Accepted
, , Received Date: 2023-10-09
Abstract +
Quantum entanglement is a crucial resource for performing quantum computing and constructing quantum communication networks. The preparation and manipulation of entangled light field are the basic elements of quantum communication. With the development of science and technology, multicolor multipartite entanglement is becoming a kind of special resource for quantum information, quantum networks, and quantum memory. In this paper, we propose a scheme of generating quadripartite entanglement among four output beams from a two-port frequency doubling resonator, in which a type-II phase matching nonlinear crystal is placed. We make two fundamental-frequency pump beams with the same frequency and vertical polarization pass through the nonlinear crystal to produce two frequency-doubling beams. There is a quadripartite entanglement between the frequency-doubling beams, which are output at two ports of the optical resonator, and the incident fundamental beams. Based on the transmission matrix from the coupled wave equation, the self-consistent equations of the intracavity modes and the corresponding noise properties of the output modes can be obtained. Then, the quadripartite entanglement produced from two second harmonic beams and two reflected fundamental-frequency pump beams, is verified by using the positive partial transposition criterion, in a wide range of pumping power and analysis frequency. The setup proposed in this work is compact and experimentally feasible. It is also convenient to separate the four entangled beams spatially, with different wavelengths and polarizations. When the beam wavelengths are matched with 1560 nm (low loss window of fiber) and 780 nm (atomic absorption line of Rb), this scheme can be more useful in both quantum communication and quantum memory.
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Owing to the complete spin-polarization of electronic states near Fermi energy, half-metallic ferromagnets, especially two-dimensional half-metallic ferromagnets, have garnered significant attention in the field of spintronics. However, the practical application of these materials is greatly hindered by their low Curie temperatures. Therefore, the exploration of high Curie temperature half-metallic ferromagnets poses a necessary and challenging task. In this study, we predict a two-dimensional transition metal oxide, CrO2 monolayer, and employed first-principles calculations to investigate the crystal structure, electronic properties, magnetic ground state, and ferromagnetic phase transition. The calculations on phonon spectrum, elastic constant, and molecular dynamics simulations indicate that CrO2 monolayer is dynamically, thermally, and mechanically stable. The convex hull diagram of Cr-O systems shows the hull energy of the predicted CrO2 layer is only 0.18 eV, further confirming the structural stability and large possibility for experimental fabrication. More importantly, the electronic and magnetic properties of CrO2 monolayer demonstrate that it is a two-dimensional ferromagnetic half-metal with wide band gap. Five d suborbitals are divided into eg and t2g orbitals because of the crystal field of Cr atom at the center of O tetrahedron, and the spin-polarization of eg orbitals make a major contribution to the moment around Cr atom. The ferromagnetic coupling along Cr-O-Cr chain is dominated by the superexchange interaction bridged by O 2p orbital, similar to the typical Mn-O-Mn superexchange model. The magnetic behavior of the Cr spin lattice in a CrO2 monolayer is described by a two-dimensional Heisenberg model, in which the exchange coupling anisotropy is ignored and the single ion anisotropy is the main consideration. By solving the Heisenberg model using the Monte Carlo simulation method, the Curie temperature is determined to be over 400 K. The high Curie temperature ferromagnetism is rare in two-dimensional ferromagnetic materials and even rarer in semi-metallic materials, which makes it an ideal material for the fabrication of spintronic devices and the study of spin quantum effects.
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The pulsed high current linear driving device operates under extreme working conditions, and various forms of metal damage will reduce the service life of the device. At present, the multi-physics coupling mechanism of pulsed high current linear driving device is still unclear, and the multi-parameter diagnosis method in the laboratory environment is limited. Therefore, it is urgent to clarify the evolution process of multiple physical parameters through numerical modeling methods, so as to guide the optimization of the overall performance and improve the service life of the device. In this paper, mathematical and physical models of electromagnetic field, temperature field and structural field under dynamic conditions are established. The local solution is carried out by using the characteristics of rail reverse motion and the invariant physical quantities at the distal end of the contact. The constraint equations of the non-equipotential surface of the rail entrance and the armature-rail interface conditions under the technical framework are derived. The constraint equations applied by the penalty function method. The model also takes into account practical factors such as the temperature dependence of the material properties, thermal stresses, and the frictional heat of the contact surface. The finite element discrete format of the electromagnetic field and the temperature field is solved in the form of Euler's backward differentiation, and the structural field is solved by the Newmark method. The reliability of the model is verified by comparing the calculation results with the numerical tools EMAP3D and Comsol under the same configuration and input conditions, as well as related experiments. Through the numerical simulation of the C-type armature, the typical evolution process of the corresponding multi-parameter is obtained. During sliding electrical contact, the velocity skin effect becomes more pronounced with increasing velocity. The current is gradually concentrated on the surface of the rail, and the highest current density is found at the rear edge of the contact surface and at the edge of the outer arm of the armature. Moreover, the magnetic induction intensity at the tail of the contact surface continues to shrink over time. The heat-concentrated region appears at the top edge of the contact surface, and over time it extends along the sliding and bottom directions of the armature. In addition, there is peak stress at the front of the rail contact and significant stress at the armature throat. When the local stress at the throat of the armature exceeds the corresponding yield strength, it can cause severe deformation or even fracture of the armature.
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Perovskite solar cells have been widely recognized as the most promising new type of photovoltaic technology due to its rapid development of power conversion efficiency from 3.8% to over 26% in merely fifteen years. However, the high performances were achieved mainly on small area cells with active area lower than 0.1 cm2. When enlarging the active area of perovskite solar cells, the efficiency fell dramatically. How to reduce the gap between performances of small area and large area cells gradually becomes a critical point in the path towards the commercialization of perovskite photovoltaic technology. Herein, a strategy to pre-grow a thin layer of TiO2 on rough FTO substrate by atomic layer deposition method before spin-coating SnO2 nanoparticles was developed. The FTO substrate could be covered completely by TiO2 due to the intrinsic conformal film growth mode of atomic layer deposition, thus the direct contact between local protuberance of FTO and perovskite layer could be prevented and the current leakage phenomenon could be prevented. X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and dark current measurement further proved this point. Thanks to the approach, the repeatability and consistency of the small area cell fabrication technology on the same substrate were ameliorated obviously. The improved electron transport process revealed by photoluminescence results and incident light management process revealed by external quantum efficiency results also brought better solar cell performances. More importantly, highly efficient 0.5 cm2 large area perovskite solar cells were fabricated through optimization of TiO2 thicknesses. When growing 200 cycles TiO2 (~9 nm thickness) using atomic layer deposition technology, the champion large area perovskite solar cell possessed a power conversion efficiency as high as 24.8% (certified 24.65%). The device performances also showed excellent repeatability between different fabrication batches. The perovskite solar cell with atomic layer deposited TiO2 as buffer layer could retain over 95% of its initial efficiency after storage for 1500 hours under nitrogen atmosphere. The technique proposed in this paper could be helpful for the fabrication of perovskite solar cell modules in the realistic photovoltaic market and could be potentially extended to the large area fabrication of other perovskite optoelectronic devices such as light emitting diode, laser and detector.
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Cross-linked polyethylene (XLPE) has been widely applied in the field of power cables because of its excellent mechanical properties and insulating properties. However, during the operation of high voltage cables, XLPE will inevitably be affected by electrical aging, thermal aging and electric-thermal joint aging, which makes the performance and life of the material decline. Therefore, it is necessary to enhance the aging resistance of XLPE without affecting its mechanical properties and insulating properties, so as to achieve the purpose of extending its service life. In this paper, the structural characteristics and cross-linking mechanism of XLPE are introduced, the aging process and influencing mechanism are systematically analyzed, and the life decay problems of XLPE due to aging are explored by using methods such as the temperature Arrhenius equation and the inverse power law of voltage. The regulatory strategies such as grafting, blending, and nano-particle modification, etc., can be applied to improve the thermal stability, antioxidant performance, and thermal aging resistance of XLPE, thereby extending its service life. Finally, the future direction of the strategies for regulating the service life of XLPE cable insulation materials is envisioned, which provides theoretical guidance for the further improvement and long-term stable operation of XLPE cable insulation materials.
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Abstract +
The high-confinement mode (H-mode) significantly enhances the energy and particle confinement in fusion plasmas compared to the low-confinement mode (L-mode), and it is the basic operation scenario for ITER and CFETR. Edge localized modes (ELMs) often appear in H-mode, helping to expel impurities to maintain a longer stable state. However, the particle and energy bursts from ELM eruptions can severely damage the first wall of fusion devices, necessitating control of ELMs. Experiments in EAST and HL-2A tokamaks have been conducted with ELM mitigation by lower hybrid wave (LHW), confirming the effect of LHW on ELMs, but the physical mechanism of ELM mitigation by LHW is still not fully understood. This paper investigates the influence of LHW injection on the linear and nonlinear characteristics of peeling-ballooning mode (P-B mode) in the edge pedestal region of H-mode plasmas in tokamaks, based on the BOUT++ code. The simulations consider both the conventional main plasma current driven by LHW and the three-dimensional perturbed magnetic field generated by the scrape-off layer helical current filament (HCF) on the P-B modes. The linear results show that the core plasma current driven by LHW moves the linear toroidal mode spectrum towards higher mode numbers and lower growth rates by reducing the normalized pressure gradient and magnetic shear of the equilibrium. Nonlinear simulations indicate that due to the broadening of the linear mode spectrum, the core current driven by LHW can reduce the pedestal energy loss caused by ELMs by globally suppressing different toroidal modes of the P-B mode, the three-dimensional perturbed magnetic field generated by LHW-driven HCF can reduce the energy loss caused by ELMs by promoting the growth of modes other than the main mode and enhancing the coupling between different modes. The study finds that the P-B mode promoted by the three-dimensional perturbed magnetic field generated by HCF have a mode number threshold, and when the dominant mode of the P-B mode is far from the mode number threshold driven by the three-dimensional perturbed magnetic field, the energy loss due to ELMs is more significantly reduced. These results contribute to a deeper understanding of the physical mechanisms in ELM control experiments by LHW.
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In the two-dimensional boron structure, the ordered vacancy distribution of high concentrations enhances structural stability and significantly modulates material properties. Based on recent experimental progress, herein we have particularly focused on the two-dimensional boron structures with a striped distribution of hexagonal vacancies, aiming to explore the formation of long-period boron structures. Utilizing the SAGAR (Structures of Alloy Generation And Recognition) program developed by our group, we have eliminated duplicate structures according the structural symmetry to reduce computational costs. An effective model system is proposed to investigate the effect of vacancy distribution on the stability of the system, where the interactions between vacancies are utilized for the estimation of total energy. By selecting structures with appropriate concentrations and combining first-principles calculations, the parameters in the model are fitted for different vacancy neighbor interactions, which is further used to predict stable structures at various vacancy concentrations. The feasibility of model analysis is emphasized for structural screening, showing the good agreement between the parameterized model and the first-principles calculations. Interestingly, under the same vacancy concentration, stable boron structures with different cell sizes exhibit distinct vacancy distributions, indicating a trend of long-period distribution for ground state structures. To address this phenomenon, as the stable candidate structures from the 1/6 series are the most numerous within the computable range, enabling a clearer observation of changes in neighbor statistics, we selected structures from this concentration series for detailed calculations. The calculation results indicate that the convergence of the average energy is primarily influenced by the contributions from the fourth and sixth nearest neighbor interactions. When considering only these two neighbors, the system energy changes with the increase in cell size as follows: the average energy of structures with a cell size being an even multiple of the minimum cell size remains unchanged, while the average energy of structures with a cell size being an odd multiple of the minimum cell size gradually decreases, eventually converging to a stable value. Upon including interactions from the ninth and tenth nearest neighbors, the average energy of structures with a cell size being an even multiple of the minimum cell size also decreases gradually. The average energy decreases with oscillations, with the magnitude gradually diminishing and eventually stabilizing. This discovery reveals that the enhanced stability of long-period structures is attributed to the competitive interactions among different neighboring vacancies.
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In recent decades, significant progress has been made in the precise theoretical investigation of gas-phase chemical reactions. Presently, a major challenge in the field of quantum dynamics is the development of precise methodologies for studying chemical reactions involving more than four atoms. As a typical multi-atomic reaction system, the F+CH4 reaction and its isotopic substitution reactions have attracted widespread attention from both experimental and theoretical perspectives in recent years. Experimental studies on the reaction of F+CHD3 revealed that the stretching vibration excitation of the C-H bond inhibits the bond dissociation, favoring the formation of DF+CHD2 products channel. In this study, we employed a seven-dimensional quantum time-dependent wave packet method to investigate the dynamics of the F+CHD3 reaction in both the reactant vibrational ground state and the first stretching excited state of the C-H bond. The paper analyzed reaction probabilities under different vibrational conditions, revealing that when the collision energy is below 0.06 eV, the reaction probability curves exhibit numerous fast-oscillating peaks, supporting the experimentally suggested phenomena of dynamic resonance. At collision energies ranging from 0.06 eV to 0.3 eV, the reaction probability for the HF product channel in the vibrational excited state was lower than that in the ground state, consistent with experimental observations. Through the analysis of the time-independent wave functions of product channels under low-energy collision conditions, we have found that for reactions involving vibrational ground states, the HF products in the product asymptotic region and the reaction transition state region are in the v'=2 and v'=3 excited states of stretching vibration, respectively, consistent with previous experimental observations and six-dimensional quantum wave packet simulations. For reactions involving the first excited state of C-H stretching vibration, the HF products in the product asymptotic region and the reaction transition state region are both in the v'=3 excited state of stretching vibration, consistent with the results obtained based on energy analysis. Simulation results indicated that, in the case of low-energy collisions, the time-independent wave function for the C-H stretching vibrational excited state tends to be closer to the D atom side in the transition state region. This phenomenon was attributed to the more significant energy advantage of the vibrational excited state potential energy surface in the large collision angle region, explaining the inhibitory effect of stretching vibration excitation on the HF product channel. This study offers crucial theoretical support for interpreting experimental results and contributes to a deeper understanding of the influence of vibrational mode excitations on the dynamical processes in poly-atomic reactions.
, , Received Date: 2023-10-20
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The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.
, , Received Date: 2023-10-17
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Monte Carlo (MC) method is a powerful tool for solving particle transport problems. However, it is extremely time-consuming to obtain results that meet the specified statistical error requirements, especially for large-scale refined models. This paper focuses on improving the computational efficiency of neutron transport simulations. Specifically, this study presents a novel method of efficiently calculating neutron fixed source problems, which has many applications. This type of particle transport problem aims at obtaining a fixed target tally corresponding to different source distributions for fixed geometry and material. First, an efficient simulation is achieved by treating the source distribution as the input to a neural network, with the estimated target tally as the output. This neural network is trained with data from MC simulations of diverse source distributions, ensuring its reusability. Second, since the data acquisition is time consuming, the importance principle of MC method is utilized to efficiently generate training data. This method has been tested on several benchmark models. The relative errors resulting from neural networks are less than 5% and the times needed to obtain these results are negligible compared with those for original Monte Carlo simulations. In conclusion, in this work we propose a method to train neural networks, with MC simulation results containing importance data and we also use this network to accelerate the computation of neutron fixed source problems.
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, , Received Date: 2023-09-14
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Positron annihilation technique is an atomic-scale characterization method used to analyze the defects and microstructure of materials, which is extremely sensitive to open volume defects. By examining the annihilation behaviour of positrons and electrons in open volume defects, local electron density and atomic structure information around the annihilation site can be obtained, such as the size and concentration of vacancies, and vacancy clusters. In recent years, positron annihilation spectroscopy has evolved into a superior tool for characterizing features of material compared with conventional methods. The coincident Doppler broadening technique provides unique advantages for examining the local electronic structure and chemical environment (elemental composition) information about defects due to its effectiveness describing high momentum electronic information. The low momentum portion of the quotient spectrum indicates the Doppler shift generated by the annihilation of valence electrons near the vacancy defect. Changes in the peak amplitudes and positions of the characteristic peaks in the high momentum region can reveal elemental information about the positron annihilation point. The physical mechanism of element segregation, the structural features of open volume defects and the interaction between interstitial atoms and vacancy defects are well investigated by using the coincidence Doppler broadening technology. In recent years, based on the development of Doppler broadening technology, the sensitivity of slow positron beam coincidence Doppler broadening technology with adjustable energy has been significantly enhanced at a certain depth. It is notable that slow positron beam techniques can offer surface, defect, and interface microstructural information as a function of material depth. It compensates for the fact that the traditional coincidence Doppler broadening technique can only determine the overall defect information. Positron annihilation technology has been applied to the fields of second phase evolution in irradiated materials, hydrogen/helium effect, and free volume in thin films, as a result of the continuous development of slow positron beam and the improvement of various experimental test methods based on slow positron beam. In this paper, the basic principles of the coincidence Doppler broadening technique are briefly discussed, and the application research progress of the coincidence Doppler broadening technique in various materials is reviewed by combining the reported developments: 1) the evolution behaviour of nanoscale precipitation in alloys; 2) the interaction between lattice vacancies and impurity atoms in semiconductors; 3) the changes of oxygen vacancy and metal cation concentration in oxide material. In addition, coincident Doppler broadening technology has been steadily used to estimate and quantify the sizes, quantities, and distributions of free volume holes in polymers.
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The control of microscopic particle behavior based on specific external fields has always been a research hotspot in the field of physics. Many studies have been exploring various methods to manipulate and control the behavior of particles at the microscopic level. In this study, we investigate the phenomenon of single-particle squeezing induced by frequency jumping in a two-dimensional rotating harmonic oscillator potential. Squeezing, as a quantum mechanical phenomenon, has attracted significant attention due to its potential applications in various fields. It refers to the reduction of fluctuations in certain physical quantities, allowing for more precise measurement results. Squeezing phenomena have been extensively studied in different physical systems, including optics, atomic physics, and solid-state physics. However, there have been few reports on the quantum state squeezing phenomenon induced by frequency jumping in a rotating harmonic oscillator potential. Therefore, our study aims to fill this gap and shed light on this intriguing phenomenon. To explore the squeezing phenomenon induced by frequency jumping, we focus on the fluctuations and squeezing of the single particle's cyclotron radius coordinate and center-guided coordinate in the two-dimensional rotating harmonic oscillator potential. Through numerical simulations and theoretical analysis, we aim to understand the influence of frequency jumping on the degree of squeezing and reveal the underlying physical mechanism of squeezing evolution. In this work, we first investigate the influence of frequency jumping on the squeezing evolution of the cyclotron radius mode. By carefully selecting appropriate jumping moments, we analyze the impact of frequency jumping on the degree of squeezing. Our research results show that the degree of squeezing in the cyclotron radius coordinate remains unchanged at the jumping moment. However, we observe a stronger squeezing phenomenon in the subsequent evolution process. This suggests that frequency jumping plays a crucial role in the squeezing evolution of the cyclotron radius mode. Furthermore, we focus on the squeezing evolution of the center-guided mode during frequency jumping. By selecting suitable parameters, we analyze the squeezing and evolution of two squeezing modes: the divergent mode and the oscillatory mode. Interestingly, we discover the existence of a critical potential trap aspect ratio, which is determined by the rotation angular velocity of the external potential. When the aspect ratio approaches this critical value, the squeezing mode undergoes a transition, and a significant squeezing phenomenon appears in the oscillatory mode. This finding provides valuable insights into the origin and control of squeezing phenomena. Finally, we discuss the potential applications of these squeezing phenomena. Squeezing has significant implications in the fields of quantum sensing and quantum information processing. By gaining a deeper understanding of the squeezing evolution process induced by frequency jumping, we can better harness the control of microscopic particle behavior by external fields. This knowledge opens up new possibilities for future physics research and technological applications.
, , Received Date: 2023-09-04
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MXene materials have received increasing attention due to their unique properties and potential applications. Ti2CO2, as a typical MXene material that has been prepared, has been widely studied. The adsorption characteristics of two-dimensional materials for gas molecules can be significantly improved through transition metal modification. However, there are few studies on the use of transition metals to modify Ti2CO2. In this work, the adsorption processes of different harmful gases (CO, NH3, NO, SO2, CH4, H2S) on the surfaces of these two materials, i.e. Ti2CO2 and metal Sc modified Ti2CO2, are studied and analyzed based on first-principles density functional theory and generalized gradient method. The geometric optimization calculation of the metal-modified adsorption harmful gas structure is carried out, and the kinetic energy cutoff energy of the plane wave basis set is taken as 450 eV. The calculation results show that the structure in which Sc atoms are located above the C atoms in the hollow position has a large binding energy, but it is smaller than the experimental value of the cohesive energy of solid Sc (3.90 eV). Sc atoms can effectively avoid clustering. Surface Sc metal provides active sites for gas adsorption. By analyzing the optimal adsorption points, adsorption energy and other parameters of different gases, the adsorption effects of metal Sc-modified Ti2CO2 on these gases are analyzed. Among them, the adsorption effect of SO2 is better, the adsorption energy is increased from -0.314 eV to -2.043 eV, and the adsorption effects of other gases are improved. Due to the introduction of new atoms on the surface of Ti2CO2, the carrier density and carrier mobility of the material are increased, thereby improving the charge transfer on the surface of the material, which is beneficial to its sensitivity to gas molecules. The results of density of states and work function further verify that the carrier density and carrier mobility of Sc-Ti2CO2 are increased, which is beneficial to gas adsorption. It is expected that the metal Sc-modified Ti2CO2 becomes an excellent gas-sensing material for the detection of CO, NH3, NO, SO2, CH4 and H2S, and the present work can provide a reference for theoretically studying the gas-sensing performance of metal Sc-modified Ti2CO2 materials.
, , Received Date: 2023-11-14
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Accurate description of the free energy landscape (FES) is the basis for understanding complex molecular systems, and for further realizing molecular design, manufacture and industrialization. Major challenges include multiple metastable states, which usually are separated by high potential barriers and are not linearly separable, and may exist at multiple levels of time and spatial scales. Consequently FES is not suitable for analytical analysis and brute force simulation. To address these challenges, many enhanced sampling methods have been developed. However, utility of them usually involves many empirical choices, which hinders research advancement, and also makes error control very unimportant. Although variational calculus has been widely applied and achieved great success in physics, engineering and statistics, its application in complex molecular systems has just begun with the development of neural networks. This brief review is to summarize the background, major developments, current limitations, and prospects of applying variation in this field. It is hoped to facilitate the AI algorithm development for complex molecular systems in general, and to promote the further methodological development in this line of research in particular.
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