Vol. 61, No. 17 (2012)



Fabrication and characterization of single Nb/NbxSi1-x/Nb Josephson junction for voltage standard
Cao Wen-Hui, Li Jin-Jin, Zhong Qing, Guo Xiao-Wei, He Qing, Chi Zong-Tao
2012, 61 (17): 170304. doi: 10.7498/aps.61.170304
Abstract +
The core device of Modern programmable Josephson voltage standard is Josephson junction array. The most advantageous Josephson junction array is Nb/NbxSi1-x/Nb material array. The advantages of Nb/NbxSi1-x/Nb material Josephson junction are that three-layer film production process is simple, Nb and NbxSi1-x etching processes are the same and NbxSi1-x potential barrier layer components can be easily adjusted. In this paper, we investigate the NbxSi1-x/Nb single Josephson junction in National Institute of Metrology. Through measuring the dc current-voltage characteristics under low temperature (4.2 K), superconducting tunneling current and a zero voltage state jumping to voltage state are observed clearly, finally the measurement results are discussed. The work is the first study on Nb/NbxSi1-x/Nb single Josephson junction in China.


Laser-induced ultrafast spin transfer in linear magnetic molecular ions
Li Chun, Zhang Shao-Bin, Jin Wei, Georgios Lefkidis, Wolfgang Hübner
2012, 61 (17): 177502. doi: 10.7498/aps.61.177502
Abstract +
The demagnetization, spin switching, and spin transfer in magnetic molecular systems can be theoretically achieved via the laser-induced Λ process. In the present work, both quantum chemistry ab initio calculations and our self-written programs are adopted to investigate the spin-transfer behavior of the magnetic molecular ions with linear configuration. It is shown for the first time that the Λ process based ultrafast spin transfer can be achieved in a linear two-magnetic-center molecular ion [Fe-O-Co]+, and the fidelity of the population transfer exceeds 90%. The present theoretical prediction shows that the magnetocrystalline anisotropy of a molecular system can be enhanced by properly adjusting the direction of the applied magnetic field, which is shown to be a better way to improve the spin-transfer ability of the molecular system than by increasing additional bridging atoms since it could avoid the complexity of the latter situation in practical applications. At the same time, the present spin-transfer scenario indicates that the fidelity of the population transfer could also be evidently increased.


Random walks on spatial networks
Dou Fei-Ling, Hu Yan-Qing, Li Yong, Fan Ying, Di Zeng-Ru
2012, 61 (17): 178901. doi: 10.7498/aps.61.178901
Abstract +
In this paper, we construct a cost constrained spatial network by adding long-range connections to the one-dimensional circle. The probability for a long-range connection between nodes i and j is pij dij- ( 0), where dij is the lattice distance and the total length of the long-range connections is set to be =cN(c 0), where c is a positive constant and N is the network size. According to the simulation and numeric results, we find an optimal power-law exponent 0 such that the mean first-passage time is shortest. Furthermore, the shortest mean first-passage time has the power law relationships with the network size N. With the increase of network size N and the total cost , the optimal power-law exponent 0 increases monotonically and approaches 1.5.
Research on non-linear beam-wave interaction of W-band gyro-TWT with helical waveguide
Xue Zhi-Hao, Liu Pu-Kun, Du Chao-Hai, Li Zheng-Di
2012, 61 (17): 170201. doi: 10.7498/aps.61.170201
Abstract +
Gyro-TWT is one of the most promising candidates for the application of the transmitter microwave source of the next-generation imaging radar; meanwhile, it plays an important role in national security. Gyro-TWT with helical waveguide is capable of generating broad-bandwidth radiation, as well as high stable ability. In this paper, we derive the dispersion equation of helical waveguide and the non-linear theory of calculating the beam-wave interaction. Numerical stimulation results accord with the experimental results. We design a W-band gyro-TWT operating at a 80 keV, 5 A electron beam, which can produce an output power of 142 kW, with 3 dB bandwidth 4.5%, cantral frequency 95 GHz and saturation gain 52 dB. Finally, we calculate the effect of variation of voltage and current on the output performance of gyro-TWT with helical waveguide.
Experimental investigation of different Bell-type inequality in three-qubit Greenberger-Horne-Zeilinger states
Zhao Jia-Qiang, Cao Lian-Zhen, Wang Xiao-Qin, Lu Huai-Xin
2012, 61 (17): 170301. doi: 10.7498/aps.61.170301
Abstract +
We investigate experimentally the violation of the Bell, Mermin and Svetlichny inequalities to local realism prediction. Using the maximum-likelihood technique to construct the density matrix and calculate the fidelity of the Greenberger-Horne-Zeilinger (GHZ) state. The values of the three inequalities are measured. The experimental resuls show that the three inequalities have different violation degrees to local realism prediction. So we can choose a suitable method of nonlocality to estimate the security of quantum channel.
Properties and coherence-controlling of entanglement of a two-qubit Heisenberg XY chain with intrinsic decoherence
Jiang Chun-Lei, Liu Xiao-Juan, Liu Ming-Wei, Wang Yan-Hui, Peng Zhao-Hui
2012, 61 (17): 170302. doi: 10.7498/aps.61.170302
Abstract +
By solving the Milburn equation, we investigate the thermal entanglement properties of a two-qubit Heisenberg XY chain in the presence of intrinsic decoherence. The controls of nonuniform magnetic field, the initial state of two qubits, the relative phases and the amplitudes of the polarized qubits on thermal entanglement are studied. The results show that for a particular initial state, the thermal entanglement can be increased by the external magnetic field. The time behavior of the entanglement exhibits a strong dependence on the initial state of two qubits, and it can be manipulated by changing the relative phase and the amplitudes of the polarized qubits. It is also notable that stable entanglement, which is dependent on initial state of the qubit, occurs even in the presence of decoherence. The magnetic field may have a constructive effect on the stable entanglement for a certain initial state, and the Bell-diagonal state turns out to be a dark state of the system in the absence of the magnetic field.
Broadcast and multicast in quantum teleportation internet
Zhou Xiao-Qing, Wu Yun-Wen
2012, 61 (17): 170303. doi: 10.7498/aps.61.170303
Abstract +
The physical principle of broadcast and multicast is given. When Alice needs to send out an information, she sends out a conjunction claim signal to the switch that connects with hers. After receiving the conjunction claim the signal, the switch pass to examines the purpose address to judge whether it is a point-to-point communication or broadcast and multicast. If the purpose address is A or B or C address, then the switch carries on the correspondence operation of point-to-point; if the purpose address is a D address or an address (local or appointed network webcasting) with special meaning, then the switch carries on the correspondence operation of broadcast and multicast. With broadcast and multicast, the fidelity decreases with the increase of receiving terminal number, and its limit is 2/3.
Free particle geodesic affine parameter time-space coordinate systems
Bian Bao-Min, Lai Xiao-Ming, Yang Lin, Li Zhen-Hua, He An-Zhi
2012, 61 (17): 170401. doi: 10.7498/aps.61.170401
Abstract +
Taking the time-series t as independent variable, the parameter equations {Xi(t)} of free particle space geodesic can be given. By transforming affine parameter R(t) we achieve homogeneous geodesic differential equations, and derive the first-order differential equations which are satisfied by affine parameter R and the sequence of analytical solutions R marked by rational number Cu. In light of R we define the distance unit of flat four-dimensional coordinate system {t,r,θ,φ}, and then establish a free particle geodesic affine parameter time-space coordinate system {t,ξ,θ,φ}. By the study of the diagonalization process of special relativity time-space interval model metric tensor g in {t,ξ,θ,φ}, we find the spatial and temporal line characteristic quantities t1(t,ξ), τ1(τ,ξ),tt(t,τ,ξ) and ττ1(t,τ,ξ) corresponding to diagonal metric. Derived from these quantities, the dimension of time-space coordinate system is less than 4.
The resonant behavior of an over-damped linear fractional Langevin equation
Zhong Su-Chuan, Gao Shi-Long, Wei Kun, Ma Hong
2012, 61 (17): 170501. doi: 10.7498/aps.61.170501
Abstract +
By choosing the internal noise as a fractional Gaussian noise, we obtain the fractional Langevin equation. We explore the phenomenon of stochastic resonance in an over-damped linear fractional Langevin equation subjected to an external sinusoidal forcing. The influence of fluctuations of environmental parameters on the dynamics of the system is modeled by a dichotomous noise. Using the Shapiro-Loginov formula and the Laplace transformation technique, we obtain the exact expressions of the first and second moment of the output signal, the mean particle displacement and the variance of the output signal in the long-time limit t→∞. Finally, the numerical simulation shows that the over-damped linear fractional Langevin equation reveals a lot of dynamic behaviors and the stochastic resonance (SR) in a wide sense can be found with internal noise and external noise.
Suprathreshold stochastic resonance of a non-linear multilevel threshold neuronal networks system
Guo Yong-Feng, Tan Jian-Guo
2012, 61 (17): 170502. doi: 10.7498/aps.61.170502
Abstract +
Suprathreshold stochastic resonance can be used to explain some biologic phenomena. In this paper, the suprathreshold stochastic resonance of a non-linear multilevel threshold neuronal network system is studied. The expression of the mutual information is derived, and the effects of the noise intensity and the system parameter on mutual information are discussed. The numerical computation results show that in the process of information transmission the selection of the threshold is very important according to the different effects of additive noise and multiplicative noise on system. Moreover, we also find that the more the number of devices, the more apparent the phenomenon of suprathreshold stochastic resonance is.
Brownian motion in a harmonic trap: magnetic tweezers experiment and its simulation
Ran Shi-Yong
2012, 61 (17): 170503. doi: 10.7498/aps.61.170503
Abstract +
The Brownian motion in a harmonic trap is studied by magnetic tweezers experiment and computer simulation. The results of the experiment and simulation validate the theory. Then the theory is used to analyze the experimental results including the effect of persistent length of DNA on the displacement distribution of the bead and the error in force measurements. It can be concluded that the variation of the persistent length affects more the Brownian motion along the DNA chain than in the other direction; under a small force, a considerable error of the force measurement will occur.
Feature analysis in frequency domain of Duffing system based on general local frequency
Tang You-Fu, Liu Shu-Lin, Lei Na, Jiang Rui-Hong, Liu Ying-Hui
2012, 61 (17): 170504. doi: 10.7498/aps.61.170504
Abstract +
Owing to the limitations of the concept of frequency for power spectrum and the inherent defects of Fourier transform, a novel concept of general local frequency is proposed. Based on a approach to adaptive peak decomposition, the dynamic feature in frequency domain varying with parameter r of Duffing system driven by periodic signal is investigated. And a phenomenon of frequency bifurcation is found. Moreover, coninuous frequency bands exist near the central frequency of chaos time seriers at different values of parameter r and their shapes are similar. By demodulation analysis of Hilbert transform, the modulation characteristic and modulation similarity of chaos time seriers are summarized. The above study shows that the proposed approach to general local frequency based on adaptive peak is effective for freature extraction in frequency domain of Duffing system. It provides a new method to observe a continunous distribution of frequency bands for the non-linear system in chaotic state.
Stabilities and bifurcations of sine dynamic equations on time scale
Hu Wen, Zhao Guang-Hao, Zhang Gong, Zhang Jing-Qiao, Liu Xian-Long
2012, 61 (17): 170505. doi: 10.7498/aps.61.170505
Abstract +
A time scale is a nonempty closed subset of the real numbers R. Recently, the dynamic equations on time scale have received much attention, which have the generalized forms of differential and differential dynamic equations. In this paper, we study the stabilities of fixed points and bifurcations of the sine dynamic equations on time scale. The results show that the solutions of the sine dynamic equations become different with the time scale parameter changing. And n-period-doubling bifurcations and splits of fixed points are observed. Moreover, the chaotic parameter spaces of the dynamic equations are expanded by the increase of complexity of time scale but without increasing the system parameter, thus providing a potential advantage for chaos encryption, radar waveform design and other application areas.
Study on period doubling bifurcation in current-mode SEPIC converter
Li Guan-Lin, Li Chun-Yang, Chen Xi-You, Mu Xian-Min
2012, 61 (17): 170506. doi: 10.7498/aps.61.170506
Abstract +
Choosing the current of the front inductor as control object, the discrete time model of single-ended primary-inductor converter (SEPIC) converter working in the discontinuous conduction mode (DCM) is established in this paper, and the stability of the fixed point is analyzed. By studying the phase portrait, power spectrum and bifurcation diagram of the circuitry, a special period-doubling bifurcation phenomenon is found in this circuitry. The circuit experiences period, double period, quadruple period and then double period, quadruple period, finally enters into chaotic condition. The experimental results accord with the simulation results, which confirms that the special period-doubling bifurcation exists in the SEPIC converter working in the DCM.
Bounded damped oscillatory solutions of Fisher equation
Li Xiang-Zheng
2012, 61 (17): 170507. doi: 10.7498/aps.61.170507
Abstract +
To research the bounded damped oscillatory solutions of nonlinear evolution equation, we choose the Fisher equation as an example. The solutions with negative amplitudes of Fisher equation may become meaningful in the context of nonscalar models describing excitable media (e.g. the Belousov-Zhabotinsky (BZ) reaction). The theory of planar dynamical systems is used to study the existence conditions of bounded traveling wave solutions of Fisher equation. The bounded approximate damped oscillatory analytic solution is given by using LS method and linearization method. And its error is also estimated.
A novel method of estimating parameter and its application to blind separation of chaotic signals
Wang Shi-Yuan, Feng Jiu-Chao
2012, 61 (17): 170508. doi: 10.7498/aps.61.170508
Abstract +
To estimate effectively parameters of nonlinear mapping, a cubature rule is used to approximate the weighted integral of this mapping. In this paper, based on these parameters modeled by a state-space model, a novel parameter estimation is proposed. Blind separation of chaotic signals is a challenging problem. The proposed method is used to solve this problem to achieve the effective reconstruction of chaotic signals. Simulation results indicate that the proposed method has a faster convergence speed and a higher numerical accuracy, and can effectively separate original chaotic signals.
Information entropies and dynamics in the stochastic ecosystem of two competing species
Xie Wen-Xian, Cai Li, Yue Xiao-Le, Lei You-Ming, Xu Wei
2012, 61 (17): 170509. doi: 10.7498/aps.61.170509
Abstract +
Using the models of stochastic population dynamics, the competitions and interactions of interspecies and between species and the stochastic environment are studied. In this paper, the stochastic ecosystems (in Itô or Statonovich model) of two competing species are investigated through evaluating probability densities and information entropy fluxes and productions of two species. The formulas of entropy flux (i.e. expectation of divergence) and entropy production are educed for numerical calculations, through the corresponding Fokker-Planck equation with its condition and the definition of Shannon entropy. The nonlinear characteristics of entropy fluxes are captured and the relationships are found between the extremal points of entropy productions and the rapid transitions or bifurcations. The numerical results obtained with path integration method show that the probability densities and Shannon entropies of these two stochastic models (in Itô or Statonovich meaning) have the same evolutional tendency but with different points of extrema.
Detecting unstable periodic orbits from continuous chaotic dynamical systems by dynamical transformation method
Ma Wen-Cong, Jin Ning-De, Gao Zhong-Ke
2012, 61 (17): 170510. doi: 10.7498/aps.61.170510
Abstract +
Detecting unstable periodic orbits (UPOs) from chaotic dynamic systems is a challenging problem. For a large number of complex systems, we can collect some experimental time series data but cannot find theoretical models to describe them. Thus, detecting unstable periodic orbits from experimental data can help us understand the chaotic properties of physical phenomenon without using theoretical models. We, in this paper, first use the dynamical transformation (DT) algorithm to detect unstable periodic orbits from chaotic systems, and find that the original DT algorithm can detect the UPOs from the time series of chaotic discrete map, but it is infeasible for the time series from continuous chaotic flow. In this regard, we then propose an improved DT algorithm that is based on the Poincare section method to detect the UPOs from continuous chaotic flow. In particular, we transform the continuous flow data into discrete map time series in terms of Poincare section, and then detect unstable periodic orbits from the transformed discrete map time series. In addition, we take Rössler and Lorenz chaotic systems as examples to demonstrate the effectiveness of our proposed method.
Saturation property of mean growth of initial error for chaos systems
Yang Jin-Hui, Song Jun-Qiang
2012, 61 (17): 170511. doi: 10.7498/aps.61.170511
Abstract +
The saturation property of mean growth of initial error and the relation between saturation value and predictability limit of chaos system are studied in a frame of the nonlinear error growth dynamics. Firstly, the saturation property of mean relative growth of initial error (RGIE) of Lorenz96 system is investigated. It is found that there exists a simple linear relationship between the logarithm of saturation value of mean RGIE and initial error. The sum of logarithms of the two is constant that is independent of the magnitude of the initial error. It is proven by experiment that this conclusion is suitable for other chaotic systems too. With this conclusion, once the constant sum has been determined, the saturation values of mean RGIE at any magnitude of initial error can be calculated easily. Furthermore, to make the study of the relation between error growth saturation and the predictability limits more convenient, just as the definition of the mean RGIE, a definition of the mean absolute growth of initial error (AGIE) is introduced and theoretical analysis reveals that the AGIE has a similar saturation property as RGIE. The saturation value of mean AGIE is constant, which means for a given chaos system, once the control parameters of the system has been determined, the saturation of AGIE is determined. Finally a model for calculating predictability limit quantitatively is given as follows: Tp=1/∧ln(Es/δ0)+c, where Es is the saturation value of mean AGIE. It is shown that this model can work with complicated and high dimension chaos system very well.
Controllability of complex networks based on propagation immunization
Lü Tian-Yang, Piao Xiu-Feng, Xie Wen-Yan, Huang Shao-Bin
2012, 61 (17): 170512. doi: 10.7498/aps.61.170512
Abstract +
Control of complex network reflects humans' comprehension of complex system and the ability to reform it. Up-to-date research establishes the controllability theory of the complex networks based on linear system control theory. The theory could find a minimal set of driver nodes which controls all nodes' state in a linear time invariant complex network with any topology. However, this theory does not take into account the immune node or failure node which blocks the control signal. While inheriting the advantages of the theory, in the paper we first refine the complex network control model based on propagation immunization. Second we adopt four methods which belong to random immunization strategy and targeted immunization strategy to determine the immune nodes, and analyze the controllability of 14 real networks. The experimental results show that when the nodes which have higher degrees, betweeness or closeness are treated as immune nodes, the control of complex networks will become more difficult.
Effect of weighted scheme on synchronizability based on different network structures
Wang Dan, Jing Yuan-Wei, Hao Bin-Bin
2012, 61 (17): 170513. doi: 10.7498/aps.61.170513
Abstract +
Motivated by the observations that the weight of a link in realistic networks is strongly correlated with the product of the corresponding degrees, we present an asymmetrical weighted scheme relating to the degrees of starting point and terminal point, and investigate the effects of weighted scheme on synchronizability in light of different network structures. The results show that the more heterogeneous the degree distribution of network, the more remarkable the effect on enhanced synchronizability by the weighted method is. However, there is little effect on enhanced synchronizability by the weighted method for the homogeneous random network. It is shown that no matter whether small world networks or scale-free networks, the synchronizability is maximal when the total strength of all in-links of every node is equal to one.
Complex spatiotemporal behaviors in a transmission line system terminated by an N-channel metal oxide semiconductor (NMOS) inverter
Zou Jian-Long, Shen Yao, Ma Xi-Kui
2012, 61 (17): 170514. doi: 10.7498/aps.61.170514
Abstract +
Based on the traveling wave theory, a nonlinear discrete map of a transmission line system terminated by an N-channel metal oxide semiconductor (NMOS) inverter is established. After simulating by the nonlinear discrete map, it is found that the change of the reflection coefficient may lead to spatiotemporal bifurcation and chaos, and that the initial distribution significantly affects the spatiotemporal pattern of steady state. For the zero initial distribution, the spatiotemporal pattern is very regular, whereas the complex spatiotemporal pattern may appear when the initial distribution is nonzero. The analysis results demonstrate that the complex spatiotemporal behaviors originate from the infinite-dimensional essence of the transmission line and the nonlinear voltage-ampere characteristics of the NMOS inverter.
Complexity-based pattern recognition and diagnosis for engineering simulation system of HTR-PM
Fang Chao, Sun Jun, Lai Yu-Yang
2012, 61 (17): 170515. doi: 10.7498/aps.61.170515
Abstract +
Complexity-based pattern recognition and diagnosis for engineering simulation system of HTR-PM are carried through analyzing 12 state parameters of the nuclear reactor. It is shown that the complexities and robustnesses of HTR-PM simulation system are remarkably discrepant in three different operating modes (normal operation, increased ambient temperature and sudden leakage in primary loop). The effectivenesses of coupling calculations and feedback of multi systems of the HTR-PM simulator are also embodied in system robustness. All these results demonstrate that the method of complexity and robustness can indicate the availability of different parameters for recognizing and diagnosing the operating conditions of the HTR-PM.
Redundancy-test-based hyper-parameters selection approach for support vector machines to predict time series
Yu Yan-Hua, Song Jun-De
2012, 61 (17): 170516. doi: 10.7498/aps.61.170516
Abstract +
The selection of hyper-parameters is a crucial point in support vector machine modeling. Different from previous method of choosing an optimal model by using basic statistics of residuals in, the new approach selects hyper-parameters by checking whether there is redundant information in residual sequence. Furthermore, omni-directional correlation function (ODCF) is used to test redundancy in residual, and the accuracy of the method is proved by theoretical analysis and numerical simulation. Experiments conducted on benchmark time series, annual sunspot number and Mackey-Glass time series, indicating that the proposed method has better performance than the recorded in the literature.
Precision spectroscopy of helium using a laser-cooled atomic beam
Sun Yu, Feng Gao-Ping, Cheng Cun-Feng, Tu Le-Yi, Pan Hu, Yang Guo-Min, Hu Shui-Ming
2012, 61 (17): 170601. doi: 10.7498/aps.61.170601
Abstract +
The 23P0,1,2 fine structure interval of 4He can be determined to 10-8 accuracy both theoretically and experimentally. It can be used either to determine the fine structure constant or to test the quantum electrodynamics theory. To reach this goal, it is necessary to measure the fine structure splitting to sub kHz accuracy by increasing the signal-to-noise ratio and eliminating the systematic deviations. In the experimental configuration of present study, transverse laser cooling is used to obtain an intense metastable helium atom beam. The triple state metastable atoms are also bent from the original atomic beam to reduce the background noise. The spectral scanning will be accomplished by tuning the sideband of a frequency-locked diode laser to maintain sufficient frequency stability during the scan. The experimental method has been tested on the setup recently built, and the analysis shows that a sub-kHz precision is feasible.
Study on characteristics and fluid sensing of unparallel wall fiber micro-cavity Mach-Zehnder interferometer fabricated by femtosecond laser micromachining
Yin Li-Mei, Zhang Wei-Gang, Xue Xiao-Lin, Bai Zhi-Yong, Wei Shi-Lei
2012, 61 (17): 170701. doi: 10.7498/aps.61.170701
Abstract +
It is found that two walls of fiber micro-cavity fabricated by femtosecond laser micromachining are not perpendicular to the fiber axis. Interference spectrum of the unparallel wall fiber micro-cavity Mach-Zehnder interferometer (MZI) shows abnormal characteristics, such as optical path difference decreasing linearly with wavelength increasing and the total loss decreasing with wavelength increasing. In this regard, we propose an unparalleled wall fiber micro-cavity MZI model and establish analytical theory. By using new models and theories, the new micro-cavity interferometer characteristics are studied, including that the effects of corner and depth on spectral peak wavelength are numerically analysed and transmission loss, absorption loss, insertion loss, infrared absorption loss of material as well as how they affect the interference fringe contrast are theoretically studied. Theoretical analyses and experimental results are in agreement with each other. For fluid sensing, a high-quality unparallel wall fiber micro-cavity MZI is fabricated. The interference fringe contrast of the fiber micro-cavity reaches up to 35 dB in water. Experimental results show that the sensor exhibits an ultrahigh RI sensitivities, as high as——12937.31 nm/RIU in aqueous solution of sucrose.
Observation of transitions in strontium triplet state and its application in optical clock
Gao Feng, Wang Ye-Bing, Tian Xiao, Xu Peng, Chang Hong
2012, 61 (17): 173201. doi: 10.7498/aps.61.173201
Abstract +
In this paper we present a method of observing triplet state transitions of strontium. The intercombination transition is employed to pump the atom population from singlet-state (5s2)1S0 to triplet-state (5s5p)3P1 by a laser at 689 nm. Then 688 nm laser is also employed to divide atom population into the two other triplet-state states (5s5p)3P0 and (5s5p)3PP2. We can obtain the absorption signals of triplet-state transition (5s6s)3S1 → (5s5p)3P0 and (5s6p)3S1 → (5s5p)3P3P2. And these atomic absorption signals can be used for stabilizing the repumping light 679 nm and 707 nm directly to transition line of strontium. This method can be used in the Doppler cooling of strontium atoms.
Mass shift effect on relativistic high-order harmonic generation
Guo Fu-Ming, Chen Ji-Gen, Yang Yu-Jun, Zeng Si-Liang
2012, 61 (17): 173202. doi: 10.7498/aps.61.173202
Abstract +
By solving the time-dependent Dirac and Schrödinger equations, we investigate the super high-order harmonic generation (HHG) from a one-dimensional model atom in a relativistic intense laser pulse. Our numerical simulations show that the relativistic results can be degraded to non-relativistic ones in the weak laser field, and the clear effect of the relativistic mass-shift can be gradually observed as the increase of the laser field intensity. Furthermore, the cutoff frequency and the emission efficiency of the HHG spectrum are analyzed by the wavelet time-frequency analysis and the relativistic "three-step" model.
The P-branch emission spectral lines in the 2Δ3/2-12Δ3/2 system of VO molecule
Liu Wei-Ning, Fan Qun-Chao, Sun Wei-Guo, Feng Hao, Hu Shi
2012, 61 (17): 173301. doi: 10.7498/aps.61.173301
Abstract +
The accurate P-branch emission spectra of the (0, 0) band in the 2Δ3/2-12Δ3/2 system of the VO molecule are studied in this work using the analytical formula derived by Sun group in their previous work. The calculated results generate correct values of the unknown spectral lines up to J=80.5 which are not available experimentally for this band, as well as reproduce all known experimental spectral lines accurately.
STM study of single cobalt atoms and clusters adsorbed on Rh (111) and Pd (111)
Feng Wei, Zhao Ai-Di
2012, 61 (17): 173601. doi: 10.7498/aps.61.173601
Abstract +
We investigate the adsorption and electronic properties of single cobalt atoms and clusters adsorbed on Rh (111) and Pd (111) with scanning tunneling microscopy and scanning tunneling spectrum (STM/STS). It is found that there are two apparent heights for individual cobalt atoms on Rh (111), corresponding to Co atoms adsorbed hcp and fcc hollow sites. The Co atoms on both sites exhibit a notable peak near the Fermi energy, and the two peaks have a slight difference in peak shape. By fitting the dI/dV spectrum to the Fano lineshape, we find that the peak cannot be simply ascribed to the Kondo model. The peak position and full width at half maximum of the peak suggest that the magnetic impurity is in the mixed-valence regime rather than in the Kondo regime. And the peak can be interpreted as a combination of the Kondo resonance and bare d resonance. For Co dimers and trimers on Rh (111), there is no observable feature in their dI/dV spectra near the Fermi level. This is speculated to be due to the magnetic exchange interaction and orbital hybridization between Co atoms. For Co single atoms adsorbed on Pd (111) surface, only one apparent height is found, suggesting that it is due to a different interaction from Co on Rh (111) surface. We do not find notable feature near the Fermi level in the dI/dV spectra of all Co monomers, dimers and trimers.
Tunneling mode in symmetrical one-dimensional photonic crystal of single-negative material
Li Wen-Sheng, Luo Shi-Jun, Huang Hai-Ming, Zhang Qin, Fu Yan-Hua
2012, 61 (17): 174101. doi: 10.7498/aps.61.174101
Abstract +
Electrical single-negative material A and magnetic single-negative material B are used to constitute a symmetrical one-dimensional photonic crystal. Numerical calculation results indicate that a tunneling mode will appear in its band gap. With material layers increasing, the width of the tunneling mode narrows sharply, but its position remains unchanged. The position and width of the tunneling model are not sensitive to the incident angle. With the geometry thickness of the material reducing, the position of the tunneling mode is blue-shifted, but its width remains unchanged. With μA and εB increasing, the location of tunneling mode is red-shifted and the width decreases. Using the properties of the tunneling mode, it is possible to realise dynamic regulation for electromagnetic wave propagation.
The beam propagation factor and the kurtosis parameter of a Gaussian vortex beam
Zhou Guo-Quan
2012, 61 (17): 174102. doi: 10.7498/aps.61.174102
Abstract +
Based on the definition of the second-order moment of intensity, the analytical expression for the beam propagation factor, namely the M2 factor, of a Gaussian vortex beam is derived, which is uniquely determined by the topological charge n. The numerical result indicates that the M2 factor of a Gaussian vortex beam increases with the increase of topological charge n. By means of the higher-order moment of intensity, the analytical expression for the kurtosis parameter of a Gaussian vortex beam passing through a paraxial ABCD optical system is also presented, which depends on topological charge n, parameter δ, transfer matrix elements A and D. When propagating in free space, the kurtosis parameter of a Gaussian vortex beam is determined by topological charge n and parameter δ. With the increase of parameter δ, the kurtosis parameter of a Gaussian vortex beam in free space first decreases and finally tends to a minimal value. Moreover, the kurtosis parameter of a Gaussian vortex beam in free space decreases with the increase of topological charge n. This research is helpful for the practical application of the Gaussian vortex beam.
Analysis of the facula of partially coherent vortex beam in propagation
Ding Pan-Feng, Pu Ji-Xiong
2012, 61 (17): 174201. doi: 10.7498/aps.61.174201
Abstract +
The propagation law of the cross spectrum density is employed to derive the analytical expression of the elements of the cross spectrum density matrix in the observation plane for partially coherent vortex beam after propagation under the condition of paraxial approximation. Based on the derived result, the intensity distribution in the observation plane is analyzed. It is shown that different from the completely coherent vortex beam, the partially coherent votex beam has an intensity of the center-point in the observation plane, which gradually becomes prominent after propagation, and the intensity distribution in the observation plane tends to the distribution of Gaussian-like type with the increase of propagation length. The evolution of intensity distribution depends on the topological charge and correlation length of the source beam. On the condition that other parameters of the source beam are invariable, the beam will evolve fast if the topological charge is small and the correlation length is short. Finally, for the first-order partially coherent vortex beam, the detail of the evolution of the beam shape is investigated by studying the extremum of the intensity in the observation plane. And the theoretical proof is presented for the rule of the evolution of the beam.
Propagation of helical beams through amplitude diffractive optical elements and the measurement of topological charge
Xin Jing-Tao, Gao Chun-Qing, Li Chen, Wang Zheng
2012, 61 (17): 174202. doi: 10.7498/aps.61.174202
Abstract +
A new method is proposed to generate amplitude diffractive optical elements such as Young's double-slits and triangular apertures from a spatial light modulator (SLM). With the help of plane wave angular spectrum diffraction formula and Collins formula, the propagation property of helical beams passing through those two amplitude optical elements is studied, and the measurement of topological charge of helical beams is also achieved with those two amplitude diffractive optical elements. Due to the fact that the SLM can flexibly and accurately change the configurations and locations of optical elements, the measurements can be conveniently achieved.
Diffraction efficiency measurement of large aperture multilayer dielectric grating and its application in the fabrication process
Zhou Xiao-Wei, Liu Ying, Xu Xiang-Dong, Qiu Ke-Qiang, Liu Zhen-Kun, Hong Yi-Ling, Fu Shao-Jun
2012, 61 (17): 174203. doi: 10.7498/aps.61.174203
Abstract +
Large aperture multilayer dielectric grating (MDG) is one of the key elements of high-power laser systems. In order to meet the requirements for large aperture MLG in a high-power laser system, a diffraction efficiency measurement system is developed for characterizing the diffraction efficiency of large MDG at 1064 nm and Littrow mounting. Through analyzinge the main factors such as detector signal-to-noise ratio and operating staff, which influence the testing results during measurement, their difference is obtained to be less than ±1%, which proves that the method can be used to measure the diffraction efficiency of MDG. Based on a large number of experimental data, the optical characterization of MDG in a negative first-order Littrow configuration could provide some information about grating microstructure. So the diffraction efficiency measurements have a number of important applications in the fabrication process, as a nondestructive grating inspection tool, such as determining the suitable photoresist grating mask which meet the ion beam etching requirements, on line end-point detection during ion beam etching, and judging whether another etching is needed. Based on above techniques, a number of MDGs have been achieved, each of which has mean diffraction efficiency greater than 95% line density 1740 lines/mm, and aperture up to 430 mm×350 mm. The results show that this method can satisfy the requirements for simple operation, testing speediness and preciseness, in which any inspection is not necessary for the MDG microstructure paramters.
Experimental study of 8-bits information transmission system based on orbital angular momentum of light beams
Qi Xiao-Qing, Gao Chun-Qing, Xin Jing-Tao, Zhang Ge
2012, 61 (17): 174204. doi: 10.7498/aps.61.174204
Abstract +
An experimental setup is built to realize 8-bits information transmission based on orbital angular momentum (OAM) of light beams. In the modulation section, a controllable laser diode array and a stable phase modulator are used to superpose and encode the OAM states. A binary amplitude grating is used to detect the OAM states in the demodulation part. This system transmits 8-bits data successfully.
Ionization currents and terahertz emission from the interaction of few-cycle laser pulses with gas targets
Du Hai-Wei, Chen Min, Zhang Kai-Yun, Sheng Zheng-Ming, Zhang Jie
2012, 61 (17): 174205. doi: 10.7498/aps.61.174205
Abstract +
Based on a theoretical model and numerical simulations, the ionization currents and subsequent terahertz (THz) emission induced by the interaction of a few-cycle laser pulses with He gas targets are studied. It is shown that owing to the large transverse current generated by field ionization with few-cycle laser pulses, strong THz emission can be generated. The change of the carrier phase of the few-cycle laser pulses leads to the variation of the ionization currents. Correspondingly, the THz emission amplitude shows the characteristic as a periodic function of the carrier phase, which is also confirmed by one-dimensional particle-in-cell simulations. For a given carrier phase, the THz emission amplitude is not proportional to the laser amplitude. It shows at least two peaks at certain laser amplitudes. When the gas density profile is not uniform, the emission amplitude has a similar dependence on laser amplitude and carrier envelope phase, but the THz pulse duration and spectrum are quite different.
Off-focus generation of strong super-continuum emission in fused silica using high power femtosecond laser pulses
Feng Liu-Bin, Lu Xin, Liu Xiao-Long, Ge Xu-Lei, Ma Jing-Long, Li Yu-Tong, Chen Li-Ming, Dong Quan-Li, Wang Wei-Min, Teng Hao, Wang Zhao-Hua, Sheng Zheng-Ming, Wei Zhi-Yi, He Duan-Wei, Zhang Jie
2012, 61 (17): 174206. doi: 10.7498/aps.61.174206
Abstract +
We report on the generation of millijoules supercontinuum, which covers waveband from ~ 400 nm to ~ 900 nm wavelength, by intense femtosecond laser pulse propagating in 3 mm thick fused silica. The fused silica is placed in front of the geometric focus of incident laser pulse to prevent the breakdown. The dependences of supercontinuum spectrum on incident pulse energy and off-focus distance of medium are investigated in detail. Our work demonstrates that strong supercontinuum can be generated using off-focus pump of solid transparent medium with energetic incident pulses.
Spatially induced Airy-Bessel light bullets
Ren Zhi-Jun, Wu Qiong, Zhou Wei-Dong, Wu Gen-Zhu, Shi Yi-Le
2012, 61 (17): 174207. doi: 10.7498/aps.61.174207
Abstract +
Based on wave equation, we theoretically study the effect of spatially induced group velocity dispersion (SIGVD) of pulsed Bessel light beam propagation in free space. The results show that the third-order SIGVD can make pulsed Bessel beam gradually evolve into temporally Airy distribution. Airy-Bessel wave packet is such an exotic localized optical wave packet that it can possibly serve versatile tool in the research of light-matter interactions and has extensive applications. Hence we demonstrate the realization scheme of Airy-Bessel light bullets in free space by compensating the second-order SIGVD through utilizing dispersion management technique. To analyze the spatiotemporal propagation properties, we numerically simulate this light bullet propagation in a dispersion medium. The results show that it can retain spatial diffraction-free and temporal dispersion-free propagation in medium.
Measurements of transient reflectance spectra of dynamic gratings by optical frequency modulation
Xu Pan, Hu Zheng-Liang, Ma Ming-Xiang, Jiang Nuan, Hu Yong-Ming
2012, 61 (17): 174208. doi: 10.7498/aps.61.174208
Abstract +
The characteristics of dynamic gratings greatly affect the linewidth and mode stability of ultra-narrow linewidth erbium-doped fiber (EDF) lasers. In this paper, we propose a novel method to measure the temporal evolution of the reflectance spectra of the dynamic gratings recorded in EDF based on the transient effect of the erbium ions by applying optical frequency modulation on the written light. The transient reflectance spectra of the linear configuration dynamic gratings are measured, and the influences of the written optical power and the terminal reflectivity on the response characteristics of the gratings are also studied. Experimental results show that the first order zero point frequency of the gratings formed in a 3-m-long erbium doped fiber is 30 MHz which accords with the value obtained from the steady state theory. The relative reflectivity change decreases with the increase of input optical power or terminal reflectivity. The measured maximal changes occur at low input power or terminal reflectivity. The grating building time also decreases with the increase of optical power, and it is less than 1 ms when input power is larger than 4 times the saturable power. This phenomenon can be explained by the process of two-wave mixing.
A scheme for improving optical signal-to-noise ratio of multi-carrier source based on recirculating frequency shifter
Liu Na, Xi Li-Xia, Li Jian-Ping, Zhang Xiao-Guang, Tian Feng, Zhou Hao
2012, 61 (17): 174209. doi: 10.7498/aps.61.174209
Abstract +
The output quality of frequency-locked multi-carrier source based on recirculating frequency shifter (RFS) technology is easily influenced by the inherent high-order harmonic crosstalk of modulator, especially the third-order crosstalk. In order to reduce the third-order crosstalk, a scheme that another RF signal whose frequency is 3fm is loaded on both ports of I/Q modulator is adopted. The first-order signal it produces is used to suppress the third-order crosstalk. A multi-carrier source whose number is 24 and maximal power difference is smaller than 0.1 dB is achieved through theoretical analysis and simulation research. The effective optical signal-to-noise ratio (OSNR) of multi-carrier source is improved by 2 dB compared with that before using the scheme of suppressing third-order crosstalk.
Investigation of the collapse of laser-induced bubble near a cone boundary
Li Bei-Bei, Zhang Hong-Chao, Han Bing, Chen Jun, Ni Xiao-Wu, Lu Jian
2012, 61 (17): 174210. doi: 10.7498/aps.61.174210
Abstract +
The effect of the cone boundary on collapse of the laser-induced bubble is investigated under the assumption of virtual plane, and the methods of shadow photography, optical beam deflection and simulation are also used to obtain the effect of cone angle on the bubble dynamics. The results indicate that the effects of the cone angle on the shape of bubble, collapse time and the formation of liquid jet are obvious. The degree of departure of bubble shape from spherical shape and the collapse time are found to increase with the increase of cone angle, and the valid liquid jet is able to form at a lager cone angle. The experimental value and theoretical value of collapse are in good agreement with each other, and the assumption of virtual plane and the modification of dimensionless distance parameter proposed in this study are valid.
Fabricating three-dimensional periodic micro-structure with planar defects via a single exposure
Chen Xiao-Jun, Zhang Zi-Li, Ge Hui-Liang
2012, 61 (17): 174211. doi: 10.7498/aps.61.174211
Abstract +
In this paper, we introduce a method to incorporate a planar defect into the fcc-like photonic crystal structure by utilizing a negative photoresistor SU8. This method in which multi-coating and a single exposure are used simplifies the experiment much more than other methods. In the paper, we exhibit the SEM images for the intact and defective structures. Corresponding to each structure, the reflection spectrum in (1 1 1) direction fabricated shows obviously characteristic peaks and pits. For the intact structure, the spectrum contains two peaks whose wavelengths approach to 1.2 μm and 2.2 μm. These two peaks corresponds to two optical forbidden gaps. For the structure with planar defect, a pit which splits the optical forbidden gap is considered to be a defect mode exhibited on spectral curve. The structure parameters are extracted from the SEM image and used to simulate the reflectance spectra via FDTD program. The simulation results almost match the experiment data accurately.
The method of quantitative characterization using reflection pulsed thermography
Tao Ning, Zeng Zhi, Feng Li-Chun, Zhang Cun-Lin
2012, 61 (17): 174212. doi: 10.7498/aps.61.174212
Abstract +
In this paper, a new method using reflective pulsed thermography to measure defect depth, thermal wave reflection coefficient and thermal diffusivity is presented. First, a brief description of the pulsed thermography in terms of theoretical background and quantitative measurement is given. One stainless steel 304 structure machined several flat-bottom holes in which it is filled with different materials are used as experimental sample, the measured results of defect depth, thermal diffusivity and reflection coefficients at defect interface under different conditions are given. The agreement between the results obtained by using pulsed thermography and the value presented in the literature or measured by other techniques appears satisfactory within errors of 5%, and possible reasons for affecting the measurement precision are discussed.
Analytical model for acoustic multi-relaxation spectrum in gas mixtures
Zhang Ke-Sheng, Wang Shu, Zhu Ming, Hu Yi, Jia Ya-Qiong
2012, 61 (17): 174301. doi: 10.7498/aps.61.174301
Abstract +
To identify the correlation between sound propagation and molecular multimode vibrational relaxation in polyatomic gas mixture, an analytical model that constructs acoustic multi-relaxation spectrum is presented. The frequency-dependent effective specific heat of gas is formulated from the micro view of vibrational mode energy transfer as well as the macro view of relaxation process due to vibrational-vibrational mode energy coupling. With the aid of the general relaxation equations of multimode vibrational energy transfer, the analytical expressions to calculate acoustic relaxation absorption and dispersion, which reflect both primary and secondary relaxation processes, are developed from the effective specific heat. The constructed absorption spectra of various gas mixtures, consisting of carbon dioxide, methane, nitrogen, and oxygen, accord with the experimental data very well. Especially, the peak errors of those results are less than 1%. Moreover, the simulation results illustrate that less than two single processes with higher strength appear generally in a multi-relaxation absorption spectrum. Compared with the existing models, the analytical model can directly obtain the analytical expressions of characteristic points in the relaxation spectrum of gas mixtures, which makes it advantageous to analyze the spectral characteristics qualitatively and quantitatively. Consequently, the model provides an effective approach to analyzing the relationship between sound propagation and molecular vibrational relaxation of gas mixtures.
Application of leading structure on thermal protection of nosetip
Sun Jian, Liu Wei-Qiang
2012, 61 (17): 174401. doi: 10.7498/aps.61.174401
Abstract +
The structure of embedded high thermal conductivity carbon material leading thermal protection is considered as thermal protection system to prevent hypersonic vehicle nosetip which requires sharp figure during hypersonic flying, from being seriously aerodynamically heated. By fluid structure interaction method, we analyze leading thermal protection of nosetip and validate that embedded high thermal conductivity carbon material structure has a good effect on thermal protection. The maximal temperature of the nosetip which uses leading thermal protection structure is reduced by 21.9% and the lowest temperature of aft is increased by 15.2% when Mach number is 9. The transfer of heat from head to after-body is achieved, the thermal load of the front head is weakened and the ability of leading-edge thermal protection is strengthened. The influences of structure parameter and material attributes of coating and thermal conductivity of high conductivity carbon material on thermal protection are discussed. The highest temperature of the nosetip decreases with thermal conductivity of coating increasing to a steady value and descends with blackness level of coating ascending and increases with the thickness of coating increasing, and it descends in parabola form as thermal conductivity of carbon materials increases.
Simulation of spectrum of doped Ar in indirectly driven implosion target
Qiao Xiu-Mei, Zheng Wu-Di, Gao Yao-Ming, Ye Wen-Hua
2012, 61 (17): 175201. doi: 10.7498/aps.61.175201
Abstract +
X-ray spectrum of tracer in ICF implosion target is usually used to infer electron temperature, density and mix of fuel. As the plasma in fuel is in non-local thermodynamic equilibrium (non-LTE), a line transfer code Alpha is developed. Taking the electron temperature and density provided by radiation hydrodynamic as input condition, atomic kinetics and radiation transfer equation are self-consistently solved with the detailed configuration atom (DCA) model. The opacity for specified frequency intervals is obtained, and X-ray spectrum in the image plane is also presented.As application of Alpha program, the spectrum of doped Ar in implosion target on SG Ⅱ laser facility is simulated. The effect of self-absorption of K α line is studied. And it is shown that self-absorption of K α line affects both the intensity and shape of the spectrum, and it should be considered in simulating X-ray spectrum of Ar. And as the spectrum of local thermodynamic equilibrium (LTE) simulation gives large intensity and different shapes compared with the non-LTE results, non-LTE simulation is necessary in such a simulation.
The effect of the proton layer initial size on the proton beam characteristic in target normal sheath acceleration
Yu Jin-Qing, Zhou Wei-Min, Jin Xiao-Lin, Li Bin, Zhao Zong-Qing, Cao Lei-Feng, Dong Ke-Gong, Liu Dong-Xiao, Fan Wei, Wei Lai, Yan Yong-Hong, Qian Feng, Yang Zu-Hua, Hong Wei, Gu Yu-Qiu
2012, 61 (17): 175202. doi: 10.7498/aps.61.175202
Abstract +
The proton beam accelerated by the interaction of laser with plasma has practical applications in radiography of dense plasma, fast ignition in inertial confinement fusion, and cancer treatment. The application domain is determined by the characteristic of the proton beams, which is affected by a lot parameters. In order to investigate the effect of the initial size of the proton layer, the two-dimensional Particle-In-Cell (2D-PIC) code Flips2D is used. The total energy of proton beam vs. time is studied, and the relation between the duration of acceleration and the period of laser pulse is obtained. The effects of the proton layer initial width and thickness on the divergence angle and the energy spectrum of the proton beam are investigated. The relation between the proton beam characteristics and proton layer initial size is obtained.
The study of meteorological effects and time variations of the fair weather atmospheric electric field near ground in YBJ, Tibet
Xu Bin, He Hua, Yang Xiao-Yan, Bie Ye-Guang
2012, 61 (17): 175203. doi: 10.7498/aps.61.175203
Abstract +
Based on the fair weather atmospheric electric field recorded by YBJ Cosmic Ray Station located at the YBJ High Altitude Cosmic Ray Laboratory (90° 31'50" E, 30° 06'38" N, 4300 m a.s.l., 606 g/cm2), Tibet, China, its meteorological effects and time variations are studied. The analysis in this paper is to look into how the factors like atmospheric pressure, temperature and relative humidity in the presence of the meteorological factors in four seasons govern the fair weather atmospheric electric field at the site of measurement. The results of meteorological effects show that there is a strong positive correlation between the fair weather atmospheric electric field and temperature, while a moderate positive correlation is seen between the fair weather atmospheric electric field and the atmospheric pressure, also the relative humidity. The long-term variations of the fair weather atmospheric electric field are affected by seasons. The mean value is higher in summer and fall, but lower in winter and spring. The daily variation of the fair weather atmospheric electric field is modulated by solar diurnal, semi-solar diurnal, and its third and fourth harmonic component. The higher the harmonic frequency, the lower the modulate amplitude is. The daily variation curves show bimodal variation with evening peak and sunrise peak. Two minima can also be seen in early morning and evening. Sunrise peak value is highest during summer and fall, but lowest in spring and winter morning. Sunrise peak occurs at around noon, and evening peak is modulated by the season effects.
Investigation of a energy conversion silicon chip based on 63Ni radio-voltaic effect
Gao Hui, Luo Shun-Zhong, Zhang Hua-Ming, Wang He-Yi
2012, 61 (17): 176101. doi: 10.7498/aps.61.176101
Abstract +
Aiming at the main problem encountered in the research of radioisotope microbattery based on β radio-voltaic effect-enhancement of energy transfer efficiency and radiation damage as mutually constraining sides of a contradiction, an investigation of radiation-induced defects in different silicon wafers by low-energy electron irradiation was carried out and the electrical characteristic measurement for two types of PIN structures indicated that P+I (N-) N+ device in I zone with a dopant concentration of 2× 1012 cm-3 agreed with the predicted result of P, N type silicon radiation damage effect. This was then taken as the prototype device, on which test of 63Ni radiation output characteristics was performed. The test result was compared with the experimental data of Wisconsin University and the major factors causing low energy transfer efficiency were analysed. Adoption of three-dimensional PIN junction structure, increasing the proportion of energy deposition in depletion region, matching I (N-) zone width and deposition depth and controlling the leak current under an order of magnitude of Picoampere were considered to enhance the energy transfer efficiency, based on which energy transfer structure was designed and ultimately structure parameters such as multi-hole PIN structure, radiation source thickness, depletion region width were determined, thus the energy transfer structure optimization was accomplished.
Helicities and thermostabilities of Ni nanowires in the carbon nanotubes
Du Yu-Guang, Zhang Kai-Wang, Peng Xiang-Yang, Jin Fu-Bao, Zhong Jian-Xin
2012, 61 (17): 176102. doi: 10.7498/aps.61.176102
Abstract +
Molecular dynamics calculations are performed to study the structures of armchair carbon nanotubes (CNTs) encapsulating Ni nanowires and their helicities and thermal stabilities are discussed. It is found that Ni nanowires are of multiple helical tubular structure and each layer is composed of several Ni atom chains. Different layers of Ni nanowires have different helicities and the helicities of Ni nanowires in the inner layer are greater than those of the outer one. What is more, the helicity will be greatest when the number of the Ni atom chains is an integer multiple of three. As the increases of the diameters of the CNTs, the helicities of Ni nanowires change periodically. The structure and helicity only have tiny variations even at high temperature as the CNTs can protect the Ni nanowire.
Effect of high pressure treatment on microstructure and resistivity of Cu75.15Al24.85 alloy
Chen Yan, Liu Lin, Liu Jian-Hua, Zhang Rui-Jun
2012, 61 (17): 176103. doi: 10.7498/aps.61.176103
Abstract +
The microstructures of Cu75.15Al24.85 alloy before and after treatment at 15 GPa pressure and 750℃ for 15 min are analyzed by optical metallograph, X-ray diffraction, scanning electron microscopy and differential scanning calorimeter, and its resistivity is also obtained through resistivity measuring instrument. According to the obtained results, the effects of high pressure treatment on the microstructure and resistivity are investigated. The results show that high pressure treatment can refine the microstructure, and increase the resistivity of the alloy. When the pressure is 3 GPa, the refinement is more remarkable and the resistivity reaches a maximal value.
Lattice constant deviation from Vegard's law in GeSn alloys
Su Shao-Jian, Cheng Bu-Wen, Xue Chun-Lai, Zhang Dong-Liang, Zhang Guang-Ze, Wang Qi-Ming
2012, 61 (17): 176104. doi: 10.7498/aps.61.176104
Abstract +
Three layers of fully-strained Ge1-xSnx alloys with x=0.025, 0.052, and 0.078 from bottom to up are grown on a Si (001) substrate using a high-quality, strain-relaxed Ge thin film as buffer layer. Five relaxed Ge1-xSnx samples (x=0.005, 0.016, 0.044, 0.070, and 0.155) are grown directly on Si (001) substrates as well. The compositions and lattice constants of the Ge1-xSnx alloys are measured by Rutherford backscattering spectra, high-resolution X-ray diffractions, and X-ray reciprocal space mapping. The experimental results reveal a quite large positive deviation from Vegard's law with a bowing parameter b=0.211 Å.
First-principles study of p-type ZnO by Te-N codoping
Yao Guang-Rui, Fan Guang-Han, Zheng Shu-Wen, Ma Jia-Hong, Chen Jun, Zhang Yong, Li Shu-Ti, Su Shi-Chen, Zhang Tao
2012, 61 (17): 176105. doi: 10.7498/aps.61.176105
Abstract +
The crystal structure, density of states and electronic structures of Te-N doped ZnO are investigated from the first-principles pseudo-potential approach based on density functional theory. It is found that the incorporation of N into ZnO induces contraction of lattice, while Te incorporation will cause expansion of lattice. Thus, the co-doping of both Te and N is conducible to the incorporation of N with minimum lattice strain. Besides, Te atoms is positively charged because the electronegativity of Te is smaller than that of O. Consequently, Te atom is expected to act as an isoelectronic donor in ZnO. Moreover, the acceptor level of N doped ZnO is narrow and deep. While in the Te-N doped ZnO system, N-impurity bandwidth at the top of valence band becomes larger, while tends to delocalize the hole. Meantime, the system obtains shallower acceptor levels and lighter mass of acceptors. The results suggest that the codoping of Te-N is an effective p-type doping method in ZnO.
The influence of deuteration and helium-implantation on the surface morphology and phase structure of scandium thick film
Peng Shu-Ming, Shen Hua-Hai, Long Xing-Gui, Zhou Xiao-Song, Yang Li, Zu Xiao-Tao
2012, 61 (17): 176106. doi: 10.7498/aps.61.176106
Abstract +
The effects of deuteration and helium-implantation on the surface morphology and phase structure of scandium (Sc) thick film are investigated by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) methods. The preferred orientation of (002) crystal plane is observed for both films deposited on substrates of monocrystalline-silicon and polished-molybdenum. A great number of holes appear on surface after deuteration and the grain size of ScD2 is bigger than that of Sc, whereas a few ScD0.33/Sc grains with relatively small grain size retain inside crystal. The surface morphologies of scandium and scandium deuteride films are slightly affected by helium-implantation. The formation of helium bubbles with a preferred orientation in the crystal lattice of scandium and scandium deuteride, generated by the aggregation of ion-implanted helium, causes the corresponding diffraction peaks of scandium and scandium deuteride phase to shift towards smaller angles.
Research on the total dose effects for domestic VDMOS devices used in satellite
Gao Bo, Liu Gang, Wang Li-Xin, Han Zheng-Sheng, Zhang Yan-Fei, Wang Chun-Ling, Wen Jing-Chao
2012, 61 (17): 176107. doi: 10.7498/aps.61.176107
Abstract +
Total dose effects of domestic VDMOS devices used in satellite under different bias conditions are investigated. The dependences of the typical electrical parameters such as threshold voltage, breakdown voltage, on-state resistance, and leakage current on total dose are discussed. The experimental results show that the electrical parameters of the irradiated domestic VDMOS devices fulfill the design requirements. These devices also meet the work demand in a complex ionizing total dose irradiation environment. In addition, our experimental results are meaningful and important for further improving the design and the process of the others types of domestic radiation hardened VDMOS devices.
The study on the stress and the friction coefficient of tetrahedral amorphous carbon films bombarded by energetic Ar ion
Han Liang, Ning Tao, Liu De-Lian, He Liang
2012, 61 (17): 176801. doi: 10.7498/aps.61.176801
Abstract +
The ta-C films with sp3 bonds more than 80% in fraction are deposited by FCVA technique, and then they are bombarded by Ar ions. The composition and structures of the ta-C films before and after the bombardment of energetic Ar ions are analyzed by X-ray photoelectron spectroscopy. The surface morphology is investigated by AFM. The result shows that the bombardment of Ar ions induces the conversion of sp3 bond into sp2 bond, and the fraction of sp2 bonds increases with the energy of Ar ion increasing. The stress of the film decreases with the increase of the Ar ion energy. The RMS and etching pits on the surface of film increase with the increase of Ar ion bombarding energy.The friction test indicates that Ar ion bombardment has an important influence on initial friction coefficient, but just has little influence on steady state friction coefficient. The steady state friction coefficient of film keeps about 0.1, which shows a good antiwear property.
Helium effect on the stability of the interface of Cu/W nanomultilayer
Liu Wang, Wu Qi-Qi, Chen Shun-Li, Zhu Jing-Jun, An Zhu, Wang Yuan
2012, 61 (17): 176802. doi: 10.7498/aps.61.176802
Abstract +
In this paper, Cu/W nanomultilayers with different modulation periods are prepared in pure Ar and mixing atmosphere (He and Ar) by radio frequency magnetron sputtering method. The helium content, cross-section morphology and phase structure of Cu/W nanomultilayer are characterized by EPBS, SEM and XRD, repectively. The results show that the stable interface of the nanomultilayer is the prerequisite for resisting and the guarantee against the damage of helium. The nanomultilayer can suppress the growth of helium bubbles under the appropriate modulation.
The electronic theory study on high-temperature oxidation mechanism of TiAl alloy
Liu Gui-Li, Li Yong
2012, 61 (17): 177101. doi: 10.7498/aps.61.177101
Abstract +
In order to reveal the physical nature of high temperature oxidation of titanium-aluminous alloy from the electronic level, the atom embedded energy, affinity energy, binding energy and other electronic structure parameters are calculated by using recursive method combining with Castep, and the alloy oxidation mechanism is explored. The results show that there is a larger oxygen solubility in titanium and oxygen atoms can aggregate into titanium matrix near surface, and gradually spread into the deep matrix. Oxygen and titanium have a strong affinity to form a titanium oxide film. Aluminum can form clusters with mutual attraction between aluminum atoms in titanium matrix. Titanium atoms in aluminum clusters are mutually repulsive and form chemical compounds with aluminum atoms. Because of the closing affinity energy between aluminum and titanium with oxygen, the preferential oxidation of aluminum cannot occur, but titanium oxide and aluminum oxide form. The binding energy of Al2O3 is slightly lower than that of TiO2, therefore Al2O3 is more stable. Aluminum in TiO2 has a greater solubility, which can replace titanium to form more stable oxide Al2O3.
Study on the electronic structures and energy band properties of Cd-doped wurtzite BeO
Zheng Shu-Wen, Fan Guang-Han, He Miao, Yao Guang-Rui, Chen Jun, He Long-Fei
2012, 61 (17): 177102. doi: 10.7498/aps.61.177102
Abstract +
The electronic structures and energy band properties of the Cd-doped wurtzite BeO are investigated by plan-wave pseudopotential method with the generalized gradient approximation in the frame of density functional theory. The theoretical results show that the valence band maximum is determined by O 2p states and the conduction band minimum is occupied by Cd 5s and Be 2s orbitals based on the total density of states and partial density of states of Be1-xCdxO alloy. With the Cd content x of Be1-xCdxO increasing, the repulsion effect between Cd 4d and O 2p states is more enhanced and the bandgap of Be1-xCdxO is reduced. At the same time, the bandgap undergoes the direct-indirect-direct transition. In order to obtain the theoretical values in accord with the experimental results, the bandgaps of Be1-xCdxO are corrected. Moreover, the relations among energy bandgap, bowing parameter and lattice constant of the wurtzite BeO-ZnO-CdO ternary alloy are analyzed.
Transport properties of nonlinear chains with random dimer nonlinearity parameters
Hu Dong-Sheng, Zhang Yan-Ling, Yin Xiao-Gang, Xu Jiang
2012, 61 (17): 177103. doi: 10.7498/aps.61.177103
Abstract +
By using the recursion relation of discrete Schrödinger equation we investigate the transport properties of nonlinear chains with random dimer nonlinearity parameters. It is shown that there is a resonance state, which is just the product of the nonlinearity and the square of the incident wave amplitude modulus. The transmission coefficients are calculated in two conditions. One is that the transmission wave amplitude is a certain value, the other is that the incident wave amplitude modulus is a certain value. There are obvious differences in non-resonant states between the two kinds of conditions. The transmission is a single value function of the electronic energy for the former. However, it will be multi-stability for the latter. It is pointed out that the influence of the nonlinearity parameters on the transport properties can be exactly reflected only when the modulus of incident wave is set to be a certain value.
First principles study of the elastic, electronic and optical properties of MgS under pressure
Chen Zhong-Jun
2012, 61 (17): 177104. doi: 10.7498/aps.61.177104
Abstract +
The structural, elastic, electronic and optical properties of MgS B2 crystal under high pressure are studied by accurate first-principles plane wave pseudo-potential method which is based on the density functional theory. Our results show that the conduction band of the structure has a shift tendency toward higher energy, and the valence band has a shift tendency toward lower energy under high pressure. We analyze the optical properties associated with the partial density of states and the shift of energy level under high pressure. At the same time, the absorption spectrum of the MgS B2 crystal has an evident blue shift.
Effects of the improved hetero-material-gate approach on sub-micron silicon carbide metal-semiconductor field-effect transistor
Song Kun, Chai Chang-Chun, Yang Yin-Tang, Jia Hu-Jun, Chen Bin, Ma Zhen-Yang
2012, 61 (17): 177201. doi: 10.7498/aps.61.177201
Abstract +
Based on the device operation mechanism and physical model, effects of the improved hetero-material-gate (HMG) approach on deep sub-micron silicon carbide (SiC) metal-semiconductor field-effect transistor (MESFET) are analyzed. By comparing with the conventional MESFET, it is shown that the improved HMG approach induces a multi-stepped distribution of the potential in the channel, leading to an enhanced electric field at the source. Meanwhile, the position of the maximum of the channel potential is changed to the drain side compared with the dual-material-gate (DMG) device, thus the carriers in the channel are accelerated more efficiently and the variation of potential caused by drain voltage is eliminated to a certain degree, resulting in a better restraint in short-channel effect. Also, different technological parameters are designed to study the dependence of the device performance and an optimization plan is obtained, leading to a decreased sub-threshold swing. In addition, asymmetric gate structures are designed for high power application, achieving an improved distribution of the electric field at the gate edge and an enhanced breakdown voltage of the small scale device.
The influence of gate voltage on electron transport in the graphene field-effect transistor under strong laser field
Liu Jiang-Tao, Huang Jie-Hui, Xiao Wen-Bo, Hu Ai-Rong, Wang Jian-Hui
2012, 61 (17): 177202. doi: 10.7498/aps.61.177202
Abstract +
The influence of gate voltage on electron transport in the graphene field-effect transistor under strong laser field is studied by using the finite-difference time-domain method. The perfect tunneling in graphene can be strongly suppressed by the strong laser field induced optical stark effect. This suppression depends not only on the laser field but also on the width and the height of the gate voltage. The electron transport through a non-square barrier is investigated. We find that a barrier with a small incline has little effect on the electron transport, but if the barrier has a large incline, the tunneling probability changes remarkably.
Study of stability of persistent spin helix in two-dimensional electron gases with spin-orbit coupling
Zhang Lei, Li Hui-Wu, Hu Liang-Bin
2012, 61 (17): 177203. doi: 10.7498/aps.61.177203
Abstract +
We study theoretically the stability of persistent spin helix in two-dimensional electron gases with spin-orbit coupling by a semi-classical spin density matrix method. The possible influences of disordering scattering, temperature, spin-orbit coupling strength and external electric field on the lifetime of persistent spin helix state are investigated. The theoretical results are found to be in agreement with some recent relevant experimental results.
Study on the self-heating effect in silicon-on-insulator devices with SOANN buried oxide
Cao Lei, Liu Hong-Xia
2012, 61 (17): 177301. doi: 10.7498/aps.61.177301
Abstract +
In this paper, we present a new silicon-on-insulator (SOI) buried oxide structure, i.e., silicon on aluminum nitride with nothing (SOANN). In the novel structure, the traditional SiO2 is replaced by A1N, and gas cavity is constructed in the SOI buried oxide. The self-heating effect of novel SOI device is analyzed. The result shows that using A1N as a buried oxide, the temperature of lattice and the effectively restrained self-heating effect can decrease. In addition, the gas cavity in the buried oxide can provide a heat emission passage and reduce the dielectric constant. The coupling effect of electric field lines from drain to source is weakened, and the drain induced barrier lowering effects is effectively restrained. Therefore, this new SOANN structure can improve the performance of the SOI devices, and provide high reliability as well.
Reliability of SiN-based MIM capacitors in GaN MMIC
Wang Xin-Hua, Wang Jian-Hui, Pang Lei, Chen Xiao-Juan, Yuan Ting-Ting, Luo Wei-Jun, Liu Xin-Yu
2012, 61 (17): 177302. doi: 10.7498/aps.61.177302
Abstract +
Reliability assessment of SiN MIM capacitors in GaN MMIC is performed by constant voltage stress test. Two kinds of failure modes, critical charge density at which the dielectric breaks down and mean time prior to failure are investigated. The trap energy level in SiN dielectric is obtained by temperature dependent current characteristics. The degradation mechanism of SiN MIM capacitor is analyzed. The research shows that new donor-like traps are generated at dominant position during the stress. And the trap energy level becomes deeper after stress. The increased trap accelerates the scattering of the carrier, which leads to the decrease of leakage current in the end. The investigation on the failure mechanism of SiN MIM capacitor provides a reference for reinforcing the dielectric capacitors.
Electromagnetic wave scattering by a topological insulator prolate spheroid particle
Zeng Lun-Wu, Zhang Hao, Tang Zhong-Liang, Song Run-Xia
2012, 61 (17): 177303. doi: 10.7498/aps.61.177303
Abstract +
Using Maxwell equations, we expand electromagnetic field components in spherical coordinate frame by using spherical vector wave function. According to the constitute relations of the topological insulator (TI), we modify transmittance fields and scattering fields. Using boundary conditions of topological insulator, we obtain the scattering electromagnetic fields. Numerical result show that when the time reversal symmetry is broken, the scattering cross sections are influenced by the topological magneto-electric polarizability.
Study on SQUID method and Campbell method measure superconductors
Guo Zhi-Chao, Suo Hong-Li, Liu Zhi-Yong, Liu Min, Ma Lin
2012, 61 (17): 177401. doi: 10.7498/aps.61.177401
Abstract +
The two magnetic measurement methods of superconductors, SQUID method and Campbell method, are introduced briefly. Superconducting properties of MgB2 bulk samples are prepared by the high-pressure powder in tube (PIT) sintering method. The two measurement methods are employed to measure the critical current density of the sample. The Jc-B curves of the MgB2 samples are obtained with each method. The SQUID method can be used to measure the magnetic strength field up to 6 T and the material is in normal state, and the result is used to scale F(b) of the pinning through necessary calculation which is used to study the magnetic characteristic of the material. The critical current density (Jc) measured by the SQUID method is the average of the materials different parts. Jc is measured and estimated by the Campbell's method. The magnetic field is only to 0.4 T while the frequency of the AC parts magnetic field is varied from 37 Hz to 797 Hz. The critical current density obtained by Campbell's method is smaller than that obtained by the SQUID measurement, which is due to the existence of various faults and the decrease of the electrical connectivity.
Effects of Gd and Co doping on the electrical and ferromagnetism properties of BiFeO3 ceramics
Song Gui-Lin, Zhou Xiao-Hui, Su Jian, Yang Hai-Gang, Wang Tian-Xing, Chang Fang-Gao
2012, 61 (17): 177501. doi: 10.7498/aps.61.177501
Abstract +
Multiferroic Bi0.95Gd0.05Fe1-xCoxO3 (x= 0, 0.05, 0.1, 0.15, 0.2) ceramics were prepared by rapid liquid phase sintering method. We studied effect of Gd and Co doping on the structure, electrical and ferromagnetism properties of BiFeO3 ceramics. The structure and morphology of BiFeO3 ceramics are characterized by X-ray diffraction (XRD). The results show that all the peaks for Bi0.95Gd0.05Fe1-xCoxO3 (x= 0, 0.05, 0.1, 0.15, 0.2) samples can be indexed according to the crystal structure of pure BiFeO3. And X-ray diffraction analysis reveals a phase transition in Gd-Co codoped BiFeO3 ceramics when x is larger than 0.1.The current densities of all samples measured at room temperature are approximately three orders of magnitude lower than that of BFO ceramic, and the leakage current of the ceramics at room temperature exhibits two distinctive conduction behaviors: Ohmic conduction and space charge limited (SCL) conduction mechanism.For all the samples studied here, the dielectric constant and dielectric loss decrease with the increase of frequency in a range from 1 kHz to 1 MHz. The dielectric constants of Bi0.95Gd0.05Fe1-xCoxO3 (x= 0, 0.05, 0.1, 0.15,0.2) samples are nearly 1.9, 2.68, 3.85, 5.3, and 6 times larger than that of pure BiFeO3 (εr= 61.2) ceramic at 1 kHz, respectively. And the dielectric losses of Bi0.95Gd0.05Fe1-xCoxO3 samples become smaller than that of BFO ceramic.The magnetic measurements show that all the samples possess strong ferromagnetism at room temperature expect BiFeO3 and Bi0.95Gd0.05FeO3 which are weakly ferromagnetic. Under an external magnetic field of 30 kOe, the values of Mr of Bi0.95Gd0.05Fe1-xCoxO3 are 34, 60, 105, 103 and 180 times that of BiFeO3, respectively.
Influence of core-shell structure on dielectric behaviour in relaxor ferroelectrics
Shu Ming-Fei, Shang Yu-Li, Chen Wei, Cao Wan-Qiang
2012, 61 (17): 177701. doi: 10.7498/aps.61.177701
Abstract +
Dominant features of relaxor ferroelectrics are dielectric dispersion and the nonlinear relationship between reciprocal dielectric constant and temperature. The result of the analysis of the thermal dynamic function for core-shell structure in a grain shows that the core-shell structure doped with dopant in linear gradient descending ingredients can remain high dielectric constant at low temperatures, but cannot lead to the nonlinear relationship between reciprocal dielectric constant and temperature. By comparing diffusion transitions with different doping ingredients, it is suggested that the concentration of ingredient will affect the inhomogeneity of the doping ingredient. A wide distribution of the ingredient between grains by high doping concentration will result in the nonlinear relationship between reciprocal dielectric constant and temperature, and therefore the coexistence of grains in paraelectric phase and in ferroelectric phase in the peak area of dielectric constant. The change of measurement temperature will affect the ratio of the grains in two phases and the change in ferroelectric domains, which results in ferroelectric dielectric dispersion. The core-shell structure will increase the dielectric dispersion. Ferroelectric ceramics, doping species and their concentrations, and sintering temperature all can influence the inhomogeneity of core-shell structure and dielectric dispersion.
Spectroscopic and thermal properties of Yb doped CaF2-SrF2 laser crystal
Wu Ye-Qing, Su Liang-Bi, Xu Jun, Chen Hong-Bing, Li Hong-Jun, Zheng Li-He, Wang Qing-Guo
2012, 61 (17): 177801. doi: 10.7498/aps.61.177801
Abstract +
Yb: CaF2-SrF2 crystals are grown by Bridgman technique. The spectroscopic properties, thermal diffusion coefficients and thermal expansion coefficients at different temperatures are studied. Thermal conductivity at 300 K and thermal expansion coefficient are calculated. The absorption spectra, fluorescence spectra, thermal properties of crystals are analyzed by comparison method. The results show that the absorption and emission cross sections are larger in the high concentration SrF2 disordered crystal. And also the emission cross section is large and wide at 1040 nm in the Yb: CaF2-SrF2 (19%) crystal. It demonstrates that for different ratios of CaF2 and SrF2 in the disordered crystals, the spectroscopic properties are different. The main possible reason is the different disorders, low symmetry, low symmetry optical centers in the disordered crystal. It can also be seen that the disordered crystal has a good thermal conductivity.
Ultrafast spectroscopy of the Mn3+ dd transition in YMnO3 film
Xu Yue, Jin Zuan-Ming, Li Gao-Fang, Zhang Zheng-Bing, Lin Xian, Ma Guo-Hong, Cheng Zhen-Xiang
2012, 61 (17): 177802. doi: 10.7498/aps.61.177802
Abstract +
Photoinduced carrier dynamic behavior of the Mn3+ 3d resonance excitation of YMnO3 thin film is studied by the femtosecond time resolved spectroscopy. The photon energy of the pump pulse is tuned to 1.70 eV, which is corresponding to the Mn3+ 3d energy level at room temperature. With resonant excitation, the transient transmission signals at the zero-delay time gradually increase with temperature increasing. The temperature dependent transmission change results from the blue shift of the Mn3+ 3d energy level, which is believed to originate from the short-range antiferromagnetic order in YMnO3 film. In addition, the fast and slow relaxations of the transient signal arise from electronic-phonon and phonon-spin interactions, respectively. When the temperature is lower than TN, the relaxation time of the fast process increases significantly, which indicates that the strength of electronic-phonon coupling is restrained by the long-range antiferromagnetic order.
Investigation of the cathode electric field at the initial stage of explosive electron emission
Zuo Ying-Hong, Wang Jian-Guo, Zhu Jin-Hui, Niu Sheng-Li, Fan Ru-Yu
2012, 61 (17): 177901. doi: 10.7498/aps.61.177901
Abstract +
To investigate a complex physical phenomenon and its evolution law at the cathode surface at the initial stage of the explosive electron emission process in a high-power diode, in this paper we present a model of a planar vacuum diode with a field emission cathode. The model is one-dimensional and nonstationary. To study the space charge effect of the emitted electrons on the electric field at the cathode surface, Poisson's equation is solved numerically by using our developed code, and the time-dependences of the electric field at the cathode surface for different cases are obtained. The results show that the electric field at the cathode surface first oscillates and finally yields a steady state. The absolute value of the steady electric field at the cathode surface is higher for the higher field enhancement factor at a given applied electric field, and the applied electric field has the same effects on the steady value at a certain field enhancement factor. The electric field at the cathode surface completely determines the extracted field emission current density from the cathode, and at the same time, the electric field at the cathode surface is influenced by the emitting current density.
Electronic properties of disordered bilayer hexagonal boron nitride quantum films
Xiao Hua-Ping, Chen Yuan-Ping, Yang Kai-Ke, Wei Xiao-Lin, Sun Li-Zhong, Zhong Jian-Xin
2012, 61 (17): 178101. doi: 10.7498/aps.61.178101
Abstract +
Based on the Anderson tight-binding model, the electronic properties of disordered bilayer hexagonal boron nitride quantum films are investigated. Our numerical results show that the electrons in a disordered bilayer hexagonal boron nitride quantum film are localized, presenting an insulating property. However, for the monolayer disordered bilayer hexagonal boron nitride quantum film, the energy spectrum has persistent mobility edges which are independent of the disorder strength. This indicates that a metal-insulator transition occurs in the monolayer disorder structure. This is similar to the case in an order-disorder separated quantum film. The results could offer useful information for understanding and manipulating the electronic properties of bilayer hexagonal boron nitride quantum films.
Calculation of beam-wave interaction of coupled-cavity TWT using equivalent circuit model
Bai Chun-Jiang, Li Jian-Qing, Hu Yu-Lu, Yang Zhong-Hai, Li Bin
2012, 61 (17): 178401. doi: 10.7498/aps.61.178401
Abstract +
A nonlinear model for the numerical simulation of coupled-cavity traveling wave tube is described. The model is based on the Curnow equivalent circuit from which the matrix equations for the fields of any single cavity can be obtained. And the model can be divided into the matrix equations, the motion equations and the space-charge field equations, in which the sever, dynamic tapering in the circuit and the multiple-frequency signal amplification can be also considered. The simulation is used to analyze the nonlinear AM-PM distortion and the intermodulation products of a coupled cavity TWT whose working frequencies range from 59 GHz to 64 GHz. And the analysis of the saturated output power within the frequency band is also included. Comparing the simulation results with the measurements, the percentage difference between the calculation results and the test results is less than 5%.
Design and simulation of 140 GHz high power staggered double vane traveling-wave tube
Lai Jian-Qiang, Wei Yan-Yu, Xu Xiong, Shen Fei, Liu Yang, Huang Min-Zhi, Tang Tao, Gong Yu-Bin
2012, 61 (17): 178501. doi: 10.7498/aps.61.178501
Abstract +
Staggered double vane slow wave structure (SWS) and sheet electron beam are employed to investigate a 140 GHz high power traveling wave tube. Numerical calculation of eigenmode shows that the SWS has a good characteristic of dispersion and interaction impedance. The transition structure, input/output coupler and concentrated attenuator are especially proposed for the circuit to ensure that the tube will work well. Particle-in-cell simulation results demonstrate that the traveling wave tube can provide over 300 W of peak power in a frequency range of 132-152 GHz with a maximum of 546 W and a corresponding gain of 37.37 dB at 138 GHz assuming a beam power to be 5.115 kW and input power to be 0.1 W. The output power of the tube can exceed 440 W in a frequency range of 128-152 GHz with a corresponding interaction efficiency of over 8.6% when the input powers range from 0.027 W to 0.46 W. Such a traveling wave tube has a great significance and a potential application in high power short millimeter wave field.
Highly color-stability flexible white organic light-emitting devices fabricated by color conversion method
Wu Xiao-Ming, Shen Li-Ying, Hua Yu-Lin, Dong Mu-Sen, Mu Xue, Bai Juan-Juan, Bi Wen-Tao, Yang Xiao-Yan, Yin Shou-Gen
2012, 61 (17): 178502. doi: 10.7498/aps.61.178502
Abstract +
We demonstrate flexible white organic light-emitting diode (WOLED) with high color stability, which combines vacuum deposited blue flexible organic light-emitting diode (FOLED) with red surface color conversion material (CCM). Firstly, we utilize the novel multiple quantum well (MQW), which consists of the alternate layers of 2, 3, 5, 6-Tetrafluoro-7, 7, 8, 8-tetracyano-quinodimethane (F4-TCNQ) and 4, 4', 4"-tris-(N-3-methylphenyl-N-phenylamino) tripheny-lamine (m-MTDATA) as hole injection layer (HIL), through combining a new blue emitting material of N6, N6, N12, N12-tetrap-tolylchrysene-6, 12-diamine (NCA), to fabricate highly efficient blue FOLED. Then, the CCM of 4-(dicyanomethylene)-2-tert-butyl-6-(1, 1, 7, 7-tetramethyljul-oli-dyl-9-en-yl)-4H-pyran (DCJTB) is deposited on the outside of the ITO flexible substrate. Finally, the thickness of the color conversion film (CCF) is optimized to improve the color purity of flexible WOLED. The results demonstrate that at a driving voltage of 7 V, the CIE coordinates of (0.33, 0.27) which are very close to the white equal-energy area are obtained with the thickness of DCJTB of 120 nm. Moreover, the CIE coordinate migrations of the flexible WOLED are less than (±0.02, ±0.02) in a wide range of driving voltages (from 6 V to 11 V) for the device, indicating the flexible WOLED has excellent color stability.
Efficiency droop in blue InGaN/GaN light emitting diodes on Si substrate
Liu Mu-Lin, Min Qiu-Ying, Ye Zhi-Qing
2012, 61 (17): 178503. doi: 10.7498/aps.61.178503
Abstract +
InGaN/GaN based light emitting diodes (LEDs) suffer from decrease in efficiency at a high injection current level which is called efficiency droop. In this paper, blue InGaN/GaN multiple quantum well light emitting diodes on Si (111) substrates are prepared, and their electroluminescence spectra are tested. Comparing the experimental measurements with the simulating results based on simple ABC model, the cause for quantum efficiency droop is investigated. The results show that the light emitting diode has worse electron spreading and less hole concentration with temperature decreasing, and the electrons will overflow frome the well after filled up in higher and higher state for their inhomogeneous distribution, thus efficiency droop will happen at a lower injection more severely for electron leakage under lower injection, and experimental measurements are in disagreement with simulation results of Auger recombination at high injection current levels under different temperatures. The results confirm that the main factor for efficiency droop is not Auger nonradiative recombination but electron leakage, and the essential cause for electron leakage is severe carrier localization.
The investigation of performance improvement of GaN-based dual-wavelength light-emitting diodes with various thickness of quantum barriers
Chen Jun, Fan Guang-Han, Zhang Yun-Yan
2012, 61 (17): 178504. doi: 10.7498/aps.61.178504
Abstract +
The electrical and optical characteristics of GaN-based dual-wavelength light-emitting diodes (LEDs) with the specific design of various thick barriers are investigated numerically. The simulation results show that the thickness of barrier plays a regulatory role in emission spectrum of the dual-wavelength LED. The internal quantum efficiency droop is improved and the two peaks of spectrum become uniform due to the thickness of barriers gradually decreasing from the n-side to the p-side in a specific way. The balanced distribution of carrier concentration and the enhancement of electron confinement could be the major physical mechanism behind these improvements. It is also shown that the better optical performance is achieved at the large current injection level.
Magnetization of degenerate and relativistic electron gas
Wang Zhao-Jun, Zhu Chun-Hua, Huo Wen-Sheng
2012, 61 (17): 179701. doi: 10.7498/aps.61.179701
Abstract +
The dense electron gas interior neutron star is high degenerate and relativistic. The observation of neutron star about its thermal and magnetic effects depends on transport properties of the electron gas which is thought as the magnetic carrier. Its Landau levels in magnetic field are quantized and highly degenerate. The energy difference of an electron gas between in and not in magnetic field determines the magnetization of the gas, and the corresponding susceptipilities can be obtained through the thermodynamic calculation. When the magnetic field is weak, the susceptipility is 10-3 having similar order as in white dwarf. While in strong field the magnetization has the de Haas-van Alphen fluctuant effect like in microtherm metals. The differential susceptipilities can equal or exceed critical for high order harmonic frequency. Correspondingly, there is probably the phase-instability occuring in dense electron gas and the stable state is consisted of two different magnetization phases similar as the first-order phase transition of water. But if, there is a surface energy at the boundary then there is metastable state of homogeneous magnetization. The phase transition of interior neutron star can be observed through its electromagnetic radiation.This electromagnetic radiation may provide the extra energy in starquake model which was proposed to explain the giant flash of a magnetar.