bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal stability of MoS2 encapsulated by graphene
Authors: Liu Le  , Tang Jian  , Wang Qin-Qin  , Shi Dong-Xia  , Zhang Guang-Yu  ,   Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   School of Physics, University of Chinese Academy of Sciences, Beijing 100190, China;,   Beijing Key Laboratory for Nanomaterials and Nanodevices, Beijing 100190, China;,   Collaborative Innovation Center of Quantum Matter, Beijing 100190, China
Published: December 2020

Monolayer molybdenum disulfide (MoS2), a semiconductor material with direct band gap, is considered to be an important fundamental material for the future development of the semiconductor industry. In order to apply the material to semiconductor devices, we have to investigate the electrical, optical and thermal properties of MoS2. People have always been concerning about the electrical and optical properties, but pay little attention to the thermal properties of MoS2, especially thermal stability. It is well known that semiconductor device generates a lot of heat when it works, sometimes even running in high temperature environment. The above conditions all require the material which has good thermal stability. So we focus on how to improve the thermal stability of MoS2. In this paper, we report the construction of the van der Waals heterostructures of graphene and MoS2 by encapsulating monolayer MoS2 with graphene, and dissect the thermal stability of encapsulated MoS2 in argon (Ar) and hydrogen (H2) atmosphere respectively. The results show that in Ar atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, while the exposed MoS2 is decomposed almost completely at 1000 ℃. In H2 atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, but the exposed MoS2 is decomposed completely at 800 ℃. In conclusion, the thermal stability of MoS2 encapsulated by graphene can be improved significantly. We analyze the reason why MoS2 encapsulated by graphene gains good thermal stability. Firstly, the covered graphene provides additional van der Waals forces, which increases the decomposition energy of MoS2, making it more stable at high temperature environment. Secondly, graphene separates MoS2 from the external environment, preventing MoS2 from contacting and reacting with external gas, which greatly improves the thermal stability of MoS2 at high temperature environment. Meanwhile, graphene covers the active defect site on MoS2, making it difficult to react at defects. In summary, the monolayer MoS2 devices can work normally at high temperature when MoS2 is encapsulated by graphene. In addition, our work also provides a feasible approach to improving the thermal stability of other two-dimensional materials.
Analysis of particle size distribution and scattering characteristics of water cloud in condensation and coalescence growth
Authors: Song Yue-Hui  , Zhou Yu-Dong  , Wang Yu-Feng  , Li Shi-Chun  , Gao Fei  , Li Bo  , Hua Deng-Xin  ,   School of Mechanical and Instrumental Engineering, Xi'an University of Technology, Xi'an 710048, China
Published: December 2020

Study of scattering characteristics of water cloud is of great significance for weather forecasting, meteorological disaster warning, weather modification and research of radiation transmission in the lower atmosphere. In this paper, particle size distribution and scattering characteristics of water cloud in condensation and coalescence growth are studied by numerical simulation. The particle size distribution model of water cloud in the condensation growth and coalescence growth are established respectively. The dynamic process of the particle size distribution of water cloud in the condensation growth, the coalescence growth and the condensation-coalescence combination growth are analyzed. Then the scattering characteristics of water cloud in the droplet growth are studied with the Mie theory. The results show that with the condensation growing the full width at half maximum of particle size distribution, the effective radius and mode radius of water cloud increase continuously. The effective radius increases in the coalescence growth process and there are multiple peaks in the particle size distribution in the coalescence growth anaphase. The average growth rate of the effective radius of cloud droplets is 8 nm/s in the condensation-coalescence combination growth. The extinction coefficient and scattering coefficient of water cloud increase linearly with time increasing during the condensation growth or the coalescence growth. In the condensation-coalescence combination growth, the extinction coefficient and scattering coefficient increase exponentially with time increasing except at a wavelength of 3.2 mm; the asymmetric factors at the wavelengths of 1.064, 2.2, 3.7, 12 and 22 μm tend to be stable, while the asymmetric factor at the wavelength of 3.2 mm remains the same basically. Meanwhile, the lidar ratio at each of the wavelengths of 1.064 μm and 2.2 μm fluctuates near 20 sr, and that at the wavelength of 3.7 μm fluctuates greatly. In the growth process of cloud droplet, the single scattering albedo of water cloud decreases gradually at each of the wavelengths of 1.064, 2.2 and 3.7 μm, while it increases gradually at each of the wavelengths of 12, 22, 200 and 3.2 mm. The absolute value of Ångström exponent decreases gradually, which means that the wavelength-dependence of extinction coefficient decreases with cloud droplet growing. These research results reveal the change law of particle size distribution and the scattering characteristics of water cloud in condensation and coalescence growth. The results provide important reference for forecasting weather, studying earth-atmosphere radiation balance and correcting remote sensing data.
High order harmonics generation by relativistically circularly polarized laser-solid interaction
Authors: Cai Huai-Peng1\2  , Gao Jian1\2  , Li Bo-Yuan1\2  , Liu Feng1\2  , Chen Li-Ming1\2\3  , Yuan Xiao-Hui1\2  , Chen Min1\2  , Sheng Zheng-Ming1\2\4\5  , Zhang Jie1\2\3  ,   Key Laboratory for Laser Plasmas(Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China;,   IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China;,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Department of Physics, University of Strathclyde, Glasgow G40 NG, UK;,   Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240, China
Published: December 2020

Coherent extreme ultra-violet (XUV) and soft X-ray light with attosecond duration enable the time-resolved study of electron dynamics in a completely new regime. High order harmonic generation (HHG) from the highly nonlinear process of relativistically intense laser interactions with solid-density plasma offers a very new way to generate such a light source. In this paper, we study the HHG by a relativistically circularly polarized femtosecond laser interacting with solid-density plasma. The experiment is carried out by using a 200 TW Ti:sapphire laser system at the Laboratory for Laser Plasmas in Shanghai Jiao Tong University, China. The laser system can deliver laser pulses at 800 nm with a pulse duration (full width at half maximum, FWHM) of 25 fs and repetition rate of 10 Hz. The circularly polarized laser beam with an energy of 460 mJ is used in the experiment and focused by an F/4 off-axis parabolic mirror at an incidence angle of 40 with respect to the glass target. The focal spot diameter is 6 m (FWHM) with 25% energy enclosed, giving a calculated peak intensity of 1.61019 W/cm2. We detect high order harmonics by a flat-field spectrometer. The experimental results show that high order harmonic radiation can also be efficiently generated by a circularly polarized laser at a lager incidence angle, which provides a straightforward way to obtain a circularly polarized XUV light source. Different plasma density scale lengths are obtained by introducing a prepulse with different delays. We study the dependence of HHG efficiency on plasma density scale length by the circularly polarized laser, and find an optimal density scale length to exist. The influence of laser polarization and plasma density scale length on HHG are studied by two-dimensional (2D) PIC simulations. The good agreement is found between the 2D PIC simulations and experimental results. We plan to measure the polarization characteristics of high order harmonic produced by the interaction of circularly polarized lasers with solid target in the future. It is expected to obtain a compact coherent circularly polarized XUV light source, which can be used to study the ultra-fast dynamic process of magnetic materials.
New progress of FeSe-based superconducting single crystals and films: Spin nematicity, electronic phase separation, and high critical parameters
Authors: Dong Xiao-Li  , Jin Kui  , Yuan Jie  , Zhou Fang  , Zhang Guang-Ming  , Zhao Zhong-Xian  ,   National Lab for Supreconductivity, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Key Laboratory for Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, China;,   State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
Published: December 2020

High-quality superconducting single crystals and thin films play an important role in the basic research and application of high-Tc superconductivity. In these two aspects, iron-based superconductors feature the merit of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). By developing ion-exchange and ion-de-intercalation method, we successfully synthesize a series of high-quality and sizable (Li,Fe)OHFeSe and FeSe single crystal samples. We observe Ising spin nematicity (below Tsn), and the universal linear relationship between Tc and Tsn in FeSe single crystals, indicating that the superconductivity is closely related to the spin nematicity driven by stripe antiferromagnetic spin fluctuations. In (Li,Fe)OHFeSe single crystals, we observe the coexistence of an AFM state (below Tafm~125 K) together with the SC state. We explain the coexistence by electronic phase separation, similar to that in high-Tc cuprates and iron arsenides, and establish a complete phase diagram for (Li,Fe)OHFeSe system. Here, we also make a brief introduction about our latest progress in growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe. The film is prepared by a hydrothermal epitaxial method. The high crystalline quality of the film is demonstrated by x-ray diffraction results, showing a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. Based on systematic magnetoresistance measurements, the values of upper critical field Hc2 are estimated at 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at~20 K. Such a (Li,Fe)OHFeSe film is not only important for the fundamental research for understanding the high-Tc mechanism, but also promises the high-Tc superconductivity applications, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.
Numerical simulation of recombination rate effect on development of equatorial plasma bubbles
Authors: Jiang Chun-Hua , Zhao Zheng-Yu ,   School of Electronic Information, Wuhan University, Wuhan 430072, China
Published: December 2020

Plasma bubbles in the ionosphere have a significant effect on radio wave communication and navigation. Under the worst condition, it will fail to function the systems relying on the ionosphere. On the other hand, the physical mechanisms of evolution and diurnal variations of the plasma bubbles in the ionosphere are still unclear. Therefore, it is still worthy to simulate the plasma bubbles in the ionosphere for radio wave propagation and science. In this study, the equatorial plasma bubbles induced by one- and two- dimensional disturbance in the ionosphere are simulated, where traveling ionospheric disturbance (TID) is used to produce the two-dimensional disturbance in the ionosphere. The dispersion relationship of gravity wave is used to represent the corresponding wavelength of TID in this study. More importantly, we investigate the effect of recombination rate on the development of plasma bubbles by numerical simulation. The simulation results show that the recombination rate of the plasma in the ionosphere has a significant effect on the development of plasma bubbles. The greater the recombination rate, the more time it takes to produce the plasma bubbles. For one-dimensional disturbance in the ionosphere, there is no significant effect in the structure of plasma bubbles for the recombination rate of the plasma. However, the recombination rate plays a significant effect on the structure of plasma bubbles induced by TID. When the recombination rate is less in numerical simulation, the complex structure including bifurcation, plume-like structures, and pinching of plasma bubbles can occur in the development of bubbles. In contrast, the structure of plasma bubbles is simpler when the recombination rate is greater in simulation. As a result, the recombination rate of the plasma is a significant factor for simulating the plasma bubbles in the ionosphere. The greater recombination rate can result in the slowing down of equatorial plasma bubbles and the simplifying the structure of the bubbles as well. In addition, it is found that not all of plasma bubbles at the bottom of the ionosphere can grow to the top of the ionosphere when many bubbles occur on the bottom side. The direction of the polarization electric field near the bubbles can be changed in the non-linear development of the bubbles. Therefore, only the bubbles where the polarization electric field is always eastward can develop to the topside.
Intrinsic stability of organic-inorganic hybrid perovskite
Authors: Zhang Yu , Zhou Huan-Ping ,   College of Engineering, Peking University, Beijing 100871, China
Published: December 2020

The power conversion efficiency of organic-inorganic hybrid perovskite solar cell has exceeded 24%. The rapid increase in efficiency coupled with its cost-effective fabrication has attracted tremendous attention toward the commercialization of perovskite solar cells. The biggest challenge that hinders the commercialization of perovskite solar cells is the long-term instability of materials and the corresponding devices, which cannot compete with other commercialized solar cells, such as Si cells, in terms of lifetime. The intrinsic instability of perovskite material itself is the most critical challenge faced by researchers. In this study, we discuss the intrinsic instability of organic-inorganic hybrid perovskite materials from the aspects of both chemical instability and phase instability. Suggestions for improving the stability of perovskite solar cell are provided from the perspective of composition design and fabrication process.
Effect of H on elastic properties of Mg2Si by the first principles calculation
Authors: Fu Zheng-Hong , Li Ting , Shan Mei-Le , Guo Kang , Gou Guo-Qing ,   Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China
Published: December 2020

The mechanical properties of Al-Mg-Si-type aluminum alloys may degenerate due to the hydrogen damage during servicing in hydrogen environment. The Mg2Si is the main strengthening phases in Al-Mg-Si-type aluminum alloys. Therefore, the mechanical properties of Mg2Si directly determine the strengths of Al-Mg-Si-type aluminum alloys. In this work, the effects of hydrogen atoms on the mechanical properties of Mg2Si are investigated by first principle calculation, which is based on the density function theory. First of all, we calculate the single crystal elasticity constants of C11, C12 and C44. Then the elasticity modulus, Poisson’s ratio and hardness of polycrystalline are calculated by using the crystal elasticity constants. Furthermore, we also calculate the tensile properties of Mg2Si with and without H atoms. The difference between the densities of states with and without H atoms is used to investigate the change of Mg2Si induced by H atoms. The results show that hydrogen atoms significantly reduce the shear modulus and elastic modulus of Mg2Si, resulting in the strength and hardness decreasing, but the toughness increasing. The calculations of tensile properties indicate that H atoms reduce the fracture strength but enhance the fracture elongation of Mg2Si. The analysis of density of states indicates that hydrogen atoms will induce the properties of Mg2Si to transform from semiconductor to metal properties. The calculated results in this paper can provide a reference basis for revealing the mechanism of strength reduction of Mg2Si materials in a hydrogen environment.
A novel method of heart rate variability measurement
Authors: Shao Shi-Liang , Wang Ting , Song Chun-He , Cui E-Nuo , Zhao Hai , Yao Chen ,   School of Computer Science and Engineering, Northeastern University, Shenyang 110819, China,   State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China,   Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169, China
Published: December 2020

The complex fluctuation of heart rate variability reflects the autonomous regulation function of the heart. In this paper, a novel method of measuring the heart rate variability is proposed. Firstly, the heart rate variability signal is decomposed by the improved complete ensemble empirical mode decomposition with adaptive noise method, and the multiple intrinsic mode functions are obtained, and the bubble entropy of each intrinsic mode function is calculated to obtain an entropy value vector. Then, the vector is mapped to a network based on a limited penetrable horizontal visibility graph method. By calculating various characteristic parameters of the network, the coupling relationship between the nonlinear features of heart rate variability in different time-frequency scale states are studied. The characteristic parameters include mean value of aggregation coefficient (MC), the characteristic path length (CL), the topological entropy of network (TE), the network level weighted bubble value (WB), and the pseudo mean value of node weight (PW). Firstly, the heart rate variabilities of 29 patients with congestive heart failure and 29 normal sinus heart rhythm subjects are analyzed by time domain, frequency domain and ICBN analysis method, the T test is used for statistical analysis, and Fisher discriminant method is used for classification. The results show that the time domain triangular index HRVTI, frequency domain index LF/HF, WB, PW and CL in ICBN have statistical differences. The accuracy rate of recognition model based on WB, CL, frequency domain index LF/HF and Fisher discriminant method is 89.66%. Secondly, the heart rate variabilities of 43 patients with atrial fibrillation arrhythmia and another 43 normal sinus heart rhythm subjects are analyzed by the same methods, including the time domain analyzed method, frequency domain analyzed method, and ICBN analyzed method. Then, the T test is also used for statistical analysis, and Fisher discriminant method is used for classification. The results show that using the time domain index pNN5 and RMSSD, frequency index LF/HF, ICBN index WB and PW as the feature vectors, and the Fisher discriminant mode as the classifier, the accuracy rate of recognition for atrial fibrillation arrhythmia is 91.86%. From these results it is concluded that the ICBN method provides a new idea for the heart rate variability measurement.
Isolated attosecond pulse generation from polarizationgating pulse with 10 fs duration
Authors: Song Hao , Lü Xiao-Yuan , Zhu Ruo-Bi , Chen Gao ,   School of Science, Changchun University of Science and Technology, Changchun 130022, China
Published: December 2020

Isolated attosecond pulses make it possible to study and control the ultrafast electron processes in atoms and molecules. High order harmonic generation (HHG) is the most promising way to generate such pulses, which is benefited from the broad plateau structure of the typical HHG spectrum. In previous HHG studies on the polarization gating pulse with longer pulse duration, one needs to dramatically increase the separation in time between the two counter-rotating circularly-polarized pulses to generate the nearly-linear half-cycle " polarization gate”. This leads to a low harmonic conversion efficiency because the field outside the polarization gate is much stronger than inside the polarization gate. In this paper, by using Lewenstein model, we theoretically simulate the high-order harmonic generation from helium atom subjected to the polarization gating pulse with 10 fs pulse duration. It is found that high-order harmonic spectra each with a higher efficiency and regular structure can still be obtained by reasonably adjusting the delay time ratio and the amplitude ratio of electric fields between the two counter-rotating pulses. Further, a single 175 as pulse in the time domain is obtained by Fourier transforming the 80th order harmonics into the 172nd order harmonic without compensating for the harmonic chirp. This scheme has two main advantages. First, the adjustment of the polarization gate width from half optical cycle into nearly one cycle ensures higher intensity of the synthesized electric field inside the polarization gate. Second, the suitable adjustment of the amplitude ratio between two electric fields ensures the low ionization probability before the polarization gate, and thus further fulfills the harmonic phase matching condition in the process of the propagation.
Nonlinear acoustic response of two bubble oscillators
Authors: Mo Run-Yang , Wang Cheng-Hui , Hu Jing , Chen Shi ,   Shaanxi Key Laboratory of Ultrasonics, Shaanxi Normal University, Xi’an 710062, China
Published: December 2020

It is of great importance to investigate the dynamics of the multiple bubble system for revealing the mechanism of cavitation. Because of the secondary radiation of the oscillating bubbles, the coupled vibration of neighboring bubbles arises. Previous studies have reported that time delays appear to be more important when the coupled bubbles are close to each other. In this paper, we investigate the acoustical response of two bubble oscillators theoretically and numerically. Firstly, we modify the dynamic model equation by use of Taylor series being accurate up to terms of second order in radial displacement of bubbles. Based on the perturbation theory, the eigenmodes of the coupled-bubble system are analyzed, and two different resonant frequencies are obtained. Secondly, the effects of time delays on the coupled oscillation are analyzed numerically by use of phase diagram. When bubbles are driven by low-intensity ultrasound, we can neglect the effect of the time delay for the coupled-bubble system. Thirdly, the theoretical and numerical curve of amplitude versus frequency are compared with each other. There are two peaks on each curve on which present are two resonant regions. The relative position of the resonant peaks of the two bubbles in each region is similar for the two analytical methods. Finally, the effect of equivalent radius of bubble, equivalent radius ratio, bubble center distance, and driving pressure amplitude on the radial motion are numerically explored. With the increase of the intensity of the acoustic wave, the resonant peaks shift toward the low-frequency region. The coupled oscillation of the two bubbles of different radii could be intensified when these conditions are satisfied, such as resonant driving, equal radius, and the range of center distance smaller than 10R10. We can observe a transition phenomenon and out-of-phase fluctuation of the bubble oscillation in the strong coupling region. Therefore, bubbles play an important role of energy translator in the ultrasound applications.
Statistical compressive sensing based on convolutional Gaussian mixture model
Authors: Wang Ren , Guo Jing-Bo , Hui Jun-Peng , Wang Ze , Liu Hong-Jun , Xu Yuan-Nan , Liu Yun-Fo ,   China Academy of Launch Vehicle Technology R&D Center, Beijing 100076, China,   Department of Electrical Engineering, Tsinghua University, Beijing 100084, China
Published: December 2020

Statistical compressive sensing needs to use the statistical description of source signal. By decomposing a whole image into a set of non-overlapping or overlapping patches, the Gaussian mixture model (GMM) has been used to statistically represent patches in an image. Compressive sensing, however, always imposes compression on the whole image. It is obvious that the entire image contains much richer information than the small patches. Extending from the small divided patches to an entire image, we propose a convolutional Gaussian mixture model (convGMM) to depict the statistics of an entire image and apply it to compressive sensing. We present the algorithm details by learning a convGMM from training images based on maximizing the marginal log-likelihood estimation. The learned convGMM is used to perform the model-based compressive sensing by using the convGMM as a model of the underlying image. In addition, aiming at the problem of high-dimensional image that makes learning, estimation and optimization suffer high computational complexity, all of the training and reconstruction process in our method can be fast and efficiently calculated in the frequency-domain by two-dimensional fast Fourier transforms. The performance of the convGMM on compressive sensing is demonstrated on several image sets.
Single-molecule probes revealed dynamics of confined nano-regions in miscible polymer blends
Authors: Zhang Guo-Feng , Li Bin , Chen Rui-Yun , Qin Cheng-Bing , Gao Yan , Xiao Lian-Tuan , Jia Suo-Tang ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Collaborative Innovation Centre of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

Miscible mixtures of polymer blends have physical properties that are often linked simply to the blend composition, thus offering an inexpensive and convenient method to achieve new high performance polymers. Confinement effect has been found in various polymer blend systems by the ensemble methods, but further understanding the confinement effect still requires large efforts both in experiment and in theory. Single molecule spectroscopy has the potential to provide an in-depth insight to the dynamic information by directly coupling their reorientation to the segmental relaxation of the surrounding polymer matrix. We investigate the confinement effects in polystyrene and oligostyrene blend films by using single-molecule defocused wide-field fluorescence microscopy. According to the observation for dynamic behaviors of probe molecules in the blend films of 75 wt.% and 25 wt.% polystyrene, we find that there are two types of single molecules in the blend films: rotational molecules and immobile molecules. The experimental temperature of 296 K is between the glass transition temperature (Tg) values of two pure components and also is far from the two Tg values. At the temperature, oligostyrene component is trapped by the frozen polystyrene component, but they still move locally. Therefore, the rotational and immobile molecules should couple to the oligostyrene component and polystyrene component, respectively. The distribution of rotational single molecules reveals that the confined regions randomly distribute across miscible polymer blends. The length scale of confined region is estimated to be close to that of the probe molecule by taking into account the rotational dynamics of single molecules. The local relaxation of blend film is also investigated by the rotational correlation time which can be estimated by fitting the autocorrelation curve of 〈cos(Φ)〉 with a Kohlrausch-Williams-Watts stretched exponential function. The histograms of the rotational correlation times in the blend films of 75 wt.% and 25 wt.% polystyrene are obtained respectively, which reveal the characteristic of local dynamic distribution in the confined nano-regions. We find that the dynamic behavior in the blend film of 75 wt.% polystyrene is faster than that of 25 wt.% polystyrene, indicating there is a confinement effect in the blend due to the increased constraints imposed by the polystyrene component at a higher concentration of polystyrene. All results observed in the experiment can be explained qualitatively by the self-concentration model. Our work indicates that the single molecule defocused wide-field fluorescence microscopy is a powerful tool to study the complex dynamic features in the polymer blends.
Optoelectronic properties of vertical-cavity surface-emitting laser at low temperature
Authors: Qin Lu , Ren Jie , Xu Xing-Sheng ,   State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors,Chinese Academy of Sciences, Beijing 100083, China,   Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The vertical-cavity surface-emitting laser (VCSEL) is usually used as an 850nm short wavelength source for short-distance optical interconnection at normal temperature. In this study, the characterization of the VCSEL at low temperature was mainly studied. The laser spectra and the P-I-V curves are obtained with direct current and pulse current with 10% duty-cycle at different temperatures. It indicates that the VCSEL can work at 10K temperature environment. When the VCSEL laser is driven by direct current in a temperature range from 295 K to 10 K, the central wavelength of the laser is first red-shifted and then blue-shifted due to the change of environmental temperature and thermal effect on the device. With a pulsed-current driven source, the smaller the duty cycle, the less the heat generated by the device will be. The laser spectrum shows a blue-shift trend in the cooling process. The spectral width remains approximately stable in the cooling process. With temperature decreasing, the laser threshold current increases, and the lower the temperature, the larger the threshold current will be. It shows that the cavity mode and the gain spectrum shift with temperature changing. The cavity mode and the gain spectrum both shift to red with temperature increasing, and they shift to blue with temperature decreasing. But their shifting speeds are different. The mismatch between the cavity mode and the gain curve causes the device to need more energy for lasing. So the laser will work at a higher current driven at low temperature. The laser can work at low temperature as a stable light source. Therefore, the VCSEL has potential applications in optical interconnection system as a source at low temperature.
In situ observation of lithiation mechanism of SnO2 nanoparticles
Authors: Xiong Yu-Wei , Yin Kui-Bo , Wen Yi-Feng , Xin Lei , Yao Li-Bing , Zhu Chong-Yang , Sun Li-Tao ,   SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China
Published: December 2020

Tin oxide (SnO2) has attracted a lot of attention among lithium ion battery anode materials due to its rich reserves, high theoretical capacity, and safe potential. However, the mechanism of the SnO2 nano materials in the lithiation-delithiation reaction, especially whether the first-step conversion reaction is reversible, is still controversial. In this paper, SnO2 nanoparticles with an average particle size of 4.4 nm are successfully prepared via a simple hydrothermal method. A nanosized lithium ion battery that enables the in situ electrochemical experiments of SnO2 nanoparticles is constructed to investigate the electrochemical behavior of SnO2 in lithiation-delithiation process. Briefly, the nanosized electrochemical cell consists of a SnO2 working electrode, a metal lithium (Li) counter electrode on a sharp tungsten probe, and a solid electrolyte of lithium oxide (Li2O) layer naturally grown on the surface of metal Li. Then, the whole lithiation-delithiation process of SnO2 nanocrystals is tracked in real time. When a constant potential of –2 V is applied to the SnO2 with respect to lithium, lithium ions begin to diffuse from one side of the nanoparticles, which is in contact with the Li/Li2O layer, and gradually propagate to the other side. Upon the lithiation, a two-step conversion reaction mechanism is revealed: SnO2 is first converted into intermediate phase of Sn with an average diameter of 4.2 nm which is then further converted into Li22Sn5. Upon the delithiation, a potential of 2 V is applied and Li22Sn5 phase can be reconverted into SnO2 phase when completely delithiated. It is because the interfaces and grain boundaries of nano-sized SnO2 may impede the Sn diffusing from one grain into another during lithiation/delithiation and then suppress the coarsening of Sn, and enable the Li2O and Sn to be sufficiently contacted with each other and then converted into SnO2. This work provides a valuable insight into an understanding of phase evolution in the lithiation-delithiation process of SnO2 and the results are of great significance for improving the reversible capacity and cycle performance of lithium ion batteries with SnO2 electrodes.
Numerical simulation of deuterium-tritium fusion reaction rate in laser plasma based on Monte Carlo-discrete ordinate method
Authors: Chen Zhong , Zhao Zi-Jia , Lü Zhong-Liang , Li Jun-Han , Pan Dong-Mei ,   School of National Defense Science and Technology, Southwest University of Science and Technology, Mianyang 621010, China,   College of Liberal Arts and Science, National University of Defense Technology, Changsha 410073, China,   University of Science and Technology of China, Hefei 230027, China
Published: December 2020

Inertial confinement fusion (ICF) is one of the possible ways to realize controlled thermonuclear fusion. The fusion neutron source term is one of the important parameters in the physical design and analysis of laser plasma. The accuracy of the fusion neutron source term directly affects the reliability of the analysis results. At present, the neutron source term of deuterium-tritium fusion reaction in ICF is mainly based on formula method. It has limited applications in temperature and reaction type. Because of a large quantity of data, it is impossible to simulate the fusion reaction of each particle. In this paper, the concept of particle cloud is introduced, that is, the collection of the like particles with the same position and speed, and it is considered that the action of particle cloud is the same reaction. Because the particles should satisfy the Maxwell velocity distribution at a certain temperature and the direction is all around the circumference angle, the collision cross sections between the incident particle and different target particles are different. Therefore, the design program takes all the possible velocities, polar angles and direction angles, reads the collision cross sections between deuterium and tritium and makes corrections, and obtains the multi-temperature differential correction cross sections of deuterium and tritium fusion with Doppler energy broadening. On these bases, Monte Carlo method and discrete ordinate method method are used. A numerical simulation program for the fusion rate of D-T particles in laser plasma is developed in this paper. It is found that there are significant differences between the DT, DD, TD cross sections and the original cross sections after Doppler broadening. In a range of plasma temperature between 20 keV and 100 keV, the simulation results are more consistent with the cross section data of ENDF/B-VI and ENDF/B-VII databases of deuterium-tritium fusion reaction than those from the analytical formula method. There is a large error between the numerical simulation results and the analytical formula method in the low energy region. It may be caused by the difference of calculation methods and too big difference among the used fusion cross sections at low temperature.
Photorefractive effect of low-temperature-grown aluminum gallium arsenide
Authors: Zhong Zi-Yuan , He Kai , Yuan Yun , Wang Tao , Gao Gui-Long , Yan Xin , Li Shao-Hui , Yin Fei , Tian Jin-Shou ,   Key Laboratory of Ultra-fast Photoelectric Diagnostics Technology, 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,   Rocket Force Academy, Beijing 100101, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

The ternary compound aluminum gallium arsenide is an important material that can be used in all-optical solid-state ultrafast diagnostic technology. The low-temperature-epitaxially-grown AlGaAs (LT-AlGaAs) not only has the characteristics of ultra-short carrier lifetime of low-temperature-grown gallium arsenide (LT-GaAs), but also possesses the advantage of adjustability of band gap, which will provide great flexibility for the design of ultra-fast diagnostic systems. We use low-temperature epitaxial growth technology to grow AlGaAs on a GaAs substrate. The low-temperature-grown AlGaAs can effectively absorb 400 nm pump light to generate excess carrier. Therefore, we use a femtosecond laser with a wavelength of 800 nm and a pulse width of 200 fs as a light source to generate 400-nm pump light after passing through the BBO crystal, and 800 nm light without frequency doubling as the probe light. Using such a light source, we build a pump probe experimental platform to test the LT-AlGaAs. We normalize the experimental results and deconvolute it with the normalized laser pulses to obtain the response function of the semiconductor to the pump light. Therefore, we know that the nonequilibrium carrier relaxation time is less than 300 fs, and the nonequilibrium carrier recombination time is 2.08 ps. Due to the special passivation process, the effect of surface recombination on the carrier decay process is greatly reduced. The As clusters introduced by low-temperature epitaxial growth form deep level defects are the main factor for accelerating carrier recombination. In order to understand the complex process of photogenerated nonequilibrium carriers in depth, we use the indirect recombination theory of single recombination center to calculate the carrier recombination process, and establish an LT-AlGaAs carrier evolution model. Thus we obtain the key physical parameter related to the recombination rate, which is the carrier trapping area. We also use a theoretical model of carrier-regulated refractive index to calculate the effect of carrier concentration on the amount of change in refractive index. Combining our AlGaAs carrier evolution model, we simulate the refractive index change process of LT-AlGaAs after being illuminated by pump light. The simulation results are in good agreement with the experimental results. The method can be used for the quantitative analysis of carrier evolution characteristics of semiconductor materials, and it can conduce to the optimization and improvement of ultra-fast response semiconductor materials.
Constraints of neutron star on new interaction ofspin-dependent axial-vector coupling
Authors: Gao Peng-Lin , Zheng Hao , Sun Guang-Ai ,   School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China,   Key Laboratory of Neutron Physics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

It is predicted in many theories beyond the standard model that the new interaction relevant to spin is existent. The exchange of an axial vector particle will result in attractive dipole-dipole interaction which can be viewed as an effective magnetic potential that looks quite different from those expected from electromagnetism. In this work, we demonstrate that, instead of the laboratory spin source, stringent constraints can be set on these attractive spin-spin interactions from polarized nuclear matters within neutron stars which have extremely strong magnetic fields (up to 1015 G in some cases). By considering such an exotic interaction within the framework of relativistic mean field model, we find that the stability of infinite nuclear matter can be influenced significantly when the ratio of coupling strength to boson mass become larger than \begin{document}$g_{\rm A}^2/m_{\rm Z'}^2 \sim {\cal O}(10 \, {\rm GeV^{-2}})$\end{document}. Furthermore, based on the curvature matrix approach, when \begin{document}$g_{\rm A}^2/m_{\rm Z'}^2 > 130 \, {\rm GeV^{-2}}$\end{document}, phase transition inside low-density nuclear matter will no longer take place before the pressure of nuclear matter becomes zero, which forbids core-crust transition at the inner edge separating the liquid core from the solid crust of neutron stars. Thus bare neutron stars without any crusts are predicted. However, observations of pulsar glitches, i.e., the occasional disruptions of the extremely regular pulsations from magnetized, rotating neutron stars, imply the existence of crusts inside these dense objects. This in turn constrains the strength of the exotic interaction. In fact, in the case of dipole-dipole force on a length scale between µm to cm, the highest value of these constraints can be 8 orders of magnitude higher than those from existing laboratory results.
Mixed injection mechanism assisted cascaded laser wakefield accelerator
Authors: Tan Fang , Zhang Xiao-Hui , Zhu Bin , Li Gang , Wu Yu-Chi , Yu Ming-Hai , Yang Yue , Yan Yong-Hong , Yang Jing , Fan Wei , Dong Ke-Gong , Lu Feng , Gu Yu-Qiu ,   Institute of Modern Physics, University of Science and Technology of China, Hefei 030006, China,   Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

Femtosecond electron bunches can be produced by laser plasma wakefield accelerators, with energy tunable from tens of MeV to a few GeV. In order to produce stable mono-energetic electron bunches, a critical issue is to control the injection of electron into the wakefield. The ionization injection is one of the most effective methods of controlling the injection, which is usually a continuous process. So, the electron bunches produced through ionization injection usually possess large energy spread. In order to optimize the ionization injection technique and produce stable monoenergetic wakefield electron beams, experimental studies are conducted on our 45 TW laser facility. In this work, a mixed injection mechanism assisted cascaded laser wakefield accelerator is presented. Based on a double-nozzle cascaded accelerator, the influences of ionization injection, shock wave front injection and their combination are experimentally studied. The results show that the lower threshold of the injection can be substantially reduced. The ionization injection is restricted within the shock wave front. As a result, mono-energetic electron bunches with reduced absolute energy spread can be stably produced. Under the most optimal conditions, the central energy and energy spread are (63.24 ± 6.12) MeV and (13.0 ± 3.9) MeV. The charge quantity of the electron bunches is (5.99 ± 3.10) pC. The minimum emitting anglular spread is (3.6 × 3.8) mrad.
Electromagnetic particle-in-cell simulation of high-power single-hole post-hole convolute
Authors: Wu Han-Yu , Zeng Zheng-Zhong , Qiu Meng-Tong , Zhang Xin-Jun , Guo Ning , Wei Hao ,   State Key Laboratory of Intense Pulsed Radiation Simulation and Effect, Northwest Institute of Nuclear Technology, Xi’an 710024, China
Published: December 2020

The post-hole convolutes (PHCs) are used in pulsed high-power generators to add the output currents of the magnetically insulated transmission lines (MITLs) and deliver the combined current to a single MITL. Then the single MITL delivers the combined current to the load. Magnetic insulation of electron flow is lost near the post-hole convolute (PHC) in the high-power generator. Although cathode plasma and anode ions are widely considered as the factors of the magnetic insulation collapse, there are some other factors that are needed to study. In this paper, the cathode negative ions are considered in the PIC simulation of a single-hole PHC. In this work, we examine the evolution and dynamics of the negative ions in the PHC. The simulation results demonstrate that there are no current losses while the cathode emits only electrons, little current losses (10 kA out of a total current of 900 kA) while the cathode emits plasma including electrons and ions, and obvious current losses (20 kA out of a total current of 900 kA) while the cathode emits plasma including the electrons, ions and negative ions. The results also indicate that the velocity of the negative ions is about 10 cm/μs, larger than that of the cathode plasma including the electrons and the ions. All results suggest that the cathode negative ions can play an important role in the magnetic insulation collapse, and should be considered carefully in experiment.