Vol. 64, No. 19 (2015)
2015, 64 (19): 194201. doi: 10.7498/aps.64.194201
Surface plasmon polaritons (SPPs) are a hybrid mode of a light field and metallic collective electrons oscillated resonantly and excited at the metal/dielectric interface. Recently extensive research has been carried out due to its technological potential in nano-optics. The SPPs coupling, focusing, waveguiding and resonance enhancement are hot spots in this field. In particular, to find a simple method that can focus SPPs into a highly confined spot with the size beyond the diffraction limit is still a big challenge. In this work, we have fabricated the Archimedes' spiral structures with different structural parameters on an Au film by using focused ion beam etching technique. Through changing the chiralities of the incident circularly polarized light and the spiral structure, we have studied theoretically and experimentally the focusing properties of the Archimedes spiral structures with different parameters. We find that besides the chiralities of the incident light and the spiral structure, the pitch of screw of the spiral structure and the wavelength of the excited light also affect the surface plasmon field. The resulting surface plasmon fields inside the structure are the zero-order, first-order, and high-order evanescent Bessel beams. By using a phase analysis and a finite-difference time-domain simulation method, we calculate the electric field and phase distribution in different spiral structures. A near-field vortex mode with different spin-dependent topological charges can be obtained in the structures. Furthermore, the results of the scanning near-field optical microscopy measurements verify the theory and simulation results. The method of using an Archimedes' spiral structure to focus SPPs provides a new route to manipulate the SPPs optical field in nanoscale. Based on theoretical calculation and FDTD simulation in this work, we have studied the physical process of the optical field manipulation in spiral structures. The significant and innovated points of this work are: a) We have developed the phase theory, and analyzed the field manipulation process of spiral structures with different parameters and chiralities at different circular polarization and wavelengths. b) A more effective and convenient way is used for SPPs focusing in linearly polarized light and circularly polarized light. c) A near-field vortex surface mode with different spin-dependent topological charges is obtained for the structure. This work can be considered to have applications in SPPs tweezers, highly integrated photonic devices.
2015, 64 (19): 198401. doi: 10.7498/aps.64.198401
Compressive sensing of binary signals is corresponding to the problem of binary symbol detection in the faster-than-Nyquist signaling systems, which has significant research value. Traditional compressive measurement of a binary signal is based on Gaussian matrix, and l1 minimization is a classic algorithm for signal reconstruction. However, stochastic matrix such as the Gaussian matrix can hardly be realized by a digital circuit, and the reconstruction performance of l1 minimization is not well enough for binary signals. Thus, it is of great meaning to construct a new kind of measurement matrix as well as a better reconstruction algorithm for binary signals. This paper constructs a chaotic circulant measurement matrix based on Cat chaotic map (CCMM), and proposes a brand new algorithm for binary signal reconstructionsmooth function approximation method (SFAM). Chaotic sequence has characteristics of both internal certainty and external randomness, while a circulant matrix requires less elements and can be realized through fast Fourier transform. CCMM conbines the advantages of both chaotic sequence and circulant matrix, so that it not only satisfies the RIPless property required by the compressive measurement matrix because of external randomness, but also has the power to resist the effect of low signaling efficiency and low SNR due to the internal certainty. Moreover, the circle structure gives CCMM the potential to be digital realized in the future. In SFAM, we first use a non-convex function to approximate the original discontinuous objective function, in order to transfer the original combinatorial optimization problem into an optimization problem with equality constraints which can be solved much easier. Then we use the interior point method to solve this optimization problem. Furthermore, sparse Bayesian learning algorithm is used to correct the reconstruction error for a more accurate result. Compressive measurement and reconstruction of binary signals in additive Gaussian white noise channel are operated. Result of numerical experiments shows that CCMM is much better than the traditional Gaussian matrix for compressive measurement, especially in the condition of low signaling efficiency and low SNR, and SFAM is much better than l1 minimization for binary signal reconstruction. At the end of this paper, we explain the essential reason why CCMM performs better than the traditional Gaussian matrix, through calculating the autocorrelation function of compressive measurement vector in various conditions.
2015, 64 (19): 190601. doi: 10.7498/aps.64.190601
Ultra-stable lasers at optical communication wavelengths have important applications in developing optical frequency transfer via optical fibers. We report the recent development of a 1550 nm stable laser system built at National Time Service Center and its preliminary application in optical frequency transfer via laboratory fibers. In the experiment, the conventional Pound-Drever-Hall(PDH) frequency stabilization technology is implemented to achieve the ultra-stable laser at the wavelength of 1550 nm. The output of a single laser source is split and locked onto the resonant frequency of two independent reference cavities, of 344000 and 296000 respectively. The frequency of the laser source is actively stabilized to the first reference cavity by piezo and external frequency shifters simultaneously and the total control bandwidth is measured to be 50 kHz. Then the laser frequency is shifted and stabilized to the second reference cavity by an acousto-optical modulator. A 5 m long single-mode fiber is used to bring the first laser beam to the second reference cavity which unfortunately induces unexpected phase noise by environmental distortions. The laser linewidth broadened is determined to be 0.27 Hz by the beat note measurement between the input and output beams of the fiber. To evaluate the frequency stability of the laser, the frequency control signal within the control bandwidth of the second stable laser system is analyzed by a spectrum analyzer and a frequency counter. The control signal shows a Lorentz linewidth of 2.7 Hz and a frequency stability of 2.510-14/s, corresponding to a single laser linewidth of 1.9 Hz with a frequency stability of 1.710-14/s if the two stable lasers have similar frequency stability. Applying this ultra-stable laser system as the laser source for the fiber-based optical frequency transfer, a short-term frequency transfer stability of 7.510-17/s is demonstrated through a 50 km-long fiber spool, while a frequency transfer stability of 2.410-16/s is achieved by a similar setup except that the laser source is a kHz-level linewidth laser. In the experiment an Agilent 53232 A frequency counter is applied to record the beat note signal in the auto mode. In the end, we discuss the possible improvements of the stable laser system, including the miniaturization of the optical setup, optimization of the control bandwidth and shortening of the response time of control loop.
2015, 64 (19): 190501. doi: 10.7498/aps.64.190501
We study the quantum pseudocritical points in the unbounded quasiperiodic transverse field Ising chain of finite-size systematically. Firstly, we study the derivatives of averaged magnetic moment and the averaged concurrence with transverse fields. Both of them show two visible peaks, with are nearly not raised when the length of chain is increased. Moreover, the places where the peaks occur in the transverse field are obviously different from that of the quantum phase transition point in the thermodynamic limit. These results are very different from those of the bounded quasiperiodic transverse field Ising chain and the disordered transverse field Ising chain. Then, we analyze the origin of the two visible peaks. For that we study the derivative of magnetic moment for each spin with transverse field. For all spins, the single magnetic moment only show one peak. However, the places where the peaks occur are not random. The peaks always occur in two regions. Thus, the derivatives of averaged magnetic moment reveal two peaks. Furthermore, we study the probability distribution of the pseudocritical points through finding out the peaks of the single magnetic moment in 1000 samples. The distribution is not Guassian. This result is obviously different from that of the disordered case. Besides, the pseudocritical points nearly do not occur at the quantum phase transition point. Finally, we analyze the origin of the pseudocritical points. For that we study the relationship between the spin places and the corresponding places of pseudocritical points. It is found that the pseudocritical points are caused by the two groups that exist in the nearest neighboring interactions of the unbounded quasiperiodic structure. When a spin is in one group, this group will decide the probable place of the pseudocritical point. Through this study, we find that although the quantum phase transition behaviors of the unbounded quasiperiodic transverse field Ising chain and the disordered transverse field Ising chain belong to the same universal class in the thermodynamic limit, the thermodynamic behaviors of the two Ising chains are very different as in finite sizes. The differences are caused by the special structure in the unbounded quasiperiodic system.
2015, 64 (19): 190701. doi: 10.7498/aps.64.190701
Real-time, point-by-point localization of magnetic targets such as ferrous unexploded ordnance can be achieved by the cube magnetic gradiometer system designed by the Naval Surface Warfare Center. The localization method uses the Frobenius norm of the magnetic gradient tensor to calculate the location of the magnetic target. This method assumes that the potential field of the Frobenius norm of the magnetic gradient tensor is a prefect sphere. But the Frobenius norm of the magnetic gradient tensor has an asphericity parameter, and its potential field is an ellipsoid, which can cause asphericity error. Since the current localization method can be affected seriously by the asphericity error, an improved method is proposed in this paper to eliminate the asphericity error. The improved method is based on a new invariant, which does not contain asphericity parameter. The new invariant can be obtained by the combination of the Frobenius norm and eigenvalues of the magnetic gradient tensor. In detail the procedure is as follows: first, the magnetic gradient tensor of the center point of the regular hexahedron's six planes can be measured by the cube magnetic gradiometer system, then these eigenvalues can be calculated and combined according to a certain relationship to eliminate the asphericity parameter, then the new invariants of the six planes can be obtained, and the spatial gradient of the new invariant can be calculated from the six new invariants, then the localization of the magnetic target can be calculated from the spatial gradient of the new invariant. This improved method can overcome the asphericity error effectively, and it can be used for real-time, point-by-point localization and detection of unexploded ordnance. Simulation experiments show that the localization error of the improved method is much smaller than that of the original method, the average relative error can be reduced by 10.9%. The improved method can be deployed on the highly maneuverable platform. The platform motion will not be constrained, and the improved method will be made more effectively in detection and localization of the magnetic targets. Thus the improved method can be widely applied in naval mines localization, mineral exploration, ferrous resources exploration, moving magnetic target tracking, and other fields.
Study of plant fluorescence properties based on laser-induced chlorophyll fluorescence lifetime imaging technology
2015, 64 (19): 190702. doi: 10.7498/aps.64.190702
Plant fluorescence is a susceptible signal in plant fluorescence remote sensing detection. In order to solve this problem, a technique for plant chlorophyll fluorescence lifetime imaging is presented to evaluate living status for plant growth and environmental monitoring. A concave lens is used to expand laser beam at a wavelength of 355 nm, and the living plant is exposed in this laser light source to excite chlorophyll fluorescence. And the chlorophyll fluorescence signals are detected by an intensification charge coupled device. Time resolved measurement method is used in this article, so that every time the same fluorescence signals can be excited by the same laser pulse. Meanwhile, the delay time needed for triggering intensification charge coupled device should be changed consecutively, and the whole discrete fluorescence signal can be obtained. The discrete fluorescence signals from the particular location points of the plant are fitted. An improved method of forward iterative deconvolution is used to retrieve the corresponding fluorescence lifetime, and the high-precision fluorescence lifetime can be obtained. Furthermore, the fluorescence lifetime values at all the location points are retrieved to obtain the distribution map of chlorophyll fluorescence lifetime. This method can give the chlorophyll fluorescence image efficiently. The distribution map of fluorescence lifetime can more effectively reflect the plant chlorophyll concentration than the fluorescence intensity image does. The physical property of chlorophyll fluorescence lifetime from living plants has been studied preliminarily, indicating that the plant physiological status is related to its fluorescence lifetime to a certain extent; and the chlorophyll fluorescence lifetime and plant environment have a subtle and complex correlation. In the future, the relationship between chlorophyll fluorescence lifetime and plant environment will be expected to study with the cooperation of biophysicist.
2015, 64 (19): 190101. doi: 10.7498/aps.64.190101
Influence maximization modeling and analyzing is a critical issue in social network analysis in a complex network environment, and it can be significantly beneficial to both theory and real life. Given a fixed number k, how to find the set size k which has the greatest influencing scope is a combinatory optimization problem that has been proved to be NP-hard by Kempe et al. (2003). State-of-the-art random algorithm, although it is computation efficient, yields the worst performance; on the contrary, the well-studied greedy algorithms can achieve approximately optimal performance but its computing complexity is prohibitive for large social network; meanwhile, these algorithms should first acquire the global information (topology) of the network which is impractical for the colossal and forever changing network. We propose a new algorithm for influence maximization computing-RMDN and its improved version RMDN++. RMDN uses the information of a randomly chosen node and its nearest neighboring nodes which can avoid the procedure of knowing knowledge of the whole network. This can greatly accelerate the computing process, but its computing complexity is limited to the order of O(klog(n)). We use three different real-life datasets to test the effectiveness and efficiency of RMDN in IC model and LT model respectively. Result shows that RMDN has a comparable performance as the greedy algorithms, but obtains orders of magnitude faster according to different network; in the meantime, we have systematically and theoretically studied and proved the feasibility of our method. The wider applicability and stronger operability of RMDN may also shed light on the profound problem of influence maximization in social network.
2015, 64 (19): 190301. doi: 10.7498/aps.64.190301
The core of optical field theory at finite temperature is how to introduce the thermo-vacuum state which is the basis of comprehensive investigation of electromagnetic field by virtue of quantum statistic method. Based on the spirit of thermo-field dynamics initiated by Takahashi and Umezawa, we first employ the integration method within the ordered product of operators to search for thermo-vacuum state for the optical negative binomial state (NBS)s=s+1(1-)n|n,+,+,which takes the form of pure negative binomial state. The newly found thermo-vacuum state brings convenience for evaluating the Wigner function of NBS and the fluctuation of photon numbers in NBS.
ATOMIC AND MOLECULAR PHYSICS
High-order harmonics and attosecond pulse generation of a He+ ion by a chirped two-color inhomogeneous laser field
2015, 64 (19): 193201. doi: 10.7498/aps.64.193201
We theoretically study high-order harmonics generation (HHG) and isolated attosecond pulse (IAP) generation in a spatially inhomogeneous chirped two-color (5 fs/800 nm and 12 fs/1600 nm) laser field by solving numerically the time-dependent Schrdinger equation(TDSE) for a one-dimensional (1D) model of He+ ion by the splitting-operator fast-Fourier transform technique. Results show that the inhomogeneity of the laser field plays an important role in the HHG process. The harmonic spectra exhibit a two-plateau structure, and the cutoff of high-order harmonics is extremely extended to 851th order and the smooth supercontinuum harmonic spectrum is obtained in a chirped two-color inhomogeneous laser field. To further understand the physical mechanism of HHG, we give a reasonable explanation for the extension of harmonic plateau by using the semi-classical three-step model, the time-frequency profile of the time-dependent dipole, and the classical electron trajectories. Explicitly, the harmonic order as a function of the ionization time and emission time can be given by the semi-classical three-step model. If we define the path with earlier ionization time and later emission time as a ongelectronic trajectory, and the path with later ionization time and earlier emission time as a short electronic trajectory, then, there exist a few electronic trajectories that contribute to the harmonics in cutoff region. Numerical results show that the short quantum path is enhanced, and the long quantum path is suppressed in spatially inhomogeneous fields, and this is advantageous to generate an IAP. We find that the quantum path can be controlled by increasing inhomogeneity parameter of the laser field. Effects of the time delay on HHG is also discussed. We find that the smooth supercontinuum harmonic spectrum is obtained by adjusting the time delay. When the time delay is t0=1.6up/1, the cutoff of the harmonics can be extended remarkably. By synthesizing the 600th to 680th (80th) order harmonics in the continuum region, an isolated 32 attosecond pulse can be generated by a spatially inhomogeneous chirped two-color laser field with parameters =0.25, =0.00105, t0=1.6/1.
2015, 64 (19): 193601. doi: 10.7498/aps.64.193601
(MgO)12 in a tube structure is one of the magic number clusters of (MgO)n and exhibits particular stability. To study the electric field effect on the hydrogen storage properties of (MgO)12, the H2 adsorption behavior on the surface of the tube (MgO)12 in an external electric field is explored at the level of B3LY/6-31G**. In the external electric field, the (MgO)12 keeps the frame of tube structure but with little distortion, implying that the (MgO)12 cluster can sustain the strong electric field for hydrogen storage. The NBO analysis also indicates that (MgO)12 is polarized by the external electric field; and its dipole momenta increase to 9.21 and 19.39 Debye at the field intensities of 0.01 and 0.02 a.u., respectively. Without the external electric field, H2 can be adsorbed on Mg atoms in the end on modes, while on O atoms in the top on modes. When the external electric field is applied, whether H2 is adsorbed on Mg or O atoms, the stable adsorption structures are all top on modes and the molecular orientation of H2 is turned to the direction of the external electric field. Because (MgO)12 and H2 are effectively polarized by the external electric field, the adsorption strength of H2 on some adsorption sites are enhanced remarkably. The adsorption energies of H2 on the three-coordinated Mg/O are promoted from 0.08/0.06 eV in free field to 0.12/0.11 eV and 0.20/0.26 eV at field intensities of 0.01 a.u. and 0.02 a.u., respectively. Electronic structure analysis reveals that when H2 is adsorbed on Mg atoms, this process denotes charges moving to the cluster, and the improvement of adsorption interaction of H2 on Mg atoms is mainly due to the polarization effect. While the adsorption on O atoms, on the one hand implies the polarization effect of O anion is stronger than that of Mg cation, on the other hand, H2 receives charges from (MgO)12 and its valence orbitals also take part in the bonding with the valence orbitals of the cluster. Thus the structures of H2 adsorbed on O atoms are more stable. In an external electric field, (MgO)12 can adsorb sixteen H2 molecules at most, and the corresponding mass density of hydrogen storage reaches 6.25wt%.
2015, 64 (19): 193101. doi: 10.7498/aps.64.193101
Indium oxide with its wide gap is a multifunctional semiconductor material, which has gained application in many areas. Indium oxide films show high electrical property and high transparency, which have been applied in OLED display, flat-panel display, thin film solar cells, etc. However, the mechanisms of both high electrical and high transparent properties are still not clear up to now. So in this paper, the electronic structures of the In2O3 crystals are studied by GGA, GGA+U, HSE06 and G0W0 corrections. The mechanisms of optical transition and formation of transparent electrode in In2O3 crystals are studied using Hedin's G0W0 approximation and the Bethe-Salpeter equation. The complex refractive index, complex dielectric function and optical absorption spectrum of the In2O3 crystal have been obtained, which are in good agreement with experimental results. By analyzing the quasi-particle band structures, optical transition matrix and optical absorption spectrum, the mechanisms of optical transition and formation of transparent electrode in In2O3 can be interpreted. BSE (Bethe-Salpeter equation) calculation results show that the transition from 8 to 1 is permitted, however, the transition probability is far less than that from 10 to 1. This is because, for 8 to 1 transition, there are three even symmetry bands and two odd symmetry bands, in which only the transition from two odd symmetry bands to the conduction band is permitted. Other causes for this phenomenon are that in the In2O3 primitive cell there exist some overlapping bands, which result in the false transition. Therefore, this work argues that in the In2O3 crystals optical band gap is 4.167 eV, which corresponds to the direct transition from 10 to 1. This result will help understand the mechanisms of optical transition and the transparent electrode in In2O3.
2015, 64 (19): 193102. doi: 10.7498/aps.64.193102
The three-body Schrdinger equation is approximately solved in the hyperspherical coordinates and the binding energies of the three-body weakly bound systems are calculated with the purpose to find if He-He-Ba trimers could exist. Using the special feature of the B-spline function like the flexible and highly localized properties, hypersphercial potentials are obtained by modifying the knots distribution of the B-spline basis of different weakly bound three-atom systems. Employing the best empirical interaction potentials between each pair of particles, we obtain that in the ground state binding energies of the weakly bound typical three-atom systems, the bindings of the molecules, 4He-4He-138Ba, 4He-3He-138Ba and 3He-3He-138Ba are possible. The binding energies of these systems are shown in the order of 1 Kelvin, each system could support only one bound state. These weakly bound molecules can exist only in a very cold environment. To get insight into the geometry of the molecules, the features of the channel functions associated with the hyperspherical potential curves of each system are investigated.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (19): 194202. doi: 10.7498/aps.64.194202
Large planar plasma sheets, generated by a linear hollow cathode in pulse discharge mode under magnetic confinement, can be used in the field of plasma antenna, plasma stealth, and simulation of a plasma layer surrounding vehicles traveling at hypersonic velocities within the Earth's atmosphere. Firstly, to investigate the propagation properties of electromagnetic waves at different frequencies and polarization, the transverse field transfer matrix method is introduced. Secondly, the measured electron density temporal and spatial distribution and the transverse field transfer matrix method are utilized to calculate the reflection, transmission and absorption of electromagnetic waves by large planar plasma sheets with different currents. Finally, 1 GHz (less than the critical cut-off frequency) electromagnetic waves and 4 GHz (greater than the critical frequency) electromagnetic waves are chosen to investigate the evolution of propagation properties during the pulsed discharge period. Results show that both the reflection and absorption of the electromagnetic waves are greater for their polarization direction parallel to that of magnetic field, and their frequencies lower than the critical cut-off frequency, and as the discharge currents rise, the reflection increases while the absorption decreases. However both the reflection and absorption of the electromagnetic waves with their polarization direction perpendicular to the magnetic field direction and their frequency greater than the critical cut-off frequency become less, and as the discharge currents rise, both the reflection and absorption will increase. For the electromagnetic waves with their polarization direction perpendicular to the magnetic field direction, there is an upper hybrid resonance absorption band near the upper hybrid resonance frequencies, in which the absorption is significant but the absorption peak value is not affected by the discharge current. The propagation characteristics of the electromagnetic waves with polarization direction perpendicular to the magnetic field direction are the same as that of the electromagnetic waves with the polarization direction parallel to the magnetic field direction, except the upper hybrid resonance absorption. During the pulse discharge period, the propagation characteristic of the electromagnetic waves experiences an unstable phase before reaching steady states. The transition time is about 100 s and increases as the discharge current rises. The upper hybrid resonance absorption is significant during the phase of steady state for waves with frequency lower than the critical cut-off frequency and polarization direction parallel to the magnetic field direction. For the applications of a large planar plasma sheet to reflect electromagnetic waves effectively and steadily, the pulse discharge period should be larger than 100 s, and its discharge current should be large enough to make the critical cut-off frequency greater than the frequency of incident wave, and its polarization direction should be parallel to the magnetic field direction.
2015, 64 (19): 194208. doi: 10.7498/aps.64.194208
The proton and neutron irradiation and annealing experiments are carried out on a domestic buried channel CCD (charge-coupled devices), Monte Carlo method being applied to calculate the energy deposition of scientific CCD irradiated by proton and neutron, and the radiation damage mechanism of the device is analyzed. The displacement damage dose in N+ buried channel is simulated. During irradiation and annealing experiments, the main parameter (dark signal) is investigated. Results show that the dark signal of the buried channel CCD irradiated by 10 MeV proton and 1 MeV neutron rises obviously. With the same fluence, the increase of dark signal and the displacement damage dose in N+ buried channel caused by 10 MeV proton is larger than that by 1 MeV neutron. Dark signal caused by proton irradiation is divided into surface dark signal and bulk dark signal. Oxide-trapped-charges and interface states may be caused by ionization-generated surface dark signal, and the bulk defects may be caused by displacement-generated bulk dark signal. Neutron irradiation only affects the bulk dark signal. Defects and their annealing temperature are studied. The dark signal of CCD irradiated by proton is greatly reduced after annealing, this phenomenon means that the dark signal is mainly affected by ionization. The proportion of bulk dark signals in total dark signals can be calculated by the remainder of dark signal after annealing, and it is at most about 20% or less. From the formula, the position of energy level of bulk defects has an obvious influence on the bulk dark signal. The energy level in the middle of the forbidden band can provide effective hot carriers. Combining the results of experiment and simulation, when the displacement damage doses in N+ buried channel are the same, the bulk dark signal produced by proton is nearly the same as that produced by neutron. This phenomenon means that the defect levels in the forbidden band gap caused by proton and neutron irradiation have the same contributions to dark signal generation. Effect of proton and neutron irradiation on the bulk dark signal is homogeneous. The displacement damage dose can be used to characterize the degradation degree of the bulk dark signal in CCD after irradiation.
2015, 64 (19): 194301. doi: 10.7498/aps.64.194301
A two-dimensional phononic crystal (PC) composed of a triangular array of square iron cylinders embedded in water is designed, in which the accidental degeneracy of the Bloch eigenstates is utilized to realize a semi-Dirac point at the Brillouin zone center. In the vicinity of the semi-Dirac point, the dispersion relation is linear along the Y direction but quadratic along the X direction. Rotating the iron cylinders around their axis by 45 and slightly tuning the side length of the cylinders, a new semi-Dirac point can be realized at the Brillouin zone center, where the dispersion relation is quadratic along the Y direction but linear along the X direction. To gain a deeper understanding of the semi-Dirac point, a k p perturbation method is used to investigate this peculiar dispersion relation and study how the semi-Dirac point is formed. The linear slopes of dispersion relations along any direction around the semi-Dirac point can be accurately predicted by the perturbation method, and the results agree very well with the rigorous band structure calculations. Furthermore, the mode-coupling integration between the degenerate Bloch eigenstates is zero in one direction but non-zero in the perpendicular direction, and this is the ultimate reason for the forming of a semi-Dirac point. With the help of the perturbation method, an effective Hamiltonian can be constructed around the semi-Dirac point, so that the Berry phase can be calculated, which is found to be zero. Actually, the different values of Berry phase indicate an important distinction between the semi-Dirac points and Dirac points. In addition, the acoustic wave transmission through the corresponding PC structure has been studied, and a switch-like behavior of the transmittance is observed along different directions. Along some particular direction, there exist deaf bands around the semi-Dirac point, and these bands cannot be excited by the externally incident plane waves due to the mismatch in mode symmetry. But the situation is different along the other direction, where the bands are active ones and therefore can be excited by the incident plane waves. Actually, such properties of the bands can be easily changed as long as the iron cylinders are rotated around their axis. The work described in this paper is helpful to the understanding of semi-Dirac point in phononic crystals and suggests possible applications in diverse fields.
The Lagrangian hydrodynamics algorithm using staggered mesh is one of the most important algorithms for engineering design and science computing. Some questions need to use the conservative scheme. Shashkov gave the idea how to get conservation with a material. He defined the conservative scheme to be a compatible algorithm. When we perform a numerical simulation with two or more materials, we should use sliding line or contact-impact algorithm. In this case, the Wilkins algorithm is used mostly. But this algorithm is not conservative. This paper presents a conservative method for sliding line based on the compatible Lagrangian hydrodynamics algorithm and Wilkins sliding algorithm. The conservation of total energy can be got by the local modification through the idea of contact force and contact work. This method can ensure the symmetric property and conservative property, and improve the numerical accuracy. In this paper, we give the detail in how to design the conservative sliding algorithm and how to impose the slave's edge artificial viscosity. We also gave some numerical simulations to prove that our scheme is right and useful.
Semi-analytical solution of the dry baroclinic Lagrange primitive equation and numerical experiment of a non-linear density current
2015, 64 (19): 194702. doi: 10.7498/aps.64.194702
To solve atmospheric primitive equations, the finite difference approach would result in numerous problems, compared to the differential equations. Taking the semi-Lagrange model as an example, there exist two difficult problemsthe particle trajectory computation and the solutions of the Helmholtz equations. In this study, based on the substitution of atmosphere pressure, the atmospheric primitive equations are linearized within an integral time step, which are broadly seen as ordinary differential equations and can be derived as semi-analytical solutions (SASs). The variables of SASs are continuous functions of time and discretized in a special direction, so the gradient and divergence terms are solved by the difference method. Since the numerical solution of the SASs can be calculated via a highly precise numerical computational method of exponential matrixthe precise integration method, the numerical solution of SASs at any time in the future can be obtained via step-by-step integration procedure. For the SAS methodology, the pressure, as well as the wind vector and displacement, can be obtained without solving the Helmholtz formulations. Compared to the extrapolated method, the SAS is more reasonable as the displacements of the particle are solved via time integration. In order to test the validity of the algorithms, the SAS model is constructed and the same experiment of a non-linear density current as reported by Straka in 1993 is implemented, which contains non-linear dynamics, transient features and fine-scale structures of the fluid flow. The results of the experiment with 50 m spatial resolution show that the SAS model can capture the characters of generation and development process of the Kelvin-Helmholtz shear instability vortex; the structures of the perturbation potential temperature field are very close to the benchmark solutions given by Straka, as well as the structures of the simulated atmosphere pressure and wind field. To further test the convergence of the numerical solution of the SAS model, the 100 m spatial resolution experiment of the non-linear density current is also implemented for comparison. Although the results from both experiments are similar, the former one is better and the property of mass-energy conservation is comparatively reasonable, and furthermore, the SAS model has a convergent property in the numerical solutions. Therefore, the SAS method is a new tool with efficiency for solving the atmospheric primitive equations.
Simulation and experimental investigation of tooth tissue in photothermal radiometry dynamic response induced by modluated laser
2015, 64 (19): 194203. doi: 10.7498/aps.64.194203
The photon-density wave is generated in tooth tissue due to the scattering induced by modulated laser beams, and furthermore, thermal-wave will form because of photothermal effect. A one-dimensional thermal-wave model for three-layer tooth tissue using modulated laser stimulation is developed based on 1D diffusion approximation of the radiative transfer theory in combination with 1D heat conduction equation. Effects of photothermal properties (i.e. light absorption coefficient, scattering coefficient and thermal diffusivity coefficient), enamel depth and caries depth on the photothermal radiometry (PTR) dynamic responses are investigated based on the 1D thermal wave model coupling with photon-density wave. The PTR amplitude and phase delay (the phase difference between the PTR signal and reference signal) are strongly dependent on the photothermal parameters of the dental enamel caries layers (DECLs). PTR amplitude and phase delay increase with increasing DECL absorption coefficient, scattering coefficient and thermal diffusivity. Additionally, PTR amplitude may also increase due to the larger thickness of caries layer, and the PTR phase peak value is generated at low frequencies. The inhomogeneous photothermal properties of dental enamel healthy layer (DEHL) also have obviously influenced PTR amplitude and phase. Increasing DEHL scattering coefficient leads to the increase of PTR amplitude, but has no apparent effect on the PTR phase. While the PTR phase delay increases with increasing DEHL absorption coefficient. The delay of PTR amplitude and phase is enlarged at the high value of DEHL thermal diffusivity. However, the DEHL layer thickness has no apparent effect on the PTR amplitude and phase.The PTR signal of tooth tissue induced by the 808 nm diode laser is monitored using an infrared detector (HgCdTe, spectral width 2.012.0 m), and the PTR amplitude and phase response are obtained using lock-in amplifier (SR830). Through frequency-scanning experiments of dental tissue, PTR dynamic responses can be measured and employed to characterize the inhomogeneity and caries of the tooth tissue. The photothermal parameters and caries characteristic of the tooth issue can be simultaneously obtained by multi-parameters statistic best-fit.Simulation and experimental results show that the PTR dynamic response has the advantages of high sensitivity and high contrast for inhomogeneity and caries of the tooth tissue.
2015, 64 (19): 194204. doi: 10.7498/aps.64.194204
In image processing, most of the anisotropic diffusion models based on partial differential equation use gradient information to detect image edge. If the image edge is seriously polluted by noise, these methods would not be able to detect image edge, so the edge features cannot be retained. Pulse coupled neural network (PCNN) has the property that similar input neurons can generate pulse at the same time; this property is used to process the noisy image, and we can get an image entropy sequence. The image entropy sequence which will be used as an edge detecting operator is introduced into the diffusion equation, and this will not only reduce the defects produced when the gradient is used as an edge detecting operator so it is easily affected by the noise, but the area image information can also retain more completely. Then, we will use the rule of minimum cross entropy to search for a minimum threshold, which would satisfy the condition that the information difference between noisy image and denoised image is the minimum. The optimal threshold designed will control diffusion intensity reasonably, and the anisotropic diffusion model based on pulse coupled neural network and image entropy (PCNN-IEAD) can be established. Analysis and simulation results show that the proposed model preserves more image information than the classical ones. It removes the image noise and at the same time protects the edge texture details of the image; the proposed model retains the area image information more completely, the performance indexes can also confirm the superiority of the new model. In addition, the operating time of the proposed model is shorter than that of the classical models, therefore, the proposed model may be the ideal one.
Dark pulses and harmonic mode locking in graphene-based passively mode-locked Yb3+-doped fiber laser with all-normal dispersion cavity
2015, 64 (19): 194205. doi: 10.7498/aps.64.194205
Graphene has recently been proposed as an attractive material in saturable absorption (SA) applications due to its broad operation range, low saturation power, easy fabrication, high reliability, and quick recovery time. In this paper, we use laser-induced deposition to prepare graphene saturable absorber, and apply it in a mode-locked all-normal-dispersion (ANDi) Yb-doped fiber laser to experimentally investigate different operational states. By adjusting a polarization controller (PC) and the pump power, bright pulses, dark-bright pulse pairs and their second-harmonic pulses, as well as dark pulses and their second, third-harmonic pulses can all be obeserved. In particular, it is the first time to our knowledge to report on the formation of dark-bright pulse pairs, dark pulses and their harmonic mode locking (HML) counterparts in graphene-based passively mode-locked Yb-doped fiber laser with ANDi cavity. Accoding to simulation, the main causes of these pulses are different cavity nonlinear effects which result from the fiber mode-lock members including graphene. Bright pulses, dark pulses and dark-bright pulse pairs are determined both by the laser structure and their own initial signals. Bright pulse harmonic generation is ascribed to the noise gains which form new components. However, it is found that the multiple-time repetition rate of dark pulses is a result of square pulse splitting of each component. This consequence may be of potential application in new type mode-locked fiber lasers.
2015, 64 (19): 194206. doi: 10.7498/aps.64.194206
The microwave modulation induced by liquid crystals is determined by the orientation of liquid crystal molecules under an external applied voltage. The anchoring of substrate has an important effect on the liquid crystal orientation, which results in the change of microwave modulation. In this paper, the microwave modulation property of 90 twisted nematic liquid crystals with weak anchoring without chiral dopant is studied. Based on the elastic theory of liquid crystals and the variational theory, the equations of equilibrium state and the boundary condition are given, and the variations of phase-shift per unit-length with voltage for different anchoring energy coefficients and pre-tilt angles are also simulated using the finite-difference iterative method. Results are as follows: (1) The influence of pre-tilt angle on microwave phase-shift is related to the applied voltage. When the voltage applied to the liquid crystal cell is from 0.5 to 1.6 V, with increasing pre-tilt angle, the microwave phase-shift per unit-length and the phase-shift difference relative to the strong anchoring 90 twisted nematic liquid crystal with pre-tilt angle 0 will all increase, and the applied voltage for the maximum phase-shift difference decreases. When the applied voltages are from 1.6 to 3.0 V, the microwave phase-shift per unit-length and the phase-shift difference all decrease with increasing pre-tilt angle. When the applied voltages are near 1.6 V or larger than 3.0 V, the phase-shift per unit-length has little change. (2) The anchoring energy strength has a great influence on microwave phase-shift. As the anchoring strength decreases, the microwave phase shift per unit-length and the phase-shift difference will increase, also the tunable range of microwave phase-shift increases more and more obviously. This research provides a theoretical foundation for the design of the liquid crystal modulator.
Full-vectorial analysis of a polarization demultiplexer using a microring resonator with silicon-based slot waveguides
2015, 64 (19): 194207. doi: 10.7498/aps.64.194207
Photonic integrated circuits (PICs) based on silicon-on-insulator (SOI) platform with the advantages of high-index-contrast and CMOS-compatible process can efficiently reduce the component sizes and densely integrate them at a chip scale. To meet the ever-increasing demand for the optical interconnect capacity, various multiplexing techniques have been used. However, it should still be proposed to effectively reduce the component size accompanied with the reasonable performance and wavelength division multiplexing (WDM) compatibility. To the best of our knowledge, there has no attempt so far to design a polarization demultiplexer based on a microring resonator in slot waveguide structures. In this paper, a compact silicon-based polarization demultiplexer is proposed, where two regular silicon-based waveguides are used as the input/output channels and a microring in slot waveguide structures is used as the polarization/wavelength-selective component. A full-vectorial finite-difference frequency-domain method is utilized to study the modal characteristics of the regular and slot silicon-based waveguides, where the effective indices and coupling for transverse magnetic (TM) and transverse electric (TE) modes are presented. With the unique modal characteristics of slot waveguides and the strong polarization-dependent features of microring resonator, we can show that the field distributions and the effective indices of the TM mode between the regular and slot waveguides are similar, while those of the TE mode show clearly different. As a result, the input TM mode outputs from the drop port at the resonant wavelength, while the input TE mode outputs from the through port directly with nearly neglected coupling, thus the two polarizations are separated efficiently. A three-dimensional finite-difference time-domain method is utilized to study the spectrum and transmission characteristics of the proposed device. From the results, a polarization demultiplexer with a radius of 3.489 m is achieved with the extinction ratio and insertion loss of ～ 26.12(36.67) dB and ～ 0.49(0.09) dB respectively for the TM(TE) mode at the wavelength of 1.55 m by carefully optimizing the key structural parameters. In addition, taking the fabrication errors into account during the practical process, the fabrication tolerances to the proposed device are analyzed in detail and the performance is assessed by the extinction ratio and insertion loss. For demonstrating the transmission characteristics of the designed polarization (de) multipexing (P-DEMUX) device, the evolution along the propagation distance of the input mode through the designed P-DEMUX is also presented. The present polarization demultiplexer is compatible with the WDM systems on-chip based on microring resonators and can be easily introduced into the WDM system to further increase the optical interconnect capacity.
Propagation characteristics of power line harmonic radiation into the stratified anisotropic ionosphere
2015, 64 (19): 194101. doi: 10.7498/aps.64.194101
Power line harmonic radiation (PLHR), which specifically refers to the electromagnetic wave radiation observed in ionosphere or magnetosphere, is radiated by the transmission lines of power systems on the ground. PLHR is shown as a parallel spectrogram between 400 Hz and 5 kHz in frequency-time power spectrogram of electromagnetic field. And the frequency spacing of the parallel spectrogram is 50/100 Hz or 60/120 Hz. As an artificial pollution source in the near earth space, PLHR has attracted more and more attention. However, so far, there have been little proposed quantitative researches on the formation mechanism. This paper studies the propagation model for the electromagnetic waves generated by the electric dipole source above the non-ideal conductive ground in the stratified anisotropic ionosphere. Based on the method by Lehtinen(2008), a new full-wave finite element method is give to solve the problem. By recursively calculating reflection coefficients and mode amplitudes, the method contains no index increasing items. So it can effectively overcome the numerical overflow in programming calculations. In order to verify the correctness of the method, comparison are made between the existing analytical solutions and the solutions obtained from the proposed method, and they are in excellent agreement. Further more, using the present model, the new method and the associated parameters about practical power lines, ground and ionosphere, we have studied the effects of the frequency of dipole source, the bottom boundary height of ionosphere, the earth conductivity, and the geomagnetic field direction on PLHR propagation in the ionosphere. Results show that when the frequency of radiation source equals the cut off frequency of earth-ionosphere waveguide-guided wave modes, the strength of PLHR for penetrating into the ionosphere becomes larger. Keeping the harmonic current constant, a smaller ground conductivity would be accompanied by a larger power of PLHR. PLHR propagates along the direction of the geomagnetic field in the ionosphere. Therefore, it is much easier for a high-order harmonic radiation of transmission lines to penetrate into the ionosphere along the direction of the geomagnetic field in the areas with low ground conductivity under a certain condition. Results obtained in this paper may have important implications to explain the formation mechanism of PLHR.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2015, 64 (19): 195201. doi: 10.7498/aps.64.195201
The electron thermal transport in fluid theory would be inaccurate when the collisionality is not enough, and the Fokker-Planck (FP) simulations are usually employed to resolve the inadequacies. In this paper, the one-dimensional Fokker-Planck code is extended to handle the cylindrical and spherical geometries in which the electron distribution functions are solved in the reference frame of the ion fluid. The FP code is validated in the fluid limit by comparing with fluid (MULTI) simulations. Then, the expansions of plasmas in different spatial geometries are simulated with the FP and fluid codes. As the main characters of nonlocal transport, the electron thermal transport inhibition and preheating are investigated in expanding plasmas. The spherical nonlocal theory can give the thermal transport inhibition and preheating phenomenon, which is exploited to fit the heat flux with variation of fitting parameter . The spherical nonlocal theory will reproduce Spizer-Hrm expression as = 0. Then we analyze the heat flux after the plasma expanding 200 ps with a uniform initial temperature T = 100 eV and density ne= 1 1021 /cm3. By comparing the heat flux computed by spherical nonlocal thermal transport theory and FP simulation, it is found that (n-1)/r term in Eq. (3a) cannot be neglected when the radius is small and the geometrical curvature effect will decrease the nonlocality of transport in outer expanding plasmas. The geometrical curvature effect leads to a smaller thermal transport inhibition and preheating in the expanding plasmas as comparing the spherical case with the planar one. The expansions of plasmas in different spatial geometries are also simulated with the FP and fluid codes under the initial conditions which are similar to the inertial confinement fusion. The same influence of geometrical curvature on nonlocal electron thermal transport are also obtained.
2015, 64 (19): 195202. doi: 10.7498/aps.64.195202
We have studied various complex motions of the irregular dust grains immersed in non-uniformly magnetized plasma. The cylindrical magnet that we used for experiments significantly alters the radial distribution of the sheath potential which confines the negatively charged grains. Grains are horizontally illuminated by a 50 mW, 532 nm laser sheet and imaged by a CCD camera from the upper transparent electrode. Hypocycloid and epicycloid motions of grains are observed for the first time as far as we know. Cuspate cycloid motions, circle motion, wave motion, and stationary grains are also observed. Their trajectories can be obtained by using long-time exposure, and the characteristic parameters of the grain movement are measured by using the image processing with MATLAB. Though the dust grains can move around the magnet steadily in various trajectories, the induced magnetic field is too weak to give rise to cycloid motions of grains. Then we propose a new mechanism that an inverse Magnus force induced by the spin of the irregular grains plays an important role in their cycloid motions. The pollen pini we used for experiment is not a regular microsphere, there is a symmetry in the shape. On the basis of Bernoulli principle, the pressure difference between the left and right side of the forward moving grains produces the inverse Magnus effect. Additional comparison experiments with regular microspheres are also performed to confirm that the cycloid motions are distinctive features of an irregular dust grain immersed in the plasma. The periodical change of the cyclotron radius as the grain travels would result in the (cuspate) cycloid motions, and the maximal value of angular velocity of spin is about 105 rad/s. Our experimental observations can be well explained based on the force analysis in 2D horizontal plane.
Capsule illumination uniformity illuminated by direct laser-driven irradiation from several tens of directions
2015, 64 (19): 195203. doi: 10.7498/aps.64.195203
Capsule illumination uniformity obtained by direct driving lasers from several tens of directions is studied systematically. The best polar angles of the three focal spot rings on the capsule are determined to be 22.4, 47.7, and 73.6by a spherical-harmonic mode analysis and a numerical simulation. Based on the configuration of indirect laser driven facility, we have optimized the beam re-directions and the focal spot distributions for polar direct drive, which smooth successfully the illumination distribution on the capsule.Laser driven inertial confinement fusion is an important way to achieve controllable nuclear fusion for human beings, which includes two laser-driven schemesdirectly driving and indirectly driving scheme. Since the indirect driving scheme considerably relaxes the strict requirements for laser performance and decreases the engineering difficulties, the main laser facilities around the world have adopted the indirect driving scheme, such as the National Ignition Facility in the U. S., the Laser Megajoule in France, and the SG series laser drivers in China.Meanwhile, scientists keep developing the key technologies for directly driving and have made great progress. For example, the fast ignition and shock ignition are two new methods to achieve fusion ignition in the direct driving scheme, which attracted lots of attention in the past few years. However, the main laser drivers for inertial confinement fusion research are configured as indirect drivers, which are not suitable for direct driving experiments. So a compromising suggestion was proposed that by redirecting the lasers, changing the laser energy distributions, designing new type of targets, and so on, a radiation field which is very close to a direct driving radiation field can be simulated in a laser facility that is configured as an indirect driver. This is the so called polar direct drive method that provides a feasible way for primary researches on direct driving technologies in an indirect laser driver. Such experiments have already been conducted in the National Ignition Facility.In China, the large indirect laser driver with an output capability in the level of hundreds kilojoule will finish its engineering construction and routinely operate for physical experiments soon. To achieve a good polar direct drive performance in this laser facility is much more difficult than in previous smaller laser drivers. In this paper, capsule illumination uniformity by directly driving laser from several tens of directions is studied systematically. The best polar angles of the three focal spot rings on the capsule are determined to be 22.4, 47.7, and 73.6 by a spherical-harmonic mode analysis and a numerical simulation. Based on the configuration of indirect driving laser facility, we have optimized the beam re-directions and the focal spot distributions for polar direct drive, which successfully smoothes the illumination distribution on the capsule.
2015, 64 (19): 195204. doi: 10.7498/aps.64.195204
Gliding discharges driven by microsecond-pulse power supply can generate non-thermal plasmas with high energy and high power density at atmospheric pressure. However, the flowing air significantly influences the characteristics of the microsecond-pulse gliding discharges in a repetitive mode. In this paper, in order to obtain the characteristics of the microsecond-pulse gliding discharges in a needle-to-needle gap, a microsecond-pulse power supply with an output voltage up to 30 kV, a pulse width 8 s, and a pulse repetition frequencies 1 Hz 3000 Hz is used to investigate the electrical characteristics of gliding discharges by analyzing the voltage-current waveforms and obtaining the discharge images. Experimental results show that there are three typical discharge modes in the microsecond-pulse gliding discharges as the applied voltage increases, i.e. corona discharge, diffuse discharge, and gliding-like discharge. Both voltage-current waveforms and the discharge images at different discharge modes have significantly different behaviors. Corona discharge only exists near the positive electrode with a small radius of curvature. Diffuse discharges behave as the overlapped plasma channels bridge the entire gap. The channel of diffuse discharge is full of gap, which starts from the positive electrode, spreads in all directions, and ends at the negative electrode. Gliding-like discharge behaves as a continuous spark channeling, showing a continuous spark, which is discharging strongly and influenced by flow rates. Furthermore, both pulse repetition frequency (PRF) and flow rate remarkably affects the characteristics of microsecond-pulse gliding discharges. When the flow rate is small (2 L/min), the spark channels of gliding-like discharge gradually concentrate with the increase of the PRF. However, when the flow rate is larger (16 L/min), the spark channels of gliding-like discharge behave dispersively when the PRF increases. In our opinion, different characteristics of microsecond-pulse gliding discharge at different flow rates are closely related to the memory effect of the residual particles in the discharges and the state of the air flow. When the flow rate is small (2 L/min), the air flow is stable, and the discharge is generated in a laminar flow state. In this case, the memory effect of particles in repetitive microsecond-pulse gliding discharges dominates the formation of the discharges. These particles could enhance the electric field strength for the next pulse. Because the time interval between two pulses at high PRF is shorter than that at low PRF, there are fewer particles leaving the air gap at high PRF. Thus, memory effect is more significant at high PRF. As a result, the channel of spark discharge concentrates with the increase of the PRF. When the flow rate increases to 16 L/min, the calculated Reynolds number increases to 2864, indicating the transition from laminar state to turbulence state. The residual particles are more likely to escape from the gap. Thus, memory effect slightly affects the characteristics of the microsecond-pulse gliding discharges. In this case, the state of the air flow dominates the formation of the discharge. The spark channels spread towards the top in the direction of the gas flow, making the region of the spark channels gradually disperse as the PRF increases.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (19): 196101. doi: 10.7498/aps.64.196101
By using nonequilibrium Green's functions in combination with the first principles density functional theory, for the similar right triangle graphene devices as the research object, we take the zigzag graphene as electrodes, to investigate the B(N) doping and B-N co-doping effect, i.e. mainly the influence of doping on the transport properties of similar right triangle graphene devices, as well as the asymmetric doping effect on the rectifying behaviors in similar right triangle graphene devices. Calculated results show that the system conductivity is increased when the vertex carbon atom of a similar right triangle graphene is substituted by a boron or nitrogen atom, and a novel rectifying effect appears. The rectification behavior can be observed because of an asymmetric movement on the molecular-level in B(N) doping in the similar right triangle graphene devices under positive and negative biases and the asymmetry in the spatial distribution of the frontier orbitals. Most importantly, when the vertex carbon atoms of the right and left similar right triangle graphenes are simultaneously doped with boron and nitrogen atoms, the rectifying effect of the system is significantly enhanced and appears also a negative differential resistance effect.
2015, 64 (19): 196201. doi: 10.7498/aps.64.196201
Clarifying the effect of rare earth (RE) elements on the microstructure and properties of glass ceramics is technically and theoretically important for the further development. Thus the glass ceramics of the CaO-Al2O3-MgO-SiO2 with 04 wt% La2O3 are fabricated from Bayan Obo Mine tailing and fly ash by means of the conventional melting method. Effect of the existence form and the concentration variation of La3+ ions on the crystallization behavior, microstructure and properties, such as bending strength, chemical resistance and density of the glass ceramics, are investigated by DTA, XRD, SEM, TEM and EDS. Results show that both the glass transition and crystallization peak temperature of the samples shift to high temperatures with increasing La2O3 content. Augite [Ca(Mg, Al, Fe)(Si, Al)2O6] is the only crystalline phase in all the five samples. Augite crystals in the form of column are distributed uniformly within the residual glass, and their average size is below 100 nm. The crystallinity of augite has been effectively enhanced by the addition of 1 wt% of La2O3. Owing to the similar ion radius of La3+ and Ca2+, Ca2+ ions within augite have been partially substituted by La3+. Such a substitution can serve as one of the key factors to the enhancement of bending strength of the investigated material with 1 wt% of La2O3 because of the stronger bonding energy of La-O than Ca-O. With further increase of La2O3 from 1 to 4 wt%, the Ca3La6 (SiO4)6 secondary phase forms on the boundary between augite grains and residual glass phase in the form of irregular-shaped particles and this in turn hinders the growth of augite crystals. The crystallinity of augite will be decreased gradually since then. Meanwhile, the formation of this La-riched phase (Ca3La6(SiO4)6) may also prevent augite grains from growing through consuming Ca2+ and Si4+ ions which are two key constituent elements of augite grains. Therefore, there are two forms of La3+ ions in the glass ceramics developed from Bayan Obo Mine tailing: one is the substitution of Ca2+ ion by La3+ in augite crystalline phase, and the other is the forming of secondary crystalline phase La-riched Ca3La6 (SiO4)6. The glass ceramic sample with 1 wt% of La2O3 shows the optimum properties. Its density is 3.18 g/cm3, the bending strength is 198 MPa, and the weight loss in 20 wt% NaOH of this sample is lower than 1 wt%.
2015, 64 (19): 196801. doi: 10.7498/aps.64.196801
In order to further explore the oscillation mechanism of constrained droplets in microgravity and extend the application and management of space fluid, the small-amplitude self-excited oscillation processes of droplets that are pinned on a confined substrate are investigated. The substrate has a 5 mm diameter contact circle, which is implemented through the use of a drop tower and high-speed photography technology. Oscillation is a recovery procedure for droplet configuration in microgravity with the confined effect at the boundary, making the contact line and diameter unchanged throughout the entire process. A self-excited oscillation could be divided into two stages: a morphological change process and a small-amplitude damping attenuation oscillation. The first stage is a morphological change process, where the heights of high and low oscillations rise gradually, which in turn correspond to the variation of gravity. And the deformation rate is inversely proportional to the droplet size. The second stage is the small-amplitude damping attenuation oscillation around the equilibrium position until it reaches the final steady state in microgravity. At this stage, the frequency is nearly constant and the attenuation of amplitude represents an exponential damping, like the free oscillation of isolated viscous droplets. The pinning contact line makes the oscillation waveform deviate from sine curve and in the process there exists a period when the heights keep constant at some positions, such as the highest, lowest and others. Studies confirm the hypothesis that the oscillation occurs with the similar second-order mode of free drop when the height fluctuates, and the third-order mode when the height is immobile. This is in agreement with the spectral analysis. Furthermore, when the liquid volume varies within this experimental system, the pinning constraint with fixed contact area on the confined substrate can generate droplets with various static contact angles and undisturbed radii. The deformation stage and oscillation mode of the droplets remains stable, although the concrete courses differ in some ways. In the case of bigger drops, the phenomenon of height unchanging should be in the middle position and vanishes with time. However, the smaller one shows no signs for this condition, and the waveform remains consistent all around. In the second stage, the amplitude decay damping rate and non-dimensional frequency of small droplet are higher.
Simulation studies on the transport properties of Cu-H2O nanofluids based on water continuum assumption
2015, 64 (19): 196601. doi: 10.7498/aps.64.196601
Nanofluid is a kind of new engineering medium which is created by dispersing small quantity of nano-sized particles in the base fluid. The dispersion of solid nanoparticles in conventional fluids changes their transport properties remarkably. Molecular dynamics simulation (MDS) is an important approach to study the transport properties of nanofluids. However, the computation amount is huge, and it is very difficult to use the normal MDS to capture the transient flow and heat processes in Cu-H2O nanofluids if the regions in the simulation reach 149.6443 nm3 or 299.2883 nm3, and the number of Cu nano-particles reaches 6-64. Further study by means of simulation on the effects on effective transport properties of nanofluids is also difficult. In this paper, the water-based fluid region of 149.6443 nm3 or 299.2883 nm3 is assumed as continuum phase because of the very low Knudsen number of fluid, and the effects of water on nano-particles are fitted into the Cu-Cu potential parameters. Using the proposed method, the computation amount is significantly reduced. The effective thermal conductivity and dynamic viscosity coefficient of Cu-H2O nanofluids under the stationary condition are simulated and the results are verified with existing experimental data. The motion and aggregation processes of nano-particles in the water-based fluids at different velocity shear rate are simulated. Effects of velocity shear rate, fluid velocity, temperature gradient, and average temperature on the effective thermal conductivity and the dynamic viscosity of Cu-H2O nanofluids in the processes of flow and heat transfer are studied. Three conclusions can be drawn from the obtained results. Firstly, the proposed method is feasible to capture the transient flow and heat processes in Cu-H2O nanofluids, and is also capable to further study the transport properties of Cu-H2O nanofluids. Secondly, the velocity shear rate acting on a nanofluid can effectively prevent the aggregating process of nano-particles, and therefore reduce the diameter of particle-aggregations. Finally, the velocity shear rate and the average temperature of Cu-H2O nanofluids have much more effects on the transport properties, while the fluid velocity and temperature gradient have less effects; the velocity shear rate increases the effective thermal conductivity of a nanofluid but decreases its dynamic viscosity. A rise of average temperature increases the effective thermal conductivity but decreases the dynamic viscosity.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (19): 197201. doi: 10.7498/aps.64.197201
Wide gap semiconductors as the thermoelectric (TE) candidates have been increasingly interested because of their inherent high Seebeck coefficients and low thermal conductivities. Ga2Te3 is one of the typical defect compounds (Eg=1.65 eV) among the A2IIIB3VI type semiconductors, in which there are periodically self-assembled 2D vacancy planes that wrap the nanostructured domains. The vacancy planes scatter phonons highly effectively and are responsible for reducing the lattice thermal conductivity. Hence Ga2Te3 might be a good TE candidate. In the phase diagram of Ga-Te, Ga2Te3 is involved in the eutectoid and peritectic reactions at the critical temperatures (CTs) of 680 10 K and 757 10 K respectively. These reactions would lead to the generation of enthalpies of reactions, and induce the alteration of some thermo-physical properties. In the present work, we have not observed the phase transformations at CTs in the Ga2Te3-based materials with sulfur isoelectronic substitution for Te, which are prepared by powder metallurgy with the spark plasma sintering (SPS) technique, but can observe the generation of assumed enthalpies of reactions near CTs, which directly gives rise to the critical acoustic charge transport behaviors. The critical behaviors involve the remarkable increase of heat capacities and Seebeck coefficients and, at the same time, reductions of thermal diffusivities (thermal conductivities) and electrical conductivities. For example, the Seebeck coefficient () at x=0.05 increases rapidly from 376.3(VK-1) to 608.2(VK-1) when the temperature rises from 596 to 695 K, and then decreases to 213.8(VK-1) at 764 K. Similarly, all the S-doped samples, which have lowest electrical conductivities ( ) of 2.12102 (x=0.05), 0.25102 (x=0.1), 0.12102 -1m-1 (x=0.2) and 0.14102 -1m-1 (x=0.3) at 696725 K, undergo dramatic changes when the temperature rises to about 750 K, and then the electrical conductivities begin to decrease, and the changes tend to slow down. It is notable that both the Seebeck coefficients and electrical conductivities exhibit a typical zigzag temperature dependence in the temperature range from 596 to 812 K. These behaviors reveal the remarkable alterations in scattering mechanism of both phonons and carriers at temperatures near the CTs. Although the materials with these critical behaviors near CTs do not have satisfactory thermoelectric performance (ZTmax=0.17 at 793 K for x=0.3) as compared with the known binary Cu2Se, Ag2Se(S) or ternary based AgCrSe2 alloys, however, the findings of such critical transport behaviors have a great significance for future researches.
2015, 64 (19): 197304. doi: 10.7498/aps.64.197304
The field emission current variation law of carbon nanotube in a large electric field range (0-32 V m-1) is analyzed in depth by combining the density functional theory with metal electron theory. The results show that their emission current densities are determined by their densities of states, the pseudogap, the length and the local electric field, showing the different variation laws in the different electric field ranges. In the lower electric field (corresponding macroscopic field is less than 18 Vm-1), when their density of states increases, their pseudogap decreases: the two trends are opposite, the former increases the number of electrons for emission, and the latter improves the ability to transfer electrons, they all turn to the increase of the emission current, so their field-emission current density increases linearly with increasing electric field in this range. But in the higher electric field (corresponding macroscopic field is less than 32 Vm-1 and more than 18 Vm-1), their densities of states and the pseudogaps take on the same decrease and increase, so do they in the opposite change case, therefore the emission current density behaves as a non-periodic oscillation in the increasing electric field, moreover the higher electric conductivity lead to the rising of current density, the combined effect of the emitter current density exhibits an oscillatory growth in this electric field range, and the carbon nanotubes behave as ionizing radiation. So the too high electric field may cause the emission current to be instable. The electric conductivity variation law of the metallic carbon nanotube is further studied in this paper. In the lower electric field (corresponding macroscopic field is less than 5 Vm-1), the electric conductivity of CNT increases linearly with increasing electric field; when the macroscopic electric field increases up to a value in a range from 5 to 14 Vm-1, the electric conductivity only changes like a slight concussion in (6.3-9.9)1017Sm-1 range, when the macroscopic electric field increases to a value in a range from 16 to 32 Vm-1, the electric conductivity appears as a sharp oscillation growth trend. Additionally, the specific binding energy of CNT is enhanced with increasing electric field, accordingly the structural stability turns better and the cone-capped carbon nanotubes could be used for emission cathode material. The calculation results are consistent with the experimental results of the literature.
Enhancement of perpendicular magnetic anisotropy and thermal stability in Co/Ni multilayers by MgO/Pt interfaces
2015, 64 (19): 197501. doi: 10.7498/aps.64.197501
Co/Ni multilayers with Pt and MgO/Pt underlayer have been grown by means of magnetron sputtering and the perpendicular magnetic anisotropy (PMA) of the samples is studied using anomalous Hall effect (AHE). The Co/Ni multilayer has to be thermally stable to stabilize the PMA, which is studied by annealing treatment. In early researches of Co/Ni multilayes, the optimum sample with Pt underlayer was obtained as Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) with PMA in good performance. Thermal stability of the sample is studied in this paper by the Hall loop measurement of it after annealing. Results show that the remanence ratio and rectangular degree of the sample are kept well and the Hall resistance (RHall) has little change at the annealing temperature of 100 ℃. As the annealing temperature rising above 100 ℃, the PMA of Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) becomes weakened. Its coercivity (Hc) decreases rapidly and RHall reduces greatly. So the thermal stability of Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) will be poor and the PMA cannot be enhanced by annealing treatment. A series of samples with MgO/Pt underlayer are prepared with the thickness of Pt being fixed at 2 nm and that of MgO ranging from 1 to 5 nm. Thus the interface between amorphous insulation layer and metal layer is added to be used to enhance the PMA of the sample for the strong electron additive scattering. Magnetization reversal can be very rapid and the rectangular degree is kept very well, and furthermore, the remanence ratio of the samples can reach 100% so they all show good PMA.The Hc increases with increasing MgO underlayer and reaches the maximum value as the MgO thickness arrives at 4 nm, and the Hc of the sample MgO(4 nm)/Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) is 2.3 times that of Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm), the RHall is up to 9% correspondingly. The roughnesses of Pt(2 nm)/Co(0.2 nm)/ Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) and MgO(4 nm)/Pt(2 nm)/Co(0.2 nm) /Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) are 0.192 nm and 0.115 nm respectively, as tested by AFM. Result shows that the roughness of the Co/Ni multilayer is greatly reduced so the PMA of the Co/Ni multilayer is enhanced remarkably after the addition of 4 nm MgO. The thermal stability of MgO(4 nm)/Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) is also studied. When the annealing temperature rises up to 200 ℃, the Hc reaches its maximum value i.e. 1.5 times that of the sample without MgO, and it is 3.5 times that of the sample with Pt underlayer only. This sample also show good thermal stability. Higher temperatures will result in intermixing of Co and Ni and diminish the PMA. After annealing at 400 ℃, the easy axis of the sample becomes in-plane. The anisotropy constant Keff of MgO(4 nm)/Pt(2 nm)/Co(0.2 nm)/Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm) is 8.2106 erg/cm3, and it has an increase of 15% in Pt(2 nm)/Co(0.2 nm)/ Ni(0.4 nm)/Co(0.2 nm)/Pt(2 nm), which shows that the sample has an excellent PMA.
2015, 64 (19): 197502. doi: 10.7498/aps.64.197502
Magnetic recording has now played an important role in the development of non-volatile information storage technologies, so it becomes essential to quantitatively understand the magnetization distribution in magnetic microstructures. In ferromagnetic disks, squares and triangles with submicron sizes, it is energetically favorable for the magnetization to form a closed in-plane vortex and a perpendicular vortex core at the center. This vortex magnetic structure is a new candidate for future magnetic memory device because both the vortex chirality and the core polarity can be manipulated by applying an external magnetic field or a spin-polarized current. Further development of vortex-based memory devices requires quantitative measurement of vortex domain structures, which is still lacking.In this paper, magnetization configuration in a vortex structure has been quantitatively studied by scanning transmission X-ray microscope (STXM) utilizing X-ray magnetic circular dichroism (XMCD) effect in Shanghai Synchrotron Radiation Facility. Samples have been fabricated on the 100 nm silicon-nitride membranes. The patterns are first transferred to PMMA photoresist using e-beam lithography, then a 50 nm thick Ni80Fe20 film is deposited by e-beam evaporation. Magnetic vortex configurations are characterized with the X-ray energy at Fe L3 absorption edge and Ni L3 absorption edge, respectively. The image taken at Fe edge shows greater contrast than that at Ni edge. Experimental results indicate that the magnetic vortex state remains stable in permalloy circle, square and triangle structures with diameters from 2 to 5 m. The STXM images indicate that the magnetization in circular geometry changes continuously along the concentric circles without clear domain boundaries. In contrast, magnetization in square geometry consists of four distinct domains with clear diagonal domain boundaries. Similarly, three domains can be observed in triangle geometry. In order to quantify the in-plane magnetization configuration in magnetic vortices, we also use micromagnetic simulation to calculate the magnetization distributions of these three geometries. By extracting Mx along the circular profiles in both experimental and simulated vortex images, we find that the experimental magnetic profiles in the STXM images are consistent with the simulation data quantitatively. These magnetic structures are also studied by magnetic force microscopy (MFM). Since MFM is only sensitive to the dipolar magnetic field around the domain boundary, the MFM images show different configurations from the STXM images.
2015, 64 (19): 197302. doi: 10.7498/aps.64.197302
Tantalum nitride with a face-centered cubic structure (TaN1-) has received much attention due to its high hardness, good wear resistance, chemical inertness, thermodynamic stability, and low temperature coefficients of resistivity. First-principles calculations have indicated that cubic-TaN possesses metallic energy band structure, and the experimental results show that the carrier concentration in TaN1- films are comparable to that of normal metals. However, semiconductor-like temperature behavior of resistivity is often observed in polycrystalline TaN1- film. In the present paper, we systematically study the crystal structures and electrical transport properties of a series of TaN1- thin films, deposited on quartz glass substrates at different temperatures by the rf sputtering method. Both X-ray diffraction patterns and scanning electron microscope images indicate that the films are polycrystalline and have face-centered cubic structure. It is also found that the mean grain sizes of the films gradually increase with increasing depositing temperature. The temperature dependence of resistivity is measured from 350 K down to 2 K. The films with large grain sizes have a superconductor-insulator transition below ～ 5 K, while the films with small grain sizes retain the semiconductor characteristics down to the minimum measuring temperature, 2 K. These phenomena are similar to that observed in superconductor-insulator granular composites. Above 5 K, the temperature coefficients of the resistivities of the films are all negative. In the temperature range between 10 and 30 K, the films show hopping transport properties which are often seen in metal-insulator granular systems, i. e. the logarithm of the resistivity (log ) varies linearly with T-1/2, where T represents the measured temperature. The thermal fluctuation-induced tunneling conductive mechanism dominates the temperature behaviors of resistivities from 70 K up to 350 K. It can be seen that the thermal fluctuation induced tunneling conductive mechanism is also the main conductive mechanism in metal-insulator granular systems in the higher temperature regions. Our results indicate that the electrical transport properties of the polycrystalline TaN1- films are similar to that of metal-insulator granular films with different volume fractions of metal, where the metal possesses superconductivity at low temperatures. Hence the high resistivity and negative temperature coefficient of resistivity of TaN1- polycrystalline film can be reasonably ascribed to the similarity in microstructures between TaN1- polycrystalline film and metal-insulator granular film.
2015, 64 (19): 197303. doi: 10.7498/aps.64.197303
This paper investigates the changes of electron transport properties in AlxGa1-xN/GaN with an inserted AlN layer. The polarization charge density and two-dimensional electron gas (2DEG) sheet density in AlxGa1-xN/AlN/GaN double heterojunction high electron mobility transistors (HEMT) affected by the spontaneous polarization and piezoelectric polarization in AlxGa1-xN and AlN barrier are studied. Relations of interface roughness scattering and alloy disorder scattering with the AlN thickness are systematically analyzed. It is found that the alloy disorder scattering is the main scattering mechanism in AlxGa1-xN/GaN heterojunction high-electron-mobility transistors, while the interface roughness scattering is the main scattering mechanism in AlxGa1-xN/AlN/GaN double-heterojunction structure. It is also known that the 2DEG sheet density, interface roughness scattering and alloy disorder scattering are depended on the thickness of the inserted AlN layer. The 2DEG sheet density increases slightly and the mobility increases obviously by inserting an AlN layer about 13 nm. Taking Al mole fraction of 0.3 as an example, if without AlN layer, the 2DEG sheet density is 1.47 1013 cm-2 with the mobility limited by the interface roughness scattering of 1.15 104 cm2V-1-1, and the mobility limited by alloy disorder scattering of 6.07 102cm2V-1-1. After inserting an AlN layer of 1 nm, the 2DEG sheet density increases to 1.66 1013cm-2, and the mobility limited by the interface roughness scattering reduces to 7.88 103cm2V-1-1 while the mobility limited by alloy disorder scattering increases greatly up to 1.42 108 cm2V-1-1.
2015, 64 (19): 197301. doi: 10.7498/aps.64.197301
The channel current model is used to analyse the behavior of uniaxially strained Si NMOSFET device and circuit. With the development of mobility and threshold voltage model, starting from the basic drift-diffusion equation, the channel current model for an uniaxially strained Si NMOSFET device is developed under different bias conditions. Especially, the stress intensity is explicitly included in the mobility and threshold voltage model, and this makes the model convenient to directly reflect the relationship between the device channel current and the stress intensity. Moreover, in terms of the subthreshold current model, the charge of weak inversion rather than the normal effective channel thickness approximation is involved. In this way, the model accuracy can be improved. Furthermore, this model is implemented by using verilogA language and is applied to the strained Si circuit's SPICE simulation, the model parameters extraction tool ParamPlus++ is developed at the same time. As a result, the simulation of uniaxial-strained Si NMOSFET device and circuit can be achieved; the simulation data fits the experimental results or TCAD simulation results very well, and this proves the accuracy of the model. Meanwhile the simulation results of the threshold voltage and subthreshold current with respect to stress intensity are obtained and analyzed. The results show that with increasing stress intensity the subthreshold current is increased while the threshold voltage is decreased.
2015, 64 (19): 197801. doi: 10.7498/aps.64.197801
In this paper, the lattice vibration modes of ammonium dihydrogen phosphate NH4H2PO4 (ADP) and its deuterated analog DADP are studied by using polarized Raman spectra and the first-principles calculation in the framework of the density functional theory. The vibration modes of ADP crystal, in which the basic structural units are the NH4+ and H2 PO4-groups, have been simulated by using a plane-wave pseudo potential method. Result shows that the peaks of 921 and near 3400 cm-1 are assigned as the vibrational H2 PO4-and NH4+ groups, respectively. In order to investigate the deuteration process, the polarized Raman spectra are obtained in different polarization configurations and recorded in the range of 200-4000 cm-1, so that we can study the variation of Raman peaks at 921 and 26003400 cm-1. For example, in the scattering geometryX(YY)X , with increasing degree of deuterated, the peak of 921 cm-1 red shifts and decreases in intensity, while the peaks ranging from 2600 to 3000 cm-1 decrease in intensity. Moreover, a new broadened line appears in the range of 20002600 cm-1, and the intensity of the new line increases with the degree of deuterated, but no change occurs in the peak position. With Lorentz analysis, it can be inferred that the deuterated of NH4+ group is prior to that of H2 PO4-group. We also study the spectra for other Raman scattering geometry of these crystals, and the result shows that the site symmetries of NH4+ (ND4+) and H2 PO4-(D2 PO4-) groups are determined to be C2, which means that the site symmetry of the two groups in crystals have no change during the deuteration process. These results will be a foundaton for optimizing the growth and property of this kind of material.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Optical modulation characteristics of VO2 thin film due to electric field induced phase transition in the FTO/VO2/FTO structure
2015, 64 (19): 198101. doi: 10.7498/aps.64.198101
VO2 thin films have been studied for their semiconductor-metal reversible transition from the monoclinic to the rutile structure, where the electrical and optical properties undergo a drastic change by increasing the temperature or by applying a voltage. VO2 film is becoming a promising material for optical switch, optical storage, optical modulator, smart window, and micro-bolometer. The preparation procedures of the FTO/VO2/FTO structure in detail are as follows: First, the F-doped SnO2 conductive glass (FTO) substrates are cleaned sequentially in acetone, ethanol, and deionized water for 10 min using an ultrasonic cleaning equipment at a frequency of 20 kHz. When the FTO substrates was cleaned, they are dried with nitrogen. Second, the dried FTO substrates are placed in the chamber of a DC magnetron sputtering system equipped with a high-purity metal target of V (99.9%). After argon (99.999%) of 80 sccm flux was discharged with the current of 2 A and the voltage of 400 V for 2 min, the vanadium films are deposited on the FTO substrates. Third, the prepared vanadium films are annealed for different annealing time in an atmosphere composed of different proportions of nitrogen-oxygen. Then another layer thickness of 350 nm of FTO conductive film is deposited on the VO2 thin film by using the plasma enhanced chemical vapor deposition method. Finally, different sizes of the FTO/VO2/FTO structure are prepared by using photolithography and chemical etching processes. The effect of different annealing time and different proportions of nitrogen-oxygen atmosphere on the VO2 thin films has been studied. X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and spectrophotometer are then used to test and analyze the crystal structure, surface morphology, surface roughness, the relative content of the surface elements, and transmittance of the VO2/FTO composite films. Results show that a relatively single component VO2 thin film can be obtained under the optimum condition. The current abrupt change can be seen at the threshold voltage when the FTO/VO2/FTO structure is applied to voltage on both the transparent conductive films of the VO2 thin film. The threshold voltage is 1.7 V when the contact area is 3 mm×mm, and the threshold voltage increases as the contact area increases. When the contact area is 6 mm × 6 mm, the threshold voltage of the thin film phase transition is 4.3 V; when the contact area is 8 mm × 8 mm, the threshold voltage of the thin film phase transition is 9.3 V. Compared with the no voltage situation, the infrared transmittance difference of the FTO/VO2/FTO structure under the effect of voltage is up to 28% before and after the transition. The structure remains stable with a strong electrochromic capacity when it is applied with voltage repeatedly. This brings about many new opportunities for optoelectronic devices and industrial production.
2015, 64 (19): 198402. doi: 10.7498/aps.64.198402
Strong front wall clutter has serious impacts on the target detection and imaging in the through-wall radar (TWR) system. A method of robust wall clutter suppression based on the entropy of an expanded antenna source for ultra-wide-band through-wall radar is presented in this paper. The model of TWR scenario consists of four layers. Assume that the first and the third layers are air space, while the second and the fourth layers are composed of uniform flat concrete wall. The circular target, assumed to be a perfect electric conductor, is located in the third layer. Along the measurement line which is parallel to the front wall, the transceiver antenna scans uniformly. The echo signals that come from the target and walls are processed into discrete data at first, so that the calculation of probability space is subsequently implemented and the discrete data are expanded as well. And then the entropy of the expanded data that contain robust wall clutter and echo of target is calculated. Taking into consideration the amplitude of target signal varying in each scan, while that of clutter signal is not, it is evident that the entropy can be utilized to discriminate the signals between the target and wall. According to the difference between the entropy of the wall clutter and that of the target, a certain threshold can be set and the optimum tolerance threshold is adaptively selected on the basis of target-to-clutter ratio. With the optimum tolerance threshold, process of clutter suppression is conducted. Finally, back projection is employed for imaging of target. In this paper, data of through-wall radar for simulation are provided by GprMax2D/3D, based on the finite difference-time domain methsd. The clutter suppression and imaging are separately conducted by the method based on data entropy and the method proposed in this paper. Comparing the results of simulations, it is shown that the gain of target-to-clutter ratio for the former is 15.51 dB, and that for the latter is 19.74 dB. It is obvious that the proposed method can provide imaging with higher quality for the same measurement, and it requires fewer scans with the same quality of imaging as well. Computational complexity of the proposed method and the method based on entropy can be expressed as O(M NL) and O(M N) , respectively
Effect of the intrinsic in-plane shape anisotropy on the oscillation characteristics of zero-field spin torque oscillator
2015, 64 (19): 198502. doi: 10.7498/aps.64.198502
The spin-torque oscillator, which can generate an AC voltage oscillation with the same frequency, have attracted considerable attention due to its potential applications in the frequency-tunable transmitters and receivers for wireless communication and the recording heads of high-density hard disk drives. However, from the energy-balance equation's point of view, in the absence of in-plane shape anisotropy of spin torque oscillator, the energy supplied by the spin torque is always larger than the energy dissipation due to the Gilbert damping, thus, a finite magnetic field applied perpendicular to the plane is required for a steady-state precession. This feature has limited its potential applications. In this paper, the influence of the intrinsic in-plane shape anisotropy on the magnetization dynamics of spin torque oscillator consisting of an in-plane polarizer and an out-of-plane free layer is studied numerically in terms of the Landau-Lifshitz-Gilbert-Slonczewski equation. It is demonstrated that the additional in-plane shape anisotropy plays a significant role in the energy balance between the energy accumulation due to the spin torque and the energy dissipation due to Gilbert damping, which can stabilize a steady-state precession. Therefore, a stable self-oscillation in the absence of the applied magnetic field can be excited by introducing additional in-plane shape anisotropy. In particular, a relatively large current region with zero-field self-oscillation, in which the corresponding microwave frequency is increased while the threshold current still maintains an almost constant value, can be obtained by introducing a relatively large intrinsic in-plane shape anisotropy. Our results suggest that a tunable spin transfer oscillator without an applied magnetic field can be realized by adjusting the intrinsic in-plane shape anisotropy, and it may be a promising configuration in the future wireless communications.
Investigation of emergence of target wave and spiral wave in neuronal network induced by gradient coupling
2015, 64 (19): 198701. doi: 10.7498/aps.64.198701
Distinct rhythm and self-organization in collective electric activities of neurons could be observed in a neuronal system composed of a large number of neurons. It is found that target wave can be induced in the network by imposing continuous local periodical force or introducing local heterogeneity in the network; and these target waves can regulate the wave propagation and development as pacemaker' in the network or media. A regular neuronal network is constructed in two-dimensional space, in which the local kinetics can be described by Hindmarsh-Rose neuron model, the emergence and development of ordered waves are investigated by introducing gradient coupling between neurons. For simplicity, the center area is selected by the largest coupling intensity, which is gradually decreased at certain step with increasing distance from the center area. It is found that the spiral wave and/or the target wave can be induced by appropriate selection of gradient coupling, and both waves can occupy the network, and then the collective behaviors of the network can be regulated to show ordered states. Particularly, the ordered wave can be effective to dominate the collective behavior of neuronal networks, even as the stochastic values are used for initial states. These results associated with the gradient coupling on the regulating collective behaviors could be useful to understand the self-organization behaviors in neuronal networks.
Investigation of post-annealing enhancement effect of passivation quality of hydrogenated amorphous silicon
2015, 64 (19): 198801. doi: 10.7498/aps.64.198801
The excellent surface passivation scheme for suppression of surface recombination is a basic prerequisite to obtain high efficiency solar cells. Particularly, the HIT (heterojunction with intrinsic thin-layer) solar cell, which possesses an abrupt discontinuity of the crystal network at an interface between the crystalline silicon (c-Si) surface and the hydrogenated amorphous silicon (a-Si:H) thin film, usually causes a large density of defects in the bandgap due to a high density of dangling bonds, so it is very important for high energy conversion efficiency to obtain millisecond (ms) range of minority carrier lifetime (i. e. 2 ms). The a-Si:H, due to its excellent passivation properties obtained at low deposition temperatures and also mature processing, is still the best candidate materials for silicon HIT solar cell. Deposition of a transparent conductive oxide (TCO), such as indium tin oxide (ITO), has to be used to improve the carrier transport, since the lateral conductivity of a-Si:H is very poor. Usually, ITO is deposited by magnetron sputtering, but damage of a-Si:H layers by sputtering-induced ion bombardment inevitably occurs, thus triggering the serious degradation of the minority carrier lifetime, i. e., a loss in wafer passivation. Fortunately, this damage can be often recovered by some post-annealing. In this paper, however, the situation is different, and it is found that the minority carrier lifetime of ITO/a-Si:H/c-Si/a-Si:H heterojunction has been drastically enhanced by post-annealing after sputtering ITO on a- Si:H/c-Si/a-Si:H heterojunction (from 1.7 ms to 4.0 ms), not just recovering. It is very important to investigate how post-annealing enhances the lifetime and its physics nature. Combining the two experimental ways of HF treatment and vacuum annealing, three possible reasons for this enhancement effect (the field effect at the ITO/a-Si:H interface, the surface reaction-layer resulting from annealing in air, and the optimization of a-Si:H material itself) have been studied, suggesting this is irrelevant to the first two. The influence of post-annealing on a-Si:H/c-Si/a-Si:H heterojunction deposited at different temperatures has also been investigated. It is found that the remarkable enhancement effect of post-annealing is for low growth temperature(175 ℃) and not for high growth temperature(200 ℃), with the confirmation of an effective way for high quality passivation using growth at low temperature and then annealed at high temperature. Moreover, the configuration of a-Si:H at different growth temperatures between afore and after annealing has been discussed by an application of Fourier transform infrared (FTIR) spectroscopy. It is shown that the large increase of the lifetime of the heterojunction after annealing results from the improvement of microstructure of a-Si:H itself, which is essentially a competitive balance of the dominant role of some micro-factors, including hydrogen content, hydrogen bonding and network disorder in amorphous silicon film determined by the optimized matching between the growth temperature of a-Si:H materials and the annealing temperature of the heterojunction. An optimum control for this balance point is the essential cause of lifetime enhancement.
2015, 64 (19): 198901. doi: 10.7498/aps.64.198901
The degree correlation of nodes is known to considerably affect the network dynamics in systems with a complex network structure. Thus it is necessary to generate degree correlated networks for the study of network systems. The assortatively correlated networks can be generated effectively by rewiring connections in scale-free networks. However, disassortativity in scale-free networks due to rewiring has not been studied systematically.In this paper, we present the effectiveness of generating disassortative scale-free networks by rewiring the already formed structure of connections which are built using the evolving network model. In the rewiring, two randomly selected links are cut and the four ends are connected randomly by two new links. The rewiring will be reserved if the disassortativity changes to the direction we need, otherwise it will be aborted. However, if one or both of the new links already exist in the network or a node is connected to itself, the rewiring step is aborted and two new links are selected. Our result shows that the rewiring method can enhance the disassortativity of scale-free networks. However, it is notable that the disassortativity measured by the Pearson correlation coefficient cannot be tuned to-1 which is believed to be the complete disassortativity. We obtain that the minimum value of the Pearson correlation coefficient depends on the parameters of networks, and we study the effect of network parameters on the degree correlation of the rewired networks, including the network size, the connection density of the network, and the heterogeneity of node degrees in the network. The result suggests that the effect of rewiring process is poorer in networks with higher heterogeneity, large size and sparse density. Another measurement of degree correlation called Kendall-Gibbons' coefficient is also used here, which gives the value of degree correlation independent of the network size. We give the relation of Kendall-Gibbons' coefficient to network sizes in both original scale-free networks and rewired networks. Results show that there is no obvious variance in rewired networks when the network size changes. The Kendall-Gibbons' coefficient also shows that rewiring can effectively enhance the disassortativity of the scale-free network.We also study the effectiveness of rewiring by comparing it with two sets of data of real Internets. We use the evolving network model to generate networks which have the same parameters as the real Internet, including network sizes, connection density and degree distribution exponents. We obtain that the networks generated by rewiring procedure cannot reach the same degree correlation as the real networks. The degree distribution of real networks diverges from the model at the largest degree or the smallest degree, which provides a heuristic explanation for the special degree correlation of real networks. Therefore, the difference at the end of the distribution is not negligible.
2015, 64 (19): 198201. doi: 10.7498/aps.64.198201
The dynamics of spiral waves in the two-layer excitable media is studied by using the Br-Eiswirth model. The two media adopts the inhibitory and excitatory asymmetric couplings. Numerical results show that the excitatory asymmetric coupling can promote the frequency-locking of two spiral waves with different frequencies. The two spiral waves can achieve frequency-locking even if the frequency difference between them is large. The coupling causes the two spiral waves to have the strongest ability of frequency-locking; when the coupling between the two media is the inhibitory asymmetric coupling, the two spiral waves can achieve frequency-locking only when the frequency difference of the initial spiral waves is small. Furthermore, the range of frequency-locking is smaller than that of the general feedback coupling, and the frequency-locking ability of spiral waves reaches the minimum level. When the coupling strength and control parameters are chosen appropriately, the inhibitory and excitatory asymmetric coupling can keep the spiral wave unchanged in one medium and result in the transition from spiral wave to the resting state or target wave with low-frequency in the other. The coupling also induces the meandering of spiral waves or leads to the transition from two spiral waves to two target waves in the two-layer media. Finally the generated target waves either disappear or develop into the plane-wave-like oscillation patterns. Furthermore, the oscillation of the patterns is in antiphase. In addition, the locally intermittent frequency-locking of the two spiral waves is observed. These results can help understand the complicated phenomena occurring in the cardiac system.
2015, 64 (19): 198501. doi: 10.7498/aps.64.198501
As a wide bandgap semiconductor material, ZnO has huge potential in applications such as light emitting devices and sensors. Compared with GaN and SiC, ZnO has a bandgap of 3.37 eV and exciton binding energy of 60 meV at room temperature, indicating it is a promising candidate of UV detector. ZnO based metal-semiconductor-metal photoconductive ultraviolet detector has the advantages of high optical gain and strong responsivity. However, due to the photoconductive relaxation and surface effect of the ZnO material, a ZnO-based photoconductive UV detector has a slow response which is defective for practical application. The intrinsic defects typically generated during the synthesis of ZnO, e.g. oxygen vacancy, should be responsible for the slow response. Therefore, we have fabricated the high-resistive ZnO thin film based UV detector and studied its UV response characteristic. High resistance ZnO thin film is fabricated on glass by RF magnetron sputtering and followed by lift-off photolithography to form Al interdigital electrodes. SEM and XRD images show that the as-fabricated ZnO thin film grows with preferential orientation along c-axis. A linear I-V curve under UV illumination indicates the ohmic contact between Al and ZnO. From these results, we can calculate the resistivities to be 3.71×109 Ω · cm and 7.20×106 Ω · cm respectively when in the dark and under 365 nm UV light of 303 μW/cm2. The light-to-dark current ratio is up to 516 with bias of 40 V. Besides, the ZnO thin film detector shows a stable, rapid, repeatible and reproducible response with a rise time of 199 ms and a fall time of 217 ms when exposed to periodically switched UV light illumination at a bias voltage of 40 V. Moreover, the detector has a high selectivity for 365 nm UV light and the responsivity is 0.15 mA/W with the intensity of 303 μW/cm2. Furthermore, the transient response process is analyzed using the theory of surface recombination and bulk recombination of ZnO semiconductor. For a high resistance ZnO thin film based UV detector, the surface recombination process is weakened ascribed to the decrease of intrinsic defects and the bulk recombination process plays a leading role, resulting in the fast response. Results show that high resistivity ZnO thin film based UV detectors have outstanding UV photoresponse characteristics for potential applications in UV/radiation detection.
Transmission mode of a single layer graphene and its performance in the detection of the vibration spectrum of gas molecular
2015, 64 (19): 198102. doi: 10.7498/aps.64.198102
Since its successful preparation in 2004, graphene has attracted a great deal of attention, and the sensing application is an important research field. But nearly all the researches about graphene sensors focus on low frequency band, of which the mechanism is mainly dependent on the detection of charge carrier concentration and conductivity variation induced by the absorption of molecules. However, due to the fact that most of the molecules absorbed on the surface of graphene will induce the change of conductivity, this method is incapable of distinguishing different molecules. Transmission mode of a single molecular layer is studied based on Kubo formula and combined with a numerical method. The relation between transmission properties and effective mode index is analyzed, and the broadband localization capability of the waveguide mode is demonstrated. Meanwhile, the variation of the transmission intensity which is due to the interaction between the first order waveguide mode and the gas is adopted to retrieve the vibration spectrum of molecules. Taking the sensing of SO2, CO and C7H8 as examples, the effectiveness of this method is verified based on eigenmode analysis. Results show that the transmission spectrum is consistent with the variation spectrum of gas molecules; besides, in the transmission direction, the larger the interaction range, the greater the attenuation of mode transmission intensity will be. This study has provided a theoretical foundation for the realization of the detection and identification of gas moleculan fingerprints.
2015, 64 (19): 198104. doi: 10.7498/aps.64.198104
It is well known that diamond is one of the most ideal cutting tool for materials, but the rapid tool wear can make surface integrity of the machined surface decline sharply during the nanometric cutting process for a single crystal silicon. Thus, a research on the wear mechanism of the diamond tool is of tremendous importance for selecting measures to reduce tool wear so as to extend service life of the tool. In this paper, the molecular dynamics simulation is applied to investigating the wear of the diamond tool during nanometric cutting for the single crystal silicon. Tersoff potential is used to describe the CC and SiSi interactions, and also the Morse potential for the CSi interaction. The rake and flank faces are diamond (111) and (112) planes respectively. A new method, by the name of 6-ring, is proposed to describe the bond change of carbon atoms. This new method can extract, all the worn carbon atoms in diamond tool, whose accuracy is higher than the conventional coordination number method. Moreover, the graphitized carbon atoms in the diamond tool also can be extracted by the combination of these two methods. Results show that during the cutting process, the CC bond's breaking in the surface layer of the diamond tool leads to the transformation of hybrid structure of the carbon atoms at both ends of the broken bond, from sp3 to sp2. Following to the bond breaking, the bond angle between the surface carbon atoms increases to 119.3 whose hybrid structure has changed, and the length between nearest neighboring atoms quickly decreases to 0.144 nm, indicating that the space structure formed by these carbon atoms has changed from 3D net structure of diamond to plane structure of graphite. Hence, the carbon atoms in the tool surface whose space structure has changed due to bond breaking should be defined as worn carbon atoms, but not only the carbon atoms whose hybrid structure has changed. The structure defects at both edges of the diamond tool are much more serious, which make the energy of CC bonds at the edges weakened with the enhancement of defects. The bonds with lower energy are broken under the effect of high temperature and shear stress, which also produces the tool wear. The graphitization occurs at the tool of the cutting tool because the structure defects there are the most serious. The reconstruction of the carbon atoms with lower coordination number causes its neighboring region to produce serious distortion, which leads to easy breaking of CC bonds in this region.
2015, 64 (19): 198702. doi: 10.7498/aps.64.198702
Expression for the formation of the pixel value of fast neutron radiography has been derived. The contrast inequality for the photograph has been established using the derived expression; then the relationships of the image contrast with the source intensity, the exposure time, and the scattering have therefore been obtained through the acquired inequality. A simulation on the process of fast neutron radiography is carried out based on the pixel value analysis, and the spatial resolution and image contrast have also been considered. Simulation results show that the spatial resolution is better than that from experiments and the effect of image contrast is equivalent to that of the experiments. Finally, various samples, such as Pb samples, with slits, Fe samples with square holes and multiple materials-combined samples, are used to test the performance of the simulation. Results demonstrate that the simulations are in agreement with the experiments, thus providing a reference to the future experimental design and engineering application.
Effect of temperature on the (100) surface features of type Ib and type IIa large single crystal diamonds
2015, 64 (19): 198103. doi: 10.7498/aps.64.198103
In this paper, by choosing FeNiMnCo alloy as a catalyst and the (100) face of a seed crystal as the growth face, high quality type Ib and type IIa large diamond single crystals (diameter about 3-4 mm) can be successfully synthesized using temperature gradient method, at 5.6 GPa pressure and different temperatures between 1250-1340 ℃. To control the diamond crystal morphology, the growth temperature should be adjusted. Then the morphology of the synthesized large diamonds is plate-like at low temperatures, tower-like at medium temperatures, and spire tower-like at high temperatures. For the same crystal morphology, the synthetic temperature of type IIa diamond single crystals is about 30 ℃ higher than that of type Ib. The central and angularity regions of the top (100) surface, for the synthesized samples of type Ib and type IIa large diamond single crystals at different temperatures, are examined by laser Raman microscope respectively. It is found that the black lines of the type Ib and type IIa large diamond single crystals become dimmed and dense on the same top surface from center to the edge. It is indicated that the priority growth mechanism is in the angularity regions, compared with the central regions. Namely the solute of carbon is primarily precipitated in the angularity regions of the (100) surface. With increasing synthesis temperature, the black lines on the top surface (100) of type Ib diamond single crystals become gradually denser, and the characteristics of the lines are transformed from irregular distribution to typical dendritic distribution. The reason of the above results is that the rate of carbon deposition (the growth rate of diamond crystal), which is along the direction of the diamond crystal , will gradually rise as the synthesis temperature of the crystal is increased. The characteristics of the lines on the top surfaces (100) of type IIa large diamond single crystals, which are synthesized under different temperatures, are similar to that of type Ib. However, the lines on the top (100) surface of type IIa diamonds are not so obvious and denser than that of type Ib diamonds at different synthesis temperatures. Similar characteristics of lines on the top (100) surface of both types of diamond single crystals can be explained by the axis and radial growth rate variation at different temperatures. These different characteristics of the lines are due to the fact that the growth rate of type IIa diamonds is slower than that of type Ib diamonds, and the nitrogen concentrations in type IIa diamonds are lower than those of type Ib diamonds. Finally, the full width at half maximum (5.554 cm-1) of the tower-like type IIa diamond is narrower than that (5.842 cm-1) of tower-like type Ib diamond from the test of Raman spectra. It is shown that the quality of type IIa diamond single crystals is better than that of type Ib.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Nonlinear solitary Rossby waves with external heating source and effect topographic effect in stratified flows described by the inhomogeneous Schrdinger equation
2015, 64 (19): 199201. doi: 10.7498/aps.64.199201
Rossby waves are intrinsic in the large-scale systems of fluids, so they are the most important waves in the atmosphere and ocean. Theory and observation show that their basic characteristic is to satisfy the quasi-geostrophic and quasi-static equilibrium approximations. In stratified fluids, we discuss the long waves in a homogenous atmosphere and obtain the KdV equation, but the analysis is limited to the case that the velocity shear is small compared with a basic uniform zonal motion, and it gives no insight pertaining to the kinds of stream-line-flow patterns accompanying these waves. Here, the -plane approximation f= f0+ 0 y (0 is a constant) is extended into f= f0+ (y) y, which includes a nonlinear function (y) taking the place of in the -plane approximation. Such an approximation can depict more precisely the motion of the atmosphere and ocean, especially in the middle and high latitude regions. It generalizes the theory developed by Helfrich and Pedlosky for time-dependent coherent structures in a marginally stable zonal flow by including forcing. Such forcing could be due to topography or external source. We take the basic flow to be a shear and the Visl-Brunt frequency N a function of variable z. For the stratified fluids, based on the lower boundary with external heating source and topography, as well as the quasi-geostrophic potential vorticity equation with external heating source, an inhomogeneous nonlinear Schrdinger equation (including topographic forcing and an external heating source) is derived by using the perturbation method and stretching transforms of time and space. It is found that the external heating source, effect and topography effect are the important factors that could induce the nonlinear solitary Rossby by inspection of the evolution of the amplitude of Rossby waves. On the assumption that nonlinear topographic effects and the dissipation of external heating source are balanced, an inhomogenous equation in which the coefficients depend on (y), u(y,z) and N(z) is derived. Results show that the topography, external heating source and Rossby waves will interact with a basic stream function that has a shear. In stratified fluids, the inhomogeneous nonlinear Schrdinger equation is obtained for describing the evolution of the amplitude of solitary Rossby envelop solitary waves as the change of Rossby parameter (y) with latitude y, topographic forcing and the external heating source.
2015, 64 (19): 199401. doi: 10.7498/aps.64.199401
Since the concept of hypersonic flight was proposed, progress of the related theory, experiments and simulations has been gained. As an important component of the scramjet engine, the isolator plays a key role in the engine performance and flight success. The flow mechanism it involves is very complicated. In the view point of experimental research, this paper reviews the recent progress of scramjet isolator studies, analyzes the features of the isolator flow based on fine flow diagnosis technique (nano-tracer planar laser scattering, NPLS), including the three-dimensional structures of the shock train flow field, turbulent characteristics, hysteresis motions, unstart flow and shock train leading edge detection. Studies of the isolator flow can be classified and discussed according to the wind tunnel facility, isolator design and measurement techniques. Based on this, suggestions for further research can be proposed.