Design and analysis of biconvex liquid lens with circular hole plate electrode structure
Kong Mei-Mei, Xue Yin-Yan, Xu Chun-Sheng, Dong Yuan, Liu Yue, Pan Shi-Cheng, Zhao Rui
Abstract +
In this paper, based on the research of zoom liquid lens with parallel plate electrode and the principle of dielectrophoresis, a model of the biconvex liquid lens with circular hole plate electrode structure is proposed, which is a novel three-layer liquid lens structure. The dielectrophoretic effect refers to the phenomenon that free dielectric molecules will be polarized and moved by the force in a non-uniform electric field, thus deforming the dielectric liquid. In the dielectrophoretic liquid lens, only two insulating liquid materials with large refractive index difference and dielectric constant difference need to be selected, which can increase the selection range of liquid materials. The liquid lens structure mainly consists of a piece of double-sided conductive flat plate ITO glass with a circular hole and two pieces of single-sided conductive flat plate ITO glass, which respectively form two sets of flat electrode structures to control the upper interface and lower interface of the liquid droplet. In this structure, the influences of the intermediate glass plate on the focus and imaging are reduced by using the flat plate electrode with circular hole. The theoretical analysis of the structure is carried out with simulation software. Firstly, the models of the biconvex liquid lens with circular hole plate electrode under different voltages are built with Comsol software, the data of upper interface and lower interface of the liquid droplet are exported. Then by using Matlab, the surface shapes of the upper interface and lower interface of the droplet are fitted and the corresponding aspheric coefficients are obtained. Finally, the optical models are built with Zemax software, the imaging optical paths and the variation range of focal length under different voltages are analyzed. On the basis of the simulation, the corresponding device is made, and the specific experimental analysis is carried out. The surface patterns of the upper interface and lower interfaces of the droplet of the biconvex liquid lens under different voltages are recorded, the focal length and imaging resolution of the liquid lens are measured. When the operating voltage is in a range of 0–260 V, the focal length varies from 23.8–17.5 mm, which is basically consistent with the simulation results (22.6–15.9 mm). The feasibility of the structure of the biconvex liquid lens with circular hole plate electrode structure is verified experimentally. The imaging resolution can reach 45.255 lp/mm. The results show that this proposed novel three-layer liquid structure of the biconvex liquid lens has the characteristics of simple structure, easy-to-realize and good imaging quality. Therefore, the research of this biconvex liquid lens can provide a new idea for expanding the high-resolution imaging research of liquid lenses and their applications.
Research on pressure transmission and sealing performance of pyrophyllite in a cubic large chamber static high-pressure device
Tian Yi, Du Ming-Hao, Zhang Jia-Wei, He Duan-Wei
Abstract +
Owing to the need for a hydrostatic high-pressure cubic large cavity (hexahedral top) press used in high-pressure research and production of superhard material, two kinds of pyrophyllite powder compacts (A and B) from pyrophyllite mine in South Africa are prepared, and compared with the domestic yellow pyrophyllite powder compacts (Mentougou, Beijing) produced by the same process, to establish experimental methods and physical criteria for evaluating the pressure transmission and sealing performance of Pyrophyllite. During the experiment, standard pressure materials such as Bi, Tl, and Ba are used to in-situ calibrate the pressure at the central positions and sealing edges of the pyrophyllite pressure chambers from the three aforementioned compacts under normal pressure conditions. Additionally, the silver melting point method is employed to obtain the corresponding relationship between chamber pressure at high temperature and system loading when using these three types of pyrophyllite as load-transmitting sealing materials. The results show that under the same hydraulic pressure loading, the difference in pressure at the central position between South African pyrophyllite B powder blocks and domestically produced pyrophyllite powder blocks does not exceed 0.1 GPa. Furthermore, in pressurization process and depressurization processe, the differences in pressure between the central position and the sealing edge of the pyrophyllite blocks are notably similar. Compared with South African pyrophyllite A powder blocks, pyrophyllite B powder blocks exhibit a closer resemblance to domestically produced pyrophyllite powder blocks in terms of high-temperature load transmission and sealing performance. Pyrophyllite B powder blocks from South Africa have the potential to serve as a substitute for domestically produced pyrophyllite without changing the existing superhard material synthesis process, making them promising candidates for use as load-transmitting media and sealing materials. These research findings hold significant academic importance in the realms of high-pressure research and superhard material production. They provide valuable insights into the selection of suitable transmission and sealing materials and the optimization of high-pressure experimental conditions. Additionally, this study presents robust method and criteria for experimental procedures and performance assessment.
Optical fiber high-temperature pressure sensor with weak temperature sensitivity
Wang Wei, Li Jin-Yang, Mao Guo-Pei, Yang Yan, Gao Zhi-Qiang, Ma Cong, Zhong Xiang-Yu, Shi Qing
Abstract +
In aerospace, petrochemical, gas turbines and other high-temperature environments, pressure measurement of equipment has always been a challenge to be solved. The electrical high temperature pressure sensor has the problem of component failure in high temperature environment, and it is difficult to use in the high temperature environment for a long time. The detection device of the optical fiber sensor does not include electrical components, so it has the advantages of high working temperature, high measurement accuracy, anti-electromagnetic interference and so on. In order to use a sensor to measure pressure in high temperature environment, a temperature-weakly sensitive optical fiber micro-electro-mechanical system (MEMS) pressure sensing technology is proposed. The technique uses extrisic Fabry-Pérot interference (EFPI) model. It uses the MEMS pressure chip to passively modulate the optical signal of the interference, and then realizes the pressure signal measurement. Among them, MEMS pressure sensitive chip is the core component of the sensor. The MEMS pressure sensitive chip adopts the design method of all solid state vacuum absolute pressure. Change in environmental pressure will deform the membrane. This phenomenon can cause change in the cavity of the EFPI cavity. Therefore, stress information can be obtained by measuring changes in EFPI cavity. The thermal stress and temperature parasitical response introduced by thermal expansion of the material are calculated by simulation. The influence of temperature signal on chip displacement is analyzed by the above results. On this basis, a prototype of high temperature pressure sensor is developed by combining the sub-micron white light interference response technology and low thermal stress packaging technology. In order to test the ability of the sensor to implement actual measurement, this paper carry out the pressure test and high temperature test respectively. When the pressure changes from 0 kPa to 100 kPa, the spectral intensity of the sensor output has a linear relationship with the pressure. During the temperature changing from 20–400 ℃, the spectral intensity of the sensor output does not change significantly. The experimental test results show that the pressure measurement of 0–100 kPa can be satisfied in the range of 20–400 ℃, and the measurement error introduced by temperature change is less than 4%. Therefore, the fiber pressure sensor can be used to measure the pressure in high temperature environment.
Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate
Wu Hang, Chen Liao, Li Shuai, Du Yu-Fan, Zhang Chi, Zhang Xin-Liang
Abstract +
Orbital angular momentum (OAM) lasers have potential applications in large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus due to the advantages of simple structure, low cost and high peak power. At present, OAM mode femtosecond fiber lasers have made some breakthroughs in key parameters such as repetition frequency, pulse width, spectrum width, but it is difficult to achieve good overall performance. Besides, the repetition rate is tens of MHz at present. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. In coupler, the first order mode coupler with 3 dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3 dB polarization dependent loss is made by the technology of weak fused biconical taper. By combining the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber laser with over 100 MHz repetition rate is built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Comparing with traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indexes of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8 μm, which is the same as that of a single mode fiber. Comparing with DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required, which ensures a shorter overall cavity length and achieves high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101 nm 10-dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100 nm 10 dB bandwidth. Compared with the reported schemes, our scheme has good performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with excellent performance is expected to be widely used in OAM communication, particle manipulation and other research fields.
Progress in protein pKa prediction
Luo Fang-Fang, Cai Zhi-Tao, Huang Yan-Dong
Abstract +
The pH value represents the acidity of the solution and plays a key role in many life events linked to human diseases. For instance, the β-site amyloid precursor protein cleavage enzyme, BACE1, which is a major therapeutic target of treating Alzheimer’s disease, functions within a narrow pH region around 4.5. In addition, the sodium-proton antiporter NhaA from Escherichia coli is activated only when the cytoplasmic pH is higher than 6.5 and the activity reaches a maximum value around pH 8.8. To explore the molecular mechanism of a protein regulated by pH, it is important to measure, typically by nuclear magnetic resonance , the binding affinities of protons to ionizable key residues, namely $ {\mathrm{p}}{{\mathrm{K}}}_{{\mathrm{a}}} $ values, which determine the deprotonation equilibria under a pH condition. However, web-lab experiments are often expensive and time consuming. In some cases, owing to the structural complexity of a protein, $ {\mathrm{p}}{K}_{{\mathrm{a}}} $ measurements become difficult, making theoretical $ {\mathrm{p}}{K}_{{\mathrm{a}}} $ predictions in a test laboratory more advantageous. In the past thirty years, many efforts have been made to accurately and fast predict protein $ {\mathrm{p}}{{\mathrm{K}}}_{{\mathrm{a}}} $ with physics-based methods. Theoretically, constant pH molecular dynamics (CpHMD) method that takes conformational fluctuations into account gives the most accurate predictions, especially the explicit-solvent CpHMD model proposed by Huang and coworkers (2016 J. Chem. Theory Comput. 12 5411) which in principle is applicable to any system that can be described by a force field. However, lengthy molecular simulations are usually necessary for the extensive sampling of conformation. In particular, the computational complexity increases significantly if water molecules are included explicitly in the simulation system. Thus, CpHMD is not suitable for high-throughout computing requested in industry circle. To accelerate $ {\mathrm{p}}{{\mathrm{K}}}_{{\mathrm{a}}} $ prediction, Poisson-Boltzmann (PB) or empirical equation-based schemes, such as H++ and PropKa, have been developed and widely used where $ {\mathrm{p}}{K}_{{\mathrm{a}}} $ values are obtained via one-structure calculations. Recently, artificial intelligence (AI) is applied to the area of protein $ {\mathrm{p}}{{\mathrm{K}}}_{{\mathrm{a}}} $ prediction, which leads to the development of DeepKa by Huang laboratory (2021 ACS Omega 6 34823), the first AI-driven $ {\mathrm{p}}{{\mathrm{K}}}_{{\mathrm{a}}} $ predictor. In this paper, we review the advances in protein $ {\mathrm{p}}{{\mathrm{K}}}_{{\mathrm{a}}} $ prediction contributed mainly by CpHMD methods, PB or empirical equation-based schemes, and AI models. Notably, the modeling hypotheses explained in the review would shed light on future development of more powerful protein $ {\mathrm{p}}{K}_{{\mathrm{a}}} $ predictors.
Study on total X-ray dose effect of graphene field effect transistor
Li Ji-Fang, Guo Hong-Xia, Ma Wu-Ying, Song Hong-Jia, Zhong Xiang-Li, Li Yang-Fan, Bai Ru-Xue, Lu Xiao-Jie, Zhang Feng-Qi
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In this paper, the total dose effect of graphene field-effect transistors (GFET) of different structures and sizes was studied. The irradiation experiments were carried by 10-keV X-ray irradiation platform with a dose rate of 200 rad(Si)/s. Positive gate bias (VG=+1 V, VD=VS=0 V) was applied during irradiation. Using a semiconductor parameter analyzer, the transfer characteristic curves of top-gate and back-gate GFETs were obtained both before and after irradiation. At the same time, the degradation condition of the Dirac voltage VDirac and the carrier mobility μ are extracted from the transfer characteristic curve. The experimental results demonstrate that VDirac and carrier mobility μ degrade with increasing dose. the depletion of VDirac and carrier mobility μ is caused by the oxide trap charge generated in the gate oxygen layer during X-ray irradiation. Compared with the back-gate GFETs, the top-gate GFETs show more severe degradation of VDirac and the carrier mobility, therefore Top-gate GFETs are more sensitive to X-ray radiation at the same cumulative dose than back-gate GFETs. The analysis found that, the degradation of top-gate GFET is mainly caused by the oxide trap charge. And in contrast to top-gate GFET, oxygen adsorption contributes to the irradiation process of back-gate GFET, which somewhat mitigates the impacts of radiation damage. Furthermore, a comparison was made between the electrical property deterioration of GFETs of varying sizes between the pre-irradiation and post-irradiation. The back-gate GFET, which had the size of 50 μm*50 μm, and the top-gate GFET, which had the size of 200 μm*200 μm were found to have sustained the most damage. In the case of the top-gate GFET, the larger the radiation area, the more oxide trap charges generated, the more serious the damage. In contrast, back-gate GFETs have a larger oxygen adsorption area during irradiation and a more noticeable oxygen adsorption effect, which partially offsets the damage produced by irradiation. Lastly, the oxide trap charge mechanism was simulated using TCAD simulation tool. TCAD simulation reveals that the trap charge at the interface of Al2O3 and graphene is the primary cause of top-gate GFET property degradation. It has a significant impact on the the investigation of the radiation effect and radiation reinforcement of GFETs.
Quantum-enhanced optical phase tracking via squeezed state
Sun Xiaocong, Li Wei, Wang Yajun, Zheng Yaohui
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Quantum-enhanced optical phase tracking is a quantum optical technique for high-precision tracking and measurement of optical phases. It has important applications in areas such as laser interferometry, spectral analysis, and optical measurements. In this paper, we propose a quantum-enhanced optical phase tracking protocol based on squeezed state optical fields. By using a continuous solid-state laser source with a central wavelength of 1064 nm, combined with second harmonic generation, optical parametric oscillator, and PDH (Pound-Drever-Hall) locking technology, we prepare an initial squeezed state with a squeezing level of 8.0±0.2 dB. Through signal modulation and demodulation techniques, we achieve control over the phase of the squeezed state optical field, thereby realizing quantum-enhanced tracking of optical phases within the range of 0-2π. Compared to classical protocols, this protocol can suppress the noise fluctuations of phase tracking to at least 6.27 dB below the shot noise limit, improving the phase tracking accuracy by more than 76.4%. Due to the high requirements for phase measurement accuracy in applications such as angle estimation, phased array radar, and phased array sonar, this protocol is expected to improve the accuracy of phase estimation beyond the shot noise limit. It provides compressed light sources for related fields and lays a theoretical and experimental foundation for higher-precision spatial localization and quantum ranging techniques. To determine the probe made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by a gene and encoded in the genetic code. This can happen either before the protein is used in the cell, or as part of control mechanisms.
Displacement damage simulation of InP induced by proton and α in Low Earth Orbit
Yurong Bai, Pei Li, Huan He, Fang liu, Wei Li, Chaohui He
Abstract +
Indium phosphide (InP) material has the advantages of large band gap, high electron mobility, high photoelectric conversion efficiency, high temperature resistance, radiation resistance better than silicon (Si), gallium arsenide (GaAs). Meanwhile InP is widely used in optical communications, high-frequency millimeter waves, optoelectronic integrated circuits, satellite communications, space solar cells and other fields. Radiation particles incident in InP devices to produce displacement atoms through elastic process. And these displacement atoms continue cascade collisions to generate lattice defects which are vacancies, interstitials and clusters. These defects capture electrons-holes by introducing defective energy levels in the energy band. And then they resulting in a decrease in the life of minority carriers which is the reason of degradation of InP devices. The process of degradation of InP devices induced by lattice defects from ion-irradiation is called displacement damage effect (DDE). The non-ionizing loss energy (NIEL) scaling is a useful method to predict the degradation of devices caused by DDE of radiation particles. Abundant studies have shown that NIEL is linearly related to the damage coefficient of InP devices. Previous study of radiation damage effect of InP devices are mainly focused on single-energy protons, electrons, and neutrons. Low Earth Orbit (LEO) consists of most protons and a little of α and electrons while the electrons NIEL is too small and its DDE is negligible. The InP NIEL induced by proton and α energy spectrum in LEO has not been studied in detail. Therefore, this paper uses Monte Carlo software Geant4 to study the non-ionizing loss energy (NIEL), damage energy distribution with depth and annual total non-ionization loss energy generated by protons and α particles in LEO in 500/1000/5000 μm InP materials. The shielding of 150 μm SiO2 and 2.54 mm Al for proton and α are considered as InP solar cell and InP devices in spacecraft respectively. We found that the energy spectrum determines the non-ionizing damage energy T_"dam" distribution, and then influence the NIEL value which increase with the increase of T_"dam" and the decrease of InP materials thickness. And α NIEL is larger than proton, the single particle DDE of InP devices induced by α should be focused. The annual non-ionizing damage energy of proton accounts for 98%, which means proton is the main factor of the degradation of InP devices in LEO.
Bound states in the continuum of double resonant ring metal metasurfaces
Wang Yue, Wang Hao-Jie, Cui Zi-Jian, Zhang Da-Chi
Abstract +
Metasurfaces have found extensive application in microwave, terahertz, and optical range, serving purposes such as filters, sensors, slow light devices, and nonlinear devices due to their distinctive electromagnetic response characteristics. Recent advancements demand metasurface devices to exhibit enhanced monochromaticity and stronger light interaction. Consequently, there is a growing interest in designing metasurfaces with high-quality factor (Q-factor) resonances, considering their crucial role in achieving sharp resonances through constructing bound states in the continuum (BIC) mode. The utilization of BIC has emerged as a prominent method for designing metasurfaces with high Q-factor resonances. Due to the fact that changes in the structural parameters of metasurfaces can simultaneously affect the resonance of two components of q-BICs, it is difficult to achieve on-demand design of operating frequency, bandwidth, and Q-factor. In this work, we have investigated a novel THz metasurfaces supporting q-BIC resonance. We optimize the geometric parameters of two split ring resonators (SRRs) to tailor the operating frequencies of the intrinsic resonance, and tuning the coupling between different resonance modes to form the q-BIC mode resonance. The dominant modes are demonstrated by the results of multipolar decomposition calculations of the electromagnetic field distributions and scattered power at different resonant operating frequencies. In the incident electromagnetic wave along x and y polarization, the normalized coupling strength ratio between the two modes is 0.54% (x polarization) and 4.42% (y polarization) respectively calculated through the Jaynes-Cummings model, which explains the law of the resonant frequency of different modes with the change of structural parameters of SRRs device. In order to analyze the refractive index sensing capability of our designed metasurfaces under the incident electromagnetic waves with different polarizations. We investigated the variation of the transmitted spectra of the metasurfaces under different refractive index of matters. The calculated results show that the sensitivity of the metasurface is 151 GHz/RIU when the incident wave is y-polarized and 108 GHz/RIU when the incident wave is x-polarized. We realize the effective control of the operating frequency, bandwidth, and Q-factor of the q-BIC mode resonance in the transmission spectrum of the metasurface, which provides a new idea for the practical design of terahertz metasurfaces with high Q-factor.
Quark matter and quark star in color-flavor-locked phase
Chu Peng-Cheng, Liu He, Du Xian-Bin
Abstract +
In this work, we investigate the thermodynamical properties of the strange quark matter (SQM) and color-flavorlocked (CFL) quark matter under strong magnetic fields within quasiparticle model. We calculate the energy density and the corresponding anisotropic pressure of SQM and CFL quark matter. Our results indicate that CFL quark matter is more stable than SQM, and the pressure of CFL quark matter increases with the energy gap constant ∆. We also find that the oscillation effects coming from the Lowest Landau Level can be reduced when considering increasing the energy gap constant ∆, which cannot be found in SQM at the identical strong magnetic field. The equivalent quark mass for u, d, and s quark and the chemical potential for each flavor of quarks decrease with the energy gap constant ∆, which matches the conclusion that CFL quark matter is more stable than SQM. From the calculation of the magnetars with SQM and CFL quark matter, we find the maximum mass of the magnetars increases with the energy gap constant ∆ for both the longitudinal magnetic field orientation distribution and the transverse magnetic field orientation distribution, and the tidal deformability of the magnetars increases with the increment of ∆. On the other side, the central baryon density of the maximum mass of the magnetars decreases with the increment of ∆. The results also indicate that the mass-radius lines of the CFL quark star can also satisfy the new estimates of the mass-radius region from PSR J0740 + 6620, PSR J0030 + 0451, and HESS J1731-347.
Electronic Properties and Modulation Effects on Edge-Modified GeS2 Nanoribbons
Li Jing-Hui, Cao Sheng-Guo, Han Jia-Ning, Li Zhan-Hai, Zhang Zhen-Hua
Abstract +
GeS2 monolayer has been successfully prepared. Here, to further expand their applications and discover new physical property, we construct armchair-type GeS2 nanoribbons (AGeS2NR) and use different concentrations of H and O atoms for the edge modification, as well as their structural stability, electronic properties, carrier mobility, and physical field modulation effects are studied deeply. The results show that the edge-modified nanoribbons have a higher energy and thermal stability. The bare edge nanoribbons are nonmagnetic semiconductors, while the edge modification can change the bandgap of AGeS2NR and make them become wide or narrowed bandgap semiconductor, or a metal, which is closely related to the elimination or partial elimination of the edge states or the creation of hybridization bands. Thus edge modification extends the application range of nanoribbons in the field of electronic devices and optical devices. In addition, the carrier mobility is found to be very sensitive to the edge modification, the carrier mobility (electrons and holes) of nanoribbons can be tuned to have a difference up to one order of magnitude, and the carrier mobility polarization up to one order of magnitude occurs. Strain effect studies reveal that the semiconducting nanoribbons are robust in keeping the electronic phase unchanged over a wide strain range, which is useful for maintaining the stability of the electron transport in the related devices. Most of the semiconducting nanoribbons have the stability to keep the semiconducting properties unchanged under high external electric field, but the bandgap can be reduced significantly with the increase of the electric field. In short, this study provides a theoretical analysis and reference for understanding the property of GeS2 nanoribbons and developing related devices.
Ultralow Switching Threshold Optical Bistable Devices based on Epsilon-near-zero Ga2O3-SiC-Ag Multilayer Structures
Hu Sheng-Run, Ji Xue-Qiang, Wang Jin-Jin, Yan Jie-Yun, Zhang Tian-Yue, Li Pei-Gang
Abstract +
Optical bistability has attracted much attention due to its enormous potentials in all-optical operation and signal processing. However, the weak nonlinear responses typically require huge pump power to fulfill the threshold of the optical bistability, thus hindering the real applications. In this study, we propose an efficient optical bistable metamaterial that composes multi-layered Ga2O3-SiC-Ag metal-dielectric nanostructures. We not only achieve the substantial field enhancement by utilizing the epsilon – near - zero (ENZ) with SiC-Ag thin layers, but also incorporate the SiC material leveraging its significant optical nonlinear coefficient. In the structural design, the introduction of Ga2O3 layers facilitate the light field concentration, contributing to the further reduction in threshold power for optical bistability, and also help to improve the physical and chemical stability of the device. The influences of the thickness and length of the ENZ layer on the optical bistability are systematically investigated using the finite element method. The results demonstrate that optical bistability becomes more pronounced with the increase of the thickness and length of ENZ layer, exhibiting a bistability switching threshold as low as~10-6 W/cm2 in the telecommunication band. Compared with the previously reported optical bistability based on ENZ mechanism, it shows a significant reduction by 9 orders of magnitude, demonstrating great application potentials in the fields of semiconductor devices, and photonic integrated circuits.
A weak pulse signal detection method based on improved strongly coupled oscillators
Li Yuan-Fang, Jiang Yuan, Zhao Lei
Abstract +
A strongly coupled oscillator can be used to detect weak pulse signals and recover waveforms, but its detection frequency of weak pulse signals is limited by the system’s built-in frequency. With a fixed built-in frequency, the system can only effectively detect and recover pulse signals within a certain frequency range, and waveform distortion occurs when pulse signals of higher frequencies are detected. In this paper, the relationship between the built-in frequency of the coupled oscillator and the frequency detection range of weak pulse signals is analyzed, and two kinds of improved strongly coupled oscillator structures are proposed to extend the frequency detection range of weak pulse signals. By introducing the nonlinear restoring force coupling term, the nonlinear restoring force strongly coupled oscillator can effectively retain the high-frequency component of the signal, and can also better retain the signal characteristics when the pulse signal is input at a higher frequency. By introducing the Van der Pol-Duffng oscillator, the two-oscillator strong coupling system strengthens the stability of the internal structure of the system, and also achieves the effect of expanding the frequency detection range of the pulse signal. In addition, based on the variable iteration step size and frequency correlation of chaos detection, a detection method for unknown frequency pulse signals is proposed. Instead of changing the built-in frequency of the system for frequency scanning, the method of changing the iteration step size is used. And using the frequency correlation of chaos detection, the correlation coeffcient of the received signal and the recovered signal is compared with the correlation coeffcient of the pure noise input case, then the pulse signals can be effectively detected based on the apparent difference between the two correlation coeffcients. It is verified by simulation experiments that the proposed method can effectively detect the pulse signal of unknown frequency, and the proposed improved strong coupling oscillator has a greater performance improvement than that of the strong coupling oscillator.
Two-dimensional ferromagnetism induced by Jahn-Teller distortion and orbital order
Zhang Jun-Ting, Wu Zong-Shuo, Shen Xiao-Fan
Abstract +
As the dimension of the system decreases, the quantum confinement effect and electronic correlation interaction inside the material will be correspondingly enhanced, often resulting in some novel physical properties. Recently, the freestanding perovskite oxide films down to the monolayer limit have been successfully prepared and can be transferred to any desired substrate, providing a great opportunitiy to explore the functional properties of two-dimensional perovskites. In perovskite materials, Jahn-Teller distortion and orbital order often cause a variety of correlated electronic behaviors. However, unlike van der Waals materials that retain their structural and chemical bonding characteristics when reduced to the monolayer limit, perovskite materials may undergo structural reconstruction when reduced to two dimensions. Therefore, whether Jahn-Teller distortion and related effects exist in the perovskite monolayer limit, and whether two-dimensional perovskite can exhibit some new properties different from its bulk phase, have become urgent issues to be solved. In this work, perovskite fluoride KCuF3 and its monolayer have been comparatively studied by means of first-principles calculation, symmetry analysis and Monte Carlo simulation methods, to reveal the change in lattice dynamics, structural, electronic, and magnetic properties caused by dimensionality reduction in perovskites. The results show that the cooperative Jahn-Teller distortion and the in-plane staggered orbital order occurring in the KCuF3 bulk can be retained to the monolayer limit. However, unlike the bulk phase, the Jahn-Teller distortion mode appears as a soft mode of the prototype phase in the monolayer, and the insulating property of the monolayer does not rely on the emergence of the Jahn-Teller distortion, but is related to the enhancement of the electronic correlation effect. The staggered orbital order causes the nearest-neighbor exchange interaction to be ferromagnetic, resulting in the monolayer being a two-dimensional ferromagnetic insulator, different the antiferromagnetic phase in the bulk. Monte Carlo simulations predict that the Curie temperature of the monolayer is about 5 K, which is much lower than the NNéel temperature of the bulk phase, indicating that the disappearance of interlayer coupling leads to a significant reduction in the magnetic phase transition temperature. This work provides guidance and reference for the study of two-dimensional perovskite materials and the design of perovskite-based two-dimensional ferromagnets.
Transition state searching for complex biomolecules: algorithms and machine learning
Yang Jian-Yu, Xi Kun, Zhu Li-Zhe
Abstract +
Transition state is a key concept for chemists to understand and fine-tune the conformational changes of large biomolecules. Due to its short residence time, it is difficult to capture a transition state via experimental techniques. Characterizing transition states for a conformational change therefore is only achievable via physics-driven molecular dynamics simulations. However, unlike chemical reactions which involve only a small number of atoms, conformational changes of biomolecules depend on numerous atoms and therefore the number of their coordinates in our 3D space. The searching for their transition states will inevitably encounter the curse of dimensionality, i.e. the reaction coordinate problem, which invokes the invention of various algorithms for solution. Recent years, new machine learning techniques and the incorporation of some of them into the transition state searching methods emerged. Here, we first review the design principle of representative transition state searching algorithms, including the collective-variable (CV)-dependent gentlest ascent dynamics, finite temperature string, fast tomographic, travelling-salesman based automated path searching, and the CV-independent transition path sampling. Then, we focus on the new version of TPS that incorporates reinforcement learning for efficient sampling, and we also clarify the suitable situation for its application. Finally, we propose a new paradigm for transition state searching, a new dimensionality reduction technique that preserves transition state information and combines GAD.
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