Vol. 65, No. 8 (2016)
2016, 65 (8): 080201. doi: 10.7498/aps.65.080201
Brikhoff system is a kind of basic dynamical system. The theory and method of Brikhoff system dynamics have been applied to the hadron physics, quantum physics, relativity and rotational relativistic system. The properties of gradient system not only play an important role in revealing the internal structure of dynamical system, but also help to explore the dynamical behavior of the system. In this paper, two kinds of generalized gradient representations for generalized Birkhoff system are studied. First, two kinds of generalized gradient systems, i. e., the generalized skew gradient system and the generalized gradient system with symmetric negative definite matrix, are proposed and the characteristics of the systems are studied. Second, the relations of stability between these two kinds of gradient system and the dynamical system are discussed. Third, the condition under which a generalized Birkhoff system can be considered as one of the two generalized gradient systems is obtained. Fourth, the gradient discrimination method of stability of the generalized Brikhoff system is given, and the characteristics of the generalized gradient systems can be used to study the stability of the generalized Birkhoff system. Finally, some examples are given to illustrate the application of the result. Therefore, once the mechanical system is expressed as the generalized gradient system, the stability and the asymptotic stability can be conveniently studied by using the properties of generalized gradient system. The difficulty in constructing Lyapunov functions is avoided, and a convenient method of analyzing the stability of mechanical system is provided.
2016, 65 (8): 080202. doi: 10.7498/aps.65.080202
As frequency modulated (FM) signals widely exist in the natural world as well as in different artificial applications, it is of great practical significance to explore the ways to extract such signal components in the complex and noisy environment. To extract one component from the noisy multicomponent signal effectively, a component extraction method based on polynomial chirp Fourier transform (PCFT) is presented in this paper. First, the physical meanings of Fourier transform (FT) and fractional Fourier transform (FRFT) are analyzed and their internal relations are expounded from the perspective of signal energy accumulation. Essentially, the FT accumulates signal energy along the time-frequency beelines parallel to the time axis and obtains an energy-concentrated spectrum from the narrow-band stationary signals whose frequency does not change, whereas it fails to process non-stationary signals with changeable frequencies. By rotating the time-frequency axis, the FRFT changes the energy accumulation mode of the signal in the old time-frequency plane and achieves a more concentrated spectrum for the linear frequency modulated (LFM) signal, but with larger error or even invalidation when dealing with nonlinear frequency modulated (NLFM) signal. Using FT and FRFT, in this paper we attempt to improve the energy accumulation mode of the conventional transform method and propose the PCFT. In this transform, the beeline families in the traditional transform, independent of time (or v) axes, are replaced by a family of polynomial chirping curves in the time-frequency plane. These polynomial chirping curves are capable of approaching more closely to the instantaneous frequency curve of FM signal so as to obtain a more concentrated transform spectrum and thereby extend the application of PCFT from LFM signal to NLFM signal. When selecting the polynomial chirping curve, we build up a nonlinear optimization model guided by the principle of energy spectrum concentration and in this way convert the problem of determining the polynomial curve families into the one of optimizing the polynomial parameters. Then particle swarm optimization algorithm is employed to search for the optimal polynomial parameters so as to concentrate the energy of one component in the new transform domain, i.e., the polynomial chirp Fourier domain. After doing that, each component is separated into its concentrated spectrum with a narrow-band filter and reconstructed with the inverse PCFT. Moreover, to extract components from a noisy multicomponent signal successfully, an iteration involving parameter estimation, PCFT, filter and recovery is introduced. To verify the effectiveness of the PCFT-based method, a series of examples, including simulated and real-world signals, is chosen for simulations and experiments. The experimental results indicate that compared with FT and FRFT, the proposed method overcomes the shortcoming of distributed energy spectrum for NLFM components in the traditional transforms and obtains a concentrated energy spectrum in the polynomial chirp Fourier domain, therefore realizing component separation and time-frequency characteristic extraction. The PCFT-based method not only has the capability of dealing with the extraction of LFM components, but also performs well in the separation of crossed NLFM components, and with little extraction error.
2016, 65 (8): 080301. doi: 10.7498/aps.65.080301
Nonorthogonal coded agreements and decoy state method can effectively protect the photon number against splitting attack. Owing to the fact that the component of single-photon in the coherent-state superposition (CSS) is as high as 90%, CSS has recently emerged as an alternative to single-photon qubits for quantum information processing and metrology. The approximate CSS of small amplitudes is generated by the subtraction of photons from a squeezed vacuum state, and the approximate CSS of large amplitude is generated from Fock state by using a single homodyne detection. Here, we combine both of the methods and propose a new protocol by using the CSS as a light source. We derive the secure key generation rate, the lower bound of count rate and upper bound of error rate of single-photon. We simulate the curves relationship between secure key generation rate and safety transmission distance in the case of an infinite number of decoy states by using matlab. The parameters are given according to the Gobby-Yuan-Shields (GYS) experiment. We infer that the safety transmission distance achieves 147.4 km and the secure key generation rate is much higher than those of other schemes. We also simulate the relationship between key generation rate and safety transmission distance in the case of a limited number of decoy states by using matlab. The parameters are given according to the GYS experiment too. When the N is 1010, the safety transmission distance achieves 144 km; when the N is 109, the safety transmission distance achieves 139 km; when the N is 108, the safety transmission distance achieves 125.9 km. In this paper, we propose the use of CSS as the light source. Combining SARG04 agreements and decoy state, the scheme has the following advantages: first, the scheme which combines SARG04 agreements and decoy state method can effectively resist PNS; second, nonorthogonal decoy-state quantum key distribution based on coherent-state superpositions has a longer safety transmission distance and higher secure key generation rate than nonorthogonal decoy-state quantum key distribution based on weak coherent pulse and nonorthogonal decoy-state quantum key distribution based on conditionally prepared down-conversion source; third, nonorthogonal decoy-state quantum key distribution based on coherent-state superpositions is easier to prepare, which just needs one decoy state, than other schemes that require several decoy states. Obviously, our scheme can enhance the performance of quantum key distribution. Nonorthogonal decoy-state quantum key distribution based on coherent-state superpositions will have a very good application with the further development of preparation technology of CSS.
2016, 65 (8): 080302. doi: 10.7498/aps.65.080302
Quantum walks have been proven to be a useful framework in designing new quantum algorithms, of which the search algorithm is the most notable. Besides a general search for a special vertex, recent researches have shown that quantum walks can also be used to find structural anomalies. Suppose a vertex of complete graph KN is attached to a second graph G, then the kind of structure anomaly will break the symmetry of the complete graph. The search algorithm based on scattering quantum walk model is presented to speed up locating this kind of structure anomaly. The concepts of scattering quantum walk model and collapsed graphs are presented. The definition of the evolutionary operator, which is different from that of a general search, is given. Based on the specific definition of evolutionary operator and the obvious symmetry of complete graph, it is shown that the search space is confined to a low-dimensional collapsed space, and the initial state is chosen to lie in this subspace. To illustrate the evolutionary process of the search algorithm, an example is given in the case that G is a single vertex. Taking advantage of our earlier study on the evolutionary operator of coined quantum walks with Grover coin, calculations of the unitary operator in the collapsed space are greatly simplified. To quantify the evolutionary process of the algorithm, we use the matrix perturbation theory involving a perturbative approach to find the eigenvalues and eigenstates. It is the degenerate zeroth-order eigenvalue 0 = 1 that leads to the interesting parts of the Hilbert space. Most of the recent researches of searching the structure anomalies focus on star graph SN with an unspecified graph G attached to one of its external vertices, where the overall graph is divided into two parts by the central vertex. It is shown that quantum speedup will occur if and only if the eigenvalues associated with these two parts in the N limit are the same. In this paper, we find that the collapsed graph of complete graphs can also be divided into two parts by a single collapsed vertex. As these two parts roughly correspond to the initial state and the desired state respectively, the techniques and results in star graphs can be generalized to the collapse graph on complete graph. What is more, under our definition of unitary evolution operator these two parts in the N limit will always share the same eigenvalue, i.e. 0 = 1, no matter what the structure of graph G is. Based on this, we prove that the search algorithm can find the target vertex in O(N) time steps with a success probability of 1-O(1N). That is to say, the quantum search algorithm gains a quadratic speedup over classical counterpart.
2016, 65 (8): 080303. doi: 10.7498/aps.65.080303
Entanglement is a vital resource for many quantum information processes. However, the unavoidable interaction between quantum system and its environment will lead to quantum decoherence. So protecting remote entanglement against decoherence is of great importance for realizing quantum information and quantum communication. In fact, there are many types of decoherences. Besides the depolarization and phase damping, amplitude damping is a typical decoherence mechanism. If we monitor the environments to guarantee that no excitation escapes from the system, the amplitude damping is modified into a weak measurement induced amplitude damping of the system. Amplitude damping decoherence can affect both single-qubit quantum states and multipartite entangled states. However, in most of previous quantum state protection schemes, the authors only pay attention to the single-qubit system or two-qubit system. Compared with bipartite entangled states, multipartite entangled states possess many advantages, but the entanglement property of multipartite entangled state is much more complicated than bipartite entanglement, so bipartite entanglement reversal (protection) scheme may not be suitable for multipartite case. Thus, in this paper, according to local pulse series and weak measurement, we propose an effective scheme for protecting two multipartite entangled states against amplitude damping, and these two multipartite states are Cluster state and Maximal slice (MS) state. Cluster state and MS state are two typical classes of multipartite entangled states, which play important roles in quantum computation and communication, respectively. These two states cannot be converted into each other with local operation and classical communication. Owing to its good operational and computable properties, here we choose negativity as a measure to quantify the multipartite entanglement. For the case of MS sate, no matter what the initial parameter is, when the local pulses are exerted on all qubits, the entanglement can be fixed around the entanglement of the initial state. Similarly, in the four-qubit cluster state case, if a series of flip operations is exerted on all qubits, it is shown that the multipartite entanglement can be recovered to the maximum 1.0. All these results show that this protocol can protect remote multipartite entanglement effectively. The physical mechanism behind this scheme is that the weak measurement combining with flip operation can balance the weight of different terms of the state, and move the entanglement toward the initial value. To summarize, our scheme is much simpler and feasible, which may warrant its experimental realization. Moreover, our scheme could be extended to protect other multipartite states.
2016, 65 (8): 080601. doi: 10.7498/aps.65.080601
The principle of interference imaging spectrometer is presented. According to the drift of recovery spectral line position, two representative methods of calibrating the laboratory spectral line position are proposed, and the calibration results and their comparative analyses are given. One method of calibration is to correct the principle, which embarks from parameter selection of interference imaging spectrometer and the analysis of the reason why the spectral line position is drifted. Aiming at the problem that the position of spectral line changes with row, the correction scheme is given to improve the accuracy of spectral line position. For four given laser wavelengths, which are 543.5 nm, 594.1 nm, 612 nm, and 632.8 nm, the root-mean-square (RMS) error of spectral line position is reduced from 28.3914 to 5.5371 after calibration. For the interferometer system which has no dispersion, the accuracy of calibration is better than the dispersion system, and can be the same at all detected wavelengths. In this article, the calibration accuracy of long wave is better than that of short wave, which is dependent on the selection of the initial correction wavelength. This method achieves a kind of universality for interference imaging spectrometer and its calibration parameters provide a convenient way to analyze the instrument indexes. Another calibration method is data processing. It makes up the deficiencies of the method mentioned above: a large number of data are needed and the effect of calibration at short wave is not good enough. The RMS error of spectral line position is reduced to 0.9178, which proves that the calibration has a really high precision. This method is simple and can correct all the detected wavelengths and spectral lines by using two united formula. Though this method is not applicable for all the interference imaging spectrometers, the idea that makes hard things simple is deserving of our attention. We can use it in many other fields. The essence of the method is to change a variable quantity into a slowly varying quantity by algorithms, and then establish the relationship between the slowly varying quantity and the standard value. This idea can always make a substantial increase in efficiency of calibration and has a satisfied accuracy. Each of the two methods has advantages and disadvantages: which method we choose to use is dependent on the effect we want to achieve, and it is better to make their combination. This study provides a theoretical and practical guidance for study, design, modulation, experiment and engineering of interference imaging spectrometers.
2016, 65 (8): 080602. doi: 10.7498/aps.65.080602
Large-scale absolute distance measurement system with high accuracy plays a significant role in science and engineering applications. In many fields such as aerospace technology, large-scale manufacture, geodetic survey and civil engineering, absolute distance measurement systems with a range of up to kilometers and accuracy of better than several micrometers are generally required. Traditional laser ranging methods such as the time-of-flight method and the interferometry method are difficult to achieve both large scale and high accuracy. With the development of femtosecond optical frequency comb technology, several ranging methods with larger range and higher accuracy are developed. In the frequency domain, the optical frequency comb has a large number of stable mode lines, or the longitudinal modes, at regular intervals, which generates the inter-mode beat signal. In this study, based on the inter-mode beat of a femtosecond laser, an absolute distance measurement system using multi-wavelength interferometric method is demonstrated. It has a simple experimental setup with high accuracy but in a limited range of 2.5 m due to the 2-period of phase detection. To achieve a large-scale measurement system, the measurement range of the experimental system is extended by using the synthetic wavelength generated by tuning the repetition frequency of the laser. With a repetition frequency change of 0.2 MHz, a synthetic wavelength of up to 1.5 km is realized, thus the measurement range of the experimental setup can be extended to 0.75 km. Besides the reference and measurement path beams, a monitor path beam and two alternately opened mechanical shutters are used to measure and compensate for the phase drift due to the unbalanced drift of the electronic circuit. By using this method, the standard deviation of the phase measurement results in 30 min is 0.022 in the experiment, and the phase drift can be compensated for very well. The measurement results from the experimental system are compared with the results from a commercial heterodyne interferometer, and the comparison between results shows a precision of better than 50 m in a displacement of 1125 mm. In the experiment, the repeatability of absolute distance measurement using the range extending method is better than 3 m, thus the range of the distance measurement system can be theoretically extended up to 7.5 km. In conclusion, we demonstrate that a large-scale absolute distance measurement system using inter-mode beat of a femtosecond laser, has a range of up to 7.5 km, an accuracy of better than 50 m and a repeatability of better than 3 m. The accuracy of the experimental system can be further improved by using photodetectors with higher bandwidth so that a higher inter-mode beat and a shorter wavelength can be used.
2016, 65 (8): 080603. doi: 10.7498/aps.65.080603
The high precision measurement has been a focus in the field of manufacturing and microelectronics in this year. The micro/nano probe for coordinate measuring machine (CMM) acts as a key characteristic because it can measure the high-aspect-ratio components with high precision. Various micro/nano-CMM probes with different principles and different structures have been developed in the last decade. However, most of these studies focused on the sensing principle and measurement methods. There is little research on the behavior of the surface interaction between the probe tip and the workpiece. And the measurement accuracy and reliability of the current probe, especially those of the low stiffness probe, are limited by interaction forces including capillary force, van der Waals force, electrostatic force and Casimir force. Therefore, it becomes a challenge to reduce the effect of the surface interaction forces for the Micro/nano CMM probe. A new trigger probe based on the vibrating principle is analyzed and an optimal method for the appropriate vibrating parameters is presented in this paper. The structure and principle of the probe are briefly described in the first part. In this system, a tungsten stylus with a tip-ball is fixed to the floating plate, which is supported by four L-shape high-elasticity leaf springs. The fiber Bargg grating (FBG) sensors are used in the probe for micro-CMM due to their superiority in t of small size, high sensitivity, large linear measuring range, immunity to electromagnetic interference, and low cost. One end of FBG is attached to a floating plate, and the other end to a retention plate which is connected with the piezoelectric ceramic actuator (PZT). The probe is driven by the PZT vibrating. Assuming that the driving forces can offset the surface interaction forces, then the probe can be described as a forced vibration model of the spring oscillator. Therefore, the equivalent model of the probe is set up. In the second part, a relationship between the vibration parameters of the probe and the surface interaction can be confirmed. Through theoretical analysis and numerical simulation, the appropriate vibrating parameters including resonance amplitude, velocity and frequency of the probe are designed, which can offset the surface interaction forces. In the third part, a probe is designed based on the above theories and an experimental system is set up to verify its rationality. The results show that the resonant micro/nano probe after optimizing its parameters can effectively reduce the influence of surface forces and improve the measurement accuracy.
Characterization and preliminary application of top-gated graphene ion-sensitive field effect transistors
2016, 65 (8): 080701. doi: 10.7498/aps.65.080701
Graphene, a 2-dimensional material, has received increasing attention due to its unique physicochemical properties (high surface area, excellent conductivity, and high mechanical strength). Field-effect transistor is shown to be a very promising candidate for electrically detecting chemical and biological species. Most of the reports on graphene field-effect transistors show that solution-gated graphene field effect transistors have been used so far. Although the traditional solution-gated graphene field effect transistor has high sensitivity, but the graphene channel is contaminated easily. The stability of the device is reduced so that the device cannot be reused. Only very recently, has the top-gated graphene, which is potentially used for pH sensors, been reported. In the top-gated graphene the dielectrics is deposited at the top of graphene. However, the sensitivity is lower than other sensors. To improve the properties, we design and fabricate a top-gated graphene ion-sensitive field effect transistor by using large-area graphene synthesized by chemical vapor deposition. At the top of graphene, HfO2/Al2O3 thin film is deposited by atomic layer deposition. The Al2O3 film plays a role of sensitive membrane, and the HfO2/Al2O3 thin film protects the graphene from contamination of the solution. After depositing the top-gate, because of the shield of the insulation, the boundary between the graphene and the substrate is not clear. And the Raman spectrum indicates the presence of a defective top layer accompanied by an increase in the Raman D peak. After a series of electrical characterizations, compared with solution-gated graphene field effect transistor which directly contacts the graphene channel with the solution, the top-gated graphene ion-sensitive field effect transistor has a high resistance. This increase relative to uncovered grapheme, is attributed to the participation of the top -orbitals in van der Waals bonds to the insulation. The graphene -orbitals contributing to van der Waals bonds have less overlaps and thus result in reduced conductivity. However the output curves and transfer curves show that the top-gated graphene ion-sensitive field effect transistor has higher signal-to-noise ratio and better stability. In view of the biochemical detection, in this paper we also examine the adsorption of single-stranded DNA. Silane functionalization of metal oxide system is a versatile technique that can be used in DNA microarray and nanotechnology. The DNA immobilization process we have developed contains several steps: silanization (APTES), crosslinker attachment (EDC and NHS), reaction with carboxyl-DNA and removal of non-covalently bound DNA. We characterize the process with carboxyl-quantum dots. We also measure the transfer curves before and after the adsorption of DNA, and demonstrate the effectiveness of the functionalized process and the feasibility that the top-gated graphene ion-sensitive field effect transistor is used as the biosensor.
2016, 65 (8): 080702. doi: 10.7498/aps.65.080702
Polarization is one of the basic properties of electromagnetic wave conveying valuable information about signal transmission and sensitive measurements. Manipulations of polarization state and amplitude have aroused a lot of research interest in many different fields, especially in the terahertz (THz) regime. Although many researches on THz polarization controller have been carried out, their transmission losses are still difficult to lower in a broad bandwidth. And there are few reports on THz polarization controller which can rotate the polarization state and split beams at the same time. Multifunctional THz devices are required to meet the needs of the progress of THz technology and its applications. In order to overcome this constraint, semiconductor silicon is integrated into the proposed structure to manipulate the polarization state and the amplitude, because of its optical properties with the external pump light. Here, according to the electromagnetic resonance between split rings and silicon rings in Fabry-Prot-like cavity, we propose a metasurfaces-based terahertz polarization controller. The unite cell structure is composed of metal grids-split ring/Si ring-metal grids spaced by silica layers. By using the finite element method in CST Microwave Studio, we simulate the transport and polarization properties under different conditions. The results show that a linear polarization state can be nearly perfectly converted into its orthogonal one from 0.39 to 1.11 THz with a transmission loss of 1 dB, which fits well to the one of multiple-beam interference theory. Then we study the properties of the proposed metasurface structure for oblique incidence. The property of rotating polarization basically keeps stable even at an incident angle of 60 from 0.52 to 1.05 THz. At the end of the paper, the polarization splitting feature of the device is discussed in the THz regime. The results demonstrate that the transmitted and reflected beam power of the device can be tuned by changing the pump light power. The modulation depths of two beams reach more than 90% at 0.5 THz. It is worth noting that the proposed structure can not only rotate the polarization state of transmitted light in a broad bandwidth of 0.72 THz, but also modulate the transmitted and reflected beam power with a wide modulation depth. It can be used as a broad-band, low-loss and tunable terahertz polarization controller which is also insensitive to the incident angle. So it meets the requirements in THz communication, spectrum detection and imaging systems.
2016, 65 (8): 080703. doi: 10.7498/aps.65.080703
In order to obtain accurate image, spectrum and polarization state of target by the interferometric channeled spectropolarimeter, the interferogram and the image need to be separated. Although it can be achieved by digital image processing technology, heavy computations with approximation would be introduced. In the application of channeled spectropolarimetry, an inevitable crosstalk will be present between channels of the interferogram formed on the CCD. Spatial filtering in the optical path difference domain will generate a loss of spectral resolution and the distortion of the recovered spectrum. To overcome these drawbacks, a static imaging channeled spectropolarimeter based on division of aperture and field of view is presented. The aperture is divided by a polarization array, which consists of two polarizers with their transmission axes perpendicular to each other. The field of view is divided by a pair of lenses with the same focal lengths. The spectral modulation module is composed of an achromatic quarter wave plate, a retarder and a polarization array. The interference system consists of an achromatic half wave plate, a Wollaston prism, and a Savart polariscope. Two pairs of in-phase and anti-phase interferogram with different intensity modulations can be obtained simultaneously on a single detector array. The pure image of the target is acquired by the summation of the four interferograms. The background intensity is removed by the subtraction of the interferograms with in-phase and anti-phase, and the pure interference fringes can be acquired. By the summation and subtraction of the two pure interference fringes, the single channeled interference fringes corresponding to spectrum of intensity and linear polarization state can be obtained. Spectral and polarization information of the target are acquired by Fourier transform of the single channeled interference fringes. Compared with previous instruments, the described model has the significant advantage that the background intensity can be removed from the hardware of the layout, and thus avoiding the spatial filtering in the optical path difference domain. The obtained spectra have the same resolutions as those obtained from the interference system, and the distortion of the recovered spectrum can also be vanished. Since there is neither rotating part nor moving part, the system is relatively robust. In the present paper, the principle of the instrument is described, and the interference fringe intensity distribution formula is obtained and analyzed. The performance of the system is demonstrated through a numerical simulation. This work will provide an important theoretical basis and the practical instruction for designing a new type of imaging sepctropolarimeter and its engineering applications.
ATOMIC AND MOLECULAR PHYSICS
2016, 65 (8): 083201. doi: 10.7498/aps.65.083201
The general formula of the angular distribution of photoelectron is derived by using the density matrix theory and Racah algebra method. For comparing with the experimental data, the general formula in this paper is matched to the parametric formula and the non-dipole parameters of the photoelectron angular distribution associated with the terms of the second order for both unpolarized and polarized incident light are given explicitly. From the formula of these parameters we can see that the contribution to the non-dipole parameter is from the interference between dipole amplitude and multipole amplitude. And then, the relativistic calculation program for photoelectron angular distribution is further developed with the help of the program packages GRASP2K and RATIP which are based on the multi-configuration Dirac-Fock method. By using this program, the dipole and non-dipole angular-distribution parameters for neon 2s and 2p photoelectrons are calculated concretely. The good agreement between the results of this paper and the available theoretical data is obtained in a 50-5000 eV photoelectron-energy range studied. On this basis, the angular photoelectron distributions for neon 2s and 2p are calculated with and without considering the second non-dipole terms at the photoelectron energy E=600 eV and E=5000 eV, respectively. Special attention is paid to the effects of the polarization property of incident light and the non-dipole terms of photo-electron interaction on the angular distribution of photoelectrons. The results show that 1) the dipole and non-dipole parameters of the photoelectron angular distribution are sensitive to the ionized electron orbital, it can bring out considerable diversities among the photoelectron angular distributions of the different shells; 2) non-dipole effects make the photoelectron forward distribution in the direction of incident light, the polarization property of incident light will strengthen the asymmetric distribution of photoelectrons.
2016, 65 (8): 083202. doi: 10.7498/aps.65.083202
We theoretically study the electron detachment of negative hydrogen ions in a three-cycle linearly polarized laser field with a wavelength of 2150 nm in the context of the strong field approximation (SFA). The numerical integration and the saddle-point (SP) methods are both used in our calculations. The results show that both the energy spectra and the momentum spectra of the photoelectrons detached from negative hydrogen ions, obtained from these two methods, accord very well with each other for the laser intensities of 1.31011 W/cm2 and 6.51011 W/cm2, respectively. It is found that there is an obvious stripe-like structure along the vertical direction of the momentum spectra when the laser intensity is 6.51011 W/cm2. To explore the main origin which leads to the specific structures of the momentum spectra, we divide the interferences of the electronic wave packets emitted at different times during the laser pulse into the intra-cycle interference and the inter-cycle interference based on the SP method. Inter-cycle interference arises from the coherent superposition of electron wave packets released at complex times during different optical cycles, whereas intra-cycle interference comes from the coherent superposition of electron packets released in the same optical cycle. It is found that when only considering the inter-cycle interference, the main structures of the momentum spectra accord well with the above-threshold detachment (ATD) rings, which indicates that the inter-cycle interference corresponds to ATD rings of the photoelectron spectrum. But when only considering the intra-cycle interference, there are stripe-like structures with left-right asymmetry along the vertical direction of the momentum spectra. So the main structures of the momentum spectra of the photoelectrons are attributed to the interplay of the intra-and inter-cycle interferences. In addition, to intuitively explain the reason why the momentum spectra depend on the intensity of the laser field, we analyze the influence of the intensity of the laser field on the inter-cycle interference of quantum wave packets. It is found that the phase difference of the inter-cycle interference depends on the intensity of the laser field, which may lead to the difference among the momentum spectra of the photoelectrons at different laser intensities. Moreover, the influences of the intra-and inter-cycle interferences on the energy spectrum of the photoelectrons are also analyzed. It is found that the main oscillatory patterns and the peak positions of the energy spectra are mainly determined by the inter-cycle interference. Finally, the effects of the duration of laser pulse on the intra-and inter-cycle interferences are discussed. It seems that the main structures of the momentum spectra accord well with the ATD rings in multi-cycle laser pulses. So it is concluded that in multi-cycle laser pulses, the inter-cycle interference dominates while the intra-cycle interference is suppressed. The work in this paper is meaningful for further understanding the quantum interference effect and the optical control of the laser-induced photodetachment of negative ions.
2016, 65 (8): 083301. doi: 10.7498/aps.65.083301
Using the three-dimensional classical ensemble model, nonsequential double ionization (NSDI) of aligned molecules by the few-cycle laser pulse at the low intensity is investigated. Here the two electrons involved in NSDI finally are ionized through a transition doubly excited state induced by the recollision. The results show that the electron correlation behavior in NSDI is strongly dependent on the molecular alignment and the carrier-envelope phase (CEP) of the laser pulse. There are more anti-correlated emissions for the perpendicular molecules than those for the parallel molecules regardless of CEP. The dependence of the electron correlation behavior on molecular alignment can be well explained by the potential energy curves of molecules. That is because the suppressed potential barrier for perpendicular molecules is higher and the electron is more difficult to ionize than for parallel molecules. Thus for perpendicular molecules the ionization of the two electrons has longer time delay, which results in more anticorrelated emissions. Additionally, because the potential barrier for the perpendicular molecules is higher than that for the parallel molecules, the ionization yield of NSDI is about an order of magnitude smaller than that for the parallel molecules. With CEP increasing from 0 to , the anti-correlated emission first increases and then decreases. For parallel alignment, the correlated emission is always dominant at all CEPs. However, for perpendicular alignment, the dominant correlation behavior depends on the CEP of the laser pulse. When the CEP is in a range from 0.3 to 0.7, the anti-correlated emission is dominant. At other CEPs, the correlated emission is dominant. The dependence of the electron correlation behavior on the CEP of the laser pulse is well explained by the dependence of the returning energy of the electron on the CEP of the laser pulse. For different CEPs, the single ionization times resulting in NSDI and the corresponding acceleration electric field are different, which leads to at some CEPs the returning energy of the electron being large and at some other CEPs the returning energy of the electron being small. When those CEPs are available where the returning energy of the electron is larger, the doubly excited state induced by the recollision is more energetic. Thus at those CEPs the emissions of the two electrons from the doubly excited state have smaller time delays and more correlated emissions occur. On the contrary, at those CEPs where the returning energy of the electron is small, more anti-correlated emissions are produced.
Inverstigation on loading of the Dimple optical trap based on a magnetically levitated large-volume crossed optical dipole trap
2016, 65 (8): 083701. doi: 10.7498/aps.65.083701
Optical trapping techniques and the ability to tune the atomic interactions both have made the unprecedented progress in the quantum gas research field. The major advantage of the optical trap is that the atoms are likely to be trapped at various sub-levels of the electronic ground state and the interaction strength can be controlled by Feshbach resonance. Optical trapping methods in combination with magnetic tuning of the scattering properties directly lead to the experimental achievements of Bose-Einstein condensation (BEC) of Cesium, which at first failed by using magnetic trapping approaches due to the large inelastic collision rate. The rapid loss of cesium atoms due to the inelastic two-body collisions greatly suppresses the efficient evaporative cooling to obtain a condensate. For optical production of cesium atomic BEC, it is necessary to prepare a large number of Cs atoms at specified state in an optical trap for condensation, especially for an efficient forced evaporation cooling. In this paper, we demonstrate our research on enhancing the loading rate of the atoms by using a dimple trap combined with a large-volume optical dipole trap (reservoir trap). In our work, the cold cesium atoms are prepared by a three-dimensional degenerated Raman sideband cooling, and then loaded into a large-volume crossed dipole trap by using the magnetic levitation technique. Effective load of the dimple optical trap is realized by superposing the small-volume dimple trap on the center of the largevolume optical trap. The theoretical analyses are performed for the magnetically levitated large-volume crossed dipole trap in variable magnetic field gradients and uniform bias fields. Optimal experimental values are acquired accordingly. The combined potential curve of the dimple trap, which is superimposed on the magnetically levitated large-volume dipole trap, is also given. The loading of precooled atoms from Raman sideband cooling into the magnetically levitated large-volume optical trap is measured in variable magnetic field gradients and uniform bias fields. Different loading results of the dimple trap are investigated, including direct loading after Raman sideband cooling, the large-volume optical trap and the magnetically levitated large-volume dipole trap without anti-trapping potential. Comparatively, the atomic number density is enhanced by a factor of ~15 by loading the atomic sample from the magnetically levitated large-volume dipole trap into the dimple optical trap. The experimental results lay a sound basis for the further cooling and densifying the atomic cloud through the evaporating cooling stage. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for atomic species with large atom mass.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (8): 084201. doi: 10.7498/aps.65.084201
The physics-based methods that can effectively improve the image contrast in turbid media while truly preserving all the detailed information, have received great attention in recent years. The range-gated imaging (RGI), polarization difference method (PD) and polarization-based range-gated technology (PRG) are three effective methods of enhancing the contrast. However, the relationship between the extent of contrast enhancement and the imaging distance for each method has not been revealed. In this paper, a compact disc (CD) plate is set to be in the intralipid with different concentrations contained in a glass cell and imaged by RGI, PD, PRG and raw intensity imaging (RI). The Indian ink is used as the absorber which eliminates the multiple scattered photons and achieves the range-gated technology. In order to investigate the number of the scattered photons filtered out by the 4 methods, the image intensity curves are acquired while the imaging distance, the distance between the target surface and the front surface of the cell, is set to be 26 mm. The results indicate that PRG filters out the largest number of the scattered photons, followed by PD and RGI because the long imaging distance results in more multiple scattering photons. Then the influence of the imaging distance on the image intensity is investigated by the 4 methods. The image intensity is recorded while the imaging distance varies from 22 mm to 30 mm with even increments. Then four sets of intensity curves are plotted against the imaging distance corresponding to RI, RGI, PD and PRG respectively. Based on the RI, three sets of image intensity difference curves of RGI, PD and PRG are also calculated. The tendencies of the curves show that these imaging methods have their own imaging distance thresholds. It implies that the numbers of the photons filtered out by these methods are all constant when their imaging distances exceed their thresholds of 22 mm, 30 mm and 30 mm, respectively. Finally, the effect of the imaging distance on the contrast variation is studied in turbid media with two different scattering coefficient 0.714 cm-1 and 1.19 cm-1. The results show that PRG is superior to other methods in contrast enhancement. In addition, the imaging distances of the 4 methods under the same image contrast are obtained, showing that under the same contrast increment, the PRG presents the largest imaging distance enhancement, followed by PD, RGI and RI. The increase of scattering coefficient could also cause the decrease of the imaging distance. These results can be very useful to understand the mechanism of imaging in turbid media and are of great significance for improving the ability to recognize the target.
Concentration and pressure measurement of water vapor in sealed glass containers based on tunable diode laser absorption spectroscopy
2016, 65 (8): 084202. doi: 10.7498/aps.65.084202
Water vapor in a sealed glass container processed by lyophilization is a main factor for drug metamorphism. The key to knowing whether there is a leakage occurring in the container is how to detect water concentration and pressure in the sealed container quickly and accurately. In the present paper, a strong absorption line of H2O near 1.39 m is carefully selected to avoid the interference of neighboring transitions. A distributed feedback laser semiconductor laser near 1.396 m is employed as the light source with a power of 10 mW and typical linewidth of 2 MHz by combining with tunable diode laser absorption spectroscopy technique, the concentrations and pressures of water vapor in the sealed container are successfully detected under static condition and dynamic condition. In order to isolate the interference absorption from the ambient water vapor in the air, a differential absorption technique is employed in our experiment, which makes our system simpler than routine nitrogen purging based system. During the measurement, the second harmonic signal is utilized for measuring the concentration and pressure, the concentration is retrieved by the peak value while the pressure is calculated by the full width at half maximum. For the measurement of concentration ranging from 0.2% to 12%, the linear correlation coefficient between the real values and the inversed values and the standard deviation ratio are 0.9978 and 4.81%, respectively. For the measurement of pressure, the correlation coefficient and standard deviation ratio are 0.982 and 5.6%, respectively. The minimum detection limits of the concentration and pressure are 400 ppm and 2.5 Torr, respectively. Moreover, in order to test the system for on-line applications in the pharmaceutical industry, measurements are performed in vials which are placed on a rotary stage to simulate the process of the assemble line. In particular, the amplitude of sinuous signal without absorption is used as the reference signal to validate whether the vial is in the optical path. Besides, this amplitude is also utilized to normalize the laser power. The results show that our system can handle about 300 bottles in one minute, which can meet well the requirements for rapid and real-time detections. This system can be applied directly to the medicine bottle on-line detection, and multicomponent detection can also be realized by employing two or more lasers (e.g. H2O, oxygen, etc.). In the future, we plan to build a system for detecting water vapor and oxygen simultaneously, as oxygen is another import indicator for drug metamorphism.
Output pulse compressibility of the chirped pulse fiber amplification based on the dissipative solitons
2016, 65 (8): 084203. doi: 10.7498/aps.65.084203
The all-normal-dispersion mode locked fiber laser can produce the dissipative soliton pulses because the laser can tolerate much more nonlinear phase shift than the other mode locked fiber lasers. Such large energy mode locked fiber lasers are excellent seed pulse sources for generating very large-energy ultrashort pulses with fiber chirped pulse amplification (CPA) systems. However, the spectral amplitude modulation carried by the dissipative soliton pulses will severely restrict the compressibility of the output pulses from the typical CPA system. Therefore, it is necessary to investigate and design a suitable CPA system for improving the compressibility of the output pulses according to the properties of dissipative solitons. In this paper, using the dissipative solitons generated by the all-normal-dispersion fiber laser with different spectral filter bandwidths as the input seed pulses, the compressible properties of the pulses for the CPA system with both the grating pair stretcher and the fiber stretcher are investigated. Our simulation results show that, for such a large-energy dissipative soliton seed pulse, when the grating pair stretcher is used in the CPA system, the spectral amplitude modulation of the seed pulse can be mapped to the temporal amplitude modulation by the stretcher, and amplified by the subsequent fiber amplifier, which introduces additional nonlinear phase, finally restricts the compressibility of the output pulses; when the normal-dispersion fiber stretcher is used, the interaction between the group velocity dispersion and the self-phase modulation can not only eliminate the influence of the modulated spectrum of the dissipative soliton on the compressible properties of the pulses, but also make it possible to evolve the pulse self-similarity in the fiber stretcher, and thus improve the compressibility of the output pulses of the CPA system. For the normal-dispersion fiber stretcher CPA system, the compressibility of the output pulses is mainly determined by the fiber stretcher length. If the fiber length is too short, the compressibility of the output pulses may be affected by the uncompleted self-similar evolution of the pulse, while the pulse compressibility is also restricted because the pulse spectral width may exceed the amplifier gain bandwidth due to the self-similar evolution process if the fiber length is too long. Moreover, for the dissipative soliton seed pulses, both the compressibility of the output pulses and the energy ratio of the main pulse to the total pulse for the CPA system with the fiber stretcher are better than those with the grating pair stretcher when the normal fiber stretcher length is suitably optimized.
Comparison and validation of band residual difference algorithm and principal component analysis algorithm for retrievals of atmospheric SO2 columns from satellite observations
2016, 65 (8): 084204. doi: 10.7498/aps.65.084204
Remote sensing technology provides an unprecedented tool for the continuous and real-time monitoring of atmospheric SO2 from volcanic eruption and anthropogenic emission. The Global Ozone Monitoring Experiment (GOME), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), and Ozone Monitoring Instrument (OMI) have high SO2 monitoring capability. The OMI, which was launched on the EOS/Aura platform in July 2004, has the same hyperspectral measurements as the GOME and SCIAMACHY, but offers the improved spatial resolution at nadir (1324 km2) and daily global coverage for short-lifetime SO2. For OMI operational SO2 planetary boundary layer (PBL) retrieval, the previous band residual difference (BRD) algorithm has been replaced by principal component analysis (PCA) algorithm, which effectively reduces the systematic biases in SO2 column retrievals. However, there are few studies on the evaluations and validations of PCA SO2 retrievals over China, and the long-term comparisons with BRD SO2 retrievals also need to be conducted. In this study, the accuracies of PCA and BRD SO2 retrievals are validated by using ground-based multi axis differential optical absorption spectroscopy (MAX-DOAS) located in Beijing, and regional atmospheric modeling system, community multi-scale air quality (RAMS-CMAQ) modeling system model which can simulate the vertical distribution of atmospheric SO2. Moreover, BRD and PCA SO2 retrievals from oceanic area, eastern China and Reunion volcanic eruption are compared to find the long-term trend and spatiotemporal differences between SO2 columns. Finally, the uncertainty of SO2 retrieval, caused by measurement errors, band selection and input parameter errors in radiative transfer model, are analysed to understand the limitations of BRD and PCA algorithms. Results show that both PCA and BRD SO2 retrievals over Beijing are lower than ground-based MAX-DOAS measurements of SO2. PCA and BRD SO2 retrievals over eastern China are lower than the simulated SO2 columns from RAMS-CMAQ in winter 2008, but in July and August BRD SO2 columns are higher than RAMS-CMAQ simulations. The values of SO2 columns from BRD over China are more consistent with those from ground-based MAX-DOAS and RAMS-CMAQ model than from PCA. Although PCA algorithm effectively reduces the noise in SO2 column retrieval, SO2 columns from PCA over China are lower than those from BRD. For oceanic area where SO2 amount is nearly zero, the standard deviation of PCA results is lower than that of BRD, but the absolute value of averaged PCA SO2 column is larger than that of BRD. In the case of Reunion volcanic eruption with SO2 columns larger than 25 DU, the BRD SO2 columns are lower than PCA retrievals. Meanwhile, with the increase of SO2 column, the difference between BRD and PCA SO2 retrievals increases. Detailed uncertainty analysis shows the influences of measurement errors, band selection and inputs of radiative transfer model on the retrieval results. This study is important for developing the retrieval algorithm, and can also improve the application of OMI SO2 products.
2016, 65 (8): 084205. doi: 10.7498/aps.65.084205
Lanthanide-doped upconverting fluoride nano-and micro-materials have aroused much research interest due to their potential applications in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging. In order to synthesize Ln3+ doped crystals with favorable optical properties, such as high upconversion (UC) efficiency and controllable emission profile, the two major parameters that affect luminescence processes including host materials and lanthanide activator ions should be selected appropriately in the synthesis process. Majority of scientists deem that lanthanide doped fluoride nano-and micro-materials with low phonon energy are currently the efficient UC host materials. In this work, Yb3+ and Er3+ ions codoped NaYF4 and LiYF4 microcrystals are synthesized by a facile hydrothermal method with ethylene diamine tetraacetic acid (EDTA) as a chelator. The NaYF4:Yb3+/Er3+ and LiYF4:Yb3+/Er3+ microcrystals are characterized by X-ray diffraction (XRD), scanning electron microscope(SEM), and the photo-luminescence spectra method. The influences of EDTA on the crystal phase, shape and upconversion luminescence are explored in detail. According to the results of XRD and SEM, the pure hexagonal phased NaYF4:Yb3+/Er3+ rod-like microcrystals each with smooth surface are all around 12 m in the length. While the pure tetragonal phased LiYF4: Yb3+/Er3+ microcrystals each with smooth surface are octahedral in shape, and their average size is around 12 m. Under near infrared (NIR) 980 nm excitation, the two dominant emission peaks of Er3+ ions at 544 nm and at 650 nm are observed in NaYF4 and LiYF4 microcrystals, which can be assigned to the transitions of (2H11/2/4S3/2)4I15/2 and 4F9/24I15/2, respectively. It is found that the upconversion luminescence intensity of NaYF4:Yb3+/Er3+ microcrystals is about two times that of LiYF4:Yb3+/Er3+ microcrystals under the same excitation conditions. The ratio of red-to-green emission of Er3+ ions in LiYF4 microcrystals is higher than that of the NaYF4:Yb3+/Er3+microcrystals. The changes of the spectra in the different hosts could stem from two sources: one is that the nonradiation relaxation probability relative to phonon energy of matrix, the other is that the radiative transition probability relative to the site symmetry of the crystal field acting on the ion. The ratios between 5D07F1 and 5D07F2 transitions of Eu3+ ions in NaYF4:Yb3+/Eu3+ and LiYF4:Yb3+/Eu3+ microcrystals are employed to compare and elucidate the site symmetry of the crystal field for Ln3+ ions. Note that the ratio of 5D07F1 and 5D07F2 transitions in NaYF4:Yb3+/Eu3+ microcrystals is smaller than that of the LiYF4:Yb3+/Eu3+ microcrystals, which indicates a much higher radiative relaxation rate in NaYF4 microcrystals than in LiYF4 microcrystals. The organic ligands of EDTA on the surface of microparticles affect the properties of luminescence through changing the nonradiative relaxation rate, resulting in the different R/G ratios in NaYF4 and LiYF4 microcrystals. This result can be further supported by the comparison between NaYF4 and LiYF4 microcrystals without EDTA added in the preparation process. The micro-sized luminescence materials usually present stronger upconversion luminescence because of their higher degree of crystallinity and less surface quenching centers. Thus, the Er3+ codoped NaYF4 and LiYF4 microcrystals exhibit strong green upconversion emission, which has potential applications in full-color displays and microelectronic devices.
2016, 65 (8): 084206. doi: 10.7498/aps.65.084206
According to the statistical theory, the influence of laser linewidth on the performance of optical heterodyne system is studied. Also, the effect of laser linewidth on the visibility of fringe pattern is discussed. The expressions of self-correlation function power-spectrum and definition of visibility of fringe patterns are obtained in this paper. Based on the analytical expressions, the numerical simulation is performed. The obtained results demonstrate that the laser linewidth influences the visibility of fringe patterns according to the result shown in Fig. 3, and that the intermediate frequency can be still detected by heterodyne detection technique as the laser linewidth increases. For different linewidths, the measurement of intermediate frequency is accurate without the influence of noise as the delay d between the received signal and locally generated signal is less than the coherence time c of laser source. If the delay d is greater than the coherence time c, the full width at half maximum of intermediate frequency in the frequency spectrum of the output signal of photodetector will broaden as the laser linewidth increases. However, for a wide linewidth, the measurement of intermediate frequency is inaccurate due to the influence of noise when the delay d is greater than the coherence time c. The wider the linewidth, the less accurate the measurement of intermediate frequency will be. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1 MHz, which has an 8.1 km channel path. In our experimental set-up, a cooperative target is employed to modulate and reflect the transmitted beam. In this way, an echo signal is received. The mixing process of the received signal and local signal on the photodetector surface produces an electrical current known as the photomixing current. A spectrum analyzer is used to observe the output signal of detector. The obtained spectrum shows that intermediate frequency can be checked, which is in agreement with the theoretical result. In this work, the obtained conclusions can be directly used to choose a proper laser for optical heterodyne system. According to the target characteristics and measurement requirements, and by following the conclusions obtained in this paper, the laser linewidth can be evaluated.
2016, 65 (8): 084207. doi: 10.7498/aps.65.084207
Free space optical-communication (FSO) has gained significant importance due to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power and low mass requirement. However, the performance of FSO is degraded in the turbid and turbulent atmosphere, dramatically. Various techniques are proposed to cope with the turbid media and turbulence in atmosphere, e. g. aperture averaging, diversity, adaptive optics, modulation and coding and orbital angular momentum. However, in the above systems with point-to-point optical communication structure, there exist obvious drawbacks or they are complex and expensive, and thus difficult to use in practice. In this article, array-to-point optical communication (APOC) with good performance in turbid and turbulent atmosphere is demonstrated. The strongly disturbed communication channel can be expressed as a circular complex Gaussian transmission matrix, and the transmitted field is described as a linear combination of the fields coming from different and independent segments of the digital micro-mirror device (DMD), so that the cross terms are averaged on the surface of bucket detector. Instead, the contributions of all segments for each light field nearly becomes equally weighted. Turbulence and other noises are reduced for the incoherence with sampling matrix based on the second-order correlation which has widely been used in ghost imaging and LIDAR. Consequently, narrow-band optical filter is not required at the receiver. The decoding algorithm is a new signal processing strategy from information technology, compressed sensing, which discards low frequency components in sampling process and recovers the signal by conducting convex optimization. Numerical simulations and experiments with binary and multi-bits level signals are demonstrated to show that the bit error rate of the proposed method APOC is approximately 10-4-10-2, which is feasible for the optical communication in such complex communication channels. The communication rate, limited by the frequency of the DMD and the sampling rate of the receiver, could reach hundreds of kbit/s, and with improved technology a rate of Mbit/s should be attainable. This proposed APOC could realize optical communication in turbid and turbulent atmosphere and thus offers a very effective approach to promoting the implementation in military and rescue.
2016, 65 (8): 084208. doi: 10.7498/aps.65.084208
In order to study the computer-aided alignment for the off-axis reflective zoom optical system, the wavefront aberration of the off-axis reflective zoom system across the field of view needs to be detected. Obtaining the wavefront aberration across the field of view could improve the accuracy of the computer-aided alignment for the off-axis reflective zoom optical system. Restricted by the current wavefront aberration detection technology, only the wavefront aberration at a field degree of 0 could be detected. To solve this problem, a new method to detect the wavefront aberration of off-axis reflective zoom system across the field of view is proposed. According to the traditional autocollimation interferometry method, we improve the detection method by substituting the scan of standard plane mirror with the deformable mirror, replacing the interferometer with Shark-Hartmann sensor and employing the accurately calibrated laser source array to realize the wavefront aberration detection at multiple field of view simultaneously. The simulation shows that the residual wavefront aberration root-mean-square values after compensating for the deformation mirror in the following 6 fields of view (0, 3), (0, 4.2), (0, 5.5), (0, 7), (0, 9.8), and (0, 14) are 0.00039, 0.00075, 0.00024, 0.00017, 0.00053, and 0.0057, respectively. It shows that the detection method we proposed is suitable for the computer-aided alignment technology for the off-axis reflective zoom system.
2016, 65 (8): 084209. doi: 10.7498/aps.65.084209
Silicon devices are extensively used in space and other radiation-rich environments. They must withstand radiation damage processes that occur over wide range of time and length. Ion implantation technique, one of the most important process in the fabrication of integrated circuits, can also create the displacement damage in silicon lattice. Exposure of silicon wafer or silicon device to radiation causes the creations of variety of defects and has adverse effects on the electrical properties of devices. Although phenomenological studies on the radiation effects in silicon have been carried out in the past decades, the features of multi-scale of displacement damage make it difficult to characterize the defect production and evolution experimentally or theoretically. Recently, the silicon device with ultra-low leakage current was shown to be very sensitive to the permanent displacement damage induced by single particles, called single particle displacement damage (SPDD) event. To the best of our knowledge, the investigation of single particle displacement damage (SPDD) event in silicon device by the coupling molecular dynamics (MD) and kinetic Monte Carlo (KMC) techniques has not yet been reported so far. In this paper, MD simulations are combined with KMC simulations to investigate the formation and evolution of SPDD event in silicon. In MD simulations, Tersoff potential is used to describe the Si-Si atomic interactions. The potential smoothly joins to Ziegler-Biersack-Littmark potential that describes the energetic short range interactions well. All atoms in the MD cell are allowed to evolve 0.205 ns to track the damage production and short-term evolution. A multi-phase simulations are performed to improve the simulation efficiency. Then the nearest neighbor criterion is employed to identify the configurations and spatial distributions of interstitials and vacancies, which are used as input in KMC simulations to study the thermal diffusion and interactions of those defects in the time interval from 0.205 ns to 1000 s. The results show that no defects are missing when transferring from MD to KMC simulation and the whole damage obtained in MD simulations is reproduced in KMC simulations. Since the production and evolution of defects are simulated, the SPDD current could be calculated based on Shockley-Read-Hall theory. We derive the formula to calculate the SPDD current and its annealing factor related to interstitials and vacancies in the depletion region. The calculated annealing factors of defects are compared with the annealing factors of SPDD currents and also with the experimental results. The results show that an annealing factor of defects has the same value as the annealing factor of an SPDD current when only one type of defect is considered in the calculations, while there are some differences between these two annealing factors when two and more types of defects are considered. The annealing factors of defects can be used to represent the annealing behaviors of SPDD currents since the divergences between these two annealing factors are not significant. Finally, SPDD current annealing factor based MD simulation results obtained with Tersoff potential are compared with the results in our previous study in which the Stillinger-Weber potential is used, and also compared with experimental results. The comparisons show that the simulation results with considering both Stillinger- Weber potential and Tersoff potential are in good agreement with experimental results. Compared with the calculated results with considering the Tersoff potential, the results with considering the Stillinger-Weber potential are closer to experimental results.
2016, 65 (8): 084501. doi: 10.7498/aps.65.084501
We investigate the response property of a linear system that is excited by the base excitation. The linear system contains the ordinary damping or the fractional-order damping. In our studies, the base excitation is in the harmonic form or in the general periodic form. When the base excitation is in the harmonic form, we obtain the dynamic transfer coefficient by the undetermined coefficient method. When the base excitation is in the general periodic form, we first expand the excitation into the Fourier series, then, according to the linear superposition principle, we obtain the dynamic transfer coefficient that is induced by each harmonic component in the excitation. By expanding the general periodic excitation into the Fourier series, we can solve the non-differentiable problem that is induced by the periodic base excitation for the numerical calculations. Based on the Grnwald-Letnikov definition, the discretization formula for the fractional-order system is obtained explicitly. The analytical results are in good agreement with the numerical simulations, which verifies the validity of the analytical results. Both the analytical and the numerical results show that the dynamic transfer coefficient depends on the fractional-order of the damping closely. The dynamic transfer coefficient can be controlled by tuning the value of the fractional-order. For the general periodic excitation, when the frequency is fixed, the dynamic transfer coefficient that is induced by the high-order harmonic component may be stronger than that induced by the low-order harmonic component in the base excitation. Hence, the effect of the high-order harmonic component in the excitation cannot be ignored although its amplitude is small. Further, when the base excitation is in the full sine form, or the square form, or the triangular form, the response property of the system can be described by center frequency, resonance peak, cutoff frequency, and the filter bandwidth. For a fixed fractional-order, the center frequencies of each order corresponding to the response, obtained by the three kinds of the periodic base excitations mentioned above, are identical. However, the corresponding resonance peaks are different. The resonance peak and the filter bandwidth are both maximal when the base excitation is in the square form. The resonance peak and the filter bandwidth are both minimal when the base excitation is in the triangular form. We believe that our results are useful for solving the vibration problem in the engineering field such as the vibration isolation and the vibration control.
Systematic experimental study on inclined orifice flow and the measurement of the angle of repose are carried out in this work. The inclined orifice flow is formed by glass beads in an inclined channel. The flow is discharged near the bottom of the channel under gravity. The flow rates are measured at various inclination angles of the channel and opening sizes of the orifice. We then record the inclination angle when the rate becomes zero. We compare this zero-rate inclination angle with the repose angle of glass-beads, and the internal friction angle is determined by the yield stress obtained from a direct shear experiment. It is interesting to find that the experimental values at these three measured critical angles are equal within the experimental errors: 1) the supplementary angle of the extrapolating inclined angle at which the flow rate becomes zero and the inclined hole of diameter approaches infinitely large value (i. e. D), s= 180-c, where c is the critical angle for the inclined hole of diameter D and cc(D); 2) the repose angle r of a cone-shaped pile, which is formed when particles fall from the top point of the heap onto a smooth bottom plate; and 3) the internal friction angle that is measured by direct shear experiment. This result intends to support that the solid-liquid transitions occurring in the inclined orifice flow and free surface of granular heap, and the Coulomb yield occurring in the bulk of the granular solid all originate from the same critical property. Owing to the fact that the internal stresses and strains of samples in the three cases all have complicated and nonuniform distributions so that they cannot be analyzed quantitatively at present, Only some qualitative discussion on this issue is given in this paper.
A hybrid traffic flow model with considering the influence of adaptive cruise control vehicles and on-ramps
2016, 65 (8): 084503. doi: 10.7498/aps.65.084503
Recently, autonomous vehicles and the relevant studies have attracted much attention. Adaptive cruise control (ACC), which is a kind of cruise control system for vehicles, automatically adjusts the vehicle speed to maintain a safe distance from vehicles ahead. Since the vehicle with ACC (called ACC vehicles) is semi-autonomous, the performance of ACC vehicle must be quite different from that of manual vehicle. The characteristics of traffic flow with ACC vehicles should be carefully investigated, especially when the traffic system is a bit complicated, such as on-ramp system. The primary objective of this paper is to propose a traffic flow model to simulate the traffic flow with considering the influence of ACC vehicles and on-ramps. Based on the model proposed by Yuan in 2009 [Yuan Y M 2009 Ph. D. Dissertation (Hefei: University of Science and Technology of China)], a hybrid traffic flow model with considering the influence of ACC vehicles and on-ramps is developed. Considering the differences between ACC and manual vehicles, a car-following sub-model based on constant time headway principle is developed for ACC vehicles, while an MCD cellular automata sub-model is proposed for manual vehicles. Besides, a new parameter, , is introduced to show different psychologies of drivers when changing lane from on-ramp to main road. The lane-changing model for vehicles on-ramp is developed as well. At the end, numerical simulation is demonstrated to study the influence of ACC vehicles on traffic flow at on-ramp, and to reveal the influence of parameters on the proposed hybrid model (i.e., the length of merge area, the desired time headway of ACC vehicle and ) on model performance. The results of this paper are as follows. 1) When the ACC vehicles exist in a traffic system, the performance of traffic flow in a on-ramp area is improved: the influence of merged vehicles on main road is reduced, and the average speed and volume are increased. 2) The increase of ACC vehicles can help to alleviate traffic congestion in both congestion duration and scope aspects. 3) The newly proposed hybrid model is sensitive to the length of merge area lw, the desired time headway of ACC vehicle Hd and lane-changing psychology parameter : the decrease of Hd and the increase of can both improve the average speed and volume of traffic flow. In addition, when the volume of on-ramp is small, the speed and volume of main road can be improved by enlarging lw. When the volume of on-ramp is large, a small lw will be better for traffic flow.
Numerical simulation of laminar flow past two side-by-side cylinders by discontinuous Galerkin method
2016, 65 (8): 084701. doi: 10.7498/aps.65.084701
Investigations of vortex dynamics about two circular cylinders in a side-by-side arrangement help the understanding of flows around more complex structures, which are found to have many engineering applications. These applications involve offshore structures, power generation, micro-turbine engines, cooling towers, and paper machine forming fabrics, etc. Therefore, two-dimensional compressible laminar flows over two cylinders in side-by-side arrangement are numerically investigated at low Reynolds number. The high-order discontinuous Galerkin method is employed to simulate the flow, which combines the advantages associated with finite element and finite volume methods. As in classical finite element method, the spatial accuracy can be obtained by the high-order polynomial approximation within an element rather than by stencils as in finite volume method. The curved triangle is used to represent the wall boundary of cylinder to maintain the high-order accurate simulation. Then the characteristics of the wake flow are identified by capturing the vortex structure. After verifying the rationality of the method, the influences of gap spacing on vortex shedding and mechanical characteristics are analyzed. The results reveal that the flow depends to a large extent on the gap spacing between the two cylinders, which can change the vortex shedding pattern. At the gap spacing S*=1.1, wake flow pattern resembles the vortex street of a single bluff body. The flow in the gap is too weak to affect the wake pattern, leading to the complete suppression of vortices shed on the gap sides of both cylinders. At the gap spacing S*=1.4, the results reveal that the gap flow is deflected from one cylinder to another. Meanwhile, the wakes represent randomly flip-flopping between two states of the gap flow direction, which is called the flip-flopping wake pattern. The flow is no longer periodic but becomes drastically unsteady. Anti-symmetric flow pattern is predicted for gap spacing S*=2.5, indicating that two parallel vortex streets are anti-symmetric with respect to the centerline. With further increasing the gap spacing to S*=4, the symmetric flow pattern is observed. Furthermore, the flow preserves its structure very far downstream without any distortion. With the increase of the cylinder spacing, the average drag coefficients are declined significantly, and the absolute value of average lift coefficient decreases simultaneously. The Reynolds number has a little influence on the average drag coefficient. As the Reynolds number increases, the average lift coefficient decreases, while the vortex shedding frequency increases.
2016, 65 (8): 084702. doi: 10.7498/aps.65.084702
The process of the droplet impact onto the liquid film, as one of the basic multiphase problems, is very important in many fields of science and engineering. On the other hand, the problem is also very complicated since there are many parameters that may influence the process of the droplet impact on the liquid film. To clearly understand the physical phenomena appearing in the process droplet impact on the liquid film, a parametric study on this problem is conduced based on a recently developed lattice Boltzmann method in which a lattice Boltzmann model is used to solve the Navier-Stokes equations, and the other is adopted to solve the Cahn-Hilliard equation that is used to depict the interface between different phases. In this paper, we mainly focus on the effects of the Reynolds number (Re), the Weber number (We), the relative thickness of film (h) and the surface tension () on the dynamic behavior of interface between different phases, and the velocity and pressure fields are also presented. It is found that with the increase of Re and We, the phenomena of crown and entrainment can be observed obviously during the process of droplet impact onto the liquid film, and the radius of the crown seems not dependent on the We and Re where the relative thickness of film and surface tension are fixed to be 0.5 and 0.003. However, when Re becomes much larger, the splashing phenomenon is produced, and the small droplets caused by the splashing can fall and then impact onto the liquid film again. We also find that if the relative thickness of film is small, the surface tension, Re and We are set to be 0.003, 480 and 500, the film can break up during the process of the droplet impact onto the liquid film, while with the increase of relative thickness, although more liquid are induced in the splashing process, the film cant break up. In addition, with the increase of surface tension, the resistance which prevents the change of interface becomes large, and thus the change of interface is not large when the droplet impacts onto liquid film, as expected. And finally, a quantitative study on the relation between the radius of crown (formed by droplet impact onto liquid film) and the time is also performed, and the expression r/(2R) Ut/(2R) where the parameter is about 1.0 and is also independent of We and Re, can be used to describe the relation.
Numerical investigation on the influence of gas-particle two-way coupling to the shock fluid in the two-dimensional Lagrangian framework
2016, 65 (8): 084703. doi: 10.7498/aps.65.084703
When an extreme shock wave releases from the free surface of the material, some high speed particulate matters will be ejected from the material body and enter into the background gas. This induced multiphase mixing phenomenon is known as the ejecta mixing. Ejecta mixing is one of the most important problems in the scope of inner explosive compression engineering, and it is also a frontier research subject of the impact dynamics, multiphase fluid dynamics, computational mathematics, etc. The properties of ejecta mixing have been investigated experimently and analytically for many years. However, the results of numerical simuliation are very rare. At present, the ejecta mixing study mainly focuses on the gas particle one-way coupling, that is, the interests of existing works are in the characteristics of the ejected particulate matter transport in the gas. In fact, after a large number of particles entering into the gas, the gas and the particles will interact with each other. So it is necessary to consider the feedback of particles to the gas. In this paper, the theoretical modeling of gas particle two-way coupling, the discrete algorithm of the mathematical model and the particle phase feedback effects on the gas shock wave propagation are investigated in the framework of Lagrangian coordinates. In order to obtain the details of ejecta movement, the particle trajectory model is chosen as the basic model, and then the governing equations including interactions between gas phase and particle phase are derived. For giving the specific calculation formula, the physical meanings of the coupled interaction source terms in the Lagrangian framework are analyzed and a stable numerical scheme is given based on the staggered strategy. We also devise two different computing models of ejecta mixing, the planar and the column configurations, and then the numerical simulations are carried out. The phenomenon of gas shock speed acceleration caused by particle feedback is found, and the distributions of the physical quantities, such as density, velocity, specific internal energy, pressure, in the gas area are changed. Especially for the convergent configuration, the feedback effects will be amplified further by the geometrical shrinkage, which may have a significant influence on the performance of the inner explosion compression, owing to the obvious uniformity variation of the gas flow field and the gas shock rebound in advance. The mathematical model, the numerical method and the new physical findings in this paper, will provide an important theoretical support for the in-depth understanding of the ejecta mixing and also for the solving of the corresponding engineering application problems.
2016, 65 (8): 084704. doi: 10.7498/aps.65.084704
Analytical investigations are performed for pressure driven flow of an electrically conducting, incompressible and viscous fluid in a polyelectrolyte-grafted nanotube by using Bessel functions. Nanofluidic tubes whose walls are covered by polyelectrolyte materials, named the fixed charge layer (FCL), are identified as soft nanotubes. The flow relies on an externally imposed pressure gradient and an induced reverse electroosmotic force produced by the streaming potential field which is spontaneously developed due to the ionic charge migration with the fluid flow. Many parametrical ranges are determined to ensure the validity of Debye-Hckel approximation. The analysis is based on the solutions of the linearized Poissson-Boltzmann equation and modified Navier-Stokes equation. To obtain the streaming potential, we use a numerical treatment to solve an integral equation governing the streaming potential. Finally, the electrokinetic energy conversion efficiency is studied. The result shows that both the streaming potential and energy conversion efficiency monotonically increase with the FCL thickness d increasing. However, they present a monotonic decrease trend with the increase of K, which is the ratio of the characteristic scale of the mobile charges to the fixed charge within the FCL. We compare the results in a soft nanotube with those in a rigid one, whose zeta potential is equal to the electrostatic potential at the solid-polyelectrolyte interface of the soft nanotube. We find that the electric potential in a soft nanotube is higher than that in the corresponding rigid nanotube, which results in a larger streaming potential in the soft nanotue. Moreover, for the parameter ranges considered in this work, our results show that the electrokinetic energy conversion efficiency in a soft nanotube is 1.5-3 times higher than that in a rigid nanotube. These findings are important for investigating the streaming potential and electrokinetic energy conversion efficiency in soft nanotubes. They can be used as a kind of new method to enhance the energy conversion efficiency of the electrokinetic transport in nanotube.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2016, 65 (8): 085201. doi: 10.7498/aps.65.085201
Based on the theory of conservation of momentum, a theoretical method of calculating the shock pressure induced by laser loading via diagnosing plasma recoil momentum is presented. When a high-power laser irradiates a solid target surface, the plasma jet with high velocity induced by laser has a recoil effect on the target. Then the plasma recoil momentum induced by laser irradiating solid target can be calculated by the distribution of electron plasma. At the same time, the subcritical electron plasma density could be measured by interferometry and the supercritical plasma density could be fitted into exponential function form. So the variation of shock wave pressure could be calculated via diagnosing plasma recoil momentum. This method does not consider the relationship between D and u, nor uses the window material nor needs the steady shock propagation. It is a useful method of studying the material property under high strain rate and isentropic compression. Numerical simulation results using one-dimensional radiation hydro code called MULTI for laser intensities ranging from 51012 W/cm2 to 51013 W/cm2 are presented. The electron temperature is nearly equal to the ion temperature for the laser pulse duration 2 ns but much greater than the ion temperature for = 1 ns. This means for that ns pulse duration, the difference between electron and ion temperature could be ignored in general. And in order to fit the shock pressure value more exactly, the density of ablation surface nabl = n0exp(-1) is used in the simulations. The simulation results indicate that the value of calculating shock pressure obtained via diagnosing plasma recoil momentum is similar to the shock pressure calculated by MULTI simulation for ns pulse duration. And the value of calculating shock pressure is also similar to the experimental value for pulse duration = 5 ns. From the simulation results, it is obvious that the method of calculating the shock pressure via diagnosing plasma recoil momentum is effective and feasible.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (8): 086101. doi: 10.7498/aps.65.086101
The X-ray timing and polarization telescope proposed in China is for imaging spectroscopy in an energy range of 1-30 keV. To obtain the high energy spectrum response with a large effective area, W/Si multilayer mirrors each with a mirror thickness of only 0.3 mm are used. This makes the figure accuracy of the mirror and the distortion caused by the multilayer stress an important issue during the telescope development. W/Si multilayer mirror is an important component of X-ray telescope for astronomical observation. To reduce the effect of the multilayer stress and maintain a high reflectivity at the same time, the W/Si multilayers prepared by magnetron sputtering deposition are annealed at low temperatures of 150 ℃, 175 ℃ and 200 ℃, respectively, for 3 h. The stress of the multilayer is determined based on the surface figure measurements of each sample before and after annealing. The X-ray reflectance and layer structure of the multilayer are characterized by the grazing incidence X-ray reflectometry (GIXR) and the reflectance fitting curves. The first Bragg peak reflectivity of the as-deposited sample is 67% at 8.04 keV and the multilayer stress is around -260 MPa. After annealing at 150 ℃ for 3 h, the first Bragg peak reflectivity and the layer structure are almost the same as before annealing, while the stress reduces 27%. The fitting results display almost the same interface widths of the multilayer before and after annealing. As the temperature increases to 175 ℃, the first Bragg peak reflectivity drops by about 2%. The multilayer structure begins to deteriorate and the W/Si interface widths increase from 0.346 nm/0.351 nm to 0.356 nm/0.389 nm, according to the fitting results, while the stress reduces about 50%. After annealing at 200 ℃ for 3 h, the stress reduces 60% and the stress decreases down to about -86 MPa. However, the first Bragg peak reflectivity drops by 17%, and the layer structure undergoes significant change after annealing. The W/Si interface widths increase from 0.352 nm/0.364 nm to 0.364 nm/0.405 nm. The GIXR results also show that the d-spacing between the multilayers decreases after annealing, and a higher annealing temperature causes a larger decrease. The stress reduction should be mainly caused by the enhanced atomic diffusions at the interface and inside the layer structure during the annealing. The enlarged interface and the possible compound formation contribute to the decrease of X-ray reflectance and the layer compactness. These results provide important guidance for developing low-stress X-ray multilayer mirrors.
Characteristics of acoustic phonon transport and thermal conductance in multi-terminal graphene junctions
2016, 65 (8): 086301. doi: 10.7498/aps.65.086301
By using non-equilibrium Greens function method, we investigate the transmission rate of acoustic phonon and thermal conductance through a parallel multi-terminal graphene junctions, the relationship between the thermal-transport property in each terminal and the number of quantum terminals, the relationship between the thermal-transport property in each terminal and the relative position of quantum terminals in quantum structure, and also study the thermaltransport property in each terminal and the rough degree of edge structure. The results show that when the graphene chains (dimer lines) across the ribbon width are fixed, the increase of the number of the parallel multi-terminal graphene junctions can reduce the transmission rate of the phonons and the thermal conductance of each output terminal as well. This is because the increase of the number of the graphene junctions can lead to the decrease of the transverse dimension of the each output terminal, which enlarges the strength of the phonon scattering and results in the reduction of the phonon transmission. Owing to long distance scattering, the transmission rate of the phonons of the furthest distant output terminal is the smallest, and also the thermal conductance of the furthest output terminal is the smallest. On the contrary, the strength of the phonon scattering is the weakest for the closest output terminal. So the transmission rate of the phonons is the biggest, which induces the thermal conductance to be the biggest. The thermal conductance of the middle-output terminal depends sensitively on the structural parameters of each terminal. This is because mainly the relative position between the middle-output terminal and the phonon-input terminal is related closely to the structural parameters of each terminal, which can influence the strength of the phonon scattering and the transmission rate of the phonons. However, the thermal conductances in the top and bottom output terminals are just sensitively dependent on the structural parameters of the respective output terminal. This is because the relative position between the top (or bottom) output terminal and the phonon-input terminal is only related to the structural parameters of the respective output terminal. The rough edge structure can reduce obviously the transmission rate of the phonons, and the thermal conductance of the closest output terminal as well. The rough edge structure can modulate slightly the transmission rate of the phonons and the thermal conductance of the other output terminal. The total thermal conductance is related closely to the number of total graphene chains, the number of the multi-terminal graphene junctions, and the rough degree of edge structure. These results shed new light on the understanding of the thermal transport behaviors of multi-terminal junction quantum devices based on graphene-based nanomaterials in practical application.
The wettabilities of molten metals on ceramics are poor normally. In order to improve the wettability, all existing ceramic brazing methods introduce a compound transition layer that is formed by the reaction of active metal and ceramic. The transition layer between brazing seam and ceramic however creates negative effect on the properties of brazing joints. This paper reports our study of the wetting effect of sputtered Al particles on AlN, which enables the direct brazing of AlN using deposited Al-based films as fillers, thereby eliminating the need of a transition layer. The results show that under the bombardment of energetic sputtered Al particles, Al-N chemical bonding is formed at the interface between Al film and AlN, which typically requires temperatures above 850 ℃, much higher than the melting point of Al. The bonding remained intact even after the Al film has been melted, achieving the wetting effect on AlN. As a result, the direct brazing of AlN without the need of a transition layer becomes feasible. The shear strength of Al/AlN joint using this process reaches 104 MPa. The addition of 3.8 at.% Cu to film fillers increases the shear strength to 165 MPa. The fracture is generated in metallic brazing seam in both cases. When Cu content increases to 9.1 at.%, the segregation of Cu at the interface between the brazing seam and the ceramic reduces the shear strength of the joint to 95 MPa. With Al-20 at.% Ge, the brazing temperature can be lowered to 510 ℃, although the segregation of Ge at interface results in a low shear strength of 48 MPa. Instead of the traditional use of molten metals, utilization of the metallic vapor particles to bombard AlN achieves the wetting and the direct brazing of ceramics, with no negative effect of transition layers. This breakthrough method provides a brand new perspective to the technique of ceramic brazing.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
First-principle studies of the electronic structures and optical properties of diamond crystal co-doped with B and N
2016, 65 (8): 087101. doi: 10.7498/aps.65.087101
On the basis of our previous experimental results for the diamond synthesized in FeNi-C system by B and N co-doping under high pressure and high temperature conditions, the crystals doped with B or N atom, and co-doped with B and N atom are investigated separately by using the first principle density functional theory based on the stable structure of diamond. The formation energies corresponding to the all doping diamond crystals are studied while the most stable co-doping structures with the lowest energy are obtained. Furthermore, the absorption spectra, the densities of states, and the electronic structures of the doping diamond crystals are discussed. The absorption spectra show obviously the red shift, which is consistent with the experimental result. The electronic results show that the doping of N atom is prior to the doping of B atom while the system co-doped with nearly neighboring B and N atoms creates the most stable structure with the lowest formation energy due to the synergistic effect.
2016, 65 (8): 087102. doi: 10.7498/aps.65.087102
Since the spin-transfer effect was predicted in 1996, the direct-current-switched magnetic storage has received much attention. A slender nanopillar with high spin-polarized ratio of the conductive electrons is the most favorable for realizing the direct-current-switched magnetic memory. Wang et al. (Sup. Mic. 2015 86 493) showed a supercell idea used to design the nanopillar array in a semiconductor matrix. Based on this idea, in this paper, the Ni-based single atomic chains are designed in the semiconductive CoTiSb matrix by continuously substituting Ni for Ti, Sb, or Ti-Sb in the  crystallographic direction. These single atomic chains are uniformly distributed in the matrix. We investigate the electronic structures and magnetic properties of CoTiSb supercells with the Ni-based single atomic chains by using the first-principle calculations. The calculation results show that the single atomic chains of Ni-Sb (achieved by substituting Ni for Ti) have a high spin polarization and hole conduction properties. The single atomic chain of Ni-Ti (achieved by substituting Ni for Sb) and Ni-Ni single atomic chain (achieved by substituting Ni for Ti and Sb) both have a 100% spin polarization ration at the Fermi level. The Ni-based single atomic chain has an effect on the electronic structures of other atoms surrounding it in about a lattice length and forms a nanopillar with the center of the Ni-based single atomic chain. We predict that CoTiSb matrixes with the Ni-Ti and Ni-Ni single atomic chains will be good candidates for the direct-current-switched magnetic storage.
2016, 65 (8): 087301. doi: 10.7498/aps.65.087301
At different frequencies of light pulse field, the coupling between plasmon resonance and electron transport of one-dimensional sodium atomic chain is investigated by using time-dependent density functional theory. Light pulse field, whose frequency is in about 0.8 eV range around the plasmon resonance point, can stimulate plasmon resonance of the system. Plasmon resonance intensity magnitude which is stimulated by these different frequency light pulse fields are in the same order of magnitude. The more closely the external field frequency approaches to plasmon resonance frequency, the larger plasmon resonance amplitude stimulated by the external field fields will be. With regard to the nonlinear excitation phenomenon of linear atomic chain plasmon, using a classical harmonic oscillator model, we gives a qualitative explanation.
2016, 65 (8): 087302. doi: 10.7498/aps.65.087302
Switching behavior in Nd0.7Sr0.3MnO3 ceramic is investigated widely due to its close association with the new storage Resistive random access memory. In this work, we discuss the transport characteristic of the electrode-bulk interface and boundary/phase interface, and explain the differences between the two interfaces. Firstly, the Nd0.7Sr0.3MnO3 ceramic samples are prepared by solid-phase reaction and high-energy milling methods, respectively. And the transport properties of the two interfaces are investigated respectively by the two-line and four-line measurements. The results show that the Ag electrode-bulk interfaces exhibit nonlinear and hysteretic I-V characteristics and a stable resistance switching effect, and the stability of resistance switching behavior is reduced gradually with the increase of temperature. For the boundaries/phase interfaces, however, it does not exhibit resistance switching effect, although a nonlinear and hysteretic I-V behavior can also be observed under the four-line measurement mode. Various defects in the two interfaces act as traps and regulate the interfacial transports and result in the nonlinear and hysteretic I-V behaviors in the two interfaces. Additionally, the simulation experiments reveal that a large number of boundaries/phase interfaces and larger leakage conductance resulting from the complex connections of boundaries/phase interfaces are the main responsibilities for the fact that the boundaries/(phase) interfaces do not exhibit EPIR behavior as the electrode-bulk interface.
2016, 65 (8): 087801. doi: 10.7498/aps.65.087801
In this paper, the refractive index of GdTaO4 crystal is measured by the auto-collimation method. GdTaO4 crystal is processed into three rectangular prisms, their planes with longer right angle side are planes a, b and c of the crystal, respectively, and their normal directions are the directions of crystal plane axes a, b and c, respectively. with a side by angle . Plane enclosed by hypotenuse and the longer right angle side is subjected to fine polishing, while the surface plating for the latter is subjected to Al reflectance coating, so that the light is incident along the hypotenuse plane with minimum angle of deviation (), reflects on the plane with longer right angle and returns along the original path. The rectangular prisms processed by GdTaO4 crystal are placed on the platform of 32 J goniometer with an accuracy of arc seconds. The 473, 532 nm YAG double frequency laser, 633 nm He-Ne laser and 1064 nm YAG laser with stable light intensity are used as a measuring light source, light will refract into the light perpendicular to the longer right angle side when the laser of measuring light source shoots towards the bevel of a prism with a minimum angle of deviation (). The refractive indexes nx, ny, and nz of a crystallographic axis directions can be measured by , and the relationship between refractive index ellipsoid section and prism refraction of light. The constants Ai, Bi, Ci, and Di (i=x, y, z) are given in Sellmeier's equation ni2 =Ai+Bi/(2-Ci)-Di2, and the values of angle Vz included between light axis and refractive index at wavelengths of 473, 532, 632.8 and 1064 nm are calculated to be 22.5, 22.5, 21.9 and 22.0, respectively. It is proved that GdTaO4 crystal is optically positive biaxial crystal.
2016, 65 (8): 087802. doi: 10.7498/aps.65.087802
The performance of the ZnO film that is an indispensable part of pin-type Si-based thin-film solar cells, plays a crucial role in high-efficiency thin-film solar cells and also forms a significant part in photovoltaic research and development. In this paper, low resistivity and wide broadband spectrum transmittance vanadium (V) doped ZnO (VZO) films are successfully fabricated on Corning XG substrates at various substrate temperatures (STs). The properties of VZO films are investigated by the radio-frequency magnetron sputtering technique and plane wave pseudo-potential method based on the density-functional theory. The experimental results demonstrate that all the VZO flms have (002) preferred orientation with the c-axis perpendicular to the substrate, and the crystalline quality is found to increase with the substrate temperature (ST) rising up to 280 ℃ and decrease when the ST increases further. The optimal VZO film is achieved at 280 ℃ with a resistivity of 3.810-3 cm and an average transmittance of more than 85% in a range of 500-2000 nm. The theoretical result shows that after incorporation of V the Fermi level goes through the conduction band, showing a typical n-type metallic characteristic. The carriers originate from the orbits of V 3d and O 2p. The calculated lattice constants and mobility for VZO film are in agreement well with the experimental results. The consistency of the theoretical results with the experimental results shows that the VZO thin film has a great potential application as a front contact in high-efficiency thin film solar cells.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (8): 088101. doi: 10.7498/aps.65.088101
Oxygen-defect vacancies that routinely exist in wet production of VO2 material or on the surface of VO2 single crystal after surface treatment have significant influence on the metal-insulator phase transition features mainly due to their enhanced effect of doping on V 3d electronic structure. The removal of the surface oxygen defects is highly desired for investigating the VO2 intrinsic electronic properties. In this work, we propose a charge transfer doping method by using strong electric affinity molecule tetrafluorotetracyanoquinodimethane (F4TCNQ) adsorption rather than the normal thermal annealing in oxygen atmosphere to heal the surface oxygen defects of VO2 crystalline film. The healing effect is probed by the electronic structure evolution at the F4TCNQ/VO2 interface. The VO2 crystalline film is grown by an oxygen plasma assisted molecular beam epitaxy method on an Al2O3(0001) substrate. Surface oxygen defects on VO2 film are produced after a mild sputtering with an ionic energy of 1 keV and a thermal annealing in vacuum at 100 ℃. The influence of F4TCNQ molecule adsorption on the electronic structure of the sputtered VO2 film is studied by using in-situ synchrotron-based photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). XPS and XAS results demonstrate convincingly that V3+ species of sputtered VO2 are oxidized into the V4+ and simultaneously negative molecular ions form at F4TCNQ/VO2 interface resulting from the electron transfer from VO2 to the F4TCNQ layer. The preferred adsorption on surface defects and the strong electron withdrawing function of F4TCNQ molecules may account for the effective elimination of the electron doping effect of oxygen defects on VO2 surface. This charge transfer effect at interface recovers the electronic properties of VO2. Compared with thermal annealing in oxygen environment, the healing of oxygen defects by the molecular adsorption can prevent the surface from over oxidating VO2 into V2O5, which opens a new route to surface defect healing.
2016, 65 (8): 088401. doi: 10.7498/aps.65.088401
In recent decades, nonlinear Kalman filtering based on Bayesian theory has been intensively studied to solve the problem of state estimation in nonlinear dynamical system. Under the Gaussian assumption, Bayesian filtering can provide a unified recursive solution to the estimation problem that is described as the calculation of Gaussian weighted integrals. However it is typically intractable to directly calculate these integrals. The numerical integration methods are required from a practical perspective. Therefore, nonlinear Kalman filters are generated by different numerical integrations. As a representative of nonlinear Kalman filter, cubature Kalman filter (CKF) utilizes a numerical rule based on the third-degree spherical-radial cubature rule to obtain better numerical stability, which is widely used in many fields, e.g., positioning, attitude estimation, and communication. Target tracking can be generalized as the estimations of the target position, the target velocity and other parameters. Hence, nonlinear Kalman filters can also be used to perform target tracking, effectively. Since the CKF based on the third-degree cubature rule has a limited accuracy of estimation, it is necessary to find a CKF based a cubature rule with higher accuracy in the case of target tracking system with a large uncertainty. High-degree cubature Kalman filter is therefore proposed to implement state estimation due to its higher numerical accuracy, which is preferred to solve the estimation problem existing in target tracking. To improve the filtering accuracy and robustness of high-degree cubature Kalman filter, in this paper we present a new filtering algorithm named Huber-based high-degree cubature Kalman filter (HHCKF) algorithm. After approximating nonlinear measurements by using the statistical linear regression model, the measurement update is implemented by the Huber M estimation. As a mixed estimation technique based on the minimum of l1-norm and l2-norm, the Huber estimator has high robustness and numerical accuracy under the assumption of Gaussian measurement noises. Therefore, the Huber-based high-degree cubature Kalman tracking algorithm is generated by combining the state prediction based on the fifth-degree spherical radial rule. In this paper, the influence of tuning parameter on the tracking performance is discussed by simulations. Simulations in the context of bearings only tracking and reentry vehicle tracking demonstrate that the new HHCKF can improve the tracking performance significantly.
Influence of etching AlN buffer layer on the surface roughening of N-polar n-GaN grown on Si substrate
2016, 65 (8): 088501. doi: 10.7498/aps.65.088501
Light extraction efficiency of thin-film GaN-based light-emitting-diode (LED) chip can be effectively improved by surface roughening. The film transfer is an indispensable process in the manufacture of thin-film LED chip, which means transferring the LED film from the growth substrate to a new substrate, and then removing the growth substrate. After the growth substrate is removed, the buffer layer is used to cushion the mismatch between the substrate and the n-GaN exposed, which has a significant influence on the roughening behavior of n-GaN. Unlike the GaN buffer layer grown on sapphire substrate, AlN buffer layer is usually used when n-GaN is grown on Si substrate. In this paper, the surface treatment of the AlN buffer layer by reactive ion etching (RIE) is used to improve the surface roughening effect of N-polar n-GaN grown on the silicon substrate in the hot alkali solution (85 ℃, 20% KOH mass concentration of solution), and the mechanism of the influence of the surface treatment on the roughening behavior is discussed by X-ray photoelectron spectroscopy (XPS) and other advanced methods. The degree of etching surface AlN buffer layer is detected by energy dispersive spectrometer (EDS), the sample surface state after RIE etching is analyzed by XPS, the morphology of the surface roughening is observed by scanning electron microscope (SEM) and the effect of surface roughening on the optical power of LED devices is verified by the photoelectric performance test. The EDS results show that the AlN buffer layer remains after RIE etching 10-30 min and the AlN disappears after RIE etching for 40 min. The SEM results show that surface states of AlN buffer layer have a great influence on the roughening behavior of n-GaN in KOH solution. The sample with part of AlN buffer layer has a good roughening effect and proper size hexagonal pyramid distributing uniformly. In addition, the rate of coarsening is too fast for the samples with AlN buffer layer completely removed, while the rate is too slow for the samples without any etching process. In summation, using RIE etching to remove a part of the AlN buffer layer can effectively improve the roughening effect of N-polar n-GaN in KOH solution. We believe that lots of N-vacancies are produced on the surface of the sample after RIE etching, which provides the electrons, thereby causing the surface Fermi level to be elevated. The XPS analysis shows that the RIE etching can improve the electronic binding energy of Al 2p of AlN buffer layer, resulting in a shift of the surface Fermi level near to the conduction band, and reducing the Schottky barrier between the KOH solution and the surface of the sample, which is beneficial to the surface roughening. To remove a part of the AlN buffer by using plasma etching layer can improve the roughening effect of N-polar n-GaN in KOH solution, resulting in the output power of the corresponding LED device being improved obviously.
2016, 65 (8): 088901. doi: 10.7498/aps.65.088901
In this paper, we analyze the diffusion patterns of cascading failure, which happen in the express hypernetwork and electronic hypernetwork respectively. The cascading failure of the express hypernetwork is diffused by the node, and the cascading failure of the electronic hypernetwork is diffused by the hyper-edge. According to hyper-graph theory, we propose two methods to characterize these cascading failures, which are 2-section graph analytical method and line-graph analytical method. We analyze the characteristics of the cascading failures based on node by using the 2-section graph analytical method and based on hyper-edge by using line-graph analytical method, respectively. We construct a k uniform scale-free hypernetwork and analyze the cascading failure process of this hypernetwork based on the couple map lattice according to our methods. The simulation results show that the scale-free hypernetworks are both robust and vulnerable for attack. It is found that the cascading failure based on the node of k uniform scale-free hypernetwork is associated with the hyper-degree distribution of nodes, and the scale-free hypernetwork is robust for random attack and vulnerable for deliberate attack. The more nodes a hyper-edge has, the better robustness the hypernetwork has.The cascading failure based on the hyper-edge is different from the cascading failure based on the node. The cascading failure based on the hyper-edge is associated with the hyper-edge degree distribution. The hyper-edge degree distribution of the scale-free hypernetwork is not entirely the power-low distribution. When the cascading failure is diffused by the hyper-edge, the hypernetwork is vulnerable for random attack and robustness for deliberate attack if there are 3 or 5 nodes in a hyper-edge. Moreover, the hypernetwork becomes robust for the random attack if there are 7 nodes in a hyper-edge. Furthermore, the k uniform scale-free hypernetwork is more robust than the same size Barabasi-Albert scale-free network for the same attack. The cascading failure process based on the hyper-edge is slower than based on the node. We find that the edge number is another influential factor of robustness. The network is more robust if it has more edges for fixed node number. The line-graph has more edges than the 2-section graph in the same size scale-free hypernetwork, so the cascading failure of node is slower than that of hyper-edge.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2016, 65 (8): 089201. doi: 10.7498/aps.65.089201
Owing to its destructive power to porous structures such as buildings and rocks, salt weathering has attracted considerable attention in the community of civil engineers and geomorphologists, who devote their efforts to conservations of architecture and engineering structures afflicted by salt attack, and to the investigation of natural landscape caused by the same group of processes, respectively. Precipitation of dissolved salts is a direct cause of salt weathering effect. Crystallization phenomena in salt weathering can be crudely categorized under efflorescence and subflorescence with respect to the distinct precipitation sites, and the latter is believed to be able to cause more destructions to porous structure. In contrast to subflorescence for which even models of statistical dynamics have been well-established, efflorescence has drawn less attention, partly because of the complexity of constructing a sound theoretical model to describe the mass transport process there involved. As a serie of sodium salts is the main culprit of salt weathering, the current work deals with experimental study of efflorescences of the aqueous NaCl, NaNO3 and Na2SO4 solutions on the surface of porous silica gel particles. We investigate the influences of salt concentration and pore size on the crystal morphology arising in efflorescence by using scanning electron microscopy. It is found that though Na2SO4 effloresces on the specimen surface, its inclination towards subflorescence makes the whiskers appear on specimen with smaller pore radii at low concentrations, which differs obviously from the cases of NaCl and NaNO3. Moreover, unlike the upright growths of NaCl and NaNO3 crystals, the whiskers of Na2SO4 are always oblique to the specimen surface, and the large lateral stress to the specimen thus induced may become another factor of its destructive power apart from the subflorescing trend. The crystallization behaviors of Na2SO4, i.e., both the oblique whiskers and regular crystallites, indicate that mirabilite (Na2SO410H2O) is the main precipitation, which is consistent with the high relative humidity employed in this article. Remarkably, the thinnest whiskers of NaNO3 exhibit the branching and ball-chain structures, indicating that plateau-Rayleigh instablility occurs in the growth process. Our results are expected to inspire more deliberate studies for the full understanding of detailed processes and mechanism involved in efflorescence of aqueous salt solutions.