中国物理B

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Multiple exp-function method for soliton solutions ofnonlinear evolution equations

Yakup Yıldırım, Emrullah Yašar

2017 (7): 70201-070201. doi: 10.1088/1674-1056/26/7/070201

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We applied the multiple exp-function scheme to the (2+1)-dimensional Sawada–Kotera (SK) equation and (3+1)-dimensional nonlinear evolution equation and analytic particular solutions have been deduced. The analytic particular solutions contain one-soliton, two-soliton, and three-soliton type solutions. With the assistance of Maple, we demonstrated the efficiency and advantages of the procedure that generalizes Hirota's perturbation scheme. The obtained solutions can be used as a benchmark for numerical solutions and describe the physical phenomena behind the model.

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A new two-mode thermo-and squeezing-mixed optical field

周军, 范洪义, 宋军

2017 (7): 70301-070301. doi: 10.1088/1674-1056/26/7/070301

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We propose a new two-mode thermo-and squeezing-mixed optical field, described by the new density operator ρ=(1–e^f–|g|²) e^{ga^†b^†}e^{fa^†a}|0>_bb < 0|e^{g^*ab}, where|0>_bb < 0|is the b-mode vacuum, e^{fa^†a} represents the thermo-field, and e^{ga^†b} indicates squeezing. The photon statistics for ρ is studied by virtue of the method of integration within ordered product (IWOP) of operators. Such a field can be generated when a two-mode squeezed state passes through a one-mode dissipation channel.

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Thermal entanglement of the spin-1 Ising–Heisenberg diamond chain with biquadratic interaction

郑一丹, 毛竹, 周斌

2017 (7): 70302-070302. doi: 10.1088/1674-1056/26/7/070302

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We investigate the thermal entanglement of the spin-1 Ising–Heisenberg diamond chain, which can be regarded as a theoretical model for the homometallic molecular ferrimagnet[Ni₃(C₄H₂O₄)₂-(μ₃-OH)₂(H₂O)₄]_n·(2H₂O)_n. Two cases, i.e., the isotropic Heisenberg (Ising–XXX) coupling model and anisotropic Heisenberg (Ising–XXZ) coupling model, are discussed respectively. The negativity is chosen as the measurement of the thermal entanglement. By means of the transfer-matrix approach, we focus on the effects of biquadratic interaction parameters on the negativity of the infinite spin-1 Ising–Heisenberg diamond chain. In the Ising–XXX coupling model, it is shown that for the case with ferromagnetic coupling the thermal entanglement can be induced by the biquadratic interaction, but the external magnetic field will suppress the occurrence of the entanglement induced by the biquadratic interaction. In the Ising–XXZ coupling model, for the case with antiferromagnetic coupling, due to the biquadratic interaction the effect of the anisotropy parameter on the entanglement will be suppressed at near-zero temperature. Moreover, the biquadratic interaction makes the threshold temperature increase. The effects of the external magnetic field on the thermal entanglement are also discussed, and it is observed that the entanglement revival phenomena exist in both models considered.

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Tracking consensus for nonlinear heterogeneous multi-agent systems subject to unknown disturbances via sliding mode control

张翔, 王金环, 杨德东, 徐勇

2017 (7): 70501-070501. doi: 10.1088/1674-1056/26/7/070501

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We investigate the tracking control for a class of nonlinear heterogeneous leader–follower multi-agent systems (MAS) with unknown external disturbances. Firstly, the neighbor-based distributed finite-time observers are proposed for the followers to estimate the position and velocity of the leader. Then, two novel distributed adaptive control laws are designed by means of linear sliding mode (LSM) as well as nonsingular terminal sliding mode (NTSM), respectively. One can prove that the tracking consensus can be achieved asymptotically under LSM and the tracking error can converge to a quite small neighborhood of the origin in finite time by NTSM in spite of uncertainties and disturbances. Finally, a simulation example is given to verify the effectiveness of the obtained theoretical results.

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Collective transport of Lennard–Jones particles through one-dimensional periodic potentials

何健辉, 温家乐, 陈沛荣, 郑冬琴, 钟伟荣

2017 (7): 70502-070502. doi: 10.1088/1674-1056/26/7/070502

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The surrounding media in which transport occurs contains various kinds of fields, such as particle potentials and external potentials. One of the important questions is how elements work and how position and momentum are redistributed in the diffusion under these conditions. For enriching Fick's law, ordinary non-equilibrium statistical physics can be used to understand the complex process. This study attempts to discuss particle transport in the one-dimensional channel under external potential fields. Two kinds of potentials–-the potential well and barrier–-which do not change the potential in total, are built during the diffusion process. There are quite distinct phenomena because of the different one-dimensional periodic potentials. By the combination of a Monte Carlo method and molecular dynamics, we meticulously explore why an external potential field impacts transport by the subsection and statistical method. Besides, one piece of evidence of the Maxwell velocity distribution is confirmed under the assumption of local equilibrium. The simple model is based on the key concept that relates the flux to sectional statistics of position and momentum and could be referenced in similar transport problems.

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Molecular dynamics simulation of decomposition and thermal conductivity of methane hydrate in porous media

郭平, 潘意坤, 李龙龙, 唐斌

2017 (7): 73101-073101. doi: 10.1088/1674-1056/26/7/073101

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The hydrate has characteristics of low thermal conductivity and temperature sensitivity. To further analysis the mechanism of thermal conductivity and provide method for the exploitation, transportation and utilization of hydrate, the effect of decomposition and thermal conductivity of methane hydrate in porous media has been studied by using the molecular dynamics simulation. In this study, the simulation is carried out under the condition of temperature 253.15 K–273.15 K and pressure 1 MPa. The results show that the thermal conductivity of methane hydrate increases with the increase of temperature and has a faster growth near freezing. With the addition of porous media, the thermal conductivity of the methane hydrate improves significantly. The methane hydrate-porous media system also has the characteristics of vitreous body. With the decrease of the pore size of the porous media, thermal conductivity of the system increases gradually at the same temperature. It can be ascertained that the porous media of different pore sizes have strengthened the role of the thermal conductivity of hydrates.

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Electronic transport properties of lead nanowires

张力舒, 周毅, 代新月, 赵珍阳, 李辉

2017 (7): 73102-073102. doi: 10.1088/1674-1056/26/7/073102

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Lead nanowire occupies a very important position in an electronic device. In this study, a genetic algorithm (GA) method has been used to simulate the Pb nanowire. The result shows that Pb nanowires are a multishell cylinder. Each shell consists of atomic rows wound up helically side by side. The quantum electron transport properties of these structures are calculated based on the non-equilibrium Green function (NEGF) combined with the density functional theory (DFT), which indicate that electronic transport ability increases gradually with the atomic number increase. In addition, the thickest nanowire shows excellent electron transport performance. It possesses great transmission at the Fermi level due to the strongest delocalization of the electronic state. The results provide valuable information on the relationship between the transport properties of nanowires and their diameter.

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Restraint of spatial distribution in high-order harmonic generation from a model of hydrogen molecular ion

夏昌龙, 张军, 苗向阳, 刘学深

2017 (7): 73201-073201. doi: 10.1088/1674-1056/26/7/073201

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The spatial distribution in high-order harmonic generation (HHG) is theoretically investigated by using a few-cycle laser pulse from a two-dimensional model of a hydrogen molecular ion. The spatial distribution in HHG demonstrates that the harmonic spectra are sensitive to the carrier envelope phase and the duration of the laser pulse. The HHG can be restrained by a pulse with the duration of 5 fs in the region from the 90th to 320th order. This characteristic is illustrated by the probability density of electron wave packet distribution. The electron is mainly located near the nucleus along the positive-x direction from 3.0 o.c. to 3.2 o.c., which is an important time to generate the HHG in the plateau area. We also demonstrate the time–frequency distribution in the region of the positive-and negative-x direction to explain the physical mechanism.

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Pressure-broadened atomic Li(2s-2p) line perturbed by ground neon atoms in the spectral wings and core

Sabri Bouchoucha,Kamel Alioua,Moncef Bouledroua

2017 (7): 73202-073202. doi: 10.1088/1674-1056/26/7/073202

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Full quantum calculations are performed to investigate the broadening profiles of the atomic lithium Li(2s-2p) resonance line induced by interactions with ground Ne(2s²2p⁶) perturbers in the spectral wings and core. The X²Σ⁺, A²Π, and B²Σ⁺ potential-energy curves of the two first low lying LiNe molecular states, as well as the corresponding transition dipole moments, are determined with ab initio methods based on the SA-CASSCF-MRCI calculations. The emission and absorption coefficients in the wavelength range 550–800 nm and the line-core width and shift are investigated theoretically for temperatures ranging from 130 K to 3000 K. Their temperature dependence is analyzed, and the obtained results are compared with the previous experimental measurements and theoretical works.

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Asymmetrical mirror optimization for a 140 GHz TE_{22, 6} quasi-optical mode converter system

夏冬, 金铭, 白明

2017 (7): 74101-074101. doi: 10.1088/1674-1056/26/7/074101

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We introduce an asymmetrical mirror design to a 140 GHz TE_22,6 quasi-optical (QO) mode converter system to correct the asymmetry of the beam's field distribution caused by the Denisov launcher. By such optimization, the output beam with better symmetrical distribution is obtained at the system's output window. Based on the calculated results, the QO mode converter system's performance is already satisfying without iterative phase correction. Scalar and vector correlation coefficients between the output beam and the fundamental Gaussian beam are respectively 98.4% and 93.0%, while the total power transmission efficiency of the converter system is 94.4%. The assistance of optical ray tracing to the design of such QO mode converters is introduced and discussed as well.

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Laser beam shaping with magnetic fluid-based liquid deformable mirrors

吴智政, 张柱, 孔祥会, 吴君秋, 王韬, 刘梅, 谢少荣

2017 (7): 74201-074201. doi: 10.1088/1674-1056/26/7/074201

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A new controllable laser beam shaping technique is demonstrated, where a magnetic fluid-based liquid deformable mirror is proposed to redistribute the laser phase profile and thus change the propagation property of the beam. The mirror is driven by an inner miniature actuator array along with a large outer actuator. The inner actuator array is used for deforming the magnetic fluid surface, while the outer actuator is used to linearize the fluid surface response and amplify the magnitude of the deflection. In comparison to other laser beam shaping techniques, this technique offers the advantages such as simplicity, low cost, large shape deformation, and high adaptability. Based on a fabricated prototype of the liquid deformable mirror, an experimental AO system was set up to produce a desired conical surface shape that shaped the incident beam into a Bessel beam. The experimental results show the effectiveness of the proposed technique for laser beam shaping.

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Simple and universal method in designs of high-efficiency diffractive optical elements for spectrum separation and beam concentration

徐文琪, 林冬风, 许信, 叶佳声, 王新柯, 冯胜飞, 孙文峰, 韩鹏, 张岩, 孟庆波, 杨国桢

2017 (7): 74202-074202. doi: 10.1088/1674-1056/26/7/074202

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Diffractive optical elements (DOEs) with spectrum separation and beam concentration (SSBC) functions have important applications in solar cell systems. With the SSBC DOEs, the sunlight radiation is divided into several wave bands so as to be effectively absorbed by photovoltaic materials with different band gaps. A new method is proposed for designing high-efficiency SSBC DOEs, which is physically simple, numerically fast, and universally applicable. The SSBC DOEs are designed by the new design method, and their performances are analyzed by the Fresnel diffraction integral method. The new design method takes two advantages over the previous design method. Firstly, the optical focusing efficiency is heightened by up to 10%. Secondly, focal positions of all the designed wavelengths can be designated arbitrarily and independently. It is believed that the designed SSBC DOEs should have practical applications to solar cell systems.

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Analytical solutions for a doubly driven two-level atom

刘晋允, 贾凤东, 李晓康, 吕双飞, 许祥源, 薛平, 钟志萍

2017 (7): 74203-074203. doi: 10.1088/1674-1056/26/7/074203

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We have deduced analytical solutions of an energy level diagram of the doubly driven/dressed atom for a two-level atom exposed to a strong near-resonant bichromatic laser field in a special case, i.e., the bichromatic field with frequencies ω₁ and ω₂, and Rabi frequencies Ω₁ and Ω₂, in which the first coupling field of \varOmega₁ acts on the bare atomic levels, and then the resulting singly dressed states are driven by the second coupling field of Ω₂, thus resulting in the doubly dressed atom. We have measured the probe absorption spectra of a doubly driven two-level atom. The system consists of 5²S_1/2,F=2 and 5²P_3/2,F'=3 states of ⁸⁷Rb atoms in a magneto-optical trap (MOT) as well as the cooling/trapping beams and an additional coupling field. As for the spectroscopic properties of the doubly driven two-level atom, theoretical analytical solutions are in general agreement with the experimental spectrum as a whole.

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High quality electromagnetically induced transparency spectroscopy of ⁸⁷Rb in a buffer gas cell with a magnetic field

成红, 王汉睦, 张珊珊, 辛培培, 罗军, 刘红平

2017 (7): 74204-074204. doi: 10.1088/1674-1056/26/7/074204

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We have studied the phenomenon of electromagnetically induced transparency (EIT) of ⁸⁷Rb vapor with a buffer gas in a magnetic field at room temperature. It is found that the spectral lines caused by the velocity selective optical pump effects get much weaker and wider when the sample cell is mixed with a 5-Torr N₂ gas while the EIT signal is kept almost unchanged. A weighted least-square fit is also developed to remove the Doppler broadening completely. This spectral method provides a way to measure the Zeeman splitting with high resolution, for example, the Λ -type EIT resonance splits into four peaks on the D₂ line of ⁸⁷Rb in the thermal 2-cm vapor cell with a magnetic field along the electric field of the linearly polarized coupling laser. The high-resolution spectrum can be used to lock the laser to a given frequency by tuning the magnetic field.

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Quantum feedback cooling of two trapped ions

张硕, 吴伟, 吴春旺, 李风光, 李坦, 汪翔, 鲍皖苏

2017 (7): 74205-074205. doi: 10.1088/1674-1056/26/7/074205

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We present a sub-Doppler cooling scheme of a two-trapped-ion crystal by quantum feedback control method. In the scheme, we obtain the motional information by continuously measuring the spontaneous emission photons from one single ion of the crystal, and then apply a feedback force to cool the whole chain down.We derive the cooling dynamics of the cooling scheme using quantum feedback theory and quantum regression theorem. The result shows that with experimentally achievable parameters, our scheme can achieve lower temperature and faster cooling rate than Doppler cooling.

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Quantum interference between heralded single photon stateand coherent state

杨磊, 马晓欣, 李小英

2017 (7): 74206-074206. doi: 10.1088/1674-1056/26/7/074206

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Balanced homodyne detection has been introduced as a reliable technique of reconstructing the quantum state of a single photon Fock state, which is based on coupling the single photon state and a strong coherent local oscillator in a beam splitter and detecting the field quadrature at the output ports separately. The main challenge associated with a tomographic characterization of the single photon state is mode matching between the single photon state and the local oscillator. Utilizing the heralded single photon generated by the spontaneous parametric process, the multi-mode theoretical model of quantum interference between the single photon state and the coherent state in the fiber beam splitter is established. Moreover, the analytical expressions of the temporal-mode matching coefficient and interference visibility and relationship between the two parameters are shown. In the experimental scheme, the interference visibility under various temporal-mode matching coefficients is demonstrated, which is almost accordant with the theoretical value. Our work explores the principle of temporal-mode matching between the single photon state and the coherent photon state, originated from a local oscillator, and could provide guidance for designing the high-performance balanced homodyne detection system.

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Controllable double electromagnetically induced transparency in a closed four-level-loop cavity–atom system

郭苗迪, 苏雪梅

2017 (7): 74207-074207. doi: 10.1088/1674-1056/26/7/074207

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We present a theoretical study of an optical cavity coupled with single four-level atoms in closed loop formed via applied control lasers.The transmitted probe field from the cavity is analyzed.We show that the electromagnetically induced transparency (EIT) in the cavity and the normal mode splitting will be very different with changing the closed interaction phase and the intensity of the free-space control laser.This coupled cavity–atom system presents a variational double-EIT that comes from modulating the splitting of the dark state,which means that we could realize the gradual transfer between one EIT peak and two EIT peaks by adjusting the applied control lasers,and the normal mode splitting sidebands will shift slightly by changing the free-space control laser.This means that we could control the output cavity probe field more freely and it is easer to realize optical switch controlled by more parameters.We also depict the angular dispersion of the intracavity probe field in different free-space control laser.The large phase shift (–π→π) of the reflected intracavity probe field will be very useful for optical temporal differentiation and quantum phase gate.

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Spectral dynamical behavior in two-section, quantum well, mode-locked laser at 1.064μm

魏思航, 马奔, 陈泽升, 廖永平, 郝宏玥, 张宇, 倪海桥, 牛智川

2017 (7): 74208-074208. doi: 10.1088/1674-1056/26/7/074208

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In this study, two-section mode-locked semiconductor lasers with different numbers of quantum wells and different types of waveguide structures are made. Their ultrashort pulse features are presented. The spectral dynamical behaviors in these lasers are studied in detail. In the simulation part, a two-band compressive-strained quantum well (QW) model is used to study thermally induced band-edge detuning in the amplifier and saturable absorber (SA). A sudden blue shift in laser spectrum is expected by calculating the peak of the net gain. In the experiment part, the sudden blue shift in the emission spectrum is observed in triple QW samples under certain operating conditions but remains absent in single QW samples. Experimental results reveal that blue shift phenomenon is connected with the difference between currents in two sections. Additionally, a threshold current ratio for blue-shift is also demonstrated.

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Stable single-mode operation of 894.6 nm VCSEL at high temperatures for Cs atomic sensing

向磊, 张星, 张建伟, 宁永强, 王立军

2017 (7): 74209-074209. doi: 10.1088/1674-1056/26/7/074209

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In this paper, stable single-mode operation at high temperatures is produced by the surface-relief-integrated vertical cavity surface emitting laser (VCSEL). The gain-cavity mode detuning technique is employed to realize high operating temperatures for the VCSEL. The surface relief is etched in the centre of the top side as a mode discriminator for the fundamental mode output, and the threshold current minimum is 1.94 mA at high temperatures by the gain-cavity mode detuning technique. Maximum single-fundamental-mode output power of 0.45 mW at 80℃ is obtained, and the side mode suppression ratios (SMSRs) are more than 30 dB with increasing temperature and current, respectively.

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Supercontinuum generation in seven-core photonic crystal fiber pumped by a broadband picosecond pulsed fiber amplifier

苏宁, 李平雪, 肖坤, 王晓晓, 刘建国, 邵月, 苏盟

2017 (7): 74210-074210. doi: 10.1088/1674-1056/26/7/074210

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We report a supercontinuum source generated in seven-core photonic crystal fibers (PCFs) pumped by a self-made all-fiber picosecond pulsed broadband fiber amplifier. The amplifier's output average power is 60 W at 1150 nm with spectral width of 260 nm, and its repetition rate is 8.47 MHz with pulse width of 221 ps. With two different lengths of seven-core PCF, different output powers and spectra are obtained. When a 10 m long seven-core PCF is chosen, the output supercontinuum covers the wavelength range from 620 nm to 1700 nm, with the output power of 11.7 W. With only 2 m long seven-core PCF used in the same experiment, the wavelength of the supercontinuum spans from 680 nm to 1700 nm, with the output power of 20.4 W. The results show that the pulse width is 385 ps in the 10 m long seven-core PCF and 255 ps in the 2 m long one, respectively, due to the normal dispersion of the PCF.

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Design and performance of a composite Tm: YAG laser pumped by VBG-stabilized narrow-band laser diode

戴殊韬, 黄见洪, 黄海舟, 吴丽霞, 李锦辉, 邓晶, 葛燕, 林文雄

2017 (7): 74211-074211. doi: 10.1088/1674-1056/26/7/074211

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A 2-μm composite Tm:YAG laser pumped with a narrow-band laser diode was presented. The temperature distribution and thermal lens in the Tm:YAG were numerically simulated by a finite element method and the results were used to design the special cavity, in order to achieve a high efficiency and stable output. With a 25-W incident pump power, we obtained a maximum output power of 11 W at 2018.5 nm, corresponding to a slope efficiency of 51.3% and an optical-to-optical efficiency of 44.5%, respectively. The beam quality was measured to be M_x²=1.8 and M_y²=1.6.

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Observation of stable bound soliton with dual-wavelength in a passively mode-locked Er-doped fiber laser

郑煜, 田金荣, 董自凯, 徐润亲, 李克轩, 宋晏蓉

2017 (7): 74212-074212. doi: 10.1088/1674-1056/26/7/074212

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A phase-locked bound state soliton with dual-wavelength is observed experimentally in a passively mode-locked Er-doped fiber (EDF) laser with a fiber loop mirror (FLM). The pulse duration of the soliton is 15 ps and the peak-to-peak separation is 125 ps. The repetition rate of the pulse sequence is 3.47 MHz. The output power is 11.8 mW at the pump power of 128 mW, corresponding to the pulse energy of 1.52 nJ. The FLM with a polarization controller can produce a comb spectrum, which acts as a filter. By adjusting the polarization controller or varying the pump power, the central wavelength of the comb spectrum can be tuned. When it combines with the reflective spectrum of the fiber Bragg grating, the total spectrum of the cavity can be cleaved into two parts, then the bound state soliton with dual-wavelength at 1549.7 nm and 1550.4 nm is obtained.

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Intense supercontinuum generation in the near-ultraviolet range from a 400-nm femtosecond laser filament array in fused silica

李东伟, 张兰芝, 宋鹤, 郝作强, 奚婷婷, 高勋, 林景全

2017 (7): 74213-074213. doi: 10.1088/1674-1056/26/7/074213

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An intense supercontinuum (SC) in the near-ultraviolet range is generated from filamentation by focusing a 400-nm laser into fused silica with a microlens array (MLA). The spectrum of the SC is shown to be sensitive to the distance between the MLA and fused silica. In our optimal conditions, the near-ultraviolet SC can cover a range of 350–600 nm, where a bandwidth of approximately 55 nm above the 1μ J/nm spectral energy density and 20 nm bandwidth with tens ofμJ/nm are achieved. In addition, the energy conversion efficiency of the 400 nm laser for SC generation is further analyzed. A maximum conversion efficiency of 66% is obtained when the entrance face of fused silica is set around the focus of the MLA.

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Characterization of the pairwise correlations in different quantum networks consisting of four-wave mixers and beamsplitters

祁健, 忻俊, 王海龙, 荆杰泰

2017 (7): 74214-074214. doi: 10.1088/1674-1056/26/7/074214

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We investigate the performances of the pairwise correlations (PCs) in different quantum networks consisting of four-wave mixers (FWMs) and beamsplitters (BSs). PCs with quantum correlation in different quantum networks can be verified by calculating the degree of relative intensity squeezing for any pair of all the output fields. More interestingly, the quantum correlation recovery and enhancement are present in the FWM+BS network and the repulsion effect phenomena (signal (idler)–frequency mode cannot be quantum correlated with the other two idler (signal)–frequency modes simultaneously) between the PCs with quantum correlation are predicted in the FWM+FWM and FWM+FWM+FWM networks. Our results presented here pave the way for the manipulation of the quantum correlation in quantum networks.

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Theoretical study of the odd–even-order harmonic generation for asymmetric ions in non-Born–Oppenheimer approximation

岳生俊, 杜洪川, 吴红梅, 薛山, 赵家琛, 胡碧涛

2017 (7): 74215-074215. doi: 10.1088/1674-1056/26/7/074215

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We calculated the harmonic spectra generated from the asymmetric molecules of HD⁺ and HeH²⁺. It is found that HD⁺produces only odd harmonics, while HeH²⁺ produces both odd and even harmonics. Further analysis reveals that for both HD⁺ and HeH²⁺, the nuclear dipole acceleration can generate even harmonics, but it is three orders of magnitude lower than that of the electron. Hence, the electronic dipole acceleration dominates the harmonic generation. For HD⁺, the electronic dipole acceleration only contributes to the generation of odd harmonics, but for HeH²⁺ it contributes to the generation of both odd and even harmonics. Besides, one concept of the broken degree of system-symmetry is proposed to explain the different odd-even property between the harmonic spectra of HD⁺ and HeH²⁺.

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Spatiotemporal evolution of continuous-wave field and dark soliton formation in a microcavity with normal dispersion

胡晓鸿, 张伟, 刘元山, 冯野, 张文富, 王擂然, 王屹山, 赵卫

2017 (7): 74216-074216. doi: 10.1088/1674-1056/26/7/074216

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Stable dark soliton and dark pulse formation in normally dispersive and red-detuned microcavities are investigated by numerically solving the normalized Lugiato-Lefever equation. The soliton essence is proved by fitting the calculated field intensity profile with the analytical formula of a dark soliton. Meanwhile, we find that a dark soliton can be generated either from the nonlinear evolution of an optical shock wave or narrowing of a locally broad dark pulse with smoother fronts. Explicit analytical expression is obtained to describe the oscillatory fronts of the optical shock wave. Furthermore, from the calculation results, we show that for smaller frequency detunings, e.g., α<3, in addition to the dark soliton formation, a single dark pulse with an oscillatory dip can also arise and propagate stably in the microcavity under proper pump detuning and pump strength combination. The existence region together with various field intensity profiles and the corresponding spectra of single dark pulse are demonstrated.

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Spectroscopic and radiation-resistant properties of Er, Pr: GYSGG laser crystal operated at 2.79μm

赵绪尧, 孙敦陆, 罗建乔, 张会丽, 方忠庆, 权聪, 李秀丽, 程毛杰, 张庆礼, 殷绍唐

2017 (7): 74217-074217. doi: 10.1088/1674-1056/26/7/074217

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We demonstrate the spectroscopic and laser performance before and after 100 Mrad gamma-ray irradiation on an Er,Pr:GYSGG crystal grown by the Czochralski method. The additional absorption of Er,Pr:GYSGG crystal is close to zero in the 968 nm pumping and 2.7–3μm laser wavelength regions. The lifetimes of the upper and lower levels show faint decreases after gamma-ray irradiation. The maximum output powers of 542 and 526 mW with the slope efficiencies of 17.7% and 17.0% are obtained, respectively, on the GYSGG/Er,Pr:GYSGG composite crystal before and after the gamma-ray irradiation. These results suggest that Er,Pr:GYSGG crystal as a laser gain medium possesses a distinguished anti-radiation ability for application in space and radiant environments.

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Bifurcated overtones of one-way localized Fabry–Pérot resonances in parity-time symmetric optical lattices

沈亚西, 彭玉桂, 武爱民, 张鹏, 祝雪丰

2017 (7): 74218-074218. doi: 10.1088/1674-1056/26/7/074218

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Since the first observation of parity-time (PT) symmetry in optics, varied interesting phenomena have been discovered in both theories and experiments, such as PT phase transition and unidirectional invisibility, which turns PT-symmetric optics into a hotspot in research. Here, we report on the one-way localized Fabry–Pérot (FP) resonance, where a well-designed PT optical resonator may operate at exceptional points with bidirectional transparency but unidirectional field localization. Overtones of such one-way localized FP resonance can be classified into a blue shifted branch and a red shifted branch. Therefore, the fundamental resonant frequency is not the lowest one. We find that the spatial field distributions of the overtones at the same absolute order are almost the same, even though their frequencies are quite different.

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Mechanically tunable metamaterials terahertz dual-band bandstop filter

胡放荣, 胥欣, 李鹏, 徐新龙, 王月娥

2017 (7): 74219-074219. doi: 10.1088/1674-1056/26/7/074219

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We experimentally demonstrate a mechanically tunable metamaterials terahertz (THz) dual-band bandstop filter. The unit cell of the filter contains an inner aluminum circle and an outside aluminum Ohm-ring on high resistance silicon substrate. The performance of the filter is simulated by finite-integration-time-domain (FITD) method. The sample is fabricated using a surface micromachining process and experimentally demonstrated using a THz time-domain-spectroscopy (TDS) system. The results show that, when the incident THz wave is polarized in y-axis, the filter has two intensive absorption peaks locating at 0.71 THz and 1.13 THz, respectively. The position of the high-frequency absorption peak and the amplitude of the low-frequency absorption peak can be simultaneously tuned by rotating the sample along its normal axis. The tunability of the high-frequency absorption peak is due to the shift of resonance frequency of two electrical dipoles, and that of the low-frequency absorption peak results from the effect of rotationally induced transparent. This tunable filter is very useful for switch, manipulation, and frequency selective detection of THz beam.

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Asymmetrical plasmon reflections in tapered graphene ribbons with wrinkle edges

杨翠, 陈闰堃, 贾玉萍, 郭丽伟, 陈佳宁

2017 (7): 74220-074220. doi: 10.1088/1674-1056/26/7/074220

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Asymmetrical graphene plasmon reflection patterns are found in infrared near-field images of tapered graphene ribbons epitaxially grown on silicon carbon substrates. Comparing experimental data with numerical simulations, the asymmetry of these patterns is attributed to reflection of plasmons by wrinkled edges naturally grown in the graphene. These graphene wrinkles are additional plasmon reflectors with varying optical conductivity, which act as nanometer scale plasmonic modulators and thus have potential applications in photoelectric information detectors, transmitters, and modulators.

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Low power consumption 4-channel variable optical attenuator array based on planar lightwave circuit technique

任梅珍, 张家顺, 安俊明, 王玥, 王亮亮, 李建光, 吴远大, 尹小杰, 胡雄伟

2017 (7): 74221-074221. doi: 10.1088/1674-1056/26/7/074221

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The power consumption of a variable optical attenuator (VOA) array based on a silica planar lightwave circuit was investigated. The thermal field profile of the device was optimized using the finite-element analysis. The simulation results showed that the power consumption reduces as the depth of the heat-insulating grooves is deeper, the up-cladding is thinner, the down-cladding is thicker, and the width of the cladding ridge is narrower. The materials component and thickness of the electrodes were also optimized to guarantee the driving voltage under 5 V. The power consumption was successfully reduced to as low as 155 mW at an attenuation of 30 dB in the experiment.

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A tunable XUV monochromatic light source based on the time preserving grating selection of high-order harmonic generation

牛永, 梁红静, 刘燚, 刘方圆, 马日, 丁大军

2017 (7): 74222-074222. doi: 10.1088/1674-1056/26/7/074222

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We accomplish a laboratory facility for producing a femtosecond XUV coherent monochromatic radiation with a broad tunable spectral range of 20 eV–75 eV. It is based on spectral selected single-order harmonics from intense laser driven high harmonic generation in gas phase. The time preserving for the selected harmonic radiation is achieved by a Czerny–Turner type monochromator designed with a conical diffraction grating mount for minimizing the time broadening caused by grating diffraction and keeping a relatively high diffraction efficiency. Our measurement shows that the photon flux of the 23-order harmonic (H23) centered at 35.7 eV is 1×10⁹ photons/s approximately with a resolving power E/ΔE≈36. This source provides an ultrashort tunable monochromatic XUV beam for ultrafast studies of electronic and structural dynamics in a large variety of matters.

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Bubble acoustical scattering cross section under multi-frequency acoustic excitation

时洁, 杨德森, 张昊阳, 时胜国, 李松, 胡博

2017 (7): 74301-074301. doi: 10.1088/1674-1056/26/7/074301

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The acoustical scattering cross section is usually employed to evaluate the scattering ability of the bubbles when they are excited by the incident acoustic waves. This parameter is strongly related to many important applications of performance prediction for search sonar or underwater telemetry, acoustical oceanography, acoustic cavitation, volcanology, and medical and industrial ultrasound. In the present paper, both the analytical and numerical analysis results of the acoustical scattering cross section of a single bubble under multi-frequency excitation are obtained. The nonlinear characteristics (e.g., harmonics, subharmonics, and ultraharmonics) of the scattering cross section curve under multi-frequency excitation are investigated compared with single-frequency excitation. The influence of several paramount parameters (e.g., bubble equilibrium radius, acoustic pressure amplitude, and acoustic frequencies) in the multi-frequency system on the predictions of scattering cross section is discussed. It is shown that the combination resonances become significant in the multi-frequency system when the acoustic power is big enough, and the acoustical scattering cross section is promoted significantly within a much broader range of bubble sizes and acoustic frequencies due to the generation of more resonances.

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Acoustic-electromagnetic slow waves in a periodical defective piezoelectric slab

李小娟, 葛欢, 范理, 张淑仪, 张辉, 丁劲

2017 (7): 74302-074302. doi: 10.1088/1674-1056/26/7/074302

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Coupled slow waves, slow acoustic waves, and electromagnetic waves are simultaneously achieved based on a piezoelectric material, in which a line defect is created within a honeycomb lattice array of cylindrical holes etched in a LiNbO₃ slab. Finite element simulations in frequency domain and time domain demonstrate that a highly localized slow mode is obtained in the defect. Owing to the piezoelectricity of LiNbO₃, acoustic and electromagnetic waves are coupled with each other and transmit along the line defect. Therefore, in addition to a slow acoustic wave, an electromagnetic wave with a group velocity even lower than conventional acoustic waves is achieved.

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Relationship between the real contact area and contact force in pre-sliding regime

宋保江, 阎绍泽

2017 (7): 74601-074601. doi: 10.1088/1674-1056/26/7/074601

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The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To investigate the relationship between contact status and contact force in pre-sliding friction, an optical experimental method is presented in this paper. With this method, the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique. A novel setup, which includes a pair of rectangular trapezoidal blocks, is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface. The improved Otsu's method is used for measurement. Through an experimental study performed on polymethyl methacrylate (PMMA), the quantity of contact asperities is proven to be the dominant factor that affects the real contact area. The relationship between the real contact area and the contact force in the pre-sliding regime is studied, and the distribution of static friction at the contact interface is qualitatively discussed. New phenomena in which the real contact area expands along with increasing static friction are identified. The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure, and the distribution of friction stress decreases from the leading edge to the trailing edge.

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Buckling analysis of nanobeams with exponentially varying stiffness by differential quadrature method

S Chakraverty,Laxmi Behera

2017 (7): 74602-074602. doi: 10.1088/1674-1056/26/7/074602

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We present the application of differential quadrature (DQ) method for the buckling analysis of nanobeams with exponentially varying stiffness based on four different beam theories of Euler–Bernoulli, Timoshenko, Reddy, and Levison. The formulation is based on the nonlocal elasticity theory of Eringen. New results are presented for the guided and simply supported guided boundary conditions. Numerical results are obtained to investigate the effects of the nonlocal parameter, length-to-height ratio, boundary condition, and nonuniform parameter on the critical buckling load parameter. It is observed that the critical buckling load decreases with increase in the nonlocal parameter while the critical buckling load parameter increases with increase in the length-to-height ratio.

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Experimental study and theoretical analysis of fluid resistance in porous media of glass spheres

王彤, 郑坤灿, 贾宇鹏, 付承鹭, 龚志军, 武文斐

2017 (7): 74701-074701. doi: 10.1088/1674-1056/26/7/074701

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Porous media have a wide range of applications in production and life, as well as in science and technology. The study of flow resistance in porous media has a great effect on industrial and agricultural production. The flow resistance of fluid flow through a 20-mm glass sphere bed is studied experimentally. It is found that there is a significant deviation between the Ergun equation and the experimental data. A staggered pore-throat model is established to investigate the flow resistance in randomly packed porous media. A hypothesis is made that the particles are staggered in a regular triangle arrangement. An analytical formulation of the flow resistance in random porous media is derived. There are no empirical constants in the formulation and every parameter has a specific physical meaning. The formulation predictions are in good agreement with the experimental data. The deviation is within the range of 25%. This shows that the staggered pore-throat model is reasonable and is expected to be verified by more experiments and extended to other porous media.

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Radiative properties of matter based on quantum statistical method

Rukhsana Kouser,G Tasneem,Muhammad Saleem Shahzad,S Sardar,Amjad Ali,M H Nasim,M Salahuddin

2017 (7): 75201-075201. doi: 10.1088/1674-1056/26/7/075201

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We present the preliminary results of our code OPAQS (opacity calculation using quantum statistical model) that is based on the self consistent Hartree–Fock–Slater model for the average atom. The code is capable of performing robust calculations of average charge state, frequency-dependent and mean opacities. The accuracy of the atomic model is verified by comparing the calculations of average charge state with various published results. The monochromatic opacities for iron computed at different sets of temperatures and densities are compared with LEDCOP. The Rosseland and Planck opacities for iron and aluminum are validated with some state-of-the-art codes. The results are in good agreement with the published data.

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Magneto-Rayleigh–Taylor instability in compressible Z-pinch liner plasmas

杨学, 肖德龙, 丁宁, 刘杰

2017 (7): 75202-075202. doi: 10.1088/1674-1056/26/7/075202

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In this paper, the characteristics of magneto-Rayleigh–Taylor (MRT) instability of liner plasmas in MagLIF is theoretically investigated. A three-region slab model, based on ideal MHD equations, is used to derive the dispersion relation of MRT instability. The effect of compressibility on the development of MRT instability is specially examined. It is shown that the growth rate of MRT instability in compressible condition is generally lower than that in incompressible condition in the presence of magnetic field. In the case of zero magnetic field, the growth rate in compressible assumption is approximately the same as that in incompressible assumption. Generally, MRT instability in (x,y) plane can be remarkably mitigated due to the presence of magnetic field especially for short-wavelength perturbations. Perturbations may be nearly completely mitigated when the magnetic field is increased to over 1000 T during liner implosions. The feedthrough of MRT instability in liner outer surface on inner surface is also discussed.

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Influence of a centered dielectric tube on inductively coupled plasma source: Chamber structures and plasma characteristics

毕振华, 洪义, 雷光玖, 王帅, 王友年, 刘东平

2017 (7): 75203-075203. doi: 10.1088/1674-1056/26/7/075203

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A high-density RF ion source is an essential part of a neutral beam injector. In this study, the authors attempt to retrofit an original regular RF ion source reactor by inserting a thin dielectric tube through the symmetric axis of the discharge chamber. With the aid of this inner tube, the reactor is capable of generating a radial magnetic field instead of the original transverse magnetic field, which solves the E×B drift problem in the current RF ion source structure. To study the disturbance of the dielectric tube, a fluid model is introduced to study the plasma parameters with or without the internal dielectric tube, based on the inductively coupled plasma (ICP) reactor. The simulation results show that while introducing the internal dielectric tube into the ICP reactor, both the plasma density and plasma potential have minor influence during the discharge process, and there is good uniformity at the extraction region. The influence of the control parameters reveals that the plasma densities at the extraction region decrease first and subsequently slow down while enhancing the diffusion region.

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Numerical simulation of the initial plasma formation and current transfer in single-wire electrical explosion in vacuum

王坤, 史宗谦, 石元杰, 白骏, 吴坚, 贾申利, 邱爱慈

2017 (7): 75204-075204. doi: 10.1088/1674-1056/26/7/075204

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In this paper, a computational model is constructed to investigate the phenomenon of the initial plasma formation and current transfer in the single-wire electrical explosion in a vacuum. The process of the single-wire electrical explosion is divided into four stages. Stage I:the wire is in solid state. Stage Ⅱ:the melting stage. Stage Ⅲ:the wire melts completely and the initial plasma forms. Stage IV:the core and corona expand separately. The thermodynamic calculation is applied before the wire melts completely in stages I and Ⅱ. In stage Ⅲ, a one-dimensional magnetohydrodynamics model comes into play until the instant when the voltage collapse occurs. The temperature, density, and velocity, which are derived from the magnetohydrodynamics calculation, are averaged over the distribution area. The averaged parameters are taken as the initial conditions for stage IV in which a simplified magnetohydrodynamics model is applied. A wide-range semi-empirical equation of state, which is established based on the Thomas–Fermi–Kirzhnits model, is constructed to describe the phase transition from solid state to plasma state. The initial plasma formation and the phenomenon of current transfer in the electrical explosion of aluminum wire are investigated using the computational model. Experiments of electrical explosion of aluminum wires are carried out to verify this model. Simulation results are also compared with experimental results of the electrical explosion of copper wire.

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Atomic pair distribution function method development at the Shanghai Synchrotron Radiation Facility

周晓娟, 陶举洲, 郭瀚, 林鹤

2017 (7): 76101-076101. doi: 10.1088/1674-1056/26/7/076101

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The atomic pair distribution function (PDF) reveals the interatomic distance in a material directly in real-space. It is a very powerful method to characterize the local structure of materials. With the help of the third generation synchrotron facility and spallation neutron source worldwide, the PDF method has developed quickly both experimentally and theoretically in recent years. Recently this method was successfully implemented at the Shanghai Synchrotron Radiation Facility (SSRF). The data quality is very high and this ensures the applicability of the method to study the subtle structural changes in complex materials. In this article, we introduce in detail this new method and show some experimental data we collected.

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Laser-induced fabrication of highly branched CuS nanocrystals with excellent near-infrared absorption properties

杨汝雨, 张中义, 徐林林, 李爽, 焦扬, 张华, 陈明

2017 (7): 76102-076102. doi: 10.1088/1674-1056/26/7/076102

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We report on the successful fabrication of highly branched CuS nanocrystals by laser-induced photochemical reaction. Surprisingly, the single-crystalline nature with preferential alignment of the (107) orientation can be well improved during the moderate growth process. The branch length drastically increases from about 5 nm to 6μm with an increase of photochemical reaction time (0–40 min). The absorption spectra of as-prepared CuS nanodendrites show that localized surface plasmon resonance (LSPR) peaks can be modulated from about 1037 nm to 1700 nm with an increase of branch length. Our results have a promising potential for photodynamic therapy and biological imaging application.

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Crystal structures and electronic properties of solid fluorine under high pressure

吕欠欠, 靳锡联, 崔田, 庄全, 李颖, 王友春, 包括, 孟醒

2017 (7): 76103-076103. doi: 10.1088/1674-1056/26/7/076103

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As the previously proposed structures of C2/m and C2/c possess similar enthalpies and x-ray diffraction patterns, the space group of fluorine at ambient pressure is in controversy. We successfully obtain its thermodynamically stable low-pressure phase, which shares the same structure as the earlier known C2/c. Further investigations on phonon spectra reveal the instability of the C2/m structure with imaginary frequency in the Brillouin zone and confirm the dynamically stable property of the C2/c structure at the same time. Compressing fluorine up to 8 GPa, the C2/c phase is found to undergo a phase transition to a new structure with a space group of Cmca. Electronic energy band structures indicate the insulating feature of C2/c and Cmca with no bands across the Fermi level. The infrared (IR) and Raman spectra of C2/c and Cmca at selected pressures are calculated to provide useful information to future experiments.

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Electronic structure & yield strength prediction for dislocation–Mo complex in the γ phase of nickel-based superalloys

刘凤华, 王崇愚

2017 (7): 76104-076104. doi: 10.1088/1674-1056/26/7/076104

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Molybenum's effects when added in the γ phase of nickel-based superalloys were studied using the lattice Green's function multiscale method. The electronic structure of the dislocation–Mo complex was analyzed and hybridization was found to contribute to the strengthening. Moreover, by combining the interaction energies calculated from two scales, the yield stress was theoretically predicted at 0 K and finite temperature.

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Optical waveguide in Nd: Bi₁₂SiO₂₀ crystal produced bymulti-energy C ion implantation

刘涛, 孔伟金, 任莹莹, 成燕

2017 (7): 76105-076105. doi: 10.1088/1674-1056/26/7/076105

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We report the fabrication of a planar waveguide in the Nd:Bi₁₂SiO₂₀ crystal by multi-energy C ions at room temperature. The waveguide is annealed at 200℃, 260℃, and 300℃ in succession each for 30 min in an open oven. The effective refractive index profiles at transverse electric (TE) polarization are stable after the annealing treatments. Damage distribution for multi-energy C ion implanted in Nd:Bi₁₂SiO₂₀ crystal is calculated by SRIM 2010. The Raman and fluorescence spectra of the Nd:Bi₁₂SiO₂₀ crystal are collected by an excitation beam at 633 nm and 473 nm, respectively. The results indicate the stabilization of the optical waveguide in Nd:Bi₁₂SiO₂₀ crystal.

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Molecular dynamics simulations of cascade damage near the Y₂Ti₂O₇ nanocluster/ferrite interface in nanostructured ferritic alloys

孙祎强, 赖文生

2017 (7): 76106-076106. doi: 10.1088/1674-1056/26/7/076106

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A comparative study of cascades in nanostructured ferritic alloys and pure Fe is performed to reveal the influence of Y₂Ti₂O₇ nanocluster on cascades by molecular dynamics simulations. The cascades with energies of primary knock-on atom (PKA) ranging from 0.5 keV to 4.0 keV and PKA's distances to the interface from 0 Å to 50 Å are simulated. It turns out that the Y₂Ti₂O₇ nanocluster can absorb the kinetic energy of cascade atoms, prevent the cascade from extending and reduce the defect production significantly when the cascades overlap with the nanocluster. When the cascade affects seriously the nanocluster, the weak sub-cascade collisions are rebounded by the nanocluster and thus leave more interstitials in the matrix. On the contrary, when the cascade contacts weakly the nanocluster, the interface can pin the arrived interstitials and this leaves more vacancies in the matrix. Moreover, the results indicate that the Y₂Ti₂O₇ nanocluster keeps stable upon the displacement cascade damage.

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First-principles study of the new potential photovoltaic absorber: Cu₂MgSnS₄ compound

Belmorsli Bekki,Kadda Amara,Mohammed El Keurti

2017 (7): 76201-076201. doi: 10.1088/1674-1056/26/7/076201

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The structural, electronic, optical, and elastic properties of Cu₂MgSnS₄ in four crystalline phases (wurtzite–stannite (WS), stannite (ST), kesterite (KS), and primitive-mixed CuAu (PMCA)) are investigated using density functional theory (DFT) in the framework of the full potential linearized augmented plane wave plus local-orbitals (FP-LAPW+lo) method within the generalized gradient approximation based on the Perdew 2008 functional (GGA-PBEsol). For each phase, the structural parameters, bulk modulus, and its pressure derivative are calculated. The relative stability of these phases is also discussed. In addition, the elastic constants have been calculated in order to investigate the mechanical stability of all phases. Moreover, the anisotropy factor, shear modulus, Young's modulus, Lame's coefficient, and Poisson's ratio have been estimated from the calculated single crystalline elastic constants. For the band structure, the density of states and optical properties of the exchange and correlation effects are treated by the Tran–Blaha modified Becke–Johnson potential to give a better description of the band-gap energies and optical spectra. The obtained results are compared with available experimental data and to other theoretical calculations.

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Effects of the Be₂₂W phase formation on hydrogen retention and blistering in mixed Be/W systems

曹金利, 赫丙龄, 肖伟, 王立根

2017 (7): 76801-076801. doi: 10.1088/1674-1056/26/7/076801

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We have performed first-principles density functional theory calculations to investigate the retention and migration of hydrogen in Be₂₂W, a stable low-W intermetallic compound. The solution energy of interstitial H in Be₂₂W is found to be 0.49 eV lower, while the diffusion barrier, on the other hand, is higher by 0.13 eV compared to those in pure hcp-Be. The higher solubility and lower diffusivity for H atoms make Be₂₂W a potential beneficial secondary phase in hcp-Be to impede the accumulation of H atoms, and hence better resist H blistering. We also find that in Be₂₂W, the attraction between an interstitial H and a beryllium vacancy ranges from 0.34 eV to 1.08 eV, which indicates a weaker trapping for hydrogen than in pure Be. Our calculated results suggest that small size Be₂₂W particles in hcp-Be might serve as the hydrogen trapping centers, hinder hydrogen bubble growth, and improve the resistance to irradiation void swelling, just as dispersed oxide particles in steel do.

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Physical implications of activation energy derived from temperature dependent photoluminescence of InGaN-based materials

杨静, 赵德刚, 江德生, 陈平, 刘宗顺, 朱建军, 李翔, 刘炜, 梁锋, 张立群, 杨辉, 王文杰, 李沫

2017 (7): 77101-077101. doi: 10.1088/1674-1056/26/7/077101

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Physical implications of the activation energy derived from temperature dependent photoluminescence (PL) of InGaN-based materials are investigated, finding that the activation energy is determined by the thermal decay processes involved. If the carrier escaping from localization states is responsible for the thermal quenching of PL intensity, as often occurs in InGaN materials, the activation energy is related to the energy barrier height of localization states. An alternative possibility for the thermal decay of the PL intensity is the activation of nonradiative recombination processes, in which case thermal activation energy would be determined by the carrier capture process of the nonradiative recombination centers rather than by the ionization energy of the defects themselves.

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Coherent charge transport in ferromagnet/semiconductor nanowire/ferromagnet double barrier junctions with the interplay of Rashba spin–orbit coupling, induced superconducting pair potential, and external magnetic field

黄立捷, 刘恋, 王瑞强, 胡梁宾

2017 (7): 77201-077201. doi: 10.1088/1674-1056/26/7/077201

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By solving the Bogoliubov–de Gennes equation, the influence of the interplay of Rashba spin–orbit coupling, induced superconducting pair potential, and external magnetic field on the spin-polarized coherent charge transport in ferromagnet/semiconductor nanowire/ferromagnet double barrier junctions is investigated based on the Blonder–Tinkham–Klapwijk theory. The coherence effect is characterized by the strong oscillations of the charge conductance as a function of the bias voltage or the thickness of the semiconductor nanowire, resulting from the quantum interference of incoming and outgoing quasiparticles in the nanowire. Such oscillations can be effectively modulated by varying the strength of the Rashba spin–orbit coupling, the thickness of the nanowire, or the strength of the external magnetic field. It is also shown that two different types of zero-bias conductance peaks may occur under some particular conditions, which have some different characteristics and may be due to different mechanisms.

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Phase diagram of a family of one-dimensional nearest-neighbor tight-binding models with an exact mobility edge

巩龙延, 赵小新

2017 (7): 77202-077202. doi: 10.1088/1674-1056/26/7/077202

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Recently, an interesting family of quasiperiodic models with exact mobility edges (MEs) has been proposed (Phys. Rev. Lett. 114 146601 (2015)). It is self-dual under a generalized duality transformation. However, such transformation is not obvious to map extended (localized) states in the real space to localized (extended) ones in the Fourier space. Therefore, it needs more convictive evidences to confirm the existence of MEs. We use the second moment of wave functions, Shannon information entropies, and Lypanunov exponents to characterize the localization properties of the eigenstates, respectively. Furthermore, we obtain the phase diagram of the model. Our numerical results support the existing analytical findings.

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Plasmonic Mach–Zehnder interferometric sensor based on a metal–insulator–metal nanostructure

沈萌, 王鸣, 杜澜, 潘庭婷

2017 (7): 77301-077301. doi: 10.1088/1674-1056/26/7/077301

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A plasmonic Mach–Zehnder interferometric sensor based on a semicircular aperture-slit nanostructure patterned on a metal–insulator–metal film is proposed and demonstrated by finite difference time domain (FDTD) simulation. Due to the interference between two different surface plasmon polariton modes in this design, the transmission spectra exhibit oscillation behaviors in a broad bandwidth, and can be readily tailored by changing the SPP path length and core layer thickness. Based on this principle, the characteristics of refractive index sensing are also demonstrated by simulation. This structure is illuminated with a collimated light source from the back side to avoid impacts on the interference. Meanwhile, these results show that the proposed structure is promising for portable, efficient, and sensitive biosensing applications.

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Abundant photoelectronic behaviors of La_0.67Sr_0.33MnO₃/Nb: SrTiO₃ junctions

黄海林, 王登京, 张洪瑞, 张慧, 熊昌民, 孙继荣, 沈保根

2017 (7): 77302-077302. doi: 10.1088/1674-1056/26/7/077302

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Temperature dependence on rectifying and photoelectronic properties of La_0.67Sr_0.33MnO₃/Nb:SrTiO₃ (LSMO/STON) junctions with the thickness values of LSMO film varying from 1 nm to 54 nm are systematically studied. As shown experimentally, the junctions exhibit good rectifying properties. The significant differences in photoemission property among the LSMO/STON junctions are observed. For the junction in a thicker LSMO film, the photocurrent shows a monotonic growth when temperature decreases from 300 K to 13 K. While for the junction in an ultrathin LSMO film, the behaviors of photocurrent are more complicated. The photocurrent increases rapidly to a maximum and then smoothly decreases with the decrease of temperature. The unusual phenomenon can be elucidated by the diffusion and recombination model of the photocarrier.

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Analytical capacitance model for 14 nm FinFET considering dual-k spacer

郑芳林, 刘程晟, 任佳琪, 石艳玲, 孙亚宾, 李小进

2017 (7): 77303-077303. doi: 10.1088/1674-1056/26/7/077303

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The conformal mapping of an electric field has been employed to develop an accurate parasitic capacitance model for nanoscale fin field-effect transistor (FinFET) device. Firstly, the structure of the dual-layer spacers and the gate parasitic capacitors are thoroughly analyzed. Then, the Cartesian coordinate is transferred into the elliptic coordinate and the equivalent fringe capacitance model can be built-up by some arithmetical operations. In order to validate our proposed model, the comparison of statistical analysis between the proposed calculation and the 3D-TCAD simulation has been carried out, and several different material combinations of the dual-k structure have been considered. The results show that the proposed analytical model can accurately calculate the fringe capacitance of the FinFET device with dual-k spacers.

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High holding voltage SCR for robust electrostatic discharge protection

齐钊, 乔明, 何逸涛, 张波

2017 (7): 77304-077304. doi: 10.1088/1674-1056/26/7/077304

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A novel silicon controlled rectifier (SCR) with high holding voltage (V_h) for electrostatic discharge (ESD) protection is proposed and investigated in this paper. The proposed SCR obtains high V_h by adding a long N+ layer (LN+) and a long P+ layer (LP+), which divide the conventional low voltage trigger silicon controlled rectifier (LVTSCR) into two SCRs (SCR1:P+/Nwell/Pwell/N+ and SCR2:P+/LN+/LP+/N+) with a shared emitter. Under the low ESD current (I_ESD), the two SCRs are turned on at the same time to induce the first snapback with high V_h (V_h1). As the I_ESD increases, the SCR2 will be turned off because of its low current gain. Therefore, the I_ESD will flow through the longer SCR1 path, bypassing SCR2, which induces the second snapback with high V_h (V_h2). The anti-latch-up ability of the proposed SCR for ESD protection is proved by a dynamic TLP-like (Transmission Line Pulse-like) simulation. An optimized V_h2 of 7.4 V with a maximum failure current (I_t2) of 14.7 mA/μ m is obtained by the simulation.

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Electronic structure of heavy fermion system CePt₂In₇ from angle-resolved photoemission spectroscopy

沈兵, 俞理, 刘凯, 吕守鹏, 贾小文, 张艳, 王晨露, 胡成, 丁颖, 孙璇, 胡勇, 刘静, 高强, 赵林, 刘国东, 许祖彦, 陈创天, 卢仲毅, 周兴江

2017 (7): 77401-077401. doi: 10.1088/1674-1056/26/7/077401

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We have carried out high-resolution angle-resolved photoemission measurements on the Ce-based heavy fermion compound CePt₂In₇ that exhibits stronger two-dimensional character than the prototypical heavy fermion system CeCoIn₅. Multiple Fermi surface sheets and a complex band structure are clearly resolved. We have also performed detailed band structure calculations on CePt₂In₇. The good agreement found between our measurements and the calculations suggests that the band renormalization effect is rather weak in CePt₂In₇. A comparison of the common features of the electronic structure of CePt₂In₇ and CeCoIn₅ indicates that CeCoIn₅ shows a much stronger band renormalization effect than CePt₂In₇. These results provide new information for understanding the heavy fermion behaviors and unconventional superconductivity in Ce-based heavy fermion systems.

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Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

沈兵, 冯中沛, 黄建伟, 胡勇, 高强, 李聪, 徐煜, 刘国东, 俞理, 赵林, 金魁, 周兴江

2017 (7): 77402-077402. doi: 10.1088/1674-1056/26/7/077402

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We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF₂ substrate by pulsed laser deposition (PLD) method. Measurements on FeSe/CaF₂ samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140–160 K, much higher than~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF₂ films by PLD method with the FeSe/SrTiO₃ films by molecular beam epitaxy (MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF₂ is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF₂ films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO₃ films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.

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Magnetic properties of Sn-substituted Ni–Zn ferrites synthesized from nano-sized powders of NiO, ZnO, Fe₂O₃, and SnO₂

M A Ali,M M Uddin,M N I Khan,F U Z Chowdhury,S M Hoque,S I Liba

2017 (7): 77501-077501. doi: 10.1088/1674-1056/26/7/077501

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A series of Ni_0.6-x/2Zn_0.4-x/2Sn_xFe₂O₄ (x=0.0, 0.05, 0.1, 0.15, 0.2, and 0.3) (NZSFO) ferrite composities have been synthesized from nano powders using a standard solid state reaction technique. The spinel cubic structure of the investigated samples has been confirmed by x-ray diffraction (XRD). The magnetic properties such as saturation magnetization (M_s), remanent magnetization (M_r), coercive field (H_c), and Bohr magneton (μ) are calculated from the hysteresis loops. The value of M_s is found to decrease with increasing Sn content in the samples. This change is successfully explained by the variation of A–B interaction strength due to Sn substitution in different sites. The compositional stability and quality of the prepared ferrite composites have also been endorsed by the fairly constant initial permeability (μ') over a wide range of frequency. The decreasing trend of μ' with increasing Sn content has been observed. Curie temperature T_C has been found to increase with the increase in Sn content. A wide spread frequency utility zone indicates that the NZSFO can be considered as a good candidate for use in broadband pulse transformers and wide band read-write heads for video recording. The composition of x=0.05 shows unusual results and the possible reason is also mentioned with the established formalism.

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Modified magnetomechancial model in the constant and low intensity magnetic field based on J–A theory

刘清友, 罗旭, 朱海燕, 刘建勋, 韩一维

2017 (7): 77502-077502. doi: 10.1088/1674-1056/26/7/077502

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The existing magnetomechancial models cannot explain the different experimental phenomena when the ferromagnetic specimen is respectively subjected to tension and compression stress in the constant and low intensity magnetic field, especially in the compression case. To promote the development of magnetomechancial theory, the energy conservation equation, effective magnetic field equation, and anhysteretic magnetization equation of the original Jiles–Atherton (J–A) theory are elucidated and modified, an equation of the local equilibrium status is employed and the differential expression of the modified magnetomechancial model based on the modified J–A theory is established finally. The effect of stress and plastic deformation on the magnetic parameters is analyzed. An excellent agreement is achieved between the theoretic predictions by the present modified model and the previous experimental results. Comparing with the calculation results given by the existing models and experimental results, it is seen indeed that the modified magnetomechanical model can describe the different magnetization features during tension-release and compression-release processes much better, and is the only one which can accurately reflect the experimental observation that the magnetic induction intensity reverses to negative value with the increase of the compressive stress and applied field.

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Electrical analysis of inter-growth structured Bi₄Ti₃O₁₂–Na_0.5Bi_4.5Ti₄O₁₅ ceramics

江向平, 江亚林, 江兴安, 陈超, 涂娜, 陈云婧

2017 (7): 77701-077701. doi: 10.1088/1674-1056/26/7/077701

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Inter-growth bismuth layer-structured ferroelectrics (BLSFs), Bi₄Ti₃O₁₂–Na_0.5Bi_4.5Ti₄O₁₅ (BIT–NBT), were successfully synthesized using the traditional solid-state reaction method. X-ray diffraction (XRD) Rietveld refinements were conducted using GSAS software. Good agreement and low residual are obtained. The XRD diffraction peaks can be well indexed into I2cm space group. The inter-growth structure was further observed in the high-resolution TEM image. Dielectric and impedance properties were measured and systematically analyzed. At the temperature range 763–923 K (below T_c), doubly ionized oxygen vacancies (OVs) are localized and the short-range hopping leads to the relaxation processes with an activation energy of 0.79–1.01 eV. Above T_c, the doubly charged OVs are delocalized and become free ones, which contribute to the long-range dc conduction. The reduction in relaxation species gives rise to a higher relaxation activation energy~1.6 eV.

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Accuracy design of ultra-low residual reflection coatingsfor laser optics

刘华松, 杨霄, 王利栓, 焦宏飞, 季一勤, 张锋, 刘丹丹, 姜承慧, 姜玉刚, 陈德应

2017 (7): 77801-077801. doi: 10.1088/1674-1056/26/7/077801

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Refractive index inhomogeneity is one of the important characteristics of optical coating material, which is one of the key factors to produce loss to the ultra-low residual reflection coatings except using the refractive index inhomogeneity to obtain gradient-index coating. In the normal structure of antireflection coatings for center wavelength at 532 nm, the physical thicknesses of layer H and layer L are 22.18 nm and 118.86 nm, respectively. The residual reflectance caused by refractive index inhomogeneity (the degree of inhomogeneous is between –0.2 and 0.2) is about 200 ppm, and the minimum reflectivity wavelength is between 528.2 nm and 535.2 nm. A new numerical method adding the refractive index inhomogeneity to the spectra calculation was proposed to design the laser antireflection coatings, which can achieve the design of antireflection coatings with ppm residual reflection by adjusting physical thickness of the couple layers. When the degree of refractive index inhomogeneity of the layer H and layer L is –0.08 and 0.05 respectively, the residual reflectance increase from zero to 0.0769% at 532 nm. According to the above accuracy numerical method, if layer H physical thickness increases by 1.30 nm and layer L decrease by 4.50 nm, residual reflectance of thin film will achieve to 2.06 ppm. When the degree of refractive index inhomogeneity of the layer H and layer L is 0.08 and –0.05 respectively, the residual reflectance increase from zero to 0.0784% at 532 nm. The residual reflectance of designed thin film can be reduced to 0.8 ppm by decreasing the layer H of 1.55 nm while increasing the layer L of 4.94 nm.

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Enhancing redshift phenomenon in time-resolved photoluminescence spectra of AlGaN epilayer

李维, 金鹏, 王维颖, 毛德丰, 潘旭, 王晓亮, 王占国

2017 (7): 77802-077802. doi: 10.1088/1674-1056/26/7/077802

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AlGaN epitaxial layer has been studied by means of temperature-dependent time-integrated photoluminescence (PL) and time-resolved photoluminescence (TRPL). An enhancing redshift phenomenon in TRPL spectra with increasing temperature was observed, and the localized excitons behaved like quasi two-dimensional excitons between 6 K and 90 K. We demonstrated that these behaviors are caused by a change in the carrier dynamics with increasing temperature due to the competition of carriers' localization and delocalization in the AlGaN alloy.

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Profile function properties & optical transition formulae

张庆礼, 孙贵花, 高进云, 孙敦陆, 罗建乔, 刘文鹏, 张德明, 施朝淑, 殷绍唐

2017 (7): 77803-077803. doi: 10.1088/1674-1056/26/7/077803

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Profile function properties with different variables are discussed, the formulae of stimulated absorption, spontaneous and stimulated emission, absorption and emission coefficients, and cross sections are deduced, and some confusing issues are clarified.

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Guided mode resonance in planar metamaterials consistingof two ring resonators with different sizes

俞禛, 陈航, 刘建军, 井绪峰, 李向军, 洪治

2017 (7): 77804-077804. doi: 10.1088/1674-1056/26/7/077804

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We proposed and experimentally investigated a two-ring-resonator composed planar hybrid metamaterial (MM), in which the spectra of guided mode resonance (GMR) and Fano resonance or EIT-like response induced by coherent interaction between MM resonance and GMR can be easily controlled by the size of the two rings in the terahertz regime. Furthermore, a four-ring-resonator composed MM for polarization-insensitive GMRs was demonstrated, where GMRs of both TE and TM modes are physically attributed to the diffraction coupling by two ±45° tilting gratings. Such kind of device has great potential in ultra-sensitive label-free sensors, filters, or slow light based devices.

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Electromechanical actuation of CNT/PVDF composite films based on a bridge configuration

谷孝刚, 夏小刚, 张楠, 肖卓建, 范庆霞, 杨丰, 肖仕奇, 陈辉亮, 周维亚, 解思深

2017 (7): 78101-078101. doi: 10.1088/1674-1056/26/7/078101

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Bridged strips consisting of carbon nanotubes and poly(vinylidene fluoride) are developed, which exhibit notable deflection in response to very low driven voltages (< 1 V), because of both the excellent conductivity of the unique carbon nanotube film and the powerful thermal expansion capability of the polymer. The actuators demonstrate periodic vibrations motivated by the alternating signals. The amplitude of displacement is dependent not only on the driven voltage but also on the applied frequency. The mechanism of actuation is confirmed to be the thermal power induced by the electrical heating. By accelerating the dissipation of heat, the vibration response at higher frequencies can be significantly enhanced. The useful locomotion shows great promise in potential applications such as miniature smart devices and micro power generators.

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Growth and characterization of AlN epilayers using pulsed metal organic chemical vapor deposition

吉泽生, 汪连山, 赵桂娟, 孟钰淋, 李方政, 李辉杰, 杨少延, 王占国

2017 (7): 78102-078102. doi: 10.1088/1674-1056/26/7/078102

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We report the growth of AlN epilayers on c-plane sapphire substrates by pulsed metal organic chemical vapor deposition (MOCVD). The sources of trimethylaluminium (TMAl) and ammonia were pulse introduced into the reactor to avoid the occurrence of the parasitic reaction. Through adjusting the duty cycle ratio of TMAl to ammonia from 0.8 to 3.0, the growth rate of AlN epilayers could be controlled in the range of 0.24 m/h to 0.93 m/h. The high-resolution x-ray diffraction (HRXRD) measurement showed that the full width at half maximum (FWHM) of the (0002) and (10-12) reflections for a sample would be 194 arcsec and 421 arcsec, respectively. The step-flow growth mode was observed in the sample with the atomic level flat surface steps, in which a root-mean-square (RMS) roughness was lower to 0.2 nm as tested by atomic force microscope (AFM). The growth process of AlN epilayers was discussed in terms of crystalline quality, surface morphology, and residual stress.

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Low-temperature-cured highly conductive composite of Ag nanowires & polyvinyl alcohol

何松, 张祥, 杨兵初, 徐晓梅, 陈辉, 周聪华

2017 (7): 78103-078103. doi: 10.1088/1674-1056/26/7/078103

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Flexible conductive films were fabricated from a low-temperature-cured, highly conductive composite of silver nanowires (as conducting filler) and polyvinyl alcohol (PVA, as binder). Sheet resistance of 0.12 Ω/sq, conductivity of 2.63×10⁴ S/cm, and contact resistance of 1.0 Ω/cm² were measured in the films, along with excellent resistance to scratching and good flexibility, making them suitable electrical contact materials for flexible optoelectronic devices. Effects of curing temperature, curing duration, film thickness, and nanowire length on the film's electrical properties were studied. Due to the abundance of hydroxyl groups on its molecular chains, the addition of PVA improves the film's flexibility and resistance to scratching. Increased nanowire density and nanowire length benefit film conductance. Monte Carlo simulation was used to further explore the impact of these two parameters on the conductivity. It was observed that longer nanowires produce a higher length-ratio of conducting routes in the networks, giving better film conductivity.

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Conductivity and applications of Li-biphenyl-1, 2-dimethoxyethane solution for lithium ion batteries

褚赓, 刘柏男, 罗飞, 李文俊, 陆浩, 陈立泉, 李泓

2017 (7): 78201-078201. doi: 10.1088/1674-1056/26/7/078201

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The total conductivity of Li-biphenyl-1,2-dimethoxyethane solution (Li_xBp(DME)_9.65, Bp=biphenyl, DME=1,2-dimethoxyethane, x=0.25, 0.50, 1.00, 1.50, 2.00) is measured by impedance spectroscopy at a temperature range from 0℃ to 40℃. The Li_1.50Bp(DME)_9.65 has the highest total conductivity 10.7 mS/cm. The conductivity obeys Arrhenius law with the activation energy (E_a(x=0.50)=0.014 eV, E_a(x=1.00)=0.046 eV). The ionic conductivity and electronic conductivity of Li_xBp(DME)_9.65 solutions are investigated at 20℃ using the isothermal transient ionic current (ITIC) technique with an ion-blocking stainless steal electrode. The ionic conductivity and electronic conductivity of Li_1.00Bp(DME)_9.65 are measured as 4.5 mS/cm and 6.6 mS/cm, respectively. The Li_1.00Bp(DME)_9.65 solution is tested as an anode material of half liquid lithium ion battery due to the coexistence of electronic conductivity and ionic conductivity. The lithium iron phosphate (LFP) and Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) are chosen to be the counter electrode and electrolyte, respectively. The assembled cell is cycled in the voltage range of 2.2 V–3.75 V at a current density of 50 mA/g. The potential of Li_1.00Bp(DME)_9.65 solution is about 0.3 V vs. Li⁺/Li, which indicates the solution has a strong reducibility. The Li_1.00Bp(DME)_9.65 solution is also used to prelithiate the anode material with low first efficiency, such as hard carbon, soft carbon and silicon.

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Negative-index dispersion and accidental mode degeneracy inan asymmetric spoof–insulator–spoof waveguide

田莉莉, 刘建龙, 周可雅, 高扬, 刘树田

2017 (7): 78401-078401. doi: 10.1088/1674-1056/26/7/078401

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It has been recently demonstrated that negative-index dispersion and mode degeneracy can be achieved by manipulating a spoof–insulator–spoof (SIS) waveguide. In this paper, we propose a new SIS waveguide, which is composed of two spoof surface plasmon polaritons (SSPPs) waveguides drilled with periodic rhomboidal grooves. Both the symmetric and asymmetric cases are investigated. Our simulation results show that the asymmetric SIS waveguides are more significant. By tailoring the tilt of the rhomboidal grooves, the negative-index dispersion can be achieved and the microwave band gap (MBG) can be effectively modulated. At a critical tilt, there appears an accidental mode degeneracy at the edge of the first Brillouin zone. The excitation and propagation of the two coupled modes sustained by the asymmetric SIS waveguides are also demonstrated.

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Bridge-free fabrication process for Al/AlO_x/Al Josephson junctions

张珂, 李蒙蒙, 刘强, 于海峰, 于扬

2017 (7): 78501-078501. doi: 10.1088/1674-1056/26/7/078501

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We fabricate different-sized Al/AlO_x/Al Josephson junctions by using a simple bridge-free technique, in which only single-layer E-beam resist polymethyl methacrylate (PMMA) is exposed at low accelerate voltage (below 30 kV) and the size of junction can be varied in a large range. Compared with the bridge technique, this fabrication process is very robust because it can avoid collapsing the bridge during fabrication. This makes the bridge-free technique more popular to meet different requirements for Josephson junction devices especially for superconducting quantum bits.

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Double-gate-all-around tunnel field-effect transistor

张文豪, 李尊朝, 关云鹤, 张也非

2017 (7): 78502-078502. doi: 10.1088/1674-1056/26/7/078502

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In this work, a double-gate-all-around tunneling field-effect transistor is proposed. The performance of the novel device is studied by numerical simulation. The results show that with a thinner body and an additional core gate, the novel device achieves a steeper subthreshold slope, less susceptibility to the short channel effect, higher on-state current, and larger on/off current ratio than the traditional gate-all-around tunneling field-effect transistor. The excellent performance makes the proposed structure more attractive to further dimension scaling.

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Evaluation of current and temperature effects on optical performance of InGaAlP thin-film SMD LED mounted on different substrate packages

Muna E. Raypah,Mutharasu Devarajan,Fauziah Sulaiman

2017 (7): 78503-078503. doi: 10.1088/1674-1056/26/7/078503

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The relationship between the photometric, electric, and thermal parameters of light-emitting diodes (LEDs) is important for optimizing the LED illumination design. Indium gallium aluminium phosphide (InGaAlP)-based thin-film surface-mounted device (SMD) LEDs have attracted wide attention in research and development due to their portability and miniaturization. We report the optical characterization of InGaAlP thin-film SMD LED mounted on FR4, 2 W, and 5 W aluminum (Al) packages. The optical and thermal parameters of LED are determined at different injection currents and ambient temperatures by combining the T3ster (thermal transient tester) and TeraLED (thermal and radiometric characterization of power LEDs) systems. Analysis shows that LED on a 5 W Al substrate package obtains the highest luminous and optical efficiency.

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Image reconstruction for cone-beam computed tomography using total p-variation plus Kullback–Leibler data divergence

蔡爱龙, 李磊, 王林元, 闫镔, 郑治中, 张瀚铭, 胡国恩

2017 (7): 78701-078701. doi: 10.1088/1674-1056/26/7/078701

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Accurate reconstruction from a reduced data set is highly essential for computed tomography in fast and/or low dose imaging applications. Conventional total variation (TV)-based algorithms apply the L1 norm-based penalties, which are not as efficient as Lp(0 <p <1) quasi-norm-based penalties. TV with a p-th power-based norm can serve as a feasible alternative of the conventional TV, which is referred to as total p-variation (TpV). This paper proposes a TpV-based reconstruction model and develops an efficient algorithm. The total p-variation and Kullback–Leibler (KL) data divergence, which has better noise suppression capability compared with the often-used quadratic term, are combined to build the reconstruction model. The proposed algorithm is derived by the alternating direction method (ADM) which offers a stable, efficient, and easily coded implementation. We apply the proposed method in the reconstructions from very few views of projections (7 views evenly acquired within 180°). The images reconstructed by the new method show clearer edges and higher numerical accuracy than the conventional TV method. Both the simulations and real CT data experiments indicate that the proposed method may be promising for practical applications.

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Detection of invisible phonon modes in individual defect-free carbon nanotubes by gradient-field Raman scattering

杨丰, 纪英露, 张霄, 范庆霞, 张楠, 谷孝刚, 肖卓建, 张强, 王艳春, 吴晓春, 李俊杰, 周维亚

2017 (7): 78801-078801. doi: 10.1088/1674-1056/26/7/078801

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We provide an effective method to investigate the field gradient effect in nanoconfined plasmon–matter interaction. Aligned ultralong SWNTs without defects were grown on marked substrates, followed by assembling gold nanoparticle clusters around individual nanotubes. The Raman scattering behavior of a nanotube placed in an atomic scale nanogap between adjacent nanoparticles was studied. In addition to the expected plasmon-induced Raman enhancement up to 10³, the defect-free D-mode of an individual SWNT induced by gradient field is found for the first time. When the light is confined at atomic scale, gradient field Raman scattering becomes significant and dipole-forbidden phonon modes can be activated by quadrupole Raman tensor variation, indicating breakdown of the Raman selection rules.

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Feedback arcs and node hierarchy in directed networks

赵金华, 周海军

2017 (7): 78901-078901. doi: 10.1088/1674-1056/26/7/078901

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Directed networks such as gene regulation networks and neural networks are connected by arcs (directed links). The nodes in a directed network are often strongly interwound by a huge number of directed cycles, which leads to complex information-processing dynamics in the network and makes it highly challenging to infer the intrinsic direction of information flow. In this theoretical paper, based on the principle of minimum-feedback, we explore the node hierarchy of directed networks and distinguish feedforward and feedback arcs. Nearly optimal node hierarchy solutions, which minimize the number of feedback arcs from lower-level nodes to higher-level nodes, are constructed by belief-propagation and simulated-annealing methods. For real-world networks, we quantify the extent of feedback scarcity by comparison with the ensemble of direction-randomized networks and identify the most important feedback arcs. Our methods are also useful for visualizing directed networks.

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Density functional theory investigation of carbon monoxide adsorption on the kaolinite (001) surface

赵健, 何满潮, 胡祥星, 高炜

2017 (7): 79101-079101. doi: 10.1088/1674-1056/26/7/079101

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Carbon monoxide (CO) is a gaseous pollutant with adverse effects on human health and the environment. Kaolinite is a natural mineral resource that can be used for different applications, including that it can also be used for retention of pollutant gases. The adsorption behavior of carbon monoxide molecules on the (001) surface of kaolinite was studied systematically by using density-functional theory and supercell models for a range coverage from 0.11 to 1.0 monolayers (ML). The CO adsorbed on the three-fold hollow, two-fold bridge, and one-fold top sites of the kaolinite(001) was tilted with respect to the surface. The strongest adsorbed site of carbon monoxide on the kaolinite (001) surface is the hollow site followed by the bridge and top site. The adsorption energy of CO decreased when increasing the coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighboring CO molecules. In addition to the adsorption structures and energetics, the lattice relaxation, the electronic density of states, and the different charge distribution have been investigated for different surface coverages.

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