Accepted
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Abstract +
Separated flow is a typical complex turbulent phenomenon, the fully development of small-scale structures is of great importance for accurate numerical simulation. However, these small-scale structures can be easily damped by the inherent dissipation of numerical methods. Therefore, based on the 5th order dissipative compact scheme(DCS), we proposed an adaptive dissipative compact scheme (ADCS), which can adjust the numerical dissipation with self-adaptive capability by incorporating a flow related adjusting parameter. Cooperated with the delay detached eddy simulation(DDES), the ADCS can reduce the influence of numerical dissipation in LES region to improve the resolution of small-scale structures, and simulate with the normal dissipation in RANS region to keep numerical stability. In the process of numerical simulation, the approximate dispersion relation(ADR) is carried out firstly, it shows that the ADCS method can effectively reduce the influence of dissipation in high wave number region, without contaminating the dispersive performance, which is contribute to enhance the resolution of turbulent structures. Secondly, the advection of vortical structure is simulated, compared with DCS, ADCS can reach the theoretical accuracy of order in an efficient way and acquire more advanced resolution of vortical structure even on a relatively coarse mesh scale, which proves that ADCS reduce the negative influence of dissipation to vortical structure. The third case is the decay of isotropic turbulence, the energy spectral curves stay close to the reference before cut-off number, it shows that more small-scale structures can be distinguished by ADCS method, and most of the vortical dominated region is solved at a near minimum level. The forth case is plate channel turbulent flow, both of DCS and ADCS method show acceptable results, the ADCS method perform with optimal dissipation in the near wall region and decrease dissipative level in vortex dominated region, it also exhibits that the ADCS method can stay stability for flow with high gradient and avoid divergence. Finally, the sub-critical Re = 3900 circular cylinder is simulated, the fully separated flow is developed in the wake. The turbulent fluctuation near wall is sensitive to the effect of numerical dissipation, the contour of flow field shows that the ADCS method obtain more small-scale structures, as for the pressure coefficient and mean velocity, ADCS method shows an acceptable accuracy, considering the Reynolds stress profile, which can be easily affected by the dissipation, the ADCS exhibits more accurate results than the traditional DCS method. Generally, the ADCS method can reduce the influence of dissipation and is beneficial to acquire more accurate results in separated flow.
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Abstract +
$\pm1$$\pm 1/2$ fillings and at charge neutrality point of magic-angle TBG are just valley polarized states with compensating current loops in the moiré supercell, which induce staggered orbital to be magnetized on the moiré length scale. If $C_{2z}$ symmetry is broken due to the alignment of hexagonal boron nitride substrate, then a valley-polarized ground state will be a moiré orbital ferromagnetic state, which exhibits not only (quantum) anomalous Hall effect, but also novel magneto-optical and nonlinear optical responses.">We study the electronic structures, topological properties and orbital magnetism in twisted bilayer (TBG) and multilayer graphene systems. Moiré pattern is formed in twisted bilayer graphene due to the mutual twist of the two graphene layers. We show that the moiré potential induced by the twist can generate opposite pseudo magnetic fields in the moiré supercell, which are coupled with the Dirac fermions and generate two sets of pseudo Landau levels with opposite Chern numbers $\pm1$ . The two flat bands for each valley each spin of TBG are just equivalent to the two zeroth pseudo Landau levels with opposite Chern numbers and opposite sublattice polarizations. Such a pseudo-Landau-level representation has significant implications of the quantum anomalous Hall states observed at integer fillings of the flat bands in TBG at the magic angle. The origin of the magic angle can also be naturally explained by using the pseudo-Landau-level picture. We further study twisted multilayer graphene systems, and find that topological flat bands generally exist in the twisted multilayer graphene systems. These topological flat bands have nonzero valley Chern numbers, which can be described by a succinct formula under certain approxmations. These topological flat bands in twisted bilayer and multilayer graphene systems are associated with orbital magnetism. We propose that a valley polarized state in the twist graphene system is an orbital magnetic state with nontrivial current-loop pattern in the moiré supercell. We predict that the experimentally observed correlated insulating states at $\pm 1/2$ fillings and at charge neutrality point of magic-angle TBG are just valley polarized states with compensating current loops in the moiré supercell, which induce staggered orbital to be magnetized on the moiré length scale. If $C_{2z}$ symmetry is broken due to the alignment of hexagonal boron nitride substrate, then a valley-polarized ground state will be a moiré orbital ferromagnetic state, which exhibits not only (quantum) anomalous Hall effect, but also novel magneto-optical and nonlinear optical responses.
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Abstract +
Mesoscale eddy is a frequently occurred marine phenomenon in deep ocean that will disturb the sound speed within the upper water layer. As a result, mesoscale eddies will influence the propagation of wing-generated noise and cause the variation of the noise field. This paper investigates the effect of mesoscale eddies on the vertical spatial characteristics, including the noise vertical directionality and the noise vertical coherence, of wind-generated noise at the horizontal center of the eddy. In the study, the Gaussian eddy model is used to describe the sound speed fluctuation, and the ray and parabolic equation theories are used to describe the propagation of noise in near field and far field, respectively. Simulations suggest that: (1) at the depth of the eddy center, a clod-core eddy causes the decrease of the width of the horizontal notch and the decrease of the noise vertical coherence, while the effect of a warm-core eddy is contrary; (2) at the depth far from the eddy center, the effect of eddies is reduced, a cold-core and a warm-core eddy only lead the peak at the down edge of the horizontal notch in the noise directionality to rise and fall, respectively, and do not influence the noise vertical coherence; (3) the effect of a eddy becomes severe as its absolute strength becomes higher. The ray reversion method based on the principle of reciprocity is used to explain the physical reason behind the above phenomena. The method lunches rays from the noise receiving point and analyzes the polar angle and the strength of the noise arriving reversely along the ray paths. It is shown that the change of the polar angle and the strength of the noise arriving reversely along the surface reflected ray paths in the presence of eddies is the main cause of the change of the noise vertical spatial characteristics. Furthermore, simulations show that the analysis and conclusions in the study are still approximately valid when the receiving point deviate from the eddy center but the horizontal distance between them is short.
, , Received Date: 2020-03-22
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Thermoelectric (TE) material is a kind of energy conversion material, which can be used for power generation and refrigeration. Until now, traditional inorganic TE materials have shown high dimensionless thermoelectric figure of merit (ZT) values. But their expensive raw material and high processing cost, heavy metal pollution and poor processability limit their broad applications. Poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymers possess some excellent features, such as high electrical conductivity, low thermal conductivity, flexibility, low cost, abundance, and light weight. More and more attention has recently been paid to the TE properties of PEDOT polymers and PEDOT polymer based nanocomposites. Ascorbic acid (VC) is used as a reducing agent to tune the PEDOT-Tos-PPP film. The PEDOT-Tos-PPP films via VPP technique are treated with VC solutions with different concentrations. The TE properties of the films before and after being treated with VC at different concentrations are measured. The effect of concentration of VC aqueous solution on the thermoelectric properties and stabilities of the film are studied. The results indicate that the power factor of the film after being treated with 20% VC is 55.6 μW·m–1·K–2, which is 1.7 times as high as that of the pristine PEDOT-Tos-PPP film (34.4 μW·m–1·K–2). The maximum ZT value at room temperature is 0.032. After the VC treatment, the conductivity and Seebeck coefficient of the PEDOT film show unstable characteristics in the air, which is mainly due to the further oxidation of the neutral state on the PEDOT film surface in the air.
, , Received Date: 2020-02-19
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Recently, the measurement scheme of quantum dot qubit decocoherence quantized by the longitudinal optical (LO) phonon spontaneous emission rate has attracted the attention and discussion of many researchers. However, it is not difficult to see that the above-mentioned measurement scheme still has some insufficient and imperfect aspects that are to be studied urgently. Considering from the physical mechanism, the essence of the above scheme is to quantify the decoherence time of qubit by using the excited state decay time or excited state lifetime of the polaron. However, so far, there is little research on how the ground state decay time and ground state lifetime of two-state polaron affect the coherence of qubit. There is no doubt that this is an equally important research topic. This is because, firstly, for the coherence of the quantum state of polaron, both the decay of the excited state and the decay of the ground state will destroy or attenuate the qubit coherence, secondly, the transition rate of the two-state polaron from the ground state to the excited state after absorbing an LO phonon is also a function quantifying the qubit decoherence time of two-state system of which the inverse is called the ground state decay time or the ground state lifetime. It may be called a measure of qubit decoherence time quantized by the ground state decay time or ground state lifetime of polaron. In this article, the ground-state and excited-state energy and wave function of the magnetopolaron in a donor-center quantum dot with asymmetric Gaussian potential are derived by Lee-Low-Pines transformation and Pekar-type variational methodd, and then the two-level structure for a qubit is constructed. The measure of qubit decoherence time of quantum dots quantified by ground state decay time of two-state polaron is established, which is compared with the well-known measure of qubit decoherence time of quantum dots quantified by polaron excited state decay time, and their physical mechanisms are revealed. By studying the influence of dielectric constant ratio, electro-phonons coupling constant, temperature and electromagnetic field on the ground state lifetime of magnetopolaron in the donor-center quantum dots with asymmetric Gaussian potential, the influences of material properties, temperature, electromagnetic field and other environmental factors on qubit decoherence of quantum dots are revealed, thereby revealing the mechanism of qubit decoherence caused by LO phonon effect.
, , Received Date: 2020-01-17
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In this paper, a near-infrared laser heterodyne spectrometer developed by the laboratory is used to investigate the inversion of greenhouse gas column concentration and approximately evaluate the system measurement errors based on the optimal estimation algorithm. Firstly, the spectral database and the calculation results from the reference forward model are compared with the ground-based FTIR results, thereby selecting the detection window, the corresponding laser and detector. Secondly, the optimal estimation concentration inversion algorithm based on the reference forward model is established, and the Levenberg-Marquardt (LM) iterative method is adopted to realize the inversion of the concentration and vertical distribution profile of atmospheric CO2 column in the whole layer, and the long-term observation comparative experiment is carried out to verify the feasibility of this algorithm. Finally, by simulating the selected detection window spectrum in different white noise, the approximate corresponding relationship between the system signal-noise-ratio (SNR) and CO2 column concentration measuring error is eventually obtained. This research is an indispensable theoretical calculation part of the detection system and will conduce to improving the application of laser heterodyne technology in atmospheric observations.
, , Received Date: 2020-01-28
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Self-organization represents a ubiquitous transition from disorder to order. It plays a critical role in forming crystalline materials and functional structures in biology. Functional structures are generally hybrid on a multiple scale in which nano-structures are often organized in a specific way such that they can perform functions. There are two typical functional structures: static equilibrium structures and dynamic non-equilibrium structures. In this review, recent advances in understanding and mimicking functional structures are summarized. Although great advances have been achieved, it is still a big challenge to realize dynamic non-equilibrium structures. In this case, we suggest that the controlling of self-organization in active systems may be a route toward interactive and adaptive structures.
, , Received Date: 2019-11-27
Abstract +
Cold collision frequency shift is one of the major systematic effects which limit the frequency uncertainty of the cesium fountain atomic clock. It is proportional to the effective atomic density, which is defined as the average density over the initial spacial and velocity distribution. The measurement of the frequency shift is based on a differential method, in which the fountain clock is operated with two different atomic densities, i.e. high density and low density, in turn. The clock frequency without collision shift can be achieved by linear extrapolation with the frequencies and density ratios of two states. For the density ratio is estimated with the atom number, it plays a crucial role in generating atoms with same density distribution for reducing systematic uncertainty in cold collision frequency shift estimation. The rapid adiabatic passage method is used in Cesium fountain clock to realize homogeneous transition probability, which modulates the amplitude and frequency of microwave continuously to prepare atom sample. To investigate the precision of this method, theoretical analysis and experimental measurement are both used here. An equation of deviation is derived from the time evolution of Bloch vector. The vector rotates at angular speed Ω with the rotation axis processing at lower angular speed. The deviations in the two directions on the surface of Bloch sphere are determined by the equations which are similar to wave equations, and can be simplified into wave equations when the deviations are sufficiently small. It is shown in the equations that the deviations are stimulated by angular velocity and angular acceleration of the precession, and is inversely proportional to the square of Ω. Further calculation shows that the deviation becomes smaller when the amplitude of microwave frequency and Rabi frequency are close to each other. It is then confirmed experimentally. The effects of some other parameters, such as the pulse length and time delay, on transition probability are also measured, showing that the RAP method is insensitive to these parameters up to a large scope. The precision of RAP method is dominated by three factors. The first factor is the product of rotating angular speed Ω and pulse length T, i.e. ΩT: The increase of ΩT can reduce the uncertainty to a satisfactory degree. The second factor is the uncertainty of resonant frequency, so the measurement is required to be precise. The third factor is the unexpected atoms which are not selected by the microwave, and may be attributed to pulling light. After optimizing the parameters, the ratio of low density to high density can approach to 0.5 with 3 × 10–3 uncertainty, which leads to a systematic relative uncertainty of cold collision shift up to 1.6 × 10–16.
, , Received Date: 2019-11-07
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, , Received Date: 2020-02-19
Abstract +
Vortex beams with orbital angular momenta with different mode numbers are mutually orthogonal to each other, which makes it possible to improve the information transmission efficiency in space optical communication system. Nevertheless, the implementation of this strategy is limited by the orbital angular momentum crosstalk caused by atmospheric turbulence. Focused Laguerre-Gaussian vortex beams are less affected by atmospheric turbulence due to their lager intensity density. Consequently, focused Laguerre-Gaussian vortex beams can be used as the carriers to reduce the orbit angular momentum crosstalk and increase the channel capacity of information transmission. In this paper, based on the spiral spectrum analysis theory, the analytical expression of spiral spectrum of focused Laguerre Gaussian beam propagating in anisotropic atmospheric turbulence is derived. The influences of turbulence and beam parameters on the received power of focused and unfocused Laguerre Gaussian beam are investigated via numerical calculations. Finally, the multi-phase screen method is used for verificating the simulation. The research findings are as follows. First, with the increase of transmission distance, turbulence intensity and topological charge, the receiving power of orbital angular momentum decreases, that is, the orbital angular momentum crosstalk turns more serious. Second, the larger the turbulence inner-scale, anisotropy index and beam wavelength are, the smaller the orbital angular momentum crosstalk is. Third, when the receiving aperture reaches a certain value, its influence on the orbit angular momentum crosstalk is very small. Fourth, different parameters have different effects on crosstalk, and the orbit angular momentum crosstalk of the focused vortex beam is less than that of the unfocused vortex beam. Therefore, in the vortex optical communication, the focused vortex beams can be used as the signal light to reduce the crosstalk between the orbit angular momentum modes, and thus improving the communication quality. These results have some theoretical reference values for reducing crosstalk in free-space optical communication.
Abstract +
Recent years, with the rapid development of information technology, the information security has received more and more attention. A variety of encryption methods to protect the information have been reported. Visual cryptography is one of the encryption methods, which has highly security because of its threshold feature. And the cryptographic information can be explained by a naked eye in the decryption process. In the application of visual cryptography, however, each shared image is limited to transparency films and overlapping on computer. In our previous work, we proposed the scheme of invisible visual cryptography and developed the visual-cryptography-based optical hiding system(VCOH), which transformed the conventional visual cryptography shares into diffraction optical elements(DOEs). It not only increases the application range of visual cryptography, but also enhances security. In this paper, we propose an optical information hiding system based on the extended visual cryptography, which inherits the concept of invisible visual cryptography. In contrast to our previous work, the method proposed in this work can hide a meaningful image instead of text messages. Meanwhile, the capacity and imperceptibility of the method are greatly increased. The hiding process of the system contains two steps. Firstly, the secret image is converted into meaningful shares through the extended visual cryptography algorithm. Secondly, the meaningful shares are able to hide in phase-keys through an iterative phase retrieval algorithm, such as Gerchberg-Saxton algorithm and Yang-Gu iterative algorithm. Then the phase-keys can be made into diffraction optical elements(DOEs) to store and transport in a physical way. In the decryption process, DOEs are illuminated with the laser beam to reconstruct the meaningful shares. The secret image can be explained by the direct overlapping of the reconstructed shares without any optical or cryptographic knowledge. The simulation and optical experimental results show that the proposed method has good performance of security and validate the feasibility of the proposed method. Besides, in this paper the robustness and security issues are also analyzed. This system has a high security because of its indistinguishability under adaptive chosen ciphertext attack(IND-CCA2) security. Additionally, this system is relatively less robust than the VCOH because it shares meaningful images with highly complex and detailed structures.
, , Received Date: 2020-03-08
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In recent years, lanthanide-containing upconversion nanoparticles (UCNPs) have aroused the extensive interest in bioimaging due to their unique upconversion fluorescent properties, such as the high tissue penetration depth, good biocompatibility, low auto-fluorescence, and high imaging sensitivity. In this work, we synthesize a series of NaYF4:Yb, Tm@NaYF4 core-shell structured nanoparticles with various shell thickness. A “hot injection” strategy is introduced to fabricate the core-shell UCNPs through using high boiling-point mixtures (sodium/rare-earth trifluoroacetates dissolved in oleic acid and octadecene at 150 °C) as shell precursor solutions. The as-synthesized UCNPs are characterized by transmission electron microscope, particle size analysis and fluorescence spectra. The experimental results show that the shell thickness of UCNPs can be well controlled within a range from 4.2 nm to 32.6 nm by simply tuning the added quantity of the shell precursors. Meanwhile, the upconversion luminescence intensity of NaYF4:Yb, Tm@NaYF4 shows tens times higher than that of NaYF4:Yb, Tm owing to the effective suppression of surface quenching. The optimized thickness of the shell is determined to be 22.7 nm. An ultrathick inert shell (>22.7 nm) is not beneficial to upconversion luminescence mainly due to a strong scattering effect. In addition, the in vitro upconversion luminescent bioimaging application is demonstrated by using the as-synthesized core-shell structured UCNPs. Typically, the prepared OA capped UCNPs are dispersed in HCl solution to obtain hydrophilic ones, followed by polyethylene glycol (PEG) modification to improve their biological compatibility. The hydrophilic NaYF4:Yb, Tm@NaYF4@PEG nanostructures (denoted as UCNP@PEG) show a good biocompatibility with HeLa cells, as the viability of HeLa cells do not decrease obviously when the concentration of UCNP@PEG increases to 0.2 mg/mL. Then, we evaluate the upconversion luminescent signals of UCNP@PEG in HeLa cells under the excitation of 980 nm laser. An obviously increasing upconversion luminescent signal can be observed in HeLa cells with the incubation time increasing from 0.5 h to 6 h, indicating that the UCNP@PEG can be used as an excellent luminescence probe for cell imaging and monitoring the cell endocytosis process. All in all, we offer an efficient “hot injection” strategy of fabricating the core-shell structured UCNPs with various shell thickness for improving the upconversion efficiency of UCNPs, which will pave the way for new bioimaging and medical applications.
, , Received Date: 2020-02-20
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Cardiovascular disease, especially hypertension, has become one of the biggest killers of human life and health. Therefore, early detection and prevention of cardiovascular diseases are of great significance for people’s health. In this paper, we explore the relationship between aortic valve heart sound signal and blood pressure, and propose a method of non-invasively estimating blood pressure based on heart sound signals. First, according to the relationship between blood pressure and the heart sound signal, both the time interval between the peak point of the first heart sound and that of the second heart sound and the kurtosis of the second heart sound are extracted as features. Then the time interval between the first heart sound and the second heart sound, and the kurtosis of the second heart sound are linearly fitted to the measured blood pressure. Finally, according to the linear relationship between heart sound and blood pressure, a blood pressure evaluation formula based on the heart sound is established. The experimental results show that the time interval between the peak point of the first heart sound and that of the second heart sound, and the kurtosis of the second heart sound can be used as the characteristic parameters of blood pressure evaluation, which have a good linear relationship with blood pressure. The goodness of fit is 0.801 and 0.765, separately. The average error between the blood pressure calculated from the blood pressure calculation formula and the blood pressure measured by a commercial electronic sphygmomanometer is less than 5 mmHg, and the standard deviation is less than 8 mmHg. For hypertensive patients, the time interval between the peak point of the first heart sound and the second heart sound is shortened, and the kurtosis of the second heart sound is increased, which is a typical feature of heart sounds in patients with hypertension. Compared with the traditional blood pressure calculation method, the blood pressure assessment method proposed in this paper only needs to collect heart sound signals to effectively assess the blood pressure. The method is convenient to operate and can be used for continuously monitoring the blood pressure, and is especially suitable for monitoring the blood pressures of infants, disabled patients with limbs, and disabilities in certain medical environments.
, , Received Date: 2020-03-24
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Owing to their excellent performances, Te-based thermoelectric materials have been extensively concerned. However little attention has been paid to the bonding interfaces with electrodes, which play an important role in their practical applications. Excessive element mutual diffusion occurs across the bonding interfaces when Te is connected with metallic electrode, such as copper, aluminum, iron, etc, which will impair its transport performance and life especially when they serve in the higher temperature environments. Seeking proper barriers is the key to solving the interface problem. In this work, a gradient bonding structure of Te/FeTe/Fe is prepared in one step by the spark plasma sintering (SPS) method, in which a metallic layer of FeTe, referred to as β(FeTe) phase, is introduced as barrier. The interface microstructure, element distribution, and new phases are analyzed, and the joint properties including contact resistance and shearing strength after being aged are evaluated. The results show that the introduction of β(FeTe) phase can promote the boding of Fe/β(FeTe)/Te and thus inhibiting the excessive element diffusion across the interfaces, which is due to the formation of ε(FeTe2) phase between β(FeTe) phase and Te. The contact resistance of Fe/β(FeTe) and β(FeTe)/Te are 4.1 μΩ·cm2 and 7.54 μΩ·cm2, respectively, and the shearing strength are 16.11 MPa and 15.63 Mpa, respectively. The annealing temperature has significant effect on the performance of the gradient bonding structure. It has been indicated that the whole joint still owns good performance after being annealed at 553 K for 15 days, while it decreases sharply when the temperature is increased to 573 K. Hence, the optimal service temperature of Te/β(FeTe)/Fe should not be higher than 553 K. The gradient bonding structure is successfully achieved, thus attaining the purposes of inhibiting interface elements from excessively diffuse, reducing interface residual stress, and improving interface working stability and service life. So the design ideas and research results in this work have great reference significance for the study on semiconductor devices.
, , Received Date: 2020-04-09
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To suppress the effect of dark count noise on single photon avalanche diode (SPAD) detector, the mechanism and method of reducing the dark count rate (DCR) of SPAD device by using a polysilicon field plate is studied in this paper. Based on the 0.18-μm standard CMOS process, a polysilicon field plate located between the P+ active region and shallow trench isolation (STI) is deposited to reduce the dark count noise for a scaleable P+/P-well/deep N-well SPAD structure. Test results show that the DCR of SPAD device decreases by an order of magnitude after the deposition of polysilicon field plates, and its dark count performance at high temperature is even better than that of device without polysilicon field plate at room temperature. The TCAD simulation further indicates that the peak electric field in the guard ring region of the SPAD device is introduced into the STI by the field plate, and the overall electric field in the guard ring region is reduced by 25%. Finally, through modeling and calculating the DCR, the polysilicon field plate weakens the electric field of the guard ring region with high trap density, hence the trap-related DCR is significantly reduced. Therefore, the dark count performance of SPAD detector is effectively improved.
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