Homogeneous transparent device and its layered realization
Arbitrarily shaped electromagnetic transparent devices with homogeneous, non-negative, anisotropic and generic constitutive parameters are proposed based on linear transformation optics, which provides the flexibility for device design that is applicable for the practical fabrication. To remove the anisotropic property, a layered structure is developed based on effective medium theory. Simulation results show that with sufficient layers, the performance of the layered transparent device is nearly as perfect as an ideal device, and it is able to protect an antenna without sacrificing its performance. The feasibility of designing a transparent device by using natural isotropic materials instead of metamaterials would dramatically reduce the difficulty of fabrication and further promote the practicality of the device.
Polarization ratio characteristics of electromagnetic scattering from sea ice in polar areas
In the global climate system, the polar regions are sensitive indicators of climate change, in which sea ice plays an important role. Satellite remote sensing is a significant tool for monitoring sea ice. The use of synthetic aperture radar (SAR) images to distinguish sea ice from sea water is one of the current research hotspots in this topic. To distinguish sea ice from the open sea, the polarization ratio characteristics of sea ice and sea water are studied for L-band and C-band radars, based on an electromagnetic scattering model of sea ice derived from the integral equation method (IEM) and the radiative transfer (RT) model. Numerical experiments are carried out based on the model and the results are given as follows. For L-band, the polarization ratio for sea water depends only on the incident angle, while the polarization ratio for sea ice is related to the incident angle and the ice thickness. For C-band, the sea water polarization ratio is influenced by the incident angle and the root mean square (RMS) height of the sea surface. For C-band, for small to medium incident angles, the polarization ratio for bare sea ice is mainly determined by the incident angle and ice thickness. When the incident angle increases, the RMS height will also affect the polarization ratio for bare sea ice. If snow covers the sea ice, then the polarization ratio for sea ice decreases and is affected by the RMS height of snow surface, snow thickness, volume fraction and the radius of scatterers. The results show that the sea ice and the open sea can be distinguished by using either L-band or C-band radar according to their polarization ratio difference. However, the ability of L-band to make this differentiation is higher than that of C-band.
Generation of Mathieu beams using angular pupil modulation
By using an amplitude-type spatial light modulator to load angular spectrum of Mathieu function distribution along a narrow annular pupils, the Durnin's experimental setup is extended to generate various types of Mathieu beams. As a special type of Mathieu beams, Bessel beams are also generated using this optical setup. Furthermore, the optical morphology of the Mathieu beams family are also presented and analyzed.
Polarization-based range-gated imaging in birefringent medium:Effect of size parameter
We have investigated the effect of size parameter of the scatterer on the image quality obtained with polarization-based range-gated imaging in birefringent turbid medium. Both linearly and circularly polarized light were utilized for imaging. The simulated results indicate that the improvement of visibility is more pronounced using circularly polarized light for the birefringent medium composed of smaller-sized scatterers at lower values of optical thickness and the birefringent medium comprising larger-sized scatterers. In contrast, linearly polarized light provides better image quality for the birefringent medium composed of smaller-sized scatterers at larger values of optical thickness. The evolution of the polarization characteristics of backscattered light and target light under the conditions mentioned above was measured to account for these numerical results.
Geometrical condition for observing Talbot effect in plasmonics infinite metallic groove arrays
The plasmonics Talbot effect in metallic layer with infinite periodic grooves is presented in this study. Numerical approach based on the finite element method is employed to verify the derived Talbot carpet on the non-illumination side. The groove depth is less than the metallic layer thickness; however, for specific conditions, surface plasmons polaritons (SPPs) can penetrate through grooves, propagate under the metallic layer, and form Talbot revivals. The geometrical parameters are specified via groove width, gap size, period, and wavelength, and their proper values are determined by introducing two opening ratio parameters. To quantitatively compare different Talbot carpets, we introduce new parameters such as R-square that characterizes the periodicity of Talbot images. The higher the R-square of a carpet, the more coincident with non-paraxial approximation the Talbot distance becomes. We believe that our results can help to understand the nature of SPPs and also contribute to exploring this phenomenon in Talbot-image-based applications, including imaging, optical systems, and measurements.
Tunable plasmon-induced transparency based on asymmetric H-shaped graphene metamaterials
We propose and numerically demonstrate a tunable plasmon-induced transparency (PIT) phenomenon based on asymmetric H-shaped graphene metamaterials. The tunable PIT effect is realized through varying the applied polarization angles rather than changing the structure geometry. By simply adjusting the polarization angle, the transmission spectra can be controlled between the switch-on state and switch-off state. The physical mechanism of the induced transparency is revealed from magnetic dipole inductive coupling and phase coupling. Importantly, by varying the Fermi energy of the graphene or the refractive index of the substrate, the resonant position of the PIT can be dynamically controlled and the maximum modulation depths can reach up to 60.7%. The sensitivity (nm/RIU) of the graphene structure, which is the shift of resonance wavelength per unit change of refractive index, is 5619.56 nm/RIU. Moreover, we also extend our research to the x-axis symmetric H-shaped structure, and the tunable PIT transmission window can also be realized. The physical mechanism of the induced transparency is revealed from the electric dipole hybridization coupling. Our designed H-shaped graphene-based structures is a promising candidate for compact elements such as tunable sensors, switches and slow-light devices.
Dynamic properties of atomic collective decay in cavity quantum electrodynamics
We theoretically study the collective decay of two atoms trapped in a single mode cavity and describe the evolution of the population of Dicke states. We show that the collective decay property is strongly dependent on the phase of atomic radiation and the speeding up of collective decay can be observed in bad cavity regime. For in- or out-phase case, it occurs due to the quantum interference enhancement no matter which atom is excited initially. For π/2 phase, the speeding up of collective decay takes place if the first atom is excited at the beginning. However, it disappears due to the quantum interference cancelation if the second atom is excited. Compared with the in-phase and out-phase cases, we also show that the speeding up of collective decay can be significantly enhanced in strong coupling regime for π/2 phase although one atom is decoupled to the cavity in this condition. The study presented here is helpful to understand the physical mechanism of collective decay in cavity quantum electrodynamics, and provide a useful method to control the collective decay phenomenon via quantum interference effect.
Room-temperature continuous-wave interband cascade laser emitting at 3.45 μm
We report a type-Ⅱ GaSb-based interband cascade laser operating a continuous wave at room temperature. The cascade region of interband cascade laser was designed using the ‘W’ configuration of the active quantum wells and the ‘Carrier Rebalancing’ method in the electron injector. The devices were processed into narrow ridges and mounted epitaxial side down on a copper heat sink. The 25-μm-wide, 3-mm-long ridge without coated facets generated 41.4 mW of continuous wave output power at T=15℃. And a low threshold current density of 267 A/cm2 is achieved. The emission wavelength of the ICL is 3452.3 nm at 0.5 A.
16-channel dual-tuning wavelength division multiplexer/demultiplexer
A 16-channel dual tuning wavelength division multiplexer/demultiplexer based on silicon on insulator platform is demonstrated, which is both peak wavelength tunable and output optical power tunable. The wavelength division multiplexer/demultiplexer consists of an arrayed waveguide grating for wavelength division multiplexing/demultiplexing, a heater for peak wavelength tuning and a variable optical attenuator based on p-i-n carrier-injection structure for optical power tuning. The experimental results show that the insertion loss on chip of the device is 3.7 dB-5.7 dB and the crosstalk is 7.5 dB-9 dB. For the tunability of the peak wavelength, 1.058-nm wavelength tunability is achieved with 271.2-mW power consumption, and the average modulation efficiency is 3.9244 nm/W; for the tunability of the optical power, the optical power equalization is achieved in all 16 channels, 20-dB attenuation is achieved with 144.07-mW power consumption, and the raise/fall time of VOA is 35 ns/42 ns.
Thermoacoustic-reflected focusing lens based on acoustic Bessel-like beam with phase manipulation
We report the realization of broadband reflected acoustic focusing lenses based on thermoacoustic phased arrays of Bessel-like beams, in which the units of phase manipulation are composed of three rigid insulated boundaries and a thermal insulation film in air with different temperatures. Based on these units, we realize a reflected focusing lens which can focus reflected acoustic energy on a line, and its fractional bandwidth can reach about 0.29. In addition, we discuss the influences of the base angle of Bessel-like beam, the number of basic unit, and the variation of unit temperature on focusing performances in details. Furthermore, the reflected focusing lens for the cylindrical acoustic wave based on the Bessel-like beam is also demonstrated. The proposed focusing lens has the advantages of a broad working bandwidth, large focus size, and high robustness, which may provide possibilities for the design and application of acoustic lenses.
Cellular automaton modeling of pedestrian movement behavior on an escalator
As a convenient passenger transit facility between floors with different heights, escalators have been extensively used in shopping malls, metro stations, airport terminals, etc. Compared with other vertical transit facilities including stairs and elevators, escalators usually have large transit capacity. It is expected to reduce pedestrian traveling time and thus improve the quality of pedestrian's experiences especially in jamming conditions. However, it is noticed that pedestrians may present different movement patterns, e.g., queuing on each step of the escalator, walking on the left-side and meanwhile standing on the right-side of the escalator. These different patterns affect the actual escalator traffic volume and finally the passenger spatiotemporal distribution in different built environments. Thus, in the present study, a microscopic cellular automaton (CA) simulation model considering pedestrian movement behavior on escalators is built. Simulations are performed considering different pedestrian movement speeds, queuing modes, and segregation on escalators with different escalator speeds. The actual escalator capacities under different pedestrian movement patterns are investigated. It is found that walking on escalators will not always benefit escalator transit volume improvement, especially in jamming conditions.
Traffic flow velocity disturbance characteristics and control strategy at the bottleneck of expressway
In the three-phase traffic flow studies, the traffic flow characteristic at the bottleneck section is a hot spot in the academic field. The controversy about the characteristics of the synchronized flow at bottleneck is also the main contradiction between the three-phase traffic flow theory and the traditional traffic flow theory. Under the framework of three-phase traffic flow theory, this paper takes the on-ramp as an example to discuss the traffic flow characteristics at the bottleneck section. In particular, this paper mainly conducts the micro-analysis to the effect of lane change under the two lane conditions, as well as the effect of the on-ramp on the main line traffic flow. It is found that when the main road flow is low, the greater the on-ramp inflow rate, the higher the average speed of the whole road section. As the probability of vehicles entering from the on-ramp increases, the flow and the average speed of the main road are gradually stabilized, and then the on-ramp inflow vehicles no longer have a significant impact on the traffic flow. In addition, this paper focuses on the velocity disturbance generated at the on-ramp, and proposes the corresponding on-ramp control strategy based on it, and the simulation verified that the control strategy can reasonably control the traffic flow by the on-ramp, which can meet the control strategy requirements to some extent.
Gas flow characteristics of argon inductively coupled plasma and advections of plasma species under incompressible and compressible flows
In this work, incompressible and compressible flows of background gas are characterized in argon inductively coupled plasma by using a fluid model, and the respective influence of the two flows on the plasma properties is specified. In the incompressible flow, only the velocity variable is calculated, while in the compressible flow, both the velocity and density variables are calculated. The compressible flow is more realistic; nevertheless, a comparison of the two types of flow is convenient for people to investigate the respective role of velocity and density variables. The peripheral symmetric profile of metastable density near the chamber sidewall is broken in the incompressible flow. At the compressible flow, the electron density increases and the electron temperature decreases. Meanwhile, the metastable density peak shifts to the dielectric window from the discharge center, besides for the peripheral density profile distortion, similar to the incompressible flow. The velocity profile at incompressible flow is not altered when changing the inlet velocity, whereas clear peak shift of velocity profile from the inlet to the outlet at compressible flow is observed as increasing the gas flow rate. The shift of velocity peak is more obvious at low pressures for it is easy to compress the rarefied gas. The velocity profile variations at compressible flow show people the concrete residing processes of background molecule and plasma species in the chamber at different flow rates. Of more significance is it implied that in the usual linear method that people use to calculate the residence time, one important parameter in the gas flow dynamics, needs to be rectified. The spatial profile of pressure simulated exhibits obvious spatial gradient. This is helpful for experimentalists to understand their gas pressure measurements that are always taken at the chamber outlet. At the end, the work specification and limitations are listed.
Numerical simulation of metal evaporation based on the kinetic model equation and the direct simulation Monte Carlo method
Metal evaporation on the basis of the kinetic model equations (BGK and S-model) and the direct simulation Monte Carlo (DSMC) method was investigated computationally under the circumstances of collimators existing or not. Numerical data of distributions of number density, bulk velocity and temperature were reported over a wide range of evaporation rate. It was shown that these results reached a good agreement for the case of small evaporation rate, while the deviations became increasingly obvious with the increase of evaporation rate, especially when the collimators existed. Moreover, the deposition thickness over substrate obtained from the kinetic model equations were inaccurate even though the evaporation rate was small. All of the comparisons showed the reliability of the kinetic model equations, which require less computational cost at small evaporation rate and simple structure.
Drag reduction characteristics of heated spheres falling into water
We experimentally investigate the drag reduction characteristics of heated spheres falling into water by using a high-speed camera. In 25-℃ water, with the increase of the sphere temperature the average velocity increases to a maximum value at a temperature of 400℃ and then decreases until the temperature reaches 700℃, the average velocity will increase while the sphere temperature continually rises until the temperature reaches 900℃. The average and the maximum velocity of the heated sphere are larger than those of the room-temperature sphere. The flow separates at the rear of the heated sphere, leading to low pressure drag. The drag reduction effect of the stable film boiling is lower than that of the nucleate boiling. In the nucleate boiling regime, the average velocity decreases with the increase of water temperature, the drag of the sphere with gentle boiling intensity is smaller. The vapor layer formed in the stable film boiling regime can improve the stability of the fall trajectory. The intense turbulence caused by the nucleate boiling can make the sphere largely deviate from rectilinear motion.
Numerical simulations of dense granular flow in a two-dimensional channel:The role of exit position
Molecular dynamics simulations have been performed to elucidate the influence of exit position on a dense granular flow in a two-dimensional channel. The results show that the dense flow rate remains constant when the exit is far from the channel wall and increases exponentially when the exit moves close to the lateral position. Beverloo's law proves to be successful in describing the relation between the dense flow rate and the exit size for both the center and the lateral exits. Further simulated results confirm the existence of arch-like structure of contact force above the exit. The effective exit size is enlarged when the exit moves from the center to the lateral position. As compared with the granular flow of the center exit, both the vertical velocities of the grains and the flow rate increase for the lateral exit.
Preliminary computation of the gap eigenmode of shear Alfvén waves on the LAPD
Characterizing the gap eigenmode of shear Alfvén waves (SAWs) and its interaction with energetic ions is important to the success of magnetically confined fusion. Previous studies have reported an experimental observation of the spectral gap of SAW on the on Large Plasma Device (LAPD) (Zhang et al. 2008 Phys. Plasmas 15 012103), a linear large plasma device (Gekelman et al. 1991 Rev. Sci. Instrum. 62 2875) possessing easier diagnostic access and lower cost compared with traditional fusion devices, and analytical theory and numerical gap eigenmode using ideal conditions (Chang 2014 Ph.D Thesis at Australian National University). To guide experimental implementation, the present work models the gap eigenmode of SAWs using exact LAPD parameters. A full picture of the wave field for previous experiment reveals that the previously observed spectral gap is not global but an axially local result. To form a global spectral gap, the number of magnetic mirrors has to be increased and stronger static magnetic field makes it clearer. Such a spectral gap is obtained for the magnetic field of B0(z)=1.2+0.6cos[2π (z-33.68)/3.63] with 7.74-m magnetic beach. By introducing two types of local defects (corresponding to Eθ(z0)=0 and Eθ'(z0)=0 respectively), odd-parity and even-parity discrete eigenmodes are formed clearly inside the gap. The strength of these gap eigenmodes decreases significantly with collision frequency, which is consistent with previous studies. Parameter scans show that these gap eigenmodes can be even formed successfully for the field strength of B0(z)=0.2+0.1cos[2π (z-33.68)/3.63] and with only four magnetic mirrors, which are achievable by the LAPD at its present status. This work can serve as a strong motivation and direct reference for the experimental implementation of the gap eigenmode of SAWs on the LAPD and other linear plasma devices.
Numerical simulation of the multiple reversed shear Alfvén eigenmodes associated with the triangularity Alfvén gap
It was found that there are multiplicity of low shear toroidicity-induced Alfvén eigenmodes in a zero beta limit if the inverse aspect ratio is larger than the magnetic shear at the mode location (Candy 1996 Phys. Lett. A 215 299). Because the reversed shear Alfvén eigenmode (RSAE) and even the RSAE associated with the non-circular triangularity-induced Alfvén eigenmode (NAE) gap (NAE-RSAE) usually reside near the shear-reversal point, the condition that the inverse aspect ratio is larger than the magnetic shear is naturally satisfied. For this reason, we numerically investigate the existence of multiplicity of core-localized NAE-RSAEs and mode characteristics in the present work. We firstly verify the existence of the multiplicity for zero beta plasma by using a D-shaped equilibrium. It is pointed out that, for a given toroidal mode number, the Alfvén cascade spectrum accommodates down-sweeping and up-sweeping modes above and below the NAE range of frequencies. An analytical model for the existence of multiple RSAE modes is in good agreement with the simulation results. One notices that the triangularity has a greater effect on the odd-type modes than that on the even-type modes:the odd-type modes come into existence because of the plasma triangularity.
First-principles investigations on structural stability, mechanical, and thermodynamic properties of LaT2Al20 (T=Ti, V, Cr, Nb, and Ta) intermetallic cage compounds
First principles calculations were used to explore the structural stability, mechanical properties, and thermodynamic properties of LaT2Al20 (T=Ti, V, Cr, Nb, and Ta) intermetallics. The calculated formation enthalpy and phonon frequencies indicate that LaT2Al20 intermetallics exhibit the structural stability. The elastic moduli (B, G, E, and Hv) indicate that these intermetallics possess the better elastic properties than pure Al. The values of Poisson's ratio v and B/G demonstrate that LaT2Al20 intermetallics are all brittle materials. The anisotropy of elasticity and Young's modulus (three- and two-dimensional figures) indicate that LaT2Al20 compounds are anisotropic. Importantly, the calculated thermal quantities demonstrate that LaT2Al20 intermetallics possess the better thermal physical properties than pure Al at high temperatures.
Shock temperature and reflectivity of precompressed H2O up to 350 GPa:Approaching the interior of planets
Using a combination of static precompression and laser-driven shock compression, shock temperature and reflectivity of H2O have been measured up to 350 GPa and 2.1×104 K. Here, two calibration standards were applied to enhance temperature measurement reliability. Additionally, in temperature calculations, the discrepancy in reflectivity between active probe beam wavelength and self-emission wavelength has been taken into account to improve the data's precision. Precompressed water's temperature-pressure data are in very good agreement with our quantum molecular dynamics model, suggesting a superionic conductor of H2O in the icy planets' deep interior. A sluggish slope gradually approaching Dulong-Petit limit at high temperature was found at a specific heat capacity. Also, high reflectivity and conductivity were observed at the same state. By analyzing the temperature-pressure diagram, reflectivity, conductivity and specific heat comprehensively at conditions simulating the interior of planets in this work, we found that as the pressure rises, a change in ionization appears; it is supposedly attributed to energetics of bond-breaking in the H2O as it transforms from a bonded molecular fluid to an ionic state. Such molecular dissociation in H2O is associated with the conducting transition because the dissociated hydrogen atoms contribute to electrical properties.
Simulation and experimental investigation of low-frequency vibration reduction of honeycomb phononic crystals
The honeycomb phononic crystal displays good performance in reducing vibration, especially at low frequency, but there are few corresponding experiments involving this kind of phononic crystal and the influence of geometric parameters on the bandgap is unclear. We design a honeycomb phononic crystal, which is assembled by using a chemigum plate and a steel column, calculate the bandgaps of the phononic crystal, and analyze the vibration modes. In the experiment, we attach a same-sized rubber plate and a phononic crystal to a steel plate separately in order to compare their vibration reduction performances. We use 8×8 unit cells as a complete phononic crystal plate to imitate an infinite period structure and choose a string suspension arrangement to support the experiment. The results show that the honeycomb phononic crystal can reduce the vibrating plate magnitude by up to 60 dB in a frequency range of 600 Hz-900 Hz, while the rubber plate can reduce only about 20 dB. In addition, we study the effect of the thickness of plate and the height and the radius of the column in order to choose the most superior parameters to achieve low frequency and wide bandgap.
Exact solitary wave solutions of a nonlinear Schrödinger equation model with saturable-like nonlinearities governing modulated waves in a discrete electrical lattice
In this paper, we introduce and propose exact and explicit analytical solutions to a novel model of the nonlinear Schrödinger (NLS) equation. This model is derived as the equation governing the dynamics of modulated cutoff waves in a discrete nonlinear electrical lattice. It is characterized by the addition of two terms that involve time derivatives to the classical equation. Through those terms, our model is also tantamount to a generalized NLS equation with saturable; which suggests that the discrete electrical transmission lines can potentially be used to experimentally investigate wave propagation in media that are modeled by such type of nonlinearity. We demonstrate that the new terms can enlarge considerably the forms of the solutions as compared to similar NLS-type equations. Sine-Gordon expansion-method is used to derive numerous kink, antikink, dark, and bright soliton solutions.
Multilayer graphene refractive index tuning by optical power
Graphene's optical absorption coefficient increases linearly with the number of layers making it more effective in the construction of optical tuning graphene-based devices. Refractive index (RI) is one of the important optical parameters of the graphene for accurately describing its optical characteristics and further applications. In view of the RI research of the multilayer graphene is lacking and existing measurement methods are complicated. Optical power tuning RI of multilayer graphene is investigated using a simple measurement and no temperature cross sensitivity all optical fiber sensing structure. Optical power tuning RI characteristics of multilayer graphene are studied by tuning the introducing broad band light power from 0.57 mW to 22.7 mW. Different thickness graphene coating shows different tuning efficiency. At 4.86-μm thickness, a 3.433-nm Bragg wavelength shift is obtained with 156.2-pm/mW wavelength versus optical power tuning sensitivity corresponding to 3.25×103 RI change and 0.154 URI/W (URI, unit of RI) RI optical power tuning efficiency.
Effects of dielectric decrement on surface potential in a mixed electrolyte solution
Surface potential is an important parameter related to the physical and chemical properties of charged particles. A simple analytical model for the estimation of surface potential is established based on the Poisson-Boltzmann theory with the consideration of the dielectric decrement in mixed electrolyte. The analytical relationships between surface potential and charge density are derived in different mixed electrolytes with monovalent and bivalent ions. The dielectric decrease on the charged surface strongly affects the surface potential at a high charge density with different ion strengths and concentration ratios of counter-ions. The surface potential based on the Gouy-Chapman model is underestimated because of the dielectric decrement on the surface. The diffuse layer can be regarded as a continuous uniform medium only when the surface charge density is lower than 0.3 C·m-2. However, the surface charge densities of many materials in practical applications are higher than 0.3 C·m-2. The new model for the estimation of surface potential can return to the results obtained based on the Gouy-Chapman model at a low charge density. Therefore, it is implied that the established model that considers the dielectric decrement is valid and widely applicable.
Effects of the band-filling and Fe/Mo disorder on physical properties of Ca2FeMoO6
Both the band filling effect and Fe/Mo disorder have a close correlation with the physical properties of the double perovskite Ca2FeMoO6. Two series of Ca2FeMoO6 and Nd0.3Ca1.7FeMoO6 ceramics sintered at (1050℃, 1200℃, and 1300℃) were specially designed to comparatively investigate the band-filling effect and Fe/Mo disorder on the physical properties of Ca2FeMoO6. The x-ray diffraction indicates that Fe/Mo disorder is sensitive to the sintering temperature. The magnetization behavior is mainly controlled by the Fe/Mo disorder not by the band filling effect, manifested by a close correlation of saturated magnetization (Ms) with the Fe/Mo disorder. Interestingly, magnetoresistance (MR) property of the same composition is dominantly contributed by the grain boundary strength, which can be expressed by the macroscopic resistivity values. However, the band filling effect caused by the Nd-substitution can decrease the spin polarization, and thus suppress the MR performance fundamentally. Contrary to the MR response, the Curie temperature (TC) shows an obvious optimization due to the band filling effect, which increases the carrier density near the Fermi level responsible for the ferromagnetic coupling interaction strengthen. Maybe, our work can provoke further research interests into the correlation of the band-filling effects and Fe/Mo disorder with the physical properties of other Fe/Mo-based double perovskites.
Temporal pulsed x-ray response of CdZnTe:In detector Hot!
The temporal response of cadmium-zinc-telluride (CZT) crystals is evaluated at room temperature by using an ultrafast-pulsed x-ray source. The dynamics of carrier relaxation in a CZT single crystal is modeled at a microscopic level based on a multi-trapping effect. The effects of the irradiation flux and bias voltage on the amplitude and full width at half maximum (FWHM) of the transient currents are investigated. It is demonstrated that the temporal response process is affected by defect level occupation fraction. A fast photon current can be achieved under intense pulsed x-ray irradiation to be up to 2.78×109 photons mm-2·s-1. Meanwhile, it is found that high bias voltage could enhance carrier detrapping by suppressing the capture of structure defects and thus improve the temporal response of CZT detectors.
A simulation study of field plate termination in Ga2O3 Schottky barrier diodes
In this work, the field plate termination is studied for Ga2O3 Schottky barrier diodes (SBDs) by simulation. The influence of field plate overlap, dielectric material and thickness on the termination electric field distribution are demonstrated. It is found that the optimal thickness increases with reverse bias increasing for all the three dielectrics of SiO2, Al2O3, and HfO2. As the thickness increases, the maximum electric field intensity decreases in SiO2 and Al2O3, but increases in HfO2. Furthermore, it is found that SiO2 and HfO2 are suitable for the 600 V rate Ga2O3 SBD, and Al2O3 is suitable for both 600 V and 1200 V rate Ga2O3 SBD. In addition, the comparison of Ga2O3 SBDs between the SiC and GaN counterpart reveals that for Ga2O3, the breakdown voltage bottleneck is the dielectric. While, for SiC and GaN, the bottleneck is mainly the semiconductor itself.
Enhanced performance of a solar cell based on a layer-by-layer self-assembled luminescence down-shifting layer of core-shell quantum dots
In this paper, core-shell quantum dots (QDs) with two polar surface functional groups (ZnSe/ZnS-COOH QDs and ZnSe/ZnS-NH2 QDs) are synthesized in an aqueous phase. Photoluminescence (PL) and absorption spectra clearly indicate luminescence down-shifting (LDS) properties. On the basis of QDs, surface functional group multilayer LDS films (M-LDSs) are fabricated through an electrostatic layer-by-layer (LBL) self-assembly method. The PL intensity increases linearly with the number of bilayers, showing a regular and uniform film growth. When the M-LDS is placed on the surface of a Si-based solar cell as an optical conversion layer for the first time, the external quantum efficiency (EQE) and short-circuit current density (Jsc) notably increases for the LDS process. The EQE response improves in a wavelength region extending from the UV region to the blue region, and its maximum increase reaches more than 15% between 350 nm and 460 nm.
Dyson-Maleev theory of an X X Z ferrimagnetic spin chain with single-ion anisotropy
We use the mean-field approximation of Dyson-Maleev representation to study an XXZ Heisenberg ferrimagnetic spin chain with single-ion anisotropy. By solving the self-consistent equations with different anisotropies, λ and D respectively, the energy spectrums, internal energy, static susceptibility and specific heat are calculated. Especially, the quantum phase transition of the magnetization plateau induced by single-ion anisotropy D is obtained in the model of the ferrimagnetic spin chain by using Dyson-Maleev mean-field theory.
Realization of artificial skyrmion in CoCrPt/NiFe bilayers
Skyrmion, as a quasi-particle structure, has attracted much attention due to its potential applications in future spintronic devices. Artificial skyrmion structure has aroused great interest as it can be stabilized at room temperature, without needing to incorporate materials with Dzyaloshinskii-Moriya interaction (DMI) into it. In this paper, it is found that the artificial skyrmion structure can be realized in CoCrPt/NiFe bilayers by micromagnetic simulations. The critical magnetic field of the core decreases as the diameter of the NiFe soft magnetic layer increases. The artificial skyrmion has excellent topological protection, and the critical magnetic field of plane is about 76 mT (760 Oe, 1 Oe=79.5775 A·m-1). The external magnetic field plays a key role in determining the core diameter of the skyrmion, and the artificial skyrmion can be realized in CoCrPt/Cu/CoCrPt/NiFe four-layer with a diameter of 13 nm.
Effect of an electric field on the electrocaloric response of ferroelectrics
The electrocaloric effect of the model ferroelectric BaTiO3 was investigated using phenomenological theory. The results indicate that the applied electric field strength is a key factor for the induced electrocaloric response and there are two distinguishing electrocaloric responses. When a moderate electric field is applied, the electrocaloric temperature variation is small but the electrocaloric strength is high. In contrast, the electrocaloric temperature variation is large but electrocaloric strength is low when a very high electric field is applied. These results are consistent with the experimental observations on BaTiO3 based bulk and thin film ferroelectric materials.
Epitaxially strained SnTiO3 at finite temperatures
By combining the effective Hamiltonian approach and direct ab initio computation, we obtain the phase diagram of SnTiO3 with respect to epitaxial strain and temperature. This demonstrates the complex features of the phase diagram and provides an insight into this system, which is a presumably simple perovskite. Two triple points, as shown in the phase diagram, may be exploited to achieve high-performance piezoelectric effects. Despite the inclusion of the degree of freedom related to oxygen octahedron tilting, the ferroelectric displacements dominate the structural phases over the whole misfit strain range. Finally, we show that SnTiO3 can change from hard to soft ferroelectrics with the epitaxial strain.
Broadband microwave absorption properties of polyurethane foam absorber optimized by sandwiched cross-shaped metamaterial
The effect of a sandwiched cross-shaped metamaterial absorber (CMMA) on microwave absorption properties of the double-layered polyurethane foam absorber (PUFA) is investigated. Combining with the sandwiched CMMA, the bandwidth of -10-dB reflection loss for PUFA is broadened from 7.4 GHz to 9.1 GHz, which is attributed to the overlap of two absorption peaks originating from CMMA and PUFA, respectively. The values of the two absorption peaks located at 10.15 GHz and 14.7 GHz are -38.44 dB and -40.91 dB, respectively. Additionally, distribution of surface current, electromagnetic field and power loss density are introduced to investigate the absorption mechanism of the CMMA. The electromagnetic field distribution of the double-layered PUFA and the three-layered hybrid absorber are comparatively analyzed to ascertain the influence of CMMA. The results show that the proposed hybrid absorber can be applied to the anti-electromagnetic interference and stealth technology.
Influences on oxidation voltage and holding time on poly(3-methylthiophene) film for electrochromic stability
In this study, we report the influences of oxidation potential and holding time on the electrochromic (EC) stability of poly(3-methylthiophene) (P3MT) film during the electrochemical reaction. The cycle stability and transmittance changes of the film were investigated by optimizing the oxidation potential, and its chemical compositions were measured by x-ray photoelectron spectra after multiple electrochemical cycles. High oxidation potentials can increase the P3MT film color contrast and decrease its cycle stability because of accelerating chemical decomposition. Moreover, the holding time with potential pulsing was analyzed by using the optical memory of P3MT at an optimized oxidation potential, which revealed the reduced voltage duration saved energy consumption by 11.6% and improved the EC cycle stability without changing in color contrast.
Influence of carrier gas H2 flow rate on quality of p-type GaN epilayer grown and annealed at lower temperatures
In this work, we study the influence of carrier gas H2 flow rate on the quality of p-type GaN grown and annealed at lower temperatures. It is found that the concentration of H atoms in Mg-doped GaN epilayer can effectively decrease with appropriately reducing the carrier gas H2 flow rate, and a high-quality p-type GaN layer could be obtained at a comparatively low annealing temperature by reducing the carrier gas H2 flow rate. Meanwhile, it is found that the intensity and wavelength of DAP peak are changed as the annealing temperature varies, which shows that the thermal annealing has a remarkable effect not only on the activation of acceptors but also on the compensation donors.
Suppression of indium-composition fluctuations in InGaN epitaxial layers by periodically-pulsed mixture of N2 and H2 carrier gas
Indium-composition fluctuations in InGaN epitaxial layers are suppressed by using periodically-pulsed mixture (PPM) of N2 and H2 carrier gas. Photoluminescence, optical transmission, reciprocal space map and space-resolved cathodoluminescence are employed to characterize the InGaN epilayers. It is shown that the lateral In-fluctuations mainly occur as hillock-like In-rich regions. Both the number and size of In-rich regions are reduced by introducing the PPM carrier gas. Moreover, the measurements first experimentally demonstrate that the H2 carrier gas has a stronger decomposition effect on the In-rich region. As the duration time of the PPM carrier gas increases, the reduction of In-content in the In-rich region reaches up to 12%, however, only 2% for the In-homogeneous region. These factors lead to the suppression of In-fluctuations.
Large magnetic moment at sheared ends of single-walled carbon nanotubes
In this work we report that after single-walled carbon nanotubes (SWNTs) are sheared with a pair of titanium scissors, the magnetization becomes larger than that of the corresponding pristine ones. The magnetization increases proportionally with the number of SWNTs with sheared ends, suggesting that there exist magnetic moments at the sheared ends of SWNTs. By using the coefficient of this linear relation, the average magnetic moment is estimated to be 41.5±9.8 μB (Bohr magneton) per carbon atom in the edge state at temperature of 300.0 K, suggesting that ultrahigh magnetic fields can be produced. The dangling sigma and pi bonds of the carbon atoms at sheared ends play important roles in determining the unexpectedly high magnetic moments, which may have great potential applications.
Enhanced dielectric and optical properties of nanoscale barium hexaferrites for optoelectronics and high frequency application
M-type barium hexaferrites with chemical composition Ba1-xDyxFe12-yCryO19 (x=0.0, 0.1, 0.2, and y=0.0, 0.4, 0.5) were synthesized via sol-gel auto-combustion method. The samples were pre-sintered at 400℃ for 3 h and sintered at 950℃ for 5 h. The changes in the structural, dielectric, and optical properties were studied after the substitution of Dy3+ and Cr3+ ions. X-ray diffraction (XRD) analysis confirms the formation of single phase hexaferrites with the absence of secondary phase. FTIR analysis gives an idea of the formation of hexaferrites with the appearance of two peaks at 438 cm-1 and 589 cm-1. The field emission scanning electron micrographs (FESEM) show a combination of crystallites with large shapes close to hexagonal platelet-like shape and others with rice or rod-like shapes, whereas EDX and elemental analysis confirm the stoichiometry of prepared samples. The calculated band gap from UV-vis NIR spectroscopy spectra was found to decreases with increase in Dy3+-Cr3+ substitution. The dielectric properties were explained on the basis of Maxwell-Wagner model. Enhancement of dielectric constant at higher frequencies was observed in all the samples. Low dielectric loss is also observed in all the samples and Cole-Cole plot shows that grain boundary resistance (Rgb) contribute most to the dielectric properties. The prepared samples exhibit properties that could be useful for optoelectronics and high frequency application.
1.3-μm InAs/GaAs quantum dots grown on Si substrates
We compare the effect of InGaAs/GaAs strained-layer superlattice (SLS) with that of GaAs thick buffer layer (TBL) serving as a dislocation filter layer. The InGaAs/GaAs SLS is found to be more effective than GaAs TBL in blocking the propagation of threading dislocations, which are generated at the interface between the GaAs buffer layer and the Si substrate. Through testing and analysis, we conclude that the weaker photoluminescence for quantum dots (QDs) on Si substrate is caused by the quality of capping In0.15Ga0.85As and upper GaAs. We also find that the periodic misfits at the interface are related to the initial stress release of GaAs islands, which guarantees that the upper layers are stress-free.
Imaging the diffusion pathway of Al3+ ion in NASICON-type (Al0.2Zr0.8)20/19Nb(PO4)3 as electrolyte for rechargeable solid-state Al batteries Hot!
Among all-solid-state batteries, rechargeable Al-ion batteries have attracted most attention because they involve three-electron-redox reactions with high theoretic specific capacity. However, the solid Al-ion conductor electrolytes are less studied. Here, the microscopic path of Al3+-ion conduction of NASICON-type (Al0.2Zr0.8)20/19Nb(PO4)3 oxide is identified by temperature-dependent neutron powder diffraction and aberration-corrected scanning transmission electron microscopy experiments. (Al0.2Zr0.8)20/19Nb(PO4)3 shows a rhombohedral structure consisting of a framework of (Zr,Nb)O6 octahedra sharing corners with (PO4) tetrahedra; the Al occupy trigonal antiprisms exhibiting extremely large displacement factors. This suggests a strong displacement of Al ions along the c axis of the unit cell as they diffuse across the structure by a vacancy mechanism. Negative thermal expansion behavior is also identified along a and b axes, due to folding of the framework as temperature increases.
Nonlinear fast-slow dynamics of a coupled fractional order hydropower generation system
Internal effects of the dynamic behaviors and nonlinear characteristics of a coupled fractional order hydropower generation system (HGS) are analyzed. A mathematical model of hydro-turbine governing system (HTGS) with rigid water hammer and hydro-turbine generator unit (HTGU) with fractional order damping forces are proposed. Based on Lagrange equations, a coupled fractional order HGS is established. Considering the dynamic transfer coefficient e is variational during the operation, introduced e as a periodic excitation into the HGS. The internal relationship of the dynamic behaviors between HTGS and HTGU is analyzed under different parameter values and fractional order. The results show obvious fast-slow dynamic behaviors in the HGS, causing corresponding vibration of the system, and some remarkable evolution phenomena take place with the changing of the periodic excitation parameter values.
“Refractivity-from-clutter” based on local empirical refractivity model
Constructing sophisticated refractivity models is one of the key problems for the RFC (refractivity from clutter) technology. If prior knowledge of the local refractivity environment is available, more accurate parameterized model can be constructed from the statistical information, which in turn can be used to improve the quality of the local refractivity retrievals. The validity of this proposal was demonstrated by range-dependent refractivity profile inversions using the adjoint parabolic equation method to the Wallops'98 experimental data.
Influence of characteristics' measurement sequence on total ionizing dose effect in PDSOI nMOSFET
The influence of characteristics' measurement sequence on total ionizing dose effect in partially-depleted SOI nMOSFET is comprehensively studied. We find that measuring the front-gate curves has no influence on total ionizing dose effect. However, the back-gate curves' measurement has a great influence on total ionizing dose effect due to high electric field in the buried oxide during measuring. In this paper, we analyze their mechanisms and we find that there are three kinds of electrons tunneling mechanisms at the bottom corner of the shallow trench isolation and in the buried oxide during the back-gate curves' measurement, which are:Fowler-Nordheim tunneling, trap-assisted tunneling, and charge-assisted tunneling. The tunneling electrons neutralize the radiation-induced positive trapped charges, which weakens the total ionizing dose effect. As the total ionizing dose level increases, the charge-assisted tunneling is enhanced by the radiation-induced positive trapped charges. Hence, the influence of the back-gate curves' measurement is enhanced as the total ionizing dose level increases. Different irradiation biases are compared with each other. An appropriate measurement sequence and voltage bias are proposed to eliminate the influence of measurement.
Efficiency of collective myosin Ⅱ motors studied with an elastic coupling power-stroke ratchet model
We proposed a modified ratchet model including power-stroke and elastic coupling to study the efficiency of collective non-processive motors such as myosin Ⅱ in muscle. Our theoretical results are in good agreement with the experimental data. Our study not only reveals that the maximum efficiency depends on elasticity and is independent of transition rates but also indicates that the parameters fitted to fast muscle are different from those fitted to a slow one. The latter may imply that the structure of the fast muscle is different from that of the slow one. The main reason that our model succeeds is that velocity in this model is an independent variable.
The determinant factors for map resolutions obtained using CryoEM single particle imaging method
The CryoEM single particle structure determination method has recently received broad attention in the field of structural biology. The structures can be resolved to near-atomic resolutions after model reconstructions from a large number of CryoEM images measuring molecules in different orientations. However, the determining factors for reconstructed map resolution need to be further explored. Here, we provide a theoretical framework in conjunction with numerical simulations to gauge the influence of several key factors to CryoEM map resolutions. If the projection image quality allows orientation assignment, then the number of measured projection images and the quality of each measurement (quantified using average signal-to-noise ratio) can be combined to a single factor, which is dominant to the resolution of reconstructed maps. Furthermore, the intrinsic thermal motion of molecules has significant effects on the resolution. These effects can be quantitatively summarized with an analytical formula that provides a theoretical guideline on structure resolutions for given experimental measurements.
Controlled generation of cell-laden hydrogel microspheres with core-shell scaffold mimicking microenvironment of tumor
Development of an in vitro three-dimensional (3D) model that closely mimics actual environment of tissue has become extraordinarily important for anti-cancer study. In recent years, various 3D cell culture systems have been developed, with multicellular tumor spheroids being the most popular and effective model. In this work, we present a microfluidic device used as a robust platform for generating core-shell hydrogel microspheres with precisely controlled sizes and varied components of hydrogel matrix. To gain a better understanding of the governing mechanism of microsphere formation, computational models based on multiphase flow were developed to numerically model the droplet generation and velocity field evolution process with COMSOL Multiphysics software. Our modeling results show good agreement with experiments in size dependence on flow rate as well as effect of vortex flow on microsphere formation. With real-time tuning of the flow rates of aqueous phase and oil phase, tumor cells were encapsulated into the microspheres with controllable core-shell structure and different volume ratios of core (comprised of alginate, Matrigel, and/or Collagen) and shell (comprised of alginate). Viability of cells in four different hydrogel matrices were evaluated by standard acridine orange (AO) and propidium iodide (PI) staining. The proposed microfluidic system can play an important role in engineering the in vitro micro-environment of tumor spheroids to better mimic the actual in vivo 3D spatial structure of a tumor and perfect the 3D tumor models for more effective clinical therapies.