## Vol. 64, No. 13 (2015)

##### 2015-07-05

###### GENERAL

2015, 64 (13): 130501.
doi: 10.7498/aps.64.130501

Abstract +

In order to investigate the influence of structures of substrates on the dynamic properties of a discrete growth model, the restricted solid-on-solid model for Koch lattice and Koch curve fractal substrates, which have different fractal dimensions and spectrum dimensions but the same walk dimensions, is studied by means of numerical simulations. Surface width and distribution of the extremal height of the saturated surface are calculated. Results show that the random walk exponent plays the determinative part in the saturated regime. Although the fractal substrates have different fractal dimensions and spectral dimensions, the value of roughness exponents for the two substrates are almost the same within the error. The data of maximal height distributions (minmal height distribution) on the width of the saturated surface for the two fractal substrates can be well collapsed together and fitted by Asym2Sig distribution.

2015, 64 (13): 130502.
doi: 10.7498/aps.64.130502

Abstract +

In gradient computations of the variational data assimilation (VDA) by the adjoint method, in order to overcome a lot of shortcomings such as low accuracy, difficult implementation, and great complexity, etc., a novel data assimilation method is proposed based on the dual-number theory. The important advantages are that the coding of adjoint models and reverse integrations are not necessary any more, and the values of cost functional and its corresponding gradient vectors can be attained simultaneously only by one forward computation in dual-number space. Furthermore, the accuracy of gradient can be close to the computer machine precision without other error sources. The paper is organised as follows. Firstly, the dual-number theory and algorithm rules are introduced. Then, the issues of gradient analysis and computation in VDA are transformed into the processes of calculating the cost functional numerically in dual-number space, and the gradient vectors can be obtained at the same time in an easy, efficient and accurate way. Secondly, the new algorithm for data assimilation in nonlinear physical systems is developed by combining accurate gradient information from the dual-number method with classical optimization algorithm. Thirdly, numerical experiments on sensitivity analysis for an ENSO nonlinear air-sea coupled oscillator are implemented, and the results are presented to demonstrate the important advantages of the dual-number method in the calculation of derivative information. Finally, numerical simulations for data assimilation are carried out respectively for the typical Lorenz 63 chaotic systems, the specific humidity evolving equation with physical “on-off” process at a single grid point, and a parabolic partial differential equation. Some conclusions can be drawn from the numerical experiments. The newly proposed method may be suited to many kinds of optimization problems with ordinary or partial differential equations as constraints, such as data assimilation, parameter estimation, inverse problems, sensitivity analysis etc. Results show that the new method can reconstruct the initial conditions or parameters of a nonlinear dynamical system very conveniently and accurately. Its another advantage is being very easy to implement with a high accuracy in gradient computation, so it is robust in the process of numerical optimization. The estimated initial states or parameters are convergent to real value in the cost of no more computations, when there are noises in the observations. But many tests are still needed to demonstrate the validity and advantages of the new data assimilation method, especially in more complex and realistic numerical prediction models of atmosphere and ocean.

2015, 64 (13): 130503.
doi: 10.7498/aps.64.130503

Abstract +

In this paper, the piecewise smooth state equation of a two-stage photovoltaic grid-connected (TPG) inverter is established and studied; based on the solution to the piecewise smooth state equation of the TPG inverter, effects of the photovoltaic array voltage on nonlinear dynamical behaviors of the TPG inverter are analyzed by using bifurcation diagram, folded diagram, 3D phase diagram, and Poincaré section. Then the nonlinear dynamical behaviors of TPG inverter are compared with the conventional one. And a strategy of expanding the input voltage range for the TPG inverter is explored. Finally, the nonlinear dynamical behaviors in it caused by the variation of main circuit parameters: such as the output inductance and capacitance of the front-stage, as well as those of the second-stage, are discussed through slow-scale bifurcation diagrams. Studies have found that it is effective to expand the input voltage range of the TPG inverter by segment control of the photovoltaic array voltage, and the chaotic phenomena in the TPG inverter can be avoided by increasing the parameter values of inertial devices such as output inductance and capacitance in the front-stage appropriately, but the values of output inductance and capacitance in the second-stage should be away from the multiple noncontiguous region, since it can cause chaotic behavior. The above work may have important guiding significance and application for improving the stability and efficiency of two-stage photovoltaic grid-connected inverter based photovoltaic power generation system.

2015, 64 (13): 130504.
doi: 10.7498/aps.64.130504

Abstract +

A random fractal exhibits self-similarity over the scaling region, this is different from the regular fractal. The scaling region obtained by the proper method for the exact fractal dimension is very important. And the correlation dimension is one of the fractal dimensions which is used widely in many fields. Therefore, it is necessary and timely to identify the scaling region that plays a critical role in calculating the correlation dimension accurately in various chaotic systems. Visual identification is widely used to determine the scaling region as a quick and simple subjective method. However, this method may lead to an inaccurate result in Grassberger Procaccia algorithm. In order to reduce the error caused by human factors from computing the correlation dimension, a novel method of identifying the scaling region based on simulated annealing genetic fuzzy C-means clustering algorithm is proposed. This new method is based on the fluctuating characteristics that the second-order derivative of the curve within the scaling region is zero or nearly zero. Firstly, the second-order differential of the double logarithm correlation integral discrete data is calculated. Secondly, the simulated annealing genetic fuzzy C-means clustering method is used for dividing the data into three groups: positive fluctuation data, zero fluctuation data, and negative fluctuation data. The zero fluctuation data are selected to retain, the rest is excluded. Thirdly, the 3 σ criteria are used for excluding gross errors to retain those valid from the zero fluctuation data. Fourthly, the data of the consecutive nature point interval are chosen from the retained data. Finally, the regression analysis and statistical test are used to identify the scaling region from the data valid. In order to verify the effectiveness of the proposed method, it is used to simulate the Lorenz and Henon systems. The calculated results are in good agreement with the theoretical values. Experimental results show that the proposed new approach is easy to operate, more efficient and more accurate than the subjective recognition, K-means method, and 2-means method in identifying the scaling region.

2015, 64 (13): 130601.
doi: 10.7498/aps.64.130601

Abstract +

The optical lattice clock with neutral atoms occupies an outstanding position in the research field of atomic clocks, demonstrating the great potential of its performance (like the uncertainty and the stability). At present, the optical lattice clock has realized a 10-18 level of its uncertainty. In this paper, we present the realization of loading bosonic atoms 88Sr (strontium, alkaline-earth metals) into a one-dimensional (1D) optical lattice in our laboratory. The optical lattice where the atoms are trapped can make the energy level shift, called Stark shift. But there is the special optical lattice operating at the “magic” wavelength for clock transitions (5s2) 1S0-(5s5p) 3P0, which can make the same Stark light-shift for both of them, indicating a zero light-shift relative to the clock. In our experiment, Sr atoms are cooled in a two-stage cooling and its temperature can be as low as 2 μK. Then these cold atoms are confined in the Lamb-Dicke region by the lattice laser output from an amplified diode laser operating at the “magic” wavelength, 813 nm. Experimentally, it is straightforward to provide 850 mW of lattice power focused to a 38 μm beam radius. After the cold atoms have trapped in the optical lattice, the lifetime of atoms in 1D optical lattice is measured to be 270 ms. The temperature and the number are about 3.5 μK and 1.2×105 respectively. Besides, effects of the power of the lattice laser on both the number and temperature are analyzed. The number changes linearly with the laser power, while there is no obvious influence on the temperature by the power. This original and special approach for atoms trapped in the optical lattice can provide a long interrogation time for probing the clock transition. Furthermore, it may be the foundation for developing our optical lattice clock of strontium atoms.

2015, 64 (13): 130702.
doi: 10.7498/aps.64.130702

Abstract +

Circulant measurement matrix has been widely used in compressive sensing because of its high-speed discrete convolution algorithm. The typical work of optimizing circulant measurement matrix was introduced by Wotao Yin in Reference [16]. Motivated by his work, the construction of circulant measurement matrix in this paper is explored from the view point of generating elements' amplitudes and phases; and the optimal construction procedures are proposed based on alternately optimizing amplitudes in conjunction with chaotic stochastic phases of the matrix generating elements. The main idea of this paper is based on two innovations: The first one is to reduce the mutual coherence between column vectors of equivalent dictionary by alternately optimizing the generating elements' amplitudes, thus improving the recovery performance of the circulant measurement matrix. From the different expressions of the circulant matrixes of one-dimensional and two-dimensional signal, by setting the Welch bound for the coefficient of mutual coherence between the column vectors of equivalent dictionary as the approximation objective, two novel unified mathematical models are derived from the optimizing function for generating elements' amplitudes of the two different matrixes. Optimal solutions for generating elements' amplitudes are gained by alternately optimizing method. The second innovation is to construct the generating elements' phases of circulant measurement matrix by utilization of a chaotic sequence with independent property. The chaotic stochastic phase of the circulant measurement matrix generating elements are generated by taking advantage of the chaotic internal certainty, which means an independent identically-distributed randomness sequence can be produced by the chaotic map with the initial value at certain sampling distance. At the same time, the external randomness of chaotic sequence can satisfy the stochastic requirement of circulant measurement matrix. This paper presents the method of constructing chaotic stochastic phase using Cat chaotic map. Experimental results of one-dimensional and two-dimensional signals in the optimized circulant measurement matrix are studied in this paper, which has a better performance as compared with the results of conventional circulant measurement matrixes, such as Gaussian circulant matrix and optimized circulant matrix proposed by Wotao Yin. The column vectors of equivalent dictionary in the optimized circulant measurement matrix have lower mutual coherence, this is the essence of the superiority of the optimized circulant matrix.

## EDITOR'S SUGGESTION

2015, 64 (13): 130704.
doi: 10.7498/aps.64.130704

Abstract +

Orbital angular momentum (OAM) of photons has both classical and quantum applications due to its feature of optical vortex and infinite dimension. OAM discrimination is one of the basic problems, which has been paid much attention recently. Here we present an interferometer method in which a Sagnac interferometer with a Dove prism is placed on each arm to separate the different OAM of photons into different output ports, namely, OAM sorters. We demonstrate experimentally the feasibility of OAM sorter by dividing different OAM states into different output ports. Using the cascade interferometers, we also sort the superposition state successfully. Experimental results are in good agreement with the theoretical predictions. Compared with other methods, this method is more stable and can be used to separate superposition states into single photon levels. Furthermore, this method can also be used to couple OAM modes with spatial modes, a very important method for manipulating OAM states. It is a useful method and has potential applications in high-capacity optical communication, quantum entanglement, quantum cryptography, quantum computation and quantum information.

2015, 64 (13): 130705.
doi: 10.7498/aps.64.130705

Abstract +

Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) has a very high detection sensitivity, nevertheless the quantitative results retrieved by the traditional concentration retrieval method are affected by the calibration error of mirror reflectivity. Therefore, in this paper we present another concentration retrieval method based on the measurements of atmospheric O2-O2 absorption. In this method the optical cavity of IBBCEAS is equivalent to a multi-reflection cell, in which the optical path length is independent of the wavelength, i.e. a constant. First, we get the slanting column concentrations of atmospheric O2-O2 and other trace gases measured by using the differential optical absorption spectroscopy (DOAS) to fit the corrected cross sections of the measured gases to the optical density from the IBBCEAS absorption spectra and reference spectra. Second, the optical path length of an equivalent cell is determined by the known concentrations of O2-O2 in the atmosphere. Third, the concentrations measured for trace gases are retrieved by the deduction of absorption optical path length from the obtained slanting column concentrations. The above method is demonstrated by measuring the atmospheric NO2 with an IBBCEAS instrument in the range of 454-487 nm. Atmospheric NO2 concentrations retrieved by this method are compared with those by the traditional method, and the difference between them is shown to be less than 7%. Experimental results show that the absorption of atmospheric O2-O2 can be used to quantify other trace gases when measured by IBBCEAS, and, above all, the quantitative results are almost insensitive to the calibration error from mirror reflectivity.

2015, 64 (13): 130505.
doi: 10.7498/aps.64.130505

Abstract +

In this paper, we investigate the finite-temperature properties and phase transition of the Dicke model. Converting the atomic pseudo-spin operator to the two-mode Fermi operators, we obtain the partition function in terms of the imaginary-time path integral. The atomic population and average photon number as analytic functions of the atom-photon coupling strength are found from the thermodynamic equilibrium equation, which leads to the stationary state at a finite temperature and is determined by the variation in an extremum-condition of the Euclidean action with respect to the bosonic field. In particular we study the phase transition from normal to superradiation phase at a fixed low-temperature, in which the phase transition is dominated by quantum fluctuations. The phase transition induced by the variation of the atom-photon coupling strength indeed obeys the Landau continuous phase-transition theory, in which the average photon-number can serve as an order parameter with non-zero value that characterizes the superradiation phase. In the zero temperature limit our results recover exactly all those obtained from the quantum phase transition theory at zero temperature. In addition, we discuss the thermodynamic properties and compare the difference between finite-temperature phase transition and zero-temperature quantum phase transition. It is discovered that the average photon-number and mean energy in the low-temperature stationary state coincide with the corresponding values of zero-temperature in the strong coupling region. The entropy of the superradiation phase decays rapidly to zero with the increase of coupling strength.

2015, 64 (13): 130701.
doi: 10.7498/aps.64.130701

Abstract +

A highly sensitive NO2 optical sensor has been designed by means of combining the electrical modulation cancellation method (E-MOCAM) and off-beam quartz enhanced photoacoustic spectroscopy (QEPAS). A high power multimode blue laser diode emitting at around 450 nm is used as the excitation light source of the photoacoustic signal. In the E-MOCAM, the balance signal is generated from a dual-channel function generator and introduced to the pin of the quartz tuning fork (QTF) to balance out the huge background noise. The principle of the E-MOCAM is explained in detail from the perspective of equivalent circuit of QTF, and the background noise of the high power LD-based QEPAS sensor is analyzed. Results show that stray light noises coming from the LD beam and blocked by the resonator and the photoacoustic cell are dominated in all the noises. Gas flow noise of QEPAS sensor is also estimated, and excessive noise could be introduced by the gas flow even at a rate below 200 sccm. The gas flow noise is measured at different gas flow rate, from 60 to 200 sccm. Compared with the QEPAS sensor based on wavelength modulation, the sensor based on amplitude modulation, especially in the case of high power light source, is more sensitive to the gas flow. The ultimate background noise of the off-beam QEPAS sensor can be reduced by 269 times after the E-MOCAM is applied. The performance of the NO2 QEPAS sensor is evaluated in the NO2/N2 mixtures of different concentrations, ranging from ppb to ppm levels. In the case of the 2.85 ppm NO2 measurement, the SNR of 630 is achieved. A linear fitting is implemented to evaluate the response of the sensor, resulting in an R square value of 0.999. Allan plot is used to investigate the long term stability of the sensor. The original background noise produced from the off-beam QEPAS configuration is less than that from the on-beam QEPAS configuration, thus the combination of off-beam QEPAS configuration and E-MOCAM shows a better stability. A detection limit of 0.34 ppb (1, 46 s integration time) for NO2 in N2 at atmospheric pressure can be achieved, which corresponds to a normalized noise equivalent absorption coefficient of 2.210-8 cm-1W/Hz1/2.

2015, 64 (13): 130703.
doi: 10.7498/aps.64.130703

Abstract +

In a stepped-mirror-based static Fourier transform infrared spectrometer, the collimation lens is located adjacent to the light source and the thermal radiation would lead to the partial temperature increase, and the refractive index of the infrared material unavoidably changes. Then the light beam passing through the collimation lens will induce a divergence angle, directly affecting the resolution of the recovered spectrum. Meanwhile, the angular divergence results in a displacement in the interference signal, making increasing difficulties in the interferogram processing and the spectrum recovery. In this paper, the distribution of temperature in different areas of the collimation lens is studied under the working condition, and the defocusing value of the collimation lens is 0.153 mm that is caused by the refractive index gradient of the infrared material along with the temperature. In addition, the divergence angle induced by defocusing is calculated, its distribution being nonuniform but symmetrical within the sample area. Moreover, the divergence angle brings about additional optical path difference, its effect on the recovered spectrum is analyzed. Compared with the ideal recovered spectrum, much noise emerges and the peak value is reduced in the real recovered spectrum. The spectrum-construction error of the real recovered spectrum is 18.72%, indicating that the recovered spectrum is seriously distorted, and the resolution at the center wavelength in the ideal recovered spectrum and the real spectrum are 4.71 cm-1 and 5.57 cm-1, respectively. This indicates that the spectrum resolving power is weakened. Furthermore, the reasonable temperature range is obtained by analyzing the curve of spectrum-construction error versus temperature of the collimation lens. When a spectrum-construction error of less than 5% is demanded, the temperature difference between the front len of collimation and the ambient must be less than 8 ℃. Finally, experiments are performed using a SiC rod as the light source, and interferograms are made by four steps including dark current electric noise elimination, spatial gain correction, image signal averaging, and spectrum recovery. Results show that the spectrum-construction errors (SCE) from the cold light source and non-cold light source are 8.48% and 21.51%, respectively. Although they are all larger than the theoretical value, the SCE of cold light source decreases by 13.03% compared to cooling-free light source. Hence, it is necessary to reduced the thermal radiation influence on collimation lens from the light source. This result is helpful in solving the analogous problems.

###### ATOMIC AND MOLECULAR PHYSICS

2015, 64 (13): 133201.
doi: 10.7498/aps.64.133201

Abstract +

The dynamical process of photoexcitation and photoionization of alkali atoms is studied with three-step laser pulses, focusing on the similarities and differences between Li and Cs atoms on their properties by making a comparison of them. Based on several excitation schemes, the present work not only establishes the rate equations of atom population for all related transition states, but also obtains the analytical solutions of photoexcitation and photoionization process. The mathematical solutions are simplified significantly by restricting the most general case to the several special cases, either designed or selected carefully, in order to highlight the main factors and obtain the physical insight underlying the complicated mathematical expressions. With self-programming, the possible impact of time configuration of laser pulses on the three-step laser excitation process of the photoexcitation and photoionization is calculated and studied systematically. Variation of the ionization efficiency with the laser parameters is investigated and discussed. With the same time configuration of laser pulses, the dependences of atom population for all the related transition states on the two different excitation paths are compared and analyzed, reflecting the impact of changing the atomic parameters. Under the similar excitation conditions, the differences of ionization efficiency between Li and Cs atoms are explored. Finally, based on the present study, several means for optimizing ionization efficiency are proposed.

2015, 64 (13): 133101.
doi: 10.7498/aps.64.133101

Abstract +

Based on the ab initio coupled-cluster CCSD(T) method in quantum mechanics, the charge distribution of Ar atom and its vib-rotational interaction with H2 molecule are calculated using augmented correlation consistent basis sets aug-cc-pV5Z and 3s3p2d1f1g Gaussian bonding function, and the basis set superposition error (BSSE) is eliminated using Boy and Bernardi's full counterpoise method. Afterwards, the analytical expression of the interaction potential of the Ar-H2 system is fitted with Tang-Toennies potential function. With this interaction potential, the scattering cross section of Ar-H2(D2, T2) collision system is calculated by using close-coupling method when the incident energy of Ar atoms is 83 meV. The calculated differential cross section of Ar-D2 collision system is consistent with the experimental results. Calculated result and analysis show that the dispersion energy plays a key role in the long-range attractive potential scattering, and the exchange energy plays an important role in the short-range repulsive potential scattering. The direction of the radial dipole of the Ar-H2 (D2, T2) collision system is turned twice in the range of impact parameters from 0.27 to 0.47 nm.

2015, 64 (13): 133102.
doi: 10.7498/aps.64.133102

Abstract +

The first-principles method has been used to explore how to minimize the over-erase phenomenon in charge trapping memory. Over-erase phenomenon originates from the nitrogen vacancy due to its weak localization of charge on Si atoms. Therefore, we develop a defect model for studying Si3N4 supercells. The defect model consists of an N vacancy and a substitutional atom on the Si site. The substitutional atoms can be C, N, and O atoms, respectively. The Si site belongs to the N vacancy. Then, the Bader charge distribution after program/erase operation, the interaction energy and density of states are calculated for the model so as to analyze the effects of the substitutional atoms on the over-erase phenomenon. The obtained results of the Bader charge distribution show that the substitution of O for the 128th Si can minimize the over-erase phenomenon in Si3N4, and the replacement of the 128th Si by C can also reduce the over-erase phenomenon. However, the model represents a weak localization of charge due to the replacement by C, which is not preferable for charge storage. And the results also reveal that the substitution of N for the 128th Si completely fails to reduce the over-erase phenomenon. With regard to the 162th and 196th Si sites, the substitutions of the three atoms for the two sites cannot minimize the over-erase phenomenon. Furthermore, the analysis of the interaction energies indicates that the combination of each of the three atoms with the N vacancy can form stable clusters on the 128th site in the model. In particular, the attractive interaction between O and N vacancy is the weakest of the three so that the injected charge can temporarily break the stability of the O cluster to rearrange the charge distribution, realizing the localization of charge around the O cluster. And then, the results of the density of states designate that subtitutional O atom at the 128th Si atom site produces a deep-level trap in the band gap, which has a powerful ability to localize the charge. The above results suggest that substitution of O for Si is an excellent solution for the minimization of over-erase phenomenon in Si3N4. This work can provide a method for the minimization of over-erase phenomenon in charge trapping memory and also can be helpful to the improvement of charge retention and optimization of memory window in the charge trapping memory.

2015, 64 (13): 133202.
doi: 10.7498/aps.64.133202

Abstract +

Rydberg atom, with a large principal quantum number n, has big size, long lifetime, strong long-range interactions, and so on. These properties make Rydberg atoms potential candidate of quantum gate and single-photon source. Rydberg electron can interact with nearby ground-state atom, which is polarized by the Rydberg electron and is bound to the orbit of Rydberg electrons forming Rydberg molecule. As the kinetic energy of the Rydberg electron is very low, only the lowest partial waves will contribute to the molecular potential.#br#In this paper, the low electron-atom scattering with the semi-classical approximation is introduced, and the pseudopotential of interaction between Rydberg electron and ground-state atom is used to describe the long-range Rydberg molecular potential. Molecular potential curves for cesium (nS, n=30-60) are plotted according to the results of numerical computation, from which the outermost potential depth De and the equilibrium distance r0 of long-range cesium Rydberg molecule are deduced. Potential curves of cesium Rydberg molecules are consistent with the distribution curves in radial probability densities of cesium Rydberg electrons. Dependences of De and r0 on the principal quantum number n are investigated, this has an important role for the experimental measurements. The size of a Rydberg molecule depends on the equilibrium distance r0 and is proportional to the square of effective principal quantum number (n-δ )2. The calculated outermost potential depth De of Rydberg molecule becomes smaller with the increase of principal quantum number n. Rydberg molecule is very sensitive to the external field and can be used to measure and monitor weak signals.

2015, 64 (13): 133203.
doi: 10.7498/aps.64.133203

Abstract +

In the present paper, we investigate the quantum control of the XUV photoabsorption spectrum of helium atoms via the carrier-envelope-phase (CEP) of an infrared (IR) laser pulse by numerically solving the time-dependent one-dimensional (1D) two-electron Schrödinger equation. The advantage of the 1D model is that the associated time-dependent Schrodinger equation (TDSE) can be solved numerically with high precision as taking full account of the interaction between the electrons and without making any assumptions about the dominant physical mechanisms. In our study, a single attosecond XUV pulse with broad bandwidth is used to create a wave packet consisting of several doubly-excited states. Helium atoms subjected to the XUV pulse can be ionized through two different pathways: either direct ionization into the continuum or indirect ionization via the autoionization of doubly excited states. The interference of these two paths gives rise to the well-known Fano line shape in the photoabsorption spectrum, which is determined by the ratio and relative phases of the two paths. In the presence of an IR laser pulse, however, we find that the Fano line profiles are strongly modified, in good agreement with recent experimental observations [C. Ott et al., Science 340, 716 (2013); C. Ott et al., Nature 516, 374 (2014)]. At certain time delays, we can observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections, indicating that the XUV light can be amplified during the interaction with atoms. We fit the absorption spectra with the Fano line profiles giving rise to the CEP-dependent Fano q parameters, which are modulated from extremely large positive value to extremely large negative value. Since the q parameter is proportional to the ratio between the dipole matrix of the indirect ionization path and the dipole matrix of the direct ionization path; these results indicate that the quantum interference between the two ionization paths can be efficiently controlled by the CEP of an ultrashort laser pulse, thus offering another possibility (in addition to the laser intensity and the time delay between the XUV pulse and the IR laser) of manipulating the extreme ultrafast electronic motion in atoms. Our predictions can be experimentally verified easily with the present experimental technique.

###### ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2015, 64 (13): 134201.
doi: 10.7498/aps.64.134201

Abstract +

Based on the generalized Huygens theory and the unified theory of coherence and polarization, study is made on the module value of complex degree of coherence of partially coherent radially polarized beam (PCRPB) which changes with transmission distance at different reference point. Results show that the module value distribution of the complex degree changing with the transmission distance is different for different reference point while PCRPB propagates in a free space. When the reference point is at the origin, with the increase of the transmission distance, μxx and μxy hold a symmetric distribution, and the distribution range increases. When the reference point is confined on the x-axis, μxx changes from single peak to three peaks, and the two valleys lie symmetrically; and μxy changes from two peaks to four peaks, and the four valleys lie symmetrically. The transmission distance of the evolution is related to the distance between the reference point and the origin: the closer the distance between the reference point and the origin, the shorter the transmission distance is needed to achieve the evolution process. When the reference point lies on the y-axis, μxx holds a symmetric distribution, its distribution range increases, and μxy changes from two peak values to four peaks and four valleys which are in symmetric form. The transmission distance is related to the spacing between the reference point and the origin, the closer the distance between the reference point and the origin: the shorter the transmission is needed to achieve the evolution process. In addition, when the reference point lies at other positions on the observation plane, the module value distribution of μxx and μxy is obtained by combining the distribution rules of reference point at x-axis and y-axis: i.e., when the reference point lies at other positions of the observation plane, the module values μxx and μxy can be composed of three peaks and four peaks with the increase of transmission distance, respectively.

2015, 64 (13): 134202.
doi: 10.7498/aps.64.134202

Abstract +

As a major branch and research focus in the image processing field, the study on image restoration is always of practical significance and application value. Blind image restoration has always been one of the most difficult problems in image restoration. In view of the image motion blurring induced by the relative motion between the camera and the subject, we present a blind image restoration method based on guided filter. We first estimate the point spread function by using the iteration algorithm in the frequency domain. And then, because the guided filter has the edge-preserving smoothing property, we restore the target image by the non-blind image restoration algorithm based on the guided filter. By iterating the above two steps, we can obtain the original clear image. In order to verify the effectiveness of the proposed algorithm, we give several groups of experiments. Experimental results show that the proposed algorithm can not only effectively eliminate the noise and suppress the ringing, but also well preserve the edge and texture details. Therefore, the proposed algorithm can restore the image with high quality.

2015, 64 (13): 134204.
doi: 10.7498/aps.64.134204

Abstract +

A scheme of two-dimensional atomic localization of the Λ-type quasi-four-level atoms based on quantum-coherent-controlled absorption is proposed. Using the perturbation theory of the density matrix, the filter function is derived for the position probability distribution of atoms, which is determined by the imaginary part of the optical susceptibility. Because of the space-dependent interaction between atoms and fields, the position information is contained in the filter function, which provides an approach to explore the spacial position probability distribution of a single atom. Effect of the initial state of the atom under coherent control on the atomic localization is analyzed. It is found that the atomic localization is related to the initial atom distribution and the dipole moment between two lower levels under the coherent field control. When probing field and coupling field are under the configuration of the electromagnetically induced transparency, the position of atoms can be localized in the domain of sub wavelength; when the electromagnetically induced transparency is not satisfied, an atom can be measured in a sub wave region with the probability of 100% by changing the traveling wave amplitude in the controlling field and the detuning in the probing field.

## EDITOR'S SUGGESTION

2015, 64 (13): 134205.
doi: 10.7498/aps.64.134205

Abstract +

The two-photon spectroscopy of 1S-2S transition in atomic hydrogen needs a narrow linewidth laser at the wavelength of 243 nm. In order to reduce the linewidth to several tens hertz level, a free operation CW ECDL 972 nm laser has been locked to a high fineness ultralow expansion reference cavity by using the Pound-Drever-Hall technique. And the part of 972 nm laser output is set into the tapered amplifier and the two enhanced doubling frequency stages to obtain the output of purple light at 243 nm. It is estimated that such a narrow linewidth laser system at 243 nm can be used well in the detection of the 1S-2S transition of hydrogen.

2015, 64 (13): 134206.
doi: 10.7498/aps.64.134206

Abstract +

Optical fiber communication systems are going to adopt the use of advanced modulation formats. It is thus important to develop measurement techniques and solutions capable of quantifying such signals. Linear optical sampling is an effective technique to characterize the quality of an advanced modulation format for high-speed optical signal with high fidelity, while the passively mode-locked fiber laser is an enabling module to implement the linear optical sampling. In this paper, we obtain a trade-off relationship between the repetition rate of passively mode-locked fiber laser and the linewidth of high-speed signal under test, after the introduction of operation principle for linear optical sampling. It is found that, for the quadrature phase shift keying (QPSK) signal, when the ratio of the linewidth of the signal under test to the repetition rate of passively mode-locked fiber laser is less than 1.5×10-3, the linear optical sampling-induced impairments can be ignored when there occurs phase noise. Therefore, the phase estimation can be successfully made by using the Viterbi-Viterbi algorithm applied to the block of samples corresponding to the modulation format phase states. Next, we use an optical sampling pulse with a repetition rate of 95.984 MHz, and carry out the optical linear sampling to a 28 Gbaud QPSK signal with a linewidth of 100 kHz. The error vector magnitude (EVM) has long been a commonly used parameter for quantifying the quality of advanced modulation signals. Using the standard coherent detection algorithm, we can successfully recover the constellation with the error vector magnitude (EVM) error less than 1%. Theoretical investigations agree well with the experimental characterization. Such a conclusion is helpful to optimize the design of passively mode-locked fiber laser for optical sampling application.

2015, 64 (13): 134501.
doi: 10.7498/aps.64.134501

Abstract +

For a holonomic system in relative motion, the conformal invariance and the conserved quantity of Mei symmetry with Appell equations are investigated. First, by using the infinitesimal one-parameter transformation group and the infinitesimal generator vector, the definitions of Mei symmetry and the conformal invariance with Appell equations in a holonomic system in relative motion are given, and the determining equations of the conformal invariance of Mei symmetry for the system are derived. Relationship between the conformal invariance and Mei symmetry for the system is mainly studied. Then, by means of the structural equation which the gauge function satisfies, the expression of Mei conserved quantity deduced from Mei symmetry for the system is obtained. Finally, an example is given to illustrate the application of the result.

2015, 64 (13): 134601.
doi: 10.7498/aps.64.134601

Abstract +

Four kinds of unpoled lead zirconate titanate (PZT95/5) ferroelectric ceramics were fabricated in a range of different porosity levels by systematic additions of added pore formers. By using the non-contact digital image correlation (DIC) optical technique to measure the full-field strain, the response of unpoled PZT95/5 ferroelectric ceramics to statically applied uniaxial stresses was investigated. The influences of porosities on the mechanical behavior, domain switching, and phase transformation of the porous unpoled PZT95/5 ferroelectric ceramics were explored. All the measured stress versus strain curves for the tested porous unpoled PZT95/5 ferroelectric ceramic samples can be divided into three stages: the initial linear elastic region, the approximate plateau region, and the second linear elastic region, similar to the behavior of foam or honeycomb materials. However, the deformation mechanism of porous unpoled PZT95/5 ferroelectric ceramics should be attributed to the domain switching and phase transformation processes, but not related to the collapse of voids. With the increase of porosity, the elastic modulus, fracture strength and fracture strain of the porous unpoled PZT95/5 ferroelectric ceramics would decrease. Effect of dispersed voids does not improve plasticity of the porous unpoled PZT95/5 ferroelectric ceramics, which is mainly attributed to no effect of the pores on the obstacle and proliferation of crack propagation during the axial splitting failure processes. Critical stresses of the domain switching and phase transformation decrease linearly with increasing porosity. The macroscopic critical volumetric strain needed for phase transformation is independent of the porosity in the unpoled PZT95/5 ferroelectric ceramics.

2015, 64 (13): 134703.
doi: 10.7498/aps.64.134703

Abstract +

HiWay (or channel) fracturing has been a new technology for development of unconventional oil and gas resources in recent years. It has been carried out more than 4000 times worldwide, and obtained good performance in oil and gas recovery. HiWay fracturing improves the flow conductivity of fractures by constructing inhomogeneous distributions of proppant and stable, open flow channel in hydraulic fractures. However, the mechanism and impact factors of high flow conductivity of HiWay fractures are not very clear. To the best of our knowledge, there are no relevant research reports available for such analysis. In this paper, it is first assumed that the fluid flow in proppant clusters follows the Darcy's law and the flow in the channels with proppant clusters is laminar viscous flow, which can be described using Stokes equation. However, the coupling of Darcy-Stokes equations is difficult, and some untrivial interface conditions at the interface between the porous and free-flow regions should be introduced, this will increase greater complexity in numerical computation. As an alternative approach, the Darcy-Brinkman equation is often used for this coupling flow problem, which provides a unified equation with continuous variable coefficients in the two different flow regions. Therefore, there is not necessary to introduce specific interface conditions any more. In this work, we first applied the Darcy-Brinkman equation to model the fluid flow in hydraulic fractures, and then the upscaling of Darcy-Brinkman equation is conducted to evaluate the equivalent permeability of a fracture by using homogenization theory and finite element numerical simulation. Finally, various impact factors of flow conductivity of a hydraulic fracture, such as the cluster shape, cluster distribution, cluster size, etc., are analyzed based on the equivalent permeability. Results show that the permeability of a hydraulic fracture is considerably greater than thst of proppant cluster when the free-flow region is well connected in the fracture, and the geometric properties of proppant clusters are also the key influencing factors for the flow conductivity. Therefore, in HiWay fracturing process, how to construct the well-connected free-flow region in hydraulic fractures is most important, and the flow conductivity of proppant cluster is not the keypoint. However, the surface roughness and stress sensitivity of the hydraulic fractures have not been considered in this work, it will be considered in the future work.

2015, 64 (13): 134705.
doi: 10.7498/aps.64.134705

Abstract +

Self-propellant Janus microsphere is a special class of active particles with a regular shape and irregular surface characteristic. With the self-propulsion of 2 μm diameter Pt-SiO2 Janus microsphere near the wall, we have measured the relationship of self-propellant velocity VJanus versus the observed time Δtobs. A diffusiophoretic force-dominated motion, which can be deemed as a quasi-1 D motion with the characteristics of both force free and torque free, is distinguished from the entire motion process. At the same time, it is also observed that the Janus microsphere is deflected about the vertical direction with an angle ψ. The deflection angle ψ is found to decrease with the increase of H2O2 concentration in the solution. For the 2.5%-10% H2O2 solution in this experiment, the angle ψ ranges from 20° to 7° approximately. A numerical model, involving viscous force, diffusiophoretic force and the effective gravity, is created with a reference frame, this quasi-1 D self-propellant motion can be solved to satisfy the conditions of the force and torque balance simultaneously. We have studied the changes of angle ψ and separation distance δ of the microsphere from the substrate under different conditions, including the concentrations of H2O2 solution, the material density, and the diameter of the microsphere. For the self-propulsion velocity VJanus and the deflection angle ψ, numerical results show good agreement with the published experimental observation results. Moreover, it is found that the lower density or the smaller diameter of the microsphere will generate the smaller distance δ, while the higher concentration of H2O2 in the solution will result in a larger distance δ. The predicted δ is 2-8 μm. With the obtained data, we further discuss the effect of near wall on the characteristic time τR of rotational diffusion of the Janus microsphere. Because the predicted values of δ are relative high, the near wall effect can be neglected, indicating that this effect should not be a significant factor to cause a big discrepancy of τR in different references. The present work will be beneficial to the understanding of the mechanism of self-propulsion and the development in its potential applications.

2015, 64 (13): 134203.
doi: 10.7498/aps.64.134203

Abstract +

Coherent population beating (CPB) phenomenon occurs in a typical three-level system. When the frequency difference between two coherent pumping laser fields has a certain detuning from the ground-state hyperfine splitting, the excited state population will experience a transient oscillation before reaching equilibrium, and the oscillation frequency is equal to the detuning. The CPB phenomenon enables us to directly obtain the beat frequency between the measured radio frequency (RF) signal and the atomic transition frequency. Then we can get the standard frequency by compensating the beat frequency to the RF. We propose a scheme to implement atomic clock based on the CPB phenomenon in 2009, and the scheme has been implemented. When this effect is used to achieve an atomic clock, the frequency stability is directly related to the amplitude and SNR (signal to noise ratio) of the CPB signal. Influence of the ground-state hyperfine sublevels' coherence on CPB signal is theoretically simulated and experimentally investigated in this paper. A formula of the CPB signal is derived by using the semi-classical model of the interaction of atoms with light, and the theoretical simulation is done using the formula obtained. In the experiment two coherent pumping laser fields are used to interact with 87Rb atoms. A CPB process includes the coherence build-up and the CPB stimulation. The coherence of the ground-state hyperfine sublevels is achieved by controlling the pumping time of the coherent laser fields that are resonant to the ground-state hyperfine sublevels. With this method, the relationship between CPB signal and coherence of the ground-state hyperfine sublevels can be observed. Result shows that the amplitude of CPB signal is proportional to the ground-state hyperfine sublevels' coherence. The hign quality CPB signal can be achieved when the CPB stimulation is started with a pure coherent population trapping (CPT) state. In the CPB process, the coherence build-up rate is approximately equal to the coherence decay rate. So a 50% duty cycle square wave can be used to modulate the RF, and the period of the square wave had better be twice of the decay time of the ground-state hyperfine sublevels' coherence. To improve the SNR of CPB signal and the stability of atomic frequency standard, the ground-state hyperfine sublevels' coherence must be built up, improved, and maintained before the CPB stimulation. The feasibility of applying CPB phenomenon to the weak magnetic field measurement and other applications is also discussed in this paper.

2015, 64 (13): 134207.
doi: 10.7498/aps.64.134207

Abstract +

No reports of measuring the focusing anisoplanatism of laser guide star were mentioned before because the guide star instantaneous wavefront measurement technique by using sodium is not mature. As directly verifying the confidence of artificial sodium guide star which takes the place of the natural star to become the reference of atmosphere adaptive optic correction, in this paper a new method is put forward that the focusing anisoplanatic error of sodium guide star can be measured via synchronous pulse detection if the atmosphere Greenwood time is constant. Results of correlation analysis show that it is feasible to establish an experimental system for the measurement. A high-precision wavelength and linewidth sodium guide star laser and a 1.3 m caliber telescope underly the experimental system for the technology research on the focus anisoplanatism. Experiment of detecting sodium guide star has been carried out. And synchronous detection about the returning light matrix of visible light of sodium and the natural star is achieved when both are in a coincident imaging path. In the experiment, the recovered single frame wavefront of sodium guide star is acquired, and experimental data analysis is carried out in several aspects: contrasting the coherence of the two types of wavefront images, comparing the Zernike coefficients, and simulating the imaging calibration. Results show that the imaging contrast in the light matrix of the plused sodium guide star is poor because of the limited experimental condition so that the precision of the recovered wavefront is restricted, but the two groups of wavefronts the sodium guide star wavefront and the natural star wavefront always keep nice comparability, which we can see from the recoverd synchronous wavefront images. And the Zernike coefficients of both are close in the first 36 orders, which we deduce from the Zernike anisoplanatic error curves. Simulation result shows that the acquired sodium wavefront data would contribute to calibrate the far-field images of natural star in astronomical observation activities. And the measured results, below 0.1 rad2, while wiping off the wavefront recovering error are close to the theoretical result. Result shows also that the low SNR of sub aperture imaging restricts the wavefront recovery precision to some extent, but the effective atmospheric wavefront distorted information has been obtained by sodium guide star experiment and it can help to correct the natural star images.

2015, 64 (13): 134301.
doi: 10.7498/aps.64.134301

Abstract +

Statistical energy analysis (SEA) is widely used in predicting dynamic response of complex coupled systems. This paper studies the bending wave propagation in periodic rib-stiffened plates in the framework of SEA. Effect of frequency band gap property of the rib-stiffened plate and wave filtering characteristics of the stiffened ribs on the prediction results of SEA is analyzed by using the wave approach and Bloch theory. It is found that due to the fact that classic SEA ignores an energy “tunneling mechanism” between subsystems that are not physically connected, large error up to almost 40 dB is generated in the subsystems of the plate compared with the results calculated from the finite element method. This tunneling mechanism mainly results from the wave filtering effects caused by the periodic arrangement of the ribs and it plays a significant role on the subsystem response at high frequencies. However, this is not incorporated in the modelling of classic SEA thus large errors can occur. To solve this problem, an advanced statistical energy analysis (ASEA) is used to consider the transition, transmission and transport of energy between unconnected subsystems. ASEA divides the energy of each subsystem into two parts: available energy which is the modal energy that could transmit into connected subsystems, and unavailable energy that dissipates within the subsystem; therefore the energy cannot propagate further away. Then the ray tracing algorithm is used to track the power flow across subsystems. By using ASEA, the accuracy of the prediction results can be greatly improved so that the error is reduced to less than 5 dB in most frequency bands. An experimental set-up is also designed to support the plate by simulating the simply-supported boundary conditions along the edges. The test results agree well with the finite element method, and it is sufficient to validate the theoretical models.

2015, 64 (13): 134302.
doi: 10.7498/aps.64.134302

Abstract +

According to the thermoelastic equations and heat conduction equations of laser-generated acoustic surface wave (SAW), the SAW propagation images are obtained by taking finite element solutions to the equations. When the SAW propagates through the aluminum plate with the near-surface defects at different depths, producing an oscillating effect between the surface acoustic wave and the near-surface defects, and then there is a gradual decrease after the gradual decrease of the oscillation waves as getting through the near-surface defect. When the SAW propagates through the aluminum plate with the near-surface defect at different depth, there is a certain variation of the center frequency of the oscillation waves. Simulation results show that, when the depth of near-surface defect changes from 0.1 to 0.5 mm, the center frequency of the oscillation signal generated by the oscillation effect changes from 0.4 to 0.76 MHz, and the center frequency of the oscillation signal is seriously attenuated with increasing width of the near-surface defect, thus providing a theoretical basis for quantitative detection of the near-surface defects.

2015, 64 (13): 134303.
doi: 10.7498/aps.64.134303

Abstract +

As a newly-developed method, acoustic cloak made of pentamode materials is on its speedway to the promising potential application. However, physical fabrication of pentamode cloak with continuously varying material parameters can be a tough work, if not totally impossible. Layering is a natural compromise to bypass this quandary. Researches on layering effects of inertial cloak are ample. However, researches on layering pentamode acoustic cloak are relatively limited. Among these researches Scandrett extends the effective bandwidth through optimization of material parameters[2010 J. Acoust. Soc. Am. 127 2856, 2011 Wave Motion. 48 505].#br#The present work concerns the layering effects of pentamode acoustic cloak. By comparing with precedent results, the present paper has two major innovations: Firstly, cylinder is chosen to be the basic geometry. This is of obvious advantage since cylinder is the basic geometry of acoustic cloak's important potential host. Secondly, effects of layers' number and thickness distribution on the stealth effect are analyzed. The two are key parameters to be determined in the layering process. This paper is organized as follows: Firstly, analytical expression of the scattering pressure field of layered cloak is deduced by means of variables separation. In this process Fourier expansion plays a key role. And the harmonic assumption of the incident acoustic wave is made. Secondly, typical cases are calculated to verify the validation of the theoretical analysis. First let material parameters tend towards that of water, and compare the scattering field with that of the bare rigid object when the cloak is replaced by water. Second let the layering number goes to infinity, and compare the scattering field with that of the continuous cloak. Phenomena conforming with basic physical laws are observed. And validity of the theory and codes is confirmed. Thirdly, effects of layers' number and thickness distribution on the stealth character are theoretically and numerically analyzed. One can easily see from the computational results that a critical number N exists. When layers' number exceeds N, improvement of the stealth effect becomes less efficient by further adding layers' number. One can also see from the computational results that a wise distributional strategy that helps improve the stealth effect indeed exists. And the optimization iteration can be utilized to further improve it.#br#As a summary, the present paper concerns the layering process of cloaking. Qualitatively and quantitatively, several significant results are obtained. This paper offers a useful reference for future fabrication of realistic acoustic pentamode cloak.

2015, 64 (13): 134502.
doi: 10.7498/aps.64.134502

Abstract +

Granular material is a kind of soft condensed matter, which gathers up a large number of particles, and the relation between its microstructure and macroscopic mechanical properties is very complex. In this paper, the lateral stress distribution of the two-dimensional vertically stacked lattice of granular material under a pressure in the vertical direction has been investigated experimentally. The steering behavior of the vertical pressure in a granular system is discussed and analyzed in detail based on the experimental results. Results show that in the process of slow compression, the vertical pressure increases slowly in a nonlinear form at first and gradually transforms into a linear increase. This phenomenon corresponds to the dynamic processes of friction-slip-extrusion. This kind of behavior is more significant in the particle system of the same size. In the initial stage of pressing, the vertical force of the stepping motor is mainly used to overcome the friction between the particles and the sliding friction between the particle and the wall. As the friction in the granular system is related to the geometry of the particulate deposits, the material of particles, the roughness of the wall surface, and other relevant factors, the front-end of vertical pressure displays nonlinear characteristics. Continuing the squeeze and push forward, a force chain is formed among particles through self-organization. The vertical force is mainly used to overcome the elastic pressing force between the particles and the force to the wall, so later on the vertical pressure performs linear growth. For the system of particles with an established packed structure, the vertical pressure applied in the vertical direction steers along the force chain between the particles, and the value of horizontal pressure is different at different stacking heights. That is, the pressure in the middle is greater than that at the top and the bottom. The saturated value of steering coefficient k decreases with the stacking angle θ. As the stacking angle increases, the vertical component of the stress becomes more pronounced than its horizontal component. The expression of steering coefficients against stacking angle has been obtained through careful analysis of the geometrical structure and the force distribution of the granular pile, and the theoretical value fit well with the experimental results.

2015, 64 (13): 134503.
doi: 10.7498/aps.64.134503

Abstract +

For a vertically vibrating column filled with binary mixtures consisting of big copper beads and small glass beads, the phenomenon of periodic segregation (PS) is observed experimentally, in which distinct segregation patterns of Brazil nut effect (BNE), reversed Brazil nut (RBN) and sandwich (SW) are emerged successively under a certain vibration condition. The periodic time increases with increasing vibration frequency or decreasing acceleration, and the SW pattern holds 90% duration of a cycle. Since the three segregation patterns emerging sequentially in a cycle are all well defined, the energy dissipation power for each segregation pattern is measured under the same vibration condition. It is found that the dissipation power is the largest in RBN pattern and the smallest in BNE pattern during a cycle. Moreover, in the periodic segregation region the same patterns (BNE, RBN or SW) emerging at different vibration accelerations have almost the same dissipation power within the experimental error. Based on the viewpoint of competition between condensation and percolation from Hong, the periodic segregation phenomenon can be explained qualitatively by combining with our measurements of energy dissipation power.

2015, 64 (13): 134701.
doi: 10.7498/aps.64.134701

Abstract +

Precondition for simulating low-speed turbulence is studied in this paper. Against the stiffness of the time-dependent scheme applied to low-speed turbulence, the precondition based on conservative variables is developed, which adopts an implicit iterative method for solving main control equations coupled with turbulence transport equations. In order to ensure the iterative solution stable, a reference Mach number, the dual-time stepping no-matrix scheme, and the method for processing implicitly the source terms of turbulence equations etc. have been developed reasonably, making the software platform unified for all-speed turbulence. Reference Mach number is defined in terms of global and local velocity by a single parameter, and the parameter can be used to control stability, numerical result accuracy, and switch of the precondition. The dual-time stepping LU-SGS method based on conservative variable precondition is developed, realizing no-matrix iterative solution for unsteady flow problems. Against the stiffness in solving the main control equations coupled with turbulence transport equations, the dissipation term of the turbulence equations is processed implicitly, which can enhance main diagonal dominance of the turbulence equations and make the iteration with greater stability. In simulating the turbulence in a nozzle and around a square cylinder or an airfoil, the precondition depicts correctly the structural character of the flowfield; and the computational results are in good agreement with those of theory and experiment etc., and its iterative convergence and numerical accuracy is excellent. It is shown that the precondition in this paper for low-speed turbulence is very effective.

2015, 64 (13): 134702.
doi: 10.7498/aps.64.134702

Abstract +

In this paper, we present a modified smoothed particle hydrodynamics (SPH) method. SPH is a Lagrangian meshfree particle method, and it is attractive in dealing with free surfaces, moving interfaces, and deformable boundaries. The improved SPH method modifies the kernel gradient in the traditional SPH method with a new kernel function and a modified SPH discrete form. Use of improved smoothed particle hydrodynamics is made to carry out numerical analysis on micro liquid drop oscillation process. The study focuses on the relation between the micro liquid drop oscillation damping and the oscillating period and amplitude in different aspect ratio and Re number. It is shown that for the micro liquid drop oscillation process with aspect ratio λ≤ 4, under the circumstance of constancy of other parameters, the larger the Re number, the more intense the change of liquid drop's shapes, the weaker the damping effect, and the longer the period of liquid drop's oscillation. Under the circumstance of constancy of Re number, as the initial aspect ratio of liquid drop increases, the amplitude of liquid drop oscillation is stronger, and the period of liquid drop's oscillation is longer.

2015, 64 (13): 134704.
doi: 10.7498/aps.64.134704

Abstract +

Droplets impact on surfaces exist widely in industrial equipments, such as spraying cooling, ink jet printing, oil drops impact on walls in combustion chamber, brine droplets impact on heat transfer tubes in horizontal-tube falling film evaporators etc. In particular, for the droplets impinging on heated surfaces, the contact scale and the heat transfer flux affect the cooling of the hot surfaces greatly. In this work, evaporation processes of water and ethanol droplets impact on a heated surface are observed using a high-speed digital camera with a capacity of 106 frames per second. The corresponding evaporation parameters including the contact diameter, the droplet height, the contact angle, and heat flux are analyzed. The initial liquid temperature keeps constant at 20 ℃, and the initial surface temperature varies in the range of 68-126 ℃. Diameters of single water droplets and ethanol droplets are 2.07 and 1.64 mm, respectively. The impact Weber number of water droplets ranges from 2 to 44 while that of ethanol droplets ranges from 3 to 88. The present results show that due to the coupled effects of gravity, surface tension, fluid flow and evaporation processes, the height of water droplets reduces continuously while the contact diameter almost does not change during the most part of evaporation time. In the later stage of evaporation, the contact diameter, height and contact angle of water droplets oscillate, mainly because of droplet retraction. The critical contact angle for water droplets retraction is in the range of 4-8. The contact angle of ethanol droplets first reduces and then remains constant, while the contact diameter and the height decrease continuously. The droplet evaporation time depends on liquid properties and the surface temperature, and the Weber number effect is minor. The evaporation time decreases with the increase in the surface temperature. At the same time, with increasing surface temperature, the ratio between the sensible heat and the total heat increases, and this part of heat cannot be neglected from the total heat transfer calculation. Based on the present experimental conditions, the average heat flux for the water droplets ranges from 0.014 to 0.110 Wmm-2 in this work.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2015, 64 (13): 136104.
doi: 10.7498/aps.64.136104

Abstract +

Ionizing-radiation-induced oxide-trapped charges and interface states cause the current and 1/f noise degradation in bipolar junction transistors. In order to better understand these two degradation mechanisms and develop hardening approaches for a specific process technology, it is necessary to measure the effect of each mechanism separately. In recent years, several techniques have been developed, but no charge-separation approach based on 1/f noise for NPN bipolar junction transistors is available. In this paper, the effects of ionizing-radiation-induced oxide trapped charges and interface states on base current and 1/f noise in NPN bipolar junction transistors are studied in detail. Firstly, a new model of base surface current of NPN bipolar junction transistors is presented with some approximations, based on an available model for the base surface current under certain conditions; this model can identify the physical mechanism responsible for the current degradation. Secondly, combining the theory of carrier number fluctuation and the new model of base surface current another model is developed which can well explain the 1/f noise degradation. This model suggests that the induced oxide-trapped charges would make more carriers, involving the dynamic trapping-detrapping, which leads to the 1/f noise to increase; and the induced oxide-trapped charges and interface states can also bring about an increase in base surface current which can also cause the l/f noise increase. These two models suggest that the current and1/f noise degradations can be attributed to the same physical origin, and these two kinds of degradations are the result of accumulation of oxide-trapped charges and interface states. According to these two models, simple approaches for quantifying the effects of oxide-trapped charges and interface states are proposed. The base surface current can be extracted from the base current using the available method. The oxide-trapped charge density is estimated using the amplitude of 1/f noise (10-100 Hz) and the base surface current. Given the estimated oxide-trapped charge density, the interface state density can be estimated using the base surface current. These methods are simple to implement and can provide insight into the mechanisms and magnitudes of the radiation-induced damage in NPN bipolar junction transistors.

2015, 64 (13): 136401.
doi: 10.7498/aps.64.136401

Abstract +

Due to their unusual electrical conductivity, carbon nanotubes as the ideal candidates for making future electronic components have extensive application potentiality. In order to meet the requirements in space electronic components for carbon nanotubes, effect of 170 keV proton irradiation on structure and electrical conductivity of multi-walled carbon nanotubes (MWCNTs) film is investigated in this paper. Surface morphologies and microstructure of the carbon nanotube films are examined by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy, respectively. Electrical conductivities of the carbon nanotube films before and after 170 keV proton irradiation are measured using four-point probe technique. SEM analysis reveals that when proton irradiation fluence is greater than 51015 p/cm2, the surface of the carbon nanotube film becomes rough and loose, and obvious bending, shrinkage, and entanglement of nanotubes are observed. Moreover, the shrinkage phenomenon of MWCNTs caused by proton irradiation is found the first time so far as we know. Based on Raman and XPS analyses, it is confirmed that 170 keV protons can improve the ordered structure of the MWCNTs, and irradiation fluence plays a key role in reducing the disorder in the MWCNTs. Improvement of the irradiated MWCNTs by 170 keV protons can be attributed to restructuring of defect sites induced by knock-on atom displacements. On the other hand, carbon impurities on surface of the MWCNT film are reduced due to the effect of sputtering by the 170 keV proton irradiation, which is also helpful to the improvement of the structure of carbon nanotubes. EPR spectra show that the electrons delocalized over carbon nanotubes decrease with increasing irradiation fluence, implying that the carbon nanotube film is not sensitive to ionizing radiation induced by the 170 keV protons, and the electrical conductivities of the MWCNTs films may be decreased. Four-point probe technical analysis shows that with increasing irradiation fluence, electrical properties of the carbon nanotubes film deteriorate, which can be attributed to the changes in electronic properties and morphology of the MWCNT films induced by 170 keV protons. Acquired results could be beneficial to tailoring of structure and properties for the carbon nanotubes film irradiated by protons to develop nanoelectronics of radiation-resistant systems.

2015, 64 (13): 136402.
doi: 10.7498/aps.64.136402

Abstract +

This paper investigates the permeability of microcracked porous solids incorporating random crack networks in terms of continuum percolation theory. Main factors of permeability include the geometry of crack networks, permeability of porous matrix, and crack opening. For the two-dimensional random crack networks, a new connectivity factor is defined to take into consideration the spanning cluster of cracks, fractal dimension of networks, and the size of a finite domain. For an infinite domain, the connectivity factor around a percolation threshold observes the scaling law, so this definition of connectivity is proved to be consistent with the percolation concepts. Geometric analysis reveals that the local clustering will not necessarily contribute to the global connectivity of networks. It is also found that too strong a local clustering of cracks will decrease the probability of the global percolation, and this adverse aspect of the local clustering effect has never been reported in the literature. The percolation threshold changes with the crack pattern of networks and the scaling exponents of percolation are not constant but depend on the fractal dimension of the crack networks. On the basis of connectivity and tortuosity of crack networks, the scaling law for permeability is established, K=K0(Km,b)(-c), taking into consideration the geometris characteristics through (-c), the permeability of porous matrix Km, and the crack opening aperture b. Then the permeability of a solid incorporating random crack networks is solved by finite element methods: all the cracks are idealized as 2-node elements and the matrix is divided into 6-node triangle elements. The fluid is assumed to be incompressible and Newtonian. With these assumptions the effective permeability of numerical samples is evaluated through Darcy's law. The scaling exponents of the permeability obtained numerically are very near to the theoretical values, and the impact of crack opening is less important as the crack density is far below the percolation threshold and the effect of crack opening becomes significant only as the crack density approaches the percolation threshold. Influence of crack opening on the permeability is strongly dependent on the opening aperture of the cracks. Finite element simulation results show that K0 depends on b through a power law near the percolation threshold and this dependence disappears as the ratio between the local permeability of crack and the matrix permeability exceeds 106.

2015, 64 (13): 136403.
doi: 10.7498/aps.64.136403

Abstract +

Multifractal detrended fluctuation analysis (MFDFA) and multifractal detrended moving average (MFDMA) algorithm have been established as two important methods to estimate the multifractal spectrum of the one-dimensional random fractal signals. They have been generalized to deal with two-dimensional and higher-dimensional fractal signals. This paper gives a brief introduction of the two algorithms, and a detail description of the numerical experiments on the one-dimensional time series by using the two methods. By applying the two methods to the series generated from the binomial multiplicative cascades (BMC), we systematically carry out comparative analysis to get the advantages, disadvantages and the applicability of the two algorithms, for the first time so far as we know, from six aspects: the similarities and differences of the algorithm models, the statistical accuracy, the sensitivities of the sample size, the selection of scaling range, the choice of the q-orders, and the calculation amount. For one class of signals, the larger the sample size, the more accurate the estimated multifractal spectrum. Selection of appropriate scaling range affects the statistical accuracy in comparison of the two methods for almost all examples. The presence of scale invariance should be checked by first running the two methods over a large scaling range (e.g., from 10 to (N+1)/11 in this paper) and then plot log10 (Fq (scale)) against log10 (scale). In the MFDFA-m (m is the polynomial order, and in this paper m=1) method, the scaling range can be selected from {m + 2, 10} to N/10, N is the sample size of the time series. In the MFDMA algorithm, the scaling range should be from 10 to (N+1)/11. It is favorable to have an equal spacing between scales and the number of the scales should be larger than 10 and usually be selected from 20 to 40. The q-orders should consist of both positive and negative q's. When |q| = 5, the calculated results will not be sensitive with the increase of Δq from 0.05 to 1. If Δq = 0.1, the calculation error will be relatively small when 0 q|≤ 10. With the increase of |q|, the width of the multifractal spectrum will obviously become wider when 0 q|≤10 and the change will be smaller when |q|≥20. If |q| continues to increase, the local fluctuations will approach zero when the scale is small. The critical steps exist in the calculation of local trends for the MFDFA-m and the running moving average for the MFDMA. If the sample size N is fixed and the scale is relatively small, the runtime of the critical steps of MFDFA-1 will be longer than that of MFDMA. When the scale increases from 4 to N/4, it will be shorter than that of MFDMA. Results provide a valuable reference on how to choose the algorithm between MFDFA and MFDMA, and how to make the schemes of the parameter setting of the two algorithms when dealing with specific signals in practical applications.

2015, 64 (13): 136101.
doi: 10.7498/aps.64.136101

Abstract +

Combined with the microstructure evolution in amorphous alloys under the external load, a fractional order viscoelastic constitutive model is first derived by replacing a Newtonian dashpot in the classical Zener model with the fractional derivative Abel dashpot. Based on the Hertzian theory and the fractional order viscoelastic constitutive model, a relationship between displacement and load (or time) for an instrumental nanoindentation test with a spherical indenter is then proposed. Finally, a series of nanoindentation test data for an Fe-base bulk amorphous alloy are employed to verify the derived model, and its viscoelastic behavior in the apparent elastic region is analyzed in detail. Results show that the fractional order rheological model has higher fitting accuracy than that of the integer order model, and the fitting parameters of the proposed model are more suitable to reflect the effect of the loading rate on the viscoelastic behavior in the alloy studied. Variation of the above-mentioned fitting parameters exhibits a strong correlation with the microstructure evolution during the loading of this Fe-base amorphous alloy.

2015, 64 (13): 136102.
doi: 10.7498/aps.64.136102

Abstract +

Single event transients (SETs) in a 100 series 0.18 m partially- depleted silicon-on-insulator (PDSOI) complementary metal oxide semiconductor (CMOS) inverter chain are studied by using pulsed laser. In this paper, effects of struck transistor type and struck locations on the threshold laser energy and the pulse width of SETs are investigated. Results show that the threshold laser energies at different locations are similar, but the threshold laser energies of n-channel metal-oxide-semiconductor (NMOS) transistors are much smaller than that of p-channel metal-oxide-semiconductor (PMOS) transistors. The SET pulse width of n-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) is 427.5 ps as measured at the output terminal when the 2nd stage is irradiated, and 287.4 ps when the 100th stage is irradiated; the SET pulse width of p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET) is 295.9 ps as measured at the output terminal when the 1st stage is irradiated, and 150.5 ps when the 99th stage is irradiated. Both broadening rates are about 1.4 ps/stage. When the struck locations are close to the output terminal of the chain, the SET pulse is narrowed; however, when the struck nodes are close to the input terminal, the SET pulse is broadened. SET pulses are progressively broadened up when propagating is along inverter chains. A similar broadening rate in neither NMOSFET nor PMOSFET, indicates that the SET pulse broadening effect is caused by propagation, independent of the type of struck transistors. Through analysis, the charge of floating body-induced threshold voltage hysteresis in PDSOI transistors is the main cause of pulse broadening. The positive SET pulse observed on the oscilloscope, contrary to the expectation, is due to charging and discharging of the output node capacitor. Also, the observed sub-rail-to-rail swings of the SET pulses are due to the voltage division between the internal resistance of the oscilloscope and the resistance of the PMOS transistor.

2015, 64 (13): 136103.
doi: 10.7498/aps.64.136103

Abstract +

With the rapid development of the space technology, operational amplifier is widely used as the basic liner circuit in a satellite system. There are many charged particles trapped in the earth's magnetosphere, most of the particles are protons and electrons. In BJTs, the damage caused by electrons causes both bulk recombination and surface recombination to increase and subsequently current gain to decrease. Transistor gain degradation is the primary cause of parametric shifts and functional failures in linear bipolar circuits. The severity of electron radiation response correlates with electron's energy and flux, therefore it is important to understand the electron radiation response in different conditions. In this paper, the tested devices used in this study are NPN-input bipolar operational amplifiers commercial-off-the-shelf (COTS) manufactured by Texas Instruments (TI). NPN-input bipolar operational amplifiers LM108 are irradiated with different energy and different beam current electrons respectively under different bias conditions to study the electron radiation damage effect. Experiment using 60Coγ-ray radiation is conducted to compare the different radiation damages between 60Coγ-ray and electron radiation. The total radiation experiments are carried out with the 60Coγ-ray source (Xinjiang Technical Institute of physics and chemistry). The radiation dose rates for the test samples are 1 Gy (Si)/s, and the total accumulated dose is 1000 Gy (Si). Subsequently, room temperature and high temperature annealings are conducted to analyze the parametric failure mechanism of LM108 caused by a total dose radiation for different biases. Result shows that 0.32 Gy(Si)/s beam current electrons can induce more damage than that caused by 1.53 Gy(Si)/s electrons with the same energy; 1.8 MeV electrons can induce more damages than 1 MeV electrons with the same electron beam current because the former produces more displacement damage than the latter. Comparison between zero and forward biased devices shows that different biased devices have different radiation sensibility, and radiation produces more damages in zero biased devices than in forward biased devices with the same electron energy and beam current. This is because forward biased BJT will suppress the edge electric field, thus leading to the decrease of oxide-trapped charge and interface-trapped charge. During high-temperature annealing, degradation of the devices obviously can be recovered and almost return to the initial value finally. This result indicates that the 1.8 MeV and 1 MeV electron radiation mainly induces ionization damage in bipolar operational amplifier LM108.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2015, 64 (13): 137101.
doi: 10.7498/aps.64.137101

Abstract +

Defects and weak bonds generated in the fabricating process of amorphous InGaZnO(a-IGZO) films distribute non-uniformly in the band gap of the a-IGZO film in the form of traps. These traps would capture the charges induced by gate voltage, and affect the linear region mobility, channel carrier density and so on, then the electrical properties in the linear region of a-IGZO thin film transistor. The model used is based on the mobility in linear region which is in direct proportion to the ratio of the free charge to the total induced charge in the channel, and then the free charge and the trapped charge are separated. From the ratio of the density of free carriers to that of the trapped, a direct relationship with the derivative of the free charge with respect to surface potential, and the derivative of the trapped charge with respect to surface potential is calculated by bringing in the gate voltage that serves as an intermediate variable between the linear region mobility and the total induced charge. In this way, the free carrier density and the trapped carrier density can be separated by using the transfer characteristic and capacitor-voltage characteristic. Poisson's equation and Gauss theorem are applied to the interface between the channel layer and the insulating layer. In consideration of the non-uniform characteristic between the surface potential and the gate voltage, the relationship between the free carrier density and the surface potential, the trapped carrier density and the surface potential are obtained. Finally, the density of states in the linear region could be gained by differentiating the trapped carrier density with respect to surface potential.

2015, 64 (13): 137201.
doi: 10.7498/aps.64.137201

Abstract +

The influence of spin Hall effect on magnetization dynamics is one of the hottest topics in spintronics. In this paper, the magnetization dynamics driven by the spin Hall effect-induced torque in a ferromagnet /heavy metal bilayer structure has been investigated theoretically. By linearizing the Landau-Lifshitz-Gilbert equation which includes the spin Hall effect torque term, and taking stability analysis, the phase diagrams in the plane defined by the current density and external magnetic field have been obtained. Under the control of the current density and external magnetic field, several magnetic states, such as in-plane stable state, in-plane precession and bistable states can be realized. With the external magnetic field oriented within a certain range, the magnetization reversal and precession can be realized through adjusting the current density. In addition, the dynamic evolutions of these magnetic states are demonstrated by solving the temporal evolutive equations numerically.

2015, 64 (13): 137501.
doi: 10.7498/aps.64.137501

Abstract +

In this paper, the nanocomposites are synthesized by the non-equal precipitation method to study the effect of the metal ions doped in antiferromagnetic matrix on the exchange bias. XRD patterns and SEM images reveal that the as-synthesized CuO nanocomposites have uniform size (～80 nm), and the ferrimagnetic particles MFe2O4 (M=Cu, Ni) are embedded in the antiferromagnetic (AFM) CuO matrix by doping of magnetic metal ions Ni and Fe. And the ferrimagnetic phase MFe2O4 (M=Cu, Ni) is formed through the addition of a small amount of Fe that reacts with Cu and Ni ions. Effects of different doping amount of Ni on exchange bias are different. A small doping amount of Ni can induce magnetic disorder at the interface of both phases, then the spin-glass-like phase may be formed. The spin-glass-like phases enhance the pinning effect on the magnetic moments of ferrimagnetic phase. Meanwhile, during field cooling process the antiferromagnetic phase splits into domains, which are aligned either with cooling field or in the original antiferromagnetic configuration. The domain wall serves as pinning sites for the magnetic moments of ferromagnetic phase, and the exchange bias effect is increased. The AFM NiO grains with high anisotropic energy are generated, this also increases the exchange bias effect when continuous doping of Ni ions. In the process of field cooling (FC), upward shift occurs in all hysteresis loops, which is perpendicular to the exchange bias. As x=0.08 (x is the concentration of Ni) the perpendicular displacement is 3.6%, this behavior also proves that under FC measurements, the spin-glass-like phase can be formed between the antiferromagnetic nanopaticles. It is the magnetic exchange coupling at the interface between the ferrimagnetic phase and the spin-glass-like phase that result in an upward shift in the entire measurement range. The plot of M versus T under zero field cooling (ZFC) and field cooling (FC) indicates that the exchange bias effect in these composites is ascribed to the exchange coupling at the interface between the ferrimagnetic particles and the spin-glass-like phase. With continuous introduction of magnetic Ni ions, the exchange bias field first increases slowly, then at x=0.08 it increases sharply. The existence of AFM NiO with high anisotropic energy and the domain structure in AFM matrix are the causes of the result.

2015, 64 (13): 137601.
doi: 10.7498/aps.64.137601

Abstract +

We have investigated the photocatalysis efficiency and electron transfer of the znic prophyrin/TiO2 composite photocatalyst, a sensitised hybrid porphyrin material, and prepared it successfully by sol-gel method. The UV-visible spectroscopy and electron paramagnetic resonance spectra are used to analyze and characterize the znic prophyrin/TiO2 composite photocatalyst. The proportions 0%, 0.2%, 0.5% and 0.9% (mass ratio) of zinc porphyrin, and the photocatalytic mechanism of TiO2 sensitized by zinc porphyrin are reasonably explained by ultraviolet visible spectroscopy and electron paramagnetic resonance spectroscopy (EPR). Different samples' UV-visible spectra show that the degradation efficiency of methyl orange solution by TiO2 may be improved via adding appropriate amount of zinc porphyrin sensitizer. Prohibitive incorporation of the sensitizer would cause excess of particles in the mixed catalyst, leading to the TiO2 surface covered by the sensitizer, thus affecting the absorption of photons, and the light degradation rate of TiO2 may be lowered, even lower than the pure TiO2. EPR spectra show the excited state of electrons in zinc prophyrin generated by irradiation of light can promote the generation of Ti3+ with strong oxidizing and superoxide radicals when using UV-visible light to irradiate the powder samples, thus effectively enhancing the separation of photogenerated electron-hole pairs, and improving the photocatalytic performance of TiO2.

2015, 64 (13): 137701.
doi: 10.7498/aps.64.137701

Abstract +

To suppress the secondary electron multipactor on dielectric surfaces of a dielectric load accelerator under an electromagnetic field in TM mode, the method of adopting both groove structure and external axial magnetic field is introduced. As the electric field distribution of the TM mode is composed of both normal and tangential components, it is different from that under the condition of dielectric window in HPM. Thus, theoretical analysis and numerical simulation are employed to study the movement of electrons under different conditions: such as dielectric surface shapes, electric field strength, and magnetic field strength etc. Based on the particle-in-cell (PIC) simulation, the collision energy and transmit-duration of secondary electrons in different groove structures and axial magnetic fields are compared with one another. Results show that the magnetic field is useful for suppressing the development of secondary electron on dielectric surface, while it is not very efficient under high electric field strength. The method of introducing groove structure and certain axial magnetic field on dielectric surface at the same time is capable of affecting the movement of electrons in electric field of different strength. So it is great helpful in improving the ability of multipactor suppression, which is significant for improving the threshold of breakdown on dielectric surface and the power of cavity. However, a too high or too low magnetic field is not very useful for the suppression of multipactor. Furthermore, employing only one of the two parts of the method is also less effective in suppressing the multipactor.

2015, 64 (13): 137103.
doi: 10.7498/aps.64.137103

Abstract +

In this paper, compressive behavior of electrodeposited nano-crystalline (nc) Ni at various temperatures and strain rates is studied using a low temperature mechanic test system. Plastic deformation mechanisms of nc Ni caused by compression are characterized by the strain rate sensitivity index, the activation volume, and examined by scanning electron microscopy and high resolution transmission electron microscopic analysis. Results show that at low temperatures, the plastic deformation of nc Ni is mainly dominated by grain boundary accommodated dislocations. In other words, during plastic deformation of nc Ni at low temperatures, the intrinsic dislocation at the grain boundary bends up and expands without obstacles to the opposite grain boundary in the inner grain dislocation-free zone, until the occurrence of similar cutting forest-dislocation behavior appearing at opposite grain boundary. Moreover, the residual dislocations in the grain boundary bending out during plastic deformation could increase the strain compatibility and decrease the stress concentration. At room temperature, the plastic deformation mechanism of nc Ni is controlled by the deformation of grain boundary accommodated dislocations and grain slipping/rotating. Based on the above analyses, differences in compressive behavior of nc Ni at various temperatures and strain rates can be revealed by the correlation of deformation mechanisms of grain boundary accommodated dislocations and residual dislocation movement, temperature and defects in nc Ni.

2015, 64 (13): 137104.
doi: 10.7498/aps.64.137104

Abstract +

The p-type porous silicon layer with the aperture about 1.5 microns and hole depth about 15 microns is prepared by electrochemical etching of a p-type monocrystalline silicon wafer with a resistivity 10-15 cm and along [100] orientation in a double-tank cell which consists of the electrolyte (volume ratio HF: DMF=1:2). Silver nanoparticles film with different thickness has been deposited on porous silicon by the electroless deposition for different deposition times. Morphology and microstructure of the silver nanoparticles/porous silicon composite and ere studied by scanning electron microscope and X ray diffracmeter. Result indicates that the silver nanoparticles are uniformly distributed on the surface of porous silicon and the deposition time has an important influence on the morphology of the composite. The gas-sensing properties of the silver nanoparticles/porous silicon composite to NH3 are tested at room temperature by the static volumetric method. Results show that the deposition time has a significant impact on the gas-sensing properties of the silver nanoparticles/porous silicon. In a short deposition time, the composite with an appropriate amount of silver nanoparticles doped on the porous silicon shows good gas-sensing properties to NH3 with high sensitivity, fast response-recovery characteristic due to the high specific surface area and special microstructure. At room temperature, the gas sensor has a sensitivity of about 5.8 to 50 ppm NH3.

## EDITOR'S SUGGESTION

2015, 64 (13): 137801.
doi: 10.7498/aps.64.137801

Abstract +

Based on the electro-optical energy transfer process, the pseudo-spectral-luminous efficiency function (QD) of quantum dots (QDs) is introduced, the equations of chromaticity coordinates, luminous efficiency, and the QDs' mass of the white-light-emitting devices with CdSe, CuInS2 or CdS:Mn QDs are obtained, and the calculated results are in agreement with the experimental data. For a certain luminescent peak wavelength, when the full width at half maximum of the QD's photoluminescence becomes larger, the QD value becomes smaller while the chromaticity coordinates become more different from those of the corresponding monochromatic light. It is indicated that the color rendering index (CRI) of the devices is strongly dependent on the photoluminescent position and width of the QDs, and the CRI value can be increased towards 98 when a certain kind of CdS:Mn QDs is added into the traditional white-light LEDs.

2015, 64 (13): 137102.
doi: 10.7498/aps.64.137102

Abstract +

The p-type porous silicon layer with the aperture about 1.5 microns and hole depth about 15-20 microns is prepared by electrochemical etching of a p-type monocrystalline silicon wafer with a resistivity 10-15 Ω·cm and along [100] orientation in a double-tank cell which consists of the electrolyte (volume ratio HF: DMF=1:2). Silver nanoparticles film with different thickness has been deposited on porous silicon by the electroless deposition for different deposition times. Morphology and microstructure of the silver nanoparticles/porous silicon composite are studied by scanning electron microscope and X ray diffracmeter. Result indicates that the silver nanoparticles are uniformly distributed on the surface of porous silicon and the deposition time has an important influence on the morphology of the composite. The gas-sensing properties of the silver nanoparticles/porous silicon composite to NH3 are tested at room temperature by the static volumetric method. Results show that the deposition time has a significant impact on the gas-sensing properties of the silver nanoparticles/porous silicon. In a short deposition time, the composite with an appropriate amount of silver nanoparticles doped on the porous silicon shows good gas-sensing properties to NH3 with high sensitivity, fast response-recovery characteristic due to the high specific surface area and special microstructure. At room temperature, the gas sensor has a sensitivity of about 5.8 to 50 ppm NH3.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2015, 64 (13): 138101.
doi: 10.7498/aps.64.138101

Abstract +

The Ba5SiO4Cl6: Yb3+, Er3+, Li+ phosphor has been prepared by high temperature solid state reaction, and their upconversion (UC) luminescence properties and mechanisms are investigated. The UC emission bands located at 525 nm (2H11/24I15/2), 548 nm (4S3/24I15/2), and 662 nm (4F9/24I15/2) due to Er3+ are observed under the excitation of 980 nm. UC luminescence of Ba5SiO4Cl6: Yb3+, Er3+ phosphors is increased with increasing Er3+ and Yb3+ concentration due to the energy transfer enhancement of Er3+ and Yb3+. Based on the relations of UC luminescence intensity and excitation light power, the UC luminescence mechanisms are discussed. At a low excited power (below 0.8 W), the two-photon processes are involved in both green and red UC emission mechanisms. When the power exceeds 0.9 W, the green and red UC emission is a four-photon process. One new and interesting UC emission mechanism may occur in the Ba5SiO4Cl6: Yb3+, Er3+ phosphors. Both green and red UC emissions at a higher pumping power are generated by photon avalanche UC process. Influence of Li+ doping on the UC luminescence of Ba5SiO4Cl6: Yb3+, Er3+ phosphors is investigated. Result demonstrates that Li+ ion doping could enhance the UC luminescence of Ba5SiO4Cl6: Yb3+, Er3+, which is attributed to the distortion of the local symmetry around Er3+.

2015, 64 (13): 138102.
doi: 10.7498/aps.64.138102

Abstract +

A method of calculating coherent X-ray diffraction from a bent nanowire, simulated by the molecular dynamics technique under the bent periodic boundary condition, is reported. The segment of nanowire under the X-ray beam consists of the central box and 2N image boxes. X-ray diffraction from this segment of nanowire is obtained from a single calculation of the amplitude of diffraction from the atoms in the central box according to the kinematic theory. Contributions from the image boxes are then obtained by rotations of this amplitude in the reciprocal space and additional phase factors to take into account the position of the image boxes with respect to the central box. This method will be called rotation in the reciprocal space (RRS). Comparison between the RRS and the full calculation of the diffracted amplitude from all the atoms in the central box and the 2N image boxes (full kinematic sum) is done in the Cu nanowire case. The bending of an FCC Cu nanowire oriented along a direction with an equilibrium shape made up of {100} and {111} facets is calculated by using the SMA (The second-moment approximation of the density of states in the tight-binding formalism) potential. The Cartesian x, y, and z axes correspond, respectively, to [112], [111] and [110] directions. The bending occurs in the y-z plane. The calculation time of the RRS method is about 1/(2N+1) times that obtained by doing the full kinematic sum, the RRS method being more efficient when the number of image boxes N is a bigger one. A very small difference in the calculated intensity between the RRS and the full kinematic sum comes from the interpolation in the reciprocal space. So the RRS method is more accurate, when there are more points calculated in the reciprocal space. Similarly, the RRS method can be applied to tension, compression and torsion of the nanowires, When using the molecular dynamics simulation under periodic boundary conditions. In the cases of tension and compression, it is simpler as only the phase factors have to be considered. Results are also reported in this paper.

2015, 64 (13): 138103.
doi: 10.7498/aps.64.138103

Abstract +

In this paper, a new phase-field model based on diffusion interface is put forward to describe the epitaxial growth including island nucleation, growth, and ripening. Thermodynamics and kinetics play an important role in epitaxial morphology evolution. This model includes combined effects of the following processes, such as elastic field, surface energy, deposition, diffusion, desorption, and energy barrier etc. We use the classical BCF model to describe the atomic diffusion and nucleation processes, and use a new free energy function, including elastic strain energy, to obtain a phase-field equation that can describe the growth of dynamic multi-island by variation method. This model can effectively simulates the complex morphology in epitaxial growth. The nonlinear coupled equations can be solved by finite difference scheme. Numerical result shows that this model can reproduce the real multilayer epitaxial growth structure, and the simulation results are consistent with the experimental results. At the same time we also simulate the complex growth stress with morphology evolution. Results show that, accompanied with the epitaxial growth, a complex stress distribution is produced, and the stress reaches a local maximum on the boundaries of the island, which is consistent with the experimental results. Most importantly, the stress significantly affects the atomic diffusion process. While the stress exists, the epitaxial structure will change faster. These results can make a significance effect on the research of physical mechanism in epitaxial growth.

2015, 64 (13): 138104.
doi: 10.7498/aps.64.138104

Abstract +

A series of uniform and transparent Cr-O films were synthesized on the silicon and quartz glass substrates at different bias voltages by pulsed bias arc ion plating. Effects of bias voltage on surface morphology, phase structure, composition, chemical valence states, hardness and optical property of the films were investigated by field emission scanning electron microscopy, grazing incident X-ray diffraction, X-ray photoelectron spectroscopy, nanoindentation and ultraviolet-visible spectrophotometer, respectively. Results indicate that the bias voltage can improve the quality of the films significantly and plays an important role in the film properties. Macroparticles and holes are observed on the surface of the films if without application of bias voltage, while the films prepared with bias voltage are uniform and smooth. The crystalline phase of the film is of amorphous structure if without bias voltage. While the bias voltage applies and increases from -100 V to -500 V, the Cr2O3 phase appears and changes into CrO phase. The crystal plane (104), (116) of the Cr2O3 phase and (200) of the Cr phase are observed in the film at the bias voltage of -100 V. When the bias voltage is above -200 V, the crystal planes (311) and (400) of the CrO phase can be observed. In order to further obtain the structure information, a detailed XPS study is performed. Chromium in the films shows different valence states, namely metallic Cr, Cr2+, Cr3+ and Cr6+. Thereby, the main components of the polycrystalline films are Cr2O3 and CrO phases, meanwhile, and the films also contain a small amount of CrO3 and metal Cr phases. The films under different bias voltage show good mechanical properties and the hardness of all the films is above 19 GPa. With the increase of bias voltage the hardness first increases and then decreases, reaching a maximum value of 24.4 GPa at the bias voltage of -300 V. The films show good optical transmittance and its highest value can be up to 72%. As the bias voltage rises, it is observed first the red shift and then blue shift of the absorption edge. And the optical band gap reaches the maximum value of 1.88 eV when the bias voltage is -200 V. Therefore, Cr-O functional films can be synthesized by pulsed bias arc ion plating and the phase structure and properties can be effectively adjusted.

2015, 64 (13): 138701.
doi: 10.7498/aps.64.138701

Abstract +

For complicated structural components with wider X-ray attenuation ranges, the projection information is short of single-energy computed tomography (CT) imaging because the attenuation thickness of the components along the orientation of X-ray penetration exceeds the limit of the dynamic range of the CT imaging system. Then under this circumstance, in the conventional imaging mode of fixed ray energy the phenomenon of overexposure and underexposure easily occurs. The whole structure information cannot be obtained, and the projection information is seriously lacking, hence it will affect the quality of CT reconstruction. This paper proposes a CT imaging method with varying energy based on logarithm demodulation.#br#For the incomplete projection due to the limited dynamic range, we collect images at various rotation angles, as in standard CT; also, at each angle we collect a series of images at different energies and fuse them into a single high dynamic range (HDR) image. Based on this HDR image, the conventional CT reconstruction methods can get the full construction information. However, the CT quality is poor, because of noise amplification in the fusion process of X-ray image sequences which are gathered during varying the X-ray energy. To solve this problem, this paper proposes to use logarithm transformation to reduce the dynamic range for the varying energy image sequences. This can change the weight coefficients' access in the projection sequences to suppress the noise. Also in the transformation sequences, we can also use the fusion method, which is based on image gray consistency, to compute the weighting coefficient, to obtain the HDR projection, because the logarithm transformation does not change the image structure, and the overlap area between image sequences is definite. Finally we use the conventional CT reconstruction algorithm to make the CT imaging with the complicated structural components.#br#An accompanying experiment with a complicated instrument demonstrates that the new method can extend the dynamic range of X-ray imaging system and provide a complete representation of the internal structure of the complicated structural components as compared with the conventional CT imaging method. So, it will be of important significance and may be applied to the potential system, so as to improve the detecting sensitivity of the system, and make the high-quality reconstruction about complex products.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2015, 64 (13): 139801.
doi: 10.7498/aps.64.139801

Abstract +

In this paper, we collect the 37 years' data observed for BL Lac object OJ287 in the optical band, and construct its light curve from 1972 to 2009. Using the method of Lomb-Scargle Fourier transform, we analyze the middle time-scale periodic behavior and discover the primary periodic compositions of 52.0±1.1 day and 255±23 day. Then, we simulate the quasi-periodic signal of period doubling bifurcations and compare it with the observation data at different time-interval in their phase diagram and correlation dimension. Result indicates that OJ287 has multi-period scale features. At different time-intervals, there are different harmonic period components in the quasi-periodic light curve. Variation of harmonic period component is related with the disturbance of accretion flow which is from the secondary black hole to the primary black hole, and the periodicity variability may bifurcate the harmonic period component when the primary accretion flow is influenced by a strong disturbance.