Vol. 67, No. 14 (2018)
Effects of plastic deformation in current collector on lithium diffusion and stress in bilayer lithium-ion battery electrode
2018, 67 (14): 140201. doi: 10.7498/aps.67.20180148
Lithium-ion batteries (LIBs) have already become indispensable energy storage devices, as they can meet urgent requirements for higher energy and power density in the applications ranging from portable electronics to electric vehicles. However, in the process of charging and discharging of LIB, the diffusion-induced stress associated with inhomogeneous Li concentration in the electrode may cause the electrode material to damage, and then further degrade storage capacity and cycling performance of LIB. Therefore, it is important to quantitatively understand the mechanism relating to the stress evolution in electrode during electrochemical cycling, which will be conducive to developing effective methods of relieving the diffusion induced stress. In this work, a bilayer electrode model is proposed by taking into account Li diffusion, built-in stress, concentration-dependent material properties and elastoplastic deformation of current collector. Based on the established model, the influences of the possible plastic deformation in the current collector on the lithium diffusion and stress evolution of bilayer electrode during charging are investigated. The numerical results show that the plastic deformation of current collector can weaken the constraint between current collector and active layer, which leads to a smaller electrode curvature and more homogeneous lithium concentration in the active layer. The relaxation effect of the plastic deformation not only significantly relieves the stresses at the bottom and top surface of active layer, but also promotes the diffusion of lithium into active layer, which can improve the structural reliability of the electrode and increase the effective capacity of the active layer. Furthermore, the influences of the yield strength and plastic modulus of the current collector are discussed. The results indicate that the constraint between the current collector and active layer becomes weaker with reducing yield strength and plastic modulus of current collector, respectively. In other words, the further stress relaxation in the electrode indicates that the capacity can be enhanced upon reducing the yield strength and plastic modulus of current collector, respectively. Considering our results, it is expected that a bilayer electrode composed of the current collector with smaller mechanical strength enjoys simultaneous improvement in battery usable capacity and structural reliability. Consequently, the results of this paper provide a route to improving the cycle performance of bilayer lithium-ion battery electrode.
2018, 67 (14): 140302. doi: 10.7498/aps.67.20180040
Quantum metrology is a subject of studying quantum measurement and quantum statistical deduction, and the precision of parameter estimation can be enhanced by quantum properties. In general, the process of parameter estimation includes four steps:preparation of probe state, parameterization process, measurement, and data processing. Of these four steps, the preparation of probe state is the most crucial. However, in practical applications, in the process of preparing quantum probe state, the probe system will couple to its environment, which will inevitably cause the quantum properties of the probe system to deteriorate, and thus reducing the precision of quantum parameter estimation. The dynamics of quantum Fisher information (QFI) for W state and Greenberger-Horne-Zeilinger (GHZ) state have been studied in decoherence channels. Because W state and GHZ state have different entanglement properties, the studies of the dynamics of QFI for the superposition of W state and GHZ state are of practical significance in quantum metrology field. In this paper, the dynamics of QFIs for the superposition of W state and GHZ state in three typical decoherence channels (depolarization channel, amplitude damping channel and phase damping channel) are studied. In the four steps of quantum parameter estimation, our major attention is paid to the first step (i.e., the preparation of probe state). For comparison, the QFIs of different probe states are studied, with the other three steps fixed, i.e., all the probe states will undergo the same parameterization, measurement and estimation process. The parameterization process involved here is a quantum spin operation (specified by the spin rotation direction), which is chosen to maximize the QFI of the probe state. The initial probe states under consideration are the superpositions of W state and GHZ state of three-particle and five-particle systems, and the QFI dynamics of those probe states are studied in the three different typical decoherence channels. By using the operator-sum (Kraus) representation of those three typical decoherence channels, the QFI dynamics of the probe state can be analytically derived in three different decoherence channels. The results show that in the depolarization channel, the maximum QFI of the probe state decreases with the decoherence evolving to zero in the end; in the amplitude damping channel, the QFI of the probe state decreases to the minimum with the decoherence evolution and then increases to the shot noise limit; in the phase damping channel, the QFI of the probe state decreases with the evolution of decoherence, but the final stable value is not zero. Further analyses show that W state component of the superposition plays a role in resisting phase damping and the GHZ state component plays a role in resisting amplitude damping. These results can help us to choose the optimal probe state for maximizing the estimation precision in practice.
2018, 67 (14): 140702. doi: 10.7498/aps.67.20180207
In the paper, under 5.6 GPa and 1200-1400℃, the type Ib diamond single crystals on defect-free -oriented seed crystals are synthesized in a cubic anvil under high pressure and high temperature when the crack problem of diamond single crystal appears frequently. Highpurity Fe-Ni-Co solvents are chosen as the catalysts. Highpurity graphite powder (99.99%, purity) is selected as a carbon source. The effects of cooling process on the qualities of Gem-diamond single crystals are studied carefully. First, in order to study the common crack defects of diamond single crystals, using scanning electron microscope (SEM), the surface morphologies of high quality diamond single crystals and crack crystals are obtained respectively. Our SEM test results show that the surfaces of the crack crystals and the high quality crystals are all very smooth. Therefore, the crack crystal problem is not directly caused by the unordered accumulation of carbon. Second, the concentrations of nitrogen in the high quality diamonds and crack crystals are measured by Fourier transform infrared. In our studies, the nitrogen content of the diamond single crystal with crack is similar to the nitrogen content of high quality single crystal, so the appearance of crystal crack is not caused by high impurity content. According to the test results and the regularity of the occurrence of crack crystals, the reasons for the occurrence of crack crystals are analyzed seriously. When the weather conditions such as seasonal change, wind, rain or snowfall are not very stable, the probability of crack crystal problem to appear will increase greatly. In our opinion, the decrease of diamond crystal quality caused by the fluctuation of external growth conditions is the internal cause of crack crystal problem appearing. After growing diamond crystals, choosing the traditional power failure mode and slowing cooling process respectively, the effect of cooling process on the quality of diamond single crystal is investigated. In the season of the crack problem occurring frequently, choosing power failure cooling process, cracks appear in both diamond crystals with 1.3 mm or 6.0 mm in diameter. With the slow cooling process, the synthetic diamond crystals with 1.2 mm or 5.8 mm in diameter are all high-quality single crystals with no cracks inside. The research results show that the slow cooling process can effectively restrain the occurrence of crack crystal problems. In addition, the mechanism problems of crack crystals and the mechanisms of the effects of slow cooling process on diamond crystal qualities are discussed in detail. We believe that the slow cooling process is effective in solving the crack crystal problem, which is mainly attributed to the following two aspects:on the one hand, the slow cooling makes the internal stress of diamond single crystal growing effectively released, which improves the compressive strength of the crystal and the crystal quality as well; on the other hand, the slow cooling makes the solidification process of the catalyst melt slowly, which provides enough time for the crystal to balance the external stress of the catalyst and the equipment, so that the crystals, which are not affected by the unbalanced external stress, are not cracked.
2018, 67 (14): 140704. doi: 10.7498/aps.67.20172722
Quantitative phase microscopy, as a non-destructive and non-invasive measurement technique, can indirectly reflect three-dimensional (3D) morphology and optical properties of transparent microstructure object by measuring phase information. In recent years, this kind of technique has been widely used to detect and investigate the characteristics of biological cells and it has become more and more important in the field of modern biomedical and life science. In this paper, only by using a single cube beamsplitter interferometer, a simple single-shot dual-channel quantitative phase microscopic measurement technique is demonstrated for 3D quantitative phase imaging of biological cells. In the proposed method, a conventional non-polarized cube beamsplitter is the most pivotal element. Unlike its traditional application method, the cube beamsplitter is tilted in a nonconventional configuration and the illumination beam is only incident on the left (or right) half of the cube beamsplitter (just the one side of central semi-reflecting layer), and a very small angle is introduced between the central semi-reflecting layer and the optical axis of incident beam. Based on the light splitting characteristic of the cube beamsplitter, two replicas of incident beam are generated. These two generated replicas (transmission beam and reflection beam) are of symmetry with respect to each other, and they will encounter and form interference when the direction of the incident beam meets a certain condition. Adjust the sample to a suitable position and make it only contact one half of incident beam, and the modulated beam will be seen as the object beam and the remaining clean half of incident beam as the reference beam. When the interference phenomenon occurs, two interference channels with a relative π (rad) phase-shift in one interferogram are acquired simultaneously only using one digital camera, and the higher spatial frequency of interference fringes can be achieved by adjusting a relatively big angle between the central semi-reflecting layer and the optical axis of incident beam. Because of the off-axis interference mode, we only need to record one interferogram to gain the continuous phase information and avoid using complex phase-shift techniques. At the same time, this proposed method is of simple structure and easy to operate due to using less ordinary off-the-shelf optical elements. All these simplify the structure of the system and reduce the cost of the system as much as possible. Finally, the phase information of paramecium is successfully obtained from different interference channels respectively. Furthermore, according to the characteristic of π (rad) phase-shift, we also realize the calibration and determination of ultimate precise phase information of sample by using the method of averaging between these two channels. The experimental results show that our proposed method is suitable for 3D surface morphology measurement of small transparent samples.
2018, 67 (14): 140301. doi: 10.7498/aps.67.20172401
Much interest has been aroused in quantum metrology such as quantum interferometric radar, due to its application in sub-Raleigh ranging and remote sensing. Generally, the quantum signal emitted by quantum radar will be affected by atmosphere medium. For instance, both atmospheric loss and atmospheric scintillation seriously affect the sensitivity and resolution of quantum radar. In fact, the effects of atmospheric loss on the sensitivity and resolution of quantum interferometric radar have been investigated thoroughly and completely in the past decades. However, the investigation about the influence of atmospheric scintillation is lacking until now. To realize practical quantum interferometric radar, the perturbation coming from turbulent atmosphere must be considered, thus it is necessary to investigate how the atmospheric scintillation affects the performance of quantum radar.In this paper, the influence of intensity fluctuation which is caused by atmospheric scintillation on the performance of quantum interferometric radar with entangled coherent states (ECS) is thoroughly investigated. We first introduce the physical model of quantum interferometric radar, and the dynamic evolution of quantum light field in atmosphere is obtained by solving the master equation of dissipation channel. Considering the dissipation and fluctuation caused by atmospheric scintillation, we regard the turbulent atmosphere as so-called dissipation-fluctuation channel. Moreover, according to classical statistical theory of turbulence, we derive the explicit expression of probability distribution of transmission coefficient P(T), this probability distribution of transmission cofficient, which is determined by average transmission coefficient TD and scintillation index βD2 plays a crucial role in the studying of atmospheric scintillation.The results of investigation show that atmospheric scintillation leads to the degradation of the sensitivity and resolution of ECS quantum interferometric radar at lower atmospheric loss. Under the higher lossy condition of atmosphere, atmospheric scintillation can greatly enhance the performance of quantum interferometric radar. Furthermore, the critical atmospheric transmission coefficient which determines the lower and higher loss of atmosphere keeps increasing with the increase of average photon number per pulse. Increasing the atmospheric scintillation, rather than introducing noise and degrading the performance of quantum radar, can improve the sensitivity and resolution.This anomalous phenomenon can be explained only by quantum decoherence theory. As is well known, the supersensitivity and super-resolution of quantum radar are based on the nonlocal characteristic of quantum light field, while the dissipation process will induce decoherence that leads to the loss of nonlocal characteristic, and finally degrades the performance of quantum radar. However, there have been several researches indicating that the dissipation-fluctuation channel can alleviate the decoherence effect and maintain the nonlocal characteristic of quantum light field compared with pure dissipation channel. For the evolution of quantum light field in dissipation medium, the loss of amplitude plays a crucial role at a lower loss, while the decoherence will play a dominant role at a higher loss. Consequently, the fluctuation may induce extra noise and degrade the performance of quantum radar at lower loss. For higher loss, the fluctuation can prevent the decoherence process and maintain the quantum characteristic of light field, thus the atmospheric scintillation finally improves the sensitivity and resolution of quantum radar.
Enhancement of quantum Fisher information of quantum teleportation by optimizing partial measurements
2018, 67 (14): 140304. doi: 10.7498/aps.67.20180330
The purpose of quantum teleportation is to achieve perfect transmission of quantum information from one site to another distant site. In the teleportation process, the quantum system is inevitably affected by its surrounding environment, causing the system to lose its coherence, which will result in distortion of the transmitted information. In recent years, weak measurement and measurement reversal have been proposed to suppress the decoherence of quantum entanglement and protect some quantum states. On the other hand, quantum Fisher information (QFI) is an important physical quantity in quantum metrology, which can give the optimal value estimating the accuracy of parameters. As is well known, QFI is highly susceptible to environmental noise and can lead its measurement accuracy to decrease. Therefore, it is of great importance to examine how to protect QFI from being influenced by the external circumstance during the teleportation procedure. In this paper, we study how to improve the QFI of teleporting a single-qubit state via a Greenberger-Horne-Zeilinger state in a finite temperature environment with the technique of weak measurement and weak measurement reversal. According to different qubit transmission cases of three quantum teleportation schemes, we consider their respective QFIs in detail. After constructing the quantum logic circuit of each teleportation scheme, we first analyze the variance trend of QFI against the generalized amplitude damping noise parameters. Then by introducing weak measurement and measurement reversal on each noise particle of the three schemes, we optimize the related partial measurement parameters and explore the corresponding improved QFI, namely, the difference between the QFI with optimal partial measurements and that without partial measurements. We find that optimizing partial measurements can efficiently enhance the QFI of the teleported state for the three kinds of teleportation schemes at finite temperature. Moreover, with the value of p fixed, the lower the environment temperature, the larger the value of the improved QFI is. Our results could be useful in further understanding the applications of weak measurement and measurement reversal to the quantum communication process and may shed light on estimating some relevant quantum parameters and implementing quantum information tasks.
Influence of surge movement in non-uniform water flow on performance of underwater quantum communication
2018, 67 (14): 140305. doi: 10.7498/aps.67.20180078
Quantum communication is brand new way of communication in which quantum entanglement is used to transmit information. It is an interdisciplinary subject combining quantum informatics with modern communication theory. Motivated by the communication requirements for underwater sensor networks, submarines, etc., underwater optical communication has been developing rapidly in recent years due to the ideal information security of quantum communication. However, the research on the performance of underwater quantum communication in sea has not yet been fully developed because of a series of factors such as surge, salinity and seaweed and so on. In this paper, the influence of surge in non-uniform water flow on the underwater quantum communication is studied theoretically and experimentally. Firstly, a new Boussinesq equation with a given flow function is derived based on the horizontal and vertical wave velocity of the free surface to represent the free surface boundary conditions. On the other hand, In view of the nonlinear motion of movement, the complexity of change and the randomness of the distribution, the spectrum is used for numerically calculating the surge. The characteristics of wave motion are described by wave height, period and wavelength. Secondly, the influence of surge on the entanglement of underwater quantum channel is analyzed. It is proved that the wave height of surge and the change of the cycle affect quantum communication due to the destruction of the quantum coherence and the reduction in quantum entanglement degree. Thirdly, the influence of surge motion on the quantum channel capacity is studied. The influence of the relation between the wavelength and the transmission cycle on the quantum channel capacity is simulated. The relationship between the physical characteristics of surge wave and the capacity of depolarized channel is established. Fourthly, the influence of surge motion on error rate in quantum key distribution is studied. The simulation results show that when the sea surface wind speed changes in a range of 0-20.5 m/s, the propagation cycle is increased gradually. The channel entanglement is increased from 0.0012 to 0.8426, and the channel capacity is reduced from 0.8736 to 0.1024. In the key distribution process, the quantum bit error rate increases from 0.1651 to 0.4812. Therefore, in underwater quantum communication, the parameters of the system should be adjusted adaptively according to the varying degree of the surge movement.
2018, 67 (14): 140501. doi: 10.7498/aps.67.20180035
The center wavelength of the distribution feedback semiconductor laser is about 1550 nm, and it is in the lowest loss window of the optical fiber communication. A distribution feedback semiconductor laser (DFB-SL) can generate wideband chaotic signals under external disturbances such as optical feedback, optical injection, etc. Thus, due to the simple structure, DFB-SLs with the optical feedback are widely applied to many fields, including information security, lasers radar, and physical entropy sources for generating physical random numbers. However, optical feedback can cause weak periodicity in chaotic signals from the semiconductor laser, and increase the time delay characteristics of chaotic laser, moreover reduce the quality of random numbers generated by using chaotic signals. Meanwhile, to meet the needs of the current high speed and large capacity communication, the DFB-SL, which can generate wideband chaotic laser with low time delay characteristics, has received wide attention and become a hot research subject.In this paper, we present a new scheme for suppressing the time delay characteristics and investigating the bandwidth (BW) of chaotic signals from the semiconductor laser. In this scheme, we build a system that is a distribution feedback semiconductor laser with double phase modulated optical feedback (DFB-SL-DPMOF). In this system, two phase modulators driven by the pseudorandom signals are respectively added to the two optical feedback cavities to eliminate the weak periodicity of the generated chaotic signals. For this system, we numerically investigate the influence of the system parameter, such as the delay time, feedback coefficient, etc., on the time delay characteristic of the chaotic laser. In this paper, the time delay characteristic of chaotic signal is expressed by the maximum value of the time delay signature (TDS) peak of the autocorrelation function curve. Then, to illuminate the effectiveness of this system, other two systems, i.e., DFB-SL with double optical feedback (DFB-SL-DOF) and DFB-SL with single phase modulated optical feedback (DFB-SL-SPMOF) are considered. We study the suppression effect of the system on the TDS among DFB-SL-DPMOF, DFB-SL-DOF and DFB-SL-SPMOF. For these three systems, we give and analyze the simulation curves of the time delay characteristic values with the feedback coefficient and the pumping factor respectively. The results indicate that our proposed scheme has the best suppression effect. Moreover, we numerically investigate the BW of chaotic signals from DFB-SL-DPMOF based on the parameter conditions suppressing TDS effectively. The results show that BW becomes large with the pumping factor and feedback coefficient increasing, and the maximum BW value of the obtained chaotic laser is about 7.2 GHz. Therefore the effectiveness of the presented scheme is numerically clarified. And the conclusions of this paper are useful for applying the chaotic laser to the secure communication field.
2018, 67 (14): 140701. doi: 10.7498/aps.67.20180182
Control and administration of various dangerous gases existing in the environment is very important both for safety in the workplace and for quality of daily life, such as acetone and ethanol, etc. Zinc oxide, a well-known n-type semiconductor with a direct wide band-gap of 3.37 eV, is a very promising gas sensing material. However, zinc oxide's limited selectivity, relatively long response/recovery time, high-power consumption, and lack of long-term stability have restricted its applications in high-standard gas detection. Therefore, increasing gas sensing selectivity is a crucial issue for ZnO application in the gas sensing field. So far, many researches have reported and discussed the effects of morphologies, structures, doping of gas sensing materials, on its sensing performance. In this work, we intend to investigate and theoretically analyze how the polarization of the external electric field affects gas sensing performance and selectivity. Zinc oxide nanoparticles, as a testing gas sensing material, are synthesized by simple precipitation method. Then they are pressed into a disc and polarized under an external electric field with different electric field intensities at different temperatures. The structure and Raman activity for each of the unpolarized ZnO and the polarized ZnO are characterized using X-ray diffraction and Raman spectrometry, respectively. The gas sensing performances of unpolarized and polarized ZnO based sensors to ethanol and acetone are carefully examined using a chemical gas sensing system. The mechanism of external electric field polarization effect on gas sensitivity is discussed. The results reveal that there exists a threshold value for each of voltage and temperature for ZnO polarization under an external electric field. When the voltage and temperature are over 9375 V·cm-1 and 150℃, respectively, the leakage of electricity in ZnO disk happens and the polarization effect gradually disappears. Within the above voltage and temperature limits, Raman peak intensity of the polarized ZnO at 437 cm-1 obviously decreases after external electric field polarization. The response of the polarized ZnO sensor to acetone increases with external electronic field and polarization temperature increasing, while the response to ethanol decreases, which indicates that external electric field polarization can effectively adjust the gas sensing selectivity of nano zinc oxide. Raman analysis indirectly shows that the enhanced gas sensing selectivity of ZnO by the polarization effect of the external electric field is due to oxygen vacancy and zinc vacancy directionally moving under the action of an external electric field. Thus it can be seen that the polarization of the external electric field acting on gas sensing material is a promising effective method to improve gas sensing selectivity.
Simulation of Doppler velocity measurement based on Doppler asymmetric space heterodyne spectroscopy
2018, 67 (14): 140703. doi: 10.7498/aps.67.20180063
Doppler asymmetric spatial heterodyne spectroscopy (DASH) technique with the advantages of high spectral resolution and high phase sensitivity can be considered as a combination of the spatial heterodyne spectroscopy (SHS) technique and the Michelson interferometer technique, which is very suitable for high-precision passive measurement of Doppler velocity. Since a larger optical path difference offset in one of the spectrometer arms corresponds to a higher phase shift sensitivity while suffering a lower contrast of the interferogram, there is an optimum path difference offset for measuring the phase shift and thus the Doppler shift is most sensitive. By comprehensively considering the trade-off between the contrast and the phase shift sensitivity of the interferogram, in this paper we carry out theoretical analysis on the optimum path difference offset. Based on the efficiency function which is defined as the product of the optical path difference and the contrast of the interferogram, the mathematical expressions of the optimum path difference offset for the Gaussian and Lorentz type emission spectral lines are theoretically deduced, respectively. In order to verify these two mathematical expressions, a simulation analysis about the phase shifts of the interference fringes for a single Gaussian type emission spectral line is carried out. The simulation result is consistent with the theoretical value calculated by the deduced mathematical expression. In addition, concerning the complexity of the traditional data processing method for resolving the Doppler velocities of multiple spectral lines, a simplified data processing method based on partial interference fringes is proposed. In general, a single spectral line should be distinguished and isolated from multiple spectral lines in the traditional data processing method. If the distribution of the spectral lines in the passband is too dense, a DASH spectrometer with high enough spectral resolution will be needed. The proposed processing method, retrieving the Doppler velocity from multiple spectral lines without isolating a single line in frequency domain, can not only effectively reduce the calculation of data processing, but also lower the requirement for the spectral resolution of the DASH spectrometer. Combining it with the adaptive frequency-tracing algorithm, the simulation calculations of the Doppler velocity measurement process of the single and multiple spectral lines are conducted. The results show that without taking the noise into account, the maximum resolving errors derived from the proposed data processing method for single and multiple spectral lines are similar, both within 0.005 m/s. It indicates that the proposed data processing method can fully meet the accuracy requirement of practical application and shows the prospect of wide applications in the field of passive Doppler velocity measurement.
2018, 67 (14): 140303. doi: 10.7498/aps.67.20172755
In this paper, we study the quantum coherence of one-dimensional transverse XY model with Dzyaloshinskii-Moriya interaction, which is given by the following Hamiltonian:HXY=∑i=1N((1+γ/2) σixσi+1x+(1-γ/2) σiyσi+1y-hσiz) ∑i=1ND(σixσi+1y-σiyσi+1x).(8)Here, 0 ≤ γ ≤ 1 is the anisotropic parameter, h is the magnitude of the transverse magnetic field, D is the strength of Dzyaloshinskii-Moriya (DM) interaction along the z direction. The limiting cases such as γ=0 and 1 reduce to the isotropic XX model and the Ising model, respectively. We use the Jordan-Winger transform to map explicitly spin operators into spinless fermion operators, and then adopt the discrete Fourier transform and the Bogoliubov transform to solve the Hamiltonian Eq.(8) analytically. When the DM interactions appear, the excitation spectrum becomes asymmetric in the momentum space and is not always positive, and thus a gapless chiral phase is induced. Based on the exact solutions, three phases are identified by varying the parameters:antiferromagnetic phase, paramagnetic phase, and gapless chiral phase. The antiferromagnetic phase is characterized by the dominant x-component nearest correlation function, while the paramagnetic phase can be characterized by the z component of spin correlation function. The two-site correlation functions Grxy and Gryx (r is the distance between two sites) are nonvanishing in the gapless chiral phase, and they act as good order parameters to identify this phase. The critical lines correspond to h=1, γ=2D, and h=√4D2 -γ2 + 1 for γ>0. When γ=0, there is no antiferromagnetic phase. We also find that the correlation functions undergo a rapid change across the quantum critical points, which can be pinpointed by the first-order derivative. In addition, Grxy decreases oscillatingly with the increase of distance r. The correlation function Grxy for γ=0 oscillates more dramatically than for γ=1. The upper boundary of the envelope is approximated as Grxy~r-1/2, and the lower boundary is approximately Grxy~r-3/2, so the long-range order is absent in the gapless chiral phase. Besides, we study various quantum coherence measures to quantify the quantum correlations of Eq.(8). One finds that the relative entropy CRE and the Jensen-Shannon entropy CJS are able to capture the quantum phase transitions, and quantum critical points are readily discriminated by their first derivative. We conclude that both quantum coherence measures can well signify the second-order quantum phase transitions. Moreover, we also point out a few differences in deriving the correlation functions and the associated density matrix in systems with broken reflection symmetry.
Design of beam shaping assembly based on 3.5 MeV radio-frequency quadrupole proton accelerator for boron neutron capture therapy
2018, 67 (14): 142801. doi: 10.7498/aps.67.20180380
Boron neutron capture therapy (BNCT) is expected to be an effective method of improving the treatment results on malignant brain glioma and malignant melanoma, for which no successful treatment has been developed so far. The beam shaping assembly (BSA) of accelerator-based boron neutron capture therapy (A-BNCT) consists of a moderator, a reflector, gamma and thermal neutron shielding and a collimator. The BSA moderates the fast neutron produced in target to epithermal energy range. Design of BSA is one of the key jobs in BNCT project. An optimized study was conducted to design a beam shaping assembly for BNCT facility based on 3.5 MeV 10 mA radio-frequency quadrupole proton accelerator at Dongguan Neutron Science Center. In this simulation work, the neutron produced from the 7Li (p, n) 7Be reaction by 3.5 MeV proton is adopted as a neutron source term. In order to search for an optimized beam shaping assembly for accelerator-based BNCT, Monte Carlo simulation is carried out based on the parameters of moderator material and structure, the Gamma shielding, and the thermal neutron filter in the beam shaping assembly. The beam shaping assembly in this work consists of various moderator materials, teflon as reflector, Bi as gamma shielding, 6Li as thermal neutron filter, and lithium polyethylene as collimator. After comparing the simulation results of Fluental and LiF moderator materials, the beam shaping assembly configuration based on sandwich Fluental-LiF configuration is proposed. The sandwich Fluental-LiF configuration is made up of Fluental and LiF layer by layer, like a sandwich structure, and each layer is 2 cm thick. According to the beam quality requirement of the IAEA-tecdoc-1223 report, the optimized epithermal neutron flux in air at the exit of BSA of the sandwich Fluental-LiF configuration is 9.14×108 n/(cm2·s), which is greater than those of the Fluental configuration (7.81×108 n/(cm2·s)) and LiF configuration (8.79×108 n/(cm2·s)), when the ratio of fast neutron component to gamma ray component to thermal neutron is less than the limiting value of IAEA recommendation. Subsequently, the depth distribution of the equivalent doses in the Snyder head phantom is calculated to evaluate the treatment characteristic. The simulation results show that the therapy rate of the beam shaping assembly based on the sandwich Fluental-LiF configuration is basically equal to that of the Fluental configuration and better than that of the LiF configuration, and the therapy time is less than that of the Fluental configuration. This means that the beam shaping assembly based on the sandwich Fluental-LiF configuration is one of the suitable options for our accelerator-based BNCT.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (14): 144101. doi: 10.7498/aps.67.20180266
A method for the super-resolution imaging of two-dimensional (2D) high-contrast targets is presented. There are two main methods to reconstruct unknown targets with super resolution. One is to illuminate the targets with specific incident fields and transform the information about the evanescent waves into the propagation waves, and the other is to adopt non-linear inversion methods where the multiple scattering within the objects are considered. For the specific-incident-field method, it has been proved that the orbital-angular-momentum (OAM)-carrying electromagnetic (EM) waves can be employed to image unknown targets with super resolution. In fact, OAM-carrying EM waves can transform the information about the evanescent waves into the propagation waves. Thus the resolution of imaging results can break the Rayleigh limit, namely super resolution. At present, the application of OAM-based super-resolution algorithm is only valid for weak scatters based on Born approximation. For the non-linear inversion methods, the contrast source inversion (CSI) is widely used to reconstruct unknown targets, including large-contrast or complex ones. In the CSI method, the information about the evanescent waves is naturally involved since the EM coupling within the objects is taken into account. Thus super resolution can also be achieved by the CSI method. This paper demonstrates a novel algorithm for super resolution of large-contrast targets by combining the OAM-based super-resolution technique and the CSI method. And the better resolution is achieved than by the CSI method. Firstly, 2D OAM EM waves are generated using uniform circular array of line source, and the region of interest is illuminated by the OAM beams of different topological charges. So the information about the evanescent waves can be converted into the propagation waves. Secondly, Born approximation is used to obtain the starting value of the contrast. In the process of evaluating the contrast, the super-resolution information is fully utilized. Thirdly, the starting value of the contrast source is evaluated using the starting value of the contrast. Then the CSI method starts to be iterated. Since the information about the evanescent waves is always involved in the iterating process, super-resolution reconstruction can be obtained and is better than that obtained by the CSI method. Numerical experiments show the accuracy of the algorithm by testing different scenarios. The resolution and outline of the target are reconstructed accurately even when the measurement data are corrupted by noise. To sum up, to reconstruct unknown targets with super resolution, one should firstly transform the information about the evanescent waves into the propagation waves, and secondly make full use of the super-resolution information in the inversion methods. The conclusion of this paper may provide an insight into the super resolution in EM inverse scattering.
2018, 67 (14): 144202. doi: 10.7498/aps.67.20180030
The flat-topped beam is a special beam with wide applications in the directional backlight autostereoscopic display, and it is used as a directional backlight in the horizontal direction. However, it is still challenge to white light flat-topped beams with the traditional flat-topped beam shaper. In this paper, it is proposed that diffraction mask with butterfly-shaped hole arrays and cylindrical lens be used to produce the horizontal flat-topped white beams. The surface of the LCD backlight mask is covered by a layer of diffraction mask, where the butterfly-shaped holes are arranged in line along the vertical direction. Simultaneously, the height and width, hole center height are kept identical, and the ratio between the center depth and the perimeter of butterfly-shape hole is defined as the concavity. A flat convex cylindrical lens is placed in front of diffraction mask gaplessly. The uniform light field from LCD backlight is transformed into the white light flat-topped beams and projected on the receiving screen by the diffraction mask and cylindrical lens. Based on the Huygens-Fresnel diffraction integral, the intensity distribution formula of diffraction of the single wavelength light source on the receiving screen is derived. Furthermore, the intensity distribution formula on the screen is derived by super positioning the multiple wavelengths. The proposed method is verified by both numerical simulation and experimental validation. Numerical simulations elucidate the effects of the different transmission distance and butterfly hole concavity on the white light flat-topped beam flat-topped factor. The stimulated results show that the propagation distance does not influence the white light beam transverse intensity distribution characteristics of flat top. With the beam propagation distance increasing, the horizontal width of flat-topped beam becomes larger. When the concavity of the butterfly hole decreases, light intensity distribution shifts from Gaussian to flat type. However, the flat-topped factor decreases when the butterfly concavity is too small. The optimal concavity varies from 0.4 to 0.6, where the flattened factor of the transverse flat-topped beams reaches 0.89. In the experiments, films are produced with the diffraction of butterfly hole array mask. The height and width of butterfly are both 48 μm, and the concavities of the butterfly are 1, 0.83, 0.66 and 0.83 respectively. The cylindrical lens adopts PMMA cylindrical lens grating plate, with a thickness of 8 mm, a grating density for 18 line/inch, and the cylindrical lens curvature radius R is 2.67 mm. The experimental results show that the beam transmission is consistent with the result of numerical simulation. When concavity of the butterfly is 0.5, the flat factors of the white light horizontal of flat-topped beams are higher than 0.89 in a range from 500 mm to 2000 mm. Moreover, we also discuss the dispersion effects of shaft flat-topped beams and off-axis flat-topped beams, showing that the refraction and dispersion of the cylindrical lens can cancel out each other, so that the horizontal flat-toped white beams is basically dispersionless.
2018, 67 (14): 144203. doi: 10.7498/aps.67.20180140
One of the most outstanding limitations in the evolution of the power scaling of fiber laser with near diffraction limited beam quality has been the mode instability since it was found in 2010. For a long time, researchers have focused on the dynamic mode instability (DMI) theoretically and experimentally, and it was not until 2016 that a new analytical model called quasi-static mode instability (QSMI) was proposed. Unlike DMI, because of the one-way energy transfer characteristic on a specific time scale, QSMI will show no apparent fluctuations with respect to the time domain traces. In this paper, based on a counter-pump few-mode fiber amplifier schematic system, the output power, beam quality and time traces of the amplifier under changing seed laser power are measured to investigate its mode instability effect. The ytterbium-doped fiber of the amplifier has a core diameter of 25 μm and inner cladding diameter of 400 μm, which can support 4-5 modes to be transmitted in the amplifier. The experimental results reveal that QSMI happens in the few-mode fiber amplifier. Taking 234 W seed power for example, it is found that when the output power reaches 2030 W, the optical-to-optical efficiency begins to fell from 86% to 32%, and at the same time the M2 value has an abrupt degradation from 2.2 to 2.8, which indicates that MI happens. On the other hand, it can be seen from the time traces of the output laser that there exist no rapid fluctuations, and the Fourier analysis shows no sign of DMI characteristic frequency components either. Quoting the definition of drifting ratio σ, when the output power is 2030 W under 234 W seed power, it is only 4%, and thus verifying that it is QSMI instead of DMI. The experiment also indicates that increasing the seed power has an effective influence on enhancing the mode instability power. When the seed power is raised from 86 W to 528 W, the corresponding threshold power is increased from 1560 W to 3090 W. And for 528 W seed power, when the output laser surpasses 3000 W, the optical-to-optical efficiency does not decline as fast as other relatively low seed power. To sum up, the mode instability effect represents a kind of quasi-static property in these large core diameter few-mode fiber amplifiers, which needs further studying.
Generation and quantum characterization of miniaturized frequency entangled source in telecommunication band based on type-II periodically poled lithium niobate waveguide
2018, 67 (14): 144204. doi: 10.7498/aps.67.20180329
The frequency entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) possess wide applications in quantum optics and relevant fields.To facilitate the practical quantum information technologies,particularly in optical fiber links,a frequency entangled source at telecommunication wavelength with features of compactness,portability,high efficiency and miniaturization is highly desired.In this paper,we report the experimental generation of a miniaturized frequency entangled source in telecommunication band from a 10 mm-long type-Ⅱ periodically poled lithium niobate (PPLN) waveguide pumped by a 780 nm distributed Bragg reflector (DBR) laser diode.The frequency entangled photon pairs generated by SPDC possess wide applications in quantum optics and relevant fields.When the DBR laser diode is driven by a current of 170 mA at a temperature controlled to 20℃,the output power is measured to be 70.4 mW with a central wavelength of 780.585 nm.Under this pump,the orthogonally-polarized photon pairs are generated and output from the PPLN waveguide.After filtering out the remaining pump by three high-performance long-pass filters mounted on an adjustable U-type fiber bench,the photon-pair generation rate,spectral and temporal properties of the generated frequency entangled source are measured.The results show that the generation rate of the photon pairs,after being corrected for the detection efficiencies of the single photon detectors and the optical losses,is achieved to be 1.86×107 s-1 at a pump power of 44.9 mW (coupled into the waveguide).Optimizing the working temperature of the waveguide and fixing it at 46.5℃,the frequency degeneracy of the SPDC generated photon pairs is realized.Based on the coincidence measurement setup together with two infrared spectrometers,the spectra of the signal and idler photons are obtained with their center wavelengths of 1561.43 nm and 1561.45 nm,and their 3-dB bandwidths of 3.62 nm and 3.60 nm respectively.The joint spectrum of the photon pair is observed,showing a joint spectrum bandwidth of 3.18 nm.The degree of frequency entanglement is quantified to be 1.13 according to the bandwidth ratio between the single photon spectrum and the joint spectrum.Furthermore,based on the Hong-Ou-Mandel (HOM) interferometric coincidence measurement setup,a visibility of about 96.1% is observed,which indicates the very good frequency indistinguishibility of the down-converted biphotons.The measured 3-dB width of the HOM dip is 1.47 ps and shows good agreement with the measured single-photon spectral bandwidth.The experimental results lay a solid foundation for developing portable,miniaturized frequency entangled sources at telecommunication band for the further applications in quantum information areas,such as quantum time synchronization.
Optical microcavity can confine light into a small volume by resonant recirculation. Devices based on optical microcavities are already indispensable for a wide range of applications and studies. They not only apply to traditional optics, but also have broad application prospects in quantum information and integrated optoelectronic chips. In quantum optical devices, microcavity can cause atoms or quantum dots to emit spontaneous photons in a desired direction or can provide an environment where dissipative mechanisms such as spontaneous emission are overcome so that quantum entanglement of radiation and matter is possible. For better application in quantum communication, optical microcavity needs to have a high quality factor and a low mode volume. Considering the beam coupling, spot shape and experimental production and others, the Fabry-Perot (F-P) microcavity has been widely applied to the field of optoelectronics. However, the Q-factor of the F-P microcavity is generally low, and the mode volume is large, so it needs to be improved.In addition, high Q-factor microcavity can also play a large role in detecting particles and biological macromolecules.In this paper, through the theory of wave optics, the eigenmodes of a new type of cone-top cylindrical optical micro-cavity are analyzed, and the resonant wavelength expression of the resonant cavity is obtained. We discuss the effects of the top mirror angle on the resonator performance and application of COMSOL simulation software to verify the proposed cone-top cylindrical microcavity. The optimized design and simulation results show that the quality factor of the new resonator can be increased by 22.4% to 49928.5 and the effective mode volume of the resonator can be reduced by 47.8% compared with the traditional parallel resonator. In this case, the corresponding new cavity length is 4.51 μm and the diameter is 3.13 μm. In this article its fabrications are also discussed.
2018, 67 (14): 144501. doi: 10.7498/aps.67.20172752
With the rapid development of vehicular technology, hi-tech manufacturing facilities are equipped in intelligent vehicles to improve road capacity and traffic safety. However, freeway diverge segment has significant influence on current traffic flow, and could affect the heterogeneous traffic flow consisting of manual and intelligent vehicles. The primary objective of this study is to evaluate how intelligent vehicles affect traffic flow at an off-ramp bottleneck.In order to depict the car-following dynamics of manual vehicles, the modified comfortable model, one of the most classic cellular automata models, is employed to distinguish intelligent vehicles. In this paper, intelligent vehicles consist of adaptive cruise control (ACC) vehicles cooperative adaptive cruise control (CACC) vehicles. The ACC and CACC model are proposed by partners for advanced transportation technology (PATH), which are validated by real experimental data. Besides, vehicles equipped with CACC will degrade ACC vehicle if the leading vehicle is driven manually. From the perspective of vehicle's lateral movement, two novel lane-changing models, including the discretionary lane-change (DLC) model and mandatory lane-change (MLC) model, are developed to model the future behaviors of intelligent vehicles. A risk factor λ is introduced into the DLC model to distinguish vehicles from conventional ones. Based on environment perception technology, a five-step MLC decision-making model is designed specifically for intelligent vehicles exiting to off-ramp. It is comprised of environment perception, safe gap computation, measured gap ranking, measured gap classification and lane-changing gap selection. Based on the proposed hybrid traffic flow model, numerical simulations are conducted to study the influences of intelligent vehicles on the traffic flow near an off-ramp. Apart from the market penetration of intelligent vehicles, parameters considered in this paper include the demands of mainlines and off-ramp, range of environment perception, length of lane-changing area, and level of lane-changing risk.Analytical studies and simulation results are as follows. 1) The integration of car-following model and lane-changing model for the off-ramp system enables vehicles to have reasonable dynamic characteristics. 2) The capacity ascends to the peak after an initial decrease as CACC vehicle penetration increases. The maximum capacity obtained in 100% CACC vehicle scenario is improved by over 50%, compared with that in 50% CACC penetration scenario. 3) Enlarging the ranges of environment perception and lane-changing areas, and enhancing the lane-changing risk can significantly dissipate congestion upstream of the off-ramp and improve the efficiency of mainlines. However, they have little influence on traffic flow at off-ramp. 4) The worst performance of the system occurs in the scenario of 50% CACC penetration, where deterioration caused by degraded ACC vehicles suggests that enough patience and public confidence should be paid for the development of intelligent vehicles.
2018, 67 (14): 144701. doi: 10.7498/aps.67.20180371
In 2005, a bicontinuous arrangement of domains was explored by large-scale computer simulations. In a binary liquid host, the behaviors of neutrally wetting particles were simulated following an instantaneous quench into the demixed region. As the two mutually immiscible liquids phase separate, particles can be swept up by the freshly created interface and jam together as the domains coarsen, forming a particle-stabilized interface between two continuous liquid phases. This type of material is known as “bicontinuous interfacially jammed emulsion gel” (Bijel), and has been demonstrated experimentally using water-lutidine mixture in 2007. It is believed that Bijels have rich potential applications in diverse areas including healthcare, food, energy and reaction engineering due to their unique structural, mechanical and transport properties.As a new class of soft materials, Bijels have received great attention in recent years, and have been developed by using different liquids and non-spherical particles. However, a wide gap remains between the experimental systems and the industrial applications. This short review will critically assess current progress of Bijels and relevant studies including the attempts and challenges to use them in industry; the creation of Bijels by direct mixing at room temperature will be highlighted specifically.Chapter 1 presents the theoretical background. For binary-liquid systems containing dispersed colloidal particles, arrested composites can be created via the stabilization of convoluted fluid-fluid interfaces. Based on this, different morphologies of Pickering emulsions would be obtained. Chapter 2 first focuses on some complex emulsions, including Janus droplets and multiple emulsions, and then induces the bi-continuous structures. Such structures were originally formed through spinodal decomposition, which catches the phase demixing of an initially single-phase liquid mixture containing a colloidal suspension, and normally needs to control the temperature carefully. In Chapter 3, the mechanism of spinodal decomposition is presented. Chapter 4 shows some recent research progress of Bijels, including the studies with different liquid systems, nonspherical particles and some chemical property measurements. This chapter also summarizes the challenges in using Bijels in industry. In Chapter 5, a new method of creating Bijels by direct mixing at room temperature is demonstrated. This method simply needs high viscosity liquids, nanoparticles and a surfactant; it not only bridges the gap between conventional Bijel production (see Chapter 3) and that of particle stabilized bicontinuous structures using bulk polymers, but also bypasses the careful particle modification and phase separation steps for conventional Bijels. In Chapter 6 some conclusions are drawn and a general outlook is also provided.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (14): 145201. doi: 10.7498/aps.67.20180366
Cusped field thruster is a new kind of thruster which confines plasma by magnetic mirror effect to produce thrust. It is characterized by long lifespan and adjustable thrust in a large range, which makes it have great potential applications in drag free satellites and commercial space satellites. It was put forward first by THALES Electron Devices in Germany and sponsored from European Space Agency. There are several institutions are engaged in the research of this thruster, including Massachusetts Institute of Technology, Stanford University and Technische Universiteit Delft. Now the test experiments on the cusped field thruster using Xe, Kr and Ar are being carried out in the laboratory of plasma propulsion of Harbin Institute of Technology to ascertain the ionization regulations of different propellants under the high voltage and strong magnetic field conditions. On this basis, it is significant to know the mechanism about how the performances change with propellant and provide the foundation for the cusped field thruster using different propellants. In this paper, the principle and design process of this thruster are presented. Then it can be found that the thruster can be ignited easily by using Xe compared with by using Kr and Ar under the same volume flux, which is caused by their differences in ionization energy and ionization section. Experiments show that the cusped field thruster can be ignited under 200 V while it cannot be ignited by using Kr and Ar even under 1000 V under the same volume flux. Then the performances of cusped field thruster using three propellants are tested. It can be found that there are obvious differences in anode current, thrust, efficiency and impulse using three propellants under the same conditions. The diagnosing of plume using Faraday probe shows that the propellant utilization causes the difference in performance which is related to ionization process. The experiments show that the utilization rate of Xe is over 90 percent, while the utilization rate of Kr is less than 60 percent and the utilization rate of Ar is less than 20 percent. The obvious difference in ionization voltage can reflect the difference in performance. The experimental results under the same flux show that the utilization rates of Kr and Ar can be improved by increasing flow density and reducing the collision free path between atoms. Experiments show that the peak utilization rate of Ar can be improved to 50 percent approximately. In the aspect of plume structure, the results of Faraday probe show that the hollow plume can be observed and the angle linked with peak ion current density decreases with atom mass decreasing.
2018, 67 (14): 145202. doi: 10.7498/aps.67.20180274
Optically pumped metastable rare-gas laser (OPRGL) have been proposed to overcome the shortcomings of diode-pumped alkali-vapor laser in the recent years. The OPRGL promises to realize high-scale output. But how to achieve enough particle density of metastable atoms is still an open problem. Usually, plasma produced by discharge serves as a gain medium of the OPRGL. Here in this paper, we are to reveal the effects of different discharge parameters on the plasma properties, such as particle density of metastable argon atoms. Gas discharge at a radio frequency of 13.56 MHz is adopted to excite argon atoms. Emission spectrum is employed to study argon and helium radio frequency discharge of optically pumped argon laser at high pressure, different powers of discharge and various content of argon. Gas temperature is obtained by analyzing rotational spectrum (A2∑+ → X2Π) of OH radical generated by residual water vapor and comparing simulated spectrum with the measured spectrum. The electronic excitation temperature relating to electron temperature is obtained by the method of Boltzmann's plot. Stark broadening of the spectrum is used to determine the electron density. The results show that gas temperature rises slightly with the increase of pressure and varies little with content and discharge power changing. The electronic excitation temperature increases with the decrease of pressure evidently and decreases slightly with the increase of content. The electron density is on the order of 1015 cm-3 under various conditions controlled by us. Long time discharge test reveals that residual water vapor can lead to the decrease of electron temperature, and thus reducing the yield of argon metastable state. In conclusion, considering that the higher gas temperature can improve the collision relaxation rate of helium and argon, and the higher electron temperature can improve the rate of production of argon metastable state. Thus a proposal is put forward that appropriately heating gas and reducing gas pressure can obtain higher particle density of metastable argon. Furthermore, It can be found from these results that heating and cleaning the gas during discharge may be candidate methods to obtain and sustain the higher particle density in the plasma.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2018, 67 (14): 146103. doi: 10.7498/aps.67.20172542
Radiation effect of deep submicron semiconductor device has been extensively studied in recent years. However, fewer researches laid emphasis on the degradation characterization induced by total ionizing dose (TID) damage in nano-device. The purpose of this paper is to analyze the TID effect on the 65 nm commercial complementary metal oxide semiconductor transistor. The n-type and p-type metal oxide semiconductor field effect transistors (NMOSFET and PMOSFET) with different sizes are irradiated by 60Co γ rays at 50 rad (Si)/s, and TID is about 1 Mrad (Si). Static drain-current ID versus gate-voltage VG electrical characteristics are measured with semiconductor parameter measurement equipment. The irradiation bias of NMOSFET is as follows:the ON state is under gate voltage VG=+1.32 V, drain voltage VD is equal to source voltage VS (VD=VS=0), and the OFF state is under drain voltage VD=+1.32 V, gate voltage VG is equal to source voltage VS (VG=VS=0). The irradiation bias of PMOSFET is follows:the ON state is under gate voltage VG=0 V, drain voltage VD is equal to source voltage VS (VD=VS=1.32 V), and the OFF state is under VD=VG=VS=+1.32 V. The experimental results show that the negative shifts in the threshold voltage are observed in PMOSFET after irradiation. Besides, for PMOSFET the degradation of the ON state during radiation is more severe than that of the OFF state, whereas comparatively small effect are present in NMOSFET. Through experimental data and theoretical analysis, we find that the changes in the characteristics of the irradiated devices are attributed to the building up of positive oxide charges in the light doped drain (LDD) spacer oxide, rather than shallow trench isolation oxide degradation. The positive charges induced by TID in PMOSFET LDD spacer oxide will lead to the change of hole concentration in channel, which causes the threshold voltage to shift. What is more, the difference in electric field in the LDD spacer is the main reason for the difference in the radiation response between the two radiation bias conditions. Radiation-enabled technology computer aided design used to establish two-dimensional mode of the transistor. The simulation results of ID-VG curves are in good agreement with the experimental results. Combining theoretical analysis and numerical simulation, the radiation sensitive regions and the damage physical mechanism and radiation sensitivity regions of PMOSFETs are given. This work provides the helpful theory guidance and technical supports for the radiation hardening of the nano-devices used in the radiation environments.
2018, 67 (14): 146102. doi: 10.7498/aps.67.20180276
Brittle materials have many excellent properties for structural applications, whereas the brittleness and disorder due to defects and micro-cracks cause failure. Fragmentation may occur and often lead to a catastrophic damage, bring dangers to the users especially when brittle materials suffer dynamic loads like impact and explosion. The impact fragmentation of brittle material belongs to the continuum/discretization domain. The combined finite and discrete element method (FDEM) is used to investigate the impact fragmentation of disordered material in detail. In this work, structural disorder in the brittle material is not considered, and the disorder is only reflected in the strength heterogeneity. Assuming that the mesoscopic fracture parameters of brittle materials obey the Weibull distribution, the degree of disorder can be quantified by the Weibull modulus k. The impact of a brittle sphere against a rigid plate is simulated using the FDEM. The dynamic response can be classified into damage and fragmentation zones. In sphere with low material disorder, cracking pattern is mainly dominated by single or more penetrating cracks. Increasing the disorder degree by smaller k, branch cracks emerge. Finally, it changes into a global branch crack in highly disordered sphere. Besides, mass index analysis indicates that higher disordered sphere has a higher critical velocity in impact events, in which the critical impact velocities equal 10, 15, 40 and 80 m/s when the values of m are 10, 5, 2 and 1, respectively. Furthermore, the principal component analysis is adopted for digging the crack features from fragments morphology description. The statistics of two fragment shape indexes shows that fragments coming from the highly disordered spheres have greater variability with a rougher surface and higher flatness overall, corresponding to the fracture pattern. Finally, we conclude that the effects of disorder on impact fragmentation can be ascribed to the dominant cracking mechanism-specifically, the proportion of shear failure mechanism grows with the disorder degree, implying more non-penetrating branch cracks existing in the fragments. We demonstrate that the effect of disorder on impact fragmentation is probably a consequence of a continuous phase nucleation-avalanche-percolation transition as well.
2018, 67 (14): 146101. doi: 10.7498/aps.67.20180062
Stainless steels with excellent hardness and corrosion resistance performance have been widely used in industrial production. Ternary Fe-Cr-Ni alloys, as a model alloy of nickel chromium stainless steels, are of great importance in the fields of material science. Under non-equilibrium solidification condition, alloys may have new microstructure and improved performance. In this paper, two liquid ternary Fe-Cr-Ni alloys are deeply undercooled and rapidly solidified in a 3-m drop tube to investigate the microstructure evolution and solute distribution of alloy droplets with different sizes. In the drop tube experiments, the Fe-Cr-Ni alloy samples with a mass of 1.5 g are placed in a φ16 m mm×150 mm quartz tube with a 0.5-mm-diameter orifice at its bottom and heated by induction heating device in a high vacuum chamber. Then the samples are melted and overheated to 200 K above their liquidus temperatures for several seconds. The alloy melt is ejected out of the small orifice and dispersed into numerous droplets after adding high pressure helium gas flow. The alloy droplets with diameters ranging from 68 μm to 1124 μm are achieved. After experiments, the alloy droplets with different sizes are mounted respectively. Then they are polished and etched. The drop tube technique provides an efficient way to study the rapid solidification mechanism of alloys. Besides the experiments, the dendrite growth velocities of primary phase in two Fe-Cr-Ni alloys are calculated theoretically using the modified LKT/BCT model. As droplet size decreases, both cooling rate and undercooling increase exponentially and the morphologies of two alloys become well refined. Under the near-equilibrium solidification condition with a cooling rate of 10 K/min, both alloys consist of coarse lath-like α phase. After rapid solidification of Fe81.4Cr13.9Ni4.7 alloy droplets during free fall, the microstructure presents a lath-like α phase, resulting from the solid-solid phase transition. As undercooling increases, the primary δ phase is converted from the coarse dendrite with long trunk into equiaxed grain. For Fe81.4Cr4.7Ni13.9 alloy, the microstructure is composed of α phase grains. The transition of primary γ phase from coarse dendrite with long trunk to refined equiaxed grain occurs as the undercooling increases. Meanwhile, both dendrite trunk length and secondary dendrite arm spacing decrease drastically, suggesting that the rapid solidification is the main reason for grain refinement. Moreover, the relative segregation degree of solute Cr and Ni inside α phase grain also decreases obviously with the increase of undercooling, and the microsegregation of Ni is more remarkable than that of Cr. This suggests that the high cooling rate and undercooling cause the solute to be distributed evenly. Compared with that of γ phase, the dendrite growth velocity of δ phase is large and its dendrite tip radius is small. The two phase transform from solute diffusion controlled growth into thermal diffusion controlled growth as undercooling increases to 8 K. When undercooling is larger than 8 K and within the experimental undercooling range, the dendrite growth of both Fe-Cr-Ni alloys is controlled by thermal diffusion.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Temperature dependent excitonic transition energies and linewidths of monolayer MoS2 probed by magnetic circular dichroism spectroscopy
2018, 67 (14): 147801. doi: 10.7498/aps.67.20180615
Layered transition metal dichalcogenides (TMDs), as a new class of two-dimensional material, have received wide attention of scientific community due to their peculiar electronic and optical properties. Monolayer TMDs such as MoS2, MoSe2, WS2 and WSe2 are semiconductors with band gap energies in the visible and near-infrared region, which promises the applications in logic nano-devices, ultra-high speed photoelectric detectors and nano-lasers. Temperature has strong influences on the electronic and optical properties of semiconductors, and their applications in photonic and optoelectronic devices. Thus, the research on the temperature dependence of the energy band of monolayer TMDs is important and meaningful. Monolayer MoS2, as a prototype of TMDs, displays a weak absorption line with a strong background in original reflection or absorption spectra. The strong background has a tremendous influence on the determination of excitonic transition energy and linewidth. In this work, we adopt the reflection magnetic circular dichroism (MCD) spectroscopy in which reflection spectra and MCD spectra can be simultaneously obtained. We demonstrate that the background disturbance is eliminated in the MCD spectra, in contrast to the reflectivity spectra. And we discuss the optimization of our home-built experimental setup in detail. Through the elaborate analysis of the MCD theory, we demonstrate that the excitonic transition energy and linewidth can be directly and accurately extracted from the MCD spectrum. We perform the reflection MCD measurements on monolayer MoS2 in a temperature range of 65–300 K. The transition energies and linewidths of A and B excitons of monolayer MoS2 are extracted, respectively. Those functional parameters that describe the temperature dependence of the energy and linewidth of both excitonic transitions are evaluated and analyzed. We find that the broadening of the linewidth is related to the LO phonon scattering, and the linewidth of A exciton is clearly narrower than that of B exciton. The linewidth difference between A and B excitons might result from the stronger inter-valley coupling of B exciton. Our results indicate that MCD spectroscopy, as a modulated spectroscopy by magnetic fields, provides an easy tool to determine the features of monolayer excitons.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2018, 67 (14): 148702. doi: 10.7498/aps.67.20180441
Magnetic tweezers are a high precision single-molecule manipulation instrument. A gradient magnetic field is used to generate a force on the order of pN, acting on biomolecule-tethered superparamagnetic beads and to manipulate them. By tracking the bead with an inverted microscope, an imaging system and an image process software, one can obtain the extension length information of the biomolecules, thus can study the mechanism and dynamics of the molecules at a single molecule level. Magnetic tweezers include transverse magnetic tweezers (TMT) which are cheap and simple, and longitudinal magnetic tweezers (LMT) which are expensive and complicated. As the traditional TMT can only track the long biomolecule-tethered beads and their spatial resolution is poorer than that of the LMT according to the error theory of magnetic tweezers and the experimental results, the TMT is not so widely used. To solve this problem, we utilize a light sheet to illuminate the beads only in TMT, and then observe the bead sticking on the lateral surface. The tracking error on the extension axis is 4 nm, which is very small. Then we track and obtain the “folding-unfolding” state transition trace of a hairpin DNA. The hairpin DNA is inserted into a 0.5 μm dsDNA. This experiment proves its ability to study short DNA, RNA or protein. Instead of the fully folded and unfolded state, we observe a semi-stable state at the 1/3 length of the hairpin. The semi-stable state is precisely at the place of the CG rich area of the hairpin, so the CG rich area should be the reason for the semi-stable state. Then we use the 16 μm λ -DNA to further test the novel TMT system. Having obtained the stretching curve of the dsDNA, we fit the length-force data with the worm-like-chain model. The fitted persistence length of the dsDNA is (47±2) nm, which is consistent with the result in the literature. Finally, we compare the noise of traditional TMT, novel TMT and LMT with that of short and long dsDNA at weak and strong force, and we find that at weak force, the novel TMT distinctly enhances the resolution to the LMT level; while at strong force, the resolution of the novel TMT is about half that of the LMT. The results above prove that (1) the short DNA, RNA or protein can be studied by the novel TMT, which extends the application scope of the instrument; (2) the resolution of TMT is enhanced distinctly under weak and strong force, making the novel TMT competent of more experiments.
2018, 67 (14): 148703. doi: 10.7498/aps.67.20180099
The proapoptotic protein tBid is a member of Bcl-2 family, and it plays an important role in apoptosis by inducing mitochondrial outer membrane permeabilization (MOMP) and lysosomal membrane permeabilization (LMP). Previous studies have shown that the mechanism of tBid-dependent MOMP and LMP depends on tBid interacting with membranes. Researchers hold different opinions about whether tBid itself could induce MOMP and LMP. Some of the researchers insist that tBid must trigger other proteins like Bax or Bak inserting into the membrane, and assembly of tBid itself could not form pores large enough to release cytochrome c. Some others think that tBid just like Bax, can permeabilize mitochondrial outer membrane releasing cytochrome c and lysosomal membrane with the leakage of lysosomal cathepsin B. Here, we want to know whether the tBid itself can induce membrane permeabilization in our model system at low concentration. We use 3 ways to observe tBid and membranes interactions. They are confocal imaging of GUVs (giant unilamellar vesicles), traditional single molecular fluorescence assay, and a recently developed approach, single molecular surface-induced fluorescence attenuation (sm-SIFA). So we can obtain information from single vesicle level and single molecule level. At single vesicle level, we can directly find out whether the GUVs are permeabilized and at the same time the shape of the GUVs is changed. At a single molecule level, we can know the properties of one protein. Especially by using the sm-SIFA, we can obtain the insertion depth of exact residue. Combining the results obtained from different ways under the same conditions, we find that tBid itself can induce the model membrane to permeate, releasing the fluorescent molecules, by oligomerization. What is more, we suggest that the mechanism is that in oligomers some tBids can be inserted deep into the membrane although in oligomers not all the proteins have the same insertion depth. It is indicated that the conformations of tBids in oligomers are diversified. We also prove that the ways we use here are efficient. The GUVs and supported lipid bilayers are indeed tenable model systems. Sm-SIFA has a grand future in the study of protein and membrane interactions.
2018, 67 (14): 148901. doi: 10.7498/aps.67.20172490
Complex networks are widely used in many problems of the financial field. It can be used to find the topological structure properties of the financial markets and to embody the interdependence between different financial entities. The correlation is important to create the complex networks of the financial markets. A novel approach to incorporating textual mutual information into financial complex networks as a measure of the correlation coefficient is developed in the paper. We will symbolize the multivariate financial time series firstly, and then calculate correlation coefficient with textual mutual information. Finally, we will convert it into a distance. To test the proposed method, four complex network models will be built with different correlation coefficients (Pearson's and textual mutual information's) and different network simplification methods (the threshold and minimum spanning tree). In addition, for the threshold networks, a quantile method is proposed to estimate the threshold automatically. The correlation coefficients are divided into 6 equal parts. And the midpoint of the 4th interval will be taken as the threshold according to our experience, which can make the MI methods and Pearson methods have the closest number of edges to compare the two methods. The data come from the closing prices of Chinese regional indexes including both Shanghai and Shenzhen stock market. The data range from January 4, 2006 to December 30, 2016, including 2673 trading days. In view of node correlation, the numerical results show that there are about 20% of the nonlinear relationships of the Chinese regional financial complex networks. In view of the network topology, four topological indicators for the regional financial complex network models will be calculated in the paper. For average weighted degree, the novel method can make the reserved nodes closely compared with Pearson's correlation coefficient. For network betweenness centralization, it can improve the betweenness importance of reserved nodes effectively. From the perspective of modularity, the novel method can detect better community structures. Finally, in dynamic network topology features, the data of regional indexes will be equally divided yearly for constructing complex network separately. The simplification method used in the section is the threshold method. The numerical results show that the proposed methods can successfully capture the two-abnormal fluctuation in the sample interval with the dynamics of the small-world and the network degree centralization. In addition, we find that the proposed regional financial network models follow the power-law distribution and are dynamically stable. Some developing regions are more important than the developed ones in the regional financial networks.
2018, 67 (14): 148201. doi: 10.7498/aps.67.20180630
Each organism has its own set of chromatin proteins to protect the stable structure of DNA and thus maintain the stability of genes. Sso7d is a small nonspecific DNA-binding protein from the hyperthermophilic archaea Sulfolobus solfataricus. This protein has high thermal and acid stability. It stabilizes dsDNA and constrains negative DNA supercoils. Besides, the Sso7d binds in a minor groove of DNA and causes a sharp kink in DNA. By observing the interaction between chromatin protein and DNA structure, we can understand the function and mechanism of chromatin protein. Sulfolobus solfataricus can survive at high temperature. To understand why the DNA of Sulfolobus solfataricus retains activity at high temperature, we investigate the interaction between Sso7d and DNA by atomic force microscope (AFM) and magnetic tweezers. Atomic force microscope and magnetic tweezers are advanced single molecule experimental tools that can be used to observe the interaction between individual molecules. The experimental result of AFM reveals the process of interaction between Sso7d and DNA. The DNA structure changes at a different concentration of Sso7d and depends on reaction time. At a relatively low concentration of Sso7d, DNA strand forms a kink structure. When the concentration of Sso7d is increased, DNA loops appear. Finally, DNA becomes a dense nuclear structure at a high concentration of Sso7d. If the time of the interaction between Sso7d and DNA is increased, DNA structure tends to be more compact. These results indicate that high concentration of Sso7d is important for the compact structure of DNA. We design an experiment to find out the formation of the looped structure on DNA. Moreover, we measure the angle of kinked DNA and compared it with previous result. Through the experiment of magnetic tweezers, we measure the forces of unfolding the double-stranded DNA complexed with Sso7d at different concentrations. The experimental results show that the binding between Sso7d and DNA increases the force of unfolding the double-stranded DNA. The binding energy between Sso7d and dsDNA is 3.1kBT which is calculated from experimental data. It indicates that DNA base pairs are more stable when chromatin protein Sso7d exists. These results can explain the survival of Sulfolobus in high temperature environment.
2018, 67 (14): 148701. doi: 10.7498/aps.67.20180378
With the development of technology and the widespread use of high static magnetic fields (SMFs) in medical diagnosis, such as MRI (magnetic resonance imaging) in hospitals, patients have more and more chances to encounter high SMFs (higher than 1 T), which invokes increasing public concerns about human health. However, due to the experimental limitations, there are very few studies of high SMFs (above 1 T) on animals and human bodies. In contrast, cell, as a basic unit of various organisms, is the primary research target for most researches of the biological effects under the action of magnetic fields. However, due to the differences in magnetic field parameter, exposure condition and cell type, there are diverse experimental outcomes reported by individual studies in the literature. Here in this review, we summarize the results about the cellular effects under SMFs above 1 T, including changes of cell orientation, cell proliferation, microtubule and mitotic spindle orientation, DNA and cell cycle. Moreover, we also compare and analyze the factors that could cause these experimental variations, including the differential effects of high SMFs on cell type, such as cancer and non-cancer cells, as well as magnetic field intensity-induced experimental variations. The most well studied cellular effects are SMF-induced cell and polymer orientation changes, and the cellular composition is a key factor that determines the exact orientation of a cell in an SMF. For example, the normal red blood cell is aligned parallelly to the SMF direction, but the whole bull sperm is aligned perpendicularly to the SMF direction. Among the magnetic field parameters, the magnetic field intensity is especially critical. The red blood cells can only be partially aligned by 1 T SMF, but an 8 T SMF could align the red blood cells 100% along the magnetic direction. Overall, the biological research of high SMFs above 1 T, especially above 10 T, is still at an initial stage. Biological experiments in high SMFs above 20 T are especially lacking. This review could help provide some biological bases for future high SMF investigations, which is important not only for the basic understanding of the biological effects of high SMFs, but also for the applications of high SMFs in medicine, such as high field MRI.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2018, 67 (14): 149601. doi: 10.7498/aps.67.20172717
Liquid gallium and its alloy with low melting point, low toxic and high electrical conductivity are used extensively in burgeoning microfluidic and flexible electronic devices. The key to producing these devices is to effectively control the wettability and morphology of liquid metal on the solid interface in different manufacturing processes. Based on the Lennard-Jones (L-J) potential describing the solid-liquid interaction, the wettabilities of liquid gallium film on the smooth and rough graphene surfaces are effectively investigated by molecular dynamics simulation which is an available and powerful option in this field. Different regimes of wetting are discovered by changing the depth of the L-J potential, and the stable contact angle increases with Ga-C potential depth decreases. The results show that the equilibrium contact angle and the retraction velocity increase with the decrease of the L-J potential between the gallium and graphene, showing that some properties change from complete wetting to hydrophilic and to hydrophobic. The L-J potential depth obtained from the simulation results can be effectively employed to describe the interaction between the liquid gallium and the substrate because the resulting wetting angle is extremely close to the experimental value. When employing the most appropriate L-J potential, it is found that although the initial retraction velocity increases with the proportional decrease of the thickness of the liquid Ga film, there are a few of differences in equilibrium contact angle and final retraction velocity in virtue of the competition between the surface tension of the Ga film and Ga-C interaction. It means that for the wetting state the film thickness is not the crux for changing the equilibrium contact angle and retraction velocity based on a similar conversion of potential energy into kinetic energy. Finally, we investigate the effects of the L-J potential on three rough surfaces which are patterned into three types of nanopillars with different top morphologies respectively. Specifically, it is shown that in spite of similar surface roughness, the wetting morphologies of liquid gallium deposited on various nano-textured graphene surfaces range from hydrophobic to dewetting state, suggesting that not only the roughness but also the morphology of surface can exert an available influence on the wettability of liquid. The wetting transition between the wetting and dewetting state can be achieved dynamically by adjusting the morphologies of nanopillars involved although we still need to go into more detail on the configurable way to fulfill the changing requirements.