Search for Majorana zero mode in the magnetic vortex of artificial topological superconductor
The search for new states that exhibit topological order is currently a very active and exciting area of research. Like a topological insulator, superconducting order can also exhibit topological order, which is different from that of a conventional superconductor. This superconductor is so-called "topological superconductor", which has a full pairing gap in the bulk and gapless surface state. Majorana Fermions obey non-Abelian fractional statistics, and have been proposed to construct topological qubits, so there is a great prospect of scientific research and application in topological quantum computing. It is very interesting that Majorana Fermions are predicted to exist in topological superconductors. However, natural topological superconductor is very rare. Inspired by the realization of topological insulators, theoretical physicists have proposed that via the fabrication of the s-wave superconductor/topological insulator heterostructure, Majorana Fermions may exist in the superconducting topological insulator induced by proximate effect. Due to various kinds of topological insulators and conventional s-wave superconductors, heterostructures constructed by this method can greatly increase the variety of artificial topological superconductors. In this paper we review the experimental progress in the heterostructure composed of the Bi2Te3-type topological insulator and the conventional s-wave superconductor NbSe2. Using molecular beam epitaxy, atomically flat topological insulator film can be fabricated at the top of superconductor substrate. The spatial distribution of Majorana Fermions on the surface of topological insulator can be directly observed by in situ scanning tunneling microscopy/spectroscopy. In the center of a magnetic vortex, Majorana Fermions will appear as the Majorana zero mode, a zero-energy peak inside the superconducting gap. Although the energy gap between low energy quasiparticle excitation and the Majorana zero mode is very small, the evidences such as zero bias conductance anomaly, Y-shape splitting of zero-bias conductance, spin-selective Andreev reflection are self-consistent and reveal that the Majorana zero mode exists in the center of a magnetic vortex. These experiments have led to a new insight into superconductivity. It may open a door to probing the novel physics of Majorana fermions.
Mechanism of interaction between interface and radiation defects in metal
High-energy particles' radiation produces a large number of radiation defects in material, such as interstitial atoms, vacancies, dislocation loops, voids and helium bubbles. The formation and evolution of massive radiation defects cause the instability of microstructure in metal, which further degrades its mechanical performance. Interface engineering is an effective method to tune the radiation resistance of metal and alloy. By introducing a large number of grain boundaries, phase interfaces, free surfaces, etc., the recombination probability of radiation-induced vacancies and interstitial atoms increases, thereby reducing the accumulation of radiation defects, improving the structural stability of the metal and eliminating the harmful effects of radiation. In this paper, we briefly review the recent progress of the mechanisms of interactions between several typical interfaces and various types of irradiation defects. The influence of interface structure, irradiation condition and defect character on their interaction behavior are reviewed and discussed. We also propose some critical questions about the radiation damage to material which remain to be understood. It is necessary to combine multidisciplinary techniques, knowledge and theories in order to fully understand the mechanism of radiation damage and design the advanced radiation-tolerant materials.
First principle study of occupancy, bonding characteristics and alloying effect of Zr, Nb, V in bulk α-Fe(C)
The volume change rate, total energy, binding characteristics, state density, charge distribution number and mechanical properties of cells formed by solid solution of Zr, Nb and V in α-Fe(C) are calculated by using the first-principles method. Thus, the effect of Zr, Nb, V on α-Fe(C) are studied in this paper. The results show that V displaces Fe atoms which is at the apex angle of α-Fe (C) cells preferentially, while Zr and Nb displace Fe atoms at the body center of α-Fe(C) cells. Zr and Nb reduce the stability of ferrite, but Zr is more difficult to solidly solubilize in α-Fe(C) than Nb. Solid solution of V increases the binding energy of crystal cells, meanwhile the toughness of crystal cell is mainly improved. After solid is solubilized in ferrite, Zr and Nb atoms only form metal bonds with Fe atoms while V and Fe atoms form the metal bonds and Fe-V ionic bonds. The ionic bonds of Fe-V are stronger than metal bonds of Zr and Nb atoms with Fe atoms, which is the main factor of the cell increasing. Zr and Nb mainly improve the mechanical properties of steel material by means of dispersion strengthening. To some extent, V solid solution can improve the toughness of ferrite, which is the main reason for improving the mechanical properties.
First-principles study of lattice dynamical and thermodynamic properties of Li2NH
One of the key issues for scale applications of hydrogen energy is the availability of safe, efficient and ecnomicical hydrogen storage technologies. In the past few years, light metal hydrides have attracted considerable attention due to their high hydrogen capacity. With a hydrogen capacity up to~6.5 wt%, Li2NH is regarded as one of the most promising hydrogen storage materials. Although the hydrogen physical and thermodynamic properties of Li2NH have been studied, the electronic structure, phonon vibration mode and thermodynamic properties of Li2NH have not yet been resolved. In this paper, by using the first principles based on the density functional theory (DFT), we investigate the electronic structure, lattice dynamical and thermodynamic properties of Li2NH in detail.
Firstly, the structure of Li2NH is optimized and the lattice parameters and total energy of the crystals are calculated. As shown by the calculation results, the lattice parameters are in good agreement with previous theoretical and experimental results. Our lowest-energy structure of Li2NH has orthorhombic Pnma symmetry at T=0 K for all of the proposed structures. Secondly, the electronic band-structure studies reveal that Li2NH has a small band gap of about 2.0 eV. The analysis of total and partial density of states of Li2NH show that the bonding between the N and H has a covalent character. Thirdly, the lattice dynamical properties of Li2NH are investgated at the corresponding equilibrium states. These results show that only the phonon dispersion curves of Li2NH (Pnma) without negative frequencies are calculated along the high-symmetry points. The optical modes of phonon frequencies at Γ point are assigned as Raman and Infrared-active modes. Based on the calculated phonon density of states, the thermodynamic properties are computed, such as the Helmholtz free energy, internal energy, entropy and the constant-volume specific heat versus temperature. The calculation results may explore the applications in areas of hydrogen storage for Li-N-H, which is of great importance forusing hydrogen in the future.
The characteristics of multiple isothermal tempered Fe-based alloy ribbons
The magnetic properties of Fe-based alloy ribbons are sensitive to stress, and it's an interesting scientific question whether stress-induced magnetic anisotropy during annealing procedure can be eliminated by tempering. In this paper, the synchrotron radiation technique was used to observe the microstructure of Fe73.5Cu1Nb3Si13.5B9 amorphous ribbons annealed at 540℃ for 30 minutes under 394.7MPa stress and tempered several times at the same temperature. The macroscopic elongation of the samples during stress annealing and tempering was recorded by SupereyesB011 microcamera, and the magnetic anisotropy of the samples was measured by HP4294A impedance analyzer. After fitting the experimental data, it is found that:(a) The lattice anisotropy, macroscopic strain and magnetic anisotropy of the sample show negative exponential attenuation with the tempering times, and their final residual are 19.04%. 98.27% and 31.65%. (b) Multiple tempering can not completely eliminate lattice anisotropy, macroscopic strain and magnetic anisotropy induced by stress annealing. (c) The magnetic anisotropy of the sample has a linear relationship with the lattice anisotropy, but the intercept between the reverse extension line of the relation curve and the longitudinal coordinate is not zero. When the lattice anisotropy is zero, there is still 16.36% magnetic anisotropy. This is different from Ohnuma's conclusion that lattice anisotropy is the direct cause of magnetic anisotropy. (d) The structure anisotropy caused by the residual stress after stress annealing is the main cause of magnetic anisotropy, but it is not the only reason. The directional congregation of agglomerated nanocrystalline grains caused by creep of amorphous substrates during stress annealing is also an important cause of magnetic anisotropy induced by stress annealing. Moreover, the magnetic anisotropy induced by the directional congregation of agglomerated nanocrystalline grains due to the creep of amorphous substrates during stress annealing can not be completely eliminated by tempering.
Quantum calculation of the influence of trivalent praseodymium ions doping on the magneto-optical properties of terbium gallium garnet crystal
Compared with those materials with superior magneto-optical properties, such as YIG, Ce:YIG and Ba3Tb(PO4)3, pure terbium gallium garnet (TGG) crystal has comparative low Verdet constant and cannot meet the requirements of some high-power devices. Doping Pr3+ ions in TGG crystal can remarkably enhance its magneto-optical properties and expand its application scope, but there are still lack of systematic theoretical calculations to clarify this phenomenon. Based on the quantum theory, this paper presents the influence of doping Pr3+ ions on the magneto-optical performance and the corresponding quantitative calculation results. Firstly, taking various effects on Tb3+ ions and Pr3+ ions in the crystal into consideration, the Hamiltonian is modeled and discussed in detail. The secular equations are solved by applying the perturbation method, and then the energy level shifts and wave functions of the Tb3+ ions and Pr3+ ions are worked out, where the spin-orbit coupling, crystal field, effective field and super-exchange interaction between the two types of ions are considered. Furthermore, the transition dipole moments of Tb3+ ions and Pr3+ ions from the 4f ground state to higher level 5d, together with the distribution probability at each energy level and the average magnetic moment, are resolved. Finally, the Verdet constants and magnetic susceptibilities of pure TGG crystal and Pr:TGG crystal are calculated and compared with each other. Moreover, the relationship between the Verdet constant of Pr:TGG crystal and the Pr3+-doping amount is derived. The results show that the Faraday rotation angle caused by Pr3+ ions is larger than that of Tb3+ ions, meanwhile, the strong super-exchange between Tb3+ ions and Pr3+ ions causes further splitting of the 4f energy level, resulting in a significant increasement of the Verdet constant of the Pr:TGG crystal, which reaches 313.4 rad/m·T, 191.2 rad/m·T and 60.4 rad/m·T at the wavelengths of 532 nm, 632.8 nm and 1064 nm, respectively. In addition, doping Pr3+ ions inside the crystal improves the internal effective magnetic moment, which can reach 9.92 μB at 10 K. At the same time, the magnetic susceptibility increases, while the temperature interdependency decreases. The linear relationship between the reciprocal of magnetic susceptibility and temperature reduces from 4.41/K to 3.92/K. The Verdet constant of the Pr:TGG crystal is linear with the amount of Pr3+ ions doping. When the contents of Tb3+ ions and Pr3+ ions inside the crystal are equal, the maximum value is reached, which is about 2913.4 rad/m·T. The calculation results in this paper are in good agreement with the existing experimental data.
Enhancement of graphene three-channel optical absorption based on metal grating
As an emerging new material, graphene has aroused the great research interest. How to improve its absorption efficiency is one of the hot research topics. However, currently most of the studies concentrate in THz band or middle-to-far-infrared region:the research in the visible and near-infrared regions is rare, which greatly limits the applications of graphene in opto-electric fields. In order to improve the absorption efficiency of single-layered graphene in visible and near-infrared band and realize multi-channel optical absorption enhancement, we propose a hybrid structure consisting of graphene-metal grating-dielectric layer-metal substrate. The proposed structure can realize three-channel light absorption enhancement at wavelengths λ1=0.553 μm, λ2=0.769 μm, and λ3=1.130 μm. The maximum absorption efficiency of graphene is 41%, which is 17.82 times that of single-layered graphene. The magnetic field distributions of the hybrid structure at three resonance wavelengths are calculated respectively. It can be found that for the resonance peak λ1, the energy of light field is distributed mainly on the surface of metal grating, which is the characteristic of surface plasmon polariton (SPP) resonance. Therefore, it can be judged that the enhancement of graphene absorption in this channel is due to the SPP resonance stimulated by metal grating. For the resonance peak λ2, the energy of the optical field is mainly confined into the metal grating groove, which is the remarkable resonance characteristic of the Fabry-Pérot (FP) cavity, it can be concluded that the enhancement of the optical absorption of graphene at the resonance peak λ2 is due to the resonance of the FP cavity. When the resonance peak is λ3, the energy of the light field mainly concentrates on the upper and lower edges of the metal grating and permeates into the SiO2 layer, and it can be observed that there are energy concentration points (reddish) at the left end and the right end of the metal grating edge, which is a typical magnetic polariton (MP) resonance feature. Therefore, the enhancement of absorption of graphene at the resonance peak λ3 is caused by the MP resonance induced by the metal grating. We also analyze the absorption characteristic (resonance wavelength and absorption efficiency) dependence on structure parameters by using the finite-difference time-domain (FDTD) simulation. Our study reveals that by increasing grating width, all the three resonance wavelengths are red-shifted, and the absorption efficiency at λ2 and λ3 are both enhanced whereas the absorption efficiency at λ1 almost keeps unchanged. By increasing dielectric layer thickness, λ2 will be red-shifted and λ3 will be blue-shifted, whereas the absorption efficiency at the three resonance wavelengths all remain constant. By increasing graphene chemical potential, none of the wavelengths of the three absorption peaks is shifted, and the absorption efficiency at λ3 decreases. According to our findings, we optimize structure parameters and achieve the light absorption efficiency larger than 97% at the three channels simultaneously, which can make metamaterial absorbers.
First-principles study of rare-earth-doped cathode materials Li2MnO3 in Li-ion batteries
Although Li-ion batteries (LIBs) have had great success in portable electronic devices and electrical vehicles, the improvement of the performances has received intensive attention. Generally, doping is an effective method to modify the battery performance, such as cycling performance. Appropriate doping can effectively reduce the structural deformation of electrode materials during charging and discharging, thus improving the cycling performace of LIBs. Because of the large radius, large charge and strong self-polarization ability of rare earth ions, rare earth element is a promising candidate for doping modification. Motivated by this, we study the structural, electronic and ionic diffusion properties of rare-earth-doped cathode material Li2MnO3 by using first-principles calculations based on density functional theory as implemented in Vienna ab initio simulation package. After the doping of rare earth elements (La, Ce, Pr, Sm), the lattice constants and cell volumes increase with respect to the undoped one. The cell volume of La-doped Li2MnO3 has the biggest change, while the cell volume of Sm-doped one has the smallest change. Due to the different ionic valence states, the electronic structures of the doped Li2MnO3 are various. La-doped Li2MnO3 exhibits metallic characteristic, whereas Ce-, Pr-, and Sm-doped structures are semiconducting with smaller band gap than that of the undoped case. The Li diffusion energy barrier in Li2MnO3 shows complicated variation when the La and Ce are doped. At the sites far away from the rare earth ions, the Li diffusion barriers are lower than that of undoped one. The reason is that the diffusion channels, which are determined by the distance between neighboring O-layers, are enlarged due to the implantment of rare earth ions. However, the situations are various at the sites close to the rare earth ions. The Li diffusion barriers increase essentially when Li ions diffuse from the nearest sites to rare earth ions. Such a result is closely related to the huge changes of local structures around the rare earth ions. In addition, the effect of La doping on the Li diffusion barrier is more obvious than that of Ce doping, which is due to the local structure change around rare earth ions.
Rheological properties of ring and linear polymers under start-up shear by molecular dynamics simulations
We analyze the structure and rheological properties of ring and linear polymers under shear byusing the non-equilibrium molecular dynamics simulation. The simulation results show that compared with the ring chains, the linear polymers do not present prominent stress over shoot phenomenon. Since the overshoot reflects the maximum flow-induced deformation of the polymer, this qualitative observation already implies that the ring experience less deformation than its linear precursor in simple shear flow. This is consistent with the recent experimental result. In order to further study the molecular mechanism of this phenomenon, the segmental structure and orientation angle distribution as a function of strain under the different Weissenberg numbers are given in this study. The weak overshoot of the stretching of the ring polymers proves that the weak shear thinning and peak strain are due to the weak deformation of the segment chain of the ring in the shear flow. The rheological properties of linear and ring system are extracted from the stress-strain curves, can be used further to analyze the data. The peak strain γmax as afunction of WiR follows a power-law with an exponent of 0.3 for linear polymer at WiR>1, however, for the ring system thepeak strain follows a power-law with an exponent of 0.1. The parameter ηmax/ηsteady is also the measure of the effective chain deformation at a steady state. The data show its progressive increase with WiR increasing, and follows a power-law with a scaling slop of 0.13 and 0.08 for linear and ring polymers, respectively. The peak stress σmax as a function of WiR is also extracted from stress-strain curve. The two investigated systems both obey the scaling law with an exponent of 0.5. The normalized steady-state shear viscosity obeys a shear thinning slop of -0.86 for the linear polymer, the ring polymer obeysa shear thinning slop of -0.4. According to the gyration tensor and orientation angle, the power-law relationship between stretching and orientation is also given in this work.
Design and fabrication of low-noise superconducting quantum interference device magnetometer
Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor known, which is widely used in biomagnetism, low-field nuclear magnetic resonance, geophysics, etc. In this paper, we introduce a high-sensitivity SQUID magnetometer, which consists of an SQUID and a flux transformer. The SQUID is first-order gradiometer configuration, which is insensitive to interference noise. The flux transformer includes a multi-turn spiral input coil and a large-sized pickup coil. And the input coil is inductively coupled to the SQUID through mutual inductance. We present an SQUID magnetometer fabricated with Nb/Al-AlOx/Nb Josephson junction technology on a 4-inch silicon wafer at our superconducting electronics facilities. We develop a fabrication process based on selective niobium etching process consisting of five mask levels. In the first two mask levels, the trilayer is patterned by a dry etch to define base electrode, contact pads, and interconnects. The shunt resistor and a dielectric insulating layer are then deposited and patterned by using lift-off and dry etchant, respectively. Finally, the niobium wiring layer is deposited and patterned by using reactive ion etching to define input, pickup and feedback coils. The measurement of the SQUID magnetometer is performed inside a magnetically shielded room. The operating temperature is realized by immersing the SQUID into the liquid helium (4.2 K). Moreover, a superconducting niobium tube is employed to protect the SQUID from being disturbed by external environments. A homemade readout electronics instrument with low input voltage noise is used to characterize the SQUID magnetometer. The results of low-temperature measurements indicate that the magnetometer has a magnetic field sensitivity of 0.36 nT/Φ0 and a white flux noise of 8 μΦ0/√Hz,corresponding to a white field noise of 2.88 fT/√Hz. This kind of SQUID magnetometer is suitable for multi-channel systems, e.g., magnetocardiography, magnetoencephalography, etc. Although the SQUID process development benefits from the rapid advance of semiconductor process technology, the uniformity of the SQUID on one wafer is fluctuated due to the film deposition. Now, we have realized a best SQUID yield of 50% on a 4-inch wafer. In the future, the SQUID chip yield should be improved by well controlling the optimizing process. The device yield is expected to reach as high as 80%.
Current source reconstructing and magnetic imaging of cardiac electrical activity during P-wave
The current source reconstruction and magnetic imaging is a new technique to non-invasively obtain spatial information regarding cardiac electrical activity using magnetocardiogram (MCG) signals measured by the superconducting quantum interference device (SQUID) on the human thorax surface. Using MCG signals to reconstruct distributed current sources needs to solve the inverse problem of magnetic field. The beamforming is a type of spatial filter method that has been used for distributed source reconstruction and source imaging in electroencephalogram (EEG) and magnetoencephalogram (MEG). In this paper, the dipole moment of distributed current source is estimated with corresponding each spatial filter based on the cardiac source field model. The purpose is to enhance the intensity contrast of the dipole moment of distributed current sources in distributed source spatial spectrum estimation with beamforming, so that the reconstructed-pseudo sources beyond the heart can be removed for imaging cardiac electric activity well.
A new beamforming method of improving intensity contrast (ⅡC) of distributed source spatial spectrum estimation is developed for imaging cardiac electric activity in P-wave, due to cardiac magnetic signals in P-wave lower than that of the peak value of R-wave, which has a relatively low signal-to-noise ratio (SNR). For enhancing the accuracy of current source reconstruction in P-wave, the ⅡC divided into two steps:firstly, to introduce the lead-field matrix, which represents the measurement sensor-array sensitivity to magnetic field current sources, into a weight matrix of the spatial filter for making the output estimation of the filter more sensitive to the current source distribution, so as to improve the intensity contrast of the reconstructed distributed sources.
Secondly, by setting a threshold of source intensity from experience, to extract the reconstructed source with locally-maximal dipole strength at each time for eliminating the relatively weak pseudo sources in other locations, so as to enhance the accuracy of current source reconstruction during P-wave. In this paper, the ⅡC and three other methods, including minimum variance beamforming (MVB), suppressing spatial filter output noise-power gain (SONG) and trust region reflective (TRR), are compared by using the theoretical analysis and simulation experiments of MCG current source reconstruction during P-wave.
The results show that the ⅡC has higher intensity contrast of the single source spatial spectrum estimation, and possesses better accuracy of the current source reconstruction. The 61-channel MCG signals of two healthy subjects and their imaging of cardiac electrical activity during P-wave also are analyzed. The result shows that the ⅡC is better than the other three methods. It is indicated that two healthy subjects have stronger electrical activity in the atrium than that in the ventricle at Ppeak time, also that the electrical activity has the direction feature when the right-atrium is depolarized during P-wave.
In summary, the ⅡC is useful for imaging the cardiac electrical activity. However, it is needed to carry out a further research on patients with local myocardial ischemia and left or right coronary artery stenosis, and to establish the evaluation index for imaging of cardiac electrical activity in such patients.
Eavesdropping and countermeasures for backflash side channel in fiber polarization-coded quantum key distribution
Nowadays, the practical security of quantum key distribution (QKD) is the biggest challenge. In practical implementation, the security of a practical system strongly depends on its device implementation, and device defects will create security holes. The information leakage from a receiving unit due to secondary photon emission (backflash) is caused by a single-photon detector in the avalanche process. Now studies have shown that the backflash will leak the information about time and polarization and the eavesdropping behavior will not generate additional error rate in the communication process. An eavesdropping scheme obtaining time information by using backflash is proposed. Targeting this security hole for backflash leaking polarization information, an eavesdropping scheme for obtaining polarization information by using backflash is proposed in free-space QKD; however, it has not been reported in fiber QKD. In this study, the eavesdropping scheme and countermeasures for obtaining information by using backflash in fiber polarization-coded QKD is proposed. Since the polarization state of the fiber polarization-coded QKD system is easy to change, the scheme is proposed based on the time-division multiplexing polarization compensation fiber polarization-coded QKD system. In theory, the eavesdropper in this scheme obtaining the key information by using the backflash is theoretically deduced, and corrects the polarization change of the backflash by time-division multiplexing polarization compensation method, thus obtaining the accurate polarization information. The probability of backflash in the fiber polarization-coded QKD is measured to be 0.05, and the information leakage in the proposed eavesdropping scheme is quantified. The lower limit of the information obtained by the eavesdropper is 2.5×10-4. Due to the fact that the polarization compensation process increases invalid information in actual operation, the information obtained by the eavesdropper will be further reduced, thus obtaining the lower limit of information leakage. The results show that the backflash leaks a small amount of key information in a time-multiplexed polarization-compensated fiber polarization-coded QKD system. The wavelength characteristics of the backflash can be utilized to take corresponding defense methods. Backflash has a wide spectral range, and the count of backflash has a peak wavelength. So, tunable filters and isolators can be used to reduce backflash leakage, thereby reducing the information leakage.
Continuous time random walk model with advection and diffusion as two distinct dynamical origins
Modeling the solute transport in geological porous media is of both theoretical interest and practical importance. Of several approaches, the continuous time random walk method is a most successful one that can be used to quantitatively predict the statistical features of the process, which are ubiquitously anomalous in the case of high Péclet numbers and normal in the case of low Péclet numbers. It establishes a quantitative relation between the spatial moment of an ensemble of solute particles and the waiting time distribution in the model. However, despite its success, the classical version of this model is a "static" one in the sense that there is no tuning parameter in the waiting time distribution that can reflect the relative strength of advection and diffusion which are two mechanisms that underlie the transport process, hence it cannot be used to show the transition from anomalous to normal transport as the Péclet numbers decreases. In this work, a new continuous time random walk model is established by taking into account these two different origins of solute particle transport in a geological porous medium. In particular, solute transitions due to advection and diffusion are separately treated by using a mixture probability model for the particle's waiting time distribution, which contains two terms representing the effects of advection and diffusion, respectively. By varying the weights of these two terms, two limiting cases can be obtained, i.e. the advection-dominated transport and the diffusion-dominated transport. The values of scaling exponent β of the mean square displacement versus time, (△x)2~tβ, are derived for both cases by using our model, which are consistent with previous results. In the advection dominant case with the Péclet number going to infinity, the scaling exponent β is found to be equal to 3-α where α ∈ (1, 2) is the anomaly exponent in the advection-originated part of the waiting time distribution that ω1 (t)~t-1-α. As the Péclet number decreases, the diffusion-originated part of the waiting time distribution begins to have a stronger influence on the transport process and in the limit of the Péclet number going to 0 we observe a gradual transition of β from 3-α to 1, indicating that the underlying transport process changes from anomalous to normal transport. By incorporating advection and diffusion as two mechanisms giving rise to solute transport in the continuous time random walk model, we successfully capture the qualitative transition of the transport process as the Péclet number is varied, which is, however, elusive from the classical continuous time random walk model. Also established are the corresponding macroscopic transport equations for both anomalous and normal transport, which are consistent with previous findings as well. Our model hence fully describes the transition from normal to anomalous transport in a porous medium as the Péclet number increases in a qualitative and semi-quantitative way.
Dynamic analysis of symmetric behavior in flux-controlled memristor circuit based on field programmable gate array
The lack of the relationship between flux and charge has been made up for by the memristor which is suitable to constructing chaotic circuits as a nonlinear element. Commonly, the memristor-based chaotic systems are constructed by introducing the model of memristor into various classical nonlinear circuits, and more special and abundant dynamic behaviors are existent in these memristive systems. With the deepening of research, several novel nonlinear phenomena of memristor circuits have been found, such as hidden attractors, self-excited attractors and anti-monotonic characteristic. Meanwhile, multistability of a memristor-based circuit explained by the coexistence of multiple attractors with different topological structures is a typical phenomenon in a nonlinear system, and it is also one of the hotspots in this field. In addition, the chaotic sequences generated by the memristive circuits are used as additional signals for information transmission or image encryption. Therefore, the study of modeling memristor systems and analyzing various nonlinear behaviors is of certain valuable.
In this paper, a four-dimensional flux-controlled memeristive circuit is constructed by introducing an active memristor with absolute value into an improved Chua's circuit, and the special dynamic behaviors are observed. Through the bifurcation diagrams and Lyapunov exponent spectra, the symmetric bifurcations are shown, and the symmetric system states in parameter mappings are found. Besides, the distribution maps of memristive circuit are used to analyze the multistability in a symmetrical attraction domain, and the corresponding phase diagrams are depicted to confirm the existence of multistability. Furthermore, the circuit experiments of the flux-controlled memeristive circuit are implemented by the field programmable gate array simulation, and the experimental results are obtained on a digital oscilloscope, which proves the physical implementability of the memristor-based system.
Generation and application of novel Chua multi-scroll chaotic attractors
Chaos has great potential applications in engineering fields, such as secure communication and digital encryption. Since the double-scroll Chua's circuit was developed first by Chua, it has quickly become a paradigm to study the double-scroll chaotic attractors. Compared with the conventional double-scroll chaotic attractors, the multi-scroll chaotic attractors have complex structures and rich nonlinear dynamical behaviors. The multi-scroll chaotic attractors have been applied to various chaos-based information technologies, such as secure communication and chaotic cryptanalysis. Hence, the generation of the multi-scroll chaotic attractors has become a hot topic in research field of chaos at present. In this paper, a novel Chua multi-scroll chaotic system is constructed by using a logarithmic function series. The nonlinear dynamical behaviors of the novel Chua multi-scroll chaotic system are analyzed, including symmetry, invariance, equilibrium points, the largest Lyapunov exponent, etc. The existence of chaos is confirmed by theoretical analyses and numerical simulations. The results show that the rich Chua multi-scroll chaotic attractors can be generated by combining the logarithmic function series with the novel Chua double-scroll chaotic system. The generation mechanism of the Chua multi-scroll chaotic attractors is that the saddle-focus equilibrium points of index 2 are used to generate the scrolls, and the saddle-focus equilibrium points of index 1 are used to connect these scrolls. Then, three recursive back-stepping controllers are designed to control the chaotic behavior in the novel Chua multi-scroll chaotic system. The recursive back-stepping controllers can control the novel Chua multi-scroll chaotic system to a fixed point or a given sinusoidal function. Finally, a new method of detecting a weak signal embedded in the Gaussian noise is proposed on the basis of the novel Chua multi-scroll chaotic system and the recursive back-stepping controllers. The immunity of the novel Chua multi-scroll chaotic system to the Gaussian noise with the zero mean is analyzed by using the stochastic differential equation theory. The results show that the proposed new method of detecting the weak signal can detect the frequencies of the multi-frequency weak periodic signal embedded in the Gaussian noise. In addition, the novel Chua multi-scroll chaotic system has strong immunity to any Gaussian noise with the zero mean. The proposed method provides a new thought for detecting the weak signal.
Long-range, high-precision absolute distance measurement technology based on alternately oscillating optoelectronic oscillator
Absolute distance measurement plays an important role in many areas, such as aerospace and scientific research. Traditional measurement methods generally cannot meet requirements for long-range and high-precision at the same time. In this paper, an absolute distance measurement method based on alternately oscillating optoelectronic oscillator is proposed. This method places the distance to be measured in the loop of optoelectronic oscillator and takes advantage of accumulative magnification effect to achieve high accuracy. The measurement and the reference optoelectronic oscillators are established and selected by an optical switch, and a microwave switch is used to choose the high-order or low-order oscillating frequency. The high-order frequency and low-order oscillating frequency of the measurement and reference optoelectronic oscillators are measured in turn by frequency counter to calculate the loop lengths of two optoelectronic oscillators. The low-order frequencies are used to measure the fundamental frequency roughly and the high-order frequencies are used to calculate loop length precisely. Although the mode hopping occurs in the measurement process, it does not affect the loop length calculation by substituting the corresponding oscillating mode number. Note that the loop length measurement moments of two optoelectronic oscillators are different due to the switching order of optical switch and microwave switch. In order to calculate the absolute distance, which is the length difference between two optoelectronic oscillators at the same moment, the measured loop lengths should be averaged.In this way, systematic error accumulation caused by slow drift of environment can be eliminated, and this method does not need to control the length of reference optoelectronic oscillator. Meanwhile, the measurement system is simple. In the experiment, 1 km, 5 km and 8 km fibers are placed in a common part of the measurement and reference optoelectronic oscillators to simulate different long-range distances in space. A high-resolution optical delay line is placed in the measurement optoelectronic oscillator to verify the performance of the measurement system. The experimental results show that the measurement error is 3.5 μm with a 3.5 μm maximum standard deviation of each measurement distance at an emulated round trip distance of 6 km. The relative measurement accuracy reaches 5.8×10-10. This method provides a feasible idea for solving the technical problems of long-range and high-precision absolute distance measurement.
Online learning method based on artificial neural network to optimize magnetic shielding characteristic parameters
Magnetic shielding plays an important role in magnetically susceptible devices such as cold atom clocks, atomic interferometers and other precision equipment. The residual magnetic field in a magnetic shield under a varying external magnetic field can be calculated by the Jiles-Atherton (J-A) hysteresis model and magnetic shielding coefficient. According to the calculation results, the variation of internal magnetic field can be compensated for the active compensation coils. However, it is difficult to practically obtain the exact values of the five magnetic-shielding-related parameters in the J-A hysteresis model and the other two magnetic-field-attenuation-related parameters. It usually takes a lot of time to match the parameters manually according to the measured hysteresis loop and it is difficult to ensure that the final parameters are the global optimal values. The machine learning method based on artificial neural network has been used as an efficient method to optimize the parameters of complex systems. Owing to the powerful computing capability of modern computers, using the artificial neural network to optimize parameters is usually much faster than manual optimization method, and has a greater probability of finding the global optimal parameters. In this paper, the five J-A parameters and the other two parameters relating to magnetic field attenuation are optimized by the method of online learning based on artificial neural network, and the residual magnetic field in the magnetic shield is predicted under the simulated satellite magnetic field environment. By comparing the measured residual magnetic field with the predicted value, it is found that the machine learning method can optimize the magnetic shielding characteristic parameters more quickly and accurately than the manual optimization method. This result can not only help us to compensate for the magnetic field better and optimize the parameters of our cold atom system, but also validate the application of neural network in a multi-parameter physical system. This proves that the in-depth learning neural network can be conveniently applied to other physical experiments with multi-parameter interaction, and can quickly determine the optimal parameters needed in the experiment. This application is especially effective for remote experiments with slow response to parameter adjustment, such as scientific experiments carried out on satellites or deep space.
High temperature sample environment upgrade of ISIS engineering materials in-situ diffraction experiment
The ISIS Neutron Facility of Rutherford Appleton Laboratory (RAL) in the UK plays an important and world leading role in in-situ engineering materials testing, one of the most typical neutron diffractometers known as Engin-X, used to measure residual stress and phase transformation and to do micromechanics research, through using different sample environment equipment, such as mechanical fatigue loading frame, cryogenic temperature furnace of cooling the sample down to 1.5 K and particularly high temperature furnace of heating the sample up to 1100℃ under loading condition. The present maximum heating capability of the Engin-X high temperature furnace at ISIS can be increased to above 1100℃, that would allow more extremely challenging high temperature engineering problems around the world to be investigated. With this ambition in mind, in this paper we use TracePro software initially to optimize the geometry of the present Engin-X furnace reflectors and their configurations' arrangement. One is to use ellipse-sphere combination and the other is to use ellipse-sphere-ellipse combination to replace the present Engin-X high temperature furnace's half ellipse reflector geometry. The results show that the former plus further reflector surface coating and reasonable side shielding arrangement result in a total increase of 109% of energy absorption by the sample. The latter makes a further 6% of increase of energy absorption by the sample. Such results are further checked by subsequent ANSYS thermal analysis to investigate the temperature distributions within the centre portion of the sample. The ANSYS simulation results further reveal that both the ellipse-sphere and ellipse-sphere-ellipse configurations are able to increase the maximum capability of the Engin-X high temperature furnace at ISIS from the present 1100℃ to 1399℃ and 1423℃, respectively. In this paper, we present the details of the simulations and all the configurations of the Engin-X high temperature furnace.
Molecular dynamics simulation of void nucleation, growth and closure of nano-Cu/Al films under cyclic loading
In this paper, molecular dynamics method is used to simulate the evolution mechanism of void nucleation, growth and closure of diffusion-welded copper/aluminum bilayer film under cyclic loading condition with a strain-to-width ratio of R=-1. It is found that under cyclic loading condition, the voids mainly nucleate inside the aluminum side of the copper/aluminum bilayer film, and two kinds of evolution modes of voids I and Ⅱ are found. The void I nucleates at the position of the gap defect produced by the Kirkendall effect when the copper-aluminum diffuses to form the bilayer film. Under this nucleation mode, after the gap defects have become void, the void moves into the area where copper atoms are relatively dense inside the OTHER structure on the aluminum side. When gaps accumulate to form voids, the voids grow at a fixed position. The void Ⅱ on the aluminum side nucleates at the position of the gap defect formed by overcoming the stair-rod dislocation and then remains motionless in the process of nucleation, growth and closure. Comparing with the void I, the stress corresponding to the nucleation of void Ⅱ is large, the growth speed of the void Ⅱ is fast and the size of the void Ⅱ is slightly large in the process of strain loading. The void Ⅱ closure speed is also faster in the strain unloading stage. The two kinds of voids have two common characteristics in the process of nucleation, growth and closure. 1) Both kinds of voids nucleate at the position of the gap defect inside OTHER structure on the aluminum side. 2) In the process of voids growth and closure, both kinds of voids have the same shape changes. In the void growth stage, both kinds of voids first grow along the strain loading direction, then expand in the direction perpendicular to the strain loading direction, and finally, the shapes of two kinds of voids tend to become spherical. In the stage of void closure, the two kinds of voids are first compressed into ellipsoidal shape along the strain loading direction, and then disappear from both ends of the void to the center of the void in the direction perpendicular to the strain loading direction. In the subsequent cyclic loading process, none of new voids appears again at the position where the voids disappearred, but the nucleation of voids at other position of gap defect forms inside the other structure located on the aluminum side.
Exact solution to a kind of multi-level Rosen-Zener models
Exact solution to the driven quantum system with an explicitly time-dependent Hamiltonian is not only an issue of fundamental importance to quantum mechanics itself, but also a ubiquitous problem in the design for quantum control. In particular, the nonadiabatic transition induced by the time-dependent external field is often involved in order to target the quantum state for the atomic and molecular systems. In this paper we investigate the exact dynamics and the associated nonadiabatic transition in a typical driven model, the Rosen-Zener model and its multi-level extension, by virtue of the algebraic dynamical method. Previously, this kind of driven models, especially of the two-level case, were solved by converting the corresponding Schrödinger equation to a hypergeometric equation. The property of the dynamical transition of the system was then achieved by the asymptotic behavior of the yielded hypergeometric function. A critical drawback related to such methods is that they are very hard to be developed so as to treat the multi-level extension of the driven model. Differing from the above mentioned method, we demonstrate that the particular kind of the Rosen-Zener model introduced here could be solved analytically via a canonical transformation or a gauge transformation approach. In comparison, we show that the present method at least has two aspects of advantages. Firstly, the method enables one to describe the evolution of the wavefunction of the system analytically over any time interval of the pulse duration. Moreover, we show that the method could be exploited to deal with the multi-level extensions of the model. The explicit expression of the dynamical basis states, including the three-level system and the four-level system, is presented and the transition probabilities induced by the nonadiabatic evolution among different levels are then characterized for the model during the time evolution. In addition, our study reveals further that the dual model of the driven system can be constructed. Since the dynamical invariant of a solvable system can always be obtained within the framework of the algebraic dynamical method, the general connection between the dual model and the original one, including the solvability and their dynamical invariants, are established and characterized distinctly.
Initial configuration design of off-axis reflective optical system based on vector aberration theory
The traditional method of designing the initial configuration of off-axis reflective optical system is to first obtain the initial configuration of coaxial reflective optical system, and then achieve the unobscured design with an offset aperture stop or a biased input field, or both. Because the aberration distribution of coaxial reflective optical system is not applicable to the off-axis reflective optical system, the obtained unobscured off-axis reflective optical system has large aberration, and the unobscured design process is complicated. In this paper we present a method of designing an initial configuration of off-axis reflective optical system based on vector aberration theory. With this design method, a good unobscured initial configuration of off-axis reflective optical system can be directly obtained by using an offset aperture stop or a biased input field, or both. Based on the vector aberration theory and gaussian brackets, the third-order aberration coefficient is derived for off-axis reflective optical system. Initial configuration performance is important for optical design, especially for the complicated optical system design. The selection of initial configuration highly affects the final system imaging performance, fabrication difficulty and alignment difficulty. An error function is established to evaluate the performance of off-axis reflective optical system, and it consists of aberration coefficients and other constraints. The genetic algorithm is a highly parallel, random and adaptive global optimization algorithm. To obtain a good initial configuration for the off-axis reflective optical system, the genetic algorithm is used to search for the initial configuration with minimum residual aberration. This method can obtain a good initial configuration of off-axis reflective optical system for further optimization. The benefit of this design method is demonstrated by designing an off-axis three-mirror optical system. For the focal plane array, a long-wave infrared off-axis three-mirror optical system is designed. A good initial configuration is obtained with the proposed method, which achieves the unobscured design by using an offset aperture stop and a biased input field. To improve the performance of initial configuration, the obtained initial configuration is optimized with the optical design software. The designed optical system has good imaging quality. As the mirrors are free from the tilts and decenters, the designed optical system is aligned easily.
Single-axial-mode Nd:YAG laser with precisely controllable laser pulse output time
In recent years, high-energy single-axial-mode Q-switched lasers have been widely studied and applied because of their wide applications such as in nonlinear optics, laser spectroscopy and light detection and ranging (LIDAR). Many applications require a Q-switched pulse that has not only single axial mode but also can be synchronized with an external system. But two most commonly used methods (the build-up time reducing technique and ramp fire technique) are difficult to achieve single-axial mode operation. In this work, we apply the ramp-hold-fire technique to an injection-seeded Nd:YAG laser. The slave oscillator is a self-filtering unstable resonator (SFUR). The SFUR oscillator can achieve a smooth spatial profile TEM00 transverse mode. An RTP electro-optical crystal is adopted for intracavity phase modulator to modify the effective optical path length of the slave oscillator cavity. The seed-injection locking is realized by the ramp-hold-fire technique. The laser driver generates a pumping pulse. After a suitable time delay the driver is fired, a linear ramp voltage is applied to the RTP crystal. A photodiode detector monitors the interference signal. As soon as the interference peak is detected, the controlling electronics produces a stop signal. The ramp voltage on the RTP crystal is stopped and held at a fixed value. Then the Q-switch is fired at a set time, and finally single axial mode laser is demonstrated. Combining the advantages of intracavity phase modulation and Q-switch exact synchronization of the ramp hold fire technique, we obtain a narrow linewidth single-axial-mode laser pulse with precisely controllable output time. The laser is capable of generating 1064 nm pulse energy large than 50 mJ. The pulse build-up time is reduced by 31 ns to 48 ns. The pulse firing time is precisely controlled with jitter less than 1%. Then the frequency spectrum of the 1064 nm laser is measured with a commercial Fizeau wavemeter HighFinesse WS7. The multi-beam interference patterns of the pulse are shown to be smooth in the wavemeter. The wavelength is measured to be 1064.40416 nm and the linewidth is less than 0.5 pm which is limited by the instrument resolution. Meanwhile, the frequency stability is measured to be less than 0.1 pm (V-V) over 1700 pulses with a working frequency of 0.1 Hz.
Development and application of MoS2 and its metal composite surface enhanced Raman scattering substrates
Surface-enhanced Raman scattering is an ultra-sensitive molecular detection technology, and the exploration of its mechanism and the improvement of sensitivity, uniformity and stability have always been significant challenge to researchers. In this paper, the development of surface-enhanced Raman scattering mechanism and its research progress, and thus review the mechanism, research status and existing problems of single metal substrate, molybdenum disulfide substrate and metal/molybdenum disulfide composite substrate are summarized; The preparation method of the molybdenum disulfide substrate including hydrothermal/solvothermal method, micromechanical peeling method, chemical meteorological deposition method, and preparation method of metal/molybdenum disulfide composite substrate are briefly introduced, in which the electrochemical method, thermal reduction method, seed-mediated growth method, and electron beam lithography method are covered, and the advantages and disadvantages of the above preparation methods are evaluated; The research progress of the applications of molybdenum disulfide and its metal composite substrates in food testing, biomedicine, environmental pollution monitoring, etc. are briefly overviewed The surface-enhanced Raman scattering study is extended to other transition metal binary compounds and their metal composite structures. Therefore, the metal/molybdenum disulfide composite substrate expands the types of surface-enhanced Raman scattering substrates, thereby making up for the deficiency of low reproducibility, poor stability, and weak adsorption. Moreover, it has the advantages of fluorescence quenching effect, high sensitivity, wide detection range, and it can be combined with on-site rapid separation technology, and thus has widespread application prospects. Finally, the shortcomings of surface-enhanced Raman scattering technology and prospects for its development are also pointed out.
Microwave photonic frequency up-converter with LO doubling based on carrier suppression single-sideband modulation
Frequency up-converter as an essential component of the transmitter, which is used to implement the frequency up-conversion by mixing a low-frequency intermediate frequency (IF) signal with a local oscillator (LO) signal. However, only the 1st-order sideband of the LO signal and the IF signal are used in the tradtioanal microwave photonic up-converser, thus the frequency of the up-conversion signal is ωLO + ωIF. In this case, an LO with a higher frequency is needed for generating a high-frequency up-converted signal. In order to reduce the frequency requirement of the LO signal, the high-order LO singals or secondary modulation can be used to achieve high-frequency up-conversion. A microwave photonic up-converter with LO doubling based on carrier suppressing single-sideband modulation is proposed based on the cascaded structure of a Mach-Zehnder modulator (MZM) and a dual-parallel Mach-Zehnder modulator (DPMZM). The MZM is driven by an LO signal biased at the minimum transmission point for carrier suppressing double-sideband (CS-DSB) modulation. A fiber Bragg grating (FBG) is used to separate the +1st-order from -1st-order of the LO signal. The -1st-order of LO signal is then sent to a DPMZM for the secondary modulation, and the carrier suppressing single-sideband (CS-SSB) modulation is realized in order to generate the -1st-order of the IF signal by using an electrical 90° hybrid coupler. The modulated IF signal is then combined with the +1st-order LO signal reflected by the FBG and sent into the photodetector (PD) to implement the photoelectric detection. The upconverted signal with a frequency of 2ωLO + ωIF can be detected by a PD. The experimental results show that the spur suppression ratio of the optical spectrum and the up-converter signal reach 22.5 dB and 23.6 dB, respectively. The spurious-free dynamic range of the system is 96.1 dB·Hz2/3. The proposed system can effectively reduce the frequency requirement of LO signal, and the purity of the electrical spectrum is largely improved which benefits from the CS-SSB modulation. The proposed microwave photonic up-converter provides an effective way for high-frequency emissions in systems such as radio-over-fiber and optically controlled phased array radar.
Generation of turbulence phase screen based on gravitational search algorithm
The new techniques in adaptive optics, free space optical(FSO) communication rely on the use of numerical simulations for atmospheric turbulence to evaluate the performance of the system. The simulation of turbulence phase screen is the heart of numerical simulations which produces random wavefront phase perturbations with the correct statistical properties corresponding to models of optical propagation through atmospheric turbulence. The phase-screen simulation techniques can be roughly divided into fast Fourier transform (FFT) method and matrix-based method. Because of a better performance in computation time, the FFT method is generally used for modeling the performance of a real system. But the classical FFT method has a main deficiency of oversample in low frequency region, which leads to the lost of accuracy. To overcome this deficiency, many methods have been proposed for compensating for the oversample of low frequency components, in the last decades. Essentially, these methods achieve a higher accuracy at the expense of computation time. A good compensation method should take into consideration both accuracy and computation time.
To achieve higher accurcy and lower computational cost simultaneously, we develop a hybrid method to generate turbulence phase screen, i.e. the classical FFT model is mixed with the sparse spectrum model. We first extract the low frequency region from the frequency grid of FFT model, and resample this region with 16 samples. It is found that the accuracy of phase screen is related to the distribution of these samples, and there must be an optimum distribution that can minimize the relative error between expected structure function and theoretical structure function in the low frequency region. So it permits one to use optimization algorithm to find the optimized distribution of low frequency samples. Here an improved gravity search algorithm is adopted in which the memory of each particle is taken into consideration. The optimization parameters are determined after a lot of tests, and the robustness testing shows that the algorithm is effective. To compare with existing subharmonic method, we choose the same parameters of phase screen as those used in the expanded subharmonic method, generate 1000 phase screens for each method, compute the phase structure function, and we also compare our results with those from the theoretical structure function. The comparison result shows that the curve of phase structure function generated by our method is nearly consistent with the theoretical one, the maximum relative error in low frequency region is about 0.063% which is much better than that from the expanded subharmonic method 5%. Finally in this paper, the computational cost is analyzed, showing that the generation speed for our method is at least 4.5 times as fast as that for the Johansson's method.
Experimental study of supercontinuum generation in Yb3+-doped microstructure fiber pumped by femtosecond pulses
The nonlinear effects and supercontinuum generation by the concept of wavelength conversion and amplification are experimentally studied in two Yb3+-doped microstructure fibers (Yb3+-MSFs), with the Ti:sapphire femtosecond pulses used as pump. Firstly, two Yb3+-MSFs are pumped by continuous wave separately to obtain the emission spectrum. The relationship between the luminous efficiency and the deviation of pump light from the Yb3+ absorption peak is studied for each of the two fibers. The experimental results indicate that the luminous efficiency decreases as the deviation increases. However, both fibers still have high luminous efficiency even when the deviation reaches to 85 nm. Secondly, the supercontinuum spectrum is generated by the femtosecond laser pumping the cores of the two fibers. The influence of the pump power, relative position between emission light and zero-dispersion wavelength λ0, pump wavelength and fiber length on the supercontinuum generation are studied. The results demonstrate that the amplified emission light at 1035 nm is first captured by the pump light to evolve into ultrashort pulse, and nonlinear effects are subsequently generated. As the pump power increases, for Yb3+-MSF1 whose λ0 is located near the emission light of Yb3+ irons, the fundamental soliton is generated and further shifts toward red region under Raman effect. Compared with Yb3+-MSF1, the Yb3+-MSF2 has a small core, which means that its λ0 is short and the emission light is located in its anomalous dispersion region far from the λ0. Experimental results reveal that higher-order soliton and soliton fission are more likely to happen and supercontinuum spectrum can be formed. However, the further broadening of the supercontinuum spectrum is limited by OH-absorption at 1380 nm. Either increasing the deviation of pump light from the Yb3+ absorption peak or shortening the fiber length reduces the accumulated power of the emission light, so the experimental results show that red-shift of Raman soliton is reduced and the supercontinuum spectrum is narrowed for both fibers. The supercontinuum generation efficiency in the output spectrum can reach 98% when the effect of pump light coupling efficiency and microstructure fiber loss are neglected. It means that almost all the residual pump light and emission light of Yb3+ contribute to the generation of supercontinuum. Finally, the Yb3+-MSF2s are tapered to different taper lengths to study their influence on supercontinuum generation. The results indicate that the leakage after tapering weakens the energy of the Raman soliton, which further results in the decrease of red-shift. Eventually, the red edge of supercontinuum spectrum shrinks seriously with theincrease of the taper length. However, the decreasing of λ0 at the taper waist leads to blue-shift of dispersive wave that satisfies the phase matching condition with Raman soliton. This contributes to the blue-shift of the short wavelength boundary and widens the range of supercontinuum spectrum at short wavelength. Therefore, tapering is a promising method of expanding supercontinuum spectrum towards short wavelength. In conclusion, the supercontinuum spectrum is generated in Yb3+-doped microstructure fiber pumped by the Ti:sapphire femtosecond laser. The output spectrum can be adjusted flexibly by combining the merit of high peak power and wavelength tunability of Ti:sapphire femtosecond laser and the characteristics of wavelength conversion and amplification of Yb3+ irons. Thus, the method presented in the paper provides a promising way to obtain tunable supercontinuum spectrum.
Coupled resonance of bubbles in spherical cavitation clouds
The interaction between bubbles in bubble group mainly acts on the other bubble through radiation sound pressure between the bubbles. In this paper, based on the bubble dynamics equation in bubble clouds, the equation of bubble wall motion is linearly reduced, the expression of bubble resonance frequency in spherical bubble group is obtained and the correction coefficient of bubble resonance frequency and single bubble are given. Furthermore, the effects of the initial radius, the number of bubbles and the distance between bubbles on the resonance frequency are discussed. The results show that the phase of bubbles is taken into account. Considering the interaction between bubbles, the resonance frequency of bubbles in spherical bubble group is obviously less than that of single bubble. With the decrease of the number of bubbles in bubble group, the distance between bubbles increases, the interaction between bubbles in bubble group decreases, and the resonance frequency of bubbles returns to the resonance frequency of Minnaert single bubble. At the same time, the resonance frequency of bubbles in bubble group changes gradient with the increase of the distance between bubbles and the number of bubbles. However, when the number of bubbles increases a certain value, the resonant frequency of the bubble is almost constant. When the bubble group has a certain radius, the more the number of bubbles, the smaller the resonance frequency of the bubble is, but there exists a critical value. It is also found that a smaller correction coefficient is held by the bubble group with larger initial radius, which indicates the same number of bubble groups. Under the same bubble spacing, the interaction of small bubbles with smaller bubbles is more significant, and the resonance frequency of the bubble is obviously affected. Because the frequency and amplitude of driving sound pressure can only be given values in ultrasonic cavitation, the resonant frequency of cavitation bubbles will be reduced by properly injecting air bubbles into liquid, which makes most of cavitation bubbles undergo intense non-linear oscillating steady-state cavitation. Therefore, the occurrence of cavitation can be effectively suppressed.
Sound insulation performance of thin-film acoustic metamaterials based on piezoelectric materials
Aiming at the isolation of low-frequency sound, a kind of thin-film acoustic metamaterialis designed and obtained by implanting PZT into thin film. The finite element method (FEM) of the structure is built, and 1st-14th order eigenfrequencies and transmission loss between 20-1200 Hz are calculated. The reliability of finite element calculation is verified experimentally and the existence of adjustable sound insulation peak is monitored in the experiment. The results show that the acoustic metamaterial has good sound insulation performance in a frequency range between 20 and 1200 Hz, and has two sound insulation peaks of more than 50 dB, and there is a sound insulation peak which can be changed by adjusting the parameters of the outer circuit. By analyzing the first resonance mode of simple structure and building its equivalent model, the effect of structural parameter on the sound insulation performance of thin film acoustic metamaterial is investigated theoretically, and the rationality of the equivalent model is verified by the finite element calculation. The sound insulation mechanism of the structure is further illustrated by taking into consideration the eigenfrequencies, transmission loss curve and vibration mode diagrams at various frequencies. It is found that at the resonance frequency, the flapping motion of the film will cause the sound wave in the subsequent propagation to cancell the interference, therefore realizing the attenuation of the sound wave. Based on Fano resonance theory, the reasons for the different characteristics of transmission loss curves at different resonance points are investigated. The PZT and outer circuit can form a LC oscillator. At the resonant frequency of the oscillator, the vibration of the piezoelectric material can absorb the energy of sound wave to cause a sound insolation peak. The resonant frequency of the circuit can be adjusted by changing the parameters of the outer circuit, thereby realizing the adjustability of the sound insulation performance. The influence of eccentricity of piezoelectric mass block on sound insulation performance of material is explored, proving that the sound insulation performance can be further optimized by improving structure. And through the finite element calculation, it is proved that the sound insulation performance of material is adjustable by changing the parameters of the outer circuit. The results provide a theoretical reference for designing the thin film acoustic metamaterials.
Bottom reverberation for large receiving depth in deep water
Ocean reverberation is an important issue in underwater acoustics, which usually influences the working performance of the active sonars significantly. The deep-water reverberation data are collected from the South China Sea experiment including the reverberation signals at large receiving depths near the bottom, showing that the wave intensity increases obviously at some moments with time increasing. To analyze in depth the data, a uniform bottom-reverberation model is proposed based on the ray theory, which can calculate monostatic and bistatic reverberation intensity and explain the generation process of deep-water reverberation. The mesh method is first used in this model by dividing bottom scatterers into a number of grids. Then reverberation is calculated based on the exact time of generating the scattering signal from each grid. Due to the exact arrival time, the presented model can provide more accurate result than classical models, in which scatterers are usually treated as circular rings or elliptical rings. Numerical results are compared with experimental reverberations at different receiving distances and depths. The simulated and experimental results agree well overall for large receiving depths, whereas agreement extent decreases for the case of receiving depth close to the sea surface. The analytical results indicate that the applied scattering coefficient is suitable for this experimental sea area, and meanwhile verify that this scattering model is more accurate for low-angle bottom backscatters corresponding to the reverberation at large receiving depths.
Identification of interference normal mode pairs of low frequency sound in shallow water
The interference characteristics of normal modes in low-frequency broadband sound can be applied to source localization and environmental parameter inversion in shallow water. However, the identification ambiguity of interference normal mode pairs generally occurs in practical applications due to unknown source position, some weakly-excited normal modes, mismatched environmental model, etc. For the applications of a horizontal line array, a model-based processing approach is proposed to determine the orders of the interference normal mode pairs based on the intrinsic dispersion characteristics of interference normal mode pairs in the received signals and the range-independent properties of the array beam output angles. Firstly, the normal mode pair filtering is achieved by using the WARPING transform of the signal autocorrelation function in the element domain of the horizontal line array. Then, the arrival angles of the filtered interference normal mode pairs are estimated by using array beamforming. Finally, the estimated beam output angles are matched with the replica values computed by sound field model. The approach is verified by using the explosive pulse signals received by the seafloor-deployed 32-element horizontal line array at the North Yellow Sea in 2011. Furthermore, some simulations are involved to analyze the effects of environmental parameter mismatches including water sound speed profile, sea bottom parameters and water depth on the identification performance of interference normal mode pairs. The results show that the water depth is a major factor influencing the extracted values of the beam output angles of interference normal mode pairs. The approach might fail when the water depth mismatch exceeds 14% of the practical value. However, the effects of water sound speed profile mismatch and sea bottom parameters mismatch are negligible. The effect of signal-to-noise ratio in the element domain on a horizontal line array is also simulated in order to analyze the limitation of identification performance, which shows that the required signal-to-noise ratio in the element domain should be more than 2 dB.
Vertical correlations of sound field at large depths in deep water
The research on the vertical correlation characteristics of sound field in deep water has important implications for enhancing the vertical array gain and improving the ability to detect the underwater target. Based on a deep-water experiment conducted in the South China Sea, the vertical coherence of sound fields in the direct zone, shadow zone and convergence zone are analyzed with the sound signals received by a vertical line array that covers the maximal depth to 1,866 m. The numerical analysis based on the ray theory is carried out to provide corresponding theoretical explanations to the variations of the vertical correlation characteristics at different ranges and depths. The vertical correlation coefficients in the direct zone are higher than 0.707 for the whole depth and drop very little with the increase of the vertical depth. It is because the main contributions come from direct arrival ray and sea surface reflection ray. The pulse structure is relatively simple, and the time delays of the two rays increase with the space between two receivers increasing. In the shadow zone, sound energy mainly comes from bottom reflection. Therefore, the vertical correlation coefficients are relatively low. Multi-path arrival is observed obviously. Vertical correlation coefficients drop quickly with depth increasing. With range increasing, the time delays of the multi-path pulses decrease. The vertical correlation coefficients at the same depth will increase a little with range increasing. Near the first convergence zone, vertical correlations oscillate periodically with the increase of vertical separation, and share the same distribution pattern with the sound energy along the vertical direction, which is caused by the periodical oscillation of two groups of the refracted rays from water volume. The refracted rays have the same amplitude, therefore, the time delays of the two group of rays increase with receiver depth increasing, and the phase of sound filed fluctuates in $The research on the vertical correlation characteristics of sound field in deep water has important implications for enhancing the vertical array gain and improving the ability to detect the underwater target. Based on a deep-water experiment conducted in the South China Sea, the vertical coherence of sound fields in the direct zone, shadow zone and convergence zone are analyzed with the sound signals received by a vertical line array that covers the maximal depth to 1,866 m. The numerical analysis based on the ray theory is carried out to provide corresponding theoretical explanations to the variations of the vertical correlation characteristics at different ranges and depths. The vertical correlation coefficients in the direct zone are higher than 0.707 for the whole depth and drop very little with the increase of the vertical depth. It is because the main contributions come from direct arrival ray and sea surface reflection ray. The pulse structure is relatively simple, and the time delays of the two rays increase with the space between two receivers increasing. In the shadow zone, sound energy mainly comes from bottom reflection. Therefore, the vertical correlation coefficients are relatively low. Multi-path arrival is observed obviously. Vertical correlation coefficients drop quickly with depth increasing. With range increasing, the time delays of the multi-path pulses decrease. The vertical correlation coefficients at the same depth will increase a little with range increasing. Near the first convergence zone, vertical correlations oscillate periodically with the increase of vertical separation, and share the same distribution pattern with the sound energy along the vertical direction, which is caused by the periodical oscillation of two groups of the refracted rays from water volume. The refracted rays have the same amplitude, therefore, the time delays of the two group of rays increase with receiver depth increasing, and the phase of sound filed fluctuates in[0, 2π] periodically. The periodicity causes the sound intensity and the vertical correlation coefficients to have the same oscillation structures. If the rays have the same phases, the main contribution comes from refraction rays, the structure of the pulses is relatively simple and causes vertical correlation to be higher. Otherwise, the main contribution comes from bottom reflected rays, the structure of the pulses is complex, and vertical correlation drops down.
Analysis of the granular pressure and velocity field of hourglass flow based on the local constitutive law
Granular medium is ubiquitous in nature, and is an important issue in many infrastructural construction projects. In particular, the gravity discharge of fine particles from a silo constitutes an important problem of research, because of its many industrial applications. However, the physical mechanism of this system remains unclear. In this work, we study the discharge of silo from the bottom or lateral orifice, by performing pseudo-three-dimensional (3D) continuum simulations based on the local constitutive theory. The simulation is two-dimensional (2D), in order to study the 3D silo, we add the lateral frictional force in the averaged momentum equation. For a rectangular silo with an orifice of height D and the silo thickness W, we study the influence of the orifice size (W and D) on the granular pressure and velocity. The force analysis and simulation results reveal that for the relation between the granular pressure and the orifice size, there exist two regimes:when D/W is small enough, the pressure near the orifice varies only with D; when D/W is large enough, the pressure varies only with W. These scaling laws are the same for both bottom and lateral orifice. Somewhat surprisingly, the simulation results also show that when the orifice is at the bottom, the scaling law of the vertical velocity is different from that of the pressure; when it is on the lateral side, the scaling law of the horizontal velocity is consistent with that of the pressure. This observation contradicts a hypothesis that the flow rate of discharge is controlled by the granular pressure near the orifice, and validates the recent experimental results reported in the literature. Furthermore, the relationship between the vertical velocity and the orifice size reveals that when the orifice is at the bottom, the critical value of D/W for the transition of regime is much larger than the lateral orifice case, the flow rate will depend only on W when D/W>>50. This condition is hardly satisfied in practice, so the new scaling law has not yet been observed for the bottom orifice case in the literature. Furthermore, this work demonstrates that the stagnant zone has an important effect on the discharge of silo, especially for the lateral orifice case. Since a non-local constitutive law can well describe the quasi-static flow, it will be interesting to modify the local constitutive model into a non-local constitutive model, and to compare the results from the two models.
The dynamics of self-propelled Janus microspheres near obstacles with different geometries
Self-propelled particles exhibit interesting behavior when approaching boundaries or obstacles, which has been drawn a lot of attention due to its potential applications in areas of cargo delivery, sensing and environmental remediation. However, our understanding on the mechanism of how they interact with boundaries or obstacles is still limited. Here, using video particle-tracking microscopy, we experimentally studied the dynamics of self-propelled Janus microsphere driven by H2O2 near obstacles. The Janus particles used are sulfuric polystyrene (PS) microspheres (hydrodynamic diameter is 3.2 μm) with only half surface being sputter-coated with a five-micron-thick platinum layer. Two different types of obstacles are used. One is cylindrical post and the other is PS microsphere. To understand the size effect of obstacles, cylindrical posts with three different diameters (3 μm, 10 μm and 20 μm), and PS microspheres with four different diameters (1.0 μm, 1.8 μm, 2.4 μm and 7.2 μm) are tested, respectively. The results show that when obstacles are larger than a critical size, the self-propelled Janus microspheres will be captured and orbit around them. The retention time and the orbiting speed of the Janus particles increase with the concentration of H2O2, as well as with the diameter of obstacles no matter whether cylindrical posts or PS microspheres are used as obstacles. However, we found that under the same concentration of H2O2, compared with the case of PS microspheres as obstacles, when Janus particles orbit around cylindrical posts, the retention time is larger and the average speed is smaller. These results indicate that the self-propelled behavior of Janus particles near obstacles is closely dependent on the geometrical properties of obstacles. Our results of Janus spheres are different from earlier work on Au-Pt Janus rods[Takagi D, Palacci J, Braunschweig A B, Shelley M J, Zhang J 2014 Soft Matter 10 1784]. By comparing the speed of Janus particles before and after they are captured by spherical obstacles, for our case, the speed of Janus spheres is reduced, while for the case of Au-Pt rods, the speed of Au-Pt rods doesn't change much. Such discrepancies may originate from different driven mechanisms in these two systems (electropheoresis mechanism for Au-Pt micro-rods and diffusiophoresis mechanism for PS-Pt Janus microspheres), which are then resulted in different flow fields and different distributions of catalytic solutions. But to test this hypothesis, further work is needed. Our study provides us a better understanding on the dynamic behavior of self-propelled particles near obstacles, which will be helpful for applications in, for example, designing micro-structures to guide the motion of self-propelled particles.
Effective quintessence from string landscape
Recently, Vafa et al. proposed two string swampland criteria, and studying the constraints imposed by the two string swampland criteria on cosmology, they found that the inflationary models are generally difficult to be compatible with these two criteria. Applying these two criteria to the accelerated expansion of the universe during the current period, it was found that the specific quintessence model can satisfy these constraints while satisfying the constraints imposed by the current observations. Applying the gravitational theory of large scale Lorentz violation to cosmology, the vacuum energy density is not the only cause of the accelerated expansion of the universe. The large scale Lorentz violation combined with the cosmological constant term results in the observed accelerated expansion of the late universe. The vacuum energy density is a bit like a naked cosmological constant. The equivalent energy density considering the large scale Lorentz violation effect is the effective cosmological constant that determines the evolution of the universe. In this way, we find that the negative cosmological constant in the string landscape can also accelerate the expansion of the universe, and compared with the Λeff model, it leads to a cosmological constant as an effective vacuum energy density. Effective vacuum energy density behaves as a monotonically decreasing quintessence potential energy for the string landscape, for most of the naked positive vacuum energy densities in the swampland, the evolution of effective cosmological constant with time will show a local minimum. Comparing the calculated results of the distance modulus withthe astronomical observations, we can obtain that a negative cosmological constant also accelerates the expansion of the universe. Thus, the vacuum energy density derived from the string landscape will give quintessence potential that satisfies the swampland criterion, while the evolution of vacuum energy density given by the swampland model of the metastable dS vacuum is not quintessence potential, so it cannot satisfy the second de Sitter criterion. Therefore, the effective potential leading to the accelerated expansion of the late universe can only come from the string landscape, which is naturally UV completion. Therefore, it gives that the accelerated expansion of the late universe is the feature of early quantum gravity. It is not necessary to use the metastable de Sitter vacuum to explain the accelerated expansion of the late universe. The difficulty of incompatibility between the swampland model and the accelerated expansion of the late universe caused by the swampland conjecture will be eliminated.
Statistical analysis of shock properties driven by coronal mass ejections based on observations of type Ⅱ radio bursts
In this paper, we investigate 77 type-Ⅱ radio burst events' data observed by Wind/WAVES and STEREO/SWAVES from January 2007 to December 2015. By fitting the frequency-time profile to obtain the corresponding shock velocity, we study the relationship between the parameters of shock and those of coronal mass ejection (CME), solar flare and the associated SEP events, and explore the influences of type Ⅱ radio enhancement on these relationships. Our findings are as follows. 1) In general, at the onset time of type Ⅱ radio bursts within deca-hectometric (DH) waveband, the shock front is about 0.4Rs ahead of the leading edge of CME (shock standoff distance), and this distance increases as the CME propagates outward. In the low and high corona, the relationship between shock standoff distance and CME speed indicates a significant difference; the shock standoff distance is correlated with the CME speed positively at the low altitude, but negatively at high altitude. 2) The CME speed of the events with radio enhancement is significantly larger than that with no radio enhancement; and comparing with the events with no radio enhancement, the correlation coefficient between the shock speed and the mass and kinetic energy of the associated CME is significantly high for the events with radio enhancement. 3) There is no correlation between the duration of type Ⅱ radio burst in DH waveband with enhancement and the speed, mass and kinetic energy of CME. However, it presents a positive correlation for the events with no radio enhancement. 4) Usually the speed of shock that can produce SEP event is obviously higher than that with producing no SEP event. The probability of the SEP generated by the events associated with radio enhancement is slightly higher than with no radio enhancement (73.5% > 67.4%), but for the large SEP events, the generation probability (67.6%) associated with radio enhancement is about one-order of magnitude higher than that with no radio enhancement (37.2%). This conclusion indicates that the type Ⅱ radio enhancement can be used as one of the signatures of the shock or the radio source that more probably produces a large SEP event.