## Vol. 66, No. 9 (2017)

##### 2017-05-05

###### GENERAL

2017, 66 (9): 090501.
doi: 10.7498/aps.66.090501

Abstract +

Neuron is a typical dynamic system, therefore, it is quite natural to study the firing behaviors of neurons by using the dynamical system theory. Two kinds of firing patterns, i.e., the periodic spiking and the periodic bursting, are the limit cycle oscillators from the point of view of nonlinear dynamics. The simplest way to describe the limit cycle is to use the phase of the oscillator. A complex state space model can be mapped into a one-dimensional phase model by phase transformation, which is helpful for obtaining the analytical solution of the oscillator system. The response characteristics of the oscillator system in the motion state of the limit cycle to the external stimuli can be characterized by the phase response curve. A phase response curve illustrates the transient change in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received. Now it is widely believed that the phase response curve provides a new way to study the behavior of the neuron. Existing studies have shown that the phase response curve of the periodic spiking can be divided into two types, which are closely related to the bifurcation mechanism of neurons from rest to repetitive firing. However, there are few studies on the relationship between the phase response curve and the bifurcation type of the periodic bursting. Clearly, the first prerequisite to understand this relationship is to calculate the phase response curve of the periodic bursting. The existing algorithms for computing the phase response curve are often unsuccessful in the periodic bursting. In this paper, we present a method of calculating the phase response curve, namely the direct algorithm with square wave perturbation. The phase response curves of periodic spiking and periodic bursting can be obtained by making use of the direct algorithm, which is verified in the four neuron models of the Hodgkin-Huxley, FitzHugh-Nagumo, Morris-Lecar and Hindmarsh-Rose. This algorithm overcomes the limitations to other algorithms in the application. The calculation results show that the phase response curve of the periodic spiking is determined by the bifurcation type. We find a suprathreshold periodic oscillation starting from a Hopf bifurcation and terminating at a saddle homoclinic orbit bifurcation as a function of the applied current strength in the Morris-Lecar model, and its phase response curve belongs to Type II. A large amount of calculation indicates that the relative size of the phase response and its positive or negative value depend only on the time of imposing perturbation, and the phase response curve of periodic bursting is more complicated than that of periodic spiking.

2017, 66 (9): 090702.
doi: 10.7498/aps.66.090702

Abstract +

Large volume cubic press is one of the most popular high pressure devices which can produce pressures up to about 7 GPa. It is well known experimentally that the enhancing of the maximum pressure generated in the large volume cubic press has attracted wide attention among scientists and engineers because the higher pressure is capable of synthesizing some materials with interesting properties. In the large volume cubic press, pyrophyllite is typically used as a pressure-transmitting medium. A specimen immersed in such a solid experiences a generalized stress state. The pressure distribution in pyrophyllite is an important parameter for characterizing the sample environment and designing the experiments at high pressure. There is a need for the quantitative measurement of pressure gradients in the pyrophyllite pressure medium, so that the accurate experimental data under high pressure can be obtained.
In the large volume cubic apparatus (68 MN), we put a circuit into the high pressure cubic cell, so that the pressures at various positions can be measured by using the phase transitions in Bi, Tl and Ba. In the present work, the relationship between the total press load and the press load allocated to the anvil face, and the relationship between the total press load and the press load allocated to gaskets are established at room temperature. The results show that with the increase of the total press load, the load allocated to the gaskets is increased sharply, while the curve of load allocated to the anvil face versus total press load reaches a plateau, which results in the cell pressure reaching upper limit when the cell pressure reaches up to about 5 GPa. According to the experimental results, the stress state of the cubic cell under high pressure is analyzed and the reason why the pressure generated in the large volume cubic chamber is difficult to exceed 7 GPa is explained. Based on the geometrical structure of the cubic cell, the scheme to increase the upper pressure limit for cubic cell by using the material with high bulk modulus as the pressure transmitting medium and the material with low bulk modulus as the gasket, is proposed. Additionally, the method of calculating the pressure values at different positions along the axis of symmetry in the cubic cell is given through the quantitative calibration of the pressure gradient in the axial direction of the cubic cell. This method can provide more accurate pressure data for high pressure experiments.

2017, 66 (9): 090502.
doi: 10.7498/aps.66.090502

Abstract +

The optimization performance of transiently chaotic neural network (TCNN) is affected by various factors such as chaotic characteristic, model parameters, and annealing function, and its capacity of global optimization is limited. It is demonstrated that the non-monotonic activation function can generate richer chaotic characteristic than the monotonic activation function in the TCNN model. Besides, the activation function involving neurobiological mechanism can not only reflect the rich brain activity in brain waves, but also enhance the non-linear dynamic characteristic, which may further improve the global optimization ability. Hence, a novel chaotic neuron model is proposed with the non-monotonic activation function based on the neurobiological mechanisms from the electroencephalogram.
The electroencephalogram consists of five brain waves (i.e., , , , , and waves) which are defined by the quality and intensity of brain waves with different frequency bands ranging from 0.5 Hz to 100 Hz. The brain wave with a higher frequency and a lower amplitude represents a more active brain. Researches demonstrate that the five brain waves can be simplified into sinusoidal waves with different frequencies. Hence, a frequency conversion sinusoidal (FCS) function which has the consistent frequency range and features with brain waves is designed based on the above neurobiological mechanisms. Then a novel chaotic neuron model with non-monotonic activation function which is composed of the FCS function and sigmoid function, is proposed for richer chaotic dynamic characteristic. The reversed bifurcation and the Lyapunov exponent of the chaotic neuron are given and the dynamic system is analyzed, indicating that the proposed FCS neuron model owns richer chaotic dynamic characteristic than transiently chaotic neuron model due to its special non-monotonic activation function.
Based on the neuron model, a novel transiently-chaotic neural networkfrequency conversion sinusoidal chaotic neural network (FCSCNN) is constructed and the basis of model parameter selection is provided as well. To validate the effectiveness of the proposed model, the FCSCNN is applied to nonlinear function optimization and 10-city, 30-city, 75-city traveling salesman problem. The experimental results show that 1) the FCSCNN has a good performance under the condition of moderate a, smaller cA(0) and 2(0); 2) on the basis of the appropriate model parameters, the FCSCNN has better global optimization ability and optimization accuracy than Hopfield neural network, TCNN, improved-TCNN due to its richer chaotic characteristic in complicated combinational optimization problem, especially in middle and large scale problem.

2017, 66 (9): 090503.
doi: 10.7498/aps.66.090503

Abstract +

As is well known, people has been suffering noise interference for a long time, and more and more researches show that a lot of weak signals such as pulse signal are embedded in the strong chaotic noise. The purpose of weak signal detection and recovery is to retrieve useful signal from strong noise. It is very difficult to detect and estimate the weak pulse signal which is mixed in the chaotic background interference. Therefore, the detection and recovery of weak signal are significant and have application value in signal processing area, especially for the weak pulse signal detection and recovery. By studying various methods of detecting and estimating the weak pulse signal in strong chaotic background noise, in this paper, we propose an efficient hybrid processing technique. First, based on the short-term predictability and sensitivity to the tiny disturbance, a new method is proposed, which can be used for detecting and estimating the weak pulse signals in chaotic background that the nonlinear mapping is unknown. We reconstruct a phase space according to Takens delay embedding theorem; then we establish the local linear autoregressive model to predict the short-term chaotic signal and obtain the fitting error, and judge whether there are weak pulse signals. Second, we establish a single-jump model for pulse signals, and combine the local linear autoregressive model with it to build a double local linear (DLL) model for estimating the weak pulse signal. DLL model contains two parameters, and the two parameters affect each other. We use the back-fitting algorithm to estimate model parameters and ultimately recover the weak pulse signals. Detecting and estimating the pulse signals in chaotic background turns into estimating the parameters of DLL model. The minimum fitting error criterion is used as the objective function to estimate the parameters of the DLL model. To make the estimation more exact, we can use the formula of mean square error. The new algorithm presented here in this paper does not need to know the prior knowledge of the chaotic background nor weak pulse signal, and this algorithm is also simple and effective. Finally, the simulation results show that the method is effective for detecting and estimating the weak pulse signals based on the chaotic background noise. Specifically, the weak pulse signal can be extracted well with low SNR and the minimum mean square error or the minimum normalized mean squared error is very low.

2017, 66 (9): 090504.
doi: 10.7498/aps.66.090504

Abstract +

Time delay frequently appears in many phenomena of real life and the presence of time delay in a chaotic system leads to its complexity. It is of great practical significance to study the synchronization control of fractional-order chaotic systems with time delay. This is because it is closer to the real life and its dynamical behavior is more complex. However, the chaotic system is usually uncertain or unknown, and may also be affected by external disturbances, which cannot make the ideal model accurately describe the actual system. Moreover, in most of existing researches, they are difficult to realize the synchronization control of fractional-order time delay chaotic systems with unknown terms.
In this paper, for the synchronization problems of the different structural fractional-order time delay chaotic systems with completely unknown nonlinear uncertain terms and external disturbances, based on Lyapunov stability theory, an adaptive radial basis function (RBF) neural network controller, which is accompanied by integer-order adaptive laws of parameters, is established. The controller combines RBF neural network and adaptive control technology, the RBF neural network is employed to approximate the unknown nonlinear functions, and the adaptive laws are used to adjust corresponding parameters of the controller. The system stability is analyzed by constructing a quadratic Lyapunov function. This method not only avoids the fractional derivative of the quadratic Lyapunov function, but also ensures that the adaptive laws are integer-order. Based on Barbalat lemma, it is proved that the synchronization error tends to zero asymptotically. In the numerical simulation, the uncertain fractional-order Liu chaotic system with time delay is chosen as the driving system, and the uncertain fractional-order Chen chaotic system with time delay is used as the response system. The simulation results show that the controller can realize the synchronization control of the different structural fractional-order chaotic systems with time delay, and has the advantages of fast response speed, good control effect, and strong anti-interference ability. From the perspective of long-term application, the synchronization of different structures has greater research significance and more development prospect than self synchronization. Therefore, the results of this study have great theoretical significance, and have a great application value in the field of secure communication.

2017, 66 (9): 090505.
doi: 10.7498/aps.66.090505

Abstract +

In recent years, nonlocal spatial solitons have attracted a great deal of attention. Optical spatial solitons result from the suppression of beam diffraction by the light-induced perturbed refractive index. For spatial nonlocal solitons, the light-induced perturbed refractive index of medium depends on the light intensity nonlocally, namely, the perturbed refractive index at a point is determined not only by the light intensity at that point but also by the light intensity in its vicinity. Such a spatial nonlocality may originate from heat transfer, like the nonlocal bright solitons in lead glass and dark solitons in liquids or gases. The perturbed refractive index n of lead glass or liquid is direct proportional to the light-induced temperature perturbation t, i.e. n=1t. The proportional coefficient 1 is positive (negative) for lead glass (liquid), and the light-induced temperature perturbation t is determined by the Poisson equation ▽2(t)=-DI, where I is the light intensity and D is a coefficient. In this paper, we investigate another type of thermal nonlinear effect, in which the perturbed refractive index n depends on the light-induced temperature perturbation t in a new way that n=1t+2(t)2. It has been indicated previously that the refractive index of a supercooled aqueous solution depends on the temperature, specifically n(t)=n0-2(t-t0)2, where n0=1.337733 for 501 nm light wave, t0=-0.1℃ and 2=310-6 K-2. So for tt0, the refractive index of aqueous solution increases with temperature rising, while tt0, it decreases with temperature increasing. In this paper, we use the numerical simulation method to investigate the propagation and interaction properties of optical solitons propagating in a supercooled aqueous solution, whose temperature on boundary is maintained at some value below t0, with the aqueous solution placed in a thermostatic chamber. Obviously, the inner temperature of the solution rises, owing to absorbing some optical energies of the light beam propagating in it, and as a consequence the inner refractive index changes according to n(t)=n0-2(t-t0)2. For a soliton with a low power, the inner temperature t of the solution is always kept below t0, so the refractive index at a point with a higher t is larger than that at another point with a lower t. In this case, the solution behaves as a self-focusing medium. A soliton with a higher power has a narrower beam width and a larger propagation constant, and the soliton takes a bell shape. However, for a soliton with a rather high power, the temperature in the core will be higher than t0 while the temperature in the periphery is still below t0. Therefore, the part of the solution in the core behaves as a self-defocusing medium while the part in the periphery behaves as a self-focusing medium. For such a case, the higher the power of the soliton, the larger the radius of the core is and the larger the beam width of the soliton, so the soliton takes a crater shape with a saturated propagation constant. Finally we also investigate the interaction between two solitons in a supercooled aqueous solution. For two neighboring beams with a rather high total power, they cannot maintain their individualities any more during the interaction, but merge into an expanding crater.

2017, 66 (9): 090701.
doi: 10.7498/aps.66.090701

Abstract +

The Shack-Hartmann wavefront sensor (SHWFS) is an optical detection device based on the measurements of wavefront slopes. It is widely used in an adaptive optics system due to its simple structure and strong environment adaptability. The measuring accuracy of the SHWFS depends mainly on the accuracy of the spot image centroid in each sub-aperture. There are many centroid algorithms including the center of gravity algorithm, Gauss fitting algorithm, and correlation algorithm. As to the simplicity, robustness, high accuracy and stability, the center of gravity algorithm is more widely used. However, the accuracy of gravity algorithm is sensitive to the noise including discretization, aliasing, photon noise, readout noise, stray light, and direct current bias. To improve the accuracy of centroid, the output signals of SHWFS must be pre-processed to suppress the noise effect by using the method of thresholding in general. Many threshold methods have been presented to reduce the error of centroid and there theoretically exists an optimum threshold which causes the minimum error of centroid based on the characteristics of SHWFS and noise. However, it is difficult to separate the signals from the noises, and the optimum threshold cannot be estimated accurately in real time in the SHWFS systems. In this paper aiming at noises in SHWFS, which vary with time and space rapidly, a method based on the noise weighted function of the mean value of pixels and the local gradient direction of image signals in the moving windows is presented according to the characteristics of the Gaussian spot and noise distributions. Moreover, the theory and parameters determination of the method are analyzed. The method utilizes the probability that the pixels in the moving windows belong to the noise, and the probability is inversely proportional to the mean value of pixels and the local gradient direction of image signals, and so the monotonically reducing probability function of pixels is constructed. Finally, the standard deviation and mean value of noise can be obtained, and the estimation value of optimum threshold is equal to the mean value of noise plus three times the standard deviation of noise. To investigate the effects of the optimum threshold estimation with the different spot sizes, spot strengths and noise levels, the proposed algorithm is compared with traditional methods. The simulation and experimental results show that the proposed method could achieve higher accuracy, and the error between the threshold obtained by the method presented in this paper and theoretical optimum threshold is less than 10%, which is less than those from the traditional methods.

2017, 66 (9): 090703.
doi: 10.7498/aps.66.090703

Abstract +

Time delay estimation (TDE) is a hot research topic in wireless location technology. Compressed sensing (CS) theory has been widely applied to image reconstruction and direction of arrival estimation since it was proposed in 2004. The sparse model can be constructed in time domain for estimating the time delay by using the CS theory. The measurement matrix plays a crucial role in the processing of signal reconstruction which is the core problem of CS theory. Therefore the research in the measurement matrix has becomes a hotspot in recent years. The existing measurement matrix is mainly divided into two categories, i.e., random measurement matrix and deterministic measurement matrix. The performance of random measurement matrix has bottlenecks. Firstly, because of the redundant measurement matrix data, the generation and storage of the random number put forward a high requirement for hardware. Secondly the random matrix can only satisfy the restricted isometry property in a statistical sense. The research of the deterministic measurement matrix is of great value under this background. The parity check matrix of low density parity check (LDPC) code has good performance in CS theory. However, the method of randomly selecting non-zero element position has a certain probability to generate a measurement matrix with a short loop structure during generating LDPC code measurement matrix. The robustness of the reconstruction performance decreases with the increase of iteration times.
A novel quasi-cyclic CS algorithm based on progressive edge-growth is constructed to estimate the time delay. The purpose of this article is to deal with the need to store a large number of data in existing measurement matrix during time delay, by using the CS theory. The algorithm presented here can achieve TDE in a high precision. First, the theoretical bridge between CS and the maximum likelihood decoding is established. And the design criterion of measurement matrix based on the LDPC code is derived. The sparse measurement matrix with quasi-cyclic structure is constructed by introducing the idea of progressive edge-growth. Finally, the orthogonal matching pursuit algorithm is used to estimate the time delay. Furthermore, the computational complexity of the algorithm and the data storage of the measurement matrix are analyzed theoretically. Simulations show that the correct reconstruction probability of the proposed approach is higher than those of the Gauss random matrix and random LDPC matrix under the same dimension. Compared with the random LDPC matrix, the proposed method can improve performance at the expense of less complexity under the condition of the same data storage.

2017, 66 (9): 090704.
doi: 10.7498/aps.66.090704

Abstract +

Nitrogen dioxide (NO2) is an important trace gas in the troposphere and plays a vital role in many aspects of the chemistry of the atmosphere. Accurate measurement of NO2 is the primary step to understand its role in atmospheric chemistry and to establish effective pollution prevention policies. Relatively few measurements of the NO2 profile in troposphere by using point-type instruments with high temporal resolution have been carried out in China. Due to the relatively poor measurement environment on airborne platform, the measurement system requires good anti-vibration ability, stability and environmental adaptability. A home-built incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) on the airborne platform is presented in this paper, and applied to high temporal resolution observations of the actual atmospheric NO2 spatial distribution. According to the strong absorption of NO2 in a wavelength range from 449 nm to 470 nm, we choose a high-power 457 nm light-emitting diode (LED) as a light source. A Peltier is used to control LED temperature and to stabilize the LED temperature at (200.1)℃. The pure PFA material optical cavity and sampling tube are used to reduce wall loss. And we choose the highly reflecting mirrors (reflectivity R0.9999@440-450 nm) to improve the effective optical path. A 2 m filter is used at the inlet of instrument to remove most of the particulate matter in the sample flows, which reduce the effect of particulate matter on the effective path length. In order to meet the requirement for time resolution in airborne measurement, we use an off-axis paraboloic mirror instead of an achromatic lens to improve the optical coupling efficiency. The reflectivity of the highly reflecting mirror is calibrated by the difference in Rayleigh scattering between He and N2. And the optimum averaging time of the IBBCEAS instrument is confirmed to be 1000 s by the Allan variance analysis. Detection limit (1) of 10 ppt for NO2 is achieved with an optimum acquisition time of 1000 s. Concentrations of NO2 are recorded and compared with data from a long path different optical absorption spectroscopy instrument, and the results show good agreement with each other. The linear correlation coefficient R2 is 0.86 in a slope of 0.92 with an offset of -0.402 ppb. The IBBCEAS system is deployed on an airborne platform, and the detection limit is 95 ppt (1) with a time resolution of 2 s. The profile of tropospheric NO2 by airborne observation is obtained over Shijiazhuang in Northern China. IBBCEAS system in the airborne platform shows good stability.

###### ATOMIC AND MOLECULAR PHYSICS

2017, 66 (9): 093101.
doi: 10.7498/aps.66.093101

Abstract +

As one of the simplest alkyl halides, methyl iodide is extensively investigated in the research fields of the photodissociation and photoionization. In the experimental investigations of ionization and dissociation, many molecular fragments, such as Iq+(q3), CHn+(n3), H+, etc., are observed in the mass spectrum of CH3I. While the mechanisms for dissociation and ionization are not completely understood. As the doubly-ionized product, CH3I2+ exhibits different isomer structures and isomerization reactions. The dissociation channels of different isomers in combination with the corresponding transition states of CH3I2+ are helpful for better understanding the dissociation and ionization dynamics of CH3I in an intense laser field.
In our present work, the dissociation channels of CH3I2+ are investigated by the density functional and couple cluster theory. The geometries and energies corresponding to the local isomers and the transition states of CH3I, CH3I+ and CH3I2+ are computed. The first and second ionization energies we measured are in good agreement with experimental values. Our computational results show that the ground state of the CH3I2+ is a triplet one with 3A2 symmetry. Totally 11 two-body and 15 three-body dissociation channels of the CH3I2+ on both the lowest singlet and the lowest triplet potential energy surfaces are computed and analyzed in detail. Our computations indicate that seven two-body dissociations channels, i.e., six singlet and one triplet ones, are exergonic, in which CH3I2+(1A')CH2++HI+(4A1) is the easiest process to achieve; four exergonic three-body dissociation channels with three on singlet potential energy surface and one on triplet potential energy surface are found. The possible mechanisms for producing the dissociative ionized fragments observed in experiments, CH3+, H+, and I+, are presented; furthermore, the dissociation channels generating other ions not observed in experiments, such as H3+ et al, are also given for further experimental study. The detailed information about dissociation channels and fragments is summarized for further experimental comparisons. In the computations, we find that the density functional theory and CCSD(T) methods give different energy orders for a few dissociation potential energy surfaces; and in this work, the analysis and discussion are performed based on the CCSD(T) results. Our computations indicate that the dissociation channels on singlet and triplet potential energy surface exhibit different behaviors.

2017, 66 (9): 093201.
doi: 10.7498/aps.66.093201

Abstract +

The three-step two-color resonant ionization method and three-step three-color isolated-core excitation (ICE) technique are used to study the spectra of the highly excited bound states systematically, either Eu 4f76snl Rydberg states or other valence states converging to the higher ionization limits. Specifically, the highly excited bound states are populated from the ground state via three different 4f76s6p intermediate states, thereby establishing the three different excitation schemes. The schemes are designed to allow us to assign a J-quantum number uniquely to a given highly excited state with the selection rules of J-quantum number for each excitation scheme by comparing their corresponding photoionization spectra, which are obtained with three-step two-color resonant ionization method. By tuning the wavelength of the second laser, the 56 highly excited bound states located in the energy region between 42250 cm-1 and 44510 cm-1 are detected. To explore their spectroscopic information, more efforts have been made 1) to judge whether an excited state is a bound Rydberg state and to observe whether it may be excited further to an autoionizing state by using the ICE technique; 2) to deduce the principal quantum number of the given bound Rydberg states, and to observe whether they are converged to the same ionization limit by calculating their quantum defects with respect to several ionization limits. Based on the above manipulations, all detected highly excited bound states can be classified as the two categories: bound Eu 4f76snl Rydberg states and other valence states converging to the higher ionization limits, such as the Eu 4f75dnl states. Specifically, to fulfill the ICE technique, it is necessary to make a resonance transition from the 4f76snl Rydberg states to the 4f76p1/2nl autoionizing states with the third dye laser whose wavelength is scanned around the Eu 4f76s+-4f76p1/2+ ionic line. Once the Eu 4f76snl Rydberg states are recognized with the ICE technique, the identification of their orbital quantum numbers is a primary task to determine their electron configurations. With all the efforts mentioned and existing information, three Rydberg states can be assigned to the 4f76s10s(8S9/2), 4f76s9d(8D9/2) and 4f76s9d(6D7/2), whereas the rest can be regarded as highly excited valence states.

2017, 66 (9): 093202.
doi: 10.7498/aps.66.093202

Abstract +

Rydberg atoms are highly excited atoms with large principal quantum number n, big sizes (~n2) and long lifetimes (~n3). Rydberg atoms are very sensitive to an external field due to the large polarizabilities of Rydberg atoms (~n7). Electromagnetically induced transparency (EIT) of Rydberg atom provides an ideal method to detect Rydberg atoms without destroying atoms, and can be used to measure the external direct current and radio frequency (RF) field.
In this paper, we study the EIT effect of a cesium ladder-type three-level atom involving Rydberg state exposed to a weak RF field. The ground state (6S1/2), the excited state (6P3/2) and Rydberg state (48D5/2) constitute the Rydberg three-level system, in which the probe laser couples 6S1/2(F=4)6P3/2(F'=5) transition, whereas the coupling laser scans across the 6P3/248D5/2 Rydberg transition. The coupling laser (510 nm laser, propagating in the z-axis direction and linear polarization in the y-axis direction) and the probe laser (852 nm laser, linear polarization in the y-axis direction) counter-propagate through a 50-mm-long cesium vapor cell at room temperature, yielding Rydberg EIT spectra. Rydberg EIT signal is detected as a function of the detuning of the coupling laser. When a weak RF (80 MHz) electric field polarized in the x-axis direction is applied to a pair of electrode plates located on both sides of the cesium cell, the EIT spectrum of Rydberg 48D5/2 shows the Stark splitting and the even order harmonic sidebands. The experimental results are analyzed by using the Floquet theory. The simulation results accord well with the experimentally measured results. Furthermore, we also investigate the influence of the self-ionization effect of Rydberg atom on the Stark spectrum by changing the RF frequency. We put forward a proposal to avoid the effect of ionization by placing electrode plates in the cesium cell. In the weak RF-field domain, mj=5/2 Stark line crosses mj=1/2, 3/2 sidebands, these cross points provide an antenna-free method of accurately calibrating the RF electric field based on Rydberg atoms.

2017, 66 (9): 093301.
doi: 10.7498/aps.66.093301

Abstract +

p-methoxybenzonitrile is an important chemical and industrial material which has been widely used in many fields, such as medicine, chemistry, photoelectron, etc. In this paper, we use the technologies of supersonic molecular beam, resonance enhanced multiphoton ionization and time-of-flight mass spectrometer to obtain the high-resolution one color resonance two-photon ionization spectra of p-methoxybenzonitrile in a vibrational wavenumber range of 0-2400 cm-1. In order to analyze the experimental results, the theoretical calculations are performed. The molecular structure, energy, and vibration frequencies at the electronic excited state S1 are computed with time-dependent density functional theory at the level of B3PW91/6-311 g++**. According to the calculated results, the observed bands are assigned by the method of Varsanyi and Szoke. The band origin of the S1S0 electronic transition of p-methoxybenzonitrile is determined to be (355492) cm-1. A lot of vibrational bands of the electronic excited state S1 are observed. The results show that the vibrational modes of 9b, 6b, 15 and 1 are very easy to activate in a wavenumber range of 0-800 cm-1. There are also a lot of intense bands in a wavenumber range of 800-1600 cm-1. In addition to the fundamental vibrations, many combined vibrations between breathing and other fundamental vibrations are found. Several vibrations in this range are located at OCH3 and CN group. Most of the bands in a range of 1600-2400 cm-1 correspond to ones in the range of 800-1600 cm-1. Except for the bands appearing at 1664 and 2156 cm-1, which are assigned to 15011301 and (CN) (CN stretching) respectively, the remaining bands in the range of 1600-2400 cm-1 are assigned as the combined vibrations between the breathing and the corresponding modes in the range of 800-1600 cm-1, i.e., the combined vibrations between the breathing overtone and other fundamental modes. Our theoretical calculations show that except for CN stretching vibration at 2162 cm-1, there is no fundamental frequency in a range of 1600-3000 cm-1, which is consistent with our experimental result and assignment. The fundamental of the breathing vibration 11 and its second overtone vibration 12 are very strong. The third overtone frequency 13 can be identified unambiguously. This is an important characteristic of p-methoxybenzonitrile, which is different from that of the usual polyatomic molecule. These results provide important reference for future researches on Rydberg states, chemical kinetics and zero kinetic energy spectroscopy of p-methoxybenzonitrile.

2017, 66 (9): 093401.
doi: 10.7498/aps.66.093401

Abstract +

In recent years, by using the etching techniques with great precision, the ion tracks in materials were converted into insulator and metal nanocapillaries. The physical and chemical properties of the inner surface on micro and nano-scales of these capillaries can be investigated by the interaction of ions with the surfaces.
Stolterfoht et al. (2002 Phys. Rev. Lett. 88 133201) have found the evidence for capillary guiding in studying the transmission of 3 keV Ne7+ ions (energy/charge E/q100 kV) through the polymer nanocapillaries. The self-organized charge-up process was thought to inhibit close contact between the ions and the inner capillary walls. Skog et al. (2008 Phys. Rev. Lett. 101 223202) investigated the guiding effect of 7 keV Ne7+ ions (E/q100 kV) transmitted through SiO2 nanocapillaries, and found the evidence of sequentially formed charge patches along the capillary. For these keV highly charged ions with E/q100 kV, the charge patches were formed in a very short time, and then the repulsive electric field rapidly becomes strong enough to deflect the ions, then the ions move along the capillary axis without charge exchange.
Zhou et al. (2016 Acta Phys. Sin. 65 103401) have investigated the transmission of 100 keV protons (E/q102 kV) through the nanocapillaries in polycarbonate (PC) membrane. It was found that the transmitted ions are located around the direction of the incident beam, rather than along the capillary axis. This indicated that the transmission mechanism of hundreds of keV protons through nanocapillaries is significantly different from that for keV highly charged ions. For 100 keV protons, several charge patches suppress the protons to penetrate into the surface, and the protons are transmitted via twice specular scattering near the surface and finally emitted along the incident direction. However, the study of the transmission of E/q101 kV ions through nanocapillaries is still lacking.
In this work, we measure the time evolution of the relative transmission rate, charge state and angular distribution as well as the full width at half maximum of 20 keV protons (E/q101 kV) transmitted through the nanocapillaries in PC membrane at a tilt angle of +1. We observe a very long time pre-guiding period before the stable guiding process is established. During the pre-guiding period the direction of the transmitted H+ ions changes to the direction of capillary axis gradually. The transmitted H0 particles are composed of two peaks:the higher and sharper one is nearly in the beam direction, the wider and lower one is around the guiding direction. With the continuous charging-up process, the intensities of the narrow and sharp peak of transmitted H0 near the beam direction will decrease and disappear at the end. The data indicate that the scattering and guiding forces are both important for E/q101 kV ions during the period of pre-guiding process, and the guiding force is dominant till a long time pre-guiding period is ended. This finding will fill in the gap between E/q100 kV and 102 kV of previous studies of ions transmitted through nanocapillaries. It is also helpful for finding the applications of nano-and micro-sized ion beams produced by tapered glass capillary with E/q101 kV.

2017, 66 (9): 093601.
doi: 10.7498/aps.66.093601

Abstract +

It is well known that carbon nanotubes (CNTs) have received much attention since they were discovered. With the rapid development of carbon-based electronics and quantum computers, CNTs are required to have their unique physical and chemical properties in many fields. However, due to their uncertain mechanism of growth, it is difficult to achieve high production of CNTs with certain controlled structures. In this paper, we construct the nuclei of specific single- and double-walled zigzag CNTs and study their structural derivatives and electronic properties by using the density functional theory. According to the study of carbon clusters, we find some stable cage-like clusters containing zigzag structure which can be used as the nucleus of the corresponding single-walled CNTs. The nucleus of the double-walled CNTs is composed of the corresponding nucleus of single-walled CNTs.
It is possible to obtain a tubular cluster by optimizing the structure of the nucleus with accumulating carbon atoms at one end. The results show that the pentagonal structure plays a key role in the growing of tubular clusters. We find that the tubular clusters are grown in the form of global reconstruction when the clusters are short, but grown by local reconstruction when the clusters are longer. It can provide a theoretical reference to realize numerous CNTs with certain structures. Furthermore, the average binding energy (Eb) of tubular clusters is studied, and we find that their Eb is more and more stable and then close to the corresponding CNTs. At the same time, the study of the thermodynamic quantities of tubular clusters shows that their structures are thermodynamically stable.
In addition, the infinite zigzag CNTs can be obtained by using the periodic boundary conditions. Furthermore, the energy bands and density of states are calculated to study their electronic properties. The results show that the energy band structures of zigzag CNTs are closely related to the chiral index n. For zigzag CNTs (n, 0) and (n, 0)@(2n, 0), they show a metal property or narrow band gap semiconductor when n=3q (q is an integer); when n3q, they show a wide band gap semiconductor, and the band gap decreases with the diameter increasing. It is interesting that the two metallic single-walled CNTs (SWCNTs) are nested to obtain metallic double-walled (CNTs) DWCNTs, while the two semiconducting SWCNTs are nested to obtain semiconducting DWCNTs. However, due to the obvious curvature effect, small-diameter CNTs (4, 0), (4, 0)@(8, 0) and (5, 0)@(10, 0) show the metal properties but CNT (6, 0)@(12, 0) shows the obvious semiconductor property.

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

2017, 66 (9): 094201.
doi: 10.7498/aps.66.094201

Abstract +

Due to the extremely high diffractive efficiency and flexible design freedom, binary optical element can realize specific function in the optical system in comparison with the traditional refractive optical element. Ptychography, which is a typical lensless optical imaging technology with simple structure, has the advantages of the extensible imaging range and high resolution. The topography of binary optical element can produce the phase difference between the illumination and transmission fields. The features of binary optical element are based on the complex amplitude modulation. So we can obtain the complex transmission function by using ptychography to realize the phase retrieval. In this paper, we propose a detection method for binary diffractive optical element based on ptychography. An improved ptychography optical system is designed by using the combination of variable aperture and lens to control the illumination field. Because the illumination field is a diverging spherical wave, the diffractive patterns can avoid the high contrast and the reconstruction result will contain more details of the sample. The proposed method can not only inspect a large region of the binary optical element, but also calibrate its feature size, such as step height. Compared with the traditional binary optical element detection methods, the proposed method can simplify the system structure, and it can be applied to special environment by using lensless imaging technology. The increasing of the diffraction pattern numbers can acquire the topography of the large size sample and improve the detection efficiency. Taking a phase step plate for sample, the simulations are conducted to analyze the influences of step height and noise on the recovery result. The results show that the detection range of step height is less than 1.5. We can realize a preferable sample reconstruction when the noise of diffraction pattern is less than 5%. A computer-generated holography (CGH) is reconstructed by using the extended ptychographic iterative engine. The diameter of illumination filed is selected to be about 2 mm in order to obtain a large detection region of the sample. The surface micro topography of CGH can be shown through the m 1.98 mm1.98 mm recovery result. More details can be obtained by changing the diameter of illumination filed about 1.6 mm. The recovery result is quite accurate and the error of step height is less than 30 nm compared with the result of white light interference detection. The simulation and experimental results verify the feasibility of this method. When the requirement for accuracy is not extremely high, the proposed method can obtain a satisfactory image quality. In addition, we hope to improve the proposed method, which can be more accurate to detect different types of optical elements in the future research.

2017, 66 (9): 094204.
doi: 10.7498/aps.66.094204

Abstract +

We report on a new kind of highly birefringent and highly nonlinear photonic crystal fiber with a row of sub-micron air hole in the fiber core. The diameters of air holes in fiber core and cladding are 0.2 μm and 6.6 μ$m respectively. The parameters of birefringence, nonlinear and dispersion coefficient of the fiber are simulated by finite element method. It is found that the birefringence of the fiber can exist at the wavelengths up to 1550 nm, which is one order of magnitude higher than that of the traditional polarization-maintaining fiber. The zero-dispersion wavelengths of the fast axis and slow axis are 1050 nm and 1080 nm respectively. This fiber has a clear advantage over conventional fiber in continuum generation. Firstly, the polarization state of the pulse traveling in the fiber can be sustained along the fiber length and the extinction ratio is more than 20 dB. In addition, the pulses travel at different group velocities along the two polarization directions, which provide a convenient way of tuning the properties of the generated supercontinuum. Using this fiber as a nonlinear medium, an efficient generation of a tunable supercontinuum is demonstrated by pumping with 15 ps pulses of 1040 nm laser radiation, which is located in the normal dispersion region. A half-wave plate is used to vary the input polarization of the light pulse launched into the fiber, and the polarization of output supercontinuum is adjusted by a Glan prism at the same time. It is experimentally found that the polarization of pulse has a significant influence on the generation of the supercontinuum. When the linear polarization of the input pulse matches with the direction of the main axis of the fiber, the supercontinuum can be broadened over wavelength range of 800-1500 nm, and the extinction ratio is 21.2 dB. The polarization direction of the output SC is found to coincide with the pump pulse. When the angle between the polarization of the input pulse and the fast axis is increased to 45 degrees, the output supercontinuum is circularly polarized and becomes narrowest, extending from 900 to 1300 nm. So we can realize the wide tuning of a supercontinuum by only changing the polarization direction of the incident pulse. Under the circumstances, the pulse in optical fiber can be broken into two components along the main axis respectively. If the input polarization direction is away from both principal axis directions, the power along the main axis and the contribution of cross phase modulation are reduced because of the walk-off effect, so the width of the supercontinuum will become narrower. It is suggested that this type of high birefringence photonic crystal fiber could be effectively applied to the generation of the tunable supercontinuum.

2017, 66 (9): 094302.
doi: 10.7498/aps.66.094302

Abstract +

Aiming at the passive impulse wideband source range problem in shallow water waveguides, a passive source range method with single hydrophone based on the matched mode processing is presented in this paper, the method is applied to the shallow water waveguide with a bottom of liquid semi-infinite space. Warping transformation is a useful tool to separate the normal modes of the received signals of the impulse source, and the frequency domain signals of each order can be obtained. The seafloor phase shift parameter is an important parameter describing the acoustic parameters of the seafloor, which contains nearly all the information about sea floor, what is more, the seafloor phase shift parameter is also an parameter that can be obtained by some experimental data easily. Each order normal mode can be represented by the expression that contains the phase shift parameter of sea floor. What is more, the influence of sound speed profile of the waveguide on eigenvalue can be approximately eliminated by jointly processing arbitrary two-order normal modes. Sound speed profile has a similar influence on eigenvalue of each order normal mode, therefore, the difference in the eigenvalues between arbitrary two-order normal modes can be approximated represented by the phase shift parameter of the sea-floor, the sea depth and the mean speed in the waveguide. In this way, the phase replica which consists of the eigenvalue difference of each two-order mode can be calculated simply and quickly, and then by constructing cost function and matching normal mode, the underwater impulse source can be located. Compared with the traditional method of processing matched mode and the method of processing matched fields, the method presented in this paper has two advantages: using warping transformation instead of hydrophone arrays to separate the normal modes; the replica can be calculated quickly and easily, depending on a small number of environmental parameters of waveguide. The effectiveness and accuracy of the method are proved by the results of numerical simulation and sea experimental data processing, in which the signals are both received by a single hydrophone. The sea experimental data contain linear frequency modulation impulse source signal and explosion sound source signal, and the mean relative error of range estimation is less than 10%. In the end of this paper, the range estimation error is analyzed, indicating that the error originates mainly from the mode phase parts besides the phase part of Hankel function. Consequently, finding the ways to reduce the range estimation error is an important project in the future.

2017, 66 (9): 094502.
doi: 10.7498/aps.66.094502

Abstract +

This paper is aimed at building a framework for string stability analysis of traffic flow mixed with different cooperative adaptive cruise control (CACC) market penetration rates. In addition to the string stability, the fundamental diagram of the mixed flow is also taken into consideration for evaluating the effect of CACC vehicles on capacity.
In order to describe the car-following dynamics of real CACC vehicles, the CACC model proposed by PATH is employed, which is validated by real experimental data. The intelligent driver model (IDM) is used as a surrogate car-following model for traditional manual driven vehicles. Based on the guidelines proposed by Ward[Ward J A 2009 Ph. D. Dissertation (Bristol:University of Bristol)], a framework is developed for the analytical investigation of heterogeneous traffic flow string stability. The framework presented considers the instability condition of traffic flow as a linear function of CACC market penetration rate. Following the framework, the string stabilities of the mixed traffic flow under different CACC market penetration rates and equilibrium velocities are analyzed. For fundamental diagram of the heterogeneous traffic flow, the equilibrium velocity-spacing functions of manual vehicles and CACC vehicles are obtained respectively based on car-following model. Then, the fundamental diagram of the density-velocity relationship of the heterogeneous traffic flow is derived based on the definition of traffic flow density. In addition, the theoretical fundamental diagram is plotted to show the property of traffic throughput. The numerical simulations are also carried out in order to investigate the effect of CACC vehicle on the characteristics of fundamental diagram. Besides, sensitivity analyses on CACC desired time gap are conducted for both string stability and fundamental diagram.
Analytical studies and simulation results are as follows. 1) The heterogeneous traffic flow is stable for different equilibrium velocities and CACC market penetration rates, if manual driven vehicles are stable. Otherwise, the instability of traditional traffic flow is improved gradually with the increase of the CACC market penetration rate. Additionally, the stability will become better when equilibrium velocity is away from the velocity range of 9.6-18.6 m/s. 2) Because CACC vehicles can travel at free-flow speed in a relatively small headway, CACC vehicles can improve the capacity of heterogeneous traffic flow. 3) The results of sensitivity analysis indicate that with the increase of the CACC desired time gap, the stable region of heterogeneous traffic flow increases. However, the capacity of the fundamental diagram drops. Therefore, the value of the desired time gap should be determined with considering the effects of the two aspects on the heterogeneous traffic flow. It is noted that the CACC model used in this paper is based on the current state-of-the-art real CACC vehicle experiments. In the future, more experimental observations will yield new CACC models. However, the framework presented in this paper can still be used for the analytical investigation of string stability of the heterogeneous traffic flow at that time.

2017, 66 (9): 094202.
doi: 10.7498/aps.66.094202

Abstract +

Fused silica is an indispensable basic element in a laser system and the weakest link in all components. When the laser interacts with fused silica, the target absorbs the laser energy so that its own temperature rises, and then it melts and vaporizes. The vaporization of the target gasification further absorbs the laser energy and produces a low density ionization reaction, resulting in the laser supported combustion wave (LSCW) phenomenon.
In this paper, taking into account the effects of temperature residual, change in target morphology, distribution of splash material, and distribution of target surface airflow condition, we model and simulate the process of LSCW in stages. The laser energy transfer process, including the inverse bremsstrahlung radiation, thermal radiation, heat conduction and convection processes, is simulated by establishing a two-dimensional axisymmetric gas dynamic model. In addition, the LSCW in the visible light band has a strong radiation characteristic, which is significantly different from the laser induced target melting and vaporization phenomenon. The LSCW is easily received and displayed by high-speed camera. Therefore, a shadow system is established to measure the expanding velocity of the combustion wave in the process of fused silica damaged by laser, and the evolution process image of the combustion wave is obtained.
The results show that under the action of parallel laser beam, the propagation of the combustion wave is in a steady-state and the gas dynamic behavior is stable. For the pulse widths of 1 ms and 3 ms, the average propagation velocity of the LSCW is calculated to be about 24 m/s, which is consistent with the experimental result in the literature available. This verifies the correctness of our theoretical model. For the pulse width of 3 ms, the average velocity of the flow field near the wavefront is calculated to be about 200 m/s. The numerical relationship between the velocity of the flow field and the propagation velocity of the LSCW is also basically consistent with the theoretical derivation result. Under the action of focused laser beam, the propagation of the combustion wave is unsteady. For the pulse widths of 1 ms, the laser intensity at the front of the plasma decreases gradually and the beam radius becomes larger. For the pulse width of 1.8 ms, both a similar pattern of mushroom cloud in the combustion wave and turbulence are observed, which is basically consistent with the evolution process of the combustion wave appearing in our experiment. The simulation results are in good accordance with the experimental results, and also provide a theoretical and experimental basis for studying the LSCW of fused silica.

2017, 66 (9): 094203.
doi: 10.7498/aps.66.094203

Abstract +

Quantum cascade lasers (QCLs) are relatively new sources of mid-infrared radiation (between 2.5 m and 25 m), and are very well suited to the application of in-field trace gas sensing, mainly due to their superiority of being robust, compact, wavelength-versatile, narrow line width and low power consumption. All these advantages make the laser absorption spectroscopy based on QCL light sources become one of the most popular technologies for the quantitative chemical detection in a variety of fields including atmospheric environmental monitoring, chemical analysis, industrial process control, medical diagnostics, security or bio-medical studies, etc.
In the present work, a highly sensitive mid-infrared gas sensor employing a single continuous-wave distributed feedback QCL and an astigmatic multi-path optical absorption cell is demonstrated for the simultaneous measurement of atmospheric carbon monoxide (CO), nitrous oxide (N2O) and water vapor (H2O). By combining with an adaptive Savitzky-Golay (S-G) filter signal processing algorithm, the detection sensitivity and spectral resolution of the QCL sensor system are significantly improved. Compared with the traditional wavelet transform based signal de-noising technique, the developed adaptive S-G smoothing filter shows obvious advantages in terms of computational efficiency and selection of the optimal filter parameters, namely only two filter parameters (the width of the smoothing window and the degree of the smoothing polynomial) need to be considered. Currently, the QCL sensor system is estimated for the long term measurement of ambient air in laboratory environment. The results show that measurement precisions of 8.20 ppb (1 ppb=10-9) for CO, 7.90 ppb for N2O, and 64.00 ppm (1 ppm=10-6) for H2O at 1 s time resolution and 1 atmospheric pressure (atm) are obtained by using the quadratic differential detection scheme, which can be further improved to 1.25 ppb (for CO), 1.15 ppb (for N2O) and 35.77 ppm (for H2O) by increasing average time up to 85 s, respectively. On the whole, the QCL sensor system has significant features of portability and low-cost, moreover, it can be easily modified for the real-time analysis of other gas molecules through the choosing of corresponding QCL light sources. The QCL gas sensor can be widely used in the field of atmospheric chemistry and other applications. Future work will focus on H2O induced broadening coefficients for CO and N2O transitions near 4.57 m, which will be updated for the developed multi-species QCL sensor system, thus resolving the influence of water vapor broadening effect and achieving the measurement of gas concentration in a high humid environment with sub-percent precision.

2017, 66 (9): 094205.
doi: 10.7498/aps.66.094205

Abstract +

Fiber-optic temperature sensors have gained much attention owing to their intrinsic features of light weight, immunity to electromagnetic interference, and capability for distributed measurement. Especially, temperature sensors based on Fabry-Perot interferometers (FPIs) are attractive owing to their advantages of compact size and convenient reflection measurement. However, due to the low thermal expansion or/and thermo-optic coefficient of fiber, the temperature sensitivities of these sensors are normally low (~10 pm/℃ or even lower). In order to improve the temperature sensitivity, a device with dual cascaded FPIs is proposed and demonstrated in this paper, which works on vernier effect and exhibits a much higher temperature sensitivity. The device is fabricated by splicing a short segment of large mode area (LMA) fiber to a short segment of capillary tube fused with a section of single-mode fiber to form an extrinsic Fabry-Perot interferometer with a glass cavity cascaded to an intrinsic FPI with a narrow air cavity. By setting the lengths of capillary tube and LMA fiber to allow similar free spectral ranges to be obtained, and superimposing of the reflection spectra of the two FPIs, the vernier effect can be generated. Firstly, the principle of temperature sensing based on vernier effect of this device is analyzed and simulated theoretically, and it is found that the temperature sensitivity can be improved significantly by using vernier effect compared with that of a single FPI with an air-cavity or glass cavity by directly tracing resonant dips/peaks. Then, the temperature responses of the FPI with single air-cavity and dual cascaded cavities are measured, respectively. Experimental results match well with the theoretical analysis carried out. The temperature sensitivity of the proposed sensor is improved greatly from 0.71 pm/℃ for a single FPI sensor with an air-cavity to 179.30 pm/℃ by employing the vernier effect. Additionally, the sensor exhibits good repeatability in a temperature range of 100-500℃. The proposed sensor has the advantages of compact size (1 mm in dimension) and high sensitivity, which makes it promising for temperature sensing in a variety of industries, such as food inspection, pharmacy, oil/gas exploration, environment, and high-voltage power systems.

2017, 66 (9): 094206.
doi: 10.7498/aps.66.094206

Abstract +

Ultra-short and ultra-intense laser is one of the hottest research spot of laser technology and strong field physics, due to its challenging and the frontier application research. As the key specification of ultrafast ultrahigh intensity laser pulse, the contrast ratio is very influential on the effect of laser-matter interaction. To perform the laser-matter interaction experiments at a high power level, the contrast is required to be as high as 1010 to prevent preplasma dynamics. To solve these problems, one has proposed many methods to improve the contrast of ultrafast laser, such as using the saturable absorbers, double chirped pulse amplification, plasma mirrors and the cross-polarized wave (XPW) generation. The XPW technology can not only enhance the contrast of the pulse by 3-4 orders of magnitude without introducing any space dispersion, but also extend the output spectrum to support shorter pulse duration. The XPW is a nonlinear filter technique in third-order nonlinear crystal with anisotropic susceptibility. Because of its simple and all-solid-state structure, the XPW technique has become one of the most effective methods to enhance the temporal pulse contrast and deliver shorter pulse duration in the field of high peak-power ultrafast lasers. This method has been used in many large laser facilities under construction or upgrades, such as the Apollon and ELI, the contrast ratio as high as 1010 has been achieved. It is known that the conversion efficiency and spectral characteristics of XPW have a strong dependence on the spatial and temporal magnitudes of the input driving pulse. In our experiment, it is found that the various changes of the driven pulse properties have different influences on the characteristics of XPW pulses. The relationship between the linear dispersion of driven pulse and temporal property of XPW is investigated theoretically. In addition, an experiment on verifying the theory is conducted by taking advantage of a programmable acousto-optic dispersion filter. The experimental results fit well to the theoretical results while some new phenomena emerge when the intensity in the BaF2 crystal reaches a saturation threshold. The spectral broadening capability of XPW becomes stronger and exceeds a theoretical upper limit. The pulse width can also be compressed to shorter than the theoretical limit. It is found that there are significant differences in spectral shape and conversion efficiency between the XPW signals by applying the opposite linear chirps to the driving pulse. A further analysis and theoretical explanation of these new phenomena are also presented.

2017, 66 (9): 094207.
doi: 10.7498/aps.66.094207

Abstract +

In oxy-fuel combustion with CO2 recycle, the non-gray gas radiative heat transfer characteristics of gaseous participating media are different from those in air-fuel combustion. Therefore, the choice of a non-gray gas radiation model should be carefully made since it plays an important role in modeling the oxy-fuel combustion system. Using the statistical narrow-band model as a benchmark, in this paper we provide a comprehensive assessment of the development of the weighted-sum-of-gray-gase (WSGG) model, which has been achieved in recent years. The results show that the predicted values obtained by the WSGG model are generally reasonably accurate, though some significant differences still exist. For the total emissivity, the WSGG models by Dorigon et al. (2013 Int. J. Heat Mass Transfer 64 863) and Bordbar et al. (2014 Combust. Flame 161 2435) are consistent well with the benchmark model, within a relative error of less than about 20%. Under the conditions of PH2O/PCO2=1 and 2, the magnitudes of radiative heat transfer between two planar plates are calculated using the discrete-ordinate method and WSGG model. It is found that the radiative source and radiative net heat flux obtained using the WSGG model parameters of Dorigon et al. and Bordbar et al. are more accurate than using other parameters developed in the literature (about 10% relative errors). It is worth noting that the WSGG model parameters of Jonhansson et al. (2011 Combust. Flame 158 893) and Bordbar et al. have a wider range of applications.

2017, 66 (9): 094301.
doi: 10.7498/aps.66.094301

Abstract +

In this paper, a new type of acoustic metamaterial with negative modulus is proposed, and the formation and broadening mechanism of the low frequency bandgap are revealed. The expression of the normalized effective modulus of the structure is derived theoretically. Since the zero value of the effective modulus is closely related to the system parameters, the appropriate parameters can be adjusted to reduce the zero point, and the lower bound of the bandgap is reduced, thus the low-frequency bandgap is realized. The theoretical results show that the elastic modulus of the system is negative and the region of the negative modulus is widened in a certain frequency range, therefore, the widening of the bandgap can be realized through the enlargement of the negative modulus region. This new mechanism for achieving low-frequency bandgap overcomes the shortcomings both in the traditional local resonance with too large additional mass, and in the inertial amplification structures with narrow bandgaps. At the same time, the transmission of this periodic structure obtained by the finite element method is highly consistent with that by the theoretical analysis, with a low-frequency band of 40-180 Hz, from which the new mechanism presented here is verified. This new idea of achieving low-frequency bandgap is of great theoretical significance for controlling low-frequency sound waves.

2017, 66 (9): 094501.
doi: 10.7498/aps.66.094501

Abstract +

The external environment affects the structural form of biological system. Many biological systems are surrounded by cell solutions, such as DNA and bacteria. The solution will offer a viscous resistance as the biological system moves in the viscous fluid. How does the viscous resistance affect the stability of biological system and what mode will be selected after instability? In this paper, we establish a super-long elastic rod model which contains the viscous resistance to model this phenomenon. The stability and instability of the super-long elastic rod in the viscous fluid are studied. The dynamic equations of motion of the super-long elastic rod in viscous fluid are given based on the Kirchhoff dynamic analogy. Then a coordinate basis vector perturbation scheme is reviewed. According to the new perturbation method, we obtain the first order perturbation representation of super-long elastic rod dynamic equation in the viscous fluid, which is a group of the second order linear partial differential equations. The stability of the super-long elastic rod can be determined by analyzing the solutions of the second order linear partial differential equations. The results are applied to a twisted planar DNA ring. The stability criterion of the twisted planar DNA ring and its critical region are obtained. The results show that the viscous resistance has no effect on the stability of super-long elastic rod dynamics, but affects its instability. The mode selection and the influence of the viscous resistance on the instability of DNA ring are discussed. The amplitude of the elastic loop becomes smaller under the influence of the viscous resistance, and a bifurcation occurs. The mode number of instability of DNA loop becomes bigger with the increase of viscous resistance.

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

2017, 66 (9): 096801.
doi: 10.7498/aps.66.096801

Abstract +

The transmittance diminishment of solar cells, caused by dust accumulation is higher than 52.54% every year (2006 Energ. Convers. Manage. 47 3192), which greatly reduces their overall efficiencies of power conversion. Any other strategy for improving the photovoltaic device cannot compensate for this loss caused by the dust. However, this critical issue has not received much attention. In this work, a kind of self-cleaning coating consisting of ZnO nanowire-silicon pyramid hierarchical structures is proposed to overcome the dust accumulation on the photovoltaic device. The principle of designing this self-cleaning is based on the Cassie-Baxter theory. Both the micron size effect for superhydrophobicity and the performance of anti-reflection of light of the substrate should be retained, which are the requirements of application of solar cell. The pyramid-like silicon (named silicon pyramid, hereafter) is fabricated by simple chemical etching. The effects of isopropanol, KOH, etching time, and etching temperature on the morphology of the silicon pyramid are investigated by using systematic statistical design and analysis method, to obtain the best distribution and size of the silicon pyramid. In the systematic statistical design and analysis method, the pick-the-winner rule is adopted. Eventually, we find that the optimized conditions for etching silicon pyramid (according the requirements of self-clean) are as follows: etching time is 60 min, etching temperature is 95℃, and mixture is 80 mL DI water, 2.9598 g KOH and 20 mL isopropanol. Moreover, ZnO nanowire-silicon pyramid hierarchical structures for the application of photovoltaic device are successfully hydrothermally grown on the substrate of silicon pyramid for the first time. The obtained self-cleaning coating consists of ZnO nanowire (with a diameter of 136 nm) and silicon pyramid (with a size of 8-11 m). The surface of this coating possesses superhydrophobic properties, i.e., a water contact angle of 154 and a contact angle hysteresis of less than 10, after being modified by heptadecafluorodecyltrimethoxysilane. Also, our obtained ZnO nanowire-silicon pyramid hierarchical structures have quite a good performance of anti-reflection, which appear gray in the normal environment. And the mechanism for it is postulated. Importantly, some new phenomena, such as high temperature improving the growth of silicon pyramid, are also revealed. Besides, the physical mechanism for high temperature improving the growth of silicon pyramid and anisotropic etching of silicon substrate is discussed. It is indicated that the anisotropic behavior is attributed to small difference in energy level (being a function of the crystal orientation) between the back-bond surface states. The method we proposed to achieve self-cleaning coating is versatile, reliable and low-cost, which is also compatible with contemporary micro-and nano-fabrication processes.

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

2017, 66 (9): 097101.
doi: 10.7498/aps.66.097101

Abstract +

Oxide indium gallium zinc thin film transistor (IGZO TFT) is a promising candidate for mass production of next-generation flat panel display technology with high performance. This is due to many merits of IGZO TFTs, such as high mobility, excellent uniformity over large area, and low cost. In recent years, IGZO TFTs with dual gate structure have attracted enormous attention. Compared with the conventional single gate IGZO TFTs, the dual gate IGZO TFTs have many advantages including increased driving ability, reduced leakage current, and improved reliability for both negative biasing stressing and positive biasing stressing. Although the measurement results of fabricated circuit samples have proven that dual gate IGZO TFTs are beneficial for the integration of digital circuit and active matrix light emitting display with in-array or external compensation schematics, there has been no proper analytic model for dual gate IGZO TFTs to date. As the analytic model is crucial to circuit simulations, there are great difficulties in circuit designs by using dual gate IGZO TFTs. Although there are some similarities between the operating principal of the dual gate IGZO TFTs and that of the dual gate silicon-on-insulator devices, the complexity of conducting mechanism of IGZO TFTs is increased due to the existence of sub-gap density of states (DOS) in the IGZO thin film. In this paper, an analytical channel potential model for IGZO TFT with synchronized symmetric dual gate structure is proposed. Gaussian method and Lambert function are used for solving the Poisson equation. The DOS of IGZO thin film is included in the proposed model. Analytical expressions for the surface potential (S) and central potential (0) of the IGZO film are derived in detail. And the proposed channel potential model is valid for both sub-threshold and above-threshold region of IGZO TFTs. The influences of geometry of dual-gate IGZO TFT, including thickness values of gate oxide layer and IGZO layer, on the device performance are thoroughly discussed. It is found that in the case of small gate-to-source voltage (VGS), as the conducting of IGZO layer is weak, both S and 0 increase linearly with the increase of VGS due to the increase of voltage division between the oxide and IGZO layer. However, the increase of S and 0 starts to saturate once VGS is larger than threshold voltage due to the shielding of electrical field by the induced electron layer of IGZO surface. With the evolution of VGS, the calculated results of S and 0 by using the proposed dual gate IGZO TFT model are in good agreement with the numerical results by technology computer aided design simulation method. Therefore, the proposed model is promising for new IGZO TFT electronics design automation tool development.

2017, 66 (9): 097301.
doi: 10.7498/aps.66.097301

Abstract +

The micro-nano-scale science is rapidly developing. In order to study the properties of micro-nano-scale particles by the optical method, we discuss the scattering effects of sub-micrometer wires and sub-micrometer balls on photo-electromagnetic wave in this paper. For the optical scattering of micro-nano-scale particles, the scattering particle size can meet the Mie scattering conditions compared with the incident light wavelength, that is to say, the scatterer and the incident wavelength have comparable size. In this article, the analysis results are clearly displayed in the form of simulation graphs obtained by the Matlab numerical simulation. The Mie scattering analysis method can be used for discussing the scattering of electromagnetic waves in the cases of layered particles which meet the size requirements and any number of scattering particles. Multi-particle scattering is analyzed to investigate the effects of scatterers at different positions on the scattering. By analyzing the differential scattering cross section and the electromagnetic field distribution of near-field scattering related to the scattering light field, we obtain the variation trend of the scattering light field with scattering angle and the effects of various factors on the scattering light field, including the polarization of incident wave, the size of the scatterer, the structure of scattering particles, the number of particles, the number of scattering particles, and some hidden factors such as the relative refractive index of scatterer and surrounding medium. The scientific significance of the paper is reflected through the fact that the sub-micron scale particle can be used as a sensor of detecting the displacement, which can be realized by optical means. So it has a certain reference value for studying the influence of particle own characteristic on the scattering light, thereby rendering the optical readout of the mechanical displacement very accurate. The obtained results have a guiding significance for studying the optical detection of mechanical vibrations of sub-micron wires.

## EDITOR'S SUGGESTION

2017, 66 (9): 097302.
doi: 10.7498/aps.66.097302

Abstract +

With the increase of integration scale, heat dissipation becomes one of the major problems in high density electronic devices and circuits. Controlling and reusing the heat energy in such miniaturized structures are essential topics for current and future technologies. With the development of microfabrication technology and low-temperature measurement technology in the last two decades, the thermoelectric measurement in low-dimensional sample has been feasible, and the thermal transport has received more and more attention. For the multi-terminal device, there is a novel thermoelectric phenomenon, called the spin Nernst effect, in which spin currents (or spin voltages) are generated perpendicularly to the temperature gradient. The spin Nernst effect has been confirmed experimentally, and has been theoretically studied in a variety of materials. In this paper, the spin and charge Nernst effect in a pair of vertically aligned quantum dots attached to four leads are studied in the Coulomb blockade regime based on the nonequilibrium Green's function technique. We focus on the influences of magnetic configuration and intra-dot (inter-dot) Coulomb interaction on the spin and charge Nernst effect. It is found that the signs and the magnitudes of spin and charge Nernst effect can be modulated by adjusting the magnetization directions of ferromagnetic electrodes. When the magnetic moments in the 1 and 3 electrodes are turned to antiparallel alignment, the pure spin Nernst (without charge Nernst) effect can occur by applying a transverse temperature gradient. Conversely, the spin and charge Nernst effect disappear if the magnetic moments of lead 1 and lead 3 are in the case of parallel configuration. Except for left and right thermal leads, we investigate the effect of the middle lead (lead 4) on the property of the Nernst effect. We find that when the normal metal lead 4 is transferred to ferromagnetic metal, the spin and charge Nernst effect both can be obtained simultaneously. In the end of the paper, we study the influences of intra-dot and inter-dot Coulomb interaction on the spin dependent Nernst coefficient. Through numerical calculations, we demonstrate that the magnitude of the Nernst effect is less dependent on the polarization strength of ferromagnetic electrodes, but can be remarkably enhanced by the Coulomb blockade. The spin Nernst coefficient is predicted to be more than two orders of magnitude larger than that of the case of zero Coulomb interaction. All the results indicate that the proposed four-terminal double quantum dot nano system is a promising candidate for spin caloritronic device.

2017, 66 (9): 097701.
doi: 10.7498/aps.66.097701

Abstract +

Epoxy resin nanocomposites have excellent properties such as the suppression of space charge accumulation, high resistivity, and high electrical breakdown strength, which play an important role in developing the direct current power equipment. However, the influencing mechanisms of filler content on trap, conductivity, and space charge of nanocomposites have not been clear to date. In the present paper, a method to calculate the densities of shallow traps and deep traps in interaction zones is proposed based on the multi-region structure model of interaction zones, and the dependence of shallow traps and deep traps on filler content is obtained. It is found that the shallow trap density increases with the increase of filler content, while the deep trap density first increases and then decreases with increasing the filler content, which is caused by the overlap of interaction zones. Then, the relation between the shallow trap controlled carrier mobility and the filler content is investigated. With the filler content increasing, the density of shallow traps increases and their mean distance decreases, leading to an increase in the shallow trap controlled carrier mobility.
Considering the charge injection from cathode into dielectrics, carrier hopping in shallow traps, charge trapping into and detrapping from deep traps, a unipolar charge transport model is established to study the conductivity and distributions of space charges and electric field in epoxy resin nanocomposites. At relatively low filler content, the charge transport is dominated by deep traps in interaction zones and the conductivity decreases with the increase of filler content. However, the charge transport is determined by shallow traps at relatively high filler content and the conductivity increases.

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

2017, 66 (9): 098501.
doi: 10.7498/aps.66.098501

Abstract +

Molecular electronics offers new possibilities for continually miniaturizing the electronic devices beyond the limits of standard silicon-based technologies. There have been significant experimental and theoretical efforts to build thiophene molecular junctions and study their quantum transport properties. However, in most of these studies Au is used as lead material. It is well known that the fabrication of the traditional molecular device is now hindered by technological difficulties such as the oxidation of metallic contacts, and the interface instability between the organic molecule and the inorganic metallic electrodes. In this paper, we use the graphene electrodes to construct a series of thiophene-based devices. The graphene electrodes proposed in this paper are able to avoid such problems. Moreover, the stability of graphene electrodes at room temperature paves the way to studying the electron transport through a single molecule under the ambient conditions. Firstly, we design a series of molecular rectifying devices based on thiophene dimer and its derivatives, in which the hydrogen atom on the thiophene monomer is substituted with a representative electron-donating group (NH2) and electron-withdrawing group (NO2). Secondly, we investigate systematically the electronic transport properties through these molecular junctions by performing the first principles calculations based on density functional theory and nonequilibrium Green's function. The calculated results show that these thiophene molecular devices substituted with NH2 and NO2 possess the rectifying behavior and negative differential resistance properties. Furthermore, we also find that the position of substituent group NH2 or NO2 has a major influence on the electronic transport properties. In order to explore the physical mechanism behind these transport properties, the electronic structures of the molecules, the transmission spectrum, and the molecular projected eigenstates are analyzed. The results reveal that the position of NH2 can adjust the intensity of the negative differential resistance. When the NH2 group is close to the molecular end, the negative differential resistance behavior in this molecular device is more prominent than in other molecules. In addition, the position of NO2 can change the direction of the rectification. When the NO2 group is close to the molecular end, the current in negative bias is larger than in positive bias, resulting in a negative rectification. In contrast, when the NO2 group is close to the molecular centre, a positive rectification occurs. Our results can provide a worthy complement to thiophene molecular experiment, and also has a guiding significance for designing other molecular electronic devices.

2017, 66 (9): 098101.
doi: 10.7498/aps.66.098101

Abstract +

Metalorganic chemical vapour deposition and molecular beam epitaxy have already been demonstrated to be successful techniques for obtaining high-quality epitaxial GaN layers with low impurity concentrations and pretty good electrical properties. However, high growth temperature employed in both of these methods give rise to some intrinsic defects of the thin films, such as high background-carrier concentrations. As a low-temperature thin film deposition method, plasma-enhanced atomic layer deposition (PE-ALD) has more unique advantages compared to both methods for epitaxial growth of GaN. In this paper, the polycrystalline GaN thin films were fabricated on Si (100) substrates at 150-300℃ by PE-ALD. Trimethylgallium and N2/H2 plasma gas mixture were used as the Ga and N precursors. The growth rate of the thin films was demonstrated by the spectroscopic ellipsometer. The crystal structrue and composition of the GaN thin films were characterized by X-ray diffractometer and X-ray photoelectron spectrometer (XPS). It is showed that the growth window for PE-ALD grown GaN thin films is 210-270℃, where the growth rate remains constant at 0.70 /cycle. And it is known that it is the self-limiting nature of PE-ALD that is ascribed to the plateau of the growth rate. Films grown at relatively higher temperature are polycrystalline with a hexagonal wurtzite structure, while films grown under relatively lower temperature are amorphous. The grazing incidence X-ray diffraction (GIXRD) patterns of the polycrystalline thin films reveal three main peaks located at 2=32.4, 34.6 and 36.9, which are corresponding to the (100), (002) and (101) reflections. It is showed that the Ga, N atoms would get higher energy for more effective migration to positions with lowest energy to form ordered crystalline arrange at higher growth temperature. The XPS results show that all the N elements of the as-grown thin films are in the form of NGa bond, indicating that all the N elements are formed into GaN thin films; and there is a little amount of the Ga elements that exist in GaO bond. The fact that there is no Ga2O3-related peaks in the GIXRD pattern suggests that there is small amount of amorphous Ga2O3 dispersed in the polycrystalline GaN thin films. In the future work, reducing the concentration of the C and O impurities may be achieved by increasing the time of the reaction and plasma pules in the process formula and replacing the inductively coupled plasma with the hollow cathode plasma, respectively.

2017, 66 (9): 098301.
doi: 10.7498/aps.66.098301

Abstract +

The design of gradient coils for a magnetic resonance imaging (MRI) system is a multiple objective optimization problem, which usually needs to deal with a couple of conflicting design objectives, such as the stored magnetic energy, power consumption, and target linear gradient distribution. These design requirements usually conflict with each other, and there is no unique optimal solution which is capable of minimizing all objectives simultaneously. Therefore, the design of gradient coils needs to be optimized reasonably with the tradeoff among different design objectives. Based on the developable property of the super-elliptical cylindrical surface and the stream function design method, the multiple objective optimization problem is analyzed by using the Pareto optimization method in this paper. The effect of proposed approach is illustrated by using the stream function method and three aforementioned coil design objectives are analyzed. The influences of the stored magnetic energy and power consumption target on linearity of gradient coil and the configuration of coils are analyzed respectively. The suitable sizes of gradient coils are discussed by analyzing the change of the stored magnetic energy. A weighted sum method is employed to produce the optimal Pareto solutions, in which the multiple objective problem reduces into a single objective function through a weighted sum of all objectives. The quantitative relationship of each design requirement is analyzed in the Pareto solution space, where Pareto optimal solutions can be intuitively found by dealing efficiently with the tradeoff among different coil properties. Numerical examples of super-elliptical gradient coil solutions are provided to demonstrate the effectiveness and versatility of the proposed method to design super-elliptical gradient coils with different coil requirements. The optimization results show that there are multiple available solutions in the convex Pareto solution space under the constraints that the linear gradient deviation is less than 5% and the magnetic stored energy and power dissipated are both no more than user-preset values. In the case that the values of summed objective functions are the same, the proposed method can intuitively see the performance of each individual target, thereby conducting to realizing the final design of gradient coils under the different design requirements. With the proposed approach, coil designers can have a reasonable overview of gradient coil design about the achievable performances of some specific properties and the competing or compatible relationships among coils properties. Therefore, a suitable design of the gradient coils for a given requirement of MRI application can be chosen reasonably.

2017, 66 (9): 098401.
doi: 10.7498/aps.66.098401

Abstract +

With the rapid development of the microwave photonic communication technology, the frequency of the microwave signal is expanded to the Ka waveband, since most of low frequency bands are occupied. However, the current commercial detectors and signal processing modules are limited by bandwidth. Therefore, the traditional method of directly detecting the microwave signals cannot meet the actual demands. It is essential to achieve the microwave photonic down-conversion from the high frequency microwave signal (~10 GHz) to the lower frequency signal (~100 MHz). Meanwhile, the down-conversion low frequency signal can be processed by the existing mature technology and low cost devices. The microwave down-conversion link can effectively avoid leaking the local oscillator, and it possesses many advantages such as high bandwidth and spurious free dynamic range, low loss and low noise.
In this paper, a microwave photonic down-conversion system is presented based on the integrated dual-parallel Mach-Zehnder modulator (DPMZM) to increase the spurious-free dynamic range as well as conversion efficient of microwave photonic link. The integrated DPMZM is mainly comprised of two intensity modulators (MZM-a and MZM-b), and a phase shifter. The radio frequency (RF) signal is loaded into DPMZM to modulate the optical signal. The local oscillator is loaded into the MZM-a to produce the 1st local oscillator sideband, and two RF signals are fed to the MZM-b to form the 1st RF signal sideband. The direct current bias of the DPMZM is adjusted to output a high carrier suppressed double sideband (DSB) signal. The erbium-doped fiber amplifier is used to increase the power of light to match the power range of the detector. The RF signal sideband and local oscillator sideband are mixed to produce the beat frequency, and the frequency down-conversion can be achieved. The principle of frequency down-conversion is elaborated by theoretical analysis. The conversion efficiency and spurious free dynamic range are analyzed and simulated. On this basis, the microwave photonic link of frequency down-conversion is built. The performance of the system is tested. The ratio of optical carrier power to sideband power of the DSB signal is 26 dB. The experimental result shows that the conversion efficiency is 7.43 dB and spurious-free dynamic range is 110.85 dB/Hz2/3. The down-conversion method based on the DPMZM can optimize the output spectrum of the sideband. The structure of system is simple and easy to implement, so it is a good option for improving the conversion efficiency and spurious-free dynamic range.

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

## EDITOR'S SUGGESTION

2017, 66 (9): 099501.
doi: 10.7498/aps.66.099501

Abstract +

The single lobe far-field patterns produced from terahertz quantum cascade lasers (QCLs) are greatly demanded for various applications, such as imaging, data transmission, etc. However, for a ridge waveguide terahertz QCL, the far-field beam divergence is large due to the fact that the waveguide aperture is far smaller than the terahertz wavelength. This is the case typically for double-metal waveguide terahertz QCL which emits terahertz photons in almost every direction in the space. Even for a single plasmon waveguide terahertz QCL, the divergence angle is as large as 30 in both horizontal and vertical direction. Here, in this work we design and fabricate a double metal third-order distributed feedback terahertz QCL emitting around 4.3 THz, and investigate the characteristics of the longitudinal and transverse modes. This work aims to achieve high beam quality for terahertz QCL by exploiting the third-order distributed feedback geometry, and in the meantime to achieve single longitudinal mode operation. The electromagnetic field distribution in the waveguide is modelled by employing a finite element method. The mode selection mechanism is studied by using the eigen frequency analysis, and the far-field beam is simulated by applying the near-field to far-field Fourier transform technique. The QCL active region used in this work is based on the resonant-phonon design, which is grown by a molecular beam epitaxy (MBE) system on a semi-insulating GaAs (100) substrate. The wafer bonding and traditional semiconductor device fabrication technology, i.e., optical lithography, electron beam evaporation, lift-off, wet and dry etching, are used to process the MBE-growth wafer into the third-order distributed feedback geometry with double-metal waveguides. By carefully designing the grating structures and optimizing the fabrication process, we achieve third-order distributed feedback terahertz QCL with quasi-single-longitudinal mode operation and single lobe far-field beam pattern with low beam divergence in both vertical and horizontal directions. The effect of grating duty cycle on the far-field beam divergence is systematically studied theoretically and experimentally. By the simulation, we finally achieve the divergence angle of 1213 for a third-order distributed feedback laser with a grating duty cycle of 12% that results in an effective refractive index close to 3. The experimental results show good agreement with the simulation. There is still room to further reduce the beam divergence of third-order distributed feedback terahertz QCL by improve the accuracy of the simulation and the fabrication.

###### NUCLEAR PHYSICS

2017, 66 (9): 092101.
doi: 10.7498/aps.66.092101

Abstract +

Cr-C system is an important protective coating material for its high hardness, good corrosion resistance and electrical conductivity. It is also a typical eutectic system, where all stable phases are involved in the eutectic reactions. According to our previous work, binary eutectic liquids satisfy the dual-cluster short-range-order structural model, i.e., a eutectic liquid is composed of two stable liquid subunits respectively issued from the two eutectic phases and each one formulates the same ideal metallic glass [cluster] (glue atom)1 or 3, where the nearest-neighbor cluster is derived from a devitrification phase. Therefore a eutectic liquid can always be formulated as two nearest-neighbor clusters plus two, four, or six glue atoms. The key step towards understanding a eutectic composition is then to obtain the right clusters from the two eutectic phases for use in the formulation of the glassy/eutectic composition, which we call the principal clusters. In this paper, Friedel oscillation and atomic dense packing theories are adopted to identify the principal clusters of Cr-C eutectic phases for the objective of establishing the dual cluster formulas for the eutectic compositions. First, clusters in eutectic phases Cr, Cr23C6, Cr7C3 and Cr3C2 are defined by assuming that all the nearest neighbors are located within the first negative potential minimum zone in Friedel oscillation, which causes a cutoff distance to be less than 1.5 times the innermost shell distance. Second, by comparing all the radial distribution profiles of total atomic density centered by each cluster in a given phase structure, the one exhibiting the most distinct spherical periodicity feature is selected as the principal cluster. Moreover, the principal clusters are the most separated from each other among all the clusters in the same phase, showing the highest degree of cluster isolation. Under the criteria of the cluster distribution following spherical periodicity order and of the cluster isolation, the following principal clusters are derived: rhombidodecahedron CN14 [Cr-Cr14] from Cr, capped trigonal prism CN9 [C-Cr9] from Cr23C6 and Cr7C3, and [C-Cr8] from Cr3C2. Via these examples, the principal cluster identification procedures are detailed. Third, the thus selected principal clusters are matched with appropriate glue atoms to construct the dual cluster formulas for the Cr-C eutectics Cr86C14 and Cr67.4C32.6, i.e., [Cr-Cr14+C-Cr9]CrC3Cr86.2C13.8 and [C-Cr9+C-Cr8]C6Cr68.0C32.0, respectively. This work proves the universality of the cluster-plus-glue-atom model in explaining the composition of binary eutectics and lays a theoretical foundation for the composition design of Cr-C based materials.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2017, 66 (9): 095201.
doi: 10.7498/aps.66.095201

Abstract +

As an important analytical tool, laser-induced breakdown spectroscopy (LIBS) has been widely used in material analysis, environmental monitoring, and other fields. In recent years, due to increasingly serious air pollution, various LIBS-based on-line air pollution detection techniques are being developed. The temporal evolution of nitrogen plasma characteristics is of great importance for investigating the atmospheric plasma dynamics and developing the LIBS-based air pollution monitoring techniques. Temperature and electron density, which are the most important parameters of a plasma state, directly influence the kinetic behaviors of plasma formation, expansion and degradation processes, as well as the energy transfer efficiency in plasma. In this paper, the temporal evolutions of continuous background radiation, molecular spectral strength, and signal-to-background ratio (SBR) are studied based on time-resolved spectra. The results show that the lifetime of the continuous background radiation is about 700 ns, the N2+(B2u+-X2g+, v: 0-0) transition line strength reaches a maximum value within 12-15 s, the SBR first increases and then stabilizes. Accordingly, the optimal observation period for N2+(B2u+-X2g+) band system based plasma temperature investigation should be selected to be between 10 and 25 s. The temporal evolution of plasma temperature is determined by fitting experimental spectra to theoretical ones simulated by LIFBASE (a spectral simulation program). As the radiation loss is less than the loss due to the collision cooling, the plasma temperature decays exponentially from ~10000 K to ~6000 K within 10-28 s. By taking into account the instrumental broadening lineshape (Voigt lineshape), the temporal evolutions of Stark broadening and Stark shift of N 746.831 nm atomic line are obtained via Nelder-Mead simplex algorithm, and then the electron density is calculated accordingly. The results show that the electron density decays between 1017 and 1016 cm-3 in magnitude. By comparing the experimental electron decay rate with theoretical values calculated from different mechanisms, it is concluded that a three-body collision recombination is the main mechanism of electron decay.

## EDITOR'S SUGGESTION

2017, 66 (9): 095202.
doi: 10.7498/aps.66.095202

Abstract +

Laboratory astrophysics is a rapid developing field studying astrophysical or astronomical processes on a high-power pulsed facility in laboratory. It has been proved that with the similarity criteria, the parameters in astrophysical processes can be transformed into those under laboratory conditions. With appropriate experimental designs the astrophysical processes can be simulated in laboratory in a detailed and controlled way. Magnetic fields play an important role in many astrophysical processes. Recently, the generation of strong magnetic fields and their effects on relevant astrophysics have attracted much interest. According to our previous work, a strong magnetic field can be induced by a huge current formed by the background cold electron flow around the laser spot when high power laser pulses irradiate a metal wire. In this paper we use this scheme to produce a strong magnetic field and observe its effect on a bow shock on the Shenguang II (SG II) laser facility. The strength of the magnetic field is measured by B-dot detectors. With the measured results, the magnetic field distribution is calculated by using a three-dimension code. Another bunch of lasers irradiates a CH planar target to generate a high-speed plasma. A bow shock is formed in the interaction of the high-speed plasma with the metal wire under the strong magnetic condition. The effects of the strong magnetic field on the bow shock are observed by shadowgraphy and interferometry. It is shown that the Mach number of the plasma flow is reduced by the magnetic field, leading to an increase of opening angle of the bow shock and a decrease of the density ratio between downstream and upstream. In addition, according to the similarity criteria, the experimental parameters of plasma are scaled to those in space. The transformed results show that the magnetized plasma around the wire, produced by X-ray emitted from the laser-irradiated planar target in the experiment, is suitable for simulating solar wind in astrophysics. In this paper, we provide another method to produce strong magnetic field, apply it to a bow shock laboratory astrophysical study, and also generate the magnetized plasma which can be used to simulate solar wind in the future experiments.

2017, 66 (9): 095203.
doi: 10.7498/aps.66.095203

Abstract +

High-power impulse magnetron sputtering (HiPIMS), a new physical vapor deposition technique which combines the advantages of the high ionization rates of the sputtered materials and control of electromagnetism, has been widely used to deposit high-performance coatings with a large density and high adhesion. However, HiPIMS has some intrinsic disadvantages such as the low deposition rate, unstable discharge, and different ionization rates for different materials thereby hampering wider industrial adoption. We have recently designed an optimized cylindrical source based on the hollow cathode effect to circumvent the aforementioned limitations. However, during the operation of the cylindrical source, the discharge is inhomogeneous and the etching stripes are nonuniform. In order to determine the underlying mechanism and optimize the electromagnetic control, the discharge in the HiPIMS cylindrical source is simulated. The tangential magnetic field distribution on the target surface of the cylindrical sputtering source is inhomogeneous and electron runaway is serious, resulting in a relatively low plasma density. Two solutions are proposed to improve the situations. The first one is electrical improvement by installing an electron blocking plate, and the second one is magnetic improvement by adding compensating magnets. Our simulation results of the first method show that a potential well is produced by the electron blocking plate to suppress electron runaway and the plasma density is improved significantly, especially around the central cross-section of the cylindrical sputtering source. The discharge becomes homogeneous, and the etching stripes are uniform albeit not full enough. The second method of magnetic improvement significantly improves the homogeneity of the tangential magnetic field distribution on the target surface and the target utilization rate. After adding the optimized compensating magnets, the shape of the effective area (the value of the tangential magnetic field in a range of 25-50 mT) on the target surface can be controlled and made zonal. The target utilization rate increases to over 80% from 60%. In order to obtain the optimal conditions, the two techniques are combined. A larger and more homogeneous etching ring is observed by adopting both the electrical and magnetic improvements as predicted and explained by the simulation results. It can be concluded that the combination of the two improvement techniques can improve and optimize the HiPIMS cylindrical source.