中国物理B

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Ranking important nodes in complex networks by simulated annealing

孙昱, 姚佩阳, 万路军, 申健, 钟赟

2017 (2): 20201-020201. doi: 10.1088/1674-1056/26/2/020201

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In this paper, based on simulated annealing a new method to rank important nodes in complex networks is presented. First, the concept of an importance sequence (IS) to describe the relative importance of nodes in complex networks is defined. Then, a measure used to evaluate the reasonability of an IS is designed. By comparing an IS and the measure of its reasonability to a state of complex networks and the energy of the state, respectively, the method finds the ground state of complex networks by simulated annealing. In other words, the method can construct a most reasonable IS. The results of experiments on real and artificial networks show that this ranking method not only is effective but also can be applied to different kinds of complex networks.

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Enhanced electron-positron pair production by frequency chirping in one- and two-color laser pulse fields

Nuriman Abdukerim, 李子良, 谢柏松

2017 (2): 20301-020301. doi: 10.1088/1674-1056/26/2/020301

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Enhanced electron-positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum along the momentum axis. The positive and negative frequency chirp parameters play the same role in increasing the pair number density. The sign change of the frequency chirp parameter at the moment t=0 leads the pulse shape and momentum spectrum to be symmetric, and the number density to be increased. The number density of produced pairs in the two-color pulse field is much higher than that in the one-color pulse field and the larger frequency chirp pulse field dominates more strongly. In the two-color pulse fields, the relation between the frequency ratio of two colors and the number density is not sensitive to the parameters of small frequency chirp added in either a low frequency strong field or a high frequency weak field but sensitive to the parameters of large frequency chirp added in a high frequency weak field.

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Optimal multi-photon entanglement concentration with the photonic Faraday rotation

周澜, 王丹丹, 王兴福, 顾世浦, 盛宇波

2017 (2): 20302-020302. doi: 10.1088/1674-1056/26/2/020302

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We put forward an optimal entanglement concentration protocol (ECP) for recovering an arbitrary less-entangled multi-photon Greenberger-Horne-Zeilinger (GHZ) state into the maximally entangled GHZ state based on the photonic Faraday rotation in low-quality (Q) cavity. In the ECP, only one pair of less-entangled multi-photon GHZ state and one auxiliary photon are required, and the concentration task can be realized by local operations. Moreover, our ECP can be used repeatedly to further concentrate the discarded items of conventional ECPs, which can increase its success probability largely. Under the practical imperfect detection condition, our protocol can still work with relatively high success probability. This ECP has application potential in current and future quantum communication.

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Round-robin differential quadrature phase-shift quantum key distribution

周淳, 张莹莹, 鲍皖苏, 李宏伟, 汪洋, 江木生

2017 (2): 20303-020303. doi: 10.1088/1674-1056/26/2/020303

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Recently, a round-robin differential phase-shift (RRDPS) protocol was proposed [Nature 509, 475 (2014)], in which the amount of leakage is bounded without monitoring the signal disturbance. Introducing states of the phase-encoded Bennett-Brassard 1984 protocol (PE-BB84) to the RRDPS, this paper presents another quantum key distribution protocol called round-robin differential quadrature phase-shift (RRDQPS) quantum key distribution. Regarding a train of many pulses as a single packet, the sender modulates the phase of each pulse by one of 0,π/2,π,3π/2, then the receiver measures each packet with a Mach-Zehnder interferometer having a phase basis of 0 or π/2. The RRDQPS protocol can be implemented with essential similar hardware to the PE-BB84, so it has great compatibility with the current quantum system. Here we analyze the security of the RRDQPS protocol against the intercept-resend attack and the beam-splitting attack. Results show that the proposed protocol inherits the advantages arising from the simplicity of the RRDPS protocol and is more robust against these attacks than the original protocol.

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Probabilistic direct counterfactual quantum communication

张盛

2017 (2): 20304-020304. doi: 10.1088/1674-1056/26/2/020304

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It is striking that the quantum Zeno effect can be used to launch a direct counterfactual communication between two spatially separated parties, Alice and Bob. So far, existing protocols of this type only provide a deterministic counterfactual communication service. However, this counterfactuality should be payed at a price. Firstly, the transmission time is much longer than a classical transmission costs. Secondly, the chained-cycle structure makes them more sensitive to channel noises. Here, we extend the idea of counterfactual communication, and present a probabilistic-counterfactual quantum communication protocol, which is proved to have advantages over the deterministic ones. Moreover, the presented protocol could evolve to a deterministic one solely by adjusting the parameters of the beam splitters.

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Creating nitrogen–vacancy ensembles in diamond for coupling with flux qubit

郑亚锐, 邢健, 常彦春, 闫智广, 邓辉, 吴玉林, 吕力, 潘新宇, 朱晓波, 郑东宁

2017 (2): 20305-020305. doi: 10.1088/1674-1056/26/2/020305

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Hybrid quantum system of negatively charged nitrogen-vacancy (NV^-) centers in diamond and superconducting qubits provide the possibility to extend the performances of both systems. In this work, we numerically simulate the coupling strength between NV^- ensembles and superconducting flux qubits and obtain a lower bound of 10¹⁶ cm^-3 for NV^- concentration to achieve a sufficiently strong coupling of 10 MHz when the gap between NV-ensemble and flux qubit is 0. Moreover, we create NV^- ensembles in different types of diamonds by ₁₄N⁺ and ₁₂C⁺ ion implantation, electron irradiation, and high temperature annealing. We obtain an NV^- concentration of 1.05×10¹⁶ cm^-3 in the diamond with 1-ppm nitrogen impurity, which is expected to have a long coherence time for the low nitrogen impurity concentration. This shows a step toward performance improvement of flux qubit-NV^- hybrid system.

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Pattern dynamics of network-organized system with cross-diffusion

郑前前, 王直杰, 申建伟

2017 (2): 20501-020501. doi: 10.1088/1674-1056/26/2/020501

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Cross-diffusion is a ubiquitous phenomenon in complex networks, but it is often neglected in the study of reaction-diffusion networks. In fact, network connections are often random. In this paper, we investigate pattern dynamics of random networks with cross-diffusion by using the method of network analysis and obtain a condition under which the network loses stability and Turing bifurcation occurs. In addition, we also derive the amplitude equation for the network and prove the stability of the amplitude equation which is also an effective tool to investigate pattern dynamics of the random network with cross diffusion. In the meantime, the pattern formation consistently matches the stability of the system and the amplitude equation is verified by simulations. A novel approach to the investigation of specific real systems was presented in this paper. Finally, the example and simulation used in this paper validate our theoretical results.

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Magnetic phase diagrams of Fe-Mn-Al alloy on the Bethe lattice

Erhan Albayrak

2017 (2): 20502-020502. doi: 10.1088/1674-1056/26/2/020502

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The magnetic behaviors of the Fe-Mn-Al alloy are simulated on the Bethe lattice by using a trimodal random bilinear exchange interaction (J) distribution in the Blume-Capel (BC) model. Ferromagnetic (J>0) or antiferromagnetic (J<0) bonds or dilution of the bonds (J=0) are assumed between the atoms with some probabilities. It is found that the second- or the first-order phase boundaries separate the ferromagnetic (F), antiferromagnetic (AF), paramagnetic (P), or spin-glass (SG) phases from the possible other one. In addition to the tricritical points, the special points at which the second- and the first-order and the spin-glass phase lines meet are also found. Very rich phase diagrams in agreement with the literature are obtained.

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Spurious symmetry-broken phase in a bidirectional two-lane ASEP with narrow entrances

田波, 姜锐, 胡茂彬, 贾斌

2017 (2): 20503-020503. doi: 10.1088/1674-1056/26/2/020503

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As one of the paradigmatic models of non-equilibrium systems, the asymmetric simple exclusion process (ASEP) has been widely used to study many physical, chemical, and biological systems. The ASEP shows a range of nontrivial macroscopic phenomena, among which, the spontaneous symmetry breaking has gained a great deal of attention. Nevertheless, as a basic problem, it has been controversial whether there exist one or two symmetry-broken phases in the ASEP. Based on the mean field analysis and current minimization principle, this paper demonstrates that one of the broken-symmetry phases does not exist in a bidirectional two-lane ASEP with narrow entrances. Moreover, an exponential decay feature is observed, which has been used to predict the phase boundary in the thermodynamic limit. Our findings might be generalized to other ASEP models and thus deepen the understanding of the spontaneous symmetry breaking in non-equilibrium systems.

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An image encryption scheme based on three-dimensional Brownian motion and chaotic system

柴秀丽, 甘志华, 袁科, 路杨, 陈怡然

2017 (2): 20504-020504. doi: 10.1088/1674-1056/26/2/020504

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At present, many chaos-based image encryption algorithms have proved to be unsafe, few encryption schemes permute the plain images as three-dimensional (3D) bit matrices, and thus bits cannot move to any position, the movement range of bits are limited, and based on them, in this paper we present a novel image encryption algorithm based on 3D Brownian motion and chaotic systems. The architecture of confusion and diffusion is adopted. Firstly, the plain image is converted into a 3D bit matrix and split into sub blocks. Secondly, block confusion based on 3D Brownian motion (BCB3DBM) is proposed to permute the position of the bits within the sub blocks, and the direction of particle movement is generated by logistic-tent system (LTS). Furthermore, block confusion based on position sequence group (BCBPSG) is introduced, a four-order memristive chaotic system is utilized to give random chaotic sequences, and the chaotic sequences are sorted and a position sequence group is chosen based on the plain image, then the sub blocks are confused. The proposed confusion strategy can change the positions of the bits and modify their weights, and effectively improve the statistical performance of the algorithm. Finally, a pixel level confusion is employed to enhance the encryption effect. The initial values and parameters of chaotic systems are produced by the SHA 256 hash function of the plain image. Simulation results and security analyses illustrate that our algorithm has excellent encryption performance in terms of security and speed.

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Room temperature NO₂-sensing properties of hexagonal tungsten oxide nanorods

武雅乔, 胡明, 田玉明

2017 (2): 20701-020701. doi: 10.1088/1674-1056/26/2/020701

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Hexagonal WO₃ nanorods were synthesized through a facile hydrothermal method. The nanorods properties were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The NO₂-sensing performances in terms of sensor response, response/recovery times and repeatability at room temperature were optimized by varying the heat treatment temperature of WO₃ nanorods. The optimized NO₂ sensor (400-℃-annealed WO₃ nanorods) showed an ultra-high sensor response of 3.2 and short response time of 1 s to 5-ppm NO₂. In addition, the 400-℃-annealed sample exhibited more stable repeatability. Furthermore, dynamic responses measurements of annealed samples showed that all the annealed WO₃ nanorods sensors presented p-type behaviors. We suppose the p-type behavior of the WO₃ nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor is exposed to NO₂ at room temperature.Therefore, the 400-℃-annealed WO₃ nanorods sensor is one of the most energy conservation candidates to detect NO₂ at room temperature.

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Effect of P impurity on NiAlΣ5 grain boundary from first-principles study

胡雪兰, 赵若汐, 罗阳, 宋庆功

2017 (2): 23101-023101. doi: 10.1088/1674-1056/26/2/023101

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First-principles calculations based on the density functional theory (DFT) and ultra-soft pseudopotential are employed to study the atomic configuration and charge density of impurity P in NiAl Σ5 grain boundary (GB). The negative segregation energy of a P atom proves that a P atom can easily segregate in the NiAl GB. The atomic configuration and formation energy of the P atom in the NiAl GB demonstrate that the P atom tends to occupy an interstitial site or substitute a Al atom depending on the Ni/Al atoms ratio. The P atom is preferable to staying in the Ni-rich environment in the NiAl GB forming P-Ni bonds. Both of the charge density and the deformation charge imply that a P atom is more likely to bond with Ni atoms rather than with Al atoms. The density of states further exhibits the interactions between P atom and Ni atom, and the orbital electrons of P, Ni and Al atoms all contribute to P-Ni bonds in the NiAl GB. It is worth noting that the P-Ni covalent bonds might embrittle the NiAl GB and weakens the plasticity of the NiAl intermetallics.

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Dirac R-matrix calculations of photoionization cross sections of Ni XII and atomic structure data of Ni XIII

R T Nazir, M A Bari, M Bilal, S Sardar, M H Nasim, M Salahuddin

2017 (2): 23102-023102. doi: 10.1088/1674-1056/26/2/023102

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We performed R-matrix calculations for photoionization cross sections of the two ground state configuration 3s²3p⁵ (²P^o_3/2,1/2) levels and 12 excited states of Ni XII using relativistic Dirac Atomic R-matrix Codes (DARC) across the photon energy range between the ionizations thresholds of the corresponding states and well above the thresholds of the last level of the Ni XIII target ion. Generally, a good agreement is obtained between our results and the earlier theoretical photoionization cross sections. Moreover, we have used two independent fully relativistic GRASP and FAC codes to calculate fine-structure energy levels, wavelengths, oscillator strengths, transitions rates among the lowest 48 levels belonging to the configuration (3s²3p⁴, 3s3p⁵, 3p⁶, 3s²3p³3d) in Ni XIII. Additionally, radiative lifetimes of all the excited states of Ni XIII are presented. Our results of the atomic structure of Ni XIII show good agreement with other theoretical and experimental results available in the literature. A good agreement is found between our calculated lifetimes and the experimental ones. Our present results are useful for plasma diagnostic of fusion and astrophysical plasmas.

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The inelastic electron tunneling spectroscopy of edge-modified graphene nanoribbon-based molecular devices

丁宗玲, 孙兆奇, 孙进, 李广, 孟凡明, 吴明在, 马永青, 程龙玖, 陈效双

2017 (2): 23103-023103. doi: 10.1088/1674-1056/26/2/023103

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The inelastic electron tunneling spectroscopy (IETS) of four edge-modified finite-size grapheme nanoribbon (GNR)-based molecular devices has been studied by using the density functional theory and Green's function method. The effects of atomic structures and connection types on inelastic transport properties of the junctions have been studied. The IETS is sensitive to the electrode connection types and modification types. Comparing with the pure hydrogen edge passivation systems, we conclude that the IETS for the lower energy region increases obviously when using donor-acceptor functional groups as the edge modification types of the central scattering area. When using donor-acceptor as the electrode connection groups, the intensity of IETS increases several orders of magnitude than that of the pure ones. The effects of temperature on the inelastic electron tunneling spectroscopy also have been discussed. The IETS curves show significant fine structures at lower temperatures. With the increasing of temperature, peak broadening covers many fine structures of the IETS curves. The changes of IETS in the low-frequency region are caused by the introduction of the donor-acceptor groups and the population distribution of thermal particles. The effect of Fermi distribution on the tunneling current is persistent.

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Uncertainty evaluation of the isotope shift factors for 2s2p^3,1P₁^o-2s²¹S₀ transitions in B II

刘建鹏, 李冀光, 邹宏新

2017 (2): 23104-023104. doi: 10.1088/1674-1056/26/2/023104

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Accurate isotope shift factors of the 2s2p ^3,1P₁-2s2¹S₀ transitions in B II, obtained with the multi-configuration Dirac-Hartree-Fock and the relativistic configuration interaction methods, are reported. We found a linear correlation relation between the mass shift factors and the energies for the transitions concerned, considering all-order electron correlations. This relation is important for estimating the uncertainty in the calculation of isotope shift factors. These atomic data can be used to extract the nuclear mean-square charge radii of the boron isotopes with halo structures or to resolve the high precise spectroscopy of B II in astronomical observation.

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MRCI+Q study of the low-lying electronic states of CdF including spin—orbit coupling

赵书涛, 闫冰, 李瑞, 武山, 王秋玲

2017 (2): 23105-023105. doi: 10.1088/1674-1056/26/2/023105

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CdF molecule, which plays an important role in a great variety of research fields, has long been subject to numerous researchers. Due to the unstable nature and heavy atom Cd containing in the CdF molecule, electronic states of the molecule have not been well studied. In this paper, high accurate ab initio calculations on the CdF molecule have been performed at the multi-reference configuration interaction level including Davidson correction (MRCI+Q). Adiabatic potential energy curves (PECs) of the 14 low-lying Λ-S states correlating with the two lowest dissociation limits Cd(¹S_g)+F(²P_u) and Cd(³P_u)+F(²P_u) have been constructed. For the bound Λ-S and Ω states, the dominant electronic configurations and spectroscopic constants are obtained，and the calculated spectroscopic constants of bound states are consistent with previous experimental results. The dipole moments (DMs) of ²Σ⁺ and ²Π are determined, and the spin-orbit (SO) matrix elements between each pair of X²Σ⁺, 2²Σ⁺, 1²Π, and 2²Π are obtained. The results indicate that the sudden changes of DMs and SO matrix elements arise from the variation of the electronic configurations around the avoided crossing region. Moreover, the Franck-Condon factors (FCFs), the transition dipole moments (TDMs), and radiative lifetimes of low-lying states-the ground state X²Σ⁺ are determined. Finally, the transitional properties of 2²Π-X²Σ⁺ and 2²Σ⁺-X²Σ⁺ are studied. Based on our computed spectroscopic information of CdF, the feasibility and challenge for laser cooling of CdF molecule are discussed.

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Parameter analysis for a nuclear magnetic resonance gyroscope based on ^bf133Cs-¹²⁹Xe/¹³¹Xe

张大伟, 徐正一, 周敏, 徐信业

2017 (2): 23201-023201. doi: 10.1088/1674-1056/26/2/023201

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We theoretically investigate several parameters for the nuclear magnetic resonance gyroscope based on ¹³³Cs-¹²⁹Xe/¹³¹Xe. For a cell containing a mixture of ¹³³Cs at saturated pressure, we investigate the optimal quenching gas (N₂) pressure and the corresponding pump laser intensity to achieve 30% ¹³³Cs polarization at the center of the cell when the static magnetic field B₀ is 5 μT with different ¹²⁹Xe/¹³¹Xe pressure. The effective field produced by spin-exchange polarized ¹²⁹Xe or ¹³¹Xe sensed by ¹³³Cs can also be discussed in different ¹²⁹Xe/¹³¹Xe pressure conditions. Furthermore, the relationship between the detected signal and the probe laser frequency is researched. We obtain the optimum probe laser detuning from the D2 (6²S_1/2→6²P_3/2) resonance with different ¹²⁹Xe/¹³¹Xe pressure owing to the pressure broadening.

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Equivalent electron correlations in nonsequential double ionization of noble atoms

董善思, 韩秋静, 张敬涛

2017 (2): 23202-023202. doi: 10.1088/1674-1056/26/2/023202

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Electron correlation is encoded directly in the distribution of the energetic electrons produced in a recollision-impact double ionization process, and varies with the laser field and the target atoms. In order to get equivalent electron correlation effects, one should enlarge the laser intensity cubically and the laser frequency linearly in proportion to the second ionization potentials of the target atoms. The physical mechanism behind the transform is to keep the ponderomotive parameter unchanged when the laser frequency is enlarged.

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Ionization in an intense field considering Coulomb correction

李健, 霍一宁, 唐增华, 马凤才

2017 (2): 23203-023203. doi: 10.1088/1674-1056/26/2/023203

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We derive a simple ionization rate formula for the ground state of a hydrogen atom in the velocity gauge under the conditions: ω<<1 a.u. (a.u. is short for atomic unit) and γ<<1 (ω is the laser frequency and γ is the Keldysh parameter). Comparisons are made among the different versions of the Keldysh-Faisal-Reiss (KFR) theory. The numerical study shows that with considering the quasi-classical (WKB) Coulomb correction in the final state of the ionized electron, the photoionization rate is enhanced compared with without considering the Coulomb correction, and the Reiss theory with the WKB Coulomb correction gives the correct result in the tunneling regime. Our concise formula of the ionization rate may provide an insight into the ionization mechanism for the ground state of a hydrogen atom.

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Theoretical study on non-sequential double ionization of carbon disulfide with different bond lengths in linearly polarized laser fields

宋凯莉, 于伟威, 贲帅, 徐彤彤, 张宏丹, 郭培莹, 郭静

2017 (2): 23204-023204. doi: 10.1088/1674-1056/26/2/023204

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By using a two-dimensional Monte-Carlo classical ensemble method, we investigate the double ionization (DI) process of the CS₂ molecule with different bond lengths in an 800-nm intense laser field. The double ionization probability presents a “knee” structure with equilibrium internuclear distance R=2.9245 a.u. (a.u. is short for atomic unit). As the bond length of CS increases, the DI probability is enhanced and the “knee” structure becomes less obvious. In addition, the momentum distribution of double ionized electrons is also investigated, which shows the momentum mostly distributed in the first and third quadrants with equilibrium internuclear distance R=2.9245 a.u. As the bond length of CS increases, the electron momentum becomes evenly distributed in the four quadrants. Furthermore, the energy distributions and the corresponding trajectories of the double-ionized electrons versus time are also demonstrated, which show that the bond length of CS in the CS₂ molecule plays a key role in the DI process.

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Controllable optical activity of non-spherical Ag and Co SERS substrate with different magnetic field

范春珍, 朱双美, 辛昊毅

2017 (2): 23301-023301. doi: 10.1088/1674-1056/26/2/023301

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We experimentally fabricate a non-spherical Ag and Co surface-enhanced Raman scattering (SERS) substrate, which not only retains the metallic plasmon resonant effect, but also possesses the magnetic field controllable characteristics. Raman detections are carried out with the test crystal violet (CV) and rhodamine 6G (R6G) molecules with the initiation of different magnitudes of external magnetic field. Experimental results indicate that our prepared substrate shows a higher SERS activity and magnetic controllability, where non-spherical Ag nanoparticles are driven to aggregate effectively by the magnetized Co and plenty of hot-spots are built around the metallic Ag nanoparticles, thereby leading to the enhancement of local electromagnetic field. Moreover, when the external magnetic field is increased, our prepared substrate demonstrates excellent SERS enhancement. With the 2500 Gs and 3500 Gs (1 Gs=10^-4 T) magnetic fields, SERS signal can also be obtained with the detection limit lowering down to 10^-9 M. These results indicate that our proposed magnetic field controlled substrate enables us to freely achieve the enhanced and controllable SERS effect, which can be widely used in the optical sensing, single molecule detection and bio-medical applications.

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Optical potential approach for positron scattering by metastable 2³S state of helium

吴锡刚, 程勇军, 刘芳, 周雅君

2017 (2): 23401-023401. doi: 10.1088/1674-1056/26/2/023401

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The momentum space coupled channels optical (CCO) method for positron scattering has been extended to study the scattering of positrons by metastable helium for impact energies in the range from the positronium threshold up to high energies. Both the positronium formation and ionization continuum channels are included in the calculations via a complex equivalent local potential. The positronium formation, ionization, elastic and 2³S-2³P excitation, and total scattering cross sections are all presented and compared with the available information.

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Electromagnetic coupling reduction in dual-band microstrip antenna array using ultra-compact single-negative electric metamaterials for MIMO application

付孝龙, 吴国成, 白渭雄, 王光明, 梁建刚

2017 (2): 24101-024101. doi: 10.1088/1674-1056/26/2/024101

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In this paper, an ultra-compact single negative (SNG) electric waveguided metamaterial (WG-MTM) is first investigated and used to reduce the mutual coupling in E & H planes of a dual-band microstrip antenna array. The proposed SNG electric WG-MTM unit cell is designed by etching two different symmetrical spiral lines on the ground, and has two stopbands operating at 1.86 GHz and 2.40 GHz. The circuit size is very compact, which is only λ₀/33.6×λ₀/15.1 (where λ₀ is the wavelength at 1.86 GHz in free space). Taking advantage of the dual-stopband property of the proposed SNG electric WG-MTM, a dual-band microstrip antenna array operating at 1.86 GHz and 2.40 GHz with very low mutual coupling is designed by embedding a cross shaped array of the proposed SNG electric WG-MTM. The measured and simulated results of the designed dual-band antenna array are in good agreement with each other, indicating that the mutual coupling of the fabricated dual-band antenna array realizes 9.8/11.1 dB reductions in the H plane, 8.5/7.9 dB reductions in the E plane at 1.86 GHz and 2.40 GHz, respectively. Besides, the distance of the antenna elements in the array is only 0.35λ₀ (where λ₀ is the wavelength at 1.86 GHz in free space). The proposed strategy is used for the first time to reduce the mutual coupling in E & H planes of the dual-band microstrip antenna array by using ultra-compact SNG electric WG-MTM.

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Metamaterial beam scanning leaky-wave antenna based on quarter mode substrate integrated waveguide structure

吴国成, 王光明, 付孝龙, 梁建刚, 白渭雄

2017 (2): 24102-024102. doi: 10.1088/1674-1056/26/2/024102

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In this paper, we first propose a metamaterial structure by etching the same two interdigital fingers on the upper ground of quarter mode substrate integrated waveguide (QMSIW). The simulated results show that the proposed QMSIW-based metamaterial has a continuous phase constant changing from negative to positive values within its passband. A periodic leaky-wave antenna (LWA), which consists of 11 QMSIW-based metamaterial unit cells, is designed, fabricated, and measured. The measured results show that the fabricated antenna achieves a continuous beam scanning property from backward -43° to forward +32° over an operating frequencyrange of 8.9 GHz-11.8 GHz with return loss better than 10 dB. The measured antenna gain keeps consistent with the variation of less than 2 dB over the operating frequency range with a maximum gain of 12 dB. Besides, the measured and simulated results are in good agreement with each other, indicating the significance and effectiveness of this method.

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Propagation factor of electromagnetic concentric rings Schell-model beams in non-Kolmogorov turbulence

宋真真, 刘正君, 周可雅, 孙琼阁, 刘树田

2017 (2): 24201-024201. doi: 10.1088/1674-1056/26/2/024201

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We derive an analytical expression for the propagation factor (known as M²-factor) of electromagnetic concentric rings Schell-model (EM CRSM) beams in non-Kolmogorov turbulence by utilizing the extended Huygens-Fresnel diffraction integral formula and the second-order moments of the Wigner distribution function (WDF). Our results show that the EM CRSM beam has advantage over the scalar CRSM beam for reducing the turbulence-induced degradation under suitable conditions. The EM CRSM beam with multi-rings far-fields in free space is less affected by the turbulence than the one with dark-hollow far-fields or the electromagnetic Gaussian Schell-model (EGSM) beam. The dependence of the M²-factor on the beam parameters and the turbulence are investigated in detail.

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Theoretical investigation of hierarchical sub-wavelength photonic structures fabricated using high-order waveguide-mode interference lithograph

王茹, 王向贤, 杨华, 祁云平

2017 (2): 24202-024202. doi: 10.1088/1674-1056/26/2/024202

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This paper presents the theoretical investigation of hierarchical sub-wavelength photonic structures with various periods and numbers of layers, which were fabricated using a high-order waveguide-mode interference field. A 442-nm laser was used to excite high-order waveguide modes in an asymmetric metal-cladding dielectric waveguide structure. The dispersion curve of the waveguide modes was theoretically analyzed, and the distribution of the interference field of high-order waveguide modes was numerically simulated using the finite-element method. The various dependences of the characteristics of hierarchical sub-wavelength photonic structures on the thickness and refractive index of the photoresist and the waveguide mode were investigated in detail. These hierarchical sub-wavelength photonic structures have various periods and numbers of layers and can be fabricated by a simple and low-cost method.

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Sub-Rayleigh imaging via undersampling scanning based on sparsity constraints

薛长斌, 姚旭日, 李龙珍, 刘雪峰, 俞文凯, 郭晓勇, 翟光杰, 赵清

2017 (2): 24203-024203. doi: 10.1088/1674-1056/26/2/024203

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We demonstrate that, by undersampling scanning object with a reconstruction algorithm related to compressed sensing, an image with the resolution exceeding the finest resolution defined by the numerical aperture of the system can be obtained. Experimental results show that the measurements needed to achieve sub-Rayleigh resolution enhancement can be less than 10% of the pixels of the object. This method offers a general approach applicable to point-by-point illumination super-resolution techniques.

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Probe gain via four-wave mixing based on spontaneously generated coherence

杨红, 张廷桂, 张岩

2017 (2): 24204-024204. doi: 10.1088/1674-1056/26/2/024204

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We have studied the probe gain via a double-Λ atomic system with a pair of closely lying lower levels in the presence of two probe and two coherent pump fields. The inversionless gain can be realized by using nondegenerate four-wave mixing under the condition of spontaneously generated coherence (SGC) owing to near-degenerate lower levels. Note that by using SGC, two probe fields can be amplified with more remarkable amplitudes, and the gain spectra of an extremely narrow linewidth can be obtained. Last but not least, our results show that the probe gain is quite sensitive to relative phases due to the SGC presence which allows one to modulate the gain spectra periodically by phase modulation, and can also be influenced by all laser field intensities and frequencies, and the angles between dipole elements.

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Tunable Nd, La: SrF₂ laser and passively Q-switched operation based on gold nanobipyramids saturable absorber

张峰, 张华年, 刘丹华, 刘杰, 马凤凯, 姜大朋, 逄思远, 苏良碧, 徐军

2017 (2): 24205-024205. doi: 10.1088/1674-1056/26/2/024205

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A novel Nd, La:SrF₂ disordered crystal is prepared, and its continuous-wave wavelength tuning operation is performed for the first time. Employing a surface plasmon resonance (SPR) based gold nanobipyramids (G-NBPs) saturable absorber, we obtain a compact diode-pumped passively Q-switched Nd, La:SrF₂ laser. The stable Q-switched pulse operates with the shortest pulse duration of 1.15 μs and the maximum repetition rate of 41 kHz. The corresponding single pulse energy is 2.24 μJ. The results indicate that G-NBPs could be a promising saturable absorber applied to the diode-pumped solid state lasers (DPSSLs).

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Efficient Nd: YVO₄ laser in-band pumped by wavelength-locked 913.9-nm laser diode and Q-switch operation

李斌, 雷鹏, 孙冰, 白扬博

2017 (2): 24206-024206. doi: 10.1088/1674-1056/26/2/024206

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An efficient 1064-nm Nd:YVO₄ laser in-band pumped by a wavelength-locked laser diode (LD) at 913.9 nm was demonstrated. The maximum continuous wave (CW) output power of 23.4 W at 1064 nm was realized with the incident pump power of 40 W, corresponding to a total optical-to-optical efficiency of 58.5%. This is to the best of our knowledge the highest total optical-to-optical efficiency and output power of Nd:YVO₄ laser in-band pumped by a 913.9-nm laser diode. The Q-switched operation of this laser was also investigated. Through a contrast experiment of pumping at 808 nm, the experimental results showed that an Nd:YVO₄ laser in-band pumped by a wavelength-locked LD at 913.9 nm had excellent pulse stability and beam quality for high repetition rate Q-switching operation.

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The influence of stimulated temperature-dependent emission cross section on intracavity optical parametric oscillator

S Samimi, A Keshavarz

2017 (2): 24207-024207. doi: 10.1088/1674-1056/26/2/024207

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In this paper, the influence of temperature on the intracavity optical parametric oscillator (IOPO) is investigated by using the stimulated temperature-dependent emission cross section of laser crystal. The rate equations under plane wave approximation have been used for simulation of signal output pulse. Results show that the signal output pulse width is decreased by increasing the laser crystal temperature. Also, the signal output energy is increased by the increasing of the laser crystal temperature. The simulation results for IOPO based on Nd:YAG and Nd:YVO₄, show that the signal pulse energies are increased by 3.2 and 5.6 times respectively when the laser crystal temperature increased from 15℃ to 300℃. The presented model indicates that the temperature sensitivity of Nd:YVO₄-based IOPOs is more than that of Nd:YAG-based IOPOs which is expected from a physical point of view.

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Band gaps structure and semi-Dirac point of two-dimensional function photonic crystals

张斯淇, 陆景彬, 梁禺, 马季, 李宏, 李雪, 刘晓静, 吴向尧, 孟祥东

2017 (2): 24208-024208. doi: 10.1088/1674-1056/26/2/024208

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Two-dimensional function photonic crystals, in which the dielectric constants of medium columns are the functions of space coordinates r, are proposed and studied numerically. The band gaps structures of the photonic crystals for TE and TM waves are different from the two-dimensional conventional photonic crystals. Some absolute band gaps and semi-Dirac points are found. When the medium column radius and the function form of the dielectric constant are modulated, the numbers, width, and position of band gaps are changed, and the semi-Dirac point can either occur or disappear. Therefore, the special band gaps structures and semi-Dirac points can be achieved through the modulation on the two-dimensional function photonic crystals. The results will provide a new design method of optical devices based on the two-dimensional function photonic crystals.

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Tunable optical filter using second-order micro-ring resonator

邓林, 李德钊, 刘子龙, 孟英昊, 郭小男, 田永辉

2017 (2): 24209-024209. doi: 10.1088/1674-1056/26/2/024209

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In this paper, we design and fabricate a silicon integrated optical filter consisting of two cascaded micro-ring resonators and two straight waveguides. Two micro-heaters are fabricated on the top of two micro-rings respectively, which are employed to modulate the micro-rings to perform the function of a tunable optical filter by the thermo-optic effect. The static response test indicates that the extinction ratio and 3-dB bandwidth are 29.01 dB and 0.21 nm respectively, the dynamic response test indicates that the 10%-90% rise and 90%-10% fall time of the filter are 16 μs and 12 μs, respectively, which can meet the requirements of optical communication and information processing. Finally, the power consumption of the device is also characterized, and the total power consumption is about 9.43 mW/nm, which has been improved efficiently.

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Degree of polarization based on the three-component pBRDF model for metallic materials

王凯, 朱京平, 刘宏

2017 (2): 24210-024210. doi: 10.1088/1674-1056/26/2/024210

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An expression of degree of polarization (DOP) for metallic material is presented based on the three-component polarized bidirectional reflectance distribution function (pBRDF) model with considering specular reflection, directional diffuse reflection and ideal diffuse reflection. The three-component pBRDF model with a detailed reflection assumption is validated by comparing simulations with measurements. The DOP expression presented in this paper is related to surface roughness, which makes it more reasonable in physics. Test results for two metallic samples show that the DOP based on the three-component pBRDF model accords well with the measurement and the error of existing DOP expression is significantly reduced by introducing the diffuse reflection. It indicates that our DOP expression describes the polarized reflection properties of metallic surfaces more accurately.

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Simplified modeling of frequency behavior in photonic crystal vertical cavity surface emitting laser with tunnel injection quantum dot in active region

Mehdi Riahinasab, Vahid Ahmadi, Elham Darabi

2017 (2): 24211-024211. doi: 10.1088/1674-1056/26/2/024211

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In this work, the characteristics of the photonic crystal tunneling injection quantum dot vertical cavity surface emitting lasers (PhC-TIQD-VCSEL) are studied through analyzing a modified modulation transfer function. The function is based on the rate equations describing the carrier dynamics at different energy levels of dot and injector well. Although the frequency modulation response component associated with carrier dynamics in wetting layer (WL) and at excited state (ES) levels of dots limits the total bandwidth in conventional QD-VCSEL, our study shows that it can be compensated for by electron tunneling from the injector well into the dot in TIQD structure. Carrier back tunneling time is one of the most important parameters, and by increment of that, the bias current dependence of the total bandwidth will be insignificant. It is proved that at high bias current, the limitation of the WL-ES level plays an important role in reducing the total bandwidth and results in rollovers on 3-dB bandwidth-I curves. In such a way, for smaller air hole diameter of photonic crystal, the effect of this reduction is stronger.

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Tunable wavelength filters using polymer long-period waveguide gratings based on metal-cladding directly defined technique

王继厚, 陈长鸣, 郑洋, 王希斌, 衣云骥, 孙小强, 王菲, 张大明

2017 (2): 24212-024212. doi: 10.1088/1674-1056/26/2/024212

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In this work, long-period waveguide grating-based tunable wavelength filters using organic-inorganic grafting poly(methyl methacrylate) (PMMA) materials are designed and fabricated by metal-cladding directly defined technique. The thermal stabilities and optical properties of the organic-inorganic grafting PMMA core materials are analyzed. Structures and performance parameters of the waveguide gratings and self-electrode heaters are designed and simulated. The contrast of the filter is about 15 dB and the resonant wavelength can be tuned by different electric powers applied to the metal-cladding self-electrode heaters. The temperature sensitivity is 3.5 nm/℃ and the switching time is about 1 ms. The technique is very suitable for realizing the optoelectronic integrated wavelength-division-multiplexing systems.

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Hot-embossing fabrication of chalcogenide glasses rib waveguide for mid-infrared molecular sensing

颜庭阳, 沈祥, 王荣平, 王国祥, 戴世勋, 徐铁峰, 聂秋华

2017 (2): 24213-024213. doi: 10.1088/1674-1056/26/2/024213

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Chalcogenide glasses have shown promise in fabricating mid infrared (MIR) photonic sensing devices due to their excellent optical properties in MIR. In addition, the glass transition temperature of chalcogenide glasses are generally low, making them ideal to create the high-throughput patterns of micro-scale structures based on hot embossing that is alternative to the standard lithographic technology. In this paper, we outline the research progress in the chalcogenide waveguide based on the hot embossing method, and discuss the problems remaining to be solved and the possible solutions.

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Study on shock wave-induced cavitation bubbles dissolution process

许欢, 范鹏飞, 马勇, 郭霞生, 杨平, 屠娟, 章东

2017 (2): 24301-024301. doi: 10.1088/1674-1056/26/2/024301

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This study investigated dissolution processes of cavitation bubbles generated during in vivo shock wave (SW)-induced treatments. Both active cavitation detection (ACD) and the B-mode imaging technique were applied to measure the dissolution procedure of biSpheres contrast agent bubbles by in vitro experiments. Besides, the simulation of SW-induced cavitation bubbles dissolution behaviors detected by the B-mode imaging system during in vivo SW treatments, including extracorporeal shock wave lithotripsy (ESWL) and extracorporeal shock wave therapy (ESWT), were carried out based on calculating the integrated scattering cross-section of dissolving gas bubbles with employing gas bubble dissolution equations and Gaussian bubble size distribution. The results showed that (i) B-mode imaging technology is an effective tool to monitor the temporal evolution of cavitation bubbles dissolution procedures after the SW pulses ceased, which is important for evaluation and controlling the cavitation activity generated during subsequent SW treatments within a treatment period; (ii) the characteristics of the bubbles, such as the bubble size distribution and gas diffusion, can be estimated by simulating the experimental data properly.

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Ultra-broadband asymmetric acoustic transmission with single transmitted beam

贾鼎, 孙宏祥, 袁寿其, 葛勇

2017 (2): 24302-024302. doi: 10.1088/1674-1056/26/2/024302

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We report both experimentally and numerically that ultra-broadband asymmetric acoustic transmission is realized by a brass plate and a right triangle reflector immersed in water. This exotic phenomenon arises from the asymmetric excitation of the leaky asymmetric zero-order Lamb mode in the brass plate induced by the incident angle of external bulk waves. The results show that the bandwidth of the asymmetric acoustic transmission could reach 2000 kHz, and the positive transmitted wave is only a single acoustic beam. The device has the advantages of ultra-broadband, single transmitted beam, and simpler structure, which has great potential applications in ultrasonic devices.

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Numerical investigation of the interaction of the turbulent dual-jet and acoustic propagation

李一明, 李宝宽, 齐凤升, 王喜春

2017 (2): 24701-024701. doi: 10.1088/1674-1056/26/2/024701

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In order to study the interaction between two independent jets, a three-dimensional (3D) transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets. The results are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600. The large eddy simulation (LES) with dynamic Smagorinsky sub-grid scale (SGS) approach is used to simulate the turbulent jet flow structure. The acoustic field is evaluated by the Ffowcs Williams-Hawkings (FW-H) integral equation. Considering the compressibility of high-speed gas jets, the density-based explicit formulation is adopted to solve the governing equations. Meanwhile, the viscosity is approximated by using the Sutherland kinetic theory. The predicted flow characteristics as well as the acoustic properties show that they are in good agreement with the existing experimental and numerical results under the same flow conditions available in the literature. The results indicate that the merging phenomenon of the dual-jet is triggered by the deflection mechanism of the Coanda effect, which sequentially introduces additional complexity and instability of flow structure. One of the main factors affecting the dual-jet merging is the aperture ratio, which has a direct influence on the potential core and surrounding flow fluctuation. The analysis on the noise pollution reveals that the potential core plays a fundamental role in noise emission while the additional mixing noise makes less contribution than the single jet noise. The overall sound pressure level (OASPL) profiles have a directive property, suggesting an approximate 25° deflection from the streamwise direction, however, shifting toward lateral direction of about 10° to 15° in the dual-jet. The conclusion obtained in this study can provide valuable data to guide the development of manufacturing-green technology in the multi-jet applications.

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Effect of electrical discharge in water on concentration of nitrate solution

F Sohbatzadeh, H Bagheri, R Safari

2017 (2): 25101-025101. doi: 10.1088/1674-1056/26/2/025101

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In this work, the effect of electrical discharge on nitrate concentration is considered in aqueous solution. The atmospheric pressure plasma was produced by a high-voltage power supply at 27 kHz using pin-to-pin configuration. Air, argon, and argon/methane mixture were used to study the working gas effect. UV-VIS spectroscopy and ion chromatography were used to analyze the effect of the electrical discharge on nitrate concentration in deionized water. Optical emission spectroscopy (OES) was applied to diagnose active species inside and on the surface of the deionized water solution. The results of the present work showed that the atmospheric pressure electric discharge with air increases nitrate concentration while it remains constant using argon and argon/methane electrical discharges. It was also revealed that in the presence of air, the electrical discharge reduces pH, acidifying the solution and increasing solution conductivity due to production of extra nitrate ions. On the other hand, argon electrical discharge increases pH and conductivity due to production of OH^- ion in water.

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Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

H Vosoughian, Z Riazi, H Afarideh, G Sarri

2017 (2): 25201-025201. doi: 10.1088/1674-1056/26/2/025201

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The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λ_p, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier-envelope phase (CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional (2D) particle-in-cell (PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.

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Production of a large area diffuse arc plasma with multiple cathode

王城, 崔海超, 李皖皖, 廖梦然, 夏维珞, 夏维东

2017 (2): 25202-025202. doi: 10.1088/1674-1056/26/2/025202

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An arc channel at atmospheric pressure tends to shrink generally. In this paper, a non-transferred DC arc plasma device with multiple cathode is introduced to produce a large area arc plasma at atmospheric pressure. This device is comprised of a 42-mm diameter tubular chamber, multiple cathode which is radially inserted into the chamber, and a tungsten anode with a nozzle in its center. In argon/helium atmosphere, a large area and circumferential homogenous diffuse arc plasma, which fills the entire cross section surrounded by the cathode tips, is observed. Results show that the uniformity and stability of diffuse arc plasma are strongly related to the plasma forming gas. Based on these experimental results, an explanation to the arc diffusion is suggested. Moreover, the electron excitation temperature and electron density measured in diffuse helium plasma are much lower than those of constricted arc column, which indicates the diffuse helium plasma probably deviates from the local thermodynamic equilibrium state. Unlike the common non-transferred arc plasma devices, this device can provide a condition for axial-fed feedstock particles. The plasma device is attempted to spheroidize alumina powders by using the central axis to send the powder. Results show that the powder produced is usually a typical hollow sphere.

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Lower order three-dimensional Burgers equation having non-Maxwellian ions in dusty plasmas

Apul N Dev

2017 (2): 25203-025203. doi: 10.1088/1674-1056/26/2/025203

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The dust acoustic (DA) shock wave with dust charge fluctuations, non-Maxwellian ions, and non-isothermal electrons is studied theoretically. The perturbation technique is employed to derive the lower order three-dimensional (3D) Burgers equation, and shock wave solution is explored by the tan-hyperbolic method. The effects of flat trapped and trapped electron distributions in the presence of Maxwellian and non-Maxwellian ions on characteristics shock waves are observed. The temperature ratio of non-Maxwellian ion temperature and non-isothermal electron temperature is found to play an important role in forming the shock-like structure.

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Detailed calibration of the PI-LCX: 1300 high performance single photon counting hard x-ray CCD camera

洪伟, 温贤伦, 魏来, 朱斌, 吴玉迟, 董克攻, 焦春晔, 伍波, 何颖玲, 张发强, 周维民, 谷渝秋

2017 (2): 25204-025204. doi: 10.1088/1674-1056/26/2/025204

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X-ray charge-coupled-device (CCD) camera working in single photon counting mode is a type of x-ray spectrometer with high-sensitivity and superior signal-to-noise performance. In this study, two single photon counting CCD cameras with the same mode (model: PI-LCX: 1300) are calibrated with quasi-monochromatic x-rays from radioactive sources and a conventional x-ray tube. The details of the CCD response to x-rays are analyzed by using a computer program of multi-pixel analyzing and event-distinguishing capability. The detection efficiency, energy resolution, fraction of multi-pixel events each as a function of x-ray energy, and consistence of two CCD cameras are obtained. The calibrated detection efficiency is consistent with the detection efficiency from Monte Carlo calculations with XOP program. When the multi-pixel event analysis is applied, the CCDs may be used to measure x-rays up to 60 keV with good energy resolution (E/ΔE≈100 at 60 keV). The difference in detection efficiency between two CCD cameras is small (5.6% at 5.89 keV), but the difference in fraction of the single pixel event between them is much larger (25% at 8.04 keV). The obtained small relative error of detection efficiency (2.4% at 5.89 keV) makes the high accurate measurement of x-ray yield possible in the laser plasma interaction studies. Based on the discrete calibration results, the calculated detection efficiency with XOP can be used for the whole range of 5 keV-30 keV.

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High sampling-rate measurement of turbulence velocity fluctuations in Mach 1.8 Laval jet using interferometric Rayleigh scattering

陈力, 杨富荣, 苏铁, 鲍伟义, 闫博, 陈爽, 李仁兵

2017 (2): 25205-025205. doi: 10.1088/1674-1056/26/2/025205

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Interferometric Rayleigh scattering diagnostic technique for the time-resolved measurement of flow velocity is studied. Theoretically, this systematic velocity-measured accuracy can reach up to 1.23 m/s. Measurement accuracy is then evaluated by comparing with hot wire anemometry results. Moreover, the distributions of velocity and turbulence intensity in a supersonic free jet from a Laval nozzle with a Mach number of 1.8 are also obtained quantitatively. The sampling rate in this measurement is determined to be approximately 10 kHz.

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New progress on beam availability and reliability of PKU high intensity CW proton ECR ion source

彭士香, 张艾霖, 任海涛, 徐源, 张滔, 张景丰, 温佳美, 郭之虞, 陈佳洱

2017 (2): 25206-025206. doi: 10.1088/1674-1056/26/2/025206

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The stability and reliability of an ion source and its beam availability are extremely significant for any accelerator, especially for those high current long term CW operation ones like ADS. Although the first high quality 306-hours continuous wave (CW) operating curve at 50 mA@35 keV has been successfully obtained with a standard compact 2.45 GHz ECR ion source at Peking University (PKU), but the uncertainties that caused beam trips before are unacceptable during an accelerator real operation and should be eliminated. Meanwhile, no permission will be given when the beam power is upgraded from 50 mA@35 keV to 50 mA@50 keV. To improve the PKU CW proton source quality, several upgrades were done recently. After those improvements, a new long term CW proton beam experiment at 50 mA@50 keV was carried out in June 2016. The total running time is 300.5 hours, including near 6 hours ion source preparation and 294 hours non-disturb continuous operation. Within the continuous 13 days operation, no beam-off happened, no spark was observed, no beam drop appeared, no interrupting action was needed, and only a few beam fluctuations caused by the air conditional failure occurred. Beam availability and reliability within the 294 hours is 100%. The root-mean-square (RMS) emittance of this 50 mA@50 keV CW proton beam is about 0.186 π.mm.mrad. A careful inspection of the ion source was done after this long term operation and no obvious damage was found. The restart experimental results obtained after the ion source inspection prove the high repeatability of PKU PMECRIS. In addition, a 130-mA H⁺ beam was obtained at 50 kV with duty factor of 10% (100 Hz/1 ms) with this source. Details will be presented in this paper.

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Microwave absorption properties of Ag naowires/carbon black composites

黄海龙, 夏辉, 郭智博, 陈羽, 李宏建

2017 (2): 25207-025207. doi: 10.1088/1674-1056/26/2/025207

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The composite that can absorb the high-performance electromagnetic (EM) wave is constructed into a sandwiched structure composed of carbon black (CB)/ethylene-vinyl acetate (EVA) and Ag naowires (AgNWs). The AgNWs sandwiched between two CB/EVA layers are used to improve the absorption properties of composite. The effects of EVA-to-CB weight ratio, concentration and diameter of AgNWs with a thickness of 0.4 mm on microwave absorption are investigated. The results indicate that for an EVA-to-CB weight ratio of 1:3, AgNW concentration of 1.0 mg/100 mL, and average diameter of 56 nm, the reflection loss (RL) of the composite is below -10 dB in a frequency range of 9.3 Ghz-18.0 GHz, with the minimum values of -40.0 dB and -25.6 dB at 13.5 GHz and 15.3 GHz, respectively. A finite element method (FEM) is used for calculating the RL of the composite. The calculated results are in agreement with the experimental data.

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Local microstructural analysis for Y₂O₃/Eu³⁺/Mg²⁺ nanorods by Raman and photoluminescence spectra under high pressure

王金华, 李泽朋, 刘波, 刘冰冰

2017 (2): 26101-026101. doi: 10.1088/1674-1056/26/2/026101

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In this paper, we investigate the Raman and photoluminescence spectra of Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods under high pressures using 514-nm and 532-nm laser light excitation. We observe transitions from the initial cubic phase to amorphous at pressures higher than 24 GPa for both Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods. In addition, Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods exhibit different distorted states after the pressure has been raised to 8 GPa. The analyses of intensity ratios, I_0-2/I_0-1 from ⁵D₀-⁷F₂ to ⁵D₀-⁷F₁ and I_0-2A/B of ⁵D₀-⁷F₂ transitions indicate that Y₂O₃/Eu³⁺/Mg²⁺ nanorods exhibit stronger local micro-surrounding characteristics for Eu³⁺ ions in a pressure-modulated crystal field. The doped Mg²⁺ ion results in reducing the crystal ionicity in the distorted lattice state under high pressures. The use of doped ions as an ion modifier can be applied to the study of small local microstructural changes through Eu³⁺ luminescence.

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Irradiation-induced void evolution in iron: A phase-field approach with atomistic derived parameters

王园园, 丁建华, 柳文波, 黄绍松, 柯小琴, 王云志, 张弛, 赵纪军

2017 (2): 26102-026102. doi: 10.1088/1674-1056/26/2/026102

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A series of material parameters are derived from atomistic simulations and implemented into a phase field (PF) model to simulate void evolution in body-centered cubic (bcc) iron subjected to different irradiation doses at different temperatures. The simulation results show good agreement with experimental observations–the porosity as a function of temperature varies in a bell-shaped manner and the void density monotonically decreases with increasing temperatures; both porosity and void density increase with increasing irradiation dose at the same temperature. Analysis reveals that the evolution of void number and size is determined by the interplay among the production, diffusion and recombination of vacancy and interstitial.

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Anomalous low-temperature heat capacity in antiperovskite compounds

郭新格, 林建超, 童鹏, 蔺帅, 杨骋, 鲁文建, 宋文海, 孙玉平

2017 (2): 26501-026501. doi: 10.1088/1674-1056/26/2/026501

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The low-temperature heat capacities are studied for antiperovskite compounds AXM₃ (A=Al, Ga, Cu, Ag, Sn, X=C, N, M=Mn, Fe, Co). A large peak in (C-γT)/T³ versus T is observed for each of a total of 18 compounds investigated, indicating an existence of low-energy phonon mode unexpected by Debye T³ law. Such a peak is insensitive to the external magnetic field up to 80 kOe (1 Oe=79.5775 A·m^-1). For compounds with smaller lattice constant, the peak shifts towards higher temperatures with a reduction of peak height. This abnormal peak in (C-γT)/T³ versus T of antiperovskite compound may result from the strongly dispersive acoustic branch due to the heavier A atoms and the optical-like mode from the dynamic rotation of XM₆ octahedron. Such a low-energy phonon mode may not contribute negatively to the normal thermal expansion in AXM₃ compounds, while it is usually concomitant with negative thermal expansion in open-structure material (e.g., ZrW₂O₈, ScF₃).

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Orbital electronic heat capacity of hydrogenated monolayer and bilayer graphene

Mohsen Yarmohammadi

2017 (2): 26502-026502. doi: 10.1088/1674-1056/26/2/026502

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The tight-binding Harrison model and Green's function approach have been utilized in order to investigate the contribution of hybridized orbitals in the electronic density of states (DOS) and electronic heat capacity (EHC) for four hydrogenated structures, including monolayer chair-like, table-like, bilayer AA- and finally AB-stacked graphene. After hydrogenation, monolayer graphene and bilayer graphene are behave as semiconducting systems owning a wide direct band gap and this means that all orbitals have several states around the Fermi level. The energy gap in DOS and Schottky anomaly in EHC curves of these structures are compared together illustrating the maximum and minimum band gaps are appear for monolayer chair-like and bilayer AA-stacked graphane, respectively. In spite of these, our findings show that the maximum and minimum values of Schottky anomaly appear for hydrogenated bilayer AA-stacked and monolayer table-like configurations, respectively.

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Novel high-K with low specific on-resistance high voltage lateral double-diffused MOSFET

吴丽娟, 章中杰, 宋月, 杨航, 胡利民, 袁娜

2017 (2): 27101-027101. doi: 10.1088/1674-1056/26/2/027101

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A novel voltage-withstand substrate with high-K (HK, k>3.9, k is the relative permittivity) dielectric and low specific on-resistance (R_on,sp) bulk-silicon, high-voltage LDMOS (HKLR LDMOS) is proposed in this paper. The high-K dielectric and highly doped interface N⁺-layer are made in bulk silicon to reduce the surface field drift region. The high-K dielectric can fully assist in depleting the drift region to increase the drift doping concentration (N_d) and reshape the electric field distribution. The highly doped N⁺-layer under the high-K dielectric acts as a low resistance path to reduce the R_on,sp. The new device with the high breakdown voltage (BV), the low R_on,sp, and the excellent figure of merit (FOM=BV²/R_on,sp) is obtained. The BV of HKLR LDMOS is 534 V, R_on,sp is 70.6 mΩ·cm², and FOM is 4.039 MW·cm^-2.

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Structural, electronic, optical, and magnetic properties of Co-doped Cu₂O

I Djabri, T Rezkallah, F Chemam

2017 (2): 27102-027102. doi: 10.1088/1674-1056/26/2/027102

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We investigate the magnetic properties of Co-doped Cu₂O. We studied first the electronic and structural properties of Cu₂O using the optimization of the lattice constant which is 4.18 Å. The calculated gap is found between 0.825 eV and 1.5 eV, these values are in good agreement with the experimental results. The Co atoms are inserted in Cu₂O by means of the density functional theory (DFT) using LSDA, LSDA+U, and LSDA+MBJ approximations in the WIEN2k code, based on the supercell model by setting up 12, 24, and 48 atoms in (1×1×2), (1×2×2), and (2×2×2) supercells respectively with one or two copper atoms being replaced by cobalt atoms. The energy difference between the ferromagnetic and anti-ferromagnetic coupling of the spins located on the substitute Co has been calculated in order to obtain better insight into the magnetic exchange coupling for this particular compound. The studied compound exhibits stable integer magnetic moments of 2 μ_B and 4 μ_B when it is doped with 2 atoms of Co. Optical properties have also been worked out. The results obtained in this study demonstrate the importance of the magnetic effect in Cu₂O.

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Structural, electronic, and magnetic properties of vanadium atom-adsorbed MoSe₂ monolayer

刘萍, 秦真真, 乐云亮, 左旭

2017 (2): 27103-027103. doi: 10.1088/1674-1056/26/2/027103

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Using the first-principles calculations, we study the structural, electronic, and magnetic properties of vanadium adsorbed MoSe₂ monolayer, and the magnetic couplings between the V adatoms at different adsorption concentrations. The calculations show that the V atom is chemically adsorbed on the MoSe₂ monolayer and prefers the location on the top of an Mo atom surrounded by three nearest-neighbor Se atoms. The interatomic electron transfer from the V to the nearest-neighbor Se results in the polarized covalent bond with weak covalency, associated with the hybridizations of V with Se and Mo. The V adatom induces local impurity states in the middle of the band gap of pristine MoSe₂, and the peak of density of states right below the Fermi energy is associated with the V-dz² orbital. A single V adatom induces a magnetic moment of 5 μ_B that mainly distributes on the V-3d and Mo-4d orbitals. The V adatom is in high-spin state, and its local magnetic moment is associated with the mid-gap impurity states that are mainly from the V-3d orbitals. In addition, the crystal field squashes a part of the V-4s electrons into the V-3d orbitals, which enhances the local magnetic moment. The magnetic ground states at different adsorption concentrations are calculated by generalized gradient approximations (GGA) and GGA+U with enhanced electron localization. In addition, the exchange integrals between the nearest-neighbor V adatoms at different adsorption concentrations are calculated by fitting the first-principle total energies of ferromagnetic (FM) and antiferromagnetic (AFM) states to the Heisenberg model. The calculations with GGA show that there is a transition from ferromagnetic to antiferromagnetic ground state with increasing the distance between the V adatoms. We propose an exchange mechanism based on the on-site exchange on Mo and the hybridization between Mo and V, to explain the strong ferromagnetic coupling at a short distance between the V adatoms. However, the ferromagnetic exchange mechanism is sensitive to both the increased inter-adatom distance at low concentration and the enhanced electron localization by GGA+U, which leads to antiferromagnetic ground state, where the antiferromagnetic superexchange is dominant.

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Temperature and hydrogen-like impurity effects on the excited state of the strong coupling bound polaron in a CsI quantum pseudodot

肖景林

2017 (2): 27104-027104. doi: 10.1088/1674-1056/26/2/027104

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With hydrogen-like impurity (HLI) located in the center of CsI quantum pseudodot (QPD) and by using the variational method of Pekar type (VMPT), we investigate the first-excited state energy (FESE), excitation energy and transition frequency of the strongly-coupled bound polaron in the present paper. Temperature effects on bound polaron properties are calculated by employing the quantum statistical theory (QST). According to the present work's numerical results, the FESE, excitation energy and transition frequency decay (amplify) with raising temperature in the regime of lower (higher) temperature. They are decreasing functions of Coulomb impurity potential strength.

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On the reverse leakage current of Schottky contacts on free-standing GaN at high reverse biases

雷勇, 苏静, 吴红艳, 杨翠红, 饶伟锋

2017 (2): 27105-027105. doi: 10.1088/1674-1056/26/2/027105

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In this work, a dislocation-related tunneling leakage current model is developed to explain the temperature-dependent reverse current-voltage (I-V-T) characteristics of a Schottky barrier diode fabricated on free-standing GaN substrate for reverse-bias voltages up to -150 V. The model suggests that the reverse leakage current is dominated by the direct tunneling of electrons from Schottky contact metal into a continuum of states associated with conductive dislocations in GaN epilayer. A reverse leakage current ideality factor, which originates from the scattering effect at metal/GaN interface, is introduced into the model. Good agreement between the experimental data and the simulated I-V curves is obtained.

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Photon-assisted and spin-dependent shot noise in magnetic-field tunable ZnSe/Zn_1-xMn_xSe structures

李春雷, 郭永, 王小明, 律原

2017 (2): 27301-027301. doi: 10.1088/1674-1056/26/2/027301

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We have investigated the photon-assisted shot noise properties in the magnetic field tunable heterostructures.} Transport properties of the model structure are strongly dependent on the oscillatory field and the magnetic field. In this structure, electrons can absorb or emit one or multi-photons to reach the quasi-bound state. As a result, the transmission properties are affected considerably by photon-assisted tunneling and these features cause the nontrivial variations in the shot noise and Fano factor. It is found that the shot noise becomes spin-dependent and can be modulated not only by the magnetic field, but also by the oscillatory field. Both the spin-up and spin-down components of the shot noise can be greatly suppressed by the magnetic field, and can also be drastically enhanced by the harmonically driven field. Furthermore, with increasing external magnetic field, it is important to note that the enhanced intensity is decreased, even suppressed. These results suggest another method to suppress the shot noise via modulating the oscillatory field at a diluted-magnetic-semiconductors/semiconductor structure.

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Effect of metal catalyst on the mechanism of hydrogen spillover in three-dimensional covalent-organic frameworks

刘秀英, 于景新, 李晓东, 刘桂成, 李晓凤, Joong-Kee Lee

2017 (2): 27302-027302. doi: 10.1088/1674-1056/26/2/027302

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Hydrogen spillover mechanism of metal-supported covalent-organic frameworks COF-105 is investigated by means of the density functional theory, and the effects of metal catalysts M₄ (Pt₄, Pd₄, and Ni₄) on the whole spillover process are systematically analyzed. These three metal catalysts exhibit several similar phenomena: (i) they prefer to deposit on the tetra (4-dihydroxyborylphenyl) silane (TBPS) cluster with surface-contacted configuration; (ii) only the H atoms at the bridge site can migrate to 2,3,6,7,10,11-hexahydroxy triphenylene (HHTP) and TBPS surfaces, and the migration process is an endothermic reaction and not stable; (iii) the introduction of M₄ catalyst can greatly reduce the diffusion energy barrier of H atoms, which makes it easier for the H atoms to diffuse on the substrate surface. Differently, all of the H₂ molecules spontaneously dissociate into H atoms onto Pt₄ and Pd₄ clusters. However, the adsorbed H₂ molecules on Ni₄ cluster show two types of adsorption states: one activated state with stretched H-H bond length of 0.88 Å via the Kubas interaction and five dissociated states with separated hydrogen atoms. Among all the M₄ catalysts, the orders of the binding energy of M₄ deposited on the substrate and average chemisorption energy per H₂ molecule are Pt₄ > Ni₄ > Pd₄. On the contrary, the orders of the migration and diffusion barriers of H atoms are Pt₄ < Ni₄ < Pd₄, which indicates that Pt₄ is the most promising catalyst for the hydrogen spillover with the lowest migration and diffusion energy barriers. However, the migration of H atoms from Pt₄ toward the substrate is still endothermic. Thus direct migration of H atom from metal catalyst toward the substrate is thermodynamically unfavorable.

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Impact of coupling geometry on thermoelectric properties of oligophenyl-base transistor

S Ramezani Akbarabadi, H Rahimpour Soleimani, M Bagheri Tagani, Z Golsanamlou

2017 (2): 27303-027303. doi: 10.1088/1674-1056/26/2/027303

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Thermal and electron transport through organic molecules attached to three-dimensional gold electrodes in two different configurations, namely para and meta with thiol-terminated junctions is studied theoretically in the linear response regime using Green's function formalism. We used thiol-terminated (-SH bond) benzene units and found a positive thermopower because the highest occupied molecular orbital (HOMO) is near the Fermi energy level. We investigated the influence of molecular length and molecular junction geometry on the thermoelectric properties. Our results show that the thermoelectric properties are highly sensitive to the coupling geometry and the molecular length. In addition, we observed that the interference effects and increasing molecular length can increase the thermoelectric efficiency of device in a specific configuration.

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Enhancement of subgap conductance in a graphene superconductor junction by valley polarization

李传新, 汪萨克, 汪军

2017 (2): 27304-027304. doi: 10.1088/1674-1056/26/2/027304

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We theoretically study the differential conductance of a graphene/graphene superconductor junction, where the valley polarization of Dirac electrons is considered in the nonsuperconducting region. It is shown that the subgap conductance will increase monotonically with the valley-polarization strength when the chemical potential μ is near the Dirac point μ≤3Δ (Δ is the superconducting gap), whereas it will decrease monotonically when μ is far away from the Dirac point, μ≥5Δ. The former case is induced by the specular Andreev reflection while the retro-reflection accounts for the later result. Our findings may shed light on the control of conductance of a graphene superconductor junction by valley polarization.

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Ballistic transport and quantum interference in InSb nanowire devices

李森, 黄光耀, 郭景琨, 康宁, Philippe Caroff, 徐洪起

2017 (2): 27305-027305. doi: 10.1088/1674-1056/26/2/027305

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An experimental realization of a ballistic superconductor proximitized semiconductor nanowire device is a necessary step towards engineering topological quantum electronics. Here, we report on ballistic transport in InSb nanowires grown by molecular-beam epitaxy contacted by superconductor electrodes. At an elevated temperature, clear conductance plateaus are observed at zero magnetic field and in agreement with calculations based on the Landauer formula. At lower temperature, we have observed characteristic Fabry-Pérot patterns which confirm the ballistic nature of charge transport. Furthermore, the magnetoconductance measurements in the ballistic regime reveal a periodic variation related to the Fabry-Pérot oscillations. The result can be reasonably explained by taking into account the impact of magnetic field on the phase of ballistic electron's wave function, which is further verified by our simulation. Our results pave the way for better understanding of the quantum interference effects on the transport properties of InSb nanowires in the ballistic regime as well as developing of novel device for topological quantum computations.

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Thermal stability and electrical transport properties of Ge/Sn-codoped single crystalline β-Zn₄Sb₃ prepared by the Sn-flux method

刘虹霞, 邓书平, 李德聪, 申兰先, 邓书康

2017 (2): 27401-027401. doi: 10.1088/1674-1056/26/2/027401

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This study prepares a group of single crystalline β-Zn₄Sb₃ with Ge and Sn codoped by the Sn-flux method according to the nominal stoichiometric ratios of Zn_4.4Sb₃Ge_xSn₃ (x=0-0.15). The prepared samples possess a metallic luster surface with perfect appearance and large crystal sizes. The microscopic cracks or defects are invisible in the samples from the back-scattered electron image. Except for the heavily Ge-doped sample of x=0.15, all the samples are single phase with space group R3c. The thermal analysis results show that the samples doped with Ge exhibit an excellent thermal stability. Compared with the polycrystalline Ge-substituted β-Zn₄Sb₃, the present single crystals have higher carrier mobility, and hence the electrical conductivity is improved, which reaches 7.48×10⁴ S·m^-1 at room temperature for the x=0.1 sample. The change of Ge and Sn contents does not improve the Seebeck coefficient significantly. Benefiting from the increased electrical conductivity, the sample with x=0.075 gets the highest power factor of 1.45×10^-3 W·m^-1·K^-2 at 543 K.

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The magnetic properties and magnetocaloric effects in binary R-T (R=Pr, Gd, Tb, Dy, Ho, Er, Tm; T=Ga, Ni, Co, Cu) intermetallic compounds

郑新奇, 沈保根

2017 (2): 27501-027501. doi: 10.1088/1674-1056/26/2/027501

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In this paper, we review the magnetic properties and magnetocaloric effects (MCE) of binary R-T (R=Pr, Gd, Tb, Dy, Ho, Er, Tm; T=Ga, Ni, Co, Cu) intermetallic compounds (including RGa series, RNi series, R₁₂Co₇ series, R₃Co series and RCu₂ series), which have been investigated in detail in the past several years. The R-T compounds are studied by means of magnetic measurements, heat capacity measurements, magnetoresistance measurements and neutron powder diffraction measurements. The R-T compounds show complex magnetic transitions and interesting magnetic properties. The types of magnetic transitions are investigated and confirmed in detail by multiple approaches. Especially, most of the R-T compounds undergo more than one magnetic transition, which has significant impact on the magnetocaloric effect of R-T compounds. The MCE of R-T compounds are calculated by different ways and the special shapes of MCE peaks for different compounds are investigated and discussed in detail. To improve the MCE performance of R-T compounds, atoms with large spin (S) and atoms with large total angular momentum (J) are introduced to substitute the related rare earth atoms. With the atom substitution, the maximum of magnetic entropy change (Δ S_M), refrigerant temperature width (T_width) or refrigerant capacity (RC) is enlarged for some R-T compounds. In the low temperature range, binary R-T (R=Pr, Gd, Tb, Dy, Ho, Er, Tm; T=Ga, Ni, Co, Cu) intermetallic compounds (including RGa series, RNi series, R₁₂Co₇ series, R₃Co series and RCu₂ series) show excellent performance of MCE, indicating the potential application for gas liquefaction in the future.

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Electronic structures and magnetic properties of Zn- and Cd-doped AlN nanosheets: A first-principles study

韩瑞林, 姜世民, 闫羽

2017 (2): 27502-027502. doi: 10.1088/1674-1056/26/2/027502

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In this paper, the magnetic properties, electronic structures and the stabilities of Zn/Cd incorporated two-dimensional AlN nanosheets are investigated by the first-principles method. Numerical results indicate that Zn and Cd substituting Al atom in AlN nanosheets introduce some holes into the 2p orbitals of the N atoms, and the holes mainly come from spin-down 2p orbitals of the N atoms. The magnetic moment of 1.0 μ_B is produced by Zn/Cd doping AlN nanosheets, and the main component of the magnetic moment of the system is contributed by the partially filled 2p states of the N atoms around the dopant. In particular, when Zn/Cd substituting Al atoms, the magnetic coupling is found to be ferromagnetic. We attribute the hole-mediated p-d interaction to the created ferromagnetic coupling. More importantly, the result of formation energy indicates that Al atom is more inclined to be replaced by Zn atom rather than Cd. This finding is beneficial to developing the spin electronic devices.

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Study of magnetic and optical properties of Zn_1-xTM_xTe (TM=Mn, Fe, Co, Ni) diluted magnetic semiconductors: First principle approach

Q Mahmood, M Hassan, M A Faridi

2017 (2): 27503-027503. doi: 10.1088/1674-1056/26/2/027503

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We present structural, magnetic and optical characteristics of Zn_1-xTM_xTe (TM=Mn, Fe, Co, Ni and x=6.25%), calculated through Wien2k code, by using full potential linearized augmented plane wave (FP-LAPW) technique. The optimization of the crystal structures have been done to compare the ferromagnetic (FM) and antiferromagnetic (AFM) ground state energies, to elucidate the ferromagnetic phase stability, which further has been verified through the formation and cohesive energies. Moreover, the estimated Curie temperatures T_c have demonstrated above room temperature ferromagnetism (RTFM) in Zn_1-xTM_xTe (TM=Mn, Fe, Co, Ni and x=6.25%). The calculated electronic properties have depicted that Mn- and Co-doped ZnTe behave as ferromagnetic semiconductors, while half-metallic ferromagnetic behaviors are observed in Fe- and Ni-doped ZnTe. The presence of ferromagnetism is also demonstrated to be due to both the p-d and s-d hybridizations between the host lattice cations and TM impurities. The calculated band gaps and static real dielectric constants have been observed to vary according to Penn's model. The evaluated band gaps lie in near visible and ultraviolet regions, which make these materials suitable for various important device applications in optoelectronic and spintronic.

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Crystallization behaviors of ultrathin Al-doped HfO₂ amorphous films grown by atomic layer deposition

马雪丽, 杨红, 项金娟, 王晓磊, 王文武, 张建齐, 殷华湘, 朱慧珑, 赵超

2017 (2): 27701-027701. doi: 10.1088/1674-1056/26/2/027701

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In this work, ultrathin pure HfO₂ and Al-doped HfO₂ films (about 4-nm thick) are prepared by atomic layer deposition and the crystallinities of these films before and after annealing at temperatures ranging from 550℃ to 750℃ are analyzed by grazing incidence x-ray diffraction. The as-deposited pure HfO₂ and Al-doped HfO₂ films are both amorphous. After 550-℃ annealing, a multiphase consisting of a few orthorhombic, monoclinic and tetragonal phases can be observed in the pure HfO₂ film while the Al-doped HfO₂ film remains amorphous. After annealing at 650℃ and above, a great number of HfO₂ tetragonal phases, a high-temperature phase with higher dielectric constant, can be stabilized in the Al-doped HfO₂ film. As a result, the dielectric constant is enhanced up to about 35. The physical mechanism of the phase transition behavior is discussed from the viewpoint of thermodynamics and kinetics.

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Semipolar (1122) and polar (0001) InGaN grown on sapphire substrate by using pulsed metal organic chemical vapor deposition

许晟瑞, 赵颖, 蒋仁渊, 姜腾, 任泽阳, 张进成, 郝跃

2017 (2): 27801-027801. doi: 10.1088/1674-1056/26/2/027801

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High indium semipolar (1122) and polar (0001) InGaN layers each with a thickness of about 100 nm are realized simultaneously on sapphire substrates by pulsed metal organic chemical vapor deposition (MOCVD). The morphology evolution, structural and optical characteristics are also studied. The indium content in the layer of the surface (1122) is larger than that of the surface (0001), which is confirmed by reciprocal space map, photoluminescence spectrum and secondary ion mass spectrometer. Additionally, the (0001) surface with island-like morphology shows inhomogeneous indium incorporation, while the (1122) surface with a spiral-like morphology shows a better homogeneous In composition. This feature is also demonstrated by the monochromatic cathodoluminescence map.

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Photoconductive multi-layer graphene photodetectors fabricated on etched silicon-on-insulator substrates

王玉冰, 尹伟红, 韩勤, 杨晓红, 叶焓, 吕倩倩, 尹冬冬

2017 (2): 28101-028101. doi: 10.1088/1674-1056/26/2/028101

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Recently, graphene-based photodetectors have been rapidly developed. However, their photoresponsivities are generally low due to the weak optical absorption strength of graphene. In this paper, we fabricate photoconductive multi-layer graphene (MLG) photodetectors on etched silicon-on-insulator substrates. A photoresponsivity exceeding 200 A·W^-1 is obtained, which enables most optoelectronic application. In addition, according to the analyses of the high photoresponsivity and long photoresponse time, we conclude that the working mechanism of the device is photoconductive effect. The process of photons conversion into conducting electrons is also described in detail. Finally, according to the distinct difference between the photoresponses at 1550 nm and 808 nm, we estimate that the position of the trapping energy is somewhere between 0.4 eV and 0.76 eV, higher than the Fermi energy of MLG. Our work paves a new way for fabricating the graphene photoconductive photodetectors.

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Electrical and dielectric characterization of Au/ZnO/n—Si device depending frequency and voltage

I Orak, A Kocyigit, Ş Alındal

2017 (2): 28102-028102. doi: 10.1088/1674-1056/26/2/028102

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Au/ZnO/n-type Si device is obtained using atomic layer deposition (ALD) for ZnO layer, and some main electrical parameters are investigated, such as surface/interface state (N_ss), barrier height (Φ_b), series resistance (R_s), donor concentration (N_d), and dielectric characterization depending on frequency or voltage. These parameters are acquired by use of impedance spectroscopy measurements at frequencies ranging from 10 kHz to 1 MHz and the direct current (DC) bias voltages in a range from -2 V to +2 V at room temperature are used. The main electrical parameters and dielectric parameters, such as dielectric constant (ε"), dielectric loss (ε"), loss tangent (tan δ), the real and imaginary parts of electric modulus (M' and M"), and alternating current (AC) electrical conductivity (σ) are affected by changing voltage and frequency. The characterizations show that some main electrical parameters usually decrease with increasing frequency because charge carriers at surface states have not enough time to fallow an external AC signal at high frequencies, and all dielectric parameters strongly depend on the voltage and frequency especially in the depletion and accumulation regions. Consequently, it can be concluded that interfacial polarization and interface charges can easily follow AC signal at low frequencies.

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Geometrically induced π-band splitting in graphene superlattices

魏艳佩, 贾甜甜, 陈刚

2017 (2): 28103-028103. doi: 10.1088/1674-1056/26/2/028103

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According to band folding analyses, the graphene superlattices can be differed by whether the Dirac points are folded to Γ point or not. In previous studies, the inversion symmetry preserved defects open bandgap in the former superlattices while they cannot in the latter ones. In this paper, by using density functional theory with generalized gradient approximation, we have carefully studied the electronic properties of the latter graphene superlattices, in which the defects would induce π-band splitting to get the π_a1-π_a2 and π_z1-π_z2 band sets. Based on our detailed studies, such splitting could be attributed to the geometrically induced bond-symmetry breaking. In addition, these band sets could be shifted toward each other by the methodology of strain engineering. A bandgap would be opened once the band sets start to overlap. Then, its gap width could be continuously enlarged by enhancing strain until reaching the maximum value determined by the defect density. These studies contribute to the bandstructure engineering of graphene-based nanomaterials, which would be interesting to call for further investigations on both theory and experiment.

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Investigation on latch-up susceptibility induced by high-power microwave in complementary metal-oxide-semiconductor inverter

张宇航, 柴常春, 于新海, 杨银堂, 刘阳, 樊庆扬, 史春蕾

2017 (2): 28501-028501. doi: 10.1088/1674-1056/26/2/028501

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The latch-up effect induced by high-power microwave (HPM) in complementary metal-oxide-semiconductor (CMOS) inverter is investigated in simulation and theory in this paper. The physical mechanisms of excess carrier injection and HPM-induced latch-up are proposed. Analysis on upset characteristic under pulsed wave reveals increasing susceptibility under shorter-width pulsed wave which satisfies experimental data, and the dependence of upset threshold on pulse repetitive frequency (PRF) is believed to be due to the accumulation of excess carriers. Moreover, the trend that HPM-induced latch-up is more likely to happen in shallow-well device is proposed.Finally, the process of self-recovery which is ever-reported in experiment with its correlation with supply voltage and power level is elaborated, and the conclusions are consistent with reported experimental results.

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Improvement of the carrier distribution with GaN/InGaN/AlGaN/InGaN/GaN composition-graded barrier for InGaN-based blue light-emitting diode

郭敏, 郭志友, 黄晶, 刘洋, 姚舜禹

2017 (2): 28502-028502. doi: 10.1088/1674-1056/26/2/028502

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InGaN light-emitting diodes (LEDs) with GaN/InGaN/AlGaN/InGaN/GaN composition-graded barriers are proposed to replace the sixth and the middle five GaN barriers under the condition of removing the electron blocking layer (EBL) and studied numerically in this paper. Simulation results show that the specially designed barrier in the sixth barrier is able to modulate the distributions of the holes and electrons in quantum well which is adjacent to the specially designed barrier. Concretely speaking, the new barrier could enhance both the electron and hole concentration remarkably in the previous well and reduce the hole concentration for the latter one to some extent along the growth direction. What is more, a phenomenon, i.e., a better carrier distribution in all the wells, just appears with the adoption of the new barriers in the middle five barriers, resulting in a much higher light output power and a lower efficiency droop than those in a conventional LED structure.

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Spin transfer torque in the semiconductor/ferromagnetic structure in the presence of Rashba effect

Javad Vahedi, Sahar Ghasab Satoory

2017 (2): 28503-028503. doi: 10.1088/1674-1056/26/2/028503

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Spin transfer torque in magnetic structure occurs when the transverse component of the spin current that flows from the nonmagnetic medium to ferromagnetic medium is absorbed by the interface. In this paper, considering the Rashba effect on the semiconductor region, we discuss the spin transfer torque in semiconductor/ferromagnetic structure and obtain the components of spin-current density for two models: (i) single electron and (ii) the distribution of electrons. We show that no matter whether the difference in Fermi surface between semiconductor and Fermi spheres for the up and down spins in ferromagnetic increases, the transmission probability decreases. The obtained results for the values used in this article illustrate that Rashba effect increases the difference in Fermi sphere between semiconductor and Fermi sphere for the up and down spins in ferromagnetic. The results also show that the Rashba effect, brings an additional contribution to the components of spin transfer torque, which does not exist in the absence of the Rashba interaction. Moreover, the Rashba term has also different effects on the transverse components of the spin torque transfer.

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Shifting curves based on the detector integration effect for x-ray phase contrast imaging

杨君, 郭金川, 雷耀虎, 易明皓, 陈力

2017 (2): 28701-028701. doi: 10.1088/1674-1056/26/2/028701

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In theory, we find that the actual function of the analyzer grating in the Talbot-Lau interferometer is segmenting the self-images of the phase grating and choosing integral areas, which make sure that each period of self-images in one detector pixel contributes the same signal to the detector. Furthermore, in the case of the lack of an analyzer grating, the shifting curves are still existent in theory as long as the number of fringes is non-integral in a detector pixel, which is a sufficient condition for creating shifting curve. The sufficient condition is available for not only the Talbot-Lau interferometer and the inverse geometry of Talbot-Lau interferometer, but also the x-ray phase contrast imaging system based on geometrical optics. In practical applications, we propose a method to improve the performances of the existing systems by employing the sufficient condition. This method can shorten the system length, is applicable to large period gratings, and can use the detectors with large pixels and large field of view. In addition, the experimental arrangement can be simplified due to the lack of an analyzer grating. In order to improve detection sensitivity and resolution, we also give an optimal fringe period. We believe that the theory and method proposed here is a step forward for x-ray phase contrast imaging.

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Highly conductive and transparent carbon nanotube-based electrodes for ultrathin and stretchable organic solar cells

范庆霞, 张强, 周文斌, 杨丰, 张楠, 肖仕奇, 谷孝刚, 肖卓建, 陈辉亮, 王艳春, 刘华平, 周维亚

2017 (2): 28801-028801. doi: 10.1088/1674-1056/26/2/028801

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In this work, we have presented a freestanding and flexible CNT-based film with sheet resistance of 60 Ω/□ and transmittance of 82% treated by nitric acid and chloroauric acid in sequence. Based on modified CNT film as a transparent electrode, we have demonstrated an ultrathin, flexible organic solar cell (OSC) fabricated on 2.5-μm PET substrate. The efficiency of OSC, combined with a composite film of poly (3-hexylthiophene) (P3HT) and phenyl-C61 butyric acid methyl ester (PCBM) as an active layer and with a thin layer of methanol soluble biuret inserted between the photoactive layer and the cathode, can be up to 2.74% which is approximate to that of the reference solar cell fabricated with ITO-coated glass (2.93%). Incorporating the as-fabricated ITO-free OSC with pre-stretched elastomer, 50% compressive deformation can apply to the solar cells. The results show that the as-prepared CNT-based hybrid film with outstanding electrical and optical properties could serve as a promising transparent electrode for low cost, flexible and stretchable OSCs, which will broaden the applications of OSC and generate more solar power than it now does.

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Performance improvement of continuous carbon nanotube fibers by acid treatment

张强, 李克伟, 范庆霞, 夏晓刚, 张楠, 肖卓建, 周文斌, 杨丰, 王艳春, 刘华平, 周维亚

2017 (2): 28802-028802. doi: 10.1088/1674-1056/26/2/028802

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Continuous CNT fibers have been directly fabricated in a speed of 50 m/h-400 m/h, based on an improved chemical vapor deposition method. As-prepared fibers are further post-treated by acid. According to the SEM images and Raman spectra, the acid treatment results in the compaction and surface modification of the CNTs in fibers, which are beneficial for the electron and load transfer. Compared to the HNO₃ treatment, HClSO₃ or H₂SO₄ treatment is more effective for the improvement of the fibers' properties. After HClSO₃ treatment for 2 h, the fibers' strength and electrical conductivity reach up to ~2 GPa and ~4.3 MS/m, which are promoted by ~200% and almost one order of magnitude than those without acid treatment, respectively. The load-bearing status of the CNT fibers are analyzed based on the downshifts of the G' band and the strain transfer factor of the fibers under tension. The results reveal that acid treatment could greatly enhance the load transfer and inter-bundle strength. With the HClSO₃ treatment, the strain transfer factor is enhanced from ~3.9% to ~53.6%.

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Simulation design of P-I-N-type all-perovskite solar cells with high efficiency

杜会静, 王韦超, 顾一帆

2017 (2): 28803-028803. doi: 10.1088/1674-1056/26/2/028803

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According to the good charge transporting property of perovskite, we design and simulate a p-i-n-type all-perovskite solar cell by using one-dimensional device simulator. The perovskite charge transporting layers and the perovskite absorber constitute the all-perovskite cell. By modulating the cell parameters, such as layer thickness values, doping concentrations and energy bands of n-, i-, and p-type perovskite layers, the all-perovskite solar cell obtains a high power conversion efficiency of 25.84%. The band matched cell shows appreciably improved performance with widen absorption spectrum and lowered recombination rate, so weobtain a high J_sc of 32.47 mA/cm². The small series resistance of the all-perovskite solar cell also benefits the high J_sc. The simulation provides a novel thought of designing perovskite solar cells with simple producing process, low production cost and high efficient structure to solve the energy problem.

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Scaling of weighted spectral distribution in weighted small-world networks

焦波, 吴晓群

2017 (2): 28901-028901. doi: 10.1088/1674-1056/26/2/028901

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Many real-world systems can be modeled by weighted small-world networks with high clustering coefficients. Recent studies for rigorously analyzing the weighted spectral distribution (WSD) have focused on unweighted networks with low clustering coefficients. In this paper, we rigorously analyze the WSD in a deterministic weighted scale-free small-world network model and find that the WSD grows sublinearly with increasing network order (i.e., the number of nodes) and provides a sensitive discrimination for each input of this model. This study demonstrates that the scaling feature of the WSD exists in the weighted network model which has high and order-independent clustering coefficients and reasonable power-law exponents.

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Effect of air breakdown on microwave pulse energy transmission

赵朋程, 郭立新, 舒盼盼

2017 (2): 29201-029201. doi: 10.1088/1674-1056/26/2/029201

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The energy transmission of the long microwave pulse for the frequency of 2.45 GHz and 5.8 GHz is studied by using the electron fluid model, where the rate coefficients are deduced from the Boltzmann equation solver named BOLSIG+. The breakdown thresholds for different air pressures and incident pulse parameters are predicted, which show good agreement with the experimental data. Below the breakdown threshold, the transmitted pulse energy is proportional to the square of the incident electric field amplitude. When the incident electric field amplitude higher than the breakdown threshold increases, the transmitted pulse energy decreases monotonously at a high air pressure, while at a low air pressure it first decreases and then increases. We also compare the pulse energy transmission for the frequency of 2.45 GHz with the case of 5.8 GHz.

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