Vol. 67, No. 19 (2018)
Topological orders and quantum phase transitions in a one-dimensional extended quantum compass model
2018, 67 (19): 190301. doi: 10.7498/aps.67.20180855
By using the infinite time evolving block decimation in the presentation of infinite matrix product states, we study an extended quantum compass model (EQCM). This model does not only include extremely rich phase diagrams due to competitions of orbital degrees of freedom and anisotropic couplings between pseudospin-1/2 operators but also have the capacity to describe property of protected qubits for quantum computation which leads to lots of attentions paid to the phase boundaries of the EQCM. However, few attentions are paid to long-range topological string correlation order parameters of the EQCM. To study order parameters, one should understand spontaneous symmetry breaking which relates to Landau quantum phase transitions theory. Once spontaneous symmetry breaking happens, there should exist some local order which can be described by a local order parameter. This order parameter can be used to distinguish the phase from others. For continuous quantum phase transitions, in the critical regime, critical exponents can be extracted. Unfortunately, the long-range topological string correlation orders are beyond Landau quantum phase transitions theory, one can not directly use two paradigms of Landau-Ginzburg-Wilson. Usually, one can define a local order parameter by local magnetization. Naturally, one can also refer to this way to define the long-range topological string correlation order parameters by long-range topological string correlations on the following conditions, i.e. the quantum system undergoes a hidden spontaneous symmetry breaking; the long-range topological string correlation order parameter can be used to distinguish the phase from others; for continuous quantum phase transitions, the long-range topological string correlation order parameter satisfies scaling law when control parameter getting close to critical point. Based on above idea, in order to characterize the topological ordered phases and quantum phase transitions in the EQCM, even/odd long-range topological string correlations are introduced based on even/odd bonds. Hereafter, fidelity per lattice site, even/odd long-range topological string correlations, the saturation behavior of odd long-range topological string correlations and order parameters are calculated. The long-range topological string correlations show three distinguished behaviors which include decaying to zero, monotonic saturation and oscillatory saturation. By the above characterizations, oscillatory/monotonic odd long-range topological string correlation order parameter is derived. Then ground-state phase diagram of order parameters is computed which includes oscillatory/monotonic odd long-range topological string correlation order phase and antiferromagnetic phase. In the critical regime, critical exponent β=1/8 extracted from monotonic odd long-range topological string correlation order parameter and local magnetization shows the phase transition belongs to Ising universality. In addition, the phase transition points, the order of the phase transitions of fidelity show consistent with the results of order parameters.
2018, 67 (19): 190302. doi: 10.7498/aps.67.20181121
The tilt angle of a cold atom gravimeter (CAG) could have a significant influence on the measurement of absolute gravity. The measurement, manipulation, and compensation of the tilt for CAG need to be conducted in order to obtain a high-accuracy absolute gravity measurement. In this paper, firstly, the influences of tilt on absolute gravity measurement under four different conditions are analyzed theoretically by taking into account the position of vacuum system relative to Raman retro-reflection mirror. Then, the experimental investigation is carried out and it is found that the measured results agree well with the theoretical prediction curves. According to the analysis above, we design a scheme for absolute gravity measurement based on two inclinometers, mainly to solve the problem of long-term tilt drift of CAG especially in harsh measurement environment. In this scheme, a high-resolution inclinometer is used to record the tilt angle of Raman retro-reflection mirror, which is fixed on a passive vibration isolation platform. Besides, another inclinometer is utilized to monitor the tilt angle of vacuum chamber of the CAG. By doing so, the vibration noise can be suppressed and the tilt data can be measured with a high precision. Finally, the experimental verification of this proposal is carried out based on our homemade compact cold atom gravimeter, and the high accuracy absolute gravity measurement is realized in a complex workshop environment. Since the vibration noise of Raman mirror is improved by using the vibration isolation platform, the sensitivity of our CAG can reach 319 μGal √Hz. Besides, we measure the long-term changes of gravity with time and find that the experimental results are consistent with the curves calculated by theoretical tidal model. Moreover, due to the precise measurement and compensation for the tilt drift, the accuracy of our CAG is estimated at 12.3 μGal. In order to evaluate this system accuracy, a comparison between our CAG and the FG5 at the same measured site is made. The absolute gravity values determined by both gravimeters coincide with each other. In this paper, we provide a feasible scheme for measuring the absolute gravity in the complex environment. The experimental demonstration of this measurement scheme is performed thereby acquiring some valuable reference data for the practical use of CAG.
2018, 67 (19): 190601. doi: 10.7498/aps.67.20180842
With the development of science and technology, the super high accurate time comparison techniques with several ten picoseconds or higher accuracy are required in many advanced and basic fields. The atomic clock system in the space station has better performance than that on the ground, but the traditional common-view time comparison method cannot be applied to the space station because there are some limitations. At first, the space station common-view time comparison principle aiming at several ten picoseconds accuracy is analyzed, and the sources of delay larger than 1 picosecond are considered. According to the space station common-view time comparison principle, the visibility of the space station is simulated based on several main geographical cities in China. The analysis results show that the time interval is short for ground station to observe the space station, and the common-view time interval is shorter. A more serious problem is shown that some areas cannot receive the signal send by the space station simultaneously, so the traditional common-view time comparison method is invalid when the ground stations are in these areas. Then the effect of space station orbit error is studied in theory and simulation based on the traditional method. The research result shows that the orbit error cannot be cancelled effectively by the traditional method, and the remnant orbit error is on the order of about several hundred picoseconds. These remnant orbit errors have a direct influence on the time comparison. A new asynchronous common-view time comparison method is proposed, and its principle and advantages are introduced. The geometric expression that describes the position relationship between the space station and two ground stations is proposed to find the observation time when the orbit errors can cancel completely. And the high stability of the space atomic clock and the ground atomic clock are advantaged to model and extrapolate the space-ground clock bias. The geometric position relationship and the modeling and extrapolating of the space-ground clock bias are combined together to solve the problems of time comparison accuracy and common-view blind area, because the optimized method does not require that two ground stations observe the space station simultaneously. Finally, the simulation experiments are done to validate the new method. The experimental result shows that the asynchronous common-view time comparison method is valid to realize the time comparison with the accuracy of several ten picoseconds. And it also shows that the new method is helpful in solving the problem of blind area that exists in traditional space station common-view time comparison method.
Influence of interface structure on nanoindentation behavior of Cu/Ni multilayer film: Atomic scale simulation
2018, 67 (19): 190202. doi: 10.7498/aps.67.20180958
The mechanical properties of metal multilayers change significantly when the modulation period decreases to a nanoscale. As is well known, the lattice misfit between Ni and Cu is~2.7%, it means that the coherent and semi-coherent interfaces can form between the Ni and Cu atomic layer. Hetero-twin interface Cu/Ni multilayer film with a modulation period of several nanometers and grown along the direction is realized experimentally, and the mechanical properties change significantly due to the effect of interfaces. In this study, molecular dynamics simulations on Cu/Ni multilayers with coherent, coherent twin, semi-coherent, and semi-coherent twin interfaces under nanoindentation are carried out to study the deformation evolutions of different interfaces and the interactions between dislocation and interfaces. Furthermore, the influence of Cu/Ni interface on the mechanical property is investigated. The simulation results show that the different interface structures exhibit different strengthening and/or softening mechanisms at different indentation depths. The hardness values of the Cu/Ni multilayer films with four different interface structures are different, and the hardness of the coherent interface is larger than the semi-coherent interface's. The hardness values of the four interface structures reside between the pure Cu and pure Ni. For the coherent twin interface, with the increase of the modulation ratio, the strengthening effect of the twin interface is enhanced. The softening effect for the coherent interface is mainly attributed to the generation of parallel dislocations and their proliferation. While for the semi-coherent interface, the mismatched networks are formed at the Cu/Ni interfaces, the softening effect on the movable dislocation is mainly the repulsion of the mismatched network, while the strengthening effect on the movable dislocation is the hindrance of the mismatched dislocation network. The strengthening of the coherent twin interface is attributed to the limited effect of twin interface on the movable dislocation within the monolayer. Unlike the coherent twin interface, the strengthening effect of the semi-coherent twin interface is mainly due to the mutual repulsion between the arched dislocation, which is generated within the twin interface, and the mismatched network. Furthermore, the pinning effect of misfit dislocation network will impede the migration of twin interfaces and will also enhance the mechanical property of Cu/Ni multilayer film.
2018, 67 (19): 190501. doi: 10.7498/aps.67.20181066
Biomolecular motors are a big family of protein, and play a very important role in transporting the organelles within cells. They can also convert chemical energy into mechanical energy. In order to study the dynamic mechanism of molecular motors in depth, a great many of Brownian ratchet models such as double-temperature ratchet, feedback control ratchet, and hand-over-hand ratchet have been proposed. By investigating different kinds of ratchets, it is better to comprehend the directed transport of Brownian particles and obtain an insight into the transport process in biomedicine. Especially, the investigation of Brownian ratchets can also be used for improving the accurate drug delivery and effectively utilizing the medicine.Until now, the directed transport of ratchet has aoused the interest of researchers. It is found that a certain driving phase can lead to the current reversal of the underdamped ratchets in theory. A large number of experiments have shown that most of the biomolecular motors in cells are enzyme protein macromolecules and they can carry the “cargos” to implement the directed transport. Interestingly, molecular motors have high efficiency usually, and some of them can even reach an efficiency close to 100% in experiment. Nevertheless, it is found that the energy conversion of Brownian motors is low as indicated by calculating the rate between the effective work of particles and the input energy of ratchets. According to a comparison between the experimental results and theoretical analyses, it is well known that the efficiency of ratchets is still far from the actual motor efficiency measured experimentally. Therefore, how to increase the efficiency of molecular motor which is pulled by loads is still a very important research topic. Owing to the fact that the molecular motors are influenced by the cellular environment during the hydrolysis of ATP in the organism, the catalytic cycles of the coupled motor proteins are out of phase. This gives us an inspiration for establishing the corresponding feedback pulsing ratchet.Due to the effect of the feedback pulse on coupled ratchets, the directed transport character of pulsing ratchets when they drag loads is explored in the present work. And the directed transport, diffusion and energy conversion efficiency of coupled particles are discussed systematically. It can be observed that the directed transport of the feedback pulsing ratchets would be futher facilitated by adjusting suitable free length and coupling strength. Meanwhile, the energy conversion efficiency of coupled particles can obtain a maximum value under a certain free length and coupling strength. In particular, there is the current reversal in an evolutive cycle under a certain pulse. Moreover, the diffusion of coupled particles can be suppressed effectively by modulating the pulsing phase, thus the corresponding directed transport of pulsing ratchets can be facilitated. In addition, the energy conversion of feedback ratchets can also be improved if the load is appropriate. The current reserval obtained in this paper can be applied to the particle separation. On the other hand, these results provide some great experimental inspirations in the aspect of medical delivery.
2018, 67 (19): 190201. doi: 10.7498/aps.67.20180534
When pedestrian and vehicle are in conflict, they will pass at a certain probability after they have made a simple judgment respectively. According to the actual situation of the conflict between pedestrian and vehicle, the concept of basic payoff, conflict loss, waiting loss and mutual avoiding loss are put forward. A game matrix of the conflict between pedestrian and vehicle is consequently established. Then the evolutionary analysis paradigm is introduced, and the dynamic model of the conflict evolution between pedestrian and vehicle is established. After that, the position and stability of the equilibrium point and the evolution mechanism of the system in different traffic situations are analyzed in detail. It is found that the relative size between conflict loss and waiting loss of pedestrian and vehicle are different, corresponding to different evolution directions of the system. The possible evolutionary directions include “vehicles first”, “pedestrians first”, “neither vehicles nor pedestrians goes first”, “vehicles and pedestrians do not yield to each other”. In addition, in this paper, we define the traffic concept of opportunity loss, and analyze the sensitivity of the system to the mutual avoiding loss and the opportunity loss of pedestrian and vehicle. It is found that the increasing of the mutually avoiding loss of pedestrian or vehicle has a positive effect on improving the probability of each passing conflict zone, but it has a negative effect on reducing the probability of each passing conflict zone. On the other hand, the effect of opportunity loss is just the opposite to the mutual avoiding loss. The dynamic model established in this paper can provide a theoretical basis for the macro control of the conflict evolution direction between pedestrian and vehicle. For instance, the current conflict situation between pedestrian and vehicle in a city is “vehicles first”. For promoting the traffic civilization, the transportation officials hope to change the current conflict situation to realize the “pedestrians first”. According to the model established in this paper, some parameters of the game matrix on the conflict between pedestrian and vehicle can be changed by formulating relevant highway traffic regulations to adjust the evolution direction of the conflict between pedestrian and vehicle.
Network structure optimization algorithm for information propagation considering edge clustering and diffusion characteristics
2018, 67 (19): 190502. doi: 10.7498/aps.67.20180395
Optimizing network structure to promote information propagation has been a key issue in the research field of complex network, and both clustering and diffusion characteristics of edges in a network play a very important role in information propagation. K-truss decomposition is an algorithm for identifying the most influential nodes in the network. We find that K-truss decomposition only considers edge clustering characteristics, without considering the diffusion characteristics, so it is easily affected by the local clustering structure in the network, such as core-like groups. There are mutually closely connected the core-like groups in the network, but the correlation between the core-like groups and the other parts of the network is less, so the information is easy to spread in the core-like groups, but not in the other parts of the network, nor over the whole network. For the reason, we propose an index to measure the edge diffusion characteristics in a network, and it is found that the diffusion characteristics of some edges in the periphery of the network are relatively high, but the clustering characteristics of these edges are relatively low, so they are not beneficial for rapid information propagation. In this paper, by considering the relationship between the clustering characteristics and diffusion characteristics of the edges, we propose a novel network structure optimization algorithm for information propagation. By measuring the comprehensive ability strength of the clustering characteristics and the diffusion characteristics of the edges, we can filter out the edges whose comprehensive ability is poor in the network, then determine whether the edges should be optimized according to the relative relationship between the clustering characteristics and the diffusion characteristics of the edges. To prove the effectiveness of the proposed algorithm, it is carried out to optimize the structures of four real networks, and verify the effective range of information propagation before and after the optimization of network structure from the classical independent cascade model. The results show that the network topology optimized by the proposed algorithm can effectively increase the range of information propagation. Moreover, the number of leaf nodes in the optimized network is reduced, and the clustering coefficient and the average path length are also reduced.
2018, 67 (19): 192101. doi: 10.7498/aps.67.20180666
Due to lack of experimental data of the inner shell ionization cross sections induced by low-energy positron, advanced theoretical models developed in recent years cannot be correctly evaluated, and the application of slow positron beam technique is greatly limited. Here we present the method of obtaining reliable experiment data of atomic inner-shell ionization cross section by positron impact. In this work, the slow positron beam device is used to generate 8-9.5 keV positron beams impacting on a pure thick Ti target, and the silicon drift detector (SDD) is adopted to collect the X-ray spectra produced by positrons impacting on thick Ti target, and the incident positron numbers are obtained by applying an HPGe detector to on-line collect annihilation photons. Then the experimental characteristic X-ray yields of Ti K shell impacted by 8-9.5 keV positron could be acquired. Meantime, the simulated characteristic X-ray yields are acquired by the PENELOPE program simulating the experiments. In the comparison between the experimental yields and the simulated yields based on two sets of different inner shell ionization cross section database in the PENELOPE code, i.e. the optical data model (ODM) and the distorted-wave Born approximation model (DWBA), there is a large difference between the simulated data from the ODM theoretical model and the experimental values, while the simulated yields from the DWBA theoretical model are in good agreement with the experimental results. Accordingly, a correction factor is introduced to modify the DWBA theoretical model database which is used in the PENELOPE, and then the experimental process is re-simulated. When the simulated yields and the experimental yields are in the highest consistence, the reliable Ti K shell ionization cross sections impacted by 8-9.5 keV positron could be obtained. The biggest advantage of using this method to obtain atomic inner-shell ionization cross section impacted by positron is that the effects of the multiple scattering of incident positrons in the thick target, from the bremsstrahlung and annihilation photons, and other secondary particles on the experimental characteristic X-rays do not need calculating (the calculation method that has been developed previously cannot give the more correct result about the contribution of the multiple scattering of incident positrons, from the bremsstrahlung and annihilation photons, and other secondary particles to characteristic X-rays).
2018, 67 (19): 192801. doi: 10.7498/aps.67.20181204
Nd2Zr2O7 pyrochlore with higher physicochemical and radiation stability has been considered as a host matrix for actinide immobilization of high level radioactive wastes. Uranium is a constituent and the decay-daughter product of high level radioactive wastes. It is necessary to study the solubility and ion-irradiation effect of uranium in Nd2Zr2O7 pyrochlore. The solubility of U is studied by the A site substitution in the pyrochlore structure. A series of uranium-doped zirconate pyrochlore compositions is prepared by the sol-gel-spray pyrolysis-high temperature sintering method. The structures of immobilization are studied by using X-ray diffraction (XRD) and Raman spectroscopy. The XRD and Raman spectroscopy studies reveal that the solubility limit of uranium in Nd2Zr2O7 is estimated at 10 at%. The lattice parameter of pyrochlore decreases with U content increasing, which is due to lower ionic radius of U. The immobilization structure changes from order pyrochlore to disorder structure. Further addition of U content leads to the separation of U3O8 phase in the immobilization. The U ions with high valance may be substituted at A or B site in Nd2Zr2O7 pyrochlore, which results in the A–O and B–O bond destruction. In order to keep the balance of charge, extra O ions should enter into the vacancy site, the structure of pyrochlore maybe transforms into a disorder structure. The radiation resistance of immobilization is investigated by ion-beam irradiation with 2 MeV Kr15+ ions at room temperature. The Nd2Zr2O7 and Nd1.9U0.1Zr2O7 are irradiated at doses of 1 dpa and 3 dpa, respectively. Analyses of the XRD and Raman spectroscopy data show that the Nd2Zr2O7 pyrochlore remains full pyrochlore structure even at a higher irradiation dose, which suggests that the Nd2Zr2O7 exhibits higher radiation resistance as potential immobilization. In contrast, the Nd1.9U0.1Zr2O7 immobilization shows the weaker radiation resistance, the pyrochlore structure completely transforms into a disorder fluorite structure. The A–O and B–O bonds of Nd1.9U0.1Zr2O7 pyrochlore structure are easy to destroy under ion irradiation conditions due to the disorder of pyrochlore. At the same time, the excess O ions are rearranged in U-rich pyrochlore after irradiation. Bond destruction and ion rearrangement of pyrochlore structure result in the full disorder fluorite structure. The actinides-doped pyrochlore structure is modified due to the change in physicochemical propertyof actinide, which results in the reductionof the solubility limit and radiation resistance.
2018, 67 (19): 194301. doi: 10.7498/aps.67.20180963
Acoustic metamateiral (AM) is an artificially structured material with the unique properties that cannot be found in nature materials, such as negative refraction, slab focusing, super-resolution imaging, cloaking, inverse Doppler effect, etc. In this paper we first review the research advances in AM in recent 20 years and then mainly discuss the properties of the meta-atom AM (MAAM), meta-molecule AM (MMAM), meta-atom cluster AM, and meta-molecule cluster AM. The MAAM consists of local resonant meta-atoms, whose resonant frequency is related to the geometry size of the structure. The MAAM presents the transmission dip and inversed phase near the resonant frequency. The meta-atoms discussed in the paper contain the split hollow sphere and hollow tube (HT), which can be used to realize the AM with single negative modulus and AM with single negative mass density near the frequency, respectively. The effective parameter of the MAAM is calculated from the transmission and reflection data in experiment according to the homogeneous-medium theory. By combining the two kinds of meta-atoms together, the assembled two-layered composite AM presents a transmission peak similar to the electromagnetic metamaterial in the overlapping resonant frequency region. The effective parameters calculated by experimental data demonstrate that the composite AM could realize simultaneously negative modulus and negative mass density near the peak frequency. In the double-negative band, this kind of double-negative AM can faithfully distinguish the acoustic sub-wavelength details (/7). Furthermore, by coupling the two kinds of meta-atoms in a structure, we design a flute-like meta-molecule structure of perforated hollow tube, which can be used to fabricate double-negative AM in high or low frequency band. The experimental results also show that the double-negative AM has the properties of flat focusing and negative refraction effect. Based on the weak interaction of the meta-atoms, the meta-atom cluster AM can be fabricated by arraying different sized meta-atoms. The meta-atom cluster AM composed of different sized meta-atoms of SHSs can realize multi-band or broadband negative modulus, and the different sized meta-atoms of HTs can realize broadband negative mass density. Similarly, the meta-molecule cluster AMs are constructed with seven kinds of flute-like perforated hollow tubes, which can overcome the limitations of arbitrary broadband negative bulk modulus and mass density to provide a region of inverse Doppler effects. It is also shown that the inverse frequency shift values will be enhanced with the increase of frequency. As the resonant unit can realize the effect of discontinuous phase, it can be used to design acoustic metasurface (AMS) to control the acoustic wavefronts at will and realize the anomalous manipulation of acoustic waves. Finally, we introduce the research status and tendency of AMS in coming years.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2018, 67 (19): 197101. doi: 10.7498/aps.67.20180956
Ti-based alloys are widely used in aerospace and medical engineering because of their excellent properties, such as good fracture toughness, high strength, good corrosion resistance, etc. However, the corrosion resistance performance of the alloys is not adequate to meet the requirements in many cases. The Ti-Cr-Nb ternary alloy system exhibits many excellent characteristics, especially the anti-corrosion ability, making it a very promising candidate for the applications in aerospace and medical engineering. The alloying element Cr can improve the corrosion resistance of Ti-based alloys as reported by many experiments. In order to understand and then predict the effect of Cr content on Ti-Nb-Cr alloy, the electronic structures, such as the cohesive energies, the formation energies, the Fermi levels and the densities of states (DOSs) of the Ti-Nb-Cr alloys with different Cr content of the alloys, are calculated by first-principles method. The calculations in this paper are carried out by VASP (Vienna ab-initio simulation package) software package, which is based on the density functional theory. The generalized gradient approximation is selected to deal with the exchange correlation energy of electrons. And the special k-point sample of the Monkhorst-Pack type is used in the Brillouin-zone integration. The effects of Cr content on the electronic stability and corrosion resistance of the alloy are discussed. In this paper, the Ti-25 at.%Nb alloy with the stable β-phase is a matrix material, and Ti12Nb4 supercell model is adopted, in which 1 to 4 Ti atoms are replaced by the Cr atoms, respectively. In energetics, the sequence of the cohesive capacity of the system is as follows:Ti12Nb4 11Nb4Cr1 10Nb4Cr2 9Nb4Cr3 8Nb4Cr4, showing that the stability of the structure decreases with Cr content increasing. While the formation energy of the system energy shows a gradual increase trend with the increase of Cr, indicating that the formation of the system becomes gradually difficult when adding more Cr atoms. The Fermi level of the ternary alloy system containing Cr element is much lower than that of Ti12Nb4 alloy and tends to decrease slightly with the increase of Cr content. That means that with increasing the Cr content, the alloy system is not easy to lose electrons, and thus the corrosion resistance is improved. And when the Cr content is around 18.75 at.%, there should be an optimal Cr concentration for corrosion resistance. The differential charge density diagrams show that with the increase of Cr content, the covalent bonding of the system is weakened, while the metal bonding is strengthened, which makes the electronic structure of the system more stable and thus the corrosion resistance is improved. The DOS shows that the Fermi level is not zero, indicating the metallic behavior of the alloy. With the increase of Cr content in the alloy system, the pseudo-energy gap gradually disappears, indicating that the structural stability of the system decreases accordingly, which is consistent with the calculation result of the density of states. The maximum value of the DOS diagram is shifted toward the lower energy level area, showing that the stability of the electronic structure of the system is improved so that the corrosion resistance of the alloy is enhanced. And the maximum value of the DOS also shows that when the Cr content is around 18.75 at.%, there is an optimal Cr concentration for corrosion resistance.
2018, 67 (19): 197502. doi: 10.7498/aps.67.20181005
There are various nonlinear solutions in the anisotropic Heisenberg spin chain model (AHSCM), such as soliton solutions. In consideration of high-order nonlinear terms, a good modified nonlinear analytical solution can be obtained under reasonable simplification conditions. The purpose of this paper is to find the nonlinear solutions other than soliton of AHSCM. We use Holstein-Primakoff representation to study the AHSCM. Under the semi-classical approximation, considering the high order nonlinear term and the periodic boundary condition, an improved nonlinear Schrodinger equation and its wave solutions of the hyper-elliptic function expressed by the combination of the inverse function of Jacobi elliptic function are obtained through using the coherent state. These solutions can be expressed by the combination of the inverse functions of the first kind of elliptic functions. In the limit case, these solutions are reduced to wave solutions of sinusoidal (or cosine) functions, or wave solutions that can be represented by hyperbolic tangent functions. The energy levels of these nonlinear solutions can be obtained theoretically by the normalized conditions, but even by using hyper-elliptic functions, it is difficult to express them as analytic expressions.
Coherent terahertz radiation via ultrafast manipulation of spin currents in ferromagnetic heterostructures
2018, 67 (19): 197202. doi: 10.7498/aps.67.20181178
The development of efficient terahertz (THz) radiation sources is driven by the scientific and technological applications. To date, as far as the radiation of THz pulses is concerned, the widely used methods are biased semiconductor, electro-optical crystal and air plasma, which are excited separately by femtosecond laser pulses. The mechanisms involved in these THz sources are photo-carrier acceleration, second order nonlinear effect, and plasma oscillations, respectively. Here, we report the generation of coherent THz radiation in the designed ferromagnetic/non-magnetic metallic W/CoFeB/Pt and Ta/CoFeB/Pt trilayers on SiO2 substrates, excited separately by ultrafast laser pulses. The transient THz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the sample. We present the THz radiation results of the bilayers, CoFeB/W, CoFeB/Pt and CoFeB/Ta, which are comprised of the trilayer heterostructures used in our experiments. We find that all experimental results are in good agreement with the results from the inversed spin-Hall effect (ISHE) mechanism. Owing to the ISHE, the transient spin current converts into a transient transverse charge current, which launches the THz electromagnetic wave. In our experiments, W or Ta has an opposite spin Hall angle to Pt. Therefore, the amplitude of the THz emission can be increased by a constructive superposition of two charge currents in metallic layers. Our results indicate that the peak-values of the THz radiation covering the 0-2.5 THz range from W/CoFeB/Pt and Ta/CoFeB/Pt are stronger than that from 0.5 mm thick ZnTe (110) crystal, under very similar excitation conditions. Finally, we investigate the dependence of peak-to-peak values for two different heterostructures on the pump fluence. The saturations of THz pulse at pump fluences of~0.47 mJ/cm2 and~0.61 mJ/cm2 are found for W/CoFeB/Pt and Ta/CoFeB/Pt heterostructures, respectively. The saturation can be generally attributed to the spin accumulation effect and laser-induced thermal effect. Our results indicate that the spin accumulation effect, by which the density of spin-polarized electrons is restricted in a non-magnetic metallic layer, is slightly less pronounced for Ta/CoFeB/Pt system at high fluences. Our findings provide a new pathway for fabricating the spintronic THz emitter, which is comparable to the conventional nonlinear optical crystals.
Refractive index sensor and filter of metal-insulator-metal waveguide based on ring resonator embedded by cross structure
2018, 67 (19): 197301. doi: 10.7498/aps.67.20180758
Continuous improvement in nanofabrication and nano-characterization capabilities have changed projections about the role that metals could play in developing the new optical devices. Surface plasmon polaritons are evanescent waves that propagate along a metal-dielectric interface. They can be laterally confined below the diffraction limit by using subwavelength metal structures, rendering them attractive to the development of miniaturized optical devices. A surface plasmon polariton refractive index sensor and filter which consist of two metal-insulator-metal (MIM) waveguides coupled to each other by a ring resonator embedded by cross structure are proposed. And the transmission characteristics of surface plasmon polaritons are studied in our proposed structure. The transmission properties of such a structure are simulated by the finite element method, and the eigenvalue wavelengths of the ring resonator are calculated theoretically. The sensing characteristics of such a structure are systematically analyzed by investigating the transmission spectrum. The results show that there are three resonance peaks in the transmission spectrum, that is, three resonance modes corresponding to the eigenvalue solutions of the first, second and third-order Bessel eigen-function equations, and each of which has a linear relationship with the refractive index of the material under sensing. Through the optimization of structural parameters, we achieve a theoretical value of the refractive index sensitivity (S) as high as 1500 nm/RIU, and the corresponding sensing resolution is 1.3310-4 RIU. More importantly, it is sensitive to none of the parameters of our proposed structure, which means that the sensitivity of the sensor is immune to the fabrication deviation. In addition, by the resonant theory of ring resonator, we find a linear relationship between the resonance wavelength and the radius of ring resonator. So the resonance wavelength can be easily manipulated by adjusting the radius and refractive index. In addition, the positions of transmission peaks can be easily modulated by changing the radius of the ring, which can be used to design band-pass filter for a large wavelength range. Moreover, the transmission intensity and the transmission bandwidth decrease as spacing distance between the MIM waveguide and ring cavity increases. These results would be helpful in designing the refractive index sensor of high-sensitivity and band-pass filters, and have guiding significance for biological sensor applications.
2018, 67 (19): 197201. doi: 10.7498/aps.67.20180382
Poly(3, 4-ethylenedioxythiophene) (PEDOT) has applications in many areas due to its exciting electrical performance and high stability. Since it has very low thermal conductivity, it is also a good organic thermoelectric material. However, the ZT value of pure PEDOT is rather low, because the electrical properties such as conductivity are still not satisfactory. It is found that the thermoelectric performance can be enhanced by adding inorganic thermoelectric materials into PEDOT to form composites. In this paper, we synthesize a composite of In2O3/PEDOT by chemical oxidation. Microstructure of the composite is studied by X-ray diffraction, infrared spectroscopy, transmission electron microscope, and positron annihilation spectroscopy. The XRD measurements show that the pure PEDOT sample is amorphous, and the crystallinity in composite sample is contributed by In2O3. Besides, the diffraction peaks become sharper with increasing the In2O3 content. Transmission electron microscope measurements confirm that the PEDOT sample is amorphous and the shapes of In2O3 particles are regular. The surfaces of the In2O3 particles are wholly coated with thin layers of PEDOT, and when the In2O3 content is higher than 22 wt%, the In2O3 particles cannot be uniformly dispersed in pure PEDOT layers. The positron annihilation measurements reveal the interface structure in the In2O3/PEDOT composite, which can capture positron and cause the lifetime of positron to increase. The relative quantity of interface increases with In2O3 content increasing. However, when the In2O3 content is more than 22 wt%, the interface structure is destroyed. All the measurements show that when the In2O3 content is lower than 22 wt%, the In2O3 nanoparticles are well dispersed in PEDOT. The electrical conductivity of In2O3/PEDOT composite increases with In2O3 content increasing. At room temperature, the electrical conductivity of PEDOT is 7.5 S/m, while in the In2O3/PEDOT sample with 12.3 wt% In2O3, a maximum electrical conductivity of 25.75 S/m is obtained. When the In2O3 content increases from 0 to 22 wt%, the power factor of the composite increases rapidly from 14.5×10-4 to 68.8×10-4 μW/m·K2. On the contrary, the thermal conductivity shows decrease compared with the thermal conductivity of pure PEDOT. The ZT value of the composite increases from 0.015×10-4 to 0.073×10-4. Our results indicate that the thermoelectric properties of In2O3/PEDOT composite can be effectively improved compared with those of the pure PEDOT
Magnetic property of CeFe2-xInx alloys and critical parameters of magnetic phase transition of CeFe1.95In0.05 alloy
2018, 67 (19): 197501. doi: 10.7498/aps.67.20180815
Magnetic properties of CeFe2-xInx alloys and scaling critical behaviors of CeFe1.95In0.05 alloy are investigated by measuring the magnetic susceptibility and isothermal magneteization. The X-ray diffraction (XRD) patterns show that the solid solubility of the In substituted for the Fe in CeFe2-xInx alloy is limited. Because the intensity diffraction peak of impurity at 2=30.75 and 35.80 in CeFe1.95In0.05 XRD pattern are very low, the effect of impurity on magnetism is not considered in this paper. Magnetic measurements indicate that using 2.5 at.% indium to substitute for Fe in CeFe2 alloy can strengthen the orbital hybridization interaction between Ce-4f and Fe-3d, but it cannot reach the critical point to make the antiferromagnetic stable. The AFM fluctuation still keeps in a value ranging from 2 K to 80 K. The second order paramagnetic-ferromagnetic transition of CeFe1.95In0.05 at TC=230 K is confirmed by Arrott plot analysis. The effective ferromagnetic moment of Fe atoms can be increased by replacing part of the Fe atoms with In atoms in the CeFe2 alloy, which can increase the paramagnetic and effective magnetic moment and the magnetic saturation magnetic moment of the alloy. For a magnetic field change of 0-50 kOe, the maximum value of the magnetic entropy change-△ SM is 3.13 J/(kgK) at 230 K and RCP is 151.3 J/kg, which are higher than the values of Ce0.95Gd0.05Fe2, Ce0.9Gd0.1Fe2, and Ce0.9Ho0.1Fe2 alloys under the same magnetic field. The high self-consistent scaling critical exponents determined by modified Arrott plot and Kouvel-Fisher methods are[=0.3212(8) and =0.9357(9)] and[=0.3304(1) and =0.9249(1)], respectively. The parameter obtained from the critical magnetization isotherm MTC=DH1/ satisfies the Widom scaling relation =1+/. Moreover, the plot of M1/ vs. (H/M) 1/ constructed by the above critical parameters completely complies with the scaling hypothesis. At the same time, the critical parameters of n and obtained by|△ SM| Hn and RCP H(1 + 1/) fitting are 0.6191(8) and 5.0559(1), respectively. In all, non-local effect of spin interaction causes a certain difference between the critical parameters and 3D-Ising model standard values (=0.325, =1.241, n=0.569, and =4.818). But these differences are small, especially for critical parameter , which suggests that the magnetic interaction in CeFe1.95In0.05 alloy is a short-range interaction.
Comparison between axial residual stresses measured by Raman spectroscopy and X-ray diffraction in SiC fiber reinforced titanium matrix composite
2018, 67 (19): 197203. doi: 10.7498/aps.67.20181157
Accurate measurement and analysis of residual stress state in the SiCf/Ti composites are crucial to optimizing their fabrication process and to understanding their failure mode, but they are still a challenge. In this work, SiCf/C/Ti17 composites with~48% fiber volume fraction, consisting of W-core SiC fibers (~100 m in diameter), turbostratic C coating (~2.5 m in thickness) and Ti17 matrix, are prepared by consolidating precursor wires fabricated by matrix-coated fiber method through hot isostatic pressing at 920℃/120 MPa/2 h; these samples are used for measuring their stresses. It is noted that turbostratic C coating, a necessary diffusion barrier layer between SiC fiber and Ti17 alloy matrix, bears the residual stress caused by the mismatch of thermal expansion coefficients between fiber and matrix during consolidation. It is found that the graphene planes are almost parallel to the axial direction of SiC fibers in the turbostratic C coating revealed by high magnification transmission electron microscope, and thus G peak position of C coating would be sensitive to stress state. Accordingly, micro-Raman spectroscopy is first used to measure the G peak positions of C coating under stress and stress-free state in the SiCf/C/Ti17 composite, respectively. Based on the position shift of G band caused by residual stress, the axial residual compressive stress of SiC fiber in SiCf/C/Ti17 composite is calculated to be~705.0 MPa. For comparison, X-ray diffraction method is also adopted to measure the interplanar spacing values of the Ti17 alloy matrix in different directions to obtain the spatial strains. During measurement, -Ti (213) high-angle diffraction peak is chosen to reduce test error, and then the different interplanar spacing values of -Ti (213) are obtained by varying the values of in three different directions at =0, 45 and 90. As three-axis-stress model is employed, the residual tensile stress of Ti17 alloy matrix in the axial direction of SiCf/C/Ti17 composite is~701.3 MPa, which is transformed through linear elastic theory into the residual compressive stress of SiC fiber of~759.4 MPa. The similar results confirm that it is reliable to characterize the residual stress in the SiCf/C/Ti17 composite with high-texture turbostratic carbon by both the Raman spectroscopy and the X-ray diffraction method.
2018, 67 (19): 197302. doi: 10.7498/aps.67.20180650
The Raman signal of adsorbed Raman probe molecule can be significantly enhanced by using metallic nanostructures with high-density hot spots as surface enhanced Raman scattering (SERS) substrates. A great effort has been devoted to the improving of the SERS detection sensitivity and reproducibility by preparing ordered metal nanostructure arrays with controlled particle size, shape and hot spot position, which are used as SERS substrates. In this paper, we prepare high-density Ag nanoparticle arrays by electrochemical deposition in anodic aluminum oxide (AAO) templates. The particle size and the nanogap between the adjacent particles can be adjusted by changing the deposition time. The structures and surface plasmons of Ag nanoparticle arrays are characterized by scanning electron microscopy and reflectance spectra. The size of the gap between the particles significantly affects the plasmon resonance and the plasmon coupling between the particles. The SERS properties of Ag nanoparticle arrays are investigated by using 1, 4-benzenedithiol (1, 4-BDT) as Raman probe molecule. The Ag nanoparticle arrays with high SERS detection sensitivity and high reproducibility (uniformity) are prepared by optimizing the deposition time (the nanogap between the adjacent particles), and the detection limit of the 1, 4-BDT can reach 10-13 mol/L. The relative standard deviation of the SERS signal intensity randomly measured from 20 spots on the Ag nanoparticle array substrate is 5.35%. The finite-difference time domain simulations confirm that the plasmon coupling between nanoparticles is strong, and that the coupling between the nanoparticles will increase as the nanogap decreases. Additionally, the local field is enhanced at the bottom of the nanoparticle and the gap between the Ag nanoparticle and the AAO template is larger. These results show that Ag nanoparticle array can be used as a high-efficiency SERS substrate.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2018, 67 (19): 198501. doi: 10.7498/aps.67.20181208
GaN plays an important role in compound semiconductor, which exhibits excellent electrical properties such as wide band gap (3.4 eV), high breakdown field strength (3.3 MV/cm), and high electron mobility (600 cm2/(Vs)). AlGaN/GaN heterojunction produces two-dimensional electron gas (2DEG) with high density (11013 cm-2) and high electron mobility (2000 cm2/(Vs)) which are caused by strong piezoelectric and spontaneous polarization. The Si-based AlGaN/GaN devices emerge as a promising candidate for the nextgeneration switching application in power system due to 2DEG of AlGaN/GaN heterojunction. Turn-on and breakdown voltage are key parameters for diodes and they have a tradeoff between each other. These two parameters affect diode loss and power handling capability. For better properties, we propose a novel p-GaN hybrid anode AlGaN/GaN diode with high-resistance-cap-layer (HRCL) to optimize turn-on voltage and breakdown characteristics. Based on the p-GaN/AlGaN/GaN material structure, an HRCL is fabricated in the channel region by self-aligned hydrogen plasma treatment to improve the breakdown voltage. Hydrogen plasma is adopted to compensate for holes in the p-GaN to release electrons from the 2DEG channel, forming a high-resistivity area. The transmission line method tests the material after passivation, showing that its sheet resistance is 570 /□ and a contact resistance is 0.7 mm. In the HRCL p-GaN diode, negative charges can appear at the interface of HR-GaN/AlGaN due to polarization effect, which increases the vertical electric field in AlGaN and reduces the lateral electric field near the cathode in the p-GaN, compared with in the p-GaN diode without HRCL. The p-GaN in the anode region is reserved to regulate the turn-on voltage by depleting the underlying 2DEG. The p-GaN structure raises conduction band beyond the Fermi level, ensuring the reduction of 2DEG. The fabricated HRCL p-GaN diode exhibits a high breakdown voltage over 1000 V at Ileakage=110-4 A/mm with a cathode-anode distance Lac of 10 m and a turn-on voltage of +1.2 V when forward current is 1 mA/mm. These results indicate that the introduction of p-GaN hybrid anode and HRCL can enhance the electrical properties of AlGaN/GaN diode effectively. However, little attention has been paid to doping concentration in p-GaN. Study of the regulation of Mg2+ doping concentration on the turn-on voltage in p-GaN will be investigated in future to achieve a low forward turn-on voltage of the p-GaN HRCL diode.
2018, 67 (19): 198101. doi: 10.7498/aps.67.20181053
To enhance backscattering, corner reflector and Luneburg lens are usually used. They can operate effectively in a broad angle range and also in a quite wide band. However, corner reflector as a typical structure of backscattering enhancement device, has obvious disadvantages in practical application. For example, it is usually made of metal material, which causes it to be too heavy and bulky. Luneburg lens is generally made of dielectric with strong loss and high cost, which is unfavorable for applications. Thus, it is necessary to explore a new way to realize wide-angle backscattering enhancement. In this paper, a phase gradient metasurface with wide-angle radar cross section (RCS) enhancement property is proposed and demonstrated, which consists of two phase gradients with equal magnitude but in opposite directions. Through designing a reflective phase profile along the surface, an equivalent wave vector can be generated, with doubled magnitude but in an opposite direction to the parallel component of the wave vector of the incident wave. At the incidence angles =-45 and 45, electromagnetic (EM) waves are reflected to the directions just opposite to the directions of incident waves. And at incidence angle =0, the incident EM wave is coupled into spoof surface wave and then guided to another region to decouple into a free space wave. These guarantee RCS enhancement property in a related angular domain. The polarization independent Jerusalem cross unit is used to design the phase gradient, and a wide-angle RCS enhancement metasurface is designed. The simulated results indicate that at the designed incidence angles, directions of the reflected waves are all opposite to the directions of incidence waves for both x and y polarized wave. In order to evaluate the RCS enhancement performances, the mono-static RCS of the designed wide-angle RCS enhancement metasurface is measured. Both the simulations and experiments are in good agreement with each other, and show that the designed metasurface obtains tremendous RCS enhancement performances in a wide-angle domain (-45-45) for both x and y polarized wave with frequencies ranging from 9 GHz to 12 GHz.
2018, 67 (19): 198401. doi: 10.7498/aps.67.20180577
A gradient leaky-wall waveguide loaded absorbing load filter structure is proposed, which is designed for harmonic suppression in super-high power transmitter of deep-space probe. The attenuation loss characteristics of the filter is analyzed according to the equivalent circuit method, and the massive structure is simulated by the electromagnetic field simulation software. The filter sample which includes one main waveguide, 288 deputy waveguides and 288 absorb loads is processed following the simulating and designing sizes. In order to prevent microwave from leaking and keep good air tightness under the condition of high power, all the components of the filter will be welded together by means of vacuum welding, and then the sample is cleaned ultrasonically. Finally, the filter sample is tested under small signal and large signal separately. According to our test results, the pass band max insertion loss of the filter is 0.3 dB, the min suppression of second harmonic is 75 dB, the min suppression of third harmonic is 50 dB, and the min suppression of fourth harmonic is 35 dB. The measured results show that they are almost the same as the simulation results, and consistent completely with the anticipated. We further conduct the high power experiment on the filter under a large signal of 100 kW, showing that the continuous wave power capacity of the filter can reach up to 100 kW through the power resistance test with the liquid-cooled system. All the test data show that the study and development are very successful. At present, the filer has been applied to a type of ground high power transmitter, and its performances and indicators behave well.
The total variation constrained data divergence minimization model for image reconstruction and its Chambolle-Pock solving algorithm
2018, 67 (19): 198701. doi: 10.7498/aps.67.20180839
Image reconstruction is an important inverse problem to reconstruct images from its transform. The two main reconstruction methods are the analytic method and the iterative method. The analytic method, for example, the filtered backprojection algorithm, needs complete projection data, so it is not competent to accurately reconstruct an image from sparse data. Thus the iterative method combined with optimization techniques has received more and more attention. The optimization-based iterative image reconstruction algorithm may accurately reconstruct images by the use of compressed sensing, low rank matrix and other sparse optimization techniques. Among them, the total variation (TV) minimization model is a simple but effective optimization model. The traditional, constrained TV model employs the data fidelity term as the constraint term and the TV regularization term as the objective function. In the present work, we study a novel, TV constrained, data divergence minimization (TVcDM) model and its solver. We derive in detail the Chambolle-Pock (CP) algorithm for solving the TVcDM model, verify the correctness of the model and its solver, analyze the convergence behavior of the algorithm, evaluate the sparse reconstruction ability of the TVcDM-CP algorithm and finally analyze the influence of the model parameters on reconstruction and the effect of algorithm parameters on convergence rate. The studies show that the TVcDM model may accurately reconstruct images from sparse-view projections. The TVcDM-CP algorithm may ensure convergence but the vibration phenomena may be observed in the convergence process. The model parameter, TV tolerance, has important influence on reconstruction quality, i. e. too big a value introduces noise whereas too small a value may smoothen the image details. Also, the studies reveal that different algorithm-parameter selections may lead to different convergence rates. The TVcDM-CP algorithm may be tailored and applied to other computed tomography scanning configurations and other imaging modalities. The necessary key work is just to design the corresponding system matrix and select the optimal model parameters and algorithm parameters according to the insights gained in the work.
2018, 67 (19): 198901. doi: 10.7498/aps.67.20181000
Complex networks are ubiquitous in natural science and social science, ranging from social and information networks to technological and biological networks. The roles of nodes in networks are often distinct, the most influential nodes often play an important role in understanding the spreading process and developing strategies to control epidemic spreading or accelerating the information diffusion. Therefore, identifying the influential nodes in complex networks has great theoretical and practical significance. Some centrality indices have been proposed to identify the influential nodes in recent years, but most of the existing algorithms are only appropriate to the identifying of single influential node. Many times, spreading process is initiated by simultaneously choosing multiple nodes as the spreading sources, such as rumors, opinions, advertisements, etc. Therefore, it is necessary to develop efficient methods of identifying the multiple influential nodes in complex networks. In this paper, a method based on region density curve of networks (RDC) is proposed to identify the multiple influential nodes in complex networks. Firstly, we rearrange all nodes of network in a new sequence, and then plot the region density curve for network. Finally, we identify the multiple influential nodes based on the valley points of region density curve. Using two kinds of spreading models, we compare RDC index with other indices in different real networks, such as degree, degree discount, k-shell, betweenness and their corresponding coloring methods. The results show that the influential nodes chosen according to our method are not only dispersively distributed, but also are relatively important nodes in networks. In addition, the time complexity of our method is low because it only depends on the local information of networks.
2018, 67 (19): 198502. doi: 10.7498/aps.67.20181155
Ge is an indirect bandgap semiconductor, which can be converted into a direct bandgap semiconductor by using the modification techniques. The carrier radiation recombination efficiency of modified Ge is high, which can be used in optical devices. The mobility of Ge semiconductor carriers is higher than that of Si semiconductor carriers, so Ge device can work fast and have good frequency characteristics in electronic device. In view of the application advantages of modified Ge semiconductors in both optical devices and electrical devices, it has been a potential material of monolithic optoelectronic integration. The Ge and GeSn as optoelectronic device materials have a great competitive advantage, but there is no mature Ge-based monolithic photoelectric integration. In order to realize Ge-based optical interconnection, the bandgap of luminous tube, detector and waveguide active layer material must satisfy the following sequence:Eg,waveguide Eg,luminoustube Eg,detector. Therefore, in order to achieve the same layer monolithic photoelectric integration, we must modulate the energy band structure of the active layer material of the device. Unfortunately, the literature in this area is lacking. The band structure is one of the theoretical foundations for the monolithic photoelectric integration of the modified Ge materials, but the work in this area is still inadequate. In this paper, this problem is investigated from three aspects. 1) Based on the generalized Hooke's law and the principle of deformation potential, a modified Ge bandgap type transformation model is established under different modification conditions, perfecting the theory of converting the indirect switching into direct band gap of Ge. 2) On the basis of establishing the strain tensor and deformation potential model, a modified Ge band E-k model is established, and the relevant conclusions can provide key parameters for LED and laser device simulation models. 3) Based on the theory of solid energy band, the bandgap width modulation scheme of the modified Ge under the uniaxial stress is proposed, which provides an important theoretical reference for realizing the Ge-based single-layer photoelectric integration. The results in this paper can provide an important theoretical basis for understanding the material physics of the modified Ge and designing the active layers of the light emitting devices in the Ge based optical interconnection.
Organic color photodetectors based on tri-phase bulk heterojunction with wide sectrum and photoelectronic mltiplication
2018, 67 (19): 198503. doi: 10.7498/aps.67.20180502
In order to obtain highly sensitive broadband organic photodetectors (OPDs) used for image sensors with the stable ability to detect three primary colors (RGB), in this paper, the spectral broadening of organic active layer based on tri-phase bulk heterojunction formed by P3HT:PCBM doped with narrow band material PBDT-TT-F which absorbs red light is investigated. The influences of PBDT-TT-F doping ratio on the morphology of active layer film and detector photoelectric properties are further analyzed. Finally, the operating mechanism of trap-assisted photoelectronic multiplication is discussed. On this basis, the detector with 350-750 nm wide spectrum is obtained where the optimum mixing ratio of P3HT:PCBM:PBDT-TT-F is 12:8:3. At a small reverse bias of 1 V, the values of responsivity and external quantum efficiency of the photodetector can reach 470, 381, 450 mA/W and 93%, 89%, 121% respectively under the illumination of three primary colors and its normalized detectivity to the RGB is close to 1012 Jones. Additionally, the maximum relative difference between each parameter and its average value is lower than 20%; the bandwidths are 5, 8, and 8 kHz respectively, which reach the imaging requirements for image sensors. The experimental results show that not only the absorption spectra of the active layer can be broadened but also the carriers collection efficiency of respective electrodes can be well maintained by adding a small quantity of spectral broadening material while keeping the microstructure of the original binary bulk heterojunction. Utilizing the reasonable combination of materials to form electron traps, photoelectronic multiplication can be realized by trap-assisted hole tunneling injection from the Al cathode into active layer, and thus improving the normalized detectivity. Moreover, in order to detect different light intensities, the hole injection barrier width should be controlled by the corresponding light intensity. The resulting OPD shows a good liner response to all three primary colors when light intensity increases from 0.1 to 10 mW/cm2. By adjusting the mixing ratio of the tri-phase materials, the stable ability to detect the primary color can be achieved. The present study paves the way for high responsivity broadband OPDs based on tri-phase bulk heterojunction.
2018, 67 (19): 198801. doi: 10.7498/aps.67.20181024
In recent years, the parameter extraction methods of solar cell have attracted a lot of research attention. The reason is that the matching solar cell parameters can effectively reduce the influences of internal and external factors on photovoltaic efficiencies. In this paper, the five-parameter extraction methods of solar cell single-diode model are discussed in detail. The five parameters are the photocurrent, the reverse diode saturation current, the ideality factor of diode, the series resistance, and the shunt resistance. In fact, the existing research methods are classified as four categories, namely, analytically extracting parameter methods, extracting parameter methods with the help of Lambert W function, constructing or using special functions to extract parameter methods, and using intelligent algorithm to extract parameter methods. In this article, we not only elaborate their main theories and approaches, but also discuss their advantages and disadvantages. The main conclusion is that the analytical method for the extraction of solar cell model parameters requires some assumptions. Therefore, this method is fast but less accurate due to various approximations. In addition, the parameter extraction using the analytical method needs a thorough calculation, and deducing the actual values of (dI/dV)|V=Voc and (dI/dV)|I=Isc and peak power point is also challenging. When the five parameters of solar cell are calculated using the Lambert W-function method, the results show that the extraction process is easier when using the consecrated software such as MATLAB, but the larger computational time is needed. Generally, the Lambert-W function provides the exact explicit expression for parameter extraction. As a result, the accuracy of approximate solution using Lambert-W function is much higher than that of the above method. It is obvious that the accuracy of using special functions to extract cell parameters is limited by those function characteristics. Of course, those special functions, such as Green's function, seem to be complex approaches. The accuracy of the extracting cell parameters by using intelligent algorithm strongly depends on the type of fitting algorithm, the fitting criterion, objective function and the starting values of the parameters. Finally, based on the conducted review, the future research trend of parameter extraction is also predicted
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2018, 67 (19): 199701. doi: 10.7498/aps.67.20181056
The tide is a very important physical factor which can significantly affect the structure and evolution of stars. The physical factors which can affect tidal synchronization and orbital circularization are explored in this paper. For stars with radiative envelopes, radiative damping mechanism is required to explain the observed synchronization and circularization of close binaries. A star can experience a range of oscillations that arise from, and are driven by, the tidal field:the dynamical tides. The dynamical tide is the dynamical response to the tidal force exerted by the companion; it takes into account the elastic properties of the star, and the possibilities of resonances with its free modes of oscillation. The dissipation mechanism acting on this kind of tide is the deviation from adiabaticity of the forced oscillation, due to the radiative damping. Several physical factors can have an influence on the process of radiative damping which is scaled with thermal timescale. These physical factors include stellar mass, initial velocity, orbital period, metallicity, overshooting, etc. According to the equations for angular momentum transfer and chemical elements diffusion, we can obtain how these physical factors affect the evolution of rotating binaries and the mixing of chemical elements in two rotating components. The results indicates that the binaries with massive stars, smaller initial spin velocities, smaller overshooting parameters, and shorter orbital periods can attain the equilibrium speed and orbital circularization early. At synchronous states, the tidal torque is zero and stellar winds continue to brake the star. Therefore, two components cannot keep the synchronous state for a long time. At the equilibrium state, the tidal torque is counteracted by wind torques. Therefore, the equilibrium speed is less than the synchronous one. The system with smaller initial spin velocities reaches the equilibrium speed and orbital circularization early because angular momentum transformation between spin and the orbit can shorten the orbital distance and increase the tidal torques. Nitrogen enrichment in binaries is weaker than the one in single stars due to tidal braking. The results reveal that the system with massive components, higher metallicities, larger overshooting parameters, and shorter orbital periods can display high nitrogen enrichment. Stellar radius is small in the star with lower mass, lower metallicities, slower spin speeds and larger overshooting parameters whereas the star with lower metallicities have higher surface effective temperature. Rapid rotating stars evolve towards low temperature and luminosity in the HR diagram.
2018, 67 (19): 199201. doi: 10.7498/aps.67.20181014
The eddy viscosity is an important parameter in the atmospheric boundary layer meteorology, and we usually cannot determine their exact values by direct measurements, but we can only obtain an approximate range by indirect approximate method. In this paper, the eddy viscosity in the stochastic general Ekman momentum approximation model is used for the retrieval research and uncertainty analysis. The main purpose of retrieval is to reduce the uncertainty and narrow the approximate range of eddy viscosity. First, the polynomial chaos-ensemble Kalman filter and the wind observations are used for eddy viscosity retrieval and uncertainty reduction. The main idea of this method is to replace the Monte-Carlo method with polynomial chaos in the uncertainty quantification of ensemble Kalman filter, and thusavoiding the consumption of computing resources brought by massive samples. The goal of uncertainty quantification is to investigate the effect of uncertainty in the eddy viscosity on the model and to subsequently provide a reliable distribution of simulation results. Then two numerical experiments are implemented, i.e. experiment I in which the eddy viscosity is assumed to be constant, and experiment Ⅱ in which the eddy viscosity is assumed to be a vertically varying random parameter. The uncertainty of eddy viscosity in experiment I is reduced quickly, at the same time the mean of eddy viscosity can converge to a reference value. The effect in experiment Ⅱ is also remarkable after 16 data assimilation steps. These results show that the polynomial chaos-ensemble Kalman filter is an effective and fast method of solving the posterior distribution of eddy viscosity and reducing the uncertainty of eddy viscosity. Finally, we calculate the prior distribution of wind speed according to the prior distribution of eddy viscosity and identify the heavy uncertainty area in wind speed. The results indicate that the posterior distribution of eddy viscosity solved with wind observations in the big uncertainty area is more accurate, which provides an important guidance for selecting the location of observation points.
ATOMIC AND MOLECULAR PHYSICS
Renormalization of photon dyadic Green function by finite element method and its applications in the study of spontaneous emission rate and energy level shift
2018, 67 (19): 193102. doi: 10.7498/aps.67.20180898
The spontaneous emission rate and the energy level shift of a quantum dot in any micro-nanostructures can be expressed by the classical dyadic Green's function. However, the real part of the dyadic Green's function is divergent, when the source point and the field point are at the same position. This leads to an unphysical divergent level shift. Theoretically, the dyadic Green's function can be decomposed into a homogeneous part and a scattering part. Traditionally, the homogeneous field contribution is introduced into the definition of the transition frequency and the only need is to consider the effect of the scattering part which is non-divergent. Another renormalization method is to average the Green tensor over the volume of the quantum dot. In this work, a finite element method is proposed to address this problem. The renormalized dyadic Green function is expressed by the averaged radiation field of a point dipole source over the quantum dot volume. For the vacuum case, numerical results of the renormalized Green tensor agree well with the analytical ones. For the nanosphere model, the renormalized scattering Green tensor, which is the difference between the renormalized Green tensor and the analytical renormalized one in homogeneous space, agrees well with the analytical scattering Green tensor in the center of the quantum dot. Both of the above models clearly demonstrate the validity and accuracy of our method. Compared with the previous scattering Green function method where two different finite element runs are needed for one frequency point, our renormalization method just needs one single run. This greatly reduces the computation burden. Applying the theory to a gap plasmonic nano-cavity, we find extremely large modifications for the spontaneous emission rate and the energy level shift which are independent of the size of the quantum dot. For frequency around the higher order mode of the nano-cavity, spontaneous emission enhancement is about Г/Г0 2.02106 and the energy level shift is about △ 1000 meV for a dipole moment 24D. These findings are instructive in the fields of quantum light-matter interactions.
2018, 67 (19): 193401. doi: 10.7498/aps.67.20180322
During the last decades, the electron impact excitation (EIE) process has aroused much interest in various research areas. This process is crucial to the diagnoses of astrophysical and laboratory plasmas. Moreover, the EIE studies play an important role in understanding the quantum electrodynamic, many-electron, and hyperfine interaction effects in heavy atomic systems. As is well known, when ions are excited by collisions with a unidirectional beam of electrons, the magnetic sublevels of the excited state may be populated with nonstatistical probability. In the decay of the excited state, the emitted radiation is found to be anisotropic and polarized. From the analysis of the polarization, valuable information can be obtained. These properties have become indispensable tools for the diagnosis of plasma state and the analysis of complex spectrum formation mechanism. Up to now, however, most of studies have dealt with the linear polarization of X-ray radiation. Fewer publications have reported the circular polarization. Moreover, theoretical studies of the characteristic X-ray emission have just dealt with ions having zero nuclear spin, or have simply omitted all contributions that arise from such a spin. It is known that some kinds of ions each have a nuclear spin I 0. Owing to the hyperfine coupling, new decay channel will be open, namely, hyperfine-induced transition. It is thus important to analyze how the hyperfine interaction affects the polarization properties of X-ray radiation. In this study, we present a systematically theoretical analysis of the polarization and angular distribution of X-ray radiation during the hyperfine-induced transition. The calculations are performed by using a fully relativistic distorted-wave method. Special attention is paid to the studies of angular correlations and polarization properties of the 1s2p 3P2 Fi=3/2 1s2 1S Ff=1/2 decay for highly charged He-like Sc19+ and 205Tl79+ ions with nuclear spin I=1/2 following impact excitation by a completely longitudinally-polarized electron beam. Two effects, i.e.the BI and the mutipole mixing between the leading M2 decay and hyperfine-induced E1 decay, on the polarization of the emitted radiation are discussed. Our results show that both the BI and the E1-M2 interference effects may significantly affect the polarization and angular emission pattern of the transition line. For example, the BI and the E1-M2 mixing lead the circular polarization to increase by about 50% and 40% for 205Tl79+ ions, respectively. With the development of the X-ray detectors, the measurement on the polarization during the hyperfine-induced transition becomes feasible. We hope that the present results would be useful in resolving some disagreement between the theories and experiments relating to the polarization properties of the X-ray radiation.
Theoretical study of spectroscopic properties of 5 -S and 10 states and laser cooling for AlH+ cation
2018, 67 (19): 193101. doi: 10.7498/aps.67.20180926
In this paper, we calculate the potential energy curves of 5 -S and 10 , which arise from the first two dissociation limits of the AlH+ cation. The calculations are done using the complete active space self-consistent field method, which combines with the valence internally contracted multireference configuration interaction plus the Davidson modification (icMRCI+Q) approach with the aug-cc-pV6Z basis set. To improve the reliability and accuracy of the potential energy curves, the core-valence correlation and scalar relativistic correction, as well as the extrapolation of potential energy to the complete basis set limit are taken into account. The spin-orbit coupling is computed using the state interaction approach with the Breit-Pauli Hamiltonian. Employing the potential energy curves obtained in this study, we evaluate the spectroscopic parameters and vibrational levels for the bound and quasi-bound 4 -S and 8 states. The computed spectroscopic constants of X2+ and A2 states are all in agreement with the available experimental data. Moreover, the present theoretical energy separations between each higher channel (Al+(3P0) + H(2S1/2), Al+(3P1) + H(2S1/2), and Al+(3P2) + H(2S1/2) and the lowest one (Al+(1S0) + H(2S1/2)) are in excellent agreement with the experimental values. The transition dipole moments are calculated using the valence internally contracted multireference configuration interaction approach with the aug-cc-pV6Z basis set for the 2(1/2) X21/2+ and A23/2X21/2+. Based on the obtained potential energy curves and transition dipole moments, highly diagonally distributed Franck-Condon factors (f00 and f11) and large vibrational branching ratios are determined for the 2(1/2)1st well (v'=0, 1) X21/2+ (v) and A23/2(v'=0,1)X21/2+(v) transitions; short spontaneous radiative lifetime and narrow radiative width for the 2(1/2)1st well (v'=0, 1) and A23/2 (v'=0, 1) are also predicted in this study, which are suitable for the rapid laser cooling of the AlH+ cation. The three required laser cooling wavelengths are all in the ultraviolet region, that is, 1) for the X21/2+(v) 2(1/2)1st well (v') transition:the main repumping laser 00=358.74 nm, two repumping lasers 10=379.27 nm and 21=374.86 nm; 2) for the X21/2+ (v) A23/2 (v') transition:the main repumping laser 00=357.43 nm, two repumping lasers 10=377.80 nm and 21=373.26 nm. In addition, the recoil temperature for the X21/2+ (v=0) 2(1/2)1st well (v'=0) and X21/2+ (v=0) A23/2 (v'=0) transitions are obtained. The results imply the feasibility of laser cooling of AlH+ cation. In addition, the spin-orbit coupling effect on the spectroscopic parameter, vibrational level, and laser cooling of AlH+ cation are evaluated.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (19): 194302. doi: 10.7498/aps.67.20180705
Microfluidic is of great significance for biomedical research and chemical engineering. The mixing of liquids is an essential and necessary procedure for the sample preparation. Due to the low Reynolds number, laminar flow is dominant in a microfluidic channel and it is difficult to mix the fluids in the microchannel quickly and effectively. To improve the mixing efficiency of the liquids in microfluidic channels, we develop an acoustic mixer based on single microbubble oscillation. By designing the cylinder structure on the bottom surface, when the fluid flows through cylinder structure with a diameter of 40 m, the microbubble can be generated by the surface tension of the liquid. The device is fabricated by using standard soft lithography and the replica moulding technique, ensuring the stability and repeatability of the mixing. A piezoelectric transducer (PZT) with a resonant frequency of 165 kHz is attached to the polydimethylsiloxane microfluidic device on the glass substrate by ultrasound coupling gel. When the microbubble is excited by the PZT at a resonant frequency of 165 kHz, microbubble oscillates immediately. To verify whether ultrasound can induce microbubble cavitation, a passive cavitation detection system is established. The results show that the higher harmonics can be detected, indicating that the stable cavitation occurs. The microstreaming induced by the oscillating microbubble disturbs the fluid dramatically, achieving the mixture of liquids. Particle image velocimetry method is utilized to characterize the microstreaming, and a pair of counter-rotating vortices in the microchannel is detected. Furthermore, to test the performance of the device, the deionized water and rhodamine B are injected into the Y-shape microchannel. Relative mixing index is used to quantitatively analyze the mixing performance by measuring the grayscale values of the optical images. The results indicate that with the increase of the input power, mixing time can be shortened correspondingly. When the input power is 14.76 W, the mixing process is ultrafast, within 37.5 ms the high mixing uniformity can be achieved to be 92.7%. With the advantages of simple design, high efficient and ultrafast mixing, and low power consumption, this oscillating microbubble-based acoustic micromixer may provide a powerful tool for various biochemical studies and applications.
2018, 67 (19): 194303. doi: 10.7498/aps.67.20180925
Laminate piezoelectric (PE)/piezomagnetic (PM) composites consisting of alternating PE and PM layers can facilitate the conversion of energy between electric and magnetic fields, i.e., they possess the magneto-electric (ME) coupling effects, which recently has attracted much attention due to the huge potential applications in the field of high technology. The PE/PM phononic crystal is an ideal material for manufacturing high-tech precision parts such as resonator components, magnetoelectric sensors, weak magnetic field detectors, electric field tunable filters and magnetic field probes. In the practical applications, the adhesive interfaces of PE/PM phononic crystals are prone to deformation and failure during their use, because of the big difference between PE and PM material. In this paper, the magneto-electro-elastic (MEE) interlayer of magneto-electro-mechanical coupling is introduced into the PE/PM phononic crystal. The thickness of the MEE interlayer, the volume fraction of the piezoelectric material in the MEE interlayer and the type of the piezoelectric materials in the MEE interlayer are changed separately, with the thickness of the unit cell kept at a fixed value. The dispersion relation between the k and the is obtained by using the transfer matrix method and Bloch theorem. The influence of MEE interlayer on the band gap characteristics of PE/PM phononic crystal is studied by the dispersion relation diagram. The results show that as the thickness of the MEE interlayer increases, the central frequency of the band gaps shifts toward a higher frequency and the width of band gap becomes wider. As the volume fraction of the piezoelectric material increases, the center frequency and the width of the first band gap decrease. However, the width of the second band gap increases, and the width of the third band gap remains unchanged. The type of piezoelectric material in the MEE interlayer has an obvious influence on both the width and the central frequency of the band gaps. The effect of MEE interlayer on the central frequency of band gap of PE/PM phononic crystal is more significant in the high frequency region than in the low frequency region. Therefore, the width and central frequency of the band gaps can be adjusted to a certain extent by adding different MEE interlayers into the phononic crystal structure when designed.
High-order delay detached-eddy simulations of cylindrical separated vortex/vortex induced noise based on transition model and acoustic analogy
2018, 67 (19): 194701. doi: 10.7498/aps.67.20172677
The numerical prediction of transition from laminar to turbulent flow has proven to be an arduous challenge to computational fluid dynamics (CFD). Few approaches can provide routine accurate results within the cost limitations of engineering applications. In the present paper described is the application of a -Re transition model in combination with the delay detached eddy simulation (DDES) and Ffowcs Williams and Hawkings (FW-H) acoustic analogy method to cylinder vortex/vortex induced noise at a subcritical Reynolds number. In the process of numerical simulation, a traditional DDES based on the full-turbulence model SST is carried out for comparison and a 7th-order weighted compact nonlinear scheme (WCNS-E8T7) is adopted to ensure that the physical models are not affected by numerical dissipation or dispersion. In the first case, single cylinder cross-flow at ReD =4.3104 and Ma=0.21, is considered as a benchmarking problem for validating turbulence models and aerodynamic noise prediction methods. Its aerodynamic coefficients, St, CL and CD at root-mean-square (rms) and averaged values are measured by Szepessy and Bearman, while an acoustic measurement was recently made at Ecole Centrale de Lyon. The traditional DDES only based on SST model (SST-DDES) delays the instability of the shear layer on the sides of the cylinder, which leads to the recirculation zone in mean flow to grow and the induced drag to increase. Moreover, the vortex shedding frequency predicted by SST-DDES is larger than the actual value, which makes the whole sound pressure level (SPL) spectrum move toward high frequency region. However, combining the -Re transition model, the DDES (called Tran-DDES in the present article) can give the results in good agreement with the experimental data. In the second case considered is an airfoil in the wake of the cylinder. The flow condition is similar to that in the first case and the experimental results are also obtained at Ecole Centrale de Lyon. The issue of SST-DDES in recirculation zone in mean flow is weakened, which relates to the interaction between the airfoil and cylinder wake, the prediction of mean flow by SST-DDES is similar to that by the Tran-DDES. But in terms of the rms values of turbulent fluctuation components and SPL, the predictions by Tran-DDES are still better than those by SST-DDES.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (19): 195202. doi: 10.7498/aps.67.20180909
Photothermal effects associated with noble metal nanostructures have shown wide potential applications in photo-thermal cancer therapy, photo-thermal imaging, nanomedicine, etc. These applications benefit from the localized surface plasmon resonance (LSPR) effect of the nanoparticles. Due to the LSPR effect, the nanoparticles exhibit unique optical properties such as strong scattering and absorption in the band ranging from visible to near-infrared region. The absorption enables the plasmonic nanoparticle to be a thermal source to increase the temperature of itself and the localized surrounding environment. Among these particels, the anisotropic core-capped nanostructures distinguish themselves by their strong polarization selectivity. The absorptions are different when the incident light is polarized in the directions vertical (90) and parallel (0) to its symmetry axis, respectively. At 90, a large red-shift can be achieved and the absorption cross section is greatly enhanced. Moreover, their absorption peaks can be flexibly manipulated by slightly adjusting one of the geometrical parameters. However, the photothermal responses to these parameters are left blank. In this paper, photothermal effects of SiO2@Au core-capped nanoparticles are studied based on the numerical finite elemental analysis method (COMSOL software). The thermal response to each of the paramenters, including shell thickness, core diameter, core-shell ratio, and metal surface coverage is achieved. The calculation shows that the temperature of these core-capped nanoparticles can be adjusted efficiently in the near infrared band by easily rotating the polarization, i.e. slightly adjusting the geometric parameters. Especially in a range between 30 and 70, the temperature varying with the polarization follows almost a linear relationship. The comparisons with other popular structures including solid sphere, core-shell and nanorod are also made. The results indicate that at a similar size, the core-capped structure can offer a higher temperature than solid spheres and core-shell structures. To obtain the same temperature variation, the core-capped one has a smaller size than a nanorod. The comparisons demonstrate that the core-capped structure can be an alternative to a high-efficient nano heat source in the photothemal applications.
Role of impurities in modifying isotope scaling law of ion temperature gradient turbulence driven transport in tokamak
2018, 67 (19): 195203. doi: 10.7498/aps.67.20180703
Tokamak experiments show that the plasma empirical energy confinement scaling law varies with plasma ion mass (Ai) in a certain range under conditions of different plasma parameters or different devices. In order to understand such a modification of the empirical energy confinement scaling law, the isotope mass dependence of ion temperature gradient (ITG, including impurity modes) turbulence driven transport in the presence of tungsten impurity ions in tokamak plasma is studied by employing the gyrokinetic theory. The effect of heavy (tungsten) impurity ions on ITG and impurity mode is revealed to modify significantly the isotope mass dependence and effective charge effect. As the charge number of impurity ions (Z) or impurity charge concentration (fz) changes, the theoretical scaling law of ITG turbulence transport varies substantially in a relatively large range. The maximum growth rate of ITG mode scales as Mi-0.48 -0.12, whilst that of impurity mode scales as Mi-0.46 -0.3. Here, Mi is the mass number of primary ion in the plasma. In both cases the fitting index with Mi deviates further away from -0.5 when impurity charge concentration fz increases. The isotope mass dependence of ITG turbulence gradually weakens when the effective charge number Zeff increases. The isotope mass dependence of impurity mode turbulence also weakens with Zeff increasing for the same impurity ion charge number (Z). In contrast, the isotope mass dependence gradually strengthens with effective charge number Zeff increasing for the same impurity charge concentration (fz). On average, the maximum growth rates of impurity mode scale roughly as max~Mi-0.35Zeff1.5 and max~Mi-0.4Zeff1, respectively, for Zeff 3 and Zeff 3. The reason for the deviation of isotope scaling law from the normal case is investigated deliberately, and it is demonstrated that the isotope scaling index deviates from -0.5 more or less due to the fact that the impurity species, charge number and impurity concentrations vary in a certain range. These results demonstrate that it is impossible to deduce a unique isotope scaling law due to the variety of micro-instabilities and various plasma parameter regimes in tokamak plasma, which is consistent with the experimental observations. These results may contribute to the transport study involving heavy (tungsten) impurity ions in ITER discharge scenario investigation.
In this paper, an improved hybrid surface plasmon nanolaser with a gain medium ridge and a layer of air gap is proposed. In order to achieve low propagation loss and sub-wavelength field confinement, a triangular air gap and a 50 nm microcavity end face silver mirror are adopted in this structure, and the combination of this particular triangular structure and silver mirror effectively improves the performance of nano-laser. In this paper, we numerically simulate the waveguide by using the finite-element method. The COMSOL multiphysics software is a superior numerical simulation software to simulate the real physical phenomena based on the finite element method. On the basic of the COMSOL multiphysics software, a two-dimensional cross-section model and a three-dimensional model are built, the transmission performance and microcavity performance of the improved structure are analyzed in detail at a working wavelength of 1550 nm. Some quantities including the electric field distribution, transmission length, normalized mode field area, average energy density, foundation modal volume, quality factor of the structure, threshold gain, quality factor, effective modal volume, and Purcell factor are considered here which are dependent on the dielectric constant and geometrical parameters. The results indicate that on a two-dimensional scale, the contradiction between transmission loss and transmission distance can be effectively solved by the guidance of Fom value, and the IHPM laser structure with optimal transmission characteristics is obtained under the guidance of quality factor and foundation modal volume. A deep sub-wavelength constraint on light is achieved:the propagation length of the electromagnetic mode reaches a millimeter level and the longest distance can reach 1.29 mm. When testing the microcavity performance of the laser separately on a two-dimensional scale and three-dimensional scale, the high quality factor, low gain threshold, ultra-small effective mode volume of 0.001092 μm3 and ultra-high Purcell factor of 8.29×105 are obtained by adjusting the structural parameters and plating a 50 nm-thick silver layer on the end face of the laser microcavity. Compared with the previous structure without air gaps, the designed structure has a low laser lasing threshold and strong micro-cavity local capability when these two structural parameters are unified. The designed hybrid surface plasmon nanolaser may serve as a fundamental building block for various functional photonic components and can have applications such as in sensing, nanofocusing, and nanolasing.
2018, 67 (19): 195201. doi: 10.7498/aps.67.20180532
In order to consider comprehensively the effects of high-energy electron radiation and space plasma on the exposed dielectrics outside a spacecraft, in this paper, a model named surface and internal coupling charging model for the exposed dielectric of spacecraft is proposed, and its numerical solution is obtained. It is based on the deep dielectric charging model, with considering the interaction between the exposed dielectric surface and the ambient plasma by adding an incident charging current into the boundary in the proposed model, and the potential of infinite plasma is regarded as the referential potential (zero potential). The determinate solution of the model is analyzed and a numerical solution in one-dimensional case is provided by using an iterative algorithm to overcome the coupling between electric field and conductivity. The solution includes the potential of spacecraft body, the distribution of dielectric potential, and the electric field. Moreover, the new model is compared with surface charging model and internal charging model. The results show that the new model has an advatage of depicting the electric field exactly with respect to the surface charging model; if the internal deposition current is equal to zero, the new model degenerates into the one depicting the surface charging. It considers the effect of surface potential on charging results compared with the internal charging model. The three kinds of currents, namely the surface incident current, the internal deposition current and the leakage current, are considered comprehensively in the new model. Among them, the leakage current is the most complicated, which is determined by the potential and the dielectric conductivity affected by the electric field, radiation dose rate, and temperature. Using this new model, the surface and internal coupling charging simulation of the exposed dielectric can be performed. Therefore, the new model can provide a more comprehensive assessment for the charging of exposed dielectric of spacecraft.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2018, 67 (19): 191101. doi: 10.7498/aps.67.20180424
With the further reform of interest rate liberalization and the increasing of interest rate derivatives, it becomes more important and urgent to model the forward rate accurately and rationally in China. In this paper, we use the quantum field theory in econophysics, which can effectively incorporate the incomplete correlations between forward interest rates with different maturities, to model the Chinese treasury bond instantaneous forward rates. Firstly, we start with the correlation structure of the instantaneous change of treasury forward rates, one of the most important variables for a quantum field, during the period from January 4, 2011 to December 30, 2016, then apply the quantum field theory to model the actual market evolution of the treasury instantaneous forward rates directly. Secondly, we also use the mainstream two-factor Heath-Jarrow-Morton (HJM) model commonly used in financial industry, which requires the particular form of forward rate volatility functions to be set in advance, to model the treasury instantaneous forward rates, then compare the results with those of the quantum field model. The empirical results show that the quantum field model based on stiff action provides a fitting accuracy of 63.23% for actual treasury bond instantaneous forward rate, but this fitting accuracy increases to 92.67% for the quantum field model with taking into account the psychological perceptive remaining time, which is also superior to the classic optimal two-factor HJM model with a fitting accuracy of 69.02%. Finally, the optimal parameters estimated are respectively substituted into the forward interest rate update equations of the quantum field model with the psychological perception time in mind and the classical two-factor HJM model to conduct the back testing of forward rates with one hundred maturities, from January 3, 2017 to December 30, 2017. From the results of average instantaneous forward rate, root mean square error and Theil inequality coefficient, we can see the superiority of using the quantum field theory to model the term structure of treasury forward rates compared with traditionally used two-factor HJM model in financial industry. In conclusion, the quantum field model we constructed, is more consistent with the actual situation, and all the parameters estimated by this model are obtained directly from the market data, without making any assumption of the specific form of forward rate volatility function, thus greatly improving the accuracy of applying the quantum field theory to finance. These findings are not only of great theoretic and practical significance for applying the quantum field theory to pricing those financial products linked to treasury bonds and for managing its relevant interest rate risk, but also have reference value for quantitatively analyzing banks and finance companies in financial field, and also for practitioners in the field of fixed-income securities.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2018, 67 (19): 196401. doi: 10.7498/aps.67.20180700
The growth of ice crystal has been widely investigated by researchers from various fields, but efficient method that can meet the experimental requirements for identifying and reproducing the ice crystal with specific orientation is still lacking. In this paper, an ice crystal can be characterized with unique orientation information, where tilt angle of optical axis α, extinction angle β and the angle γ relative to preferred orientation 〈1120〉 in the basal plane (0001) and the direction of temperature gradient G are determined based on the properties of optic polarization of hexagonal ice in the directional solidification. An integrated criterion for determining the orientation of hexagonal ice is proposed by combining the crystal optics and solidification interface morphology. Precise manipulation of the orientation of single ice crystal is achieved by using a step-by-step method via a unidirectional platform combined with a polarized optical microscope. Three coordinate systems are established to achieve the manipulation of ice. They are the microscope coordinate system termed as “A-P-L”, where A, P and L refer to the directions of analyzer, polarizer and incident beam of the optical microscope, respectively, the specimen box coordinate system named “xyz”, and the crystallographic coordinate system described by the optical axis and 〈1120〉 in the basal plane (0001). Ice crystals are all confined in a series of glass specimen boxes filled with KCl solution (0.2 mol/L) and the growth sequence of the single ice crystal from one specimen box to another is specially designed to ensure the specific orientation relations among specimen boxes, and the orientation relations among the specimen boxes are adjusted according to the integrated criterion. Single ice crystals with three typical orientations (α3=90°, β3 a=0°; α3=90°, β3b=90°; α4=90°, β4 dose not exist, γ ≈ 33°) relative to the microscope coordinate A-P-L are obtained, and their morphological characteristics of S/L interface are observed in situ under different pulling velocities (10.3 μm/s, 13.4 μm/s and 100 μm/s, respectively). In this paper we successfully solve the problem of orientation determination and manipulation of ice orientation in the study of directional solidification of ice crystal, which may provide an effective experimental approach for investigating the theoretical problems concerning ice crystal growth.