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Nonadiabatic molecular dynamics under adiabatic representation
Sun Zhen, Lü Xiang, Li Sheng, An Zhong
2024, 73 (14): 140201.
Abstract +
In this paper, we develop a nonadiabatic molecular dynamics method based on Su-Schriffer-Heeger (SSH) Hamiltonian, and this method is widely used to study the photoexcitation dynamics and polaron motion in conjugated polymers. However, in this method, the time-dependent Schrödinger equation has so far been solved in a diabatic representation, also known as site representation. In order to provide a deeper insight into the nonadiabatic molecular dynamics method, we solve the time-dependent Schrödinger equation in an adiabatic representation. The new method can directly provide the important information about the strength of nonadiabatic couplings between different molecular orbitals in the excited-state relaxation process, helping us to predict the electron and energy transfer within or between polymer chains.Solving the time-dependent Schrödinger equation in an adiabatic representation is much more complicated, it is mainly because we need to calculate the nonadiabatic couplings between different molecular orbitals. In this paper, the detailed formula derivation and actual calculation process of the nonadiabatic molecular dynamics method in an adiabatic representation are given. Using this new method, we simulate three photoexcitation processes in a conjugated polymer chain, HOMO→LUMO, HOMO–1→LUMO+1 and HOMO–2→LUMO+2. We analyze in detail the time evolutions of lattice configuration for these three photoexcitation processes, and compare these results with those obtained by diabatic representation (site representation) showing that the results obtained from these two representations are consistent with each other.
Computational reconstruction on-chip spectrometer based on reconfigurable silicon photonic filters
Zhang Zan, Huang Bei-Ju, Chen Hong-Da
2024, 73 (14): 140701.
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Spectroscopic analysis technique is an indispensable tool in many disciplines such as biomedical research, materials science, and remote sensing. Traditional benchtop spectrometers have several drawbacks; bulky, complex, and expensive, making them ineffective for emerging applications such as wearable health monitoring and Lab-on-Chip systems. Compared with bulky desktop spectrometers, integrated chip-level spectrometers find many applications in portable health monitoring, environmental sensing, and other scenarios. We design an on-chip spectrometer based on a silicon photonics platform. The device consists of a silicon photonic filter with a reconfigurable transmission spectrum.By changing the transmission spectrum of the filter, the multiple and diverse sampling of the input spectrum can be obtained. Using an artificial neural network algorithm, the incident spectrum is reconstructed from the sampled signals. The reconfigurable silicon photonic filter is composed of intercoupled Mach-Zehnder interferometer and micro-ring resonator. The introduction of thermal-optic phase shifter facilitates the reconstruction of the transmission spectrum of filter. Through this approach, a response function encompassing diverse features of broad and narrow spectra can be obtained from a single reconfigurable filter, eliminating the need for a filter array and significantly reducing the footprint of the spectrometer. Simulation results demonstrate that the designed device can achieve continuous and sparse spectrum reconstruction in a wavelength range of 1500–1600 nm, with a resolution of approximately 0.2 nm. On a test set composed of synthetic spectra, the calculated average RMSE for the reconstructed spectra is 0.0075, with an average relative error of 0.0174. Owing to the reconfigurable nature of the silicon photonic filter, this device exhibits the ability to flexibly adjust the number of sampling channels, thus enabling users to configure the chip according to specific application scenarios. This device possesses significant potential applications such as in wearable optical sensors and portable spectrometers.
K X-ray emission and kinetic energy-nuclear charge relationship of 252Cf spontaneous fission
Liu Chao, Liu Shi-Long, Yang Yi, Feng Jing, Li Yu-Zhao
2024, 73 (14): 142501.
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Experimental study of physical quantities after fission provides crucial insights into the fission process, which is an indispensable way to test the fission theory. The characteristics of primary fission products before beta decay are of great value in unraveling fission kinematics and nuclear energy applications. However, the measurement of the fragment charge has always been challenging. Multi-parameter studies related to nuclear charge remain relatively scarce. The deexcitation of the primary fission products may undergo internal conversion and is often accompanied by characteristic X-ray emissions. Therefore, the correlated measurement of fragment kinetic energy and K X-rays for 252Cf spontaneous fission is conducted. A silicon surface barrier detector is used to measure the fragment kinetic energy, while two low-energy high-pure germanium detectors are utilized for K X-ray measurement. Identification of fission fragments with Z = 39–62 is realized through characteristic K X-rays with a charge resolution of ΔZ ≈ 0.7. Fission fragment K X-ray yields exhibit a strong charge correlation, with an odd-even effect factor of about 13%. Based on K X-rays, the post-neutron-emission average kinetic energy, average total kinetic energy $(\langle \rm TKE\rangle) $, and its dispersion ($ {\sigma }_{{\mathrm{T}}{\mathrm{K}}{\mathrm{E}}} $) of fission fragments are determined each as a function of nuclear charge. The kinetic energy distribution of light fragments shows a pronounced odd-even effect, with even-Z elements exhibiting kinetic energy enhanced by about 0.48 MeV compared with odd-Z fragments. The peak of the $(\langle\rm TKE\rangle) $ distribution is nearly Z = 52–53, while the minimum of the $ {\sigma }_{{\mathrm{T}}{\mathrm{K}}{\mathrm{E}}} $ appears near Z = 56, indicating the significant influence of deformed shells in the highly asymmetric fission region. The post-neutron kinetic energy distribution of fission fragments from 252Cf (sf) is calculated by using the GEF model and CGMF model. The CGMF model effectively reproduces the overall trend of kinetic energy as a function of charge number, while the results of the GEF calculation are systematically higher than the experimental values. Nonetheless, these two phenomenological models make it difficult to quantitatively describe the kinetic energy distribution of fission fragments accurately. In this study, the insights into K X-ray emissions and kinetic energy-nuclear charge relationships provide valuable reference data for independently measuring the fission yields and verifying the theoretical models of fission.
First-principles study on the structure and stability of (H2dabco)[K(ClO4)3] under hydrostatic pressure
Li Qiao-Li, Li Shen-Shen, Xiao Ji-Jun, Chen Zhao-Xu
2024, 73 (14): 143101.
Abstract +
The crystal structure, molecular structure, electronic structure and mechanical properties of molecular perovskite high-energetic material (H2dabco)[K(ClO4)3] (DAP-2) under hydrostatic pressure ranging from 0 to 50 GPa are calculated and studied based on density functional theory. And the influences of pressure on its stability and impact sensitivity of DAP-2 are investigated. As the external pressure gradually increases, both the lattice parameters and the volume of DAP-2 crystal exhibit a monotonic decreasing trend. In the entire pressure range, the unit cell volume shrinks by up to 40.20%. By using the Birch Munnaghan equation of state to fit P-V relation, the bulk modulus B0 and its first-order derivative B0’ with respect to pressure are obtained to be 23.4 GPa and 4.9 GPa, respectively. The observations of the characteristic bond length and bond angle within the crystal indicate that the cage-like structure of organic cation H2dabco2+ undergoes distortion at 25 GPa. Further analysis of the average fractional coordinates of the center-of-mass and Euler angles for H2dabco2+ and KO12 polyhedron shows that within a pressure range from 0 to 50 GPa, both the average fractional coordinates of the center-of-mass and the Euler angles exhibit fluctuations at 25 GPa, but the overall amplitude of these fluctuations is very small. Based on this finding, it is speculated that the space group symmetry of the crystal may remain unchanged in the entire pressure range. In terms of electronic structure, with the increase of pressure, the band gap value increases rapidly and reaches a maximum value at about 20 GPa, followed by a slow decreasing trend. Based on the first-principles band gap criterion and the variation of the band gap under different pressures, it is demonstrated that below 20 GPa, the impact sensitivity of DAP-2 gradually decreases with pressure increasing; however, when the pressure exceeds 20 GPa, the impact sensitivity exhibits a slow increasing trend. In addition, the elastic constants Cij, Young’s modulus (E), bulk modulus (B), shear modulus (G), and Cauchy pressure (C12C44) all increase with pressure rising, indicating that the rigidity and ductility of the crystal under pressure are significantly strengthened. According to the mechanical stability criterion, the crystal maintains the mechanical stability throughout the pressure range.
Low-density plasmas generated by electron beams passing through silicon nitride window
Yan Shao-Qi, Gao Ji-Kun, Chen Yue, Ma Yao, Zhu Xiao-Dong
2024, 73 (14): 144102.
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In general, more attention is paid to how to improve the characteristic parameters of plasma in plasma applications. However, in some cases, it is necessary to produce plasma with low-electron density, such as in the laboratory simulation of ionospheric plasma in space science. In this study, a low-density plasma is generated by electron beams passing through a silicon nitride transmission window under low pressure condition. The transmission properties of electron beam passing through silicon nitride films are investigated by Monte Carlo simulation, and the plasma feature is studied by a planar Langmuir probe and a digital camera. It is found that the plasma exhibits a conical structure with its apex located at the transmission window. At a constant pressure, the cone angle of conical plasma decreases with the electron energy increasing. This is qualitatively consistent with the Monte Carlo simulation result. The frequency of electron-neutral collisions increases as the working pressure rising, which leads the plasma cone angle to increase. When the beam current is reduced from 10 μA to 0.5 μA at 40 keV, the electron density decreases, in a range between 105 and 106 cm–3, while the electron temperature does not change significantly but approaches 1 eV. It can be inferred that the electron density decreases with the distance z from the transmission window in the incident direction of the electron beam. A low-density plasma of less than 105 cm–3 can be obtained further away from the transmission window.
Using asynchronous optical sampling to measure timing jitter of electro-optic frequency combs
Ma Bo-Wen, Dai Wen, Meng Fei, Tao Jia-Ning, Wu Zi-Ling, Shi Yan-Qing, Fang Zhan-Jun, Hu Ming-Lie, Song You-Jian
2024, 73 (14): 144203.
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Electro-optic frequency combs (EOCs) are optical frequency combs constructed by phase modulation of single frequency lasers. The electro-optic modulated optical frequency combs have shown their unique advantages in many application fields due to their high repetition frequencies, high stabilities and other advantages, especially in precision measurement applications. Through accurate dispersion control, the electro-optical frequency combs can output ultra-short pulse laser sequences in the time domain, and their timing jitter characteristic is very important for precision measurement and other applications. This work presents a scheme to measure the timing jitter of the electro-optic combs directly in the time domain based on the principle of dual-comb asynchronous optical sampling method(ASOPS), which relies on temporal cross-correlation between the high repetition rate electro-optic combs and a low repetition rate passively mode-locked fiber laser. The ASOPS process allows timing jitter measurement in a magnified time scale where the timing jitter at a femtosecond level can be received and visualized by standard low speed electronics. We build a theoretical model for timing jitter measurement, conduct a numerical study to verify the model, and also construct an experimental system to characterize the period jitter of a 10-GHz electro-optic comb.Firstly, the theoretical model for measuring timing jitter is established. In this work, the basic theory of measuring the timing jitter is discussed by analyzing the histogram directly in time domain through using the obtained ASOPS signal. Subsequently, numerical simulations are conducted to simulate the ASOPS process after establishing a sequence of Gaussian pulse train with quantum limited timing jitter. Another pulse train without timing jitter serves as a local oscillator. Through the square law optical detection after sum-frequency generation between LO and LUT, the ASOPS process can be realized and periodic jitter can be obtained directly through histogram statistical analysis. The simulation result is consistent with the theoretical result very well. Finally, an EOC system with cascaded modulators at a repetition rate of 10 GHz is designed and built, and a timing jitter measurement system is designed and built with an all-fiber configuration. The period jitter of 10-GHz EOC is measured by using a 161-MHz mode-locked fiber laser as local oscillator. Histogram analysis shows that the period jitter of the EOC is 3.86 fs.This measurement technique does not require to use the intricate electrical phase-locked circuits or a high-speed photodetector to receive ultrashort pulses of EOC. Like the eye map analysis method commonly used in telecommunication, the histogram analysis can be used to determine the timing jitter approaching the quantum limit. This approach is easy to set up and operate, and it is anticipated to become a standard method of measuring period jitter of ultrashort pulse with high repetition frequency in a laboratory setting. It will be particularly useful for measuring timing jitters of the sources of novel high repetition rate optical frequency combs, such as micro-resonators and electro-optic frequency combs.
Analysis of GAAFET’s transient heat transport process based on phonon hydrodynamic equations
Liu Zhe, Wei Hao, Cui Hai-Hang, Sun Kai, Sun Bo-Hua
2024, 73 (14): 144401.
Abstract +
Compared to the classical Fourier’s law, the phonon hydrodynamic model has demonstrated significant advantages in describing ultrafast phonon heat transport at the nanoscale. The gate-all-around field-effect transistor (GAAFET) greatly optimizes its electrical performance through its three-dimensional channel design, but its nanoscale characteristics also lead to challenges such as self-heating and localized overheating. Therefore, it is of great significance to study the internal heat transport mechanism of GAAFET devices to obtain the thermal process and heat distribution characteristics. Based on this, this paper conducts theoretical and numerical simulation analyses on the phonon heat transfer characteristics within nanoscale GAAFET devices. Firstly, based on the phonon Boltzmann equation, the phonon hydrodynamic model and boundary conditions are rigorously derived, establishing a numerical solution method based on finite elements. For the novel GAAFET devices, the effects of factors such as surface roughness, channel length, channel radius, gate dielectric, and interface thermal resistance on their heat transfer characteristics are analyzed. The research results indicate that the larger the surface roughness, the smaller the channel length and the channel radius, the larger the interface thermal resistance leads to the higher hot spot peak temperature. The non-Fourier heat analysis method based on the phonon hydrodynamic model and temperature jump condition within the continuous medium framework constructed in this paper can accurately predict the non-Fourier phonon heat conduction process inside GAAFET and reveal the mechanisms of resistive scattering and phonon/interface scattering. This work provides important theoretical support for further optimizing the thermal reliability design of GAAFET, improving its thermal stability, and operational performance.
Spectral regulation in thermophotovoltaic devices
Xiong Jia-Cheng, Huang Zhe-Qun, Zhang Heng, Wang Qi-Xiang, Cui Ke-Hang
2024, 73 (14): 144402.
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Thermophotovoltaic (TPV) device converts thermal radiation into electricity output through photovoltaic effect. High-efficiency TPV devices have extensive applications in grid-scale thermal storage, full-spectrum solar utilization, distributed thermal-electricity cogeneration, and waste heat recovery. The key to high-efficiency TPV devices lies in spectral regulation to achieve band-matching between thermal radiation of the emitters and electron transition of the photovoltaic cells. The latest advances in nanophotonics, materials science, and artificial intelligence have made milestone progress in spectral regulation and recording power conversion efficiency of up to 40% of TPV devices. Here we systematically review spectral regulation in TPV devices at the emitter end as well as the photovoltaic cell end. At the emitter end, spectral regulation is realized through thermal metamaterials and rare-earth intrinsic emitters to selectively enhance the in-band radiation and suppress the sub-bandgap radiation. At the photovoltaic cell end, spectral regulation mainly focuses on recycling the sub-bandgap thermal radiation through optical filters and back surface reflectors located at the front and back of the photovoltaic cells, respectively. We emphasize the light-matter interaction mechanisms and material systems of different spectral regulation strategies. We also discuss the spectral regulation strategies in near-field TPV devices. Finally, we look forward to potential development paths and prospects of spectral regulation to achieve scalable deployment of future TPV devices.
Motion of a circular particle in the power-law lid-driven cavity flow
Yang Xiao-Feng, Liu Jiao, Shan Fang, Chai Zhen-Hua, Shi Bao-Chang
2024, 73 (14): 144701.
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In this paper, the motion of a circular particle in a lid-driven square cavity with the power-law fluid is studied by using the diffuse interface lattice Boltzmann method, and the study mainly considers the effects of the particle's initial position, the power-law index, the Reynolds number, and the particle size. The numerical results show that the circular particle is first in a centrifugal motion under the effect of inertia, and it finally moves steadily on the limit cycle. Furthermore, it is also found that the initial position of the particle has no influence on the limit cycle. For a shear-thinning fluid flow, the limit cycle moves towards the bottom right corner of the square cavity. Moreover, the particle velocity is small, and the period of the particle motion is long. On the other hand, in the case of shear-thickening fluid flow, the limit cycle moves towards the top left corner of the cavity. In addition, the particle velocity is large, and the period of the particle motion is short.With the increase of Reynolds number, the limit cycle moves towards the bottom right corner of the square cavity, which is caused by a strong fluid flow field. Meanwhile, the particle velocity becomes larger, and the period of the particle motion is shorter. With the increase of particle size, the effect of confinement of the cavity boundary becomes significant, and the circular particle is pushed towards the center of the cavity. In this case, the limit cycle shrinks towards the center of the cavity. The circular particle squeezes the secondary vortices, especially when the circular particle is located in the bottom left, bottom right and top left corners. Additionally, the appearance of the circular particle has a significant influence on the position of the primary vortex, which changes periodically near the position of the primary vortex without the particle. It is also observed that the influence of the circular particle becomes more significant as its size increases and the power-law index decreases.
Structural phase transition induced enhancement of carrier mobility of monolayer RuSe2
Lu Kang-Jun, Wang Yi-Fan, Xia Qian, Zhang Gui-Tao, Chen Qian
2024, 73 (14): 146302.
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Transition metal dichalcogenides (TMDs) is an important member of two-dimensional material family, which has various crystal structures and physical properties, thus providing a broad platform for scientific research and device applications. The diversity of TMD's properties arises not only from their relatively large family but also from the variety of their crystal structure phases. The most common structure of TMD is the trigonal prismatic phase (H phase) and the octahedral phase (T phase). Studies have shown that, in addition to these two high-symmetry phases, TMD has other distorted phases. Distorted phase often exhibits different physical properties from symmetric phases and can perform better in certain systems. Because the structural differences between different distorted phases are sometimes very small, it is experimentally challenging to observe multiple distorted phases coexisting. Therefore, it is meaningful to theoretically investigate the structural stability and physical properties of different distorted phases. In this study, we investigate the structure and phase transition of monolayer RuSe2 through first-principles calculation. While confirming that its ground state is a the dimerized phase ($T^\prime$ phase), we find the presence of another energetically competitive trimerized phase ($T^{\prime\prime\prime}$ phase). By comparing the energy values of four different structures and combining the results of phonon spectra and molecular dynamics simulations, we predict the stability of the $T^{\prime\prime\prime}$ phase at room temperature. Because the H phase and T phase of two-dimensional RuSe2 have already been observed experimentally, and considering the fact that $T^{\prime\prime\prime}$ phase has much lower energy than the H and T phases, it is highly likely that the $T^{\prime\prime\prime}$ phase exists in experiment. Combining the calculations of the phase transition barrier and the molecular dynamics simulations, we anticipate that applying a slight stress to the $T^\prime$ phase structure at room temperature can induce a lattice transition from $T^\prime$phase to $T^{\prime\prime\prime}$ phase, resulting in significant changes in the band structure and carrier mobility, with the bandgap changing from an indirect bandgap of 1.11 eV to a direct bandgap of 0.71 eV, and the carrier mobility in the armchair direction increasing from $ 0.82 \times 10^3 \, {\rm cm}^{2}{\cdot}{\rm V}^{-1}{\cdot}{\rm s}^{-1}$ to $3.22 \times 10^3 \, {\rm cm}^{2}{\cdot}{\rm V}^{-1}{\cdot}{\rm s}^{-1}$, an approximately threefold enhancement. In this work, two possible coexisting distorted phases in monolayer RuSe2 are compared with each other and studied, and their electronic structures and carrier mobilities are analyzed, thereby facilitating experimental research on two-dimensional RuSe2 materials and their applications in future electronic devices.
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