中国物理B

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Cluster algebra structure on the finite dimensional representations of affine quantum group U_q(Â₃)

杨彦敏, 马海涛, 林冰生, 郑驻军

2015 (1): 10201-010201. doi: 10.1088/1674-1056/24/1/010201

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In this paper, we prove one case of conjecture given by Hernandez and Leclerc. We give a cluster algebra structure on the Grothendieck ring of a full subcategory of the finite dimensional representations of affine quantum group U_q(Â₃). As a conclusion, for every exchange relation of cluster algebra, there exists an exact sequence of the full subcategory corresponding to it.

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Exact solutions and residual symmetries of the Ablowitz-Kaup-Newell-Segur system

刘萍, 曾葆青, 杨建荣, 任博

2015 (1): 10202-010202. doi: 10.1088/1674-1056/24/1/010202

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The residual symmetries of the Ablowitz-Kaup-Newell-Segur (AKNS) equations are obtained by the truncated Painlevé analysis. The residual symmetries for the AKNS equations are proved to be nonlocal and the nonlocal residual symmetries are extended to the local Lie point symmetries of a prolonged AKNS system. The local Lie point symmetries of the prolonged AKNS equations are composed of the residual symmetries and the standard Lie point symmetries, which suggests that the residual symmetry method is a useful complement to the classical Lie group theory. The calculation on the symmetries shows that the enlarged equations are invariant under the scaling transformations, the space-time translations, and the shift translations. Three types of similarity solutions and the reduction equations are demonstrated. Furthermore, several types of exact solutions for the AKNS equations are obtained with the help of the symmetry method and the Bäcklund transformations between the AKNS equations and the Schwarzian AKNS equation.

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Residual symmetry reductions and interaction solutions of the (2+1)-dimensional Burgers equation

刘希忠, 俞军, 任博, 杨建荣

2015 (1): 10203-010203. doi: 10.1088/1674-1056/24/1/010203

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In nonlinear physics, it is very difficult to study interactions among different types of nonlinear waves. In this paper, the nonlocal symmetry related to the truncated Painlevé expansion of the (2+1)-dimensional Burgers equation is localized after introducing multiple new variables to extend the original equation into a new system. Then the corresponding group invariant solutions are found, from which interaction solutions among different types of nonlinear waves can be found. Furthermore, the Burgers equation is also studied by using the generalized tanh expansion method and a new Bäcklund transformation (BT) is obtained. From this BT, novel interactive solutions among different nonlinear excitations are found.

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Hybrid natural element method for viscoelasticity problems

周延凯, 马永其, 董轶, 冯伟

2015 (1): 10204-010204. doi: 10.1088/1674-1056/24/1/010204

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A hybrid natural element method (HNEM) for two-dimensional viscoelasticity problems under the creep condition is proposed. The natural neighbor interpolation is used as the test function, and the discrete equation system of the HNEM for viscoelasticity problems is obtained using the Hellinger-Reissner variational principle. In contrast to the natural element method (NEM), the HNEM can directly obtain the nodal stresses, which have higher precisions than those obtained using the moving least-square (MLS) approximation. Some numerical examples are given to demonstrate the validity and superiority of this HNEM.

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A new optical field generated as an output of the displaced Fock state in an amplitude dissipative channel

许雪芬, 范洪义

2015 (1): 10301-010301. doi: 10.1088/1674-1056/24/1/010301

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We propose a new optical field and show that such an optical field can be generated as an output of a displaced Fock state in an amplitude dissipative channel. We derive new generating function formulas and binomial formula involving two-variable Hermite polynomials to reach this result. The photon number average in this new optical field is (m+|α|²)e^-2κt, which indicates that controlling the photon number can be realized by adjusting the value of m or |α|² or κ. The time evolution law of displaced Fock state in a thermo reservoir is thus revealed.

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Efficient error estimation in quantum key distribution

李默, Patcharapong Treeviriyanupab, 张春梅, 银振强, 陈巍, 韩正甫

2015 (1): 10302-010302. doi: 10.1088/1674-1056/24/1/010302

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In a quantum key distribution (QKD) system, the error rate needs to be estimated for determining the joint probability distribution between legitimate parties, and for improving the performance of key reconciliation. We propose an efficient error estimation scheme for QKD, which is called parity comparison method (PCM). In the proposed method, the parity of a group of sifted keys is practically analysed to estimate the quantum bit error rate instead of using the traditional key sampling. From the simulation results, the proposed method evidently improves the accuracy and decreases revealed information in most realistic application situations.

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Disordered quantum walks in two-dimensional lattices

张融, 徐韵秋,薛鹏

2015 (1): 10303-010303. doi: 10.1088/1674-1056/24/1/010303

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The properties of the two-dimensional quantum walk with point, line, and circle disorders in phase are reported. Localization is observed in the two-dimensional quantum walk with certain phase disorder and specific initial coin states. We give an explanation of the localization behavior via the localized stationary states of the unitary operator of the walker + coin system and the overlap between the initial state of the whole system and the localized stationary states.

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Coherent spin dynamics in spin-imbalanced ferromagnetic spinor condensates

邱海波, 武丽伟

2015 (1): 10304-010304. doi: 10.1088/1674-1056/24/1/010304

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We study the coherent spin dynamics of a ferromagnetic spinor Bose-Einstein condensate (BEC) in its domain formation process with an arbitrary spin configuration. Through a simplified schematic view of the domain structure, a semiclassical theory that captures the essential dynamics of the system is presented, and the coherent spin mixing dynamics can be understood in terms of oscillation in the phase space diagram. Using the phase diagram analysis method, we identify new phases, including the π phase oscillation and the running phase for the spin-imbalanced ferromagnetic spinor BEC.

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Neural adaptive chaotic control with constrained input using state and output feedback

高士根, 董海荣, 孙绪彬, 宁滨

2015 (1): 10501-010501. doi: 10.1088/1674-1056/24/1/010501

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This paper presents neural adaptive control methods for a class of chaotic nonlinear systems in the presence of constrained input and unknown dynamics. To attenuate the influence of constrained input caused by actuator saturation, an effective auxiliary system is constructed to prevent the stability of closed loop system from being destroyed. Radial basis function neural networks (RBF-NNs) are used in the online learning of the unknown dynamics, which do not require an off-line training phase. Both state and output feedback control laws are developed. In the output feedback case, high-order sliding mode (HOSM) observer is utilized to estimate the unmeasurable system states. Simulation results are presented to verify the effectiveness of proposed schemes.

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Predictive control of a chaotic permanent magnet synchronous generator in a wind turbine system

Manal Messadi, Adel Mellit, Karim Kemih, Malek Ghanes

2015 (1): 10502-010502. doi: 10.1088/1674-1056/24/1/010502

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This paper investigates how to address the chaos problem in a permanent magnet synchronous generator (PMSG) in a wind turbine system. Predictive control approach is proposed to suppress chaotic behavior and make operating stable; the advantage of this method is that it can only be applied to one state of the wind turbine system. The use of the genetic algorithms to estimate the optimal parameter values of the wind turbine leads to maximization of the power generation. Moreover, some simulation results are included to visualize the effectiveness and robustness of the proposed method.

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Robust output feedback cruise control for high-speed train movement with uncertain parameters

李树凯, 杨立兴, 李克平

2015 (1): 10503-010503. doi: 10.1088/1674-1056/24/1/010503

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In this paper, the robust output feedback cruise control for high-speed train movement with uncertain parameters is investigated. The dynamic of a high-speed train is modeled by a cascade of cars connected by flexible couplers, which is subject to rolling mechanical resistance, aerodynamic drag and wind gust. Based on Lyapunov's stability theory, the sufficient condition for the existence of the robust output feedback cruise control law is given in terms of linear matrix inequalities (LMIs), under which the high-speed train tracks the desired speed, the relative spring displacement between the two neighboring cars is stable at the equilibrium state, and meanwhile a small prescribed H_∞ disturbance attenuation level is guaranteed. One numerical example is given to illustrate the effectiveness of the proposed methods.

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Progress on accurate measurement of the Planck constant: Watt balance and counting atoms

李世松, 张钟华, 赵伟, 李正坤, 黄松岭

2015 (1): 10601-010601. doi: 10.1088/1674-1056/24/1/010601

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The Planck constant h is one of the most significant constants in quantum physics. Recently, the precision measurement of the value of h has been a hot issue due to its important role for the establishment of both a new SI and a revised fundamental physical constant system. Up to date, two approaches, the watt balance and counting atoms, have been employed to determine the Planck constant at a level of several parts in 10⁸. In this paper, the principle and progress on precision measurement of the Planck constant using watt balance and counting atoms at national metrology institutes are reviewed. Further improvement in determining the Planck constant and possible developments of a revised physical constant system in future are discussed.

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Design and test of the microwave cavity in an optically-pumped Rubidium beam frequency standard

刘畅, 王延辉

2015 (1): 10602-010602. doi: 10.1088/1674-1056/24/1/010602

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We are developing a compact rubidium atomic beam frequency standard with optical pumping and detection. The cavity for microwave interrogation is an important part of the clock. The cavity in our design is a Ramsey-type, E-bend one, which is the same as the conventional method in most cesium beam clocks. Requirements for the design are proposed based on the frequency shift associated with the cavity. The basic structure of the cavity is given by theoretical analysis and detailed dimensions are determined by means of electromagnetic field simulation with the help of commercial software. The cavity is manufactured and fabricated successfully. The preliminary test result of the cavity is given, which is in good agreement with the simulation. The resonant frequency is 6.835 GHz, equal to the clock transition frequency of ⁸⁷Rb, and the loaded quality factor is 500. These values are adjustable with posts outside the cavity. Estimations on the Ramsey line width and several frequency shifts are made.

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Thermal efficiency of the principal greenhouse gases

A. Y. Galashev, O. R. Rakhmanova

2015 (1): 10701-010701. doi: 10.1088/1674-1056/24/1/010701

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Atmospheric gases are ranked according to the efficiency with which they absorb and radiate longwave radiation. The open international HITRAN database of gaseous absorption lines of high resolution together with inverse Fourier transform were used. The autocorrelation functions of the total dipole moment of the basic greenhouse gases molecules such as H₂O, CO₂, O₃, N₂O, and CH₄ were obtained. Absorption coefficient spectra and emission power spectra of infrared radiation of these gases were calculated. Analysis of the emissive ability of all gases under consideration was carried out. Compared to CO₂, all the gases under investigation have more effective emission except ozone. An efficiency criterion of IR absorption and emission is defined and is calculated for each studied gas, and the gases are ranked accordingly as follows (from strong to weak): H₂O, CH₄, CO₂, N₂O, and O₃.

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Accurate ab initio-based analytical potential energy function for S₂ (ã¹Δ_g) via extrapolation to the complete basis set limit

张路路, 高守宝, 孟庆田, 宋玉志

2015 (1): 13101-013101. doi: 10.1088/1674-1056/24/1/013101

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The potential energy curves (PECs) of the first electronic excited state of S₂ (ã¹Δ_g) are calculated employing a multi-reference configuration interaction method with the Davidson correction in combination with a series of correlation-consistent basis sets from Dunning: aug-cc-pVXZ (X=T, Q, 5, 6). In order to obtain PECs with high accuracy, PECs calculated with aug-cc-pV(Q, 5)Z basis sets are extrapolated to the complete basis set limit. The resulting PECs are then fitted to the analytical potential energy function (APEF) using the extended Hartree-Fock approximate correlation energy method. By utilizing the fitted APEF, accurate and reliable spectroscopic parameters are obtained, which are consistent with both experimental and theoretical results. By solving the Schrödinger equation numerically with the APEFs obtained at the AV6Z and the extrapolated AV(Q, 5)Z level of theory, we calculate the complete set of vibrational levels, classical turning points, inertial rotation and centrifugal distortion constants.

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Precision frequency measurement of ¹S₀-³P₁ intercombination lines of Sr isotopes

刘辉, 高峰, 王叶兵, 田晓, 任洁, 卢本全, 徐琴芳, 谢玉林, 常宏

2015 (1): 13201-013201. doi: 10.1088/1674-1056/24/1/013201

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We report on frequency measurement of the intercombination (5s²)¹S₀-(5s5p)³P₁ transition of the four natural isotopes of strontium, including ⁸⁸Sr (82.58%), ⁸⁷Sr (7.0%), ⁸⁶Sr (9.86%), and ⁸⁴Sr (0.56%). A narrow-linewidth laser that is locked to an ultra-low expansion (ULE) optical cavity with a finesse of 12000 is evaluated at a linewidth of 200 Hz with a fractional frequency drift of 2.8× 10^-13 at an integration time of 1 s. The fluorescence collector and detector are specially designed, based on a thermal atomic beam. Using a double-pass acousto-optic modulator (AOM) combined with a fiber and laser power stabilization configuration to detune the laser frequency enables high signal-to-noise ratios and precision saturated spectra to be obtained for the six transition lines, which allows us to determine the transition frequency precisely. The optical frequency is measured using an optical frequency synthesizer referenced to an H maser. Both the statistical values and the final values, including the corrections and uncertainties, are derived for a comparison with the values given in other works.

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Non-linear spectral splitting of Rydberg sodium in external fields

高伟, 杨海峰, 成红, 张珊珊, 刘丹峰, 刘红平

2015 (1): 13202-013202. doi: 10.1088/1674-1056/24/1/013202

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We have studied highly excited sodium in various electric fields, parallel electric and magnetic fields, with one σ and π photon irradiation, and even in a magnetic field with a complex laser polarization configuration. The σ spectra shows a simple linear Stark effect with the applied electric field, while the π spectra exhibits a strong non-linear dependence on the electric field. The π transitions in parallel fields show a similar behavior to that in a pure electric field but the spectra get more smooth due to the magnetic field. The diamagnetic spectrum with laser polarization angles between 0 and π/2 proves that it can be reproduced by simple linear combination of π and σ components, indicating there is no interference between the π and σ channels. A full quantum calculation considering the quantum defects accounts for all the observations. The quantum defects, especially for the channel np, play an important role in the spectral profile.

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Lifetimes of Rydberg states of Eu atoms

荆华, 野仕伟, 戴长建

2015 (1): 13203-013203. doi: 10.1088/1674-1056/24/1/013203

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The radiative lifetimes of the Eu 4f⁷6snp (⁸P_J or ¹⁰P_J) Rydberg states with J=5/2 and 11/2 are investigated with a combination of multi-step laser excitation and pulsed electric field ionization, from which their dependence on the effective principal quantum number is observed. The lifetimes of 21 states are reported along with an evaluation of their experimental uncertainty. The influence of blackbody radiation, due to the oven temperature, on the lifetime of the higher-n states is detected. The non-hydrogen behavior of the investigated states is also observed.

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Control of electron localization in the dissociation of H₂⁺ and its isotopes with a THz pulse

贾正茂, 曾志男, 李儒新, 徐至展, 邓蕴沛

2015 (1): 13204-013204. doi: 10.1088/1674-1056/24/1/013204

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The molecular dissociation with a two-laser-pulse scheme is theoretically investigated for the hydrogen molecular ion (H₂⁺) and its isotopes (HD⁺ and HT⁺). The terahertz pulse is used to steer the electron motion after it has been excited by an ultrashort ultraviolet laser pulse and an unprecedented electron localization ratio can be achieved. With the coupled equations, the mass effect of the nuclei on the effective time of the electron localization control is discussed.

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Population inversion in fluorescing fragments of super-excited molecules inside an air filament

徐淮良, See Leang Chin

2015 (1): 13301-013301. doi: 10.1088/1674-1056/24/1/013301

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An original idea is reviewed. When a molecule is pumped into a super-excited state, one of its decay channels is neutral dissociation. One or more of the neutral fragments will fluoresce. Hence, if a lower state of such fluorescing fragments was populated through other channels but with a lower probability, population inversion of the fluorescing fragments would be naturally realized. This idea seems to be validated, so far, by comparing published work on three hydrocarbon molecules, CH₄, C₂H₂, C₂H₄, and water vapor, H₂O. After super-excitation in a femtosecond laser filament in air mixed with these molecules, the fluorescence from the CH or OH fragments exhibits population inversion, i.e., amplified spontaneous emission was observed in the backscattering direction of the filament.

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Effect of pump-1 laser on Autler–Townes splitting in photoelectron spectrum of K₂ molecule

郭玮, 路兴强, 王新林, 姚洪斌

2015 (1): 13302-013302. doi: 10.1088/1674-1056/24/1/013302

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We theoretically investigate the Autler-Townes (AT) splitting in the photoelectron spectrum of four-level ladder K₂ molecule driven by a pump 1-pump 2-probe pulse via employing the time-dependent wave packet approach. The effects of the pump-1 laser intensity and wavelength on AT splitting are studied for the first time. The magnitude of AT splitting increases with increasing the pump-1 laser intensity. The triple splitting with asymmetric profile occurs due to the nonresonant excitation. The triple structure is transformed into a double structure (near-resonant region), and then becomes a peak (far-off resonant region) progressively as the pump-1 laser is detuned from the resonance wavelength, which can be explained in terms of the asymmetric excitation/population of dressed states. The splitting between adjacent peaks and the splitting between the two sideband peaks in the triplet do not change with the pump-1 pulse wavelength. The three peaks shift toward lower energy with the same shift 1/4^*Δ₁ as the pump-1 wavelength changes in near-resonant region. The asymptotic behaviors of AT splitting with the pump-1 laser intensity are interesting in the threshold points of the near-resonant region and the far-off resonant region.

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Electron correlation in fast ion-impact single ionization of helium atoms

E. Ghanbari-Adivi, S. Eskandari

2015 (1): 13401-013401. doi: 10.1088/1674-1056/24/1/013401

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A four-body distorted-wave approximation is applied for theoretical analysis of the fully differential cross sections (FDCS) for proton-impact single ionization of helium atoms in their ground states. The nine-dimensional integrals for the partial amplitudes are analytically reduced to closed-form expressions or some one-dimensional integrals which can be easily calculated numerically. Calculations are performed in the scattering and perpendicular planes. The influence of the target static electron correlations on the process is investigated using a number of different bound-state wave functions for the ground state of the helium targets. An illustrative computation is performed for 75-keV proton-helium collisions and the obtained results are compared with experimental data and other theoretical predictions. Although for small momentum transfers, the comparison shows a reasonable agreement with experiments in the scattering and perpendicular planes, some significant discrepancies are still present at large momentum transfers in these planes. However, our results are compatible and for some cases, better than those of the other sophisticated calculations.

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Experimental demonstration of an invisible cloak with irregular shape by using tensor transmission line metamaterials

刘国昌, 李超, 方广有

2015 (1): 14101-014101. doi: 10.1088/1674-1056/24/1/014101

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We present the design and the experimental demonstration of an invisible cloak with irregular shape by using tensor transmission line (TL) metamaterials. The fabricated cloak consists of tensor TL unit cells exhibiting anisotropic effective material parameters, while the background medium consists of isotropic TL unit cells. The simulated and the measured field patterns around the cloak show a fairly good agreement, both demonstrate that the fabricated cloak can shield the cloaked interior area from electromagnetic fields without perturbing the external fields. The scattering of the cloaked perfect electric conductor (PEC) is minimized. Furthermore, the nonresonant property of the TL structure results in a relatively broad bandwidth of the realized cloak, which is clearly observed in our experiment.

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Broadband perfect polarization conversion metasurfaces

陈红雅, 王甲富, 马华, 屈绍波, 张介秋, 徐卓, 张安学

2015 (1): 14201-014201. doi: 10.1088/1674-1056/24/1/014201

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We propose a broadband perfect polarization conversion metasurface composed of copper sheet-backed asymmetric double spilt ring resonator (DSRR). The broadband perfect polarization convertibility results from metallic ground and multiple plasmon resonances of the DSRR. Physics of plasmon resonances are governed by the electric and magnetic resonances. Both the simulation and measured results show that the polarization conversion ratio (PCR) is higher than 99% for both x- and y-polarized normally incident EM waves and the fractional bandwidth is about 34.5%. The metasurface possesses the merits of high PCR and broad bandwidth, and thus has great application values in novel polarization-control devices.

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Wide-band circular polarization-keeping reflection mediated by metasurface

李勇峰, 张介秋, 屈绍波, 王甲富, 郑麟, 周航, 徐卓, 张安学

2015 (1): 14202-014202. doi: 10.1088/1674-1056/24/1/014202

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In this paper, we show that circular polarization-keeping reflection can be achieved using reflective metasurfaces. The underlying physical mechanism of the polarization-keeping reflection is analyzed using a reflection matrix. A wideband circular polarization-keeping reflector is demonstrated using N-shaped resonators. Both the simulation and experiment results show that the polarization-keeping reflection can be achieved with a high efficiency larger than 98% over the frequency range from 9.2 GHz to 17.7 GHz for both incident left- and right-handed circularly polarized waves. Under oblique incidence, the bandwidth increases as the incident angle varies from 0° to 80°. Moreover, the co-polarization reflection is independent of the incident azimuth angles.

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s-parameterized Weyl transformation and the corresponding quantization scheme

王继锁, 孟祥国, 范洪义

2015 (1): 14203-014203. doi: 10.1088/1674-1056/24/1/014203

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By extending the usual Weyl transformation to the s-parameterized Weyl transformation with s being a real parameter, we obtain the s-parameterized quantization scheme which includes P-Q quantization, Q-P quantization, and Weyl ordering as its three special cases. Some operator identities can be derived directly by virtue of the s-parameterized quantization scheme.

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Role of incoherent pumping and Er³⁺ ion concentration on subluminal and superluminal light propagation in Er³⁺-doped YAG crystal

Seyyed Hossein Asadpour, H. Rahimpour Soleimani

2015 (1): 14204-014204. doi: 10.1088/1674-1056/24/1/014204

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We study the absorption-dispersion process and group index of weak probe field in a four-level Er³⁺:YAG crystal. We find that the Er³⁺ ion concentration and incoherent pumping field can influence the absorption-dispersion process and group index of weak probe field. Moreover, our results show that Er³⁺ ion concentration plays a major role in lasing without inversion and absorption with inversion.

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Macroscopic effects in electromagnetically-induced transparency in a Doppler-broadened system

裴丽娅, 牛金艳, 王如泉, 屈一至, 吴令安, 傅盘铭, 左战春

2015 (1): 14205-014205. doi: 10.1088/1674-1056/24/1/014205

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We study the electromagnetically-induced transparency (EIT) in a Doppler-broadened cascaded three-level system. We decompose the susceptibility responsible for the EIT resonance into a linear and a nonlinear part, and the EIT resonance reflects mainly the characteristics of the nonlinear susceptibility. It is found that the macroscopic polarization interference effect plays a crucial role in determining the EIT resonance spectrum. To obtain a Doppler-free spectrum there must be polarization interference between atoms of different velocities. A dressed-state model, which analyzes the velocities at which the atoms are in resonance with the dressed states through Doppler frequency shifting, is employed to explain the results.

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A quartz-enhanced photoacoustic spectroscopy sensor for measurement of water vapor concentration in the air

龚萍, 谢亮, 漆晓琼, 王瑞, 王辉, 常明超, 杨慧霞, 孙菲, 李冠鹏

2015 (1): 14206-014206. doi: 10.1088/1674-1056/24/1/014206

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A compact and highly linear quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for the measurement of water vapor concentration in the air is demonstrated. A cost-effective quartz tuning fork (QTF) is used as the sharp transducer to convert light energy into an electrical signal based on the piezoelectric effect, thereby removing the need for a photodetector. The short optical path featured by the proposed sensing system leads to a decreased size. Furthermore, a pair of microresonators is applied in the absorbance detection module (ADM) for QTF signal enhancement. Compared with the system without microresonators, the detected QTF signal is increased to approximately 7-fold. Using this optimized QEPAS sensor with the proper modulation frequency and depth, we measure the water vapor concentration in the air at atmospheric pressure and room temperature. The experimental result shows that the sensor has a high sensitivity of 1.058 parts-per-million.

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A 1.7-ps pulse mode-locked Yb³⁺:Sc₂SiO₅ laser with a reflective graphene oxide saturable absorber

葛平广, 苏黎明, 刘杰, 郑丽和, 苏良碧, 徐军, 王勇刚

2015 (1): 14207-014207. doi: 10.1088/1674-1056/24/1/014207

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By using a reflective graphene oxide as saturable absorber, a diode-pumped passively mode-locked Yb³⁺:Sc₂SiO₅ (Yb:SSO) laser has been demonstrated for the first time. Without extra negative dispersion compensation, the minimum pulse duration of 1.7 ps with a repetition rate of 94 MHz was obtained at the central wavelength of 1062.6 nm. The average output power amounts to 355 mW under the absorbed pump power of 15 W. The maximum peak power of the mode-locking laser is up to 2.2 kW, and the single pulse energy is 3.8 nJ.

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Femtosecond filamentation induced fluorescence technique for atmospheric sensing

袁帅, 陈瑞良, 曾和平

2015 (1): 14208-014208. doi: 10.1088/1674-1056/24/1/014208

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Recent progress in filament-induced atmospheric sensing is reviewed. Self-guided propagation of ultrashort laser pulses in air induces laser filamentation. All molecules in the path of a filament can be dissociated into highly excited fragments, resulting in emission of characteristic fluorescence spectra. The fluorescence spectra provide information about the various molecules in the filaments. By using a filament-induced “fingerprinting” fluorescence technique, molecules in the atmosphere can be identified.

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Three-wavelength generation from cascaded wavelength conversion in monolithic periodically poled lithium niobate

肖坤, 张静, 陈宝琴, 张秋琳, 张东香, 冯宝华, 张景园

2015 (1): 14209-014209. doi: 10.1088/1674-1056/24/1/014209

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Tunable coherent emission is generated in a single-pass, cascaded wavelength conversion process from mode-locked laser-pumped monolithic periodically poled lithium niobate (PPLN). Three ranges of wavelength, including visible output from 628 nm to 639 nm, near-infrared output from 797 nm to 816 nm, and mid-infrared output from 3167 nm to 3459 nm, were obtained from the monolithic PPLN, which consists of a 10-mm section for 532-nm-pumped optical parametric generation (OPG) and a 7-mm section for 1064-nm-pumped sum frequency generation (SFG). A pump-to-signal conversion efficiency of 23.4% for OPG at 50 ℃ and a quantum efficiency of 26.2% for SFG at 200 ℃ were obtained.

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Fluctuations of optical phase of diffracted light for Raman-Nath diffraction in acousto-optic effect

翁存程, 章小曼

2015 (1): 14210-014210. doi: 10.1088/1674-1056/24/1/014210

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The Raman-Nath diffraction in acousto-optic effect was studied theoretically and experimentally in the paper. Up to now, each order of diffracted light in Raman-Nath diffraction was still considered simply to be just frequency-shifted and to be a plane wave. However, we find that the phase and frequency shifts occur simultaneously and individually in Raman-Nath diffraction. The findings demonstrate that, in addition to the frequency shift, the optical phase of each order of diffracted light is also shifted by the sound wave and fluctuates with the sound wave and is related to the location in the acoustic field from which the diffracted light originates. As a result, the wavefront of each order of diffracted light is modulated to fluctuate spatially and temporally with the sound wave. Obviously, these findings are significant for applications of Raman-Nath diffraction in acousto-optic effect because the optical phase plays an important role in optical coherence technology.

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Backward Raman amplification in plasmas with chirped wideband pump and seed pulses

吴朝辉, 魏晓峰, 左言磊, 刘兰琴, 张智猛, 李敏, 周煜梁, 粟敬钦

2015 (1): 14211-014211. doi: 10.1088/1674-1056/24/1/014211

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Chirped wideband pump and seed pulses are usually considered for backward Raman amplification (BRA) in plasmas to achieve an extremely high-power laser pulse. However, current theoretical models only contain either a chirped pump or a chirped seed. In this paper, modified three-wave coupling equations are proposed for the BRA in the plasmas with both chirped wideband pump and seed. The simulation results can more precisely describe the experiments, such as the Princeton University experiment. The optimized chirp and bandwidth are determined based on the simulation to enhance the output intensity and efficiency.

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Picosecond pulses compression at 1053-nm center wavelength by using a gas-filled hollow-core fiber compressor

黄志远, 王丁, 冷雨欣, 戴晔

2015 (1): 14212-014212. doi: 10.1088/1674-1056/24/1/014212

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We theoretically study the nonlinear compression of picosecond pulses with 10-mJ of input energy at the 1053-nm center wavelength by using a one-meter-long gas-filled hollow-core fiber (HCF) compressor and considering the third-order dispersion (TOD) effect. It is found that when the input pulse is about 1 ps/10 mJ, it can be compressed down to less than 20 fs with a high transmission efficiency. The gas for optimal compression is krypton gas which is filled in a HCF with a 400-μm inner diameter. When the input pulse duration is increased to 5 ps, it can also be compressed down to less than 100 fs efficiently under proper conditions. The results show that the TOD effect has little impact on picosecond pulse compression and the HCF compressor can be applied on compressing picosecond pulses efficiently with a high compression ratio, which will benefit the research of high-field laser physics.

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Crosstalk elimination in multi-view autostereoscopic display based on polarized lenticular lens array

王志远, 侯春萍

2015 (1): 14213-014213. doi: 10.1088/1674-1056/24/1/014213

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An autostereoscopic display composed of a directional backlight, an image display panel, a striped half-wave plate, and a polarized lenticular lens array is proposed. The directional backlight emitting the parallel light can redirect the cones of light to lenticular lens array and reduce the chromatic spatial-interference effect. The striped half-wave plate, located in front of the image display panel, transformed the polarization direction of the lights from the directional backlight into two mutually perpendicular directions. The polarized lenticular lens array not only can divide the light from the left and right view images to send to left and right eyes but also can reduce the crosstalk of the stereoscopic images. The proposed autostereoscopic display can produce high quality stereoscopic images without crosstalk at the optimal viewing distance.

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All-fiber optical modulator based on no-core fiber and magnetic fluid as cladding

陈耀飞, 韩群, 刘铁根

2015 (1): 14214-014214. doi: 10.1088/1674-1056/24/1/014214

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An all-fiber optical modulator, which is composed of a piece of no-core fiber spliced between two sections of single-mode fibers and uses magnetic fluid (MF) as the cladding of the no-core fiber section, is proposed and investigated experimentally. Due to the tunable refractive index and absorption coefficient of MF, the output intensity can be modulated by controlling an applied magnetic field. The dependences of the modulator's temporal response on the working wavelength, the magnetic field strength (H), and the MF's concentration are investigated experimentally. The results are explained qualitatively by the dynamic response process of MF under the action of a magnetic field. The findings are helpful for optimizing this kind of modulator.

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Sound field prediction of ultrasonic lithotripsy in water with spheroidal beam equations

张略, 王祥达, 刘晓宙, 龚秀芬

2015 (1): 14301-014301. doi: 10.1088/1674-1056/24/1/014301

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With converged shock wave, extracorporeal shock wave lithotripsy (ESWL) has become a preferable way to crush human calculi because of its advantages of efficiency and non-intrusion. Nonlinear spheroidal beam equations (SBE) are employed to illustrate the acoustic wave propagation for transducers with a wide aperture angle. To predict the acoustic field distribution precisely, boundary conditions are obtained for the SBE model of the monochromatic wave when the source is located on the focus of an ESWL transducer. Numerical results of the monochromatic wave propagation in water are analyzed and the influences of half-angle, fundamental frequency, and initial pressure are investigated. According to our results, with optimization of these factors, the pressure focal gain of ESWL can be enhanced and the effectiveness of treatment can be improved.

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Reception pattern influence on magnetoacoustic tomography with magnetic induction

孙晓冬, 王欣, 周雨琦, 马青玉, 章东

2015 (1): 14302-014302. doi: 10.1088/1674-1056/24/1/014302

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Based on the acoustic radiation theory of a dipole source, the influence of the transducer reception pattern is studied for magnetoacoustic tomography with magnetic induction (MAT-MI). Numerical studies are conducted to simulate acoustic pressures, waveforms, and reconstructed images with unidirectional, omnidirectional, and strong directional transducers. With the analyses of equivalent and projection sources, the influences of the model dimension and the layer effect are qualitatively analyzed to evaluate the performance of MAT-MI. Three-dimensional simulation studies show that the strong directional transducer with a large radius can reduce the influences of equivalent sources, projection sources, and the layer effect effectively, resulting in enhanced pressure and improved image contrast, which is beneficial for boundary pressure extraction in conductivity reconstruction. The reconstructed conductivity contrast images present the conductivity boundaries as stripes with different contrasts and polarities, representing the values and directions of the conductivity changes of the scanned layer. The favorable results provide solid evidence for transducer selection and suggest potential practical applications of MAT-MI in biomedical imaging.

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Near-field radiative heat transfer in mesoporous alumina

李静, 冯妍卉, 张欣欣, 黄丛亮, 王戈

2015 (1): 14401-014401. doi: 10.1088/1674-1056/24/1/014401

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The thermal conductivity of mesoporous material has aroused the great interest of scholars due to its wide applications such as insulation, catalyst, etc. Mesoporous alumina substrate consists of uniformly distributed, unconnected cylindrical pores. Near-field radiative heat transfer cannot be ignored, when the diameters of the pores are less than the characteristic wavelength of thermal radiation. In this paper, near-field radiation across a cylindrical pore is simulated by employing the fluctuation dissipation theorem and Green function. Such factors as the diameter of the pore, and the temperature of the material are further analyzed. The research results show that the radiative heat transfer on a mesoscale is 2～ 4 orders higher than on a macroscale. The heat flux and equivalent thermal conductivity of radiation across a cylindrical pore decrease exponentially with pore diameter increasing, while increase with temperature increasing. The calculated equivalent thermal conductivity of radiation is further developed to modify the thermal conductivity of the mesoporous alumina. The combined thermal conductivity of the mesoporous alumina is obtained by using porosity weighted dilute medium and compared with the measurement. The combined thermal conductivity of mesoporous silica decreases gradually with pore diameter increasing, while increases smoothly with temperature increasing, which is in good agreement with the experimental data. The larger the porosity, the more significant the near-field effect is, which cannot be ignored.

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Stability and Hopf bifurcation of a nonlinear electromechanical coupling system with time delay feedback

刘爽, 赵双双, 王兆龙, 李海滨

2015 (1): 14501-014501. doi: 10.1088/1674-1056/24/1/014501

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The stability and the Hopf bifurcation of a nonlinear electromechanical coupling system with time delay feedback are studied. By considering the energy in the air-gap field of the AC motor, the dynamical equation of the electromechanical coupling transmission system is deduced and a time delay feedback is introduced to control the dynamic behaviors of the system. The characteristic roots and the stable regions of time delay are determined by the direct method, and the relationship between the feedback gain and the length summation of stable regions is analyzed. Choosing the time delay as a bifurcation parameter, we find that the Hopf bifurcation occurs when the time delay passes through a critical value. A formula for determining the direction of the Hopf bifurcation and the stability of the bifurcating periodic solutions is given by using the normal form method and the center manifold theorem. Numerical simulations are also performed, which confirm the analytical results.

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A measurement method for distinguishing the real contact area of rough surfaces of transparent solids using improved Otsu technique

宋保江, 阎绍泽, 向吴维凯

2015 (1): 14601-014601. doi: 10.1088/1674-1056/24/1/014601

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An experimental method of measuring the real contact area of transparent blocks based on the principle of total internal reflection is presented, intending to support the investigation of friction characteristics, heat conduction, and energy dissipation at the contact interface. A laser sheet illuminates the contact interface, and the transmitted laser sheet is projected onto a screen. Then the contact information is acquired from the screen by a camera. An improved Otsu method is proposed to process the data of experimental images. It can compute the threshold of the overall image and filter out all the pixels one by one. Through analyzing the experimental results, we describe the relationship between the real contact area and the positive pressure during a continuous loading process, at different loading rates, with the polymethyl methacrylate (PMMA) material. A hysteresis phenomenon in the relationship between the real contact area and the positive pressure is found and explained.

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Critical deflagration waves leading to detonation onset under different boundary conditions

林伟, 周进, 范孝华, 林志勇

2015 (1): 14701-014701. doi: 10.1088/1674-1056/24/1/014701

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High-speed turbulent critical deflagration waves before detonation onset in H₂-air mixture propagated into a square cross section channel, which was assembled of optional rigid rough, rigid smooth, or flexible walls. The corresponding propagation characteristic and the influence of the wall boundaries on the propagation were investigated via high-speed shadowgraph and a high-frequency pressure sampling system. As a comprehensive supplement to the different walls effect investigation, the effect of porous absorbing walls on the detonation propagation was also investigated via smoke foils and the high-frequency pressure sampling system. Results are as follows. In the critical deflagration stage, the leading shock and the closely following turbulent flame front travel at a speed of nearly half the CJ detonation velocity. In the preheated zone, a zonary flame arises from the overlapping part of the boundary layer and the pressure waves, and then merges into the mainstream flame. Among these wall boundary conditions, the rigid rough wall plays a most positive role in the formation of the zonary flame and thus accelerates the transition of the deflagration to detonation (DDT), which is due to the boost of the boundary layer growth and the pressure wave reflection. Even though the flexible wall is not conducive to the pressure wave reflection, it brings out a faster boundary layer growth, which plays a more significant role in the zonary flame formation. Additionally, the porous absorbing wall absorbs the transverse wave and yields detonation decay and velocity deficit. After the absorbing wall, below some low initial pressure conditions, no re-initiation occurs and the deflagration propagates in critical deflagration for a relatively long distance.

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Partial slip effect on non-aligned stagnation point nanofluid over a stretching convective surface

S. Nadeem, Rashid Mehmood, Noreen Sher Akbar

2015 (1): 14702-014702. doi: 10.1088/1674-1056/24/1/014702

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The present study inspects the non-aligned stagnation point nano fluid over a convective surface in the presence of partial slip. Two types of base fluids namely water and kerosene are selected with Cu nanoparticles. The governing physical problem is presented and transformed into a system of coupled nonlinear differential equations using suitable similarity transformations. These equations are then solved numerically using midpoint integration scheme along with Richardson extrapolation via Maple. Impact of relevant physical parameters on the dimensionless velocity and temperature profiles are portrayed through graphs. Physical quantities such as local skin frictions co-efficient and Nusselt numbers are tabularized. It is detected from numerical computations that kerosene-based nano fluids have better heat transfer capability compared with water-based nanofluids. Moreover it is found that water-based nanofluids offer less resistance in terms of skin friction than kerosene-based fluid. In order to authenticate our present study, the calculated results are compared with the prevailing literature and a considerable agreement is perceived for the limiting case.

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Lattice Boltzmann simulation of liquid–vapor system by incorporating a surface tension term

宋保维, 任峰, 胡海豹, 黄桥高

2015 (1): 14703-014703. doi: 10.1088/1674-1056/24/1/014703

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In this study, we investigate the pseudopotential multiphase model of lattice Boltzmann method (LBM) and incorporate a surface tension term to implement the particle interaction force. By using the Carnahan-Starling (CS) equation of state (EOS) with a proper critical pressure-density ratio, a density ratio over 160000 is obtained with satisfactory numerical stability. The added surface tension term offers a flexible choice to adjust the surface tension strength. Numerical tests of the Laplace rule are conducted, proving that smaller spurious velocity and better numerical stability can be acquired as the surface tension becomes stronger. Moreover, by wall adhesion and heterogeneous cavitation tests, the surface tension term shows its practical application in dealing with problems in which the surface tension plays an important role.

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Surface modification of magnetic nanoparticles in biomedicine

储鑫, 余靓, 侯仰龙

2015 (1): 14704-014704. doi: 10.1088/1674-1056/24/1/014704

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Progress in surface modification of magnetic nanoparticles (MNPs) is summarized with regard to organic molecules, macromolecules and inorganic materials. Many researchers are now devoted to synthesizing new types of multi-functional MNPs, which show great application potential in both diagnosis and treatment of disease. By employing an ever-greater variety of surface modification techniques, MNPs can satisfy more and more of the demands of medical practice in areas like magnetic resonance imaging (MRI), fluorescent marking, cell targeting, and drug delivery.

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Thermodynamic study of fluid in terms of equation of state containing physical parameters

S. B. Khasare

2015 (1): 15101-015101. doi: 10.1088/1674-1056/24/1/015101

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We introduce a simple condition for one mole fluid by considering the thermodynamics of molecules pointing towards the effective potential for the cluster. Efforts are made to estimate new physical parameter f in liquid state using the equation of state containing only two physical parameters such as the hard sphere diameter and binding energy. The temperature dependence of the structural properties and the thermodynamic behavior of the clusters are studied. Computations based on f predict the variation of numbers of particles at the contact point of the molecular cavity (radial distribution function). From the thermodynamic profile of the fluid, the model results are discussed in terms of the cavity due to the closed surface along with suitable energy. The present calculation is based upon the sample thermodynamic data for n-hexanol, such as the ultrasonic wave, density, volume expansion coefficient, and ratio of specific heat in the liquid state, and it is consistent with the thermodynamic relations containing physical parameters such as size and energy. Since the data is restricted to n-hexanol, we avoid giving the physical meaning of f, which is the key parameter studied in the present work.

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Absorption of ultrashort intense lasers in laser-solid interactions

盛政明, 翁苏明, 於陆勒, 王伟民, 崔云千, 陈民, 张杰

2015 (1): 15201-015201. doi: 10.1088/1674-1056/24/1/015201

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With the advent of ultrashort high intensity laser pulses, laser absorption during the laser-solid interactions has received significant attention over the last two decades since it is related to a variety of applications of high intensity lasers, including the hot electron production for fast ignition of fusion targets, table-top bright X-ray and gamma-ray sources, ion acceleration, compact neutron sources, and generally the creation of high energy density matters. Normally, some absorption mechanisms found for nanosecond long laser pulses also appear for ultrashort laser pulses. The peculiar aspects with ultrashort laser pulses are that their absorption depends significantly on the preplasma condition and the initial target structures. Meanwhile, relativistic nonlinearity and ponderomotive force associated with the laser pulses lead to new mechanisms or phenomena, which are usually not found with nanosecond long pulses. In this paper, we present an overview of the recent progress on the major absorption mechanisms in intense laser-solid interactions, where emphasis is paid to our related theory and simulation studies.

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Cylindrical effects in weakly nonlinear Rayleigh–Taylor instability

刘万海, 马文芳, 王绪林

2015 (1): 15202-015202. doi: 10.1088/1674-1056/24/1/015202

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The classical Rayleigh-Taylor instability (RTI) at the interface between two variable density fluids in the cylindrical geometry is explicitly investigated by the formal perturbation method up to the second order. Two styles of RTI, convergent (i.e., gravity pointing inward) and divergent (i.e., gravity pointing outwards) configurations, compared with RTI in Cartesian geometry, are taken into account. Our explicit results show that the interface function in the cylindrical geometry consists of two parts: oscillatory part similar to the result of the Cartesian geometry, and non-oscillatory one contributing nothing to the result of the Cartesian geometry. The velocity resulting only from the non-oscillatory term is followed with interest in this paper. It is found that both the convergent and the divergent configurations have the same zeroth-order velocity, whose magnitude increases with the Atwood number, while decreases with the initial radius of the interface or mode number. The occurrence of non-oscillation terms is an essential character of the RTI in the cylindrical geometry different from Cartesian one.

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Tunable terahertz plasmon in grating-gate coupled graphene with a resonant cavity

闫博, 杨昕昕, 方靖岳, 黄永丹, 秦华, 秦石乔

2015 (1): 15203-015203. doi: 10.1088/1674-1056/24/1/015203

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Plasmon modes in graphene can be tuned into resonance with an incident terahertz electromagnetic wave in the range of 1-4 THz by setting a proper gate voltage. By using the finite-difference-time-domain (FDTD) method, we simulate a graphene plasmon device comprising a single-layer graphene, a metallic grating, and a terahertz cavity. The simulations suggest that the terahertz electric field can be enhanced by several times due to the grating-cavity configuration. Due to this near-field enhancement, the maximal absorption of the incident terahertz wave reaches up to about 45%.

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Studies of collisionless shockwaves using high-power laser pulses in laboratories

袁大伟, 李玉同

2015 (1): 15204-015204. doi: 10.1088/1674-1056/24/1/015204

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The remarkable experimental progress in the studies of collisionless shockwave (CS) in laboratories employing high-power lasers is briefly reviewed. The results show that CS can be generated in laser-produced plasmas due to the micro-turbulence associated with instabilities. CS is one of the most important astronomical phenomena. It has been found in supernova remnants (SNRs), Sun-Earth space, etc. This paper focuses on CS in ways relevant to SNRs. Laboratory astrophysics (LA), a new interdisciplinary frontier of astrophysics, plasma and laser physics, has developed rapidly in recent years. As an accessory to the astronomical observation, LA experimenters can closely study some astronomical events scaled-down to controllable phenomena.

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Developments in laser wakefield accelerators: From single-stage to two-stage

李文涛, 王文涛, 刘建胜, 王成, 张志钧, 齐荣, 余昌海, 李儒新, 徐至展

2015 (1): 15205-015205. doi: 10.1088/1674-1056/24/1/015205

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Laser wakefield accelerators (LWFAs) are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators. It is attributed to their high acceleration gradient which is about 3 orders of magnitude larger than the traditional ones. The past decade has witnessed the major breakthroughs and progress in developing the laser wakfield accelerators. To achieve the LWFAs suitable for applications, more and more attention has been paid to optimize the LWFAs for high-quality electron beams. A single-staged LWFA does not favor generating controllable electron beams beyond 1 GeV since electron injection and acceleration are coupled and cannot be independently controlled. Staged LWFAs provide a promising route to overcome this disadvantage by decoupling injection from acceleration and thus the electron-beam quality as well as the stability can be greatly improved. This paper provides an overview of the physical conceptions of the LWFA, as well as the major breakthroughs and progress in developing LWFAs from single-stage to two-stage LWFAs.

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Growth of PbS nanoclusters on specific sites of programmed oligodeoxynucleotides

陆颖, 滕翠娟, 李颖, 王惠, 徐春华, 胡书新, 李明

2015 (1): 16101-016101. doi: 10.1088/1674-1056/24/1/016101

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We develope a method to synthesize PbS nanoclusters (NCs) using guanine-containing oligodeoxynucleotides (ODNs) as templates. The NCs on the ODNs are ultra small (ranging from ～ 0.5 nm to 2.1 nm) and luminescent in the visible region. They are characterized by photoluminescence (PL) spectra, transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). The ODN-NC complexes can be used as customer-designed fluorophores which do not have the problem of multiple conjugations. The same method enables us to fabricate PbS quantum dot molecules and connect them into nanowires, expanding their potential applications in molecule electronics and quantum computing.

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Mechanism of single-event transient pulse quenching between dummy gate isolated logic nodes

陈建军, 池雅庆, 梁斌

2015 (1): 16102-016102. doi: 10.1088/1674-1056/24/1/016102

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As integrated circuits scale down in size, a single high-energy ion strike often affects multiple adjacent logic nodes. The so-called single-event transient (SET) pulse quenching induced by single-event charge sharing collection has been widely studied. In this paper, SET pulse quenching enhancement is found in dummy gate isolated adjacent logic nodes compared with that isolated by the common shallow trench isolation (STI). The physical mechanism is studied in depth and this isolation technique is explored for SET mitigation in combinational standard cells. Three-dimensional (3D) technology computer-aided design simulation (TCAD) results show that this technique can achieve efficient SET mitigation.

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Electron-acoustic phonon interaction and mobility in stressed rectangular silicon nanowires

朱林利

2015 (1): 16201-016201. doi: 10.1088/1674-1056/24/1/016201

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We investigate the effects of pre-stress and surface tension on the electron-acoustic phonon scattering rate and the mobility of rectangular silicon nanowires. With the elastic theory and the interaction Hamiltonian for the deformation potential, which considers both the surface energy and the acoustoelastic effects, the phonon dispersion relation for a stressed nanowire under spatial confinement is derived. The subsequent analysis indicates that both surface tension and pre-stress can dramatically change the electron-acoustic phonon interaction. Under a negative (positive) surface tension and a tensile (compressive) pre-stress, the electron mobility is reduced (enhanced) due to the decrease (increase) of the phonon energy as well as the deformation-potential scattering rate. This study suggests an alternative approach based on the strain engineering to tune the speed and the drive current of low-dimensional electronic devices.

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Giant magnetic moment at open ends of multiwalled carbon nanotubes

王钢, 陈闽江, 余芳, 薛磊江, 邓娅, 张键, 齐笑迎, 高燕, 褚卫国, 刘广同, 杨海方, 顾长志, 孙连峰

2015 (1): 16202-016202. doi: 10.1088/1674-1056/24/1/016202

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The attractions of cantilevers made of multiwalled carbon nanotubes (MWNTs) and secured on one end are studied in the non-uniform magnetic field of a permanent magnet. Under an optical microscope, the positions and the corresponding deflections of the original cantilevers (with iron catalytic nanoparticles at the free end) and corresponding cut-off cantilevers (the free ends consisting of open ends of MWNTs) are studied. Both kinds of CNT cantilevers are found to be attracted by the magnet, and the point of application of force is proven to be at the tip of the cantilever. By measuring and comparing deflections between these two kinds of cantilevers, the magnetic moment at the open ends of the CNTs can be quantified. Due to the unexpectedly high value of the magnetic moment at the open ends of carbon nanotubes, it is called giant magnetic moment, and its possible mechanisms are proposed and discussed.

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Effect of far-field flow on a columnar crystal in the convective undercooled melt

姬小建, 陈明文, 徐小花, 王自东

2015 (1): 16401-016401. doi: 10.1088/1674-1056/24/1/016401

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The growth behavior of a columnar crystal in the convective undercooled melt affected by the far-field uniform flow is studied and the asymptotic solution for the interface evolution of the columnar crystal is derived by means of the asymptotic expansion method. The results obtained reveal that the far-field flow induces a significant change of the temperature around the columnar crystal and the convective flow caused by the far-field flow accelerates the growth velocity of the interface of the growing columnar crystal in the upstream direction and inhibits its growth velocity in the downstream direction. Our results are similar to the experimental data and numerical simulations.

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Tb doping induced enhancement of anomalous Hall effect in NiFe films

朱嘉鹏, 马丽, 周仕明, 苗君, 姜勇

2015 (1): 17101-017101. doi: 10.1088/1674-1056/24/1/017101

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Tb_x(Ni_0.8Fe_0.2)_1-x films with x≤0.14 are fabricated and the anomalous Hall effect is studied. The intrinsic anomalous Hall conductivity and the extrinsic one from the impurity and phonon induced scattering both increase with increasing x. The enhancement of the intrinsic anomalous Hall conductivity is ascribed to both the weak spin-orbit coupling enhancement and the Fermi level shift. The enhancement of the extrinsic term comes from the changes of both Fermi level and impurity distribution. In contrast, the in-plane and the out-of-plane uniaxial anisotropies in the TbNiFe films change little with x. The enhancement of the Hall angle by Tb doping is helpful for practical applications of the Hall devices.

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Tight-binding electron-phonon coupling and band renormalization in graphene

张德生, 康广震, 李俊

2015 (1): 17301-017301. doi: 10.1088/1674-1056/24/1/017301

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The kink structure in the quasiparticle spectrum of electrons in graphene observed at 200 meV below the Fermi level by angle-resolved photoemission spectroscopy (ARPES) was claimed to be caused by a tight-binding electron-phonon (e-ph) coupling in the previous theoretical studies. However, we numerically find that the e-ph coupling effect in this approach is too weak to account for the ARPES data. The former agreement between this approach and the ARPES data is due to an enlargement of the coupling constant by almost four times.

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Influence of compressive strain on the incorporation of indium in InGaN and InAlN ternary alloys

赵一, 张进成, 薛军帅, 周小伟, 许晟瑞, 郝跃

2015 (1): 17302-017302. doi: 10.1088/1674-1056/24/1/017302

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In order to investigate the influence of compressive strain on indium incorporation in InAlN and InGaN ternary nitrides, InAlN/GaN heterostructures and InGaN films were grown by metal-organic chemical vapor deposition. For the heterostructures, different compressive strains are produced by GaN buffer layers grown on unpatterned and patterned sapphire substrates thanks to the distinct growth mode; while for the InGaN films, compressive strains are changed by employing AlGaN templates with different aluminum compositions. By various characterization methods, we find that the compressive strain will hamper the indium incorporation in both InAlN and InGaN. Furthermore, compressive strain is conducive to suppress the non-uniform distribution of indium in InGaN ternary alloys.

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Degradation mechanism of enhancement-mode AlGaN/GaN HEMTs using fluorine ion implantation under the on-state gate overdrive stress

孙伟伟, 郑雪峰, 范爽, 王冲, 杜鸣, 张凯, 陈伟伟, 曹艳荣, 毛维, 马晓华, 张进成, 郝跃

2015 (1): 17303-017303. doi: 10.1088/1674-1056/24/1/017303

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The degradation mechanism of enhancement-mode AlGaN/GaN high electron mobility transistors (HEMTs) fabricated by fluorine plasma ion implantation technology is one major concern of HEMT's reliability. It is observed that the threshold voltage shows a significant negative shift during the typical long-term on-state gate overdrive stress. The degradation does not originate from the presence of as-grown traps in the AlGaN barrier layer or the generated traps during fluorine ion implantation process. By comparing the relationships between the shift of threshold voltage and the cumulative injected electrons under different stress conditions, a good agreement is observed. It provides direct experimental evidence to support the impact ionization physical model, in which the degradation of E-mode HEMTs under gate overdrive stress can be explained by the ionization of fluorine ions in the AlGaN barrier layer by electrons injected from 2DEG channel. Furthermore, our results show that there are few new traps generated in the AlGaN barrier layer during the gate overdrive stress, and the ionized fluorine ions cannot recapture the electrons.

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Fano-type resonances induced by a boson mode in Andreev conductance

J. Barański, T. Domański

2015 (1): 17304-017304. doi: 10.1088/1674-1056/24/1/017304

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We study spectroscopic signatures of a monochromatic boson mode interacting with a T-shape double quantum dot coupled between the metallic and superconducting leads. Focusing on a weak interdot coupling, we find that the proximity effect together with the bosonic mode are responsible for the series of Fano-type resonances appearing simultaneously at negative and positive energies. We investigate these interferometric features and discuss their influence on the subgap Andreev conductance taking into account the correlation effects driven by the Coulomb repulsion.

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Different charging behaviors between electrons and holes in Si nanocrystals embedded in SiN_x matrix by the influence of near-interface oxide traps

方忠慧, 江小帆, 陈坤基, 王越飞, 李伟, 徐骏

2015 (1): 17305-017305. doi: 10.1088/1674-1056/24/1/017305

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Si-rich silicon nitride films are prepared by plasma-enhanced chemical vapor deposition method, followed by thermal annealing to form the Si nanocrystals (Si-NCs) embedded in SiN_x floating gate MOS structures. The capacitance-voltage (C-V), current-voltage (I-V), and admittance-voltage (G-V) measurements are used to investigate the charging characteristics. It is found that the maximum flat band voltage shift (ΔV_FB) due to full charged holes (～ 6.2 V) is much larger than that due to full charged electrons (～ 1 V). The charging displacement current peaks of electrons and holes can be also observed by the I-V measurements, respectively. From the G-V measurements we find that the hole injection is influenced by the oxide hole traps which are located near the SiO₂/Si-substrate interface. Combining the results of C-V and G-V measurements, we find that the hole charging of the Si-NCs occurs via a two-step tunneling mechanism. The evolution of G-V peak originated from oxide traps exhibits the process of hole injection into these defects and transferring to the Si-NCs.

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Applicability of the vortex-glass model for the single crystal Tl_0.4K_0.41Fe_1.71Se₂

于一, 汪春昌, 汪洪, 李秋菊, 张昌锦, 皮雳, 张裕恒

2015 (1): 17401-017401. doi: 10.1088/1674-1056/24/1/017401

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We measure the current-voltage (I-V) characteristics for the single crystal of Tl_0.4K_0.41Fe_1.71Se₂ with the superconducting transition temperature (T_C) around 30.5 K, under a 10 T magnetic field applied perpendicular and parallel to the ab plane. We find that the shapes of the I-V isotherms are very different from the description by the vortex-glass (VG) model. Combining theoretical calculations and analysis of the ρ_H⊥ab-T and ρ_H||ab-T data, we give an explicit discussion over the suitability of the VG model for the A_0.8Fe₂Se₂ superconductors, and point out the possibility of the material acting as a convenient platform for re-examination and further study of the complex vortex behaviors in the layered superconductors.

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Field-dependent resistive transitions in Yba₂Cu₃O_7-δ thin films: Influence of the pseudogap on vortex dynamics

S H Naqib, R S Islam

2015 (1): 17402-017402. doi: 10.1088/1674-1056/24/1/017402

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The nature of resistive transition of high-quality crystalline thin films of Yba₂Cu₃O_7-δ has been studied under magnetic fields (H) applied along the c direction over a wide range of doped holes, p, in the CuO₂ planes. The field- and temperature-dependent in-plane resistivity, ρ_ab(T,H), has been analyzed within the thermally assisted flux-flow (TAFF) formalism. The flux activation energy, U(T,H), has been extracted from this analysis. The low-T part of the ρ_ab(T,H) data can be described by an activation energy having the functional form of U(T,H) = (1-t)^m(H₀/H)^-β, where t = T/T_c (reduced temperature), and H₀ is a field scale that primarily determines the magnitude of U(T,H). The temperature exponent, m, shows a systematic variation with p, whereas the field exponent, β, is insensitive to the p values and is close to unity. The H₀, on the other hand, changes rapidly as p is varied. U(T,H) is linked to the pinning potential and consequently on the superconducting condensation energy. Since the normal state pseudogap directly affects superconducting condensation energy, a clear correspondence between H₀ and the PG energy scale, ε_g, is found. Possible implications of these results are discussed.

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Effects of pressure and/or magnetism on superconductivity of δ-MoN single crystal

苗博童, 王善民, 孔盼盼, 金美玲, 冯少敏, 张思佳, 郝爱民, 于晓辉, 靳常青, 赵予生

2015 (1): 17403-017403. doi: 10.1088/1674-1056/24/1/017403

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Effects of pressure and/or magnetism on the critical superconducting temperature (T_c) of δ-MoN single crystal were investigated using a Maglab system. The δ-MoN single crystal was synthesized at extreme conditions of high pressure and high temperature. The carrier density of δ-MoN single crystal as a function of applied pressure was determined using Hall coefficient measurement.

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Variational Monte Carlo study of the nematic state in iron-pnictide superconductors with a five-orbital model

郑晓军, 黄忠兵, 邹良剑

2015 (1): 17404-017404. doi: 10.1088/1674-1056/24/1/017404

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We perform a variational Monte Carlo study of the nematic state in iron-pnictide superconductors within a realistic five-orbital model. Our numerical results show that the nematic state, formed by introducing an anisotropic hopping order into the projected wave function, is not stable unless the off-site Coulomb interaction V exceeds a critical value. This demonstrates that V plays a key role in forming the nematic state in iron-pnictide superconductors. In the nematic state, the orbital order and the anisotropic spin correlations are consistent with the experimental observations. We argue that the experimentally observed anisotropic magnetic couplings and structural transition are associated with the nematic state and can be understood in a unified framework.

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Electronic, optical properties, surface energies and work functions of Ag₈SnS₆: First-principles method

卢春林, 张林, 张云望, 刘慎业, 梅杨

2015 (1): 17501-017501. doi: 10.1088/1674-1056/24/1/017501

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Ternary metal chalcogenide semiconductor Ag₈SnS₆, which is an efficient photocatalyst under visible light radiation, is studied by plane-wave pseudopotential density functional theory. After geometry optimization, the electronic and optical properties are studied. A scissor operator value of 0.81 eV is introduced to overcome the underestimation of the calculation band gaps. The contribution of different bands is analyzed by virtue of total and partial density of states. Furthermore, in order to understand the optical properties of Ag₈SnS₆, the dielectric function, absorption coefficient, and refractive index are also performed in the energy range from 0 to 11 eV. The absorption spectrum indicates that Ag₈SnS₆ has a good absorbency in visible light area. Surface energies and work functions of (4 11), (413), (211), and (112) orientations have been calculated. These results reveal the reason for an outstanding photocatalytic activity of Ag₈SnS₆.

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Effect of CoSi₂ buffer layer on structure and magnetic properties of Co films grown on Si (001) substrate

胡泊, 何为, 叶军, 汤进, Syed Sheraz Ahmad, 张向群, 成昭华

2015 (1): 17502-017502. doi: 10.1088/1674-1056/24/1/017502

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Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films. In this paper, Co films with different thickness of CoSi₂ buffer layers were grown on Si (001) substrates. In order to investigate morphology, structure, and magnetic properties of films, scanning tunneling microscope (STM), low energy electron diffraction (LEED), high resolution transmission electron microscopy (HRTEM), and surface magneto-optical Kerr effect (SMOKE) were used. The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of CoSi₂ buffer layers. Few CoSi₂ monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality. Furthermore, the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process, which is the typical phenomenon in cubic (001) films.

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High-pressure synthesis, characterization, and equation of state of double perovskite Sr₂CoFeO₆

潘跃武, 朱品文, 王欣

2015 (1): 17503-017503. doi: 10.1088/1674-1056/24/1/017503

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Double perovskite oxide Sr₂CoFeO₆ (SCFO) has been obtained using a high-pressure and high-temperature (HPHT) synthesis method. Valence states of Fe and Co and their distributions in SCFO were examined with X-ray photoelectron spectroscopy. The electric transport behavior of SCFO showed a semiconductor behavior that can be well described by Mott's law for variable-range hopping conduction. The structural stability of SCFO was investigated at pressures up to 31 GPa with no pressure-induced phase transition found. Bulk modulus B₀ was determined to be 163(2) GPa by fitting the pressure-volume data to the Birch-Murnaghan equation of state.

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Fine-grained NdFeB magnets prepared by low temperature pre-sintering and subsequent hot pressing

剧锦云, 唐旭, 陈仁杰, 汪金芝, 尹文宗, 李东, 闫阿儒

2015 (1): 17504-017504. doi: 10.1088/1674-1056/24/1/017504

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Fine-grained Nd_10.79Pr_2.8Al_0.4B_7.72Fe_78.29 magnets were prepared by low temperature pre-sintering and subsequent hot pressing. The grain size of the magnets is just about 1-3μm because the low sintering temperature results in no grain growth. The orientation degree, microstructure, and magnetic properties were studied. Some grains' easy axes deviate from the orientation direction, possibly due to grain rotation during the hot pressing. By subsequent annealing, the magnetic properties were significantly enhanced. Especially, the squareness of the demagnetization curve was improved greatly. The enhancement of coercivity by annealing can be explained by an improvement of both grain boundaries and magnetic isolation, which decouples the exchange interaction between neighboring grains.

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MnFe(PGe) compounds: Preparation, structural evolution, and magnetocaloric effects

岳明, 张红国, 刘丹敏, 张久兴

2015 (1): 17505-017505. doi: 10.1088/1674-1056/24/1/017505

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The interdependences of preparation conditions, magnetic and crystal structures, and magnetocaloric effects (MCE) of the MnFePGe-based compounds are reviewed. Based upon those findings, a new method for the evaluation of the MCE in these compounds, based on differential scanning calorimetry (DSC), is proposed. The MnFePGe-based compounds are a group of magnetic refrigerants with giant magnetocaloric effect (GMCE), and as such, have drawn tremendous attention, especially due to their many advantages for practical applications. Structural evolution and phase transformation in the compounds as functions of temperature, pressure, and magnetic field are reported. Influences of preparation conditions upon the homogeneity of the compounds' chemical composition and microstructure, both of which play a key role in the MCE and thermal hysteresis of the compounds, are introduced. Lastly, the origin of the “virgin effect” in the MnFePGebased compounds is discussed.

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Dynamics of magnetic skyrmions

刘冶华, 李有泉

2015 (1): 17506-017506. doi: 10.1088/1674-1056/24/1/017506

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We review the recent progress on the magnetic skyrmions in chiral magnetic materials. The magnetic skyrmion is a topological spin configuration with localized spatial extent, which could be thought of as an emergent rigid particle, owing to its particular topological and chiral properties. Static skyrmionic configurations have been found in various materials with different transport and thermodynamic properties. The magnetic skyrmions respond to externally applied fields in a very unique way, and their coupling to other quasiparticles in solid-state systems gives rise to the emergent electrodynamics. Being not only theoretically important, the magnetic skyrmion is also very promising to be the information carrier in next generation spintronic devices.

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Ferroelectricity in hexagonal YFeO₃ film at room temperature

张润兰, 陈长乐, 张云婕, 邢辉, 董祥雷, 金克新

2015 (1): 17701-017701. doi: 10.1088/1674-1056/24/1/017701

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In this paper we report the leakage current, ferroelectric and piezoelectric properties of the YFeO₃ film with hexagonal structure, which was fabricated on Si(111) substrate by a simple sol-gel method. The leakage current test shows good characteristics as the leakage current density is 5.4× 10^-6A/cm² under 5 V. The dominant leakage mechanism is found to be an Ohmic behavior at low electric field and space-charge-limited conduction at high electric field region. The P-E measurements show ferroelectric hysteresis loops with small remnant polarization and coercive field at room temperature. The distinct and switchable domain structures on the nanometer scale are observed by piezoresponse force microscopy, which testifies to the ferroelectricity of the YFeO₃ film further.

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Temperature-dependent Raman spectroscopic study of bismuth borate Bi₂ZnOB₂O₆

张季, 张德明, 张庆礼, 殷绍唐

2015 (1): 17801-017801. doi: 10.1088/1674-1056/24/1/017801

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A temperature-dependent Raman spectroscopic study on Bi₂ZnOB₂O₆ crystal was carried out to investigate the structure change of the crystal with the increase of temperature. Raman spectra of crystal Bi₂ZnOB₂O₆ were recorded in the spectral range 10-1600 cm^-1 at room temperature first. Compared with the vibrational spectra of the referred compounds, satisfactory assignment of most of the high-energy modes to vibrations of Bi-O, B-O, and Zn-O bonds was achieved. In particular, the Raman high-frequency peak located at 1344 cm^-1 was attributed to the B-O vibration in the BO₃ triangle. This temperature-dependent Raman spectroscopic study was carried out up to 600 ℃. It was found that all the Raman lines exhibit decreases in frequency and the widths of the Raman peaks increase with increasing temperature. No phase transition was observed under 600 ℃.

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Ordered silicon nanorod arrays with controllable geometry and robust hydrophobicity

王子文, 蔡家琦, 吴以治, 王会杰, 许小亮

2015 (1): 17802-017802. doi: 10.1088/1674-1056/24/1/017802

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Highly ordered silicon nanorod (SiNR) arrays with controllable geometry are fabricated via nanosphere lithography and metal-assisted chemical etching. It is demonstrated that the key to achieving a high-quality metal mask is to construct a non-close-packed template that can be removed with negligible damage to the mask. Hydrophobicity of SiNR arrays of different geometries is also studied. It is shown that the nanorod structures are effectively quasi-hydrophobic with a contact angle as high as 142°, which would be useful in self-cleaning nanorod-based device applications.

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Fluctuations of electrical and mechanical properties of diamond induced by interstitial hydrogen

庄春强, 刘雷

2015 (1): 18101-018101. doi: 10.1088/1674-1056/24/1/018101

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While experimental evidence demonstrates that the presence of hydrogen (H) impurities in diamond films plays a significant role in determining their physical properties, the small radius of the H atom makes detecting such impurities quite a challenging task. In the present work, first-principles calculations were employed to provide an insight into the effects of the interstitial hydrogen on the electrical and mechanical properties of diamond crystals at the atomic level. The migrated pathways of the interstitial hydrogen are dictated by energetic considerations. Some new electronic states are formed near the Fermi level. The interstitial hydrogen markedly narrows the bandgap of the diamond and weakens the diamond crystal. The obvious decrement of the critical strain clearly implies the presence of an H-induced embrittlement effect.

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High-crystalline GaSb epitaxial films grown on GaAs(001) substrates by low-pressure metal-organic chemical vapor deposition

王连锴, 刘仁俊, 吕游, 杨皓宇, 李国兴, 张源涛, 张宝林

2015 (1): 18102-018102. doi: 10.1088/1674-1056/24/1/018102

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Orthogonal experiments of GaSb films growth on GaAs(001) substrates have been designed and performed by using a low-pressure metal-organic chemical vapor deposition (LP-MOCVD) system. The crystallinities and microstructures of the produced films were comparatively analyzed to achieve the optimum growth parameters. It was demonstrated that the optimized GaSb thin film has a narrow full width at half maximum (358 arc sec) of the (004) ω-rocking curve, and a smooth surface with a low root-mean-square roughness of about 6 nm, which is typical in the case of the heteroepitaxial single-crystal films. In addition, we studied the effects of layer thickness of GaSb thin film on the density of dislocations by Raman spectra. It is believed that our research can provide valuable information for the fabrication of high-crystalline GaSb films and can promote the integration probability of mid-infrared devices fabricated on mainstream performance electronic devices.

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Measurement of micro weld joint position based on magneto-optical imaging

高向东, 陈子琴

2015 (1): 18103-018103. doi: 10.1088/1674-1056/24/1/018103

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In a laser butt joint welding process, it is required that the laser beam focus should be controlled to follow the weld joint path accurately. Small focus wandering off the weld joint may result in insufficient penetration or unacceptable welds. Recognition of joint position offset, which describes the deviation between the laser beam focus and the weld joint, is important for adjusting the laser beam focus and obtaining high quality welds. A new method based on the magneto-optical (MO) imaging is applied to measure the micro weld joint whose gap is less than 0.2 mm. The weldments are excited by an external magnetic field, and an MO sensor based on principle of Faraday magneto effect is used to capture the weld joint images. A sequence of MO images which are tested under different magnetic field intensities and different weld joint widths are acquired. By analyzing the MO image characteristics and extracting the weld joint features, the influence of magnetic field intensity and weld joint width on the MO images and detection of weld joint position is observed and summarized.

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Ultrafast solvation dynamics at internal sites of staphylococcal nuclease investigated by site-directed mutagenesis

高光宇, 李渝, 王伟, 王树峰, Dongping Zhong, 龚旗煌

2015 (1): 18201-018201. doi: 10.1088/1674-1056/24/1/018201

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Internal solvation of protein was studied by site-directed mutagenesis, with which an intrinsically fluorescent probe, tryptophan, is inserted into the desired position inside a protein molecule for ultrafast spectroscopic study. Here we review this unique method for protein dynamics research. We first introduce the frontiers of protein solvation, site-directed mutagenesis, protein stability and characteristics, and the spectroscopic methods. Then we present time-resolved spectroscopic dynamics of solvation dynamics inside cavities of active sites. The studies are carried out on a globular protein, staphylococcal nuclease. The solvation at sites inside the protein molecule's cavities clearly reveals characteristics of the local environment. These solvation behaviors are directly correlated to enzyme activity.

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GaSb p-channel metal-oxide-semiconductor field-effect transistor and its temperature dependent characteristics

赵连锋, 谭桢, 王敬, 许军

2015 (1): 18501-018501. doi: 10.1088/1674-1056/24/1/018501

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GaSb p-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) with an atomic layer deposited Al₂O₃ gate dielectric and a self-aligned Si-implanted source/drain are experimentally demonstrated. Temperature dependent electrical characteristics are investigated. Different electrical behaviors are observed in two temperature regions, and the underlying mechanisms are discussed. It is found that the reverse-bias pn junction leakage of the drain/substrate is the main component of the off-state drain leakage current, which is generation-current dominated in the low temperature regions and is diffusion-current dominated in the high temperature regions. Methods to further reduce the off-state drain leakage current are given.

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Preparation of graphene on Cu foils by ion implantation with negative carbon clusters

李慧, 尚艳霞, 张早娣, 王泽松, 张瑞, 付德君

2015 (1): 18502-018502. doi: 10.1088/1674-1056/24/1/018502

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We report on few-layer graphene synthesized on Cu foils by ion implantation using negative carbon cluster ions, followed by annealing at 950 ℃ in vacuum. Raman spectroscopy reveals I_G/I_2D values varying from 1.55 to 2.38 depending on energy and dose of the cluster ions, indicating formation of multilayer graphene. The measurements show that the samples with more graphene layers have fewer defects. This is interpreted by graphene growth seeded by the first layers formed via outward diffusion of C from the Cu foil, though nonlinear damage and smoothing effects also play a role. Cluster ion implantation overcomes the solubility limit of carbon in Cu, providing a technique for multilayer graphene synthesis.

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Excellent acetone sensing properties of porous ZnO

刘唱白, 刘星熠, 王圣蕾

2015 (1): 18503-018503. doi: 10.1088/1674-1056/24/1/018503

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Porous ZnO was obtained by hydrothermal method. The results of scanning electron microscope revealed the porous structure in the as-prepared materials. The acetone sensing test results of porous ZnO show that porous ZnO possesses excellent acetone gas sensing properties. The response is 35.5 at the optimum operating temperature of 320 ℃ to 100 ppm acetone. The response and recovery times to 50 ppm acetone are 2 s and 8 s, respectively. The lowest detecting limit to acetone is 0.25 ppm, and the response value is 3.8. Moreover, the sensors also exhibit excellent selectivity and long-time stability to acetone.

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Mechanisms of ultrasonic modulation of multiply scattered incoherent light based on diffusion theory

朱莉莉, 李晖

2015 (1): 18701-018701. doi: 10.1088/1674-1056/24/1/018701

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An analytic equation interpreting the intensity of ultrasound-modulated scattering light is derived, based on diffusion theory and previous explanations of the intensity modulation mechanism. Furthermore, an experiment of ultrasonic modulation of incoherent light in a scattering medium is developed. This analytical model agrees well with experimental results, which confirms the validity of the proposed intensity modulation mechanism. The model supplements the existing research on the ultrasonic modulation mechanism of scattering light.

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PET image reconstruction with a system matrix containing point spread function derived from single photon incidence response

樊馨, 王海鹏, 贠明凯, 孙校丽, 曹学香, 刘双全, 柴培, 李道武, 刘宝东, 王璐, 魏龙

2015 (1): 18702-018702. doi: 10.1088/1674-1056/24/1/018702

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A point spread function (PSF) for the blurring component in positron emission tomography (PET) is studied. The PSF matrix is derived from the single photon incidence response function. A statistical iterative reconstruction (IR) method based on the system matrix containing the PSF is developed. More specifically, the gamma photon incidence upon a crystal array is simulated by Monte Carlo (MC) simulation, and then the single photon incidence response functions are calculated. Subsequently, the single photon incidence response functions are used to compute the coincidence blurring factor according to the physical process of PET coincidence detection. Through weighting the ordinary system matrix response by the coincidence blurring factors, the IR system matrix containing the PSF is finally established. By using this system matrix, the image is reconstructed by an ordered subset expectation maximization (OSEM) algorithm. The experimental results show that the proposed system matrix can substantially improve the image radial resolution, contrast, and noise property. Furthermore, the simulated single gamma-ray incidence response function depends only on the crystal configuration, so the method could be extended to any PET scanners with the same detector crystal configuration.

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Detecting overlapping communities in networks via dominant label propagation

孙鹤立, 黄健斌, 田勇强, 宋擒豹, 刘怀亮

2015 (1): 18703-018703. doi: 10.1088/1674-1056/24/1/018703

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Community detection is an important methodology for understanding the intrinsic structure and function of a real-world network. In this paper, we propose an effective and efficient algorithm, called Dominant Label Propagation Algorithm (Abbreviated as DLPA), to detect communities in complex networks. The algorithm simulates a special voting process to detect overlapping and non-overlapping community structure in complex networks simultaneously. Our algorithm is very efficient, since its computational complexity is almost linear to the number of edges in the network. Experimental results on both real-world and synthetic networks show that our algorithm also possesses high accuracies on detecting community structure in networks.

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Trends in ultrashort and ultrahigh power laser pulses based on optical parametric chirped pulse amplification

徐露, 於亮红, 储玉喜, 甘泽彪, 梁晓燕, 李儒新, 徐至展

2015 (1): 18704-018704. doi: 10.1088/1674-1056/24/1/018704

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Since the proof-of-principle demonstration of optical parametric amplification to efficiently amplify chirped laser pulses in 1992, optical parametric chirped pulse amplification (OPCPA) became the most promising method for the amplification of broadband optical pulses. In the meantime, we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers. The output power and pulse duration of these systems have ranged from a few gigawatts to hundreds of terawatts with a potential of tens of petawatts power level. Meanwhile, the output pulse duration based on optical parametric amplification has entered the range of fewoptical- cycle field. In this paper, we overview the basic principles, trends in development, and current state of the ultrashort and laser systems based on OPCPA, respectively.

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Novel pressure and displacement sensors based on carbon nanotubes

Kh. S. Karimov, Khaulah Sulaiman, Zubair Ahmad, Khakim M. Akhmedov, A. Mateen

2015 (1): 18801-018801. doi: 10.1088/1674-1056/24/1/018801

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We report newly designed pressure and displacement capacitive sensors based on a flexible paper-CNT structure. The carbon nanotube (CNT) powder was deposited on a thin paper substrate and was pressed at an elevated temperature. The sheet resistance of the paper-CNT films was in the range of 2-4 kΩ /cm². The paper-CNT films were used to fabricate pressure and displacement sensors. The sensitivities of the pressure and the displacement sensors were found to be 17.3 pF·m²/kN and 0.93 10^-3 pF/μm, respectively. The experimental results were compared with the simulated data and they found good agreement with each other.

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