The high-pressure α-PbO2 phase of TiO2 has suitable band gap and photocatalytic ability in the visible light range, which is an environmentally friendly and efficient photocatalytic material. In this paper, α-PbO2 phase of TiO2 was obtained by pressured and decompressed anatase nanospheres using diamond anvil cell, Transmission Electron Microscope(TEM) observation shows that the obvious deformation of TiO2 nanospheres are observed. High-Resolution TEM shows that there are a large number of stacking faults along  direction and deformation twins in the grain. In details, the deformation twin band with lens lamellar structure is formed in the sub micron grain; The fan-shaped multiple deformation twins are formed in the nanocrystalline grains. This study shows that anatase TiO2 can be deformed under high pressure, the micro mechanism of deformation is similar with metal, mainly including deformation twins and stacking fault slip. There is obvious size effect in the formation of deformation twins. These results provide a new breakthrough point for the study of the size effect of high-pressure phase transformation of TiO2, and also provide an experimental direction for preparing of the twin high-pressure α-PbO2 phase.
The memristor is a kind of nonlinear element with nanometer size, which can enhance the complexity of a chaotic system. With the further research of chaos, several novel nonlinear phenomena have been found by scholars, such as hidden attractors, coexisting attractors and multi-stability. Meanwhile, the extremely multi-stability representation system coexists with the infinite attractors, which has become a hot spot in the field of memristor chaos research in recent years. A general method to construct a chaotic systems of multiple coexistence is to increase the number of equilibrium points of chaotic system by means of control. The introduction of memristor results in the linear distribution of the equilibrium points of chaotic system in space, which are the linear equilibrium points. The existing researches show that chaotic system with extremely multi-stability can produce better chaotic sequence, which can be used in engineering fields such as secure communication. Therefore, it is of great significance to construct chaotic systems with rich dynamic behaviors by using memristors.In order to further improve the complexity of the chaotic system, a five-dimensional memristor chaotic system is constructed by replacing the coupling parameters in the four-dimensional chaotic system based on Sprott-B with a magnetically controlled memristor. The dynamic behavior of the system is analyzed by bifurcation diagram, Lyapunov exponent spectrum, phase portrait, Poincaré map, dynamic map and other conventional means. The analysis shows that the new system has rich dynamic behaviors: when the system parameters change, the system can produce a large number of chaotic attractors with different topological structures and periodic limit cycles with different periods. When different parameters change, the dynamic characteristics of the system also change; when the system parameters are fixed, the system not only has an offset enhancement phenomenon that depends on the change of the initial conditions, but also shows a very strong sensitivity to the initial values and a great adjustment range of the initial values, which leads the infinite chaos and periodic attractors to coexist, namely extremely multi-stability appears. Finally, the digital circuit of the memristor chaotic system is implemented based on the field programmable gate array (FPGA) technology. The phase portrait captured on the oscilloscope is consistent with that from the numerical simulation, which verifies the correctness and realizability of the memristor system.
The High Altitude Detection of Astronomical Radiation (HADAR) experiment is a refracting terrestrial telescope array based on the atmospheric Cherenkov imaging technique. It is a hybrid array consisting of four water-lens telescopes and a surrounding scintillation detector array for observing Cherenkov light induced by 10 GeV–10 TeV cosmic rays and gamma rays in the atmosphere. The water-lens telescope mainly consists of a hemispherical lens with a diameter of 5 m acting as a Cherenkov light collector, a cylindrical metal tank with a 4 m radius and 7 m height, and an imaging system at the bottom of the tank. The sky region covered by HADAR is much larger than the current generation of Imaging Atmospheric Cherenkov Telescopes, and even the CTA. The field-of-view (FOV) of HADAR can reach up to 60 degrees. The HADAR experiment possesses the advantages of a large field-of-view and low energy threshold, so it can continuously scan wide portions of the sky and easily observe extragalactic gamma-ray sources. The majority of the extragalactic gamma-ray sources detected at very high energy (VHE) energies are active galactic nuclei (AGNs). In this study, we present the potential of using the HADAR experiment for detecting AGN. Based on the AGN catalog sources of the Fermi Large Area Telescope (Fermi-LAT), the observed energy is extrapolated to the VHE range. The VHE gamma rays propagating over cosmological distances can interact with the low-energy of the extragalactic background light (EBL) and produce electron-positron pairs. Therefore, we consider the absorption effects of different EBL models when calculating the expected gamma ray spectra of the AGN sample. We select the sample with redshift measurements and locations inside the FOV of HADAR from 4LAC catalog. In total, there are 375 BL Lacertae objects (BL Lacs) and 289 flat-spectrum radio quasars (FSRQs) satisfying the selection conditions. The integral gamma ray spectra are derived and compared with the sensitivity curve of HADAR, the number of sources with fluxes above the sensitivity of HADAR is counted. Further, we calculate the statistical significance of HADAR for AGN source observation based on the equi-zenith angle sky scanning analysis method. The simulation results reveal that a total of 31 sources of Fermi-LAT AGN can be detected by HADAR with a significance greater than five standard deviations over a one-year survey period, most of which are BL Lacs.
Ni-Mn-Ti-based all-d-metal Heusler alloys have become a hot research topic in the field of metal functional materials due to their excellent mechanical properties and elastocaloric effect. However, the relatively large critical stress and transition hysteresis limit its practical application. Some researchers found that doping Fe in Ni-Mn-based alloys can not only reduce hysteresis, but also greatly improve the mechanical properties of alloys. Based on this, the effects of Fe doping on phase stability, martensitic transformation and magnetic properties of Ni50-xMn37.5Ti12.5Fex (x = 3.125, 6.25, 9.375) Heusler alloys have been systematically studied by first principles calculation. The corresponding magnetic states of the austenite and martensite of the alloy systems were determined according to the results of the formation energy. The variations of the lattice constants and the phase stability of the austenite and martensite with increasing Fe content in the alloy systems were revealed, and the related mechanism was elucidated. The atomic and total magnetic moments of the austenite and martensite in the Ni50-xMn37.5Ti12.5Fex (x = 3.125, 6.25, 9.375) systems were calculated. Based on the results of electronic structure, the essential reasons for the magnetic state changes of the alloys were further explained. In Ni50-xMn37.5Ti12.5Fex alloy system, the lattice constant of austenite decreases gradually with the increase of Fe doping amount. The stability of both austenite and martensite phase decreases with the increase of Fe doping amount. Under the different composition, the formation energy of martensite is always lower than that of austenite, indicating that the alloy can undergo martensite transformation. The energy difference ΔE, electron concentration e/a and density of electrons n of the alloy show a decreasing trend, indicating that the driving force of martensitic transformation decreases, and the corresponding martensitic transformation temperature decreases with the increase of Fe atom doping. The austenite of the alloy is ferromagnetic and the martensite is antiferromagnetic. After the martensitic transformation, the distance between Mn-Mn atoms decreases, and the magnetic moments of MnMn and MnTi atoms are arranged antiparallel, resulting in the total magnetic moments being almost zero. The magnetic properties of the two phases are little affected by the amount of Fe atom doping. The peak density of electronic states in the Fermi surface of martensite phase is lower than that of austenite phase, indicating that martensite phase has a more stable electronic structure than austenite phase. During the transition from austenite to martensite, there is a Jahn-Teller splitting effect at the peak of the down-spin density of states near the Fermi surface. The aim of this paper is to provide guidance for the composition design and property optimization of the Ni-Mn-Ti-Fe alloy.
The acoustic mismatch model and diffuse mismatch model are widely used in the calculation of interfacial thermal conductance. These two models are respectively based on the assumption of extremely smooth and rough interfaces. Due to the great difference between the actual interface structure and the two hypotheses, the prediction of these two models deviate greatly from the actual interfacial thermal conductance. The recently proposed mixed mismatch model considers the effect of interface structure on the ratio of phonon specular transmission to diffuse scattering transmission, and the prediction accuracy is improved. However, this model requires molecular dynamics simulation to obtain phonon information at the interface. In this paper, the mixed mismatch model is simplified by introducing the measured roughness value, and the influence of interface structure on the contact area is taken into account to achieve a simple, fast and accurate prediction of interface thermal conductance. Based on this model, the interfacial thermal conductance of metals (aluminum, copper, gold) and semiconductors (silicon, silicon carbide, gallium arsenide, gallium nitride) are calculated and predicted. The results of Al/Si interface are in good agreement with the experimental results. This model is not only helpful to understand the mechanism of interface heat conduction, but also helpful to compare with the measurement results.
Josephson junction based on topological insulators, as a candidate device for searching for Majorana zero energy modes, has attracted much attention. One of the key issues along this direction is to fabricate Josephson junctions with high-quality interfaces, hoping to searching for 4π-period current-phase relation in topologically non-trivial Josephson junction. In this work, the Josephson junctions based on three-dimensional topological insulator nanowires Bi2Te3 and Bi2(SexTe1-x)3 were fabricated to study their superconducting proximity effect, multiple Andreev reflection (MAR) and current-phase relations (CPR). A number of interesting phenomena were observed, including the anomalous enhancement in junctions’ critical supercurrent with magnetic field, the appearance of half-integer Shapiro steps in the ac Josephson effect, etc. And, we discussed the possible origins of the observed anomalous behaviors, in particular their relation with the ferromagnetic layer of TiTe alloy formed at the interface between the topological insulator nanowires and the Ti buffer layer of the metallic electrodes. We provided the experimental evidence for the formation of a ferromagnetic TiTe alloy layer at the interface of our devices. And, we believe that the formation of such a layer in our Josephson devices breaks the time reversal symmetry, leading to the observed anomalous enhancement of the critical supercurrent with magnetic field, as well as the appearance of half-integer Shapiro steps. Our results suggest that, to study the topological non-trivial behaviors such as 4π-period current-phase relation, one still needs to improve the interface quality of the superconductor-normal metal-superconductor type of Josephson junction devices.
Since H2S is a corrosive and toxic gas pollutant, the accurate measurement of its concentration is significant. However, in the practical industrial processes, it is difficult to implement because of the disturbance caused by other emissions such as CO2 and CO. Therefore, in this paper, the concentrations of H2S, CO2 and CO were measured simultaneously based on cavity ring-down spectroscopy (CRDS) as a viable alternative to measure the concentration of H2S accurately when CO2 and CO exist. First, the wavelength of mixed gas within the range of 6336~6339 cm-1 was selected as the target region where the spectral line intensity of H2S was stronger than 10 times of that of CO2 or CO and the water absorption was extremely weak. Second, the influence of the sampling length (Tm) on the accuracy of the ring-down time was analyzed by evaluating average (accuracy), standard deviation (precision) and consumption time (speed). Third, the experiments were carried out at different pressures in order to obtain the optimal pressure condition. Fourth, the concentration of trace H2S was measured when the disturbances caused by CO2 or CO were added, and the error analysis of the measured concentration was carried out. Finally, the detection limit of CRDS-based system was calculated as 6.9 ppb by analyzing the SNR of four groups of low concentration H2S spectra, while the lower limit of detection of CRDSbased system was calculated as 2 ppb by analyzing the Allan variance of long-term data. The measured concentration and its desired value show a great linearity at different dilution ratios. Both the high linearity and the low detection limit of H2S indicate the effectiveness of the CRDS-based measurement system to measure H2S when CO2 and CO exist. The successful application of the CRDS-based system to the measurement of H2S shows its promising prospect in gas concentration measurement for practical industrial processes.
The generation of Orbital Angular Momentum (OAM) modes is of great importance in a variety of applications such as optical tweezers, quantum optics, and optical communication systems. Particularly, how can high-order OAM modes be generated efficiently in fibers with the advantage of low cost and compatible with fiber system. the Traditional method for first order to third order OAM is based on long period fiber grating (LPFG) fabricated by Carbon dioxide laser. However, high power and large focused spot of Carbon dioxide laser are unfavorable for stable and repeatable generation of higher-order OAM, which needs the LPFG with small grating pitch. In order to solve this problem, a third-order OAM mode converter based on femtosecond microfabrication is proposed and fabricated for the first time. With the advantage of 4.4μm focused spot size near the core, lower power and lower heat absorption efficiency, this method can be more stable and promising. Therefore, we firstly did mode filed analysis and simulate the intensity and phase profiles of the superposed mode field in LP odd-even mode with different scales and phases patterns to obtain OAM mode. Second, we use the coupled-mode theory to analysis and simulate the transmission spectrum of LPFG,which guide the setting of the grating parameter such as the grating pitch, the depth of modulation and the length of the grating. By experimental verification, an asymmetric modulated long-period fiber grating with a pitch setting to 194μm is fabricated on six-mode fiber. the fundamental mode can be converted to the third-order angular linear polarization mode LP31 mode with 98% mode conversion efficiency near 1550nm, and further converted to the OAM±3 modes by superposition of the odd and even LP31 mode with ±π∕2 phase difference. At the same time, this fiber gratings can also generate LP12 mode with 90% mode conversion efficiency near 1325nm. Then we can take the same approach to transform LP12 mode to OAM modes with angular first-order as well as radial second-order. The experiment is consistent with the simulation. Thus, this scheme provides an idea to generate high-order OAM modes in all-fiber systems using only one grating with high repeatability.
As a nanoscale coherent light source, semiconductor nanolaser is a key device for future optoelectronic integrated chips. The obstacle of further miniaturization of the nanolaser is that the loss increases rapidly with the decrease of cavity volume. The bound states in the continuum (BICs) can overcome the high radiative loss. Here, we proposed a nanolaser based on quasi-BIC mode supported by all-dielectric resonant waveguide grating (RWG), which can effectively reduce the threshold of nanolaser. The quasi-BIC mode of the waveguide can be excited when the traditional two-part grating became a four-part grating. The laser behavior of the quasi-BIC was studied by finite difference-time-domain (FDTD) numerical simulation. The results show that the threshold of the naolaser based on four part-grating RWG is 20.86% lower than nanolaser based on two part-grating RWG when under TE-polarized light irradiation. For TM-polarized light irradiation, the threshold is 3.3 times lower than nanolaser based on four part-grating RWG. We also find that the threshold of the nanolaser under TE-polarized light irradiation is about one order of magnitude lower than that under TM-polarized light irradiation. Because the electric field of the structure is well confined inside the waveguide layer under TE-polarized light, which can enhance the interaction between light and gain materials and reduce the threshold of nanolasers.
Ellipticity properties of high-order harmonic generation (HHG) from symmetric molecules H+2 in strong and short wavelength (less than 800nm) laser fields are numerically investigated. In this study, the ellipticity of harmonic is compared with the corresponding harmonic spectrum and dipole, and the calculation results are analyzed and compared between different laser intensities, different laser wavelengths, different internuclear distances and different orientation angles. Our numerical simulations show that the influence of laser intensity, laser wavelength, internuclear distances and orientation angle on the ellipticity of harmonic is different. Especially in the two-center interference region, the excited state plays an important role in the HHG, but the effect of the excited state on the ellipticity of harmonic is different at different orientation angles. Further analysis shows that these different effects are due to the influence of the excited state on the harmonic yields. Using the numerical scheme, it is determined that in the two-center interference region, the excited state plays an important role in the parallel harmonic spectrum, while the effect of the excited state on the perpendicular harmonics at different angles is very small, which result in different phase differences between the accurate harmonic spectrum and the harmonic spectrum only returning to the ground state. Overall, the relative yields of the accurate perpendicular harmonics are lower (higher) than the accurate parallel ones, but the intensity of the perpendicular harmonics, which only returns to the ground state, is comparable to (or farther away from) the parallel which is only to return to ground state in the two-center interference regions. Therefore, the small (large) intensity ratio between the accurate perpendicular and accurate parallel harmonics can be attributed to the contributions of the excited state to harmonics. Then we can conclude that the harmonic spectra that only transitions back to the ground state show high (small) ellipticity, whereas the accurate harmonic spectra show small (high) ellipticity, resulting in a strong angle dependence of the influence of the excited state on the ellipticity of harmonic. In addition, in the high-order harmonic plateau region, the relative yields of harmonics can be well predicted by the corresponding dipoles, indicating the applicability of tunneling pictures and plane wave approximation in the strong and short-wave laser fields. When the ellipticity of harmonic occurs in the interference region due to the two-center characteristics of the symmetric potential, the results show that the polarization measurement can also be used to detect the structure of symmetric molecules and track the dynamics of excited states.
High-temperature superconducting films can be used for the fabrication of cutting-edge high-temperature superconducting microwave devices because of their low microwave surface resistance. However, the microwave surface resistance of high-temperature superconducting materials is particularly sensitive to microstructure due to their special two-dimensional superconducting mechanism and extremely short superconducting coherence length. To investigate the correlation between microstructure and microwave surface resistance of high-temperature superconducting materials, YBa2Cu3O7-δ (YBCO) films with different thicknesses were grown on (00l)-oriented MgO single-crystal substrates using pulsed laser deposition (PLD) technique. Electrical measurements revealed that their superconducting transition temperature and room temperature resistance do not show significant difference. However, their microwave surface resistance at superconducting state display a significant difference. The characterization of the microstructure of YBCO films by synchrotron radiation three-dimensional reciprocal space mapping(3D-RSM) technique shows that the number of the grains with CuO2 face parallel to the surface (c crystals), and the consistency of grain orientation are the main causes for the difference in microwave surface resistance.
Membrane has widely applications in the field of filtration and separation, but due to the attraction or repulsion exerted by the membrane, the particles will experience directional motion. As a result, two totally opposite effects, particle enrichment and exclusion zone, take place in the vicinity of the membrane, and the underlying reason is still not clear. In the paper, colloidal particles with negative surface charge was used as a model substance, with the advantages of monitoring the particles concentration in a real time and in situ way, to investigate the influence of cellulose membrane to the movement of particles. The experimental results showed that particles enriched in the vicinity of the membrane. The diffusiophoresis effect originates from the tiny amount ions released by the film is the main reason of the directional movement of the charged particles. Based on the two mechanisms of diffusiophoresis and diffusion, we construct a model and make relevant numerical calculation, and the numerical results are qualitatively consistent with the experimental results. Moreover, in addition to the longitudinal motion of the particles towards the filter membrane, diffusio-osmotic flow and particles lateral diffusion also result in the migration of particles towards to the container wall, and further increase particles number near the wall.
The sky infrared background radiation varies greatly with spatial distribution and time. When Scanning Fourier Transform Infrared (FTIR) remote sensing imaging system scans the target gas cloud with the sky as the background, the background radiation corresponding to each scanned pixel is different, and the background does not have a constant baseline. It is extremely difficult to obtain the background spectrum of each pixel in real-time, which affects the inversion accuracy of the target gas cloud transmittance. An inversion method of target gas cloud transmittance based on atmospheric profile synthesis background is proposed. The temperature, humidity, pressure, and ozone profiles of the measured locations and the atmospheric model are used to generate the sky infrared background in order to solve the problem that it is difficult to measure the clean sky infrared background spectrum in the chemical industry park. This paper proposes that there is a continuous derivable relationship between the sky infrared background spectrum and the cosine of zenith angle at each wavenumber, so a small amount of sky infrared background spectrum with a zenith angle gradient can quickly generate sky infrared background spectrum at any elevation angle. The proposed method is verified by the Moderate Resolution Atmospheric Radiative Transfer Model (MODTRAN) software simulation and the remote sensing imaging experiment of SF6 gas. The proposed method can quickly generate the sky infrared background spectrum corresponding to any angle within the gradient elevation angle and accurately invert the target gas cloud transmittance at each pixel. The results show that the distribution trend of the column concentration of the SF6 gas cloud is consistent with the actual distribution, the correlation is 0.99979.
Trivalent rare earth erbium ion (Er3+) doped titanium oxide (TiO2) can produce a very wide range of applications due to its excellent optoelectronic properties, standing out among many rare-earth-doped luminescent crystals. However, these issues regarding local structure and electronic properties have not been finalized. To address these problems, the the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method combined with the first-principles calculations are employed, and many converged structures of Er3+-doped TiO2 was successfully obtained. Further structural optimization was performed by using the Vienna Ab Initio Simulation Package (VASP) software package, and we report for the first time that the lowest energy structure of Er3+-doped TiO2 has the P4m2 symmetry. It can be observed that the doped Er3+ ions enter the host crystal and occupy the position of Ti4+ ions, resulting in structural distortion, which eventually leads to the reduction of the local Er3+ coordination site symmetry from D2d to C2v. We speculate that there are two reasons: 1. the difference in charge between Er3+ ions and Ti4+ ions leads to charge compensation; 2. the difference in electron radius is obvious, Er3+ ions and Ti4+ ions are 0.0881 and 0.0605 nm, respectively. In addition, during the structural search process, we also find many metastable structures that may exist at a special temperature or pressure, which play an important role in the study of structural evolution. When calculating the electronic band structure of the Er3+-doped TiO2 system, we adopted the method of local density approximation (LDA) combined with the on-site Coulomb repulsion parameter U to accurately describe the strongly correlated system. For the specific value of U, we adopted 3.5 eV and 7.6 eV to describe the strong correlation of 3d electrons of Ti4+ ions and 4f electrons of Er3+ ions, respectively. According to the calculation of electronic properties, the band gap value of Er3+ doped TiO2 is about 2.27 eV, which is lower than that of the host crystal (Eg = 2.40 eV). The results show that the reduction in the band gap is mainly caused by the f state of Er3+ ions. The doping of Er ion does reduce the band gap value, but it does not change the conductivity of the system, which have great application prospect in diode-pumped laser. These findings not only provide data for further exploration of the properties and applications of Er3+:TiO2 crystals, but also provide a systematic approach for the study of other rare-earth-doped crystalline materials.
Mg2(Si,Sn)-based thermoelectric materials, which are environmentally friendly and low-cost, have great development potential in a moderate temperature range. Electronic transport properties of Mg2Si1-xSnx alloys can be optimized by doping elements. Doping is still one of the most effective methods of optimizing electronic transport performance, such as carrier concentration, mobility, and effective mass. The most effective doping elements are Sb and Bi. Much attention has been paid to the influence of element type and doping content. Different substitution sites will also greatly affect the electronic transport parameters. In this work, the defect formation energy value of Mg2Si0.375Sn0.625 alloy for substituting Sb atoms and Bi atoms for Sn sties and Si sites, respectively, are calculated by first-principles calculations. The influence on electronic transport parameters is systematically analyzed by combining the calculated results of band structures and density of states. Corresponding component Sb and Bi atoms doped Mg2Si0.375Sn0.625 alloys are prepared by rapid solidification method, and microstructures, Seebeck coefficients, and electrical conductivities of the alloys are measured. Combined with the predicted results by solving the Boltzmann transport equation, electronic transport performances are compared and analyzed. The results indicate that both Sn and Si sites are equally susceptible to Sb and Bi doping, but the Si sites are preferentially substituted due to their lower ∆Ef values. Doped Bi atoms provide a higher electron concentration, and Sb atoms offer higher carrier effective mass. Thus, the maximum σ value of the Mg2Si0.375Sn0.615Bi0.01 alloy is 1620 S/cm. The maximum S value of the Mg2Si0.365Sn0.625Sb0.01 alloy is –228 μV/K. Correspondingly, the highest PF value for this alloy is 4.49 mW/(m·K) at T = 800 K because of the dominant role of S values. Although its power factor is slightly lower, the Mg2Si0.375Sn0.615Sb0.01 alloy is expected to exhibit lower lattice thermal conductivity due to the lattice shrinkage caused by substituting Sb sites for Sn sites. The optimal doping concentration of the Bi-doped alloy is lower than that of the Sb-doped alloy. These results are expected to provide a significant reference for optimizing the experimental performance of Mg2(Si, Sn)-based alloys.