressure-induced amorphization is observed up to 10 GPa at room temperature. The high-pressure and high-temperature sintering experiments and the Raman spectrum measurement firstly were performed to suggest that the amorphization is caused by insufficient thermal energy and tilting Zn-O-Ge and Ge-O-Ge bond angles with increasing pressure, respectively. The calculated bulk modulus of Zn₂GeO₄ is 117.8 GPa from the pressure-volume data. In general, insights into the mechanical behavior and structure evolution of Zn₂GeO₄ will shed light on the micro-mechanism of the materials variation under high pressure and high temperature.

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Impurity effects on electrical conductivity of doped bilayer graphene in the presence of a bias voltage

E Lotfi, H Rezania, B Arghavaninia, M Yarmohammadi

2016 (7): 76102-076102. doi: 10.1088/1674-1056/25/7/076102

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We address the electrical conductivity of bilayer graphene as a function of temperature, impurity concentration, and scattering strength in the presence of a finite bias voltage at finite doping, beginning with a description of the tight-binding model using the linear response theory and Green's function approach. Our results show a linear behavior at high doping for the case of high bias voltage. The effects of electron doping on the electrical conductivity have been studied via changing the electronic chemical potential. We also discuss and analyze how the bias voltage affects the temperature behavior of the electrical conductivity. Finally, we study the behavior of the electrical conductivity as a function of the impurity concentration and scattering strength for different bias voltages and chemical potentials respectively. The electrical conductivity is found to be monotonically decreasing with impurity scattering strength due to the increased scattering among electrons at higher impurity scattering strength.

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Theoretical design of diamondlike superhard structures at high pressure

李全, 郑伟涛

2016 (7): 76103-076103. doi: 10.1088/1674-1056/25/7/076103

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Diamond, as the hardest known material, has been widely used in industrial applications as abrasives, coatings, and cutting and polishing tools, but it is restricted by several shortcomings, e.g., its low thermal and chemical stability. Considerable efforts have been devoted to designing or synthesizing the diamond-like B-C-N-O compounds, which exhibit excellent mechanical property. In this paper, we review the recent theoretical design of diamond-like superhard structures at high pressure. In particular, the recently designed high symmetric phase of low-energy cubic BC₃ meets the experimental observation, and clarifies the actual existence of cubic symmetric phase for the compounds formed by B-C-N-O system, besides the classical example of cubic boron nitride.

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Theoretical calculations of structural, electronic, and elastic properties of CdSe_1-xTe_x: A first principles study

M Shakil, Muhammad Zafar, Shabbir Ahmed, Muhammad Raza-ur-rehman Hashmi, M A Choudhary, T Iqbal

2016 (7): 76104-076104. doi: 10.1088/1674-1056/25/7/076104

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The plane wave pseudo-potential method was used to investigate the structural, electronic, and elastic properties of CdSe_1-xTe_x in the zinc blende phase. It is observed that the electronic properties are improved considerably by using LDA+U as compared to the LDA approach. The calculated lattice constants and bulk moduli are also comparable to the experimental results. The cohesive energies for pure CdSe and CdTe binary and their mixed alloys are calculated. The second-order elastic constants are also calculated by the Lagrangian theory of elasticity. The elastic properties show that the studied material has a ductile nature.

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Influence of nitrogen and magnesium doping on the properties of ZnO films

李东华, 王惠琼, 周华, 李亚平, 黄政, 郑金成, 王嘉鸥, 钱海杰, 奎热西, 陈晓航, 詹华瀚, 周颖慧, 康俊勇

2016 (7): 76105-076105. doi: 10.1088/1674-1056/25/7/076105

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Undoped ZnO and doped ZnO films were deposited on the MgO(111) substrates using oxygen plasma-assisted molecular beam expitaxy. The orientations of the grown ZnO thin film were investigated by in situ reflection high-energy electron diffraction and ex situ x-ray diffraction (XRD). The film roughness was measured by atomic force microscopy, which was correlated with the grain sizes determined by XRD. Synchrotron-based x-ray absorption spectroscopy was performed to study the doping effect on the electronic properties of the ZnO films, compared with density functional theory calculations. It is found that, nitrogen doping would hinder the growth of thin film, and generate the N_O defect, while magnesium doping promotes the quality of nitrogen-doped ZnO films, inhibiting (N₂)_O production and increasing nitrogen content.

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High pressure x-ray diffraction techniques with synchrotron radiation

刘景

2016 (7): 76106-076106. doi: 10.1088/1674-1056/25/7/076106

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This article summarizes the developments of experimental techniques for high pressure x-ray diffraction (XRD) in diamond anvil cells (DACs) using synchrotron radiation. Basic principles and experimental methods for various diffraction geometry are described, including powder diffraction, single crystal diffraction, radial diffraction, as well as coupling with laser heating system. Resolution in d-spacing of different diffraction modes is discussed. More recent progress, such as extended application of single crystal diffraction for measurements of multigrain and electron density distribution, time-resolved diffraction with dynamic DAC and development of modulated heating techniques are briefly introduced. The current status of the high pressure beamline at BSRF (Beijing Synchrotron Radiation Facility) and some results are also presented.

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High pressure structural phase transitions of TiO₂ nanomaterials

李全军, 刘冰冰

2016 (7): 76107-076107. doi: 10.1088/1674-1056/25/7/076107

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Recently, the high pressure study on the TiO₂ nanomaterials has attracted considerable attention due to the typical crystal structure and the fascinating properties of TiO₂ with nanoscale sizes. In this paper, we briefly review the recent progress in the high pressure phase transitions of TiO₂ nanomaterials. We discuss the size effects and morphology effects on the high pressure phase transitions of TiO₂ nanomaterials with different particle sizes, morphologies, and microstructures. Several typical pressure-induced structural phase transitions in TiO₂ nanomaterials are presented, including size-dependent phase transition selectivity in nanoparticles, morphology-tuned phase transition in nanowires, nanosheets, and nanoporous materials, and pressure-induced amorphization (PIA) and polyamorphism in ultrafine nanoparticles and TiO₂-B nanoribbons. Various TiO₂ nanostructural materials with high pressure structures are prepared successfully by high pressure treatment of the corresponding crystal nanomaterials, such as amorphous TiO₂ nanoribbons, α -PbO₂-type TiO₂ nanowires, nanosheets, and nanoporous materials. These studies suggest that the high pressure phase transitions of TiO₂ nanomaterials depend on the nanosize, morphology, interface energy, and microstructure. The diversity of high pressure behaviors of TiO₂ nanomaterials provides a new insight into the properties of nanomaterials, and paves a way for preparing new nanomaterials with novel high pressure structures and properties for various applications.

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How to detect melting in laser heating diamond anvil cell

杨留响

2016 (7): 76201-076201. doi: 10.1088/1674-1056/25/7/076201

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Research on the melting phenomenon is the most challenging work in the high pressure/temperature field. Until now, large discrepancies still exist in the melting curve of iron, the most interesting and extensively studied element in geoscience research. Here we present a summary about techniques detecting melting in the laser heating diamond anvil cell.

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Unreacted equation of states of typical energetic materials under static compression: A review

郑朝阳, 赵纪军

2016 (7): 76202-076202. doi: 10.1088/1674-1056/25/7/076202

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The unreacted equation of state (EOS) of energetic materials is an important thermodynamic relationship to characterize their high pressure behaviors and has practical importance. The previous experimental and theoretical works on the equation of state of several energetic materials including nitromethane, 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), hexanitrostilbene (HNS), hexanitrohexaazaisowurtzitane (HNIW or CL-20), pentaerythritol tetranitrate (PETN), 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), triamino-trinitrobenzene (TATB), 1,1-diamino-2,2-dinitroethene (DADNE or FOX-7), and trinitrotoluene (TNT) are reviewed in this paper. The EOS determined from hydrostatic and non-hydrostatic compressions are discussed and compared. The theoretical results based on ab initio calculations are summarized and compared with the experimental data.

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Photo-induced athermal phase transitions of HgX (X= S, Se, Te) by ab initio study

任达华, 程新路, 张红

2016 (7): 76401-076401. doi: 10.1088/1674-1056/25/7/076401

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Ab initio calculations of lattice constants, lattice stabilities of HgX (X=S, Se, Te) at different electronic temperatures (T_e) have been performed within the density functional theory (DFT). We find that the lattice constants of HgX increase and the phonon frequencies reduce as T_e increases. Especially the transverse-acoustic (TA) phonon frequencies of HgX gradually become negative with the elevation of the electron temperature. That is to say ultrafast intense laser induces lattice instabilities of HgX and athermal melting appears for the increase of laser intensity. What is more, with the X atom number increasing, the critical electronic temperatures of HgX are decreased in sequence. This result would be helpful for understanding the athermal melting processes for femtosecond laser micromachining.

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Molecular dynamics simulation of nanoscale surface diffusion of heterogeneous adatoms clusters

Muhammad Imran, Fayyaz Hussain, Muhammad Rashid, Muhammad Ismail, Hafeez Ullah, Yongqing Cai, M Arshad Javid, Ejaz Ahmad, S A Ahmad

2016 (7): 76601-076601. doi: 10.1088/1674-1056/25/7/076601

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Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters (hexamer and above), the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster, vacancy formation, filling, and shifting can be observed from the results.

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Diffusion behavior of helium in titanium and the effect of grain boundaries revealed by molecular dynamics simulation

程贵钧, 付宝勤, 侯氢, 周晓松, 汪俊

2016 (7): 76602-076602. doi: 10.1088/1674-1056/25/7/076602

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The microstructures of titanium (Ti), an attractive tritium (T) storage material, will affect the evolution process of the retained helium (He). Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics (MD) simulation for the bulk hcp-Ti system and the system with grain boundary (GB). It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic (the diffusion coefficient of the [0001] direction is higher than that of the basal plane), as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary (TB) can serve as an effective trapped region for He. The TB accelerates diffusion of He in the direction perpendicular to the twinning direction (TD), while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region.

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Skin formation in drying a film of soft matter solutions: Application of solute based Lagrangian scheme

罗凌, 孟凡龙, 张俊英,

2016 (7): 76801-076801. doi: 10.1088/1674-1056/25/7/076801

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When a film of soft matter solutions is being dried, a skin layer often forms at its surface, which is a gel-like elastic phase made of concentrated soft matter solutions. We study the dynamics of this process by using the solute based Lagrangian scheme which was proposed by us recently. In this scheme, the process of the gelation (i.e., the change from sol to gel) can be naturally incorporated in the diffusion equation. Effects of the elasticity of the skin phase, the evaporation rate of the solvents, and the initial concentration of the solutions are discussed. Moreover, the condition for the skin formation is provided.

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In-plane anisotropy in two-dimensional electron gas at LaAlO₃/SrTiO₃(110) interface

沈胜春, 洪彦鹏, 厉承剑, 薛红霞, 王欣欣, 聂家财

2016 (7): 76802-076802. doi: 10.1088/1674-1056/25/7/076802

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A systematic study of the two-dimensional electron gas at LaAlO₃/SrTiO₃(110) interface reveals an anisotropy along two specific directions, [001] and [111]. The anisotropy becomes distinct for the interface prepared under high oxygen pressure with low carrier density. Angular dependence of magnetoresistance shows that the electron confinement is stronger along the [111] direction. Gate-tunable magnetoresistance reveals a clear in-plane anisotropy of the spin-orbit coupling, and the spin relaxation mechanism along both directions belongs to D'yakonov-Perel' (DP) scenario. Moreover, in-plane anisotropic superconductivity is observed for the sample with high carrier density, the superconducting transition temperature is lower but the upper critical field is higher along the [111] direction. This in-plane anisotropy could be ascribed to the anisotropic band structure along the two crystallographic directions.

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Electronic structure, Dirac points and Fermi arc surface states in three-dimensional Dirac semimetal Na₃Bi from angle-resolved photoemission spectroscopy

梁爱基, 陈朝宇, 王志俊, 石友国, 冯娅, 伊合绵, 谢卓晋, 何少龙, 何俊峰, 彭莹莹, 刘艳, 刘德发, 胡成, 赵林, 刘国东, 董晓莉, 张君, M Nakatake, H Iwasawa, K Shimada, M Arita, H Namatame, M Taniguchi, 许祖彦, 陈创天, 翁红明, 戴希, 方忠, 周兴江

2016 (7): 77101-077101. doi: 10.1088/1674-1056/25/7/077101

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The three-dimensional (3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A₃Bi (A = Na, K, Rb). Here we report comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on the two cleaved surfaces, (001) and (100), of Na₃Bi. On the (001) surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the k_x-k_y plane and by varying the photon energy to get access to different out-of-plane k_zs. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the (100) surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the (100) plane. We directly observe two isolated 3D Dirac nodes on the (100) surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ～150 meV before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na₃Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the 3D Dirac cones, on the possible formation of surface reconstruction of the (001) surface, and on the issue of basic Brillouin zone selection for the (100) surface.

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Improving breakdown voltage performance of SOI power device with folded drift region

李琦, 李海鸥, 黄平奖, 肖功利, 杨年炯

2016 (7): 77201-077201. doi: 10.1088/1674-1056/25/7/077201

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A novel silicon-on-insulator (SOI) high breakdown voltage (BV) power device with interlaced dielectric trenches (IDT) and N/P pillars is proposed. In the studied structure, the drift region is folded by IDT embedded in the active layer, which results in an increase of length of ionization integral remarkably. The crowding phenomenon of electric field in the corner of IDT is relieved by the N/P pillars. Both traits improve two key factors of BV, the ionization integral length and electric field magnitude, and thus BV is significantly enhanced. The electric field in the dielectric layer is enhanced and a major portion of bias is borne by the oxide layer due to the accumulation of inverse charges (holes) at the corner of IDT. The average value of the lateral electric field of the proposed device reaches 60 V/μm with a 10 μm drift length, which increases by 200% in comparison to the conventional SOI LDMOS, resulting in a breakdown voltage of 607 V.

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Compact surface plasmon amplifier in nonlinear hybrid waveguide

王曙曙, 王丹青, 胡小鹏, 李涛, 祝世宁

2016 (7): 77301-077301. doi: 10.1088/1674-1056/25/7/077301

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Surface plasmon polariton (SPP), a sub-wavelength surface wave promising for photonic integration, always suffers from the large metallic loss that seriously restricts its practical application. Here, we propose a compact SPP amplifier based on a nonlinear hybrid waveguide (a combination of silver, LiNbO₃, and SiO₂), where a couple of Bragg gratings are introduced in the waveguide to construct a cavity. This special waveguide is demonstrated to support a highly localized SPP-like hybrid mode and a low loss waveguide-like hybrid mode. To provide a large nonlinear gain, a pumping wave input from the LiNbO₃ waveguide is designed to resonate inside the cavity and satisfy the cavity phase matching to fulfill the optical parametric amplification (OPA) of the SPP signal. Proper periods of gratings and the cavity length are chosen to satisfy the impedance matching condition to ensure the high input efficiency of the pump wave from the outside into the cavity. In theoretical calculations, this device demonstrates a high performance in a very compact scheme (～ 3.32 μm) and a much lower pumping power for OPA compared with single-pass pumping. To obtain a comprehensive insight into this cavity OPA, the influences of the pumping power, cavity length, and the initial phase are discussed in detail.

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Strain-induced insulator-metal transition in ferroelectric BaTiO₃ (001) surface: First-principles study

杨林, 王长安, 刘聪, 秦明辉, 陆旭兵, 高兴森, 曾敏, 刘俊明

2016 (7): 77302-077302. doi: 10.1088/1674-1056/25/7/077302

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The electronic properties of TiO₂-terminated BaTiO₃ (001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator-metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO₃ (001) surface.

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Effect of disorders on topological phases inone-dimensional optical superlattices

王志宙, 吴一东, 杜会静, 井西利

2016 (7): 77303-077303. doi: 10.1088/1674-1056/25/7/077303

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In a recent paper, Lang et al. proposed that edge states and topological phases can be observed in one-dimensional optical superlattices. They showed that the topological phases can be revealed by observing the density profile of a trapped fermion system, which displays plateaus with their positions. However, disorders are not considered in their model. To study the effect of disorders on the topological phases, we introduce random potentials to the model for optical superlattcies. Our calculations show that edge states are robust against the disorders. We find the edge states are very sensitive to the number of the sites in the optical superlattice and we propose a simple rule to describe the relationship between the edge states and the number of sites. The density plateaus are also robust against weak disorders provided that the average density is calculated over a long interval. The widths of the plateaus are proportional to the widths of the bulk energy gaps when there are disorders. The disorders can diminish the bulk energy gaps. So the widths of the plateaus decrease with the increase of disorders and the density plateaus disappear when disorders are too strong. The results in our paper can be used to guide the experimental detection of topological phases in one-dimensional systems.

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Spin resonance transport properties of a single Au atom in S-Au-S junction and Au-Au-Au junction

王方原, 李桂琴

2016 (7): 77304-077304. doi: 10.1088/1674-1056/25/7/077304

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The spin transport properties of S-Au-S junction and Au-Au-Au junction between Au nanowires are investigated with density functional theory and the non-equilibrium Green's function. We mainly focus on the spin resonance transport properties of the center Au atom. The breaking of chemical bonds between anchor atoms and center Au atom significantly influences their spin transmission characteristics. We find the 0.8 eV orbital energy shift between anchor S atoms and the center Au atom can well protect the spin state stored in the S-Au-S junction and efficiently extract its spin state to the current by spin resonance mechanism, while the spin interaction of itinerant electrons and the valence electron of the center Au atom in the Au-Au-Au junction can extract the current spin information into the center Au atom. Fermi energy drift and bias-dependent spin filtering properties of the Au-Au-Au junction may transform information between distance, bias, and electron spin. Those unique properties make them potential candidates for a logical nanocircuit.

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Topological phase in one-dimensional Rashba wire

汪萨克, 汪军, 刘军丰

2016 (7): 77305-077305. doi: 10.1088/1674-1056/25/7/077305

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We study the possible topological phase in a one-dimensional (1D) quantum wire with an oscillating Rashba spin-orbital coupling in real space. It is shown that there are a pair of particle-hole symmetric gaps forming in the bulk energy band and fractional boundary states residing in the gap when the system has an inversion symmetry. These states are topologically nontrivial and can be characterized by a quantized Berry phase ±π or nonzero Chern number through dimensional extension. When the Rashba spin-orbital coupling varies slowly with time, the system can pump out 2 charges in a pumping cycle because of the spin flip effect. This quantized pumping is protected by topology and is robust against moderate disorders as long as the disorder strength does not exceed the opened energy gap.

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Anisotropic transport properties in the phase-separated La_0.67Ca_0.33MnO₃/NdGaO₃ (001) films

张洪瑞, 刘渊博, 王拴虎, 洪德顺, 吴文彬, 孙继荣

2016 (7): 77306-077306. doi: 10.1088/1674-1056/25/7/077306

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The anisotropic transport property was investigated in a phase separation La_0.67Ca_0.33MnO₃ (LCMO) film grown on (001)-oriented NdGaO₃ (NGO) substrate. It was found that the resistivity along the b-axis is much higher than that along the a-axis. Two resistivity peaks were observed in the temperature dependent measurement along the b-axis, one located at 91 K and the other centered at 165 K. Moreover, we also studied the response of the resistivities along the two axes to various electric currents, magnetic fields, and light illuminations. The resistivities along the two axes are sensitive to the magnetic field. However, the electric current and light illumination can influence the resistivity along the b-axis obviously, but have little effect on the resistivity along the a-axis. Based on these results, we believe that an anisotropic-strain-controlled MnO₆ octahedra shear-mode deformation may provide a mechanism of conduction filaments paths along the a-axis, which leads to the anisotropic transport property.

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Effect of thermal deformation on giant magnetoresistance of flexible spin valves grown on polyvinylidene fluoride membranes

刘鲁萍, 詹清峰, 荣欣, 杨华礼, 谢亚丽, 谭晓华, 李润伟

2016 (7): 77307-077307. doi: 10.1088/1674-1056/25/7/077307

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We fabricated flexible spin valves on polyvinylidene fluoride (PVDF) membranes and investigated the influence of thermal deformation of substrates on the giant magnetoresistance (GMR) behaviors. The large magnetostrictive Fe₈₁Ga₁₉ (FeGa) alloy and the low magnetostrictive Fe₁₉Ni₈₁ (FeNi) alloy were selected as the free and pinned ferromagnetic layers. In addition, the exchange bias (EB) of the pinned layer was set along the different thermal deformation axes α₃₁ or α₃₂ of PVDF. The GMR ratio of the reference spin valves grown on Si intrinsically increases with lowering temperature due to an enhancement of spontaneous magnetization. For flexible spin valves, when decreasing temperature, the anisotropic thermal deformation of PVDF produces a uniaxial anisotropy along the α₃₂ direction, which changes the distribution of magnetic domains. As a result, the GMR ratio at low temperature for spin valves with EB||α₃₂ becomes close to that on Si, but for spin valves with EB||α₃₁ is far away from that on Si. This thermal effect on GMR behaviors is more significant when using magnetostrictive FeGa as the free layer.

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High-pressure studies on heavy fermion systems

陈晔, 翁宗法, Smidman Michael, 路欣, 袁辉球

2016 (7): 77401-077401. doi: 10.1088/1674-1056/25/7/077401

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In this review article, we give a brief overview of heavy fermions, which are prototype examples of strongly correlated electron systems. We introduce the application of physical pressure in heavy fermion systems to construct their pressure phase diagrams and to study the close relationship between superconductivity (SC) and other electronic instabilities, such as antiferromagnetism (AFM), ferromagnetism (FM), and valence transitions. Field-angle dependent heat capacity and point-contact spectroscopic measurements under pressure are taken as examples to illustrate their ability to investigate novel physical properties of the emergent electronic states.

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Hybrid crystals of cuprates and iron-based superconductors

代霞, 勒聪聪, 吴贤新, 胡江平

2016 (7): 77402-077402. doi: 10.1088/1674-1056/25/7/077402

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We propose two possible new compounds, Ba₂CuO₂Fe₂As₂ and K₂CuO₂Fe₂Se₂, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO₂ layers and antifluorite-type Fe₂X₂ (X=As, Se) layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high T_c superconductivity.

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Multiband nodeless superconductivity near the charge-density-wave quantum critical point in ZrTe_3-xSe_x

崔珊, 何兰坡, 洪晓晨, 朱相德, Cedomir Petrovic, 李世燕

2016 (7): 77403-077403. doi: 10.1088/1674-1056/25/7/077403

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It was found that selenium doping can suppress the charge-density-wave (CDW) order and induce bulk superconductivity in ZrTe₃. The observed superconducting dome suggests the existence of a CDW quantum critical point (QCP) in ZrTe_3-xSe_x near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe_3-xSe_x single crystals (x = 0.044 and 0.051) down to 80 mK. For both samples, the residual linear term κ₀/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ₀/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe_3-xSe_x, which indicates conventional superconductivity despite of the existence of a CDW QCP.

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Synthesis of large FeSe superconductor crystals via ion release/introduction and property characterization

苑冬娜, 黄裕龙, 倪顺利, 周花雪, 毛义元, 胡卫, 袁洁, 金魁, 张广铭, 董晓莉, 周放

2016 (7): 77404-077404. doi: 10.1088/1674-1056/25/7/077404

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Large superconducting FeSe crystals of (001) orientation have been prepared via a hydrothermal ion release/introduction route for the first time. The hydrothermally derived FeSe crystals are up to 10 mm×5 mm×0.3 mm in dimension. The pure tetragonal FeSe phase has been confirmed by x-ray diffraction (XRD) and the composition determined by both inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive x-ray spectroscopy (EDX). The superconducting transition of the FeSe samples has been characterized by magnetic and transport measurements. The zero-temperature upper critical field H_c2 is calculated to be 13.2-16.7 T from a two-band model. The normal-state cooperative paramagnetism is found to be predominated by strong spin frustrations below the characteristic temperature T_sn, where the Ising spin nematicity has been discerned in the FeSe superconductor crystals as reported elsewhere.

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Interplay of Rashba effect and spin Hall effect in perpendicular Pt/Co/MgO magnetic multilayers

赵云驰, 杨光, 董博闻, 王守国, 王超, 孙阳, 张静言, 于广华

2016 (7): 77501-077501. doi: 10.1088/1674-1056/25/7/077501

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The interplay of the Rashba effect and the spin Hall effect originating from current induced spin-orbit coupling was investigated in the as-deposited and annealed Pt/Co/MgO stacks with perpendicular magnetic anisotropy. The above two effects were analyzed based on Hall measurements under external magnetic fields longitudinal and vertical to dc current, respectively. The coercive field as a function of dc current in vertical mode with only the Rashba effect involved decreases due to thermal annealing. Meanwhile, spin orbit torques calculated from Hall resistance with only the spin Hall effect involved in the longitudinal mode decrease in the annealed sample. The experimental results prove that the bottom Pt/Co interface rather than the Co/MgO top one plays a more critical role in both Rashba effect and spin Hall effect.

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Effects of terbium sulfide addition on magnetic properties, microstructure and thermal stability of sintered Nd—Fe—B magnets

李向斌, 刘硕, 曹学静, 周贝贝, 陈岭, 闫阿儒, 严高林

2016 (7): 77502-077502. doi: 10.1088/1674-1056/25/7/077502

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To increase coercivity and thermal stability of sintered Nd-Fe-B magnets for high-temperature applications, a novel terbium sulfide powder is added into (Pr_0.25Nd_0.75)_30.6Cu_0.15Fe_balB₁ (wt.%) basic magnets. The effects of the addition of terbium sulfide on magnetic properties, microstructure, and thermal stability of sintered Nd-Fe-B magnets are investigated. The experimental results show that by adding 3 wt.% Tb₂S₃, the coercivity of the magnet is remarkably increased by about 54% without a considerable reduction in remanence and maximum energy product. By means of the electron probe microanalyzer (EPMA) technology, it is observed that Tb is mainly present in the outer region of 2:14:1 matrix grains and forms a well-developed Tb-shell phase, resulting in enhancement of H_A, which accounts for the coercivity enhancement. Moreover, compared with Tb₂S₃-free magnets, the reversible temperature coefficients of remanence (α) and coercivity (β) and the irreversible flux loss of magnetic flow (h_irr) values of Tb₂S₃-added magnets are improved, indicating that the thermal stability of the magnets is also effectively improved.

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Structural stability at high pressure, electronic, and magnetic properties of BaFZnAs: A new candidate of host material of diluted magnetic semiconductors

陈碧娟, 邓正, 望贤成, 冯少敏, 袁真, 张思佳, 刘清青, 靳常青

2016 (7): 77503-077503. doi: 10.1088/1674-1056/25/7/077503

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The layered semiconductor BaFZnAs with the tetragonal ZrCuSiAs-type structure has been successfully synthesized. Both the in-situ high-pressure synchrotron x-ray diffraction and the high-pressure Raman scattering measurements demonstrate that the structure of BaFZnAs is stable under pressure up to 17.5 GPa at room temperature. The resistivity and the magnetic susceptibility data show that BaFZnAs is a non-magnetic semiconductor. BaFZnAs is recommended as a candidate of the host material of diluted magnetic semiconductor.

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Control of the interparticle spacing in superparamagnetic iron oxide nanoparticle clusters by surface ligand engineering

王丹, 林兵兵, 申太鹏, 吴君, 豪富华, 夏春潮, 龚启勇, 唐惠儒, 宋彬, 艾华

2016 (7): 77504-077504. doi: 10.1088/1674-1056/25/7/077504

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Polymer-mediated self-assembly of superparamagnetic iron oxide (SPIO) nanoparticles allows modulation of the structure of SPIO nanocrystal cluster and their magnetic properties. In this study, dopamine-functionalized polyesters (DA-polyester) were used to directly control the magnetic nanoparticle spacing and its effect on magnetic resonance relaxation properties of these clusters was investigated. Monodisperse SPIO nanocrystals with different surface coating materials (poly(ε-caprolactone), poly(lactic acid)) of different molecular weights containing dopamine (DA) structure (DA-PCL2k, DA-PCL1k, DA-PLA1k)) were prepared via ligand exchange reaction, and these nanocrystals were encapsulated inside amphiphilic polymer micelles to modulate the SPIO nanocrystal interparticle spacing. Small-angle x-ray scattering (SAXS) was applied to quantify the interparticle spacing of SPIO clusters. The results demonstrated that the tailored magnetic nanoparticle clusters featured controllable interparticle spacing providing directly by the different surface coating of SPIO nanocrystals. Systematic modulation of SPIO nanocrystal interparticle spacing can regulate the saturation magnetization (M_s) and T₂ relaxation of the aggregation, and lead to increased magnetic resonance (MR) relaxation properties with decreased interparticle spacing.

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Covalent bonding and J-J mixing effects on the EPR parameters of Er³⁺ ions in GaN crystal

柴瑞鹏, 李隆, 梁良, 庞庆

2016 (7): 77601-077601. doi: 10.1088/1674-1056/25/7/077601

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The EPR parameters of trivalent Er³⁺ ions doped in hexagonal GaN crystal have been studied by diagonalizing the 364×364 complete energy matrices. The results indicate that the resonance ground states may be derived from the Kramers doublet Γ₆. The EPR g-factors may be ascribed to the stronger covalent bonding and nephelauxetic effects compared with other rare-earth doped complexes, as a result of the mismatch of ionic radii of the impurity Er³⁺ ion and the replaced Ga³⁺ ion apart from the intrinsic covalency of host GaN. Furthermore, the J-J mixing effects on the EPR parameters from the high-lying manifolds have been evaluated. It is found that the dominant J-J mixing contribution is from the manifold ²K_15/2, which accounts for about 2.5%. The next important J-J contribution arises from the crystal-field mixture between the ground state ⁴I_15/2 and the first excited state ⁴I_13/2, and is usually less than 0.2%. The contributions from the rest states may be ignored.

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Effect of substitution group on dielectric properties of 4H-pyrano [3, 2-c] quinoline derivatives thin films

H M Zeyada, F M El-Taweel, M M El-Nahass, M M El-Shabaan

2016 (7): 77701-077701. doi: 10.1088/1674-1056/25/7/077701

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The AC electrical conductivity and dielectrical properties of 2-amino-6-ethyl-5-oxo-4-(3-phenoxyphenyl)-5,6-dihydro-4H-pyrano[3, 2-c]quinoline-3-carbonitrile (Ph-HPQ) and 2-amino-4-(2-chlorophenyl)-6-ethyl-5-oxo-5,6-dihydro-4H-pyrano [3, 2-c] quinoline-3-carbonitrile (Ch-HPQ) thin films were determined in the frequency range of 0.5 kHz-5 MHz and the temperature range of 290-443 K. The AC electrical conduction of both compounds in thin film form is governed by the correlated barrier hopping (CBH) mechanism. Some parameters such as the barrier height, the maximum barrier height, the density of charges, and the hopping distance were determined as functions of temperature and frequency. The phenoxyphenyl group has a greater influence on those parameters than the chlorophenyl group. The AC activation energies were determined at different frequencies and temperatures. The dielectric behaviors of Ph-HPQ and Ch-HPQ were investigated using the impedance spectroscopy technique. The impedance data are presented in Nyquist diagrams for different temperatures. The Ch-HPQ films have higher impedance than the Ph-HPQ films. The real dielectric constant and dielectric loss show a remarkable dependence on the frequency and temperature. The Ph-HPQ has higher dielectric constants than the Ch-HPQ.

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Thermally induced native defect transform in annealed GaSb

苏杰, 刘彤, 刘京明, 杨俊, 白永彪, 沈桂英, 董志远, 王芳芳, 赵有文

2016 (7): 77801-077801. doi: 10.1088/1674-1056/25/7/077801

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Undoped p-type GaSb single crystals were annealed at 550-600 ℃ for 100 h in ambient antimony. The annealed GaSb samples were investigated by Hall effect measurement, glow discharge mass spectroscopy (GDMS), infrared (IR) optical transmission and photoluminescence (PL) spectroscopy. Compared with the as-grown GaSb single crystal, the annealed GaSb samples have lower hole concentrations and weak native acceptor related PL peaks, indicating the reduction of the concentration of gallium antisite related native acceptor defects. Consequently, the below gap infrared transmission of the GaSb samples is enhanced after the thermal treatment. The mechanism about the reduction of the native defect concentration and its influence on the material property were discussed.

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Strain-modulated excitonic gaps in mono- and bi-layer MoSe₂

籍建葶, 张安民, 夏天龙, 高坡, 揭英昊, 张倩, 张清明

2016 (7): 77802-077802. doi: 10.1088/1674-1056/25/7/077802

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Photoluminescence (PL) and Raman spectra under uniaxial strain were measured in mono- and bi-layer MoSe₂ to comparatively investigate the evolution of excitonic gaps and Raman phonons with strain. We observed that the strain dependence of excitonic gaps shows a nearly linear behavior in both flakes. One percent of strain increase gives a reduction of ～ 42 meV (～ 35 meV) in A-exciton gap in monolayer (bilayer) MoSe₂. The PL width remains little changed in monolayer MoSe₂ while it increases rapidly with strain in the bilayer case. We have made detailed discussions on the observed strain-modulated results and compared the difference between monolayer and bilayer cases. The hybridization between 4d orbits of Mo and 4p orbits of Se, which is controlled by the Se-Mo-Se bond angle under strain, can be employed to consistently explain the observations. The study may shed light into exciton physics in few-layer MoSe₂ and provides a basis for their applications.

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Spectral features and antibacterial properties of Cu-doped ZnO nanoparticles prepared by sol-gel method

Alireza Samavati, A F Ismail, Hadi Nur, Z Othaman, M K Mustafa

2016 (7): 77803-077803. doi: 10.1088/1674-1056/25/7/077803

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Zn_1-xCu_xO (x=0.00, 0.01, 0.03, and 0.05) nanoparticles are synthesized via the sol-gel technique using gelatin and nitrate precursors. The impact of copper concentration on the structural, optical, and antibacterial properties of these nanoparticles is demonstrated. Powder x-ray diffraction investigations have illustrated the organized Cu doping into ZnO nanoparticles up to Cu concentration of 5% (x= 0.05). However, the peak corresponding to CuO for x=0.01 is not distinguishable. The images of field emission scanning electron microscopy demonstrate the existence of a nearly spherical shape with a size in the range of 30-52 nm. Doping Cu creates the Cu-O-Zn on the surface and results in a decrease in the crystallite size. Photoluminescence and absorption spectra display that doping Cu causes an increment in the energy band gap. The antibacterial activities of the nanoparticles are examined against Escherichia coli (Gram negative bacteria) cultures using optical density at 600 nm and a comparison of the size of inhibition zone diameter. It is found that both pure and doped ZnO nanoparticles indicate appropriate antibacterial activity which rises with Cu doping.

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Properties of strong-coupling magneto-bipolaron qubit in quantum dot under magnetic field

白旭芳, 张颖, 乌云其木格, 额尔敦朝鲁

2016 (7): 77804-077804. doi: 10.1088/1674-1056/25/7/077804

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Based on the variational method of Pekar type, we study the energies and the wave-functions of the ground and the first-excited states of magneto-bipolaron, which is strongly coupled to the LO phonon in a parabolic potential quantum dot under an applied magnetic field, thus built up a quantum dot magneto-bipolaron qubit. The results show that the oscillation period of the probability density of the two electrons in the qubit decreases with increasing electron-phonon coupling strength α, resonant frequency of the magnetic field ω_c, confinement strength of the quantum dot ω₀, and dielectric constant ratio of the medium η ; the probability density of the two electrons in the qubit oscillates periodically with increasing time t, angular coordinate ψ₂, and dielectric constant ratio of the medium η; the probability of electron appearing near the center of the quantum dot is larger, and the probability of electron appearing away from the center of the quantum dot is much smaller.

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Preparation of few-layer graphene-capped boron nanowires and their field emission properties

张永欣, 刘飞, 申承民, 杨天中, 李军, 邓少芝, 许宁生, 高鸿钧

2016 (7): 78101-078101. doi: 10.1088/1674-1056/25/7/078101

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Large-area boron nanowire (BNW) films were fabricated on the Si(111) substrate by chemical vapor deposition (CVD). The average diameter of the BNWs is about 20 nm, with lengths of 5-10 μm. Then, graphene-capped boron nanowires (GC-BNWs) were obtained by microwave plasma chemical vapor deposition (MPCVD). Characterization by scanning electron microscopy indicates that few-layer graphene covers the surface of the boron nanowires. Field emission measurements of the BNWs and GC-BNW films show that the GC-BNW films have a lower turn-on electric field than the BNW films.

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Tunable thermoelectric properties in bended graphene nanoribbons

潘长宁, 何军, 方卯发

2016 (7): 78102-078102. doi: 10.1088/1674-1056/25/7/078102

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The ballistic thermoelectric properties in bended graphene nanoribbons (GNRs) are systematically investigated by using atomistic simulation of electron and phonon transport. We find that the electron resonant tunneling effect occurs in the metallic-semiconducting linked ZZ-GNRs (the bended GNRs with zigzag edge leads). The electron-wave quantum interference effect occurs in the metallic-metallic linked AA-GNRs (the bended GNRs with armchair edge leads). These different physical mechanisms lead to the large Seebeck coefficient S and high electron conductance in bended ZZ-GNRs/AA-GNRs. Combined with the reduced lattice thermal conduction, the significant enhancement of the figure of merit ZT is predicted. Moreover, we find that the ZT_\max (the maximum peak of ZT) is sensitive to the structural parameters. It can be conveniently tuned by changing the interbend length of bended GNRs. The magnitude of ZT ranges from the 0.15 to 0.72. Geometry-controlled ballistic thermoelectric effect offers an effective way to design thermoelectric devices such as thermocouples based on graphene.

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Depositing aluminum as sacrificial metal to reduce metal-graphene contact resistance

毛达诚, 金智, 王少青, 张大勇, 史敬元, 彭松昂, 王选芸

2016 (7): 78103-078103. doi: 10.1088/1674-1056/25/7/078103

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Reducing the contact resistance without degrading the mobility property is crucial to achieve high-performance graphene field effect transistors. Also, the idea of modifying the graphene surface by etching away the deposited metal provides a new angle to achieve this goal. We exploit this idea by providing a new process method which reduces the contact resistance from 597 Ω·μm to sub 200 Ω·μm while no degradation of mobility is observed in the devices. This simple process method avoids the drawbacks of uncontrollability, ineffectiveness, and trade-off with mobility which often exist in the previously proposed methods.

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Perfect spin filtering controlled by an electric field in a bilayer graphene junction: Effect of layer-dependent exchange energy

Kitakorn Jatiyanon, I-Ming Tang, Bumned Soodchomshom

2016 (7): 78104-078104. doi: 10.1088/1674-1056/25/7/078104

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Magneto transport of carriers with a spin-dependent gap in a ferromagnetic-gated bilayer of graphene is investigated. We focus on the effect of an energy gap induced by the mismatch of the exchange fields in the top and bottom layers of an AB-stacked graphene bilayer. The interplay of the electric and exchange fields causes the electron to acquire a spin-dependent energy gap. We find that, only in the case of the anti-parallel configuration, the effect of a magnetic-induced gap will give rise to perfect spin filtering controlled by the electric field. The resolution of the spin filter may be enhanced by varying the bias voltage. Perfect switching of the spin polarization from +100% to -100% by reversing the direction of electric field is predicted. Giant magnetoresistance is predicted to be easily realized when the applied electric field is smaller than the magnetic energy gap. It should be pointed out that the perfect spin filter is due to the layer-dependent exchange energy. This work points to the potential application of bilayer graphene in spintronics.

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Polarization-insensitive unidirectional spoof surface plasmon polaritons coupling by gradient metasurface

施宏宇, 张安学, 陈建忠, 王甲富, 夏颂, 徐卓

2016 (7): 78105-078105. doi: 10.1088/1674-1056/25/7/078105

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A polarization-insensitive unidirectional spoof surface plasmon polariton (SPP) coupler mediated by a gradient metasurface is proposed. The field distributions and average Poynting vector of the coupled spoof SPPs are analyzed. The simulated and experimental results support the theoretical analysis and indicate that the designed gradient metasurface can couple both the parallel-polarized and normally-polarized incident waves to the spoof SPPs propagating in the same direction at about 5 GHz.

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Fabrication and formation mechanism of closed-loop fibers by electrospinning with a tip collector

闫旭, 于淼, 韩文鹏, 犹明浩, 张君诚, 董瑞华, 张红娣, 龙云泽

2016 (7): 78106-078106. doi: 10.1088/1674-1056/25/7/078106

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Electrospun nanofibers with designed or controlled structures have drawn much attention. In this study, we report an interesting new closed-loop structure in individual cerium nitrate/polyvinyl alcohol (Ce(NO₃)₃/PVA) and NaCl/PVA fibers, which are fabricated by electrospinning with a nail collector. The electrospinning parameters such as voltage and Ce(NO₃)₃ (or NaCl) concentration are examined for the formation of the closed-loop structure. The results suggest that the increase of the spinning voltage or addition of Ce(NO₃)₃ (or NaCl) is favorable for the formation of the closed-loop structure, and the increase of loop numbers and the decrease of loop size. Further analyses indicate that the formation mechanism of the closed-loop fibers can be predominantly attributed to the Coulomb repulsion in the charged jets.

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Hexagonal boron nitride hollow capsules with collapsed surfaces: Chemical vapor deposition with single-source precursor ammonium fluoroborate

李肖鹏, 张军, 郁超, 刘晓喜, Saleem Abbas, 李杰, 薛彦明, 唐成春

2016 (7): 78107-078107. doi: 10.1088/1674-1056/25/7/078107

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SBA-15 (mesoporous SiO₂) is used to stabilize and transfer F^- in the NH₄BF₄ CVD reaction for the first time, and a large-scale crystalline h-BN phase can be prepared. We successfully fabricate hollow h-BN capsules with collapsed surfaces in our designed NH₄BF₄ CVD system. Optimum temperature conditions are obtained, and a detailed formation mechanism is further proposed. The successful SBA-15-assisted NH₄BF₄ CVD route is of importance and enriches the engineering technology in the h-BN single-source CVD reaction.

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A-site ordered quadruple perovskite oxides AA₃'B₄O₁₂

龙有文

2016 (7): 78108-078108. doi: 10.1088/1674-1056/25/7/078108

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The A-site ordered perovskite oxides with chemical formula AA₃'B₄O₁₂ display many intriguing physical properties due to the introduction of transition metals at both A' and B sites. Here, research on the recently discovered intermetallic charge transfer occurring between A'-site Cu and B-site Fe ions in LaCu₃Fe₄O₁₂ and its analogues is reviewed, along with work on the magnetoelectric multiferroicity observed in LaMn₃Cr₄O₁₂ with cubic perovskite structure. The Cu-Fe intermetallic charge transfer (LaCuLaCu₃³⁺Fe₄³⁺O₁₂→LaCu₃²⁺Fe₄^3.75+O₁₂) leads to a first-order isostructural phase transition accompanied by drastic variations in magnetism and electrical transport properties. The LaMn₃Cr₄O₁₂ is a novel spindriven multiferroic system with strong magnetoelectric coupling effects. The compound is the first example of cubic perovskite multiferroics to be found. It opens up a new arena for studying unexpected multiferroic mechanisms.

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Molecular dynamics study of anisotropic growth of silicon

周耐根, 刘博, 张弛, 李克, 周浪

2016 (7): 78109-078109. doi: 10.1088/1674-1056/25/7/078109

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Based on the Tersoff potential, molecular dynamics simulations have been performed to investigate the kinetic coefficients and growth velocities of Si (100), (110), (111), and (112) planes. The sequences of the kinetic coefficients and growth velocities are μ₍₁₀₀₎> μ₍₁₁₀₎> μ₍₁₁₂₎ > μ₍₁₁₁₎ and v₍₁₀₀₎> v₍₁₁₀₎ > v₍₁₁₂₎ > v₍₁₁₁₎, respectively, which are not consistent with the sequences of the interface energies, interplanar spacings, and melting points of the four planes. However, they agree well with the sequences of the distributions and diffusion coefficients of the melting atoms near the solid-liquid interfaces. It indicates that the atomic distributions and diffusion coefficients affected by the crystal orientations determine the anisotropic growth of silicon. The formation of stacking fault structure will further decrease the growth velocity of the Si (111) plane.

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Preparation of silver-coated glass frit and its application in silicon solar cells

向锋, 李碧渊, 黎应芬, 周健, 甘卫平

2016 (7): 78110-078110. doi: 10.1088/1674-1056/25/7/078110

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A simple electroless plating process was employed to prepare silver-coated glass frits for solar cells. The surface of the glass frits was modified with polyvinyl-pyrrolidone (PVP) before the electroless plating process. Infrared (IR) spectroscopy, field emission scanning electron microscopy (FESEM), and x-ray diffraction (XRD) were used to characterize the PVP modified glass frits and investigate the mechanism of the modification process. It was found that the PVP molecules adsorbed on the glass frit surface and reduced the silver ions to the silver nanoparticles. Through epitaxial growth, these nanoparticles were uniformly deposited onto the surface of the glass frit. Silicon solar cells with this novel silver coating exhibited a photoelectric conversion efficiency increase of 0.33%. Compared with the electroless plating processes, this method provides a simple route to prepare silver-coated glass frits without introducing impurity ions.

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Direct observation of λ -DNA molecule reversal movement within microfluidic channels under electric field with single molecule imaging technique

杨凤云, 王凯歌, 孙聃, 赵伟, 王海青, 何鑫, 王归仁, 白晋涛

2016 (7): 78201-078201. doi: 10.1088/1674-1056/25/7/078201

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The electrodynamic characteristics of single DNA molecules moving within micro-/nano-fluidic channels are important in the design of biomedical chips and bimolecular sensors. In this study, the dynamic properties of λ -DNA molecules transferring along the microchannels driven by the external electrickinetic force were systemically investigated with the single molecule fluorescence imaging technique. The experimental results indicated that the velocity of DNA molecules was strictly dependent on the value of the applied electric field and the diameter of the channel. The larger the external electric field, the larger the velocity, and the more significant deformation of DNA molecules. More meaningfully, it was found that the moving directions of DNA molecules had two completely different directions: (i) along the direction of the external electric field, when the electric field intensity was smaller than a certain threshold value; (ii) opposite to the direction of the external electric field, when the electric field intensity was greater than the threshold electric field intensity. The reversal movement of DNA molecules was mainly determined by the competition between the electrophoresis force and the influence of electro-osmosis flow. These new findings will theoretically guide the practical application of fluidic channel sensors and lab-on-chips for precisely manipulating single DNA molecules.

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Ceramic synthesis of 0.08BiGaO₃-0.90BaTiO₃-0.02LiNbO₃ under high pressure and high temperature

金慧, 李勇, 宋谋胜, 陈琳, 贾晓鹏, 马红安

2016 (7): 78202-078202. doi: 10.1088/1674-1056/25/7/078202

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In this paper, the preparation of 0.08BiGaO₃-0.90BaTiO₃-0.02LiNbO₃ is investigated at pressure 3.8 GPa and temperature 1100-1200 ℃. Experimental results indicate that not only is the sintered rate more effective, but also the sintered temperature is lower under high pressure and high temperature than those of under normal pressure. It is thought that the adscititious pressure plays the key role in this process, which is discussed in detail. The composition and the structure of the as-prepared samples are recorded by XRD patterns. The result shows that the phases of BaTiO₃, BaBiO_2.77, and Ba₂Bi₄Ti₅O₁₈ with piezoelectric ceramic performance generate in the sintered samples. Furthermore, the surface morphology characteristics of the typical samples are also investigated using a scanning electron microscope. It indicates that the grain size and surface structure of the samples are closely related to the sintering temperature and sintering time. It is hoped that this study can provide a new train of thought for the preparation of lead-free piezoelectric ceramics with excellent performance.

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Concentrated dual-salt electrolytes for improving the cycling stability of lithium metal anodes

刘品, 马强, 方铮, 马洁, 胡勇胜, 周志彬, 李泓, 黄学杰, 陈立泉

2016 (7): 78203-078203. doi: 10.1088/1674-1056/25/7/078203

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Lithium (Li) metal is an ideal anode material for rechargeable Li batteries, due to its high theoretical specific capacity (3860 mAh/g), low density (0.534 g/cm³), and low negative electrochemical potential (-3.040 V vs. standard hydrogen electrode). In this work, the concentrated electrolytes with dual salts, composed of Li[N(SO₂F)₂] (LiFSI) and Li[N(SO₂CF₃)₂] (LiTFSI) were studied. In this dual-salt system, the capacity retention can even be maintained at 95.7% after 100 cycles in Li|LiFePO₄ cells. A Li|Li cell can be cycled at 0.5 mA/cm² for more than 600 h, and a Li|Cu cell can be cycled at 0.5 mA/cm² for more than 200 cycles with a high average Coulombi efficiency of 99%. These results show that the concentrated dual-salt electrolytes exhibit superior electrochemical performance and would be a promising candidate for application in rechargeable Li batteries.

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Forming solid electrolyte interphase in situ in an ionic conductingLi_1.5Al_0.5Ge_1.5(PO₄)₃-polypropylene (PP) basedseparator for Li-ion batteries

吴娇杨, 凌仕刚, 杨琪, 李泓, 许晓雄, 陈立泉

2016 (7): 78204-078204. doi: 10.1088/1674-1056/25/7/078204

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A new concept of forming solid electrolyte interphases (SEI) in situ in an ionic conducting Li_1.5Al_0.5Ge_1.5(PO₄)₃-polypropylene (LAGP-PP) based separator during charging and discharging is proposed and demonstrated. This unique structure shows a high ionic conductivity, low interface resistance with electrode, and can suppress the growth of lithium dendrite. The features of forming the SEI in situ are investigated by scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). The results confirm that SEI films mainly consist of lithium fluoride and carbonates with various alkyl contents. The cell assembled by using the LAGP-coated separator demonstrates a good cycling performance even at high charging rates, and the lithium dendrites were not observed on the lithium metal electrode. Therefore, the SEI-LAGP-PP separator can be used as a promising flexible solid electrolyte for solid state lithium batteries.

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Excitation of anti-symmetric coupled spoof SPPs in 3D SIS waveguides based on coupling

田莉莉, 陈杨, 刘建龙, 郭凯, 周可雅, 高扬, 刘树田

2016 (7): 78401-078401. doi: 10.1088/1674-1056/25/7/078401

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According to the electromagnetic field distributions, there exist two kinds of coupled spoof surface plasmon polaritons (SSPPs), the symmetric and anti-symmetric modes, in the three-dimensional (3D) subwavelength spoof-insulator-spoof (SIS) waveguide. We study the dispersion and excitation of the two kinds of coupled SSPPs supported by the 3D SIS waveguide. The evolution of the dispersion with the thickness and gap width of the waveguide is numerically investigated, and we give a theoretical analysis according to the coupling mechanism. Specially, based on the coupling mechanism, we design a zipper structure, through which the excitation and propagation of the anti-symmetric coupled modes can be realized effectively.

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Power-combining based on master—slave injection-locking magnetron

袁萍, 张益, 叶文军, 朱铧丞, 黄卡玛, 杨阳

2016 (7): 78402-078402. doi: 10.1088/1674-1056/25/7/078402

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A microwave power-combining system composed of two Panasonic 2M244-M1 magnetrons based on master-slave injection-locking is demonstrated in this paper. The principle of master-slave injection-locking and the locking condition are theoretical analyzed. Experimental results are consistent with the theoretical analysis and the experimental combined efficiency is higher than 96%. Compared with the external-injection-locked system, the power-combining based on the master-slave injection-locking magnetron is superior by taking out the external solid-state driver and the real-time phase control system. Thus, this power-combining system has great potential for obtaining a high efficiency, high stability, low cost, and high power microwave source.

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Effect of cryogenic temperature characteristics on 0.18-μm silicon-on-insulator devices

解冰清, 李博, 毕津顺, 卜建辉, 吴驰, 李彬鸿, 韩郑生, 罗家俊

2016 (7): 78501-078501. doi: 10.1088/1674-1056/25/7/078501

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The experimental results of the cryogenic temperature characteristics on 0.18-μm silicon-on-insulator (SOI) metal-oxide-silicon (MOS) field-effect-transistors (FETs) were presented in detail. The current and capacitance characteristics for different operating conditions ranging from 300 K to 10 K were discussed. SOI MOSFETs at cryogenic temperature exhibit improved performance, as expected. Nevertheless, operation at cryogenic temperature also demonstrates abnormal behaviors, such as the impurity freeze-out and series resistance effects. In this paper, the critical parameters of the devices were extracted with a specific method from 300 K to 10 K. Accordingly, some temperature-dependent-parameter models were created to improve fitting precision at cryogenic temperature.

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Enhanced light extraction of GaN-based light-emitting diodes with periodic textured SiO₂ on Al-doped ZnO transparent conductive layer

招瑜, 范冰丰, 陈义廷, 卓毅, 庞洲骏, 刘振, 王钢

2016 (7): 78502-078502. doi: 10.1088/1674-1056/25/7/078502

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We report an effective enhancement in light extraction of GaN-based light-emitting diodes (LEDs) with an Al-doped ZnO (AZO) transparent conductive layer by incorporating a top regular textured SiO₂ layer. The 2 inch transparent through-pore anodic aluminum oxide (AAO) membrane was fabricated and used as the etching mask. The periodic pore with a pitch of about 410 nm was successfully transferred to the surface of the SiO₂ layer without any etching damages to the AZO layer and the electrodes. The light output power was enhanced by 19% at 20 mA and 56% at 100 mA compared to that of the planar LEDs without a patterned surface. This approach offers a technique to fabricate a low-cost and large-area regular pattern on the LED chip for achieving enhanced light extraction without an obvious increase of the forward voltage.

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Optimization-based image reconstruction in x-ray computed tomography by sparsity exploitation of local continuity and nonlocal spatial self-similarity

张瀚铭, 王林元, 李磊, 闫镔, 蔡爱龙, 胡国恩

2016 (7): 78701-078701. doi: 10.1088/1674-1056/25/7/078701

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The additional sparse prior of images has been the subject of much research in problems of sparse-view computed tomography (CT) reconstruction. A method employing the image gradient sparsity is often used to reduce the sampling rate and is shown to remove the unwanted artifacts while preserve sharp edges, but may cause blocky or patchy artifacts. To eliminate this drawback, we propose a novel sparsity exploitation-based model for CT image reconstruction. In the presented model, the sparse representation and sparsity exploitation of both gradient and nonlocal gradient are investigated. The new model is shown to offer the potential for better results by introducing a similarity prior information of the image structure. Then, an effective alternating direction minimization algorithm is developed to optimize the objective function with a robust convergence result. Qualitative and quantitative evaluations have been carried out both on the simulation and real data in terms of accuracy and resolution properties. The results indicate that the proposed method can be applied for achieving better image-quality potential with the theoretically expected detailed feature preservation.

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Physical mechanism of mind changes and tradeoffs among speed, accuracy, and energy cost in brain decision making: Landscape, flux, and path perspectives

闫晗, 张坤, 汪劲

2016 (7): 78702-078702. doi: 10.1088/1674-1056/25/7/078702

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Cognitive behaviors are determined by underlying neural networks. Many brain functions, such as learning and memory, have been successfully described by attractor dynamics. For decision making in the brain, a quantitative description of global attractor landscapes has not yet been completely given. Here, we developed a theoretical framework to quantify the landscape associated with the steady state probability distributions and associated steady state curl flux, measuring the degree of non-equilibrium through the degree of detailed balance breaking for decision making. We quantified the decision-making processes with optimal paths from the undecided attractor states to the decided attractor states, which are identified as basins of attractions, on the landscape. Both landscape and flux determine the kinetic paths and speed. The kinetics and global stability of decision making are explored by quantifying the landscape topography through the barrier heights and the mean first passage time. Our theoretical predictions are in agreement with experimental observations: more errors occur under time pressure. We quantitatively explored two mechanisms of the speed-accuracy tradeoff with speed emphasis and further uncovered the tradeoffs among speed, accuracy, and energy cost. Our results imply that there is an optimal balance among speed, accuracy, and the energy cost in decision making. We uncovered the possible mechanisms of changes of mind and how mind changes improve performance in decision processes. Our landscape approach can help facilitate an understanding of the underlying physical mechanisms of cognitive processes and identify the key factors in the corresponding neural networks.

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