Permanent magnets capable of reliably operating at high temperatures up to ~450℃ are required in advanced power systems for future aircrafts, vehicles, and ships. Those operating temperatures are far beyond the capability of Nd– Fe–B magnets. Possessing high Curie temperature, Sm–Co based magnets are still very important because of their hightemperature capability, excellent thermal stability, and better corrosion resistance. The extensive research performed around the year 2000 resulted in a new class of Sm_{2}(Co, Fe, Cu, Zr)_{17}-type magnets capable of operating at high temperatures up to 550℃. This paper gives a systematic review of the development of Sm–Co permanent magnets, from the crystal structures and phase diagrams to the intrinsic magnetic properties. An emphasis is placed on Sm_{2}(Co, Fe, Cu, Zr)_{17}-type magnets for operation at temperatures from 300℃ to 550℃. The thermal stability issues, including instantaneous temperature coefficients of magnetic properties, are discussed in detail. The significance of nanograin structure, nanocrystalline, and nanocomposite Sm–Co magnet materials, and prospects of future rare-earth permanent magnets are also given.

TOPICAL REVIEW—Fundamental research under high magnetic fields

SPECIAL TOPIC—Recent advances in thermoelectric materials and devices

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

The propagation characteristics of oblique incidence terahertz (THz) waves through non-uniform plasma are investigated by the shift-operator finite-difference time-domain (SO-FDTD) method combined with the phase matching condition. The electron density distribution of the non-uniform plasma is assumed to be in a Gaussian profile. Validation of the present method is performed by comparing the results with those obtained by an analytical method for a homogeneous plasma slab. Then the effects of parameters of THz wave and plasma layer on the propagation properties are analyzed. It is found that the transmission coefficients greatly depend on the incident angle as well as on the thickness of the plasma, while the polarization of the incident wave has little influence on the propagation process in the range of frequency considered in this paper. The results confirm that the THz wave can pass through the plasma sheath effectively under certain conditions, which makes it a potential candidate to overcome the ionization blackout problem.

We investigate the effects of the Casimir force on the output properties of a hybrid optomechanical system. In this system, a nanosphere is fixed on the movable-mirror side of the standard optomechanical system, and the nanosphere interacts with the movable-mirror via the Casimir force, which depends on the mirror-sphere separation. In the presence of the probe and control fields, we analyze the transmission coefficient and the group delay of the field-component with the frequency of the probe field. We also study the transmission intensity of the field-component with the frequency of a newly generated four-wave mixing (FWM) field. By manipulating the Casimir force, we find that a tunable slow light can be realized for the field-component with the frequency of the probe field, and the intensity spectrum of the FWM field can be enhanced and shifted effectively.

We studied the evolution of wavefront aberration (WFA) of a signal beam during amplification in a Ti:sapphire chirped pulse amplification (CPA) system. The results verified that the WFA of the amplified laser beam has little relation with the change of the pump beam energies. Transverse parasitic lasing that might occur in CPA hardly affects the wavefront of the signal beam. Thermal effects were also considered in this study, and the results show that the thermal effect cumulated in multiple amplification processes also has no obvious influence on the wavefront of the signal beam for a single-shot frequency. The results presented in this paper confirmed experimentally that the amplification in a Ti:sapphire CPA system has little impact on the WFA of the signal beam and it is very helpful for wavefront correction of single-shot PW and multi-PW laser systems based on Ti:sapphire.

Multiple Fano resonances of plasmonic nanostructures have attracted much attention due to their potential applications in multicomponent biosensing. In this paper, we propose a series of hybridized nanostructures consisting of a single nanoring and multiple nanorods to generate multiple Fano resonances. One to three Fano resonances are achieved through tuning the number of nanorods. The interaction coupling process between different components of the nanostructures is recognized as the mechanism of multiple Fano resonances. We also theoretically investigate the applications of the produced multiple Fano resonances in refractive index sensing. The specific properties of multiple Fano resonances will make our proposed nanostructures beneficial to high-sensitivity biosensors.

Vector beams with spatially varying polarization distribution in the wavefront plane have received increasing attention in recent years. The manipulation of vector beams both in intensity and polarization distributions is highly desired and under development. In this work, we study the transmission property of vector beams through warm rubidium vapor and realize controllable transmission of vector beams based on atomic dichroism. By utilizing the linearly polarized beam and vector beams as the pump and probe beams in a pump-probe configuration, a spatially-dependent dichroism can be obtained, which leads to spatially varied absorption of the probe beam. The controllable intensity distribution of the probe beam, as a two-petal pattern, can rotate with the variation of the pump beam's polarization states. We experimentally demonstrate the mechanism of dichroism with linear polarization light and provide an explanation based on the optical pumping effect. Alternatively, the varying trend of the probe beam's intensity is also interpreted by utilizing the Jones matrix. Our results are thus beneficial for providing potential applications in optical manipulation in atomic ensembles.

A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation, demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.

An all-fiber mode-locked fiber laser was achieved with a saturable absorber based on a tapered fiber deposited with layered molybdenum selenide (MoSe_{2}). The laser was operated at a central wavelength of 1558.35 nm with an output spectral width of 2.9 nm, and a pulse repetition rate of 16.33 MHz. To the best of our knowledge, this is the first report on mode-locked fiber lasers using MoSe_{2} saturable absorbers based on tapered fibers.

InGaSb/AlGaAsSb double-quantum-well diode lasers emitting around 2 μm are demonstrated. The AlGaAsSb barriers of the lasers are grown with digital alloy techniques consisting of binary AlSb/AlAs/GaSb short-period pairs. Peak power conversion efficiency of 26% and an efficiency higher than 16% at 1 W are achieved at continuous-wave operation for a 2-mm-long and 100-μm-wide stripe laser. The maximum output power of a single emitter reaches to 1.4 W at 7 A. 19-emitter bars with maximum efficiency higher than 20% and maximum power of 16 W are fabricated. Lasers with the short-period-pair barriers are proved to have improved temperature properties and wavelength stabilities. The characteristic temperature (T_{0}) is up to 140℃ near room temperature (25-55℃).

Considering liquid viscosity, surface tension, and liquid compressibility, the effects of dynamical behaviors of cavitation bubbles on temperature and the amount of oxides inside the bubble are numerically investigated by acoustic field, regarding water as a work medium. The effects of acoustic frequency, acoustic pressure amplitude, and driving waveforms on bubble temperature and the number of oxides inside the bubbles by rapid collapse of cavitation bubbles are analysed. The results show that the changes of acoustic frequency, acoustic pressure amplitude, and driving waveforms not only have an effect on temperature and the number of oxides inside the bubble, but also influence the degradation species of pollution, which provides guidance for improving the degradation of water pollution.

Underwater acoustic applications depend critically on the prediction of sound propagation, which can be significantly affected by a rough surface, especially in shallow water. This paper aims to investigate how randomly fluctuating surface influences transmission loss (TL) in shallow water. The one-dimension wind-wave spectrum, Monterey-Miami parabolic equation (MMPE) model, Monte Carlo method, and parallel computing technology are combined to investigate the effects of different sea states on sound propagation. It is shown that TL distribution properties are related to the wind speed, frequency, range, and sound speed profile. In a homogenous waveguide, with wind speed increasing, the TLs are greater and more dispersive. For a negative thermocline waveguide, when the source is above the thermocline and the receiver is below that, the effects of the rough surface are the same and more significant. When the source and receiver are both below the thermocline, the TL distributions are nearly the same for different wind speeds. The mechanism of the different TL distribution properties in the thermocline environment is explained by using ray theory. In conclusion, the statistical characteristics of TL are affected by the relative roughness of the surface, the interaction strength of the sound field with the surface, and the changes of propagating angle due to refraction.

A lattice Boltzmann (LB) theory, the analytical characteristic integral (ACI) LB theory, is proposed in this paper. ACI LB theory takes the Bhatnagar-Gross-Krook (BGK)-Boltzmann equation as the exact kinetic equation behind Navier-Stokes continuum and momentum equations and constructs an LB equation by rigorously integrating the BGK-Boltzmann equation along characteristics. It is a general theory, supporting most existing LB equations including the standard lattice BGK (LBGK) equation inherited from lattice-gas automata, whose theoretical foundation had been questioned. ACI LB theory also indicates that the characteristic parameter of an LB equation is collision number, depicting the particle-interaction intensity in the time span of the LB equation, instead of the traditionally assumed relaxation time, and the over-relaxation time problem is merely a manifestation of the temporal evolution of equilibrium distribution along characteristics under high collision number, irrelevant to particle kinetics. In ACI LB theory, the temporal evolution of equilibrium distribution along characteristics is the determinant of LB method accuracy and we numerically prove this.

The interference of a relativistic vortex laser is investigated for the case when a linearly polarized Laguerre-Gaussian pulse impinges on a double-slit solid target. Three-dimensional particle-in-cell simulation results show that the interference fringes of high-order harmonics are twisted, similar to that of the fundamental vortex laser. The twisting order of the interference pattern is determined by the order of the vortex high-order harmonics, which can be explained by the classic double-slit interference models. The usual double-slit interference has been extended to the regime of relativistic intensity, which may have potential applications for measuring the topological charge of vortex high-order harmonics.

The microscopic stripe pillar is one of the most frequently adopted building blocks for hydrophobic substrates. However, at high temperatures the particles on the droplet surface readily evaporate and re-condense on the pillar sidewall, which makes the droplet highly unstable and undermines the overall hydrophobic performance of the pillar. In this work, molecular dynamics (MD) simulation of the simple liquid at a single stripe pillar edge defect is performed to characterize the droplet's critical wetting properties considering the evaporation-condensation effect. From the simulation results, the droplets slide down from the edge defect with a volume smaller than the critical value, which is attributed to the existence of the wetting layer on the stripe pillar sidewall. Besides, the analytical study of the pillar sidewall and wetting layer potential field distribution manifests the relation between the simulation parameters and the degree of the droplet pre-wetting, which agrees well with the MD simulation results.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

It is necessary to reduce the currents of poloidal field (PF) coils as small as possible, during the static equilibrium design procedure of Experimental Advanced Superconductive Tokamak (EAST). The quasi-snowflake (QSF) divertor configuration is studied in this paper. Starting from a standard QSF plasma equilibrium, a new QSF equilibrium with 300 kA total plasma current is designed. In order to reduce the currents of PF6 and PF14, the influence of plasma shape on PF coil current distribution is analyzed. A fixed boundary equilibrium solver based on a non-rigid plasma model is used to calculate the flux distribution and PF coil current distribution. Then the plasma shape parameters are studied by the orthogonal method. According to the result, the plasma shape is redefined, and the calculated equilibrium shows that the currents of PF6 and PF14 are reduced by 3.592 kA and 2.773 kA, respectively.

Nonlinear properties of ion acoustic solitary waves are studied in the case of dense magnetized plasmas. The degenerate electrons with relative density effects from their spin states in the same direction and from equally probable up and down spinning states are taken up separately. Quantum statistical as well as quantum tunneling effects for both types of electrons are taken. The ions have large inertia and are considered classically, whereas the electrons are degenerate. The collisions of ions and electrons with neutral atoms are considered. We derive the deformed Korteweg de-Vries (DKdV) equation for small amplitude electrostatic potential disturbances by employing the reductive perturbation technique. The Runge-Kutta method is applied to solve numerically the DKdV equation. The analytical solution of DKdV is also presented with time dependence. We discuss the profiles for velocity, amplitude, and time variations in solitons for the cases when all the electrons are spinning in the same direction and for the case when there is equal probability of electrons having spin up and spin down. We have found that the wave is unstable because of the collisions between neutral gas molecules and the charged plasmas particles in the presence of degenerate electrons.

Non-shear flow can change the O-point position of a magnetic island, and thus it may play an important role in the effects of resonant magnetic perturbation (RMP) on the single tearing mode. We employ the nonlinear magnetohydrodynamics model in a slab geometry to investigate how RMP affects the single tearing mode instability with non-shear flow. It is found that the driving and suppressing effects of RMP on single tearing mode instability will appear alternately. When the flow velocity is small, the suppressing effect plays a major role through the development of the mode, and the tearing mode instability will be suppressed. With the flow velocity increasing, the driving effect will increase, while the suppressing effect will decrease. When the two effects reach equilibrium, the tearing mode will become stable.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

We prepared the isolated micrometer-sized diamond particles without seeding on the substrate in hot filament chemical vapor deposition. The diamond particles with specific crystallographic planes and strong silicon-vacancy (SiV) photoluminescence (PL) have been prepared by adjusting the growth pressure. As the growth pressure increases from 2.5 to 3.5 kPa, the diamond particles transit from composite planes of {100} and {111} to only smooth {111} planes. The {111}-faceted diamond particles present better crystal quality and stronger normalized intensity of SiV PL with a narrower bandwidth of 5 nm. Raman depth profiles show that the SiV centers are more likely to be formed on the near-surface areas of the diamond particles, which have poorer crystal quality and greater lattice stress than the inner areas. Complex lattice stress environment in the near-surface areas broadens the bandwidth of SiV PL peak. These results provide a feasible method to prepare diamond particles with specific crystallographic planes and stronger SiV PL.

Ni self-assembly has been performed on GaN (0001), Si (111) and sapphire (0001) substrates. Scanning electron microscopy (SEM) images verify that the Si (111) substrate leads to failure of the Ni assembly due to Si-N interlayer formation; the GaN (0001) and sapphire (0001) substrates promote assembly of the Ni particles. This indicates that the GaN/sapphire (0001) substrates are fit for Ni self-assembly. For the Ni assembly process on GaN/sapphire (0001) substrates, three differences are observed from the x-ray diffraction (XRD) patterns:(i) Ni self-assembly on the sapphire (0001) needs a 900℃ annealing temperature, lower than that on the GaN (0001) at 1000℃, and loses the Ni network structure stage; (ii) the Ni particle shape is spherical for the sapphire (0001) substrate, and truncated-cone for the GaN (0001) substrate; and (iii) a Ni-N interlayer forms between the Ni particles and the GaN (0001) substrate, but an interlayer does not appear for the sapphire (0001) substrate. All these differences are attributed to the interaction between the Ni and the GaN/sapphire (0001) substrates. A model is introduced to explain this mechanism.

A series of deuterated ammonium dihydrogen phosphate (DADP) crystals were grown and their structures were investigated by using powder neutron diffraction method. In the entire composition range, the deuterated level in the crystals is lower compared with the aqueous growth solution. The deuterium segregation coefficient in the crystals decreases with increasing deuterium content of the solution. The deuterium content in the NH_{4}^{+} group is higher than that in H_{2}PO_{4}^{-} group. In addition, the variations of lattice parameters are shown here.

Diamond anvil cells combined with Raman spectroscopy and angle-dispersive x-ray diffraction (ADXRD) were used to investigate the compression behavior of cinchomeronic acid (C_{7}H_{5}NO_{4}, CA), a hydrogen-bonded polymorphs. The compression of form-I at approximately 6.5 GPa caused an irreversible phase transition that produced the new polymorph form-Ⅲ. Lattice and internal modes in the Raman spectra were analyzed to determine the modifications in the local environment of CA form-I molecules. The form-Ⅲ was indexed and refined to a low-symmetry triclinic structure with space group P1. The mechanism for the phase transition involved the reconstructions in the hydrogen-bonded networks in CA form-I.

Due to many remarkable physical and chemical properties, two-dimensional (2D) nanomaterials have become a hot spot in the field of condensed matter physics. In this paper, we have studied the structural, mechanical, and electronic properties of the 2D GaInO_{3} system by first-principles method. We find that 2D GaInO_{3} can exist stably at ambient condition. Molecular dynamic simulations show that GaInO_{3}-sheet has excellent thermal stability and is stable up to 1100 K. Electronic structural calculations show that GaInO_{3}-sheet has a band gap of 1.56 eV, which is close to the ideal band gap of solar cell materials, demonstrating great potential in future photovoltaic application. In addition, strain effect studies show that the GaInO_{3}-sheet structure always exhibits a direct band gap under biaxial compressive strain, and as the biaxial compressive strain increases, the band gap gradually decreases until it is converted into metal. While biaxial tensile strain can cause the 2D material to transform from a direct band gap semiconductor into an indirect band gap semiconductor, and even to metal. Our research expands the application of the GaInO_{3} system, which may have potential application value in electronic devices and solar energy.

The electronic structures and optical properties of β-Ga_{2}O_{3} and Si- and Sn-doped β-Ga_{2}O_{3} are studied using the GGA+U method based on density functional theory. The calculated bandgap and Ga 3d-state peak of β-Ga_{2}O_{3} are in good agreement with experimental results. Si- and Sn-doped β-Ga_{2}O_{3} tend to form under O-poor conditions, and the formation energy of Si-doped β-Ga_{2}O_{3} is larger than that of Sn-doped β-Ga_{2}O_{3} because of the large bond length variation between Ga-O and Si-O. Si- and Sn-doped β-Ga_{2}O_{3} have wider optical gaps than β-Ga_{2}O_{3}, due to the Burstein-Moss effect and the bandgap renormalization effect. Si-doped β-Ga_{2}O_{3} shows better electron conductivity and a higher optical absorption edge than Sn-doped β-Ga_{2}O_{3}, so Si is more suitable as a dopant of n-type β-Ga_{2}O_{3}, which can be applied in deep-UV photoelectric devices.

A semi-empirical equation of state model for aluminum in a warm dense matter regime is constructed. The equation of state, which is subdivided into a cold term, thermal contributions of ions and electrons, covers a broad range of phase diagram from solid state to plasma state. The cold term and thermal contribution of ions are from the Bushman-Lomonosov model, in which several undetermined parameters are fitted based on equation of state theories and specific experimental data. The Thomas-Fermi-Kirzhnits model is employed to estimate the thermal contribution of electrons. Some practical modifications are introduced to the Thomas-Fermi-Kirzhnits model to improve the prediction of the equation of state model. Theoretical calculation of thermodynamic parameters, including phase diagram, curves of isothermal compression at ambient temperature, melting, and Hugoniot, are analyzed and compared with relevant experimental data and other theoretical evaluations.

LiCoO_{2} is one of the most important cathode materials for high energy density lithium ion batteries. The compressed behavior of LiCoO_{2} under high pressure has been investigated using synchrotron radiation x-ray diffraction. It is found that LiCoO_{2} maintains hexagonal symmetry up to the maximum pressure of 30.1 GPa without phase transition. The elastic modulus at ambient pressure is 159.5(2.2) GPa and its first derivative is 3.92(0.23). In addition, the high-pressure compression behavior of LiCoO_{2} has been studied by first principles calculations. The derived bulk modulus of LiCoO_{2} is 141.6 GPa.

We calculated the room-temperature phonon thermal conductivity and phonon spectrum of alkyl group-functionalized zigzag graphene nanoribbons (ZGNRs) with molecular dynamics simulations. The increase in both chain length and concentration of alkyl groups caused remarkable reduction of phonon thermal conductivity in functionalized ZGNRs. Phonon spectra analysis showed that functionalization of ZGNR with alkyl functional groups induced phonon-structural defect scattering, thus leading to the reduction of phonon thermal conductivity of ZGNR. Our study showed that surface functionalization is an effective routine to tune the phonon thermal conductivity of GNRs, which is useful in graphene thermal-related applications.

In 1805, Thomas Young was the first to propose an equation (Young's equation) to predict the value of the equilibrium contact angle of a liquid on a solid. On the basis of our predecessors, we further clarify that the contact angle in Young's equation refers to the super-nano contact angle. Whether the equation is applicable to nanoscale systems remains an open question. Zhu et al.[College Phys.4 7 (1985)] obtained the most simple and convenient approximate formula, known as the Zhu-Qian approximate formula of Young's equation. Here, using molecular dynamics simulation, we test its applicability for nanodrops. Molecular dynamics simulations are performed on argon liquid cylinders placed on a solid surface under a temperature of 90 K, using Lennard-Jones potentials for the interaction between liquid molecules and between a liquid molecule and a solid molecule with the variable coefficient of strength a. Eight values of a between 0.650 and 0.825 are used. By comparison of the super-nano contact angles obtained from molecular dynamics simulation and the Zhu-Qian approximate formula of Young's equation, we find that it is qualitatively applicable for nanoscale systems.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

This paper reports the sensitive effect of photoluminescence peak intensity and transmittance affected by B, Al, and N dopants in fluorescent 4H-SiC single crystals. The crystalline type, doping concentration, photoluminescence spectra, and transmission spectra were characterized at room temperature. It is found that the doped 4H-SiC single crystal emits a warm white light covering a wide range from 460 nm to 720 nm, and the transmittance increases from ~10% to ~60% with the fluctuation of B, Al, and N ternary dopants. With a parameter of C_{D-A}, defined by B, Al, and N concentration, the photoluminescence and transmittance properties can be adjusted by optimal doping regulation.

Two nonstoichiometric UAu_{1-x}Sb_{2} (x=0.25, 0.1) single crystals are successfully synthesized using a flux method, and their physical properties are comprehensively studied by measuring the dc-magnetization and electrical resistivity. Evidence for at least three magnetic phases is found in these samples. In zero field, both samples undergo an antiferromagnetic transition at a relatively high temperature, and with further cooling they pass through another antiferromagnetic phase, before reaching a ferromagnetic ground state. Furthermore, the magnetic order can be tuned by varying the site occupation of Au. Such a tunable magnetic order may provide an opportunity for exploring the potential quantum critical behavior in this system.

The exploration of the appropriate red phosphor with good luminescence properties is an important issue in the development of current white light-emitting diode (WLED) devices. Transition metal Mn-doped compounds are fascinating luminescent materials. Herein, we performed a systematic theoretical study of the microstructure and optical properties of K_{2}TiF_{6}:Mn^{4+} using the CALYPSO structure search method in combination with first-principles calculations. We uncovered a novel structure of K_{2}TiF_{6}:Mn^{4+} with space group P-3m1 symmetry, where the impurity Mn^{4+} ions are accurately located at the center of the MnF_{6} octahedra. Based on our developed complete energy matrix diagonalization (CEMD) method, we calculated transition lines for ^{2}E_{g}→^{4}A_{2}, ^{4}A_{2}→^{4}T_{2}, and ^{4}A_{2}→^{4}T_{2} at 642 nm, 471 nm, and 352 nm, respectively, which are in good agreement with the available experimental data. More remarkably, we also found another transition (^{4}A_{2}→^{2}T_{2}) that lies at 380 nm, which should be a promising candidate for laser action.

Spin currents, which are excited in indium tin oxide (ITO)/yttrium iron garnet (YIG) by the methods of spin pumping and spin Seebeck effect, are investigated through the inverse spin Hall effect (ISHE). It is demonstrated that the ISHE voltage can be generated in ITO by spin pumping under both in-plane and out-of-plane magnetization configurations. Moreover, it is observed that the enhancement of spin Hall angle and interfacial spin mixing conductance can be achieved by an appropriate annealing process. However, the ISHE voltage is hardly seen in the presence of a longitudinal temperature gradient. The absence of the longitudinal spin Seebeck effect can be ascribed to the almost equal thermal conductivity of ITO and YIG and specific interface structure, or to the large negative temperature dependent spin mixing conductance.

We present a semi-analytic method to study the electronic conductance of a lengthy armchair honeycomb nanoribbon in the presence of vacancies, defects, or impurities located at a small part of it. For this purpose, we employ the Green's function technique within the nearest neighbor tight-binding approach. We first convert the Hamiltonian of an ideal semi-infinite nanoribbon to the Hamiltonian of some independent polyacetylene-like chains. Then, we derive an exact formula for the self-energy of the perturbed part due to the existence of ideal parts. The method gives a fully analytical formalism for some cases such as an infinite ideal nanoribbon and the one including linear symmetric defects. We calculate the transmission coefficient for some different configurations of a nanoribbon with special width including a vacancy, edge geometrical defects, and two electrical impurities.

Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes (LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect (CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency (IQE), light extraction efficiency (LEE), and external quantum efficiency (EQE) droop of the lateral LEDs. However, questions still exist for the vertical LEDs (V-LEDs). Here firstly the current diffusion length L_{s}(I) and L_{s}(Ⅱ) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L_{s}(Ⅲ) was developed by combining L_{s}(I) and L_{s}(Ⅱ), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency (WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.

We propose a workable scheme for generating a bulk valley pump current in a silicene-based device which consists of two pumping regions characterized by time-dependent strain and staggered potentials, respectively. In a one-dimension model, we show that a pure valley current can be generated, in which the two valley currents have the same magnitude but flow in opposite directions. Besides, the pumped valley current is quantized and maximized when the Fermi energy of the system locates in the bandgap opened by the two pumping potentials. Furthermore, the valley current can be finely controlled by tuning the device parameters. Our results are useful for the development of valleytronic devices based on two-dimensional materials.

Self-heating effect in amorphous InGaZnO thin-film transistors remains a critical issue that degrades device performance and stability, hindering their wider applications. In this work, pulsed current-voltage analysis has been applied to explore the physics origin of self-heating induced degradation, where Joule heat is shortly accumulated by drain current and dissipated in repeated time cycles as a function of gate bias. Enhanced positive threshold voltage shift is observed at reduced heat dissipation time, higher drain current, and increased gate width. A physical picture of Joule heating assisted charge trapping process has been proposed and then verified with pulsed negative gate bias stressing scheme, which could evidently counteract the self-heating effect through the electric-field assisted detrapping process. As a result, this pulsed gate bias scheme with negative quiescent voltage could be used as a possible way to actively suppress self-heating related device degradation.

The transport and thermoelectric properties together with annealing of the new layered Bi-chalcogenide LaOBiHgS_{3} are studied. On the transport part, the insulating behavior of the as-grown sample is evidently depressed by post annealing. A hump-like abnormality appears around 170 K. The thermoelectric performance of the sample is observably improved by the annealing, mainly because of the enhanced electrical conductance. The present results suggest that the physical properties of LaOBiHgS_{3} are sensitive to post annealing and the possible micro adjustments that follow, indicating the layered Bi-chalcogenide family to be an ideal platform for designing novel functional materials.

Quantum criticality is closely related to the existence of two phases with unrelated symmetry breaking. In this paper, we study Néel and Kekulé valence bond state (VBS) quantum criticality in Dirac semimetals with four-fermion interactions. Our results show that all possible dynamical masses yield the same critical coupling, which exhibits the phenomenon that all possible phases meet at a multicritical point (e.g., a tricritical point for the Néel, Kekulé-VBS and semimetallic phases). In terms of the well-established Wess-Zumino-Witten field theory, we investigate the typical criticality for the transition between Néel and Kekulé-VBS phases, and the compatible Néel-Kekulé-VBS mass matrices imply the existence of a non-Landau transition between the Néel and Kekulé-VBS phases. We show the existence of mutual duality in the defect-driven Néel-Kekulé-VBS transition near the non-Landau critical point and find that this mutual duality results from the presence of a mutual Chern-Simons term. We also study the mutual duality based on dual topological excitations.

We proposed a two-degrees-of-freedom inverted piezoelectric beam with pendulum to promote the performance of vibration energy harvesting. This configuration is composed of an inverted elastic beam and a pendulum attached to its free end. The electromechanical equations governing the nonlinear system were derived. The harmonic balance method (HBM) is applied to solve the equation and the results prove that there exists a 1:3 super-harmonic resonance. The simulation results show that owing to the particular nonlinearity, there appears a special bending effect in the amplitude-frequency response, i.e., bending right for the first natural frequency and left for the second natural frequency, which is beneficial for harvesting vibration energy. The HBM results are verified by the entity simulations. Furthermore, over a relatively wide range of power spectral density, it could reach a dense jumping and give a dense high pulse voltage.

Ionic liquids have attracted a lot of research attention for their applications in novel optoelectronic structures and devices as an optical means of regulating electricity. Although the electro-optic effect of ionic liquids is mentioned in some literature, quantitative testing and analysis are hardly found in light absorption coefficients of ionic liquids under an electric field. In the present study, an experimental apparatus is designed to measure the absorption coefficients of ionic liquids under different electric fields. Five groups of imidazole ionic liquids are experimentally investigated and an inversion is performed to determine the spectral absorption coefficients of the imidazole ionic liquids under the electric fields. Different intensities with multiple interface refractions and reflections are also considered, and the various measurement errors are analyzed through uncertainties propagation analysis. Spectral absorptions of ionic liquids from 300 nm to 2500 nm are obtained and the absorption coefficients are retrieved. It is found that the absorption behavior of ionic liquids changes in some frequency bands under an applied electric field. The experimental results show that the absorption coefficient of the ionic liquid increases with the voltage increasing at 1520 nm and 1920 nm. The change rate is affected by the types of anions and cations in the ionic liquid and the diffusion rate of the ions therein. This study provides illustrations for the ionic liquid-based electro-optical regulation in terms of physical property parameters and the testing technique.

We have discussed the materials of solar cell based on hybrid organic-inorganic halide perovskites with formamidinium (NH_{2}CH=NH_{2}^{+} or FA) lead iodide. Firstly, we build the structure of formamidinium lead iodide (FAPbI_{3}) by using the material studio. By using the first-principles calculations, the energy band structure, density of states (DOS), and partial DOS (PDOS) of the hydrazine-iodide lead halide are obtained. Then, we theoretically analyze a design scheme for perovskite solar cell materials, published in[Science354, 861 (2016)], with the photoelectric conversion efficiency that can reach 20.3%. Also, we use non-toxic elements to replace lead in FAPbI_{3} without affecting its photoelectric conversion efficiency. Here in this work, we explore the energy band structure, lattice constant, light absorption efficiency, etc. After the Ca, Zn, Ge Sr, Sn, and Ta atoms replacing lead (Pb) and through comparing the spectral distributions of the solar spectrum, it can be found that FAGeI_{3}, FASnI_{3}, and FAZnI_{3} have better absorbance characteristics in the solar spectrum range. If the band gap structure is taken into account, FAGeI_{3} will become an ideal material to replace FAPbI_{3}, although its performance is slightly lower than that of FAPbI_{3}. The toxicity of Pb is taken into account, and the Ge element can be used as a substitute element for Pb. Furthermore, we explore one of the perovskite materials, i.e., FA_{0.75}Cs_{0.25}Sn_{0.25}Ge_{0.75}I_{3} whose photovoltaic properties are close to those of FA_{0.75}Cs_{0.25}Sn_{0.5}Pb_{0.5}I_{3}, but the former does not contain toxic atoms. Our results pave the way for further investigating the applications of these materials in relevant technologies.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Beverloo's scaling law can describe the flow rate of grains discharging from hoppers. In this paper, we show that the Beverloo's scaling law is valid for varying material parameters. The flow rates from a hopper with different hopper and orifice sizes (D, D_{0}) are studied by running large-scale simulations. When the hopper size is fixed, the numerical results show that Beverloo's law is valid even if the orifice diameter is very large and then the criteria for this law are discussed. To eliminate the effect of walls, it is found that the criteria can be suggested as D-D_{0} ≥ 40d or D/D_{0} ≥ 2. Interestingly, it is found that there is still a scaling relation between the flow rate and orifice diameter if D/D_{0} is fixed and less than 2. When the orifice diameter is close to the hopper size, the velocity field changes and the vertical velocities of grains above the free fall region are much larger. Then, the free fall arch assumption is invalid and Beverloo's law is inapplicable.

Pulsed metal organic chemical vapor deposition was employed to grow nearly polarization matched InAlGaN/GaN heterostructures. A relatively low sheet carrier density of 1.8×1012 cm^{-2}, together with a high electron mobility of 1229.5 cm^{2}/V·s, was obtained for the prepared heterostructures. The surface morphology of the heterostructures was also significantly improved, i.e., with a root mean square roughness of 0.29 nm in a 2 μm×2 μm scan area. In addition to the improved properties, the enhancement-mode metal–oxide–semiconductor high electron mobility transistors (MOSHEMTs) processed on the heterostructures not only exhibited a high threshold voltage (V_{TH}) of 3.1 V, but also demonstrated a significantly enhanced drain output current density of 669 mA/mm. These values probably represent the largest values obtained from the InAlGaN based enhancement-mode devices to the best of our knowledge. This study strongly indicates that the InAlGaN/GaN heterostructures grown by pulsed metal organic chemical vapor deposition could be promising for the applications of novel nitride-based electronic devices.

Granular packings under gravity in frictional and frictionless silos were simulated and the influence of the wall friction on the normal force distribution was investigated. Although there is an obvious Janssen effect in frictional silos, only a slight influence on the geometry of packing was found. The law of normal force distribution is different for frictional and frictionless walls, which is related to the pressure profile. A modified formula with consideration of the pressure profile was well fitted to the simulation results.

We theoretically investigate the excited state intramolecular proton transfer (ESIPT) behavior of the novel fluorophore bis-imine derivative molecule HNP which was designed based on the intersection of 1-(hydrazonomethyl)-naphthalene-2-ol and 1-pyrenecarboxaldehyde. Especially, the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods for HNP monomer are introduced. Moreover, the “our own n-layered integrated molecular orbital and molecular mechanics” (ONIOM) method (TDDFT:universal force field (UFF)) is used to reveal the aggregation-induced emission (AIE) effect on the ESIPT process for HNP in crystal. Our results confirm that the ESIPT process happens upon the photoexcitation for the HNP monomer and HNP in crystal, which is distinctly monitored by the optimized geometric structures and the potential energy curves. In addition, the results of potential energy curves reveal that the ESIPT process in HNP will be promoted by the AIE effect. Furthermore, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the HNP monomer and HNP in crystal have been calculated. The calculation demonstrates that the electron density decrease of proton donor caused by excitation promotes the ESIPT process. In addition, we find that the variation of atomic dipole moment corrected Hirshfeld population (ADCH) charge for proton acceptor induced by the AIE effect facilitates the ESIPT process. The results will be expected to deepen the understanding of ESIPT dynamics for luminophore under the AIE effect and provide insight into future design of high-efficient AIE compounds.

AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) have attracted considerable interest due to their wide range of application fields. However, they are still suffering from low light out power and unsatisfactory quantum efficiency. The utilization of polarization-doped technique by grading the Al content in p-type layer has demonstrated its effectiveness in improving LED performances by providing sufficiently high hole concentration. However, too large degree of grading through monotonously increasing the Al content causes strains in active regions, which constrains application of this technique, especially for short wavelength UV-LEDs. To further improve 340-nm UV-LED performances, segmentally graded Al content p-Al_{x}Ga_{1-x}N has been proposed and investigated in this work. Numerical results show that the internal quantum efficiency and output power of proposed structures are improved due to the enhanced carrier concentrations and radiative recombination rate in multiple quantum wells, compared to those of the conventional UV-LED with a stationary Al content AlGaN electron blocking layer. Moreover, by adopting the segmentally graded p-Al_{x}Ga_{1-x}N, band bending within the last quantum barrier/p-type layer interface is effectively eliminated.

A new T-shaped tunnel field-effect transistor (TTFET) with gate dielectric spacer (GDS) structure is proposed in this paper. To further studied the effects of GDS structure on the TTFET, detailed device characteristics such as current-voltage relationships, energy band diagrams, band-to-band tunneling (BTBT) rate and the magnitude of the electric field are investigated by using TCAD simulation. It is found that compared with conventional TTFET and TTFET with gate-drain overlap (GDO) structure, GDS-TTFET not only has the minimum ambipolar current but also can suppress the ambipolar current under a more extensive bias range. Furthermore, the analog/RF performances of GDS-TTFET are also investigated in terms of transconductance, gate-source capacitance, gate-drain capacitance, cutoff frequency, and gain bandwidth production. By inserting a low-κ spacer layer between the gate electrode and the gate dielectric, the GDS structure can effectively reduce parasitic capacitances between the gate and the source/drain, which leads to better performance in term of cutoff frequency and gain bandwidth production. Finally, the thickness of the gate dielectric spacer is optimized for better ambipolar current suppression and improved analog/RF performance.

Adaption of circadian rhythms in behavioral and physiological activities to the external light-dark cycle is achieved through the main clock, i.e., the suprachiasmatic nucleus (SCN) of the brain in mammals. It has been found that the SCN neurons differ in the amplitude relaxation rate, which represents the rigidity of the neurons to the external amplitude disturbance. Thus far, the appearance of that difference has not been explained. In the present study, an alternative explanation based on the Poincaré model is given which takes into account the effect of the difference in the entrainment range of the SCN. Both our simulation results and theoretical analyses show that the largest entrainment range is obtained with suitable difference in the case that only a part of SCN neurons are sensitive to the light information. Our findings may give an alternative explanation for the appearance of that difference (heterogeneity) and shed light on the effects of the heterogeneity in the neuronal properties on the collective behaviors of the SCN neurons.

We demonstrate that the new hierarchy of integrable lattice equations in Chin. Phys. B21 090202 (2012) can be changed into the integrable lattice hierarchy in Chin. Phys. B13 1009 (2004) by using a very simple transformation.

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