We investigate the dynamical behavior of hybrid virus infection systems with nonlytic immune response in switching environment, which is modeled as a stochastic process of telegraph noise and represented as a multi-state Markov chains. Firstly, The existence of unique positive solution and boundedness of the new hybrid system is proved. Furthermore, the sufficient conditions for extinction and persistence of virus are established. Finally, stochastic simulations are performed to test and demonstrate the conclusions. As a consequence, our work suggests that stochastic switching environment plays a crucial role in the process of virus prevention and treatment.
The scattering states in one-dimensional Hermitian and non-Hermitian potentials are investigated. An analytical solution for the scattering states is presented in terms of Heun functions. It is shown that for some specially chosen parameter conditions, an infinite number of the exact scattering states is obtained. In the Hermitian potentials, they correspond to the reflectionless states. In the non-Hermitian complex potentials with parity-time symmetry, they are the unidirectionally reflectionless states.
A GaN-based pin neutron detector with a 6LiF conversion layer was fabricated, and can be used to detect thermal neutrons. Measurement of the electrical characteristic of the GaN-based pin neutron detector showed that the reverse leakage current of the neutron detector was reduced significantly after deposition of a 6LiF conversion layer on the detector surface. The thermal neutrons used in this experiment were obtained from an 241Am–Be fast neutron source after being moderated by 100-mm-thick high-density polyethylene. The experimental results show that the detector with 16.9-μm thick 6LiF achieved a maximum neutron detection efficiency of 1.9% at a reverse bias of 0 V, which is less than the theoretical detection efficiency of 4.1% calculated for our GaN neutron detectors.
In the numerical studies of active particles, models consisting of a solid body and a fluid body have been well established and widely used. In this work, such an active Brownian particle (ABP) is realized in molecular dynamics (MD) simulations. Immersed in a fluid, each ABP consists of a head particle and a spherical phantom region of fluid where the flagellum of a microswimmer takes effect. Quantitative control over the orientational persistence time is achieved via an external stochastic dynamics. This control makes it possible to validate ABP’s diffusion property in a wide range of particle activity. In molecular description, the axial velocity of ABP exhibits a Gaussian distribution. Its mean value defines the active velocity which increases with the active force linearly, but shows no dependence on the rotational diffusion coefficient. For the active diffusion coefficient measured in free space, it shows semi-quantitative agreement with the analytical result predicted by a minimal ABP model. Furthermore, the active diffusion coefficient is also calculated by performing a quantitative analysis on the ABP’s distribution along x axis in a confinement potential. Comparing the active diffusion coefficients in the above two cases (in free space and in confinement), the validity of the ABP modeling implemented in MD simulations is confirmed. Possible reasons for the small deviation between the two diffusion coefficients are also discussed.
Postselected von Neumann measurement characterized by postselection and weak value has been found to possess potential applications in quantum metrology and solved plenty of fundamental problems in quantum theory. As an application of this new measurement technique in quantum optics and quantum information processing, its effects on the features of single-mode radiation fields such as coherent state, squeezed vacuum state and Schr?dinger cat sate are investigated by considering full-order effects of unitary evolution. The results show that the conditional probabilities of finding photons, second-order correlation functions, Qm-factors and squeezing effects of those states after the postselected measurement is significantly changed are comparable with the corresponding initial pointer states.
Quantum sensing has been receiving researcher’s attention these years due to its ultrahigh sensitivity and precision. However, the bandwidth of the sensors may be low, thus limiting the scope of their practical applications. The low-bandwidth problem is conquered by feedback control methods, which are widely utilized in classic control fields. Based on a quantum harmonic oscillator model operating near the resonant point, the bandwidth and sensitivity of the quantum sensor are analyzed. The results give two important conclusions: (a) the bandwidth and sensitivity are two incompatible performance parameters of the sensor, so there must be a trade-off between bandwidth and sensitivity in practical applications; (b) the quantum white noise affects the signal to be detected in a non-white form due to the feedback control.
We present a time domain hybrid method to realize the fast coupling analysis of transmission lines excited by space electromagnetic fields, in which parallel finite-difference time-domain (FDTD) method, interpolation scheme, and Agrawal model-based transmission line (TL) equations are organically integrated together. Specifically, the Agrawal model is employed to establish the TL equations to describe the coupling effects of space electromagnetic fields on transmission lines. Then, the excitation fields functioning as distribution sources in TL equations are calculated by the parallel FDTD method through using the message passing interface (MPI) library scheme and interpolation scheme. Finally, the TL equations are discretized by the central difference scheme of FDTD and assigned to multiple processors to obtain the transient responses on the terminal loads of these lines. The significant feature of the presented method is embodied in its parallel and synchronous calculations of the space electromagnetic fields and transient responses on the lines. Numerical simulations of ambient wave acting on multi-conductor transmission lines (MTLs), which are located on the PEC ground and in the shielded cavity respectively, are implemented to verify the accuracy and efficiency of the presented method.
The behaviors of helium clusters and self-interstitial tungsten atoms at different temperatures are investigated with the molecular dynamics method. The self-interstitial tungsten atoms prefer to form crowdions which can tightly bind the helium cluster at low temperature. The crowdion can change its position around the helium cluster by rotating and slipping at medium temperatures, which leads to formation of combined crowdions or dislocation loop locating at one side of a helium cluster. The combined crowdions or dislocation loop even separates from the helium cluster at high temperature. It is found that a big helium cluster is more stable and its interaction with crowdions or dislocation loop is stronger.
A new heating method is proposed to increase the cell temperature of the 6–8 type multi-anvil apparatus without reducing the volume of the sample chamber. The double-layer heater assembly (DHA) has two layers of heaters connected in parallel. The temperature of the cell was able to reach 2500 °C by using 0.025 mm rhenium foils, and the temperature limit was increased by 25% compared with that of the traditional single-layer assembly. The power–temperature relationships for these two assemblies with different sizes were calibrated by using W/Re thermocouple at 20 GPa. When the volume of samples was the same, the DHA not only attained higher temperature, but also kept the holding time longer, compared to the traditional assembly. The results of more than ten experiments showed that the new 10/4 DHA with a relatively large sample size (2 mm in diameter and 4 mm in height) can work stably with the center temperature of the sample cavity exceeding 2300 °C under the pressure of 20 GPa.
Considering an elastically coupled Brownian motors system in a two-dimensional traveling-wave potential, we investigate the effects of the angular frequency of the traveling wave, wavelength, coupling strength, free length of the spring, and the noise intensity on the current of the system. It is found that the traveling wave is the essential condition of the directed transport. The current is dominated by the traveling wave and varies nonmonotonically with both the angular frequency and the wavelength. At an optimal angular frequency or wavelength, the current can be optimized. The coupling strength and the free length of the spring can locally modulate the current, especially at small angular frequencies. Moreover, the current decreases rapidly with the increase of the noise intensity, indicating the interference effect of noise on the directed transport.
When chaotic systems are implemented on finite precision machines, it will lead to the problem of dynamical degradation. Aiming at this problem, most previous related works have been proposed to improve the dynamical degradation of low-dimensional chaotic maps. This paper presents a novel method to construct high-dimensional digital chaotic systems in the domain of finite computing precision. The model is proposed by coupling a high-dimensional digital system with a continuous chaotic system. A rigorous proof is given that the controlled digital system is chaotic in the sense of Devaney’s definition of chaos. Numerical experimental results for different high-dimensional digital systems indicate that the proposed method can overcome the degradation problem and construct high-dimensional digital chaos with complicated dynamical properties. Based on the construction method, a kind of pseudorandom number generator (PRNG) is also proposed as an application.
A high performance fast-Fourier-transform (FFT) spectrum analyzer, which is developed for measure spin noise spectrums, is presented in this paper. The analyzer is implemented with a field-programmable-gate-arrays (FPGA) chip for data and command management. An analog-to-digital-convertor chip is integrated for analog signal acquisition. In order to meet the various requirements of measuring different types of spin noise spectrums, multiple operating modes are designed and realized using the reprogrammable FPGA logic resources. The FFT function is fully managed by the programmable resource inside the FPGA chip. A 1 GSa/s sampling rate and a 100 percent data coverage ratio with non-dead-time are obtained. 30534 FFT spectrums can be acquired per second, and the spectrums can be on-board accumulated and averaged. Digital filters, multi-stage reconfigurable data reconstruction modules, and frequency down conversion modules are also implemented in the FPGA to provide flexible real-time data processing capacity, thus the noise floor and signals aliasing can be suppressed effectively. An efficiency comparison between the FPGA-based FFT spectrum analyzer and the software-based FFT is demonstrated, and the high performance FFT spectrum analyzer has a significant advantage in obtaining high resolution spin noise spectrums with enhanced efficiency.
In recent years, clinical studies have found that acetone concentration in exhaled breath can be taken as a characteristic marker of diabetes. Metal–oxide–semiconductor (MOS) materials are widely used in acetone gas sensors due to their low cost, high sensitivity, fast response/recovery time, and easy integration. This paper reviews recent progress in acetone sensors based on MOS materials for diabetes diagnosis. The methods of improving the performance of acetone sensor have been explored for comparison, especially in high humidity conditions. We summarize the current excellent methods of preparations of sensors based on MOSs and hope to provide some help for the progress of acetone sensors in the diagnosis of diabetes.
High precision atom interferometers have shown attractive prospects in laboratory for testing fundamental physics and inertial sensing. Efforts on applying this innovative technology to field applications are also being made intensively. As the manipulation of cold atoms and related matching technologies mature, inertial sensors based on atom interferometry can be adapted to various indoor or mobile platforms. A series of experiments have been conducted and high performance has been achieved. In this paper, we will introduce the principles, the key technologies, and the applications of atom interferometers, and mainly review the recent progress of movable atom gravimeters.
Metasurface is a kind of two-dimensional metamaterial with specially designed sub-wavelength unit cells. It consists of single-layer or few-layer stacks of planar structures and possesses certain superior abilities to manipulate the propagating electromagnetic waves, including the terahertz (THz) ones. Compared with the usual passive THz metasurfaces whose optical properties are difficult to be controlled after fabrication, the active materials are highly desirable to enable dynamic and tunable control of THz waves. In this review, we briefly summarize the progress of active THz metasurfaces, from their physical mechanisms on carrier concentration modulations, phase transitions, magneto-optical effects, etc., for various possible THz applications mainly with low-dimensional materials, vanadium dioxide films, and superconductors.
Up to now, at least 806 carbon allotropes have been proposed theoretically. Three interesting carbon allotropes (named Pbam-32, P6/mmm, and I4ˉ3d) were recently uncovered based on a random sampling strategy combined with space group and graph theory. The calculation results show that they are superhard and remarkably stable compared with previously proposed metastable phases. This indicates that they are likely to be synthesized in experiment. We use the factor group analysis method to analyze their Γ -point vibrational modes. Owing to their large number of atoms in primitive unit cells (32 atoms in Pbam-32, 36 atoms in P6/mmm, and 94 atoms in I4ˉ3d), they have many Raman- and infrared-active modes. There are 48 Raman-active modes and 37 infrared-active modes in Pbam-32, 24 Raman-active modes and 14 infrared-active modes in P6/mmm, and 34 Raman-active modes and 35 Raman- and infrared-active modes in I4ˉ3d. Their calculated Raman spectra can be divided into middle frequency range from 600 cm-1 to 1150 cm-1 and high frequency range above 1150 cm-1. Their largest infrared intensities are 0.82, 0.77, and 0.70 (D/?)2/amu for Pbam, P6/mmm, and I4ˉ3d, respectively. Our calculated results provide an insight into the lattice vibrational spectra of these sp3 carbon allotropes and suggest that the middle frequency Raman shift and infrared spectrum may play a key role in identifying newly proposed carbon allotropes.
With the semiclassical ensemble model, we explore the relative phase-dependent nonsequential double ionization (NSDI) of Mg by counter-rotating two-color circularly polarized (TCCP) laser pulses. The yield of Mg2+ sensitively depends on the relative phase Δφ and the intensity of TCCP laser fields. At Δφ = 1.5π, the yield of Mg2+ exhibits a pronounced peak in the 0.05 PW/cm2 laser field. This behavior results from the increase of the initial transverse velocity compensating for the drift velocity with the decreasing angle by analyzing the angular distributions of the electron pairs in four relative phases. By changing the relative phases, we find that the recollision excitation with subsequent ionization and the recollision-impact ionization mechanisms can be controlled with TCCP laser fields.
The effects of temperature and pressure on laser-induced fluorescence (LIF) of OH are numerically studied under the excitation of A–X (1,0) transition at high pressures. A detailed theoretical analysis is carried out to reveal the physical processes of LIF. It is shown that high pressure LIF measurements get greatly complicated by the variations of pressure- and temperature-dependent parameters, such as Boltzmann fraction, absorption lineshape broadening, central-frequency shifting, and collisional quenching. Operations at high pressures require a careful choice of an excitation line, and the Q1(8) line in the A–X (1,0) band of OH is selected due to its minimum temperature dependence through the calculation of Boltzmann fraction. The absorption spectra of OH become much broader as pressure increases, leading to a smaller overlap integral and thus smaller excitation efficiency. The central-frequency shifting cannot be omitted at high pressures, and should be taken into account when setting the excitation frequency. The fluorescence yield is estimated based on the LASKIN calculation. Finally, OH-LIF measurements were conducted on flat stoichiometric CH4/air flames at high pressures. And both the numerical and experimental results illustrate that the pressure dependence of fluorescence yield is dominated, and the fluorescence yield is approximately inversely proportional to pressure. These results illustrate the physical processes of OH-LIF and provide useful guidelines for high-pressure application of OH-LIF.
The nonradiative charge-transfer processes of Be3+(1s)/B4+(1s) colliding with He(1s2) are investigated by the quantum-mechanical molecular orbital close-coupling (QMOCC) method from 10 eV/u to 1800 eV/u. Total and state-selective cross sections are obtained and compared with other results available. Although the incident ions have the same number of electrons and collide with the same target, their cross sections are different due to the differences in molecular structure. For Be3+(1s) + He(1s2), only single-electron-capture (SEC) states are important and the total cross sections have a broad maximum around E = 150 eV/u. While for B4+(1s) + He(1s2), both the SEC and double-electron-capture (DEC) processes are important, and the total SEC and DEC cross sections decrease rapidly with the energy decreasing.
A novel leaky-wave antenna (LWA) utilizing spoof surface plasmon polaritons (SSPPs) excitation is proposed with continuous scanning range from endfire to forward. The designed transmission line unit supports two SSPPS modes, of which the 2nd order mode is applied in the design. A novel strategy has been devised to excite the spatial radiation of the –1st order harmonics by arranging periodic counter changed sinusoidal structures on both sides of the SSPPs transmission line. Both full-wave simulation and measurement results show that the proposed LWA presents wide scanning angle from endfire to forward. In the frequency range from 4 GHz to 10 GHz, LWAs achieve scanning from 90° to +20°, covering the entire backward quadrant continuously.
Recent progresses on quantum control of cold atoms and trapped ions in both the scientific and technological aspects greatly advance the applications in precision measurement. Thanks to the exceptional controllability and versatility of these massive quantum systems, unprecedented sensitivity has been achieved in clocks, magnetometers, and interferometers based on cold atoms and ions. Besides, these systems also feature many characteristics that can be employed to facilitate the applications in different scenarios. In this review, we briefly introduce the principles of optical clocks, cold atom magnetometers, and atom interferometers used for precision measurement of time, magnetic field, and inertial forces. The main content is then devoted to summarize some recent experimental and theoretical progresses in these three applications, with special attention being paid to the new designs and possibilities towards better performance. The purpose of this review is by no means to give a complete overview of all important works in this fast developing field, but to draw a rough sketch about the frontiers and show the fascinating future lying ahead.
We demonstrate visible-light all-fiber vortex lasers by incorporating the home-made mode selective couplers (MSCs). The MSC at green or red wavebands is fabricated by specially designing and fusing a single-mode fiber (SMF) and a few-mode fiber (FMF). The MSCs inserted into visible fiber cavities act as power splitters and mode converters from the LP01 to LP11 mode at green and red wavelengths, respectively. The red-light all-fiber vortex laser is formed by a 10-cm Pr3+/Yb3+:ZBLAN fiber, a fiber Bragg grating, a fiber end-facet mirror and the MSC at 635 nm, which generates vortex beams with OAM±1 at 634.4 nm and an output power of 13 mW. The green-light all-fiber vortex laser consists of a 12-cm Ho3+:ZBLAN fiber, two fiber pigtail mirrors, and the MSC at 550 nm, which generates vortex beams with OAM±1 at 548.9 nm and an output power of 3 mW.
Compact atomic gravimeters are the potential next generation precision instruments for gravity survey from fundamental research to broad field applications. We report the calibration results of our home build compact absolute atomic gravimeter USTC-AG02 at Changping Campus, the National Institute of Metrology (NIM), China in January 2019. The sensitivity of the atomic gravimeter reaches 35.5μGal/Hz (1 μGal = 1 × 10-8 m/s2) and its long-term stability reaches 0.8 μGal for averaging over 4000 seconds. Considering the statistical uncertainty, the dominant instrumental systematic errors and environmental effects are evaluated and corrected within a total uncertainty (2σ) of 15.3 μGal. After compared with the reference g value given by the corner cube gravimeter NIM-3A, the atomic gravimeter USTC-AG02 reaches the degree of equivalence of 3.7 μGal.
Resonance enhanced two-photon ionization process of hydrogen atom via the resonant laser pulse is studied by Bohmian mechanics (BM) method. By analyzing the trajectories and energies of Bohmian particles (BPs), we find that under the action of high frequency and low intensity multi-circle resonant laser pulses, the ionized BPs first absorb one photon completing the excitation, and then absorb another photon, completing the ionization after staying in the first excited state for a period of time. The analysis of work done by the forces shows that the electric field force and quantum force play a major role in the whole ionization process. At the excitation moment and in the excitation-ionization process, the effect of the quantum force is greater than that of the electric field force. Finally, we discuss the principle of work and energy for BPs, and find that the electric field force and quantum force are non-conservative forces whose work is equal to the increment of mechanical energy of the system. In addition, it is proved that the quantum potential energy actually comes from the kinetic energy of the system and the increment of kinetic energy is equal to that of the kinetic energy of the system.
Based on the coupled acoustic scattering of two neighboring fluid-filled thin elastic shells suspending in an unbounded viscous liquid, an analytical method is developed to calculate the acoustic radiation force (ARF) of the shells. Two physical effects are taken into account: elastic radiation scattering and the multiple interactions of shells. Numerical results reveal that the magnitude of ARF can be enhanced by the sound radiation from the elastic shell undergoing forced vibrations and two resonant peaks can be observed on the ARF function curves. The feature of the lower peak is determined by the interactions and acoustic response of the back shell. The attractive forces can be obtained in the low kR1 band for the case of radius ratio R2/R1 > 1, while the magnitude of ARF at the lower peak may be influenced to some extent by acoustic shielding phenomenon for the case of radius ratio R2/R1 < 1. Accordingly, the interactions of particles cannot be ignored. The results may provide a theoretical basis for precisive manipulation of multiple particle systems.
We proposed a vanadium dioxide (VO2)-integrated multi-functional metamaterial structure that consists of three metallic grating layers and two VO2 films separated by SiO2 dielectric spacers. The proposed structure can be flexibly switched among three states by adjusting temperature, incident direction, and polarization. In state 1, the incident wave is strongly transmitted and perfectly converted to its orthogonal polarization state. In state 2, the incident wave is perfectly absorbed. In state 3, incident wave is totally reflected back. The working frequency of the multi-functional metamaterial can be arbitrarily tuned within a broad pass band. We believe that our findings are beneficial in designing temperature-controlled metadevices.
The ionization processes of NH3 molecule are studied by photoelectron velocity map imaging technique in a linearly polarized 400-nm femtosecond laser field. The two-dimensional photoelectron images from ammonia molecules under different laser intensities are obtained. In the slow electron region, the values of kinetic energy of photoelectrons corresponding to peaks 1, 2, 3, and 4 are 0.27, 0.86, 1.16, and 1.6 eV, respectively. With both the kinetic energy and angular distribution of photoelectrons from NH3 molecules, we can confirm that the two-photon excited intermediate Rydberg state is A～1A2′′ (v2′=3) state for photoelectron peaks 2, 3, 4, and the three peaks are marked as 1223 (2 + 2), 1123 (2 + 2), and 1023 (2 + 2) multi-photon processes, respectively. Then, peak 1 is found by adding a hexapole between the source chamber and the detection chamber to realize the rotational state selection and beam focusing. Peak 1 is labeled as the 1323 (3 + 1) multi-photon process through the intermediate Rydberg state E～1A1′. The phenomena of channel switching are found in the slow electron kinetic energy distributions. Our calculations and experimental results indicate that the stretching vibrational mode of ammonia molecules varies with channels, while the umbrella vibration does not. In addition, we consider and discuss the ac-Stark effect in a strong laser field. Peaks 5 and 6 are marked as (2 + 2 + 1) and (2 + 2 + 2) above threshold ionization processes in the fast electron region.
An all-fiber dumbbell-shaped dual-amplifier mode-locked Er-doped laser that can function in dissipative soliton resonance (DSR) regime is demonstrated. A nonlinear optical loop mirror (NOLM) and a nonlinear amplifying loop mirror (NALM) are employed to initiate the mode-locking pulses. Unlike conventional single-amplifier structure, the output peak power of which remains unchanged when pump power is varied, the proposed structure allows its output peak power to be tuned by changing the pump power of the two amplifiers while the pulse duration is directly determined by the amplifier of nonlinear amplifying loop mirror. The entire distribution maps of peak power and pulse duration clearly demonstrate that the two amplifiers are related to each other, and they supply directly a guideline for designing tunable peak power DSR fiber laser. Pulse width can change from 800 ps to 2.6 ns and peak power varies from 13 W to 27 W. To the best of our knowledge, the peak power tunable DSR pulse is observed for the first time in dumbbell-shaped Er-doped all-fiber mode-locked lasers.
The electronic structures and magnetic properties of diverse transition metal (TM = Fe, Co, and Ni) and nitrogen (N) co-doped monolayer MoS2 are investigated by using density functional theory. The results show that the intrinsic MoS2 does not have magnetism initially, but doped with TM (TM = Fe, Co, and Ni) the MoS2 possesses an obvious magnetism distinctly. The magnetic moment mainly comes from unpaired Mo:4d orbitals and the d orbitals of the dopants, as well as the S:3p states. However, the doping system exhibits certain half-metallic properties, so we select N atoms in the V family as a dopant to adjust its half-metal characteristics. The results show that the (Fe, N) co-doped MoS2 can be a satisfactory material for applications in spintronic devices. On this basis, the most stable geometry of the (2Fe–N) co-doped MoS2 system is determined by considering the different configurations of the positions of the two Fe atoms. It is found that the ferromagnetic mechanism of the (2Fe–N) co-doped MoS2 system is caused by the bond spin polarization mechanism of the Fe–Mo–Fe coupling chain. Our results verify that the (Fe, N) co-doped single-layer MoS2 has the conditions required to become a dilute magnetic semiconductor.
Due to their unique characteristics, two-dimensional (2D) materials have drawn great attention as promising candidates for the next generation of integrated circuits, which generate a calculation unit with a new working mechanism, called a logic transistor. To figure out the application prospects of logic transistors, exploring the temperature dependence of logic characteristics is important. In this work, we explore the temperature effect on the electrical characteristic of a logic transistor, finding that changes in temperature cause transformation in the calculation: logical output converts from ‘AND’ at 10 K to ‘OR’ at 250 K. The transformation phenomenon of temperature regulation in logical output is caused by energy band which decreases with increasing temperature. In the experiment, the indirect band gap of MoS2 shows an obvious decrease from 1.581 eV to 1.535 eV as the temperature increases from 10 K to 250 K. The change of threshold voltage with temperature is consistent with the energy band, which confirms the theoretical analysis. Therefore, as a promising material for future integrated circuits, the demonstrated characteristic of 2D transistors suggests possible application for future functional devices.
Majorana fermions have been predicted to exist at the edge states of a two-dimensional topological superconductor. We fabricated single quintuple layer (QL) Bi2Te3/FeTe heterostructure with the step-flow epitaxy method and studied the topological properties of this system by using angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. We observed the coexistence of robust superconductivity and edge states on the single QL Bi2Te3 islands which can be potential evidence for topological superconductor.
Last decade has witnessed a rapid development of the generation of terahertz (THz) vortex beams as well as their wide applications, mainly due to their unique combination characteristics of regular THz radiation and orbital angular momentum (OAM). Here we have reviewed the ways to generate THz vortex beams by two representative scenarios, i.e., THz wavefront modulation via specific devices, and direct excitation of the helicity of THz vortex beams. The former is similar to those wavefront engineering devices in the optical and infrared (IR) domain, but just with suitable THz materials, while the latter is newly-developed in THz regime and some of the physical mechanisms still have not been explained explicitly enough though, which would provide both challenges and opportunities for THz vortex beam generation. As for their applications, thanks to the recent development of THz optics and singular optics, THz vortex beams have potentials to open doors towards a myriad of practice applications in many fields. Besides, some representative potential applications are evaluated such as THz wireless communication, THz super-resolution imaging, manipulating chiral matters, accelerating electron bunches, and detecting astrophysical sources.
Many complex networks in real life are embedded in space and most infrastructure networks are interdependent, such as the power system and the transport network. In this paper, we construct two cascading failure models on the multilayer spatial network. In our research, the distance l between nodes within the layer obeys the exponential distribution P(l) ～ exp(–l/ζ), and the length r of dependency link between layers is defined according to node position. An entropy approach is applied to analyze the spatial network structure and reflect the difference degree between nodes. Two metrics, namely dynamic network size and dynamic network entropy, are proposed to evaluate the spatial network robustness and stability. During the cascading failure process, the spatial network evolution is analyzed, and the numbers of failure nodes caused by different reasons are also counted, respectively. Besides, we discuss the factors affecting network robustness. Simulations demonstrate that the larger the values of average degree ?k?, the stronger the network robustness. As the length r decreases, the network performs better. When the probability p is small, as ζ decreases, the network robustness becomes more reliable. When p is large, the network robustness manifests better performance as ζ increases. These results provide insight into enhancing the robustness, maintaining the stability, and adjusting the difference degree between nodes of the embedded spatiality systems.
Hall thruster is an electric propulsion device for attitude control and position maintenance of satellites. The discharge process of Hall thruster will produce divergent plume. The plume will cause erosion, static electricity, and other interference to the main components, such as solar sailboard, satellite body, and thruster. Therefore, reducing the divergence of the plume is an important content in the research of thruster plume. The additional electrode to the plume area is a way to reduce the divergence angle of the plume, but there are few related studies. This paper uses the particle-in-cell (PIC) simulation method to simulate the effect of the additional electrode on the discharge of the Hall thruster, and further explains the effect mechanism of the additional electrode on parameters such as the electric field and plume divergence angle. The simulation results show that the existence of the additional electrode can enhance the potential near the additional position. The increase of the potential can effectively suppress the radial diffusion of ions, and effectively reduce the plume divergence angle. The simulation results show that when the additional electrode is 30 V, the half plume divergence angle can be reduced by 18.21%. However, the existence of additional electric electrode can also enhance the ion bombardment on the magnetic pole. The additional electrode is relatively outside, the plume divergence angle is relatively small, and it can avoid excessive ion bombardment on the magnetic pole. The research work of this paper can provide a reference for the beam design of Hall thruster.
Grouping different oxide materials with coupled charge, spin, and orbital degrees of freedom together to form heterostructures provides a rich playground to explore the emergent interfacial phenomena. The perovskite/brownmillerite heterostructure is particularly interesting since symmetry mismatch may produce considerable interface reconstruction and unexpected physical effects. Here, we systemically study the magnetic anisotropy of tensely strained La2/3Sr1/3Co1 – xMnxO2.5 + δ/La2/3Sr1/3MnO3/La2/3Sr1/3Co1 – xMnxO2.5 + δ trilayers with interface structures changing from perovskite/brownmillerite type to perovskite/perovskite type. Without Mn doping, the initial La2/3Sr1/3CoO2.5 + δ/La2/3Sr1/3MnO3/La2/3Sr1/3CoO2.5 + δ trilayer with perovskite/brownmillerite interface type exhibits perpendicular magnetic anisotropy and the maximal anisotropy constant is 3.385 × 106 erg/cm3, which is more than one orders of magnitude larger than that of same strained LSMO film. By increasing the Mn doping concentration, the anisotropy constant displays monotonic reduction and even changes from perpendicular magnetic anisotropy to in-plane magnetic anisotropy, which is possible because of the reduced CoO4 tetrahedra concentration in the La2/3Sr1/3Co1 – xMnxO2.5 + δ layers near the interface. Based on the analysis of the x-ray linear dichroism, the orbital reconstruction of Mn ions occurs at the interface of the trilayers and thus results in the controllable magnetic anisotropy.
The ε-Ga2O3 p–n heterojunctions (HJ) have been demonstrated using typical p-type oxide semiconductors (NiO or SnO). The ε-Ga2O3 thin film was heteroepitaxial grown by metal organic chemical vapor deposition (MOCVD) with three-step growth method. The polycrystalline SnO and NiO thin films were deposited on the ε-Ga2O3 thin film by electron-beam evaporation and thermal oxidation, respectively. The valence band offsets (VBO) were determined by x-ray photoelectron spectroscopy (XPS) to be 2.17 eV at SnO/ε-Ga2O3 and 1.7 eV at NiO/ε-Ga2O3. Considering the bandgaps determined by ultraviolet-visible spectroscopy, the conduction band offsets (CBO) of 0.11 eV at SnO/ε-Ga2O3 and 0.44 eV at NiO/ε-Ga2O3 were obtained. The type-II band diagrams have been drawn for both p–n HJs. The results are useful to understand the electronic structures at the ε-Ga2O3 p–n HJ interface, and design optoelectronic devices based on ε-Ga2O3 with novel functionality and improved performance.
At least four two- or quasi-one-dimensional allotropes and a mixture of them were theoretically predicted or experimentally observed for low-dimensional Te, namely the α, β, γ, δ, and chiral-α + δ phases. Among them the γ and α phases were found to be the most stable phases for monolayer and thicker layers, respectively. Here, we found two novel low-dimensional phases, namely the ε and ζ phases. The ζ phase is over 29 meV/Te more stable than the most stable monolayer γ phase, and the ε phase shows comparable stability with the most stable monolayer γ phase. The energetic difference between the ζ and α phases reduces with respect to the increased layer thickness and vanishes at the four-layer (12-sublayer) thickness, while this thickness increases under change doping. Both ε and ζ phases are metallic chains and layers, respectively. The ζ phase, with very strong interlayer coupling, shows quantum well states in its layer-dependent bandstructures. These results provide significantly insight into the understanding of polytypism in Te few-layers and may boost tremendous studies on properties of various few-layer phases.
Despite the apparent ubiquity and variety of quantum spin liquids in theory, experimental confirmation of spin liquids remains to be a huge challenge. Motivated by the recent surge of evidences for spin liquids in a series of candidate materials, we highlight the experimental schemes, involving the thermal Hall transport and spectrum measurements, that can result in smoking-gun signatures of spin liquids beyond the usual ones. For clarity, we investigate the square lattice spin liquids and theoretically predict the possible phenomena that may emerge in the corresponding spin liquids candidates. The mechanisms for these signatures can be traced back to either the intrinsic characters of spin liquids or the external field-driven behaviors. Our conclusion does not depend on the geometry of lattices and can broadly apply to other relevant spin liquids.
Compactness and miniaturization have become increasingly important in the development of high-power microwave devices. Based on this rising demand, a novel C-band coaxial transit-time oscillator (TTO) with a low external guiding magnetic field is proposed and analyzed. The proposed device has the following advantages: simple structure, short axial length, high power conversion efficiency, and low external guiding magnetic field, which are of great significance for developing the compact and miniaturized high-power microwave devices. The application of a shorter axial length is made possible by the use of a transit radiation mechanism. Also, loading the opening foil symmetrically to both ends of the buncher helps reduce the external magnetic field of the proposed device. Unlike traditional foils, the proposed opening foil has a circular-hole; therefore, the electron beam will not bombard the conductive foil to generate plasma. This makes it possible to realize long pulse and high repetition rate operation of the device in future experiments. Through numerical calculation and PIC particle simulation, the stability of the intense relativistic electron beam (IREB) and the saturation time of the device are improved by using the conductive foil. The voltage and current of the diode are 548 kV and 11.4 kA, respectively. Under a 0.4-T external guiding magnetic field, a C-band output microwave with a frequency of 4.27 GHz and power of 1.88 GW can be generated. The power conversion efficiency of the proposed device is about 30%.
In this paper, we investigate the effects of lattice strain on the electrical and magnetotransport properties of La0.7Sr0.3MnO3 (LSMO) films by changing film thickness and substrate. For electrical properties, a resistivity upturn emerges in LSMO films, i.e., LSMO/STO and LSMO/LSAT with small lattice strain at a low temperature, which originates from the weak localization effect. Increasing film thickness weakens the weak localization effect, resulting in the disappearance of resistivity upturn. While in LSMO films with a large lattice strain (i.e., LSMO/LAO), an unexpected semiconductor behavior is observed due to the linear defects. For magnetotransport properties, an anomalous in-plane magnetoresistance peak (pMR) occurs at low temperatures in LSMO films with small lattice strain, which is caused by two-dimensional electron gas (2DEG). Increasing film thickness suppresses the 2DEG, which weakens the pMR. Besides, it is found that the film orientation has no influence on the formation of 2DEG. While in LSMO/LAO films, the 2DEG cannot form due to the existence of linear defects. This work can provide an efficient way to regulate the film transport properties.
We report observation of dispersion for coupled exciton-polariton in a plate microcavity combining with ZnO/MgZnO multi-quantum well (QW) at room temperature. Benefited from the large exciton binding energy and giant oscillator strength, the room-temperature Rabi splitting energy can be enhanced to be as large as 60 meV. The results of excitonic polariton dispersion can be well described using the coupling wave model. It is demonstrated that mode modification between polariton branches allowing, just by controlling the pumping location, to tune the photonic fraction in the different detuning can be investigated comprehensively. Our results present a direct observation of the exciton-polariton dispersions based on two-dimensional oxide semiconductor quantum wells, thus provide a feasible road for coupling of exciton with photon and pave the way for realizing novel polariton-type optoelectronic devices.
We theoretically study the Josephson effect in a quantum anomalous Hall insulator (QAHI) nanoribbon with a domain wall structure and covered by the superconductor. The anomalous Josephson current, the nonzero supercurrent at the zero superconducting phase difference, appears with the nonzero magnetization and the suitable azimuth angle of the domain wall. Dependent on the configuration of the domain wall, the anomalous current peaks in the Bloch type but disappears in the Néel type because the y-component of magnetization is necessary to break symmetry to arouse the anomalous current. The phase shift of the anomalous current is tunable by the magnetization, the azimuth angle, or the thickness of the domain wall. By introducing a bare QAHI region in the middle of the junction which is not covered by the superconductor, the anomalous Josephson effect is enhanced such that the phase shift can exceed π. Thus, a continuous change between 0 and π junctions is realized via regulating the configuration of the domain wall or the magnetization strength. As long as an s-wave superconductor is placed on the top of the QAHI with a domain wall structure, this proposal can be experimentally fabricated and useful for the phase battery or superconducting quantum bit.
By applying the first principles calculations combined with density functional theory (DFT), this study explored the optical properties, electronic structure, and structure stability of triangular borophene decorated chemically, B3X (X = F, Cl) in a systematical manner. As revealed from the results of formation energy, phonon dispersion, and molecular dynamics simulation study, all the borophene decorated chemically were superior and able to be fabricated. In the present study, triangular borophene was reported to be converted into Dirac-like materials when functionalized by F and Cl exhibiting narrow direct band gaps as 0.19 eV and 0.17 eV, separately. Significant light absorption was assessed in the visible light and ultraviolet region. According the mentioned findings, these two-dimensional (2D) materials show large and wide promising applications for future nanoelectronics and optoelectronics.
We investigate the enhanced chirality of chiral molecular J-aggregates (TDBC) by the propagating surface plasmons (PSPs) in the metallic hole array structure filled with TDBC. The two ends of the hole in the metal film form a low quality factor Fabry–Perot (FP) cavity, and this cavity confines PSPs. The resonant wavelength of the metallic hole array is tuned by the lattice constant and the size of the hole. Both the resonant wavelength of Ag hole array and the volume ratio of TDBC in the hybridized structure influence on the enhancement of the circular dichroism (CD) spectrum. The curve of CD spectrum shows Fano-like line-shape, due to the interaction between the non-radiative field in the FP cavity and the radiative field in chiral TDBC. The maximum of the CD spectrum of the hybridized structure is 0.025 times as the one of the extinction spectrum in a certain structure, while the maximum of the CD spectrum of TDBC is 1/3000 times as the one of the extinction spectrum. The enhanced factor is about 75. The resonant wavelength of the metallic hole array can be tuned in a large wavelength regime, and the chirality of a series of molecular J-aggregates with different resonant wavelengths can be enhanced. Our structure provides a new method to amplify the chirality of molecular J-aggregates in experiments.
The drying of liquid droplets is a common phenomenon in daily life, and has long attracted special interest in scientific research. We propose a simple model to quantify the shape evolution of drying droplets. The model takes into account the friction constant between the contact line (CL) and the substrate, the capillary forces, and the evaporation rate. Two typical evaporation processes observed in experiments, i.e., the constant contact radius (CCR) and the constant contact angle (CCA), are demonstrated by the model. Moreover, the simple model shows complicated evaporation dynamics, for example, the CL first spreads and then recedes during evaporation. Analytical models of no evaporation, CCR, and CCA cases are given, respectively. The scaling law of the CL or the contact angle as a function of time obtained by analytical model is consistent with the full numerical model, and they are all subjected to experimental tests. The general model facilitates a quantitative understanding of the physical mechanism underlying the drying of liquid droplets.
Pre-warning plays an important role in emergency handling, especially in urban areas with high population density like Beijing. Knowing the information dissemination mechanisms clearly could help us reduce losses and ensure the safety of human beings during emergencies. In this paper, we propose the models of pre-warning information dissemination via five classical media based on actual pre-warning issue processes, including television, radio, short message service (SMS), electronic screens, and online social networks. The population coverage ability and dissemination efficiency at different issue time of these five issue channels are analyzed by simulation methods, and their advantages and disadvantages are compared by radar graphs. Results show that SMS is the most appropriate way to issue long-term pre-warning for its large population coverage, but it is not suitable for issuing urgent warnings to large population because of the limitation of telecom company’s issue ability. TV shows the best performance to combine the dissemination speed and range, and the performance of radio and electronic screens are not as satisfactory as the others. In addition, online social networks might become one of the most promising communication method for its potential in further diffusion. These models and results could help us make pre-warning issue plans and provide guidance for future construction of information diffusion systems, thus reducing injuries, deaths, and other losses under different emergencies.
The strengthening effects of alloying elements Re, Ta, and W in the  (001) dislocation core of the γ / γ′ interface are studied by first-principles calculations. From the level of energy the substitution formation energies and the migration energies of alloying elements are computed and from the level of electron the differential charge density (DCD) and the partial density of states (PDOSs) are computed. Alloying elements above are found to tend to substitute for Al sites γ′ phase by analyzing the substitution formation energy. The calculation results for the migration energies of alloying elements indicate that the stability of the  (001) dislocation core is enhanced by adding Ta, W, and Re and the strengthening effect of Re is the strongest. Our results agree with the relevant experiments. The electronic structure analysis indicates that the electronic interaction between Re-nearest neighbor (NN) Ni is the strongest. The reason why the doped atoms have different strengthening effects in the  (001) dislocation core is explained at the level of electron.
Group-V elemental nanofilms were predicted to exhibit interesting physical properties such as nontrivial topological properties due to their strong spin–orbit coupling, the quantum confinement, and surface effect. It was reported that the ultrathin Sb nanofilms can undergo a series of topological transitions as a function of the film thickness h: from a topological semimetal (h > 7.8 nm) to a topological insulator (7.8 nm > h > 2.7 nm), then a quantum spin Hall (QSH) phase (2.7 nm > h > 1.0 nm) and a topological trivial semiconductor (h > 1.0 nm). Here, we report a comprehensive investigation on the epitaxial growth of Sb nanofilms on highly oriented pyrolytic graphite (HOPG) substrate and the controllable thermal desorption to achieve their specific thickness. The morphology, thickness, atomic structure, and thermal-strain effect of the Sb nanofilms were characterized by a combination study of scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The realization of Sb nanofilms with specific thickness paves the way for the further exploring their thickness-dependent topological phase transitions and exotic physical properties.
A fluid model is employed to investigate the effect of radio frequency bias on the behavior of an argon inductively coupled plasma (ICP). In particular, the effects of ICP source power, single-frequency bias power, and dual-frequency bias power on the characteristics of ICP are simulated at a fixed pressure of 30 mTorr (1 Torr = 1.33322 × 102 Pa). When the bias frequency is fixed at 27.12 MHz, the two-dimensional (2D) plasma density profile is significantly affected by the bias power at low ICP source power (e.g., 50 W), whereas it is weakly affected by the bias power at higher ICP source power (e.g., 100 W). When dual-frequency (27.12 MHz/2.26 MHz) bias is applied and the sum of bias powers is fixed at 500 W, a pronounced increase in the maximum argon ion density is observed with the increase of the bias power ratio in the absence of ICP source power. As the ratio of 27.12-MHz/2.26-MHz bias power decreases from 500 W/0 W to 0 W/500 W with the ICP source power fixed at 50 W, the plasma density profiles smoothly shifts from edge-high to center-high, and the effect of bias power on the plasma distribution becomes weaker with the bias power ratio decreasing. Besides, the axial ion flux at the substrate surface is characterized by a maximum at the edge of the substrate. When the ICP source power is higher, the 2D plasma density profiles, as well as the spatiotemporal and radial distributions of ion flux at the substrate surface are characterized by a peak in the reactor center, and the distributions of plasma parameters are negligibly affected by the dual-frequency bias power ratio.
Two-dimensional (2D) crystals are known to have no bulk but only surfaces and edges, thus leading to unprecedented properties thanks to the quantum confinements. For half a century, the compression of z-dimension has been attempted through ultra-thin films by such as molecular beam epitaxy. However, the revisiting of thin films becomes popular again, in another fashion of the isolation of freestanding 2D layers out of van der Waals (vdW) bulk compounds. To date, nearly two decades after the nativity of the great graphene venture, researchers are still fascinated about flattening, into the atomic limit, all kinds of crystals, whether or not they are vdW. In this introductive review, we will summarize some recent experimental progresses on 2D electronic systems, and briefly discuss their revolutionizing capabilities for the implementation of future nanostructures and nanoelectronics.
We investigate structural, mechanical, thermodynamic, and thermoelectric properties of vanadium-based XVO3 (X = Na, K, Rb) materials using density functional theory (DFT) based calculations. The structural and thermodynamic stabilities are probed by the tolerance factor (0.98, 1.01, and 1.02) with the negative value of enthalpy of formation. Mechanical properties are analyzed in the form of Born stability criteria, ductile/brittle nature (Poisson and Pugh’s ratios) and anisotropy factor. To explore the electronic transport properties, we study the electrical conductivity, thermal conductivity, Seebeck coefficient and power factor in terms of chemical potential and temperature. High values of Seebeck coefficient at room temperature may find the potential of the studied perovskites in thermo-electrics devices.
We present the behaviors of both dynamical and static charge susceptibilities of doped armchair nanotubes using the Green function approach in the context of Holstein-model Hamiltonian. Specially, the effects of magnetization and gap parameter on the the plasmon modes of armchair nanotube are investigated via calculating correlation function of charge density operators. Random phase approximation has been implemented to find the interacting dynamical charge susceptibility. The electrons in this systems interacts with each other by mediation of dispersionless Holstein phonons. Our results show that the increase of gap parameter leads to decreasing intensity of charge collective mode. Also the frequency position of the collective mode tends to higher frequencies due to the gap parameter. Furthermore the number of collective excitation mode decreases with chemical potential in the presence of electron–phonon interaction. Finally the temperature dependence of static charge structure factor of armchair nanotubes is studied. The effects of the gap parameter, magnetization and electron–phonon interaction on the static structure factor are addressed in details.
HfAlO/InAlAs metal–oxide–semiconductor capacitor (MOS capacitor) is considered as the most popular candidate of the isolated gate of InAs/AlSb high electron mobility transistor (HEMT). In order to improve the performance of the HfAlO/InAlAs MOS-capacitor, samples are annealed at different temperatures for investigating the HfAlO/InAlAs interfacial characyeristics and the device’s electrical characteristics. We find that as annealing temperature increases from 280 °C to 480 °C, the surface roughness on the oxide layer is improved. A maximum equivalent dielectric constant of 8.47, a minimum equivalent oxide thickness of 5.53 nm, and a small threshold voltage of –1.05 V are detected when being annealed at 380 °C; furthermore, a low interfacial state density is yielded at 380 °C, and this can effectively reduce the device leakage current density to a significantly low value of 1 × 10-7 A/cm2 at 3-V bias voltage. Therefore, we hold that 380 °C is the best compromised annealing temperature to ensure that the device performance is improved effectively. This study provides a reliable conceptual basis for preparing and applying HfAlO/InAlAs MOS-capacitor as the isolated gate on InAs/AlSb HEMT devices.
Torque measurements were performed on single crystal samples of Ca0.73La0.27(Fe0.96Co0.04)As2 in both the normal and superconducting states. Contributions to the torque signal from the paramagnetism and the vortex lattice were identified. The superconducting anisotropy parameter γ was determined from the reversible part of the vortex contribution based on Kogan’s model. It is found that γ ? 7.5 at t = T/Tc = 0.85, which is smaller than the result of CaFe0.88Co0.12AsF γ ? 15 at t = 0.83, but larger than the result of 11 and 122 families, where γ stays in the range of 2–3. The moderate anisotropy of this 112 iron-based superconductor fills the gap between 11, 122 families and 1111 families. In addition, we found that the γ shows a temperature dependent behavior, i.e., decreasing with increasing temperature. The fact that γ is not a constant point towards a multiband scenario in this compound.
In supersonic flowing plasmas, the auto-resonant behavior of ion acoustic waves driven by stimulated Brillouin backscattering is self-consistently investigated. A nature of absolute instability appears in the evolution of the stimulated Brillouin backscattering. By adopting certain form of incident lights combined by two perpendicular linear polarization lasers or polarization rotation lasers, the absolute instability is suppressed significantly. The suppression of auto-resonant stimulated Brillouin scattering is verified with the fully kinetic Vlasov code.
Being parent materials of two-dimensional (2D) crystals, van der Waals layered materials have received revived interest. In most 2D materials, the interaction between electrons is negligible. Introducing the interaction can give rise to a variety of exotic properties. Here, via intercalating a van der Waals layered compound VS2, we find evidence for electron correlation by extensive magnetic, thermal, electrical, and thermoelectric characterizations. The low temperature Sommerfeld coefficient is 64 mJ?K-2?mol-1 and the Kadowaki–Woods ratio rKW ～ 0.20a0. Both supports an enhancement of the electron correlation. The temperature dependences of the resistivity and thermopower indicate an important role played by the Kondo effect. The Kondo temperature TK is estimated to be around 8 K. Our results suggest intercalation as a potential means to engineer the electron correlation in van der Waals materials, as well as 2D materials.
This work presents the Gaussian process tomography (GPT) based on Bayesian data analysis and its applications in soft x-ray (SXR) and absolute extreme ultraviolet spectroscopy (AXUV) diagnostics on experimental advanced superconducting tokamak (EAST). This is the first application of the GPT method in the AXUV diagnostic system in fusion devices. It is found that even if only horizontal detector arrays are used to reconstruct the two-dimensional (2D) distribution of SXR and AXUV emissivity fields, the GPT method performs robustly and extremely fast, which enables the GPT method to provide real-time feedback on impurity transport and fast magnetohydrodynamics (MHD) events. By reconstructing SXR emissivity in the poloidal cross section on EAST, an m/n = 1/1 internal kink mode has been observed, and the plasma redistribution due to the kink mode is clearly visible in the reconstructions, where m is the poloidal mode number and n is the toroidal mode number. Sawtooth-like internal disruptions extended throughout the entire plasma core and mainly driven by the m/n = 2/1 mode have been acquired. During the sawtooth-like internal disruption crash phase, the conversion from an m = 2 mode to an m = 1 mode is observed. Using the reconstructed AXUV emissivity field we were able to observe the process of impurity accumulated in the plasma core and the mitigation of core impurity due to neon injection in the plasma edge. The data from all other diagnostics involved in the analysis shows that the reconstructions from AXUV measurements are reliable.
Effective improvement in electrical properties of NO passivated SiC/SiO2 interface after being irradiated by electrons is demonstrated. The density of interface traps after being irradiated by 100-kGy electrons decreases by about one order of magnitude, specifically, from 3×1012 cm-2?eV-1 to 4×1011 cm-2?eV-1 at 0.2 eV below the conduction band of 4H-SiC without any degradation of electric breakdown field. Particularly, the results of x-ray photoelectron spectroscopy measurement show that the C–N bonds are generated near the interface after electron irradiation, indicating that the carbon-related defects are further reduced.
Two-dimensional (2D) materials received large amount of studies because of the enormous potential in basic science and industrial applications. Monolayer Pd2Se3 is a fascinating 2D material that was predicted to possess excellent thermoelectric, electronic, transport, and optical properties. However, the fabrication of large-scale and high-quality monolayer Pd2Se3 is still challenging. Here, we report the synthesis of large-scale and high-quality monolayer Pd2Se3 on graphene-SiC (0001) by a two-step epitaxial growth. The atomic structure of Pd2Se3 was investigated by scanning tunneling microscope (STM) and confirmed by non-contact atomic force microscope (nc-AFM). Two subgroups of Se atoms have been identified by nc-AFM image in agreement with the theoretically predicted atomic structure. Scanning tunneling spectroscopy (STS) reveals a bandgap of 1.2 eV, suggesting that monolayer Pd2Se3 can be a candidate for photoelectronic applications. The atomic structure and defect levels of a single Se vacancy were also investigated. The spatial distribution of STS near the Se vacancy reveals a highly anisotropic electronic behavior. The two-step epitaxial synthesis and characterization of Pd2Se3 provide a promising platform for future investigations and applications.
Inspired by recent discoveries of the quasi-Josephson effect in shunted nanowire devices, we propose a superconducting nanowire interference device in this study, which is a combination of parallel ultrathin superconducting nanowires and a shunt resistor. A simple model based on the switching effect of nanowires and fluxoid quantization effect is developed to describe the behavior of the device. The current–voltage characteristic and flux-to-voltage conversion curves are simulated and discussed to verify the feasibility. Appropriate parameters of the shunt resistor and inductor are deduced for fabricating the devices.
The circadian clock is a self-sustained biological oscillator which can be entrained by environmental signals. The cyanobacteria circadian clock is the simplest one, which is composed of the proteins KaiA, KaiB and KaiC. The phosphorylation/dephosphorylation state of KaiC exhibits a circadian oscillator. KaiA and KaiB activate KaiC phosphorylation and dephosphorylation respectively. CikA competing with KaiA for the same binding site on KaiB affects the phosphorylation state of KaiC. Quinone is a signaling molecule for entraining the cyanobacterial circadian clock which is oxidized at the onset of darkness and reduced at the onset of light, reflecting the environmental light–dark cycle. KaiA and CikA can sense external signals by detecting the oxidation state of quinone. However, the entrainment mechanism is far from clear. We develop an enhanced mathematical model including oxidized quinone sensed by KaiA and CikA, with which we present a detailed study on the entrainment of the cyanobacteria circadian clock induced by quinone signals. We find that KaiA and CikA sensing oxidized quinone pulse are related to phase advance and delay, respectively. The time of oxidized quinone pulse addition plays a key role in the phase shifts. The combination of KaiA and CikA is beneficial to the generation of entrainment, and the increase of signal intensity reduces the entrainment phase. This study provides a theoretical reference for biological research and helps us understand the dynamical mechanisms of cyanobacteria circadian clock.
The multiple ferroelectric polarization tuned by external electric field could be used to simulate the biological synaptic weight. Ferroelectric synaptic devices have two advantages compared with other reported ones: One is that the intrinsic switching of ferroelectric domains without invoking of defect migration as in resistive oxides, contributes reliable performance in these ferroelectric synapses. Another tremendous advantage is the extremely low energy consumption because the ferroelectric polarization is manipulated by electric field which eliminates the Joule heating by current as in magnetic and phase change memories. Ferroelectric synapses have potential for the construction of low-energy and effective brain-like intelligent networks. Here we summarize recent pioneering work of ferroelectric synapses involving the structure of ferroelectric tunnel junctions (FTJs), ferroelectric diodes (FDs), and ferroelectric field effect transistors (FeFETs), respectively, and shed light on future work needed to accelerate their application for efficient neural network.
In this study, the effects of quantum dot size on the binding energy, radiative lifetime, and optical absorption coefficient of exciton state in both GaN/AlxGa1-xN core/shell and AlxGa1-xN/GaN inverted core/shell quantum dot structures are studied. For the GaN/AlxGa1-xN core/shell structure, the variation trend of binding energy is the same as that of radiation lifetime, both of which increase first and then decrease with the increase of core size. For AlxGa1-xN/GaN inverted core/shell structure, the binding energy decreases first and then increases with core size increasing, and the trends of radiation lifetime varying with core size under different shell sizes are different. For both structures, when the photon energy is approximately equal to the binding energy, the peak value of the absorption coefficient appears, and there will be different peak shifts under different conditions.
We report on the temperature-dependent Schottky barrier in organic solar cells based on PTB7:PC71BM. The ideality factor is found to increase with temperature decreasing, which is explained by a model in which the solar cell is taken as Schottky barrier diode. Accordingly, the dark current in the device originates from the thermally emitted electrons across the Schottky barrier. The fittings obtained with the thermal emission theory are systematically studied at different temperatures. It is concluded that the blend/Ca/Al interface presents great inhomogeneity, which can be described by 2 sets of Gaussian distributions with large zero bias standard deviations. With the decrease of temperature, electrons favor going across the Schottky barrier patches with lower barrier height and as a consequence the ideally factor significantly increases at low temperature.
We perform a computational simulation of light emissions from two sonoluminescent bubbles in water. Our simulation includes the radii of two bubbles, radiation acoustic pressures, and light emission spectra by numerically solving the pulsing equations of a two-bubble system and the equations of gas dynamics. The simulation results demonstrate that the motion of each bubble in the two-bubble system is restrained because of the radiation acoustic pressures from the other pulsing bubble. The restrained oscillation of a bubble with a small ambient radius is stronger than that of a bubble with a large ambient radius under the same driving acoustic pressure. This effect increases when the distance between the two bubbles decreases. When compared to single-bubble sonoluminescence, the interaction between two bubbles leads to generation of different spectral characteristics.
Transient receptor potential vanilloid subtype 1 (TRPV1) is a polymodel sensory receptor and can be activated by moderate temperature (≥ 43 °C). Though extensive researches on the heat-activation mechanism revealed some key elements that participate in the heat-sensation pathway, the detailed thermal-gating mechanism of TRPV1 is still unclear. We investigate the heat-activation process of TRPV1 channel using the molecular dynamics simulation method at different temperatures. It is found that the favored state of the supposed upper gate of TRPV1 cannot form constriction to ion permeation. Oscillation of S5 helix originated from thermal fluctuation and forming/breaking of two key hydrogen bonds can transmit to S6 helix through the hydrophobic contact between S5 and S6 helix. We propose that this is the pathway from heat sensor of TRPV1 to the opening of the lower gate. The heat-activation mechanism of TRPV1 presented in this work can help further functional study of TRPV1 channel.
Diamond crystallization was carried out with CH4N2S additive in the FeNiCo-C system at pressure 6.0 GPa and temperature ranging from 1290 °C to 1300 °C. The crystallization qualities of the synthetic crystals were characterized by Raman spectra and the Raman peaks located at 1331 cm-1. Fourier transform infrared (FTIR) results showed that the hydrogen-related absorption peak of the as-grown diamond was at 2920 cm-1, respectively. Interestingly, A-center nitrogen was observed in the obtained diamond and the characteristic absorption peaks located at 1095 cm-1 and 1282 cm-1. Especially, the absorption peak at 1426 cm-1 attributing to the aggregation B-center nitrogen defect was distinctly found when the CH4N2S content reached 0.3 mg in the synthesis system, which was extremely rare in synthetic diamond. Furthermore, optical color centers in the synthesized crystals were investigated by photoluminescence (PL).
The process of in situ tumors developing into malignant tumors and exhibiting invasive behavior is extremely complicated . From a biophysical point of view, it is a phase change process affected by many factors, including cell-to-cell, cell-to-chemical material, cell-to-environment interaction, etc. In this study, we constructed spheroids based on green fluorescence metastatic breast cancer cells MDA-MB-231 to simulate malignant tumors in vitro, while constructed a three-dimensional (3D) biochip to simulate a micro-environment for the growth and invasion of spheroids. In the experiment, the 3D spheroid was implanted into the chip, and the oriented collagen fibers controlled by collagen concentration and injection rate could guide the MDA-MB-231 cells in the spheroid to undergo directional invasion. The experiment showed that the oriented fibers greatly accelerated the invasion speed of MDA-MB-231 cells compared with the traditional uniform tumor micro-environment, namely obvious invasive branches appeared on the spheroids within 24 hours. In order to analyze this interesting phenomenon, we have developed a quantitative analyzing approach to explore strong angle correlation between the orientation of collagen fibers and invasive direction of cancer cell. The results showed that the oriented collagen fibers produced by the chip can greatly stimulate the invasion potential of cancer cells. This biochip is not only conducive to modeling cancer cell metastasis and studying cell invasion mechanisms, but also has the potential to build a quantitative evaluation platform that can be used in future chemical drug treatments.
A feasible neuron model can be effective to estimate the mode transition in neural activities in a complex electromagnetic environment. When neurons are exposed to electromagnetic field, the continuous magnetization and polarization can generate nonlinear effect on the exchange and propagation of ions in the cell, and then the firing patterns can be regulated completely. The conductivity of ion channels can be affected by the temperature and the channel current is adjusted for regulating the excitability of neurons. In this paper, a phototube and a thermistor are used to the functions of neural circuit. The phototube is used to capture external illumination for energy injection, and a continuous signal source is obtained. The thermistor is used to percept the changes of temperature, and the channel current is changed to adjust the excitability of neuron. This functional neural circuit can encode the external heat (temperature) and illumination excitation, and the dynamics of neural activities is investigated in detail. The photocurrent generated in the phototube can be used as a signal source for the neural circuit, and the thermistor is used to estimate the conduction dependence on the temperature for neurons under heat effect. Bifurcation analysis and Hamilton energy are calculated to explore the mode selection. It is found that complete dynamical properties of biological neurons can be reproduced in spiking, bursting, and chaotic firing when the phototube is activated as voltage source. The functional neural circuit mainly presents spiking states when the photocurrent is handled as a stable current source. Gaussian white noise is imposed to detect the occurrence of coherence resonance. This neural circuit can provide possible guidance for investigating dynamics of neural networks and potential application in designing sensitive sensors.
The cadmium sulphide (CdS) film is grown on cadmium telluride (CdTe) nanorods (NRs) arrays by different methods such as chemical bath deposition (CBD), magnetron sputtering (MS), and homogenous precipitation (HP) techniques. The impact of various deposition methods is explored in detail on the growth of CdTe/CdS composite film, the CdTe/CdS interface property, and solar cell efficiency. Compared to the CBD and HP methods, the MS method can improve the growth of the CdS on CdTe NRs with high crystalline quality. The device based on the CdS film prepared by the MS method demonstrates excellent photovoltaic performance, which has the potential for applications in solar cells.
The aim of this study was to investigate the feasibility of detecting potassium sorbate (PS) and sorbic acid (SA) in agricultural products using THz time-domain spectroscopy (THz-TDS). The absorption spectra of PS and SA were measured from 0.2 to 1.6 THz at room temperature. The main characteristic absorption peaks of PS and SA in polyethylene and powdered agricultural products with different weight ratios were detected and analyzed. Interval partial least squares (iPLS) combined with a particle swarm optimization and support vector classification (PSO-SVC) algorithm was proposed in this paper. iPLS was used for frequency optimization, and the PSO-SVC algorithm was used for spectrum analysis of the preservative content based on the optimal spectrum ranges. Optimized PSO-SVC models were obtained when the THz spectrum from the PS/SA mixture was divided into 11 or 12 subintervals. The optimal penalty parameter c and kernel parameter g were found to be 1.284 and 0.863 for PS (0.551–1.487 THz), 1.374 and 0.906 for SA (0.454–1.216 THz), respectively. The preliminary results indicate that THz-TDS can be an effective nondestructive analytical tool used for the quantitative detection of additives in agricultural products.
Recently, neuromorphic devices for artificial intelligence applications have attracted much attention. In this work, a three-terminal electrolyte-gated synaptic transistor based on NdNiO3 epitaxial films, a typical correlated electron material, is presented. The voltage-controlled metal–insulator transition was achieved by inserting and extracting H+ ions in the NdNiO3 channel through electrolyte gating. The non-volatile conductance change reached 104 under a 2 V gate voltage. By manipulating the amount of inserted protons, the three-terminal NdNiO3 artificial synapse imitated important synaptic functions, such as synaptic plasticity and spike-timing-dependent plasticity. These results show that the correlated material NdNiO3 has great potential for applications in neuromorphic devices.
Significant electric control of exchange bias effect in a simple CoO1–δ/Co system, grown on piezoelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (110) (PMN-PT) substrates, is achieved at room temperature. Obvious changes in both the coercivity field (HC) and the exchange bias field (HE), of 31% and 5%, respectively, have been observed when the electric field is applied to the substrate. While the change of coercivity is related to the enhanced uniaxial anisotropy in the ferromagnetic layer, the change of the exchange bias field can only originate from the spin reorientation in the antiferromagnetic CoO1–δ layer caused by the strain-induced magnetoelastic effect. A large HE/HC > 2, and HE ～ 110 Oe at room temperature, as well as the low-energy fabrication of this system, make it a practical system for spintronic device applications.
By dispersing La1 – xSrxMnO3 (LSMO) granule into PbZrxTi1 – xO3 (PZT) matrix, the 0-3 type LSMO/PZT composite film is synthesized through chemical solution method. The asymmetry of the top and bottom electrodes introduces novel electrostatic screening on LSMO/PZT interface. As electric polarization is switched between upward and downward orientations, the evolution of exchange bias, diode transport, and magnetoresistance is observed. The result implies the electrostatic switch of magnetic core-shell in the present film. In detail, as the spontaneous polarization is upward or downward in the PZT matrix, the ferromagnetic/antiferromagnetic or ferromagnetic/ferromagnetic core-shell structure is formed in LSMO granule, respectively. This work would develop a novel device for spintronics and metamaterial.
The floor field model has been widely used in evacuation simulation research based on cellular automata model. However, conventional methods of setting floor field will lead to highly insufficient utilization of the exit area when people gather on one side of the exit. In this study, an extended cellular automata model with modified floor field is proposed to solve this problem. Additionally, a congestion judgment mechanism is integrated in our model, whereby people can synthetically judge the degree of congestion and distance in front of them to determine whether they need to change another exit to evacuate or not. We contrasted the simulation results of the conventional floor field model, the extended model proposed in this paper, and Pathfinder software in a same scenario. It is demonstrated that this extended model can ameliorate the problem of insufficient utilization of the exit area and the trajectory of pedestrian movement and the crowd shape of pedestrians in front of exit in this new model are more realistic than those of the other two models. The findings have implications for modeling pedestrian evacuation.
Field-driven magnetic domain wall propagation in ferromagnetic nanostrips with trapezoidal cross section has been systematically investigated by means of micromagnetic simulation. Asymmetric dynamic behaviors of domain wall, depending on the propagation direction, were observed under an external magnetic field. When the domain walls propagate in the opposite direction along the long axis of the nanostrip, the Walker breakdown fields as well as the average velocities are different. The asymmetric landscape of demagnetization energies, which arises from the trapezoidal geometry, is the main origin of the asymmetric propagation behavior. Furthermore, a trapezoid-cross-section nanostrip will become a nanotube if it is rolled artificially along its long axis, and thus a two-dimensional transverse domain wall will become a three-dimensional one. Interestingly, it is found that the asymmetric behaviors observed in two-dimensional nanostrips with trapezoidal cross section are similar with some dynamic properties occurring in three-dimensional nanotubes.
A robust electron device called the enhanced gated-diode-triggered silicon-controlled rectifier (EGDTSCR) for electrostatic discharge (ESD) protection applications has been proposed and implemented in a 0.18-μm 5-V/24-V BCD process. The proposed EGDTSCR is constructed by adding two gated diodes into a conventional ESD device called the modified lateral silicon-controlled rectifier (MLSCR). With the shunting effect of the surface gated diode path, the proposed EGDTSCR, with a width of 50 μm, exhibits a higher failure current (i.e., 3.82 A) as well as a higher holding voltage (i.e., 10.21 V) than the MLSCR.
Self-powered photodetectors based on nanomaterials have attracted lots of attention for several years due to their various advantages. In this paper, we report a high performance Cu2O/ZnO self-powered photodetector fabricated by using electrochemical deposition. ZnO nanowires arrays grown on indium-tin-oxide glass are immersed in Cu2O film to construct type-II band structure. The Cu2O/ZnO photodetector exhibits a responsivity of 0.288 mA/W at 596 nm without bias. Compared with Cu2O photoconductive detector, the responsivity of the Cu2O/ZnO self-powered photodetector is enhanced by about two times at 2 V bias. It is attributed to the high power conversion efficiency and the efficient separation of the photogenerated electron–hole pairs, which are provided by the heterojunction. The outstanding comprehensive performances make the Cu2O film/ZnO nanowires self-powered photodetector have great potential applications.
Based on angular amplitude modulation of orthogonal base vectors in common-path interference method, we propose an interesting type of hybrid vector beams with unprecedented azimuthal polarization gradient and demonstrate in experiment. Geometrically, the configured azimuthal polarization gradient is indicated by intriguing mapping tracks of angular polarization states on Poincaré sphere, more than just conventional circles for previously reported vector beams. Moreover, via tailoring relevant parameters, more special polarization mapping tracks can be handily achieved. More noteworthily, the designed azimuthal polarization gradients are found to be able to induce azimuthally non-uniform orbital angular momentum density, while generally uniform for circle-track cases, immersing in homogenous intensity background whatever base states are. These peculiar features may open alternative routes for new optical effects and applications.
Memristive devices have attracted intensive attention in developing hardware neuromorphic computing systems with high energy efficiency due to their simple structure, low power consumption, and rich switching dynamics resembling biological synapses and neurons in the last decades. Fruitful demonstrations have been achieved in memristive synapses neurons and neural networks in the last few years. Versatile dynamics are involved in the data processing and storage in biological neurons and synapses, which ask for carefully tuning the switching dynamics of the memristive emulators. Note that switching dynamics of the memristive devices are closely related to switching mechanisms. Herein, from the perspective of switching dynamics modulations, the mainstream switching mechanisms including redox reaction with ion migration and electronic effect have been systemically reviewed. The approaches to tune the switching dynamics in the devices with different mechanisms have been described. Finally, some other mechanisms involved in neuromorphic computing are briefly introduced.
Two-dimensional topological insulators (2DTIs) have attracted increasing attention during the past few years. New 2DTIs with increasing larger spin–orbit coupling (SOC) gaps have been predicted by theoretical calculations and some of them have been synthesized experimentally. In this review, the 2DTIs, ranging from single element graphene-like materials to bi-elemental transition metal chalcogenides (TMDs) and to multi-elemental materials, with different thicknesses, structures, and phases, have been summarized and discussed. The topological properties (especially the quantum spin Hall effect and Dirac fermion feature) and potential applications have been summarized. This review also points out the challenge and opportunities for future 2DTI study, especially on the device applications based on the topological properties.
The optical atomic clocks have the potential to transform global timekeeping, relying on the state-of-the-art accuracy and stability, and greatly improve the measurement precision for a wide range of scientific and technological applications. Herein we report on the development of the optical clock based on 171Yb atoms confined in an optical lattice. A minimum width of 1.92-Hz Rabi spectra has been obtained with a new 578-nm clock interrogation laser. The in-loop fractional instability of the 171Yb clock reaches 9.1 × 10-18 after an averaging over a time of 2.0 × 104 s. By synchronous comparison between two clocks, we demonstrate that our 171Yb optical lattice clock achieves a fractional instability of 4.60×10-16/τ.