中国物理B

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Accurate treatments of electrostatics for computer simulations of biological systems: A brief survey of developments and existing problems

衣沙沙, 潘聪, 胡中汉

2015 (12): 120201-120201. doi: 10.1088/1674-1056/24/12/120201

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Modern computer simulations of biological systems often involve an explicit treatment of the complex interactions among a large number of molecules. While it is straightforward to compute the short-ranged Van der Waals interaction in classical molecular dynamics simulations, it has been a long-lasting issue to develop accurate methods for the long-ranged Coulomb interaction. In this short review, we discuss three types of methodologies for the accurate treatment of electrostatics in simulations of explicit molecules: truncation-type methods, Ewald-type methods, and mean-field-type methods. Throughout the discussion, we brief the formulations and developments of these methods, emphasize the intrinsic connections among the three types of methods, and focus on the existing problems which are often associated with the boundary conditions of electrostatics. This brief survey is summarized with a short perspective on future trends along the method developments and applications in the field of biological simulations.

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The construction of general basis functions in reweighting ensemble dynamics simulations: Reproduce equilibrium distribution in complex systems from multiple short simulation trajectories

张传彪, 黎明, 周昕

2015 (12): 120202-120202. doi: 10.1088/1674-1056/24/12/120202

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Ensemble simulations, which use multiple short independent trajectories from dispersive initial conformations, rather than a single long trajectory as used in traditional simulations, are expected to sample complex systems such as biomolecules much more efficiently. The re-weighted ensemble dynamics (RED) is designed to combine these short trajectories to reconstruct the global equilibrium distribution. In the RED, a number of conformational functions, named as basis functions, are applied to relate these trajectories to each other, then a detailed-balance-based linear equation is built, whose solution provides the weights of these trajectories in equilibrium distribution. Thus, the sufficient and efficient selection of basis functions is critical to the practical application of RED. Here, we review and present a few possible ways to generally construct basis functions for applying the RED in complex molecular systems. Especially, for systems with less priori knowledge, we could generally use the root mean squared deviation (RMSD) among conformations to split the whole conformational space into a set of cells, then use the RMSD-based-cell functions as basis functions. We demonstrate the application of the RED in typical systems, including a two-dimensional toy model, the lattice Potts model, and a short peptide system. The results indicate that the RED with the constructions of basis functions not only more efficiently sample the complex systems, but also provide a general way to understand the metastable structure of conformational space.

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Thermal vacuum state corresponding to squeezed chaotic light and its application

万志龙, 范洪义, 王震

2015 (12): 120301-120301. doi: 10.1088/1674-1056/24/12/120301

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For the density operator (mixed state) describing squeezed chaotic light (SCL) we search for its thermal vacuum state (a pure state) in the real-fictitious space. Using the method of integration within ordered product (IWOP) of operators we find that it is a kind of one-and two-mode combinatorial squeezed state. Its application in evaluating the quantum fluctuation of photon number reveals: the stronger the squeezing is, the larger a fluctuation appears. The second-order degree of coherence of SCL is also deduced which shows that SCL is classic. The new thermal vacuum state also helps to derive the Wigner function of SCL.

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Dynamics of super-quantum discord and direct control with weak measurement in open quantum system

嵇英华

2015 (12): 120302-120302. doi: 10.1088/1674-1056/24/12/120302

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Super-quantum discord (SQD) with weak measurement is regarded as a kind of quantum correlation in quantum information processing. We compare and analyze the dynamical evolutions of SQD, quantum discord (QD), and quantum entanglement (QE) between two qubits in the correlated dephasing environmental model. The results indicate that (i) owing to the much smaller influence of weak measurement on the coherence of the system than that of von Neumann projection measurement, SQD with weak measurement is larger than QD, and (ii) dynamical evolution of QD or QE monotonically goes to zero with time, while SQD monotonically tends to a stable value and a freezing phenomenon occurs. The stable value after freezing mainly depends on the measurement strength and the purity of the initial quantum state.

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Decoherence of genuine multipartite entanglement for local non-Markovian-Lorentzian reservoirs

Mazhar Ali

2015 (12): 120303-120303. doi: 10.1088/1674-1056/24/12/120303

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We study decoherence effects on genuine multipartite entanglement for three and four qubits, spatially separated and subjected to local Lorentzian reservoirs. Employing recent techniques to compute genuine negativity for multipartite systems and an exact solvable model, we analyze the dynamics of genuine entanglement for different coupling bandwidths and detunings. We find that collapses and revivals can occur by varying these parameters for various multipartite quantum states.

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Quantum speed limits for Bell-diagonal states

韩伟, 江克侠, 张英杰, 夏云杰

2015 (12): 120304-120304. doi: 10.1088/1674-1056/24/12/120304

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The lower bounds of the evolution time between two distinguishable states of a system, defined as quantum speed limit time, can characterize the maximal speed of quantum computers and communication channels. We study the quantum speed limit time between the composite quantum states and their target states in the presence of nondissipative decoherence. For the initial states with maximally mixed marginals, we obtain the exact expressions of the quantum speed limit time which mainly depend on the parameters of the initial states and the decoherence channels. Furthermore, by calculating the quantum speed limit time for the time-dependent states started from a class of initial states, we discover that the quantum speed limit time gradually decreases in time, and the decay rate of the quantum speed limit time would show a sudden change at a certain critical time. Interestingly, at the same critical time, the composite system dynamics would exhibit a sudden transition from classical decoherence to quantum decoherence.

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A note on local unitary equivalence of isotropic-like states

张廷桂, 华波波, 李明, 赵明镜, 杨红

2015 (12): 120305-120305. doi: 10.1088/1674-1056/24/12/120305

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We consider the local unitary equivalence of a class of quantum states in a bipartite case and a multipartite case. The necessary and sufficient condition is presented. As special cases, the local unitary equivalent classes of isotropic state and Werner state are provided. Then we study the local unitary similar equivalence of this class of quantum states and analyze the necessary and sufficient condition.

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Fast multi-copy entanglement purification with linear optics

蔡春, 周澜, 盛宇波

2015 (12): 120306-120306. doi: 10.1088/1674-1056/24/12/120306

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We describe an entanglement purification protocol for a polarization Bell state. Different from the previous protocols, it does not require the controlled-not gate, and only uses linear optical elements to complete the task. This protocol requires multi-copy degraded mixed states, which can make this protocol obtain a high fidelity in one purification step. It can also be extended to purify the multi-photon Greenberger-Horne-Zeilinger (GHZ) state. This protocol may be useful in future long-distance communication.

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Free-space measurement-device-independent quantum-key-distribution protocol using decoy states with orbital angular momentum

王乐, 赵生妹, 巩龙延, 程维文

2015 (12): 120307-120307. doi: 10.1088/1674-1056/24/12/120307

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In this paper, we propose a measurement-device-independent quantum-key-distribution (MDI-QKD) protocol using orbital angular momentum (OAM) in free space links, named the OAM-MDI-QKD protocol. In the proposed protocol, the OAM states of photons, instead of polarization states, are used as the information carriers to avoid the reference frame alignment, the decoy-state is adopted to overcome the security loophole caused by the weak coherent pulse source, and the high efficient OAM-sorter is adopted as the measurement tool for Charlie to obtain the output OAM state. Here, Charlie may be an untrusted third party. The results show that the authorized users, Alice and Bob, could distill a secret key with Charlie's successful measurements, and the key generation performance is slightly better than that of the polarization-based MDI-QKD protocol in the two-dimensional OAM cases. Simultaneously, Alice and Bob can reduce the number of flipping the bits in the secure key distillation. It is indicated that a higher key generation rate performance could be obtained by a high dimensional OAM-MDI-QKD protocol because of the unlimited degree of freedom on OAM states. Moreover, the results show that the key generation rate and the transmission distance will decrease as the growth of the strength of atmospheric turbulence (AT) and the link attenuation. In addition, the decoy states used in the proposed protocol can get a considerable good performance without the need for an ideal source.

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Unstable and exact periodic solutions of three-particles time-dependent FPU chains

刘期怀, 邢明燕, 李新祥, 王超

2015 (12): 120401-120401. doi: 10.1088/1674-1056/24/12/120401

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For lower dimensional Fermi-Pasta-Ulam (FPU) chains, the α-chain is completely integrable and the Hamiltonian of the β-chain can be identified with the Hénon-Heiles Hamiltonian. When the strengths α, β of the nonlinearities depend on time periodically with the same frequencies as the natural angular frequencies, the resonance phenomenon is inevitable. In this paper, for certain periodic functions α(t) and β(t) with resonance frequencies, we give the existence and stability of some nontrivial exact periodic solutions for a one-dimensional α β-FPU model composed of three particles with periodic boundary conditions.

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Langevin approach with rescaled noise for stochastic channel dynamics in Hodgkin-Huxley neurons

黄艳东, 李翔, 帅建伟

2015 (12): 120501-120501. doi: 10.1088/1674-1056/24/12/120501

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The Langevin approach has been applied to model the random open and closing dynamics of ion channels. It has long been known that the gate-based Langevin approach is not sufficiently accurate to reproduce the statistics of stochastic channel dynamics in Hodgkin-Huxley neurons. Here, we introduce a modified gate-based Langevin approach with rescaled noise strength to simulate stochastic channel dynamics. The rescaled independent gate and identical gate Langevin approaches improve the statistical results for the mean membrane voltage, inter-spike interval, and spike amplitude.

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Composition and temperature dependences of site occupation for Al, Cr, W, and Nb in MoSi₂

李小平, 孙顺平, 于赟, 王洪金, 江勇, 易丹青

2015 (12): 120502-120502. doi: 10.1088/1674-1056/24/12/120502

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The composition and temperature dependences of site occupation for Al, Cr, W, and Nb in MoSi₂ are investigated by using a thermodynamics model and first principles calculations. A simple parameter measuring the substitution energy difference between Si and Mo sites reflects the nature of site occupancy. At 0 K, these elements prefer Si sites in Mo-rich and Mo sites in Si-rich, and show no site preference in stoichiometric MoSi₂. At elevated temperature, the site occupation behaviors show strong dependence on both composition and temperature. Some calculated results have been certified in previous experiments.

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Entransy analyses of heat-work conversion systems with inner irreversible thermodynamic cycles

程雪涛, 梁新刚

2015 (12): 120503-120503. doi: 10.1088/1674-1056/24/12/120503

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In this paper, we try to use the entransy theory to analyze the heat-work conversion systems with inner irreversible thermodynamic cycles. First, the inner irreversible thermodynamic cycles are analyzed. The influences of different inner irreversible factors on entransy loss are discussed. We find that the concept of entransy loss can be used to analyze the inner irreversible thermodynamic cycles. Then, we analyze the common heat-work conversion systems with inner irreversible thermodynamic cycles. As an example, the heat-work conversion system in which the working fluid of the thermodynamic cycles is heated and cooled by streams is analyzed. Our analyses show that larger entransy loss leads to larger output work when the total heat flow from the high temperature heat source and the corresponding equivalent temperature are fixed. Some numerical cases are presented, and the results verify the theoretical analyses. On the other hand, it is also found that larger entransy loss does not always lead to larger output work when the preconditions are not satisfied.

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Computational studies on the interactions of nanomaterials with proteins and their impacts

安德义, 苏计国, 李春华, 李敬源

2015 (12): 120504-120504. doi: 10.1088/1674-1056/24/12/120504

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The intensive concern over the biosafety of nanomaterials demands the systematic study of the mechanisms underlying their biological effects. Many of the effects of nanomaterials can be attributed to their interactions with proteins and their impacts on protein function. On the other hand, nanomaterials show potential for a variety of biomedical applications, many of which also involve direct interactions with proteins. In this paper, we review some recent computational studies on this subject, especially those investigating the interactions of carbon and gold nanomaterials. Beside hydrophobic and π-stacking interactions, the mode of interaction of carbon nanomaterials can also be regulated by their functional groups. The coatings of gold nanomaterials similarly adjust their mode of interaction, in addition to coordination interactions with the sulfur groups of cysteine residues and the imidazole groups of histidine residues. Nanomaterials can interact with multiple proteins and their impacts on protein activity are attributed to a wide spectrum of mechanisms. These findings on the mechanisms of nanomaterial-protein interactions can further guide the design and development of nanomaterials to realize their application in disease diagnosis and treatment.

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Border effect-based precise measurement of any frequency signal

白丽娜, 叶波, 宣美娜, 金瑜珍, 周渭

2015 (12): 120601-120601. doi: 10.1088/1674-1056/24/12/120601

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Limited detection resolution leads to fuzzy areas during the measurement, and the discrimination of the border of a fuzzy area helps to use the resolution stability. In this way, measurement precision is greatly improved, hence this phenomenon is named the border effect. The resolution fuzzy area and its application should be studied to realize high-resolution measurement. During the measurement of any frequency signal, the fuzzy areas of phase-coincidence detection are always discrete and irregular. In this paper the difficulty in capturing the border information of discrete fuzzy areas is overcome and extra-high resolution measurement is implemented. Measurement precision of any frequency-signal can easily reach better than 1× 10^-11/s in a wide range of frequencies, showing the great importance of the border effect. An in-depth study of this issue has great significance for frequency standard comparison, signal processing, telecommunication, and fundamental subjects.

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Multistability of delayed complex-valued recurrent neural networks with discontinuous real-imaginary-type activation functions

黄玉娇, 胡海根

2015 (12): 120701-120701. doi: 10.1088/1674-1056/24/12/120701

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In this paper, the multistability issue is discussed for delayed complex-valued recurrent neural networks with discontinuous real-imaginary-type activation functions. Based on a fixed theorem and stability definition, sufficient criteria are established for the existence and stability of multiple equilibria of complex-valued recurrent neural networks. The number of stable equilibria is larger than that of real-valued recurrent neural networks, which can be used to achieve high-capacity associative memories. One numerical example is provided to show the effectiveness and superiority of the presented results.

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Influence of a strong magnetic field on the hydrogen molecular ion using B-spline-type basis-sets

张月霞, 张小龙

2015 (12): 123101-123101. doi: 10.1088/1674-1056/24/12/123101

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As an improvement on our previous work [J. Phys. B: At. Mol. Opt. Phys. 45 085101 (2012)], an accurate method combining the spheroidal coordinates and B-spline basis is applied to study the ground state 1σ_g and low excited states 1σ_u, 1π_g,u,1δ_g,u,2σ_g of the H₂⁺ in magnetic fields ranging from 10⁹ Gs (1 Gs=10^-4 T) to 4.414×10¹³ Gs. Comparing the one-center method used in our previous work, the present method has a higher precision with a shorter computing time. Equilibrium distances of the states of the H₂⁺ in strong magnetic fields were found to be accurate to 3～5 significant digits (s.d.) and the total energies 6～11 s.d., even for some antibonding state, such as 1π_g, which is difficult for the one-center method to give reliable results while the field strength is B≥q10¹³ Gs. For the large disagreement in previous works, such as the equilibrium distances of the 1π_g state at B=10⁹ Gs, the present data may be used as a reference. Further, the potential energy curves (PECs) and the electronic probability density distributions (EPDDs) of the bound states 1σ_g, 1π_u, 1δ_g and antibonding states 1σ_u, 1π_g, 1δ_u for B=1, 10, 100, 1000 a.u. (atomic unit) are compared, so that the different influences of the magnetic fields on the chemical bonds of the bound states and antibonding states are discussed in detail.

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Comment on “Relativistic atomic data for W XLVII” by S. Aggarwal et al. [Chin. Phys. B 24 (2015) 053201]

Kanti M. Aggarwal

2015 (12): 123201-123201. doi: 10.1088/1674-1056/24/12/123201

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Recently, S. Aggarwal et al. [Chin. Phys. B 24 (2015) 053201] reported energy levels, radiative rates, and lifetimes for the lowest 148 levels belonging to the 3s²3p, 3s3p², 3s²3d, 3s3p3d, 3p³, 3p²3d, 3s3d², 3p3d², and 3d³ configurations of Al-like tungsten. While their calculated energies for the levels and the radiative rates for transitions are correct, the reported results for lifetimes are completely wrong. According to our calculations, errors in their reported lifetimes are up to 14 orders of magnitude for over 90% of the levels. Here we report the correct lifetimes and explain the reasons for discrepancies.

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Fast-electron-impact study on excitations of 4d electron of xenon

张鑫, 刘亚伟, 彭裔耕, 徐龙泉, 倪冬冬, 康旭, 王洋洋, 祁月盈, 朱林繁

2015 (12): 123401-123401. doi: 10.1088/1674-1056/24/12/123401

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The electron energy loss spectrum of the 4d excitations of xenon was measured at an incident electron energy of 1500 eV and a scattering angle of 6°. Besides the optically allowed transitions of 4d_5/2^-1np and 4d_3/2^-1np', the optically forbidden transitions of 4_5/2^-1ns, 4d_5/2^-1nd, 4d_3/2^-1ns', and 4d_3/2^-1nd' were observed. The measured features are assigned with the help of the calculation by the Cowan Code. The line profile parameters of both optically allowed transitions and optically forbidden ones were determined and compared with the previous available data. It is found that the natural widths of both dipole-allowed and dipole-forbidden excitations are approximately identical, which means the spectator transitions dominate the resonant Auger effect for both dipole-allowed and dipole-forbidden transitions.

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Solvation of halogen ions in aqueous solutions at 500 K-600 K under 100 atm

申昊, 郝亭, 张丰收

2015 (12): 123601-123601. doi: 10.1088/1674-1056/24/12/123601

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Structural properties of the pure water and halogen solutions at high temperatures and pressures are studied by using the molecular dynamics simulations and quantum molecular simulations. The related characters are calculated as functions of temperature and pressure. The results show that the hydrogen bonded networks become looser as temperature increases, with the collapse of the traditional tetrahedral structure. It is similar to the concentration-dependent collapse in the NaCl solutions. However, adding other halogen elements has no further effects on the already weakly bonded water molecules. At the phase changing points, the process of hydration is evident for the bigger ions, so that the bigger the ion is, the smaller a cluster is formed.

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Design of ultra wideband microwave absorber effectual for objects of arbitrary shape

宫元勋, 周忠祥, 姜建堂, 赵宏杰

2015 (12): 124101-124101. doi: 10.1088/1674-1056/24/12/124101

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In this paper, we present the design of multilayer microwave absorbers comprised of CoFe alloy nano-particles and nano-flakes as fillers. The thickness of the unite layer is optimized by using the Genetic Algorithm. Efficient microwave absorptions over a wide frequency band and a range of incident angles are achieved by using multilayer absorbers. We show that the absorbers are effective not only for a planar surface but also for arbitrarily shaped objects as well.

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Propagation of an Airy-Gaussian beam in uniaxial crystals

周美玲, 陈迟到, 陈波, 彭喜, 彭玉莲, 邓冬梅

2015 (12): 124102-124102. doi: 10.1088/1674-1056/24/12/124102

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Under the paraxial approximation, the analytical propagation expression of an Airy-Gaussian beam (AiGB) in uniaxial crystals orthogonal to the optical axis is investigated. The propagation dynamics of the AiGB is given for different ratios of the extraordinary index to the ordinary refractive index. It has been found that the continuity and the self-bending effect of AiGB become weaker when the ratio increases. From the figure of the maximum intensity of AiGB, one can see that the maximum intensity is not monotone decreasing due to the anisotropic effect of the crystals. The intensity distribution of AiGB in different distribution factors is shown. The AiGB converges toward a Gaussian beam as the distribution factor increases.

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Propagation of rotating elliptical Gaussian beams from right-handed material to left-handed material

彭喜, 陈迟到, 陈波, 邓冬梅

2015 (12): 124201-124201. doi: 10.1088/1674-1056/24/12/124201

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By applying the ABCD matrix method, we report the propagating properties of the rotating elliptical Gaussian beams (REGBs) from the right-handed material (RHM) to the left-handed material (LHM). Based on the propagation equation, we obtain the intensity distributions of the REGBs during the propagation. It is found that the rotating direction of the REGBs is opposite in the RHM and the LHM, and the rotation angles tend to be π/2 as the propagation distance is long enough. Then we analyze the relationship between the refractive index and the rotating velocity. Furthermore, the energy flow and the angular momentum (AM) of the REGBs which can rotate are also obtained.

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Increasing the range accuracy of three-dimensional ghost imaging ladar using optimum slicing number method

杨旭, 张勇, 徐璐, 杨成华, 王强, 刘越豪, 赵远

2015 (12): 124202-124202. doi: 10.1088/1674-1056/24/12/124202

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The range accuracy of three-dimensional (3D) ghost imaging is derived. Based on the derived range accuracy equation, the relationship between the slicing number and the range accuracy is analyzed and an optimum slicing number (OSN) is determined. According to the OSN, an improved 3D ghost imaging algorithm is proposed to increase the range accuracy. Experimental results indicate that the slicing number can affect the range accuracy significantly and the highest range accuracy can be achieved if the 3D ghost imaging system works with OSN.

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Dynamical properties of total intensity fluctuation spectrum in two-mode Nd:YVO₄ microchip laser

张韶辉, 张书练, 谈宜东, 孙利群

2015 (12): 124203-124203. doi: 10.1088/1674-1056/24/12/124203

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We investigate the total intensity fluctuation spectrum of the two-longitudinal-mode Nd:YVO₄ microchip laser (ML). We find that low-frequency relaxation oscillation (RO) peaks still appear in the total intensity fluctuation spectrum, which is different from a previous research result that the low-frequency RO peaks exist in the spectrum of the individual mode but compensate for each other totally in the total intensity fluctuation spectrum. Taking the spatial hole-burning effect into account, one and two-mode rate equations for Nd:YVO₄ ML laser are established and studied. Based on the theoretical model, we find that when the gains and losses for two longitudinal models are different, a low-frequency RO peak will appear in the total intensity fluctuation spectrum, while when they share the same gain and loss, the total spectrum will behave like that of a single mode laser. Theoretical simulation results coincide with experimental results very well.

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Yb-doped passively mode-locked fiber laser with Bi₂Te₃-deposited

李璐, 闫培光, 王勇刚, 段利娜, 孙航, 司金海

2015 (12): 124204-124204. doi: 10.1088/1674-1056/24/12/124204

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In this study we present an all-normal-dispersion Yb-doped fiber laser passively mode-locked with topological insulator (Bi₂Te₃) saturable absorber. The saturable absorber device is fabricated by depositing Bi₂Te₃ on a tapered fiber through using pulsed laser deposition (PLD) technology, which can give rise to less non-saturable losses than most of the solution processing methods. Owing to the long interaction length, Bi₂Te₃ is not exposed to high optical power, which allows the saturable absorber device to work in a high power regime. The modulation depth of this kind of saturable absorber is measured to be 10%. By combining the saturable absorber device with Yb-doped fiber laser, a mode-locked pulse operating at a repetition rate of 19.8 MHz is achieved. The 3-dB spectral width and pulse duration are measured to be 1.245 nm and 317 ps, respectively.

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Analytical model for thermal lensing and spherical aberration in diode side-pumped Nd:YAG laser rod having Gaussian pump profile

M H Moghtader Dindarlu, M Kavosh Tehrani, H Saghafifar, A Maleki

2015 (12): 124205-124205. doi: 10.1088/1674-1056/24/12/124205

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In this paper, according to the temperature and strain distribution obtained by considering the Gaussian pump profile and dependence of physical properties on temperature, we derive an analytical model for refractive index variations of the diode side-pumped Nd:YAG laser rod. Then we evaluate this model by numerical solution and our maximum relative errors are 5% and 10% for variations caused by thermo-optical and thermo-mechanical effects; respectively. Finally, we present an analytical model for calculating the focal length of the thermal lens and spherical aberration. This model is evaluated by experimental results.

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Effects of 946-nm thermal shift and broadening on Nd³⁺:YAG laser performance

Seyed Ebrahim Pourmand, Ghasem Rezaei

2015 (12): 124206-124206. doi: 10.1088/1674-1056/24/12/124206

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Spectroscopic properties of flashlamp pumped Nd³⁺:YAG laser are studied as a function of temperature in a range from-30 ℃ to 60 ℃. The spectral width and shift of quasi three-level 946.0-nm inter-Stark emission within the respective intermanifold transitions of ⁴F_3/2→⁴I_9/2 are investigated. The 946.0-nm line shifts toward the shorter wavelength and broadens. In addition, the threshold power and slope efficiency of the 946.0-nm laser line are quantified with temperature. The lower the temperature, the lower the threshold power is and the higher the slope efficiency of the 946.0-nm laser line is, thus the higher the laser output is. This phenomenon is attributed to the ion-phonon interaction and the thermal population in the ground state.

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Photoluminescence characteristics of ZnTe bulk crystal and ZnTe epilayer grown on GaAs substrate by MOVPE

吕海燕, 牟奇, 张磊, 吕元杰, 冀子武, 冯志红, 徐现刚, 郭其新

2015 (12): 124207-124207. doi: 10.1088/1674-1056/24/12/124207

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Excitation power and temperature-dependent photoluminescence (PL) spectra of the ZnTe epilayer grown on (100) GaAs substrate and ZnTe bulk crystal are investigated. The measurement results show that both the structures are of good structural quality due to their sharp bound excitonic emissions and absence of the deep level structural defect-related emissions. Furthermore, in contrast to the ZnTe bulk crystal, although excitonic emissions for the ZnTe epilayer are somewhat weak, perhaps due to As atoms diffusing from the GaAs substrate into the ZnTe epilayer and/or because of the strain-induced degradation of the crystalline quality of the ZnTe epilayer, neither the donor-acceptor pair (DAP) nor conduction band-acceptor (e-A) emissions are observed in the ZnTe epilayer. This indicates that by further optimizing the growth process it is possible to obtain a high-crystalline quality ZnTe heteroepitaxial layer that is comparable to the ZnTe bulk crystal.

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Tunable negative-index photonic crystals using colloidal magnetic fluids

耿滔, 王新, 王岩, 董祥美

2015 (12): 124208-124208. doi: 10.1088/1674-1056/24/12/124208

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The model of using colloidal magnetic fluid to build tunable negative-index photonic crystal is established. The effective permittivity ε_e and permeability μ_e of the two-dimensional photonic crystal are investigated in detail. For transverse magnetic polarization, both ε _e and μ_e exhibit a Lorentz-type anomalous dispersion, leading to a region where ε_e and μ_e are simultaneously negative. Then, considering a practical case, in which the thickness of photonic crystal is finite, the band structures for odd modes are calculated by the plane wave expansion method and the finite-difference time-domain method. The results suggest that reducing the external magnetic field strength or slab thickness will weaken the periodic modulation strength of the photonic crystal. Simulation results prove that the negative-index can be tuned by varying the external magnetic field strength or the slab thickness. The work presented in this paper gives a guideline for realizing the flat photonic crystal lens with tunable properties at optical frequencies, which may have potential applications in tunable near-field imaging systems.

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Strictly non-blocking 4× 4 silicon electro-optic switch matrix

周培基, 邢界江, 李显尧, 李智勇, 余金中, 俞育德

2015 (12): 124209-124209. doi: 10.1088/1674-1056/24/12/124209

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The first path-independent insertion-loss (PILOSS) strictly non-blocking 4× 4 silicon electro-optic switch matrix is reported. The footprint of this switch matrix is only 4.6 mm × 1.0 mm. Using single-arm modulation, the crosstalk measured in this test is-13 dB～-27 dB. And a maximum crosstalk deterioration of 6dB caused by two-path interference is also found.

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Acoustic radiation from the submerged circular cylindrical shell treated with active constrained layer damping

袁丽芸, 向宇, 陆静, 蒋红华

2015 (12): 124301-124301. doi: 10.1088/1674-1056/24/12/124301

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Based on the transfer matrix method of exploring the circular cylindrical shell treated with active constrained layer damping (i.e., ACLD), combined with the analytical solution of the Helmholtz equation for a point source, a multi-point multipole virtual source simulation method is for the first time proposed for solving the acoustic radiation problem of a submerged ACLD shell. This approach, wherein some virtual point sources are assumed to be evenly distributed on the axial line of the cylindrical shell, and the sound pressure could be written in the form of the sum of the wave functions series with the undetermined coefficients, is demonstrated to be accurate to achieve the radiation acoustic pressure of the pulsating and oscillating spheres respectively. Meanwhile, this approach is proved to be accurate to obtain the radiation acoustic pressure for a stiffened cylindrical shell. Then, the chosen number of the virtual distributed point sources and truncated number of the wave functions series are discussed to achieve the approximate radiation acoustic pressure of an ACLD cylindrical shell. Applying this method, different radiation acoustic pressures of a submerged ACLD cylindrical shell with different boundary conditions, different thickness values of viscoelastic and piezoelectric layer, different feedback gains for the piezoelectric layer and coverage of ACLD are discussed in detail. Results show that a thicker thickness and larger velocity gain for the piezoelectric layer and larger coverage of the ACLD layer can obtain a better damping effect for the whole structure in general. Whereas, laying a thicker viscoelastic layer is not always a better treatment to achieve a better acoustic characteristic.

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Theoretical analysis of transcranial Hall-effect stimulation based on passive cable model

袁毅, 李小俚

2015 (12): 124302-124302. doi: 10.1088/1674-1056/24/12/124302

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Transcranial Hall-effect stimulation (THS) is a new stimulation method in which an ultrasonic wave in a static magnetic field generates an electric field in an area of interest such as in the brain to modulate neuronal activities. However, the biophysical basis of simulating the neurons remains unknown. To address this problem, we perform a theoretical analysis based on a passive cable model to investigate the THS mechanism of neurons. Nerve tissues are conductive; an ultrasonic wave can move ions embedded in the tissue in a static magnetic field to generate an electric field (due to Lorentz force). In this study, a simulation model for an ultrasonically induced electric field in a static magnetic field is derived. Then, based on the passive cable model, the analytical solution for the voltage distribution in a nerve tissue is determined. The simulation results showthat THS can generate a voltage to stimulate neurons. Because the THS method possesses a higher spatial resolution and a deeper penetration depth, it shows promise as a tool for treating or rehabilitating neuropsychiatric disorders.

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Application of Arnoldi method to boundary layer instability

张永明, 罗纪生

2015 (12): 124701-124701. doi: 10.1088/1674-1056/24/12/124701

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The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory (LST) for two-dimensional (2D) disturbance on one-dimensional (1D) basic flow, comparison with LST for three-dimensional (3D) disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence (FST). Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien-Schlichting (TS) waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘angerous' in secondary instability.

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Study of hysteresis behavior in reactive sputtering of cylindrical magnetron plasma

H. Kakati, S. M. Borah

2015 (12): 125201-125201. doi: 10.1088/1674-1056/24/12/125201

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In order to make sufficient use of reactive cylindrical magnetron plasma for depositing compound thin films, it is necessary to characterize the hysteresis behavior of the discharge. Cylindrical magnetron plasmas with different targets namely titanium and aluminium are studied in an argon/oxygen and an argon/nitrogen gas environment respectively. The aluminium and titanium emission lines are observed at different flows of reactive gases. The emission intensity is found to decrease with the increase of the reactive gas flow rate. The hysteresis behavior of reactive cylindrical magnetron plasma is studied by determining the variation of discharge voltage with increasing and then reducing the flow rate of reactive gas, while keeping the discharge current constant at 100 mA. Distinct hysteresis is found to be formed for the aluminium target and reactive gas oxygen. For aluminium/nitrogen, titanium/oxygen and titanium/nitrogen, there is also an indication of the formation of hysteresis; however, the characteristics of variation from metallic to reactive mode are different in different cases. The hysteresis behaviors are different for aluminium and titanium targets with the oxygen and nitrogen reactive gases, signifying the difference in reactivity between them. The effects of the argon flow rate and magnetic field on the hysteresis are studied and explained.

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A computational modeling study on the helium atmospheric pressure plasma needle discharge

钱沐杨, 杨从影, 刘三秋, 王震东, 吕燕, 王德真

2015 (12): 125202-125202. doi: 10.1088/1674-1056/24/12/125202

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A two-dimensional coupled model of neutral gas flow and plasma dynamics is employed to investigate the streamer dynamics in a helium plasma needle at atmospheric pressure. A parametric study of the streamer propagation as a function of needle tip curvature radius and helium gas flow rate is presented. The key chemical reactions at the He/air mixing layer which drive the streamer propagation are the direct ionization via collision with electrons, the Penning effect being not so crucial. With increasing the gas flow rate from 0.2 standard liter per minute (SLM) to 0.8 SLM, however, the emissions resulting from reactive oxygen and nitrogen species change from a solid circle to a hollow profile and the average streamer propagation velocity decreases. Air impurities (backdiffusion from ambient air) in the helium jet result in a significant increase in the streamer propagation velocity. Besides, with decreasing the tip curvature radiusfrom 200 μ to 100 μ, the electron avalanche process around the near-tip region is more pronounced. However, the spatially resolved plasma parameters distributions (electron, helium metastables, ground state atomic oxygen, etc.) remain almost the same, except that around the near-tip region where their peak values are more than doubled.

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A two-dimensional model of He/O₂ atmospheric pressure plasma needle discharge

钱沐杨, 杨从影, 陈小昌, 刘三秋, 晏雯, 刘富成, 王德真

2015 (12): 125203-125203. doi: 10.1088/1674-1056/24/12/125203

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In this paper, a computational modeling study of stream propagation in the atmospheric-pressure helium plasma in ambient atmosphere (oxygen) is presented. A coupled fluid model between time-dependent plasma dynamics and steady state neutral gas flow is employed to provide a fundamental insight into the evolution of the streamers. The obtained simulation results showing that the sheath forms near the dielectric surface and shields the axial stream propagation. The stream front propagates with axial velocity in a range of 10⁴ m/s-10⁵ m/s. And, the increasing accumulated surface charge should be responsible for reducing the propagation velocity of the streamer front in the axial direction. Besides, when the gas flow rate is 1.1 standard liter per minute (SLM), we find that the concentration of oxygen drastically increases at a larger radial position near a treated surface. Therefore, Penning ionization by helium metastables and oxygen peaks at an off-axis position, corresponding to the ring-shaped emission profile in cylindrical coordinates. In this case, the simulated results show the ring-shaped ground atomic oxygen density profile near the treated surface (z=0.5 mm) at a large gas flow rate of 1.1 SLM, which is consistent with the observation in a similar experiment.

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Structural modeling of proteins by integrating small-angle x-ray scattering data

张泳辉, 彭俊辉, 张志勇

2015 (12): 126101-126101. doi: 10.1088/1674-1056/24/12/126101

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Elucidating the structure of large biomolecules such as multi-domain proteins or protein complexes is challenging due to their high flexibility in solution. Recently, an “integrative structural biology” approach has been proposed, which aims to determine the protein structure and characterize protein flexibility by combining complementary high-and low-resolution experimental data using computer simulations. Small-angle x-ray scattering (SAXS) is an efficient technique that can yield low-resolution structural information, including protein size and shape. Here, we review computational methods that integrate SAXS with other experimental datasets for structural modeling. Finally, we provide a case study of determination of the structure of a protein complex formed between the tandem SH3 domains in c-Cb1-associated protein and the proline-rich loop in human vinculin.

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Relationship between Voronoi entropy and the viscosity of Zr₃₆Cu₆₄ alloy melt based on molecular dynamics

高伟, 冯士东, 张世良, 戚力, 刘日平

2015 (12): 126102-126102. doi: 10.1088/1674-1056/24/12/126102

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Molecular dynamics simulation is used to investigate the relationship between Voronoi entropy and viscosity for rapid solidification processing of Zr₃₆Cu₆₄ binary alloy melt. The simulation results at different temperatures, cooling rates, and pressures, show that Voronoi entropy is able to accurately describe the relationship of the transition between the cluster structure and the viscosity of Zr₃₆Cu₆₄ binary alloy melt through Voronoi polyhedron analysis. That is, the higher the degree of order of the microstructure, the lower the Voronoi entropy is and the higher the viscosity is. The simulation provides an important reference for studying metallic glass with high glass-forming ability.

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Krypton ion irradiation-induced amorphization and nano-crystal formation in pyrochlore Lu₂Ti₂O₇ at room temperature

谢秋荣, 张建, 尹东明, 郭奇勋, 李宁

2015 (12): 126103-126103. doi: 10.1088/1674-1056/24/12/126103

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Polycrystalline pyrochlore Lu₂Ti₂O₇ pellets are irradiated with 600-keV Kr³⁺ ions up to a fluence of 1.45× 10¹⁶ Kr³⁺/cm². Irradiation induced structural modifications are examined by using grazing incidence x-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (TEM). The GIXRD reveals that amorphous fraction increases with the increase of fluences up to 2× 10¹⁵ Kr³⁺/cm², and the results are explained with a direct-impact model. However, when the irradiation fluence is higher than 2× 10¹⁵ Kr³⁺/cm², the amorphous fraction reaches a saturation of ～ 80%. Further TEM observations imply that nano-crystal is formed with a diameter of ～ 10 nm within the irradiation layer at a fluence of 4×10¹⁵ Kr³⁺/cm². No full amorphization is achieved even at the highest fluence of 1.45× 10¹⁶ Kr³⁺/cm² (～ 36 displacement per atom). The high irradiation resistance of pyrochlore Lu₂Ti₂O₇ at higher fluence is explained on the basis of enhanced radiation tolerance of nano-crystal structure.

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Effect of combined platinum and electron on the temperature dependence of forward voltage in fast recovery diode

贾云鹏, 赵豹, 杨霏, 吴郁, 周璇, 李哲, 谭健

2015 (12): 126104-126104. doi: 10.1088/1674-1056/24/12/126104

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The temperature dependences of forward voltage drop (V_F) of the fast recovery diodes (FRDs) are remarkably influenced by different lifetime controlled treatments. In this paper the results of an experimental study are presented, which are the lifetime controls of platinum treatment, electron irradiation treatment, and the combined treatment of the above ones. Based on deep level transient spectroscopy (DLTS) measurements, a new level E6 (E_C-0.376 eV) is found in the combined lifetime treated (CLT) sample, which is different from the levels of the individual platinum and electron irradiation ones. Comparing the tested V_F results of CLT samples with the others, the level E6 is responsible for the degradation of temperature dependence of the forward voltage drop in the FRD.

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Electronic structures and magnetisms of the Co₂TiSb_1-xSn_x (x=0, 0.25, 0.5) Heusler alloys: A theoretical study of the shape-memory behavior

王立英, 代学芳, 王啸天, 林婷婷, 陈磊, 刘然, 崔玉亭, 刘国栋

2015 (12): 126201-126201. doi: 10.1088/1674-1056/24/12/126201

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The total energy, electronic structures, and magnetisms of the AlCu₂Mn-type Co₂TiSb_1-xSn_x (x=0, 0.25, 0.5) with the different lattice parameter ratios of c/a are studied by using the first-principles calculations. It is found that the phase transformation from the cubic to the tetragonal structure lowers the total energy, indicating that the martensitic phase is more stable and that a phase transition from austenite to martensite may happen at a lower temperature. Thus, a ferromagnetic shape memory effect can be expected to occur in these alloys. The AlCu₂Mn-type Co₂TiSb_1-xSn_x (x=0, 0.25, 0.5) alloys are weak ferrimagnets in the austenitic phase and martensitic phase.

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Raman phonons in multiferroic FeVO₄ crystals

张安民, 刘凯, 籍建葶, 何长振, 田勇, 金峰, 张清明

2015 (12): 126301-126301. doi: 10.1088/1674-1056/24/12/126301

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Multiferroic materials are promising candidates for next-generation multi-functional devices, because of the coexistence of multi-orders and the coupling between the orders. FeVO₄ has been confirmed to be a multiferroic compound, since it exhibits both ferroelectricity and antiferromagnetic ordering at low temperatures. In this paper, we have performed careful Raman scattering measurements on high-quality FeVO₄ single crystals. The compound has a very rich phonon structure due to its low crystal symmetry (P-1) and at least 47 Raman-active phonon modes have been resolved in the low and hightemperature spectra. Most of the observed modes are well assigned with aid of first-principles calculations and symmetry analysis. The present study provides an experimental basis for exploring spin-lattice coupling and the mechanism of multiferroicity in FeVO₄

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Material properties dependent on the thermal transport in a cylindrical nanowire

张勇, 谢忠祥, 邓元祥, 喻霞, 李科敏

2015 (12): 126302-126302. doi: 10.1088/1674-1056/24/12/126302

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Using the elastic wave continuum model, we investigate the effect of material properties on ballistic thermal transport in a cylindrical nanowire. A comparative analysis for the convexity-shaped and concavity-shaped structure is made. It is found that in the convexity-shaped structure, the material with higher wave velocity in the convexity region can increase the thermal conductance at the lower temperature range; the thermal conductance of the nanowire with higher wave velocity in the convexity region is lower than that of the nanowire with lower wave velocity in the convexity region at the higher temperature range. However, in the concavity-shaped structure, the material properties of the concavity region have less effect on the thermal conductance at the lower temperature range; the material with higher wave velocity in the concavity region can reduce the thermal conductance at the higher temperature range. A brief analysis of these results is given.

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Effects of temperature gradient on the interface microstructure and diffusion of diffusion couples: Phase-field simulation

李永胜, 吴兴超, 刘苇, 侯志远, 梅浩杰

2015 (12): 126401-126401. doi: 10.1088/1674-1056/24/12/126401

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The temporal interface microstructures and diffusions in the diffusion couples with the mutual interactions of the temperature gradient, concentration difference and initial aging time of the alloys are studied by phase-field simulation, and the diffusion couples are produced by the initial aged spinodal alloys with different compositions. Temporal composition evolution and volume fraction of the separated phase indicate the element diffusion direction through the interface under the temperature gradient. The increased temperature gradient induces a wide single-phase region on two sides of the interface. The uphill diffusion proceeds through the interface, no matter whether the diffusion direction is up or down with respect to the temperature gradient. For an alloy with short initial aging time, phase transformation accompanying the interdiffusion results in the straight interface with the single-phase regions on both sides. Compared with the temperature gradient, composition difference of diffusion couple and initial aging time of the alloy show greater effects on diffusion and interface microstructure.

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Saturated sodium chloride solution under an external static electric field: A molecular dynamics study

任淦, 王延颋

2015 (12): 126402-126402. doi: 10.1088/1674-1056/24/12/126402

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The behavior of saturated aqueous NaCl solutions under a constant external electric field (E) was studied by molecular dynamics (MD) simulation. Our dynamic MD simulations indicated that the irreversible nucleation process towards crystallization is accelerated by a moderate E but retarded or even prohibited under a stronger E, which can be understood by the competition between self-diffusion and drift motion. The former increases with E, thereby accelerating the nucleation process, whereas the latter pulls oppositely charged ions apart under a stronger E, thereby decelerating nucleation. Additionally, our steady-state MD simulations indicated that a first-order phase transition occurs in saturated solutions at a certain threshold E_c. The magnitude of E_c increases with concentration because larger clusters form more easily when the solution is more concentrated and require a stronger E to dissociate.

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Multiple patterns of diblock copolymer confined in irregular geometries with soft surface

李颖, 孙敏娜, 张进军, 潘俊星, 郭宇琦, 王宝凤, 武海顺

2015 (12): 126403-126403. doi: 10.1088/1674-1056/24/12/126403

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The different confinement shapes can induce the formation of various interesting and novel morphologies, which might inspire potential applications of materials. In this paper, we study the directed self-assembly of diblock copolymer confined in irregular geometries with a soft surface by using self-consistent field theory. Two types of confinement geometries are considered, namely, one is the concave pore with one groove and the other is the concave pore with two grooves. We obtain more novel and different structures which could not be produced in other two-dimensional (2D) confinements. Comparing these new structures with those obtained in regular square confinement, we find that the range of ordered lamellae is enlarged and the range of disordered structure is narrowed down under the concave pore confinement. We also compare the different structures obtained under the two types of confinement geometries, the results show that the effect of confinement would increase, which might induce the diblock copolymer to form novel structures. We construct the phase diagram as a function of the fraction of B block and the ratio of h/L of the groove. The simulation reveals that the wetting effect of brushes and the shape of confinement geometries play important roles in determining the morphologies of the system. Our results improve the applications in the directed self-assembly of diblock copolymer for fabricating the irregular structures.

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Interfacial and electrical characteristics of a HfO₂/n-InAlAs MOS-capacitor with different dielectric thicknesses

关赫, 吕红亮, 郭辉, 张义门, 张玉明, 武利翻

2015 (12): 126701-126701. doi: 10.1088/1674-1056/24/12/126701

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A HfO₂/n-InAlAs MOS-capacitor has the advantage of reducing the serious gate leakage current when it is adopted in InAs/AlSb HEMT instead of the conventional Schottky-gate. In this paper, three kinds of HfO₂/n-InAlAs MOS-capacitor samples with different HfO₂ thickness values of 6, 8, and 10 nm are fabricated and used to investigate the interfacial and electrical characteristics. As the thickness is increased, the equivalent dielectric constant ε _ox of HfO₂ layer is enhanced and the InAlAs-HfO₂ interface trap density D_it is reduced, leading to an effective reduction of the leakage current. It is found that the HfO₂ thickness of 10 nm is a suitable value to satisfy the demands of most applications of a HfO₂/n-InAlAs MOS-capacitor, with a sufficiently low leakage current compromised with the threshold voltage.

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Electrical properties and microstructural characterization of Ni/Ta contacts to n-type 6H-SiC

周天宇, 刘学超, 黄维, 卓世异, 郑燕青, 施尔畏

2015 (12): 126801-126801. doi: 10.1088/1674-1056/24/12/126801

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A Ni/Ta bilayer is deposited on n-type 6H-SiC and then annealed at different temperatures to form an ohmic contact. The electrical properties are characterized by I-V curve measurement and the specific contact resistance is extracted by the transmission line method. The phase formation and microstructure of the Ni/Ta bilayer are studied after thermal annealing. The crystalline and microstructure properties are analyzed by using glance incident x-ray diffraction (GIXRD), Raman spectroscopy, and transmission electron microscopy. It is found that the transformation from the Schottky to the Ohmic occurs at 1050 ℃ and the GIXRD results show a distinct phase change from Ta₂C to TaC at this temperature. A specific contact resistance of 6.5× 10^-5Ω · cm² is obtained for sample Ni(80 nm)/Ta(20 nm)/6H-SiC after being annealed at 1050 ℃. The formation of the TaC phase is regarded as the main reason for the excellent Ohmic properties of the Ni/Ta contacts to 6H-SiC. Raman and TEM data reveal that the graphite carbon is drastically consumed by the Ta element, which can improve the contact thermal stability. A schematic diagram is proposed to illustrate the microstructural changes of Ni/Ta/6H-SiC when annealed at different temperatures.

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First-principles calculation of the electronic structure, chemical bonding, and thermodynamic properties of β-US₂

李世长, 郑远蕾, 马生贵, 高涛, 敖冰云

2015 (12): 127101-127101. doi: 10.1088/1674-1056/24/12/127101

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The electronic structure, magnetic states, chemical bonding, and thermodynamic properties of β-US₂ are investigated by using first-principles calculation through the density functional theory (DFT)+U approach. The obtained band structure exhibits a direct band gap semiconductor at Γ point with a band gap of 0.9 eV for β-US₂, which is in good agreement with the recent experimental data. The charge-density differences, the Bader charge analysis, and the Born effective charges suggest that the U-S bonds of the β-US₂ have a mixture of covalent and ionic characters, but the ionic character is stronger than covalent character. The Raman-active, infrared-active, and silent modes at the Γ point are further assigned and discussed. The obtained optical-mode frequencies indicate that the three apparent LO-TO (longitudinal optical-transverse optical) splittings occur in B_1u, B_2u, and B_3u modes, respectively. Furthermore, the Helmholtz free energy ΔF, the specific heat ΔE, vibrational entropy SM, and constant volume C_V are studied over a range from 0 K～100 K. We expect that our work can provide some valuable information for further experimental investigation of the dielectric properties and the infrared reflectivity spectrum of uranium chalcogenide.

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Spin-valley quantum Hall phases in graphene

田宏玉

2015 (12): 127301-127301. doi: 10.1088/1674-1056/24/12/127301

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We theoretically investigate possible quantum Hall phases and corresponding edge states in graphene by taking a strong magnetic field, Zeeman splitting M, and sublattice potential Δ into account but without spin-orbit interaction. It was found that for the undoped graphene either a quantum valley Hall phase or a quantum spin Hall phase emerges in the system, depending on relative magnitudes of M and Δ . When the Fermi energy deviates from the Dirac point, the quantum spin-valley Hall phase appears and its characteristic edge state is contributed only by one spin and one valley species. The metallic boundary states bridging different quantum Hall phases possess a half-integer quantized conductance, like e²/2h or 3e²/2h. The possibility of tuning different quantum Hall states with M and Δ suggests possible graphene-based spintronics and valleytronics applications.

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Spoof surface plasmons resonance effect and tunable electric response of improved metamaterial in the terahertz regime

王玥, 张丽颖, 梅金硕, 张文超, 童一静

2015 (12): 127302-127302. doi: 10.1088/1674-1056/24/12/127302

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We propose an improved design and numerical study of an optimized tunable plasmonics artificial material resonator in the terahertz regime. We demonstrate that tunability can be realized with a transmission intensity as much as ～ 61% in the lower frequency resonance, which is implemented through the effect of photoconductive switching under photoexcitation. In the higher frequency resonance, we show that spoof surface plasmons along the interface of metal/dielectric provide new types of electromagnetic resonances. Our approach opens up possibilities for the interface of metamaterial and plasmonics to be applied to optically tunable THz switching.

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Shape effects on the ground-state energy of a three-electronquantum dot

Z. D. Vatansever, S. Sakiroglu, İ. Sokmen

2015 (12): 127303-127303. doi: 10.1088/1674-1056/24/12/127303

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In this work we will theoretically study the ground-state electronic structure of three-electron polygonal quantum dots by means of the configuration interaction method. Transition from a weakly correlated regime to a strongly correlated regime is investigated for quantum dots with hexagonal, square, and triangular geometries. Our numerical results reveal that the ground-state spin and the charge density distribution of the system are sensitive to the shape of the quantum dot.

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High-k gate dielectric GaAs MOS device with LaON as interlayer and NH₃-plasma surface pretreatment

刘超文, 徐静平, 刘璐, 卢汉汉

2015 (12): 127304-127304. doi: 10.1088/1674-1056/24/12/127304

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High-k gate dielectric HfTiON GaAs metal-oxide-semiconductor (MOS) capacitors with LaON as interfacial passivation layer (IPL) and NH₃-or N₂-plasma surface pretreatment are fabricated, and their interfacial and electrical properties are investigated and compared with their counterparts that have neither LaON IPL nor surface treatment. It is found that good interface quality and excellent electrical properties can be achieved for a NH₃-plasma pretreated GaAs MOS device with a stacked gate dielectric of HfTiON/LaON. These improvements should be ascribed to the fact that the NH₃-plasma can provide H atoms and NH radicals that can effectively remove defective Ga/As oxides. In addition, LaON IPL can further block oxygen atoms from being in-diffused, and Ga and As atoms from being out-diffused from the substrate to the high-k dielectric. This greatly suppresses the formation of Ga/As native oxides and gives rise to an excellent high-k/GaAs interface.

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Influence of ultra-thin TiN thickness (1.4 nm and 2.4 nm) on positive bias temperature instability (PBTI) of high-k/metal gate nMOSFETs with gate-last process

祁路伟, 杨红, 任尚清, 徐烨峰, 罗维春, 徐昊, 王艳蓉, 唐波, 王文武, 闫江, 朱慧珑, 赵超, 陈大鹏, 叶甜春

2015 (12): 127305-127305. doi: 10.1088/1674-1056/24/12/127305

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The positive bias temperature instability (PBTI) degradations of high-k/metal gate (HK/MG) nMOSFETs with thin TiN capping layers (1.4 nm and 2.4 nm) are systemically investigated. In this paper, the trap energy distribution in gate stack during PBTI stress is extracted by using ramped recovery stress, and the temperature dependences of PBTI (90 ℃, 125 ℃, 160 ℃) are studied and activation energy (E_a) values (0.13 eV and 0.15 eV) are extracted. Although the equivalent oxide thickness (EOT) values of two TiN thickness values are almost similar (0.85 nm and 0.87 nm), the 2.4-nm TiN one (thicker TiN capping layer) shows better PBTI reliability (13.41% at 0.9 V, 90 ℃, 1000 s). This is due to the better interfacial layer/high-k (IL/HK) interface, and HK bulk states exhibited through extracting activation energy and trap energy distribution in the high-k layer.

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Investigation of trap states in Al₂O₃ InAlN/GaN metal-oxide-semiconductor high-electron-mobility transistors

张鹏, 赵胜雷, 薛军帅, 祝杰杰, 马晓华, 张进成, 郝跃

2015 (12): 127306-127306. doi: 10.1088/1674-1056/24/12/127306

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In this paper the trapping effects in Al₂O₃/In_0.17Al_0.83N/GaN MOS-HEMT (here, HEMT stands for high electron mobility transistor) are investigated by frequency-dependent capacitance and conductance analysis. The trap states are found at both the Al₂O₃/InAlN and InAlN/GaN interface. Trap states in InAlN/GaN heterostructure are determined to have mixed de-trapping mechanisms, emission, and tunneling. Part of the electrons captured in the trap states are likely to tunnel into the two-dimensional electron gas (2DEG) channel under serious band bending and stronger electric field peak caused by high Al content in the InAlN barrier, which explains the opposite voltage dependence of time constant and relation between the time constant and energy of the trap states.

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Structures and electrical properties of pure and vacancy-included ZnO NWs of different sizes

于晓霞, 周彦, 刘甲, 金海波, 房晓勇, 曹茂盛

2015 (12): 127307-127307. doi: 10.1088/1674-1056/24/12/127307

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The structures and electronic properties of ZnO nanowires (NWs) of different diameters are investigated by employing the first-principles density functional theory. The results indicate that the oxygen vacancy (V_O) exerts a more evident influence on the band gap of the ZnO NWs. However, the effect will be weakened with the increase of the diameter. In addition, the energy band shifts downward due to the existence of V_O and the offset decreases with the reduction of the V_O concentration. As the concentration of surface Zn atoms decreases, the conduction band shifts downward, while 2p electrons are lost in the oxygen vacancy, resulting in the split of valence band and the formation of an impurity level. Our findings agree well with the previous observations and will be of great importance for theoretical research based on ZnO NWs.

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Colloidally deposited nanoparticle wires for biophysical detection

Sophie C. Shen, 刘文韬, 刁佳杰

2015 (12): 127308-127308. doi: 10.1088/1674-1056/24/12/127308

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Among the techniques developed to prepare nanoparticle wires for multiple applications, the colloidal deposition method at interface has been regarded as cost-efficient and eco-friendly, and hence has attracted an increasing amount of research attention. In this report, the recent developments in preparing nanoparticle wires and integrated nanoparticle wire arrays using this technique have been reviewed. Furthermore, we have also discussed the application of these nanoparticle structures in detecting chemical and biological molecules.

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Multi-step shot noise spectrum induced by a local large spin

牛鹏斌, 石云龙, 孙祝, 聂一行

2015 (12): 127309-127309. doi: 10.1088/1674-1056/24/12/127309

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We use non-equilibrium Green's function method to analyze the shot noise spectrum of artificial single molecular magnets (ASMM) model in the strong spin-orbit coupling limit in sequential tunneling regime, mainly focusing on the effects of local large spin. In the linear response regime, the shot noise shows 2S+1 peaks and is strongly spin-dependent. In the nonlinear response regime, one can observe 2S+1 steps in shot noise and Fano factor. In these steps one can see the significant enhancement effect due to the spin-dependent multi-channel process of local large spin, which reduces electron correlations.

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First-principles simulation of Raman spectra and structural properties of quartz up to 5 GPa

刘雷, 吕超甲, 庄春强, 易丽, 刘红, 杜建国

2015 (12): 127401-127401. doi: 10.1088/1674-1056/24/12/127401

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The crystal structure and Raman spectra of quartz are calculated by using first-principles method in a pressure range from 0 to 5 GPa. The results show that the lattice constants (a, c, and V) decrease with increasing pressure and the a-axis is more compressible than the c axis. The Si-O bond distance decreases with increasing pressure, which is in contrast to experimental results reported by Hazen et al. [Hazen R M, Finger L W, Hemley R J and Mao H K 1989 Solid State Communications 725 507-511], and Glinnemann et al. [Glinnemann J, King H E Jr, Schulz H, Hahn T, La Placa S J and Dacol F 1992 Z. Kristallogr. 198 177-212]. The most striking changes are of inter-tetrahedral O-O distances and Si-O-Si angles. The volume of the SiO₄^4- tetrahedron decreased by 0.9% (from 0 to 5 GPa), which suggests that it is relatively rigid. Vibrational models of the quartz modes are identified by visualizing the associated atomic motions. Raman vibrations are mainly controlled by the deformation of the SiO₄^4- tetrahedron and the changes in the Si-O-Si bonds. Vibrational directions and intensities of atoms in all Raman modes just show little deviations when pressure increases from 0 to 5 GPa. The pressure derivatives (dv_i/dP) of the 12 Raman frequencies are obtained at 0 GPa-5 GPa. The calculated results show that first-principles methods can well describe the high-pressure structural properties and Raman spectra of quartz. The combination of first-principles simulations of the Raman frequencies of minerals and Raman spectroscopy experiments is a useful tool for exploring the stress conditions within the Earth.

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Study of Nb/Nb_xSi_1-x/Nb Josephson junction arrays

曹文会, 李劲劲, 钟源, 贺青

2015 (12): 127402-127402. doi: 10.1088/1674-1056/24/12/127402

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Owing to the adjustable characteristics and superior etching properties of co-sputtered Nb_xSi_1-x film, we are trying to fabricate Nb/Nb_xSi_1-x/Nb Josephson junction arrays for voltage standard. It is important to find the suitable Nb_xSi_1-x barrier for the junctions. Josephson junctions with different barrier content are fabricated. Current-voltage characteristics are measured and analyzed. It is demonstrated in this paper that critical current can be adjusted by using different barrier content and thickness. Shapiro steps of five hundred junctions in series are observed.

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Observation of spin glass transition in spinel LiCoMnO₄

陈红, 杨旭, 张培松, 梁磊, 洪源泽, 魏英进, 陈岗, 杜菲, 王春忠

2015 (12): 127501-127501. doi: 10.1088/1674-1056/24/12/127501

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Spinel LiCoMnO₄ is prepared by solid-state reaction and its magnetic properties are comprehensively studied by direct current (DC) and alternating current (AC) susceptibilities, isothermal remanent magnetizations, and magnetic hysteresis. Fitting to the Curie-Weiss law by using high-temperature zero-field-cooled susceptibility confirms a low-spin state of Co³⁺ with S=0. Both the fitting parameters first increase and then tend to be saturated at high magnetic fields through using isothermal remanent magnetizations, which suggests a spin glass transition at low temperature. AC susceptibility study also supports this conclusion since the frequency dependence of peak position and intensity follows the tendency of a spin glass transition. The origin of the spin-glass transition in LiCoMnO₄ might be attributed to a spatial segregation between non-magnetic Co³⁺ regions and spin glass ordered regions of Mn⁴⁺ ions.

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Structure, morphology, and magnetic properties of high-performance NiCuZn ferrite

何学敏, 颜士明, 李志文, 张星, 宋雪银, 乔文, 钟伟, 都有为

2015 (12): 127502-127502. doi: 10.1088/1674-1056/24/12/127502

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High-performance submicron-scaled NiCuZn ferrites are prepared by the solid-state reaction method through using CuO as additive. In the synthesis process, a mixture of superfine powder is sintered at 900 ℃ for 3 h, and the obtained product is NiZn-ferrite with spinel structure. We observe that the particle size increases with raising the sintering temperature. The NiCuZn ferrite with relatively uniform size and granular shape has the best performance: its coercivity is 14 Oe (1 Oe=79.5775 A·m^-1) and saturation magnetization is 48 emu/g. We also study the effects of particle size, magnetocrystalline anisotropy, and microstructure on coercivity. The method presented here is convenient and economical for producing the high-permeability ferrite powders.

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Fabrication and magnetic properties of 4SC(NH₂)₂-Ni_0.97Cu_0.03Cl₂ single crystals

陈丽敏, 郭颖, 刘旭光, 解其云, 陶志阔, 谌静, 周玲玲, 刘春生

2015 (12): 127503-127503. doi: 10.1088/1674-1056/24/12/127503

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Single crystals of 4SC(NH₂)₂-Ni_1-xCu_xCl₂ (x=0.03) (Cu-DTN) containing spin S=1/2 Cu²⁺ and S=1 Ni²⁺ cations are synthesized by slow evaporation methods. Structural characterization demonstrates that the Cu-DTN is of a tetrahedral structure with lattice parameter c being 9.0995 Å, which is 1.32% expansion compared with that of parent material DTN due to the larger radius of the Cu ion. Direct current (DC) susceptibility measurements show that both the antiferromagnetic exchange interaction at low temperature and the large anisotropy of susceptibilities are suppressed after doping the Cu ion, which could be related to the structural distortion and the increase of the super-exchange paths in Cu-DTN.

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Magnetocaloric effects in RTX intermetallic compounds (R=Gd-Tm, T=Fe-Cu and Pd, X=Al and Si)

张虎, 沈保根

2015 (12): 127504-127504. doi: 10.1088/1674-1056/24/12/127504

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The magnetocaloric effect (MCE) of RTSi and RTAl systems with R=Gd-Tm, T=Fe-Cu and Pd, which have been widely investigated in recent years, is reviewed. It is found that these RTX compounds exhibit various crystal structures and magnetic properties, which then result in different MCE. Large MCE has been observed not only in the typical ferromagnetic materials but also in the antiferromagnetic materials. The magnetic properties have been studied in detail to discuss the physical mechanism of large MCE in RTX compounds. Particularly, some RTX compounds such as ErFeSi, HoCuSi, HoCuAl exhibit large reversible MCE under low magnetic field change, which suggests that these compounds could be promising materials for magnetic refrigeration in a low temperature range.

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Novel magnetic vortex nanorings/nanodiscs: Synthesis and theranostic applications

刘晓丽, 杨勇, 吴建鹏, 张艺凡, 樊海明, 丁军

2015 (12): 127505-127505. doi: 10.1088/1674-1056/24/12/127505

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Recent discoveries in the synthesis and applications of magnetic vortex nanorings/nanodiscs in theranostic applications are reviewed. First, the principles of nanomagnetism and magnetic vortex are introduced. Second, methods for producing magnetic vortex nanorings/nanodiscs are presented. Finally, theranostic applications of magnetic vortex nanorings/nanodiscs are addressed.

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Self-assembled superparamagnetic nanoparticles as MRI contrast agents–A review

苏红莹, 吴昌强, 李丹阳, 艾华

2015 (12): 127506-127506. doi: 10.1088/1674-1056/24/12/127506

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Recent progress of the preparation and applications of superparamagnetic iron oxide (SPIO) clusters as magnetic resonance imaging (MRI) probes is reviewed with regard to their applications in labeling and tracking cells in vivo, in diagnosis of cardiovascular diseases and tumors, and in drug delivery systems. Magnetic nanoparticles (NPs), especially SPIO nanoparticles, have long been used as MRI contrast agents and as an advantageous nanoplatform for drug delivery, taking advantage of their unique magnetic properties and ability to function at the molecular and cellular levels. Due to advances in nanotechnology, various means to control SPIO NPs' size, composition, magnetization and relaxivity have been developed, as well as ways to usefully modify their surface. Recently, self-assembly of SPIO NP clusters in particulate carriers–such as polymeric micelles, vesicles, liposomes, and layer-by-layer (LbL) capsules–have been widely studied for application as ultrasensitive MRI probes, owing to their remarkably high spin-spin (T₂) relaxivity and convenience for further functionalization.

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Al-doping-induced magnetocapacitance in the multiferroic AgCrS₂

刘荣灯, 何伦华, 闫丽琴, 王志翠, 孙阳, 刘蕴韬, 陈东风, 张森, 赵永刚, 王芳卫

2015 (12): 127507-127507. doi: 10.1088/1674-1056/24/12/127507

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In this paper, multiferroics and magnetocapacitive effect of triangular-lattice antiferromagnet AgAl_0.02Cr_0.98S₂ are investigated by magnetic, ferroelectric, pyroelectric current and dielectric measurement. We find that it is a multiferroic material and the magnetocapacitive effect reaches a factor of up to 90 in an external field of 7 T. The results imply the further possibility of synthesizing the magnetocapacitive materials by modifying the frustrated spin structure in terms of a few B-site doping nonmagnetic ions.

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Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca₃CoNb₂O₉

代佳, 周萍, 王朋帅, 庞斐, Tim J. Munsie, Graeme M. Luke, 张金珊, 于伟强

2015 (12): 127508-127508. doi: 10.1088/1674-1056/24/12/127508

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We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca₃CoNb₂O₉, in which the effective spin of Co²⁺ is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant θ_CW～-55 K and the low Neel temperature T_N～ 1.45 K give a frustration factor f=|θ_CW/T_N|≈ 38, suggesting that Ca₃CoNb₂O₉ resides in strong frustration regime. Slightly below T_N, deviation between the susceptibility data under zero-field cooling (ZFC) and field cooling (FC) is observed. A new magnetic state with 1/3 of the saturate magnetization M_s is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca₃CoNb₂O₉ is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.

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Multifold polar states in Zn-doped Sr_0.9Ba_0.1TiO₃ ceramics

郭艳艳, 郭云均, 魏通, 刘俊明

2015 (12): 127701-127701. doi: 10.1088/1674-1056/24/12/127701

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We investigate the effect of Zn doping on the dielectricity and ferroelectricity of a series of polycrystalline Sr_0.9-xZn_xBa_0.1TiO₃ (0.0% ≤ x ≤ 5.0%) ceramics. It is surprisingly observed that the Zn doping will produce the multifold polar states, i.e., the Zn-doped ceramic will convert a reduced polar state into an enhanced polar state, and eventually into a stabilized polar state with increasing the doping level x. It is revealed that in the background of quantum fluctuations, the competition between the Zn-doping-induced lattice contraction and the Ba-doping-induced lattice expansion is responsible for both the reduced polar state and the enhanced polar state coming into being. Also, the addition of the antiferrodistortive effect, which is the antipolar interaction originating from the opposite tilted-TiO₆ octahedra rotation, represents the core physics behind the stabilized polar state.

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First-principles study of the relaxor ferroelectricity of Ba(Zr, Ti)O₃

杨丽娟, 武灵芝, 董帅

2015 (12): 127702-127702. doi: 10.1088/1674-1056/24/12/127702

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Ba(Zr, Ti)O₃ is a lead-free relaxor ferroelectric. Using the first-principles method, the ferroelectric dipole moments for pure BaTiO₃ and Ba(Zr, Ti)O₃ supercells are studied. All possible ion configurations of BaZr_0.5Ti_0.5O₃ and BaZr_0.25Ti_0.75O₃ are constructed in a 2× 2× 2 supercell. For the half-substituted case, divergence of ferroelectric properties is found from these structures, which greatly depends on the arrangements of Ti and Zr ions. Thus our results provide a reasonable explanation to the relaxor behavior of Ba(Zr, Ti)O₃. In addition, a model based on the thermal statistics gives the averaged polarization for Ba(Zr, Ti)O₃, which depends on the temperature of synthesis. Our result is helpful to understand and tune the relaxor ferroelectricity of lead-free Ba(Zr, Ti)O₃.

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Comparative research on the optical properties of three surface patterning ZnO ordered arrays

侯凯, 朱亚彬, 乔璐

2015 (12): 127703-127703. doi: 10.1088/1674-1056/24/12/127703

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We fabricate three surface patterning zinc oxide (ZnO) ordered arrays on glass substrates by using nanosphere lithography technique and dc magnetron sputtering technique. The crescent, tube and honeycomb surface morphologies of the samples are observed by scanning electron microscopy. The transmittance, fluorescence and confocal Raman spectra of the sample are measured. Obviously, when the angle between the plume and the substrate is 90°, the honeycomb arrays have a better transmission. Additionally, the PL intensity of honeycomb arrays is superior. With the increasing of the angle between the substrate and the sputtering plume, the fluorescence peak shows blue shift. The Raman peak located at 438 cm^-1 belongs to ZnO E₂ (high) mode, which corresponds to the characteristic band of the hexagonal wurtzite phase. The tube arrays have the best Raman spectrum intensity.

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Ultrahigh frequency tunability of aperture-coupled microstrip antenna via electric-field tunable BST

杜洪磊, 薛倩, 高小洋, 姚凤蕊, 卢世阳, 汪业龙, 刘春恒, 张永成, 吕跃广, 李山东

2015 (12): 127704-127704. doi: 10.1088/1674-1056/24/12/127704

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A composite ceramic with nominal composition of 45.0 wt%(Ba_0.5Sr_0.5)TiO₃-55.0 wt%MgO (acronym is BST-MgO) is sintered for fabricating a frequency reconfigurable aperture-coupled microstrip antenna. The calcined BST-MgO composite ceramic exhibits good microwave dielectric properties at X-band with appropriate dielectric constant ε_r around 85, lower dielectric loss \tanδ about 0.01, and higher permittivity tunability 14.8% at 8.33 kV/cm. An ultrahigh E-field tunability of working frequency up to 11.0% (i.e., from 9.1 GHz to 10.1 GHz with a large frequency shift of 1000 MHz) at a DC bias field from 0 to 8.33 kV/cm and a considerably large center gain over 7.5 dB are obtained in the designed frequency reconfigurable microstrip antenna. These results demonstrate that BST materials are promising for the frequency reconfigurable antenna.

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Variation of efficiency droop with quantum well thickness in InGaN/GaN green light-emitting diode

刘炜, 赵德刚, 江德生, 陈平, 刘宗顺, 朱建军, 李翔, 梁锋, 刘建平, 杨辉

2015 (12): 127801-127801. doi: 10.1088/1674-1056/24/12/127801

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InGaN/GaN multiple quantum well (MQW) green light-emitting diodes (LEDs) with varying InGaN quantum well layer thickness are fabricated and characterized. The investigation of luminescence efficiency versus injection current reveals that several physical mechanisms may jointly influence the efficiency droop, resulting in a non-monotonic variation of droop behavior with increasing quantum well (QW) thickness. When the QW is very thin, the increase of InGaN well layer thickness makes the efficiency droop more serious due to the enhancement of polarization effect. When the QW thickness increases further, however, the droop is alleviated significantly, which is mainly ascribed to the enhanced non-radiative recombination process and the weak delocalization effect.

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Optical properties of F-and H-terminated armchair silicon nanoribbons

鲁道邦, 濮春英, 宋玉玲, 潘群娜, 周大伟, 徐海如

2015 (12): 127802-127802. doi: 10.1088/1674-1056/24/12/127802

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The optical properties of F-and H-terminated silicon nanoribbons with armchair edges (F-and H-terminated ASiNRs) are compared by using the first-principles within the density function theory (DFT) framework. The results show that compared with for H-terminated 7-ASiNR, the dielectric function for the F-terminated 7-ASiNR has a red shift, which is mainly attributed to the narrower band gap because of the σ-π mixing effect of F-Si bonds in F-terminated 7-ASiNR. The peaks in the energy loss spectra for both systems represent the characteristics associated with the plasma resonance, which correspond to the trailing edges in the reflection spectra. These properties show that the different terminated atoms in 7-ASiNRs affect mainly the optical properties in the low energy range. Because of the rich optical properties, the 7-ASiNR could be a potential candidate for photoelectric nanodevice.

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Dielectric and magnetic properties of (Zn, Co) co-doped SnO₂ nanoparticles

Rajwali Khan, 方明虎

2015 (12): 127803-127803. doi: 10.1088/1674-1056/24/12/127803

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Polycrystalline samples of (Zn, Co) co-doped SnO₂ nanoparticles were prepared using a co-precipitation method. The influence of (Zn, Co) co-doping on electrical, dielectric, and magnetic properties was studied. All of the (Zn, Co) co-doped SnO₂ powder samples have the same tetragonal structure of SnO₂. A decrease in the dielectric constant was observed with the increase of Co doping concentration. It was found that the dielectric constant and dielectric loss values decrease, while AC electrical conductivity increases with doping concentration and frequency. Magnetization measurements revealed that the Co doping SnO₂ samples exhibits room temperature ferromagnetism. Our results illustrate that (Zn, Co) co-doped SnO₂ nanoparticles have an excellent dielectric, magnetic properties, and high electrical conductivity than those reported previously, indicating that these (Zn, Co) co-doped SnO₂ materials can be used in the field of the ultrahigh dielectric material, high frequency device, and spintronics.

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Charge trapping in surface accumulation layer of heavily doped junctionless nanowire transistors

马刘红, 韩伟华, 王昊, 杨香, 杨富华

2015 (12): 128101-128101. doi: 10.1088/1674-1056/24/12/128101

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We investigate the conductivity characteristics in the surface accumulation layer of a junctionless nanowire transistor fabricated by the femtosecond laser lithography on a heavily n-doped silicon-on-insulator wafer. The conductivity of the accumulation region is totally suppressed when the gate voltage is more positive than the flatband voltage. The extracted low field electron mobility in the accumulation layer is estimated to be 1.25 cm²·V^-1·s^-1. A time-dependent drain current measured at 6 K predicts the existence of a complex trap state at the Si-SiO₂ interface within the bandgap. The suppressed drain current and comparable low electron mobility of the accumulation layer can be well described by the large Coulomb scattering arising from the presence of a large density of interface charged traps. The effects of charge trapping and the scattering at interface states become the main reasons for mobility reduction for electrons in the accumulation region.

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Label-free surface-enhanced infrared spectro-electro-chemical analysis of the Redox potential shift of cytochrome c complexed with a cardiolipin-containing lipid membrane of varied composition

刘丽, 武烈, 曾丽, 姜秀娥

2015 (12): 128201-128201. doi: 10.1088/1674-1056/24/12/128201

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In this study, a lipid membrane was fabricated by fusing cardiolipin-phosphatidylcholine (CL_PC, 1:4) vesicles onto a hydrophobic surface of 1-dodecanethiol (DT) preadsorbed on a nanostructured gold film. By changing the concentration of the DT adsorption solution, we constructed a series of CL_PC-DT bilayers with different hydrophobicity to study the effects of lipid membrane characteristics on the adsorption conformation of cytochrome c (Cyt c). Electrochemical analysis showed that the formal potential is 0.24 V for Cyt c-CL_PC-DT(10), 0.2 V for Cyt c-CL_PC-DT(20), and 0.16 V for Cyt c-CL_PC-DT(40)–a gradual positive shift with the decreasing DT concentration–relative to the potential of native cyt c (0.02 V). Potential-induced surface-enhanced infrared adsorption difference spectroscopy revealed that the gradual positive shift of the formal potential of CL-bound cyt c is determined by the environment with the gradually lowered dielectric constant for the heme cofactor in CL-bound cyt c (Fe³⁺).

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Computational prediction of over-annotated protein-coding genes in the genome of Agrobacterium tumefaciens strain C58

于家峰, 隋天翔, 王红梅, 王春玲, 荆莉, 王吉华

2015 (12): 128202-128202. doi: 10.1088/1674-1056/24/12/128202

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Agrobacterium tumefaciens strain C58 is a type of pathogen that can cause tumors in some dicotyledonous plants. Ever since the genome of A. tumefaciens strain C58 was sequenced, the quality of annotation of its protein-coding genes has been queried continually, because the annotation varies greatly among different databases. In this paper, the questionable hypothetical genes were re-predicted by integrating the TN curve and Z curve methods. As a result, 30 genes originally annotated as “hypothetical” were discriminated as being non-coding sequences. By testing the re-prediction program 10 times on data sets composed of the function-known genes, the mean accuracy of 99.99% and mean Matthews correlation coefficient value of 0.9999 were obtained. Further sequence analysis and COG analysis showed that the re-annotation results were very reliable. This work can provide an efficient tool and data resources for future studies of A. tumefaciens strain C58.

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Optimal satisfaction degree in energy harvesting cognitive radio networks

李赞, 刘伯阳, 司江勃, 周福辉

2015 (12): 128401-128401. doi: 10.1088/1674-1056/24/12/128401

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A cognitive radio (CR) network with energy harvesting (EH) is considered to improve both spectrum efficiency and energy efficiency. A hidden Markov model (HMM) is used to characterize the imperfect spectrum sensing process. In order to maximize the whole satisfaction degree (WSD) of the cognitive radio network, a tradeoff between the average throughput of the secondary user (SU) and the interference to the primary user (PU) is analyzed. We formulate the satisfaction degree optimization problem as a mixed integer nonlinear programming (MINLP) problem. The satisfaction degree optimization problem is solved by using differential evolution (DE) algorithm. The proposed optimization problem allows the network to adaptively achieve the optimal solution based on its required quality of service (Qos). Numerical results are given to verify our analysis.

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Real-space observation of individual skyrmions in helimagnetic nanostripes

金驰名, 杜海峰

2015 (12): 128501-128501. doi: 10.1088/1674-1056/24/12/128501

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Controllable formation and manipulation of domain walls in one-dimensional (1D) nanostripes underpins a promising type of emergent spintronic device. Magnetic skyrmion is topologically stable whirlpool-like spin texture and is expected to replace familiar domain wall phenomena to build such devices, owing to its prominent features including small size, topological stability and the small critical current required to move it. It is thus essential to understand skyrmions' properties in such a nanostructured element. In this paper, we mainly give fundamental insight into this issue. Experimental achievements in the formation and stability of individual skyrmions in the nanostripe are outlined in detail.

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Knowledge-based potentials in bioinformatics: From a physicist's viewpoint

郑伟谋

2015 (12): 128701-128701. doi: 10.1088/1674-1056/24/12/128701

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Biological raw data are growing exponentially, providing a large amount of information on what life is. It is believed that potential functions and the rules governing protein behaviors can be revealed from analysis on known native structures of proteins. Many knowledge-based potentials for proteins have been proposed. Contrary to most existing review articles which mainly describe technical details and applications of various potential models, the main foci for the discussion here are ideas and concepts involving the construction of potentials, including the relation between free energy and energy, the additivity of potentials of mean force and some key issues in potential construction. Sequence analysis is briefly viewed from an energetic viewpoint.

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A multi-field approach to DNA condensation

冉诗勇, 贾俊丽

2015 (12): 128702-128702. doi: 10.1088/1674-1056/24/12/128702

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DNA condensation is an important process in many fields including life sciences, polymer physics, and applied technology. In the nucleus, DNA is condensed into chromosomes. In polymer physics, DNA is treated as a semi-flexible molecule and a polyelectrolyte. Many agents, including multi-valent cations, surfactants, and neutral poor solvents, can cause DNA condensation, also referred to as coil-globule transition. Moreover, DNA condensation has been used for extraction and gene delivery in applied technology. Many physical theories have been presented to elucidate the mechanism underlying DNA condensation, including the counterion correlation theory, the electrostatic zipper theory, and the hydration force theory. Recently several single-molecule studies have focused on DNA condensation, shedding new light on old concepts. In this document, the multi-field concepts and theories related to DNA condensation are introduced and clarified as well as the advances and considerations of single-molecule DNA condensation experiments are introduced.

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Theoretical studies on sRNA-mediated regulation in bacteria

常晓雪, 徐留芳, 史华林

2015 (12): 128703-128703. doi: 10.1088/1674-1056/24/12/128703

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Small RNA(sRNA)-mediated post-transcriptional regulation differs from protein-mediated regulation. Through base-pairing, sRNA can regulate the target mRNA in a catalytic or stoichiometric manner. Some theoretical models were built for comparison of the protein-mediated and sRNA-mediated modes in the steady-state behaviors and noise properties. Many experiments demonstrated that a single sRNA can regulate several mRNAs, which causes crosstalk between the targets. Here, we focus on some models in which two target mRNAs are silenced by the same sRNA to discuss their crosstalk features. Additionally, the sequence-function relationship of sRNA and its role in the kinetic process of base-pairing have been highlighted in model building.

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Catch-bond behavior of DNA condensate under tension

李伟, Wong Wei-Juan, Lim Ci-Ji, 车海鹏, 李明, 严洁, 王鹏业

2015 (12): 128704-128704. doi: 10.1088/1674-1056/24/12/128704

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Toroid formation is an important mechanism underlying DNA condensation, which has been investigated extensively by single-molecule experiments in vitro. Here, the de-condensation dynamics of DNA condensates were studied using magnetic tweezers combined with Brownian dynamics simulations. The experimental results revealed a surprising non-monotonic dependence of the unfolding rate on the force applied under strong adhesion conditions, resembling the catch-bond behavior reported in the field of ligand-receptor interactions. Simulation results showed that the different unfolding pathways of DNA condensate under large forces derive from the force-dependent deformation of the DNA toroid, which explains the catch-bond behavior of DNA condensate in the magnetic tweezers experiments. These results challenge the universality of the regular toroidal DNA unwrapping mechanism and provide the most complete description to date of multivalent cation-dependent DNA unwrapping under tension.

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Comparison of ligand migration and binding in heme proteins of the globin family

Karin Nienhaus, G. Ulrich Nienhaus

2015 (12): 128705-128705. doi: 10.1088/1674-1056/24/12/128705

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The binding of small diatomic ligands such as carbon monoxide or dioxygen to heme proteins is among the simplest biological processes known. Still, it has taken many decades to understand the mechanistic aspects of this process in full detail. Here, we compare ligand binding in three heme proteins of the globin family, myoglobin, a dimeric hemoglobin, and neuroglobin. The combination of structural, spectroscopic, and kinetic experiments over many years by many laboratories has revealed common properties of globins and a clear mechanistic picture of ligand binding at the molecular level. In addition to the ligand binding site at the heme iron, a primary ligand docking site exists that ensures efficient ligand binding to and release from the heme iron. Additional, secondary docking sites can greatly facilitate ligand escape after its dissociation from the heme. Although there is only indirect evidence at present, a preformed histidine gate appears to exist that allows ligand entry to and exit from the active site. The importance of these features can be assessed by studies involving modified proteins (via site-directed mutagenesis) and comparison with heme proteins not belonging to the globin family.

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Al-doping influence on crystal growth of Ni-Al alloy: Experimental testing of a theoretical model

荣曦明, 陈骏, 李菁田, 庄军, 宁西京

2015 (12): 128706-128706. doi: 10.1088/1674-1056/24/12/128706

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Recently, a condensing potential model was developed to evaluate the crystallization ability of bulk materials [Ye X X, Ming C, Hu Y C and Ning X J 2009 J. Chem. Phys. 130 164711 and Peng K, Ming C, Ye X X, Zhang W X, Zhuang J and Ning X J 2011 Chem. Phys. Lett. 501 330], showing that the best temperature for single crystal growth is about 0.6 T_m, where T_m is the melting temperature, and for Ni-Al alloy, more than 6 wt% of Al-doping will badly reduce the crystallization ability. In order to verify these predictions, we fabricated Ni-Al films with different concentrations of Al on Si substrates at room temperature by pulsed laser deposition, and post-annealed the films at 833, 933, 1033 (～ 0.6 T_m), 1133, and 1233 K in vacuum furnace, respectively. The x-ray diffraction spectra show that annealing at 0.6 T_m is indeed best for larger crystal grain formation, and the film crystallization ability remarkably declines with more than 6-wt% Al doping.

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Application of self-consistent field theory to self-assembled bilayer membranes

张平文, 史安昌

2015 (12): 128707-128707. doi: 10.1088/1674-1056/24/12/128707

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Bilayer membranes self-assembled from amphiphilic molecules such as lipids, surfactants, and block copolymers are ubiquitous in biological and physiochemical systems. The shape and structure of bilayer membranes depend crucially on their mechanical properties such as surface tension, bending moduli, and line tension. Understanding how the molecular properties of the amphiphiles determine the structure and mechanics of the self-assembled bilayers requires a molecularly detailed theoretical framework. The self-consistent field theory provides such a theoretical framework, which is capable of accurately predicting the mechanical parameters of self-assembled bilayer membranes. In this mini review we summarize the formulation of the self-consistent field theory, as exemplified by a model system composed of flexible amphiphilic chains dissolved in hydrophilic polymeric solvents, and its application to the study of self-assembled bilayer membranes.

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One-dimensional chain of quantum molecule motors as a mathematical physics model for muscle fibers

司铁岩

2015 (12): 128708-128708. doi: 10.1088/1674-1056/24/12/128708

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A quantum chain model of multiple molecule motors is proposed as a mathematical physics theory for the microscopic modeling of classical force-velocity relation and tension transients in muscle fibers. The proposed model was a quantum many-particle Hamiltonian to predict the force-velocity relation for the slow release of muscle fibers, which has not yet been empirically defined and was much more complicated than the hyperbolic relationships. Using the same Hamiltonian model, a mathematical force-velocity relationship was proposed to explain the tension observed when the muscle was stimulated with an alternative electric current. The discrepancy between input electric frequency and the muscle oscillation frequency could be explained physically by the Doppler effect in this quantum chain model. Further more, quantum physics phenomena were applied to explore the tension time course of cardiac muscle and insect flight muscle. Most of the experimental tension transient curves were found to correspond to the theoretical output of quantum two- and three-level models. Mathematical modeling electric stimulus as photons exciting a quantum three-level particle reproduced most of the tension transient curves of water bug Lethocerus maximus.

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Firing dynamics of an autaptic neuron

王恒通, 陈勇

2015 (12): 128709-128709. doi: 10.1088/1674-1056/24/12/128709

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Autapses are synapses that connect a neuron to itself in the nervous system. Previously, both experimental and theoretical studies have demonstrated that autaptic connections in the nervous system have a significant physiological function. Autapses in nature provide self-delayed feedback, thus introducing an additional timescale to neuronal activities and causing many dynamic behaviors in neurons. Recently, theoretical studies have revealed that an autapse provides a control option for adjusting the response of a neuron: e.g., an autaptic connection can cause the electrical activities of the Hindmarsh-Rose neuron to switch between quiescent, periodic, and chaotic firing patterns; an autapse can enhance or suppress the mode-locking status of a neuron injected with sinusoidal current; and the firing frequency and interspike interval distributions of the response spike train can also be modified by the autapse. In this paper, we review recent studies that showed how an autapse affects the response of a single neuron.

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Energy dependence on the electric activities of a neuron

宋欣林, 靳伍银, 马军

2015 (12): 128710-128710. doi: 10.1088/1674-1056/24/12/128710

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A nonlinear circuit can be designed by using inductor, resistor, capacitor and other electric devices, and the electromagnetic field energy can be released from the circuit in the oscillating state. The generation of spikes or bursting states in neurons could be energetically a costly process. Based on the Helmholtz's theorem, a Hamilton energy function is defined to detect the energy shift induced by transition of electric modes in a Hindmarsh-Rose neuron. It is found that the energy storage is dependent on the external forcing, and energy release is associated with the electric mode. As a result, the bursting state and chaotic state could be helpful to release the energy in the neuron quickly.

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Linear-fitting-based similarity coefficient map for tissue dissimilarity analysis in T₂^*-w magnetic resonance imaging

余绍德, 伍世宾, 王浩宇, 魏新华, 陈鑫, 潘万龙, Hu Jiani, 谢耀钦

2015 (12): 128711-128711. doi: 10.1088/1674-1056/24/12/128711

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Similarity coefficient mapping (SCM) aims to improve the morphological evaluation of T₂^* weighted magnetic resonance imaging (T₂^*-w MRI). However, how to interpret the generated SCM map is still pending. Moreover, is it probable to extract tissue dissimilarity messages based on the theory behind SCM? The primary purpose of this paper is to address these two questions. First, the theory of SCM was interpreted from the perspective of linear fitting. Then, a term was embedded for tissue dissimilarity information. Finally, our method was validated with sixteen human brain image series from multi-echo T₂^*-w MRI. Generated maps were investigated from signal-to-noise ratio (SNR) and perceived visual quality, and then interpreted from intra- and inter-tissue intensity. Experimental results show that both perceptibility of anatomical structures and tissue contrast are improved. More importantly, tissue similarity or dissimilarity can be quantified and cross-validated from pixel intensity analysis. This method benefits image enhancement, tissue classification, malformation detection and morphological evaluation.

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Bayesian-MCMC-based parameter estimation of stealth aircraft RCS models

夏威, 代小霞, 冯圆

2015 (12): 129501-129501. doi: 10.1088/1674-1056/24/12/129501

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When modeling a stealth aircraft with low RCS (Radar Cross Section), conventional parameter estimation methods may cause a deviation from the actual distribution, owing to the fact that the characteristic parameters are estimated via directly calculating the statistics of RCS. The Bayesian-Markov Chain Monte Carlo (Bayesian-MCMC) method is introduced herein to estimate the parameters so as to improve the fitting accuracies of fluctuation models. The parameter estimations of the lognormal and the Legendre polynomial models are reformulated in the Bayesian framework. The MCMC algorithm is then adopted to calculate the parameter estimates. Numerical results show that the distribution curves obtained by the proposed method exhibit improved consistence with the actual ones, compared with those fitted by the conventional method. The fitting accuracy could be improved by no less than 25% for both fluctuation models, which implies that the Bayesian-MCMC method might be a good candidate among the optimal parameter estimation methods for stealth aircraft RCS models.

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