Research on indoor channel measurement and simulation at 340 GHz
Ultra-broadband terahertz communication systems are expected to help satisfy the ever-growing need for unoccupied bandwidth. Due to high attenuation of terahertz wave, it can be widely used in indoor WLAN data communication. Future THz WLANs will rely on not only the line-of-sight (LOS) but also the nonline-of-sight (NLOS) channels to perform data communication. Hence, both kinds of channels have to be characterized. In this paper, we present the measures of ultra-broadband channel at 340 GHz for an indoor scenario. The measured channel transfer function is compared with a ray tracing simulation performed with the indoor scenario. Additionally, we show the reflection losses of some building and plastic materials which could be required as input data for the ray tracing algorithm.
Design and study of a multi-funtional electromagnetic device with functions of field rotating and concentrating
A novel multi-functional electromagnetic (EM) device named rotary-concentrator is designed based on transformation optics theory. For its ability to manipulate the EM wave in a special manner, it can rotate the propagation direction of the EM field in the core region to a fixed angle, as well as concentrate the EM energy into the core region simultaneously. For the proposed three equivalent configurations of the rotary-concentrator, the corresponding constitutive parameter expressions are derived respectively, and the full-wave simulations using the finite element software are also carried out. The simulated results validate the derived constitutive parameter expressions. For the three different kinds of configurations, the first two kinds consist of three layers of media, and the last one is simplified to a two-layer configuration. For a given arbitrary rotating angle and an energy concentration ratio, the three configurations can perform propagation direction rotating and energy concentrating in equivalent effect. These results contribute to further understanding of the mechanism of rotator and concentrator, and provide a fuller theoretical basis for the design of multi-functional devices. The proposed rotary-concentrator has potential applications in the design of high efficient receiving antennas and special circuit package interconnecting devices.
Resonance experiment on a microwave resonator system
A microwave resonator system is made, which has a tapered resonant cavity, a microwave source, and a transmission device. Because of the electromagnetic pressure gradient on the tapered resonant cavity, a net electromagnetic force along the axis of the cavity may be observed, which is needed to verify experimentally the use of the independent microwave resonator system. It is also needed to keep the independent microwave resonator system in resonating state, which is the important procedure to demonstrate the possibility of net electromagnetic force. Thus, a low-signal resonating experiment on the tapered resonant cavity combined with resonating parts is completed to accurately find out the resonant frequency of 2.45 GHz and to analyze the influence of temperature on the resonant state. Experimental result shows that the resonant frequency and quality factor of the independent microwave resonator system are 2.44895 GHz and 117495.08 respectively. When the temperature of the tapered resonant cavity wall rises, the resonant frequency will be decreased and the quality factor changed separately.
Electric and magnetic polarization singularities of focused Gaussian vortex beams in the focal region
Taking the Gaussian vortex beam as an example, the variation of electric and magnetic polarization singularities of focused Gaussian vortex beams is studied. It is shown that in the focal region there exist two- and three-dimensional (2D and 3D) electric and magnetic polarization singularities of focused Gaussian vortex beams, which do not coincide in general. By suitably varying the distance from the focal plane and the truncation parameter, the motion, creation, and annihilation of circular polarization singularities will take place. The critical valuse of the parameters at which a pair of circular polarization singularities of 2D and 3D electric and magnetic fields will annihilate, are not the same. In the 2D electric field the V-point may appear at the focal plane.
Investigation of the scattering characteristics from discrete random scatterers based on recursive aggregate T-matrix algorithm
In this paper, we derive in vector form the recursive aggregate T-matrix algorithm based on the principles of electromagnetic wave multipole expansion of vector spherical wave functions and the vector addition theorem. After that we establish a three-dimensional electromagnetic scattering model for multiple spherical scatterers by simulating the scattering of subsurface discrete random scatterers using the derived algorithm. Calculating the scattering from different sizes, randomly distributed spherical scatteres and analyzing the high-order scattering effects, we can conclude that the vector recursive aggregate T-matrix algorithm has a high computation accuracy, and contains the interaction effects among multiple scatterers, therefore we can calculate the total scattering effects accurately from multiple scatterers. The established model can be served as a powerful tool in applications for retrieving the impact caused by the scattering of subsurface discrete random scatterers in soil moisture from radar measurements.
Conversion of cylindrical vector beams on the higher-order Poincaré sphere
A conversion approach for cylindrical vector beams on the high-order Poincaré sphere based on half-wave plate is studied theoretically and experimentally in this paper. The theoretical analysis using the Jones matrix method shows that the latitudes of two corresponding points produced by the cylindrical vector beams on the high-order Poincaré sphere before and after conversion are situated oppositely, and the longitudes are changed with different azimuth angles of the half-wave plate. Finally, an experimental system for the generation and conversion of the cylindrical vector beams is established with the spatial light modulator, and the experimental results demonstrate the feasibility of this approach.
A method of vertical and horizontal plus cubic spline interpolation for Mie scattering lidar profile data
Since the traditional spatial data interpolation method has its limits in describing the RHI (range-height indicator) graph for Mie scattering lidar scanning profile, it cannot be applied to the visualization of Mie scattering lidar space-time scan data, and it is also difficult to achieve the continuity in atmospheric parameters evolution process. This paper analyzes the space-position relations in lidar scanning profile data based on the scanning elevation and spatial distribution characteristics of the atmospheric parameters, chooses the reference data rationally of vertical and horizontal positions, and gives the position relevant weight. The smoothness of data is revised by a cubic spline function and the missing data are replenished by interpolation finally. Results show that this method can improve effectively the accuracy of atmospheric lidar scanning data space interpolation, produce few errors; and the RHI graph becomes smooth, thus it conforms to the overall change rule of atmospheric parameters.
Research on speckle denoising by lensless Fourier transform holographic imaging with angular diversity
The signal-to-noise ratio and resolution of the reconstructed image is seriously influenced by speckle noise in digital holography, so it is essential to reduce the speckle noise and improve the image quality. Intensity correlation between two speckle patterns with the rectangle speckle spot is analyzed and deduced, and the minimal angular difference of illumination beams is given quantitatively under a special situation. A lensless Fourier transform holographic imaging system with angular diversity is designed based on the lens property, in which the direction of the wave can be changed by shifting the fiber instead of conventionally rotating the mirror, and the formor can change the direction of illumination with a fixed illumination size and location. Eighty one holograms with uncorrelated speckle patterns are recorded at different illumination angles by shifting the fiber. Then a digital reconstruction is achieved only by a fast Fourier transform, and the multiple reconstructed images are averaged. Experimental results show that the speckle contrast of the averaged reconstructed image can be reduced to 14.6% that from a single reconstructed image, and the signal-to-noise ratio is improved 6.9 times. This proposed digital holographic imaging method can suppress the speckle noise greatly, has a simple setup, and is easy to operate.
Research on spectral peaks thermal-drifting in multi-wavelength infrared laser diode
To meet the requirements, requested in infrared-laser testing techniques, in spectral range and peaks precision of multi-spectral source, we present a method for preparing multi-wavelength infrared laser diode with a high precision, and design a packaging structure which can integrate four kinds of laser chips with wavelengths 860 nm, 905 nm and 1064 nm (pulse/single). The 3D heat conduction differential equations of central-substrate are given based on the above packaging structure. According to the solutions of numerical temperature field distribution, which are solved through a mathematic-modeling tool, the processing technique of central-substrate is optimized. And the prototype of multi-wavelength laser diode is prepared, and the experimental apparatus is built which can be used to observe the phenomenon of spectral peak thermal-drifting. Experimental results show that only two spectral peaks are drifting slightly 1–3 nm. The drifting amount is within the half width range of their spectral peak. This phenomenon proves that the output spectra of multi-wavelength laser diode have a high precision and a well thermal stability.
Analysis of the tunable filtering properties of a photonic crystal with symmetric dual defects
A dual channel tunable filter structure has been proposed based on the local characteristic of photonic crystals and the mesoscopic calender effect of photonic crystals. The optical transmission characteristics of the filter have been derived theoretically using the transfer matrix method, and the relationship between the transmission spectrum and structural parameters of photonic crystals has been established. Problems how the mesoscopic calender effect influences the transmission spectrum of photonic crystals with dual defects have been discussed and the structure of photonic crystals has been numerically simulated. The emulational results show that defect modes have a blue shift with the increase of the incident angle, while they have a red shift with the increase of dielectric layers' refractive index or geometrical thickness. When in photonic crystals occurs axial stretched strain, the locations of defect modes will move towards long wavelength side, but the values of defect peaks keep constant generally. Thereby, the filter's tunable property has been verified. This photonic crystal filter with good tunability has a compact structure, which may provide a certain theoretical reference for the design of photonic crystal lasers and sensors.
Laser diode end-pumped continuous-wave Nd：YVO4 self-Raman laser at 1175 nm
In this paper, an LD (laser diode) end-pumped continuous-wave Nd:YVO4 self-Raman laser at 1175 nm is reported. The doping concentration and structure of the self-Raman crystals are optimized to reduce the thermal effects of the crystal, and a high-efficient diode-end-pumped continuous-wave self-Raman laser operated at 1175 nm is demonstrated. Finally, the thermal effects are efficiently improved by using a double-end diffusion-bonded composite Nd:YVO4 crystal as a gain medium. An output power up to 3.4 W of the first-order Stokes line 1175 nm is achieved at the incident diode pump power of 25.5 W, corresponding to a diode-to-Stokes optical conversion efficiency of 13.3% and a slope efficiency of 14.6%. The Raman threshold is as low as 2.21 W of diode power at 808 nm.
Doubled temperature measurement range for a single micro-ring sensor
A novel SOI (silicon on insulator) temperature sensor based on a U-shaped waveguide-coupled single micro-ring is proposed in this paper. Refractive index and length of the temperature-sensing part will change as the temperature changes, leading to the shift of the output spectrum of the sensor. The transfer function of the U-shaped waveguide coupled with a single micro-ring is obtained based on the theory of coupling modes and transfer matrix method. And we have studied the output spectrum properties of the system from different temperature-sensing parts. Results show that the whole structure of U-shaped waveguide coupled with the single micro-ring proves to be the best temperature-sensing part as the output spectrum with no spurious modes and a deep extinction ratio of 31 dB. Compared with the traditional two waveguides coupled with a single micro-ring resonator, when the distance between the two coupling points of the U-shaped waveguide is an integer multiple of circumference of the micro-ring, the free spectrum range (FSR) can be expanded to 56 nm, the sensitivity can achieve 89.2 pm/℃, and the measurement range be 298–720 K, achieving a high temperature measurement of SOI micro-ring resonator.
Experimental demonstration on triangular-shaped pulse train generation based on harmonic fitting
We demonstrate an approach for triangular-shaped pulse train generation experimentally based on harmonic fitting. The operation principle is that a Mach-Zehnder modulator is firstly employed to suppress modulation of the optical carrier. Thus a periodically variable lightwave can be obtained at the output. Then the signal is coupled into a section of dispersive fiber. Due to the dispersion-induced power fading, the undesired 4th order harmonics in the optical intensity can be fully removed. By adjusting modulation index to an optimum value (m=2.305), the generated harmonics of the optical intensity can be made corresponding to the Fourier components of typical periodic triangular pulses. Finally, the triangular-shaped pulse train at a repetition rate two times of the driving frequency can be obtained. In the experiments, 19.724 Gb/s and 15.356 Gb/s triangular-shaped pulse trains are generated by using 9.862 GHz and 7.678 GHz driving signals respectively. Besides, the repetition rate can be switched to another value by using a different fiber dispersion (β 2L). It is found that the experimental data agree well with theoretical results.
Synthesis and upconversion luminescent properties of BaMgF4：Er3+, Yb3+ nanocrystals
BaMgF4:Er3+, Yb3+ nanocrystals in rod-shape are synthesized by means of the reverse co-precipitation. They emit green and red light under excitation of near-infrared light (980 nm). The green and red emissions may be attributed to the 2H11/2–4I15/2, 4S3/2–4I15/2 and 4F9/2–4I15/2 transitions of Er3+. Dopant ions Yb3+ as sensibilizers can improve the upconversion transformation efficiency. The emission intensity is the strongest when the contents of Er3+ and Yb3+ are 3% and 10%, respectively. With increasing concentration of Yb3+, the red emission intensity increases while the green emission reduces. And the conversion fitting curve between the luminous intensity and pump current indicates that the upconversion process of the green and red light of BaMgF4: Er3+, Yb3+ is due to two-photon absorption.
Research of the characteristics of photonic crystals based on air holes sub-wavelength imaging
Negative refractivity has been extensively studied, especially in the perfect imaging photonic crystal slab, for its fantastic characteristics. The photonic crystals sub-wavelength imaging is investigated by finite-difference time-domain (FDTD) simulation and numerical analysis. Impact of wavelength and temperature drift on the sub-wavelength imaging of photonic crystal with negative refraction has been studied in this paper.
Study on unidirectional transmission of photonic crystal diodes based on heterostructure interface optimization
In recent years, all-optical diodes based on photonic crystal heterostructures have attracted much attention, and their good characteristics of one-way transmission are the long pursued target. In this paper, different optimized designs are proposed by modifying the photonic crystal structure at the heterostructure interface. And the all-optical diodes with high contrast, which have both efficient unidirectional transmission and beam splitting, are realized in wide bandwidths.
Widely tunable frequency-doubling microwaves generated using Brillouin-assisted carrier phase shift
An optically tunable frequency-doubling microwave generation technique based on stimulated Brillouin scattering (SBS) in optical fibers is proposed and experimentally demonstrated. Due to the strong dispersion characteristics in SBS, when a πup/2 phase shift is imposed on the optical carrier of an amplitude-modulated signal by SBS, only a frequency-doubling microwave signal from the beating between the optical carrier and the ± 1st sidebands is generated. Due to the inherent narrowband character of SBS and the phase shift being only imported on to the optical carrier while the sidebands are kept unchanged, the frequency-doubling with large frequency tunability is realized, the operational bandwidth is just limited by other optical device deployed. In addition, all the required optical signals and pumps can be generated from the same laser source, the influence from the wavelength drifting is eliminated, so the stability of the system is established.
Dynamical mechanical property of viscoelastic materials and its effect on acoustic absorption of coating
Viscoelastic macromolecular materials are widely used in underwater acoustic fields because of their favorable acoustic performance. Dynamical mechanical properties of viscoelastic materials, such as Young's modulus, shear modulus, and relevant loss factors, are important in the forecast of acoustic properties, which connect acoustical design of underwater coating with materials prescription design. Dynamic Young's modulus of some macromolecular materials is measured with dynamic mechanical apparatus (DMA), and then the basic acoustic parameters are expanded from a narrow band to a broad band by using principles of time-temperature superposition (TTS). When applying the basic parameters to calculating characteristics of the uniform layer sample by using the finite element method, a reasonable agreement of sound absorption coefficients is obtained between the calculations and measurements in the acoustic pipe. Furthermore, the underwater acoustic absorption properties of the coating with a local resonant structure are discussed. Finally, some suggestions are given about how to improve the underwater absorption performance at low frequencies.
Ultrasonic imaging for appearance of vertical slot by reverse time migration
The problem in traditional industrial ultrasonic imaging methods is the difficulty to obtain the appearance or the shape of defects inside solid materials although the methods have the ability to detect and determine the position and lateral dimensions. These special defects, like the vertical slot or crack, are typical examples. Based on the multi-element array ultrasonic technique, the numerical and experimental studies on reverse time migration (RTM) ultrasonic imaging for metalic materials are carried out. In this paper, the objects in detecting and imaging experiments are aluminum samples with slot intersecting the bottom or interior slot, which cannot be effectively detected by traditional ultrasonic methods. First the single component ultrasonic field for RTM imaging is studied to obtain RTM ultrasonic imaging results based on numerical simulation and experimental measurements using a multi-element array ultrasonic testing system. Then, the RTM imaging techniques based on multi-component ultrasonic displacement field detection and converted shear wave separation are further studied, and a new approach using a multi-component laser interferometer is proposed. Numerical simulation results verify that the multi-component RTM imaging reconstructive method can overcome the shortcomings of single component method, and obtain better image quality.
A new robust adaptive beamforming and the one-dimensional search strategy
Adaptive beamforming methods will be degraded sharply in the presence of steering vector errors. The design methods of robust adaptive beamforming become more flexible when the convex optimization technique is used. However, this leads to high computational-complexity and more difficulties for engineering applications. To solve these problems, a robust adaptive beamforming based on the least square estimation is proposed, and a laconic solution method using one-dimensional search is derived. The standard Capon beamformer (SCB) is converted to a robust least-square problem based on the principle of generalized sidelobe canceller, and is then changed into a problem of second-order program. In order to reduce the amount of computation, a one-dimensional search method is deduced using the relationship between the primal and dual problems of second-order program, and Newton iteration method is adopted to obtain the optimal solution. The computational complexity of the proposed algorithm is in the same order of magnitude as that of the SCB. Simulation results demonstrate the robustness of the proposed algorithm in the case of steering vector mismatch and snapshot deficiency.
Bezier interpolation method for the dynamics of rotating flexible cantilever beam
The Bezier interpolation is introduced as a new discretization method for the rotating flexible beam deformation. First, the model of the rotating flexible beam is built. Then, the rigid-flexible coupling dynamic equations are established via employing the second kind of Lagrange's equation. The longitudinal deformation and the transverse deformation of the flexible beam are considered, and the coupling term of the deformation which is caused by the transverse deformation is included in the total longitudinal deformation; and a software package for the dynamics simulation of the flexible beam is developed. Finally, the simulation results of the Bezier interpolation are compared with those of the assumed method and the finite element method. Simulation results demonstrate that the computational efficiency of the Bezier interpolation is the highest, the computational accuracy of Bezier interpolation is in accordance with the needs of engineering, and is higher than the one of assumed mode method in high-speed case. The Bezier interpolation method is better than the assumed mode method in dealing with the large deformation dynamics problem. So the Bezier interpolation method will be hopeful to win popularity in the field of multi-body system dynamics.
Simulation study on the propagation of solitary waves in a one-dimensional composite granular chain
The propagation of solitary wave in a one-dimensional composite granular chain with heavy and light particles by turns is investigated by using molecular dynamics simulation. Under the condition of larger or smaller mass ratio of light to heavy particles, scattering effect is weaker and both particle velocity and solitary wave velocity decay slowly. In the intermediate range of mass ratio, the scattering effect becomes stronger, resulting in a faster decay of particle velocity and solitary wave velocity. Moreover, effect of increasing velocity happens when teh solitary wave travels across the heavy-light interface, indicating that the solitary wave velocity is increased. Effect of increasing velocity is enhanced when the mass ratio of light to heavy particles decreases. Due to the combined action of scattering effect and the effect of increasing velocity, the traveling time of solitary waves can be modulated by altering the mass ratio of light to heavy particle.
Quasi-static finite element calculation of interaction between graphene and nanoprobe
Probes of nano scale are a type of important tools for the study on nano-film material. Dynamic explicit method accompanied by the intermittent feeding of probe to dissipate the energy is applied to avoid the difficulty of convergence in the finite element model for a system of probe, graphene, and substrate. And the results of a static state are obtained from this strategy. The functions of interface interaction forces are deduced from adhesion energy and the potential between atoms. The force functions are implanted into subroutines in Abaqus code to simulate the interactions among graphene layers, probe, and substrate. Results of simulations show good consistency with the data of experiments.
Analysis of the tunable liquid gradient index based on optofluidics
A new type of tunable gradient-index (GRIN) microfluidic lens is proposed, which is based on the convection-diffusion process. By using the finite element method, the spatial distribution of the refractive-index (RI) determined by the concentration distribution in the microchannel is analyzed. Results show that the distribution of RI in the microchannel can be tuned by different flow rates of the core and cladding liquid streams. Furthermore, although taking into consideration the existence of the viscous resistance between the channel wall and the flowing stream, the RI distribution is still dependent on the combination of flow rates of the core and cladding streams. This microfluidic lens can be applied to dynamic adjustment of focusing, splitting or bending light beams. It therefore may have extensive applications in the optofluidic optical detecting system and microscale imaging system.
Stability of heated liquid film on an uneven substrate
This paper studies mainly the evolution and linear stability of the nonlinear surface waves of a two-dimensional viscous liquid film along an uneven inclined non-uniformly heated wall. A long wave perturbation method is used to derive zero- and first-order evolutions equations of the nonlinear surface wave flowing on an uneven substrate. Based on the obtained evolution equations, the diagram of evolution progress for film surface wave on a sinusoidal corrugated substrate is drawn, the linear stability analysis is also studied, and the effect of various parameters on the flow stability of liquid membrane is analysed. Theoretical results demonstrate that the free surface of the film shows sine wave and has the same frequency as the substrate, and the film thickness will decrease gradually along the flow direction. Marangoni number gives a stabilizing effect, the stable zone increases with the increase of Marangoni number. While, Peclet number and the angle theta are unstable factors, the stable region decreases with the increase of them. Besides, the trends that Marangoni number, Peclet number and angle theta may impact on the stability of the film whice are consistent with one another, so a liquid film is easy to destabilize at the trough of the substrate.
Corner vortex characteristics at the reversal of large scale circulation in turbulent Rayleigh-Bénard convection
The two-dimensional Rayleigh-Bénard convection is calculated by DNS method. A large scale circulation and two corner vortices with reverse rotation are presented in soft turbulent convection, and the reversal phenomenon of the large scale circulation appears. Continuous temperature contour and the streamline chart describe the whole process of the reversal clearly. In the process of the reversal, the changes of the corner vortex size play an important role. Analysis of the corner vortex size change shows that the changes appear violent oscillation in the thermal flow field with the reversal, but only slight pulsation is found without the reversal. Corner vortex size, velocity at the typical position, and the temperature near the angle location in the process of reversal are discussed. The reversal is found to be done in an instant, and the velocity pulse is smaller and the temperature is higher in the corner vortex. The corner vortex size and the vertical velocity on the side wall vary synchronistically before the reversal.
Study on thermal characteristics of phonons in graphene
Phonons are the main energy carriers for heat conduction in graphene. One of the most important and basic thermal properties is the relaxation time. In this paper, phonon relaxation times are investigated by a normal mode decomposition method to reveal the distinctions of the different phonon modes. The method is based on equilibrium molecular dynamics simulation. In the simulations, the heat current autocorrelation functions are obtained for each single phonon, and the relaxation times are extracted by fitting the functions. In addition, the relations among relaxation time, wave vector, frequency, and temperature are examined. It is found that the variation tendency of the relaxation time with wave vector is close to that of the dispersion with wave vector. For frequency and temperature, they are in agreement with the theoretical model: 1/τ=νnTm. It is shown that“n” is 1.56 for acoustic phonons, while for optical phonons, it varies slightly with frequencies; and “m” is slightly different for each mode. Finally, the contributions of different phonon modes to thermal conductivity are investigated. It is found that low frequency phonons dominate the heat conduction process because of the relatively high relaxation time and density of states.
Deformation modeling of InSb IRFPAs under liquid nitrogen shock
The deformation appearing in InSb infrared focal plane arrays (IRFPAs) as subjected to liquid nitrogen shock tests, is an important criterion to assess the reliability of the structure designed and to predict the number of thermal cycling after which no cracks appear in InSb IRFPAs. After analyzing both the deformation distribution and the deformation running directions appearing in InSb IRFPAs at 77 K, we assume that the thermal strain accumulated in the liquid nitrogen shock test is completely relaxed. Based on this assumption and according to the temperature rising curve, we may obtain the deformation distribution in InSb IRFPAs at room temperature, which is identical in the deformation charactristics to the photograph of InSb IRFPAs taken at room temperature. After comparing the simulated liquid nitrogen shock tests (which InSb IRFPAs experience), with its fabrication process, we can infer that the square checkerboard buckling pattern appearing in the top surface of InSb IRFPAs originates from the residual stress and strain generated in the process of insufficient cures. And the deformation amplitude decreases with decreasing temperature of InSb IRFPAs in the nitrogen liquid shock tests. At 77 K, the deformation amplitude reduces to zero. This state corresponds to our assumption, that the accumulated stress and strain disappears. When the temperature of InSb IRFPAs increases from 77 K to room temperature, the square checkerboard buckling pattern will reappear in the top surface of InSb IRFPAs. These findings are beneficial to the optimization of the structure of InSb IRFPAs and to the improvement of the number of thermal cycling experienced by InSb IRFPA without cracks generated from liquid nitrogen shock tests.
A nonlinear plate theory for the monolayer graphene
In the present paper, the kinematic equation of a monolayer graphene is proposed based on a plate theory, and the nonlinear elasticity stress-strain relations are obtained from experiments. The equation includes cubic and quintic nonlinearities. The bending produced when subjected to a concentrated force at the center of the plate and the static buckling arising from edge in-plane axial uniform loads are investigated using Ritz methods for a simply-supported rectangular plate. Results suggest that the plate theory with nonlinear constitutive equation may characterize the mechanical property of a monolayer graphene appropriately, and the quintic nonlinearities have a significant effect on the bending deformations of the graphene.
Investigation on the bonding behavior of the interface within the supersonic plasma sprayed coating system based on the fractal theory
In order to investigate the relationship between the interfacial morphology of the coating system and its adhesion strength, the supersonic plasma spraying equipment is employed to fabricate Fe-based alloy coating. The (Ni, Al) coating is prepared as the undercoating with different flow of Ar gas, aiming at obtaining various rough morphologies. Interfacial morphologies of the coating system are quantificationally characterized by fractal dimension(FD). The pull-off method is used to test the adhesion strength. Result shows that the adhesion strength is obviously improved by fabricating an undercoating, and the FD of the interfacial morphology decreases with the increase of the flow rate of Ar gas, while the adhesion strength will be raised at the first beginning and then decreased to a certain value.
Study on the annealing growth of Ge dots at high deposition rate by using magnetron sputtering technique
The 14 nm thick Ge thin films are firstly deposited on Si substrate at 350 ℃ by using the magnetron sputtering technique, then the Ge/Si dots are successfully fabricated by annealing those Ge films. According to the morphology and phonon vibration information obtained by AFM and Raman spectroscopy, the formation and evolution mechanism are studied in detail. Experimental results indicate that the amorphous Ge films have been converted to Ge dots with a density of 8.5×109 cm-2 after 675 ℃ annealing for 30 min. By using Ostwald ripening theory, surface diffusion model, and calculation of the activation energy, the surface transfer and the dot formation behavior of Ge atoms can be well interpreted. Based on the fabrication technique of Ge/Si nanodots at a high deposition rate combined with the thermal annealing, we have provided a theoretical support for the experiment on self-assembled growth of Ge quantum dots.
Investigation on the electrical properties of anatase and rutile Nb-doped TiO2 by GGA(+U)
Crystal structure, electronic properties, and stability of anatase and rutile Nb-doped TiO2 (Nb:TiO2) compounds with different doping concentrations are studied by the combination of GGA and GGA+U methods within the density functional theory based first-principle calculation. And the main research work and contents are listed as follows: The anatase Nb:TiO2 appears as a degenerated semiconductor which behaves as an intrinsic metal. Its metallic property arises from Nb substitution into the Ti site, providing electrons to the conduction band. In contrast, the rutile Nb:TiO2 shows insulating behaviors. Ionization efficiency of Nb in anatase Nb:TiO2 is higher than that in rutile. We expect that anatase Nb:TiO2 is a potential material for transparent conducting oxide (TCO) while rutile Nb:TiO2 is not. The doped systems show different electronic characteristics, such as band structure, Fermi energy, and effective mass of carriers at different doping levels. In higher dopant concentration nNb, the ionization efficiency decreases slightly. Calculated defect-formation energy shows that Ti-rich material growth conditions are not in favor of the introduction of Nb while Nb can be easily doped in Nb:TiO2 under O-rich growth conditions. Nb dopant is difficult to be doped at higher doping level for both anatase and rutile Nb:TiO2.
Research of the synergistic effects in Cu/N co-doped TiO2 surface：A DFT calculation
First principles density functional theory calculations are carried out to investigate the interactions between implanted copper and nitrogen atoms at the anatase TiO2 (001) surface. The doped configurations and formation energies of Cu on TiO2 (001) and TiO2 (101) surfaces, N on TiO2 (001) and Cu/TiO2 (001) surfaces have been considered, and the perfected structures are obtained. Compared with the S/TiO2 (001) perfected structure, the analyses of the band structure and density of states of Cu/N-TiO2 (001) show that the band gap is decreased obviously when the CuO2 state occurrs; this could improve the photocatalytic activity significantly.
Phase separation of bilayered perovskite manganite (La1-xGdx)4/3Sr5/3Mn2 O7 (x=0, 0.05)
La1-xGdx)4/3Sr5/3Mn2 O7 (x=0, 0.05) polycrystalline samples have been prepared by solid state reaction method, and the phase separation phenomena in this samples are investigated by measuring the magnetization-temperature (M-T) curve, electron spin resonance (ESR) curve and resistivity-temperature (ρ-T) curve. For both samples, experimental results suggest there exists competition between ferromagnetic and antiferromagnetic interactions in low temperature range, which reflects a characteristic of cluster spin glass. A Griffiths-like phase is observed in temperature ranges 125–375 K and 100–375 K for x=0 sample and x=0.05 sample, respectively. It is found that doping contributes to the decrease of three-dimensional long-range ferromagnetic ordering temperature (from Tc03D ≈ 125 K for x=0 to Tc13D ≈ 100 K for x=0.05), but has no obvious effect on the Griffiths-like temperature (TG ≈ 375 K). Above TG ≈ 375 K, a pure paramagnetic phase appears in both samples. The ρ-T curves reveal two insulator-metal transitions in the entire temperature range for x=0 sample, which is caused by coexistence of the two phases in perovskite manganese oxides. For x=0.05 sample, however, there exhibits a single insulator-metal transition, indicating that doping can hinder the coexistence phenomenon. It can be seen from the fitted ρ-T curves that the electron conduction mechanism in high temperature range is in accordance with the three-dimensional variable range of hopping conduction.
Magnetic properties of the Cu-doped ZnO：experiments and theory
Cux Zn1-xO were synthesized via the solid-state reaction route. Ferromagnetism was detected when the Cu percentage was bigger than 3%. The compounds were found to be the N-type semiconductors with a carrier concentration of 1015 cm-3. The DFT+U method was employed to calculate the magnetic exchange coupling of the Cu2+–O2-–Cu2+, Cu2+–Vo–Cu2+, Cu2+–Vo+–Cu2+, Cu2+–Vo++–Cu2+ in the CuZnO system, where Vo denoted the vacancy of oxygen. Different bound charge transfer schemes between the Vo and Cu2+ ions were revealed. The origin of the ferromagnetism was determined within the framework of the Cu2+–Vo++–Cu2+ bound magnetic polarons.
Microstructure and photoluminescence of ZnO：Cd nanorods synthesized by hydrothermal method
High-quality ZnO and Cd-doped ZnO nanorods with different Cd-doping concentrations are synthesized by using the hydrothermal method. Microstructures and photoluminescence of the samples are systematically investigated by SEM, X-ray diffraction (XRD), Raman scattering spectrum and photoluminescence (PL) spectrum. Results of XRD analysis indicate that ZnO and ZnO:Cd crystallites exhibit a hexagonal wurtzite structure. SEM shows that the nanorods become smaller due to Cd doping. There is an internal tension which induces the decrease of optical band gap in Cd-doped nanorods. Cd-doping increases the intensity of violet emission peak near 2.90 eV and the blue emission peak located at 2.67 eV appears when the doping concentration is up to 2%. This study can be used for developing blue-violet-emitting devices.
Near ultraviolet luminescence characteristics of ZnO nanoparticle film
In this paper, ZnO nanoparticle film is synthesized by using a sol-gel method. Then ITO/ZnO nanoparticles/MEH-PPV/LiF/Al heterostructure devices are fabricated. Next, the emission spectra and electrical properties of the devices are measured for different thickness of the ZnO nanoparticle films. Under DC bias, ultraviolet (UV) electroluminescence (EL) from ZnO band edge emission is observed. When the voltage is higher than 12 V, the UV electroluminescence at 390 nm from ZnO band edge emission can be observed clearly. The EL mechanisms are discussed in terms of carrier tunneling process.
Influence of polarization voltage on piezoelectric performance of polypropylene piezoelectret films
Piezoelectrets are a kind of space-charged electret material with a void-structure. The piezoelectric effect in such a material is related with its microstructure and space charge. In this paper the micro-structure of polypropylene (PP) films is first modified by using a pressed gas expansion process to enhance the charging capability of the films, and then a direct contact charging is carried out to polarize the expanded films. The relationship between the applied voltage and the space charge density, and the influence of the applied voltage on the piezoelectric performance of PP films are investigated. Results show that for 100 μm thick modified PP films, the critical voltage necessary for the build-up of the “macro-dipoles” in the inner voids is approximately 2 kV; once the “macro-dipoles” are built up, the PP films will exhibit piezoelectric effect. With increasing polarization voltage, the space charge density gradually increases, resulting in significant enhancement of piezoelectric effect. For the PP films polarized at a peak voltage of 8 kV, the space charge density, d33 coefficient, and the figure of merit FOMv(d33· g33) are 0.56 mC/m2, 379 pC/N and 8.6 GPa-1, respectively. Since not only the FOMv of the PP films is almost two orders of magnitude larger than that of PVDF, but also the acoustic impedance in such a material is very low (～ 0.025 MRayl), the PP films have an obvious advantage as applied in airborne ultrasonic transmit-receive or pulse-echo systems.
Investigations of high-quality aluminum film with large-area uniformity for large-size echelle grating
Large-size echelle grating can have extremely high spectral resolution due to its large aperture and high diffractive order. To achieve high-performance large-size echelle grating, the preparation of high-quality aluminum film with large-area uniformity is one of the most important factors. In this paper, for the first time so far as we know, we report the preparation process of high-quality aluminum with large-area uniformity in details. First, we simulate theoretically the influence of the position and emission characteristic of the evaporation source, as well as the fixture height, on aluminum film uniformity. Then, we study the influence of some key parameters of the evaporation process (such as the evaporation height and rate) on aluminum film quality and uniformity. Finally, under the optimal conditions, we prepared successfully the high-quality aluminum film with its thickness being larger than 10 μm and uniformity fluctuations less than 1% within a diameter of 700 mm.
Separation and parameter estimation of single channel sinusoidal frequency modulated signal mixture sources based on particle filtering
A signal separation and parameter extraction method based on particle filtering for single channel sinusoidal frequency modulated (SFM) signals is put forward. By assuming that the frequency of SFM signals mixture is continuous, a phase-difference de-aliasing arithmetic based on particle filtering is proposed. And the dimension of state space is reduced by using phase-difference between source signals. A likelihood function model suitable for high dimensional state space is proposed. Particles weight is accurately measured by comparing error between estimated values and true values of particles with fixed length. The problem of particle diversity reduction in the static parameters situation is solved by the introduction of Markov-chain Monte Carlo (MCMC) transfer after re-sampling, and the speed of particle filter iteration convergence is also effectively improved. Single channel SFM signal parameters are extracted and signals are separated by reconstructing signals only with the prior knowledge of modulation type. Finally, the simulation results indicate that this method can separate the multi-component signal sources and estimate the parameters effectively.
A flexible microstructure based on graphene for harvesting weak energy
A novel microstructure of flexible substrate/graphene/ZnO nanowires/graphene multilayer film for harvesting weak energy is for the first time presented as far as we know in this paper. First, the design of this microstructure and its operational principle is discussed theoretically. Next, we study the key technology in the preparation process of this microstructure and carry out the whole preparation process. Finally, the microstructure is successfully achieved and tested. Results show that the output voltage of the microstructure can be up to several hundreds of millivolt. In a word, the theoretical and experimental research of this microstructure provides a basis for self-powered micro-nano systems, and is significant to the practical development of the integrated micro-nano systems.
Analysis of cascading dynamics in complex networks with an emergency recovery mechanism
A model of cascading failures in complex networks with an emergency recovery mechanism is proposed in this paper, and the cascading dynamics is investigated by running the proposed model on nearest-neighbor coupled network, Erdos-Renyi random graph network, Watts-Strogatz small-world network and Barabasi-Albert scale-free network respectively. New concepts in emergency recovery mechanism and the efficiency of networks are defined. And the effects of the parameters on the network efficiency and failure rate are investigated. Results demonstrate that the increase of the emergency recovery probability would reduce the network efficiency decreasing speed and the failure rate growing speed, and also improve the resilience of the network. And the greater the load capacity of the nodes in the network, the slower the speeds of network efficiency reducing and failure rate growing. Meanwhile, with the decrease of the overload node failure probability, the reducing speed of network efficiency and the growing speed of failure rate would reduce gradually. Furthermore, the changes of the network efficiency and failure rate during the process of cascading failures in different network topologies are analyzed. It is found that the rise of the heterogeneity of degree distribution increases the reducing speed of network efficiency and the growing speed of failure rate. All these results can help analyze the cascading dynamics in complex networks with an emergency recovery mechanism, and may provide a guidance for the controling of cascading failures and protecting against them in real-life complex networks.
A complex network evolution model for network growth promoted by information transmission
In many real complex networks, information transmission occurs all the time. To study the effects of information transmission on the complex network evolution, we propose a new model for network growth promoted by the information transmission. The model includes three major steps: (i) New links attached to the nodes on the information transmission path, whose source point is chosen preferentially; (ii) the first link of the new node attached to the nodes in the local-world; (iii) other links of the new node attached to the nodes on the information transmission path, whose source point is the new node. The process of information transmission is simulated by self-avoiding random walk, and by considering the local information including its degree and distance; selective connection is established between the nodes on the information transmission path. Theoretical analysis and numerical simulation results show that the proposed model can not only reproduce small-world and scale-free network characteristics, but also indicate that “shift power-law distribution” and “truncated power law” function may form for different parameters which have some non-power-law features, such as exponential cutoff, and saturation for small variables. Moreover, in our model, the clustering coefficient is tunable without changing the degree distribution, and the model can also construct a network with assortative or disassortative mixed pattern.
Study on orbital angular momentum of Laguerre-Gaussian beam in a slant-path atmospheric turbulence
Atmospheric turbulence can cause random variations of the refractive index, resulting in a spatial inhomogeneity. When a Laguerre-Gaussian beam is propagating through the atmospheric turbulence, spatial inhomogeneity can bring about the change of photon wave function that causes variations in the orbital angular momentum. This article discusses how turbulence media change the orbital angular momentum of photons as to form different photon states, when the Laguerre-Gaussian beam is propagating in a slant-path atmospheric turbulence, by calculating the weight of the spiral harmonic component of the beam energy. Analysis of the variations of orbital angular momentum of Laguerre-Gaussian beam in the turbulent medium has been carried out.
Stability of dipolar soliton in crossed linear and nonlinear optical lattices
Stability of a dipolar Bose-Einstein condensate (BEC) soliton in crossed linear and nonlinear optical lattices is investigated using variational approximation. The Euler-Lagrange equations for variational parameters and the effective potential are derived by means of a cylindrically symmetric Gaussian ansatz, while the equilibrium widths are determined by minimization of the effective potential. In the presence of a periodic spatial variation of short-range contact interaction, the localized bound states can exist for both attractive and repulsive dipolar interactions. And the domain of stable dipolar BEC solitons is illustrated in a phase plot of the nonlinearities. Finally, we give the evolution of the variational width for different values of the nonlinearities.
Numerical simulations of dynamic properties of the restricted solid-on-solid model on fractal substrates
In order to investigate the effect of the structure of a non-complete substrate on the dynamic behaviors of a growing surface, the restricted solid-on-solid model on Sierpinski arrowhead and Crab fractal substrates, which have the same fractal dimensions but of different spectrum dimensions, are extensively studied by means of numerical simulations. The surface width and the maximal height of the saturated surface are calculated. It is found that the microscopic structure of the substrates affects significantly the dynamic properties of the surfaces. Although the restricted solid-on-solid model evolving on two kinds of fractal substrates exhibits dynamic scaling behavior, the standard Family-Vicsek scaling is still satisfied for different dynamic scaling exponents. The maximal height of the width of saturated surface can be fitted by Asym2Sig distribution, not by the three kinds of usual extreme statistical distribution, i.e. Weibull, Gumbel, and Frechet distributions.
Conformal invariance of isoheight lines of the (2+1)-dimensional etching surfaces
In order to study the statistical properties of the surface fluctuations in the Etching model more comprehensively and effectively, based on the Schramm Loewner evolution (SLEκ) theory, the contour lines of the saturated surface in the (2+1)-dimensional Etching model are investigated by means of numerical simulations. Results show that the isoheight lines of the (2+1)-dimensional Etching surfaces are conformally invariant and can be described in the frame work of the SLEκ theory with diffusivity κ=2.70± 0.04, which belongs to the κ=8/3 universality class. The corresponding fractal dimensions of the isoheight lines are df =1.34± 0.01.
Time-varying singularity spectrum distribution of sea clutter based on wavelet leaders
Singularity spectrum analysis of sea clutter is the key technology of detecting radar for sea target, which can discover the dynamic mechanism of the sea surface theoretically. In this paper, based on wavelet leaders the time-varying singularity spectrum distribution of sea clutters is proposed, which introduces time information to the traditional singularity spectrum, and displays the time-varying characteristic of singularity spectrum analytically. In theory, by way of self-windowed fractal signal, we introduce the time information to the traditional singularity spectrum, and realize multifractal spectrum distribution of sea clutters. In algorithm, based on the wavelet leaders, we adapt the process of embodying chirp-type and cusp-type singularities, and obtain the time-varying singularity spectrum distribution of sea clutters by the Legendre transform of the time-varying scaling function. In practice, we analyze the classical multifractal model–random wavelet series and the real sea clutter data of continuous wave Doppler radar in level III sea state. Experiments indicate that (1) the time-varying singularity spectrum distribution based on wavelet leaders can trace the time-varying scale characteristic and display the time-varying singularity spectrum distribution of sea clutters; (2) the algorithm possesses good statistical convergence, low computational cost, and passive moment property. The time-varying singularity spectrum distribution based on wavelet leaders may serve as a reference sample for nonlinear dynamics and multifractal signal processing.
Studies on weighted scale-free topology in energy heterogeneous wireless sensor network
In view of the problems about imbalance of node energy consumption and obtaining the load of node or edge efficiently in wireless sensor network, a kind of weighted scale-free topological evolution model for energy heterogeneity is put forward within the scope of local areas. By modelling the relationship among the node energy, load, energy consumption, and the weight to node and edge, we then give the evolution of the network through combining node weight and weighted model, deduce the power-law distribution of the node weight, the edge weight, and node degree, respectively, and next analyze the load and energy consumption according to the node weight and edge weight. Simulation results show that the proposed model not only can make accurate calculation for node and edge loads, but also can alleviate the node energy consumption imbalance in scale-free network.
Prediction of methane PVT relations at high temperatures by a simplified virial equation of state
In order to meet the demand of describing the supercritical gas under high temperature and medium-high pressure conditions, such as in detonation circumstance, a simplified virial equation of state (EOS), named Han-Long (HL), is presented, which is based on Lennard-Jones potential function. One hundred and twelve sets of theoretical data for methane above 1000 K are calculated using HL EOS. We obtain that the volume average absolute deviation (AAD) is about 1% and the maximum error is 3.28%; this error is far lower than the calculation deviation of DMW (Duan-Moller-Weare) EOS and BS (Belonoshko-Saxena) EOS. The shockwave data of methane is also calculated by HL EOS and the AAD are less than 3%. Results show that HL EOS can well describe the thermodynamic state of CH4 at high temperatures.
Study on phase-shift control in dispersion decreasing fibers
Phase-shift control can effectively avoid soliton interactions. With symbolic computation and Hirota's bilinear method, analytic studies are made on nonlinear Schrödinger equation, which can be used to describe the propagation of solitons in dispersion decreasing fibers. Analytic two-soliton solutions are obtained. With the obtained solutions, when the variable group-velocity dispersion function of dispersion decreasing fibers is a Gaussian one, the phase-shift control is achieved, soliton interactions are avoided, and the pulse quality in optical communication systems can be improved. Moreover, influences of parameters in dispersion decreasing fibers on the phase-shift control are discussed. Results are also helpful for the logic gates and optical switches.
Discrepancy between the interactions of nucleons in nuclear matter due to different projection choices of invariant amplitudes
Dependence of self-energy components and cross sections of nucleons in-medium on the relative momentum of nucleons in nuclear matter from different projections choices of invariant amplitudes is studied within the framework of Dirac-Brueckner-Hartree-Fock model. Special attention is paid to the discrepancy between the self-energies and cross sections for different choices in various separate chances. Our results indicate that the self-energy of nucleons in the states with spin S = 1 and isospin triplets T = 1 is larger than those with S = 0 and T = 0 at a specified relative momentum. The dependence of the self-energies of nucleons in various reaction channels on the pseudo-scalar (PS) choice are more pronounced than that on the complete pseudo vector (CPV) choice, which is mainly due to the states with smaller total momentum quantum numbers J. Results of the in-medium differential cross sections show that the difference between dσ/dΩ for the neutron-proton and neutron-neutron (or proton-proton) in various choices is larger for smaller relative momentum of nucleons and smaller scattering angles in mass center reference frame However, the total cross section σ total in the CPV choice is always larger than that in the PS choice. All these discrepancies will disappear with increasing incident energy.
A novel method of generating qausi-non-diffracting Mahtieu beam based on axicon
A novel method of generating zero order non-diffracting Mathieu beam with an axicon is proposed. To create quasi non-diffracting Mahtieu beam, an axicon is used to focus a plane wave modulated by elliptical Gaussian amplitude. Based on the formula of diffraction integral of a plane wave modulated by elliptical Gaussian amplitude propagating through the axicon, the intensity of quasi non-diffracting beam is simulated numerically. The maximum propagation distance of the quasi-non-diffracting Mathieu beam is calculated according to a geometrical optical model. To verify the results of the theory, an experimental setup is designed.Using a cylindrical lens and a collimating and expanding system, a circular Gaussian beam can be converted in to a plane wave modulated by elliptical Gaussian amplitude. Focusing the plane wave using an axicon, a qausi-non-diffracting Mathieu beam can be generated. The exoerimental results are consistent with theoretical calcuations and numerical simulations.
Shielding design of the multi-purpose reflectometer of China spallation neutron source
The shielding design employing the Monte Carlo simulation code of MCNPX 2.5.0 for multi-purpose reflectometer of China spallation neutron source (CSNS) is introduced, including the shielding needs, radiation source terms, calculating methods and simulation results, etc. Various radiation source terms such as the moderator leakage source term, and the neutron guide loss source term etc. are taken into account in the simulation, and step-by-step calculation method and variance reduction methods such as source angular biasing, source energy biasing, and geometry splitting, are adopted to improve the computing speed while the precision of the results is guaranteed. In the shielding design of instrument transport beamline, second shutter, and scattering room etc., the final shielding parameters are determined by comparing the shielding need under different operating conditions, to ensure the dose rate below the 2.5 μSv/h limitation in accessible locations outside the shielding.
First-principles calculations of the mechanical properties of IrB and IrB2
We have employed ab-initio plane-wave pseudopotential density functional theory to calculate the equilibrium lattice parameters, elastic constants, under the hydrostatic pressures from 0 to 100 GPa for P1 -IrB with Pnma space group and P5 -IrB2 with Pmmn structures. Results show that the P1 -IrB structure is stable, and the incompressibility is enhanced with the increase of pressure. And the elastic constants, bulk modulus, shear modulus for P5 -IrB2 structure exhibit the regular changes under the hydrostatic pressures from 0 to 100 GPa. But when the pressure becomes 50 GPa, the Young's modulus and the lattice constant in the direction b for P5 -IrB2 structure will change exceptionally. Results show that both are not of obvious band gaps in P1 -IrB and P5 -IrB2 electronic structures under zero pressure, because of the covalent effect between Ir and B atoms. The analysis of band structure and the figure of density of states for P1 -IrB and P5 -IrB2 indicate that the two kinds of structure have metal properties.
First-order distorted wave Born approximation for single ionization of Ar by electron impact in a coplanar doubly symmetric geometry
The first-order distorted wave Born approximation (DWBA) has been performed for single ionization of Ar(3p) by electron impact in coplanar doubly symmetric geometry from near threshold to intermediate energies. Theoretical triple differential cross sections (TDCS) are compared with the latest experimental data. It is found that when the incident electron energies are 40 eV higher than the ionization threshold, for argon, the binary and recoil collision mechanism will dominat as the energies increase, but not in the near threshold energy regime, where the distortion effects are of great significance. In order to completely describe the electron impact ionization of argon, it is imperative that more scattering mechanisms should be considered in the theoretical models.
Molecular dynamics study of cascade damage at SiC/C interface
Continuous silicon carbide (SiC) fiber-reinforced SiC (SiCf/SiC) composites have been considered to be used as structural materials in advanced nuclear reactors for its excellent properties. Their mechanical properties have been greatly improved during the last decade. But the radiation damage at the SiC and pyrolytic carbon interface would degrade the mechanical integrity of the composites, while the mechanism of degradation is remaining unknown at present. In this study, molecular dynamics simulations have been used to model the irradiation cascade of five SiC/C composite systems. According to the angle between the graphite layer and the interface, the models are marked as M0, M28, M56, M77 and M90, in which the number represents the angle. Forty primary knock-on atoms (PKAs) at different positions in each composite system are used to bombard the interface. In each run a collision cascade may be initiated by giving one of the 40 atoms 1.5 keV kinetic energy. The relationships between the distribution of defects and simulation time and PKA position are systematically studied, and compared with those in bulk SiC, which are marked as MW. Results show that the radiation damage resistance of SiC/C interface is significantly lower than bulk SiC, and the interface structure has an impact on the number of defects. Radial distribution function (RDF) is employed to examine the coordination of interfacial atoms. The results show that the higher the density of graphite atoms in the interface, the larger impact the irradiation on the RDF and coordination.
Zeeman slowing and magneto-optically trapping of lithium atoms in atomic interferometry experiments
To prepare cold lithium atoms for atomic interferometry experiments, we have carried out experimental researches on Zeeman deceleration and magneto-optical trap (MOT) of lithium atoms. We have also designed and implemented a compact adjustable Zeeman slower with an inner water cooling chamber, to decelerate the velocity of the 7Li atom beam from 600 m/s down to 60 m/s, and load them into the MOT. The loading rate is 5×108 /s, the total trapped atom number is 1×109 , and the lowest temperature of the atom cloud is 220 ± 30 μK. Then we investigate the dependence of lifetime of 7Li atoms in optical molasses on the detuning of trapping laser beams. The above results lay a foundation for further sub-Doppler cooling, optical trap based on evaporative cooling, and atomic interferometry experiments.
Simulation study of interface instability in metals driven by cylindrical implosion
Simulation of metal instability with the initial sine perturbation on the inside of the metal shell driven by cylindrical implosion is made, and the simulation results is in accordance with the experiments. By comparing with the simulation result without considering the strength of the metals, the analysis shows that the strength of unmelted metal has a strong inhibitory effect to the metal instability, and under certain loading conditions the growth rate of the perturbation will decrease with the increase of the perturbation mode number. After that, the laws of the metal instability under explosive-driven conditions are summarized. Before the implosion reflected wave arrives at the shell, RM instability plays a dominant role. After the implosion reflected wave is applied to the shell, RT instability is significantly enhanced, the effect combined with the strength of the perturbations shows a nonlinear evolution. Under both RM and RT instability condition, the strength of metal could cause the cutoff wavelength to exist in unmelted state.
Efficient simulation of three-dimensional marine controlled-source electromagnetic response in anisotropic formation by means of coupled potential finite volume method
A coupled potential finite volume method for simulation of three-dimensional marine controlled-source electromagnetic (CSEM) response in anisotropic formation is developed. To circumvent ill-conditioning and convergence problems, Maxwell's equations are reformulated into coupled scalar-vector potentials with Coulomb gauge and its complement by applying a Helmholtz decomposition to the electric field. Yee's staggered girds, finite volume averaging and interpolation techniques are used to make the Helmholtz equations discrete. The resulting sparse and complex linear system in large-scale models is solved by a direct solver PARDISO. In order to improve the accuracy of the near field results without significantly reducing the computational efficiency, a method using difference fields is proposed to reduce the source singularity effect of anisotropic formation. The anisotropic modeling examples show that marine CSEM response is predominantly sensitive to reservoir vertical resistivity, not to reservoir horizontal resistivity, provided that the reservoir are thin and high-resistive; but the marine CSEM response is sensitive to both horizontal and vertical resistivity of the overburden on top of the reservoir.