Ultra-wideband low radar cross-section metasurface and its application on waveguide slot antenna array
A novel approach devoted to achieving ultra-wideband radar cross section reduction (RCSR) of a waveguide slot antenna array (WGSAA) while maintaining its radiation performance is proposed. Three kinds of artificial magnetic conductors (AMCs) tiles consisting of three types of basic units resonant at different frequencies are designed and arranged in a novel quadruple-triangle-type configuration to create a composite planar metasurface. The proposed metasurface is characterized by low radar feature over an ultra-wideband based on the principle of phase cancellation. Both simulated and measured results demonstrate that after the composite metasurface is used to cover part of the antenna array, an ultra-wideband RCSR involving in-band and out-of-band is achieved for co-and cross-polarized incident waves based on energy cancellation, while the radiation performance is well retained. The proposed method is simple, low-cost, and easy-to-fabricate, providing a new method for ultra-wideband RCSR of an antenna array. Moreover, the method proposed in this paper can easily be applied to other antenna architectures.
Modified physical optics algorithm for near field scattering
A novel modified physical optics algorithm is proposed to overcome the difficulties of near field scattering prediction for classical physical optics. The method is applied to calculating the near field radar cross section of electrically large objects by taking into account the influence of the distinct wave propagation vector, the near field Green function, and the antenna radiation pattern. By setting up local reference coordinates, each partitioned facet has its own distinct wave front curvature. The radiation gain for every surface element is taken into consideration based on the modulation of the antenna radiation pattern. The Green function is refined both in amplitude and phase terms and allows for near field calculation. The scattered characteristics of the near field targets are studied by numerical simulations. The results show that the approach can achieve a satisfactory accuracy.
β-BaB2O4 with special cut-angle applied to single crystal cascaded third-harmonic generation
High-efficiency single crystal cascaded third-harmonic generation (THG) was realized in β-BaB2O4 (BBO) material with special cut-angle. By analyzing effective nonlinear optical coefficient (deff) of the cascaded THG process, which was composed by type-Ⅱ frequency doubling and type-I sum-frequency, the optimum phase matching (PM) direction in BBO crystal was determined to be (θ=32.1°, φ=11°). With an optimized 9-mm long sample which was processed along this direction, the highest cascaded THG conversion efficiency reached 42.3%, which is much superior to the similar components reported previously, including ADP, KDP, and GdxY1-xCOB crystals.
Modulation transfer spectroscopy based on acousto-optic modulator with zero frequency shift
We present a modulation transfer spectroscopy (MTS) configuration based on an acousto-optic modulator by using a variant of the typical double pass structure. One beam is modulated by using an acousto-optic modulator in opposite diffraction order to cancel the carrier frequency shift and produce a modulated pump beam. The line shape performance is investigated theoretically and experimentally. Laser frequency stabilization of the proposed configuration is demonstrated for the 133Cs|62S1/2, F=4> →|62 P3/2, F'=5> transition. The Allan deviations, which are measured by using beat note signals and the three-cornered hat method, are 3.6×10-11 in an integration time of 100 s and approximately 4×10-11 in a longer integration time.
Terahertz two-pixel imaging based on complementary compressive sensing
A compact terahertz (THz) imaging system based on complementary compressive sensing has been proposed using two single-pixel detectors. By using a mechanical spatial light modulator, sampling in the transmission and reflection orientations was achieved simultaneously, which allows imaging with negative mask values. The improvement of THz image quality and anti-noise performance has been verified experimentally compared with the traditional reconstructed image, and is in good agreement with the numerical simulation. The demonstrated imaging system, with the advantages of high imaging quality and strong anti-noise property, opens up possibilities for new applications in the THz region.
Nonlinear coherent perfect photon absorber in asymmetrical atom-nanowires coupling system
Coherent perfect absorption provides a method of light-controlling-light and has practical applications in optical communications. Recently, a cavity-based nonlinear perfect photon absorption extends the coherent perfect absorber (CPA) beyond the linear regime. As nanowire-based system is a more competitive candidate for full-optical device, we introduce a nonlinear CPA in the single two-level atom-nanowires coupling system in this work. Nonlinear input-output relations are derived analytically, and three contributions of atomic saturation nonlinearity are explicit. The consociation of optical nonlinearity and destructive interference makes it feasible to fabricate a nonlinear monoatomic CPA. Our results also indicate that a nonlinear system may work linearly even when the incoming lights are not weak any more. Our findings show promising applications in full-optical devices.
On the nonclassical dynamics of cavity-assisted four-channel nonlinear coupler
The non-classical properties of light propagating in four-channel Kerr waveguides, confined in an optical cavity, are studied. The solution to the Hamiltonian of field operators is obtained semi-analytically by using symmetrically ordered phase-space representation. Full quantum analysis of the input coherent fields displays a strong transition of photon property between the super-Poissonian and sub-Poissonian statistics. It is found that the cavity-assisted multichannel system exhibits enhanced squeezing both in single-and compound-mode. This multichannel system may be utilized as an efficient quantum-light generator.
Generation of sustained optimal entropy squeezing of a two-level atom via non-Hermitian operation
We investigate the entropy squeezing of a two-level atom in the Jaynes-Cummings model, and provide a scheme to generate the sustained optimal entropy squeezing of the atom via non-Hermitian operation. Our results show that the squeezing degree and the persistence time of entropy squeezing of atomic polarization components greatly depend on the non-Hermiticity intensity in non-Hermitian operation. Especially, under a proper choice of non-Hermiticity parameters, the sustained optimal entropy squeezing of the atom can be generated even though the atom is initially prepared in a no entropy squeezing state.
Energy scaling and extended tunability of a ring cavity terahertz parametric oscillator based on KTiOPO4 crystal
A wide terahertz tuning range from 0.96 THz to 7.01 THz has been demonstrated based on ring-cavity THz wave parametric oscillator with a KTiOPO4 (KTP) crystal. The tuning range was observed intermittently from 0.96 THz to 1.87 THz, from 3.04 THz to 3.33 THz, from 4.17 THz to 4.48 THz, from 4.78 THz to 4.97 THz, from 5.125 THz to 5.168 THz, from 5.44 THz to 5.97 THz, and from 6.74 THz to 7.01 THz. The dual-Stokes wavelengths resonance phenomena were observed in some certain tuning angle ranges. Through the theoretical analysis of the dispersion curve of the KTP crystal, the intermittent THz wave tuning range and dual-wavelength Stokes waves operation during angle tuning process were explained. The theoretical analysis was in good agreement with the experiment results. The maximum THz output voltage detected by Golay cell was 1.7 V at 5.7 THz under the pump energy of 210 mJ, corresponding to the THz wave output energy of 5.47 μJ and conversion efficiency of 2.6×10-5.
Carboxyl graphene oxide solution saturable absorber for femtosecond mode-locked erbium-doped fiber laser
The carboxyl-functionalized graphene oxide (GO-COOH) is a kind of unique two-dimensional (2D) material and possesses excellent nonlinear saturable absorption property and high water-solubility. In this paper, we prepare saturable absorber (SA) device by depositing GO-COOH nanosheets aqueous solution on a D-shaped fiber. The modulation depth (MD) and saturable intensity of the SA are measured to be 9.6% and 19 MW/cm2, respectively. By inserting the SA into the erbium-doped fiber (EDF) laser, a passively mode-locked EDF laser has been achieved with the spectrum center wavelength of 1562.76 nm. The pulse duration, repetition rate, and the signal-to-noise ratio (SNR) are 500 fs, 14.79 MHz, and 80 dB, respectively. The maximum average output power is measured to be 3.85 mW. These results indicate that the GO-COOH nanosheets SA can be used as a promising mode locker for the generation of ultrashort pulses.
Surface plasmon polariton at the interface of dielectric and graphene medium using Kerr effect
We theoretically investigate the control of surface plasmon polariton (SPP) generated at the interface of dielectric and graphene medium under Kerr nonlinearity. The controlled Kerr nonlinear signal of probe light beam in a dielectric medium is used to generate SPPs at the interface of dielectric and graphene medium. The positive, negative absorption, and dispersion properties of SPPs are modified and controlled by the control and Kerr fields. A large amplification (negative absorption) is noted for SPPs under the Kerr nonlinearity. The normal/anomalous slope of dispersion and propagation length of SPPs is modified and controlled with Kerr nonlinearity. This leads to significant variation in slow and fast SPP propagation. The controlled slow and fast SPP propagation may predict significant applications in nano-photonics, optical tweezers, photovoltaic devices, plasmonster, and sensing technology.
Generation of breathing solitons in the propagation and interactions of Airy-Gaussian beams in a cubic-quintic nonlinear medium
Using the split-step Fourier transform method, we numerically investigate the generation of breathing solitons in the propagation and interactions of Airy-Gaussian (AiG) beams in a cubic-quintic nonlinear medium in one transverse dimension. We show that the propagation of single AiG beams can generate stable breathing solitons that do not accelerate within a certain initial power range. The propagation direction of these breathing solitons can be controlled by introducing a launch angle to the incident AiG beams. When two AiG beams accelerated in opposite directions interact with each other, different breathing solitons and soliton pairs are observed by adjusting the phase shift, the beam interval, the amplitudes, and the light field distribution of the initial AiG beams.
Effect of Hf4+ doping on structure and enhancement of upconversion luminescence in Yb: Tm: LiNbO3 crystals
A series of Yb:Tm:LiNbO3 crystals doped with x mol% Hf4+ ions (x=2, 4, and 6) were grown by the Czochralski method. The dopant occupancy and defect structure of Hf:Yb:Tm:LiNbO3 crystals were investigated by x-ray diffraction and infrared transmission spectra. The influence of Hf4+ ions concentration on UV-VIS-NIR absorption spectra of Hf:Yb:Tm:LiNbO3 crystals was discussed. The upconversion luminescence of Hf:Yb:Tm:LiNbO3 crystals was obtained under 980 nm excitation. Strong emissions were observed at 475 nm in the blue wavelength range and 651 nm in the red wavelength range. Remarkably, enhancement of the red and blue upconversion luminescence was achieved by tridoping Hf4+ ions.
Polymer waveguide thermo-optical switch with loss compensation based on NaYF4: 18% Yb3+, 2% Er3+ nanocrystals
A polymer waveguide thermo-optical switch with loss compensation based on NaYF4:18% Yb3+, 2% Er3+ nanocrystals, fabricated by traditional semiconductor processes, has been investigated. NaYF4:18% Yb3+, 2% Er3+ nanocrystals were prepared by a pyrolysis method. The morphology and luminescent properties of the nanocrystals were characterized. The nanocrystals were doped into SU-8 as the core material of an optical waveguide amplifier. The size of the device was optimized for its optical and thermal fields as well as its transmission characteristics. The device was fabricated on a silica substrate by spin coating, photolithography, and wet etching. The insertion loss of the switch device is~15 dB. The rise and fall times of the device are 240 μs and 380 μs, respectively, as measured by application of a 304 Hz square wave voltage. The extinction ratio of the device is about 14 dB at an electrode-driving power of 7 mW. When the pump light power is 230 mW and the signal light power is 0.1 mW, the loss compensation of the device is 3.8 dB at a wavelength of 1530 nm. Optical devices with loss compensation have important research significance.
Supercontinuum manipulation based on the influence of chirp on soliton spectral tunneling
The soliton spectral tunneling (SST) effect, as a soliton spectral switching phenomenon, enables a soliton to tunnel through a spectrally limited regime of normal dispersion in the fiber with multiple zero dispersion wavelengths (ZDWs). Since initial chirp can affect the behavior of pulse evolution, we numerically study the influence of chirp on the SST in the process of supercontinuum (SC) occurring in a photonic crystal fiber (PCF) with three ZDWs. The linear chirp is imposed by a phase modulation of input pulse while maintaining a constant pulse duration. Interestingly, it is found that the spectral range and flatness can be flexibly tuned by adjusting the initial chirp value. More specifically, positive chirp facilitates soliton self-frequency shifting (SSFS), making the soliton quickly transfer from one anomalous dispersion regime to another accompanied by the generation of dispersive waves (DWs). In this case, the SST effect further expands the spectral range by enhancing both the red-shift of the fundamental soliton and the blue-shift of DWs, thus generating a broader SC. However, negative chirp suppresses the SST effect, resulting in a smoother SC at the expense of bandwidth. Therefore, the findings in this work provide interesting results relating to the influence of initial chirp on the SST to generate a considerably smoother and broader SC, which is extremely useful in many applications, such as wavelength conversion and SC generation.
Effect of graphene/ZnO hybrid transparent electrode on characteristics of GaN light-emitting diodes
In order to reduce the Schottky barrier height and sheet resistance between graphene (Gr) and the p-GaN layers in GaN-based light-emitting diodes (LEDs), conductive transparent thin films with large work function are required to be inserted between Gr and p-GaN layers. In the present work, three kinds of transparent conductive oxide (TCO) zinc oxide (ZnO) films, Al-, Ga-, and In-doped ZnO (AZO, GZO, and IZO), are introduced as a bridge layer between Gr and p-GaN, respectively. The influence of different combinations of Gr/ZnO hybrid transparent conducting layers (TCLs) on the optical and thermal characteristics of the GaN-LED was investigated by the finite element method through COMSOL software. It is found that both the TCL transmittance and the surface temperature of the LED chip reduce with the increase in Gr and ZnO thickness. In order to get the transmittance of the Gr/ZnO hybrid TCL higher than 80%, the appropriate combination of Gr/ZnO compound electrode should be a single layer of Gr with ZnO no thicker than 400 nm (1L Gr/400-nm ZnO), 2L Gr/300-nm ZnO, 3L Gr/200-nm ZnO, or 4L Gr/100-nm ZnO. The LEDs with hybrid TCLs consisting of 1L Gr/300-nm AZO, 2L Gr/300-nm GZO, and 2L Gr/300-nm IZO have good performance, among which the one with 1L Gr/300-nm GZO has the best thermal property. Typically, the temperature of LEDs with 1L Gr/300-nm GZO hybrid TCLs will drop by about 7 K compared with that of the LEDs with a TCL without ZnO film.
First-principles study on the mechanics, optical, and phonon properties of carbon chains
Besides graphite, diamond, graphene, carbon nanotubes, and fullerenes, there is another allotrope of carbon, carbyne, existing in the form of a one-dimensional chain of carbon atoms. It has been theoretically predicted that carbyne would be stronger, stiffer, and more exotic than other materials that have been synthesized before. In this article, two kinds of carbyne, i.e., cumulene and polyyne are investigated by the first principles, where the mechanical properties, electronic structure, optical and phonon properties of the carbynes are calculated. The results on the crystal binding energy and the formation energy show that though both are difficult to be synthesized from diamond or graphite, polyyne is more stable and harder than cummulene. The tensile stiffness, bond stiffness, and Young's modulus of cumulene are 94.669 eV/Å, 90.334 GPa, and 60.62 GPa, respectively, while the corresponding values of polyyne are 94.939 eV/Å, 101.42 GPa, and 60.06 GPa. The supercell calculation shows that carbyne is most stable at N=5, where N is the supercell number, which indicates that the carbon chain with 10 atoms is most stable. The calculation on the electronic band structure shows that cumulene is a conductor and polyyne is a semiconductor with a band gap of 0.37 eV. The dielectric function of carbynes varies along different directions, consistent with the one-dimensional nature of the carbon chains. In the phonon dispersion of cumulene, there are imaginary frequencies with the lowest value down to-3.817 THz, which indicates that cumulene could be unstable at room temperature and normal pressure.
Antiferromagnetic–ferromagnetic transition in zigzag graphene nanoribbons induced by substitutional doping Hot!
Using first-principles calculations based on density functional theory, we show that the ground state of zigzag-edged graphene nanoribbons (ZGNRs) can be transformed from antiferromagnetic (AFM) order to ferromagnetic (FM) order by changing the substitutional sites of N or B dopants. This AFM-FM transition induced by substitutional sites is found to be a consequence of the competition between the edge and bulk states. The energy sequence of the edge and bulk states near the Fermi level is reversed in the AFM and FM configurations. When the dopant is substituted near the edge of the ribbon, the extra charge from the dopant is energetically favorable to occupy the edge states in AFM configuration. When the dopant is substituted near the center, the extra charge is energetically favorable to occupy the bulk states in FM configuration. Proper substrate with weak interaction is necessary to maintain the magnetic properties of the doped ZGNRs. Our study can serve as a guide to synthesize graphene nanostructures with stable FM order for future applications to spintronic devices.
Phase diagram characterized by transmission in a triangular quantum dot
We propose a theoretical model to detect the quantum phase transition in a triangular quantum dot molecule with frustration. The boundaries of the phase diagram are accurately determined by the transmission. For small frustration t, as the interdot Coulomb repulsion V increases, the system undergoes a Kosterlitz-Thouless (KT) transition from the Kondo resonance state with a transmission peak at zero energy to the Coulomb blocked state with zero transmission, which is followed by a first transition to the V-induced resonance (VIR) state with unitary transmission. For large frustration t, as V increases, the orbital spin singlet without transmission transits to the VIR state through a KT transition.
Magnetism induced by Mn atom doping in SnO monolayer
The structural, magnetic properties, and mechanism of magnetization of SnO monolayer doped with 3d transition metal Mn atom were studied using first-principles calculations. The calculated results show that the substitution doping is easier to realize under the condition of oxygen enrichment. Numerical results reveal that the spin-splitting defect state of the Mn doped system is produced in the band gap and the magnetic moment of 5.0 μB is formed. The induced magnetic moment by Mnsub is mostly derived from the 3d orbital of the doped Mn atom. The magnetic coupling between magnetic moments caused by two Mn atoms in SnO monolayer is a long-range ferromagnetic, which is due to the hole-mediated p-p and p-d interactions. The calculated results suggest that room-temperature ferromagnetism in a SnO monolayer can be induced after substitutional doping of a Mn atom.
Fabrication and characterization of Ge–Ga–Sb–S glass microsphere lasers operating at~1.9 μm
We report the fabrication and characterization of germanium gallium antimony sulfide (Ge-Ga-Sb-S or 2S2G, doped with Tm3+ ions) microsphere lasers operating at~1.9-μm spectral band. Compared to the chalcogenide glasses that are used in previous microsphere lasers, this 2S2G glass has a lower transition temperature and a higher characteristic temperature. This implies that 2S2G microspheres can be fabricated at lower temperatures and the crystallization problem in the sphere-forming process can be alleviated. We show that hundreds of high-quality microspheres (quality factors higher than 105) of various diameters can be produced simultaneously via a droplet sphere-forming method. Microspheres are coupled with silica fiber tapers for optical characterizations. We demonstrate that Whispering Gallery mode (WGM) patterns in the 1.7-2.0 μm band can be conveniently obtained and that once the pump power exceeds a threshold, single-and multi-mode microsphere lasers can be generated. For a typical microsphere whose diameter is 258.64 μm, we demonstrate its laser threshold is 0.383 mW, the laser wavelength is 1907.38 nm, and the thermal sensitivity of the microsphere laser is 29.56 pm/℃.
Temperature-dependent Raman spectroscopic study of ferroelastic K2Sr(MoO4)
Raman scattering measurements of K2Sr(MoO4)2 were performed in the temperature range of 25-750℃. The Raman spectrum of the low-temperature phase α-K2Sr(MoO4)2 that was obtained by first-principle calculations indicated that the Raman bands in the wavenumber region of 250-500 cm-1 are related to Mo-O bending vibrations in MoO4 tetrahedra, while the Raman bands in the wavenumber region of 650-950 cm-1 are attributed to stretching vibrations of Mo-O bonds. The temperature-dependent Raman spectra reveal that K2Sr(MoO4)2 exhibits two sets of modifications in the Raman spectra at~150℃ and~475℃, attributed to structural phase transitions. The large change of the Raman spectra in the temperature range of 150℃ to 475℃ suggests structural instability of the medium-temperature phase β-K2Sr(MoO4)2.
Photodynamics of GaZn-VZn complex defect in Ga-doped ZnO
The wide-band-gap II-VI compound semiconductor ZnO is regarded as a promising single-photon emission (SPE) host material. In this work, we demonstrate that a (GaZn-VZn)- complex defect can readily be obtained and the density can be controlled in a certain range. In analogy to nitrogen vacancy centers, such a defect in ZnO is expected to be a new single photon source. The optical properties of the (GaZn-VZn)- complex defect are further studied by photoluminescence and time-resolved photoluminescence spectra measurements. The electron transitions between the defect levels emit light at~650 nm with a lifetime of 10-20 nanoseconds, indicating a good coherent length for SPE. Finally, a two-level emitter structure is proposed to explain the carrier dynamics. We believe that the photodynamics study of the (GaZn-VZn)- complex defect in this work is important for ZnO-based quantum emitters.
Nanocrystalline and nanocomposite permanent magnets by melt spinning technique
Mn-based permanent magnets
Mn-based intermetallic compounds have attracted much attention due to their fascinating structural and physical properties, especially their interesting hard magnetic properties. In this paper, we have summarized the magnetic and structural properties of Mn-based intermetallic compounds (MnX, where X=Al, Bi, and Ga). Various methods for synthesizing single phases of MnAl, MnBi, and MnxGa were developed in our lab. A very high saturation magnetization of 125 emu/g, coercivity of 5 kOe, and maximum energy product (BH)max of 3.1 MG·Oe were achieved at room temperature for the pure τ-Mn-Al magnetic phase without carbon doping and the extrusion process. Low temperature phase (LTP) MnBi with a purity above 95 wt.% can be synthesized. An abnormal temperature coefficient of the coercivity was observed for the LTP MnBi magnet. Its coercivity increased with temperature from 100 K to 540 K, reached a maximum of 2.5 T at about 540 K, and then decreased slowly to 1.8 T at 610 K. The positive temperature coefficient of the coercivity is related to the evolution of the structure and magnetocrystalline anisotropy field of the LTP MnBi phase with temperature. The LTP MnBi bonded magnets show maximum energy products (BH)max of 8.9 MG·Oe (70 kJ/m3) and 5.0 MG·Oe (40 kJ/m3) at room temperature and 400 K, respectively. Ferrimagnetic MnxGa phases with L10 structures (x < 2.0) and D022 structures (x > 2.0) were obtained. All of the above structures can be described by a D022 supercell model in which 2a-Ga and 2b-Mn are simultaneously substituted. The tetragonal D022 phases of the MnxGa show high coercivities ranging from 7.2 kOe for low Mn content x=1.8 to 18.2 kOe for high Mn content x=3 at room temperature. The Mn1.2Ga sample exhibits a room temperature magnetization value of 80 emu/g. The hard magnetic properties of coercivity iHc=3.5 kOe, remanence Mr=43.6 emu/g, and (BH)max=2.5 MG·Oe were obtained at room temperature. Based on the above studies, we believe that Mn-based magnetic materials could be promising candidates for rare earth free permanent magnets exhibiting a high Curie temperature, high magnetocrystalline anisotropy, and very high coercivity.
Rare earth permanent magnets prepared by hot deformation process
Hot deformation is one of the primary methods for fabricating anisotropic rare earth permanent magnets. Firstly, rapidly quenched powder flakes with a nanocrystal structure are condensed into fully dense isotropic precursors using the hot-pressing process. The prepared isotropic precursors are then hot-deformed to produce high-anisotropy uniaxial bulk rare earth permanent magnets and a highly textured structure is produced via this process. The resulting magnets possess many advantages such as near-net-shape, outstanding corrosion resistance, and ultrafine-grain structure. The influence of the preparation parameters utilized in the hot-pressing and deformation processes on the magnetic properties and microstructure of the permanent magnets are systemically summarized in this report. As a near-net-shape technique, the hot deformation process has notable advantages with regard to the production of irregular shapes, especially for radially oriented ring-shaped magnets with high length-diameter ratios or thin walls. The difficulties associated with the fabrication of crack-free, homogeneous, and non-decentered ring-shaped magnets are substantially resolved through an emphasis on mold design, adjustment of deformation parameters, and application of theoretical simulation. Considering the characteristics of hot-deformed magnets which include grain shape and size, anisotropic distribution of intergranular phases, etc., investigation and improvement of the mechanical and electric properties, in addition to thermal stability, with the objective of improving the application of hot-deformed magnets or ring-shaped magnets, is of practical significance.
Excellent thermal stability and thermoelectric properties of Pnma-phase SnSe in middle temperature aerobic environment
SnSe is considered to be a promising thermoelectric material due to a high ZT value and abundant and non-toxic composition elements. However, the thermal stability is an important issue for commercial application. In particular, thermoelectric materials are in the high temperature for a long time due to the working condition. The present work investigates the thermal stability and oxidation resistance of single crystal SnSe thermoelectric materials. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that the internal of SnSe crystal is not easily oxidized, while the x-ray photoelectron spectroscopy (XPS) results indicate that the surface of SnSe is slight oxidized to SnO2. Even if the surface is oxidized, the SnSe crystal still exhibits stable thermoelectric properties. Meanwhile, the crystallization quality of SnSe samples can be improved after the appropriate heat treatment in the air, which is in favor of the carrier mobility and can improve the electrical conduction properties of SnSe. Moreover, the decrease of defect density after heat treatment can further improve the Seebeck coefficient and electrical transport properties of SnSe. The density functional theory (DFT) calculation verifies the important role of defect on the electrical conductivity and electron configuration. In summary, appropriate temperature annealing is an effective way to improve the transmission properties of SnSe single crystal thermoelectric materials.
Electronic, optical property and carrier mobility of graphene, black phosphorus, and molybdenum disulfide based on the first principles
The band structure, density of states, optical properties, carrier mobility, and loss function of graphene, black phosphorus (BP), and molybdenum disulfide (MoS2) were investigated by the first-principles method with the generalized-gradient approximation. The graphene was a zero-band-gap semiconductor. The band gaps of BP and MoS2 were strongly dependent on the number of layers. The relationships between layers and band gap were built to predict the band gap of few-layer BP and MoS2. The absorption showed an explicit anisotropy for light polarized in (100) and (001) directions of graphene, BP, and MoS2. This behavior may be readily detected in spectroscopic measurements and exploited for optoelectronic applications. Moreover, graphene (5.27×104 cm2·V-1·s-1), BP (1.5×104 cm2·V-1·s-1),and MoS2(2.57×102 cm2·V-1·s-1) have high carrier mobility. These results show that graphene, BP, and MoS2 are promising candidates for future electronic applications.
Dual-polarized lens antenna based on multimode metasurfaces
We propose a dual-polarized lens antenna system based on isotropic metasurfaces for 12 GHz applications. The metasurface lens is composed of subwavelength unit cells (0.24λ0) with metallic strips etched on the top and bottom sides of the unit cell, and a cross-slots metallic layer in the middle that serves as the ground. The multimode resonance in the unit cell can realize a large phase shift (covering 0°-360°), and the total transmission efficiency of the lens is above 80%. The feed antenna at the focal point of the lens is a broadband dual-polarized microstrip antenna. Both the simulated and the measured results demonstrate that the dual-polarized lens antenna system can realize a gain of more than 16.1 dB, and an input port isolation of more than 25.0 dB.
Design and development of radio frequency output window for circular electron-positron collider klystron
This paper presents the first phase of design, analysis, and simulation for the klystron coaxial radio frequency (RF) output window. This study is motivated by 800 kW continuous wave (CW), 650 MHz klystrons for the future plan of circular electron-positron collider (CEPC) project. The RF window which is used in the klystron output section has a function to separate the klystron from the inner vacuum side to the outside, and high RF power propagates through the window with small power dissipation. Therefore, the window is a key component for the high power klystron. However, it is vulnerable to the high thermal stress and multipacting, so this paper presents the window design and analysis for these problems. The microwave design has been performed by using the computer simulation technology (CST) microwave studio and the return loss of the window has been established to be less than-90 dB. The multipacting simulation of the window has been carried out using MultiPac and CST particles studio. Through the multipacting analysis, it is shown that with thin coating of TiN, the multipacting effect has been suppressed effectively on the ceramic surface. The thermal analysis is carried out using ANSYS code and the temperature of alumina ceramic is lower than 310 K with water cooling. The design result successfully meets the requirement of the CEPC 650 MHz klystron. The manufacturing and high power test plan are also described in this paper.
Analysis of tail bits generation of multilevel storage in resistive switching memory
The tail bits of intermediate resistance states (IRSs) achieved in the SET process (IRSS) and the RESET process (IRSR) of conductive-bridge random-access memory were investigated. Two types of tail bits were observed, depending on the filament morphology after the SET/RESET operation. (i) Tail bits resulting from lateral diffusion of Cu ions introduced an abrupt increase of device resistance from IRS to ultrahigh-resistance state, which mainly happened in IRSS. (ii) Tail bits induced by the vertical diffusion of Cu ions showed a gradual shift of resistance toward lower value. Statistical results show that more than 95% of tail bits are generated in IRSS. To achieve a reliable IRS for multilevel cell (MLC) operation, it is desirable to program the IRS in RESET operation. The mechanism of tail bit generation that is disclosed here provides a clear guideline for the data retention optimization of MLC resistive random-access memory cells.
Impact of variations of threshold voltage and hold voltage of threshold switching selectors in 1S1R crossbar array
The impact of the variations of threshold voltage (Vth) and hold voltage (Vhold) of threshold switching (TS) selector in 1S1R crossbar array is investigated. Based on ON/OFF state I-V curves measurements from a large number of Ag-filament TS selectors, Vth and Vhold are extracted and their variations distribution expressions are obtained, which are then employed to evaluate the impact on read process and write process in 32×32 1S1R crossbar array under different bias schemes. The results indicate that Vth and Vhold variations of TS selector can lead to degradation of 1S1R array performance parameters, such as minimum read/write voltage, bit error rate (BER), and power consumption. For the read process, a small Vhold variation not only results in the minimum read voltage increasing but it also leads to serious degradation of BER. As the standard deviation of Vhold and Vth increases, the BER and the power consumption of 1S1R crossbar array under 1/2 bias, 1/3 bias, and floating scheme degrade, and the case under 1/2 bias tends to be more serious compared with other two schemes. For the write process, the minimum write voltage also increases with the variation of Vhold from small to large value. A slight increase of Vth standard deviation not only decreases write power efficiency markedly but also increases write power consumption. These results have reference significance to understand the voltage variation impacts and design of selector properly.
Unidirectional rotation of circles driven by chiral active particles
The dynamics of two-dimensional rigid circles filled with chiral active particles are investigated by employing the overdamped Langevin dynamics simulations. Unidirectional rotation of rigid circles is observed, and the rotational angular velocity (ω') relies mainly on the length (l), the number (nB), and tilt angle (γ) of boards, and the angular velocity (ω) and area fraction (ρ) of chiral active particles. There are optimum values for these parameters at which the average angular velocity of circle reaches its maximum. The center-of-mass mean square displacement for circles drops by about two orders of magnitude for large angular velocity ω of chiral active particles with oscillations in the short-time regime. Our work demonstrates that nanofabricated objects with suitable designs immersed in a bath of chiral active particles can extract and rectify energy in a unidirectional motion.
Mechanochemical model for myosin II dimer that can explain the spontaneous oscillatory contraction of muscle
The spontaneous oscillatory contraction (SPOC) of myofibrils is the essential property inherent to the contractile system of muscle. Muscle contraction results from cyclic interactions between actin filament and myosin Ⅱ which is a dimeric motor protein with two heads. Taking the two heads of myosin Ⅱ as an indivisible element and considering the effects of cooperative behavior between the two heads on rate constants in the mechanochemical cycle, the present work proposes the tenstate mechanochemical cycle model for myosin Ⅱ dimer. The simulations of this model show that the proportion of myosin Ⅱ in different states periodically changes with time, which results in the sustained oscillations of contractive tension, and serves as the primary factor for SPOC. The good fit of this model to experimental results suggests that the cooperative interaction between the two heads of myosin Ⅱ dimer may be one of the underlying mechanisms for muscle contraction.
Effects of 3.7 T–24.5 T high magnetic fields on tumor-bearing mice
Since high magnetic field (MF) intensity can improve the image quality and reduce magnetic resonance imaging (MRI) acquisition time, the field intensity of MRIs has continued to increase over the past few decades. Although MRIs in most current hospitals are 0.5 T-3 T, there are preclinical studies have been carried out using 9.4 T MRI, and engineers are also putting efforts on building MRIs with even higher MFs. However, the accompanied safety issue of high-field MRIs is an emergent question to address before their clinical applications. In the meantime, the static magnetic field (SMF) has been shown to inhibit tumor growth in previous studies. Here, we investigated both the safety issue and the anti-tumor potentials of 3.7 T-24.5 T SMFs on GIST-T1 gastrointestinal stromal tumor-bearing nude mice. We followed up the mice three weeks after their exposure to high SMF and found that none of the mice died or had severe organ damage, except for slightly decreased food intake, weight gain, and liver function. Moreover, the tumor growth was inhibited by 3.7 T-24.5 T SMFs (up to~54%). It is interesting that the effects are more dependent on MF gradient than intensities, and for the same gradient and intensity, mice responded differently to hypogravity and hypergravity conditions. Therefore, our study not only demonstrated the safeness of high SMFs up to 24.5 T on mice but also revealed their anti-tumor potentials in the future.
Probing conformational change of T7 RNA polymerase and DNA complex by solid-state nanopores
Proteins are crucial to most biological processes, such as enzymes, and in various catalytic processes a dynamic motion is required. The dynamics of protein are embodied as a conformational change, which is closely related to the flexibility of protein. Recently, nanopore sensors have become accepted as a low cost and high throughput method to study the features of proteins. In this article, we used a SiN nanopore device to study the flexibility of T7 RNA polymerase (RNAP) and its complex with DNA promoter. By calculating full-width at half-maximum (FWHM) of Gaussian fits to the blockade histograms, we found that T7 RNAP becomes more flexible after binding DNA promoter. Moreover, the distribution of fractional current blockade suggests that flexibility alters due to a breath-like change of the volume.
High capacity sodium-rich layered oxide cathode for sodium-ion batteries Hot!
Sodium-ion batteries have attracted significant recent attention currently considering the limited available lithium resource. However, the energy density of sodium-ion batteries is still insufficient compared to lithium-ion batteries, mainly because of the unavailability of high-energy cathode materials. In this work, a novel sodium-rich layered oxide material (Na2MnO3) is reported with a dynamical stability similar to that of the Li2MnO3 structure and a high capacity of 269.69 mA·h·g1, based on first-principles calculations. Sodium ion de-intercalation and anionic reaction processes are systematically investigated, in association with sodium ions migration phenomenon and structure stability during cycling of NaxMnO3 (1 ≤ x ≤ 2). In addition, the charge compensation during the initial charging process is mainly contributed by oxygen, where the small differences of the energy barriers of the paths 2c→4h, 4h→2c, 4h→4h, 2c→2b, and 4h→2b indicate the reversible sodium ion occupancy in transitional metal and sodium layers. Moreover, the slow decrease of the elastic constants is a clear indication of the high cycle stability. These results provide a framework to exploit the potential of sodium-rich layered oxide, which may facilitate the development of high-performance electrode materials for sodium-ion batteries.
Coordinated chaos control of urban expressway based on synchronization of complex networks
We investigate the problem of coordinated chaos control on an urban expressway based on pinning synchronization of complex networks. A node coupling model of an urban expressway based on complex networks has been established using the cell transmission model (CTM). The pinning controller corresponding to multi-ramp coordinated controller was designed by using the delayed feedback control (DFC) method, whose objective is to realize periodical orbits from chaotic states. The concrete pinning control nodes corresponding to the subsystems of regulating the inflows from the on-ramps to the mainline were obtained and the parameters of the controller were optimized by using the stability theory of complex networks to ensure the network synchronization. The validity of the proposed coordinated chaos control method was proven via the simulation experiment. The results of the examples indicated that the order motion on urban expressway can be realized, the wide-moving traffic jam can be suppressed, and the operating efficiency is superior to that of the traditional control methods.