Vol. 64, No. 14 (2015)
2015, 64 (14): 140101. doi: 10.7498/aps.64.140101
In wireless sensor network, the lager interference makes the data transmission failed, thus leading to data retransmission of nodes. This situation exacerbates the energy consumption of retransmission. As a result, some nodes will prematurely fail to work, thus reducing the network lifetime. In order to deal with the above issue, this paper takes full advantage of the topology and route information to design a novel load model of nodes. Then, a lifetime model of each node is constructed based on the load model. Subsequently, the path gain, intersecting interference and node lifetime are integrated into a utility function to construct a channel allocation game model called channel allocation based-game (CABG) by taking advantage of the game theory. The theoretical analysis proves the existence of the Nash Equilibrium of CABG. And then, using the best response dynamics, a channel allocation game algorithm for anti-interference and lifetime optimization (CAGLO) is established based on CABG. This algorithm CAGLO makes nodes avoid selecting the same channel as the large intersecting interference nodes and shorter-lifetime nodes in the process of channel selection, thus realizing the channel selection with less interference, less and balanced energy consumption. The theoretical analysis and simulation results show that the algorithm CAGLO could converge to the Nash Equilibrium with a good convergence speed finally. And the algorithm CAGLO has less message complexity. As a result, the algorithm itself has less energy consumption of communication. At the same time, it has good characteristics of anti-interference, energy consumption equilibrium and channel equalization. The algorithm CAGLO proved in this paper prolongs the network lifetime effectively.
Effect of horizontal temperature difference on Marangoni-thermocapillary convection in a shallow annular pool
2015, 64 (14): 140202. doi: 10.7498/aps.64.140202
The surface tension driven convection with the bidirectional temperature differences plays a very important role in many natural processes. However, most of the previous researches have focused only on the convection induced by a unidirectional temperature difference. In this paper, under the coexistence of bidirectional temperature differences, we conduct a series of numerical simulations to investigate the effect of horizontal temperature difference on the Marangoni-thermocapillary convection in a shallow annular pool. The critical values of bottom heat flux Qcri for transition from an axisymmetric steady flow to a three-dimensional unsteady flow at different values of Ma are determined. The result shows the horizontal temperature difference has a negative effect on the stability of Marangoni-thermocapillary convection. The simulation predicts two new state evolutions which do not appear in the convection with a unidirectional temperature difference. When Q is less than the Qcri value of 2.4×10-3, the Marangoni convection without horizontal temperature difference is steady and axisymmetric. When a small horizontal temperature difference is imposed, the convection called basic flow keeps steady and axisymmetric. When the value of Ma exceeds a certain threshold value Macri, the convection becomes a three-dimensional unsteady flow. After this unsteady flow happens, with the increase of Ma, the surface temperature fluctuation evolves from a punctate wave to a hydrothermal wave, and finally to a chaotic wave. Accordingly, the temperature oscillation with time is a periodically regular oscillation at first, then turns into a chaotic mess. When Q is larger than the corresponding Qcri value of 2.4×10-3, without a horizontal difference, the convection is unsteady and no basic flow exists in the variation process of Ma. With the increase of Ma, the surface temperature fluctuation evolves from a double hydrothermal wave to a single hydrothermal wave, and finally to a chaotic wave. The vertical heat transfer and horizontal temperature difference have different effects on the fluid, and their separate roles in driving fluid are determined. The bottom heat flux causes the surface fluid to flow in two opposite radial directions as the highest surface temperature is located in the middle region, while the horizontal temperature difference induces the surface fluid to flow in a single radial direction as the highest surface temperature appears at the hot wall. The combined action of these two forces generates different flows. The increase of horizontal temperature difference leads to the highest surface temperature, which originally appears in the middle region due to the bottom heat flux, and moves toward the hot wall. In this process, the horizontal temperature difference has a positive effect on the enhancement of flow near inner wall but it has a negative effect on the flow near outer wall.
2015, 64 (14): 140301. doi: 10.7498/aps.64.140301
We propose a method of generating the three-photon W state. The method uses parametric down-conversion process and hybrid entanglement swapping from multiphoton spin-entangled states to multiphoton orbital angular momentum (OAM) entangled states, with the aid of a pair of polarization photons. They generate W state entangled in different degrees of freedom of polarization and OAM with a high-dimensional Hilbert space. By simply changing the methods of generating a polarization-OAM-linear momentum entangled W state. Our method produces two mutually symmetric W states with strong entanglement and high dimension, which is expected to realize the secure communication of extending quantum bits.
2015, 64 (14): 140201. doi: 10.7498/aps.64.140201
In the last 15 years, noncommutative effects have received much attention and have been extensively studied in the fields of quantum mechanics, field theory, condensed matter physics, and astrophysics. The aim of this paper is to investigate the thermodynamic properties of a harmonic oscillator system in noncommutative phase space. For an example, the effects of noncommutativity between positions and that between momenta in the phase space on thermodynamic properties of two- and three-dimensional harmonic oscillator system are studied by a statistical method. First, in the commutative phase space, the thermodynamic state functions are obtained from the partition functions of the harmonic oscillator system which satisfies Boltzmann statistics. Then, in the noncomummutative phase space, both noncommutative positions and noncommutative momenta are represented in terms of the commutative positions and momenta of the usual quantum mechanics by linear transformation method. Meanwhile, the other physical quantities such as the volume element, the number of microstates, and partition function in the noncommutative phase space are represented in terms of commutative positions and momenta. Finally, the thermodynamic and statistical state functions for the system in the noncommutative phase space are derived from the partition function, and the thermodynamic state functions in noncummutative and commutative phase spaces are compared with each other. The results show that the noncommutative effect changes the values of microscopic functions such as the partition function and entropy with the correction terms including noncummutative parameters. As the noncommutative parameters vanishes, i.e., reaches the commutative limit, the partition and entropy functions of the system coincide with the results of usual thermodynamics and statistical physics. Moreover, the macroscopic state functions such as the internal energy and heat capacity, remain constant. The results imply that the correction terms in the partition function and entropy may result from the corrections of the number of microstates and potential energy of the system by noncommutativity of the position and momentum. In conclusion, the method used in the paper is corresponding to the classical system that satisfies Boltzmann statistics, and the results derived here can provide a starting point for further studying the quantum system that satisfies Fermi-Dirac and Bose-Einstein statistics.
2015, 64 (14): 140203. doi: 10.7498/aps.64.140203
In order to improve the ability to optimize artificial immune algorithm, the memory mechanism of non-genetic information is introduced into optimization algorithm. An immune memory optimization algorithm based on the non-genetic information is proposed. Emulating human society education and experiential inheritance mechanism, the algorithm takes, stores and uses non genetic information in the evolutionary process of the population. By setting up a separate memory base, the algorithm stores non genetic information, and guides the subsequent search process. The algorithm uses the short-term memory of the prior knowledge and guides the subsequent evolution, which can increase the intelligence of search and reduce the blind search and repeat the search. The immune memory optimization algorithm based on the non-genetic information includes key operators: mutation operator, crossover operator and complement operator. The mutation operator is able to efficiently use non genetic information of grandparents to search, which can speed up the local search efficiency. In addition, the threshold to control the search depth of single dimension can avoid falling into local optimal solution making the evolutionary standstill. Through calculating comprehensive information about contemporary populations of all antibodies, complementary operator produces new antibodies containing excellent gene fragment in the global solution space. With small probability rules, crossover operator happens in an interval of multi generation, choosing the optimal antibody and a random antibody to exchange information about a single dimension. Crossover operator and complement operator can both be conducive to jumping out of optimal location. In simulation experiment, the immune memory optimization algorithm based on the non-genetic information uses four standard test functions: Ackley function, Griewank function, Rastrigin function, and transformed Rastrigin function. In order to better compare with contrast algorithm, in the case of high dimension the values of dimension are 20 and 30, and the experiment tests the four functions to make the statistical analysis of the results. On the other hand, to further test optimal performance of the algorithm in a more global massive space, multiple random experiment is carried out in the case of dimension 100. Compared with other intelligent algorithm, the simulation experiment with standard test functions of high dimension indicates that the new algorithms are superior in convergence speed, convergence precision and robustness comparison algorithm. In addition, the simulation results in the super high dimension show that the new algorithm has the global searching ability in high-dimensional solution space.
2015, 64 (14): 140302. doi: 10.7498/aps.64.140302
We propose an approach to measuring non-Markovianity of an open two-level system from quantum coherence perspective including l1 norm of coherence and quantum relative entropy of coherence, and derive corresponding non-Markovian conditions. Further, as a particular application, non-Markovian conditions of an open two-level system undergoing phase damping channel, random unitary channel and amplitude damping channel, respectively are investigated. Specifically speaking, for the three channels we obtain non-Markovian conditions based on l1 norm of coherence at any initial state of system, and find that non-Markovian conditions are the same as the conditions of other measurements, i.e., information back-flow, divisibility and quantum mutual entropy for the phase damping channel and amplitude damping channel, but non-Markovian conditions new and different from the conditions of other measurements for random unitary channel. On the other hand, for phase damping channel we obtain non-Markovian conditions based on quantum relative entropy of coherence at any initial state of system, which are the same as the conditions of other measures, i.e., information back-flow, divisibility and quantum mutual entropy. However, for the random unitary channel and amplitude damping channel we obtain non-Markovian conditions at maximally coherent state of system.
2015, 64 (14): 140304. doi: 10.7498/aps.64.140304
We propose a long distance measurement-device-independent (MDI) quantum-key-distribution (QKD) with quantum memory, and analyze the relationship between the key generation rate and the storage efficiency of quantum memory. Our protocol is considered and compared with MDI-QKD without quantum memory. We present general formulas for our protocol with three-intensity decoy states. The simulation results show that the maximum secure distance supported by MDI-QKD with quantum memory is about 500 km, while the maximum secure distance of MDI-QKD without quantum memory is only 216 km. With certain limits, prolonging the time of maintaining the necessary quantum fidelity can increase security key transmission distance. Furthermore, the protocol is robust against device imperfection such as quantum memory decoherence effects, which can be easily applied to practical QKD system.
Three-dimensional optical modeling of vertical alignment mode color filter liquid-crystal-on-silicon microdisplays
2015, 64 (14): 140701. doi: 10.7498/aps.64.140701
A three-dimensional (3D) optical model is developed for small color pixels in vertical alignment nematic (VAN) mode color filter liquid-crystal-on-silicon (CF-LCoS) microdisplays. First of all, the electromechanical characteristics of color liquid crystal (LC) cells are analyzed to achieve the orientation structure of the LC director. Then, the optical reflectance of the cell is calculated by using the extended Jones matrix. Finally, the optical reflectance on the pixel array is reduced into the displayed color image by adopting a standard RGB representation. In order to verify the accuracy of this 3D optical model, the simulation result and the observed result are compared. The simulated optical reflectance is in good agreement with the experimental result.
A new technique of full polarization hyperspectral imaging based on acousto-optic tunable filter and liquid crystal variable retarder
2015, 64 (14): 140702. doi: 10.7498/aps.64.140702
In order to achieve all Stokes parameters of spectral image with high spectral resolution, high spatial resolution, high polarization accuracy, high signal-to-noise ratio and good stability, taking into account the orthogonal characteristic of ±1 order diffraction light which diffracts from a acousto-optic tunable filter (AOTF), a new technique of full polarization hyperspectral imaging is presented. It uses one AOTF to diffract the incident light, one liquid crystal variable retarder (LCVR) to modulate the light retardation, and two CCDs to image the ±1 order diffraction light, respectively. According to the Muller matrixes of all optical elements in the system, the basic working principle of the new technique is that LCVR sequentially provides the retardation 2π, 1.5π, π and 0.5π for each spectral channel, so the CCD obtains corresponding images. After analyzing these images, the all Stokes parameters are obtained; the precision of this system for polarization imaging is determined mainly by polarization modulation device LCVR. Considering the azimuth of LCVR fast axis and retardation precision at the same time, it is unveiled that LCVR has no effect on the accuracy of the first Stokes parameter, and the relative errors of other latter 3 Stokes parameters are less than 0.064%, 0.31% and 3.97%; then, our prototype system is used to do the outdoor experiments in a summer sunny morning, images data for 26 spectral channels with spectral bandwidth of 10 nm, which are from 450 nm to 700 nm, are acquired, the imaging quality is very fine. Firstly, LCVR are not assembled in our prototype system, and AOTF works in the sweeping frequency mode. The spectrum from each CCD proves that the diffraction efficiency of AOTF ± 1 order diffraction light is not completely the same, and the difference must be considered in polarized image processing. Then another experiment is done after LCVR has been assembled. The image data of the incident light of 600 nm are taken for example to discuss its all Stokes parameters in detail. The results show that the principle of the new technique is correct and the new scheme is feasible. This study provides a new theory and implementation scheme for the polarization spectral imaging technology.
2015, 64 (14): 140303. doi: 10.7498/aps.64.140303
Macro-micro entanglement originates from the Schrodinger's Cat paradox. The paradox has been attracting the interest of the physicists since it was proposed. Schrodinger's Cat paradox is a thought experiment that entangles a cat with some decay atoms, in which the entanglement between the macroscopic object and the microscopic atoms is established. Mac-micro entanglement relates to some important problems in quantum physics. It is more likely to interact with the surroundings for the quantum system as its size increases, which is the reason why we hardly observe the macroscopic superposition state. Can the superposition state theory of quantum physics be used in macro domain? Is there a limitation to the scale for the objects in the superposition states? These questions need studying and verifying in experiment. In addition, the preparation of the macro-micro entanglement state provides a new possibility to study the decoherence model. Macro-micro entanglement can be realized in many physical systems, such as atomic ensembles, superconducting circuits, electro-mechanical and opto-mechanical systems. Here in this paper we will introduce the development of macro-micro entanglement in optical system. The initial approach to creating the macro-micro entanglement in the context of optical system is quantum cloning by simulating the emission. Then the quantum-injected optical parametric amplification is used to amplify single photon to a macroscopic level. Afterwards, the displacement in phase space is proposed to create the macro-micro entanglement. Since the photon number of the macro-micro entanglement with the optical parametric amplification approach can be about 104, the studies towards the detection of this type of entanglement with human eyes have been extensively conducted. But it is realized that the coarse-grained measurements, such as those with the human eye, generally cannot judge whether macro-micro entanglement exists, and hence cannot be used to prove the considered type of micro-macro entanglement. A way of overcoming this difficulty is to invert the amplification process, bringing the macro system back to the micro level. The entanglement can then be verified by using single-photon detectors. Because local operation and classical communication cannot create entanglement, the de-amplification process will not increase the entanglement and the presence of the entanglement in the end shows that entanglement is present between the amplification and de-amplification process. Inspired by this thought, two groups create and verify mac-micro entanglement between one photon and 108 photons. What they used to amplify the micro states is the displacement operation in phase space, which can be realized by combining a single photon state and a coherent state with a highly asymmetric beam splitter. Because the entanglement is a precondition for a secure quantum key distribution, and the macro-micro entanglement has more photons than the traditional micro entanglement, we will discuss the possibility whether the macro-micro entanglement can be used in quantum key distribution and improve the distance of the quantum key distribution. We point out that the mac-micro entanglement and the binary reverse reconciliation continuous variable quantum key distribution protocol are the same in physics essence. We will introduce a quantum key distribution scheme with two phase entangled coherent states. Although the security proof of the scheme is not complete, it still provides us with the possibility to use the macro-micro entanglement in quantum key distribution.
2015, 64 (14): 140501. doi: 10.7498/aps.64.140501
Chaotic secure communication is an active research field of chaotic application. A novel method for chaotic secure communication is proposed based on strong tracking filter (STF) in this study. STF is an extended Kalman filter with suboptimal fading factors, especially suitable for estimating the state and parameter of nonlinear time-varying stochastic systems. The main idea of the proposed method is summarized below. At the emitting end, the chaotic mapping and the information symbol are modeled as a nonlinear state space model, and the information symbol is modulated by additive chaos masking or multiplicative chaos masking and then is outputted through the channel. At the receiving end, the driving signal is received, and the message symbol is recovered dynamically by STF with Bayesian classifier. Simulation tests of the logistic chaotic mapping show that STF can restore the information symbols in chaotic signals when information symbols are binary code, with either additive or multiplicative chaos masking modulation. Compared with STF, the conventional Kalman filter has poor ability to track the discrete information symbol. It is difficult to restore the information symbols in the chaotic mapping, and the bit error rate is high. Therefore, the STF-based chaotic secure communication method is effective.
ATOMIC AND MOLECULAR PHYSICS
Resonance-like enhancement in high-order above-threshold ionzation of argon at different wavelengths
2015, 64 (14): 143201. doi: 10.7498/aps.64.143201
Quantum S-matrix theory and “uniform approximation” method are used to study the resonance-like enhancement (RLE) structures in photoelectron spectrum of high-order above-threshold ionization (HATI) for argon atoms subjected to strong laser fields at different wavelengths. Our results show that both in the near infrared and mid-infrared fields, the RLE structures in the photoelectron spectra will appear, which manifests as a group of adjacent HATI peaks that show a significant enhancement when the laser intensity increases only a few percent. The RLE occurs precisely when the laser intensity satisfies the channel-closing (CC) condition, and this further confirms the explanation of CC mechanism of the RLE. More importantly, we find that with increasing laser wavelength, the resonance-like enhancement and suppression will appear alternately in the photoelectron energy spectrum, and this alternation phenomenon will be more pronounced as the intensity increases. This phenomenon may be attributed to the interference of “quantum orbital” of electrons which collide with the core at different return time. Since in the condition of long wavelength, the alternation phenomenon of the RLE is more pronounced, the RLE is distributed from the low-energy regime to the cutoff-regime in the photoelectron energy spectrum, thus making the RLE broader than that in the case of short wavelength. This may be used to explain the experimentally observed extension of the RLE energy region at longer wavelength. In addition, it is also shown that similar to the case of the near infrared laser fields, two types of RLE structures are also found in strong mid-infrared laser fields, where type-Ⅰ enhancement occurs in the region 5%-10% below even CC for Ar atom whose ground state has an odd parity, and its intensity dependence is comparatively smooth; and type-Ⅱ enhancement appears exactly at the channel closing and has a particularly sharp intensity dependence. And both types of enhancements are due to the constructive interference of a large amount of quantum orbits.
Direct measurement of the rotational constant of 0u+(6S1/2+6P1/2) long-range state via double-pass spectroscopy
2015, 64 (14): 143302. doi: 10.7498/aps.64.143302
In this paper, we obtain the rotational constant and the distortion constant of v=187 belonging to 0u+ state below the 6S1/2+6P1/2 disassociation limit. In our experiment, we first prepare the ultra-cold cesium sample in the MOT (magneto-optical trap) by six beams of pumping laser, one beam of repumping laser, and a pair of anti-Helmholtz coils. Then we construct a high-resolution frequency reference using the double-pass photoassociation technique. The double-pass photoassociation technique is a creative and robust method. We use a polarization beam splitter to split one laser beam from the laser to two beams-Laser Ⅰ and Laser Ⅱ; Laser Ⅱ then passes twice through an acousto-optic modulator (AOM) whose central frequency is 110 MHz, using a reflecting mirror and a convex lens before illuminating the MOT. We use two shutters-S1 and S2 to control Laser Ⅰ and Laser Ⅱ. Open S1 while keep S2 close to make Laser I interact with the MOT; and after the rotational spectroscopy of J=0-6 is observed, turn off S1 and turn on S2 immediately. Let laser II interact with MOT and obtain another part of spectroscopy that is exactly the same with J=6; we define this part of spectroscopy as J'=6. The frequency interval between J=6 and J'=6 is exactly 220 MHz for the scan process is strictly linear, and that can be an accurate frequency interval in our experiment. The laser intensities of these two laser beams have to be strictly equal in case of the laser-induced frequency shift. Using the frequency interval of 220 MHz, we can calculate the frequency interval of J=0-6. The detection method we used here is the trap loss spectroscopic technology by modulating fluorescence of cold atoms in the MOT, which allows a direct spectroscopy detection at the rovibrational levels for a very weak transition probability. With the frequency intervals of each rotational quantum number, we can fit the frequency intervals to the non-rigid model to derive the rotation constant B and distortion constant D which are crucial to precisely measure the full molecule potential curves as well as deepen our understanding of molecular formation. This kind of double-pass photoassociation technique not only can direct obtain the precise value of rotation constant B and distortion constant D as compared with the traditional photoassociation method, but also can obtain a relatively accurate potential energy curve. And another great advantage is that we are able to calculate the frequency intervals easily without the wavelength meter which is rather expensive and difficult to control.
2015, 64 (14): 143301. doi: 10.7498/aps.64.143301
Measuring the vibration dephasing time in molecular vibration is the free-mark method for detecting molecules harmlessly. Since molecular vibration refund processes are associated with molecular environment change, molecular vibration dephasing time also may reflect the substance's molecular environment change, which can be used to study the interaction between a certain molecule and its neighboring molecules. The molecular vibration spectrum and vibration dephasing time are obtained from the time-resolved coherent anti-stokes Raman scattering (CARS) simultaneously. Benzonitrile and methanol are used as samples for studying, the vibration dephasing time changes for the main vibration spectra when the environment changes. With benzonitrile mixed with anhydrous alcohol, its vibration dephasing time changes with environment are measured in three typical benzonitrile molecular vibrations 1017 cm-1, 2247 cm-1 and 3085 cm-1. For adjoining methanol molecular vibrations 2851 cm-1, and 2960 cm-1, vibration dephasing time changes are measured under environmental conditions. Results show that significant changes of molecular vibration dephasing time will take place in different environments. For a unidirectional molecular environment change, the molecular vibration dephasing time of benzonitrile is a one-way change, while the methanol molecule is of non-unidirectional vibration dephasing time change. But methanol molecules with vibration intensity ratios between two unidirectional changes with environment for I2851/I2960 are of a one-way change. By experimental measurement the vibration dephasing time of the main vibration mode of benzonitrile and methanol molecules varies with the changes in the environment, further understanding of differences on vibration dephasing time of molecular vibration spectra of adjacent and non-adjacent variations can explain the variation of vibration dephasing time of benzonitrile molecules. This method has the ability of detecting molecular environment change and molecular interactions, and has an important application prospect in the field of life science, molecular biology, and material science etc..
Theoretical investigation of femtosecond-resolved photoelectron spectra of three-level ladder K2 molecules
2015, 64 (14): 143303. doi: 10.7498/aps.64.143303
We investigate the effect of delay time, pulse width and pump wavelength on photoelectron spectra and wave packet forming process of the three-level K2 molecules via time-dependent wave packet approach. There is no Autler-Townes splitting for weaker pump intensity or shorter pulse width. Delay time and pump wavelength can affect peak structure, position, and relative height. The vibration period of wave packet does not vary with pump wavelength, while the oscillating amplitude decreases with increasing pulse width. Results may provide important basis for realizing the optical control of molecules experimentally.
2015, 64 (14): 143401. doi: 10.7498/aps.64.143401
Prediction of transport properties of noble gases requires the calculation of collision integrals, which depend on interatomic potentials as the input. However the accuracy of transport properties depends largely on the accuracy of interaction potentials. So different interatomic potentials of noble gases are compared in order to get the accurate transport properties. The forms and characteristics of Lennard-Jones, exponential repulsive, Hartree-Fock-Dispersion-B (HFD-B), and phenomenological model potentials that are used to describe the atomic interactions between noble gases are analyzed in this paper. Then the calculation method of transport properties is presented. Viscosities and thermal conductivities of noble gases based on these four potentials are obtained using Chapman-Enskog method in the temperature range for computation from 300 to 5000 K. It can be seen from the results that the interaction potentials have a great influence on the calculated results of transport properties. There are great differences between the results obtained using different interaction potentials. These differences of the calculated results can be explained according to the performance of interaction potentials. Results calculated with Lennard-Jones potential are always much lower in the high temperature range due to its overestimated repulsive part, and the exponential repulsive potential gives unreasonable results at low temperatures because there is no attractive well in this potential. Therefore, the accurate interatomic potentials for noble gases can be obtained only by comparing the calculated results with published experimental and theoretical data of other researchers. It can be found that the results obtained by HFD-B potential agree well with previously experimental and theoretical data. So it is apparent that the HFD-B potential in light of Hartree-Fock repulsion and dispersion theory can provide a realistic description of the trends and features of interatomic potentials, allowing accurate theoretical calculations to be made for transport properties of noble gases.
2015, 64 (14): 143101. doi: 10.7498/aps.64.143101
The electronic transport, the storage capacity, and the service life of the anode material for lithium ion batteries will be reduced seriously in the event of the material layering or cracking, so the anode material must have strong mechanical reliability. Firstly, in view of the traditional molecular dynamics limited by the geometric scales of the model of silicon functionalized graphenen (SFG) as lithium ion battery anode material, some full atomic models of SFG are established by using Tersoff potential and Lennard-Jones potential, and used to calculate the modulus and the adhesion properties. What is more, according to the mechanical equilibrium condition and energy conservation and by combining with calculations from full atomic model through adopting the bead-spring structure, the SFG coarse-grain model and its system energy reservation equation are established. Finally, the validity of the SFG coarse-grain model is verified by comparing the tensile property of coarse-grain model with full atoms model.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Theoretical and experimental study of the electric resonant coupling between two metamaterial resonators
2015, 64 (14): 144101. doi: 10.7498/aps.64.144101
In this paper, we realize the electrically coupled resonances between two metamaterial resonators based on two metal split-ring resonators gap-to-gap placed. The theoretical analysis and numerical calculation of the microwave equivalent circuit of the electrically coupled metamaterial resonators are performed. The results show that there are two resonance frequencies produced by the two coupled metamaterial resonators. For the two resonance frequencies, one gradually shifts towards the lower frequency with the coupling strength increasing, while the other is fixed at the resonance frequency of the single metamaterial resonator. The measured and simulated results of the microwave transmission spectra show that the two resonance peaks move respectively towards the lower and higher frequency with the coupling strength increasing. The analysis shows that the lower resonance frequency is mainly determined by the electrical coupling strength between the two metamaterial resonators, and the difference between the higher resonance frequency and the resonance frequency of the single resonator is mainly caused by the inevitable magnetic coupling between the two resonators. Moreover, the smaller the coupling space, the greater the influence of magnetic coupling is. The proposed dual resonance property and its tunability based on the electromagnetic coupling between the two metamaterial resonators greatly enhance the scopes of the design and application for metamaterials.
2015, 64 (14): 144102. doi: 10.7498/aps.64.144102
The earlier research showed that circularly polarized laser pulses with peak intensities in a range of 1022-1025 W/cm2 can directly accelerate and generate GeV-TeV monoenergetic electron beams with a linear energy scaling with the laser intensity. To obtain higher energy electron beams, a scheme is proposed to use an electron beam with an initial energy E0 along the laser propagation direction. This scheme can overcome the linear energy scaling with E0=0 obtained previously and enhance the beam energy by E0 folds. This is because an electron beam with an initial energy can move with the laser pulse together and therefore obtain a longer acceleration distance. Two-dimensional particle-in-cell simulation shows that this scheme is effective only for the electron beams initially with low energy on the order of MeV. With overhigh energy, electrons will miss the optimum acceleration field because the electron acceleration distance is much longer than the Rayleigh distance and the laser intensity is significantly attenuated.
2015, 64 (14): 144201. doi: 10.7498/aps.64.144201
In Zernike modal wavefront reconstruction algorithm, because of a certain number of the low Zernike modes is selected subjective for making up the reconstruction model, the modes confusion and modes coupling will produced, the accuracy reduction of wavefront reconstruction is deduced. According to the minimal description length principal, a principle can balance the model error against model size so as to prevent the model from overfitting. The reconstruction model is selected by top-down method, and Levenberg-Marquardt nonlinear optimization algorithm is applied to the solution of reconstruction coefficients, then the wavefronts reconstruction is realized. Under different noise conditions, the reconstruction algorithm is validated with the numerical simulation of the wavefront reconstruction of uniform amplitude incidence beam, the results indicate: Zernike modal wavefront reconstruction algorithm based on the model selection can improve the wavefront reconstruction accuracy, and algorithm possesses the property of resistance to noise.
2015, 64 (14): 144202. doi: 10.7498/aps.64.144202
For the application requirements of resonant micro-cavity, an original three-ring resonant micro-cavity structure is proposed in this paper. Like electromagnetically induced transparency in an atomic system, the coupled resonator-induced transparency (CRIT) phenomenon in a new optical micro-cavity system is proven experimentally. Up to now, most of the resonators based on CRIT are just in the theoretical exploration stage, and the analysis of the double-ring structure has been relatively common. The CRIT effect of a resonator has a significant relationship with the coupler insertion loss, the ring circumference, and the multiples of the rings, which need high requirements in the structural design and preparation process. In order to reduce the difficulty in design and preparation, we propose a new three-ring cascade resonator structure with the same cavity size on silicon. According to the transfer matrix method and coupled mode theory, we find a CRIT effect after theoretical analysis. Our devices are fabricated on an SOI wafer. By using the micro-cavity measurement platform to repeat and analyze the tests of single ring and three-ring cascade resonator structure, we obtain a grating coupler efficiency of 30%. By applying the antireflection coating, the coupling efficiency of the grating coupler is up to 34%. During the test, the mutual interference between annular cavities of the three-ring resonators produces two transmission peaks, the output spectrum of the resonator with a narrow transparency peak at a low group velocity, which is verified in CRIT phenomenon. Compared with the traditional single-ring structure, the resonator has a quality factor increasing four times, reaching a value of up to 0.65×105, the through and drop transmission spectra of the resonator are reconciled well with each other. At the same time, in order to obtain the sensitivity of the resonator to temperature, we conduct tuning tests of the resonator temperature characteristics, the resonant peak is moved to the large wavelength direction with temperature increasing, and detuning wavelength of the resonance can be controlled by changing temperature, which is called red-shift. Therefore, the original three-ring cascaded resonators have significant applications in the rotation sensing, optical filters, optical storage and temperature sensing elements.
2015, 64 (14): 144204. doi: 10.7498/aps.64.144204
A long-term stable sub-8 fs Ti:sapphire oscillator based on domestic chirped mirrors is reported. It outputs 300 mW mode-locked pulses at 86 MHz under 4 W pump power. The second order and third order of dispersion introduced by the components of the oscillator are analyzed. Two pairs of domestically designed and fabricated chirped mirrors are utilized to compensate the dispersion introduced by the crystal and the air in cavity. By precisely controlling the dispersion of chirp mirrors, the output pulses have an ultra-broad bandwidth exceeding 150 nm (FWHM) without the insertion of wedges. With the assistance of extra-cavity dispersion compensation, a pulse duration of 7.9 fs is achieved. This is the shortest pulse duration ever reported by using domestic chirped mirrors, and the shortest pulse duration achieved without intra-cavity wedges, to the best of our knowledge. Structure design and electronic feed-back loops are employed to improve the stability of the oscillator passively and actively, respectively. With the assistance of piezoelectric ceramic, the power stability within 24 h is measured to be 0.6%, which is significantly better than that without them.
2015, 64 (14): 144205. doi: 10.7498/aps.64.144205
A simple analytical method is proposed to obtain the exact propagation constant and distribution of electric field intensity of optical waveguides with graded refractive index profile. The method is based on the Wenzel-Kramers-Brillouin (WKB) solution, variational method, modified eigen-equations and discretized scalar wave equation for planar optical waveguide. The expressions of the distribution of electric field intensity based on the conventional WKB method, which diverge around the turning point, have been demonstrated to be very exact in the region beyond the turning point where the refractive index profile varies slowly. The proposed method uses the conventional WKB method to calculate the values of electric field intensity at two adjacent positions beyond the turning point and then the electric field intensity profile for the whole region is obtained by making use of the two calculated values. Two simple and explicit formulas are deduced from the discretized scalar wave equation, which provide a relationship among the values of electric field intensity at three adjacent positions. If the effective refractive index of optical waveguide and the refractive index profile for the whole region are known, we can obtain the value of electric field intensity at any position according to the corresponding values at the adjacent positions by using the two formulas aforementioned. By using the two values calculated by WKB method, the electric field intensity profile for the whole region can be determined through the iterative use of the two formulas. The accuracy of the electric field intensity profile determined by the proposed method is greatly dependent on the accuracy of the applied value of the effective refractive index. To achieve exact propagation constant and distribution of electric field intensity, the variational method and modified eigen-equations are employed in the proposed method. Variational method is a very useful method to improve the accuracy of the propagation constant in the analysis of optical waveguide with step-asymmetrical graded refractive index profile. By combining the traditional variational method and calculation of electric field intensity profile by the proposed method, the improved variational method is presented to obtain the exact propagation constant of optical waveguide. The value of propagation constant calculated by WKB method and the corresponding electric intensity field profile determined by the proposed method are chosen as the initial trial value and trial function in the variational method. Propagation constant and the corresponding electric field intensity profile with better accuracy can be achieved by the variational calculation and then are regarded as the new trial value and trial function. By the iterative use of the variational method and calculation of electric field intensity profile by the proposed method at finite times, quite accurate results are obtained. The modified eigen-equations in combination with the proposed method is another approach to calculating accurate propagation constants of optical waveguides with both the step-asymmetrical and symmetrical graded index profile. In comparison with other published methods, the proposed method has the advantages of the simplicity and considerable accuracy.
Fiber nonlinearity tolerance research of coherent optical orthogonal frequency division multiplexed system based on digital coherent superposition
2015, 64 (14): 144203. doi: 10.7498/aps.64.144203
Fiber nonlinearity of optical orthogonal frequency division multiplexed (OFDM) system restricts the capacity improvement of optical fiber transmission. In this paper, we propose a novel digital coherent superposition (DCS) scheme to improve the tolerance to fiber nonlinearity in a coherent optical orthogonal frequency division multiplexed system. In simulation, 71.53 Gbit/s orthogonal frequency division multiplexed signal per channel with Hermitian symmetry is transmitted over 400 km standard single mode fiber in a wave division multiplexed-polarization-division multiplexed-coherent optical orthogonal frequency division multiplexed system with five channels. The 4-quadrature amplitude modulation is used for symbol mapping. For the receiver, after the conventional OFDM signal processing, we conduct DCS for OFDM subcarrier pairs, which requires only conjugation and summation in the x, y polarization direction, respectively. Firstly, the channel spacing is 25 GHz, the maximum signal-to-noise ratio improvement is 9.05 dB or 6.02 dB with or without symmetric dispersion compensation compared with a conventional orthogonal frequency division multiplexed system. The optimum launch power is increased by 2 dB. Secondly, the channel spacing is changed to 50 GHz to investigate the nonlinearity tolerance at different channel spacings in the wave division multiplexed system, the maximum signal-to-noise ratio improvement is 8.75 dB or 4.9 dB with or without symmetric dispersion compensation, respectively. Theoretical and simulation analysis show that the proposed method in this paper can effectively mitigate the first-order nonlinear distortions and hence improve the tolerance of coherent optical orthogonal frequency division multiplexed system with different channel spacings to fiber nonlinear effects.
Sound field simulation of ultrasonic processing to fabricate carbon nanotubes reinforced AZ91D composites
2015, 64 (14): 144302. doi: 10.7498/aps.64.144302
The sound field in the melt processed by 20 kHz ultrasonic to fabricate CNT-AZ91D is investigated by numerical simulation. Firstly, the distribution of sound pressure in the AZ91D melt is calculated by the finite element method after the model of the sound filed of the ultrasonic processing has been built. The simulation results show that a radial sound field forms under the ultrasonic probe, which means that the sound pressure decreases with increasing distance from the sound source. After the sound field is revealed, we study the ultrasonic cavitation in the AZ91D melt with a single-bubbly-change model and examine the bubble change rule under different sound pressures by solving the Rayleigh-Plesset equation. The relationship between sound pressure amplitude and the ultrasonic cavitation in the melt is also discovered. The higher the sound pressure amplitude, the smaller the threshold radius for the bubble collapse is, and thus the ultrasonic cavitation in AZ91 melt happens more easily. Secondly, the sound fields of the melt with different immersed depths of the ultrasonic probe are calculated. The results show the optimal immersed depth is about 30 mm for the same crucible size used in the present study. Furthermore, the corresponding optimal immersed depth can be calculated for different crucible sizes or different melts by the present numerical method, which is important for the practical ultrasonic processing. After analyzing the calculated results for sound field and the cavitation rule of in the AZ91D synthetically, we find that the volume of the effective cavitation zone rapidly increases with the smaller ultrasonic power and then rises almost linearly with the ultrasonic power larger than 500 W. Finally, to verify the simulation method of the sound field, the contrast study between the simulation and experiment of ultrasonic processing using glycerol-water solution is performed. The simulation result of the sound field in glycerol-water solution is similar to that in AZ91D melt. The highest sound pressure occurs near the end face of ultrasonic probe, while the experimental observation shows that the strongest cavitation also happens near the end face of ultrasonic probe, which indicates that the highest sound pressure occurs in the zone.
2015, 64 (14): 144701. doi: 10.7498/aps.64.144701
Thrombosis caused by all kinds of cardiovascular diseases, has been influencing people's health. In a blocked blood vessel, pulsation flows have a positive effect on thrombus. Because of the blood viscosity and the inertias of red blood cells and the fluid, the frequency of pulsation flow influences the effect of dredge blood clots. Under the condition of low differential pressure, conduction of the another flow pipe reducing the pressure variation causes the effect of dredging blood clots not to be ideal in the bifurcated pipe model. So we will increase the differential pressure and improve the amplitude of pulsation flow, reduce the influence of the conduction effect of expedite tube, then study the effect of dredge blood clots which is caused by the pulsation flow in the bifurcated pipe. We find that low frequency pulsation flow has a good effect on dredge blood clots for a long time. Relatively, high frequency pulsation flow needs less time, but the effect of dredge blood clots is not obvious if the frequency is higher than a certain value. The friction between cells and the walls of the tube also has an effect on dredge blood clots.
Influences of environmental factors on low frequency abnormal sound transmission through sea-air interface
2015, 64 (14): 144301. doi: 10.7498/aps.64.144301
In view of low frequency abnormal sound transmission of sound source in the sea at the sea-air interface, according to the two-layer medium sound transmission model, we analyze the relationships of the sound speed and the density of the atmosphere with the atmospheric pressure, the air temperature, the humidity; we also analyze the relationships of the sound velocity and the density of seawater with the sea surface temperature (SST) and salinity; we investigate the low-frequency abnormal sound transmissions influenced by temperature, pressure, salinity, humidity and other environmental factors; we analyze the influences of various factors on the sound transmission. The obtained results are as follows. 1) the sound power in the air, obtained by the sound transmission of sound source in the shallow sea, is negatively correlated with atmospheric temperature and humidity, and positively correlated with the SST, salinity, and atmosphere pressure. 2) The sound power that is radiated into the sea by the monopole and horizontal dipole source at the sea, is negatively correlated with SST and salinity, while the sound power that is radiated into the sea by the vertical dipole sound source, is positively related to SST and salinity. 3) The sound transmission directivity is positively related to SST and negatively correlated with atmospheric temperature. 4) The air temperature and SST have the greatest influence on low-frequency abnormal sound transmission, while the effects of air pressure and humidity on them are smaller than that of salinity. The effect of temperature on the low frequency abnormal sound transmission of vertical dipole sound is greater than those of the horizontal dipole and a monopole sound source.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2015, 64 (14): 145201. doi: 10.7498/aps.64.145201
Mechanisms that electrons are directly accelerated by the laser-plasma interaction in non-resonant cases are studied. First, by use of a linearly polarized Gaussian laser beam, a three-dimensional model is presented to demonstrate that the frequency and the amplitude of electron oscillations can be significantly modulated by the transverse ponderomotive force, within the confinement of an underdense plasma channel. On the one hand, the transverse ponderomotive force can felicitously make electrons to experience the large amplitude oscillations and push them to the regions at a low dephasing rate. On the other hand, when the electrons oscillate across the channel with small amplitudes, the dephasing rate also can be effectively reduced by the nonlinear modulation arising from the transverse ponderomotive force. These kinds of modulations can lead electrons to stay in phase with the laser field for a longer time and thus enhance their energy gain, which also enables the mechanism of transverse ponderomotive modulation being in direct laser acceleration. This mechanism is determined by the plasma density and the laser intensity and radius. Detailed numerical results are also given which show that the electron acceleration induced by this ponderomotive modulation quite distinguishes from the parametric instability and the resonance from a driving force. Moreover, a theoretical model for the parametric amplification, which makes up the restriction of the quasi-two-dimensional model, is provided to verify that non-resonant direct laser acceleration can come from the parametric instability in the three-dimensional case.
2015, 64 (14): 145203. doi: 10.7498/aps.64.145203
To simulate the radiation transport of the spherical hohlraum with octahedral six laser entrance holes and to study the capsule radiation uniformity, a Monte Carlo method is introduced. For simple analytical models, with different hohlraumto-capsule radius ratios, the capsule radiation uniformity variation rules are studied, and the Monte Carlo calculation results can match the analytical results obtained by the “view factor” method. For more complicated models, such as the hohlraum with shields, it's difficult for an analytical method to be calculated, but is straightforward in the Monte Carlo method. Two models with different radius of the shield have been simulated. Simulated result indicates that the shield greatly influences the distribution of X-rays on the capsule surface, and an appropriate shield can increase the utilized efficiency of X-rays and improve the capsule radiation uniformity remarkably, otherwise, the uniformity might be destroyed badly. So the location and the radius of the shields must be designed carefully in a spherical hohlraum. This research supports the Monte Carlo method that is applicable in the radiation transport simulation of a complicated spherical holhraum.
2015, 64 (14): 145202. doi: 10.7498/aps.64.145202
Energy loss of protons with energy 100 keV penetrating the partially ionized hydrogen plasma target was measured. The plasma target was created by electric discharge in the hydrogen gas, the state of the plasma target was diagnosed by using the laser interferometry method: the free electron density is up to 1016 cm-3, temperature is about 1-2 eV, and the plasma target may exist at the microsecond level. It is found that the energy loss of protons is closely related to the free electron density, and the energy loss data enable us to infer the value of the Coulomb logarithm (10.8) for the stopping power of the free electrons. This agrees well with the theoretical prediction which is 4.3 times higher than that given by the Bethe formula for neutral hydrogen, which is a little bigger than Hoffmann's result but much smaller than Jacoby's result. Comparing our result with Hoffmann's, the energy we used is only 100 keV, much lower than 1.4 MeV/u, and the low-energy regime we applied could be the cause of the increase in the enhancement factor. However, in the comparison between our result and the Jacoby's, the effective charge for protons is almost constant, unlike the Kr+ impact in wihch the enhanced ion charge state induces the giant enhancement factor. Compared to the gas target, the energy loss enhancement factor in plasma target is 2.9.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Low energy phonon instabilities and magnetic abnormalities in ordered crystalline state alloys of Fe3Pt at high pressure
2015, 64 (14): 146301. doi: 10.7498/aps.64.146301
The ordered crystalline Fe3Pt invar alloy is in a special magnetic critical state under which the lattice dynamic stability of the system is extremely sensitive to the external pressure. Using projection augmented plane wave method based on the density functional theory, we calculate the dependences of enthalpy and magnetism of different crystalline alloys of Fe3Pt on external pressure. The results reveal that the P4/mbm structure is thermodynamically stable under pressures below 18.54 GPa. The total magnetic moments of Pm3m, I4/mmm and DO22 structures decrease rapidly and oscillate near the ferromagnetic collapse critical pressure. Furthermore in I4/mmm and DO22 structures the atomic magnetic moment of Fe1 reverses near the critical pressure. At a pressure of 43 GPa, the micro-ferrimagnetic property in DO22 structure becomes apparently strengthened while its volume increases rapidly. The lattice dynamic calculation shows that the spontaneous magnetization of the system in ferromagnetic states induces the softening of the transverse acoustic phonon TA1(M) in the Fe3Pt of Pm3m structure, and there exists a strong spontaneous volume magnetostriction at pressures below 26.95 GPa. Especially, in the pressure range between the ferromagnetic collapse critical pressure 41.9 GPa and the magnetism completely disappearing pressure 57.25 GPa, the lattice dynamic stability is more sensitive to the pressure. The pressure induces the stability of the phonon spectrum of the system at pressures above 57.25 GPa.
2015, 64 (14): 146501. doi: 10.7498/aps.64.146501
The rapid development of nanotechnology makes it possible to further understand nanoscale heat conduction. Theoretical analysis and experimental measurement have demonstrated the size-dependence of thermal conductivity on a nanoscale. As dielectric material (such as silicon), phonons are the predominant carriers of heat transport. Phonon ballistic transport and boundary scattering lead to the significant reduction of thermal conductivity. Various models, in which only one geometrical constraint of phonon transport is considered, have been proposed. In engineering situations the phonon transport can be influenced by multiple geometrical constraints, especially for material with long intrinsic phonon mean free path. However, at present a phonon thermal conductivity model in which the multiple geometrical constraints of phonon transport are taken into account, is still lacking. In the present paper, a multi-constrained phonon thermal conductivity model is obtained by using the phonon Boltzmann transport equation and modifying the phonon mean free path. The geometrical constraints are dealt with separately, and the effects of these constraints on thermal conductivity are then combined by the Matthiessen's rules. Different boundary conditions can lead to different influences on the phonon transport, so different methods should be used for different boundary constraints. The differential approximation method is utilized for the constraint in the direction of heat flux, while phonon scatterings on side surfaces are characterized by modifying the phonon mean free path. The model which characterizes various nanostructures including nanofilms(in-plane and cross-plane) and finite length rectangular nanowires, can well agree with the Monte Carlo simulations of different Knudsen numbers. The model with the Knudsen number Knx equal to 0 can well predict the experimental data for the in-plane thermal conductivity of nanofilm. When the Knudsen numbers Kny and Knz vanish, the model corresponds to the cross-plane thermal conductivity of nanofilm. Moreover, with Knx=0 and Kny=Knz, the model corresponds to the square nanowires of infinite length, and the similar slopes between the model and the experimental data of nanowires can be achieved.
2015, 64 (14): 146801. doi: 10.7498/aps.64.146801
Based on molecular dynamics method and in-situ scanning electron microscopy (SEM) observations, the damping efficiency of the porous metal coating is discussed in this paper. Molecular dynamics simulation is performed to study the plastic deformations of Cu films with vibration. In the simulation, embedded atom method (EAM) is selected and in the method an interatomic potential function is used. And porous copper coating is carried out for calculating by using velocity-verlet algorithm. The plastic deformation is due to the dislocation nucleation near free surfaces, and the dislocation is shaped into forward emission in the crystal orientation near the defects. At the same time, the change curves of stress and strain are drawn by origin software. Damping factor (η) is calculated by using the time of strain lagging stress. The regulation of elastic potential energy attenuation is obtained by energy calculation. On the other hand, in-situ tensile/compression experiment is conducted by the FEI Quanta 200 SEM with a maximum load capacity of 2 kN at room temperature. A copper layer is deposited on the surface of the polyimide film by the electron beam evaporation deposition method. The thickness of the copper layer is 10 μm and the thickness of polyimide is 175 μm. Using the scanning electron microscope, microstructures of the coating are observed. It could be seen that the coating and the polyimide film are both better in compactness. Using in-situ testing machine at SEM, the samples with and without copper coatings are respectively tested under tensile and unloading. The rate of displacement loading is 2 mm/min, the results of load (F) and displacement (l') are printed every 0.1 s. The loading direction is horizontal. During in-situ tensile/compression test, the straining is stopped several times in order to make the observations and take micrographs. The digital SEM images are directly transferred to a computer via a direct memory access type A/D converter, which can rapidly capture clear images of the 1024×943 pixel frames. The simulations and experimental results indicate that the dislocation near defects get rid of weak pinning points and limit to the strong pinning point, the internal friction is generated due to the change of dislocation and the relative sliding near grain boundary, and the stored elastic potential energy is consumed, which causes the damping effect of the film.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (14): 147102. doi: 10.7498/aps.64.147102
BiTiO3 of C1 C1 structure is found to be the most stable phase according to our first-principles calculations for nine possible structures, with corresponding optimized crystal parameters of a=b=5.606 Å, c=9.954 Å; α=β=105.1°, γ=61.2°. Subsequently, we have investigated the electronic and optical properties of BiTiO3 in C1C1 structure. It is found that BiTiO3 is a semiconductor with an indirect band gap with its energy band near Fermi level being dominated by O-2p and Ti-3d levels. Additionally, the dielectric function, refractive index, and reflectivity of BiTiO3 are also calculated, and it is shown that the optical properties of BiTiO3 are nearly isotropic.
Effect of bipolarons on spin polarized transport in magnetic permeated sublayer of organic spin device
2015, 64 (14): 147104. doi: 10.7498/aps.64.147104
According to the permeation phenomenon of magnetic atoms in organic device, such as Co/organic semiconductor (OSC)/La0.7Sr0.3MnO3, the evolution of spin polarons and spinless bipolarons are calculated with the drift-diffusion equations to investigate the effect of polaron-bipolaron interaction on spin polarized transport in a magnetic permeated sublayer (MPS). It is found that the MPS has different spin-flip time and mobility from those in pure organic semiconductor. The splitting of spin-flip time will be adjusted by the effect of the magnetization of the impurity atoms. Mobilities of spin carriers in the MPS will be reduced due to the scattering of the Co atoms. Both the spin-flip time and the mobility will affect the polaron-bipolaron interaction and further influence the spin polarized transport. It is found that the splitting of spin-flip time is the main factor responsible for the spin relaxation, while the polaron-bipolaron interaction is the secondary factor.
First-principles study of the electronic structure, magnetism, and spin-polarization in Heusler alloy Co2MnAl(100) surface
2015, 64 (14): 147301. doi: 10.7498/aps.64.147301
Using the first-principles calculations within the generalized gradient approximation frame work, we systematically investigate the atomic relaxation, electronic structure, magnetism, and spin-polarization in L21 and B2 structure of Heusler alloy Co2MnAl (100) surface. Due to the difference of Co–Mn and Co–Al bonding, surface atoms in the L21 and B2 structure prefer to move toward and away from the vacuum. By comparison with the bulk, the spin magnetic moments of surface Co and Mn atoms are obviously enhanced due to the surface effects. Our electronic structure calculations show that the gap in L21 structure of bulk Co2MnAl has been destroyed by the surface states and the spin-polarization of CoCo atomic terminated surface in both structures decreases. However, the spin-polarization of MnAl atomic terminated surface is not significantly affected by the surface effects and has a large value in both structures, and this may be the potential in application to magnetic tunneling junctions.
2015, 64 (14): 147302. doi: 10.7498/aps.64.147302
To achieve the surface plasmon resonance (SPR) mode splitting in infrared wavelength band, and to improve the figure-of-merit (FOM) of grating based SPR sensor, in this article we present a new composite grating structure, which consists of double metal gratings, and study the gas sensing performance. Split modes of SPR in composite metal grating are observed by using the finite difference time domain method. The original structure symmetry is broken and changed with increasing relative displacement between the double gratings, as a result, the resonant modes move to opposite directions. Calculated electric field distribution of the two separate resonant modes displays two different degrees of coupling effect between the double gratings. When the relative displacement is further increased till the double gratings are connected to form a new symmetrical single grating, the separate resonant modes will merge into another single resonant mode. If the refractive index of analyte (na) is in a range 1.01≤na≤1.05 and the relative displacement of double gratings is zero, the wavelength sensitivity based on composite metal grating gas sensor reaches 1207.5 nm/RIU (per refractive index of unit) and the FOM is obtained to be 1290.7, while the relative displacement of the double gratings is 100 nm, for the double split modes the wavelength sensitivities are 1205.0 nm/RIU and 1210.0 nm/RIU, respectively, and the corresponding FOMs are 1295.4 and 762.3. Therefore, the high FOM of the composite grating based on SPR sensor possesses great potential applications in biochemical sensing.
2015, 64 (14): 147802. doi: 10.7498/aps.64.147802
The Dirac cones in photonic crystals have aroused much interest in the last few years. Annular photonic crystals have also been well studied for designing and controlling the band gap because they have more parameters than usual photonic crystal. In this paper, we study a two-dimensional square lattice dielectric annular photonic crystal to explore the formation of the photonic Dirac cone by the accidental degeneracy method. The theoretical tool is the plane wave expansion method. The results show that this system can provide a Dirac point in the center of the Brillouin-zone in the photonic band if both the outer radius and the inner radius of each scatterer are chosen to be appreciate values when the dielectric refractive index of the annular rod is fixed. For example, there is a Dirac point at the photonic normalized frequency f=0.438(c/a) when n=3.4, RO=0.42a, RI=0.305a, where f is the frequency, c is the light speed in vacuum, a is the lattice constant, n is the refractive index, RO is the outer radius, and RI is the inner radius. It is also found that within a confined region of outer radius RO(0.37aROa), when a Dirac point is realized in the annular photonic crystal (n>1.4), the inner radius RI and the outer radius RO obey a relation of RI=-1.104+8.167RO+(-11.439)RO2, which is unrelated to the refractive index n of the dielectric annular rod. If n is less than 1.4, this rule is not valid. At the same time, the normalized frequency at which the Dirac point is realized, decreases with increasing both refractive index n and outer radius RO. Especially, the curves of the relation between photonic frequency f and outer radius RO almost do not change their profiles but only be shifted up and down with changing the refractive index n. Based on this, we also design and predict the annular photonic crystal which provides a Dirac point. The goal is to obtain the other relative parameters (frequency f, outer radius RO and the inner radius RI) of the photonic crystal system if the refractive index n is fixed. The values of the prediction agree very well with the values obtained by the rigid theoretical calculation within a relative error of only 4%.
2015, 64 (14): 147103. doi: 10.7498/aps.64.147103
The method of co-doping is very useful to improve the photocatalytic performances of titanium dioxide nanotubes. The absorption capacity to the visible light of the titanium dioxide nanotubes can be improved significantly in experiment by doping both N and F in titanium dioxide nanotubes, but the theoretical explanations are still not clear. Doping the atom N alone, the atom F alone, and both N and F in titanium dioxide nanotubes respectively, their atomic structures, electronic properties and optical performance are studied by the first principles method based on the density functional theory. It is found that formation energies are lower in titanium-rich environment than that in oxygen-rich environment. In titanium-rich environment, the N-F co-doped TiO2 nanotube has the low formation energy and stable thermodynamic system compared with the N alone and the F alone doped TiO2 nanotube. Besides, the O3C can be replaced more easily than the O2C when doping N alone, F alone and co-doping N-F in TiO2 nanotube. By analyzing the energy band, we can find that the band gap changes little with doping N and the change of the band gap for the co-doping N-F case is the most prominent, which reduces by 0.557 eV compared with that for the un-doped TiO2 nanotube case, and this is mainly from the contributions of the impurity level near the top of the valence band. Besides, the different charges are calculated and it is indicated that the ability to gain electrons of N is stronger than that of F, and through analyzing the photocatalytic performance, it is found that though the gap of the nanotube is larger than that of the body, the reducibility of nanotube is better than that of the body. Both the reducibility and the oxidability of the nanotube are reduced but its activity is not lost when co-coping the atoms of N and F in titanium dioxide nanotubes. Moreover, the optical absorption spectrum shows that the red shift phenomenon is obvious for doped system and also for the co-doped system. Therefore, co-doping both N and F in titanium dioxide nanotubes is the most useful method to improve the photocatalytic performances of the TiO2 nanotubes.
Influences of shear deformation on electronic structure and optical properties of B, N doped carbon nanotube superlattices
2015, 64 (14): 147304. doi: 10.7498/aps.64.147304
Carbon nanotubes, one of the most advanced nanoscale materials, have attracted much research attention since they exhibited semiconductor, metal or insulator properties depending on their geometric structures. Carbon nanotubes have great potential in various applications in electronic and optical device. Dopants to the carbon nanotubes intentionally could offer a possible route to change and tune their electronic, optical properties. Another important and effective method is to deform the carbon nanotubes structure. Superlattice structures can offer extra degrees of freedom in designing electronic, optical devices. To understand the involved mechanism, in this paper, the geometry structures, electronic structures and optical properties of the armchair carbon nanotube superlattices doped cyclic alternately with B and N under different shear deformations are investigated by the first-principles method through using the CASTEP code in MS 6.0. It is found that the structures of carbon nanotube superlattices can be dramatically changed by shear deformation. When the shear deformation is less than 9%, the optimization geometry structures of carbon nanotube superlattices are still similar to tubular structures, when the shear deformation is greater than 12%, the geometry structures of these systems have large distortions. The results about the binding energy show that the shear deformation changes the stability of the armchair doped carbon nanotube superlattice. The larger the shear deformation, the lower the stability of the doped carbon nanotube superlattices is. The analysis of charge population show that the covalent bond and ionic bond coexist in the armchair carbon nanotube superlattices doped cyclically alternately with B and N. The band gap of the carbon nanotube superlattice is affected by N, B dopants, as a result, the carbon nanotube superlattice changes from a metal to a semiconductor. Compared with the (5, 5) nanotube superlattices, the band gaps of the (7, 7), (9, 9) doped carbon nanotube superlattices increase. With increasing the shear deformation, the band gap of the doped carbon nanotube superlattices decreases gradually, when the shear deformation is greater than 12%, the band gap changes into 0 eV, the carbon nanotube superlattice changes back into a metal from a semiconductor. The analysis of density of states obtains the same conclusions as the energy band analysis. In optical properties, compared with the armchair carbon nanotube superlattices doped cyclically alternately with B and N without shear deformation, those systems under shear deformation have the peaks of the absorption coefficient and the reflectivity that are all reduced, and are all red-shifted.
Experimental study of photon correlation spectroscopy for the long-range fluctuation of polarization in ferroelectrics
2015, 64 (14): 147801. doi: 10.7498/aps.64.147801
Based on the theory of random fluctuation of polarization clusters and the model of Wiener random process, the relaxation law of long-range fluctuation of polarization and the possible forms of light intensity autocorrelation function g2(τ) measured from photon correlation spectroscopy (PCS) experiments have been derived. The relationship between relaxation mechanisms of microscopic polarization clusters and macro relaxation laws is expounded. This research supplies a theoretical model for the application of PCS in researching the relaxation process of polarization clusters in ferroelectrics. Based on the improved He-Ne laser PCS experimental set-up, the relaxation process of long-range fluctuation of polarization clusters in BaTiO3 and 0.71Pb (Mg1/3Nb2/3) O3-0.29PbTiO3 single crystals near phase transition temperature is studied. As for BaTiO3, the dual relaxation processes of long-range fluctuation of polarization clusters are observed at temperatures above TC+4 K, which may be related to its order-disorder mechanism of phase transition. For 0.71Pb (Mg1/3Nb2/3) O3-0.29PbTiO3, the dual relaxation processes exist on both sides of the cubic-tetragonal phase transition temperature. The PCS experimental results are fitted well by the derived theoretical model, and the characteristic relaxation times of long-range fluctuation of polarization clusters are extracted. Two relaxation times, τs and τl corresponding to short and long relaxation time, respectively, are initially observed, where τs is several microseconds, and τl is tens of microseconds. The abrupt increase of relaxation times at phase transition temperature and the phenomenon of critical slowing down can be observed in the two samples.
2015, 64 (14): 147101. doi: 10.7498/aps.64.147101
How to construct an accurate interatomic potential function is an important and basic problem in the simulation procedure. Using first-principles method, the single atom energies in different lattice constants are calculated to achieve the ground state curves of Au and Ag. These energies are calculated in the Perdew and Zunger form of the local-density approximation ultra-soft pseudo potentials. The cut-off energies of the plane wave bases of Au and Ag are set to be 320 eV and 300 eV respectively, which are sufficient for their full converge. The Brillouin zone is all sampled with a 12×12×12 Monkhorst-Pack mesh of k points for Au and Ag. Allowable error in total energy is smaller than 1×10-6 eV per atom. The lattice cohesive energies in different lattice constants are calculated to achieve the lattice energy and atom distance curves after subtracting the value of ground state energy from each of these energy. Then the accurate inversion potential curves are obtained according to the Chen-Möbius inversion theory and self-compiled program. Based on the fitting consequences of inversion potential curves, using different potential function formulas, a double exponential potential function to fit the inversion potential is presented. This function provides the accurate formulas and parameters for the following research. Moreover, the phonon spectra and the densities of states of Au and Ag are calculated respectively by using the inversion potential data, the embedded atom method (EAM) potential theory and first principles method to verify the reliability of the inversion potential. The comparison of the results among the three methods shows that the tendencies of these curves are similar. But they still have some deviations especially in the range of high frequency. However these curves indicate that the inversion potential can reasonably reflect the interaction between atoms. Meanwhile, the inversion potential method has great advantage in calculation quantity compared with the EAM potential method. The inversion method needs less time in calculation. In addition, the thermal expansion coefficients, the elastic moduli and the Grüneisen constants of Au and Ag are also calculated based on the fitting formulas and parameters. The results agree well with the experimental data, which implies that these inversion potentials are effective and accurate.
2015, 64 (14): 147303. doi: 10.7498/aps.64.147303
Photoinduced enhancement effect of the metal nanoparticle is one of the hot topics in the field of nanomaterial. Interaction between one molecule and a number of metal nanoparticles in different configurations in an applied external field is theoretically investigated in the scheme of density matrix theory, where the molecule and metal nanoparticles are excited simultaneously, and the subsequent charge transfer dynamics is simulated. Besides, the Coulomb interactions between the molecule and metal nanoparticles are calculated in the framework of dipole-dipole approximation. Parameters for metal nanoparticles with a 10 nm radius are used in the text and the polarization of the molecule has the same direction as that of external laser field. It is found that plasmon enhancement is closely related to the relative positions between the molecule and metal nanoparticles. Effects of enhancement due to the surface plasmon is discussed in detail for different configurations of the molecule and metal nanoparticles, and the surface plasmon hybridization, as well as the molecular excitation energy and the frequency of external field applied. Plasmon hybridization levels are formed when metal nanoparticles have strong enough interactions between themselves. The blue shift of the resonant frequency can be found for shorter distance of different metal nanoparticles. In the case that the centers of mass of metal nanoparticles and the molecule are on the same plane, it is found that the population in excited state of the molecule at a resonance frequency increases for a shorter distance between metal nanoparticles and the molecule. On the contrary, in the case that the centers of mass of four metal nanoparticles are located in a plane which is parallel to the x-y plane and above it by 10 nm, the population in the excited state of the molecule on resonant frequency will decrease at a shorter distance between the four metal nanoparticles.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Growth behaviors and forced modulation characteristics of dendritic sidebranches in directional solidification
2015, 64 (14): 148101. doi: 10.7498/aps.64.148101
Growth behaviors of dendritic sidebranches under random noises and characteristics of sidebranches modulated by external forces in directional solidification are studied by using phase field simulations. Simulation results show that, under random noises no regular sidebranches appears all the time, but sidebranches can be formed within a suitable range of frequencies. Moreover, strongly correlated sidebranches are formed at a fixed frequency and in a certain period, usually called a burst, to appear. There is no correlation between different bursts, but the frequency of every sidebranch within a burst is the same as its precursor, and this fixed frequency is consistent with the peak frequency of the whole spectrum of sidebranch. By introducing a time-periodic external force with a frequency in the range of the whole spectrum, regular dendritic sidebranches can be induced, and they can be most developed if the frequency of the external force is the same with that in a burst. The simulation results can provide guidance to control dendritic morphologies in solidification.
2015, 64 (14): 148502. doi: 10.7498/aps.64.148502
GaN based light-emitting diodes (LEDs) have been attracting a great deal of interest due to their capability in emitting a spectrum from ultraviolet to green and their applications in traffic signals, displays and solid-state lighting. However, the high efficiency of LED is still obstructed by light-extraction efficiency. In this work, we propose that light-extraction efficiency of GaN-based blue LED should be improved by a self-assembled monolayer of polystyrene spheres. The GaN-based LED grown on sapphire substrate emits the light mainly from the indium tin oxide (ITO) transparent electrode. And the hexagonal closely-packed polystyrene sphere monolayer is formed onto the ITO layer. In order to study the light-extraction efficiency affected by the size of nanosphere, nanosphere monolayers of different sizes are prepared onto the ITO layer, and the diameters of the polystyrene spheres are 250, 300, 450, 600 and 950 nm, respectively. The electroluminescence results show that using polystyrene sphere monolayer can improve the light-extraction efficiency compared with using the conventional LEDs, and the light-extraction efficiency reaches a maximum when the average size of spheres (450 nm) approximates to the wavelength (465 nm) of that light. The light output power of the LED with polystyrene sphere of the optimum size is experimentally enhanced by 1.34 and 1.25 times under the injection currents of 20 and 150 mA, respectively. In order to explain the physical mechanism of the light-extraction enhancement, we carried out the three-dimensional finite difference time-domain simulation thereby calculate the transmission spectrum of the structure. The results of simulation show that the incident light beyond the critical angle can be partly extracted when the surface of LED has a polystyrene sphere monolayer, leading to an enhanced light-extraction efficiency. So the nanosphere monolayer acts as a two-dimensional diffraction lattice which behaves as a light scattering medium for the light propagating in a waveguiding mode within the LED. Furthermore, the polystyrene nanosphere has the advantages of low-cost and high-precision, and is very suitable for large area preparation on LEDs. So this method is a simple and cost-effective method to improve the light-extraction efficiency from LED.
2015, 64 (14): 148102. doi: 10.7498/aps.64.148102
The plasticity of material is associated closely with the movement and proliferation of dislocation. Therefore, in the deformation and plasticity theory the dislocation kinetics is an important topic. In the case of no magnetic field, the conventional dislocation kinetics normally focuses on the dislocation microstructure, nucleation and mobility, and the inherent relationship between electron spin and plasticity is seldom concerned. As a matter of fact, the electron rotation is directionless and unordered in the absence of magnetic field, so the electron behavior will not take an apparent effect on the microstructure and properties of material. Nevertheless, in the presence of magnetic field the case is different. The magnetic field will influence the electron spin and, therefore, atomic rearrangement. The dislocation behavior and plasticity will also be affected by the magnetic field, which is called the magnetoplastic effect. In this paper, on the basis of magnetoplastic effect the dislocation kinetics involving dislocation stress, mobility and others is discussed both qualitatively and quantitatively. It has rarely reported currently in the literature. The pulsed magnetic field is first utilized to process solid nanometer alumina particulates reinforced aluminum matrix composites. The experimental results demonstrate that the dislocation density increases with magnetic induction intensity increasing from zero to 3 T. The phenomenon reveals the characteristic of plastic deformation in a processed sample. The further theoretical analysis displays that the generated magnetic force is not large enough to activate the dislocation movement. The fundamental reason lies in the magnetoplastic effect, that is, the magnetic field brings about the transition of electron spin in the radical pairs between paramagnetic dislocation cores and obstacles. The radical pairs tend to be conversed from the singlet state with high bonding energy to the triplet state with low bonding energy, therefore, the prerequisite energy for dislocation to surmount the obstacles will be lowed and the depinning tendency will be apparent. In a period of dislocation movement, the rate limiting consists in the dislocation stopping at the obstacle; on the contrary, the electron excitation and atomic arrangement governed by the magnetic field take negligible time. Hence, it can be seen that the performance of magnetic field is highly efficient. The critical magnetic induction intensity is calculated to be 3 T. That is, when the intensity is lower than 3 T, the magnetoplastic effect becomes strong with the increase of magnetic induction intensity and action time; when the intensity is higher than 3 T, the effect changes gently. Under this critical magnetic induction intensity, the dislocation velocity is deduced to be on the order of 10-3 m/s. Moreover, the dislocation length will be increased by two orders of magnitude. The displacement of dislocation is proportional to the square of magnetic induction intensity and action time of magnetic field. To sum up, the magnetic field treatment has been proved to be an efficient approach to improve the plasticity of material. The prospective research will focus on the mechanical properties of alloys or composites subjected to magnetic field, together with tensile stress so as to acquire the effect of magnetic field parameters of macro plasticity of materials.
2015, 64 (14): 148103. doi: 10.7498/aps.64.148103
High-density (～1010 cm-2) silicon nanowires are grown directly from n-(111) single crystal silicon based on solid-liquid-solid mechanism by using Au-Al films as metallic catalyst. The results indicate that the optimal parameters to realize Si nanowires with high density and uniform distribution are as follows. The thickness of Au-Al film is between 5 and 15 nm, the temperature is 1100℃, and the flow of N2 is 1.5 L/min. The diameters and lengths of the formed Si nanowires are 100 nm and from several micrometers to sereral tens of micrometerss, respectively. Then Eu-doped Si nanowires are studied. The influences of the different lengths of Si nanowires, doping temperature (900-1100℃), and doping time (15-60 min) on the luminescence of Eu3 + are experimentally investigated. The morphologies and microstructures of the SiNWs, the photoluminescence properties and growth crystall orientations are characterized and analyzed by the scanning electron microscopy, the Hitachi F-4600 fluorescence spectrophotometer and X-ray powder diffraction. The results show that the Eu-doped Si nanowires have a stronly red luminescencent with an emission peak position at 619 nm (5D0→7F2) when the doping temperature is 1000℃, the grow time of SiNWs is 30 min, and the optimal excitation wavelength is 395 nm. At the same time, there are four emission bands of 576 nm (5D0→7F0), 596 nm (5D0→7F1), 658 nm (5D0→7F3), and 708 nm (5D0→7F4) that are observed. Compared with the scenario of the silicon substrate, the Eu-doped Si nanowires present strong red light emission. The photoluminescence properties of Eu-doped Si nanowires have potential applications in the lighting and the silicon optoelectronic integration. However, the parameters of Si nanowires such as diameter, density, surface morphology have great influences on the photoluminescence properties of Eu-doped Si nanowires, which are necessary to be further studied.
2015, 64 (14): 148501. doi: 10.7498/aps.64.148501
Memristor is defined as the fourth basic electronic element, the studies on its models exhibit diversity. Now, the matching extent between memristor model and natural memristor has received researchers' wide attention. A new memristor model is proposed by changing the ion diffusion term of the WOx memristor, namely, adding another two internal state variables τ and μ which denote the relaxation time and retention, respectively, and the improved model can simulate natural memristor better. Firstly, the new one is able to not only describe the general characteristics of a memrsitor, but also capture the memory loss behavior. In addition, the new memristor can be considered as a neural synapse, under the action of the input pulses with different amplitudes, duration and intervals, the spike rate dependent plasticity, short-term plasticity (STP), and long-term plasticity (LTP) are analyzed, and the ''learning experience'' phenomenon which is very similar to the biological system is discovered, most of which is due to the back diffusion of the oxygen vacancies during the intervals of the input pulses which are caused by the concentration difference. Moreover, an exponential decay equation is built to describe the relaxation process of STP. Finally, taking into consideration the relationship between temperature and ion diffusion coefficient, the effect of temperature on the relaxation process of STP is discussed. Experimental results show that the new memristor model can better match the actual behavior characteristics, and more suitably acts as a synapse for being applied to neuromorphic systems.
Simulation on relationship between power/phase stability of low frequency oscillatory potentials and activity of dipole current
2015, 64 (14): 148701. doi: 10.7498/aps.64.148701
The physical parameters, e.g. power and phase, are usually employed in the neural analysis of brain rhythms, which are important in brain function and disease diagnosis. Though there has been extensive work, how both parameters are related to the electrical properties of brain tissue and the sources of brain rhythms has not been fully understood. To address the issue, a simulation is done based on the theory of dipole current. When referring to the solution to the forward problem in electroencephalograph, the brain is regarded as a homogenous sphere model, the electrical features of brain tissue are described by an isotropic electrical conductivity. The source of brain rhythms is simulated by the quasi-static dipole current whose activity is described as a sine oscillation at low frequency. The electrical field generated by the dipole current is considered to be quasi-static. By changing the amplitude and the phase time course of oscillatory dipole current, the distribution of potentials produced by the dipole current at a time-point could be calculated by applying the finite element method to the sphere model. Over a time period of sine oscillation, the oscillatory potentials regarded as the brain rhythms could be produced. Instantaneous power and phase of simulated rhythms are estimated by Hilbert transform, and then a method of phase stability in narrow-band is developed for a single oscillator. To highlight this method, three manners are employed to describe it, i.e., mean relative phase value termed phase preserved index, histogram on rose plane, and phase sorting with the help of EEGLAB. Finally the relationship between two physical parameters and the electrical features of brain tissue/the source activity of brain rhythms is investigated under the conditions of (an) isotropy of conductivity, linear or nonlinear phase dynamics and amplitude, eccentricity of dipole current, etc. The statistical methods of t-test and bootstrapping technology are performed respectively to show the significance of power and phase stability. It is obtained that the power of simulated rhythms decreases with the increase of electrical conductivity, and it is not only proportional to the square of the amplitude of dipole current, but also correlated with the anisotropy of conductivity and the locations of dipole current as well as meshes on the sphere model, however no relevance to other factors. On the contrary, the phase stability of simulated rhythms is correlated only with the non-linear time course of their own phase dynamics. The results imply that the power of brain rhythms is related to many factors such as brain tissue and amplitude of rhythm generator as well as placements of recording electrodes, but the phase stability is related only to the non-linear phase dynamics of brain rhythms. Thus, the electrical significance of the power is more complicated than that of the phase stability. This work might be helpful for understanding in depth the significance of both physical parameters from the perspective of electricity. The narrow-band phase stability of simulated rhythms could highlight the non-linear phase dynamics. It is hypothesized that the phase stability could not only map the synchrony in the neural activity as a custom means of phase coherence, but also reflect directly the non-linearity in phase dynamics, and the more divergent the phase dynamics, the lower the phase stability is, and vice verse. Therefore it is suggested that the phase stability of brain rhythms could be related closely to the non-linear factors to affect the phase dynamics of brain rhythms, e.g., the non-linear phase dynamics of rhythm generators. It is also suggested that both parameters of power and phase stability would offer more neural information.
Investigation of photo-induced phenomenon in the silicon nanowires made by chemical etching in HF/Fe(NO3)3 solution
2015, 64 (14): 148201. doi: 10.7498/aps.64.148201
The photo-induced phenomenon in the silicon nanowires made by chemical etching in HF/Fe(NO3)3 solution is investigated systematically by using monocrystal n-type silicons with different doping concentrations as substrates, silver as catalyst, and iron nitrates with different concentrations as oxidants. It is found that the length of silicon nanowires is determined not only by the doping concentration of substrate and the mass of oxidant, but also by the photo-induced effect. The prepared silicon nanowires may have potential applications in green energy storage device and the substrate material for sensor. In this paper, we discuss the formation mechanism from the band structure, electrochemical characterization and photoluminescence in depth. The results in this paper provide physical theoretical evidence for the development of the method, and have important guiding significance to promote the technology.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Raman spectra based pressure calibration of the non-gauge sapphire anvil cell at high temperature and high pressure
2015, 64 (14): 149101. doi: 10.7498/aps.64.149101
Gem anvil cell is on important tool in high pressure experimental research, and the key of its application is the accurate calibration of the pressure in sample chamber. To date, the pressure has been routinely calibrated by the extra gauge such as ruby. This may increase the difficulty in building a setup and changing the chemical environment, even chemical reaction happens with the sample, thereby degrading the experimental results. In this study, using the synthesized pale sapphire and the heatable Zha-Bassett type cell, the relationships between Raman shift of sapphire-anvil interface and the pressure, and also temperature in chamber are investigated by the confocal Raman microscope at 0-6.3 GPa and 300-573 K, which is used to establish a non-gauge sapphire anvil cell system. The result shows that the pressure induced Raman shift of sapphire anvil at room temperature is 1.6443 cm-1/GPa and the temperature induced shift at room pressure is -0.0198 cm-1/K. We fit the experimental data at simultaneous high temperature and high pressure (HTHP) and find that: ∂ν12/∂T=-0.01913-0.00105×P, ∂ν12/∂P=1.9158-0.00105×T. The effect between the pressure and temperature can be described by ∂ν12/∂P∂T=-0.00105. After this calibration: P=(Δλ-0.01913×ΔT)/(1.9158-0.00105×ΔT), the pressure in the sample chamber can be calculated by the Raman shift of the interface of anvil cell in the HTHP experiment, which can be directly used in hydro-thermal reaction system and has great importance in physics, material science and geoscience.
Co-phasing detecting method based on grating dispersed fringe for Fizeau optical interferometric telescope
2015, 64 (14): 149501. doi: 10.7498/aps.64.149501
For the Fizeau optical interferometric telescope system, the co-phasing detection of piston errors between sub-apertures plays an important role in realizing the high resolution of system. In this paper, the relationship between piston error and the main peak displacement of monochromatic interferogram of two sub-aperture system is analyzed based on physical principles, then the piston error detecting method is developed and clarified based on their linear relationship in the range of one wavelength. Furthermore, an innovative co-phasing detecting method based on grating dispersed interferogram with bandwidth light source is proposed, and its feasibility, detecting precision and dynamic range are analyzed in theory and studied in simulation. The results prove that with this method, the piston error between the two sub-apertures of the system can be soundly detected and its measuring error is less than 20 nm while the piston error is not more than 50 μm. In addition, the novel method solves the problems of 2π ambiguity and direction determination that might exist within some other detecting methods. Besides its millimeter level dynamic range, this new co-phasing detecting method provides a new way and an effective reference for in-depth research of co-phasing detecting techniques.
2015, 64 (14): 149201. doi: 10.7498/aps.64.149201
Striation in the ocean is a research frontier in physical oceanography. Interestingly, it has some “sisters and brothers” in Mother Nature, such as the Jovian belts, subtropical jet streams in the atmosphere, and zonal flows in plasma. This meso-scale oceanic phenomenon is, however, concomitant with but covered up by the macro-scale ocean currents or circulations. In order to unveil such zonal jet-like structures, a spatial filtering must be applied to the commonly available time-average data. Previous studies mostly focused on prominent features of striations, such as banded structures, and the generation mechanism; however, the differences revealed by applying different types of filtering methods have not received enough attention. In this paper we present a comprehensive study on the effectiveness of the different detection approaches to unveiling the striations. Three one-dimensional filtering methods: Gaussian smoothing, Hanning and Chebyshev high-pass filtering, are used to analyze SODA data and LICOM model outputs. The first two methods have been used in many previous studies; on the other hand, the Chebyshev filter is a newcomer for this purpose. Our results show that all three methods can reveal ocean banded structures, but the Chebyshev filtering is the best choice. The Gaussian smoothing is not a high pass filter, and it can merely bring regional striations, such as those in the Eastern Pacific, to light. The Hanning high pass filter can introduce a northward shifting of stripes, so it is not as good as the Chebyshev filter. In addition, a cutoff frequency is often needed in applying the high-pass filter, and this frequency depends on the spectrum analysis of the original data. In this paper, we discuss the filtering output and its spatial power spectra of three normalized cutoff-frequencies, 0.1, 0.3 and 0.7. When the cutoff-frequency is too low, the filtering is insufficient; on the other hand, if the cut-off frequency is too high, excessive filtering can happen. Our study shows that for analyzing the global ocean striations, the best normalized cutoff frequency domain is between 0.1 and 0.4. In addition, the bandwidth of striation for using the Chebyshev high pass filter to analyze the SODA data in a depth of 300 m is 150-300 km. In the general case, we propose to use the Chebyshev filter in lieu of Hanning or other methods for investigating ocean striations.
Simulation study on the correlation between the ground cosmic rays and the near earth thunderstorms electric field at Yangbajing (Tibet China)
2015, 64 (14): 149202. doi: 10.7498/aps.64.149202
Coincident study on the intensity change of the ground cosmic rays during thunderstorms is very important for understanding the acceleration mechanism of secondary charged particles caused by atmospheric electric field. It is found that the strength of the near earth thunderstorm electric field can be up to 1000 V/cm or even higher from ARGO-YBJ (where YBJ stands for Yangbajing, 4300 m a.s.l., Tibet, China) data in 2012. In this paper, Monte Carlo simulations are performed by using CORSIKA program to study the correlations between the intensity of the ground cosmic rays and the near earth thunderstorm electric field at YBJ. When the atmospheric electric field strength is higher than the threshold field strength (ERB) for the development of a runaway breakdown process, the total number of electrons and positrons is exponentially increases. At an electric field strength of 1500 V/cm, the number increases exponentially and reaches a maximum value at an atmospheric depth of ～520 g/cm2, where the electric field is slightly stronger than the threshold field strength. These results are consistent with the theoretical results of relativistic runaway electron avalanche (RREA) which was proposed by Gurevich et al. (Gurevich A V, Milikh G M, Roussel-Dupre R 1992 Phys. Lett. A 165 463) and also supports Dwyer's theory. The total number of electrons and positrons increases with the strength of the field in the negative field or in the positive field greater than 600 V/cm, while a certain degree of decline occurs in the positive field less than 400 V/cm. In the range 400-600 V/cm, the energy of the primary proton should be taken into account. For the primary energy that is lower than 80 GeV, the total number of electrons and positrons increases. And it does not change obviously when the energy is between 80 GeV and 120 GeV. For the primary energy exceeds 120 GeV, the number drops off, and the decrease is of ～4%. During thunderstorms, a short duration occurs in which the single particle counting rate increases as energy lowers, while a decrease happens with energy becoming higher than that from ARGO-YBJ data. Our preliminary results can give reasonable explanations to the experimental observations of ARGO-YBJ.
2015, 64 (14): 149701. doi: 10.7498/aps.64.149701
Lots of research work on the X-ray pulsar navigation detector has been carried out, but the rigorous quantitative calibration results of these detectors have never been given out. In order to further promote the research process in X-ray pulsar navigation field, a kind of high-precision X-ray source which could be tunable in energy distribution and photon flux is necessary. A new method has been proposed to generate such an X-ray source, in which a conventional X-ray tube which is called the original level tube is used to excite a special fluorescence target to generate fluorescence X-ray source. Firstly, the primary experiment system is built, with which the intensity of the fluorescence X-ray source is measured in atmospheric environment. The result indicates that the fluorescence X-ray source can be implemented in vacuum system to calibrate the detectors. Then an integral fluorescence X-ray source is fabricated which can be used in vacuum system and its intensity is measured under the condition of Dx=300 cm and Ia=200 μA. The photo fluxes are obtained to be 19.57 phs/s at 4.51 keV, 25.22 phs/s at 5.41 keV, and 33.27 phs/s at 8.05 keV. These data demonstrate the correctness of our method and that this new fluorescence X-ray source can be used to calibrate the x-ray pulsar navigation detector.