中国物理B

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Painlevé integrability of the supersymmetric Ito equation

岑锋杰, 赵燕丹, 房霜韵, 孟欢, 俞军

2019 (9): 90201-090201. doi: 10.1088/1674-1056/ab38a7

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A supersymmetric version of the Ito equation is proposed by extending the independent and dependent variables for the classic Ito equation. To investigate the integrability of the N=1 supersymmetric Ito (sIto) equation, a singularity structure analysis for this system is carried out. Through a detailed analysis in two cases by using Kruskal's simplified method, the sIto system is found to pass the Painlevé test, and thus is Painlevé integrable.

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Attacking a high-dimensional quantum key distribution system with wavelength-dependent beam splitter

杜舸海, 李宏伟, 汪洋, 鲍皖苏

2019 (9): 90301-090301. doi: 10.1088/1674-1056/ab343c

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The unconditional security of quantum key distribution (QKD) can be guaranteed by the nature of quantum physics. Compared with the traditional two-dimensional BB84 QKD protocol, high-dimensional quantum key distribution (HD-QKD) can be applied to generate much more secret key. Nonetheless, practical imperfections in realistic systems can be exploited by the third party to eavesdrop the secret key. The practical beam splitter has a correlation with wavelength, where different wavelengths have different coupling ratios. Using this property, we propose a wavelength-dependent attack towards time-bin high-dimensional QKD system. What is more, we demonstrate that this attacking protocol can be applied to arbitrary d-dimensional QKD system, and higher-dimensional QKD system is more vulnerable to this attacking strategy.

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Wigner function for squeezed negative binomial state and evolution of density operator for amplitude decay

吕恒云, 王继锁, 张晓燕, 吴孟艳, 梁宝龙, 孟祥国

2019 (9): 90302-090302. doi: 10.1088/1674-1056/ab38a6

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Using the thermal-entangled state representation and the operator-ordering method, we investigate Wigner function (WF) for the squeezed negative binomial state (SNBS) and the analytical evolution law of density operator in the amplitude decay channel. The results show that the analytical WF is related to the square of the module of single-variable Hermite polynomials, which leads to a new two-variable special function and its generating function, and the parameters s and γ play opposite roles in the WF distributions. Besides, after undergoing this channel, the initial pure SNBS evolves into a new mixed state related to two operator Hermite polynomials within normal ordering, and fully loses its nonclassicality and decays to vacuum at long decay time.

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Dipole-dipole interactions enhance non-Markovianity and protect information against dissipation

Munsif Jan, 许小冶, 王琴琴, 陈哲, 韩永建, 李传锋, 郭光灿

2019 (9): 90303-090303. doi: 10.1088/1674-1056/ab37f2

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Preserving non-Markovianity and quantum entanglement from decoherence effect is of theoretical and practical significance in the quantum information processing technologies. In this context, we study a system S that is initially correlated with an ancilla A, which interacts with the environment E via an amplitude damping channel. We also consider dipole-dipole interactions (DDIs) between the system and ancilla, which are responsible for strong correlations. We investigate the impact of DDIs and detuning on the non-Markovianity and information exchange in different environments. We show that DDIs are not only better than detuning at protecting the information (without destroying the memory effect) but also induce memory by causing a transition from Markovian to non-Markovian dynamics. In contrast, although detuning also protects the information, it causes a transition from non-Markovian to the Markovian dynamics. In addition, we demonstrate that the non-Markovianity grows with increasing DDI strength and diminishes with increasing detuning. We also show that the effects of negative detuning and DDIs can cancel out each other, causing a certain loss of coherence and information.

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Geometrical quantum discord and negativity of two separable and mixed qubits

刘堂昆, 刘飞, 单传家, 刘继兵

2019 (9): 90304-090304. doi: 10.1088/1674-1056/ab37f6

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We studied quantum correlation and quantum entanglement of a quantum system in which a coherent state light field interacts with two qubits that are initially prepared in a separable and mixed state. The influence of mean photon number of the coherent field and distribution probability of the atom on the geometrical quantum discord and the negativity are discussed. Our results show that the mean photon number of light field and distribution function of the atom can regulate and control the quantum correlation and quantum entanglement.

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Chaotic analysis of Atangana-Baleanu derivative fractional order Willis aneurysm system

高飞, 李文琴, 童恒庆, 李喜玲

2019 (9): 90501-090501. doi: 10.1088/1674-1056/ab38a4

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A new Willis aneurysm system is proposed, which contains the Atangana-Baleanu(AB) fractional derivative. we obtain the numerical solution of the Atangana-Baleanu fractional Willis aneurysm system (ABWAS) with the AB fractional integral and the predictor-corrector scheme. Moreover, we research the chaotic properties of ABWAS with phase diagrams and Poincare sections. The different values of pulse pressure and system order are used to evaluate and compare their effects on ABWAS. The simulations verify that the changes of pulse pressure and system order are the significant reason for ABWAS' states varying from chaotic to steady. In addition, compared with Caputo fractional WAS (FWAS), ABWAS shows less state that is chaotic. Furthermore, the results of bifurcation diagrams of blood flow damping coefficient and reciprocal heart rate show that the blood flow velocity tends to stabilize with the increase of blood flow damping coefficient or reciprocal heart rate, which is consistent with embolization therapy and drug therapy for clinical treatment of cerebral aneurysms. Finally, in view of the fact that ABWAS in chaotic state increases the possibility of rupture of cerebral aneurysms, a reasonable controller is designed to control ABWAS based on the stability theory. Compared with the control results of FWAS by the same method, the results show that the blood flow velocity in the ABWAS system varies in a smaller range. Therefore, the control effect of ABWAS is better and more stable. The new Willis aneurysm system with Atangana-Baleanu fractional derivative provides new information for the further study on treatment and control of brain aneurysms.

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Characteristic signal extracted from a continuous time signal on the aspect of frequency domain

杜志凡, 张瑞浩, 陈红

2019 (9): 90502-090502. doi: 10.1088/1674-1056/ab344a

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Extracting characteristic signal from a continuous signal can effectively reduce the difficulty of analyzing the running states of a single-variable nonlinear system. Whether the extracted characteristic signal can accurately reflect the running states of the system is very important. In this paper, a method called automatic sampling method (ASM) for extracting characteristic signals is investigated. The complete definition is described, the effectiveness is proved theoretically, and the general formulas of the extracted characteristic signals are derived for the first time. Furthermore, typical Chua's circuit is used to accomplish a lot of experimental research on the aspect of frequency domain. The experimental results show that ASM is feasible and practical, and can automatically generate a characteristic signal with the change of the original signal.

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Quasi-periodic events on structured earthquake models

李斌全, 吴枝喜, 王圣军

2019 (9): 90503-090503. doi: 10.1088/1674-1056/ab3449

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There has been much interest in studying quasi-periodic events on earthquake models. Here we investigate quasi-periodic events in the avalanche time series on structured earthquake models by the analysis of the autocorrelation function and the fast Fourier transform. For random spatial earthquake models, quasi-periodic events are robust and we obtain a simple rule for a period that is proportional to the choice of unit time and the dissipation of the system. Moreover, computer simulations validate this rule for two-dimensional lattice models and cycle graphs, but our simulation results also show that small-world models, scale-free models, and random rule graphs do not have periodic phenomena. Although the periodicity of avalanche does not depend on the criticality of the system or the average degree of the system or the size of the system, there is evidence that it depends on the time series of the average force of the system.

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Hybrid-triggered consensus for multi-agent systems with time-delays, uncertain switching topologies, and stochastic cyber-attacks

陈侠, 尹立远, 刘永泰, 刘皓

2019 (9): 90701-090701. doi: 10.1088/1674-1056/ab38a8

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We propose a new approach to discuss the consensus problem of multi-agent systems with time-varying delayed control inputs, switching topologies, and stochastic cyber-attacks under hybrid-triggered mechanism. A Bernoulli variable is used to describe the hybrid-triggered scheme, which is introduced to alleviate the burden of the network. The mathematical model of the closed-loop control system is established by taking the influences of time-varying delayed control inputs, switching topologies, and stochastic cyber-attacks into account under the hybrid-triggered scheme. A theorem as the main result is given to make the system consistent based on the theory of Lyapunov stability and linear matrix inequality. Markov jumps with uncertain rates of transitions are applied to describe the switch of topologies. Finally, a simulation example demonstrates the feasibility of the theory in this paper.

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Adsorption and desorption phenomena on thermally annealed multi-walled carbon nanotubes by XANES study

Camile Rodolphe Tchenguem Kamto, Bridinette Thiodjio Sendja, Jeannot Mane Mane

2019 (9): 93101-093101. doi: 10.1088/1674-1056/ab3437

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The multi-walled carbon nanotubes (MWCNTs) studied in this work were synthesized by the catalytic chemical vapor deposition (CCVD) process, and were thermally annealed by the hot filament plasma enhanced (HF PE) method at 550℃ for two hours. The x-ray absorption near edge structure (XANES) technique was used to investigate the adsorption and desorption phenomena of the MWCNTs at normal and grazing incidence angles. The adsorbates were found to have different sensitivities to the thermal annealing. The geometry of the incident beam consistently gave information about the adsorption and desorption phenomena. In addition, the adsorption of non-intrinsic potassium quantitatively affected the intrinsic adsorbates and contributed to increase the conductivity of the MWCNTs. The desorption of potassium was almost 70% greater after the thermal annealing. The potassium non-intrinsic adsorbates are from a physisorption mechanism whereas the intrinsic adsorbates result from chemisorption.

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The substituent effect on the excited state intramolecular proton transfer of 3-hydroxychromone

宋玉志, 刘松松, 陆佳骏, 张慧, 张常哲, 杜军

2019 (9): 93102-093102. doi: 10.1088/1674-1056/ab3434

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The excited state intramolecular proton transfer of four derivatives (FM, BFM, BFBC, CCM) of 3-hydroxychromone is investigated. The geometries of different substituents are optimized to study the substituent effects on proton transfer. The mechanism of hydrogen bond enhancement is qualitatively elucidated by comparing the infrared spectra, the reduced density gradient, and the frontier molecular orbitals. The calculated electronic spectra are consistent with the experimental results. To quantify the proton transfer, the potential energy curves (PECs) of the four derivatives in S₀ and S₁ states are scanned. It is concluded that the ability of proton transfer follows the order:FM > BFM > BFBC > CCM.

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Coherent control of fragmentation of methyl iodide by shaped femtosecond pulse train

佟秋男, 费德厚, 廉振中, 齐洪霞, 周胜鹏, 罗嗣佐, 陈洲, 胡湛

2019 (9): 93201-093201. doi: 10.1088/1674-1056/ab33f3

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Coherent control of fragmentation of CH₃I using shaped femtosecond pulse train is investigated. The dissociation processes can be modulated by changing the separation of the shaped pulse train, and the yield of I⁺ under the irradiation of the optimal pulse is significantly increased compared with that using the transform-limited pulse. We discuss the control mechanism of dissociation processes with coherent interference in time domain. A three-pulse control model is proposed to explain the counterintuitive experimental results.

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Trajectory analysis of few-cycle strong field ionization in two-color circularly polarized fields

黄燕, 秦朝朝, 张逸竹, 王新成, 阎天民, 江玉海

2019 (9): 93202-093202. doi: 10.1088/1674-1056/ab3444

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Bichromatic circularly polarized fields provide a useful tool to probe the ionization dynamics. In this work, we compare the photoelectron momentum distribution in few-cycle bichromatic field of different helicities. The spectral features are analyzed with semiclassical trajectories derived from the strong field approximation. In particular, the interference fringes in momentum distribution are investigated by tracking the ionization time and tunneling exits of released photoelectrons. Different types of trajectories that contribute to the interference fringes are elucidated.

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Coulomb exploded directional double ionization of N₂O molecules in multicycle femtosecond laser pulses

马俊杨, 林康, 季琴颖, 张文斌, 李韩笑, 孙烽豪, 强俊杰, 陆培芬, 李辉, 宫晓春, 吴健

2019 (9): 93301-093301. doi: 10.1088/1674-1056/ab38aa

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We experimentally investigate Coulomb exploded directional double ionization of N₂O molecules in elliptically polarized femtosecond laser pulses. The denitrogenation and deoxygenation channels are accessed via various pathways. It leads to distinct asymmetries in directional breaking of the doubly ionized N₂O molecules versus the instantaneous laser field vector, which is revealed by tracing the sum-momentum spectra of the ionic fragments as a recoil of the ejected electrons. Our results demonstrate that the accessibility of the Coulomb exploded double ionization channels of N₂O molecules are ruled by the detailed potential energy curves, and the directional emission of the fragments are governed by the joint effects of the electron localization-assisted enhanced ionization of the stretched molecules and the profiles of the molecular orbitals.

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Interaction of H₂⁺ molecular beam with thin layer graphene foils

李敏, 曲国峰, 王亦舟, 朱洲森, 师勉恭, 周茂蕾, 刘东, 徐子虚, 宋明江, 张俊, 白帆, 廖小东, 韩纪锋

2019 (9): 93401-093401. doi: 10.1088/1674-1056/ab33f2

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The interaction of MeV H₂⁺ molecular ions with thin layer graphene and graphite foils was studied by using a high-resolution electrostatic analyzer. A large number of fragment protons were observed at zero degree (along the beam direction) when the H₂⁺ beam was passing through the monolayer graphene foil, which indicates that the electron of the H₂⁺ molecular ions can be stripped easily even by the monolayer graphene foil. More trailing than leading protons were found in the energy spectrum, which means significant wake effect was observed in the monolayer graphene foil. The ratio of the numbers of trailing protons over leading protons first increased with the thickness for the much thinner graphene foils, and then decreased with the thickness for the much thicker graphite foils, which indicates that the bending effect of the wake field on the trailing proton varied with the foil thickness.

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Relativistic electron scattering from freely movable proton/μ⁺ in the presence of strong laser field

王宁月, 焦利光, 刘爱华

2019 (9): 93402-093402. doi: 10.1088/1674-1056/ab343e

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We have investigated the electron scattering from the freely movable spin-1/2 particle in the presence of a linearly polarized laser field in the first Born approximation. The laser-dressed state of electrons is described by a time-dependent wave function which is derived from a perturbation treatment. With the aids of numerical simulations, we explore the dependencies of the differential cross section on the laser field intensity as well as the electron-impact energy. Due to the mobility of the target, the differential cross section of this process is smaller than that of Mott scattering.

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Dynamical properties of ultracold Bose atomic gases in one-dimensional optical lattices created by two schemes

朱江, 边成玲, 王红晨

2019 (9): 93701-093701. doi: 10.1088/1674-1056/ab3448

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An optical lattice could be produced either by splitting an input light (splitting scheme) or by reflecting the input light by a mirror (retro-reflected scheme). We study quantum dynamical properties of an atomic Bose-Einstein condensate (BEC) in the two schemes. Adopting a mean field theory and neglecting collision interactions between atoms, we find that the momentum and spatial distributions of BEC are always symmetric in the splitting scheme which, however, are asymmetric in the retro-reflected scheme. The reason for this difference is due to the local field effect. Furthermore, we propose an effective method to avoid asymmetric diffraction.

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Experimental demonstration of influence of underwater turbulence on ghost imaging

殷曼倩, 王乐, 赵生妹

2019 (9): 94201-094201. doi: 10.1088/1674-1056/ab33ee

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It is difficult to obtain a clear image in underwater turbulence environment with classical imaging methods due to the absorption, scattering, and underwater turbulence on the propagation beam. However, ghost imaging (GI), a non-locally imaging technique, has shown the turbulence-free ability in atmospheric turbulence by exploiting the second-order correlation between the signal beam and the reference beam. In this paper, we experimentally investigate the imaging quality of GI affected by the underwater environment, where the underwater environment is simulated by a 1 m×0.4 m×0.4 m tank with distilled water. The water temperature is controlled by a heater inside the tank, and a temperature gradient is obtained by putting the heater at different positions of the tank. The water vibration is produced by a heavy force, and the turbid medium is obtained by dissolving very small specks of CaCO₃ in the water. A set of Hadamard speckle pattern pairs are generated and modulated on the incident beam, and then the beam illuminates on an unknown object after passing through the simulated underwater environment. With the second-order correlations, the image is reconstructed under different temperature gradients, water vibration, and turbid medium ratios. The results show that GI has the turbulence-free ability under lower temperature gradient, water vibration, and turbid media. The structural similarity image measurement (SSIM) values of the reconstructed images only start to decrease when the temperature gradient is greater than 4.0℃. The same temperature gradient produced at the different positions has a little effect on the quality of the underwater GI.

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Multiple trapping using a focused hybrid vector beam

张莉, 邱晓东, 曾令伟, 陈理想

2019 (9): 94202-094202. doi: 10.1088/1674-1056/ab33ef

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We propose a simple and efficient method that uses a single focused hybrid vector beam to confine metallic Rayleigh particles at multiple positions. We study the force mechanisms of multiple trapping by analyzing the gradient and scattering forces. It is observed that the wavelength and topological charges of the hybrid vector beam regulate the trapping positions and number of optical trap sites. The proposed method can be implemented easily in three-dimensional space, and it facilitates both trapping and organization of particles. Thus, it can provide an effective and controllable means for nanoparticle manipulation.

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CsPbBr₃ nanocrystal for mode-locking Tm-doped fiber laser

周延, 张仁栗, 李夏, 关珮雯, 贺冬钰, 侯京山, 刘玉峰, 房永征, 廖梅松

2019 (9): 94203-094203. doi: 10.1088/1674-1056/ab327c

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CsPbBr₃ nanocrystal is used as the saturable absorber (SA) for mode-locking Tm-doped fiber laser in a ring fiber cavity. The modulation depth, saturable intensity, and non-saturable loss of the fabricated SA are 14.1%, 2.5 MW/cm², and 5.9%, respectively. In the mode-locking operation, the mode-locked pulse train has a repetition rate of 16.6 MHz with pulse width of 24.2 ps. The laser wavelength is centered at 1992.9 nm with 3-dB spectrum width of 2.5 nm. The maximum output power is 110 mW with slope efficiency of 7.1%. Our experiment shows that CsPbBr₃ nanocrystal can be used as an efficient SA in the 2-μm wavelength region.

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Stable continuous-wave single-frequency intracavity frequency-doubled laser with intensity noise suppressed in audio frequency region

高英豪, 李渊骥, 冯晋霞, 张宽收

2019 (9): 94204-094204. doi: 10.1088/1674-1056/ab327b

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We demonstrated a continuous wave (cw) single-frequency intracavity frequency-doubled Nd:YVO₄/LBO laser with 532 nm output of 7.5 W and 1.06 μm output of 3.1 W, and low intensity noise in audio frequency region. To suppress the intensity noise of the high power 532 nm laser, a laser frequency locking system and a feedback loop based on a Mach-Zehnder interferometer were designed and used. The influences of the frequency stabilization and the crucial parameters of the MZI, such as the power splitting ratio of the beam splitters and the locking state of the MZI, on the intensity noise of the 532 nm laser were investigated in detail. After the experimental optimizations, the laser intensity noise in the frequency region from 0.4 kHz to 10 kHz was significantly suppressed.

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Passively Q-switched diode-pumped Tm, Ho: LuVO₄ laser with a black phosphorus saturable absorber

李林军, 李天鑫, 周龙, 范剑英, 杨玉强, 谢文强, 李莎莎

2019 (9): 94205-094205. doi: 10.1088/1674-1056/ab33f1

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We presented a passively Q-switched (PQS) diode-pumped c-cut Tm, Ho:LuVO₄ laser with a black phosphorus saturable absorber for the first time. Under PQS mode, an average output power of 0.86 W and a peak power of 2.32 W were acquired from the Tm, Ho:LuVO₄ laser with the pump power of 14.55 W, corresponding to a pulse width of 2.89 μs, a pulse repetition rate of 71.84 kHz, and a pulse energy of about 6.70 μJ.

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Forward-headed structure change of acetic acid-water binary system by stimulated Raman scattering

刘喆, 杨博, 赵洪亮, 李占龙, 门志伟, 王晓峰, 王宁, 曹献文, 汪胜晗, 孙成林

2019 (9): 94206-094206. doi: 10.1088/1674-1056/ab3279

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The acetic acid-water binary system is a classical hydroxy-carboxy mixed system, while new and interesting phenomena appear under stimulated Raman scattering (SRS). Compared with the weaker signal of the acetic acid-water binary system obtained in spontaneous Raman scattering, SRS provides a finer band and a relatively distinct structural transition point. The structural transformation points are respectively at 30% and 80% by volume ratio under the condition of spontaneous Raman spectroscopy, while they are respectively at 15% and 25% under the condition of SRS. This phenomenon is attributed to the generation of laser induced plasma and shockwave induced dynamic high pressure environment during SRS.

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Quantum interference of multi-orbital effects in high-harmonic spectra from aligned carbon dioxide and nitrous oxide

梁红静, 范鑫, 冯爽, 单立宇, 高庆华, 闫博, 马日, 徐海峰

2019 (9): 94207-094207. doi: 10.1088/1674-1056/ab3438

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We investigate experimentally multi-orbital effects in high-order harmonic generation (HHG) from aligned CO₂ and N₂O molecules by intense femtosecond laser fields with linear and elliptical polarizations. For either of the aligned molecules, a minimum in the harmonic spectrum is observed, the position of which shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser. This indicates that the minimum originates from the dynamic interference of different channels, of which the tunneling ionization and recombination are contributed via different molecular orbitals. The results show that both the highest occupied molecular orbital (HOMO) and low-lying HOMO-2 in CO₂ (or HOMO-1 in N₂O) contribute to the molecular HHG in both linearly and elliptically polarized strong laser fields. Our study would pave a way for understanding multi-electron dynamics from polyatomic molecules irradiated by strong laser fields.

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Influence of intraband motion on the interband excitation and high harmonic generation

左瑞欣, 宋晓红, 刘希望, 杨士栋, 杨玮枫

2019 (9): 94208-094208. doi: 10.1088/1674-1056/ab3446

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Tunnelling, acceleration, and collision of electrons are the basic events in the process of high harmonic generation (HHG) in strong-field interaction with atoms. However, the periodic array of atoms in semiconductor structure makes three steps become interatomic coherent process which leads to complicated carrier dynamics and two sources of high harmonic emission:interband polarization and intraband current. The difference of features of high harmonic generation between semiconductors and atoms is strongly linked to the unique presence of intraband motion which manifests itself a nontrivial role in intertwined two dynamics. Here, we review recent experimental and theoretical advances of understanding coupled interband and intraband mechanisms of HHG in semiconductors. Particularly we focus on the influence of intraband motion on the interband excitation, and on the subsequent HHG emission and attosecond pulse generation.

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Refractive index sensor based on high-order surface plasmon resonance in gold nanofilm coated photonic crystal fiber

范振凯, 方少波, 李曙光, 魏志义

2019 (9): 94209-094209. doi: 10.1088/1674-1056/ab327a

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We propose a novel kind of wide-range refractive index optical sensor based on photonic crystal fiber (PCF) covered with nano-ring gold film. The refractive index sensing performance of the PCF sensor is analyzed and simulated by the finite element method (FEM). The refractive index liquid is infiltrated into the cladding air hole of the PCF. By comparing the sensing performance of two kinds of photonic crystal fiber structures, a wide range and high sensitivity structure is optimized. The surface plasmon resonance (SPR) excitation material is chose as gold, and large gold nanorings are embedded around the first cladding air hole of the PCF. The higher order surface plasmon modes are generated in this designed optical fiber structure. The resonance coupling between the fundamental mode and the 5th order surface plasmon polariton (SPP) modes is excited when the phase matching condition is matched. Therefore, the 3rd loss peaks appear obvious red-shift with the increase of the analyte refractive index, which shows a remarkable polynomial fitting law. The fitnesses of two structures are 0.99 and 0.98, respectively. When the range of refractive indices is from 1.40 to 1.43, the two kinds of sensors have high linear sensitivities of 1604 nm/RIU and 3978 nm/RIU, respectively.

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Pancharatnam-Berry metasurface for terahertz wave radar cross section reduction

李绍和, 李九生

2019 (9): 94210-094210. doi: 10.1088/1674-1056/ab33f5

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The digital coding metasurfaces need several kinds of meta-particle structures to obtain corresponding electromagnetic wave responses and require time-consuming optimization. In this paper, we present train-symbol-shaped meta-particles with various orientations utilizing Pancharatnam-Berry (PB) phase to achieve 1-, 2-, and 3-bit digital coding metasurfaces. Terahertz wave scattering patterns of the coding metasurfaces with regular and random sequences are given and discussed. They have strongly suppressed backward scattering with approximately -13.5 dB radar cross section (RCS) reduction in a wide band range from 0.85 THz to 1.6 THz. The proposed digital coding metasurfaces provide a simple way and new opportunities for manipulating terahertz wave scattering with polarization independence.

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Resolving multi-orbital effects on high harmonic generation from aligned N₂ molecules in linearly and elliptically polarized intense laser fields

梁红静, 范鑫, 冯爽, 单立宇, 高庆华, 闫博, 马日, 徐海峰, 丁大军

2019 (9): 94211-094211. doi: 10.1088/1674-1056/ab3440

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We perform an experimental study of the multi-orbital effect on the high-order harmonic generation (HHG) from aligned N₂ molecules in both linearly and elliptically polarized intense laser fields. Measured by a home-built extreme ultraviolet (XUV) flat grating spectrometer with the pump-probe method, the angular distributions of different orders of HHG are obtained, which show distinctive behaviors for harmonics in the plateau and the cut-off regions. The ellipticity dependence of HHG is investigated by aligning the molecular axis parallel or perpendicular to the laser polarization. Our results indicate that both the highest occupied molecular orbital (HOMO) as well as the lower one (HOMO-1) contribute to the HHG of N₂ molecules, in either linearly or elliptically polarized intense laser field. The study paves the way for understanding the ultrafast electron dynamics of molecules exposed to an intense laser field.

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Atomic even-harmonic generation due to symmetry-breaking effects induced by spatially inhomogeneous field

郭月, 刘爱华, 王俊, 刘学深

2019 (9): 94212-094212. doi: 10.1088/1674-1056/ab37fa

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We ab initio investigate the interaction between the hydrogen atom and the inhomogeneous field which is induced by resonant plasmons within a metal nanostructure. Same as normal laser pulse (homogeneous field), only odd-harmonic generation occurs when the bow-tie nanostructure is utilized. For the single nanotip case, the even-harmonic generation can be distinctly found in the harmonic emission spectrum. By investigating the symmetry and trajectories of different inhomogeneous fields, we demonstrate that the breaking symmetry of system can enable even high harmonic generations.

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Diode-pumped Kerr-lens mode-locked Ti: sapphire laser with broad wavelength tunability

刘寒, 王阁阳, 杨科, 康仁铸, 田文龙, 张大成, 朱江峰, 韩海年, 魏志义

2019 (9): 94213-094213. doi: 10.1088/1674-1056/ab37fb

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We report a direct blue-diode-pumped wavelength tunable Kerr-lens mode-locked Ti:sapphire laser. Central wavelength tunability as broad as 89 nm (736-825 nm) is achieved by adjusting the insertion of the prism. Pulses as short as 17 fs are generated at a central wavelength of 736 nm with an average output power of 31 mW. The maximum output power is 46.8 mW at a central wavelength of 797 nm with a pulse duration of 46 fs.

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Comparison of three kinds of polarized Bessel vortex beams propagating through uniaxial anisotropic media

刘佳伟, 李海英, 丁炜, 白璐, 吴振森, 李正军

2019 (9): 94214-094214. doi: 10.1088/1674-1056/ab37f5

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A comparison of differently polarized Bessel vortex beams propagating through a uniaxial anisotropic slab is discussed in terms of the vector wave function expansions. The magnitude profiles of electric field components, the transformation of polarization modes, and the distributions of orbital angular momentum (OAM) states of the reflected and transmitted beams for different incident angles are numerically simulated. The results indicate that the magnitude profiles of electric field components for different polarization modes are distinct from each other and have a great dependence on the incident angle, thus the transformation of polarization modes which reflects the change of energy can be affected largely. As compared to the x and circular polarization incidences, the reflected and transmitted beams for the radial polarization incidence suffer the fewest transformation of polarization modes, showing a better energy invariance. The distributions of OAM states of the reflected and transmitted beams for different polarization modes are diverse as well, and the derived OAM states of the transmitted beam for radial polarization present a focusing effect, concentrating on the state between two predominant OAM states.

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Properties of metal-insulator-metal waveguide loop reflector

龙虎, 曾选科, 蔡懿, 陆小微, 陈红艺, 徐世祥, 李景镇

2019 (9): 94215-094215. doi: 10.1088/1674-1056/ab38a3

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A new type and easy-to-fabricate metal-insulator-metal (MIM) waveguide reflector based on Sagnac loop is designed and investigated. The transfer matrix theoretical model for the transmission of electric fields in the reflector is established, and the properties of the reflector are studied and analyzed. The simulation results indicate that the reflectivity strongly depends on the coupling splitting ratio determined by the coupling length. Accordingly, different reflectivities can be realized by varying the coupling length. For an optimum coupling length of 750 nm, the 3-dB reflection bandwidth of the MIM waveguide reflector is as wide as 1.5 μm at a wavelength of 1550 nm, and the peak reflectivity and isolation are 78% and 23 dB, respectively.

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Polarization dependence of gain and amplified spontaneous Brillouin scattering noise analysis for fiber Brillouin amplifier

穆宽林, 商建明, 唐丽华, 王正康, 喻松, 乔耀军

2019 (9): 94216-094216. doi: 10.1088/1674-1056/ab38ab

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The polarization dependences of gain and amplified spontaneous Brillouin scattering (ABS) noise for fiber Brillouin amplifier (FBA) are analyzed through theories, simulations, and experiments. Modified vector propagation equations for calculating the gain of the probe signal and the ABS noise are derived and analyzed in the Stokes spaces. In simulations and experiments, we prove that the gain of the probe signal and the ABS noise are strongly dependent on the relative state of polarization (SOP) of the pump and probe signals. The closer the relative SOP of the pump and probe signals is, the more obvious ABS noise suppression effect will be brought by increasing the power of the input probe signal.

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Quantum optical interferometry via general photon-subtracted two-mode squeezed states

侯丽丽, 薛建忠, 眭永兴, 王帅

2019 (9): 94217-094217. doi: 10.1088/1674-1056/ab38a9

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We investigate the sensitivity of phase estimation in a Mach-Zehnder interferometer with photon-subtracted two-mode squeezed vacuum states. Our results show that, for given initial squeezing parameter, both symmetric and asymmetric photon subtractions can further improve the quantum Cramér-Rao bound (i.e., the ultimate phase sensitivity), especially for single-mode photon subtraction. On the other hand, the quantum Cramér-Rao bound can be reached by parity detection for symmetric photon-subtracted two-mode squeezed vacuum states at particular values of the phase shift, but it is not valid for asymmetric photon-subtracted two-mode squeezed vacuum states. In addition, compared with the two-mode squeezed vacuum state, the phase sensitivity via parity detection with asymmetric photon-subtracted two-mode squeezed vacuum states will be getting worse. Thus, parity detection may not always be the optimal detection scheme for nonclassical states of light when they are considered as the interferometer states.

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Using Helmholtz resonator arrays to improve dipole transmission efficiency in waveguide

王力维, 全力, 钱枫, 刘晓宙

2019 (9): 94301-094301. doi: 10.1088/1674-1056/ab3447

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It is well known that the radiation efficiency of an acoustic dipole is very low, increasing the radiation efficiency of an acoustic dipole is a difficult task, especially in an ordinary waveguide. In addition, current acoustic superlenses all utilize in-phase sources to do the super-resolution imaging, it is almost impossible to realize super-resolution imaging of an acoustic dipole. In this paper, after using the Helmholtz resonator arrays (HRAs) which are placed at the upper and lower surfaces of the waveguide, we observe a large dipole radiation efficiency at the certain frequency, which gives a method to observe an acoustic dipole in the far field and offers a novel model which is promising to realize the superlens with a source of an acoustic dipole. We discuss how the arrangement of HRAs affects the transmission of the acoustic dipole.

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Strong coupling between height of gaps and thickness of thermal boundary layer in partitioned convection system

林泽鹏, 包芸

2019 (9): 94701-094701. doi: 10.1088/1674-1056/ab343d

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A direct numerical simulation (DNS) method is used to calculate the partitioned convection system with Ra number ranging from 10⁷ to 2×10⁹. Using the boundary layer thickness to normalize the height of gaps d, we find a strong consistency between the variation of the TD number (the average value of the temperature in each heat transfer channel is averaged after taking the absolute values) with the change of the height of gaps and the variation of the TD number with the change of Ra number in partitioned convection. For a given thickness of partition, heights of gaps are approximately equal to 0.5 or 1 time of the thermal boundary layer thickness λ_θ at different Ra numbers. TD number representing temperature characteristics is almost the constant value, which means that TD number is a function of d/λ_θ only. Analysis of local temperature field of area in gaps shows that the temperature distribution in the gaps are basically the same when d/λ_θ is certain. The heat transfer Nu number of the system at d/λ_θ≈ 0.5 is larger than that of d/λ_θ≈ 1, both of them have the same scaling law with Ra number and Nu~Ra^0.25.

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Fluctuation of arc plasma in arc plasma torch with multiple cathodes

张泽龙, 王城, 孙强, 夏维东

2019 (9): 95201-095201. doi: 10.1088/1674-1056/ab344b

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Fluctuation phenomena commonly exist in arc plasmas, limiting the application of this technology. In this paper, we report an investigation of fluctuations of arc plasmas in an arc plasma torch with multiple cathodes. Time-resolved images of the plasma column and anode arc roots are captured. Variations of the arc voltage, plasma column diameter, and pressure are also revealed. The results indicate that two well-separated fluctuations exist in the arc plasma torch. One is the high-frequency fluctuation (of several thousand Hz), which arises from transferring of the anode arc root. The other is the low-frequency fluctuation (of several hundred Hz), which may come from the pressure variation in the arc plasma torch. Initial analysis reveals that as the gas flow rate changes, the low-frequency fluctuation shows a similar variation trend with the Helmholtz oscillation. This oscillation leads to the shrinking and expanding of the plasma column. As a result, the arc voltage shows a sinusoidal fluctuation.

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Enhancement of corona discharge induced wind generation with carbon nanotube and titanium dioxide decoration

叶建春, 李俊, 陈晓红, 黄素梅, 欧阳威

2019 (9): 95202-095202. doi: 10.1088/1674-1056/ab33ed

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Dip-coated double-wall carbon nanotubes (DWCNTs) and titanium dioxide (TiO₂) sol have been prepared and smeared onto the tip of a conductive iron needle which serves as the corona discharge anode in a needle-cylinder corona system. Compared with the discharge electrode of a CNT-coated needle tip, great advancements have been achieved with the TiO₂/CNT-coated electrode, including higher discharge current, ionic wind velocity, and energy conversion efficiency, together with lower corona onset voltage and power consumption. Several parameters related to the discharge have been phenomenologically and mathematically studied for comparison. Thanks to the morphology reorientation of the CNT layer and the anti-oxidation of TiO₂, better performance of corona discharge induced wind generation of the TiO₂/CNT-coated electrode system has been achieved. This novel decoration may provide better thoughts about the corona discharge application and wind generation.

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First polar direct-drive exploding-pusher target experiments on the ShenGuang laser facility

余波, 杨家敏, 黄天晅, 王鹏, 尚万里, 乔秀梅, 邓学伟, 张占文, 宋仔峰, 唐琦, 彭晓世, 陈家斌, 理玉龙, 蒋炜, 蒲昱东, 晏骥, 陈忠靖, 董云松, 郑无敌, 王峰, 江少恩, 丁永坤, 郑坚

2019 (9): 95203-095203. doi: 10.1088/1674-1056/ab37f4

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Low density and low convergence implosion occurs in the exploding-pusher target experiment, and generates neutrons isotropically to develop a high yield platform. In order to validate the performance of ShenGuang (SG) laser facility and test nuclear diagnostics, all 48-beam lasers with an on-target energy of 48 kJ were firstly used to drive room-temperature, DT gas-filled glass targets. The optimization has been carried out and optimal drive uniformity was obtained by the combination of beam repointing and target. The final irradiation uniformity of less than 5% on polar direct-drive capsules of 540 μ in diameter was achieved, and the highest thermonuclear yield of the polar direct-drive DT fuel implosion at the SG was 1.04×10¹³. The experiment results show neutron yields severely depend on the irradiation uniformity and laser timing, and decrease with the increase of the diameter and fuel pressure of the target. The thin CH ablator does not impact the implosion performance, but the laser drive uniformity is important. The simulated results validate that the γ distribution laser design is reasonable and can achieve a symmetric pressure distribution. Further optimization will focus on measuring the symmetry of the hot spot by self-emission imaging, increasing the diameter, and decreasing the fuel pressure.

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Van der Waals interlayer potential of graphitic structures: From Lennard-Jones to Kolmogorov-Crespy and Lebedeva models

Zbigniew Koziol, Grzegorz Gawlik, Jacek Jagielski

2019 (9): 96101-096101. doi: 10.1088/1674-1056/ab38a5

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The experimental knowledge on interlayer potential of graphitic materials is summarized and compared with the computational results based on phenomenological models. Besides Lennard-Jones approximation, the Mie potential is discussed, as well as the Kolmogorov-Crespy model and equation of Lebedeva et al. An agreement is found between a set of reported physical properties of graphite (layer binding energies, compressibility along c-axis in a broad pressure range, Raman frequencies for bulk shear and breathing modes under pressure), when a proper choice of model parameters is taken. It is argued that anisotropic potentials, Kolmogorov-Crespy and Lebedeva, are preferable for modeling, as they provide a better, self-consistent description. A method of fast numerical modeling, convenient for the accurate estimation of the discussed physical properties, is proposed. It may be useful in studies of other van der Waals homo/heterostructures as well.

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Surperhard monoclinic BC₆N allotropes: First-principles investigations

屈年瑞, 王洪超, 李青, 李一鼎, 李志平, 缑慧阳, 高发明

2019 (9): 96201-096201. doi: 10.1088/1674-1056/ab3439

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Via structural searching methodology and first-principles calculations, we predicted two new BC₆N allotropes, a C-centered monoclinic BC₆N (Cm-BC₆N) and a primitive-centered monoclinic BC₆N (Pm-BC₆N). The lattice vibrations, elastic properties, ideal strength, theoretical hardness, and electronic structure of the predicted BC₆N were investigated systematically. Our results reveal that Cm-BC₆N is more favorable energetically than graphite-like g-BC₆N above 20.6 GPa, which is lower than the transition pressures of r-BC₆N, t-BC₆N, and Pm-BC₆N. Both Cm-BC₆N and Pm-BC₆N are indirect semiconductors with band gaps of 2.66 eV and 0.36 eV, respectively. Cm-BC₆N exhibits the excellent ideal shear strength of 53.9 GPa in (011)[011], much greater than that of Pm-BC₆N (25.0 GPa in (010)[101] shear direction), and Cm-BC₆N shows a much lower anisotropy in shear strength than Pm-BC₆N. The Vickers hardness of Cm-BC₆N is estimated to be above 80 GPa, which is more outstanding than those of t-BC₆N and r-BC₆N.

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Crystal melting processes of propylene carbonate and 1,3-propanediol investigated by the reed-vibration mechanical spectroscopy for liquids

王丽娜, 赵兴宇, 周恒为, 张丽, 黄以能

2019 (9): 96401-096401. doi: 10.1088/1674-1056/ab327d

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The melting of crystals is one of the most common and general phase transition phenomena. However, the mechanism of crystal melting is not well understood, and more experimental measurements and explorations are still needed. The mechanical spectra of propylene carbonate and 1,3-propanediol during the crystal melting processes are measured by the reed vibration mechanical spectroscopy for liquids (RMS-L) for the first time. The experimental results show that as the temperature increases, the real part of the complex Young modulus first decreases slowly, and then quickly drops to zero; meanwhile, its imaginary part increases slowly at first, then goes up and drops quickly to zero, showing a peak of internal friction. Preliminary analyses indicate that both the real and imaginary parts can present some characteristics of the melting process, such as the transition from the disconnected liquid regions to the connected liquid regions, that from the connected crystal regions to the disconnected crystal regions, and so on. In addition, the results show that the melting rate per unit volume of crystalline phase versus temperature satisfies the Arrhenius relation at the initial stage of melting, and deviates from this relation as the temperature increases to a certain value. Therefore, the RMS-L will provide an effective supplement for the further study of melting.

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Structural transitions in NaNH₂ via recrystallization under high pressure

黄艳萍, 黄晓丽, 王鑫, 张文亭, 周迪, 周强, 刘冰冰, 崔田

2019 (9): 96402-096402. doi: 10.1088/1674-1056/ab37f8

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Multiple phase transitions are detected in sodium amide (NaNH₂), an important hydrogen storage material, upon compression in diamond anvil cells (DAC) by using Raman spectroscopy and x-ray diffraction (XRD) measurements. Additional Bragg reflections appear on lower and higher angle sides of the original ones at~1.07 GPa and 1.84 GPa, accompanied by obvious changes in Raman spectroscopy, respectively. It reveals that NaNH₂ undergoes the high-pressure phase sequence (α-β-γ) up to 20 GPa at room temperature. Spectral analysis indicates an orthorhombic structure with PBAN space group for the γ phase. We also experimentally observe high pressure induced recrystallization in alkaline amide compounds for the first time.

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Laser scattering, transmittance and low thermal expansion behaviors in Y_2-x(ZnLi)_xMo₃O₁₂ by forming regular grains

刘献省, 程永光, 袁保合, 梁二军, 张伟风

2019 (9): 96501-096501. doi: 10.1088/1674-1056/ab3435

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Ceramics usually have irregular grains, cracking, or porosity, which result in their lightproof. Y₂Mo₃O₁₂ ceramics have more porosity due to the heavy hygroscopicity. Introducing ZnLi to Y₂Mo₃O₁₂ could form regular grains, reduce cracking and porosity. With increasing the content of ZnLi, the grain shapes self-assembly gradually and then the laser scattering and transmittance improve. The laser scattering property and transmittance of diverging rays become the best in ceramics Y_2-x(ZnLi)_xMo₃O₁₂ (x=1.0 and 1.2) with regular grains and low thermal expansion. The formation mechanism of regular grains is ascribed to the substitutions of Zn²⁺ and Li⁺ for Y³⁺ in Y₂Mo₃O₁₂ resulting in the preferential growth. The investigation in laser scattering, transmittance and low thermal expansion behaviors of Y_2-x(ZnLi)_xMo₃O₁₂ could pave a way to weaken the strong-laser attack from the high-power laser weapon and the other.

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Josephson effect in the strontium titanate/lanthanum aluminate junction

阳星, 陈杰, 余亚斌, 刘全慧

2019 (9): 97401-097401. doi: 10.1088/1674-1056/ab33f4

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We report theoretical studies on the newly discovered novel Josephson effect and scanning tunneling spectroscopy (STS) at the interface of strontium titanate/lanthanum aluminate (STO/LAO). With a phenomenological boson-fermion model, the density of states is calculated and the results are consistent with the STS experiments. A typical calculation of Josephson effect is performed, and it is in qualitative agreement with the experiments. The calculations indicate that the gap states come from the pairing of quasi-particles with a finite total momentum and the Josephson current comes from the tunneling of quasi-particle pairs with zero momentum. The quasi-particles are Bogoliubov quasi-particles. Moreover, the fits using Kulik's formula imply that the Josephson junction at the STO/LAO interface has a point contact with the clean superconductor limit.

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Dynamical anisotropic magnetoelectric effects at ferroelectric/ferromagnetic insulator interfaces

李耀进, Vladimir Koval, 贾成龙

2019 (9): 97501-097501. doi: 10.1088/1674-1056/ab37f7

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The interfacial magnetoelectric interaction originating from multi-orbital hopping processes with ferroelectric-associated vector potential is theoretically investigated for complex-oxide composite structures. Large mismatch in the electrical permittivity of the ferroelectric and ferromagnetic materials gives rise to giant anisotropic magnetoelectric effects at their interface. Our study reveals a strong linear dynamic magnetoelectric coupling which genuinely results in electric control of magnetic susceptibility. The constitutive conditions for negative refractive index of multiferroic composites are determined by the analysis of light propagation.

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The unique magnetic damping enhancement in epitaxial Co₂Fe_1-xMn_xAl films

李树发, 程樗元, 孟康康, 陈春雷

2019 (9): 97502-097502. doi: 10.1088/1674-1056/ab38ad

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Uniform precession dynamics and its magnetic damping are investigated in epitaxial Co₂Fe_1-xMn_xAl films by using the time-resolved magneto-optical Kerr effect under out-of-plane configuration. The decay time of uniform precession mode decreases, and thus the magnetic damping increases with the increase of external field. Moreover, the decay time decreases as x decreases, so that the enhancement of magnetic damping occurs in Fe-rich sample. Furthermore, the decay time decreases as the excitation fluence increases, which drops rapidly at low magnetic field comparing with the slow reduction at high magnetic field. This unique magnetic damping enhancement is attributed to the enhancement of homogeneous magnetization.

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Magnetic properties of the double perovskite compound Sr₂YRuO₆

N. EL Mekkaoui, S. Idrissi, S. Mtougui, I. EL Housni, R. Khalladi, S. Ziti, H. Labrim, L. Bahmad

2019 (9): 97503-097503. doi: 10.1088/1674-1056/ab3441

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We study the magnetic properties of the double perovskite ruthenate compound Sr₂YRuO₆ using Monte Carlo simulations (MCS). We elaborate the ground state phase diagrams for all possible and stable configurations. The magnetizations and the susceptibilities as a function of temperature for the studied system are also reported. The effects of the exchange coupling interactions and the crystal field are examined and discussed. On the other hand, since the compound Sr₂YRuO₆ exhibits an antiferromagnetic behavior, we find its Néel temperature, T_N ≈ 31 K, which is in good agreement with the experimental results in the literature. To complete this study, the hysteresis loops and the coercive field as a function of the external magnetic field are also obtained for fixed values of the physical parameters.

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Techniques of microwave permeability characterization for thin films

李喜玲, 王建波, 柴国志

2019 (9): 97504-097504. doi: 10.1088/1674-1056/ab3a8a

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We review the microwave methods to characterize the material properties, including the established and the emerging techniques in material characterization, especially the permeability spectra of the magnetic thin films. Almost all aspects of the microwave techniques for characterizing the permeability of thin films at microwave frequencies, including the new methods developed by our group, are presented. Firstly, the introduction part is presented. Secondly, the coaxial-line with transmission/reflection methods and the pickup coil with electromagnetic induction method are presented. Thirdly, the most widely used shorted microstrip technique is discussed in detail by the equivalent circuit method, transmission line method, and electromagnetic induction method. Fourthly, the coplanar waveguide method and the near-field probe method are also introduced. Finally, the high temperature permeability characterization by using the shorted microstrip line, the near-field microwave probe, and the shorted microstrip line probe are described in detail. This paper may be useful for researchers or engineers who will build up such measurement fixture to make full use of the existing methods or to develop original methods to meet the requirements for ever-rising measurements.

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Improvement of TE-polarized emission in type-Ⅱ InAlN-AlGaN/AlGaN quantum well

李毅, 朱友华, 王美玉, 邓洪海, 尹海宏

2019 (9): 97801-097801. doi: 10.1088/1674-1056/ab343a

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The optical properties of the type-Ⅱ lineup In_xAl_1-xN-Al_0.59Ga_0.41N/Al_0.74Ga_0.26N quantum well (QW) structures with different In contents are investigated by using the six-by-six K-P method. The type-Ⅱ lineup structures exhibit the larger product of Fermi-Dirac distribution functions of electron f_cⁿ and hole (1-f_v^Um) and the approximately equal transverse electric (TE) polarization optical matrix elements (|M_x|²) for the c1-v1 transition. As a result, the peak intensity in the TE polarization spontaneous emission spectrum is improved by 47.45%-53.84% as compared to that of the conventional AlGaN QW structure. In addition, the type-Ⅱ QW structure with x~0.17 has the largest TE mode peak intensity in the investigated In-content range of 0.13-0.23. It can be attributed to the combined effect of|M_x|² and f_cⁿ (1-f_v_Um) for the c1-v1, c1-v2, and c1-v3 transitions.

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Characteristics of urea under high pressure and high temperature

房帅, 马红安, 郭龙锁, 陈良超, 王遥, 丁路遥, 蔡正浩, 王健, 贾晓鹏

2019 (9): 98101-098101. doi: 10.1088/1674-1056/ab3445

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The properties of urea under high pressure and high temperature (HPHT) are studied using a China-type large volume cubic high-presentation apparatus (CHPA) (SPD-6×600). The samples are characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and Raman spectroscopy. By directly observing the macroscopic morphology of urea with SEM, it is confirmed that the melting point of urea rises with the increase of pressure. The XRD patterns of urea residues derived under different pressures show that the thermal stability of urea also increases with the increase of pressure. The XRD pattern of the urea residue confirms the presence of C₃H₅N₅O (ammeline) in the residue. A new peak emerges at 21.80°, which is different from any peak of all urea pyrolysis products under normal pressure. A more pronounced peak appears at 708 cm^-1 in the Raman spectrum, which is produced by C-H off-plane bending. It is determined that the urea will produce a new substance with a C-H bond under HPHT, and the assessment of this substance requires further experiments.

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The n-type Si-based materials applied on the front surface of IBC-SHJ solar cells

包建辉, 陶科, 林苡任, 贾锐, 刘爱民

2019 (9): 98201-098201. doi: 10.1088/1674-1056/ab33ec

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Interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cells exhibit excellent performance owing to the IBC and SHJ structures. The front surface field (FSF) layer composed of electric field passivation and chemical passivation has been proved to play an important role in IBC-SHJ solar cells. The electric field passivated layer n⁺-a-Si:H, an n-type Si alloy with carbon or oxygen in amorphous phase, is simulated in this study to investigate its effect on IBC-SHJ. It is indicated that the n⁺-a-Si:H layer with wider band gap can reduce the light absorption on the front side efficaciously, which hinders the surface recombination of photo-generated carriers and thus contributes to the improvement of the short circuit current density J_sc. The highly doped n⁺-a-Si:H can result in the remakable energy band bending, which makes it outstanding in the field passivation, while it makes little contribution to the chemical passivation. It is noteworthy that when the electric field intensity exceeds 1.3×10⁵ V/cm, the efficiency decrease caused by the inferior chemical passivation is only 0.16%. In this study, the IBC-SHJ solar cell with a front n⁺-a-Si:H field passivation layer is simulated, which shows the high efficiency of 26% in spite of the inferior chemical passivation on the front surface.

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Structural and dielectric properties of giant dielectric Na_1/2Sm_1/2Cu₃Ti₄O₁₂ ceramics prepared by reactive sintering methods

H Mahfoz Kotb

2019 (9): 98202-098202. doi: 10.1088/1674-1056/ab37f1

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Na_0.5Sm_0.5Cu₃Ti₄O₁₂ (NSCTO) ceramics have been prepared by reactive sintering of amorphous powder. Spark plasma sintering (SPS) for 10 min at 1025℃ and conventional sintering (CS) for 10 h at 1090℃ have been employed. X-ray diffraction measurements confirmed the pure CCTO-like phase for SPS and CS NSCTO ceramics. The SPS ceramic showed an average grain size of 500 nm, which is much smaller than that of the CS (~5 μm) sample. The impedance spectroscopy measurements revealed an electrically inhomogeneous structure in the prepared ceramics. While the resistivities of grains of both ceramic samples were in the same order of magnitude, the resistivity of grain-boundaries of the CS ceramic was three orders of magnitude greater than that of the SPS ceramic. Both of the samples showed giant dielectric constant (>10³) over wide ranges of temperatures and frequencies. Nevertheless, the room-temperature dielectric loss of the SPS NSCTO (3.2 at 1.1 kHz) ceramic sample was higher than that of the CS NSCTO (0.08 at 1.1 kHz) ceramic sample due to the reduced grain-boundary resistivity of the former. Two dielectric relaxations were detected for each sample and attributed to the relaxations in grains and grain-boundaries. The dielectric behavior of the SPS and CS NSCTO ceramics could be interpreted in terms of the internal barrier layer capacitor (IBLC) model.

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Efficient molecular model for squeeze-film damping in rarefied air

陆存豪, 李普, 方玉明

2019 (9): 98501-098501. doi: 10.1088/1674-1056/ab3443

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Based on the energy transfer model (ETM) proposed by Bao et al. and the Monte Carlo (MC) model proposed by Hutcherson and Ye, this paper proposes an efficient molecular model (MC-S) for squeeze-film damping (SQFD) in rarefied air by releasing the assumption of constant molecular velocity in the gap. Compared with the experiment data, the MC-S model is more efficient than the MC model and more accurate than ETM. Besides, by using the MC-S model, the feasibility of the empirical model proposed by Sumali for SQFD of different plate sizes is discussed. It is proved that, for various plate sizes, the accuracy of the empirical model is relatively high. At last, the SQFD of various vibration frequencies is discussed, and it shows that, for low vibration frequency, the MC-S model is reduced to ETM.

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Thermal resistance matrix representation of thermal effects and thermal design of microwave power HBTs with two-dimensional array layout

陈蕊, 金冬月, 张万荣, 王利凡, 郭斌, 陈虎, 殷凌寒, 贾晓雪

2019 (9): 98502-098502. doi: 10.1088/1674-1056/ab3436

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Based on the thermal network of the two-dimensional heterojunction bipolar transistors (HBTs) array, the thermal resistance matrix is presented, including the self-heating thermal resistance and thermal coupling resistance to describe the self-heating and thermal coupling effects, respectively. For HBT cells along the emitter length direction, the thermal coupling resistance is far smaller than the self-heating thermal resistance, and the peak junction temperature is mainly determined by the self-heating thermal resistance. However, the thermal coupling resistance is in the same order with the self-heating thermal resistance for HBT cells along the emitter width direction. Furthermore, the dependence of the thermal resistance matrix on cell spacing along the emitter length direction and cell spacing along the emitter width direction is also investigated, respectively. It is shown that the moderate increase of cell spacings along the emitter length direction and the emitter width direction could effectively lower the self-heating thermal resistance and thermal coupling resistance, and hence the peak junction temperature is decreased, which sheds light on adopting a two-dimensional non-uniform cell spacing layout to improve the uneven temperature distribution. By taking a 2×6 HBTs array for example, a two-dimensional non-uniform cell spacing layout is designed, which can effectively lower the peak junction temperature and reduce the non-uniformity of the dissipated power. For the HBTs array with optimized layout, the high power-handling capability and thermal dissipation capability are kept when the bias voltage increases.

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Performance improvement of 4H-SiC PIN ultraviolet avalanche photodiodes with different intrinsic layer thicknesses

蔡小龙, 周东, 程亮, 任芳芳, 钟宏, 张荣, 郑有炓, 陆海

2019 (9): 98503-098503. doi: 10.1088/1674-1056/ab343f

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Four 4H-SiC p-i-n ultraviolet (UV) avalanche photodiode (APD) samples PIN-0.1, PIN-0.35, PIN-0.5, and PIN-1.0 with different intrinsic layer thicknesses (0.1 μm, 0.35 μm, 0.5 μm, and 1.0 μm, respectively) are designed and fabricated. Single photon detection efficiency (SPDE) performance becomes better as the intrinsic layer thickness increases, which is attributed to the inhibitation of tunneling. Dark count origin is also investigated, an activation energy as small as 0.22 eV of the dark count rate (DCR) confirms that the trap-assisted tunneling (TAT) process is the main source of DCR. The temperature coefficient ranges from -2.6 mV/℃ to 18.3 mV/℃, demonstrating that the TAT process is dominant in APDs with thinner intrinsic layers. Additionally, the room temperature maximum quantum efficiency at 280 nm differs from 48% to 65% for PIN-0.35, PIN-0.5, and PIN-1.0 under 0 V bias, and UV/visible rejection ratios higher than 10⁴ are obtained.

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Intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in radio-frequency magnetometer cell

傅杨颖, 袁杰

2019 (9): 98504-098504. doi: 10.1088/1674-1056/ab3442

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The intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in the radio-frequency magnetometer cell are investigated. The intrinsic transverse relaxation rate of cesium atoms as a function of cell temperature is obtained. The absorption of alkali atoms by the glass wall and the reservoir effect are the main error factors which contribute to the disagreements between theory and experiments. A modified relaxation model is presented, in which both the absorption of alkali atoms by the glass wall and the reservoir effect are included. This study provides a more accurate description of the intrinsic transverse relaxation mechanisms of polarized alkali atoms, and enlightens the optimization of the cell design.

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Manipulation of superconducting qubit with direct digital synthesis

李志远, 于海峰, 谭新生, 赵士平, 于扬

2019 (9): 98505-098505. doi: 10.1088/1674-1056/ab37f9

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We investigate the XY control and manipulation of the superconducting qubit state using direct digital synthesis (DDS) for the microwave pulse signal generation. The decoherence time, gate fidelity, and other qubit properties are measured and carefully characterized, and compared with the results obtained by using the traditional mixing technique for the microwave pulse generation. In particular, the qubit performance in the state manipulation with respect to the sampling rate of DDS is studied. Our results demonstrate that the present technique provides a simple and effective method for the XY control and manipulation of the superconducting qubit state. Realistic applications of the technique for the possible future scalable superconducting quantum computation are discussed.

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Homogeneous and inhomogeneous magnetic oxide semiconductors

李小丽, 许小红

2019 (9): 98506-098506. doi: 10.1088/1674-1056/ab38ac

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Magnetic oxide semiconductors are significant spintronics materials. In this article, we review recent advances for homogeneous and inhomogeneous magnetic oxide semiconductors. In the homogeneous magnetic oxide semiconductors, we focus on the various doping techniques including choosing different transition metals, codoping, non-magnetic doping, and even un-doping to realize homogeneous substitution and the clear magnetic origin. And the enhancement of the ferromagnetism is achieved by nanodot arrays engineering, which is accompanied by the tunable optical properties. In the inhomogeneous magnetic oxide semiconductors, we review some heterostructures and their magnetic and transport properties, especially magnetoresistance, which are dramatically modulated by electric field in the constructed devices. And the related mechanisms are discussed in details. Finally, we provide an overview and possible potential applications of magnetic oxide semiconductors.

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Terahertz coherent detection via two-color laser pulses of various frequency ratios

顾新杨, 王可嘉, 杨振刚, 刘劲松

2019 (9): 98701-098701. doi: 10.1088/1674-1056/ab37fc

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The mechanism of terahertz (THz) pulse coherent detection via two-color laser pulses of various frequency ratios in gas plasma is theoretically investigated. Our investigations demonstrate that except for the commonly used frequency ratio of 2, other uncommon frequency ratios can also be utilized to detect THz pulse, such as 2n, n+1/2 (n ≤ 3, n is a positive integer). The well-developed transient photocurrent model is extended to our terahertz detection process. Based on this model, our simulation results can be explained by analyzing the process of asymmetric electron ionization and electron acceleration.

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An overview of progress in Mg-based hydrogen storage films

Lyu Jinzhe, Andrey M Lider, Viktor N Kudiiarov

2019 (9): 98801-098801. doi: 10.1088/1674-1056/ab33f0

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Mg-based hydrogen storage materials are considered to be one of the most promising solid-state hydrogen storage materials due to their large hydrogen storage capacity and low cost. However, slow hydrogen absorption/desorption rate and excessive hydrogen absorption/desorption temperature limit the application of Mg-based hydrogen storage materials. The present paper reviews the advances in the research of Mg-based hydrogen storage film in recent years, including the advantage of the film, the function theory of fabricating method and its functional theory, and the influencing factors in the technological process. The research status worldwide is introduced in detail. By comparing pure Mg, Pd-caped Mg, non-palladium capped Mg, and Mg alloy hydrogen storage films, an ideal tendency for producing Mg-based film is pointed out, for example, looking for a cheap metal element to replace the high-priced Pd, compositing Mg film with other hydrogen storage alloy of catalytic elements, and so on.

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The effect of Mn-doped ZnSe passivation layer on the performance of CdS/CdSe quantum dot-sensitized solar cells

邓云龙, 徐知源, 蔡凯, 马飞, 侯娟, 彭尚龙

2019 (9): 98802-098802. doi: 10.1088/1674-1056/ab37f3

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ZnSe as a surface passivation layer in quantum dot-sensitized solar cells plays an important role in preventing charge recombination and thus improves the power conversion efficiency (PCE). However, as a wide bandgap semiconductor, ZnSe cannot efficiently absorb and convert long-wavelength light. Doping transition metal ions into ZnSe semiconductors is an effective way to adjust the band gap, such as manganese ions. In this paper, it is found by the method of density functional theory calculation that the valence band of ZnSe moves upward with manganese ions doping, which leads to acceleration of charge separation, wider light absorption range, and enhancing light harvesting. Finally, by using ZnSe doped with manganese ions as the passivation layer, the TiO₂/CdS/CdSe co-sensitized solar cell has a PCE of 6.12%, and the PCE of the solar cell increases by 9% compared with the undoped one (5.62%).

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A new cellular automaton model accounting for stochasticity in traffic flow induced by heterogeneity in driving behavior

倪晓勇, 黄弘

2019 (9): 98901-098901. doi: 10.1088/1674-1056/ab343b

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A new reliable cellular automaon (CA) model designed to account for stochasticity in traffic flow induced by heterogeneity in driving behavior is presented. The proposed model differs from most existing CA models in that this new model focuses on describing traffic phenomena by coding into its rules the key idea that a vehicle's moving state is directly determined by a driver stepping on the accelerator or on the brake (the vehicle's acceleration). Acceleration obeys a deformed continuous distribution function when considering the heterogeneity in driving behavior and the safe distance, rather than equaling a fixed acceleration value with a probability, as is the rule in many existing CA models. Simulation results show that the new proposed model is capable of reproducing empirical findings in real traffic system. Moreover, this new model makes it possible to implement in-depth analysis of correlations between a vehicle's state parameters.

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