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CN 11-1958/O4
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Research progress of mid-and far-infrared nonlinear optical crystals
Jia Ning, Wang Shan-Peng, Tao Xu-Tang
Acta Physica Sinica, 2018, 67 (24): 244203
Energy loss of surface plasmon polaritons on Ag nanowire waveguide
Wang Wen-Hui, Zhang Nao
Acta Physica Sinica, 2018, 67 (24): 247302
Enhanced light absorption and device performances of organic photovoltaic devices with Au tetrahedra nanoparticles
Li Xue, Wang Liang, Xiong Jian-Qiao, Shao Qiu-Ping, Jiang Rong, Chen Shu-Fen
Acta Physica Sinica, 2018, 67 (24): 247201
Acta Physica Sinica  
  Acta Physica Sinica--2018, 67 (24)   Published: 20 December 2018
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INVITED REVIEW

Research progress of mid-and far-infrared nonlinear optical crystals Hot!

Jia Ning, Wang Shan-Peng, Tao Xu-Tang
Acta Physica Sinica. 2018, 67 (24): 244203 doi: 10.7498/aps.67.20181591
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High-power tunable mid-infrared (MIR) and far-infrared (FIR) lasers in a range of 3-20 μm, especially in the atmospheric windows of 3-5 μm and 8-12 μm are essential for the applications, such as in remote sensing, minimally invasive surgery, telecommunication, national security, etc. At present, the technology of MIR and FIR laser have become a research hotspot. As the core component of all-solid-state laser frequency conversion system, nonlinear optical (NLO) crystals for coherent MIR and FIR laser are urgently needed by continuously optimizing and developing. However, compared with several outstanding near infrared, visible, and ultraviolet NLO crystals, such as β-BaB2O4, LiB3O5, LiNbO3, KTiOPO4, and KBe2BO3F2, the generation of currently available NLO crystals for 3-20 μm laser is still underdeveloped. Traditional NLO oxide crystals are limited to output wavelengths ≤ 4 μm due to the multi-phonon absorption. In the past decades, the chalcopyrite-type AgGaS2, AgGaSe2 and ZnGeP2 have become three main commercial crystals in the MIR region due to their high second-harmonic generation coefficients and wide IR transparency ranges. Up to now, ZnGeP2 is still the state-of-the-art crystal for high energy and high average power output in a range of 3-8 μm. Unfortunately, there are still some intrinsic drawbacks that hinder their applications, such as in poor thermal conductivity and low laser damage threshold for AgGaS2, non-phase-matching at 1.06 μm pumping for AgGaSe2, and harmful two-photon absorption at 1.06 μm for ZnGeP2. In addition, ZnGeP2 has significant multi-phonon absorption in an 8-12 μm band, which restricts its applications in long wavelength MIR. With the development of research, several novel birefringent crystals, as well as all-epitaxial processing of orientation-patterned semiconductors GaAs (OP-GaAs) and GaP (OP-GaP), have been explored together with attractive properties, such as large NLO effect, wide transparency ranges, and high resistance to laser damage.
In this paper, from the angle of the compositions of NLO crystal materials, several kinds of phosphide crystals (ZnGeP2 CdSiP2) and chalcogenide crystals (CdSe, GaSe, LiInS2 series, and BaGa4S7 series) are summarized. In addition, the latest achievements of the orientation-patterned materials such as OP-GaAs and OP-GaP are also reviewed systematically. In summary, we review the above-mentioned attractive properties of these materials such as in the unique capabilities, the crystal growth, and the output power in the MIR and FIR region.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Buckling behavior of boron nitride nanotubes under combined axial compression and torsion via molecular dynamics simulations

Zeng Qiang, Zhang Chen-Li
Acta Physica Sinica. 2018, 67 (24): 246101 doi: 10.7498/aps.67.20180641
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Buckling behavior of boron nitride nanotubes under combined axial compression and torsion is presented by using molecular dynamics simulation. In order to study the effect of helicity and nanotube size, three groups of nanotubes are considered. The first group is a pair of boron nitride nanotubes with a similar geometry but different helicities, the second group includes three armchair naotubes having equal length but different radii, and three armchair (8, 8)-nanotubes with different lengths form the third group. The simulation is conducted by applying Nose-Hoover thermostat in a temperature range from 50 K to 1200 K. Based on the interatomic interactions given by Tersoff-type potentials, the molecular dynamics method is used to study variations of atomic interaction in initial linear deformation and postbuckling stages with various load-proportional parameters, and to determine the interactive buckling loads relationship. By comparing typical buckling modes under different loads, it is found that the boron nitride nanotube experiences complex micro-deformation processes, resulting in different variations of atomic interaction and strain energies. When the axial compressive load is relatively large, the change of atomic interaction for boron nitride nanotubes under combined loads is similar to that found under the pure axial compression. The onset of buckling leads to the abrupt releasing of strain energy. As the torsional load is relatively large, the nanotube shows torsion-like buckling behavior, no obvious reduction of strain energy is observed after the critical point. The present simulation results show that both the armchair and zigzag nanotubes exhibit nonlinear interactive buckling load relationships. Rise in temperature results in the decrease of interactive buckling load, and the effect of temperature varies with the value of load-proportional parameter. That is, the axial compressive load is relatively large, and the effect of temperature is more significant. It is found that the buckling behavior in the case of combined loading is strongly size dependent. The interactive critical axial and shear stress decrease as nanotube radius or length increases. The studies also reveal that under both simple loading and combined load condition, carbon nanotubes possess higher buckling loads than those of boron nitride nanotubes with a similar geometry, which provides valuable guidance for forming carbon and boron nitride hybrid nanotubes as well as coaxial nanotubes with superior mechanical properties.

Electrical transport properties and related mechanism of single SnO2 nanowire device

Chen Ya-Qi, Xu Hua-Kai, Tang Dong-Sheng, Yu Fang, Lei Le, Ouyang Gang
Acta Physica Sinica. 2018, 67 (24): 246801 doi: 10.7498/aps.67.20181402
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Defect engineering in a semiconductor nanowire-based device has aroused intensive attention due to its fascinating properties and the potential applications in nanoelectronics. Here in this work, in order to investigate the effect of oxygen defects on the electrical transport properties in a SnO2-nanowire-based device under normal environment, we synthesize an individual SnO2 nanowire, by a thermal chemical vapor deposition method and further construct a two-terminal Au/SnO2 nanowire/Au device by using optical lithography. The electrical transport properties of a single SnO2 nanowire device are measured under the condition of air and vacuum after hydrogen reduction. It is found that the transport performances in air are unusually different from those in vacuum. Strikingly, the reduction of electric current through the device and the increment of contact barrier of the Au/SnO2 interface in air can be observed with the I-V scan times increasing. While in vacuum, the current increases and a change from Schottky contact to ohmic contact at the interface between Au and SnO2 can be obtained by performing more scans. Our results demonstrate that the oxygen vacancy concentrations caused by the oxygen atom adsorption and desorption on the surface of nanowires play the key role in the transport properties. Furthermore, we calculate the relevant electronic properties, including energy band structure, density of states, as well as I-V characters and transmission spectrum at the interface of Au/SnO2 within the framework of density functional theory. We find that the bandgap of SnO2 nanowires decreases with oxygen vacancy concentration increasing. Also, the existence of oxygen defects enlarges the electron transmission at the interface of Au/SnO2 and enhances electrical transport. Therefore, our results provide a new strategy for designing the integrated nano-functional SnO2-based devices.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Enhanced light absorption and device performances of organic photovoltaic devices with Au tetrahedra nanoparticles Hot!

Li Xue, Wang Liang, Xiong Jian-Qiao, Shao Qiu-Ping, Jiang Rong, Chen Shu-Fen
Acta Physica Sinica. 2018, 67 (24): 247201 doi: 10.7498/aps.67.20181502
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Organic photovoltaic devices (OPVs) have attracted considerable attention because of their advantages of light-weight, low-cost, large-scale manufacturing process and mechanical flexibility. Unfortunately, in order to achieve efficient carrier extraction, the photoactive layer in OPVs must be rather thin (100 nm or less) due to its extremely low carrier mobilities for most of organic/polymer materials (on the order of 10-4 cm2/(V·s)). Such thin photoactive layers lead to a significant loss of incident sunlight, thereby improving a final low light absorption efficiency and power conversion efficiency (PCE). To promote the light absorption and thus enhance PCE of OPVs, Au tetrahedron nanoparticles (NPs) are synthesized in this work and then they are wrapped with poly (sodium 4-styrenesulfonate) (PSS) to form core-shell structure tetrahedron NPs (Au@PSS tetrahedron NPs). They are further incorporated into the interface of hole extraction layer and light photoactive layer to improve PCE of OPVs by enhancing their surface plasmon resonance effect-induced light absorption. The influences of doping concentration and PSS shell thickness of theses Au tetrahedron NPs on device performances are explored. The results indicate that the best performing PCE occurs at 6% concentration of Au@PSS tetrahedron NPs, reaching 3.08%, while it is further improved to 3.65% with an optimized PSS shell thickness of 2.5 nm, showing an enhancement factor of 22.9% compared with that of the control counterpart. The performance improvement of OPVs mainly originates from the promoted light absorption of donor due to the location of the resonant absorption peak of Au@PSS tetrahedron NPs in the absorption region of donor. Simultaneously, the introduction of the PSS shell promotes the dissociation of excitons and charge transfer. All of these contribute to the increasing of short-circuit current, fill factor and PCE of OPVs.

Capacitance scattering mechanism in lattice-matched In0.17Al0.83N/GaN heterojunction Schottky diodes

Ren Jian, Su Li-Na, Li Wen-Jia
Acta Physica Sinica. 2018, 67 (24): 247202 doi: 10.7498/aps.67.20181050
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In order to study the frequency scattering mechanism of capacitance in latticematched In0.17Al0.83N/GaN high electron mobility transistors (HEMTs), the latticematched In0.17Al0.83N/GaN heterojunction Schottky diodes with circular planar structure, which have equivalent capacitance characteristics to those of HEMTs, are fabricated and tested in this paper. The experimental curves of capacitance-voltage characteristics at different frequencies show that the capacitance of the accumulation area decreases gradually with the increase of frequency at low frequency, which accords with the capacitance frequency scattering characteristics of traditional HEMT devices. However, when the frequency is higher than 200 kHz, the capacitance of the accumulation area increases rapidly with frequency increasing, which cannot be explained by the traditional capacitance model. By comparing the reverse current and capacitance characteristics of latticematched In0.17Al0.83N/GaN Schottky diodes, it is observed that the saturation behavior of the reverse leakage current is clearly associated with full depletion of the two-dimensional electron gas at the InAlN/GaN interface, which is indicated by the rapid drop of the diode capacitance. This observation suggests that the large reverse leakage current of the lattice-matched In0.17Al0.83N/GaN Schottky diode, which reaches up to 10-4 A, should has a direct influence on the capacitance scattering. By considering the influence of leakage current, interface state and series resistance comprehensively, the capacitance frequency scattering model is modified based on the traditional model. Using various models to fit the experimental capacitance-frequency data, the results from the modified model agree well with the experimental results. According to the parameters obtained by fitting, the density and the time constant of interface defects in latticematched In0.17Al0.83N/GaN Schottky diodes, determined by equivalent interface capacitance and resistance, are about 1.66×1010 cm-2·eV-1 and 2.65μs, respectively. According to the values reported in the literature, it is suggested that the modified capacitance frequency scattering model should be reasonable for explaining the capacitance scattering phenomenon in accumulation area. In conclusion, we believe that the capacitance of latticematched In0.17Al0.83N/GaN Schottky diode scatters is a joint result of leakage current, interface state and series resistance. The interface defects in In0.17Al0.83N/GaN Schottky diodes usually have a great influence on frequency and power characteristics of devices, a correct explanation for the frequency scattering mechanism of capacitance is the basis for determining the locations and sources of defects in Ⅲ nitride devices.

Simulation and optimal design of antimony selenide thin film solar cells

Cao Yu, Zhu Xin-Yun, Chen Han-Bo, Wang Chang-Gang, Zhang Xin-Tong, Hou Bing-Dong, Shen Ming-Ren, Zhou Jing
Acta Physica Sinica. 2018, 67 (24): 247301 doi: 10.7498/aps.67.20181745
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In this paper, the wx-AMPS simulation software is used to model and simulate the antimony selenide (Sb2Se3) thin film solar cells. Three different electron transport layer models (CdS, ZnO and SnO2) are applied to the Sb2Se3 solar cells, and the conversion efficiencies of which are obtained to be 7.35%, 7.48% and 6.62% respectively. It can be seen that the application of CdS and ZnO can achieve a better device performance. Then, the electric affinity of the electron transport layer (χe-ETL) is adjusted from 3.8 eV to 4.8 eV to study the effect of the energy band structure change on the solar cell performance. The results show that the conversion efficiency of the Sb2Se3 solar cell first increases and then decreases with the increase of the χe-ETL. The lower χe-ETL creates a barrier at the interface between the electron transport layer and the Sb2Se3 layer, which can be considered as a high resistance layer, resulting in the increase of series resistance. On the other hand, when the χe-ETL is higher than 4.6 eV, the electric field of the electron transport layer can be reversed, leading to the accumulation of the photon-generated carriers at the interface between the transparent conductive film and the electron transport layer, which could also hinder the carrier transport and increase the series resistance. At the same time, the electric field of Sb2Se3 layer becomes weak with the value of χe-ETL increasing according to the band structure of the Sb2Se3 solar cell, leading to the increase of the carriers' recombination and the reduction of the cell parallel resistance. As a result, too high or too low χe-ETL can lower the FF value and cause the device performance to degrade. Thus, to maintain high device performance, from 4.0 eV to 4.4 eV is a suitable range for the χe-ETL of the Sb2Se3 solar cell. Moreover, based on the optimization of the χe-ETL, the enhancement of the Sb2Se3 layer material quality can further improve the solar cell performance. In the case of removing the defect states of the Sb2Se3 layer, the conversion efficiency of the Sb2Se3 solar cell with a thickness of 0.6 μm is significantly increased from 7.87% to 12.15%. Further increasing the thickness of the solar cell to 3 μm, the conversion efficiency can be as high as 16.55% (Jsc=34.88 mA/cm2, Voc=0.59 V, FF=80.40%). The simulation results show that the Sb2Se3 thin film solar cells can obtain excellent performance with simple device structure and have many potential applications.

Magnetoelectric coupling and external field modulation of a composite multiferroic chain

Huang Ying-Zhuang, Qi Yan, Du An, Liu Jia-Hong, Ai Chuan-Wei, Dai Hai-Yan, Zhang Xiao-Li, Huang Yu-Yan
Acta Physica Sinica. 2018, 67 (24): 247501 doi: 10.7498/aps.67.20181561
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Multiferroics, can simultaneously exhibit multiple ferroic orders, including magnetic order, electric order and elastic order. Among these orders there exist intimately coupling effects. Multiferroics is significant for technological applications and fundamental research. The interplay between ferroelectricity and magnetism allows a magnetic control of ferroelectric properties and an electric control of magnetic properties, which can yield new device concepts. Recent experimental research shows that the Fe/BaTiO3 compound exhibits a prominent magnetoelectric effect, which originates from a change in bonding at the ferroelectric-ferromagnet interface that changes the interface magnetization when the electric polarization reverses, and thus offering a new route to controlling the magnetic properties of multilayer compound heterostructures by the electric field. Motivated by recent discoveries, in this paper we investigate theoretically the thermodynamics of a finite ferroelectric-ferromagnetic chain. A microscopic Heisenberg spin model is constructed to describe magnetoelectric properties of this composite chain, in which electric and magnetic subsystem are coupled through interfacial coupling. However, this vector model is not integrable in general. Therefore, one has to resort to numerical calculations for the thermodynamic properties of such a system. A uniform discrete spin vector is adopted here to approximate the original continuous one, and then the transfer-matrix method is employed to derive the analytical expression. To verify its rationality and effectiveness, the zero-field specific heat of a classical spin chain is solved based on this simplified model, and compared with the exact solution. It demonstrates that the main characteristics obtained by previous research are well reproduced here, and the whole variant trend is also identical. And then the quantities concerned in this paper are calculated, including the magnetization, polarization, magnetoelectric susceptibility, and specific heat. The influence of interfacial coupling, external field, and single-ion anisotropy on the magnetoelectric effect of the composite chain are examined in detail. The results reveal that the interfacial coupling enhances the magnetization and polarization. And in the magnetic field driven magnetoelectric susceptibility, the large magnetoelectric correlation effects are observed, indicating that the magnetic behaviors can be effectively controlled by an external electric field. Meanwhile, it is also found that the external field and single-ion anisotropy both suppress the magnetoelectric susceptibility. In addition, interestingly, the specific heat of system presents a three-peak structure under high electric field, which stems from the thermal excitation of spin states as well as dipole moment caused jointly by electric field and temperature.

Magnetic and magnetocaloric effects of Eu0.9M0.1TiO3 (M=Ca, Sr, Ba, La, Ce, Sm) compounds

Hao Zhi-Hong, Wang Hai-Ying, Zhang Quan, Mo Zhao-Jun
Acta Physica Sinica. 2018, 67 (24): 247502 doi: 10.7498/aps.67.20181750
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EuTiO3 is a direct band-gap semiconductor material and exhibits antiferromagnetism with large magnetic entropy change around the liquid helium temperature. The ferromagnetic state can be changed into antiferromagnetic state through lattice constant change and electron doping by element substitution due to strong spin-lattice coupling coexistence of ferromagnetic coupling, and antiferromagnetic coupling. The values of magnetic entropy change can be effectively improved under low magnetic field change after changing into ferromagnetism. Samples of EuTiO3 and Eu0.9M0.1TiO3 (M=Ca, Sr, Ba, La, Ce, Sm) are prepared by the sol gel method. The Eu0.9Ca0.1TiO3 exhibits the antiferromagnetism due to similar ion radius. The ferromagnetic coupling between Eu0.9Sr0.1TiO3 and Eu0.9Ba0.1TiO3 is enhanced, for alkaline earth metal (Sr and Ba) has larger ion radius, which is beneficial to improving the magnetocaloric effect under low magnetic field. Electron doping can inhibit the antiferromagnetic coupling because the extra carrier may occupy the Ti 3d and reduce the hybridization of Eu 4f-Ti 3d-Eu 4f. When the electron doping concentration is greater than 10%, the spin polarization rate of Ti 3d state on the Fermi surface is negative, resulting in the transition from antiferromagnetic to ferromagnetic state. When the Eu ions are replaced with the Sm ions (Sm ion radius is similar to Eu ion radius), the ferromagnetic coupling is enhanced. However, when the Eu ions are replaced with the La or Ce ions, the samples show strong ferromagnetism, for the lattice constant and electron doping are increased. A giant reversible magnetocaloric effect and large refrigerant capacity for each of Eu0.9M0.1TiO3 (M=Sr, Ba, La, Ce) compounds are observed. Under the magnetic field change of 1 T, the values of maximum magnetic entropy change and refrigeration capacity are 9.8 J/(kg·K) and 36.6 J/kg for Eu0.9Sr0.1TiO3, and 10 J/(kg·K) and 45.1 J/kg for Eu0.9Ba0.1TiO3. The values of maximum magnetic entropy change of Eu0.9La0.1TiO3 and Eu0.9Ce0.1TiO3 are 10.8 J/(kg·K) and 11 J/(kg·K), respectively, which are larger than that of EuTiO3 (9.8 J/(kg·K)). The values of refrigeration capacity are 39.3 J/kg and 51.8 J/kg, which are also improved compared with those of EuTiO3. In a word, the results suggest that these compounds could be considered as good candidates for low-temperature and low-field magnetic refrigerant.

Concentration and thermal quenching of SrGdLiTeO6: Eu3+ red-emitting phosphor for white light-emitting diode

Zhao Wang, Ping Zhao-Yan, Zheng Qing-Hua, Zhou Wei-Wei
Acta Physica Sinica. 2018, 67 (24): 247801 doi: 10.7498/aps.67.20181523
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A series of SrGd1-xLiTeO6:xEu3+ (x=0.1-1) red-emitting phosphors, prepared by high-temperature solid-state reaction at 1100℃, is thoroughly investigated by means of X-ray diffraction, diffuse reflectance spectra, photoluminescence spectra, and electroluminescence spectra. These double-perovskite-type phosphors crystallize into monoclinic systems with space group P21/n(14), accommodate Eu3+ in a highly distorted C1 site symmetry without inversion center, and facilitate the enhancing of the 5D07F2 hypersensitive transition. The excitation spectra, emission spectra and decay curves indicate that the optimum doping concentration of Eu3+ is x=0.6. The SrGd0.4LiTeO6:0.6Eu3+ presents the strongest excitation peak at 395 nm, which is adequate for near-UV light-emitting diode (LED) pumping; meanwhile, it exhibits an intense red emission with chromaticity coordinates of (0.6671, 0.3284), an asymmetry ratio of 7.56, a color purity of 98.6%, and a luminous efficacy of radiation of 249 lm/W. The fluorescence lifetime is 721 μs, from which the internal quantum efficiency is determined to be 89.7% via the Judd-Ofelt analysis. The formula proposed by van Uiter (van Uitert L G 1967 J. Electrochem. Soc. 114 1048), is used to elucidate the energy transfer mechanism. However, the plot of log(I/x)-log(x) produces a confusing index s=4.26, which makes it difficult to distinguish the dipole-dipole interaction from the exchange interaction. After analyzing the reason of error, we present a new plot of log(I0'/I-1)-log(x), in which I0'=I0/x0 and I'=I/x, with x0 corresponding to the low doping content without nonradiative energy transfer. This plot gives rise to s=5.25, a more reasonable value for the dipole-dipole interaction. The integrated emission intensity at 423 K is as high as 85.2% of that at ambient temperature. The thermal activation energy is determined to be 0.2941 eV according to the model based on a temperature-dependent pathway through a charge transfer state. The prototypical LED based on it can emit a bright red light beam. In conclusion, the phosphor exhibits favorable luminous efficiency, color purity and thermal stability of luminescence, which promises solid-state lighting and display applications.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Effect of edge inclination of single crystal diamond on homoepitaxial growth

Geng Chuan-Wen, Xia Yu-Hao, Zhao Hong-Yang, Fu Qiu-Ming, Ma Zhi-Bin
Acta Physica Sinica. 2018, 67 (24): 248101 doi: 10.7498/aps.67.20181537
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Polycrystalline diamond is easy to appear at the edge of single crystal diamond grown by homogeneous epitaxial growth, which makes it difficult to enlarge the two-dimensional surface area of single crystal diamond. In this study, the microwave plasma chemical vapor deposition (MPCVD) is used, the edge of the single crystal diamond (100) crystal face is finely cut and polished to form an inclined surface which is different from the (100) crystal plane at different angles. After being pretreatment, homogeneous epitaxial growth is carried out in a double-substrate waveguide-type MPCVD device with CH4/H2 reaction gas. At the same time, the variation of plasma near the inclined plane of (100) crystal edge is analyzed by optical emission spectroscopy to study the effect of the tilting on the growth of the diamond edge. The experimental results show that the angle of the inclined surface of the edge has an effect on the quality of single crystal epitaxial growth of the edge. As the angle of the inclined surface of the single crystal diamond increases, the quantity of edge polycrystalline diamond first decreases and then increases. At an oblique angle of 3.8°, the edge exhibits complete single crystal epitaxial growth characteristics, which conduces to expand the surface area of single crystal diamond. According to the analysis, the inclined surface at different angle changes the surrounding electric field strength and plasma density of the edge, resulting in the change of carbon-containing precursors reaching the surface of the substrate. When the concentration of carbon-containing precursors on the inclined step surface is higher than the growth threshold of layered step, excessive carbon-containing precursors will constantly collide with each other and accumulate to form polycrystalline diamond on the step. When the concentration is lower than the growth threshold of layered step, the carbon-containing precursors on the surface of the substrate will be laid out to form a layered step. Therefore, the edge effect during the growth of single crystal diamond is weakened at the tilt angle of 3.8°, which leads the concentration of carbon-containing precursors on the inclined step surface to be lower than the growth threshold of layered step.

Eliminating spiral wave and spatiotemporal chaos in cardiac tissues by suppressing the rotation of spiral wave tip

Li Qian-Yun, Huang Zhi-Jing, Tang Guo-Ning
Acta Physica Sinica. 2018, 67 (24): 248201 doi: 10.7498/aps.67.20181291
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The variation of the function of sodium channel in cardiomyocyte is associated with multiple cardiac diseases. Increasing sodium channel availability can effectively increase sodium influx, leading to enhanced cardiomyocyte excitability, prolonged action potential duration and late sodium current activity, which may cause ventricular arrhythmia. On the other hand, enhancing cardiomyocyte excitability can effectively increase the conduction velocity of the medium in the rotation center of spiral wave, which can restrain the rotation of spiral wave, leading to the disappearance of spiral wave. However, how to increase the excitability of cardiomyocytes while avoiding arrhythmias has not yet been explored so far. In this paper, we study how to regulate the changes of sodium current in cardiac myocytes to control spiral wave and spatiotemporal chaos in a two-dimensional cardiac tissues by using the Luo-Rudy phase I model. We propose such a sodium current control scheme:when the cell is excited, the regulation of sodium current begins. If the absolute value of sodium current obtained from the model equation is less than the absolute value of sodium current control threshold, the sodium current is simply equal to the control threshold of sodium current. In other cases, the absolute value of sodium current cannot exceed the maximum value without control. When the membrane potential rises over-5 mV, the sodium current evolves naturally. This method of regulating sodium current ensures that all cells have almost the same amplitude of sodium current, while without obviously changing the excitation-time. All cells thus have the same excitability under the control of sodium current, so that the excitation of cell is less affected by spiral wave tip. The numerical simulation results show that as long as the control threshold of sodium current reaches a critical value, the rotation of spiral wave tip is effectively suppressed, causing spiral wave to move out of the system boundary and spatiotemporal chaos to disappear after it has evolved into a spiral wave. If the absolute value of sodium current control threshold is large enough, the spiral wave and spatiotemporal chaos would also disappear through conductive block. These results can provide a new idea for antiarrhythmic therapy.

Optical properties of niobium nitride nanowires

Wu Yang, Chen Qi, Xu Rui-Ying, Ge Rui, Zhang Biao, Tao Xu, Tu Xue-Cou, Jia Xiao-Qing, Zhang La-Bao, Kang Lin, Wu Pei-Heng
Acta Physica Sinica. 2018, 67 (24): 248501 doi: 10.7498/aps.67.20181646
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Niobium nitride (NbN) nanowires are commonly used as photosensitive materials for superconducting nanowire single-photon detectors (SNSPDs). Their optical properties are the key factors influencing the performance of SNSPD. According to the experimental data and simulation results, in this paper we systematically study the optical properties of various niobium nitride nanowire detector device structures, and characterize the reflection spectra and transmission spectra of the following four device structures:1) Backside optical devices with SiO2 as the antireflection layer, 2) backside optical devices with SiN as the antireflection layer, 3) front-facing optical devices with Au+SiN as a mirror, and 4) front-facing optical devices with distributed Bragg reflector (DBR) as a mirror. The NbN films with different thickness are grown on the basis of the four device structures, and the absorption efficiencies of the NbN films with different thickness are observed. The relationships between the optimal NbN thickness and the optical absorption rate for different device structures are found as follows:The maximum absorption rate of NbN on the SiO2 antireflection layer is 91.7% at 1606 nm, while the absorption rates of the remaining structures at the optimal thickness of NbN can reach 99% or more. The absorption rate of the SiN device, the Au+SiN device and the DBR device are 99.3%, 99.8% and 99.9%, respectively. The measured results and simulation structure of DBR device are analyzed. These results are of significance for guiding the design and development of high efficiency SNSPD.
REVIEW

Review of fabrication methods, physical properties, and applications of twisted bilayer graphene

Lin Kui-Xin, Li Duo-Sheng, Ye Yin, Jiang Wu-Gui, Ye Zhi-Guo, Qinghua Qin, Zou Wei
Acta Physica Sinica. 2018, 67 (24): 246802 doi: 10.7498/aps.67.20181432
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Graphene is a novel quasi-two-dimensional honeycomb nanomaterial. It exhibits excellent properties and modification options, and the layer-number and configuration of graphene have an important influence on its performance. The quantum state of a quasi-particle in a solid is determined by its own symmetrical nature. The twisted bilayer graphene breaks the symmetry and produces a long-period Moiré pattern due to the slight misalignment between the honeycomb lattices of each layer, which leads to a strong coupling between the layers, and thus changing some physical properties of graphene such as electronic energy band, phonon dispersion, and energy barrier and presents unique performance. For example, the superconductor phase transition can be excited by the gate voltage. The band gap can be continuously controlled in a range of 0-250 meV, and the responsiveness of the photoelectric effect is 80 times higher than that of the single-layer graphene. Therefore, it is of great significance to study the functionalization of twisted bilayer graphene. At the same time, the theoretical and experimental research progress of the transformation of the twisted bilayer layered graphene into the diamond-like carbon is also discussed, which presents the structure and performance of diamond-like carbon. It is found that hydrogenated twisted bilayer graphene bonds between layers and forms sp3 hybrid bonds, which transforms into a diamond-like structure. The number and distribution of sp3 hybrid bonds have an important influence on its performance. The twist angle of twisted bilayer graphene affects its phase transition structure and energy barrier. The effect of the twist angle of the twisted bilayer graphene on its intrinsic properties is further evaluated and reveals the behavioral characteristics of this novel nanomaterial. The unique properties of twisted bilayer graphene give rise to a wide range of applications. It is the key to the application of twisted bilayer graphene with a large area, high quality and controlled twist angle. The mechanical exfoliation method can prepare angle-controlled twisted bilayer graphene, but there are problems such as low efficiency and inability to prepare large-area twisted bilayer graphene. The large-area twisted bilayer graphene can be prepared directly by epitaxial growth and chemical vapor deposition methods, but the twist angle cannot be precisely controlled.
Finally, we mention how to control the preparation of twisted bilayer graphene, analyze its regulation mechanism, and discuss the shortcomings and development trends of those processes. Therefore, in this paper, the three aspects of the transport properties, crystal structure transformation and preparation of twisted bilayer graphene are expounded, and its potential application in the field of advanced electronic devices is also prospected.

Energy loss of surface plasmon polaritons on Ag nanowire waveguide Hot!

Wang Wen-Hui, Zhang Nao
Acta Physica Sinica. 2018, 67 (24): 247302 doi: 10.7498/aps.67.20182085
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Metal nanostructures can support surface plasmon polaritons (SPPs) propagating beyond diffraction limit, which enables the miniaturizing of optical devices and the integrating of on-chip photonic and electronic circuits. Various surface plasmon based optical components have already been developed such as plasmonic routers, detectors, logic gates, etc. However, the high energy losses associated with SPPs' propagation have largely hampered their applications in nanophotonic devices and circuits. Developing the methods of effectively reducing energy loss is significant in this field. In this review, we mainly focus on the energy losses when SPPs propagate in Ag nanowires (NWs). Researches on energy loss mechanism, measurement approaches and methods of reducing energy loss have been reviewed. Owing to their good morphology and high crystallinity as well as low loss in visible spectrum, chemically synthesized Ag NWs are a promising candidate for plasmonic waveguides. The energy losses mainly arise from inherent Ohmic damping, scattering process, leaky radiation and absorption of substrate. These processes can be influenced by excitation wavelength, the geometry of NW and the dielectric environment, especially the effect of substrate, which is discussed in the review. Longer excitation wavelength and larger NW diameter can induce decreased mode confinements and smaller Ohmic loss. The experimental methods to measure the energy loss have been summarized. Researches on reducing energy loss have been reviewed including applying dielectric layer or graphene between NW and substrate, replacing commonly used substrate with a dielectric multilayer substrate, introducing gain materials, and forming hybrid waveguides by using the semiconductor or dielectric NW. Specifically, the leaky radiation can be prevented when an appropriate dielectric layer is placed between NW and substrate, and the mode confinement can be reduced which leads to decreased Ohmic loss. The gain materials can be used to compensate for the energy loss during propagation. Compared with metal waveguides, semiconductor or dielectric NWs suffer lower energy losses while decreased field confinement. Then the hybrid waveguides constructed by metal and dielectric NWs can combine their advantages, which possesses reduced propagation loss. In addition, the plasmon modes in NWs in a homogeneous medium and a substrate are briefly discussed respectively, followed by the introduction to fundamental properties of SPPs propagation. Finally, perspectives of the future development of reducing energy loss are given. The researches on reducing energy loss are crucial for designing and fabricating the nanophotonic devices and integrated optical circuits.

GENERAL

Characteristics and expressions of entangled microwave signals

Li Xiang, Wu De-Wei, Miao Qiang, Zhu Hao-Nan, Wei Tian-Li
Acta Physica Sinica. 2018, 67 (24): 240301 doi: 10.7498/aps.67.20181595
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Entangled microwave signal is the reflection of the quantum characteristics of electromagnetic field in a GHz frequency range. Its generation is mainly dependent on superconducting circuits. Owing to the fact that there is no canonical expression to describe the format of entangled microwave signals, two expressional methods are presented on the basis of analyzing the characteristics of entangled microwave signals. One is in quantum frame, and the other is in classical frame. In quantum frame, we express entangled microwave signals in two-mode squeezed vacuum state. According to input-output relationship and parametric amplifier property in the generating process of entangled microwave signals, we describe the characteristics by two-mode squeezing operator and quantum Langevin equation. In the representation of photon number and Wigner function, we analyze the photon number distribution and the quadrature components' distribution of two-mode squeezed vacuum state, which shows the entangled two-photon correlation and the non-localized positive (negative) correlation of quadrature components. These are consistent with the characteristics of entangled microwave signals. Therefore, the results demonstrate that the entangled microwave signals can be expressed by two-mode squeezed vacuum state. In classical frame, we express entangled microwave signals in correlated random signals approximately. According to the relationship between quadrature components and the quantization of electromagnetic field, we construct the relation among electric-field intensity, input angular frequency, and squeezed parameter. The random number with Gaussian distribution is used as an input state to implement the simulation analysis. We illustrate the waveforms of entangled microwave signals after measurement and the extracted quadrature component waveform varying with time. The simulation results are consistent with the measurement results. These results show that the classical expression can reflect the one-path randomicity and two-path correlativity, which are the intrinsic characteristics of entangled microwave signals. Therefore, it is rational to express entangled microwave signals in correlated random signals. These two expressions properly reflect the continuous variable entanglement characteristics of entangled microwave signals. The expression of two-mode squeezed vacuum state is complete. Plenty of parameters that represent quantum information can be calculated by two-mode squeezed vacuum state, such as entanglement degree or the power of noise fluctuation. The merit of the expression of correlated random signals is intuitive, which makes it easier to understand the nonclassical characteristics of entangled microwave signals.

Periodic orbits of diffusionless Lorenz system

Dong Cheng-Wei
Acta Physica Sinica. 2018, 67 (24): 240501 doi: 10.7498/aps.67.20181581
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The strange attractor of a chaotic system is composed of numerous periodic orbits densely covered. The periodic orbit is the simplest invariant set except for the fixed point in the nonlinear dynamic system, it not only reflects all the characteristics of the chaotic motion, but also is closely related to the amplitude generation and change of chaotic system. Therefore, it is of great significance to obtain the periodic orbits in order to analyze the dynamical behaviors of the complex system. In this paper, we study the periodic orbits of the diffusionless Lorenz equations which are derived in the limit of high Rayleigh and Prandtl numbers. A new approach to establishing one-dimensional symbolic dynamics is proposed, and the periodic orbits based on a topological structure are systematically calculated. We use the variational method to locate the cycles, which is proposed to explore the periodic orbits in high-dimensional chaotic systems. The method not only preserves the robustness characteristics of most of other methods, such as the Newton descent method and multipoint shooting method, but it also has the characteristics of fast convergence when the search process is close to the real cycle in practice. In order to apply the method, a rough loop guess must be made first based on the entire topology for the cycle to be searched, and then the variational algorithm will bring the initial loop guess to evolving toward the real periodic orbit in the system. In the calculations, the Newton descent method is used to achieve stability. Two cycles can be used as basic building blocks for initialization, searching for more complex cycles with multiple circuits around the two fixed points requires more delicate initial conditions; otherwise, it will probably lead to nonconvergence. We can initialize the loop guess for longer cycles constructed by cutting and gluing the short, known cycles. For this system, such a method yields quite a good systematic initial guess for longer cycles. Even if we deform the orbit manually into a closed loop, the variational method still shows its powerfulness for good convergence. The topological classification based on the entire orbital structure is shown to be effective. Furthermore, the deformation of periodic orbits with the change of parameters is discussed, which provides a route to the periods of cycles. The present research may provide a method of performing systematic calculation and classification of periodic orbits in other similar chaotic systems.

Bifurcation mechanism of not increase but decrease of spike number within a neural burst induced by excitatory effect

Cao Ben, Guan Li-Nan, Gu Hua-Guang
Acta Physica Sinica. 2018, 67 (24): 240502 doi: 10.7498/aps.67.20181675
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Nonlinear dynamics is identified to play very important roles in identifying the complex phenomenon, dynamical mechanism, and physiological functions of neural electronic activities. In the present paper, a novel viewpoint that the excitatory stimulus cannot enhance but reduce the number of the spikes within a burst, the novel viewpoint which is different from the traditional viewpoint, is proposed and is explained with the nonlinear dynamics. When the impulse current or the autaptic current with suitable strength is used in the suitable phase within the quiescent state of the bursting pattern of the Rulkov model, a novel firing pattern with reduced number of spikes within a burst is evoked. The earlier the application phase of the current within the quiescent state, the higher the threshold of the current strength to evoke the novel firing pattern is and the less the number of the spikes within a burst of the novel firing pattern. Moreover, such a novel phenomenon can be explained by the intrinsic nonlinear dynamics of the bursting combined with the characteristics of the current. The nonlinear behaviors of the fast subsystem of the Rulkov model are acquired by the fast and slow variable dissection method, respectively. For the fast subsystem, there exist a stable node with lower membrane potential, a stable limit cycle with higher membrane potential, a saddle serving as the border between the stable node and limit cycle, a saddle-node bifurcation, and a homoclinic orbit bifurcation. When external simulation is not received, the bursting pattern of the Rulkov model exhibits behavior alternating between the spikes corresponding to the limit cycle of the fast subsystem and quiescent state of the fast subsystem, which is located within the parameter region between the saddle-node bifurcation point and the homoclinic orbit bifurcation point of the fast subsystem. The spikes begin with the saddle-node bifurcation and end with the homoclinic orbit bifurcation. As the bifurcation parameter turns close to the homoclinic orbit bifurcation, the disturbation or stimulus that can induce the transition from the quiescent state to the spikes becomes strong. Therefore, as the application phase of the current within the quiescent state becomes earlier, the strength threshold of the current that can induce the transition from the quiescent state to the spikes becomes stronger, and the initial phase of the spikes becomes closer to the homoclinic orbit bifurcation, which leads the parameter region of the spikes to become shorter and then leads the number of spikes within a burst to turn less. It is the dynamical mechanism of the decrease of the spike number induced by the excitatory currents. The results enrich the nonlinear phenomenon and dynamical mechanism, present a novel viewpoint for the excitatory effect, and provide a new approach to modulating the neural bursting patterns.

Modeling and simulation on interaction between pedestrians and a vehicle in a channel

Zhang Ji, Wei Yan-Fang, Dong Li-Yun
Acta Physica Sinica. 2018, 67 (24): 240503 doi: 10.7498/aps.67.20181499
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The mixed traffic flow composed of pedestrians and vehicles shows distinct features that a single kind of traffic flow does not have. In this paper, the motion of a vehicle is described by the finer deterministic Nagel-Schreckenberg model, while the motion of pedestrians is mimicked by the lattice gas model with taking the floor field into account. Then the interaction between a certain vehicle and pedestrians in a narrow channel is investigated in two cases, i.e., pedestrians move in the same as or opposite to the direction of vehicle. The direction of the pedestrian movement is determined by the floor field, and the vehicle (and its influential area), regarded as a movable obstacle, and thus causing the floor field to change. Because of the timely change of vehicle speed and the size of impact area, the floor field must be calculated at each time step. Through numerical simulation, the fundamental diagram for pedestrian flow under the typical parameters is obtained together with the average speed of the vehicle as a function of pedestrian density. It is found that there are two critical densities, i.e., ρ1 and ρ2. When ρ1ρρ2, the fundamental diagrams in the two cases are significantly different. This is due to the reverse movement of pedestrian and vehicle, the congestion ahead of the vehicle makes the average speed of pedestrians significantly lowered. In this case, the flux of pedestrians is a linear function of pedestrian density, and its slope indicates the speed at which pedestrian congestion propagates upstream. It can also represent the speed of the moving bottleneck formed by the vehicle. The slope mainly depends on the width of the vehicle and the anticipation time of pedestrians. When ρ < ρ1 and ρ > ρ2, there is no obvious difference between the two cases. We further investigate the effect of three parameters, i.e., the anticipation time of pedestrians, the width and the speed limit of the vehicle. When pedestrians have the same direction as the vehicle, these parameters only have negligible effects. However, in the case that pedestrians move oppositely to the vehicle, the width of the vehicle influences the mixed traffic significantly. When the width of the vehicle is small, even in rather high pedestrian density, the vehicle can move forward. In addition, larger anticipation time of pedestrians is helpful in improving the speed of vehicle, while the effect of the speed limit of vehicle is relatively small. The spatial distribution of pedestrians and the vehicle and the short time average speed of the vehicle are also provided to reveal more information about both pedestrians and the vehicle.

Effect of layer variation on the electronic structure of stacked MoS2(1-x) Se2x alloy

Wang Wen-Jie, Kang Zhi-Lin, Song Qian, Wang Xin, Deng Jia-Jun, Ding Xun-Lei, Che Jian-Tao
Acta Physica Sinica. 2018, 67 (24): 240601 doi: 10.7498/aps.67.20181494
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Two-dimensional transition metal dichalcogenides (TMDCs) have the extensive application prospect in multifunctional electronics and photonics due to their unique electro-optical properties. In order to further expand their application scope in micro-nano optoelectronic devices and improve the performance of devices, the band-gap and defective engineering have been studied to tune the band-gap, morphology and structure of two-dimensional semiconductor materials. The tunning of the bandgap of MoS2(1-x) Se2x alloy has been typically achieved by controlling the Se concentration. Theoretical calculations revealed that layered stacked two-dimensional alloy materials with a larger aspect ratio, exposed edges and obvious edge dangling bonds show enhanced HER activity as compared with TMDCs. In this paper, the properties of stacked MoS2(1-x) Se2x alloy grown by the chemical vapor deposition method in a quartz tube furnace are investigated by using optical microscopy (OM), atomic force microscopy (AFM), scanning tunneling microscopy (SEM), Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The OM and SEM images of the as-synthesized stacked MoS2(1-x)Se2x alloy show apparent interface between layers and their thickness is further acquired by AFM. Unlike most of single-layer or few-layer MoS2(1-x)Se2x alloys, stack-grown stepped MoS2(1-x) Se2x alloy materials all present the strong luminescence properties despite the thickness increasing from 2.2 nm (~3 layers) to 5.6 nm (~7 layers). And even till 100 nm, the emission spectrum with two luminescence peaks can still be observed. The two exciton luminescence peaks A and B are derived from the valence band splitting caused by the spin-orbit coupling, respectively. As the thickness increases, the two luminescence peaks are red-shifted and exhibit a band-bending effect that is only present when the alloy doping concentration is changed. As the sample thickness is 5.6 nm, a C-peak at 650 nm at the high energy end of the PL spectrum is observed, which may be attributed to the transition luminescence from the defect energy level introduced by Se (S) substitution, interstice or cluster. When the number of layers is small, the number of defects is small, so that the luminescence is not observed. As the number of layers increases, the defects increase to form a defect energy level. However, when the material thickness continuously increases until the bulk material is formed, the luminescence disappears in the PL spectrum because the band gap is reduced and the band gap is made smaller than the defect energy level. Raman spectroscopy gives two sets of vibration modes:like-MoS2 and like-MoSe2. The Raman peak is almost unchanged as the thickness increases, but the two vibration modes E2g (Mo-Se) and E2 g (Mo-S) in the plane gradually appear and increase. At the same time, the intensity ratio and line width of Mo-Se related vibration mode E2g/A1g increase with thickness increasing, which indicates the enhancement of the Mo-Se in-plane vibration mode and the incorporation of randomness of Se into the lattice. Obviously, the defects and stress are the main factors affecting the electronic structure of stacked MoS2(1-x) Se2x alloy, which provides a meaningful reference for preparing the special functional devices and studying the controllable defect engineering.

X-ray source with quasi-monochromatic parallel beam

Wang Rui-Rong, An Hong-Hai, Xiong Jun, Xie Zhi-Yong, Wang Wei
Acta Physica Sinica. 2018, 67 (24): 240701 doi: 10.7498/aps.67.20180861
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In inertial confined fusion experiments, an excellent-performance and high-efficiency X-ray source plays an important role in X-ray radiography schemes. Indeed, it can be used in a variety of X-ray experimental techniques. The mono-chromaticity, flux intensity, degree of collimation (the radiation can be transported long distances without loss), and spot size of the X-ray source affect the quality of imaging. Ray-tracing simulations, which are validated by experimental results, demonstrate that high-intensity collimated X-ray beams can be produced from an isotropic X-ray source. Therefore, a method of improving the performance of an X-ray source from a laser-produced plasma is presented. A spherically bent crystal is used to collimate mono-chromatic X-rays emitted from a laser-produced plasma. Here we design a spherically bent crystal spectrometer system for collimating the laser-produced X-rays. The system performance is experimentally tested at the Shenguang Ⅱ (SGⅡ) laser facility located in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. The beam divergence is measured by using a metal grid placed downstream from the crystal, the metal grid that possesses wires with 60 μm in diameter and 127 μm in period. An imaging plate (IP) is placed at various distances downstream from grid. The quality of the generated beam is monitored by measuring the dimensions of the grid image formed by the beam on IP. While the narrow range of wavelength is measured with a spherically bent crystal spectrometer. Experimental results show that the spherically bent crystal spectrometer system can produce quasi-monochromatic (10-3 < △ λ/λ <10-2) X-ray beams with a high degree of collimation (less than 2 mrad divergence), uniform spot size (~500 μm), and a relative tenability in the wide spectral range. The influences of various experimental parameters on the quality of beam collimation are evaluated in two ways. They can be investigated in test experiments by representing the beam divergence distribution as a function of Bragg angle. In another study of the effect of the aberrations, when the incident beam on the spherically bent crystal is not normal, the beam is less collimated in the tangential plane, and out of collimation in the sagittal plane. Following the ray-tracing method, we analyze the diffracted beam divergence produced by the astigmatic aberration. The qualitative conclusion is that the good agreement with the experimental results is obtained. By fully utilizing limited Bragg angle range, the spherically bent crystal spectrometer system can realize collimated diffracted X-ray beams with divergence of less than 1 mrad by using a laser-produced plasma X-ray source under the appropriately experimental parameters.
NUCLEAR PHYSICS

Regression analysis of coincidence measurements for determinating the neutron emission rate of 252Cf spontaneous fission

Li Yong-Ming, Wang Liang, Chen Xiang-Lin, Ruan Nian-Shou, Zhao De-Shan
Acta Physica Sinica. 2018, 67 (24): 242901 doi: 10.7498/aps.67.20181073
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The 252Cf isotope sources have a recommended standard neutron spectrum of spontaneous fission, and have been widely used in scientific researches, such as the detection efficiency calibration of neutron detectors, the characterization of neutron dose equivalent meters, the active analysis of special nuclear materials, etc. However, it is often necessary to correct the neutron emission rate due to its short half-life of 2.645 years. As the source age increases the contributions from 250Cf and 248Cm spontaneous fission become more significant, thus the neutron emission rate cannot be calculated simply according to the 252Cf decay law. In addition, the indirect measurement method by manganese bath activation needs a long period more than 8 hours; and it will have a large uncertainty while the source strength is lower than 104 n/s. In order to develop a more portable measurement method for larger suitable dynamic range, the comprehensive algorithms based on the neutron multiplicity counting are studied in this paper. On the basis of the measurement equations under the point model assumption, the neutron coincidence counting rate is correlated with the total neutron counting rate, and then the regression analyses with different coincidence gates and different source locations in the counter are performed. On the assumption that the average neutron die-away time is constant in the sensitive range of detection system, therefore the characteristic coefficient from the changing process can be extracted, and two kinds of methods of measuring the neutron strength are established, which are independent of the efficiency variation. The verification experiments are carried out by the JCC-51 neutron coincidence counter. It is shown that the values measured by the two regression methods are consistent with the corrected results of the nominal value within 2% deviation. Furthermore, the detection efficiency is inversed by dividing the total neutron counting rate with the neutron emission rate when the source is placed at the central axis, which accords with the result of Monte Carlo simulation by using the MCNPX code well. It can provide an accurate method of determining the neutron emission rate of 252Cf spontaneous fission, and also an approach to calibrating the detection efficiency of neutron coincidence counter while the source strength is unknown.
ATOMIC AND MOLECULAR PHYSICS

Theoretical study on potential curves and spectroscopic constants of low-lying electronic states of Na2+ cation

Wei Chang-Li, Liao Hao, Luo Tai-Sheng, Ren Yin-Shuan, Yan Bing
Acta Physica Sinica. 2018, 67 (24): 243101 doi: 10.7498/aps.67.20181690
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In this paper, high-level ab initio calculations by using multi-configuration self-consistent field method with atomic effective core potential, polarization potential, and uncontracted Gaussian basis function, are performed to compute the potential energy curves of a total of 36 low-lying ∧-S states with ∑g,u, Πg,u, △g,u symmetries of Na2+ cation associated with the lowest 9 dissociation limits Na (3s, 3p, 4s, 3d, 4p, 5s, 4d, 4f, 5p)+Na+. On the basis of the potential energy curves, the spectroscopic constants (Te, Re, ωe, ωeχe, Be, α e, De) of the bound states are determined, which are in good agreement with the existing available experimental and theoretical values. Our results indicate that 52g+-72g+, 32u+-72u+, 22Πg, 42Πg, 12u and 22u states are repulsive, which supports Berriche's results, and we report 10 electron states for the first time, that is, 82g, u+-92g, u+, 52Πg, u-72Πg, u and 32g, u. The vibrational-rotational spectroscopic constants and lowest vibrational-rotational energy levels (ν=0-20) of the bound states are also presented. Moreover, in order to illustrate the strong state interactions of adjacent states with same symmetry, the information about the avoided crossing points is shown in detail. Finally, the transition dipole moments from a few low-lying excited states (12Πu-32Πu) to the ground state X2g+ are computed. Therefore, it is expected that our computational results in the present calculations are significant for the molecular spectroscopy, ion-atom interaction and molecular cold collision fields.

Multiple ionization effect of Au induced by different ions

Liang Chang-Hui, Zhang Xiao-An, Li Yao-Zong, Zhao Yong-Tao, Zhou Xian-Ming, Wang Xing, Mei Ce-Xiang, Xiao Guo-Qing
Acta Physica Sinica. 2018, 67 (24): 243201 doi: 10.7498/aps.67.20181642
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We report the experimental data of characteristic X-ray spectra produced by the impact of H+, Ar8+, Ar12+, Kr13+ and Eu20+ ions with different kinetic energies on Au surface in the National Laboratory of Heavy Ion Research Facility in Lanzhou, China. The energy shifts of X-ray spectra are analyzed and the ratio of X-ray yield is calculated. The results show that H+ can excite the characteristic X-ray spectra of Mζ and Mα of Au, while Ar8+, Ar12+, Kr13+ and Eu20+ can excite the characteristic X-ray spectra of Mζ, Mα, Mγ and Mδ of Au, because the inner shells of target atom are multiply ionized by heavy ions impact, so that the relative intensity ratio of the X-rays changes when the heavy ions are incident. There are different energy shifts of Au M X-ray due to multiple ionization effect in collision. When the ion incident energy is lower, the degree of multiple ionization of the inner shells of the target atom is almost independent of the incident energy, when the ion energy is higher, the degree of multiple ionization increases with incident ion energy increasing. At the same time, the degree of multiple ionization also depends on the number of holes in the outer shell of the ion and its atomic number. That the ratio of X-ray yield increases with the increase of the atomic number of the incident ion further indicates that the degree of multiple ionization increases with atomic number of the incident ion increasing. The multiple ionization and electron configuration of the inner shells of the atom can be determined by the energy shift and spectra broadening. These measurements provide basic data for further studying the multiple ionization mechanism of the inner shells of the atom. But due to the limitation of the resolution of the detector, the spectral broadening data cannot be measured. It is necessary to use a higher-resolution detector to further study the multi-ionization effect of the inner shells of the atom.

Experimental investigation on ultraviolet multiphoton dissociation dynamics of methylamine

Wang Xiao-Li, Yao Guan-Xin, Yang Xin-Yan, Qin Zheng-Bo, Zheng Xian-Feng, Cui Zhi-Feng
Acta Physica Sinica. 2018, 67 (24): 243301 doi: 10.7498/aps.67.20181731
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Methylamine is the simplest alkylamine. It is a typical molecule in the field of surface physicochemistry. The basic properties of the structure and reaction activity of this molecule are essential to understand its role in many chemical reactions. Its energy state and ionic structure, ionization dissociation channel and competition have aroused the interest of astronomical and physicochemical researchers. In order to further understand the mechanism of multiphoton dissociation and ionization of methylamine in this energy region, the photodissociation channels of methylamine are studied based on the measured resonance enhanced multiphoton ionization-time-of-flight mass spectrum (TOFMS), mass-selected excitation spectra of the ionized fragment, and laser power index of each ion in a range of 280-287.5 nm. The multiphoton ionization TOFMS of methylamine molecule is obtained at the excited laser wavelength of 283 nm. After calibration, the weaker ion peaks correspond to the C+, CH+, CH2+, CH3+, NH2+, NH3+, CN+, CH2NH+(CHNH2+, CH3N+), CH3NH2+; the mass-to-charge ratio of stronger peaks except H+ ions are 27, 28 and 30, respectively, and the mass-to-charge ratio of 28 and 30 belong to CHNH+, CH2NH2+ after analysis and discussion. Combining with the mass separation excitation spectra of the parent ions, it is concluded that there is a repulsive electronic state in the single photon energy. The main dissociation channel is the resonant photodissociation of the parent molecule in the repulsive state produced by one photoabsorption, followed by the photoionization of the fragment through the (1+1) multiphoton process and the further photodissociation of the ionized fragment.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

Iterative denoising of ghost imaging based on adaptive threshold method

Zhou Yang, Zhang Hong-Wei, Zhong Fei, Guo Shu-Xu
Acta Physica Sinica. 2018, 67 (24): 244201 doi: 10.7498/aps.67.20181240
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Ghost imaging (GI) is an important technique in the fields of quantum imaging and classical optical imaging, and it can solve the problems which are difficult to solve by the traditional imaging techniques in the optically harsh environments. In this paper, we present the iterative denoising of GI based on an adaptive threshold method. This method is abbreviated as IDGI-AT, which takes the advantages of adaptive threshold, differential, binarization and iterative operation method, and can enhance image quality in GI. In addition, this method can reduce the number of measurements. As is well known, the enormous number of measurements and poor reconstruction quality are obstacles to the engineering application of GI. The correlation noise leads to low signal-to-noise ratio and low imaging efficiency in GI as well. Therefore, we establish a denoising model, which can reduce correlation noise and improve reconstruction quality. We first analyze the iterative denoising of ghost imaging (IDGI) theory, and use the adaptive threshold technique to calculate the ideal threshold associated with the correlation noise. It should be noted that the threshold can be obtained by this method under the condition without requiring prior knowledge of the object. Afterwards, we can construct the correlation noise in this denoising model. In the IDGI, the differential ghost imaging (DGI) image is taken as the initial iteration value. We use the adaptive threshold method, which is different from IDGI, to binarize the initial value of each iteration to make it closer to the original object's transmission coefficient. After three iterations, we can obtain a higher-quality reconstruction image. In order to demonstrate that the IDGI-AT is available, a GI experimental system with a pseudo-thermal light source is set up. The considerable simulation and experimental results show the advantage of our scheme in terms of removing reconstruction image background noise. Especially, the visual effects and peak signal-to-noise ratio values are improved in comparison with those from the traditional GI, DGI and IDGI. Besides, we demonstrate the role of binarization in our scheme. For a binary object, the iterative value binarization can achieve better image quality than that in the case without binarizing the iterative initial value. Therefore, this novel method is likely to provide an alternative mean for GI and further pave the way for the application fields of GI, such as lidar, biomedical engineering, etc.

Measurement of low-frequency signal based on quantum-enhanced fiber Mach-Zehnder interferometer

Cheng Jian, Feng Jin-Xia, Li Yuan-Ji, Zhang Kuan-Shou
Acta Physica Sinica. 2018, 67 (24): 244202 doi: 10.7498/aps.67.20181335
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Generation of squeezed state at telecommunication wavelength has been recently a very interesting issue due to the lowest optical power attenuation of light at a wavelength of 1550 nm in a standard telecommunication fiber. The low-frequency vacuum squeezed state at 1550 nm in combination with fiber based interferometer offers the possibility to implement quantum precision measurement beyond standard quantum limit (SQL). In this paper, we experimentally realize a quantum-enhanced fiber Mach-Zehnder interferometer (FMZI) for measuring the low-frequency phase modulation signal by using low-frequency vacuum squeezing at 1550 nm. Firstly, the low-frequency vacuum squeezed state at the telecommunication wavelength of 1550 nm is generated by using a degenerate optical parametric oscillator (DOPO). The DOPO is a semi-monolithic construction based on a type I periodically poled KTiOPO4 (PPKTP) crystal and a concave mirror. The pump threshold of DOPO is 270 mW. When the pump power is 120 mW that is below the pump threshold of DOPO and the temperature of PPKTP is controlled at 34.8℃, a vacuum squeezing of 3 dB is generated at an analysis frequency range from 10 kHz to 500 kHz. The quadrature phase vacuum squeezing is obtained by locking the squeezed quadrature angle through using a coherent control scheme, in which two acousto-optic modulators are used to shift the frequency and produce the auxiliary beam acting as a coherent control field. Based on the constructed FMZI, a quantum-enhanced FMZI is realized by injecting the generated low-frequency vacuum squeezed state at 1550 nm into the vacuum channel of FMZI. The relative phase between two injected light fields is locked at π by using the Pound-Drever-Hall (PDH) locking technology, and the relative phase between light fields of its arms in FMZI is also locked at π/2 by using the PDH locking technology. When a phase modulation signal at the frequency of 500 kHz is loaded in the signal arm of FMZI, the noise power spectrum of the output from FMZI is measured by a balance homodyne detect system. A 2 dB quantum improvement beyond shot-noise-level at the frequency of 500 kHz is obtained experimentally by using the quantum-enhanced FMZI. The experimental results demonstrate a potential application in quantum precision measurement beyond the SQL based on fiber sensor technique.

Coherent synthesis of ultrashort pulses based on balanced optical cross-correlator

Huang Pei, Fang Shao-Bo, Huang Hang-Dong, Hou Xun, Wei Zhi-Yi
Acta Physica Sinica. 2018, 67 (24): 244204 doi: 10.7498/aps.67.20181851
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Coherent synthesis of laser pulses is a major trend in the generation of ultrafast pulse field. There is no good way to compensate for the whole spectrum when the spectrum of ultrashort pulses is wide enough to reach an octave, so it is difficult to realize a sub-cycle pulse in a single-path laser system even if the spectrum range is wide enough. In this paper, 0.8 mJ, 30 fs laser pulses at 1 kHz repetition rate with 790 nm center wavelength from a Ti:sapphire chirped pulse amplifier (CPA) system are focused into hollow fiber with an inner diameter of 250 μm and a length of 1 m to produce an octave-spanning white-light supercontinuum (450-950 nm). Using this supercontinuum, we conduct two sets of comparative experiments. 1) We split the supercontinuum into two pulses with different spectrum ranges (450-750 nm and 650-1000 nm) by a dichroic mirror (HR, 500-700 nm; HT, 700-1000 nm), and we compress the two pulses by the double-chirped mirrors and wedge pairs to generate two few-cycle pulses:the long and short wavelength yielding pulses are 7.9 fs and 6.1 fs, respectively. Then we coherently synthesize two pulses by using another dichroic mirror, and controlling the relative time delay between the two pulses, and thus we synthesize a pulse of 4.1 fs. 2) We directly compress the supercontinuum by the double-chirped mirrors and wedge pairs, and obtain an optimization result of 5.3 fs, of which the pulse duration is wider than that in experiment 1. In these comparative experiments, the advantage of coherent synthesis for shorter pulse duration is preliminarily verified. Besides, the balanced optical cross-correlator technique is used to lock the relative time delay between two pulses. The root-mean-square value of relative time delay drift is less than 80 as in the case with feedback control loop, which ensures the stability of coherent synthesis system. This scheme can be adopted to accurately compensate for the dispersion and obtain the sub-cycle synthesized pulse, which is useful for generating the high harmonic and atto-second pulse.

Sound radiation from finite cylindrical shell excited by inner finite-size sources

Yang De-Sen, Zhang Rui, Shi Sheng-Guo
Acta Physica Sinica. 2018, 67 (24): 244301 doi: 10.7498/aps.67.20181716
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The study of the characteristics of noise sources in cylindrical shells is the foundation of sound field prediction. Although noise sources are usually regarded as point sources to simplify the calculation model in noise source localization and waveguide sound propagation, the approximation is limited to far-field problems. For the near-field acoustics problems in engine room and ship cabin, the radiated noise possesses the spatial directivity because of the complex vibration distribution of the noise source surface. Moreover, the sound scattering on the surface of finite-size sources makes the noise source itself act not only as the energy input of sound field, but also as the scatterer to change the structure of sound field in the environment. These factors lead to large errors when the finite-size source is simplified into a point source. In order to explore the influence of finite-size source on the acoustic field inside and outside the underwater vehicle structure, the shell coupled equation is constructed by combining thin shell theory, equivalent source and Green function. The effects of source surface scattering and directivity on the acoustic field inside and outside the cylindrical shell are studied. The results show that the accuracy of finite-size source construction is related to the equivalent source location. It proves that equivalent source allocation should be arranged in the middle of the geometric center of sources and its structural surface. Sound scattering from the surface of the finite-size source will change the sound field inside the shell, and then the resonant peaks of the cavity are shifted to the high frequencies as the source volume increases, which causes a strong sound transmission phenomenon in some frequency bands. In addition, the directivity of the finite-size source has little effect on the intensity of the sound field inside and outside the shell, which is evident in changing the far-field directivity of the radiated sound field. The research results are valuable for noise prediction and noise control.

Periodic solution and its stability of spring pendulum with horizontal base motion

Zhang Li-Juan, Zhang Hua-Biao, Li Xin-Ye
Acta Physica Sinica. 2018, 67 (24): 244302 doi: 10.7498/aps.67.20181676
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In this paper, the nonlinear dynamic response of spring pendulum with horizontal base motion is studied. The dynamical equations of the system are established by using Lagrange equation. The discrete Fourier transform, harmonic balance method and homotopy continuation method are combined to solve the periodic response of the system, which avoids the limitation of the small amplitude caused by the Taylor expansion in the traditional analytical method. The comparison with the numerical results shows that the proposed method in this paper can not only be used to solve the large amplitude vibration of spring pendulum, but also has a high accuracy. The stability of periodic response is studied by using Floquet theory. The effects of amplitude and frequency of base motion on the periodic response of the system are given, and the bifurcation characteristics of the periodic solution are analyzed. The results show that the influence curve of the base frequency on the periodic response has two peaks, and with the increase of the amplitude of the base motion, the two peaks will shift to the different sides respectively. When the base amplitude is large, the periodic response amplitude changes with the frequency of the foundation motion, and there will be two jumps. The amplitude of the periodic solution increases with the base amplitude. For some base frequencies, the amplitude of the periodic solution will jump with the change of the base amplitude. When the amplitude and frequency of the system are large, the periodic response of the system may be unstable. After the instability, the spring pendulum enters the continuous rotation state, and the amplitude in the breathing direction is great, the system will be destroyed. It is found that Hopf bifurcation may occur in the periodic response of the system corresponding to some base frequencies and amplitudes. The variation of the system response with the base frequency and amplitude after the Hopf bifurcation is studied numerically by the Runge-Kutta method. Complex dynamical behaviors such as periodic motion, almost periodic motion, torus doubling and chaos are found. It is shown that the main path of the system entering chaos is almost periodic torus rupture and paroxysmal. Finally, the influence analysis of the base frequency and amplitude is synthesized, and the transition of the response form on the plane of the basic motion parameters is given. The results of this paper provide a theoretical reference for the analysis and design of spring pendulum in engineering.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

Analysis of particle size distribution and scattering characteristics of water cloud in condensation and coalescence growth

Song Yue-Hui, Zhou Yu-Dong, Wang Yu-Feng, Li Shi-Chun, Gao Fei, Li Bo, Hua Deng-Xin
Acta Physica Sinica. 2018, 67 (24): 249201 doi: 10.7498/aps.67.20181544
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Study of scattering characteristics of water cloud is of great significance for weather forecasting, meteorological disaster warning, weather modification and research of radiation transmission in the lower atmosphere. In this paper, particle size distribution and scattering characteristics of water cloud in condensation and coalescence growth are studied by numerical simulation. The particle size distribution model of water cloud in the condensation growth and coalescence growth are established respectively. The dynamic process of the particle size distribution of water cloud in the condensation growth, the coalescence growth and the condensation-coalescence combination growth are analyzed. Then the scattering characteristics of water cloud in the droplet growth are studied with the Mie theory. The results show that with the condensation growing the full width at half maximum of particle size distribution, the effective radius and mode radius of water cloud increase continuously. The effective radius increases in the coalescence growth process and there are multiple peaks in the particle size distribution in the coalescence growth anaphase. The average growth rate of the effective radius of cloud droplets is 8 nm/s in the condensation-coalescence combination growth. The extinction coefficient and scattering coefficient of water cloud increase linearly with time increasing during the condensation growth or the coalescence growth. In the condensation-coalescence combination growth, the extinction coefficient and scattering coefficient increase exponentially with time increasing except at a wavelength of 3.2 mm; the asymmetric factors at the wavelengths of 1.064, 2.2, 3.7, 12 and 22 μm tend to be stable, while the asymmetric factor at the wavelength of 3.2 mm remains the same basically. Meanwhile, the lidar ratio at each of the wavelengths of 1.064 μm and 2.2 μm fluctuates near 20 sr, and that at the wavelength of 3.7 μm fluctuates greatly. In the growth process of cloud droplet, the single scattering albedo of water cloud decreases gradually at each of the wavelengths of 1.064, 2.2 and 3.7 μm, while it increases gradually at each of the wavelengths of 12, 22, 200 and 3.2 mm. The absolute value of Ångström exponent decreases gradually, which means that the wavelength-dependence of extinction coefficient decreases with cloud droplet growing. These research results reveal the change law of particle size distribution and the scattering characteristics of water cloud in condensation and coalescence growth. The results provide important reference for forecasting weather, studying earth-atmosphere radiation balance and correcting remote sensing data.

Processing method for the partial particles and its influence on the cloud microphysical parameters measured by the airborne cloud and precipitation image probe

Huang Min-Song, Lei Heng-Chi
Acta Physica Sinica. 2018, 67 (24): 249202 doi: 10.7498/aps.67.20181412
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As an important instrument for the cloud microphysics measurement, the airborne cloud and precipitation imaging probe plays a significant role in studying the cloud and precipitation physics and artificial weather modification. The particle image data recorded by the probe can be used to process, calculate and produce the cloud microphysical parameters, such as the cloud particle size spectra, cloud particle number concentration, cloud water content, etc. However, there are lots of partial particle images in the sampled data. This is due to the limited sample volume of the probe, the volume that contains only a part of the particles and is imaged by the probe. The number of partial particles in each sample is so large that the technique used to process these particles can have a great influence on the calculation of cloud microphysical parameters. However, there has been no perfect solution for dealing with these partial particles so far.
The three existing processing methods-“All In” method, “Center In” method, and “Diameter Reconstruction” method for the partial particles, are introduced and analysed in this study. After analyzing the advantages and disadvantages of these existing methods, a new definition and a particle shape classification for the partial particle are given, which can discriminate the circularly symmetric particles and the non-circularly symmetric particles from the partials. Then a new partial particle processing method is introduced, which combines the partial particle shape recognition technique and the traditional techniques-“Center In” method and “Diameter Reconstruction” method. The circularly symmetric partial particles are processed with the “Diameter Reconstruction method” and the non-circularly symmetric partial particles are processed with the “Center In” method.
Utilizing the historical airplane observation data from Shanxi Taiyuan, the new method presented in this study and the three traditional methods are used to calculate the cloud particle size spectra, cloud particle number concentration, and the ice water content by using the same data. The calculated results are analyzed and compared. It is found that in most cases the results from the new method are more consistent with those from the “Diameter Reconstruction” technique and can overcome the disadvantages of the existing methods, especially when the cloud has more column-shaped and needle-shaped particles, the result from the new method is more reasonable. Considering the fact that the column shape is one of the main shapes in the cloud, it is strongly recommended to use the new technique in this paper to process the data from the probes.
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