bluuebunny/crossref_metadata_2025 huggingface dataset:
Model of bit error rate for laser communication based on superconducting nanowire single photon detector
Authors: Yan Xia-Chao  , Zhu Jiang  , Zhang La-Bao  , Xing Qiang-Lin  , Chen Ya-Jun  , Zhu Hong-Quan  , Li Jian-Ting  , Kang Lin  , Chen Jian  , Wu Pei-Heng  ,   Research Institute of Superconductor Electronics, Nanjing University, Nanjing 210093, China;,   Beijing Institute of Tracking and Telecommunications Technology, Beijing 100094, China}
Published: December 2020

The high-speed deep space communication is one of the key technologies for deep space exploration. Laser communication system equipped with sensitivity of single photon will improve existing deep space communication speed. However, laser communication at single photon level needs to consider not only the effect of transmission environment, but also the performance of used single photon detector and the photon number distribution. As a new single photon detector, superconducting nanowire single photon detector (SNSPD) outperforms the traditional semiconducting SPDs at near infrared wavelengths, and has high detection efficiency, low dark count rate, low timing jitter, high counting rate, etc. The SNSPD can be used for detecting single photons efficiently, rapidly and accurately. In this paper, we introduce the system detection efficiency and dark count rate of SNSPD based on the photoelectric detecting model without considering the effect of atmospheric turbulence, establish the mathematical model of bit error, and put forward the formula of system bit error rate. What should be emphasized is that the bit error rate is an important parameter for measuring the performance of laser communication system. Error is partly from background thermal radiation and circuit electromagnetic interference; in addition, error appears when photons reach the surface of device without being absorbed to successfully produce resistance area or photons are absorbed but there occurs no response. As a result, the calculation of bit error rate includes the whole process of photoelectric conversion. In order to analyze how to affect the size of system bit error rate, first we simulate two factors of the formula, i.e., light intensity and laser pulse repetition frequency. The results show that the light intensity has the greatest influence on error bit rate. With the light intensity increasing from 0.01 to 1000 photon/pulse, the error bit rate significantly decreases from 10-1 to 10-7 level. The influence of laser pulse repetition frequency is restricted by the light intensity, which declines with the increase of pulse repetition frequency. Then we measure the error bit rate experimentally, which validates the simulation model. However, when increasing light intensity or speed, experimental bit error rate is about 10-4 times higher than simulation result. The reason may be that the insufficiency of actual communication modulation extinction ratio of optical signal to the background noise through optical fiber increases the dark count rate. The above model and experimental results could be the foundation of high-speed deep space laser communication such as moon-earth and Mars-earth based on SNSPD.
Effects of dynamic synapses, neuronal coupling, and time delay on firing of neuron
Authors: Yu Wen-Ting  , Zhang Juan  , Tang Jun  ,   School of Physics, China University of Mining and Technology, Xuzhou 221116, China
Published: December 2020

Neuronal firing plays a key role in the neuronal information transmission, and different neuronal firing patterns are reported, such as spiking, bursting. A number of neuron models are introduced to reproduce the firing patterns of single neuron or neuronal network. The key factors determining the firing pattern gain more and more attention in the study of neuron system, such as noise, network topology. Noise is able to induce sub-or super-threshold coherent neuronal firing easily, and a number of coherence resonances are reported in the noise induced firing. The network topology determines the synchronization of the firing patterns of the neuronal network, and the change of network topology may induce fruitful synchronization transitions. It is well known that synapses exhibit a high variability with a diverse origin during information transmission, such as the stochastic release of neurotransmitters, variations in chemical concentration through synapses, and spatial heterogeneity of synaptic response over dendrite tree. The collective effect of all of these factors might result in the notion of dynamic synapses. In reality, the neuronal network often involves time delay due to the ?nite signal propagation time in biological networks. Recently, neuronal networks with time delay have received considerable attention. Delay-sustained neuronal firing patterns may be relevant to neuronal networks for establishing a concept of collective information processing in the presence of delayed information transmission. According to the above-mentioned motivations, the firing dynamics of the single postsynapic neuron is investigated based on a simple postsynaptic neuron model by using numerical simulation and Fourier transform analysis. In this model, the postsynapic neuron receives dynamic synaptic currents from a population of presynaptic neurons. It is found that the firing rate resonance between the pre-and postsynaptic neuron determines the firing of the postsynaptic neuron. Stimulus currents in specific frequency range are easy to stimulate postsynaptic neuron firing. The random currents released from dynamic synapses determine the postsynaptic firing rate. Then the single postsynaptic neuron is extended to a neuronal network, in which 100 neurons connect to its 4 nearest neighbors regularly and receive delayed synaptic currents from connected neurons. All the neurons in the network receive the same dynamic synaptic currents from the presynaptic neurons. The results show that the synaptic coupling in the network is able to promote the neuron firing in the network, and time delay in the synaptic coupling could reinforce the promotion, but the mode of the promotion is not changed.
A cellular automaton model for electrocardiogram considering the structure of heart
Authors: Zhang Xue-Liang  , Tan Hui-Li  , Tang Guo-Ning  , Deng Min-Yi  ,   College of Physical Science and Technology, Guangxi Normal University, Guilin 541004, China
Published: December 2020

The electrocardiogram (ECG) has broad applications in clinical diagnosis and prognosis of cardiovascular diseases. The accurate description for the question how the ECG come from the cardiac electrical activity is helpful for understanding the corresponding relation between the ECG waveform and cardiovascular disease. Experience is the primary method of studying the ECG, but the computer simulation method makes it more convenient to explore the effect of given factor for ECG waveform. Cellular automaton is a simple and effective computer simulation method. However, the cellular automaton model considering the main structure of the heart is not yet established. Therefore, we propose a cellular automaton model for the ECG considering the atria, the ventricle, and the ventricular septum. With this model, the conduction of the myocardial electrical activation is simulated by following the field potentials under healthy and diseased conditions, and the underlying mechanisms are analyzed. Through the computer simulations and analyses the results are obtained as follows. First, the conduction process of the electrical signal in this model is the same as that in the real heart. Second, under the healthy conditions, the behavior of the field potential appears as normal ECG, in which the P wave and the QRS wave group come from the depolarization of the atria and ventricle, respectively, on the other hand, the T wave and J wave come from the repolarization of the ventricle. The computer results support the conclusion that the J wave appears just because the existence of the notch in the epicardial transmembrane potential curve. Third, the endocardium ischemia conditions result in the T wave inversion. The mechanism is that the action potential duration of the ischemic endocardial cells is shorter than that under normal conditions, which makes larger the transmembrane potential gradient between the endocardium and the subepicardium, and then contributes a more negative value to the field potential. Fourth, the epicardium ischemia leads to the higher T wave, and this is because the shorter action potential duration of the ischemic epicardial cells brings in a larger transmembrane potential gradient between the epicardium and subepicardium, which makes the field voltage larger. Fifth, the T wave appears earlier under the through-wall ischemia. The action potential durations of cells of the endocardium, the epicardium, and the subepicardium all become shorter under the through-wall ischemia, then the repolarization processes of all of these three walls are ended earlier, which leads to the earlier T wave. The cellular automaton model proposed in this paper provides a reference for the further study of ECG.
High-precision long-haul fiber-optic time transfer between multi stations
Authors: Chen Fa-Xi  , Zhao Kan  , Zhou Xu  , Liu Tao  , Zhang Shou-Gang  ,   National Time Service Centre, Chinese Academy of Sciences, Xi'an 710600, China;,   University of Chinese Academy of Sciences, Beijing 100039, China;,   Key Laboratory of Time and Frequency Standards, Chinese Academy of Sciences, Xi'an 710600, China
Published: December 2020

To achieve high-precision fiber-optic time transfer, the method of two-way transmission is usually used. Therefore in this paper we propose to develop a high-precision long-haul fiber-optic time transfer between multi stations by simultaneously transferring the 1 pluse per second signal, time code signal and 10 MHz frequency signal over single fiber with the same wavelength, and adopting the time division multi address (TDMA) as well as the purification and regeneration method at individual station. In this proposal, the equipment at each remote station has its own address, and the equipment at the local station can establish the periodic two-way time transfer with any remote station by using the TDMA method, therefore each remote station is synchronized with the local station. To avoid the superimposed effect of optical noises during propagation in fiber, the optical-electro-optical relay amplifiers are utilized. In the meantime the propagation delay of the fiber link is compensated for at each remote station. With the self-developed engineering prototypes, the experimental verifications are subsequently conducted both in laboratory and real field. In the laboratory, the experimental setup is built by cascading 11 rolls of 50 km-long fiber coils, and locating three monitoring devices at different fiber distances of 50, 300, and 550 km from the local station. The stabilities of the time transfer at these three points are achieved to be 16.7, 16.8, and 18.4 ps in standard deviation, and the time deviations are 1.78, 2.09, and 2.92 ps at an averaging time of 1000 s respectively. In the real field test, a field fiber link of 871.6 km in length is utilized, along which 11 self-developed time-frequency transceivers are set at the cascaded fiber-optic stations. Since only the 11th remote station is co-located at the local station, the performance and the time transfer between the 11th remote station and the local station are measured accurately. The time transfer is experimentally demonstrated with the time standard deviation of 29.8 ps and the time deviations of 3.85 ps/1000 s. The timing uncertainty on the field fiber link is also checked and gives a value of 25.4 ps. To further improve the long-term stability of time transfer, the more accurate thermal control of the lasers used in the system should be adopted to reduce the optical wavelength drift. By compressing the bandwidth of the phase locked loop module in each remote device, the short-term stability of time synchronization can also be better. This proposal can also be extended to the fiber networks with star-shaped and chain-shaped connections. Therefore time synchronization in even larger areas and more stations can be realized.
Theoretical studies of triple differential cross sections for electron impact ionization with neon and neon-like ions
Authors: Zhou Li-Xia  , Zhang Yan  , Yan You-Guo  ,   College of Physics Science and Technology, China University of Petroleum, Qingdao 266580, China
Published: December 2020

Electron impact single ionization of atom or molecule, the so-called (e, 2e) reaction, is one of the basic collision processes between electron and atom or molecule. The triple differential cross section (TDCS) of the collision process can provide important data for gas discharge, celestial bodies, and electron-target interaction.
A large number of experimental and theoretical studies of (e, 2e) reactions on atom targets have been carried out under different geometric conditions, such as coplanar symmetric geometry, coplanar asymmetric geometry, non-coplanar symmetric geometry, etc. However, few experimental researches of (e, 2e) reaction on ion target have been reported due to the low target source density. The difference in TDCS between atomic target and ionic target can provide more information about the (e, 2e) reaction. Thus the relevant researches on ionic targets are of significance.
In this paper, adopting distorted-wave Born approximation (DWBA), the TDCSs of 2 p orbital for Ne and neon-like ions are calculated at different outgoing electron energies (3, 5, 7.5, 10, 15, 20, 30, and 50 eV) under the condition of coplanar symmetric geometry. The results indicate that the TDCSs decrease with the increase of outgoing electron energy and nuclear charge number Z. Except Ne, the TDCSs of other ions present a new structure at an outgoing electron angle of about 150. The intensity of the new structure increases with the increase of the outgoing electron energy in a region of 10-20 eV, while it decreases with the increase of the outgoing electron energy in a region of 20-50 eV.
We propose a kind of double-binary collision process to rationalize the new structure. The incident electron ionizes the target atom and the following two outgoing electrons exit in the directions symmetric with respect to the incident electron direction. Then these two outgoing electrons are elastically scattered by the target ions and emitted in the backward directions. In order to confirm this explanation, we compare our calculation results with the previously reported experimental and theoretical results of elastic scattering between electron and Ne. Previous research results show that the elastic scattering cross section has a large intensity at a scattering angle of~150, and it reaches the largest intensity at an outgoing electron energy of 20 eV. These structural features are consistent with our calculated results, implying that our proposed process is reasonable.
Sheared-beam imaging of object with depth information
Authors: Lan Fu-Yang  , Luo Xiu-Juan  , Chen Ming-Lai  , Zhang Yu  , Liu Hui  ,   Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Sheared-beam imaging technique is a non-conventional imaging method which can be used to image remote objects through atmospheric turbulence without needing any adaptive optics. In this imaging technique, the target is coherently illuminated by three laser beams which are laterally sheared at the transmitter plane and arranged into an L shape. In addition, each beam is modulated by a slight frequency shift. The speckle intensity signals scattered from the target are received by a detector array, and then the image of target can be reconstructed by computer algorithm. By far, most of studies in this field have focused on two-dimensional imaging. In real conditions, however, the surface of targets we are concerned about reveals that different depths introduce various phase delays in the scattering signal from target. This delay causes the phase-shift errors to appear between the ideal target Fourier spectrum and the Fourier spectrum received by detector array. Finally, this would result in poor image quality and low resolution. In this study, a three-dimensional target imaging model is established based on the two-dimensional target imaging model. The influence of modulated beat frequency between sheared beam and reference beam is studied on the objects with depth information, and the result shows that large beat frequency may have an adverse effect on reconstructed images. The simulation we have developed for this three-dimensional imaging model uses three targets with different shapes. Each target is divided into several sub-blocks, and we set different depth values (within 10 m) for these blocks. Then beat frequencies are increased from 5 Hz to about 1 MHz, respectively. At each pair of frequencies, the reconstructed image is recorded. Srehl ratio is used as the measure of the imaging quality. Computer simulation results show that the Srehl ratio of reconstructed images descends with the increase of beat frequency, which is fully consistent with the theory of three-dimensional target imaging proposed before. Meanwhile, we find that the depth distribution of target also has an effect on imaging quality. As for actual space targets, the maximum depth is usually not more than 10 m. Compared with the influence caused by beat frequencies, the effect produced by depth distribution is negligible. Therefore when a space target is imaged, beat frequencies play the major role in reconstructing high-quality image. The results presented in this paper indicate that in order to achieve better imaging quality in the practical application, it is necessary to select the smallest beat frequency according to the detector performance and keep the candidate frequencies away from the low-frequency noise of the detector.
Comparison between the 1st and 3rd order mode temporal characteristics of two-sided multipactor discharge in cavity
Authors: Dong Ye  , Liu Qing-Xiang  , Pang Jian  , Zhou Hai-Jing  , Dong Zhi-Wei  ,   School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100094, China;,   Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

For investigating the influence of high order two-sided multipactor discharge on the accelerator field-building process, the temporal characteristics of the 3rd order two-sided multipactor discharge in oxygenfree copper cavity are studied numerically. The particle-in-cell and Monte-Carlo methods are used in the simulation and the characteristics of the 1st order mode are also studied for comparison. The numerical results can be concluded as follows. In the multipactor discharge evolution, the electron number, discharge current, deposited and discharge power increase exponentially and tend to be saturated. At the saturation stage of the 3rd order mode, the values of electron number, discharge current, deposited and discharge power are lower than at the saturation stage of the 1st order mode. Meanwhile, the rising time of waveform in the 3rd order mode is longer than in the 1st order mode. There is a time-delay phenomenon in the waveform of discharge current, which results in a partial charging process in multipactor discharge. The average value of the discharge power is equal to the average deposited power. The value of discharge power in the 3rd order mode is about 1% of that in the 1st order mode. Therefore, the 3rd order mode is not significant in accelerator field-building process compared with the 1st order mode. The characteristic of the 1st order two-sided multipactor discharge is the accelerated motion of single electron beam, while that of the 3rd order is the complex accelerated-decelerated-accelerated motion of multi-electron beams. When the multipactor discharge enters into the saturation stage, the space charge effect of the 3rd order mode is not stronger than that of 1st order mode.
Characteristics of H-terminated single crystalline diamond field effect transistors
Authors: Ren Ze-Yang  , Zhang Jin-Feng  , Zhang Jin-Cheng  , Xu Sheng-Rui  , Zhang Chun-Fu  , Quan Ru-Dai  , Hao Yue  ,   State Key Discipline Laboratory of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi'an 710071, China
Published: December 2020

Diamond has been considered as an ultimate semiconductor, which has great potential applications in high power, high frequency semiconductor devices. Up to now, the twodimensional hole gas (2DHG) induced on the hydrogenterminated diamond surface is used most popularly to form electric conduction in diamond semiconductor at room temperature, due to the obstacle caused by lacking of easily-ionized dopants. A 200-nm-thick single crystalline diamond is grown by microwave plasma chemical vapor deposition on the type-Ib high-pressure high-temperature synthesized diamond substrate. Then the sample is treated in hydrogen plasma atmosphere to achieve hydrogen terminated diamond surface. The sample is characterized by X-ray photoelectron spectroscopy and atomic force microscope. After that, the normally-on hydrogen-terminated diamond field effect transistors are achieved. The device with a gate length of 2 μup m delivers a saturation leakage current of 96 mA/mm at gate voltage VGS=-6 V, at which, however, the gate leakage current is too large. The saturation current reaches 77 mA/mm at VGS=-3.5 V with safety. The device shows typical long-channel behavior. The gate voltage varies almost linearly. In the saturation region of the device, the transconductance (gm) increases near-linearly to 30 mS/mm with the increase of the gate voltage in a range of 5.9 V. Analyses of the on-resistance and capacitance-voltage (C-V) data show that the 2DHG under the gate achieves a density as high as 1.99×1013 cm-2, and the extracted channel carrier density and mobility are always kept increasing with VGS negatively shifting to -2.5 V. The nearlinearly increasing of gm in a large VGS range is attributed to high 2DHG density, quite a large gate capacitance (good gate control), and increased mobility. The relevant researches of improving the carrier mobility in the channel and of finding proper gate dielectrics to improve the forward gate breakdown voltage are underway.
Theoretical research on an efficient population transfer based on two different laser pulse sequences
Authors: Zhang Lu  , Yan Lu-Yao  , Bao Hui-Han  , Chai Xiao-Qian  , Ma Dan-Dan  , Wu Qian-Nan  , Xia Ling-Chen  , Yao Dan  , Qian Jing  ,   Quantum Institute for Light and Atoms, Department of Physics and Material Science, East China Normal University, Shanghai 200062, China
Published: December 2020

A quantum gas of ultracold molecules, with long-range and anisotropic interactions, will enable a series of fundamental studies in physics and chemistry. In particular, samples of ground-state molecules at ultralow temperatures and high number densities will facilitate the explorations of a large number of many-body physical phenomena and applications in quantum information processing. However, due to the lack of efficiently cooling techniques such as laser cooling for atomic gases, high number densities for ultracold molecular samples are not readily attainable. Associating ultracold atoms to weakly bound dimer molecules via Feshbach resonance and subsequently transferring them to a wanted molecular ro-vibronic ground state by a stimulated Raman adiabatic passages (STIRAP) have proved to be an effective way in producing ideal ultracold molecular samples. As a typical illustration, in a recent study (2010 Nat. Phys. 6 265) Danzl et al. experimentally realized the preparation of Cs2 molecule into its ro-vibronic ground state via two different multi-level STIRAPs:one is based on a single conversion route and the others are based on a cascade-connected route (labeled by 4p-STIRAP and s-STIRAP, respectively). In this work, we present a theoretical study for these two STIRAP schemes, focusing on the differences in physical principle and realistic performance between them. On the one hand, according to the theoretical approach of quasi-dark eigenstates, we conclude that a highly efficient population transfer is achievable in both schemes. On the other hand, by systematically studying the influences of the relevant parameters, including the spontaneous decays and the detunings from the intermediate states, and the temporal sequence and the amplitude of the laser pulses, we disclose their respective advantages and weaknesses in the realistic implementation. We theoretically predict that for both schemes their maximal conversion efficiencies each can attain 0.97 as long as the spontaneous decays from the intermediate excited states are sufficiently suppressed. Yet considering the fact that the already implemented efficiency is only around 0.6 for both schemes, there is still room for optimization, e.g. using stable Rydberg energy levels in future experiment. Furthermore, the success of these two schemes can provide a new route to the controllable entanglement preparation, opening more applications in the fields of quantum logic gate and so on.
One-dimensional numerical investigation on the formation of Z-pinch dynamic hohlraum using the code MULTI
Authors: Wu Fu-Yuan  , Chu Yan-Yun  , Ye Fan  , Li Zheng-Hong  , Yang Jian-Lun  , Rafael Ramis  , Wang Zhen  , Qi Jian-Min  , Zhou Lin  , Liang Chuan  ,   Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China;,   E. T. S. I. Aeronautica y del Espacio, Universidad Politecnica de Madrid, Madrid 28040, Spain
Published: December 2020

Z-pinch dynamic hohlraum can effectively convert Z-pinch plasma kinetic energy into radiation field energy, which has a potential to implode a pellet filled with deuterium-tritium fuel to fusion conditions when the drive current is sufficiently large. To understand the formation process of Z-pinch dynamic hohlraum on JULONG-I facility with a typical drive current of 8-10 MA, a new radiation magneto-hydrodynamics code is developed based on the program MULTI-IFE. MULTI-IFE is a one-dimensional, two-temperature, multi-group, open-source radiation hydrodynamic code, which is initially designed for laser and heavy ion driven fusion. The original program is upgraded to simulate Z-pinch related experiments by introducing Lorentz force, Joule heating and the evolution of magnetic field into the code. Numerical results suggest that a shock wave and a thermal wave will be launched when the high speed plasma impacts onto the foam converter. The thermal wave propagates much faster than shock wave, making the foam become hot prior to the arrival of shock wave. For the load parameters and drive current of shot 0180, the calculated propagation speed of thermal wave and shock wave are about 36.1 cm/s and 17.6 cm/s, respectively. The shock wave will be reflected when it arrives at the foam center and the speed of reflected shock wave is about 12.9 cm/s. Calculations also indicate that the plastic foam will expand obviously due to the high temperature radiation environment (~30 eV) around it before the collision between tungsten plasma and foam converter. The evolution of radial radiation temperature profile shows that a pair of bright strips pointing to the foam center can be observed by an on-axis streak camera and the radiation temperature in the foam center achieves its highest value when the shock arrives at the axis. A bright emission ring moving towards the foam center can also be observed by an on-axis X-ray frame camera. The best time to capture the bright strips and bright emission rings is before the thermal wave reaches the foam center. Even though some amount of X-ray radiation in the foam is expected to escape from the hohlraum via radiation transport process, simulation results suggest that the tungsten plasma can serve as a good hohlraum wall. The radiation temperature is about 80 eV when the dynamic hohlraum is created and can rise more than 100 eV before the shock arrives at the foam center. Most of the X-rays emitted by the wire-array plasma surface have energies below 1000 eV. In this paper, the physical model of the code MULTI-IFE and the simulation results of array implosions on Saturn facility are presented as well.
Phase-field crystal modelling the nucleation processes of graphene structures on different substrates
Authors: Gu Ji-Wei  , Wang Jin-Cheng  , Wang Zhi-Jun  , Li Jun-Jie  , Guo Can  , Tang Sai  ,   State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China;,   Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany
Published: December 2020

Two-dimensional materials with unique and excellent physical and chemical properties have attracted much attention in recent years. Among the two-dimensional materials, graphene or grapheme-like materials with honeycomb structure can be mainly prepared by the chemical vapor deposition (CVD) method. The key of this method is to select the substrates and control the nucleation and growth process of honeycomb structures. Graphene prepared by CVD contains many structure defects and grain boundaries, which mainly arise from nucleation process. However, the nucleation mechanism of graphene prepared by CVD method is not very clear. In addition, more than ten kinds of metal substrates can be used as substrate materials in CVD methods, such as Cu and Ni, which have nearly always face-centered cubic (FCC) structures and similar functions in the preparation process. In order to better describe the nucleation of graphene and understand the influences of metal substrates, we introduce the structural order parameter into the three-mode phase-field crystal model to distinguish the low-density gas phase from condensed phases. Nucleation processes of graphene on substrates with different symmetries are studied at an atomic scale by using the three-mode phase-field crystal model, which can simulate transitions between highly correlated condensed phases and low-density vapor phases. Simulation results indicate that no matter whether there is a substrate in the nucleation process, firstly gaseous atoms gather to form amorphous transitional clusters, and then amorphous transitional clusters gradually transform into ordered graphene crystals, with continuous accumulation of new gaseous atoms and position adjustment of atoms. In the nucleation process, five membered ring structures act as a transitional function. When grown on the substrate with a good geometric match with the honeycomb lattice, such as (111) plane of FCC metals, the graphene island has small structural defects. However, when grown without a substrate or on the substrate with a bad geometric match, such as (100) plane of FCC metals, the graphene island contains many structural defects and grain boundaries, which are not conducive to the preparation of high quality graphene. Compared with the (100) crystal plane of the tetragonal cell, the (110) crystal plane of the rectangular cell is favorable for the preparation of graphene single crystals with less defects. Therefore, the appropriate metal substrate can promote the nucleation process of graphene and reduce the formation of distortions and defects during the nucleation and growth of graphene.
First-principles study on the adsorption of oxygen at NiTi (110) surface
Authors: Liu Kun  , Wang Fu-He  , Shang Jia-Xiang  ,   Department of Physics, Capital Normal University, Beijing 100048, China;,   School of Materials Science and Engineering, Beihang University, Beijing 100191, China
Published: December 2020

NiTi alloys with equiatomic compositions have been widely used as structural materials in aerospace, aviation and other fields due to their shape memory effects and good mechanical performances. At the same time, they are considered as excellent biomedical materials for their biocompatibilities and high fatigue resistances. As structural materials, the oxidation resistance of NiTi alloy should be improved. However, as biomedical materials, the formation of dense TiO2 layers on the surface of NiTi alloy is required to suppress the release of Ni ions in body liquid. As a result, it is of great significance to study the oxidation mechanism of NiTi alloy.
In this work, while the total number of Ti is kept the same as that of Ni atoms in the whole system, a series of defected c(22)-NiTi (110) surfaces with antisite of Ti are constructed to further understand the oxidation mechanism of NiTi alloy. The adsorption of oxygen atom at the NiTi (110) surface is investigated by the first-principles calculations. The calculated results show that the stability of the oxygen adsorption is strongly related to the enrichment of Ti atoms on the surface. The higher the enrichment of Ti atoms on the surface, the stronger the adsorption of oxygen atoms is. When the coverage of oxygen is high enough, the adsorption of oxygen atoms on the surface could cause the antisite of Ti atoms on the surface by the exchange of Ni atoms in the first layer with Ti atoms in other layers. Under the O-rich conditions (O -9.35 eV), it is the most stable that the oxygen atoms adsorbed on Ti antisite surface, with the whole Ni atoms in the first surface layer exchanged with the whole Ti atoms in the third surface layer. With the increase of the adsorbed oxygen atoms on the surface, the heights of Ti atoms in the surface layers are raised by the adsorption of oxygen. The TiO2 layer is formed by the expansive growth, while Ni atoms are enriched beneath the TiO2. As a result, the reason why the TiO2 layer is formed on the NiTi alloy surface in the experimental conditions is well explained.
Gigahertz frequency doubler based on millimeter-scale single-crystal graphene
Authors: Gao Qing-Guo  , Tian Meng-Chuan  , Li Si-Chao  , Li Xue-Fei  , Wu Yan-Qing  ,   School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China;,   Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Graphene shows great potential applications in ultrahigh speed electronics due to its high carrier mobility and velocity. Nowadays, many radio frequency circuits based on graphene have been realized. For example, graphene frequency doubler is a promising option for signal generation at high frequencies. Graphene frequency doubler can achieve excellent spectral purity, because of its ambipolar transport and highly symmetric transfer characteristics. Here, we present high performance graphene frequency doublers based on millimeter-scale single-crystal graphene on HfO2 and Si substrates. We achieve a high spectral purity degree of larger than 94% without any filtering and the conversion gain is -23.4 dB at fin=1 GHz. The high conversion gain and spectral purity can be attributed to the high-quality millimeter-scale single-crystal graphene and high-quality high- substrates. Furthermore, we investigate the relation of conversion gain to source-drain voltage Vd and input signal power Pin. The results show that the conversion gain increases with source-drain voltage increasing, and the conversion gain also increases with input signal power increasing. The dependence of conversion gain on Vd and Pin can be attributed to the transconductance increasing with Vd and Pin. We compare the conversion gains and spectral purity degrees of graphene frequency doublers with different transconductances and electron-hole symmetries at different frequencies. The result shows that the conversion gain is larger for device with higher transconductance and the spectral purity has a moderate tolerance for the electron-hole symmetry of the graphene transistor at fin=1 GHz. As the working frequency increases to 4 GHz, the spectral purity of the device with weak electron-hole symmetry decreases dramatically, while the spectral purity of the device with better electron-hole symmetry is kept around 85%. We attribute this phenomenon to the different carrier transit times and different electron-hole symmetries of graphene transistors. In conclusion, the short channel graphene transistor with ultrathin gate dielectric and high electron-hole symmetry is needed in order to achieve high performance graphene frequency doubler.
Graphene-silicon hybrid photonic integrated circuits
Authors: Xiao Ting-Hui  , Yu Yang  , Li Zhi-Yuan  ,   Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510640, China
Published: December 2020

Silicon photonics is considered as a promising technology to realize high-performance photonic integrated circuits, owing to its complementary metal oxide semiconductor-compatibility which is applicable for large-scale integration at low cost. However, due to the limitation of optoelectronic properties of silicon, the challenge to the realization of high-performance active device on the silicon integrated platform still exists. The recent development of graphene-silicon hybrid photonic integrated circuit provides a practical solution to this problem, because graphene, as a superior two-dimensional material, possesses many advantageous optoelectronic properties, such as high mobility, high electro-optical coefficient, and broadband absorption, which can be fully exploited to break through the material limitation of silicon. Moreover, compared with other active integrated materials such as germanium and compound semiconductors, graphene is cost-effective and can be conveniently integrated with silicon photonic device. Here, we review some important research progress of graphene-silicon hybrid photonic integrated circuits that include optical sources, optical waveguides, optical modulators, and photodetectors. The challenges and prospects of these devices are also analyzed, which are expected to be beneficial to the relevant research communities.
Research status and development graphene devices using silicon as the subtrate
Authors: Wu Pei  , Hu Xiao  , Zhang Jian  , Sun Lian-Feng  ,   CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nonoscience and Technology, Beijing 100190, China;,   College of Materials Science and Opt-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Graphene, a two-dimensional sheet of sp2-hybridized carbon material, possesses excellent properties, such as high carrier mobility, high electrical conductivity, high thermal conductivity, strong mechanical strength and quantum anomalous Hall effect. So graphene quickly lights the enthusiasm for its research and application due to its superior performance. The silicon-based graphene devices are compatible with traditional silicon-based semiconductor technology. The combination of silicon-based graphene devices and silicon-based devices can greatly improve the overall performances of semiconductor devices. With the optimization of graphene preparation process and transfer technology, graphene devices using silicon as the substrate will show promising potential applications.
With the scaling of device, the heat dissipation, power consumption and other issues impede the integration of silicon-based devices. Graphene provides a possible solution to these problems. In this paper, we summarize the graphene application in field effect transistor. The bandgap of graphene is zero, which will have adverse effect on the switching ratio of the device. In order to solve this problem, a variety of methods are used to open its bandgap, such as the quantum confinement method, the chemical doping method, the electric field regulation method, and the introduction stress method. In the field of optoelectronic devices, graphene can evenly absorb light at all frequencies, and its photoelectric properties have also been widespread concerned, such as photoelectric detector, photoelectric modulator, solar cell, etc. At the same time, graphene, as a typical two-dimensional material, possesses superior electrical properties and ultra-high specific surface area, and becomes the hottest material in high sensitivity sensors.
Research progress of graphene radio frequency devices
Authors: Lu Qi  , Lyu Hong-Ming  , Wu Xiao-Ming  , Wu Hua-Qiang  , Qian He  ,   Institute of Microelectronics, Tsinghua University, Beijing 100084, China;,   Rice University, Houston, TX 77005, USA;,   Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China
Published: December 2020

Graphene, the first realized two-dimensional material, has received much attention in electronic applications in recent years. With ultra-high carrier mobility and one atom thick structure, graphene becomes a promising semiconductor candidate for solving the problem of short channel effect in nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET), and exploring its applications in radio frequency devices. How to develop the advantages of graphene transistor in radio frequency is an attractive research area. The first step is to obtain high quality graphene material. In this article we summarize the graphene growth methods commonly used in electronic field, including chemical vapor deposition on metal substrates and epitaxial method on wide bandgap semiconductor and insulator substrates. Another key factor to improve graphene transistor performance is to carefully design the device structure and process flow. Multi-finger gate and T-shaped gate are widely used in MOSFET. These two structures can significantly reduce gate resistance, and result in a better radio frequency performance. Inverted process is introduced for graphene FET fabrication, which is compatible with silicon-based back-end-of-line technology. It can reduce the damages to graphene during fabrication. Another improved self-aligned gate deposition process can lead to a good gate coupling and less parasitic parameters. These newly developed process play a prominent part in increasing the cut-off frequency and maximum oscillation frequency of graphene radio frequency devices. In addition, single crystal graphene is helpful in eliminating carriers scattering and improving the radio frequency properties of graphene transistor. So far, the highest cut-off frequency of graphene transistor reaches over 300 GHz by a few groups, but the maximum oscillation frequency remains low. Record-high maximum oscillation frequency is 200 GHz when gate length is 60 nm. Further improvement of maximum oscillation frequency needs to be tried out. Several graphene radio frequency circuits are also discussed in the paper. Some of the circuits have similar structures to silicon-based circuits, and others are designed based on the unique property of graphene transistor, like ambipolar transport properties. The new concept circuits have simpler structures than conventional circuits. With the rapid development of graphene growth and related integrating technology, the potential to use graphene in radio frequency field will be further increased.
Detection of stable isotopic ratio of atmospheric CO2 based on Fourier transform infrared spectroscopy
Authors: Shan Chang-Gong  , Wang Wei  , Liu Cheng  , Xu Xing-Wei  , Sun You-Wen  , Tian Yuan  , Liu Wen-Qing  ,   School of Environment science and Optoelectronic Technology, University of Science and Technology of China, Hefei 230000, China;,   Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China;,   School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230000, China
Published: December 2020

Long-term measurement of CO2 and its stable isotopes not only obtain the CO2 sources and sink information, but also determine the contributions of different emission sources to atmospheric CO2.Fourier transform infrared spectroscopy (FTIR) is an important technique which can provide highly precise remote sensing of column abundances of atmospheric trace gases.In the study,the stable isotopes of atmospheric CO2,13CO2 and 12CO2,are retrieved from the near-infrared solar absorption spectra collected by a ground-based high-resolution Fourier transform spectrometer. Three spectral windows of 13CO2 and two spectral windows of 12CO2 are chosen to retrieve the two species.The root mean square spectral fitting residuals are about 1.2%,2.3% and 1.2% for the three spectral windows of 13CO2,and about 0.64% and 0.60% for the two spectral windows of 12CO2,respectively.The small spectral fitting residuals indicate the high-quality spectral fitting.The mean retrieval errors are (1.180.27)% and (0.890.25)% for 13CO2 and 12CO2 during the experiment,respectively.The measurement precision of carbon isotopic ratio 13C for the observation system is estimated to be about 0.041 based on the Allan variance method,comparable to the precision of in situ FTIR measurement.Moreover,long time series of atmospheric 13C in one year from September 18,2015 to September 24,2016 is obtained.The results show that atmospheric 13C varies from -7.58 to -11.66,and the mean value is about (-9.50.57) over the duration of the experiment.Also,time series of carbon isotopic signature 13C has an obvious seasonal trend,with a minimum of (-9.350.47) in winter and a maximum of (-8.730.39) in summer. The further analysis suggests that the increase of emission from the fossil fuel burning due to heating may explain the depletion of heavy isotope 13CO2 in winter.Additionally,it is revealed that the variation range of atmospheric 13C observed in Hefei area is consistent with the reported values in Nanjing area based on in situ measurement,while 13C values in summer and winter are higher than the corresponding values detected in Beijing area as indicated in recent publications,which may result from the fact that the CO2 emissions from the fossil fuel combustion in Beijing are more than those in Hefei.The experimental results demonstrate the ability of the ground-based high-resolution FTIR to detect the stable isotopes of atmospheric CO2,13CO2 and 12CO2,and carbon isotopic ratio 13C with a high precision and accuracy.
Dirac photonic crystal
Authors: Wang Hai-Xiao  , Xu Lin  , Jiang Jian-Hua  ,   Department of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China;,   Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China
Published: December 2020

Dirac Fermion, as one of the basic particles in the particle physics, nowadays have been widely used to describe the electronic states with the behavior of Dirac fermion in the topological electronics. These exotic electronic states are called Dirac point, which exhibited as a linear crossing point in the band structure. Usually Dirac point is the topological phase transition point and thus viewed as the mother state of various topological states. As an analogue of topological electronics, topological photonics, also attracted a great deal of interest due to its potential application. One of the key topic in topological photonics is to realize photonic bands with Dirac point. In this review, we briefly introduce the progress of Dirac point in the photonic system and focus on the realization method of Dirac point in photonic crystal by take advantage of lattice symmetry. We also discuss Weyl point in the photonic crystal as an extension of the Dirac point.