Vol. 61, No. 8 (2012)
The effect of the interdot Coulomb interaction on Kondo resonance in series-coupled double quantum dots
2012, 61 (8): 087203. doi: 10.7498/aps.61.087203
We theoretically investigate the effect of the interdot coulomb interaction on Kondo resonance in series-coupled double quantum dots. The Anderson Hamiltonian of our system is solved by means of the slave-boson mean-field approximation, and the variations of the density of states, the transmission probability, the occupation number and the Kondo temperature with interdot Coulomb interaction are discussed in the Kondo regime, and the densities of states are calculated in the Kondo regime for various interdot Coulomb repulsions with parallel and antiparallel lead-polarization alignments. Our results reveal that the interdot Coulomb interaction between quantum dots greatly influences the physical property of this system, and relevant underlying physics of this problem is discussed.
2012, 61 (8): 088401. doi: 10.7498/aps.61.088401
High resistivity silicon is a very common optical component in a terahertz system. However, its high relative refractive index of 3.42 causes a large impedance mismatch at the silicon-to-air interface. This severely reduces the available power in a terahertz system which motivates researchers to find a good anti-reflection solution. In the terahertz region, the lack of proper materials for broadband anti-reflection severely hinders such a research development. A photonic grating with graded refractive indices is demonstrated on silicon. Compared wich the case of planar silicon wafer, the transmission is observed to increase from 0.2 THz to over 7.3 THz for a device with 15 m period, which covers most of the terahertz band. With a striking relative 3 dB bandwidth of 116.3%, the device is polarization-independent and can be used under a wide incidence angle.
Parameterized inversion of frequency sounding data in horizontally stratified transverse isotropic medium
2012, 61 (8): 089101. doi: 10.7498/aps.61.089101
In this paper, we advance a fast iterative inversion algorithm of frequency electromagnetic sounding data to simultaneously reconstruct the horizontal and the vertical conductivities and interface each bed in the horizontally stratified transversely isotropic (TI) medium. First, applying the analytic expression of the electromagnetic dyadic Green's functions in the TI medium, we realize the forward modeling of the frequency sounding via the fast algorithm of Sommerfeld integral. Then using the perturbation principles and Fourier inversion transform, we establish an efficient computation of Frchet derivatives of the frequency sounding data with respective to all the model parameters. And we use the least square criterion, the normalization and singular value decomposition to iteratively recovery all model parameters. Finally, numerical results validate the inversion algorithms and their antinoise ability.
2012, 61 (8): 080201. doi: 10.7498/aps.61.080201
Based on some Riemnan theta function solutions to a elliptical equation, together with its Backlund transformation, infinite sequences Riemann function solutions are derived, then regarding this elliptical equation as an auxiliary equation, with the help of computer software (Mathematica), infinite sequences Riemann function solutions to the modified Korteweg de Vries sine-Gordon equation are obtained.
Approximate functional variable separation for the porous medium equation with perturbed nonlinear source
2012, 61 (8): 080202. doi: 10.7498/aps.61.080202
The approximate functional variable separation for the porous medium equation with perturbed nonlinear source is studied. Complete classification of the perturbed equation which admits approximate functional separable solutions is obtained. The main solving procedure for the approximate functional variable separation approach is shown by way of examples, and the corresponding approximate functional separable solutions to the resulting equations are then constructed.
2012, 61 (8): 080203. doi: 10.7498/aps.61.080203
A class of singularly perturbed optimal control problem is studied by direct scheme method, which is based on the boundary function method. The internal layer solution is proved to be existing, and the uniformly valid asymptotic solution for the singularly perturbed optimal control problem is constructed.
Asymptotic solving method of soliton for El Nio/La Nia-southern oscillation nonlinear disturbed model
2012, 61 (8): 080204. doi: 10.7498/aps.61.080204
A class of El Nio/La Nia-southern oscillation (ENSO) stochastic disturbed model is considered. Firstly, using the special method of undetermined coefficients, the exact solution of the non-disturbed ENSO model is obtained. Then the approximate solutions of stochastic disturbed ENSO model are constructed by using the asymptotic theory and method. And it is illustrated with example that the asymptotic solutions have the better degree of accuracy.
2012, 61 (8): 080205. doi: 10.7498/aps.61.080205
The displacement trial function is reconstructed by reproducing kernel particle shape function method with interpolation property on discrete points, then combining the principle of minimum potential energy of elasticity, the new interpolating reproducing kernel particle method to analyze the plane problem of elasticity is obtained. Because interpolation reproducing kernel particle shape function has a point interpolation property and no less than the high-order smoothness of kernel function, the difficulty for most of meshless methods to be used to deal with the essential boundary conditions is already overcome, and the high numerical accuracy is assured as well. Compared with the early meshless methods, this method has a high accuracy and a small scale of solving problem and it can be directly applied to boundary conditions. Numerical results for some typical examples of elasticity prove the proposed method to be valid.
The practical security and performance analysis of the quantum data stream cipher system by the αη protocol
2012, 61 (8): 080301. doi: 10.7498/aps.61.080301
The αη protocol is the random cipher protocol and hides information by the quantum phase noise. In this paper, the security criterion of the protocol is deduced by developing a quantum channel model and calculating the amount of information obtained by the eavesdropper. The results show the security of the protocol depends mainly on the average photon number of the signal quantum states and the number of cipher-text symbols. Considering the above factors, the secure value region of the average photon number and cipher-text symbol number is calculated, which is against the beam splitter attack. Furthermore, the effective communication distance is simulated.
2012, 61 (8): 080401. doi: 10.7498/aps.61.080401
The time-space metric is a fundamental concept of general relativity, and it is the logical foundation of cosmology and astrophysics. A time-related space scale factor is introduced into a 4-dimensional time-space interval model. The transformations among the flat metric, the Schwarzschild metric and the Robertson-Walker (R-W) metric are obtained in spherical coordinate system. Based on the time-space interval of the time-related scale factor coordinate, the solutions of R-W metric with parameter k=1 and the non-vacuum metric outside stars are deduced. A new point of view is advanced to comprehend the modern physical non-flat time-space.
2012, 61 (8): 080501. doi: 10.7498/aps.61.080501
It is of significance in engineering to achieve stochastic resonance of periodic signal with large frequency by turning the parameters of a bistable system. The normalization transform of bistable system parameter is deduced. Stochastic resonance of periodic signal with large parameters is exhibited by numerical simulation based on the proposed normalization transform, by which an arbitrary high frequency periodic signal can be processed. Analog circuit is used to verify the stochastic resonance happening in the bistable system with large parameters. The mechanism of realizing a periodic signal with large parameters by twice sampling stochastic resonance is analyzed, which is compared with normalization transform method through numerical simulation. The simulation results show that the numerical solution of twice sampling stochastic resonance is prone to being unstable with the variation of mixed input signal amplitude, while the normalization transform method possesses more stability and adaptability.
Stochastic resonance of linear harmonic oscillator subjected to simple harmonic force with frequency fluctuation
2012, 61 (8): 080502. doi: 10.7498/aps.61.080502
In view of the situation that the linear harmonic oscillator is subjected to the simple harmonic force with the frequency fluctuation, we work out the expression of the first-order moment for the system response. It is found that the system output response vibrates at the natural frequency, that the response amplitude shows bona-fide stochastic resonance as the simple harmonic excitation force frequency varies, and that the suppression and the resonance appears as the natural frequency varies.
Research of one-dimensional discrete chaotic system constructed by the hybrid circuits of single-electron transistor and metal oxide semiconductor
2012, 61 (8): 080503. doi: 10.7498/aps.61.080503
A one-dimensional discrete chaotic system is constructed based on the hybrid architecture of single-electron transistor and metal oxide semiconductor in this paper. Voltage transfer characteristics of the parallel architecture of single-electron transistor and metal oxide semiconductor are investigated, and the corresponding N-shape piecewise linear function model is established. Based on this model a one-dimensional discrete mapping system is first constructed, the dynamics of the system is then analyzed including one-dimensional mapping process, bifurcation diagram and Lyapunov exponent spectrum and the corresponding discrete chaotic system is finally designed through the electronic circuits of the hybrid architecture. The simulation result is consistent with the theoretical analysis. All these indicate that discrete chaotic system designed by the hybrid architecture of single-electron transistor and metal oxide semiconductor has some advantages such as simple circuit structure and low power dissipation, which are good for the integration and application of chaotic system.
Identification of a stochastic neural firing rhythm lying in period-adding bifurcation and resembling chaos
2012, 61 (8): 080504. doi: 10.7498/aps.61.080504
To identify non-periodic neural rhythm to be chaos or stochasticity has been an important scientific thesis. A kind of non-periodic spontaneous firing pattern, whose behavior is transition between period-k burst in a string and period-k+1 burst in a string (k=1,2), lying between period-k bursting pattern and period-k+1 bursting pattern, is found in the experimental neural pacemaker. The deterministic structures of the firing are identified by nonlinear prediction and first return map of the interspike intervals (ISIs) series. The co-existence of the period-k bursting and period-k+1 bursting is manifested in the deterministic theoretical neuronal model, Chay model. Non-periodic firing patterns similar to the experimental observation are simulated in the co-existing parameter region, implying that the firing pattern is transition between two kinds of bursts induced by noise. A binary series can be acquired by transforming two kinds of bursts to symbols 0 and 1, respectively. The stochastic dynamics within the transitions between two kinds of bursts are detected by probability analysis on the binary series. It not only shows that the rhythm is stochastic firing with deterministic structures instead of chaos, but also provides the typical examples and effective methods to intensively identify the chaotic and stochastic firing patterns in a real nervous system.
2012, 61 (8): 080505. doi: 10.7498/aps.61.080505
The complex dynamics of Chen system via impulsive force is investigated in this paper. The non-smooth bifurcation of Chen system via impulsive force is analyzed. The system can evolve to chaos by the cascading of period-doubling bifurcations. Besides, the system can evolve to chaos immediately by saddle-node bifurcations from period solutions. Finally, the Floquet theory is used to explore the non-smooth bifurcation mechanism for the periodic solutions.
2012, 61 (8): 080506. doi: 10.7498/aps.61.080506
Chaos is widespread in nature and human society, so the prediction of chaotic time series is very important. In this paper, we propose a new chaotic time series prediction model echo state network based on wavelet, which can effectively overcome the ill-posed problem that exists in traditional echo state networks. And it also has a good generalization ability. Three time series are used to test the new model, i.e., Lorenz time series, Lorenz time series with added noise and batch reactor vessel temperature time series. Results suggest that the new proposed method can achieve a higher predictable accuracy, better generalization and more stable prediction results than traditional echo state networks.
2012, 61 (8): 080507. doi: 10.7498/aps.61.080507
For multivariate chaotic time series prediction problem, a prediction based on input variable selection and extreme learning machine is proposed in this paper. The multivariate chaotic time series is reconstructed in phase space, and a mutual information based method is used to select the input variables, which have high statistics information with the output variables. The extreme learning machine is conducted to model the multivariate chaotic time series in the phase space by utilizing its approximation capability. In order to improve the prediction accuracy, a model selection algorithm is conducted for extreme learning machine to choose an expected minimum risk prediction model. Simulation results based on Lorenz, Rssler multivariate chaotic time series and Rssler hyperchaotic time series show the effectiveness of the proposed method.
A three-dimensional chaotic system is presented. The equilibrium point, the chaotic attractor and the Lyapunov exponent are investigated. According to the chaotic system, we propose a novel chaos synchronization method in which only the amplitude of chaotic signal is used to realize chaos synchronization. This feature can be used in chaotic communication system to improve the communication quality and speed. By computing the conditional Lyapunov exponents and numerical simulation the synchronization method is proved to be feasible and effective. The synchronous speed of the method is the same as that of the traditional linear feedback method.
Performance analysis of a quantum heat engine working with a particle in a one-dimensional harmonic trap
2012, 61 (8): 080509. doi: 10.7498/aps.61.080509
A heat engine model of a sing particle confined in a one-dimensional harmonic trap is established. The model, as a quantum-mechanical analog of the Carnot engine, does work to outside when both the width of the potential and the quantum state change very slowly. On the assumption that the width of the potential moves at a very slow speed and the heat leakage is included, we derive the general expressions for several performance parameters, such as work, power output, efficiency, etc. We find the optimally operating regions and the optimal values of performance parameters of the heat engine cycle.
2012, 61 (8): 080701. doi: 10.7498/aps.61.080701
In order to protect the copyright of stereoscopic images in the condition of the image quality, a zero-watermarking stereoscopic image algorithm is presented based on hyperchaotic system. In the proposed algorithm, disparity zero-watermark is constructed according to the stability of disparity between low frequency bands of wavelet decomposition of the left and the right views and direct coefficients of discrete cosine transform; in the process of zero-watermark construction, the zero-watermark position mapping according to the features of the sensitivity of hyperchaotic discrete system initial value, ampleness of parameter key space and complexity of dynamic behavior, enhances the security of algorithm. The relationship between security and watermark capacity is also analyzed. Experimental results show that the proposed algorithm is strong robust to noise, filtering, compression, and the shearing of asymmetrical and symmetrical attacks.
2012, 61 (8): 082901. doi: 10.7498/aps.61.082901
Boron-lined ionization chambers (BLICs) have gradually become one of radiation detectors which are used to monitor the neutron/-ray mixed field with large flux around reactors in recent years. In this paper a BLIC with high sensitivity is fabricated and the internal structure is detailed. By using a weak current amplifier with an accuracy of fA, the leakage current of the BLIC is less than 1.0 pA when the high voltage is below 700 V; under the irradiation by the Am-Be neutron source, on the I-V curve of the BLIC there appears a current plateau with a length of 500 V and a slope of 3.7210-4 V-1; when the operating voltage is 400 V, the leakage current of the BLIC is 0.4 pA. Experiments show that neutron signal current measured by the BLIC depends on the relative position of the BLIC with repect to the radiation source; a maximum of 2.0 pA is obtained when the source is 8 cm away from the bottom of the hole in the paraffin moderator. Under the exposure by 137Cs and 90Sr,-ray signal current measured by the BLIC is 1.02.0 pA, but the current plateau is not obvious in-ray field. The neutron sensitivity of the BLIC reaches a level of 1.010-15 Acm2s and the-ray sensitivity reaches a level of 9.010-22 Acm2seV-1. The BLIC shows small leakage current, high sensitivity, and good plateau characteristics, which can be used to monitor the neutron/-ray mixed field around reactors.
ATOMIC AND MOLECULAR PHYSICS
Numerical simulation of X-ray ionization and atmospheric temporal evolutions with high-altitude nuclear explosions
2012, 61 (8): 083201. doi: 10.7498/aps.61.083201
The X-ray ionizations and the atmospheric temporal evolutions with different altitude nuclear explosions are numerically simulated in this paper. It is concluded that the density of electrons generated by X-ray ionization has a peak value at 100 ns after the X-rays have arrived, and the peak density reduced with the increase of distance from the stripped nucleus area. The life time of electrons is long, and increases with the distance from the stripped nucleus area. O+, O2+, N2+ are generated by X-ray ionization. The distance of X-ray ionization in atmosphere is within a few tens of kilometers. The peak density of explosion at 80 km high is larger than at 120 km high in the area close to stripped nucleus area, but the scenario is opposite in the area far from stripped nucleus area.
2012, 61 (8): 083601. doi: 10.7498/aps.61.083601
The geometric structures of the MnxSny (x=2,3,4; y=18,24,30) clusters are studied using the density functional theory method. The geometric optimization shows that the favourite structures of MnxSn6x+6 (x=2,3,4) are D3d single-cage structures which encapsulate Mn atoms, i.e. Mn2Sn18, Mn3Sn24 and Mn4Sn30.However the favourite structures of MnxSn6x+12 (x=2,3) are two-cage structures, i.e. MnSn12-MnSn12 and MnSn12-Mn2Sn18.Thus, it is promising to form new one-dimensional nonawires of MnxSny heterostructures by controlling the number of Mn atoms.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2012, 61 (8): 084101. doi: 10.7498/aps.61.084101
In order to solve electromagnetic inverse scattering problem, a real-time inverse scattering method is proposed. This technique converts the inverse scattering problem into a regressed one using support vector machine (SVM). Electromagnetic inverse scattering method based on the SVM deals with nonlinearity and ill-posedness inherent in the inverse scattering problem successfully. The feasibility and the validity are tested by making use of simulating through-wall problem, and the results demonstrate that this approach can detect and position the targets behind the wall, no matter whether there exists noise or not. In the through-wall scenario, the influence of the number of sampling positions of receiving antenna on the predicted results is discussed using the predicted model of SVM. In the end, the predicted errors are analyzed and investigated in the multiple source scenario. The results show that this kind of setting is helpful for target identification in through-wall problem.
2012, 61 (8): 084102. doi: 10.7498/aps.61.084102
We present a new kind of zero-index metamaterial composed of periodic array of dendritic cells with continuous long pole, and investigate its electromagnetic characteristics. First, we study the one-level dendritic cell with continuous long pole. The retrieved effective permittivity and permeability of the unit cell simultaneously tend to be zero from their negative values at a frequency of 9.5 GHz, which leads to a near-zero refractive index. Then by changing the geometric parameters of the unit, we find the influences of the branch length and the separation angle on the electromagnetic parameters. Finally, increasing the level number of the branch gives rise to two-level and three-level dendritic cells respectively with continuous long pole. Adjusting the geometric parameters of each unit cell to appropriate values, the zero-index matematerials with zero permittivity and permeability can be obtained at the same frequency.
Isotropic negative permeability metamaterials and left-handed metamaterials based on miniature structure
2012, 61 (8): 084103. doi: 10.7498/aps.61.084103
We propose an isotropic structure based on double split-ring resonator. We investigate experimentally and numerically the electromagnetic resonant properties of the proposed structure. The result shows that when the electromagnetic wave transmits in the directions parallel and perpendicular to the plane of the split ring resonator, respectively, the resonant bands having the negative permeability both arise at the same frequencies. However, when the electromagnetic wave transmits with oblique angle, the resonant characteristic is still unchanged. That is to say, resonant characteristic of the proposed structure is independent of incidence angle. This result indicates that the proposed structure is an isotropic medium. Combining the proposed double split-ring resonator structure with the wires, the left-handed metamaterial with negative refractive index can be obtained. In addition, the proposed structure has the merit of the miniaturization due to adding the metallic via-hole, which can increase the electric length of the structure. As a result, the resonant frequency of the structure shifts toward lower frequency greatly without increasing the dimension of the structure, and then the structure is still compact in the low frequency case. The introduction of the metallic via-hole can reduce the dimension of the structure by 50%. Therefore, the proposed structure will be a good candidate in the microwave applications such as antennas, filter, among others.
2012, 61 (8): 084201. doi: 10.7498/aps.61.084201
Because of the properties of non-sphericity and multiple scattering of fog particles and the effects of geometrical optics, the light scattering through fog has become a difficult problem. In this paper, we establish an ellipsoidal model of fog particles. On the basis of radiative-transfer equation and with the consideration of the effects of geometrical optics, we obtain a formula of calculating multiple light scattering intensity through the fog particles with different sizes and different shape distributions. Our results are in agreement with the earlier results in two special cases, which verifies that our method is reliable. The calculations show that scattering spectra through the ellipsoidal fog particles present elliptical feature, which is different from circular feature for random orientation non-spherical particles. At the different orientation angles the angular distributions of the light intensity are different, and this difference becomes smaller with the shape ratio approaching to 1. The stripes of the scattering spectra progressively disappear with the increase of optical thickness, which is different from the case of single-scattering. At different orientation angles and observation angles, the scattering intensities always first increase then decrease with the increase of optical thickness for the fog particles of the different size distributions and different shape ratio distributions, and a maximal intensity occurs at =1.03.0. The calculations also indicate that the scattering spectra of actual fog particles present the turbid pattern around the central light speckle because of wide size distribution in many cases.
The ratio choice method of the pinhole aperture to the waist of the Gaussian laser in the fabrication of holographic gratings
2012, 61 (8): 084202. doi: 10.7498/aps.61.084202
In the fabrication of the holographic diffraction gratings, the scatter light which is produced by the reflection of the granule in air or the diffraction defect of the optical apparatus will be recorded simultaneously in the resist. Those noises not only reduce the diffraction efficiency of gratings, but also give rise to the scattered light in diffraction. So an appropriate spatial filter should be used to stop the higher spatial frequencies corresponding to the Gaussian beam. Because the diffraction effect after the pinhole aperture can demolish the wavefronts of laser, it is the most important to choose the optimal pinhole for adjusting the waist of the laser. On the basis of standard scalar diffraction theory, the field amplitude is given by the convolution calculation, the phase distortion of diffraction wavefronts is also analyzed, and the lower limit ratio of the aperture size with respect to the beam radius is given when the phase transitions are further away from the center. Using the energy conservation of Gaussian beam and comparing the characteristics change of diffraction beam, the effect of diffraction is presented and the upper limit ratio of the spatial filter is given by the critical condition of the diffraction. The conclusion shows that if the ratio of the aperture radius to beam waist is between 1.52 a/02.2, the phase is invariable in the area of the exposal, and the power transmitting the spatial-filter pinhole aperture is slightly more than 99%, so spatial filter performs its function only in this range of the radio.
2012, 61 (8): 084203. doi: 10.7498/aps.61.084203
In this paper, we propose a blind watermarking algorithm based on block discrete wavelet transform. The watermark is scrambled by Arnold and two pseudo random sequences are generated first, and then according to the wavelet tree structure, the significant wavelet coefficients in a block are found and quantized by pseudo random sequence. During the extraction, the quantized coefficients can be found without the original image, then the correlation between them and the pseudo random sequences are calculated to estimate the watermark bits. Moreover, in this method all high-frequency subimages are used to provide better robustness. The performance of the proposed watermarking is robust to several attacks, especially quite effective against image cropping attack.
2012, 61 (8): 084204. doi: 10.7498/aps.61.084204
We propose and experimentally demonstrate a chaotic ultra wideband (UWB) over fiber link based on optical feedback laser diode, in which UWB signals are generated and transmitted at three different bit rates of 360, 720 Mbit/s, and 1.44 Gbit/s respectively. Without utilizing any dispersion compensation module, the signal at a bit rate of 1.44 Gbit/s is detected using a digital signal processing receiver after 10 km fiber and 0.6 m wireless channel transmission. The power spectrum does not have any discrete spectral line because of the random output of the chaotic pulses, which means that the harmful effects of discrete spectral lines could be avoided with this method.
2012, 61 (8): 084205. doi: 10.7498/aps.61.084205
The linewidth characteristics of distributed feedback fiber laser (DFB-FL) is fully explored in this paper. The linewidth broadening effect of DFB-FL induced by self-pulsation, phase-shifted grating, especially lasing window, external disturbance, etc. are extensivly investigated in the paper based on the theoretical derivation and experiment. An effective linewidth compression method is proposed, and a self-injection system of DFB-FL comes up to reduce its linewidth from 35 kHz to less than 10 kHz.
2012, 61 (8): 084206. doi: 10.7498/aps.61.084206
Coherent beam combination of fiber laser array is an important technology for high-power output fiber laser systems. However, the coherent beam combination is influenced by temporal error, which always exists in the laser array. The coherent beam combination of pulse fiber lasers in the presence of temporal error is investigated in theory in this paper. The effects of pulse shape and temporal error on the coherently combined beam are studied. The characteristics of combined beam, i.e. pulse shape, peak power, far-field pattern and power in barrel (PIB) are calculated and analyzed when the pulse shapes (including square, triangle and sine) and temporal errors are different. It is revealed that for coherently combining square pulse lasers, the pulse shape of the combined beam changes acutely because of temporal error, and the far-field pattern and PIB change proportionally with temporal error increasing; for coherently combining triangle pulse lasers, the pulse shape and the peak power of the combined beam are acutely influenced by temporal error, and the far-field pattern and the PIB change acutely when temporal error is large. It is revealed that coherently combined beam of sinusoidal pulse lasers has good characteristics when two pulse laser with sinusoidal pulse shapes are coherently combined and the time delay between the two pulses is less than 10% of pulse duration.
Light intensification caused by cracks in fused silica subsurface is one of main factors of laser-induced damage to optical materials. Three-dimensional finite-difference time-domain method is used to simulate parabola-section-model lateral cracks. Moreover, the relationship between light intensification and breadth-to-depth ratioR is discussed. The results show that the morphology change after acid etching is an important cause of damage mitigation. Modulation is very weak and close to each other whenR is greater than 10.0 and it increases rapidly whenR less than 5.0. The electric field intensity reaches a maximal value whenR ranges from 1.0 to 3.0, and the maximal electric field is 4.3 V/m. The electric field intensity of more than 80% samples exceed 2 times than the incident light whenR ranges from 1.0 to 3.5. Intensified area has the skin effect with depth increasing. It is demonstrated that enhanced area lying directly below the crack firstly shifts to left and right sides, then it moves to parabola-section interface and the horizontal interface. Finally, the whole subsurface will be enhanced. In addition, electric field modulation firstly increases and then decreases in the z direction when depth is large enough.
2012, 61 (8): 084208. doi: 10.7498/aps.61.084208
A temperature model for a end-pumped thin disk laser with Gaussian beam intensity distribution and single pass absorption performance is built up according to the actual working state. Using infrared thermal image, temperature distribution, temperature variation with time and the temperature difference of the surface laser medium are measured at different pump powers. Experiment on thermal lens focal length of the thin disk laser medium is performed by utilizing Hartmann-Shack wave-front sensor. Considering the fact that thermal lens focal length of the laser medium changes with pump power, the model for analyzing the influence of thermal effect on output power of thin disk laser is presented, according to rate equations. The simulation results obtained in this paper show satisfactory agreement with experimental results. The research has the guiding significance for designing and optimizing thin disk lasers
Theoretical investigation of orthogonal dual-pump four-wave mixing in semiconductor optical amplifier
2012, 61 (8): 084209. doi: 10.7498/aps.61.084209
A comprehensive broad-band model of orthogonal dual-pump four-wave mixing (FWM) in semiconductor optical amplifier (SOA) is presented in the case of the polarization state of the input signal being arbitrary. By performing numerical simulation and taking the all-optical wavelength conversion based on orthogonal dual-pump FWM in an SOA for example, the effects of the input light powers, frequency detuning and polarization state of the input signal on the orthogonal dual-pump FWM and the performances of wavelength conversion are theoretically investigated.
Theoretical and experimental investigations of spatial optical dark soliton filamentization in nonlocal self-defocusing Kerr medium
2012, 61 (8): 084210. doi: 10.7498/aps.61.084210
In this paper, the spatial optical dark soliton filamentization in a nonlocal self-defocusing Kerr medium is investigated. Theoretically, starting from nonlocal nonlinear theoretical model, we examine the influences of the degree of nonlocality and the attenuation constant on the formation of dark soliton filaments by numerical simulation method. We find that when the input background optical intensity is determined, the greater the degree of nonlocality, the farther the initial point of the formation of dark filaments is and the less the number of dark filaments decreases with the increase of the degree of nonlocality; when the ratio of the background optical intensity to the critical optical intensity is fixed, the degree of nonlocality can hardly influence the number of dark filaments and the number of dark filaments under nonlocality is equal to that under locality. Besides, when the input background optical intensity is determined, the number of dark filaments decreases with the increase of the attenuation constant. Experimentally, by changing the concentration of dye solution and the ellipticity of background light, we discuss the influences of the concentration of sample and the ellipticity of background light on the formation of dark soliton filaments respectively and find that when the input background average optical intensity is determined, the number of dark filaments decreases with the increases of the concentration of sample and the ellipticity of background light; when the ratio of the background average optical intensity to the critical optical intensity is fixed, the concentration of sample can hardly influence the number of dark filaments. Besides, the phenomenon of optical shock wave is found in our experiment.
2012, 61 (8): 084211. doi: 10.7498/aps.61.084211
The method of sol-gel co-assembling high quality, large area silica inverse opal films is studied. Hydrolyzed sol-gel precursor solution is added in monodisperse polystyrene (PS) colloidal solution to co-assemble composite colloidal crystal film which is infiltrated with silicate gel simultaneously. PS colloidal crystal template is removed by calcining the composite colloidal crystal film to obtain the silica inverse opal film. Silica inverse opal films of different pore sizes are fabricated by this method after researching the ratio of added hydrolyzed sol-gel precursor solution, the temperature of vertical evaporation, and the sinter temperature. The structures and the elements of fabricated silica inverse opal films are characterized by scanning electron microscope and X-ray energy spectrometer, and their optical transmission spectra are measured. Research results indicate that silica inverse opal films fabricated by sol-gel co-assembly method are highly ordered in large area, and the pore sizes are controllable in a wide range; measured transmission spectra show an evident band-gap, whose central wavelength is coincident with calculated result.
2012, 61 (8): 084212. doi: 10.7498/aps.61.084212
In order to present the visual resolution effects of different integral imaging systems in three-dimensional (3D) display applications, a method of comparing the resolutions of integral imaging 3D display based on human vision is proposed. The relative resolution parameter is defined by analyzing the relation between the integral imaging 3D system and resolving power of human eyes on the best viewing condition, and its relationship with the visual resolution effect viewed in the integral imaging 3D display system is analyzed. The results indicate that the size of lens array has an important influence on the visual resolution effect in the integral imaging 3D display system. According to the theoretical results of two examples show that the difference in relative resolution parameter between the systems is 1.75 times of that of traditional one. The experimental results are consistent with the theoretical analyses. The method proposed in this paper could have a certain directive significance for 3D display resolution evaluation.
2012, 61 (8): 084213. doi: 10.7498/aps.61.084213
A novel hexangular lattice dual-concentric-core photonic crystal fiber is proposed, which is composed of a central defect core, an outer ring core by introducing small air-holes on the forth ring and double cladding circle air-holes along the direction of fiber length. Based on full vector finite element method with anisotropic perfectly matched layers, its dispersion, nonlinear, leakage loss and mode field are numerically investigated. Numerical results indicate that the proposed fiber shows higher negative dispersion tunable effect and stronger confinement ability of guided mode, which the leakage loss is close to 10-2 dBm-1. The wavelength for high negative dispersion value can be adjusted by artificially choosing the parameters of proposed fiber, i.e. , d1 and f. Both its dispersion and dispersion slope are negative, the dispersion slope values are between-1-6 pskm-1nm-2 over C band, and its negative dispersion value is-3400 pskm-1nm-1, the nonlinear coefficient is only 3.6 km-1W-1, and the corresponding area of mode field is 43 m2 at wavelength of 1.55 m, if the parameter is selected as =1.2 m, f=0.917, d1=0.515 m. Obviously, it has a good dispersion compensation, therefore it has admirable applications in the field of high-speed large-capacity high-power pulses long-distance communication system.
2012, 61 (8): 084214. doi: 10.7498/aps.61.084214
A novel scheme to realize all-optical logic gates is proposed based on nonlinear polarization rotation (NPR) in high nonlinear fiber (HNLF). Two optical signals A and B together with a continuous wave are injected into the HNLF. Due to the optical power variation in HNLF, nonlinear birefringence will be induced between the two polarization axes. Both the optical signal and the continuous wave are filtered out at the output of HNLF. By controlling the optical power and the polarization of the optical signal as well as the polarization of the polarizer with respect to the polarization of optical signal/continuous wave, multiple all-optical logic gates can be realized. The theoretical analysis of the optical logic gates based on NPR in HNLF is provided. And the feasibility of the scheme is demonstrated by realizing all opticaland,not,or,xor,nxor,A B,AB, half-adder and half-subtracter at 10 Gbit/s operation.
2012, 61 (8): 084215. doi: 10.7498/aps.61.084215
Bi-doped double cladding fiber preform is prepared by modified chemical vapor deposition and solution doping process. The absorption spectrum of preform and the near-infrared (NIR) luminescence spectrum of fiber are experimentally investigated. The luminescence spectrum of the fiber shows a peak emission band at 1140 nm with a full-width at half-maximum (FWHM) of 130 nm under 976 nm LD exciting. Under 808 or 793 nm LD exciting, an ultra-broad NIR emission with an FWHM of more than 250 nm is observed in the Bi-doped fiber, and the range of luminescence spectrum is from 1000 to 1700 nm. After the heat treatment of the Bi-doped fiber preform slice under 900 ℃ for 1 h, under 808 nm LD exciting, the fluorescence intensity is about 4 times higher than that of the perform slice without heat treatment. The results indicate that the Bi-doped double cladding fiber is a promising gain medium used for ultrashort fiber lasers and tunable fiber lasers.
Research on the temporal-spatial distributions and the physical mechanisms for the sound speed profiles in north-central Indian Ocean
2012, 61 (8): 084301. doi: 10.7498/aps.61.084301
The structure of sound speed in ocean has a strong influence on underwater sound propagation. For underwater target detection and underwater acoustic communication, it is of great significance to obtain the temporal and spatial distribution of sound speed profile. Since the limitations and applicability of vertical grads method in structural analysis of sound speed in the Indian Ocean, optimal partition method is developed to calculate the sound spring layer. The study focuses on analysing the eigenvalues of sound speed profiles (SSPs) in the north-central Indian Ocean based on the last 10 year data of array for real-time geostrophic oceanography. The vertical structure characteristics of sound speed are investigated in the Indian Ocean, and the laws of temporal and spatial variation are obtained. The distribution of the fitting accuracy with the order of empirical orthogonal functions (EOF) are found. The physical mechanisms of the temporal and spatial variation of SSPs are revealed with the marine physical environment in Indian Ocean. The simulation results show that optimal partition method is suitable for the area to judge the structure of spring layer, and the parameters of the corresponding criterion is also proposed. The regional distributions of the fitting accuracy with the order of EOF are more obvious than with the seasonal variations. The deep channel axis exists at south 5S and there are three spring layers between 15S and 25S. The structures of SSPs in the Indian Ocean can be classified into four types: single spring layer, double spring layer type Ⅰ, double spring layer type Ⅱ and three spring layer, and for seasonal models: spring model, summer model, autumn model and winter model. The analysis results of the SSPs can provide some reference value for acoustic propagation and the sonar systems.
2012, 61 (8): 084701. doi: 10.7498/aps.61.084701
As a bubble moves near different boundaries, the motion characteristic will change largely compared with the condition of free field, especially near the free surface: there will appear a complex phenomenon because of the interaction. This paper aims to investigate experimentally the phenomenon and the regularity of the interaction between bubble and free surface. Using a spark-generated device and a high-speed camera, the influences of free surface on the maximal radius, period, jet time and width of the bubble are discussed. The relation between the voltage and the maximal radius is obtained. Different typical phenomena near the free surface are observed. The factors inducing different plume phenomena are obtained. Then the regurality of maximal height of the water column varying with dimensionless distance under the same initial condition is obtained, which provides a reference to the research on the interaction between bubble and free surface.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2012, 61 (8): 086101. doi: 10.7498/aps.61.086101
CuxAg1-xSbTe2 samples withx = 00.3 are prepared by a combined process of melt-quenching and spark plasma sintering (SPS). X-ray powder diffraction (XRD) analysis indicates that single phase samples with the NaCl-type structure are obtained for the Cu-doped samples before SPS treatment, whereas a small quantity of Ag2Te impurities coexist with the main cubic phase for the sample without Cu. According to our thermoanalysis and XRD results, the substitution of Cu for Ag can effectively prevent the precipitation of Ag2Te, but this also leads to the presence of a minor amorphous phase in the melt-quenched sample. The amorphous phase crystallizes into Sb7Te metastable phase at about 540 K, which finally transforms into the stable Sb2Te3 compound. After the SPS treatment of the melt-quenched sample, the sample withx=0.1 remains a single phase with the face-centered-cubic crystal structure, while Sb7Te and Sb2Te3 are precipitated as the second phases for the samples withx = 0.2 and 0.3, respectively. The electrical conductivity increases and the Seebeck coefficient decreases with the addition of Cu due to the existence of the second phase in the samples withx = 0.2 and 0.3. Accordingly, thermal conductivities also increase with the addition of Cu, leading to the reduced thermoelectric performance of thex= 0.2 and 0.3 samples. For the sample withx = 0.1, its power factor is comparable to that of the literature reported AgSbTe2 compound. As a result of so-called alloying effect, the phonon scattering effect is enhanced due to the partial replacement of Ag by Cu, leading to the reduced thermal conductivity of thex = 0.1 sample. Therefore, the Cu0.1Ag0.9SbTe2 sample exhibits the promising thermoelectric performance and a dimensionless thermoelectric figure of merit (ZT) value of 1 is achieved at 620 K.
2012, 61 (8): 086102. doi: 10.7498/aps.61.086102
The Bi2O3 doped glasses with concentrations of (0.9-x) GeO2-xNb2O5-0.1BaO (x=0.04, 0.07, 0.1) glasses are prepared by the conventional melting method. The differential thermal analysis (DTA) curves, the absorption spectra, the fluorescence decay curve and the X-ray photoelectron spectra are measured. The difference between glass crystallization onset temperature and transition temperature (Tx-Tg) of the glasses is up to 200℃ from the DTA curve. Absorption peaks at 500, 700, and 1000 nm are observed. The absorption edges show a red-shift with the increase of Nb2O5 content x. The emission band at 1300 nm with the full width at half maximum near 200 nm is observed under the excitation of 808 nm laser. The fluorescence intensity increases with the increase of the concentration of Bi2O3. The fluorescence intensity reaches a maximal value when the concentration of Bi2O3 is about 0.01. The peaks of binding energy in XPS are located at 159.6 and 164.7 eV respectively. The binding energy peaks are located between those of Bi3+ and Bi5+ by comparing with those of Bi2O3 (Bi3+) and NaBiO3 (Bi5+). According to the XPS results, one may conclude that Bi3+ and Bi5+ ions co-exist in the glass. The near infrared broadband emission may be assigned to Bi5+ ion based on the results of emission spectra and X-ray photoelectron spectra. The broadband intensity is gradually weakened as the Nb2O5 content x increases from 0.04 to 0.1. As GeO2 is substituted by Nb2O5, complex NbGe defects are formed and the lower valence state of Bi ions will be inevitably formed to compensate the extra electric charge from Nb5+, thus resulting in the inhibition of Bi5+ and weakening the fluorescence aforementioned.
A screw dislocation interacting with a wedge-shaped crack in one-dimensional hexagonal quasicrystals
2012, 61 (8): 086103. doi: 10.7498/aps.61.086103
The interaction between a screw dislocation and a semi-infinite wedge-shaped crack in one-dimensional hexagonal quasicrystal is investigated by the conformal transformation method and the perturbation technique. Analytical solutions are obtained for the stress intensity factor of crack, and the force on dislocation. The effects of the dislocation location and the wedge angle on the image force of the dislocation are revealed. At the same time the influence of the dislocation on crack behaviour is also discussed in detail. The corresponding results of the sharp crack can be obtained as a special case of the present solutions (i.e. =1/2).
2012, 61 (8): 086801. doi: 10.7498/aps.61.086801
The dynamic scaling properties of growing surfaces with point-defects are studied by applying the dynamic renormalization-group approach to the noisy Kuramoto-Sivashinsky equation with an additional term of point-defects potential. From the roughness and the dynamic exponents and z obtained here it follows that point-defects tend to roughen the growing surface and shorten its dynamic relaxation process to a steady-growth state.
2012, 61 (8): 086802. doi: 10.7498/aps.61.086802
Using equivalent transformation and spin-rescaling methods, the Gauss model on a decorated square lattice is studied. It is found that the square decorated Gauss lattice could be transformed into a regular square Gauss lattice with nearest-neighbor, and next-nearest-neighbor interactions. By calculating the regular square-lattice Gauss model, the critical temperature of the Gauss model is obtained on a decorated square lattice, and the exact phase diagram of this system can also be obtained.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2012, 61 (8): 087101. doi: 10.7498/aps.61.087101
The electronic structures of pure and Mn-doped LiFePO4 are studied using density functional theory (DFT). The results demonstrate that the pure LiFePO4 has a band gap of 0.725 eV, while the 25% Mn doped LiFe0.75Mn0.25PO4 has the smallest band gap (0.469 eV), and the weakest FeO and LiO bond, which indicates that the electronic conductivity and the ionic conductivity of the doped LiFePO4 are improved due to doping. On the other hand, the experimental results also show that the LiFe0.75Mn0.25PO4 has the best electrochemical performance and it delivers a very high capacity of 158 mAh? g-1 and a high energy density of 551 Whkg-1.
2012, 61 (8): 087102. doi: 10.7498/aps.61.087102
Traces of nanocrystals are introduced into Zr55Al10Ni5Cu30 bulk metallic glass through isothermally annealing the glass at 715 K for 30 min. The pre-annealed samples are rolled at different strain rates up to 95% thickness reduction at room temperature. The thermal stability and the free volume of the deformed sample are examined by differential scanning calorimetry. It is found that the thermal stability does not change with deformation degree. At any strain rate, free volume content increases continuously with the increase of deformation degree without saturation. As strain rate increases, however, the free volume content with the same deformation degree first increases and then decreases, which is significantly different from the case of the metallic glass with monolithic amorphous structure under plastic deformation.
Influence of optical-phonon scattering on electron mobility in wurtzite AlGaN/AlN/GaN heterostructures
2012, 61 (8): 087201. doi: 10.7498/aps.61.087201
Adopting a numerical method of solving self-consistently the Schrdinger equation and Poisson equation through taking into account the realistic heterostructure potential, which includes the influences of energy band bending and the finite thickness of barriers, and through considering the built-in electric field induced by spontaneous and piezoelectric polarization, the eigenstates and eigenenergies of electrons in two-dimensional electron gas (2DEG) are obtained for wurtzite AlxGa1-xN/AlN/GaN heterostructures with an inserted AlN layer. Based on the continuous dielectric model and the Loudon's uniaxial crystal model, optical-phonon modes and their ternary mixed crystals effect are discussed using the transfer matrix method. Furthermore, the Lei-Ting balance equation is extended in order to investigate the distribution of 2DEG and its size effect as well as ternary mixed crystals effect on electron mobility, which under the influence of each branch of optical-phonon modes are analyzed at room temperature. The results show that the increases of the thickness of inserted AlN layer and the Al component of AlxGa1-xN in the barrier enhance the built-in electric field in the GaN layer, leading 2DEG to be much closer to the interface of a heterostructure. In addition, it can also be found that the scattering from the interface phonons is stronger than from other optical-phonons, the interface phonons play a dominant role in the total mobility. A higher electron mobility can be obtained by adjusting appropriately the thickness of inserted AlN layer and Al component.
2012, 61 (8): 087202. doi: 10.7498/aps.61.087202
Using pure metal Ag, Te, Ge and Sb powders as raw materials, (AgSbTe2)100-x-(GeTe)x (x=8090) (TAGS) are synthesized by vacuum reaction. X-ray diffraction (XRD) analysis results show that after sintering the alloys each have a rhombohedral structure. TAGS power is sintered by hot pressing method. Their phase compositions, crystal structures and morphologies are characterized by XRD and scanning electron microscope analysis methods. Their electric conductivities are measured by direct current method. Their Seebeck coefficients are measured when a temperature difference (T=14℃) is applied along two ends of sample. Through testing the thermoelectric properties of materials, the variations of different performance parameters of the sample are investigated in a temperature range of 30500℃. The results show that the sample is nano crystals and its thermoelectric properties change as the composition changes. We can see that TAGS-80 has a good thermal performance, with ZTmax=1.8 at 530℃.
2012, 61 (8): 087204. doi: 10.7498/aps.61.087204
The measurement of carrier mobility in organic semiconductor material and device is one of important study contents. The hole-only devices based on the different solvent blends of poly (3-hexylthiophene) (P3HT) and [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) as acceptor are fabricated, the structures of the devices are all ITO/PEDOT:PSS/P3HT:PCBM/Au. The hole mobilities in the blend systems with different solvents and various annealing treatments are measured by the space charge limited current method. The results show that the J-V curves of charge transfer in the devices meet Mott-Gurney equation, the hole mobilities in the active layer with different solvents are different, the active layer formed with high boiling point solvent 1, 2-dichlorobenzene possesses higher hole mobility, heat treatment contributes to the improvement of the hole mobility in the devices. The reason of change of hole mobility is analyzed.
2012, 61 (8): 087301. doi: 10.7498/aps.61.087301
With the density functional first-principles calculations, we investigate the structures and electronic properties of transition metal nickel and its (111) surface. The adsorption behavior of single C atom on Ni (111) surface and its co-adsorption with the another C atom on Ni (111) surface are studied. The calculations on band structure and density of states show that significant spin polarization exists at the Fermi level of both nickel and its (111) surface. By comparing the adsorption energy, we find that the hollow site of hexagonal close-packed on the second layer of Ni atoms is the most stable position for the first C atom adsorption, and the hollow site of face-centered cubic on the third layer of Ni atoms is the most stable site for the co-adsorption of second C atom. Charge analysis shows that 1.566e charge transfers from each C atom to the adjacent Ni atom, which is similar to the 1.68e charge transfer in the first C atom adsorption case. The calculations on magnetism show that the magnetic moments of the two C atoms in co-adsorption are 0.059B and 0.060B, respectively, which are larger than the magnetic moment 0.017B of single C atom in Ni (111) surface.
2012, 61 (8): 087302. doi: 10.7498/aps.61.087302
By using the analysis of phase, fixed point and tunneling rate between two wells, we study the nonlinear Landau-Zener transition of Fermi superfluid gases in a two-mode system. We find that the interaction between fermi pairs can affect the quantum transition. We also find that when the interaction parameter c is less than the critical value c*, in the adiabatic limit, the quantum adiabatic transition theorem is still satisfied, but when the interaction parameter c is greater than this critical value, the quantum adiabatic transition theorem will not be satisfied. Finally, we obtain the relationship between the tunneling rate and the scan rate by comparing with the linear case.
Distributions of the threshold voltage and the temperature in the channel of amorphous silicon thin film transistors
2012, 61 (8): 087303. doi: 10.7498/aps.61.087303
Based on the double exponential distributions of trap states in the channel of the hydrogenated amorphous silicon thin film transistor, characteristic temperatures of tail state and deep state are distinguished. Besides, series resistances are used to be associated with characteristic lengths of the source and the drain with trap states. By taking advantage of the Poisson equation and Gauss theorem, the expression of the threshold voltage distribution is obtained. The results show that with the increase of the distance between the point and the source, the threshold voltage decreases. Moreover, under the degradation of the self-heating effect, the distribution of the temperature in the channel is non-uniform and its variation in the channel center is the biggest.
2012, 61 (8): 087501. doi: 10.7498/aps.61.087501
It is important to study the mechanism of negative magnetoresistance (MR) in magnetic semiconductors for the correct understanding of the sp-d interactions between carriers and magnetic ions. In this work, temperature-dependent Hall effect (10300 K) and magnetic susceptibility (5300 K) are measured for the study of negative MR and paramagnetic enhancement of nondegenerate p-type Hg1-xMnxTe (x0.17) monocrystal. As temperature decreases, both negative MR and susceptibility show the same behaviors, each of which contains an exponentially changing temperature function \exp(-K/T). According to the theory of impurity energy level in semimagnetic semiconductor, magnetic field can lead to the spin-splitting of acceptor level and result in reducing the binding energy of acceptors, which is responsible mainly for the negative MR in nondegenerate p-type Hg1-xMnxTe monocrystal.
Study on conducting characteristics of electrical trees in cross-linked polyethylene cable insulation
2012, 61 (8): 087701. doi: 10.7498/aps.61.087701
The conducting characteristics of two typical electrical trees in cross-linked polyethylene (XLPE) cable insulation are studied by a combination of optical microscopy observation, partial discharge measurement and con-focal Raman spectroscopy analysis. Although they are grown under similar conditions, these two trees display very different shapes. One is a typical branch-pine tree grown at 9 kV, and the other is a branch tree grown at 11 kV. The growth and the partial discharge regularities show obvious differences. The disordered graphitic carbon is condensed in the main tree channels of the branch-pine tree. From the relative intensity of the graphitic carbon G band to D band, the graphitic domain is estimated to be about 8 nm in size. The tree channel resistance per unit length is less than 10 m-1, which is sufficient to prevent the partial discharge from developing within the tree structure. The branch-pine tree shows the features of the conducting tree. The fluorescence background is observed in the channels of branch tree, which shows the existence of the products of the material degradation, but no disordered graphitic carbon is observed in these tree channels. These tree channels display obvious non-conducting characteristics, which is not sufficient to prevent the continuous effect of the partial discharges. Finally, a single channel growth model is proposed for the conducting and non-conducting trees grown in XLPE cable insulation. Based on the equivalent circuit theory, the growth mechanisms of the two trees with different conducting characteristics in XLPE cable insulation are discussed.
The influence of variable quantity of polarization on the current intensity of the electron emission from La-doped Pb(Zr, Sn, Ti)O3 ferroelectric cathode
2012, 61 (8): 087702. doi: 10.7498/aps.61.087702
Ferroelectric cathodes exhibit huge potentials in high-power microwave tube electron beam source, panel display, and the propeller space navigation, due to their superior properties. The material properties of the ferroelectric cathode have been proved to have a significant influence on electron emission, which is indicated in recent research work. In the course of electron emission, the variation of polarization can be caused by non-shielded surface charge which is induced by high trigger voltage. A certain relationship may be found between polarization variation and current intensity of electron emission. To study the relationship between current intensity of electron emission and polarization variation in ferroelectric cathodes, the samples of lanthanum-doped lead zirconate stannate titanate ferroelectric and antiferroelectric ceramics are prepared by the method of solid state calcinations, and the polarization variations of the material under different voltages are measured in the positive half cycle test of hysteresis loop. The curve of the electron emission current intensity versus the trigger voltage is measured, and then the relationship between electron emission current intensity and polarization variation is investigated. The results show that the electron emission current intensities of the two samples are both directly proportional to the polarization variation.
2012, 61 (8): 087801. doi: 10.7498/aps.61.087801
Top-emitting white organic light-emitting diode (TEWOLED) has potential applications in lighting and full color displays. Microcavity effect in TEWOLED restrains the realization of the white emission with excellent optical and electric performances. In this paper, a ZnS film with a high refractive index used as a light outcoupling layer is introduced into the metal cathode to enhance its transmittivity to a maximal value in the blue light wavelength region. In addition, transfer matrix theory is utilized to optimize the thicknesses of the cathode and the ZnS outcoupling layer and the wide-angle interference is used to design the position of the blue emission layer inside the organic light-emitting diode. Based on the above work, the white light with relatively high luminous efficiency, good color purity, and small CIE coordinate change is acquired. The corresponding luminance and current efficicency are 9213 cd/m2 and 3 cd/A, respectively. The CIE coordinates belong to the white emission and are near the white light equal-energy point. The white emission also shows stable spectra with respect to the observation angle, with a limited CIE coordinate change of (0.02, 0) for a large observation angle change from 0 to 60.
Design and fabrication of SiO2/Si3N4 dielectric distributed Bragg reflectors for ultraviolet optoelectronic applications
2012, 61 (8): 087802. doi: 10.7498/aps.61.087802
In this paper, we design a SiO2/Si3N4 dielectric distributed Bragg reflector (DDBR) by the transfer-matrix method, which is grown by plasma-enhanced chemical vapor deposition on sapphire (0001). There exists a slight difference between theoretical and experimental results in peak wavelength (～ 10 nm). The peak wavelength is blue shifted with the number of DDBR pairs increasing. The 13-pair DDBR provides a 58 nm wide stop band centered at 333 nm with a maximal reflectivity of higher than 99%, as the refractive index ratio of Si3N4 to SiO2 is relatively high. It is proved by the scanning electron microscope and atomic force microscope measurements that the variations of thickness and roughness of Si3N4 layer with respect to SiO2 layer during growth contribute to the blue shift of peak wavelength. The X-ray reflectivity measurements indicate that the interfacial degradation of the samples has little effect on the maximum reflectivity, and the relatively poor quality of SiO2 compared with Si3N4 may be one of the reasons that cause the difference between the measurements and simulations.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
The fabrication and field emission properties of ZnO nanoparticles-doped diamond-like carbon films by electrochemical deposition
2012, 61 (8): 088101. doi: 10.7498/aps.61.088101
The formation of ZnO nanoparticles embedded in diamond-like carbon (DLC) thin film, deposited by electrochemical technique without post-processing, is observed. The effect of ZnO doping on the field emission (FE) property of DLC film is investigated. The chemical composition, the microstructure, and the surface morphologies of the sample are characterized by X-ray photoelectron microscopy, transmission electron microscopy, Raman spectrum, and atomic force microscope (AFM). It is shown that the ZnO nanoparticles are of a wurtzite structure and the content of ZnO increases with Zn source increasing in electrolyte. The ZnO doping enhances both the graphitization and the surface roughness of the DLC film, which is verified by Raman spectrum and AFM. By the ZnO doping, the FE properties of the DLC film are improved. An emission current density of 1 mA/cm2 is obtained at an electric field of 20.7 V/m for the film with a Zn/(Zn+C) ratio of 10.3at%. The improvement on the FE properties of the ZnO-doped DLC film is analyzed in the context of microstructure and chemical composition.
2012, 61 (8): 088102. doi: 10.7498/aps.61.088102
The titanium/titanium diamond-like carbon (Ti/Ti-DLC) multi-layered films are deposited by combined electron cyclotron resonance microwave plasma enhanced chemical vapor deposition method and mid-frequency magnetron sputtering method using acetylene as the reactive gas and Ti target with a purity of 99.99%. The structures, compositions and surface morphologies of the samples are characterized by X-ray diffraction, scanning electron microscope, and X-ray photoelectron spectrometer. The mechanical properties of Ti/Ti-DLC multilayer films are investigated by microhardness tester, friction wear test instrument and surface roughmeter. The results show that TiC phase appears in multilayered film. The main growth mode of the film is island mode and there exists columnar crystal structure neither in Ti layer nor Ti-DLC layer. The properties of the multi-layered film, including composite hardness, friction coefficient, surface morphology and surface roughness are affected by the modulation period . It is shown that nano-hardening effect takes place when is small while large particles appear on the surface of the film, leading to a large surface roughness and friction coefficient. The layered structure of the film is not clear when 50 nm.
2012, 61 (8): 088103. doi: 10.7498/aps.61.088103
Modeling and experimental results show that the depleted uranium (DU) and Aucocktail nanometer multilayer will improve the X-ray conversion efficiency by reducing energy loss to penetration of the X-ray into the hohlraum wall. DU/Au multilayer plane film is deposited by magnetron sputtering through alternately rotating substrate in front of separate DU and Au sources. The geometry parameter, surface topography, atomic concentration and interface structure of DU/Au multilayer are characterized by white light interferometer, scanning electronic microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Au film becomes continuous when its thickness reaches 8 nm. Combining with theoretical modeling results, 30 nm DU and 8 nm Au multilayer is chosen. The periodic thickness of DU/Au is measured to be about 37 nm. Well-defined Du/Au interface is observed by SEM. Diffusion at DU/Au interface is observed by XPS. The atomic concentration ratio of DU, Au, O is 73:26:1. The binding energy of Au 4f of 8 nm thickness Au film shifts toward high-energy tail about by 0.6 eV. Similar phenomena are unfound in 30 nm thickness DU film.
A model for releasing of stored energy and microstructure evolution during recrystallization by phase-field simulation
2012, 61 (8): 088104. doi: 10.7498/aps.61.088104
A format of f(ik,jk ) = Es (ide )2(1-(jre )2) to express cold deformed stored energy is suggested in a phase-field model for simulating recrystallization in alloys. Using AZ31 magnesium alloy as an example, the recrystallization process is simulated by the new model, and the simulation results are in good agreement with experimental measurements. The nucleation process of recrystallization is realized for the first time by the model based on the physical background. The simulation results show that the grain growth of a cold deformed alloy in the annealing process indicates automatically two stages: recrystallization driven by the restored energy and thermal growth driven by boundary energy. A theoretical time spent in finishing recrystallization, obtained by simulation, is found to be 2/3 of that obtained by industrial practice. The mechanism and the experimental results about the influence of cold deformation on subgrain size and stored energy are examined, and the experimental results are introduced into the simulation. The common experimental phenomenon shows that there is a peak at the critical strain on the curve of recrystallized grain size versus cold strain. A theoretical explaination to the mechanism of the peak occurrence is also discussed.
Analytical modeling of coil impedance based on truncated region eigenfunction expansion method in eddy current tube inspection
2012, 61 (8): 088105. doi: 10.7498/aps.61.088105
In order to calculate theoretically the coil impedance, the separation of variables method and the Cheng matrix method are used to establish the analytical model by applying the magnetic insulation condition in eddy current tube detection with encircling probes inside and outside respectively. In the established model, there exists a definite integral related to modified Bessel function. Gaussian integral algorithm is proposed to accurately perform the numerical calculation. Simulations are carried out on the thinning of a copper tube wall thickness using the presented model, traditional Dodd model and finite element model. Their results are in good agreement with each other, demonstrating that the derived analytical model is correct. Compared with the Dodd and finite element models, the improved model has some advantages such as good efficiency, easy adjustment of accuracy and so on.
Study of the effect of lithography deviation on 4H-SiC floating junction junction barrier Schottky diode
2012, 61 (8): 088501. doi: 10.7498/aps.61.088501
The breakdown voltage of 4H-SiC junction barrier schottky diode with floating junction is larger than that of traditional junction barrier Schottky diode under the condition of the same fixed on-resistance. It is a crucial technology that the alignment of lithography between p+ region of floating junction and main junction. The simulation results obtained using two-dimensional simulator ISE show that the breakdown voltage obviously drops with the deviation of lithography increasing. Although the breakdown characteristics of the dislocation and the alignment structure are similar, the series resistance of the dislocation structure is larger than the latter when the forward voltage is larger than 2 V.
Investigation of spectral regulation in dual- wavelength light-emitting diodes by using the selective p-doped barriers
2012, 61 (8): 088502. doi: 10.7498/aps.61.088502
The electrical and the optical characteristics of dual-wavelength light-emitting diode (LED) with the newly designed selective p-doped barriers are investigated numerically. The simulation results show that the selective p-doped barriers can improve the distribution equilibria of electron and hole concentrations in each quantum well (QW). The radiative recombination rate of QW is enhanced remarkably when specific number of p-doped barriers is adopted, and the electron leakage current is suppressed obviously with this new design. Therefore, the internal quantum efficiency is improved and the trend of efficiency drooping with the increase of current injection is also alleviated. Moreover, the curve peaks of the spectrum become quite uniform when the specific number of vertically-stacked QWs is adopted, and the spectral regulation of the dual-wavelength LED is more effective.
2012, 61 (8): 088503. doi: 10.7498/aps.61.088503
Using the spin diffusion-drift equation and considering the effects of the electric fields and the special carriers in organic semiconductors, the enlargement ratio of current spin polarization in a simple T-shaped organic spintronic device is theoretically studied. It is found that the significant enlargement ratio of the current spin polarization can be acquired by adjusting the electric field and the polaron ratio in organic semiconductor.
Photoelectricity performance research based on the sol-modified thin film electrode of dye-sensitized solar cells
2012, 61 (8): 088801. doi: 10.7498/aps.61.088801
Nanoporous film electrode is a crucial composition of dye-sensitized solar cells, which influences the photoelectric conversion performance. To improve the property of the photoelectrode, different modification methods by using different concentrations of TiO2 sol are investigated. The crystallite size and phase of the nanoporous TiO2 particles and the TiO2 sol after sintering are studied with X-ray diffraction. The microstructure morphologies of the conductive glass and the films are determined by the high resolution transmission electron microscopy and the field emission scanning electron microscopy. The influences on electron lifetime n and the electron transit time d are analyzed by intensity-modulated photocurrent spectroscopy and photovoltage spectroscopy from the mechanisms of electron transport and back reaction kinetics. It is found that the back reactions are well suppressed under dark conditions after sol modifications. n is effectively extended and d is also shorten correspondingly by any kind of sol treatment. The short-current density and the photoelectric conversion efficiency are increased by 10.9% and 11.9% separately, when 0.10 molL-1 sol modification is applied both to the conductive glass and to the nanoporous TiO2 film at the same time.
Analysis of organic photovoltaic devices with MoOx doped 4,4,4-tris(N-(3-methylphenyl)-N- phenylamin) triphenylamine as hole transport layer
2012, 61 (8): 088802. doi: 10.7498/aps.61.088802
MoOx doped 4,4,4-tris(N-(3-methylphenyl)-N-phenylamin) triphenylamine (m-MTDATA) is used as a hole transport layer to improve the efficiency of CuPc/C60 small molecular organic photovoltaics. A series of devices is fabricated in a high vacuum system. One of the devices with the structure of indum tin oxides (ITO)/m-MTDATA:MoOx(3:1)(30 nm)/CuPc(20 nm)/C60(40 nm)/Bphen (8 nm)/LiF(0.8 nm)/Al(100 nm) shows that the following parameters are achieved: the open circuit voltage Voc = 0.40 V, short-circuit current Jsc=6.59 mA/cm2, fill factor of 0.55, and power conversion efficiency p=1.46% under AM1.5 solar illumination. The efficiency of the device is improved by 38% compared with that of the device without hole transport layer ITO/CuPc(20 nm)/C60(40 nm)/Bphen(8 nm)/LiF(0.8 nm)/Al(100 nm). The improvement of the device performance may be attributed to the addition of m-MTDATA:MoOx (3:1) (30 nm) hole transport layer that reduces the contact resistance between the ITO electrode and the organic layer, thus reducing the overall device series resistance and improving the efficiency of the device.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2012, 61 (8): 089401. doi: 10.7498/aps.61.089401
Based on a neutral barium cloud diffusion model, considering the oxidation and ionization loss of barium atoms, the early (t 100 s) evolution characeristics, morphologies, brightnesses and electronic density distributions of barium clouds released in the ionosphere are discussed in this paper. The numerical simulation results are presented for the early dynamics of barium clouds with different released masses (1, 10 kg), different release altitudes (250, 300 km), and different initial shape factors (1, 10).
2012, 61 (8): 089501. doi: 10.7498/aps.61.089501
In order to increase the response speed of the liquid crystal phase modulator (LCPM), a new kind of fast liquid crystal material is designed and synthesized, and a corresponding LCPM, called liquid crystal on silicon (LCOS), is fabricated. We test the phase modulation of the LCOS and examine its wavefront correction ability on static and disturbed wavefront. First, the electro-optical response is tested. The response time to a phase modulation quantity of 780 nm is 2 ms. Second, the phase modulation of the LCOS is tested and linearized. Third, the 3 dB closed-loop and open-loop disturbance rejection bandwidths of the adaptive optics system (AOS) are measured to be 16 and 18 Hz, respectively. Finally, the simulated correction of 26 Hz turbulence with the open-loop AOS is conducted. After correction, the Strehl ratio increases from 0.026 to 0.225. Therefore, with our fabricated LCOS, the liquid crystal AOS is able to correct the turbulence below 30 Hz.