Vol. 64, No. 17 (2015)
2015-09-05
GENERAL
2015, 64 (17): 170502.
doi: 10.7498/aps.64.170502
Abstract +
The generalized Langevin equation with a power law memory kernel is derived via the gas/solid-surface model with fractional heat bath. Using Lapalce transformation, the dynamic evolution and long-time asymptotic behaviors of the gas particles occurring either in free or harmonic potentials are then investigated. In particular, the validity of three kinds of ergodic criteria is analyzed in detail, including the Khinchin criterion, Lee criterion, and the intrinsic and external behaviors. It is found that the Khinchin criterion holds for all ranges of diffusion and transport processes described by a generalized Langevin equation. Lee criterion is just applied to distinguish diffusion processes. Meanwhile, the intrinsic criterion and external behaviors can not only divide the nonergodicity into two classes but also reveal the underlying physical origins.
2015, 64 (17): 170503.
doi: 10.7498/aps.64.170503
Abstract +
Equivalently implementing a generalized memristor by using common components and then making a nonlinear circuit with a reliable property, are conducive to experimentally exhibit the nonlinear phenomena of the memristive chaotic circuit and show practical applications in generating chaotic signals. Firstly, based on a memristive diode bridge circuit, a new first-order actively generalized memristor emulator is constructed with no grounded restriction and ease to realize. The mathematical model of the emulator is established and its fingerprints are analyzed by the pinched hysteresis loops with different sinusoidal voltage stimuli. The results verified by experimental measurements indicate that the emulator uses only one operational amplifier and nine elementary electronic circuit elements and is an active voltage-controlled generalized memristor. Secondly, by parallelly connecting the proposed emulator to a capacitor and then linearly coupling with an RC bridge oscillator, a memristor based chaotic circuit without any inductance element is constructed. The dynamical model of the inductorless memristive chaotic circuit is established and the phase portraits of the chaotic attractor with typical circuit parameters are obtained numerically. The dissipativity, equilibrium points, and stabilities are derived, which indicate that in the phase space of the inductorless memristive chaotic circuit there exists a dissipative area where are distributed two unstable nonzero saddle-foci and a non-dissipative area containing an unstable origin saddle point. Furthermore, by utilizing the bifurcation diagram, Lyapunov exponent spectra, and phase portraits, the dynamical behaviors of the inductorless memristive chaotic circuit are investigated. Results show that with the evolution of the parameter value of the coupling resistor, the complex nonlinear phenomena of the coexisting bifurcation modes and coexisting attractors under two different initial conditions of the state variables can be found in the inductorless memristive chaotic circuit. Finally, a prototype circuit with the same circuit parameters for numerical simulations is developed, from which it can be seen that the prototype circuit has a simple circuit structure and is inexpensive and easy to practically fabricate with common components. Results of both the experimental measurements and the numerical simulations are consistent, verifying the validity of the theoretical analyses.
2015, 64 (17): 170504.
doi: 10.7498/aps.64.170504
Abstract +
A significant phenomenon in nature is that of collective synchronization, in which a large population of coupled oscillators spontaneously synchronizes at a common frequency. Nonlinearly coupled systems with local interactions are of special importance, in particular, the Kuramoto model in its nearest-neighbor version. In this paper the dynamics of a ring of Kuramoto phase oscillators with unidirectional couplings is investigated. We simulate numerically the bifurcation tree of average frequency observed and the multiple stable states in the synchronization region with the increase of the coupling strength for N4, which cannot be found for N3. Oscillators synchronize at a common frequency =0 when K is larger than a critical value of N=3. Multiple branches with 0 will appear besides the zero branch, and the number of branches increases with increasing oscillators for the system N3. We further present a theoretical analysis on the feature and stability of the multiple synchronous states and obtain the asymptotically stable solutions. When the system of N=2 reaches synchronization, the dynamic equation has two solutions: one is stable and the other is unstable. And there is also one stable solution for N=3 when the system is in global synchronization. For the larger system (N3), we study the identical oscillators and can find all the multiple branches on the bifurcation tree. Our results show that the phase difference between neighboring oscillators has different fixed values corresponding to the numbers of different branches. The behaviors in the synchronization region computed by numerical simulation are consistent with theoretical calculation very well. The systems in which original states belong to different stable states will evolve to the same incoherent state with an adiabatic decreasing of coupling strength. Behaviors of synchronization of all oscillators are exactly the same in non-synchronous region whenever the system evolves from an arbitrary branch according to the bifurcation trees. This result suggests that the only incoherent state can be attributed to the movement ergodicity in the phase space of coupled oscillators in an asynchronous region. When the system achieves synchronization, the phenomenon of the coexistence of multiple stable states will emerge because of the broken ergodicity. All these analyses indicate that the multiple stable states of synchronization in nonlinear coupling systems are indeed generically observable, which can have potential engineering applications.
2015, 64 (17): 170201.
doi: 10.7498/aps.64.170201
Abstract +
The mechanical behavior of nanoporous monocrystal aluminum subjected to uniaxial compressive loading at a rate of 2109 s- 1 along [110] crystallographic orientation is studied using molecular dynamics simulations. Subjected to such a loading, nanovoids act as the effective sources of dislocation nucleation and emission, four of the twelve {111}110 slip systems may be activated. With the same strain of 3.8%, dislocation nucleation will occur in both the sample of multiple voids and that with a single void. The configuration of multiple voids decreases the required stress for the onset of dislocation nucleation and emission in comparison with the sample with an isolated void of the same size. Because of the emission of trial partials, the accumulation of dislocation density can be changed into a piecewise linear process by the dislocation density propagation rate dd/d: in the initial stage of plastic deformation we obtain dd/d1.071018 m-2, but this changes to dd/d5.361018 m-2 at higher deformation. The velocity of dislocation is calculated to be subsonic and is a variable value during the plastic deformation. Dislocation loop pairs emit from the same void, glide and approach to each other, leading to the reduction of dislocation velocity. Then one loop of each pair continues to glide to intersect mutually and finally interact with the loops emitted from other voids, causing a strain hardening to reach the peak flow stress of 4.3 GPa. There is a post-yield softening corresponding to the onset of rapid dislocation density proliferation at higher dislocation densities. With the temperature evolution of the sample with multiple voids during plastic deformation, the density of mobile dislocations is calculated to be one magnitude lower than the total dislocation density. There is a decrease of mobile dislocation densities at large strains, showing that the mobile dislocation are diminished by the formation of dislocation forest and junctions. At the onset of their nucleation, the dislocations are all Shockley partials, however, when dislocation intersection happens, the majority are still Shockley partials, while the rest consists of Frank partials, perfect fcc dislocations and other dislocation ingredients. Voids collapse at the strain of 11.8%. No twins are found in the present simulation due to the high stacking-fault energy of aluminum. Prismatic dislocation loop emission is observed in this simulation.
EDITOR'S SUGGESTION
2015, 64 (17): 170301.
doi: 10.7498/aps.64.170301
Abstract +
Quantum Fourier transform (QFT) is a quantum analogue of the classical discrete Fourier transform. It is a fundamental quantum gate in quantum algorithms which has an exponential advantage over the classical computation and has been excessively studied. Normally, an n-qubit quantum Fourier transform could be resolved into the tensor product of n single-qubit operations, and each operation could be implemented by a Hadamard gate and a controlled phase gate. Then the complexity of an n-qubit QFT is of order O(n2). To reduce the complexity of quantum operations, optimal control (OC) method has recently been used successfully to find the minimum time for implementing a quantum operation. Up to now, two types of quantum optimal control methods have been presented, i.e. analytical and numerical methods. The analytical approach is to change the problem of efficient synthesis of unitary transformations into the geometrical one of finding the shortest paths. Numerical optimal control procedures are based on the gradient methods (GRAPE, Gradient Ascent Pulse Engineering) and Krotov methods. Notable application mainly focus on nuclear magnetic resonance fields, including imaging, liquid-state NMR, solid-state NMR, and NMR quantum computation. One obvious advantage of optimal control NMR quantum computation is that the OC unitary evolution transformation pulse sequences are normally shorter than the conventional corresponding ones. Here we use the optimal control method to find the minimum duration for implementing QFT quantum gate. A linear spin chain with nearest-neighbor Ising interaction is used to find the optimization. And the optimized pulse sequence is experimentally demonstrated on an NMR quantum information processor. By using optimal control method with numerical calculation, a three-qubit QFT in an indirect-linear-coupling chain system is optimized. The duration of the OC QFT is obviously shorter than that of conventional approaches. The OC pulse sequence has been experimentally implemented on a liquid-state NMR spectrometer. To verify the optimally controlled pulse sequence for the three-qubit QFT, different initial states are assumed. The accuracy of the OC pulse sequence could be demonstrated by the consistency of theoretical simulation spectra with the experimental results. The good consistency between the simulation and the experimental spectra demonstrates that the OC QFT is of high fidelity.
2015, 64 (17): 170501.
doi: 10.7498/aps.64.170501
Abstract +
Using the fractional calculus theory, we investigate the directional transport phenomenon in a fractional logarithm coupled system under the action of a non-periodic external force. When a Brownian particle moves in the media with memory such as viscoelastic media, the system should be modeled as a nonlinear fractional logarithm coupled one. Using the method of fractional difference, we can solve the model numerically and discuss the influences of the various system parameters on the average transport velocity of the particles. Numerical results show that: 1) The directional transport phenomenon in this fractional logarithmic coupled model appears only when the external force exists, and the value of the average transport velocity of the particles increases with increasing external force. 2) When the fractional order of the system is small enough, the damping memory has a significant impact on the average transport velocity of the particles. Furthermore, the average transport velocity of the particles has an upper bound (although it is very small), no matter how the external force, coupled force and the intensity of noise change, the average transport velocity of the particles is no more than the upper bound. When there is no external force and the damping force is big enough, the directional transport phenomenon disappears. 3) When the fractional order of the system and the external force are big enough, although the directional transport phenomenon appears, the coupled force and the intensity of noise have no impact on the system. 4) Only when the external force is small enough, could the coupled force and noise intensity influence the average transport velocity of the particles. In this situation, the directional transport phenomenon appears when the fractional order of the system is big enough, and the average transport velocity of the particles changes along with the change of the coupled force and the noise intensity.
2015, 64 (17): 170505.
doi: 10.7498/aps.64.170505
Abstract +
This paper mainly deals with the indoor wireless propagation channel under line of sight (LOS) and non-line of sight (NLOS) propagation conditions, introducing the reference model and studing the design and simulation of modeling and the relevant statistical properties. This paper will present a comprehensive and improved indoor reference channel model based on a geometric scattering model. The reference channel model assumes that infinite number of scatterers will be uniformly distributed on the two-dimensional (2D) horizontal plane of a three-dimensional space. This paper also derives analytical expressions for the probability density function (PDF) of the angle-of-arrival (AOA), the Doppler power spectral density (PSD), and the temporal autocorrelation function (ACF) of an electromagnetic arrival signal; it also analyzes the influences of the important parameters of the functions. It presents a highly efficient sum-of-cisoids (SOC) channel simulation model from the unrealistic reference model, also proposes two efficient parameter computation methods for the design of sum-of-cisoids indoor channel simulation model and compares the computing performances of both. It is shown by simulation results that the statistical properties of the sum-of-cisoids channel simulation model match perfectly the reference channel model. It turns out that the indoor reference model can be approximated by an SOC channel simulation model. Meanwhile the channel simulation model can be well applied to evaluate the performance of indoor wireless communication systems. It also extends the research for indoor wireless channel modeling while reduces the realization expenditure.
2015, 64 (17): 170202.
doi: 10.7498/aps.64.170202
Abstract +
Inverse problems in dynamics are the basic problems in astronautics, rocket dynamics, and motion planning theory, etc. Mei symmetry is a kind of new symmetry where the dynamical function in differential equations of motion still satisfies the equation's primary form under infinitesimal transformations of the group. Mei symmetry and its inverse problem of dynamics for a general holonomic system in generalized coordinates are studied. Firstly, the direct problem of dynamics of the system is proposed and solved. Introducing a one-parameter infinitesimal transformation group with respect to time and coordinates, the infinitesimal generator vector and its first prolonged vector are obtained. Based on the discussion of the differential equations of motion for a general holonomic system determined by n generalized coordinates, their Lagrangian and non-potential generalized forces are made to have an infinitesimal transformation, the definition of Mei symmetry about differential equation of motion for the system is then provided. Ignoring the high-order terms in the infinitesimal transformation, the determining equation of Mei symmetry is given. With the aid of a structure equation which the gauge function satisfies, the system's corresponding conserved quantities are derived. Secondly, the inverse problem for the Mei symmetry of the system is studied. The formulation of the inverse problem of Mei symmetry is that we use the known conserved quantity to seek the corresponding Mei symmetry. The method is: considering a given integral as a Noether conserved quantity obtained by Mei symmetry, the generators of the infinitesimal transformations can be obtained by the inverse Noether theorem. Then the question whether the obtained generators are Mei symmetrical or not is verified by the determining equation, and the effect of generators' changes on the symmetries is discussed. It has been shown from the studies that the changes of the generators have no effect on the Noether and Lie symmetries, but have effects on the Mei symmetry. However, under certain conditions, while adjusting the gauge function, changes of generators can also have no effect on the Mei symmetry. In the end of the paper, an example for the system is provided to illustrate the application of the result.
ATOMIC AND MOLECULAR PHYSICS
2015, 64 (17): 173401.
doi: 10.7498/aps.64.173401
Abstract +
Studies on the dynamical stereochemistry of the titled reaction are carried out by the quasi-classical trajectory (QCT) method based on a new accurate 4A potential energy surface constructed by Abrahamsson and coworkers (Abrahamsson E Andersson S, Nyman G, Markovic N 2008 Phys. Chem. Chem. Phys. 10 4400) at a collision energy of 0.06 eV. The distribution p(r) of the angle between k-j' and the angle distribution P(r in terms of k-k'-j' correlation have been calculated. Results indicate that the rotational angular momentum vector j' of CO is preferentially aligned perpendicular to k and also oriented with respect to the k-k' plane. Three polarization-dependent differential cross sections (2/)(d00/dt), (2/)(d20/dt), and (2/)(d22+/dt) have also been calculated. The preference of backward scattering is found from the results of (2/)(d00/dt). The behavior of (2/)(d20/dt) shows that the variation trend is opposite to that of (2/)(d00/dt), which indicates that j' is preferentially polarized along the direction perpendicular to k. The value of (2/)(d22/dt) is negative for all scattering angles, indicating the marked preference of product alignment along the y-axis. Furthermore, the influences of initial rotational and vibrational excitation on the reaction are shown and discussed. It is found that the initial vibrational excitation and rotational excitation have a larger influence on the alignment distribution of j' but a weaker effect on the orientation distribution of j' in the titled reaction. The influence of the initial vibrational excitation on the three polarization-dependent differential cross sections of product CO is stronger than that of the initial rotational excitation effect.
2015, 64 (17): 173701.
doi: 10.7498/aps.64.173701
Abstract +
In this paper, we investigate theoretically the Stark deceleration and cooling of subsonic NH3 molecular beams based on our second-generation electrostatic Stark decelerator with 180 stages. Firstly, we calculate the Stark shifts of NH3 molecules in the |J=1, K=1 ightangle states and show the stable area of longitudinal phase space for different synchronous phase angles. Secondly, we study the slowing performance of NH3 molecular beams in the traditional mode, and discuss the relationships between various parameters (such as the kinetic energy loss per stage, final velocity and the slowing efficiency) and the synchronous phase angle 0, as well as the dependence of final velocity on the applied voltages. It is found that a subsonic NH3 molecular beam can be decelerated from 280 to 6.7 m/s at 0=26.08 when the high voltages applied on the electrodes are 13 kV, corresponding to a removal of 99.9% kinetic energy. The translational temperature of the molecular packets in the moving frame is significantly reduced from 1.34 K to 80 mK. Finally, we study the slowing performance of NH3 molecules and the dependence of final velocity on the synchronous phase angle in an alternate operation mode. In this mode, a synchronous phase angle 0=0 is chosen to bunch the molecules by using the first 15 stages. The remaining 165 stages are then used to slow a subsonic molecular beam at a certain synchronous phase angle. Our result shows that a molecular beam with a mean velocity of 280 m/s can be decelerated to 20.7 m/s at 0=65.4 when the voltages applied are 6.5 kV, indicating a 99.4% kinetic energy removal, and the translational temperature of the molecular packets can be reduced from 1.34 K to 1.6 mK. By comparing the results obtained from the two operational modes, the temperature of the slowed molecular packet in the alternate mode is 50 times lower than that in the traditional mode. It is shown that our second-generation 180-stage Stark decelerator can effectively produce slow and cold molecules with relatively small electric dipole moment like NH3. These monochromatic NH3 molecular beams offer a promising starting point for high resolution spectroscopy, precision measurement, cold collisions and cold chemistry. This theoretical work provides a reliable basis in our further experimental research.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (17): 174101.
doi: 10.7498/aps.64.174101
Abstract +
The actual rough sea surface is described by Pierson-Moscowitz (PM) spectrum and Monte Carlo method; the composite electromagnetic scattering from the double rectangular crosssection conductor columns above the rough sea surface is investigated using the hybrid methodthe Method of Moment with the Kirchhoff approximation. Angular distribution curves of composite scattering coefficient are obtained; and this coefficient that varies with the parameter of sea surface, rectangular cross-section conductor columns and incident waves is calculated in detail. The characteristics of the composite scattering coefficient from the double rectangular cross-section conductor columns above the rough sea surface are also obtained. Results show that the hybrid methodthe Method of Moment with the Kirchhoff approximation, compared to the traditional Method of Moment, can obtain higher accuracy. Moreover, the hybrid method can effectively reduce the computation time and memory footprint, and the performance becomes more favorable with the increase of the sizes of rough surface.
2015, 64 (17): 174103.
doi: 10.7498/aps.64.174103
Abstract +
A single screen terahertz frequency selective surface (FS) using the improved split ring resonators (SRRs) is designed in this paper. The resonance unit of an improved SRR consists of an open seam metal patch, while the physical size of the open seam metal patch will directly affect the stepped impedance characteristics. In the paper, LC equivalent circuit model for the improved SRR unit structure is established to extract the equivalent circuit model parameters. Then the relationship between the fundamental frequency of the FSS formula and the harmonics is obtained from the basic theory of the transmission line. Compared to the traditional uniform SRR, the control of multi-band in the improved SRR is more flexible. It is an outstanding characteristic for multi-band FSS design. Based on this characteristic, the triple-band terahertz FSS centered at 0.46, 0.86 and 1.03 THz respectively is designed successively, which can be used in radio astronomy application. By using HFSS 13.0 electromagnetic software simulation, many important indicators such as the key parameters that affect the transmission characteristics of the FSS, periodic intervals, miniaturization degree and the sensitivity of the incidence angle have been studied and analyzed. Both the theoretical analysis and simulated results demonstrate the validity of the method. The triple-band FSS using the improved SRR has a lot of reformative performances. It is shown that the reflection coefficients of triple-band FSS using the improved SRR are -37.6 dB, -13 dB, and -19.6 dB, respectively. On the other hand, it owns the stable frequency response characteristics in the 0°–60° range, which is beneficial to a large incidence angle. In addition, a high degree of miniaturization and the low loss characteristics are the another two significant advantages of this FSS. This triple-band FSS with improved SRR has potential applications in the terahertz frequency radio astronomy polarizer, beam splitter, mirror and resonator mirror, etc.
2015, 64 (17): 174201.
doi: 10.7498/aps.64.174201
Abstract +
Since many industrial materials have micro or submicro structures on the surface or subsurface, utrahigh-resolution is required in the inspection of these materials. Ultrahigh-resolution optical coherence tomography uses broadband light sources to achieve axial image resolutions on the scale of a few microns. We have been investigating an ultrahigh-resolution spectral-domain optical coherence tomography (SD-OCT) system using supercontinuum sources (SC) in free space. The effective SC spectrum has a full width at half maximum of 230 nm centered around 665 nm, and the imaging setup has an ultrahigh axial resolution of 0.9 μm in air, and a lateral resolution of 3.9 μm, with the system measurement range being 0.6 mm in axial direction. At a 50 μm axial position, the sensitivity can be 63 dB with 28600 axial scans per second at 2048 pixels per axial scan. Images of polystyrene microspheres solution with an average diameter of 5 μm and different sizes of industrial abrasive papers are presented to illustrate the performance of the system.
2015, 64 (17): 174202.
doi: 10.7498/aps.64.174202
Abstract +
Photonic spin Hall effect (PSHE) of light, similar to the spin Hall effect of an electronic system, is the interaction between photon spin-orbit of spin splitting phenomenon in refractive index gradient field. PSHE provides a new way to manipulate photons and accurately measure relevant physical effects. This paper studies the photon spin splitting in the magneto-optic Kerr effect, and establishes the quantitative relationship between the magneto-optical Kerr rotation and the PSHE. We have observed the magneto-optic Kerr effect of the action of an iron film in the magnetic field. After finishing experiments, we firstly analyze the amplified shift of the spin changing with the magnetic intensity in the case of horizontal polarization and vertical polarization incidence. Then, the Kerr rotations changing with the incident angle for different magnetizations are measured separately. Finally, comparing the above two results, we obtain that the accurate magneto-optic Kerr rotation angle is 6.7×10-5o/μm. If the position sensor based on phase-locked amplifier (20 nm) is used to measure the magneto-optic Kerr rotation angle, its precision can be improved by one or two orders of magnitude. These results prove that the PSHE not only can be used to accurately measure the magneto-optic Kerr effect, but also have great significance in magnetic film research. Especially, in terms of magnetic-optic devices, PSHE can be used to achieve the superior performance of endurance cycling and data retention.
EDITOR'S SUGGESTION
2015, 64 (17): 174203.
doi: 10.7498/aps.64.174203
Abstract +
A diode pumped passively Q-switched 1123 nm laser is reported in this paper; and a mixed crystal Nd:LuYAG is selected as the gain medium. A large number of excellent properties from Nd:YAG are obtained, and the mixed crystal Nd:LuYAG has been used widely in all-solid-state lasers. Besides, compared with Nd:YAG, the Nd:LuYAG has some other wonderful advantages. For example, both the absorption bands and the fluorescence line are broadened, resulting from the crystal strong inhomogeneity. Their wide absorption makes the Nd:LuYAG lasers' pump source not rigorous in their temperature control. And the broadened fluorescence line can generally improve the laser performance in Q-switched regimes. In this paper, a concave-plane configuration cavity with its length as long as 35 mm is designed to achieve high-efficiency laser output. The rear mirror is a concave mirror with a curvature radius of 300 mm, and the output coupler is a flat mirror with a transmission of 2% at 1123 nm, 5% at 1112 nm, 4% at 1116 nm, and has high transmissions at 1064, 1319 and 1444 nm respectively. A Cr4+:YAG crystal, with its initial transmission of 97%, is used as the saturable absorber. In the continuous wave operation, the maximum average output power can reach 2.77 W, with the corresponding optical-to-optical conversion efficiency of 29.53%. In Q-switched operation, the maximum average output power is 0.94 W at 9.38 W absorbed pump power. The repetition rate is 9.40 kHz, with the corresponding single pulse energy being 100 J. The high single-pulse energy explains that the Nd:LuYAG mixed crystal is better than Nd:YAG in high energy storage. Only one wavelength can be observed in our experiment. The center wavelength is 1122.7 nm and the line width is 0.03 nm. To the best of our knowledge, this is the first time to report the Nd:LuYAG mixed crystal laser emitting at 1123 nm.
2015, 64 (17): 174205.
doi: 10.7498/aps.64.174205
Abstract +
Plasma-filled slow-wave devices provide a new way to develop high efficiency and high power vacuum-electron microwave sources, but their theoretical analysis and simulation is difficult. This paper introduces the wheel spoke antenna to excite signals for analyzing the dispersion characteristics of resonant cavity with plasma-filled metallic photonic crystal slow-wave structure (SWS). Influences of parameters of the SWS and plasma density on dispersion characteristics of the SWS are studied. Results show that there is little difference in dispersion characteristics obtained by wheel spoke antenna excitation of signals and other methods without plasma filling. When plasma fills in the SWS, the frequency of zero mode is consistent with the previous results obtained by other methods. Hence, both the results with and without plasma filling demonstrate that the wheel spoke antenna signal-excitation method is effective. Moreover, decreasing the thickness of wheel spoke antenna properly and the distance between the antenna and reflection surface of the metal plate can reduce the wheel spoke antenna influence on the cavity resonance frequency. Furthermore, thicker antenna can excite the slow wave field easily, while thinner antenna can excite the resonant mode easily. Besides, the outer radius and thickness of the SWS plate have little influence on the dispersion characteristics, while the period length and the inner radius of the SWS plate have greater influence on the dispersion characteristics. In addition, the dispersion curves of frequency and phase velocity will move to higher frequency regions with the increase of plasma density. Further, the influence of plasma filling on low-order modes is greater than that on higher order modes. It is also found that the higher-order mode operation can reduce the size of cavity and the velocity of the electron beam.
2015, 64 (17): 174301.
doi: 10.7498/aps.64.174301
Abstract +
Accurate measurement of rainfall with high spatial and temporal resolution have important significance in meteorology, hydrology, agriculture, environment, flood warning and weather forecasting, etc. Based on the rain-induced power-law attenuation, an inversion method of the path rainfall intensity is proposed by using a microwave link. Starting from the atmospheric gas absorption attenuation model, a correction model of rainfall effective attenuation and a rainfall inversion model for line-of-sight microwave links are proposed, based on the microwave rain attenuation characteristics and raindrop size distribution statistics. A line-of-sight microwave link is designed and used to measure the rainfall, and the path average rain rate is inversed by means of rainfall inversion model, which is compared with a disdrometer. Results show that the correlation coefficient of rain rate inversed by microwave link with that of disdrometer is higher than 0.6 mostly, and the maximum value is 0.9647; the error of the accumulated rain amount is less than 0.5 mm, and the minimum value is 0.0827 mm; the relative error of the accumulated rain amount is less than 15%, the minimum value is 3.3415%. Experiments confirm the feasibility and accuracy of rainfall inversion obtained using a microwave link.
2015, 64 (17): 174302.
doi: 10.7498/aps.64.174302
Abstract +
Acoustic propagation in shallow water is greatly influenced by the properties of the sea bottom. The dispersion characteristics of modes are relatively sensitive to the bottom parameters and have been used to invert the bottom parameters. Since the inversion error using a single wideband sound source increases with increasing range, a far distance inversion method based on the modal dispersion curve using a single hydrophone with two wideband sound sources is presented in this paper, in which a warping transform is applied so that it can accurately extract the modal dispersion curve from the warped signal spectrum. Experimental data used for the inversion are acquired using a hydrophone of vertical array in the South Sea of China during the Autumn in 2012. The transmitted signals are explosive signals, and the bottom sound speed and density are inverted by matching the theoretical arrival time differences of various modes and frequencies with those calculated using the experimental data. The attenuation coefficient is deduced using the transmission loss data recorded in the experiment. A genetic algorithm (GA) is used for optimization search for the parameter bounds. Inversion results demonstrate that the arrival time difference when using the bottom sound speed and density show a high consistency with those obtained using the experimental data. Moreover, the attenuation coefficient is nonlinear over the frequency band from 100 to 315 Hz. The validity of inverted parameters is evaluated by the posteriori probability distributions, and the numerical results of arrival time differences calculated using the inverted sound speed and density are in good agreement with those extracted from the other two wideband explosive signals at different distances. In addition, the theoretical transmission loss calculated using the inverted attenuation coefficient matches the experiment data very well. It is shown that the inversion scheme can provide a valid and stable environmental estimation.
2015, 64 (17): 174303.
doi: 10.7498/aps.64.174303
Abstract +
In order to solve the problem of robustness of beamforming algorithm with microphone array channel mismatch, an adaptive dynamic-weighted constrained least square algorithm-based microphone array robustness frequency invariant beamforming algorithm is proposed. In the proposed algorithm, by analyzing the microphone array model, with or without channel mismatch, the disadvantages of the constrained least square frequency invariant beamforming algorithm with channel mismatch are studied. After the probability density functions of the microphones are defined as the robustness factors and added to the constraint least square frequency invariant beamforming algorithm, the robustness is improved to a certain extent, but it is still poor. In order to further improve the robustness of the algorithm, dynamic-weighted coefficients for controlling frequency invariance in the cost function are used to regulate the sidelobe spectrum energy. The fluctuation error is defined as the ratio of the maximum to minimum value of array response formed by the same angle of arrival at different frequencies, within the frequency range of frequency invariant, to compare the proposed algorithm with the constrained least square robustness frequency invariant and minmax robustness broadband beamforming algorithm. Experimental results of the algorithms show that the fluctuation errors of the proposed algorithm are the smallest and its robustness is the best; it can effectively overcome the poor robustness of the beamforming algorithm caused by microphone array channel mismatch, and can be applied to any arbitrary array structure.
2015, 64 (17): 174501.
doi: 10.7498/aps.64.174501
Abstract +
The energy dissipation of a disc spinning on a horizontal plane is studied, as the angle α of the coin made with the horizontal plane decreases, while the angular velocity Ω of the point of contact increases. Effect of the ratio x between the thickness and diameter of an Euler disc and the α on the energy dissipation is studied. We find, by using numerical simulation, that when x is small enough, the lose of the kinetic energy and the gravitational potential energy of the mass center is dominant in energy dissipations; when x>0.4142, the rotational kinetic energy dissipation of the disc around the axis that is parallel to the disc surface, is the leading factor. The requirements in which thickness can be neglected are also obtained, and they can give some hints to the relevant theories and experiments. Our results show that when α≥10° and b/a[26] data very well. We also discuss the main energy dissipation distributed among different forms: variation of rolling friction and viscous shear of the air with x and α, also show their transition in the process of the motion. Furthermore, we find that the pure rolling friction is the unique dissipation as x=0.1733 and α>18°, which improves the results obtained before. We speculate that the dominant dissipation is the gliding friction in the final stage of the motion, because when the disc is motionless, one face of the disc lies absolutely in contact with the horizontal surface just before the disc halts. One can assume that they are in contact completely but the disc does not halt, thus axis 1 and axis Z are almost in the same direction. In this case, the energy dissipation of the Euler disc is due to the gliding friction. To some extent, this accounts for the disc final halt.
2015, 64 (17): 174102.
doi: 10.7498/aps.64.174102
Abstract +
The unique optical and physical properties of surface plasmon polaritons (SPP) has brought about a series of novel phenomena such as SPP-enhanced transmission, local resonance, etc., and SPP has become a research hotspot around the world. In this paper, the dispersion characteristics and modes of rectangular metal grating based on spoof surface plasmons (SSP) are studied theoretically and numerically. The electromagnetic fields of SSP which are below and above the grating surface are presented using eigenmode expansion method and under periodic boundary conditions, besides the fact that the SSP dispersion relations are obtained by matching the boundary conditions of electromagnetic fields both for rectangular metal grating with roofed metal plate and that without roofed metal plate. Results for these two different cases are given according to numerical calculation and it is found that the roofed metal plate can introduce an additional fast wave mode which is beyond the light line in the dispersion diagram. And the results of analytical SSP dispersion are verified by electromagnetic simulations based on the finite difference method and finite integration method. The dependence of the dispersion characteristics and mode distributions on various parameters of metal grating is studied theoretically. It is shown that the dispersion relations obtained by eigenmode expansion method agree well with the results of electromagnetic simulations. The phase velocity of SSP on the grating surface can be decreased by increasing metal grating depth or decreasing grating period. The bandwidth of electron beam-SSP interaction can be extended by increasing grating period ratio. The influence of the distance between the roofed metal plate and the grating surface on the SSP dispersion is studied and is found that the role of roofed metal plate is insensitive to the slow wave SSP mode. The SSP dispersion and modes for the 3-D metal grating which are extended from the above 2-D SSP dispersion are also given. The SSP symmetric modes and anti-symmetric modes manifest themself alternately in the dispersion diagram on the 3-D grating surface. Compared with the 2-D SSP bound mode without roofed metal plate, it is found that in the 3-D grating structure the slow wave SSP modes and fast wave SSP modes coexist. The 3-D SSP mode with various grating lateral width is studied, and the competition and degeneracy of modes are analyzed particularly. The SSP mode intervals can be enlarged by decreasing the lateral width of the grating, which is optimum for avoiding mode competitions. Studies on dispersion and modes of the 2-D and 3-D metal grating structures based on SSP will lay the foundations for further studies of electron beam-SSP interaction, and development of the novel terahertz vacuum electronic source with high-efficiency and wide-bandwidth.
2015, 64 (17): 174204.
doi: 10.7498/aps.64.174204
Abstract +
Induced by the harmonically related two-color pulse, the fourth harmonic wave in the vacuum ultraviolet range has been observed in gases. The process of the fourth harmonic generation is studied. In the case of non-ionized gas, the intensity dependence on the pump pulse reveals that the fourth harmonic generation is a third-order parametric process of 2ω+ω+ω→4ω. In the case of ionized gas, the intensity of the fourth harmonic wave can be coherently controlled by the relative phase of the pump. Employing synchronous measurements of the fourth-order harmonic and terahertz emissions, the optimized relative phases of the two emissions have been studied and compared with each other experimentally. Results agree well with those of the time-dependent Schrödinger equation (TDSE). According to the reported optimized phase for the terahertz generation, the fourth harmonic generation involves two parametric processes: 2ω+2ω±ΩTHz→4ω and 2ω+ω+ω→4ω. The modulation of the fourth harmonic wave can be understood as the interference of the two channels.
2015, 64 (17): 174206.
doi: 10.7498/aps.64.174206
Abstract +
Recently, Chan and his collaborators reported that a crossing point of bands can be achieved at the Brillouin zone center in two-dimensional (2D) dielectric photonic crystals (PhCs) by accidental degeneracy of modes. At the crossing point, the accidental threefold degeneracy of modes generates a Dirac cone and an additional flat band (longitudinal mode) intersecting the Dirac cone. This is different from that of the Dirac point at the corner of the hexagonal Brillouin zone in which only Dirac cone exists. As a result, the crossing point at the Brillouin zone center is called a Dirac-like point. If the accidental degeneracy occurs by a monopole mode and two dipolar modes, the dielectric PhCs can be mapped to a zero-refractive-index system in which the effective permittivity and permeability are zero at the Dirac-like point from the effective medium theory. According to the Maxwell equations, if the permittivity and permeability are zero, the optical longitudinal modes can exist, in additional to the well-known transverse modes. The additional flat band at the Dirac-like point is closely connected with the longitudinal mode. For a homogeneous zero-index material (ZIM), the flat band is dispersionless and the longitudinal mode cannot couple with the external light. But in a finite-sized PhC, there is always some spatial dispersion, so the flat band is not perfectly dispersionless when it is away from the zone center. Therefore, if the wave source is a Gaussian beam with non-zero k-parallel components, the longitudinal mode can be excited. And the effective wavelength of ZIM is extremely large, leading to many scattering properties. However, in a PhC which behaves as if it had a zero refractive index, it is very interesting to show how the longitudinal mode influences the wave propagations in the PhC when the longitudinal mode is excited. In this paper, the effect of longitudinal mode on the transmission properties near the Dirac-like point of PhCs is investigated by numerical simulation. The alumina dielectric rods can be moved randomly in the structure to result in the disorder of the structure. Our results show that the transmission properties at the Dirac-like point are very different from those near the Dirac-like point, when the longitudinal mode is excited. At the Dirac-like point, the transmittance decreases with increasing disorder, as a result of the influence of the longitudinal mode, which is similar to the one in the pass band. Above the Dirac-like point without the disturbance of longitudinal mode, the transmittance is insensitive to the disorder in the structure, so that the structure may mimic a near-zero index materials and have a large effective wavelength. These results may further improve the understanding about the optical longitudinal mode and the zero refractive material.
2015, 64 (17): 174207.
doi: 10.7498/aps.64.174207
Abstract +
In this paper, an all fiber acousto-optic tunable filter based on superimposed fiber Bragg gratings (SFBG-AOTF) is demonstrated and studied. Compared to the normal fiber Bragg gratings based all fiber acousto-optic tunable filter, SFBG-AOTF can modulate the two optical resonant wavelengths of the gratings synchronously. The spectrum of SFBG-AOTF at various acoustic frequencies and under acoustically induced strains, has been analyzed theoretically. Based on simulation results, one can find that each order of the secondary reflection peak is symmetrical with respect to the two primary reflection peaks with SFBG as the center, and the resonant wavelength spacing between the primary reflection peak and the secondary reflection peak which is modulated by the former, is proportional to the acoustic frequency. But the resonant wavelength between the same order secondary reflection peaks which are modulated by two different primary reflection peaks, is independent of the acoustic frequency. The acoustically induced strains mainly affect the variation of the reflectivities of the primary and secondary reflection peaks. In the experiment, the spectra of SFBG-AOTF with acoustic frequencies of 390 and 710 kHz, are measured. The variation trend of the experimental results accords well with the simulated one.
2015, 64 (17): 174304.
doi: 10.7498/aps.64.174304
Abstract +
The noise emitted by an underwater vehicle consists of several strong tones superimposed on a broad-band radiated noise component. Among them, the stable low-frequency tone noise induced by the reciprocating movements of the auxiliary machines in the underwater vehicle, carries characteristic information of the vehicle and is necessary for long-distance detection. Therefore, identification of the tone noise sources of an underwater vehicle is significant for noise reduction. On the basis of the joint information of space-time-frequency, beamspace time-frequency analysis (TFA) scheme is proposed for identification of low-frequency tone noise sources of underwater moving vehicle. First, the Doppler signals formed when the tone noise sources pass through the closest point of approach (CPA) are separated in time domain, by using superdirectivity beamforming of a small aperture circular array. The output signals can be approximated in linear form, i. e., LFM signal. After the LFM signals from the narrow beam are processed by two TFA methods of pseudo Wigner - Ville distribution and chirplet transform (CT), the time-frequency images of the noise signals are obtained. Then, the CPA time of each tone noise sources can be estimated by using peak search of the time-frequency images. At last, by converting the time coordinate to space coordinate and comparing with a reference source whose CPA time and position are known in advance, the positions of the low-frequency tone noise sources on the underwater vehicle are identified. The proposed scheme is different from the focused beamforming method, which scans the beam angle after eliminating the Doppler effect. Besides, due to no need of decorrelation usually used in the focused beamforming method, beamspace TFA scheme resolves the problem that array aperture is limited for identification of coherent noise sources of an underwater vehicle. The aperture of the used array can be reduced to meter-scale even when the frequencies of the tone noise are low. Although the array gain of superdirectivity beamforming decreases in nonisotropic noise field, the main lobe of the beam still keeps the same shape. Therefore, the performance of the proposed scheme is robust. Simulation analysis shows the following results: (1) Both the two beamspace TFA methods can precisely identify the underwater tone noise sources through a small aperture circular array, the radius of which is equal to 1.6 m, and the localization errors are less than 1 m when the signal-to-noise ratios are moderate; (2) The higher the frequencies of the tone noises are, the better the localization accuracy of beamspace TFA methods obtain; (3) The proposed scheme is less sensitive to the velocity of the underwater moving vehicle, and the localization results just have very small difference under various velocities; (4) The localization accuracy is related to distance, and decade meters is a reasonable choose for actual noise measurement; (5) Beamspace CT has better resolving accuracy when the information of measurement system is given, so the choice of the two beamspace TFA methods can be decided according to the actual measurement condition.
2015, 64 (17): 174701.
doi: 10.7498/aps.64.174701
Abstract +
Based on the principle of virtual works, a multiphase smoothed particle hydrodynamics (SPH) model is further developed from the foundation of Hu X Y et al. (2006) and Grenier N et al. (2009). In the present model, the surface tension force implementation suitable for the multiphase flows with a large density ratio is applied, and this allows a good continuity at the multiphase interface. Artificial displacement correction is applied to keep the particles distributing uniformly in the whole flow field, and therefore any artificial viscous term is never needed; this is very important in the numerical simulation of viscous flows since the introduction of artificial viscosity changes the Reynolds number. Background pressure and interface sharpness force are added in the equation of state and the equation of momentum respectively to ensure the multiphase interface stability and smoothness; this is essential in the simulation of multiphase flows with large density difference at the multiphase interface. Two types of viscosity expressions suitable for multiphase flows are introduced and analyzed; the conclusion is that the formula proposed by Morris et al. (1997) and its similarly derived forms can give more accurate results. In the numerical validations, an oscillating droplet test is applied first to confirm the accuracy of the surface tension model and good results are achieved. This demonstrates that the artificial displacement and the interface sharp force will make negligible effects to the surface tension implementation. After that, two classic quantitative benchmarks of rising bubbles are simulated and the results of SPH agree well with the reference data. Moreover, in the two numerical benchmarks, the effect of the artificial displacement, the choice of the viscosity expression, and the type of the kernel function are compared and finally an optimal combination of these numerical aspects is recommended. Based on the above numerical investigations, the splitting process of an initially circular bubble is simulated and the numerical results agree well with the experimental data. In the last numerical case, the process of chasing and merging between two rising bubbles in vertical direction is simulated, based on which the mechanisms of these interesting interactions between two rising bubbles are analyzed. It is demonstrated in the present work that further improved multiphase SPH model may provide a potential method for the research of bubble dynamics.
2015, 64 (17): 174702.
doi: 10.7498/aps.64.174702
Abstract +
The electro-osmotic flow of a non-Newtonian fluid in a slit micro-channel under the Navier's slip boundary condition is investigated. The Eyring constitutive relationship model is adopted to describe the non-Newtonian characteristics of the flow driven by the applied electric field force and pressure. In consideration of the micro-scale effects, electric field, non-Newtonian behavior and slip boundary condition, a mechanical model is built and the effects of these factors on the flow are studied. Analytical expressions are derived for the electric potential and velocity profile by solving the linearized Poisson-Boltzmann equation and the modified Cauchy equation. Approximate expressions of the velocity distribution are also given and discussed. Furthermore, by comparing the effects of electric force with that of pressure on the velocity distribution, some meaningful conclusions are drawn from the obtained graphics.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2015, 64 (17): 175201.
doi: 10.7498/aps.64.175201
Abstract +
The variable parameters like electron destiny (ne), electron collision frequency, covered-area and thickness have been regarded as the key factors for the electromagnetic scattering characteristics of the covering target. Therefore, an air inductively coupled plasma (ICP) generator of all-quartz chamber of 20 cm × 20 cm × 7 cm without magnetic confinement and grounded metal surface of substantial area is designed and conducted to study the discharge process and diagnose the parameters in this paper. The shape, E-H mode transition, and structure of inductively coupled plasma are observed, and the width and thickness change due to change of power and pressure are measured in experiments. Results show that the plasma is nearly uniformly full of the chamber in E-mode, while the shape of plasma rapidly changes to a ring in H-mode and the structure of inductively coupled plasma stratified into an electronegative core and an electropositive halo. It is observed clearly that the luminance of plasma increases slowly with the RF power in E-mode, but increases significantly in H-mode, which are proved through the relative spectral intensity variation of nitrogen 337.1 nm spectral lines due to the change of power and pressure. The width and thickness of the core region increase significantly with power, while decrease apparently with increasing pressure, which could be logically explained by the variation of RF magnetic induction amplitude distribution with power and by the theoretical diffusion analyses of electronegative gas. Since a mass of oxygen electronegative ion exists in the air inductively coupled plasma, the electron density (ne) diffusion models are different for the electronegative core and the electropositive halo. It is proved also by the theoretical drift-diffusion analyses that the electron density is distributed nearly uniformly in the electronegative core and decreased sharply in the electropositive halo. The model of electromagnetic wave propagation in the ICP generator is given and the microwave interferometry is discussed in detail. The electron density in the core region under different discharge conditions is diagnosed by microwave interferometer and the electron density of edge halo is calculated by using the high-pressure diffusion model. And the electron density increases with increasing power and pressure, which range from 0.65×1011 to 3.71×1011 cm-3. But decay rate of electron density in the halo is less affected by the power at 100 Pa, while the rate is accelerated with increasing pressure. Finally, the electronic excitation temperature of the electronegative core and the electropositive halo are diagnosed by Boltzmann graphic method using emissive spectrum of auxiliary Ar. Results show that the electronic excitation temperature of the core, which ranges from 4201 to 4390 K, increases with increasing power, but decreases with increasing pressure.
2015, 64 (17): 175202.
doi: 10.7498/aps.64.175202
Abstract +
By analyzing the breakdown threshold obtained from effective magnitude or RMS (root mean square) of microwave field, it is pointed out that the assumption of threshold is not suitable for microwave air breakdown. Variations of effective electron temperature and breakdown threshold in microwave fields, which are studied theoretically and numerically by fluid-based plasma equations coupled with the Maxwell equations, are compared with those in static fields. It is found that the effective electron temperature varies greatly with the microwave field at high pressures (electron energy transfer frequency is much larger than microwave frequency) due to its high energy transfer frequency. This causes the microwave air breakdown threshold to be smaller than that obtained from the RMS of field at high pressures because the ionization frequency varies faster than electron energy loss at an effective electron temperature. On the other hand, the effective electron temperature variation with the microwave field is insignificant at low pressures (electron energy transfer frequency is much smaller than microwave frequency) due to the negligible electron energy loss in a microwave period. In this case, the microwave air breakdown threshold is approximately equal to that obtained from the RMS one of the field. The fit formula of microwave air breakdown threshold is obtained by numerical and theoretical analysis.
2015, 64 (17): 175203.
doi: 10.7498/aps.64.175203
Abstract +
Femtosecond filament-induced breakdown spectroscopy (FIBS) is employed to qualitatively analyze the heavy metal elements (Ca, Fe and Cr) in poplar leaves, at the same age, from four representative locations in Changchun City, i.e. Changchun First Automobile Factory, Railway Station, Jingyue National Forest Park, and Changchun University of Science and Technology (CUST) in the same season. The stem and mesophyll from the same leaf are investigated as samples by the FIBS technique. Experimental results demonstrate that the concentration of Ca and Fe elements in the leaf stem are higher than those in the leaf mesophyll through comparing the spectral intensities of Ca Ⅱ 393.37 nm and Fe Ⅰ 422.87 nm. Based on the analysis of the FIBS spectral intensity of heavy metals mentioned above in the poplar leaves from the four different locations, the heavy metal elements Ca, Fe and Cr in the poplar leaves gathered from the Automobile factory area have the highest concentration among the four locations, and those from Jingyue National Forest Park are the lowest. The FIBS technique can obtain stable plasma spectrum for the poplar leaves with irregular surfaces because of the optical intensity clamping effect inside the femtosecond filament. This experiment demonstrates that FIBS can be utilized to find new applications in the environmental pollution on-site detection.
2015, 64 (17): 175204.
doi: 10.7498/aps.64.175204
Abstract +
In this paper, combined with the latest development in relevant diagnostic and modeling techniques, the intense-pulsed ion beam (IPIB) energy deposition model in solid targets is established. The power density distribution induced by IPIB is simulated by Monte Carlo method on the basis of energy balance. By taking the power density as the source term, the thermal field distribution and evolution on a 100 μm stainless steel target irradiated by IPIB are simulated using the finite element method (FEM) in a time scale of several ms. Results reveal that in a time scale of several μups after IPIB irradiation, the main feature in the induced thermal field is a thermal shock within the depth of several times of the ion range. In the time scale of ms, thermal equilibrium can be established between the front and rear surfaces of the target, and the cross-sectional temperature field profile has a similar profile to the cross-sectional energy density distribution of the ion beam. This proves that by the infrared imaging diagnostic method, high resolution cross-sectional energy density diagnostics of IPIB can be achieved with a shooting time delay in ms scale.
2015, 64 (17): 175205.
doi: 10.7498/aps.64.175205
Abstract +
In the thermal infrared (TIR) waveband, solving the target emissivity spectrum and temperature leads to an ill-posed problem in which the number of unknown parameters is larger than that of available measurements. Generally, the approaches developed for solving this kind of problems are called, by a joint name, the TES (temperature and emissivity separation) algorithm. As is shown in the name, the TES algorithm is dedicated to separating the target temperature and emissivity in the calculating procedure. In this paper, a novel method called the new MaxEnt (maximum entropy) TES algorithm is proposed, which is considered as a promotion of the MaxEnt TES algorithm proposed by Barducci. The maximum entropy estimation is utilized as the basic framework in the two preceding algorithms, so that the two algorithms both could make temperature and emissivity separation, independent of experiential information derived by some special data bases. As a result, the two algorithms could be applied to solve the temperature and emissivity spectrum of the targets which are absolutely unknown to us. However, what makes the two algorithms different is that the alpha spectrum derived by the ADE (alpha derived emissivity) method is considered as priori information to be added in the new MaxEnt TES algorithm. Based on the Wien approximation, the ADE method is dedicated to the calculation of the alpha spectrum which has a similar distribution to the true emissivity spectrum. Based on the preceding promotion, the new MaxEnt TES algorithm keeps a simpler mathematical formalism. Without any doubt, the new MaxEnt TES algorithm provides a faster computation for large volumes of data (i.e. hyperspectral images of the Earth). Some numerical simulations have been performed; the data and results show that, the maximum RMSE of emissivity estimation is 0.017, the maximum absolute error of temperature estimation is 0.62 K. Added with Gaussian white noise in which the signal to noise ratio is measured to be 11, the relative RMSE of emissivity estimation is 2.67%, the relative error of temperature estimation is 1.26%. Conclusion shows that the new MaxEnt TES algorithm may achieve high accuracy and fast calculating speed, and also get nice robustness against noise.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (17): 177301.
doi: 10.7498/aps.64.177301
Abstract +
Simulation on the properties of localized surface plasmon resonance (LSPR) of different sized hexagonal Ag nanoarrays embedded in the amorphous oxidized silicon nitride(a-SiNx:O) matrix has been carried out by using COMSOL Multiphysics and FDTD Solution simulation software. Through the calculation of the scattering and absorption cross section of Ag array with different radius, we find that the position of extinction peaks red-shift from 460 to 630 nm when the radius of nanoparticles of hexagonal Ag arrays increases from 25 to 100 nm with the distance between particles 100 nm. The enhanced scattering cross section of the localized surface plasmon (LSP) and blue-shift of the extinction peak can be obtained by tunning the distance between Ag nanoparticles from 100 to 50 nm with the radius of Ag nanoparticles fixed at 50 and 75 nm, respectively. However the mismatch between the extinction peak of hexagonal Ag nanoarrays and the blue light emission of 460 nm from a-SiNx:O films still exists. The novel overlap between the scattering cross section of LSP from hexagonal Ag arrays with a radius of 25 nm and the blue light emission of a-SiNx:O films at 460 nm further confirms that the hexagnoal Ag arrays with a radius of 25 nm is the optimal option to enhance the blue light emission from a-SiNx:O films. Therefore, strong coupling between LSP and blue light emission at 460 nm from a-SiNx:O films with a thickness of 70 nm can be realized when the radius of Ag nanoparticle is 25 nm. We also investigate the enhancement of near field radiative intensity of LSP from hexagnoal Ag arrays with a radius of 25 nm. When the excitation wavelength is 460 nm, the maximum enhancement of near field intensity of LSP from hexagnoal Ag arrays with a radius of 25 nm reaches 1.46104 V/m. The calculated polarization intensity and charge distribution of hexagonal Ag nanoparticle with a radius of 25 nm embedded in a-SiNx:O films reveal that the enhancement of electromagnetic field-intensity is through the dipolar plasmon coupling with the excitons in a-SiNx:O films in bright field mode under the excitation of 460 nm. Further calculation of perpendicular radiative intensity for LSP from the hexagonal Ag array with a radius of 25 nm embedded in a-SiNx:O films indicates that the maximum radiative intensity can be realized in a-SiNx:O matrix with an optimum thickness of 30 nm for a-SiNx:O films. Our theoretical calculations and analysis can provide valuable reference for the design of Si-base blue LED with light emission around 460 nm.
2015, 64 (17): 177501.
doi: 10.7498/aps.64.177501
Abstract +
Exchange coupling is one of the most important fundamental interactions in ferromagnetic systems. Understanding of the parameters in this interaction may help describe numerous properties of metal magnetic materials. However, in the localized electron theory or itinerant electron theory there are also certain difficulties when utilizing this approximation method to study magnetic ordering problems for multi-atom systems. In realistic magnets exchange coupling is also related to the coexistence of localized and itinerant degrees of freedom. In this case Heisenberg exchange relationship has some limitations. If the exchange relationship only depends on the structure of the magnet, and is not related to energy differences between the phases, we can better avoid the Heisenberg exchange limits. Based on this, we use the general principle of the exchange coupling theory to analyse the usual approximation, and discuss the opportunity to calculate the parameters of such coupling rigorously without specific assumptions about the range of magnetic order or any approximation about the form of magnetization density. We propose a method for calculating the exchange coupling parameter to any approximation. The range of applicability of the above relation is discussed quantitatively for real magnetic systems (magnetic metal materials Gd, Fe, Ni) and spin waves, and the relevance for the exchange coupling is also analysed. This analysis for metal magnetic system (Fe, Ni and Gd) shows that the most significant improvement is obtained for exchange coupling between nearest magnetic atoms and for spin wave spectrum at finite wave vectors. It can be described by the relationship between the exchange coupling approximation and spin wave spectrum, and also interaction between the nearest neighbor magnetic atoms in ferromagnetic systems; these will give reasonable description to the large wave vectors part of spin wave spectra in any magnet with not fully localized magnetism. This point of view from the magnetism theory is consistent with the experimental results.
2015, 64 (17): 177701.
doi: 10.7498/aps.64.177701
Abstract +
Piezoelectrets, also known as ferroelectrets, are space-charge electrets based polymer foams with strong piezoelectric effect. The piezoelectric effect in piezoelectrets originates from the regularly arranged dipolar space charges in the polymer matrix, achieved by properly charging the specific foam structure. The large figure of merit (FOM, d33·g33) in piezoelectrets implies that such kinds of materials are promising candidates in energy harvesters. In this article, the electron-irradiated cross-linked polypropylene (IXPP) foam sheets are rendered piezoelectric (i.e. to become piezoelectrets) by modification of the microstructure using hot-pressing process and polarization using corona charging at room temperature. The electromechanical properties of the fabricated IXPP piezoelectrets are investigated by measurements of quasi- and dynamic piezoelectric d33 coefficients, dielectric resonance spectrum, and isothermal decay at elevated temperatures. The energy harvesting from vibrations by using the IXPP piezoelectret films, at various vibration frequencies, load resistances, and seismic masses, are also studied. Results indicate that the quasi-static piezoelectric d33 coefficients of IXPP films up to 620 pC/N can be achieved. The variation of quasi-static piezoelectric d33 coefficient is dependent on the applied pressures, from 0.1 to 1.3 kPa, while it shows good linearity at larger pressures from 1.3 to 15 kPa. The typical values of Young's modulus in the thickness direction and the figure of merit (FOM) are 0.7 MPa and 11.2 GPa-1, respectively. The d33 coefficients will drop to 54%, 43%, 29% of the initial values after annealing the samples for 24 h at 50, 70, 90℃, respectively. At an exciting frequency of 820 Hz, the normalized output power of 65 μW/g2 is obtained from an IXPP film with an area of 3.14 cm2 and a seismic mass of 25.6 g around the optimum load resistance. Such thin, light and flexible IXPP piezoelectret films may be applied in vibration energy harvesters for powering low-power electronic devices.
2015, 64 (17): 177801.
doi: 10.7498/aps.64.177801
Abstract +
That the energy of triplet exciton in Rubrene is about half of its singlet leads to energy resonance. This resonance not only allows two triplets to annihilate into a singlet, but also makes a singlet probably fission into two triplets in different molecules. On the other hand, the π-π conjugation of two Rubrene molecules could be formed during molecules stacking, and this spatial relationship will affect the charge transport property enormously. In this article, we use organic magnetic-field effect as a convenient approach to explore the influence of the energy resonant excited states in the Rubrene molecules and the π-π conjugation between the different molecules on the luminescence property of Rubrene. Firstly, we fabricate organic light emitting diodes based on pure Rubrene and modulate the thickness of Rubrene. Experimental measurements of these devices at room temperature exhibit that the thickness can affect the devices' magneto-electroluminescence (MEL) curves substantially. Values of high-field MEL increase with the thickness of Rubrene and gradually saturate after reaching 30 nm. This can be attributed to the fact that the ratio of π-π conjugation in Rubrene molecules to the stacking will grow with increasing thickness, and then saturate at a proper thickness. Subsequently, we modulate the concentration of Rubrene by doping Buthocuproine (BCP) in the active layer. Experimental results at room temperature show that the values of high-field MEL decrease as the concentration of Rubrene decreases. These results verify that the influence of π-π conjugation is not only on the MEL curves, but also on the singlet fission. Furthermore, all the MEL curves exhibit a high-field decay at low temperatures since the endothermic fission process in the Rubrene molecules becomes weaker as the temperature decreases, and the longer triplet lifetime at lower temperatures also enhances the process of triplet annihilation. Besides, the extensively existent intersystem crossing between singlet and triplet polaron pairs may affect these devices as well. Finally, the MEL curves of 20% Rubrene device at room temperature changing with various currents are successfully fitted through the combination of two exponential functions and a Lorentzian function. By means of the fitting, we confirm that the singlet exciton fission, the triplet-triplet exciton annihilation, and the intersystem crossing between singlet and triplet polarons coexist in the devices. Therefore, the varieties of these MEL curves can be attributed to the competition of these processes. The fittings reveal that the triplet-triplet exciton annihilation rate increases more obviously than the singlet exciton fission rate with increasing current. Compared with the rates of the two bimolecular interactions given before, the change of the intersystem crossing rate could be neglected because of its small magnitude. This work is helpful to expand the understanding of the internal mechanism of organic optoelectronic devices.
2015, 64 (17): 177803.
doi: 10.7498/aps.64.177803
Abstract +
In this paper, Er3+/Eu3+ co-doped BiOCl phosphors are synthesized by the conventional solid state method at 500℃, which are characterized by XRD, SEM, absorption spectra, excitation spectra and emission spectra. XRD analysis indicates that the samples exhibit pure tetragonal phase BiOCl. In SEM pictures, the samples exhibit smooth plate-like particles. The absorption spectra indicate that Er3+/Eu3+ ion dopants result in an impurity energy level, and the excitation spectra show that the sample has excellent broadband near ultraviolet (NUV)-exciting ability, which is due to the electronic transitions of the BiOCl bandgap. Under at 380 nm excitation, the emission bands located at 410 nm (2H9/2→4I15/2), 525 nm (2H11/2→4I15/2), 554 nm (4S3/2→4I15/2), 673 nm (4F9/2→4I15/2) of Er3+ ions, and 581 nm (5D0→7F0), 594 nm (5D0→7F1), 622 nm (5D0→7F2), 653 nm (5D0→7F3), 699 nm (5D0→7F4) of Eu3+ ions can be observed, respectively. Moreover, contrary to most of the Er3+/Eu3+-activated phosphors, the Er3+/Eu3+ co-doped BiOCl phosphor shows the unique and effective emission of the violet (Er3+) and far-red (Eu3+), which results from the particular structure of BiOCl crystals. The tunability in color of emitted radiation has been visualized by using chromaticity diagram on changing the doping concentration. Results show that the Er3+/Eu3+ co-doped BiOCl is a promising phosphor for near UV white LEDs.
2015, 64 (17): 177901.
doi: 10.7498/aps.64.177901
Abstract +
Semiconductor materials exhibiting large optical nonlinearities and ultrafast nonlinear response have received extensive attention because of their potential applications in optical limiting, all-optical devices, optical telecommunication, and so on. As a direct-gap II-VI bulk semiconductor, ZnSe crystal has been exploited as the nonlinear optical devices in the regimes of nanoseconds and picoseconds during the past years. Owing to today's fast advance of laser sources with ultrashort femtosecond pulse duration, it is possible to investigate the ultrafast optical nonlinearities in the bulk ZnSe crystal. In this paper, we experimentally investigate the ultrafast dynamics of free-carriers induced by twophoton excitation in the bulk ZnSe crystal. By performing open-aperture Z-scan experiments with 41 fs laser pulses at the wavelength of 532 nm under the condition of low excitation intensity, the two-photon absorption coefficient is measured. As the excitation intensity exceeds a critical value, the interplay between third- and fifth-order nonlinear absorption processes is observed. To evaluate the ultrafast dynamics of free carriers, we have carried out femtosecond time-resolved degenerate pump-probe measurements with the same laser system used for Z-scan experiments in different levels of pump intensities. It is shown that the transient absorption signals peaked at the zero delay is a linearly increasing function of pump intensity, indicating that the observed instantaneous nonlinear absorption is dominated by the interband two-photon absorption process. At moderate irradiance, the transient absorption signals obviously indicate two components, arising from the two-photon absorption-induced free-carrier absorption, which is equivalent to the fifth-order nonlinear absorption process. Under the excitation of relatively high pump intensity, the magnitude of the reduction of free-carrier absorption signal becomes faster, suggesting that the ZnSe crystal exhibits a new effect and causes a transmittance change of the probe light. The presumable reasons are as follows: intense irradiances will result in the increase of carrier concentration and the rise of the lattice temperature as well as the narrowing of the band gap in the ZnSe crystal, which accelerates the electron-hole interband recombination process. Accordingly, the electron-hole recombination time decreases. Furthermore, when the carrier concentration is larger than 1018 cm-3, the occurrence of the electron-hole plasma is significant. At the same time, the enhancement of the scattering among the carriers results in the reduction of the free carrier absorption cross section. In summary, it is found that the free-carrier absorption cross section decreases whereas the electron-hole recombination time becomes shorter in ZnSe crystal as the excitation intensity increases, owing to both the narrowing of band gap and the occurrence of electron-hole plasma.
2015, 64 (17): 177502.
doi: 10.7498/aps.64.177502
Abstract +
The crystal field (CF)- and external magnetic field- split ground state of Dy3+ in Dy3Al5O12 (DyAG) has been calculated based on the quantum theory in this paper. The eight CF-split levels are obtained, which are all twofold degenerates and are removed by the external magnetic field. On the basis of the results, the magnetic moments and the magnetic entropy changes of DyAG are calculated in the temperature range of 3THe3Ga5O12(GdGG) is stronger at low temperatures and dependent on the temperature and external magnetic field. Besides, the variation of the adiabatic temperature change ΔT with T is theoretically anticipated and the anticipated results are comparied with that of GdGG. It is found that the maximum adiabatic temperature change ΔT of DyAG is 1.27 times larger than that of GdGG when T=11 K and He=1 T. However, it changes to 1.15 times that of GdGG when T=16 K and He=2 T. There are differences between the refrigerative properties of DyAG and GdGG when they are in different external magnetic fields and different temperature regions. At low temperature s(THe is higher, GdGG is a good selection. This study is helpful to select suitable materials for the magnetic refrigeration technology.
2015, 64 (17): 177503.
doi: 10.7498/aps.64.177503
Abstract +
Laser-triggered magnetization dynamics for diluted magnetic semiconductor (Ga, Mn)As has drawn great attention in recent years, aiming at studying the ultrafast manipulation of collective spin excitations towards spintronic information processing. In this work, different fitting methods for time-resolved magneto-optical Kerr (TR-MOKE) study of the laser-triggered magnetization dynamics in a diluted magnetic semiconductor (Ga, Mn)As are analyzed and compared. It is known that the exponentially damped cosine harmonic function and the numerical simulation based on Landau-Lifshitz-Gilbert (LLG) equation are usually applied to fit the laser-induced magnetization dynamics from TR-MOKE measurements. Under the specified experimental conditions, it is sometimes hard to fit the TR-MOKE response well with single-mode uniform precession by using the exponentially damped cosine harmonic function. Although the fitting with multiple precession frequencies may usually show much better fitting results, the numerical simulation based on LLG equation reveals that the multi-frequency precessional modes are caused by the superposition of three-dimensional trajectories of magnetization precession with different contributions from the in-plane and out-of-plane magneto-optical response in (Ga, Mn)As. Thus, the multi-frequency precessional modes obtained by adopting the fitting method with exponentially damped cosine harmonic function could be the fake ones. Meanwhile, it is important to note that though the LLG equation can be used to fit the macroscopic magnetization precession well with single frequency, the contribution of pulse-like background response from photo-generated polarized carriers at the above-bandgap excitation is strongly superimposed on the magnetization precession response, and the pulse-like background response cannot be described by LLG equation. Thus one should be cautious of applying LLG equation only to fit the entire TR-MOKE signal, especially when the excitation energy is above the band gap of (Ga, Mn)As. One may combine both fitting methods, namely, fitting with the exponentially damped cosine harmonic function and the LLG simulation by considering both the in-plane and out-of-plane magneto-optical response of (Ga, Mn)As film in order to properly fit the laser-triggered magnetization dynamic response from TR-MOKE measurements. The proper handling of fitting methods helps to extract the dynamic magnetic parameters correctly and to further understand the physical mechanisms for triggering the ultrafast manipulation of collective spin dynamics. This is fundamentally important for developing novel spintronics based on diluted magnetic semiconductor (Ga, Mn)As.
2015, 64 (17): 177702.
doi: 10.7498/aps.64.177702
Abstract +
The expression of the total polarization intensity of BaTiO3 nanoparticles based on Euler-Lagrange equation for ferroelectric particles and the form of the series solution of Bessel function are obtained, the influences of coefficient on the total polarization intensity are analyzed. According to the expression for the total polarization intensity, the ferroelectricity of BaTiO3 nanoparticles with different size below 100 nm is simulated and analyzed by MATLAB. Based on experimental data, the effect of grain size on the ferroelectricity is discussed by searching for the numerical value of the solution, and the critical grain size of BaTiO3 nanoparticles with ferroelectricity is predicted to be 6 nm subsequently; or, more specifically, based on the Euler-Lagrange equation of ferroelectric particle's total free energy, and according to the boundary condition, the equation is given in spherical coordinates, and the transformed equation has the form and characteristics of the Bessel equation, Therefore, it can be discussed according to the characteri-stics of the Bessel equation. It is considered that it has the series solution, and according to the form of deduced series solution, at the same time, under reasonable conditions, the change of polarization is simulated. By combining with the boundary conditions, the total expression of polarization of nanoparticles may be obtained. It contains some factors, first of all, it is analyzed on the whole, and the effect the factors have on the total expression of polarization of the nanoparticles is analyzed. The factors that directly affect, or indirectly affect the polarization intensity, and thus influence the trend of polarization intensity change is analyzed and identified. Then, the effect of each factor on the dielectric constant is simulated by adjusting the factors, and the numerical solution obtained is consistent with the experimental data, so the predictive value can be obtained.
2015, 64 (17): 177802.
doi: 10.7498/aps.64.177802
Abstract +
The photoluminescence properties of InGaAsP films with a bandgap energy of 1.05 eV for quadruple-junction solar cells grown by molecular beam epitaxy (MBE) are investigated. We make the excitation intensity and temperature dependence of continuous-wave photoluminescence (cw-PL) measurements. The PL peak position is 1.1 eV at 10 K, and almost independent of the excitation power, but the integrated intensity of the PL emission peaks is roughly proportional to the excitation power. The shift of peak position with temperature follows the band gap shrinking predicted by the well-known Varshni's empirical formula. These results indicate that the intrinsic transition dominates the light emission of the InGaAsP material. In addition, we also make the time-resolved photoluminescence (TRPL) measurements to determine the carrier luminescence relaxation time in InGaAsP. PL spectra suggest that the relaxation time is 10.4 ns at room temperature and increases with increasing excitation power, which demonstrates the high quality of the InGaAsP material. However, the relaxation time shows an S-shape variation with increasing temperature: it increases at temperatures lower than 50 K, and then decreases between 50–150 K, and increases again when temperature is over 150 K. According to the effect of temperature and the non-radiative recombination center concentration on the carrier relaxation time, the recombination mechanism of S-shape variation can be explained by the carrier relaxation dynamics.
2015, 64 (17): 177804.
doi: 10.7498/aps.64.177804
Abstract +
Due to the lack of GaN substrates, hetero-epitaxial growth of GaN thin films is usually carried out on a foreign substrate. There are three kinds of substrate for GaN: sapphire, silicon carbide, and silicon; the sapphire substrate is the chief one, currently. Due to the availability of large scale and low cost of Si substrates, in recent years, extensive research has been devoted to the development of gallium nitride (GaN) optoelectronic devices on silicon substrates. Because of the large lattice mismatch and thermal-expansion cofficient difference between Si and GaN, it is difficult to grow thick enough crack-free GaN LED film on Si substrates. The two main kinds of methods for overcoming the crack problem are using the patterned Si substate and the thick AlGaN buffer layer. Although the two techniques could solve the problem of crack by cooling after growth, they will lead to an increase in tensile stress for GaN on Si. When making vertical-structured LED devices by transferring the GaN-based LED thin films from Si substrate to a new submount, this tensile stress will be partially released; but few researches have been made about the stress change before and after the transfer of the film, although the stress in GaN is an important factor that alters the energy band structure and may influence the vibrational properties. In this paper, we grow the crack-free GaN-based LED films on patterned Si(111), then light-emitting diode (LED) thin films are successfully transferred from the original Si (111) substrate to the submount with a flexible layer, and then the LED films without the influence of the submount and substrate are fabricated. In the following experiments, the strain-stress variation of the LED film is determined by using nondestructive high resolution X-ray diffraction (HRXRD) in detail, and the variation of photoluminescence (PL) properties of the film is studied too. Results obtained are as follows: 1) When the LED film is transferred to the flexible submount, the huge tensile stress will turn into compressive stress, and the latter in the InGaN layers will increase. 2) The In concentration in the (InGaN/GaN) MQW (multi-quantum well) systems can be evaluated with the help of reciprocal space maps (RSM) around the symmetric (0002) and asymmetric (1015) Bragg reflections. The In concentration in (InGaN/GaN) MQW will reduce when the GaN-based LED film is transferred to the flexible submount. 3) The PL spectra of the LED films will obviously appear red shift, after they are transferred to the flexible submount.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2015, 64 (17): 178101.
doi: 10.7498/aps.64.178101
Abstract +
Polarization state of electromagnetic waves plays a significant role in the fields of signal transmission and sensitive measurements. High-efficiently manipulating and controlling polarization state by two-dimensional flat metamaterials over a wider bandwidth has been turned into hot issues in recent years. A polarization conversion metasurface based on the split elliptical ring resonator is designed, simulated, and experimentally validated in the microwave regime. The proposed metasurface can convert a linear polarization state into its orthogonal one with a high efficiency for an ultra-wide band. Theoretically, the mechanism of polarization conversion is explained by the theoretical models of high-impedance surface and multi-plasmonic resonances. The metasurface has a strong anisotropy, which behaves as a high-impedance surface, and serves as a metal sheet in orthogonal orientation in the vicinity of the resonant frequencies. The reflection phase has a delay of π for one of the two electric field components and remains unchanged for the other. As a result, the polarization angle of the synthesized reflection electric field rotates by π/2. The fourth-order plasmonic resonances are generated by the electric and magnetic resonances, which contribute to the bandwidth expansion of cross-polarization reflection. Numerically, by means of simulation and analysis on the axial ratio and flare angle of the split elliptical ring resonators, the influences of these structure parameters on the bandwidth and efficiency of the polarization conversion are clarified. And then the design method of multi-peaks and wideband polarization conversion metasurfaces with split elliptic ring resonators is proposed for different kinds of applications. Experimentally, the geometry is implemented within the currently available printing circuit techniques, and a free space method is adopted to measure the scattering coefficients. A polarization conversion ratio of the fabricated sample is larger than 85% at a relative bandwidth of 104.5%, and approximately 100% of the polarization conversion ratio can be achieved around the resonant frequencies. Experimental results are in good consistency with the simulation results. Compared with the anterior polarization conversion metasurfaces, the proposed metasurface broadens the cross-polarization bandwidth greatly with little efficiency expenses. These works provide beneficial guidance for manipulating and controlling polarization states of electromagnetic waves, and have potential applications in modern radar and communication systems, signal detection systems, and sensitivity measurement systems, etc.
EDITOR'S SUGGESTION
2015, 64 (17): 178103.
doi: 10.7498/aps.64.178103
Abstract +
Magnetic coupling properties of Gd-doped ZnO nanowires are studied theoretically by using first-principles calculations. Several positions of Zn atoms that may be substituted by Gd atoms in ZnO nanowires are discussed. Numerical results show that the magnetic coupling is ferromagnetic when the two Gd atoms doped in ZnO nanowires are near each other. Injection of suitable amount of electrons can enforce these ferromagnetic properties in Gd-doped ZnO nanowires. It is also found that s-f coupling becomes remarkable when the Gd atoms are doped in ZnO nanowires, making the ferromagnetic coupling state more stable than the anti-ferromagnetic coupling state, and this is also the mechanism to elucidate the origination of ferromagnetic state in Gd-doped ZnO nanowires in experiments. These results will give a theoretical support for those who found experimentally that Gd-doped ZnO nanowires show ferromagnetic properties.
2015, 64 (17): 178502.
doi: 10.7498/aps.64.178502
Abstract +
Infrared (IR) photodetectors have been widely used in the fields of both civil and military applications such as environmental monitoring, medical diagnostics, satellite remote sensing and missile guidance, etc. In conventional large scale focal plane array (FPA) IR imaging, the thermal mismatch between IR photodetectors and silicon readout circuits will inevitably lead to the degradation of the device performance. An up-conversion IR photodetector, which converts IR photons to short-wavelength photons for Si-CCD-based imaging, can avoid thermal mismatch caused by hybridization with silicon readout circuits, resulting in a low-cost way for large array IR imaging. The operation principle of the semiconductor up-conversion IR photodetector is based on electron transitions and carrier transportation in different functional sections including absorption section, transportation section and emission section, hence the carrier distribution in the device structure has a crucial influence on the device performance. In order to achieve low dark current, carriers are expected to be non-uniformly distributed in the up-conversion device structure. Designing and optimizing the carrier-blocking structure are usually the key issues to acquire inhomogeneous carrier distribution. In this paper, up-conversion infrared photodetectors with various hole-blocking structures are investigated both theoretically and experimentally. Firstly the carrier distributions are calculated by self-consistently solving the Schr?dinger equation, Poisson equation, current continuity equation and carrier rate equation. Then the influence of the carrierblocking structure on the device performance is analyzed by electroluminescence measurements on the corresponding epitaxial structures. According to the theoretical and experimental results, it is found that a 2-nm-thick AlAs barrier layer can block holes effectively without hampering the electron transportation, which is necessary for the up-conversion infrared photodetectors. However, other attempts to block holes, such as light n-doping in the transportation section or lowering the injection barrier, do not work well. In addition, the influences of the thickness and height of the blocking barrier and the operation temperature on the carrier distributions are also studied. When the thickness of the blocking barrier is less than 2 nm, the thicker or the higher is the barrier, the better is the blocking effect. However, when the thickness of the blocking barrier is larger than 2 nm, the blocking effect is not persistently enhanced with increasing thickness because the tunneling process is almost fully suppressed. Furthermore, with the same blocking barrier parameters, lowering the operation temperature can lead to better blocking effect. This work demonstrates the utilization and effect of carrier-blocking structures in semiconductor devices which deamnd an inhomogeneous carrier distribution.
2015, 64 (17): 178801.
doi: 10.7498/aps.64.178801
Abstract +
A major issue in organic photovoltaics (OPVs) is the poor mobility and recombination of the photogenerated charge carriers. The active layer has to be kept thin to facilitate charge transport and minimize recombination losses. However, optical losses due to inefficient light absorption in the thin active layers can be considerable in OPVs. Therefore, light trapping schemes are critically important for efficient OPVs. In this paper, high efficient OPVs are demonstrated by introducing randomly nanostructured front electrodes, which are fabricated using commercially available ZnO:Al (AZO) films by means of a wet etching method. The etched AZO front electrode induces strong diffusion and scattering of the incident light, leading to the efficient light trapping within the device and enhancement of light absorption in the active layer. Such a nanostructured electrode can achieve an improved device performance by maintaining simultaneously high open-circuit voltage and fill factor values, while providing excellent short-circuit current enhancement through efficient light trapping. The best device obtained based on the textured electrode shows a 11.29% improvement in short current density and a 8.17% improvement in power conversion efficiency, as compared with the device with a flat electrode. The improvement in PCE is directly correlated with the enhancement of light absorption in the active layer due to the light scattering and trapping effect induced by the randomly nanotextured electrodes, which is confirmed by a haze factor measurement and an external quantum efficiency characterization. The well-established contact interfaces between the etched electrodes and active layers are made, and thus reduce the impact on the open-circuit voltage and fill factor values in OPVs. We thus conclude that the method of light manipulation developed in this paper will provide a promising and practical approach to fabricate high-performance and low-cost OPVs.
2015, 64 (17): 178102.
doi: 10.7498/aps.64.178102
Abstract +
A nanometer scale mechanism for micro crack propagation under uniaxial tension in single crystals is investigated using phase field crystal (PFC) simulation. The uniaxial tensile loading is strain controlled. And three initial typical stresses of pre-existing center crack in (111) crystal plane of face centered cubic structure are chosen to study the effects of initial stress state on micro-crack propagation. Moreover, the influences of different crystal orientations, when the crystal suffers from uniaxial tension, are also investigated. Due to the influence of time scale and length scale in the PFC method, the motion of dislocations, vacancies, shear band and twinning structure should be observed and described during the propagation process of micro cracks. In addition, the free energy curves of different processes are drawn and discussed in order to explain the different behaviors of the crystal in the propagation of cracks. Simulation results show that the propagation behavior of micro cracks can be closely associated with the initial stress state. It is found that the propagation behavior mainly occurs in the 11>(111) slip system. Besides, the crystal orientation has a significant effect on the mechanism of activation and evolution. In the pre-stretching system, slip dislocation is induced near the micro-crack tip, and then its slide in [011] direction will cause the cleavage of a certain crystal plane, and promote the micro cracks to extend. However, to a certain level, the propagating direction of the micro-crack tip will turn to another slip direction [101]. As a result, zigzag edge appears. By contrast, in the pre-shear system, the tip of the micro crack propagates in a cleavage mode, and results in the appearance of slip dislocation [101] near the micro-crack tip. Afterwards, the motion of slip dislocation promotes the production of vacancies. And owing to the aggregation and combination of vacancies, secondary cracks form and propagate in the process that follows. At the same time, in a pre-deviatoric system, the micro crack propagates forward with direct cleavage of [101] slip direction near the micro-crack tip until the single crystal sample fractures. Furthermore, no slip dislocation appears during the whole process. The mechanism of micro-crack tip propagating behavior varies with crystal orientation. When the crystal orientation angle is lower, the micro-crack tip prefers to produce slip dislocation around it, and the following dislocation slide will induce vacancies, then a secondary crack also forms because of the aggregation and combination of vacancies. On the other hand, when the aggregation degree is higher, the micro-crack tip is inclined to directly propagate in a cleavage mode, and its propagating direction is nearly perpendicular to the stretching direction.
2015, 64 (17): 178501.
doi: 10.7498/aps.64.178501
Abstract +
Since metal interconnect overlayers are central components of micro/nano scaled static random access memory (SRAM), the effects of their presence on proton-induced single-event susceptibility are noteworthy. Geant4 is used to calculate the kinds and probabilities of secondary particles existing in bulk silicon, which are produced from nuclear reactions between protons of different energies (30, 100, 200 and 500 MeV) and micro/nano scaled SRAM. The probabilities of secondary particles with Z≥30 in different overlays are compared with one another; the particles are chiefly coming from nuclear reactions between 500 MeV protons and the SRAM topped with interconnect overlayers. In addition, the kinds and ranges of the secondary particles with high LETs (linear energy transfers) are also analyzed. Results show that there is an increase in the production of secondary particles with Z≥30 due to the presence of metal interconnect overlayers and the rise of proton energy. The secondary particles with Z>60 in bulk silicon are generated by proton interactions with tungsten. As another consequence of the interactions, the secondary particles with 30≤Z≤50 are produced, the probability of which is higher as the proton energy increases. The maximum LET for the secondary particles with 30≤Z≤50 is about 37 MeV·cm2/mg and the corresponding range is several microns, which may induce single event latch-up in micro/nano scaled SRAM with well depths on the order of microns. Results obtained support the theoretic analysis of proton-induced single event effects of aerospace devices in space radiation environment.
2015, 64 (17): 178802.
doi: 10.7498/aps.64.178802
Abstract +
With the development of large-scale hydro-generators, large hydro-generator cooling technology is increasingly demanded. Different cooling method will not only affect the structure of hydro-generators, also it will affect the energy consumption and reliability of the generators. The commonly large-scale hydro-generator cooling method includes: air cooling, water cooling, and evaporative cooling methods. This paper analyzes the principle of the three cooling methods and describes qualitatively the advantages and disadvantages of them. The air cooling hydro-generator structure is simple, but the generator operating temperature is high; the water-cooling method has a certain superior in cooling performance, but it requires more auxiliary equipments, and has higher equipment failure rates. The evaporative cooling method is a recently developed cooling technique. It not only has a remarkable cooling effect, but also can decrease the equipment failure rates and the cost of maintenance. In order to build a comprehensive model to assess the three hydro-generator cooling methods, this paper proposes a comprehensive evaluation method based on AHP. The method includes 11 indexes of resource consumption, energy consumption and reliability to assess the influence of cooling ways. The energy saving influence of all the 11 indicators are calculated by using the AHP comprehensive evaluation. Finally, comparison between a 400 MW air cooling and an evaporative cooling hydro-generators at Lijiaxia hydropower Station are made using the proposed method. Evaluation results indicate that in terms of daily operation, the energy saving of the evaporative cooling hydro-generator can be more than 300 tons standard coal equivalent per year as compared with that of air cooling generator. In terms of maintenance, the evaporative cooling method can save more than 5000 tons of standard coal equivalent per year. The comprehensive evaluation results show that the evaporative cooling method is significantly better than the air cooling. It can be seen that the proposed evaluation method may quantitatively calculate the merits of hydro-generator caused by cooling method, which provides guidance to select and improve cooling method of hydro-generator.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2015, 64 (17): 179201.
doi: 10.7498/aps.64.179201
Abstract +
The research of abrupt climate change is an important field in the climate change. The rapid and accurate detection of the abrupt climate change has important practical significance and major economic-social costs, which will help us understand climate change and forecast the future evolutionary trend of the climate system. The detection results of most traditional abrupt climate change depend on the selection of the time scale concerned, which may result in the fact that we cannot identify an abrupt climate change until the event has been past for a long time. Moreover, these detection methods cannot extract the dynamical changes from the observational data of the climate system. As the rapid development in nonlinear science, the abrupt climate change detection technology has also been improved gradually. This article briefly reviews several new progresses in abrupt dynamical detection methods developed on the basis of recent nonlinear technologies, and some applications in the real observational data. These new methods mainly contain the technologies based on the long-range correlation of climate systems, such as moving detrended fluctuation analysis, moving cut data-detrended fluctuation analysis, moving cut data-R/S analysis, degenerate fingerprinting, and red noise. Moreover, some abrupt dynamical detection methods developed by the complexity of the time series, namely, entropy, such as approximate entropy, moving cutting data-approximate entropy, Fisher information, and wavelet Fisher's information measure. Furthermore, there are some other abrupt dynamical detection methods based on the theory of phase space, such as the dynamics exponent Q. Climate system is a complex dynamical system with nonlinear and interactive nature, which has long-range persistence in spatio-temporal variation, thus the abrupt detection method on spatial field change is pointed out to be a promising direction for further research in future. Because the spatial field contains abundance of information about the evolution of climate system which is much more than that in a time series in single meteorological station, the detecting methods on spatial field will greatly help us detect an abrupt climate change as soon as possible. And then we will have enough time to take action and make preparations for the new challenges due to the abrupt climate change.
NUCLEAR PHYSICS
2015, 64 (17): 172901.
doi: 10.7498/aps.64.172901
Abstract +
ZnO micro/nanowires were synthesized by chemical vapor deposition method. The morphology and structure of the products have been characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and micro-Raman scattering spectrometer, etc. Results show that the surface of the highly uniform ZnO wire is smooth and the as-synthesized ZnO wires show high crystal quality. Three types of UV detector are constructed using a single ZnO nanowire with different contact characteristics, and their corresponding performances are investigated systematically by using Keithley 4200-SCS and other equipments. All of the three different devices exhibit good rectifying characters and significant responsivity to ultraviolet light. The devices show self-driven features at zero bias. Compared with the devices made from Schottky contact and ZnO/PEDOT:PSS film, the present single ZnO nanowire/p-Si film devices with heterojunctions have the best self-powered function, which can be attributed to the stronger built-in electric field as well as the smaller dark current due to the insulating layer on the p-Si film. At zero bias, the fabricated ZnO nanowire/p-Si film device can deliver a dark current of 1.210-3 nA and a high photosensitivity of about 4.5103 under UV illumination. The response of the devices made from ZnO nanowire/p-Si film to UV illumination in air is pretty fast with the rise time of about 0.7 s and the fall time of about 1 s, which could be attributed to the fact that the photo-generated electron-hole pairs in the depletion layer is quickly separated by the built-in electric field, leading to a rapid response speed and a larger photocurrent. Comparison among the three kinds of devices indicates that the devices made from ZnO nanowire/p-Si film are the best candidate for UV detectors.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (17): 176101.
doi: 10.7498/aps.64.176101
Abstract +
We use the group theory and molecular orbital theory to systematically study the origin of magnetic moment of cation-vacancy in un-doped magnetic semiconductors, and illustrate the mechanism of exchange-coupling between magnetic moments by Heisenberg model. It is found that the magnetic moment is related to the number of unpaired electrons, and the distribution of defects energy level is correlated closely with the symmetry of vacancy crystal field. The exchange-coupling coefficients J0 is estimated by the energy difference between antiferromagnetic and ferromagnetic states. And J0 can be used to predict the magnetic coupling. Positive J0 means the ferromagnetic coupling between magnetic moments, otherwise the coupling is antiferromagnetic. Moreover, we indicate that reduction of degeneracy of defect energy-level bears a direct relationship to the electron number occupied in the defect energy-level orbital, and therefore results in the structure distortion (John-Teller effect) of a cation-vacancy.
2015, 64 (17): 176601.
doi: 10.7498/aps.64.176601
Abstract +
Effects of thermal stress induced by multiple through silicon vias (TSVs) on mobility and keep out zone (KOZ) are mainly discussed in this paper. It is found that the angle and pitch between TSVs have a great effect on the carrier mobility and KOZ. In this paper, the device channel direction is set along [100]. And two types of KOZ are presented, namely the variations of electron mobility are 5% and 10% respectively. As for the two TSVs, their KOZ sizes change significantly with the angles between TSVs which change from zero to π/4, and the area of a KOZ is the minimum when the angle is π/4. But the zone for device placement is irregular, which is difficult for agreement. The area of a KOZ is the maximum when the angle is zero, and it is easy to make arrangement as the space for device distribution is regular. Based on these analyses, the effects of pitch between TSVs are presented. When the angle is zero, the area of KOZ decreases as the pitch increases and tends to be the same as that of a single TSV. For example, the KOZ, in which the variations of electron mobility are 5% and 10%, will reduce to 8.4 μm and 5.1 μm as the pitch increases to 20 μm, which is close to that of the single TSV. But when the angle is π/4, the KOZ with an electron mobility 5% increases from 5.2 to 6.4 μm as the pitch increases and tends to be the same as that of a single TSV at last. The KOZ with an electron mobility 10% will increase from 4.2 to 4.5 μm. In addition, the above analyses can be extended to the KOE of four TSVs, a more representative pattern. And two kinds of TSV displacement style including “square” and "diamond" TSV patterns are also discussed, the impact of pitch for these two patterns are also given in this paper. For the “square” TSV pattern, the KOZ decreases as the pitch increases. Under this condition, the devices can only be placed in a small square region surrounded by TSVs, but the region is regular, which is beneficial for device arranging. While for the "diamond" TSV pattern, the KOZ increases as the pitch increases. Under this condition, the area for device placement is larger than the “square” TSV pattern, but the region is irregular as it is divided into long narrow parts, which is hard for device placement.
2015, 64 (17): 176801.
doi: 10.7498/aps.64.176801
Abstract +
Superhydrophobicity of biological surfaces with micro/nanoscale hierarchical roughness has recently been given great attention and widely reported in many experimental studies due to the unique wettability. For example, the dual-scale structure of the lotus leaf not only shows high contact angle and low contact angle hysteresis but also presents good stability and mechanical properties. Though lots of experimental studies on the wettability of artificial hierarchical rough surface have been carried out, a thorough analysis on the contribution of micro- and nano-scaled roughness to the metastable wetting states and their transition is still lack. In this paper, a thermodynamic approach is applied to analyze all the wetting states (including four stable wetting states and five transition states) of a water droplet on a surface with micro/nanoscale hierarchical roughness, and the corresponding free energy expressions and apparent contact angle equations are deduced. The stable wetting states are confirmed by the principle of minimum free energy. And the calculated results by these state equations can fit well with the experimental results reported in the literature when compared with the previous models. Meanwhile, the influence of micro/nanoscale roughness on the stable wetting states and metastable-stable transition has been analyzed thermodynamically. It is found that there is a synergistic effect of micro and nanoscale roughness on wettability, which nlay result in many different wetting states. There are four wetting states during increasing relative pitch of a microscaled structure at a given nanoscaled structure, but two wetting states can be obtained as increasing relative pitch of nanoscaled structure at a given microscaled structure. The change of nondimensional energy and nondimensional energy barrier in the metastable-stable transition process of water droplet wetting micro and nanoscaled structure is quantitatively analyzed. Results indicate that the micro-scaled structure is never wetted in a special size range of the nanoscaled structure, and the special size range is of great significance to enhance superhydrophobic stability of the microscaled structure. Furthermore, the existence of microscaled structure decreases the transition energy barrier of water droplet wetting nanoscaled structure, which is helpful for understanding the experimental results reported in the literature. Finally, all possible stable wetting states of water droplet no a surface with micro/nanoscale hierarchical roughness are discribed in a wetting map. A design principle of superhydrophobic surface with micro/nanoscale hierarchical roughness is put forward, which is helpful to ensure the size of micro/nanoscale structure in the “stable superhydrophobic region” and to provide a theoretical guidance in the preparation of superhydrophobic surface.
2015, 64 (17): 176802.
doi: 10.7498/aps.64.176802
Abstract +
ZnSb-doped Ge2Sb2Te5 films have been deposited by magnetron co-sputtering using separated ZnSb and Ge2Sb2Te5 alloy targets. The concentrations of ZnSb dopant in the ZnSb-added Ge2Sb2Te5 films, measured by using energy dispersive spectroscopy (EDS), are identified to be 5.4, 9.9, 18.7 and 24.3 at. %, respectively. X-ray diffraction (XRD), in situ sheet resistance measurements, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), are used to analyze the relationships among the composition, structures and properties of the films. The sheet resistance as a function of the temperature (R-T) is in situ measured using the four-probe method in a home-made vacuum chamber. It is found that the crystallization temperature of ZnSb-doped Ge2Sb2Te5 films are much higher than that of conventional Ge2Sb2Te5 (～168℃). The higher crystallization temperature is helpful to improve the amorphous thermal stability. Data retention can be obtained by the extrapolated fitting curve based on the Arrhenius equation. It is shown that the values of 10-yr data retention for ZnSb-doped Ge2Sb2Te5 films are higher than that of conventional Ge2Sb2Te5 film (～ 88.9℃). XRD patterns of the as-deposited films when annealed at 200℃, 250℃, 300℃, and 350℃ show that ZnSb-doping can suppress the phase transition from fcc phase to hex phase. XPS spectra are further used to investigate the binding state of (ZnSb)18.7(Ge2Sb2Te5)81.3, suggesting that the Zn–Sb and Zn–Te bonds may exist in an amorphous state. In addition, we have measured the dark-field TEM images, selected area electron diffraction patterns, and high-resolution transmission electron microscopy images of the (ZnSb)18.7(Ge2Sb2Te5)81.3 films. Apparently, the films show a uniform distribution of crystalline phase with the dark areas surrounded by bright ones (Zn–Te or Zn–Sb domain). A static tester using pulsed laser irradiation is employed to investigate the phase transition behavior in nanoseconds. Results show that the ZnSb-doped Ge2Sb2Te5 films exhibit a faster crystallization speed. Among these samples, the (ZnSb)24.3(Ge2Sb2Te5)75.7 film exhibits a higher crystallization temperature of 250℃ and the 10 years data retention is 130.1℃. The duration of time for crystallization of (ZnSb)24.3(Ge2Sb2Te5)75.7 is revealed to be as short as ～64 ns at a given proper laser power 70 mW. A reversible repetitive optical switching behavior can be observed in (ZnSb)24.3(Ge2Sb2Te5)75.7, confirming that the ZnSb doping is responsible for a fast switching and the compound is stable with cycling. These excellent properties indicate that the (ZnSb)24.3(Ge2Sb2Te5)75.7 film is a potential candidate as the high-performance phase change material.
