## Vol. 65, No. 2 (2016)

##### 2016-01-20

###### GENERAL

2016, 65 (2): 020201.
doi: 10.7498/aps.65.020201

Abstract +

Yangian, as an algebra beyond the Lie algebra, is an infinite dimensional algebra and a powerful mathematical method for inVestigating the new symmetry of quantum systems which are nonlinear and integrable. Based on the su(3) symmetry of the quarK-flaVor in the meson states and the transition property of the generators in Yangian algebra, we study the applications of Yangian algebra Y(su(3)) in the decay of three mixed meson states(, K and Ki0) composed of the three positiVe and negatiVe meson states (, K, K0 and K0). As the transition operators, the eight generators (Ī, Ŭ, V, Ī3 and Ī8) of Yangian algebra Y(su(3)) are acting on the three mixed meson states, respectiVely. Then, the possible decay channels and the changes of the entanglement are studied. Results show: (i) Under the effects of Ī3 and Ī8 within the Lie algebra on the three mixed meson states, the compositions of the final states after decays of the three mixed meson states are not changed as compared with the initial state. The entanglement is not changed for the decay of the mixed meson state with the effect of Ī^8, and the others are changed. (ii) Under the effects of the other six generators (Ī, Ŭ and V) beyond the Lie algebra on the three mixed meson states, the compositions of the final states after the decay are changed compared
with the initial state. In the six possible decay channels, the two final states become single states without entanglement; two decay channels are absent; and the entanglements of the final states in the remaining two decays are changed. In addition, the entanglement of the final meson states in the possible six decay channels of the two types K mixed meson states, the charged (K+, K-) and neutral (K0, K0) meson states, are the same two by two. (iii) The three mixed meson states can be circularly transferred by the operators Ī, Ŭ and V, implying the obVious symmetry. In this paper the Yangian method is presented to study the possible decay channels of the mixed meson states and may be used to present a possible interpretation of the new unKnown or Known particle in the decay of the mixed meson.

2016, 65 (2): 020301.
doi: 10.7498/aps.65.020301

Abstract +

The concepts of pseudospin symmetry in atomic nuclei and spin symmetry in anti-nucleon are reviewed. The exploration for understanding the origin of pseudospin symmetry and its breaking mechanism, and the empirical data supporting the pseudospin symmetry are introduced. A noncentral anharmonic oscillatory potential model is proposed, in which a noncentral electric dipole and a double ring-shaped component are included. The pseudospin symmetry for this potential model is investigated by working on a complete square integrable basis that supports a tridiagonal matrix representation of the Dirac wave operator. Then, solving the Dirac equation is translated into finding solutions of the recursion relation for the expansion coefficients of the wavefunction. The angular/radial wavefunction is written in terms of the Jacobi/Laguerre polynomials. The discrete spectrum of the bound states is obtained by diagonalization of the radial recursion relation, and the property of energy equation is discussed for showing the exact pseudospin symmetry. Several particular cases obtained by setting the parameters of the potential model to appropriate values are analyzed, and the energy equations are reduced to that of the anharmonic oscillator and that of the ring-shaped non-spherical harmonic oscillator, respectively. Finally, it is pointed out that the exact spin symmetry exists also in this potential model.

2016, 65 (2): 020302.
doi: 10.7498/aps.65.020302

Abstract +

Based on the input-output relation in low-Q cavities, we propose a feasible scheme to prepare remotely a single-atom state via photonic Faraday rotation, and then the scheme is generalized to the case of remote preparation of a two-atom entangled state. Our results show that when the coefficients of the initial atomic state to be prepared are real, both remote preparation of the single-atom state and that of the two-atom entangled state can be achieved deterministically by selecting appropriate parameters of the systems for the interactions among the atom, polarized single-photon pulse, and cavity field. Compared with the existing schemes for remote preparation of atomic states, in our scheme photons are used as flying qubits to transmit quantum information, which is suitable indeed to achieve a long-distance atomic state preparation in principle. Due to the fact that the information of atomic state is encoded in two degenerate ground-state levels of a -type three-level atom confined in a unilateral dissipative cavity, and that the atoms are only virtually excited, our schemes are insensitive to both cavity decay and atomic spontaneous emission. Besides, the two schemes we proposed do not need two- or multi-particle orthogonal measurements, only product-state measurements are involved, as well as they work in low-Q regime and do not require a strong coupling condition between the atoms and the optical cavities, which greatly reduce the experimental difficulty.

2016, 65 (2): 020303.
doi: 10.7498/aps.65.020303

Abstract +

As one of the most common weathers in daily life, the rain can change the atmospheric compositions and humidity in a short time, which may cause non-ignorable attenuation in free-space quantum communication system. Besides, the absorption and scattering effects caused by raindrops can also bring huge attenuation to photon's propagation. In order to solve this burst interference caused by rain weather, optimal mean photon number per pulse and chameleon self-adaptive algorithm (CSA) are proposed based on the rainfall distribution model and decoy-state quantum key distribution. Due to the lack of producing mature ideal single photon source technology, the decoy-state protocol with highly attenuated laser becomes the most practical and most widely used quantum secure communication protocol currently. Among all the different kinds of decoy-state protocols, the vacuum+weak decoy state quantum communication secure protocol is chosen to be the basis of our research. Besides, in order to study the influence of mean photon number per signal pulse, we set the pulse ratio between signal state, decoy state and vacuum state to be fixed at 2:2:1. Since the performance of the vacuum+weak decoy state quantum communication system is closely related to the mean photon number per pulse, it is very necessary to confirm the optimal value. Combining the Weibull rainfall distribution model and Mie scattering theory, we first analyze the attenuation caused by rainfall in a free-space quantum communication system. Then the functional relationship among opt, rainfall intensity (J) and link distance (L) is built by studying the propagation of highly attenuated laser in depolarizing channel. Finally, two parameters, secure key rate and channel survival function, are chosen to evaluate the system's performance of reliability and validity. These two parameters are respectively compared between the system with and without CSA. Simulation results show that, as J=30 mm/24 h, L=30 km, the secure key generation rate rises from 210-4 up to 3.510-4 when using the CSA in the quantum communication system; as J=60 mm/24 h, L=20 km, the quantum channel survival function value increases from 0.52 to 0.63; as the quantum channel survival function value is required no lower than 0.5, the rainfall intensity in which quantum communication system can survive rises from 62 mm/24 h up to 74 mm/24 h. These results prove that there is a close relationship between opt and the channel parameters of the quantum communication system under the background of rainfall. Therefore, it is necessary for us to self-adapt the opt value by combining rainfall intensity with the CSA strategy if the reliability and survivability of free space quantum communication system are required to be improved.

2016, 65 (2): 020501.
doi: 10.7498/aps.65.020501

Abstract +

Microblog is a social media platform, based on the follower-followee relationship, that enables users to share real-time information, by which the information propagation is characterized as rapid, explosive, and immediate. The research on the information propagation and retweet prediction is very important for public sentiment analysis and product promotion. A majority of existing works adopt several traditional prediction methods to predict the future information retweet based on the features extracted from existing retweet behaviors, which are hard to reconcile accuracy, complexity, robustness and feature extensiveness. To overcome the above mentioned shortcomings in existing works, we propose in this paper a link prediction algorithm based on maximum entropy model to predict retweet behavior on microblog. In our proposed approach, firstly we abstract the retweet prediction problem to a link prediction problem. Then we analyze the retweet behaviors on microblog and determine the factors influencing the retweet behavior. We extract the features from the retweet behaviors based on these factors in the next step. Now based on these features, the retweet behavior could be predicted by the proposed approach. However, information redundancy and other issues may exist among these features. These issues will cause an increase in computational complexity or a decrease in computational accuracy. To solve the above problems, we selecte the features dominating the retweet behavior with feature selection methods such as Information Gain, IG-CHI. The proposed model requires no further independent assumption in features or intrinsic constraints, and omits the processing in relation to features, which is usually the prerequisite of other prediction methods. We take the Sina Weibo retweet records in a time span from 2009 to 2012 as an example to test the effectiveness and efficiency of our link prediction algorithm. Results show that: 1) the proposed algorithm has incomparable advantages in running time; 2) as for the predicted result, the proposed algorithm is better than other algorithms in performance evaluations; 3) the proposed algorithm runs stably for different sizes of training sets and feature sets; 4) the accuracy of the predicted results remains stable based on the selected features. The proposed approach avoids the independent restriction among features and shows better accuracy than other similar methods, thus it has reference values for resolving other prediction problems in complex networks.

2016, 65 (2): 020502.
doi: 10.7498/aps.65.020502

Abstract +

In order to reduce errors caused by human factors to identify the linear region, we propose a new method based on the fuzzy C-means clustering for calculating the maximum Lyapunov exponent from small data. The method based on the changing characteristic of divergence index curve is used to identify the linear region. Firstly, the divergence index data are calculated from the small data algorithm for the given chaotic time series. Secondly, the fuzzy C-means clustering method is used for dividing the data into two classes (unsaturated and saturated data), and the unsaturated data are retained. Thirdly, the retained data are divided by the same clustering method into three classes (positive fluctuation data, zero fluctuation data and negative fluctuation data), and the zero fluctuation data are retained. Fourthly, the 3$ criterion is used for excluding gross errors to retain the valid from the selected data. Finally, the regression analysis and statistical test are used to identify the linear region from the valid data. The effectiveness of the proposed method can be demonstrated by the famous chaotic systems of Logistic and Henon. The calculated results are closr to the theoretical values than the subjective method. Experimental results show that the proposed new approach is easier to operate, more efficient and more accurate as compared with the subjective recognition. But this method has its own shortcomings. (1) As the new method is verified by the simulation experiment, there exists no strict mathematical proof. (2) Since the difference algorithm is used in this new method, it will miss some detailed information in some cases. (3) The calculation accuracy still needs to be improved, so this method only serves as a reference to detect the linear region, it can not be applied to high precision engineering field. Considering the deficiencies of the new method, we will make further research to improve the calculation method for maximum Lyapunovexponent, so as to make it solve the real-time problem of the signal detection, and find the accurate location of abrupt climate change in the field of meteorology, to provide accurate satellite launch safety period in the field of space weather and other aspects. In short, studying the largest Lyapunov exponent from chaotic time series has a wide application prospect and practical significance.

2016, 65 (2): 020601.
doi: 10.7498/aps.65.020601

Abstract +

Large-range and high-accuracy absolute distance measurement plays an important role in many practical applications, such as industrial production, aerospace and scientific research, etc. In this paper, a method is proposed for absolute distance measurement by chirped pulse interferometry based on the femtosecond optical frequency comb. The interference spectra obtained in experiments are analyzed by means of the principle of the dispersive interference, and the distance can be determined by the shift of the widest fringe in the interference spectra. An absolute distance measurement system can be set up based on the modified Michelson interferometer with a pair of gratings to chirp the reference pulses in the reference arm. Experimental results are in agreement well within 33 m in a range up to 65 m, i.e. a relative precision of 5.110-7. In addition, the optimization of the measurement uncertainty is theoretically and experimentally performed.

2016, 65 (2): 020701.
doi: 10.7498/aps.65.020701

Abstract +

In this paper, we perform the quantitative phase-field simulations based on the surface morphology and growth regime of the hexagonal GaN spiral structure. We investigate the highly anisotropic energy, the deposition rate and the kinetic attachment and detachment effects. A regularized equation including the modified gradient coefficient is employed to study the anisotropic effect. Results show that the highly anisotropic energy modulates the equilibrium state by changing the local curvature of the tip step and thus leading to the changed spiral spacing. Under the weak anisotropy, the spiral spacing and morphology keep stable with the increase of the anisotropic strength. In the case of facet anisotropy, however, the larger anisotropic strength facilitates the spiral growth due to the local interfacial instability caused by increasing the supersaturation for the tip step. As to the effect of deposition, the deposition rate imposes the reaction on the curvature of interface due to the variations of supersaturation and step velocity. The larger rate of deposition enables the shorter spacing for both anisotropic and isotropic spirals. We carry out a convergence study of spiral spacing with respect to the step width to estimate the precision of the phase-field simulation. Results show that the larger deposition rate and the higher anisotropy give rise to the lower convergence of the spiral model. Moreover, we find that the kinetic attachment affects the instinct regime of spiral growth by changing the step spacing and the scaling exponents of spiral spacing versus deposition rate. The anisotropic spiral exhibits the more significant hexagonal structure and the lower value of step velocity by reducing the value of kinetic coefficient. The scaling exponent decreases with anisotropy increasing, but it increases with kinetic effect strengthening. The highly anisotropic energy contributes to weakening the sensitivity of the spiral spacing to the kinetic effect.

###### ATOMIC AND MOLECULAR PHYSICS

2016, 65 (2): 023101.
doi: 10.7498/aps.65.023101

Abstract +

In this paper, first-principles calculations based on the density functional theory, are performed to investigate the effects of strain field on the electronic and magnetic properties of two-bilayer gallium nitride (GaN) nanosheets. The two-bilayer GaN nanosheet without surface modification forms a planar graphitic structure, whereas that with full hydrogenation for the surface Ga and N atoms adopts the energetically more favorable wurtzite structure. Surface hydrogenation is proven to be an effective way to induce a transition from indirect to direct band gap. The bare and fully-hydrogenated GaN nanosheets are nonmagnetic semiconductors. When only one-side Ga or N atoms on the surface are hydrogenated, the semihydrogenated two-bilayer GaN nanosheets will preserve their initial wurtzite structures. The two-bilayer GaN nanosheet with one-side N atoms hydrogenated transforms into a nonmagnetic metal, while that with one-side Ga atoms hydrogenated (H-GaN) is a ferromagnetic semiconductor with band gaps of 3.99 and 0.06 eV in the spin-up and spin-down states, respectively. We find that the two-bilayer H-GaN nanosheets will maintain ferromagnetic states under a strain field and the band gaps Eg in spin-up and spin-down states are a function of strain . As the tensile strain is +6%, the band gap in spin-up state reduces to 2.71 eV, and that in spin-down state increases to 0.41 eV for the two-bilayer H-GaN nanosheets. Under the compressive strain field, the two-bilayer H-GaN nanosheets will show a transition from semiconducting to half-metallici state under compression of -1%, where the spin-up state remains as a band gap insulator with band gap of 4.16 eV and the spin-down state is metallic. Then the two-bilayer H-GaN nanosheets will turn into fully-metallic properties with bands crossing the Fermi level in the spin-up and spin-down states under a compressive strain of -6%. Moreover, the value of binding energy Eb for the two-bilayer H-GaN nanosheet decreases (increases) monotonically with increasing compressive (tensile) strain. It is found that although hydrogenation on one-side Ga atoms of the two-bilayer H-GaN nanosheets is preferred to be under compressive strain, the two-bilayer H-GaN nanosheets are still the energetically favorable structures. The physical mechanisms of strain field tuning band gaps in the spin-up and spin-down states for the two-bilayer H-GaN nanosheets are mainly induced by the combined effects of through-bond and p-p direct interactions. Our results demonstrate that the predicted diverse and tunable electronic and magnetic properties may lead to the potential application of GaN nanosheets in novel electronic and spintronic nanodevices.

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

2016, 65 (2): 024101.
doi: 10.7498/aps.65.024101

Abstract +

A novel circular porlarization (CP) antenna is designed and fabricated by loading a kind of polarization conversion metasurface. The structure of polarization conversion metasurface is composed of metal slash and copper ground sheet, which is separated by an FR4 dielectric substrates with a thickness h=3 mm. When a normal electromagnetic wave incident on the whole surface vertically, the electric field of the wave can be decomposed into two components Evi and Evi. Under the excitation of the two components, resonance is induced between the metal slash and the copper ground sheet respectively, making their own reflection phase changed to = u - v = 180, eventually making the reflection wave appear with 90 polarization rotation. By taking the peculiarity of 90polarization rotation of the polarization conversion metasurface, we can modulate the linear polarization of microstrip slot antenna into the circular polarization radiation. Through adjusting the distance between the slot antenna and the polarization rotation metasurface, we can regulate the working frequency of the circular polarization. Simulated and experimental results show that the polarization conversion metasurface makes a high-efficiency polarization rotation at 8-12 GHz. And the center working frequency of the CP antenna is 9.1 GHz, the impedance bandwidth is 8.3-10 GHz. When the distance H = 4.5 mm between the microstrip slot antenna and the polarization conversion metasurface, the 3 dB axial ratio bandwidth is 8.3-8.8 GHz, the microstrip slot antenna may realize circular polarized radiation. When H = 20 mm, the 3 dB axial ratio bandwidth is 8.8-9.3 GHz, and the slot antenna can realize circular polarized radiation. When H = 8 mm, the 3 dB axial ratio bandwidth is 9.3-10 GHz, the slot antenna can realize circular polarized radiation. Experimental results are in agreement with the simulation results, showing the validity of this design method which may become a new approach for the CP antennain designing

2016, 65 (2): 024102.
doi: 10.7498/aps.65.024102

Abstract +

Fiber-reinforced composite materials are widely used in aeronautics and automotive industries due to their excellent mechanical properties. Composites with high conductive fibers embedded have good performance of shielding effectiveness and become possible candidates to replace metals. One approach for analyzing the electromagnetic (EM) interaction of fiber-reinforced composites is to use full numerical methods, which allow precise modeling and give accurate results. However, numerical methods may lead to prohibitive computational time and memory capacity due to the strong dependence on the shielding properties from heterogeneous microstructures.In composite materials, two important parameters, effective permittivity eff and effective permeability eff, determine the interaction between the electromagnetic field and the materials. For estimating the effective parameters, homogenization techniques have been developed to describe a composite mixture in terms of a spatially homogeneous electromagnetic response, mostly under static conditions. The well-known rules are the Maxwell-Garnett (MG) formula and the Bruggeman formula.These rules are usually applied to the dilute composite materials and provide satisfactory results as long as the wavelength remains large compared to the size of the heterogeneities. Recently, revised homogenization models have been developed to extend the frequency range. Some of them are presented with the help of numerical method but still require substantial computational time and resources to be performed. One recently proposed homogenization model, called dynamic homogenization model (DHM), is an extension of quasi-static homogenization methods for microwave frequencies. It is obtained by introducing a microstructure-dependent characteristic length for the composites made of a square array of circular cylinders buried in the matrix, based on the basic inclusion problems. The DHM overcomes the limitations of standard static homogenization tools, but only applicable to low fiber volume fraction (less than 20%).In this paper, we focus on the microstructure in the case of a square array of circular 2D conductive long fibers embedded in a dielectric matrix. A revised DHM is proposed to describe the effective permittivity of the composite materials with different inclusion concentrations, including higher fiber volume fraction. Firstly, an iterative procedure is employed to estimate an effective permittivity, which is then used to modify the wavelength in the DHM. Secondly, an empirical formula-based characteristic size of the microstructure is presented by considering the current distribution of the fibers under the EM wave illumination in the case of high fiber volume fractions. Therefore, the final modified homogenization model is given for the effective permittivity of composites with arbitrary inclusion concentrations. It can be used to efficiently calculate the reflection and transmission coefficients, as well as the shielding effectiveness by classical transmission-line methods. Three infinite sheets with different physical parameters are utilized for validation. We compare the results of the shielding effectiveness obtained from this homogenization model with those obtained from a full numerical solution of the actual fiber composites. Reasonable agreements obtained demonstrate that the proposed model could define the effective permittivity of the composites with high fiber concentration over a wide frequency range including microwave frequencies. Analogous formulas also hold for the magnetic permeability with permittivity replaced by permeability wherever it appears in the proposed model.

2016, 65 (2): 024201.
doi: 10.7498/aps.65.024201

Abstract +

A tapered structure based all-fiber endoscopic probe for endoscopic optical coherence tomography (OCT) is presented in this paper. The designation and fabrication of a miniaturized high-performance probe are critical in endoscopic OCT. Compared to the conventional fiber-lens structure based endoscopic probe, the all-fiber probe has a prominent edge in size and flexibility. Due to its lower beam-divergence, the large core multi-mode fiber makes a better fit than a general single mode fiber does when utilized to replace the micro lens in a conventional endoscopic probe as the imaging component. Furthermore, a tapered fiber is introduced as a transition section between the single mode fiber and the large core multi-mode fiber in order to enhance the light transmission efficiency and reduce the rigid length of the probe simultaneously. First, in order to obtain an optimal performance, optical simulation software(Rsoft) is adopted to determine the probe's proper lengths of the tapered section and the large core multi-mode fiber. Second, the all-fiber structure based endoscopic probe is fabricated by means of large core multi-mode fiber tapering, cutting and fusing processes. The beam characterization and insertion loss of the fabricated probe are measured experimentally The probe itself is 250 m, and after covering with a stainless steel protective tube, its outer diameter becomes 325 m. The rigid length of the probe is about 1 cm, which is more flexible and easier for inserting into curved blood vessels. The insertion loss of the probe is measured to be about 0.3 dB. To the best of our knowledge, it is the lowest among all of the all-fiber endoscopic probes. Finally, the probe is integrated with a custom-built swept-source optical coherence tomography system. Imaging of human fingertip and ex-vivo chicken trachea is conducted to demonstrate the key performance parameters of our probe. The effective imaging range of the probe is up to 800 microns in air without the help of any extra mechanism to expand its depth of focus. The probe offers a compact, efficient and flexible candidate for endoscopic optical coherence tomography, which is promising in cardiovascular investigations.

2016, 65 (2): 024202.
doi: 10.7498/aps.65.024202

Abstract +

In traditional laser quads arrangement schemes for spherical hohlraum in indirect-driven laser facilities, the laser quads to bring about the laser entrance hole (LEH) to close when they are incident at a large angle (55), while the complicated cross and overlap of laser quads inside the spherical hohlraum may be generated when they are incident at a small angle (35). To overcome these problems, a novel laser quads arrangement scheme for spherical hohlraum is proposed. The laser quads injected into the single LEH are divided into two cones (the incident angle of the inner cone is 35, and that of the outer cone is 55). Furthermore, the contrast and the fractional power above the intensity have been proposed to evaluate the irradiation uniformity of single laser quad, while the dispersion degree and the duty ratio are proposed to evaluate the distribution uniformity of all laser quads on the spherical hohlraum wall. Based on the beam smoothing scheme implemented by the combination of one-dimensional smoothing by spectral dispersion, the continuous phase plate and polarization control plate, the propagation model of laser quads in the spherical hohlraum has been built up, and further used to analyze the irradiation uniformity of single laser quad and all the laser quads on the spherical hohlraum wall. On this basis, the irradiation characteristics on the LEHs and the spherical hohlraum wall, and the propagation characteristic of laser quads in the novel and traditional laser quads arrangement schemes have been analyzed and compared. Results indicate that, compared with the traditional arrangement scheme of laser quads, the novel laser quads arrangement scheme has following advantages: The irradiation uniformity on the spherical hohlraum wall of single laser quad and all laser quads remains unchanged. Not only the LEH closure problem can be alleviated, but also the complicated cross and overlap of laser quads inside the spherical hohlraum in the traditional scheme could be avoided. The novel scheme may provide useful reference for the design of spherical hohlraum structure in laser indirect-driven facilities due to its obvious advantages over the traditional scheme.

2016, 65 (2): 024203.
doi: 10.7498/aps.65.024203

Abstract +

Time fiducial laser is an important timing marker for different diagnostic instruments in high energy density physics experiments. The probe laser in velocity interferometer system for any reflector (VISAR) is also vital for precise shock wave diagnosis in inertial confinement fusion (ICF) research. Here, time fiducial laser and VISAR probe laser are generated from one source in SG-III laser facility. After generated from a 1064 nm DFB laser, the laser is modulated by an amplitude modulator driven by a 10 GS/s arbitrary waveform generator. Using time division multiplexing technology, the ten-pulse time fiducial laser and the 20 ns pulse width VISAR probe laser are split by a 12 multiplexer and then the time fiducial and VISAR pulses will be selected individually by acoustic-optic modulators. Using this technology, the cost for the system can be reduced. The technologies adopted in the system also include pulse polarization stabilization, high stable Nd: YAG amplification, high precision thermally controlled frequency conversion, fiber coupling, and energy transmission. The fiber laser system is connected to the Nd: YAG rod amplifier stage with polarizing (PZ) fibers to maintain the polarization state. The output laser of Nd: YAG amplification stage is coupled with different kinds of energy transfer fibers to propagate enough energy and maintain the pulse shape for the time fiducial and VISAR probe laser. The input and output fibers are all coupled to the rod amplifiers with high precision and being easy to plug and play for users. Since the time fiducial and imaging VISAR laser system is far from the front end room and located in the target area, the system also uses an arbitrary waveform generator (AWG) to generate the shaped ten-pulse time fiducial laser and 20 ns VISAR laser. This AWG and the other three AWGs used for the main laser pulse of SG-III laser facility will be all synchronized by 10 GHz clock inputs, realizing the smaller than 7 ps (RMS) jitter between the main laser pulse, time fiducial laser and VISAR pulse. After amplification and frequency conversion, the time fiducial laser finally generates 12 beam 2 and 4-beam 3 laserbeams, providing important reference marks for different detectors in the ICF experiments and making it convenient for the analysis of multiple diagnostic data. The VISAR laser pulse is also amplified by the Nd: YAG amplifiers and frequency-converted to 532 nm green light by a thermally controlled LBO crystal, with output energy larger than 20 mJ. Finally, the 532 nm VISAR probe laser beam is coupled with a 1-mm core diameter fused silica optical fiber, and then propagates 30 meters to the imaging VISAR system. The VISAR probe laser has been used in many high energy density physics experiments. The shock wave loading and slowdown processes are measured. Function for measuring velocity history of shock wave front movement in different kinds of materials can be also added to the SG-III laser facility.

2016, 65 (2): 024204.
doi: 10.7498/aps.65.024204

Abstract +

The polarization switching (PS) characteristics of vertical cavity surface emitting lasers(VCSELs) have received sustained attention for the past years. With the development of manufacturing technology, the performances of 1550 nm VCSELs have been improved, however the researches on the PS of 1550 nm VCSELs are relatively inadequate for the PS characteristics in the long-wavelength VCSELs may have wide application prospects in optical information processing and optical communications. In this paper, based on the extended spin-flip model (SFM), we theoretically investigate the PS with low power consumption induced by optical feedback in long-wavelength VCSELs. Results show that the PS, which is failed to realize in free-running long-wavelength VCSELs, can be achieved by introducing a moderate-strength polarization-rotation optical feedback. By comparing two different linear dispersion effects, some interesting phenomena have been found. For weak linear dispersion, the PS is relatively easy to realize for a low injection current level, and the range of feedback strength used to control the PS is wide. However, for strong dispersion effect, the PS cannot be obtained all the time since two mode-coexisting zones will appear, and the value of injection current where the PS happens is relatively high. Meanwhile, as observed in short-wavelength VCSELs, the polarization mode hopping and multiple PS have also been found in long-wavelength VCSELs, indicating that the physics nature thet induces the PS is similar for both long and short wavelength VCSELs. In addition, because the PS in long-wavelength VCSEL is more difficult to realize as compared with that in short-wavelength VCSELs, reasonable analyses and explanations may be as follows: since the linear dispersion effect in 1550 nm-VCSEL is much stronger than that of short wavelength VCSEL, the frequency difference between the two linear polarization modes is up to 60 GHz (or 0.48 nm), thus leading to the decrease of the correlation between two linear polarization modes. As a result, it is relatively difficult to obtain the PS phenomenon at low injection current level in long-wavelength VCSEL; while using suitable polarization-rotated optical feedback can partially compensate the deficiency of this correlation. We believe that the results obtained in this work will be helpful in investigation of low power consumption for all optical buffers by using long-wavelength VCSELs.

## EDITOR'S SUGGESTION

2016, 65 (2): 024205.
doi: 10.7498/aps.65.024205

Abstract +

For high-power UV laser facilities, one of the key problems limiting the maximum light influence and safe routine operation is that the UV laser induces damage to fused silica optics. The most effective mitigation protocol of the damaged optics is the CO2 laser processing that leads to make locally melt or evaporate the damage. While the mitigated damage sites possess particular morphology, which may modulate the passing laser beam and induce the downstream intensification that will ruin the neighbor optics. In this work, the morphology features of the mitigated damage pits of fused silica optics are systematically investigated. According to the measured morphology features, a 3D grid model of mitigated pit is built, and the downstream light intensity distribution of the mitigated pit model under incident 351 nm laser is studied by scalar diffraction theory and fast fourier transform (FFT) methods. Results indicate that there are two kinds of downstream intensification: off-axis and on-axis intensifications. In the former intensification, the maximum intensity is located near the output surface of the optics and comes mainly from the depth of the mitigated pit; it increases with the depth. In the alter intensification, the maximum intensity is located far from the output surface of the optics and is mainly dependent on the height of the rim structure at the fringe of the mitigated damage pit; so it increase with increasing height. In addition, it is found that the location of the maximum off-axis or on-axis intensity can approach the output surface of the optics with increasing maximum intensity. For comparison, experimental measurements of downstream intensification induced by the mitigated pits are carried out, and the experimental results are almost consistent with the numerical simulation, implying the validity of the numerical simulation of the mitigated pit model. Results of this research indicate that the downstream intensification of mitigated pits can be suppressed by controlling the morphology features of mitigated pits. this is significant for the development and improvement of the mitigated techniques of damage optics.

2016, 65 (2): 024206.
doi: 10.7498/aps.65.024206

Abstract +

Using multi-core photonic crystal fiber (PCF) has advantages of large-mode-area that can support high beam intensity and disperse heat. However, only when the beam profile in far-field and the focal point of in-phase super mode is Gaussian-shaped, the energy can be more concentrated as compared with other shapes. And this beam profile feature limits the applications of multi-core PCF.With the development of optics, there is a practical solution to improve the beam profile of multi-core PCF in which a Kagome fiber is used. This solution is to couple the in-phase super mode source (obtained from multi-core PCF) into Kagome fiber to achieve the beam combination of multi-core photonic crystal fiber, i. e. the beam profile remains to be Gaussian-shaped at any location in the optical field. The Kagome fibers have a novel hollow structure and thus will show some new properties, such as broad optical transmission bands with relatively low loss, no detectable surface modes, and high confinement of light at the core, and these features are suitable for beam combination.In this paper, a Ti: sapphire femtosecond pulsed fiber oscillator, with its center wavelength of 800 nm and output power of 550 mW, is used to pump a piece of seven-core nonlinear PCF, with an efficiency of 19%. EFL of the coupling lens is 18.40 mm and the NA is 0.15. Then the in-phase super mode source can be obtained from the 15 m multi-core PCF, with a broadband spectrum from 700 to 1050 nm. The beam profile of farfield and the focal point of in-phase super mode is Gaussian-shaped and there is a seven-core-shaped pattern at nearfield and other locations in the optical field. In order to combine the beams of multi-core fiber, the in-phase super mode source is coupled into a piece of Kagome fiber, 10 cm long, by using the coupling lens whose EFL is 13.86 mm. Its coupling efficiency is 71% and the output beam profile remains to be Gaussian-shaped at any locations in the optical field; this means that there is no seven-core-shaped pattern. It also transmits broadband spectrum with low loss. Moreover, this experiment also proves that the solution can be used for different multi-core PCFs and can have a higher coupling efficiency, 80%. Thus a reference can be given for high power applications of multi-core PCF, and inspiration may be given to some other frontier fields in fiber optics.

2016, 65 (2): 024207.
doi: 10.7498/aps.65.024207

Abstract +

In recent years, azobenzene derivates have received much attention for their potential application in optical data storage, biophotonics, holographic memories and waveguide switches optical sensors, and sensitive optical components from laser damage in both civilian and military applications. Experimental and theoretical studies demonstrate clearly the effect of the sonor-pi-acceptor (D- -A) conjugation on the steady-state and time-resolved PL spectra of azobenzene derivate films in multifarious situations, but comparatively little is concerned about the two-photon absorption and refraction involved in a single benzene ring. Furthermore, the excitation laser source on the azobenzene derivates in some investigations is continuum laser or nanosecond pulsed laser, where it is hard to avoid thermal effect on nonlinear optical (NLO) process produced by these lasers. To explore the origin of the azobenzene derivates' D- -A conjugation-dependent NLO process is a challenging task and has great signicance in describing the molecular structures of these azobenzene nanostructures as well as improving the performance of azobenzene derivates' devices. The D- -A conjugation of azobenzene functional material can be modified by mixing the azobenzene derivates with metal nanoparticles, so it is convenient to study how the D- -A conjugation affects the NLO properties by using the azobenzene derivate-metal composites. In this letter, the D- -A conjugation-dependent NLO absorption and refraction of the two kinds of azobenzene derivates 4-((4'-hydroxybenzene) azo) benzyl acid(BN) and N-(3, 4, 5-octanoxyphnyl)-N'-4-[(4-hydroxyphenyl) azophenyl]1, 3, 4-oxadiazole (AOB-t8) are investigated by Z-scan technology using 32 ps laser pulse width at 532 nm. The azobenzene derivates' surface is modified using the D- -A conjugation control and overcoating Au nanoparticles on the azobenzene derivates; and the Au/AOB-t8 composites, BN and AOB-t8 are characterized by Z-scans and absorption/fluorescence spectrum, and also calculated based on plasma resonance. The third-order NLO susceptibility of AOB-t8 is enhanced as compared with BN due to the growing conjugate chain and the increasingly extended bond. However, the third-order NLO susceptibility of AOB-t8 is decreased in the composite(Au/AOB-t8) for the cooperation of the local field effect induced by the gold nanoparticles and the extended bond of organic molecules. This work may be helpful to the understanding of the physical mechanism of the surface states and the surface-related optical nonlinearity of semiconductor QDs.

2016, 65 (2): 024208.
doi: 10.7498/aps.65.024208

Abstract +

Based on the split step Fourier method, the interaction between soliton and Airy pulse is studied in the anomalous dispersion region. And after that the strength, time-domain, and time-shift are simulated by the software of MATLAB, respectively. Results show that cross phase modulation (XPM) builds up when soliton and Airy pulse begin to overlap, which affects the properties of the two pulses. The soliton keeps its original shape but the direction of propagation is deflected by the influence of Airy pulse's self-acceleration. Airy pulse converts to soliton and the direction of propagation changes due to XPM. Therefore, the properties of Airy pulse and soliton are interacted with each other because of XPM. The time-domain of the two pulses is also influenced by XPM and their different shapes will change so as to contain a main and a secondary peaks whose structures are similar and the location and pulse width of the main and the secondary peaks are also roughly the same, which is the basis for Airy pulse to convert to soliton. In addition, the change of Airy pulse and soliton is simulated for different input intensity value of r. Simulation shows that the time-shifts of Airy pulse and soliton increase with increasing input intensity r and their variation trends are the same.

2016, 65 (2): 024209.
doi: 10.7498/aps.65.024209

Abstract +

Laser beam steering or pointing, which stabilizes the beam direction, is critical in many areas, such as optical communication systems, astronomy and directed-energy systems etc. However, the disturbances including atmospheric turbulence and mechanical jitter on platform may degrade the pointing accuracy. A proportional-integral (PI) feedback control commonly has been used in the track loop with a fast steering mirror. To compensate dynamic disturbance effectively, the laser beam steering control system must have a larger bandwidth than the disturbance bandwidth. But the control bandwidth is limited by the noise of the sensor, computing latency, and the light energy. So, a simple proportional-integral (PI) feedback controller of a piezoelectric fast steering mirror (PFSM) can only compensate the broadband disturbance of the atmospheric turbulence, but cannot effectively compensate a larger amplitude narrowband jitter because of the low control bandwidth. Moreover, when the control bandwidth is tuned to high, the mechanical resonance of the PFSM can cause the instability of the system. An improved dual two-order filter assisted high-bandwidth control algorithm to improve the pointing accuracy and error attenuation capability is proposed. This method can control a PFSM for suppression of laser beam jitter. The influence of filter parameters on frequency characteristics is analyzed, and then, a practical design method is proposed. The dual two-order filter can combine the characteristics of traditional notch filter and two-order low-pass filter, and can also obtain any desired amplitude in the interesting frequency ragion with little influence on the others. The principle of the proposed filter for suppressing the mechanical resonance of the PFSM and the narrowband disturbance is elaborated. And then, the different dual two-order filters are designed according to the frequency content of the PFSM and the narrowband disturbance. Finally, the proposed dual two-order filter assisted PI control algorithm and classic PI control algorithm are compared with each other. Experimental results show that, in the same conditions, the pointing accuracy of the proposed two-order filter assisted PI control algorithm is nearly 5 times better than that of the classic PI control algorithm, and the error attenuation bandwidth is one time higher. It also indicates that the proposed algorithm does not need an additional sensor; it is simple and effective for the suppression of the mechanical resonance of a PFSM and that of the narrowband disturbance, hence it improves the system error attenuation bandwidth and the beam pointing accuracy.

2016, 65 (2): 024210.
doi: 10.7498/aps.65.024210

Abstract +

As a key point to applying and studying magnetic photonic crystal technology, communication devices such as the magnetic photonic crystal filters with high performance and easy integration are developed. We investigate the feasibility of ferrite magnetism materials that can be used to make photonic crystal filters. The optical properties of the magnetic materials may be tuned by adjusting the magnetic field or temperature. The band gap of the magnetic photonic crystal can thus be transferred by changing the external magnetic field. This kind of magnetic photonic crystal has a great application prospect. A low insertion loss and narrow-band filter is designed based on a magnetic field-controlled ferrite defect in a photonic crystal for a terahertz (THz) wave. Ferrite is a ferromagnetic metal oxide with high dielectric constant, low saturation magnetization intensity, and high magnetic permeability at high frequencies. According to the crystal structure it can be divided into three categories: spinel, garnet and magnetic rock types. The garnet ferrite crystal can be used to realize THz band transmission, and its absorption coefficient is low (0.05-0.3) in uniform polarization. In this paper, a novel magnetic THz photonic crystal filter is proposed, in which point defects are produced by the introduction of garnet ferrite magnetic materials. Based on the coupling characteristics between the linear defect wave guide and the point defects, THz wave with a certain wave length can be well coupled by changing the radius and arrangement of the resonant cavity, so as to achieve high efficiency filter function. The permeability properties of ferrite magnetic materials are changed with the variation of the intensity of the external magnetic field, and the tuning of the frequency of the resonance mode. The optical properties of the filter are analyzed in detail by using plane waves method(PWM) in finite difference time domain(FDTD). Simulation results show that by changing the point defect structure and the radius of a certain dielectric cylinder, the insertion loss and 3 dB bandwidth of the filter are 0.0997 dB and 8.22 GHz, respectively.

2016, 65 (2): 024211.
doi: 10.7498/aps.65.024211

Abstract +

A triple-cladding quartz specialty fiber (TCQSF) temperature sensor based on cladding mode resonance is made. The sensor is fabricated by just splicing a short, few-centimeter-long segment of TCQSF between two standard single-mode fibers (SMFs), so the sensor structure is simple. In order to explain its sensing principle in detail, we assume that the TCQSF is equivalent to three coaxial waveguides based on coupling mode theory. Utilizing the scalar method and the relationship between Bessel function and mode field distribution of step-index circular symmetry waveguide, the mode field distribution of these waveguides and their characteristic equation can be easily obtained. Then the dispersion curves of each mode which is transmitted in the three waveguides can be calculated. The intersection between the fundamental core mode LP01(rod) in the rod waveguide and the cladding mode LP01(tube) in the tube waveguide I indicates that the two modes have the same propagation constant, and satisfy the phase-matching condition when the wavelength is 1563.7 nm which is known the resonant wavelength. And only when the sensor length is equal to the beatlength, can the light be coupled completely from the core to the fluorine-doped silica cladding. Thus, the cladding mode resonance phenomenon occurs and a band-stop filter spectrum will be obtained. Then the sensor is applied to the simulation calculation of the temperature sensing characteristics. With increasing temperature, both the refractive index of each layer and the sizes of the axial and radial fibers will change, which will finally lead to a big difference on the dispersion curves of LP01(rod) and LP01(tube). Therefore, the resonant wavelength shift of the sensor can be obtained by just calculating the dispersion curves of these two modes at different temperatures, and the scope of curvature sensitivity is 70.76-97.36 pm/℃. Finally, a straight forward experiment is performed to prove the temperature sensing properties. Experimental results show that the sensor has a sensitivity in temperature of 73.74 pm/℃ at 35 ℃-95 ℃, which is completely consistent with the theoreticaly calculatied results. Thus, the proposed sensor has the advantages of simple structure, easy fabrication, highly sensitivity, controlled cladding mode excitation, and so on. It can be used in industrial production, biomedical and other temperature sensing areas.

2016, 65 (2): 024212.
doi: 10.7498/aps.65.024212

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Radiation effects on four-transistor (4 T) active pixel sensor complementary metal-oxide-semiconductor (CMOS) image sensor induced by -ray are presented. The samples are 4 megapixels resolution CMOS image sensor using 11 upm pitch high dynamic 4 T pixels. They are manufactured with 0.18 upm specialized CMOS image sensortechnology. Three samples have been exposed to 200 krad(Si) 60Co -ray with different biasing condition (1# is static-biased, 2# dynamic-biased, and 3# is grounded during irradiation), and the dose rate is 50 rad(Si)/s. The influences of radiation on full well charge capacity, dark current, and conversion gain of the device are investigated. Experimental result shows that the conversion gain is not sensitive to the ionizing radiation, and it is mainly determined by the CMOS digital or analog circuits. It is known that the total ionizing dose for induced degradation in deep submicron MOSFET is negligible and so there is almost no radiation effect on the digital or analog circuits exposed to the ionizing radiation. Therefore, conversion gain does not have obvious degradation after irradiation. While full well charge capacity has a degradation after irradiation, which is due to the change of TG channel doping profile induced by the radiation. As the dose increases, dark current increases rapidly. The main source of dark current in 4 T CMOS image sensor is the current from STI interface and TG-PD overlap region. Experimental result also shows that different from 3 T CMOS image sensor, there is no biasing effect in 4 T CMOS image sensor. This is because for the 4 T CMOS image sensor most of the degradation come from STI interface and TG-PD overlap region, while biasing condition almost has no influence on both ofthem.

2016, 65 (2): 024301.
doi: 10.7498/aps.65.024301

Abstract +

Helmholtz resonators are commonly used as underwater acoustic transducers to transmit low-frequency, great power acoustic waves at fluid cavity resonant frequency. Therefore, it is an important problem in the study of how to calculate accurately the fluid cavity resonant frequency of Helmholtz resonator, especially when the Helmholtz resonator is used in underwater acoustic environment where Helmholtz transducers cannot be designed using the classical air acoustic Helmholtz resonator theory. The elasticity of the cavity wall has to be considered because it has a strong influence on the fluid cavity resonant frequency at low frequency band. In this paper, the method of calculating accurately fluid cavity resonant frequency is researched for low-frequency Helmholtz underwater transducers. A Helmholtz resonator is a slender cylindrical shell, the boundary condition of its two ends is free: one side is a radiating port, and the other side is considered as a rigid baffle. Firstly, the fluid cavity resonant frequency of the rigid wall Helmholtz resonator is given, then the radial mechanical impedance of the slender cylindrical shell is derived based on the wave equations. Elasticity of the cavity wall is introduced into the acoustic impedance of fluid cavity in the form of additional impedance. Based on the low-frequency lumped parameter model of the slender cylindrical shell, additional acoustic impedance expression of elastic cavity wall is derived, complete equivalent circuit diagram of elastic Helmholtz underwater transducers is developed, taking into account the elasticity of the cavity wall. Based on the equivalent circuit, the accurate fluid cavity resonant frequency formula has been derived; the formula shows that both the structure size and material characteristics of the cavity wall have influence on the fluid cavity resonant frequency. The thinner the cavity wall, the lower the fluild cavity resonant frequency; and the smaller the Young's modulus of the material, the lower the fluild cavity resonant frequency. To verify the accuracy of the present theory, several slender cylindrical shell models with different wall thickness, materials, and wall length are investigated by both elastic theory method and finite element method (using ANSYS software). These results reveal that the elastic theory results are in excellent agreement with the finite element simulation results. That means, compared to traditional rigid wall theory results, the results from elastic theory in this paper is much closer to the real situation. This conclusion can provide a theoretical support for the accurate design of low-frequency elastic Helmholtz underwater acoustic transducers.

2016, 65 (2): 024302.
doi: 10.7498/aps.65.024302

Abstract +

With the rapid development of the theory and algorithms for sparse recovery in finite dimension, compressive sensing (CS) has become an exciting field that has attracted considerable attention in signal processing, such as sub-Nyquist sampling systems, sound imaging and reconstruction, wavelet denoising, compressive sensor networks, and so on. Moreover, the broad applicability of CS framework has already inspired some notable investigation in the context of array processing. The problem of acoustic source identification can be investigated from a limited number of measurements delivered by a microphone array as a basis pursuit problem, which has been developed in the context of compressive sensing, and the CS beamforming can be proved to be better than the conventional beamforming even in its near-field focusing version based on spherical waves. Focused beamforming is a typical method used to localize the position of acoustic sound sources in the near field of the measurement array, and can be a jointly reconstructed source powers and positions. Many super-resolution focused beamforming approaches have been developed to overcome the Rayleigh resolution limit of conventional focused beamforming. Especially, turning to the compressive sensing (CS) framework, we are able to exploit the inherent sparsity of the underlying signal in space domains to achieve super-resolution for the focused beamforming even in a noisy and coherent environment with few snapshots.Prior research has established CS as a valuable tool for array signal processing, but it is mainly from a theoretical point of view, and its application to underwater acoustic sources localization has been developed only for very limited scenarios. In this paper, we present an underwater noise sound source near-field localization method based on a sparse representation of vector sensor array measurements. By utilizing the sparsity approach, the new localization methods can jointly reconstruct source powers and positions, and enforce sparsity by imposing penalties, based on the l1-norm, to improve the integrated performance. By comparing with other source localization methods, such as the conventional focused beamforming, MVDR focused beamforming, and the maximum likelihood focused beamforming, the performance of compressive focused beamforming and the typical focused beamforming using pressure or vector sensor array is analyzed in detail, especially under noisy conditions, and coherent sources. Simulation and experimental results demonstrate that this new approach has a number of advantages over other source localization techniques, e.g. increased resolution, improved robustness to noise, limitations in data quantity and correlation of the sources, as well as lower levels of background interference. It is feasible to apply the proposed approach for effectively localizing and identifying underwater noise sound sources.

2016, 65 (2): 024401.
doi: 10.7498/aps.65.024401

Abstract +

Based on constructal theory and entransy theory, an experimental study on + shaped high conductivity channels in a square body is carried out. Heat conduction performance comparisons of the bodies based on different optimization objectives and different layouts of the high conductivity channels are performed. In the experiment, the materials of the square body and high conductivity channel are epoxy resin and brass, respectively; the brass channel is embedded in the square body. Two square heating boards, closed at the upper and lower sides of the square body, are used to uniformly heat itself. The internal heat generation of the square body is approximately simulated by this method. The square body is placed in a thermal insulation box to reduce the heat dissipation caused by heat convection. The heat generated by the heating boards is absorbed by the outside refrigerator device. A measurement window is set at the front side of the thermal insulation box. The temperature field of the square body is measured by the infrared thermal imager. The peak temperature, average temperature difference, and entransy dissipation rate of the body can be calculated by the measured results, respectively. Experimental results are compared to those obtained by numerical calculations; the results show that for the + shaped high conductivity channels in a square body, the maximum temperatures are located between the two branches of the + shaped high conductivity channels for both experimental result and numerical calculation. The errors in the average temperature and entransy dissipation rate of the body based on the experimental result and numerical calculations are within the acceptable range. The two results verify their validity of the heat conduction constructal optimization. Compared the square body with H shaped high conductivity channel, the entransy dissipation rate of the body caused by heat conduction is reduced by adopting the first order + shaped high conductivity channel. Compared with the optimal constructs of the first order + shaped high conductivity channels based on the minimizations of entransy dissipation rate and maximum temperature difference, the entransy dissipation rate caused by heat conduction of the former construct is reduced by 5.98%, but the maximum temperature difference is increased by 3.57%. The aim of maximum temperature difference minimization helps to improve the thermal safety of a body, while that of the entransy dissipation rate helps to improve the global heat conduction performance of a body. When the thermal safety is permitted, the optimal construct based on entransy dissipation rate minimization can be adopted in the design of practical electronic device to improve its global heat conduction performance.

2016, 65 (2): 024501.
doi: 10.7498/aps.65.024501

Abstract +

Owing to their efficient penetration into elastic media, the measurement of sound waves can provide a sensitive probe of both the structural and mechanical properties of the materials through which they propagate. In this work, we first investigate the transversal and longitudinal wave velocities in granular assemblies composed of glass beads under uniaxial load by the time-of-flight method. Then the ratio G/B, (G is the shear modulus and B is the bulk modulus) as a function of pressure is analyzed, based on the theory of classical elasticity. Experimental results show that, with the pressure increasing from 10 to 100 kPa, i) the velocity of longitudinal wave (cL ) is obviously faster than that of transversal one (cT ) in the granular system(the ratio cL/cT is about 1.6), and the cL and cT of the system show power law scaling, i.e. cL p0.3817, cT p0.2809; ii) the ratio G/B decreases in the low pressure range for glass beads packing, i.e. G/B p-0.4539. It is found that the power-law exponent of G/B with pressure is very close to -1/2 (the prediction in 2015 Phys. Rev. Lett. 114 035502), suggesting that the granular system lies in glass L state within the pressure range in our experiment. Furthermore, the fast Fourier transform method is used to study the variation of acoustic attenuation and nonlinear characteristics in granular materials. Our results reveal that the acoustic attenuation coefficient () and the ratio of the second harmonic amplitude ( 2 ) to the square of fundamental amplitude ( 1 ) at the receiving end in the granular system, 2/12, both decrease in power law with the increase of pressure, i.e. p-0.1879, 2/12 p-0.866, respectively.

2016, 65 (2): 024502.
doi: 10.7498/aps.65.024502

Abstract +

A jammed state is a common phenomenon in complex granular systems, in which the relationship between the mechanical properties and the geometric structures is very complicated. The critical jammed state in a two-dimensional particle system is studied by numerical simulation. The system is composed of 2050 particles with two different radii, whose distribution is random. Initially the particles with a smaller radius are of a looser distribution in the given space. When the radius increases, a transition from the looser state to the jammed state happens. The particle dimension-radius ratio and the percentage of large particles kB play primary roles in this system, which are discussed in detail based on the statistical analysis of the average contact number, packing fraction, and contact type. By analyzing the relationship between pressure and packing fraction of the granular system, the critical jammed point for the applied pressure to the boundary can be found. Numerical simulation result shows that no obvious connection exists between the average contact number and the percentage of large particles for the case that the particle dimension-radius ratio is less than 1.4. The average contact number approximate to 4 when = 1.4, which is consistent with previous conclusions. The average contact number first decreases and then increases when the percentage of large particles become larger in the case 1.4. A minimum value C = 0.84 is obtained when kB = 0.5. When the percentage of large particles increases, the critical packing fraction decreases first and then increases in the case 1.8, but it almost keeps constant for 1.8. When the percentage of large particles is close to either 0% or 100%, the granular system is approximately mono-disperse. In this case, the average contact number and packing fraction become constant. When the percentage is close to 50%, the critical average contact number decreases all the time with larger particles-radius ratio, while the critical packing fraction decreases first and then increases. The percentage of large-small contact type is also discussed. The value varies following a quadratic function with the increase of the percentage of large particles, while the particles-radius ratio has slight impact on this variation. Specifically, we have calculated the percentage of large-small contact type based on probabilistic method, and the result agrees well with the simulation results. We give the reason why previous researchers studied the case of = 1.4 :1 and kB = 0.5 on the basis of results in this paper, and find that the values of and kB have no influence on the power-law relation around the critical jammed state.

## EDITOR'S SUGGESTION

2016, 65 (2): 024701.
doi: 10.7498/aps.65.024701

Abstract +

Under strong impact loading, metal materials will produce deformation and show ejecta behaviors. The mixing phenomenon, due to the detached matters entering into the background fluid, has a direct influence on the compression properties. According to the researches of ejecta, the damage and mixing are closely related with the loading state and the dynamic process. Up to now, many results have already been obtained under the condition of the directive impact of detonation. Further study on the metal materials response driven by detonation collision is needed. Previous studies have focused on the macro characteristics, such as the collision uplift and destruction. In this paper, we aim at the wave system's interaction process, in order to obtain the physical detail and to reveal the mechanisms of dynamic behaviors in the collision region. Investigations are carried out by means of both the numerical simulation and the shock polar theory analysis. Planer tin flying layer calculation model is designed for numerical simulation, so the sliding wave systems and shock conditions are obtained effectively. Based on the numerical results in the plane tin flying layer, the shock polar theory forecasts that the Mach reflection will occur, and the images of wave interactions given by numerical simulation also display the three-wave structure, which is the typical structure of the Mach reflection. Quantitative comparisons between the numerical results and theoretical analysis of the shock polar are in good agreement with each other. Furthermore, the critical conditions of Mach reflection in the cases of different shock conditions are given. Meanwhile typical characteristics of the histories of free surface velocity in the collision zone are analyzed. From the numerical and theoretical analyses, the shock dynamical model in the collision zone is proposed to reveal the mechanisms, and the model is very important for investigating the collision zone problem deeply in decomposition way. The results illustrate that in the collision zone there exist multiple kinds of shock loading ways, including one-dimensional once plane impact region, two-dimensional once oblique impact region, and two-dimensional twice oblique impacts region. The complex loading dynamic processes coupling with the unsteady flow field lead to the distributions of the peak pressure at different positions in the collision zone. The corresponding destroyed behaviors are shown, and thus we can establish the relationship between the reflection wave structure and the fracture morphology of the collision zone. This research results will provide an important theoretical support for the understanding and interpretation of the physical phenomena of material deformation, damage and mixing in the collision zone.

2016, 65 (2): 024702.
doi: 10.7498/aps.65.024702

Abstract +

The denotation of condensed explosives is very vulnerable to be influenced by the character of its confinement material. Confinements of different materials on the condensed explosives can remarkably change the shock locus and propagation speed of the detonation wave. Especially, when the confinement material has a higher sound-speed than the CJ velocity of explosives, some highly complicated refraction phenomena of detonation waves would take place near the explosives-material interface. This paper aims at analyzing the refraction phenomena of detonation waves in condensed explosives in theoretical and numerical ways. Firstly, an improved shock polar theory based on ZND model of detonation is built to give the styles of the refraction in detonation waves in order to provide a leading-order prediction of the confinement interaction. The improved shock polar is established at the leading shock wave of explosives detonation, and the refraction interaction is determined by the polar curve of the leading shock waves within the unreacted explosives and the polar curve of the refraction shock waves within the confinement material. Secondly, a second-order cell-centered Lagrangian hydrodynamics method, based on the characteristics theory for two-dimensional hyperbolic partial differential equations, is developed to solve the chemically reactive flow equations by the three-term ignition-growth chemistry reaction law. The main character of this method is that the finite volume discretization is adopted and an instantaneous evolution solver from an approximate Galerkin evolution operator is applied to compute the velocity and pressure of a grid vertex in order to update the grid coordinates and evaluate the numerical flux across the cell interface. A representative experiment about the propagation of a slipping detonation wave is numerically simulated. From the theoretical and numerical results about the refraction of detonation waves while the PBX9502 explosives interacting with beryllium interface, there exist four kinds of refraction styles of the detonation wave at high sound-speed material interface: the regular refraction with reflecting shock wave, the irregular refraction with bound precursor wave, the irregular refraction with twin Mach reflection, and the irregular refraction with -wave structure. In the first style, the front of the leading shock wave is straight, the flows in the detonation reactive zone and beryllium are both supersonic, and a reflecting shock wave appears behind the leading shock wave and a refracting shock wave appears within beryllium. In the second style, the front of the leading shock wave is also straight, the flow in the detonation reactive zone is supersonic but the one in beryllium is subsonic, so a reflecting shock wave appears behind the leading shock wave and a refracting shock wave appears within beryllium too; moreover, the refracting shock wave is almost perpendicular to the material interface, that is a bound precursor wave. In the third style, the front of the leading shock wave becomes forward curve, and the flows in the detonation reactive zone and beryllium are both subsonic, i.e., a Mach item is produced at some distance above the material interface where there are two Mach reflection structures on the top and the bottom of the Mach item respectively. Obviously, the bottom Mach reflection is a free precursor wave from the refracting shock wave within beryllium. In the fourth style, the forward curve range of the front of the leading shock wave becomes very broad, and accordingly, the range of subsonic flows in the detonation reactive zone becomes very wide. This makes the top Mach reflection disappear but the bottom one still exist, so the whole structure of the reflection wave seems to be like the Greek alphabet ; meanwhile, the flow within beryllium may be all in a subsonic state.

2016, 65 (2): 024703.
doi: 10.7498/aps.65.024703

Abstract +

The process of gas discharge is very complicated and experimental observations indicate that streamers in short gap under non-uniform electric fields always exhibit irregularity and self-similarity, so a dielectric breakdown model, which is the combination of the random fractal method and the traditional streamer theory, can simulate this phenomenon.In this paper, a stochastic model with the growth probability index at any point proportional to the power of the electric field is utilized to quantify the channel tortuosity, and the space charge effect is taken into account as well. The potential distribution is solved by the Poisson's equation which is calculated iteratively by finite difference method; and the box counting method is used to characterize the channel tortuosity and estimate the fractal dimensions of the discharge channels. Based on this, an idea is proposed that the analysis of the experimental results, which in turn provide the appropriate parameters for the model, can better elucidate this phenomenon.The growth probability index can always get from the previous data, but the range of the will change under different experimental condition and there will exist differences in simulation results on fractal dimensions for different , so the limitation of the previous studies is its possible lack of generalizability. In order to define the range of the growth probability index in this model, the bifurcation phenomenon of plasma channels generated by the discharge, affected by HVDC (high-voltage direct current) of short-air-gap in a needle-plate electrode, is captured by ICCD. Before estimating the fractal dimensions of discharge channels, experimental images are saved as a binarized (black and white) image, and the gray-level transformation and boundary identification algorithm will be conducted to remove the apparent thickness of the discharge channel caused by the magnitude of the flowing currents through different branches. Experimental results show that the range of fractal dimensions in the box counting method for the discharge channel is 1.40-1.55. Under the same condition that other factors remain the same but the adjusted growth probability index in this simulation model should accord with the experimental results, all the facts demonstrate that the value of must lie between 0.04 and 0.05.

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

2016, 65 (2): 025201.
doi: 10.7498/aps.65.025201

Abstract +

The deep dielectric charging (DDC) imposes a potential threat on spacecrafts. On the one hand, this kind of polymer insulator dielectric, represented by polyimide, is significantly dependent on temperature; on the other hand, during the charging process the high electric field (at the level of 107 V/m) will enhance the conductivity of the dielectric. Therefore, in order to make a precise assessment of DDC by computer simulation, the conductivity model should take into account the temperature and electric field dependences. In this field, two conductivity models are usually adopted for DDC simulation. One of them is proposed by Adamec. It puts emphasis on the enhanced conductivity due to high electric field, while its temperature dependence is based on the famous Arrhenius formula. Adamec model can make good performance versus electric field, but it is inappropriate in low temperatures. Another model combines the thermally assistant hopping conductivity and the variable-range hopping conductivity together, so it shows advantage in the temperature dependence, which is named as TAH VRH model. Although this model also can include the influence from electric fields, the effectiveness is not so good as that of Adamec model. In order to combine the advantages of these two models, i.e. the Adamec model and TAH VRH model, a new conductivity model is proposed with fewer parameters than those in TAH VRH. It is derived by replacing the Arrhenius formula in Adamec model with a simplified temperature model referred to as TAH VRH model. This formulation enables the new model to deal with a wider temperature range and keep the good performance versus high electric fields. The proposed model is verified partly by the measured data of a kind of polyimide. Satisfactory agreement is obtained in data fitting by using the new model, where the temperature dependence is better than that of Adamec model. In addition, to overcome the unreasonable increase in conductivity in low temperature and high electric field, a useful technique is proposed. By temperature mapping in the electric field correlated factors namely the carrier concentration and mobility enhancement factor, this technique can extend the feasible temperature range to a lower limit. This is done according to the assumption that the carrier concentration is small at low temperatures, and consequently the electric field influence should not be large. At high temperatures or in low electric fields, the temperature mapping is of little effect. Finally, analysis of the model's sensitivity versus several parameters is provided, demonstrating the advantage of applicability of the new model with fewer parameters.

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

2016, 65 (2): 026101.
doi: 10.7498/aps.65.026101

Abstract +

The possible geometrical and electronic structures of fullerene C20 tetramer are optimized by using the density functional theory (b3lyp) at the 6-31G level. For the ground state structures of C20 tetramer, the stability properties, natural bond orbits (NBO), the spectrum, the polarizability and the aromatic characteristics are analyzed. The calculated results show: C20 tetramer may be synthesized by [2+2] addition reaction of C20 carbon cages, which have good thermal stability. Carbon atoms in C20 tetramer are in sp2 hybridization and these atoms happen to have charge transfer. There are a lot of vibration peaks in IR and Raman spectra of C20 tetramer. The bonding interactions between atoms of C20 polymer increase with increasing number of the carbon cages. The C20 tetramer has aromaticity.

2016, 65 (2): 026102.
doi: 10.7498/aps.65.026102

Abstract +

As known for a long time, bubbles in liquid would emit light under ultrasonic waves. This experiment, called sonoluminescence, has attracted many people to research on it, but its mechanism is not yet clear. It is reasonable to think that similar phenomenon involving bubbles may happen in solid materials, but its opacity prevents researchers to detect such a kind of phenomenon. This paper investigates the change of gas bubbles by transmission electron microscope (TEM), being able to overcome the difficulty of opacity of materials. Thin film samples of pure aluminum are prepared for TEM experiment by jet chemical polishing. The samples are implanted first by deuterium or hydrogen at room temperature using ion accelerator, followed by electron irradiation under 200 kV TEM. Gas bubbles will form in aluminum after ion implantation, and then grow into larger ones or be collapsed under electron irradiation. Electron diffraction rings of polycrystals appear together with the change of gas bubbles. This kind of diffraction rings of polycrystals could be observed both in deuterium-implanted and hydrogen-implanted aluminum, but would never be found in the case of electron irradiation on the aluminum without implantation of hydrogen or deuterium. The polycrystals of aluminum are not due to the heating effect of electron beam, even electron beam could make a hole in the film sample finally. For the sample of aluminum containing no hydrogen or deuterium, only dislocation loops can be observed during electron irradiation. It may be that a kind of heat emission occurs when the gas bubbles are irradiated by electron beams, but the heat emission would not be due to deuterium fusion reaction because the electron beam-induced polycrystals occur not only in deuterium case, but also in hydrogen case, indicating that the implanted deuterium is not the necessary condition for heat emission. In addition, the energy dispersive spectrometer in TEM is used to detect the possible unique X-ray signals, but none of any special peak below 40 keV in the X-ray spectrum can be found. The plasmatization of gas in the bubbles under electron beam irradiation is used to try to explain the mechanism of such heat emission.

2016, 65 (2): 026201.
doi: 10.7498/aps.65.026201

Abstract +

Ejecta production from the metal surface under shock-loading is currently a focused issue both at home and abroad. However, the traditional experimental techniques, such as piezoelectric pin, only diagnose the ejected data for low-density ejecta but not for high-density ones, giving a poor understanding of this process. Particularly, when ejecta production increases significantly as the loaded metal melts on release or shock, the measurement carried out by the traditional piezoelectric pin becomes worse, and brings further missing knowledge in the ejecta evolution.In this paper, an Asay-F window designed earlier by the authors based on the traditional Asay-window, is employed to investigate the formation process of the ejecta from the melted Sn metal. As indicated by previous experimental findings on shocked Pb sample, the Asay-F window is a reliable and effective tool for measuring the high-density ejecta by comparing the result with those of the piezoelectric pin. The interface velocity within the Asay-F window measured by Doppler pin system, is obtained. On the basis of momentum conservation condition, the physical quantities of ejecta, such as accumulative areal mass, volume density and velocity, are derived from the interface velocity. By analyzing the experimental data diagnosed by the Asay-F window, which is placed at different offsets from the free surface of Sn sample, the expansion evolution of the ejecta is obtained. Through transforming the dynamic volume density to the static one, the picture of the ejecta density distribution changes with the spatial distance at a specific moment, which is explicitly displayed. It is found that the ejecta density distributions gained from the different offsets at the uniform moment are consistent. As a consequence, the self-similar expansion evolution of the ejecta is experimentally confirmed, which successfully avoids the unclear understanding of this process if only examined by the piezoelectric pin. This experiment may lay the foundation of the formation of the ejecta production for the metal sample subjected to high pressure loading.

2016, 65 (2): 026202.
doi: 10.7498/aps.65.026202

Abstract +

Layered MAX phase ternary compounds (M = early transition metals, A = group A elements, and X = C or N) show promise of wide applications in many applied fields because these compounds have combined ceramic and metallic properties. As an exemple of the MAX phase family, Ti3SiC2 exhibits a high melting temperature, high electrical and thermal conductivities, and an excellent resistance to oxidation and thermal shock. Particularly, it possesses unusual mechanical properties, such as easy machinability, high Young's modulus, thus it is considered as a candidate in advanced nuclear reactors.In this work, we investigate the effect of hydrogen and helium on the cleavage fracture of Ti3SiC2 in order to evaluate the reliability of Ti3SiC2 used in nuclear industry. We have performed first-principles mechanical calculations by using the density functional theory as implemented in the Cambridge Serial Total Energy Package code. Uniaxial tensile simulations along c-axis have been done to calculate the stress-strain curve and the cleavage energy for each interlayer of Ti3SiC2. It is found that Ti3SiC2 has the cleavage characteristics, and the habit cleavage plane starts from Si-Ti interlayer because of relatively weak Si-Ti bond. Hydrogen and helium always accumulate in the Si layer. Helium decreases largely the critical stress of cleavage fracture of Ti3SiC2. In contrast, hydrogen does not efficiently affect the cleavage fracture in Ti3SiC2. The difference between helium and hydrogen behaviors in Ti3SiC2 originates primarily from the difference of electronic hybridization with lattice atoms of Ti3SiC2. For helium, the neighboring Si atoms will be ejected by helium atoms, and the Si-Ti bonds will be broken, thus resulting in the cleavage fracture. However, for hydrogen, it is primarily hybridized with the s states of neighboring Si atoms, which does not severely disturb the p-d hybridization between Si and Ti atoms. Thus, the cleavage fracture from Si-Ti interlayer is hardly aggravated in the presence of hydrogen. Fortunately, Ti3SiC2 has a self-repair ability at high temperatures. It will desorb helium atoms at high helium pressure through Si layers. This behavior will alleviate the cleavage fracture induced by helium. In summary, Ti3SiC2 may be a potential material applied in light water or other fission reactors in the future.

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

2016, 65 (2): 027101.
doi: 10.7498/aps.65.027101

Abstract +

First-principles calculations of the electronic structure and thermodynamic properties of calcium sulfide (CaS) have been carried out by the plane-wave pseudopotential density functional theory method. The calculated values of lattice constant, elastic modulus and its derivative for CaS under zero pressure and zero temperature, agree well with the experimental data and some of the existing model calculations. The band structure and density of states are discussed in detail. Moreover, the dependences of the volume variation, bulk elastic modulus, thermal expansion coefficient and heat capacity on pressure have been investigated for the first time, so far as we know. It is concluded that under the condition of zero temperature (0 K) and zero pressure (0 GPa), the volume is 44.6 3 when the energy of the crystal unit cell reaches a minimum in the structural model of CaS, which is the most stable system. The energy band of CaS is mainly composed of low band gap, valence band and conduction band, the GV-XC band gap of CaS is 2.435 eV. The DOS results show that the valence band is mainly of Ca 3s and S 3p, while the conduction band is mainly of Ca 4d and a small amount of S 3p. At a certain temperature, the volume change rate, heat capacity and thermal expansion coefficient decrease with rising pressure, and the body elastic modulus B increases simultaneously. In contrast, when the pressure is constant, the volume change rate and body elastic modulus B decrease with the increase of temperature, while the thermal expansion coefficient and heat capacity increase as the temperature rises. When the temperature is higher than a certain value, the heat capacity CV is close to the Dulong-Petit limit, and the effect of temperature on the heat capacity is minimal. Furthermore, under the condition of low pressures, the influence of temperature on thermal expansion coefficient is greater than that of the pressure on it.

2016, 65 (2): 027501.
doi: 10.7498/aps.65.027501

Abstract +

In traditional views, the magnetic ordering of oxides may be explained using magnetic superexchange (SE) or double exchange (DE) interaction models. Both models are based on an assumption that the valences of all oxygen ions be -2. For example, both La and Mn in LaMnO3 are assumed to be trivalent, in which antiferromagnetic spin structure is explained using the SE interaction between Mn3+ cations mediated by oxygen anions. In La1-xSrxMnO3, there exists a part of Mn4+ cations with the content ratio of Mn4+/Mn3+ being x/(1-x), in which spin structure and electronic transport properties are explained by DE interaction. However, there is a part of monovalent oxygen ions existing in oxides. Cohen [Nature 358 136] has calculated the densities of states for valence electrons in the perovskite oxide BaTiO3 using density functional theory. Results indicate that the average valence of Ba is +2, being the same as that in the traditional one, but the average valences of Ti and O are +2.89 and -1.63 respectively, agreeing with the results obtained using ionicity investigation [Rev. Mod. Phys. 42 317] and X-ray photoelectron spectra (XPS) analysis, but different from the conventional results +4 and -2. In this paper, three samples with the nominal composition La0.95Sr0.05MnO3 are prepared by different thermal-treatments. Likewise, there are only Mn2+ and Mn3+ cations, but no Mn4+ cations in La0.95Sr0.05MnO3, a result obtained by XPS analysis, and the average valence of Mn in La0.95Sr0.05MnO3 samples increases with increaseing thermal-treatment. Although the crystal structures of the samples are the same, the magnetic moments per formula are obviously different. This magnetic structure cannot be explained using the conventional SE and DE interaction models. Using the O 2p itinerant electron model for spinel ferrites proposed recently by our group, we can explain this magnetic structure. The variation trend of the average valences of Mn cations calculated using the magnetic moments per formula of the samples at 10 K, is in accordance with the experiment results of XPS. The O 2p itinerant electron model is based on an assumption that there is a part of monovalent oxygen ions in the oxides, which is the fundamental difference from SE and DE interaction models.

2016, 65 (2): 027502.
doi: 10.7498/aps.65.027502

Abstract +

We have investigated the quantum magnetic oscillations of graphene subjected to the spin-orbit interaction(SOI) in the presence of crossed uniform electric and magnetic fields and scattered from impurities at finite temperatures. Landau levels are shown to be modified in an unexpected fashion by the spin-orbit interaction, the electrostatic potential and magnetic confinement; this is strikingly different from the non-relativistic 2D electron gas. Furthermore, we derive the analytical expressions of the thermodynamic quantities subject to the SOI, such as density of states, thermodynamic potential, magnetization, and magnetic susceptibility etc. At finite temperatures, the magnetization and magnetic susceptibility can both be predicted to oscillate periodically as a function of reciprocal field 1/B and shown to be modulated through the SOI and the dimensionless parameter ( = E/ F B). As approaches unity, the values of magnetization and magnetic susceptibility finally move to infinity, indicating a transformation of closed orbits into open trajectories, thereby, leading to the vanishing of magnetic oscillations. And, the magnetic susceptibility depends largely on the external fields, suggesting that graphene should be a non-linear magnetic medium. Besides, the associative effect of impurity scattering and temperature may make the standard 2D electron gas be deemed as the consequence of the relativistic type spectrum of low energy electrons and holes in graphene. Also, we comment on a possibility of using magnetic oscillations for detecting a gap that may open in the spectrum of quasiparticle excitations due to the SOI.

2016, 65 (2): 027701.
doi: 10.7498/aps.65.027701

Abstract +

The phase gradient metasurface has strong abilities to manipulate electromagnetic waves on a subwavelength scale and has a potential to enhance the antenna gain. Based on the single multi-resonance metallic patch srtucture, we propose a new kind of ultra-thin broadband unit cell to manipulate electromagnetic waves and enhance the gain. It has been demonstrated that anomalous reflection can be achieved by utilizing the magnetic resonance between metallic patch and ground plane. Moreover, it is believed that resonance with low quality factor (Q factor) is useful in extending the working bandwidth. In order to extend the bandwidth of phase modulation, it is necessary to design a kind of low-Q unit cell. Besides, we need to extend the phase shift to cover the entire range [0, 360] to achieve the focusing effect. Thus we design a suitable symmetrical unit cell composed of ring and cross metallic patterns to control the phase of reflected waves. The symmetrical structure is useful for decreasing the Q factor so as to get a kind of low-Q unit cell. Theoretically, ring and cross metallic patch can be regarded as multi-resonance unit cells, which can cover the entire scope [0, 360]. The unit cell operates at 15-18 GHz with a thickness of 1 mm and the sides of 0.3 0( 0=20 mm). Furthermore, we design a phase gradient metasurface composed of the designed unit cell to verify the broadband anomalous reflection and focusing effects in CST Microwave Studio; the effect can be clearly illustrated in the simulation results obtained at 15-18 GHz. Due to the successful conversion from plane wave to quasi-spherical wave, we can place the Vivaldi antenna at the focal point of the metasurface as a feed source to transform the quasi-spherical wave to plane wave to enhance antenna gain. The simulation results are in good agreement with the theoretical analysis. Meanwhile, the designed metasurface and Vivaldi antenna have been fabricated and applied to enhance the gain of Vivaldi antenna. Both simulation and test results show that the peak gain has been averagely enhanced by 11 dB during the -1 dB gain bandwidth of 15-18 GHz and the fractional bandwidth is 18.2%. Moreover, due to the thin thickness, light weight and broad band, the designed unit cell may open up a new route for the applications of phase gradient metasurfaces in the microwave band region, and may also used as an alternative of high-gain antenna.

2016, 65 (2): 027801.
doi: 10.7498/aps.65.027801

Abstract +

Recently, fused silica has been used to prepare the optical windows in the inertial confinement fusion (ICF) equipment. Challenge of application of fused silica is due to the defect-related optical absorption which is considered as the main mechanism of laser-induced damage process. However, due to structural complexity, calculation of the defect-related absorption from the first principles is only limited to small clusters, and a full treatment using the state of art GW and Bathe-Salpeter equation (BSE) method is still lacking.In this work, density functional theory calculations are performed to study the defect structure of the peroxy linkage (POL) and the neutral oxygen vacancy (NOV) defects in amorphous silica. Firstly, well relaxed structure is generated by using a combination of the bond switching Monte Carlo technique and the DFT-based structure optimization. Secondly, the defect structures are generated and studied in both the ground singlet (S0) and the first excited triplet (T1) states. Finally, the electronic and optical properties of the considered structures are studied by applying the self-consistent quasi-particle GW (sc-QPGW) and the BSE methods in Tamm-Dankoff approximation.In the ground state S0, the POL defect is found to be stable and shares a similar local structure to the H2O2 molecule. However, in T1 state, the POL defect breaks into a pair of E' center ( - Si ) and peroxy oxygen radial ( O-O-Si-). For the NOV defect, the optimized Si-Si bond length in the ground state is 2.51 with a variation of 0.1 due to the structural disorder. In comparison to the ground state, the optimized Si-Si bond length in T1 state increases to 3.56 .The scGW/BSE calculation on the defect free structure predicts a quasi particle band gap of 10.1 eV and an optical band gap of 8.0 eV, which are consistent well with the available experimental results. For the POL defect, the scGW/BSE calculation reveals a weak exciton peak at 6.3 eV. Below 6.3 eV, no new exciton peak is found, implying that the experimentally suggested 3.8 eV peak could not be attributed to the POL defect. Calculations of the NOV defect gives a strong and highly polarized optical absorption peak at 7.4 eV which is close to the previous experimental result at 7.6 eV. The structural relaxation induced by NOV also contributes to another absorption peak at 7.8 eV.

2016, 65 (2): 027901.
doi: 10.7498/aps.65.027901

Abstract +

The X-ray emission spectra produced by 2.4-6.0 MeV Xe20+ ions impacting on vanadium surface were measured. The V K-shell X-ray production cross sections were extracted from the experimental yield data and compared with the theoretical predictions of the binary encounter approximation (BEA), the plane wave born approximation (PWBA), and the energy-loss coulomb-repulsion perturbed-stationary-state relativist (ECPSSR). In order to predict reasonably the inner-shell ionization induced by highly charged heavy ions during the asymmetric collisions at near the Bohr velocity, the corrections of BEA model are discussed. It is found that the X-ray production cross section induced by highly charged heavy ions moving at near the Bohr velocity is on the magnitude of 1 barn, which is almost four orders of magnitude larger than that induced by proton. The ECPSSR, which is regarded as the best model to simulate the inner-shell ionization by light ions, may underestimate the experimental data at least three orders of magnitude. The PWBA model presents a prediction to the results on an order of magnitude better than the ECPSSR simulation, but gives a worse tendency than the BEA model. The BEA calculations, corrected both by Coulomb repulsion and effective nuclear charge, present the best agreement with the experimental results. It is proposed, that in the energy region near the Bohr velocity, during the asymmetric collisions of Xe20+ ions with V atoms, the K-shell electron of V is ionized by direct ionization, and that it can be described by the binary encounter process between the xenon ions and the bound electrons. The X-ray production cross section can be simulated by BEA model, but the corrections of Coulomb repulsion and effective nuclear charge must be considered.

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

2016, 65 (2): 028401.
doi: 10.7498/aps.65.028401

Abstract +

In real-world applications, wireless sensor networks often consist of a large number of sensor nodes with constraint battery resources. How to reduce the power consumption of sensor nodes and maximize the network life, becomes the most important goal of topology control schemes in wireless sensor networks. During the operation of networks, sensor nodes may spend different levels of energy, and result in the uneven distribution of residual energy of sensor nodes. In order to extend the network life, it is essential to adjust the network burden of sensor nodes dynamically, so as to achieve energy balance among nodes under the consideration of different energy levels at nodes. In this paper, we introduce the game theory and the concept of game potential. By synthetically considering the factors of the residual energy and transmission power of nodes, a potential game based mathematical model of topology control is constructed. We prove the existence of Nash equilibrium. Through designing a payoff function, which takes into account both network connectivity and energy balance of nodes, the connectivity of sensor networks can be maintained while the power of sensor nodes is reduced. By increasing the average value of residual energy of neighbors, it enables to select nodes with more energy that reserves in neighborhood as neighbors, to improve the energy balance among nodes. Based on that, a distributed energy-balanced topology control algorithm (DEBA) is proposed. Theoretical analysis proves that the algorithm can maintain network connectivity. Compared with other existing game theory based algorithms DIA and MLPT, the topologies formed by the proposed algorithm have fewer bottleneck nodes which feature heavy traffic load and low residual energy, and smaller variance of node residual energy, thus achieving a longer life.

## EDITOR'S SUGGESTION

2016, 65 (2): 028501.
doi: 10.7498/aps.65.028501

Abstract +

This paper presents a new high speed gate driver circuit driven by In-Zn-O thin film transistors. Two methods are employed to improve the speed of this dirver: First, the input stage multiplex structure is adopted, one input stage drives three output stages; this could reduce the quantity of thin film transistors and also could achieve the narrow bezels in the AMOLED or AMLCD displays. Even the work frequency of the input stage becomes 1/3 of the output stage. When the speed of the circuit increass, there is enough time for input stage charging and discharging. So this kind of driver is suitable for high speed driving method. Second, three times the capacitance coupled effect generated in the gate driver can pull up the voltage level of the key nodes in the circuit, ensuring the signal integrity, While the first time the effect generated in the input stage is to reduce the charge time of the cascade signal and improve the speed of input stage. The second time that generated between input stage and output stage contrbutes to the integrity of cascade ouput signal and output control signal. A diode-connected thin film transistor applied to connect the output control signal and the gate of pull-up thin film transistors in output stage generates the three time capacitance coupled effects. Since the capacitance coupled effect can pull up the gate voltage of the pull-up thin film transistors during output period, the driving ability of the pull-up thin film transistors and the working speed could be promoted effectively. Simulation result shows that the capacitance coupled effect of each key node can pull up the voltage level considerably and the gate driver can normally work at the speed of 4 s. Finally, ten stage gate driver circuits have been fabricated successfully including ten input stages and thirty output stages. The test result shows that the proposed gate driver could work normally with a load of R=5 k and C=100 pF. Furthermore, the high speed test result shows that the output signal pulse width of the circuit is 2 s meeting the driving demands of the 4 k8 k display at the frame rate of 120 Hz. The power consumption of the gate dirver circuit is measured in different resolutions under the frame frequencies of 60 and 120 Hz respectively.

2016, 65 (2): 028701.
doi: 10.7498/aps.65.028701

Abstract +

Studying global dynamics and stability of biological network is of importance in order to understand its function and behavior. In this paper, we consider the p53-Mdm2 oscillator module with PDCD5 as a core part of p53 signaling pathway after the DNA damage, and explore the dynamics and stability of the tumor suppressor p53. The dynamics of p53 may decide the cell fate after the DNA damage, while the oscillation of p53 may induce cell cycle arrest and so promote the repair of DNA, and the high levels of p53 can trigger apoptosis. However, p53 activity may be inhibited by its negative regulator Mdm2 in some cancer cells, as Mdm2 is of overexpression due to the increase in Mdm2 production rate. So we first investigate the effect of Mdm2 production rate on the kinetics of p53 through bifurcation analysis. after the DNA damage. With the increase in Mdm2 production rate, p53 can display a steady state, a stable-limit cycle and the coexistence of a stable-limit cycle and a stable steady state. Furthermore, the potential landscapes for oscillation show that the lower concentration of p53 means a stronger stability, whereas those for bistability of the higher steady state and the oscillatory state illustrate that stability of the higher steady state increases with the increasing Mdm2 production rate. In addition, noise strength can greatly affect the stability of p53 oscillations, so we explore the effect of noise strength on potential landscapes, barrier heights and periods. A smaller noise strength leads to a higher barrier height associated with more stable-limit cycle, and the harmonic oscillation with more uniform period and smaller variance is helpful to have more stable maintainance. Our results may be useful for understanding regulation of p53 signaling pathway after DNA damage.

2016, 65 (2): 028801.
doi: 10.7498/aps.65.028801

Abstract +

In recent years, solar cells, especially the bulk heterojunction (BHJ) polymer solar cells (PSCs), have attracted considerable attention. BHJ PSCs have several advantages such as easy fabrication, light weight, low cost and flexibility. The research on ternary BHJ PSCs will become a hot topic since incorporating near infrared region (NIR) low bandgap polymer materials into the donor/acceptor system can easily extend the absorption spectral range and improve the photon harvesting. In this paper, we investigate the ternary PSCs based on poly{4, 8-bis[(2-ethylhexyl)-oxy]benzo[1, 2-b:4, 5-b']dithiophene-2, 6-diyl-alt-3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3, 4-b]thiophene-4, 6-diyl} (PTB7); Bis adduct of phenyl-C71-butyric acid methyl ester (Bis-PC70BM); [6, 6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). The performance of PSCs based on PTB7 and PC70BM may be improved by doping with Bis-PC70BM which is used as an electron-cascade acceptor material. Ternary blend PSCs with 3% Bis-PC70BM exhibit a power conversion efficiency (PCE) of 7.00%, higher than that (6.07%) of the PTB7 :PC70BM binary blend. The open-circuit voltage (VOC) is 0.77 V, the short-circuit current (JSC) is 13.92 mA cm-2 and the fill factor (FF) is 65%. However, in our research, the absorption spectra for the films with different amount of Bis-PC70BM are hardly changed, implying that doping with Bis-PC70BM would not improve the photon harvesting. The LUMO (HOMO) energy levels of PTB7, Bis-PC70BM and PC70BM are -3.49 eV (-5.31 eV), -3.80 eV (-6.10 eV) and -3.91 eV (-6.20 eV), respectively. Due to the higher LUMO energy levels of Bis-PC70BM relative to PC70BM, the VOC increases when Bis-PC70BM is used. The cascade-like energy levels of ternary blend PSCs can facilitate the charge transfer at the donor/acceptor interface owing to the bridging effect. There are three routes for charge transfer (PTB7-Bis-PC70BM, Bis-PC70BM-PC70BM and PTB7-PC70BM) in ternary PSCs, more than that one in the binary PTB7:PC70BM counterpart. Moreover, PC70BM can provide a driving force to transfer the electrons on the LUMO of Bis-PC70BM to a lower energy orbital (the LUMO of PC70BM), which can facilitate charge transfer from PTB7 to Bis-PC70BM. Atomic force microscopy (AFM) images show that when 3% Bis-PC70BM is used, the film of the ternary blend active layer becomes smoother and the root-mean-square (RMS) roughness decreases from 1.87 nm to 1.80 nm. The decreased roughness is likely good for the contact between the PEDOT:PSS and the active layer, improving the transport rate. We have fabricated hole-only devices using a high-work-function material (Au) as the cathode to block the back injection of electrons in order to investigate charge carrier transport and collection in the PSCs. Result shows that doping with Bis-PC70BM may not change the hole mobility in the device. Besides, the Jph-Veff characteristics shows that doping with 3% Bis-PC70BM can facilitate exciton dissociation and charge collection at a low voltage. Our results indicate that using Bis-PC70BM as an electron-cascade acceptor material in PTB7 :PC70BM blend to fabricate ternary blend PSCs is a promising way to improve the PCE.

2016, 65 (2): 028901.
doi: 10.7498/aps.65.028901

Abstract +

Measurements of node centrality are based on characterizing the network topology structure in a certain perspective. Changing the network topology structure would affect the accuracy of the measurements. In this paper, we employ the Holme-Kim model to construct scale-free networks with tunable clustering, and consider the four measurements of classical centrality, including degree centrality, closeness centrality, betweenness centrality and the eigenvector centrality. For comparing the accuracy of the four centrality measurements, we simulate the susceptible-infected-recovered (SIR) spreading of the tunable clustering scale free networks. Experimental results show that the degree centrality and the betweenness centrality are more accurate in networks with lower clustering, while the eigenvector centrality performs well in high clustering networks, and the accuracy of the closeness centrality keeps stable in networks with variable clustering. In addition, the accuracy of the degree centrality and the betweenness centrality are more reliable in the spreading process at the high infectious rates than that of the eigenvector centrality and the closeness centrality. Furthermore, we also use the reconnected autonomous system networks to validate the performance of the four classical centrality measurements with varying cluster. Results show that the accuracy of the degree centrality declines slowly when the clustering of real reconnected networks increases from 0.3 to 0.6, and the accuracy of the closeness centrality has a tiny fluctuation when the clustering of real reconnected networks varies. The betweenness centrality is more accurate in networks with lower clustering, while the eigenvector centrality performs well in high clustering networks, which is the same as in the tunable clustering scale free networks. According to the spreading experiments in the artificial and real networks, we conclude that the network clustering structure affects the accuracy of the node centrality, and suggest that when evaluating the node influence, we can choose the degree centrality in the low clustering networks, while the eigenvector centrality and the closeness centrality are still in the high clustering networks. When considering the spreading dynamics, the accuracy of the eigenvector centrality and the closeness centrality is high, but the accuracy of the degree centrality and the betweenness centrality is more reliable in the spreading process at high infectious rates. This work would be helpful for deeply understanding of the node centrality measurements in complex networks.

2016, 65 (2): 028902.
doi: 10.7498/aps.65.028902

Abstract +

From the study of multilayer networks, scientists have found that the properties of the multilayer networks show great difference from those of the traditional complex networks. In this paper, we derive strictly the spectrum of the super-Laplacian matrix and the synchronizability of two-layer star networks by applying the master stabi- lity method. Through mathematical analysis of the eigenvalues of the super-Laplacian matrix, we study how the node number, the inter-layer and the intra-layer coupling strengths influence the synchronizability of a two-layer star net-work. We find that when the synchronous region is unbounded, the synchronizability of a two-layer star network is only related to the intra-layer coupling strength between the leaf nodes or the inter-layer coupling strength of the entire network. If the synchronous region of a two-layer star network is bounded, not only the inter-layer coupling strength of the network and the intra-layer coupling strength between the leaf nodes, but also the intra-layer coupling strength between the hub nodes and the network size have influence on the synchronizability of the networks. Provided that the same inter-layer and intra-layer coupling strengths are concerned, we would further discuss the opti-mal ways of strengthening the synchronizability of a two-layer star network. If the inter-layer and intra-layer coupling strengths are far less than unity, changing the intra-layer coupling strength is the best way to enhance the synchronizability no matter what the synchronous region is. While if the coupling strengths are the same as, less than or more than unity, there will be different scenarios for the network with bounded and unbounded synchronous regions. Besides, we also discuss the synchronizability of the multilayer network with more than two layers. And then, we carry out numerical simulations and theoretical analysis of the two-layer BA scale-free networks coupled with 200 nodes and obtain very similar conclusions to that of the two-layer star networks. Finally, conclusion and discussion are given to summarize the main results and our future research interests.