First principle study of nonlinear optical crystals
Nonlinear optical (NLO) crystal is one of the important opt-electrical functional materials that can convert laser frequency and obtain wide band tunable coherent sources, thus it possesses crucial strategic and application value in military and civil fields. On the basis of more than 30 years' efforts, the NLO crystals in visible and near infrared region, including β-BaB2O4 LiB3O5 and KTiOPO4, have been basically mature. However, there are still many shortcomings for those NLO crystals used in deep ultraviolet (DUV) and mid/far-infrared (IR) regions, thus putting forward more requirements for high performance crystals. For DUV KBe2BO3F2 (KBBF) crystals, the main shortcomings are the use of toxic BeO raw materials and strong layer growth tendency. Wide transparent region and high second harmonic generation (SHG) effect are also expected in new developed DUV NLO crystals. More importantly, a large enough birefringence is highlighted to satisfy the phase-matchable condition and DUV harmonic generation capacity below 200 nm. On the other hand, the main requirement for mid/far-infrared NLO crystals is to maintain the balance between high laser damage threshold and strong SHG response. Indeed, it is a very difficult task to search for good NLO crystals through the traditional “trial and error” experimental methods. Theoretical studies, especially first principles calculations, can provide an efficient way to investigate and design new NLO materials with superior properties. In this paper, the recent progress of deep-UV and mid-IR NLO crystals is summarized. In addition, the crucial role of first principles calculations in new material exploration and design is highlighted by introducing several typical new NLO crystals, including defect diamond-like compound AgZnPS4, trigonal alkaline metal fluorooxoborate KB4O6F and alkaline earth fluorooxoborate SrB5O7F3. Moreover, some advanced analysis tools are introduced, such as real space atomic cutting method, SHG-weighted mapping, flexible dipole moment model, and non-bonding atomic orbitals analysis, and used to investigate the structure-property relationship in langasite La3SnGa5O14, metal cyanurate Ca3(C3N3O3)2, vanadium-carbonate K3[V(O2)2O]CO3, etc. Further, the flow chart of high-throughput first principles calculations of NLO crystal is proposed. According to the known or predicted crystal structure, we can obtain the chemical stability, band gap, NLO coefficient, birefringence and phase-matchable capacity quickly, thus easily judging the research potential of a new NLO material. On the basis of these ideas, a great blueprint for NLO crystal “material genome engineering” is highly put forward. Finally, the difficulties in research and challenges in NLO material investigations are discussed, and the direction of future research priorities based on first principles calculations are pointed out.
First-principle calculation on electronic structures and optical properties of hybrid graphene and BiOI nanosheets
Photocatalytic technology has wide potential applications in the fields of energy generation and pollutant purification due to its advantages of low cost and environmental friendliness. Besides traditional photocatalysts of TiO2 and ZnO, the developing of new photocatalyst with novel properties of strong oxidation, reduction ability, and visible light response has received more attention. Bismuth compounds, such as BiOX (X=Cl, Br, I), exhibit highly efficient photocatalytic activity because of its layered structure and electronic characteristics. The special layered structure, resulting in built-in-field, is favorable for the separation and migration of photogenerated electrons and holes. Among BiOX compounds, BiOI has the best optical absorption characteristics in the range of visible light, and also has the best photocatalytic activity for the degradation of organic pollutants under visible light irradiation. Graphene is an ideal two-dimensional crystal with zero band gap and a high specific surface area. Many researches have shown that graphene can effectively reduce recombination probability of hole and electron because of its unique electron transport property, and it can improve the photocatalytic activity and light stability of the composite catalytic materials.
In this paper, by constructing BiOI nanosheets and hybrid graphene/BiOI, the nanocomposite photocatalytic materials each with a high specific surface area and good photocatalytic activity are obtained. First-principle calculation based on density functional theory is used to investigate the electronic and optical properties of single/double layer BiOI nanosheets and their nanocomposites with graphene. Three kinds of vacancy defects, such as Bi, O and I in BiOI, are also considered. The calculated results show that the spontaneous charge transfer from graphene to BiOI takes place, forming electron-hole puddle because of the interface interaction between graphene and BiOI. Additionally, the hybrid graphene/BiOI complex displays an enhanced optical absorption behavior in the visible light region, improving its photocatalytic activity. The calculated results about the vacancy defects show that the Bi vacancy enhances the charge transfer between BiOI and graphene and forms more electron-hole puddles. In contrast, O and I defects restrain the charge separation between two layers and reduce the formation of electron-hole puddles.
First principle study of electron and band structure of BixBa1-xTiO3
Some perovskite structured catalysts have narrower forbidden band widths than pure TiO2, and they have been widely used in a number of photo-catalytic reactions. The ions in the perovskite may be replaced by other ions while maintaining the structure unchanged for its tailorable character. Bi–Ti–O can form into the typical perovskite composite oxide BiTiO3 under specific preparation conditions. The regulation of the energy gap of the perovskite BaTiO3 can be realized by substituting Bi for Ba to form the BixBa1-xTiO3 perovskite structure to improve its photo-catalytic activity. But the improvement mechanism and the electron and band structures of BixBa1-xTiO3 are still not very clear.
In this study, we exhibit a detailed theoretical investigation to predict the electronic structure, band gap and optical absorption properties of BixBa1-xTiO3 structures based on the first-principles plane-wave ultrasoft pseudopotential method. The exchange and correlation interactions are modeled using the generalized gradient approximation and the Perdew-Burke-Ernzerhof exchange-correlation functional. The cutoff kinetic energy of the electron wave function is 340 eV, and the k-point sampling sets 3×3×3 division of the reciprocal unit cell based on the Monkhorst-Pack scheme. In the geometrical optimization, all forces on atoms are converged into less than 1×10-5 eV/atom, the maximum ionic displacement is within 0.001 Å and the total stress tensor decreases to the order of 0.05 GPa.
The DFT calculation results reveal that the symmetry and binding energy decline in the BixBa1-xTiO3 structure, and the bond lengths of Ba–O and Ti–O decrease a little after Ba has been substituted by Bi atom, except for the structure of Bi0.5Ba0.5TiO3. The photo-catalysts of BixBa1-xTiO3 are direct band gap semiconductors, and the substitution Bi can regulate the band gaps of BixBa1-xTiO3. The band gaps become wider from x=0.125 to x=0.750 with the carrier concentration decreasing, and then decreases with the higher carrier concentration increasing when x=0.875. It is predicted that the band width of Bi-based perovskite will be much lower than that of Ba-based perovskite. In the case of the density of states we reveal that the top of the valence band is hybrided by O-2p and Bi-6s and the bottom of the conduction band state is mainly constituted by the Ti-3d state. The electron transport properties and carrier types are mainly determined by Ti-3d, O-2p state and Ba-5p electronic states in BaTiO3 and Ti-3d, O-2p, Bi-6s and Bi-6p electronic states in BixBa1-xTiO3 respectively. The absorption spectra indicate that the ultraviolet absorption performance can be improved in BixBa1-xTiO3 system, which may effectively improve the photo-catalytic activity of BaTiO3.
First principle study of weak Dzyaloshinsky-Moriya interaction in Co/BN surface
Based on density functional theory calculations, we elucidate the atomic and electronic structures of Co atom of hexgonal BN (Co/h-BN). The interaction between magnetic moments of Co atoms is realized through Co-N_α-B_β grid, which is indicated by the analysis of spin charge contour plot and partial density of states of each atom, where α and β denote the site of B or N atom close to and away from Co atom, respectively. Then the dispersion relations E(q) and E(-q) (q denotes the direction vector of spin spiral) between energy and wave vector of spin spiral in the opposite directions are calculated with generalized Bloch equations. In the incommensurate spin spiral calculations, all the magnetic moments of Co atom are arranged in the same plane that is perpendicular to the Co/h-BN film. The difference between E(q) and E(-q) is caused by the interface of Co/h-BN, where the symmetry of space perpendicular to the film is broken. Moreover, the effective Heisenberg exchange interaction (HBI) and Dzyaloshinsky-Moriya interaction (DMI) parameters between different neighbors (Ji and di) are derived by well fitting the ab initio magnon dispersion E(q) to HBI with DMI model and E(q)-E(-q) to DMI model, respectively. The J1 has a negative value and plays a major role, J3 is one order of magnitude smaller than J1, and other parameters are close to zero. Hence, Co/h-BN is triangular antiferromagnetic material with the q at k point in the first Brillouin zone. However, the spin spiral with the q at M point is only 2 meV larger than the basic state with the only negative J1 and smaller positive J2. The DMI is not shown in this interface with d1 and d2 close to zero. Based on the non DMI character and its stability in air, h-BN can be capped on other DMI interfaces. The reason that the DMI in Co/h-BN is much smaller than in Co/Gra is much larger height between Co and h-BN. It is 0.192 nm for h-BN but it is 0.156 nm for Co/Gra. We may reduce the height to enhance the DMI by other ways, such as adding electrical and magnetic fields in the future.
Electrically controlled quantum spin Hall in narrow zigzag graphene nanoribbon
Using the tight binding Kane-Mele model including the self-consistent on-site Coulomb interactions (O-CIs), we study the influence of transverse electric field in the narrow zigzag graphene nanoribbon (ZGNR) plane on the edge band structure in order to investigate the way to control the type of quantum spin Hall (QSH) system in the ZGNR. The theoretical results show that when applying weak electric field intensity, the direction of electric field can adjust these two spin-down edge bands moving along the different directions in one-dimensional q space, which leads to the two different types of degenerative breakdown of two pure spin-down edge states at q=0.5. When applying positive electric field the energy of spin-down edge band at edge site 1 is higher than that at edge site 8. On the contrary, when applying negative electric field the energy of spin-down edge band at edge site 8 is higher than that at edge site 1. It shows that we can use the direction of electric field to control the two spin-down edge currents occurring at two different energies. Further, when the electric field intensity increases above 0.69 V/nm, the increased large band gap between the two spin-down edge bands leads to the inversion of these two spin-down edge bands. That is to say, there is a spin-down band gap, however, there is not a band gap for spin-up edge band in the region of spin-down band gap. Thus the system becomes half-metallic, and the QSH does not belong in the type B any longer. Specially, when the electric field intensity reaches 1.17 V/nm in the region of spin-down band gap, the pure spin-up edge state appears at q=0.5, which shows that the strong pure spin-up edge current along the edge site 8 can occur. With increasing the intensity of electric field, the QSH system undergoes three processes from the type B to the type C. When the electric field intensity is more than 1.42 V/nm, the two spin-up edge bands also present band inversion and turn into the conduction band and the valence band, respectively. Thus the system becomes semiconducting and the QSH system does not belong in the type C, ordinary quantum Hall system. Finally, according to the results discussed above, we can expect that using the direction and the intensity of the transverse electric field in plane we can adjust the properties of edge current, and control the type of QSH system varying from the type B to the type C.
Demonstration of four-state memory structure with perpendicular magnetic anisotropy by spin-orbit torque
Current, instead of magnetic field, induced magnetization switching is very important for future spintronics in information storage or/and information processing. As one of the effective current-induced magnetization methods, spin-orbit torque (SOT) has aroused considerable interest because it has low-power consumption and can improve the device endurance. Normal metal (NM)/ferromagnetic metal (FM) are the common materials used for SOTs, where the NM denotes the materials with strong spin-orbit coupling such as Pt, Ta, W, etc. Owing to the spin Hall effect, the in-plane current in NM layer can be converted into a vertical spin current that exerts torques on the adjacent FM layers. Spin current can also come from the NM/FM interface charge-spin conversion due to interfacial asymmetry, exerting torques on the adjacent FM layers. Materials with in-plane and perpendicular magnetic anisotropy are used to study the SOT-induced magnetization switching. Compared with the memories using the in-plane ferromagnetic films, the magnetic memories using NM/FM multilayers with perpendicular magnetic anisotropy can have much high integration density. Currently the used information storage was based on the two-state memory cell. Owing to more than two states contained in one memory cell, multiple states memory manipulated by electric current could further benefit the higher-density memory.
In this paper, a four-state memory unit is demonstrated by the influence of TaOx buffer layer on the magnetic anisotropy of Pt/Co/Pt multilayers. The memory unit consists of two regions. One is directly deposited on thermal oxide Si substrate[Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)] and the other has a buffer layer of TaOx[TaOx(0.3 nm)/Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)], thus leading to the difference in magnetic property between these two regions. According to the Z axis magnetic hysteresis loops of two regions, measured by polar magneto-optical Kerr effect, the coercivity of the region with TaOx is obtained to be 23 Oe and that without TaOx is 11 Oe. At the junction between two regions, the magnetic hysteresis loop shows the superposition of hysteresis loops of two regions, resulting in switching four times as the magnetic field changes. Under a fixed magnetic field along the current direction, the magnetization orientation of region with TaOx and that of region without TaOx are switched by spin-orbit torques with threshold currents of 5 mA and 1.5 mA respectively. The switching direction can be changed as the in-plane magnetic field changes to the opposite direction, which is one of the typical features of SOTs-induced magnetization switching. At the junction between two regions, through applying different-form current pulses to one conductive channel of the device, the magnetic state of the memory cell can be switched between four clear states. This kind of structure provides a new idea to design SOT multi-state memory devices.
Effect of lattice mismatch stress on magnetic domain of epitaxial single crystal (BiTm)3(GaFe)5O12 film
Yttrium iron garnet (YIG) film is a kind of magnetic film and has been investigated extensively because of its excellent magnetic properties and various applications in different fields. Generally, the easy-axis of the film is in-plane and can be changed from in-plane to out-of-plane by introducing some Bi3+ ions into the dodecahedral sites as it has big uniaxial anisotropy, which will be very important in magnetic bubble memories, magneto-optical devices and the new development of spin-wave logic devices. In comparison with many other preparation techniques, the liquid phase epitaxy (LPE) has been consider as a potential method of realizing perpendicular magnetization film due to its big growth-induced anisotropy. However, the LPE technique has more stringent requirements for lattice match between garnet film and gadolinium gallium garnet (GGG) substrate, especially in the growth of thick film. The lattice match is the key factor in LPE growth if the aim of experiment is to achieve a perfect quality and thick film. In most of experiments, there always exists the lattice mismatch between the film and substrate. Owing to the film and substrate have different chemical compositions, their lattice mismatch stress is unavoidable. The purpose of this paper is to investigate the effect of the stress on the anisotropy and then the magnetic domain of (BiTm)3(GaFe)5O12 single crystal film.
In our experiment, the monocrystalline (BiTm)3(GaFe)5O12 films are prepared on (111)-oriented GGG substrates by LPE technique and the effect of lattice mismatch stress on the uniaxial anisotropy and magnetic domain are investigated. It is found that the lattice constant of the film is mainly determined by the content of Bi3+ in the film composition. and the increase of Bi3+ content leads to the increase of the film lattice constant, which affects the lattice mismatch stress between film and substrate. The lattice mismatch stress can adjust the perpendicular anisotropy of film which is the main reason for the domain changes. As the mismatch stress changes from tensile stress to compressive stress gradually, the magnetic bubble domain is transformed first into maze domain, and then into the partially striped domain, finally into the completely striped domain. The mismatch tensile stress is an effective method to enhance perpendicular anisotropy, when the growth-induced perpendicular anisotropy is not large enough. The bubble domain can only appear on the film with large tensile stress. The domain size is closely related to the stress. The domain width becomes wider as the mismatch stress becomes larger and it has the smallest domain size as the stress is minimum. These experimental results are very useful in controlling the uniaxial anisotropy and magnetic domain based on the change of the lattice mismatch stress in the growth process.
Near-infrared absorption of graphene-metal nanostructure based on magnetic polaritons
As a two-dimensional material with single-layer carbon atoms, the absorptivity of graphene is only about 2.3% in visible and near-infrared region, which restricts its applications in photoelectric detection, modulation and solar cells. A way to enhance the graphene absorption in this wavelength region is to combine graphene with grating nanostructure. The grating nanostructure can generate strong near-field localization by magnetic polaritons (MPs). However, the existing structures based on MPs are facing some problems, such as sensitivity to the polarization direction of the incoming wave and difficulty in processing the deep grating. Moreover, the modulation effect of the hybrid nanostructure based on MPs combining graphene with nano-grating has not been studied. In this work, a hybrid two-dimensional shallow grating nanostructure is proposed to modulate the absorptivity of graphene based on MPs. The finite element simulation is conducted to calculate the absorptive properties. The equivalent circuit model is used to predict the resonance conditions. The current and field distributions further confirm the excitation of magnetic resonance. The influences of structural parameters and the chemical potential on absorption property are studied. The results show that the magnetic polaritons derived from the hybrid two-dimensional shallow grating structure can obviously improve the absorption of graphene in the near-infrared region. Under the specific structure, the overall absorptivity of the structure is 85%, and the absorptivity of graphene in the structure is 55%, which is over 24 times higher than that of free-standing monolayer graphene. The absorption spectra of the hybrid grating nanostructure for different geometric parameters are calculated. The results show that the absorption peak presents an obvious blue-shift as the thickness of the dielectric layer, the grating period or the width of the silver nanoparticles decrease. Numerical simulation results show that by adjusting the chemical potential of graphene, the overall absorptivity of the structure can be tuned dynamically. The reflection modulation depths of hybrid two-dimensional nanostructure under different structural parameters are calculated. By controlling the chemical potential of graphene in a range from 0.1 eV to 1 eV, the reflection modulation depths of 54.8% (1040 nm), 50.3% (890 nm) and 46.8% (750 nm) are obtained, respectively. Compared with the existing structures based on MPs, the present structure is insensitive to the incidence and polarization direction of the incident electromagnetic wave due to the symmetry in two-dimensional directions. Considering the design of shallow silver grating, the structure is easier to implement in the process. The research results provide good theoretical reference for graphene-based photoelectric detection and modulation.
Dynamic multifunctional control of terahertz beam based on graphene coding metamaterial
Effect of anisotropic surface tension on morphological stability of lamellar eutectic growth in directional solidification
Lamellar eutectic solidification is very important in the development of new materials in which the periodic multiphase structures each may have a remarkable or enhanced functionality. The morphological instability during solidification may lead to various eutectic microstructures and greatly affect the physical and mechanical properties of final solidification products. In this paper, the morphological stability of lamellar eutectic growth with the anisotropic surface tension is studied by using the matched asymptotic expansion method and multiple variable expansion method. We assume that the process of solidification is viewed as a two-dimensional problem, The anisotropic surface tension is a four-fold symmetry function. The solute diffusion in the solid phase is negligible, and there is no convection in the system. On the basis of the basic state solution for the lamellar eutectic in directional solidification, the asymptotic solution for the perturbed interface shape of lamellar eutectic growth under the anisotropic surface tension is obtained in the case where the Peclet number is small, and then the quantization conditions of interfacial morphology for lamellar eutectic crystal is obtained. A dispersion relation between the wave number and the perturbation amplification rate, and the stability criterion of lamellar eutectic growth under the anisotropic surface tension are also obtained.
The result shows that the anisotropic surface tension has a significant effect on lamellar eutectic growth in directional solidification. It shows that comparing the directional solidification system of isotropic surface tension, the interface morphological stability of anisotropic surface tension also involves two types of global instability mechanisms: the ‘exchange of stability’ induced by the non-oscillatory, unstable modes and the global wave instability caused by four types of oscillatory unstable modes, namely antisymmetric antisymmetric (AA)-, symmetric antisymmetric (SA)-, antisymmetric symmetric (AS)-, and symmetric symmetric (SS)- modes. The linear stability analysis reveals that the stability of lamellar eutectic growth depends on stability critical number ε*. When ε > ε*, the eutectic growth system is unstable; When ε ≤q ε*, the eutectic growth system is stable. The anisotropic surface tension, by reducing the corresponding stability critical number ε*, stabilizes both the ‘exchange of stability’ mechanism and the global instability mechanism for the AA-, SA- and SS-modes. It implies that the anisotropic surface tension parameter tends to reduce the stability zone. However, by increasing the corresponding stability critical number ε*, the anisotropic surface tension destabilizes the global instability mechanism for the AS-mode. It implies that the anisotropic surface tension parameter tends to increase the stability zone for AS-mode.
T7 helicase unwinding and stand switching investigated via single-molecular technology
Single-molecule fluorescence resonance energy transfer (smFRET) and magnetic tweezers are widely used to study the molecular motors because of their high resolution and real-time observation. In this work, we choose these two techniques as the research means. The bacteriophage T7 helicase, as the research object, serves as a model protein for ring-shaped hexameric helicase that couples deoxythymidine triphosphate (dTTP) hydrolysis to unidirectional translocation. The DNA strand separation is 5'-3'-along one strand of double-stranded DNA. Using smFRET and magnetic tweezers to study the unwinding process of T7 helicase, we can have more in depth understanding of the unwinding and strand switching mechanisms of the ring-shaped hexameric helicases. First, by designing DNA substrates with different 3'-tail structures, we find that the 3'-tail is required for T7 helicase unwinding process, no matter whether it is single-stranded or double-stranded. These results confirm an interaction between T7 helicase and 3'-tail. Second, examining the dependence of unwinding process on GC content in DNA sequence, we find that as GC content increases, T7 helicase has higher chances to stop and slips back to the initial position by annealing stress or dissociating from DNA substrate. As the GC content increases to 100%, 79% helicases could not finish the unwinding process. Third, by further analysing the experimental data, two different slipping-back phenomena of T7 helicase are observed. One is instantaneous and the other is slow. The results from the experiment on magnetic tweezers also confirm this slow slipping-back phenomenon. This instantaneous slipping-back results from the rewinding process of unwound single-stranded DNA as studied previously. When T7 helicase cannot continue unwinding because of the high GC content in DNA sequence, it dissociates from the single-stranded DNA or slips back to the initial position very quickly because of the annealing stress. However, this slow slipping-back phenomenon cannot be explained by this reason. According to previous researches, T7 helicase can only be translocated or unwound from 5' to 3' along one strand of double-stranded DNA because of the polarity principle. We suggest that this slow slipping-back is induced by the strand switching process of T7 helicase. Through this strand switching process, T7 helicase binds to the 3'-strand and are translocated along it from 5' to 3' to the initial position, results in this slow slipping-back phenomenon. This is the first time that the slow slipping-back phenomenon has been observed, which strongly suggests the strand switching process of T7 helicase. Based on our results and previous researches, we propose the model of this strand switching process and this model may be extended to all ring-shaped hexameric helicases.
Rectifying performances of oligo phenylene ethynylene molecular devices based on graphene electrodes
With the experimental advances in microscale fabrication technology, the designing of functional devices by using single molecules has become one of the most promising methods for the next generation of electronic devices. Molecular rectifier, as a basic component almost for any electronic device, has become a research hotspot in molecular electronics. Recently, one-dimensional graphene nanoribbons (GNRs) which cut off from the novel two-dimensional material-graphene were used as the electrodes for several molecular devices due to their unique electronic structures and transport characteristics. The GNRs have less serious contact problems than metallic electrode materials like gold. In this paper, we investigate the rectifying performances of oligo phenylene ethynylene molecular devices based on graphene electrodes by using the density-functional theory and the non-equilibrium Green's function method. The effect of functional group on the rectifying performances of molecular device is discussed. The results show that the functional group plays a significant role in determining the rectifying performances of oligo phenylene ethynylene molecular device. The rectifying ratio can be effectively tuned by the functional group: adding the donor group (NH2) can lead to the positive rectifying phenomenon, adding the acceptor group (NO2) can trigger the negative rectifying phenomenon, and simultaneously adding NH2 and NO2 groups can bring about an alternate phenomenon between positive and reverse rectifying . The physical mechanism of the rectifying behavior is explained based on the transmission spectra and molecular projected self-consistent Hamiltonian. The transmission spectra of four models (M1-M4) bias voltages in range from-1.0 V to 1.0 V are given. The main transmission peak of M1 for positive bias is similar to that for negative bias, resulting in a weak rectification ratio. However, for M2 and M3, the main transmission peaks for positive and negative bias are significantly different from each other, which shows obviously a rectifying behavior. For M4, the main transmission peak is higher for the bias of (±0.44-±0.83 V) and also for the bias (±0.95-±1.00 V), showing an alternate phenomenon between positive and reverse rectifying. The maximum rectification ratio reaches 2.71 by adding an acceptor group (NO2), which suggests that this system has attractive potential applications in future molecular circuit.
Field effect transistor photodetector based on graphene and perovskite quantum dots
Graphene is an attractive optoelectronic material for various optoelectronic devices, especially in the field of photoelectric detection due to its high carrier mobility and fast response time. However, the relatively low light absorption cross-section and fast electron-hole recombination rate can lead to rapid exciton annihilation and small light gain, which restrict the commercial applications of pure graphene-based photodetector. The perovskite has attracted much attention because of its high photoelectric conversion efficiency in the field of solar cells. The perovskite has the advantages of long carrier diffusion distance and high optical absorption coefficient, which can effectively make up for the shortcomings of pure graphene-based field-effect transistor. In this work, a field-effect transistor photodetector is demonstrated with the combination of graphene and halide perovskite quantum dots (CsPbI3) serving as conductive channel materials. The graphene is prepared by plasma enhanced chemical vapor deposition, and the quantum dots are CsPbI3 perovskite. The electrical properties of graphene and pure graphene-based field-effect transistor are detected and analyzed by using the Raman spectrum. The results show that the graphene has good intrinsic electrical properties. Unlike previous report in which bulk perovskite was used, the perovskite quantum dot field-effect transistor photodetector has an obvious light response to 400 nm signal light, and shows the excellent photoelectrical performance. Under the illumination of 400 nm light, the signal light could be detected steadily and repeatedly by the graphene-perovskite quantum dot photodetector and converted into photocurrent. The photocurrent of the photodetector has a rapid rise, and the maximum value can reach 64 μA at a light power of 12 μW. The corresponding responsivity is 6.4 A·W-1, which is two orders of magnitude higher than that of the general single graphene photodetector (10-2 A·W-1), and it is also higher than that of perovskite-based photodetector (0.4 A·W-1). In addition, the photoconductive gain and detectivity arrive at 3.7×104 and 6×107 Jones (1 Jones=1 cm·Hz1/2·W-1), respectively. The results of this study demonstrate that the graphene-perovskite quantum dot photodetector can be a promising candidate for commercial UV light detectors.
Motor imagery based on adaptive parameterless empirical wavelet transform and selective integrated classification
Improving recognition rate of motor imagery (MI)-related electroencephalography (EEG) is of great importance for numerous brain computer interface (BCI) applications. However, the performance of a typical BCI system greatly relies on the effectiveness of the extracted features from raw EEG signals and the ability of the classifier to correctly identify different MI patterns. Therefore, in this paper, a new recognition method based on adaptive parameterless empirical wavelet transform (APEWT) and selective integrated classification model is proposed to enhance the classification accuracy of MI-related EEG signal. First, the APEWT is used to decompose EEG signals from different MI patterns into several intrinsic mode functions (IMFs), each of which contains different rhythm information over different frequency bands. Then several related modes are optimally selected based on the correlation coefficients calculated between each IMF component and the original signal to reconstruct EEG signals. Next, in order to further extract useful pattern information from both the time domain and frequency domain, the energy spectrum features of multiple time segments from the reconstructed signals and marginal spectrum features of different frequency bands corresponding to those selected modes are investigated, respectively. Finally, the extracted multiple features from time domain and frequency domain are input into the selective integrated classification model to build an MI recognition system. The selective integrated classification model consists of several extreme learning machines (ELMs) as the basic classifiers, different weights are assigned, respectively, to ELM basic classifiers based on their corresponding classification performances, and several basic ELM classifiers with good performances are selected to construct the final integrated model. The proposed method is evaluated on two public datasets, including BCI Competition Ⅱ dataset Ⅲ and BCI Competition IV dataset 2 b, and is compared with four different combination methods where different features in time domain or frequency domain in the feature extraction stage and different ELMs based classification models are considered. Experimental results demonstrate that the proposed method outperformed four combination methods and the existing methods recently reported in the literature using the same datasets in terms of classification accuracy and area under the ROC curve receiver operating characteristic metric. Specifically, our proposed method achieves the highest average classification accuracy (89.95%) in the compared methods, which indicates its better classification performance and generalization capability. In addition, the proposed method exhibits high computational efficiency, thus providing a new solution for online recognition of MI-related BCI and having the potential to be embedded in a practical system for controlling an external device.
Progress of Cu2O/ZnO oxide heterojunction solar cells
Recent progress of low cost Cu2O/ZnO hetero-junction solar cells is reviewed in this paper. The Cu2O used as an absorbing layer in photovoltaic cells is a direct bandgap semiconductor, exhibiting natural p-type conductivity. The source material of Cu2O-based solar cells is abundant and environmentally friendly. The main device structure of Cu2O/ZnO solar cells presents a planar and nano-wire/nano-rod configuration. The nanostructured Cu2O architecture conduces to charge collection in the device. The planar Cu2O absorbing layer with large grain size, achieved through the thermal oxidation of Cu sheets, exhibits high quality of the Cu2O/ZnO solar cells. The interface buffer layer (like i-ZnO, a-ZTO and Ga2O3) and back surface field (BSF, such as p+-Cu2O) can effectively improve energy band alignment match and increase carrier transport. The Cu2O paired with a 10-nm-thick Ga2O3 layer provides a nearly ideal conduction band offset and thus reduces the interface recombination. The Ga2O3 is a highly suitable buffer layer for enhancing the Voc (Voc value reaches 1.2 V) and conversion efficiency of Cu2O-based solar cells. The p+-Cu2O like N-doped Cu2O (Cu2O:N) and Na-doped Cu2O (Cu2O:Na) can reduce back-contact resistance and create an electron-reflecting back surface field in the Cu2O based solar cells. When a p-type Cu2O: Na acts as an absorbing layer and a zinc-germanium-oxide (Zn1-xGex-O) thin film is used as an n-type layer (buffer), Cu2O hetero-junction solar cell with the device structure MgF2/Al-doped ZnO (ZnO:Al)/Zn0.38Ge0.62-O/Cu2O:Na shows an efficiency of 8.1%. The oxide hetero-junction solar cells have a great potential application in the future photovoltaic field.
Quasi-soliton solution of Korteweg-de Vries equation and its application in ion acoustic waves
Investigation of interaction between solitons and their background small amplitude waves has been an interesting topic in numerical study for more than three decades. A classical soliton accompanied with oscillatory tails to infinite extent in space, is an interesting quasi-soliton, which has been revealed in experimental study and really observed. However, analytical solution of such a special quasi-soliton structure is rarely considered. In this paper, two branches of soliton-cnoidal wave solution as well as the two-soliton solution of the Korteweg-de Vries (KdV) equation are obtained by the generalized tanh expansion method. The exact relation between the soliton-cnoidal wave solution and the classical soliton solution of the KdV equation is established. By choosing suitable wave parameters, the quasi-soliton behavior of the soliton-cnoidal wave solution is revealed. It is found that with modulus of the Jacobi elliptic function approaching to zero asymptotically, the oscillating tails can be minimized and the soliton core of the soliton-cnoidal wave turns closer to the classical soliton solution. In addition, the quasi-soliton structure is revealed in a plasma physics system. By the reductive perturbation approach, the KdV equation modeling ion acoustic waves in an ideal homogeneous magnetized plasma is derived. It is confirmed that the waveform of the quasi-soliton is significantly influenced by the electron distribution, temperature ratio of ion to electron, magnetic field strength, and magnetic field direction. Interestingly, the amplitude of the quasi-soliton keeps constant due to the Ω-independence of nonlinear coefficient A. The width of the soliton core and the wavelength of the surrounded periodic wave become constant with the further increase of Ω. The explicit soliton-cnoidal wave solution with quasi-soliton behavior obtained here is applicable to many physical scenarios. For instance, the quasi-soliton structure can be viewed as a classical soliton with perturbations, and can correct the classical soliton in both theoretical and experimental study.
Motion-compensated coherent integration of constant moving target echoes with arbitrary complex envelope
Signal integration, as an effective method of detecting weak target, is widely used in areas of radar, sonar, etc. In previous studies of long-time coherent integration, researchers usually established a multi-pulse echo model with linear frequency modulation (LFM) signal due to its good compression performance and large Doppler tolerance. Then, perfect analytical formula can be deduced to compensate for range migration and Doppler spread, which is helpful in analyzing the mechanism of long-time coherent integration in depth. However, besides LFM, a wide variety of signal waveforms are also used in modern sonar and underwater guidance system to meet the requirements for diverse applications. For instance, continuous wave (CW) pulse is often used in signal detection, high resolution direction of arrival (DOA) estimation, and velocity estimation, while large time-bandwidth product waveforms such as modulated signal, coded signal, and pseudo-random signal are utilized for special tasks like anti-interference detection, channel matching, and active concealed detection. Therefore, the formulas and corresponding instructive conclusions deduced by LFM have no generality when other sonar waveforms are used in pulse integration. In this paper, we focus on long-time coherent integration for arbitrary signal reflected by underwater target moving with a uniform velocity and propose a motion-compensated coherent integration method for arbitrary complex envelop signal. A kind of general ambiguity function (GAF) for transmitted signal is defined to present a unified expression based on GAF for the output of the matched filter. The operation not only helps us to describe and calculate the pulse compression form of the arbitrary complex envelop by using a general mathematical model, but also provides information about the range migration and Doppler frequency shift of the multi-pulse echo, which is needed in pulse range alignment and FFT integration. For the matched filter output expressed by the GAF, Keystone transform is utilized to correct the complex envelop of the multi-pulse echo and eliminate the range walk. Then, Doppler frequency shift is compensated for by performing FFT transform, and the long-time coherent integration for arbitrary complex envelop is realized. To verify the correctness of the proposed method, we carry out the computer simulation on both signal integration and detection performance by using four sonar waveforms, i.e., CW signal, LFM signal, m-sequence phase-coded signal, and Costas frequency hop coded signal. The simulation results show that the proposed motion-compensated coherent integration method is applicable to arbitrary complex envelop signal. We also design an anechoic water tank experiment scheme which can successfully obtain the multi-pulse echoes of constant moving target. The motion-compensated coherent integration of the experimental data of the above-mentioned four waveforms further validates the effectiveness of the proposed method.
Geometric quantum discord in Tavis-Cummings model
Quantum entanglement plays a key role in quantum information and quantum computation and thus attracts much attention in many branches of physics both in theory and in experiment. But recent studies revealed that some separable states (non-entangled state) may speed up certain tasks over their classical counterparts and may also possess certain kinds of quantum correlations. For example, geometric quantum discord, which is a more general quantum correlation measure than entanglement, can be nonzero for some separable states. From a practical point of view, it is proposed that the geometric quantum discord be responsible for the power of many quantum information processing tasks. In order to capture such correlations, Ollivier and Zurek introduced quantum discord, which measures the discrepancy between two natural yet different quantum analogues of two classically equivalent expressions of mutual information. However, the calculation of quantum discord is based on numerical maximization procedure, and there are few analytical expressions even for a two-qubit state. In order to obtain the analytical results of quantum discord, a geometric measure of quantum discord which measures the quantum correlations through the minimum Hilbert-Schmidt distance between the given state and zero discord state is introduced. Geometric quantum discord is defined as an effective measure of quantum correlation, and the geometric quantum discord through the minimal distance between the quantum state and the set of zero-discord states in a bipartite quantum system can be worked out. In this paper, by using the geometric quantum discord measurement method, the geometric quantum discord in Tavis-Cummings model is investigated, and the influences of the initial state purity, entanglement degree, dipole-dipole coupling intensity between two atoms, and field in the Fock state on the evolution characteristic of geometric quantum discord are analyzed. The results show that the geometric quantum discord appears periodically. It initially decreases to a minimum value, and then turns out to be increased for different initial states. The rigorous analysis and numerical results reveal that when we take a suitable initial state, the geometric quantum discord of two atoms can be kept in correlation. When the atoms are in the different initial states, the quantum properties of the system are significant. The photon number of the field can lead the quantum discord to be weakened. Geometric quantum discord can be increased by increasing the cavity photon number and the dipole-dipole coupling intensity. Geometric quantum discord can be enhanced obviously by increasing the strength of the dipole-dipole coupling interaction. The conclusions may conduce to the understanding of quantum correlation for the other systems from the view of geometric quantum discord.
Ground state of spin-orbit coupled rotating two-component Bose-Einstein condensate in gradient magnetic field
Two-component Bose-Einstein condensate offers an ideal platform for investigating many intriguing topological defects due to the interplay between intraspecies and interspecies interactions. The recent realization of spin-orbit coupling in two-component Bose-Einstein condensate, owing to coupling between the spin and the centre-of-mass motion of the atom, provides possibly new opportunities to search for novel quantum states. In particular, the gradient magnetic field in the Bose-Einstein condensate has brought a new way to create topologically nontrivial structures including Dirac monopoles and quantum knots. Previous studies of the gradient magnetic field effect in the Bose-Einstein condensate mainly focused on the three-component case. However, it remains unclear how the gradient magnetic field affects the ground state configuration in the rotating two-component Bose-Einstein condensate with spin-orbit coupling. In this work, by using quasi two-dimensional Gross-Pitaevskii equations, we study the ground state structure of a rotating two-component Bose-Einstein condensate with spin-orbit coupling and gradient magnetic field. We concentrate on the effects of the spin-orbit coupling and the gradient magnetic field on the ground state. The numerical results show that increasing the strength of the spin-orbit coupling can induce a phase transition from skyrmion lattice to skyrmion chain in the presence of the gradient magnetic field. Unlike the study of skyrmion in rotating two-component Bose-Einstein condensate with only spin-orbit coupling, the skyrmion chain can occur under the isotropic spin-orbit coupling when the gradient magnetic field is considered. It is worth noting that the skyrmion chain here is arrayed along the diagonal direction. Next we examine the effect of the gradient magnetic field on spin-orbit coupled two-component Bose-Einstein condensate. For the case of weak spin-orbit coupling and the slow rotation, a phase transition from a single plane-wave to half-skyrmion is found through increasing magnetic field gradient strength. For the case of strong spin-orbit coupling and the fast rotation, the nature of the ground state is shown to support the formation of a hidden vortex as the gradient magnetic field is enhanced. These hidden vortices have no visible cores in density distributions but have phase singularities in phase distributions, which are arrayed along the diagonal direction. This result confirms a new method of creating the hidden vortices in the two-component Bose-Einstein condensate. These topological structures can be detected by using the time-of-flight absorption imaging technique. Our results illustrate that the gradient magnetic field not only provides an opportunity to explore the exotic topological structures in spin-orbit coupled spinor Bose-Einstein condensate, but also is crucial for realizing the phase transitions among different ground states. This work paves the way for the future exploring of topological defect and the corresponding dynamical stability in quantum systems subjected to a gradient magnetic field.
Bursting oscillations and mechanism of sliding movement in piecewise Filippov system
Since the wide applications in science and engineering, the dynamics of non-smooth system has become one of the key research subjects. Furthermore, the interaction between different scales may result in special movement which can be usually described by the combination of large-amplitude oscillation and small-amplitude one. The influence of multiple scale on the dynamics of non-smooth system has received much attention recently. In this work, we try to explore the bursting oscillations and the mechanism of non-smooth Filippov system coupled by different scales in the frequency domain. Taking the typical periodically excited Duffing's oscillator for example a Filippov system coupled by two scales in the frequency domain is established when the difference in order between the excited frequency and the system natural frequency is obtained by introducing the piecewise control into the state variable and choosing suitable parameters. For the case in which the exciting frequency is far less than the natural frequency, the whole exciting term can be considered as a slow-varying parameter, also called slow subsystem, which leads to a generalized autonomous system, i.e., the fast subsystem. The equilibrium branches and the bifurcations of the fast subsystem along with the variation of the slow-varying parameter in different regions divided according to non-smooth boundary, can be derived. Two typical cases are taken into consideration, in which different distributions of the equilibrium branches and the relevant bifurcations of the fast subsystem may exist. It is pointed out that the variations of the parameters may influence not only the properties of the equilibrium branches, but also the structures of the bursting attractors. Furthermore, since the governing equation alternates between two subsystems located in different regions when the trajectory passes across the non-smooth boundary, the sliding movement along the non-smooth boundary of the trajectory can be observed under the condition of certain parameters. By employing the transformed phase portrait which describes the relationship between the state variable and the slow-varying parameter, the mechanisms of different bursting oscillations and sliding movements are investigated. The results show that bursting oscillations may exist in a non-smooth Filippov system coupled by two scales in the frequency domain. The alternations of the governing equation between different subsystems located in the two neighboring regions along the non-smooth boundary may result in a sliding movement of the trajectory along the non-smooth boundary.
Memristor-based multi-scroll chaotic system and its pulse synchronization control
The memristor is a nonlinear element and intrinsically possesses memory function. When it works as nonlinear part of a chaotic system, the complexity and the randomness of signal will be enhanced. In this paper memristor is introduced into a three-dimensional chaotic system based on the augmented Lü system. The interesting and promising behaviors of complex single, double and four-scroll chaotic attractors generated only by varying a parameter have not been reported in memristive chaotic system and thus they deserve to be further investigated. It is also obvious that such a simple change of one parameter could be used to generate a variety of quite complex attractors. Therefore, as a nonlinear device the memristor plays an important role in this system. Firstly, some basic dynamical properties of the memristive chaotic system, including symmetry and in-variance, the existence of attractor, equilibrium, and stability are investigated in detail. By numerically simulating the power spectrum, Lyapunov exponent, Poincare map and bifurcation diagram, in this paper we verify that the proposed system has abundant dynamical behaviors. The sensitivities of system parameters to the chaotic behaviors are further explored by calculating, in detail, its Lyapunov exponent spectrum and bifurcation diagrams. The results of simulation and experiment are in good agreement, thereby proving the veracity of analysis. The memristive chaotic circuit is designed using the memristor, operational amplifier, analog multiplier and other conventional components. The circuit implementation of the memristive system is simulated using SPICE (simulation program with integrated circuit emphasis). The SPICE simulation results and the theoretical analysis are found to be in good agreement, and thus verifying that the system can produce chaos. Pulse synchronization has the following characteristics: low energy consumption, fast synchronization and easy-to-implement single-channel transmission. Therefore, it is more practical in chaotic secure communication. Subsequently the pulse chaos synchronization is realized from the perspective of the maximum Lyapunov exponent, and numerical simulations show the existence of new memristive chaotic system and the feasibility of pulse synchronization control, and also provide an experimental basis for further studying the applications of the memristive chaotic system in voice secure communication and information processing.
Density functional theory of InCn+(n=1–10) clusters
Small indium-doped carbon clusters InCn+(n=1–10) are systematically studied by the density functional theory at the B3 LYP/LANL2 DZ level. The computed properties include equilibrium geometries, electronic energies, vibrational frequencies, dipole moments and rotational constants for individual species. The calculation results show that the open-chain linear isomers with the indium atom bound to the end of the carbon chain are the most stable geometry in all cases. There must exist a cyclic or fan structure in the metastable or the third stable structure of cluster. The bigger the size of the cluster, the more obvious the stability of the structure is. The electronic ground state is found to be alternately a triplet for even n and a singlet for odd n with the only exception of InC+. It is generally observed that the spin contamination is not serious for all electronic ground states because the <s2> values are uniform and in general deviate slightly from the pure spin values, and the B3 LYP wave functions are nearly spin-pure. It is also found that in the lowest-energy linear structure, the In–C bond is longer (from 2.319 Å to 2.850 Å) than the corresponding C–C bonds in a range from 1.268 Å to 1.360 Å. The C–C distances can be assimilated to moderately strong double bonds underlying a clear π bonding in the corresponding structures. In addition, we observe a clear alternation in C–C distances. The Codd–Ceven distances are shorter than the Ceven–Codd ones which mainly results from the charge distribution and spin density. According to the calculation and analysis of the incremental binding energy and the second difference we can deduce an even-odd alternation in the cluster stability for the linear InCn+, with their n-odd members being more stable than the adjacent even-numbered ones. This parity effect also appears in the adiabatic ionization potential curves. The analysis of magnetic properties shows the even-odd alternation with n-even clusters presenting higher values of magnetic moment than n-odd ones. The study of the polarizability indicates that the average values of both the polarization tensors and the anisotropic invariants increase with the size of cluster increasing.
Influence of laser linewidth on light-induced drift velocity of atom
Light-induced drift has many applications in astrophysics, semiconductor physics, and isotope separation. Light-induced drift velocity is a key parameter to characterize the effect of light-induced drift. Laser linewidth exerts a great influence on light-induced drift velocity through influencing the velocity selectivity of atomic excitation, so it is an important factor that cannot be ignored in the study of light-induced drift. However, in existing theoretical studies, the influence of laser linewidth is seldom considered and the exciting light is always treated as monochromatic light. Furthermore, in a few theoretical studies about laser linewidth, the numerical model adopted does not include all the factors of light-induced drift, such as energy level degeneracy, hyperfine structure, and collision model, which will cause the error of calculation. In order to study the influence of laser linewidth on light-induced drift velocity, a four-level rate equation model is established to describe the atomic energy level transition in the process of light-induced drift. In the theoretical model, we introduce strong collision model to describe collisions between atoms and buffer gas. The influences of energy level degeneracy and hyperfine structure are also taken into account. Numerical method is used to calculate the four-level rate equation. According to the calculation results, the influence of laser linewidth on drift velocity of alkali metal atoms is analyzed. The results show that as the linewidth increases, the value of drift velocity first increases and then decreases. There is an optimal linewidth that maximizes the drift velocity. For the best light-induced drift effect, the laser should work under the optimal linewidth condition. When the laser linewidth fluctuates near the optimum linewidth, the laser linewidth should be set to be slightly wider than the optimal linewidth. This can reduce the influence of fluctuation and obtain a better drift effect. In addition, as the laser linewidth increases, the optimum power density corresponding to the maximum drift velocity decreases. When the laser linewidth is narrow, small fluctuations near the optimal laser power density will not have great influence on drift velocity. When the laser linewidth is wide, the power density should be set strictly. The optimum linewidth is related to laser power density, temperature and buffer gas pressure. As the laser power density increases, the value of optimum linewidth first increases rapidly and then decreases slowly. The value of optimal linewidth also increases linearly with the increase of temperature, and it decreases with the increase of buffer gas pressure. In conclusion, the laser linewidth does play a key role in the process of light-induced drift. The results of this study can provide a theoretical basis for future experiments, and be a good reference to the selection of exciting light.
Influence of eikonal-initial-state on ionization of atom by proton
Apart from its fundamental importance, ionization phenomenon of atoms by impact of energetic charged particles has practical applications in various kinds of plasmas, in radiation physics and in the study of penetration of charged particles through matter. Compared with other processes, this particular reaction helps to reveal many details about the dynamical process and the level population, and, in fact, can provide a new insight into and a promising route to studying the e-p interactions in the presence of Coulomb field. The development of ion sources producing multiply charged ions and of antiproton beams allow us to change the potentials and hence the whole final momentum distribution. A great variety of experimental conditions allowed by changing the projectile charge and velocity constitute a stringent test for theory. The continuum-distorted-wave eikonal-initial-state (CDW-EIS) approximation model has emerged as a reliable method to compute cross sections for different projectile/target combinations from intermediate to high non-relativistic impact energies. This model is of the first order in a distorted-wave series. It takes into account the long-range behaviour of the Coulomb potential and includes the distortion of the target states in both the initial and final channels. In the present work, the single different cross sections (SDCS), double different cross sections (DDCS), and total cross sections for single ionization of 1s, 2s and 2p shell of Ne atom by impact of proton are calculated in the framework of continuum-distorted-wave (CDW) method and the CDW-EIS approximation model, respectively. The influence of the eikonal-initial-state on the cross section, and the mechanism of the proton-atom collision ionization are discussed in detail. Moreover, the structures of the SDCS and DDCS of each shell are studied and the ionization mechanism of soft collision, electron capture to the continuum state, binary encounter collision are demonstrated. Our results show that for the 2p shell of Neon, as the incident proton energy increases, the region of the SDCS becomes larger and the soft ionization turns dominant in the low energy region. The eikonal-initial-state effect on the cross section is obvious in the lower energy region, yet smaller as the incident energy increases. These effects on the DDCS are greater than on the SDCS. The present CDW-EIS and CDW results are compared with the experimental data available in the energy range of 1-5000 keV/u for H+ on Ne in the literature, showing that they are quantitatively in good agreement. In general, the CDW-EIS describes well the multiple ionization above 50 keV/u, showing a clear tendency to coalesce with the CDW at high energies.
Chip-based controllable Ioffe-typed electrostatic mirotrap for cold molecules
Trapping particles (atoms or molecules) allows long interaction time and therefore potentially high resolution in precision measurements. Moreover, the particles in the trap are thermally isolated from the outside world and can be cooled to very low temperatures. As a result, the atomic (or molecular) traps have been widely used in many research areas. However, the molecules in these traps exhibiting zero field in the trap center undergo nonadiabatic transitions, which is the major loss of particles. The loss of atoms in this type of trap seriously hinders the generation of the first BEC (Bose-Einstein condensates). In this paper, we propose a chip-based controllable Ioffe-type electrostatic mirotrap, in which nonadabatic loss can be avoided due to the non-zero electric field. The mirotrap is composed of a pair of L-typed gold wires, which is 1 μm in height and deposited on a glass substrate. The non-zero potential well originated in the microsize electrodes offers a steep gradient enable to trap low-field-seeking state polar molecules. The electric field strength in the trap center can be changed in a wide range by adjusting the applied voltage or/and the widths of the electrodes. For instance, under the conditions in the paper, the electric field strength in the trap center can be changed from 0.15 to 5.5 kV/cm. The height of the potential well is about 10 μm above the chip and can also be tuned in a large range by adjusting the parameters of the electrodes. Under the conditions in the paper, the height of the potential well can be adjusted from 6.0 to 17.0 μm. The electric fields of the microtrap near the surface of the chip are calculated by using a finite element software. Monte-Carlo simulations of the loading and the trapping processes are also carried out in order to justify the feasibility of our scheme. Taking ND3 molecules for example, the loading efficiency of molecules as a function of longitudinal velocity of molecular packet is studied. Our proposed surface microtrap can be used not only for integrating the molecular chips but also for producing the quantum degenerate gas near the chip surface. It offers a platform for many research fields such as precision measurements, quantum computing, surface cold collisions and cold chemistry.
Analysis of orbital angular momentum spectra of Hankel-Bessel beams in channels with oceanic turbulence
Beams with different-mode-number (l) orbital angular momenta (OAMs) are mutually orthogonal to each other, which makes it possible to enlarge the channel capacity in an OAM multiplexed underwater optical communication (UOC) system. Nevertheless, the implementation of this strategy is limited by oceanic turbulence. Hankel-Bessel (HB) vortex beams carrying OAM are relatively less affected by atmospheric turbulence due to their ability to propagate without changing the intensity profile (non-diffraction nature) and remarkable ability to be reconstructed after encountering an obstacle (self-healing mechanism). Consequently, HB vortex beams can be used as the carriers to increase the channel capacity of information transmission. In this paper, based on the Rytov approximation theory, the analytical expressions of OAM spectra for HB vortex beams under weak horizontal oceanic turbulent channels are derived. The influences of oceanic turbulence parameters on the OAM spectra of HB vortex beams are investigated via numerical calculations. The results indicate that oceanic turbulence leads to the decline of detection probability of transmitted OAM mode and the broadening of OAM spectra as well. Similarly, the spatial coherence length in oceanic turbulence decreases with increasing propagation distance and the dissipation rate of mean-squared temperature and with decreasing the dissipation rate of turbulent kinetic energy, which lead to the decline of detection probability of transmitted OAM mode for HB vortex beams. On the other hand, beams with larger OAM mode numbers each have a wider beam spreading after propagating in the turbulence, which results in the decrease of the detection probability for transmitted OAM modes of HB vortex beams. And the HB vortex beams are more affected by salinity fluctuation than by temperature fluctuations, which indicates that salinity fluctuations are much more effective than temperature fluctuations in determinating the effect of oceanic turbulence. In addition, for weak turbulence and a distance of several tens of meters, the transmission performance of HB vortex beams is worse than that of Laguerre-Gaussian vortex beams with the optimal waist setting. These results provide references for the realization of optical communication links in the marine environment.
Separate estimation of laser micro-Doppler parameters based on maximum likelihood schemes
Laser micro-Doppler (MD) effect is capable of obtaining obvious modulation in weak vibration detection. It helps to estimate target micro-motion parameters with high precision, which may extend the application field of MD to subtle identification and recognition. In laser detection, the multiple scattering points in the field of view will generate the single-channel multi-component (SCMC) signal. Moreover, the micro-Doppler features of each component will be overlapped in the time-frequency domain because of the similar micro-motion parameters. The overlapped SCMC signal makes the estimation of the MD parameters a very difficult problem, and there has been no good method so far. In this paper, a separate parameter estimator based on the maximum likelihood framework is proposed to deal with this underdetermined problem. First, the detailed period scanning method is presented to improve the estimation accuracy of micro-motion frequency from the singular value ratio (SVR) spectrum. Further, the amplitude ratio information of each component is extracted from the SVR spectrum. Then, the closed-form expressions of the maximum likelihood estimation (MLE) for the remaining micro-motion parameters are derived, where the mean likelihood estimation is used to approximate to the performance of MLE. The high nonlinearity and multi-peak distribution shape of the likelihood function (LF) in laser MD signal will lead to incorrect estimation result. To this end, a new LF based on the energy spectrum characteristics is designed. The new LF acts as a smoothing filter to the probability density function, through which the ideal PDF distribution form that has only one smooth peak is obtained. With this modification, the requirements for the initialization are reduced and the robustness in low SNR situation is increased. The Markov chain Monte Carlo sampling is employed to implement the MLE. The Gibbs method is chosen to solve the multi-dimensional parametric problems, and the detailed process is listed. In the end, the simulation results prove the feasibility and high efficiency of the proposed method. The accuracy of parameter estimation reaches the Cramer-Rao boundary. The inverse Radon transform is used as a comparison with the experiment, and the results show the precise estimation advantage of the presented method.
Propagation of elastic waves in saturated porous medium containing a small amount of bubbly fluid
It is very important to understand the acoustical properties of porous medium. To study the relationship between acoustical and other physical properties of porous medium will help us to use acoustical tools for determining the physical properties of porous medium. Many researchers have paid much attention to the properties of acoustic wave propagation in the gassy marine sediments based on the Biot model which is popularly used to predict the dispersion and attenuation of sound in saturated porous medium. The patchy model which contains gas inside the spherical water predicts that the existence of gas just has little effect on the propagation of acoustic wave in porous medium when the gas content is very small. However, the presence of a small number of bubbles in a fluid saturated sediment will lead to different acoustic responses. As is well known, the bubble vibration theory proposed by Keller and Miksis shows that a small number of bubbles existing in the liquid will have a great influence on sound velocity and attenuation. Therefore, in order to study the effect of a small amount of gas existing in fluid saturated porous medium on the property of acoustic wave propagation, we investigate a bubbly liquid saturated porous medium and consider the case of the bubbles vibrating linearly under the action of sound waves. First, we derive the continuity equation of the seepage according to the mass conservation of the pore fluid and the relationship between porosity differentiation and pore fluid pressure differentiation. Then, the bubble linear vibration theory given by Commander is used to deal with the time derivative of gas volume fraction in the continuity equation of the seepage, The bubble linear vibration theory gives the relationship between instantaneous bubble radius and background pressure of the medium. Through this relationship, we obtain the equation of time derivative of gas volume fraction and time derivative of pore fluid pressure. Then, we combine the obtained equation with the continuity equation of seepage, and obtain the modified continuity equation of seepage whose form is similar to that of Biot model. Finally, the modified Biot's equations for fluid saturated porous medium containing a small amount of bubbly fluid is obtained. As is well known, an effective density fluid model for acoustic propagation in sediments, derived from Biot theory, just can predict the acoustic properties of the fast compressional waves. However, the present model can predict the acoustic properties of fast, slow compressional waves and shear waves propagating in sediments. Through numerically calculating the dispersion, attenuation, amplitude ratios of pore fluid displacement to solid displacement for fast and slow compressional waves, it is found that the existence of a small number of bubbles has an influence on the acoustic properties of both the fast compressional waves and the slow compressional waves, especially the velocity of the fast compressional wave. In addition, the low-frequency speed approximation formula for the fast compressional wave is also presented. The approximate formula directly indicates the relationship between the velocity of fast compressional wave and the parameters of porous medium such as the gas volume fraction and the bubble radius. This study shows that the influence of a small number of bubbles in fluid saturated on acoustic wave propagation is noticeable. The modified Biot model presented in this paper provides one model to study the properties of acoustic waves in fluid saturated porous medium with a small number of bubbles.
Numerical simulation of deformation and rupture process of bubble in an oil film impacted by an oil droplet
Impact of oil droplet on oil film usually takes place in the lubrication process of rotating mechanical parts and machinery which can easily lead to bubble entrainment. Bubbles have important influences on the motion process of the oil droplet impacting on the oil film and also on the formation quality of the oil film layer. An oil droplet impacting on the oil film which contains a bubble is simulated numerically based on the coupled level set and the method of determining volume fraction. The bubble deformation process in the oil film during an oil droplet impacting on the oil film is investigated by the simulation method. The influences of the bubble size and the bubble position on the bubble deformation characteristic are also analyzed. The dynamic mechanism of the bubble rupture is discussed. The numerical results show that as the oil droplet impacts on the oil film, the bubble may rupture on the free surface, presenting stable deformation, or rupture in the oil film, which is greatly influenced by the bubble size. When the bubble diameter is in a range between 10 μm and 20 μm, the bubble deformation becomes more serious with the increase of bubble diameter, and the rupture of bubble on the free surface may occur over time. When the bubble diameters are in a range between 20 μm and 30 μm, the bubble rupture occurs in a short time after the bubble has reached a maximum deformation, and there is no obvious relationship between the maximum bubble deformation and the bubble diameter. The diameter of 20 μm is a critical value for a bubble to rupture on a free surface or inside an oil film, with which a bubble can keep stable in an oil film layer. As the bubble position changes, the bubble deformation process changes correspondingly. Under the same impact conditions, bubbles at the top of the oil film are more likely to deform than those in the center of the oil film. Bubbles at the bottom of the oil film have the smallest total deformation and finally attach to the wall. The bubble rupture is caused by the instability of the gas-liquid interface and the surface tension. The viscous shear force also plays an important role when the bubble rupture takes place in the oil film.
Dynamics of evaporating drop on heated surfaces with different wettabilities
The dynamics of evaporating sessile drop on a uniformly heated, horizontal, solid substrate is considered. On the basis of lubrication theory and Navier slip condition, an evolution equation for the height of the two-dimensional drop is established. The numerical results show that the drop evolution is governed by capillary force, gravity, thermal capillary force and evaporation. Gravity exerts a promoting effect on drop spreading, while capillary force and thermal capillary force inhibit drop spreading. The typical dynamic features including contact line pinning or partial pinning modes during the drop evaporation are illustrated by changing the temperature-sensitive coefficient in the present model, and the drop lifetime of contact pinning mode is found to be shorter than that of contact line partial pinning mode. Under the same temperature-sensitive coefficient of three interfaces, the drop evolution is indicated with three typical stages: 1) spreading stage, 2) contact line pinning stage, and 3) both contact line and contact angle decreasing stage. As interface tension of liquid-gas or liquid-solid is more sensitive to temperature, the drop evolution is divided into two typical stages: 1) spreading stage and 2) contact line pinning stage. The equilibrium contact angle tends to be smaller and the substrate wettability is improved, leading to the increased spreading speed, the prolonged time of the contact line to reach pinning: the faster the evaporation rate, the shorter the lifetime of drop is. Additionally, the same effect of sensitivity of liquid-gas and liquid-solid interface tension to temperature on the wettability of substrate is observed.
When the interface tension of solid-gas is more sensitive to temperature, the drop evolution is characterized in four typical stages: 1) spreading stage, 2) contact line pinning stage, 3) contact line de-pinning and constant contact angle stage, and 4) both contact line and contact angle decreasing stage. The equilibrium contact angle tends to be greater and the substrate wettability is deteriorated, leading the spreading speed to decrease. Hence, it is more effective to manipulate the drop movement in the presence of evaporation by regulating the temperature-sensitive coefficient of the solid-gas interface.