Vol. 70, No. 6 (2021)
2021-03-20
SPECIAL TOPIC—Toward making functional devices at an atomic scale: Fundamentals and frontiers
COVER ARTICLE
SPECIAL TOPIC—Toward making functional devices at an atomic scale: Fundamentals and frontiers
EDITOR'S SUGGESTION
2021, 70 (6): 060702.
doi: 10.7498/aps.70.20201870
Abstract +
Rydberg atoms have large electric dipole moments in the microwave and terahertz frequency band. The detection of electromagnetic field intensity in this frequency band can be achieved by using quantum interference effects. Theoretically, this detection method can have a sensitivity much higher than the traditional detection methods. Therefore, electromagnetic field detection and precision measurement technology based on Rydberg atomic quantum effects has great application prospects in terahertz field strength and power measurement, terahertz communication and imaging. In this paper, we review the basic theory and experimental methods to realize the self-calibration and traceability measurement of electromagnetic field based on Rydberg atomic quantum effects. The principle and technical scheme of high-sensitivity terahertz field strength measurement, terahertz near-field high-speed imaging and terahertz digital communication based on Rydberg atom are introduced in detail. Finally, the processing terahertz detection work based on Rydberg atom by our research team is also mentioned briefly.
EDITOR'S SUGGESTION
2021, 70 (6): 060703.
doi: 10.7498/aps.70.20201924
Abstract +
Measurement technology with nanometer scale or higher level precision is the basis and guarantee for developing atomic and close-to-atomic scale manufacturing. Optical measurement has the advantages of high precision, wide range and real-time measurement. The precision of localizing a single imaging spot’s center is not limited by the diffraction limit and could reach nanometer scale. However, the shot noise of light and the dark current noise of the detector bring about a precision limit for optical measurement. Based on the Cramer-Rao lower bound theory, a precision limit estimation method for general imaging profiles is developed in this paper. Taking the typical Airy spot for example, the influences of the parameters such as signal-to-noise ratio, energy concentration and processing method on the positioning precision limit are analyzed, and suggestions and conclusions for improving the measurement precision are given. The precision limit of a laboratory imaging spot is calculated, which verifies that the conclusions are also suitable for the imaging profiles similar to the Airy spot. The research provides the analytical method and theoretical guidance for the application and optimization of optical measurement in atomic and close-to-atomic scale manufacturing.
2021, 70 (6): 064701.
doi: 10.7498/aps.70.20201613
Abstract +
Since the discovery of graphene, a large number of two-dimensional (2D) materials have been found and studied. The charge carriers of 2D materials are restrained in a 1 nm physical space, which results in high sensitivity of charge carriers to chemical or electrical doping. It brings a technical innovation into a biosensing field. No matter what sensing mechanism the biosensor process is based on, it includes the process of detecting object recognition and signal transformation. The target recognition is normally realized by nano-bioprobes at the sensing interfaces of the devices. After the recognition, 2D materials at the biosensing interface can realize signal output. Constructing bioprobes and 2D materials at an atomic level at the biosensing interface can modulate the physical and chemical activity precisely in the process of sensing, which improves the sensing performances of devices. Here, we review the recent progress of constructing the 2D biosensing interfaces. Especially, we discuss various biosensing mechanisms and different nano-bioprobes. We also suggest the further research direction of this field.
2021, 70 (6): 066801.
doi: 10.7498/aps.70.20202072
Abstract +
Atomic scale characterization and manipulation is one of the physical bottlenecks, which needs to be broken when realizing atom manufacturing. The aberration-corrected transmission electron microscopy (TEM) is a powerful tool for structural characterization due to its exceptional spatial resolution. Therefore, it is very crucial to co-characterize atomic-scale three-dimensional structure and properties of atomic manufacturing materials by using TEM, which allows us to further understand the physics mechanism of atomic manipulation of materials. Nano-clusters and nanoparticles are two of the main objects in the studies of atomic manufacturing materials and devices, and possess rich physical and chemical properties and high manoeuverability. In this paper, we summarize the recent progress of quantitatively determining three-dimensional structures and magnetic properties of nanocluster, nanoparticles and nanograins, as well as their dynamic evolutions under the working conditions. The methodological breakthrough and development of electron microscopy techniques provide a solid foundation for precisely controlling atomic manufacturing materials.
2021, 70 (6): 068101.
doi: 10.7498/aps.70.20201917
Abstract +
Atomic-scale fabrication is an effective way to realize the ultra-smooth surfaces of semiconductor wafers on an atomic scale. As one of the crucial manufacturing means for atomically precise surface of large-sized functional materials, chemical mechanical polishing (CMP) has become a key technology for ultra-smooth and non-damage surface planarization of advanced materials and devices by virtue of the synergetic effect of chemical corrosion and mechanical grinding. It has been widely used in aviation, aerospace, microelectronics, and many other fields. However, in order to achieve ultra-smooth surface processing at an atomic level, chemical corrosion and mechanical grinding methods commonly used in CMP process require some highly corrosive and toxic hazardous chemicals, which would cause irreversible damage to the ecosystems. Therefore, the recently reported green chemical additives used in high-performance and environmentally friendly CMP slurry for processing atomically precise surface are summarized here in this paper. Moreover, the mechanism of chemical reagents to the modulation of materials surface properties in the CMP process is also analyzed in detail. This will provide a reference for improving the surface characteristics on an atomic scale. Finally, the challenges that the polishing slurry is facing in the research of atomic-scale processing are put forward, and their future development directions are prospected too, which has profound practical significance for further improving the atomic-scale surface accuracy.
2021, 70 (6): 068102.
doi: 10.7498/aps.70.20202014
Abstract +
At present, owing to the inherent limitations of the material characteristics of Si based semiconductor materials, Si based semiconductors are facing more and more challenges in meeting the performance requirements of the rapidly developing modern power electronic technologies used in semiconductor devices. As a new generation of semiconductor material, SiC has significant performance advantages, but it is difficult to process the SiC wafers with high-quality and high-efficiency in their industrial application. Reviewing the research progress of ultra-precision machining technology of SiC in recent years, we introduce plasma oxidation modification based highly efficient polishing technology of SiC in this paper. The material removal mechanism, typical device, modification process, and polishing result of this technology are analyzed. The analysis shows that the plasma oxidation modification possesses high removal efficiency and atomically flat surfaces without surface or subsurface damages. Furthermore, aiming at step-terrace structures produced during SiC surface processing with different polishing technologies, the generation mechanism and control strategy of periodic atomic layer step-terrace structures are discussed. Finally, the development and challenge of plasma-assisted polishing technology are prospected.
EDITOR'S SUGGESTION
2021, 70 (6): 068201.
doi: 10.7498/aps.70.20201689
Abstract +
Atomic and atom-like manufacturing has thoroughly investigated by researchers from physical science and materials science in recent years. Several novel properties which cannot be explained by classical theories can be revealed by materials in the case of the manufacturing scale progressing from micron and nanometer to atomic level gradually, so that researchers from related fields have shown the constant pursuit of ultimate manufacturing scales and subversive properties. As an advanced method of precisely manipulating the structural units on a nanoscale, DNA nanotechnology has brought a new insight into nano/atomic manufacturing during its evolution. Meanwhile, the DNA origami technique has proposed the solutions for the accurate fabrication of matters based on its remarkable programmability in design process and might create opportunities for precise construction under more minute scale and more arbitrary shape for multiple matters and materials. In this review, we first briefly summarize the fundamentals, evolutions and several representative researches of DNA origami technique, and then we further summarize some corresponding investigations of nano-fabrications based on the DNA origami structures according to the fabrication strategies. Finally, we put forward some considerations of the potential feasibility in utilizing DNA origami structures for atomic manufacturing and give some prospects for the future development of this field.
EDITOR'S SUGGESTION
2021, 70 (6): 063201.
doi: 10.7498/aps.70.20201401
Abstract +
We present a high-sensitivity weak microwave measurement and communication technology by employing the Rydberg beat technique. The Rydberg cascade three-level system is composed of a cesium ground state $6{\rm{S}}_{1/2}$ , an excited state $6{\rm{P}}_{3/2}$ , and a Rydberg state $n{\rm{D}}_{5/2}$ in a room-temperature cesium cell. A two-photon resonant Rydberg electromagnetic induced transparency (EIT) is used to optically detect the Rydberg level, in which a weak probe laser is locked at the resonant transition of $|6{\rm{S}}_{1/2}\rangle \rightarrow |6{\rm{P}}_{3/2}\rangle$ , and a strong coupling laser drives the transition of $|6{\rm{P}}_{3/2}\rangle \rightarrow |n{\rm{D}}_{5/2}\rangle$ . Both lasers are locked with a high-precision Fabry-Perot cavity. Two E-fields are incident into the vapor cell to interact with Rydberg atoms via a microwave horn, one is a strong microwave field with frequency 2.19 GHz, acting as a local field ($E_{{\rm{L}}}$ ) and resonantly coupling with two Rydberg energy levels, $|68{\rm{D}}_{5/2}\rangle$ and $|69{\rm{P}}_{3/2}\rangle$ , and the other is a weak signal field ($E_{{\rm{S}}}$ ) with frequency difference ${\text{δ}} f$ , interacting with the same Rydberg levels. The wave-absorbing material is placed around the vapor cell to reduce the reflection of microwave field. In the presence of the local field, the Rydberg atoms are employed as a microwave mixer for reading out the difference frequency ${\text{δ}}f$ oscillation signal, which is proportional to the amplitude of weak signal field. The minimum detectable field of $E_{0} = 1.7$ μV/cm is obtained when the lock-in output reaches the base noise. We also measure the frequency resolution of the Rydberg mixer by changing the ${\text{δ}} f$ with fixed $ f_{\rm ref} $ , thus achieving a frequency resolution better than 1 Hz. For neighboring fields with 1 Hz away from the signal field, an isolation of 60 dB is achieved. Furthermore, we use the Rydberg atom as an antenna to receive the baseband signals encoded into the weak microwave field, demonstrating that the receiver has a transmission bandwidth of about 200 MHz. The demonstration of sensitivity of Rydberg atoms to microwave field is particularly useful in many areas, such as quantum precise measurement and quantum communications. In general, this technique can be extended to the detection of electromagnetic radiation from the radio-frequency regime to the tera-hertz range and is feasible for fabricating a miniaturized devices, thereby providing us with a way to receive the information encoded in tera-hertz carriers in future work.
EDITOR'S SUGGESTION
2021, 70 (6): 068501.
doi: 10.7498/aps.70.20201848
Abstract +
The nitrogen-vacancy center structure of diamond has attracted widespread attention due to its high sensitivity in quantum precision measurement. In this paper, a coupled phonon field is used to resonantly regulate the atomic spins of the nitrogen-vacancy center for improving the spin transition efficiency. Firstly, the interaction between phonons and lattice energy is analyzed based on the relationship between the wave function and the lattice displacement vector. The spin transition mechanism is investigated based on phonon resonance regulation, and the strain-induced energy transferable phonon-spin interaction coupling excitation model is established. Secondly, the coefficient matrix satisfying Bloch’s theorem is adopted to develop the phonon spectrum model of the first Brillouin zone characteristic region for different axial nitrogen-vacancy centers. Considering the thermal expansion, the thermal balance properties of phonon resonance system are analyzed and its specific heat model is studied based on the Debye model. Finally, the structure optimization model of different axial nitrogen-vacancy centers under the phonon model is built up based on the molecular dynamics simulation software CASTEP and density functional theory for first-principles research. The structural characteristics, phonon characteristics, and thermodynamic properties of nitrogen-vacancy centers are analyzed. The research results show that the evolution of phonon mode depends on the occupation of the nitrogen-vacancy center. A decrease in thermodynamic entropy accompanies the strengthening of the phonon mode. The covalent bond of diamond with nitrogen-vacancy center is weaker than that of a defect-free diamond. The thermodynamic properties of a defect-free diamond are more unstable. The primary phonon resonance frequency of diamond with nitrogen-vacancy centers are on the order of THz, and the secondary phonon resonance frequency is about in a range of 800 and 1200 MHz. A surface acoustic wave resonance mechanism with an interdigital width of 1.5 μm is designed according to the secondary resonance frequency, and its center frequency is about 930 MHz. The phonon resonance control method can effectively increase the spin transition probability of nitrogen-vacancy center under suitable phonon resonance control parameters, and thus realizing the increase of atomic spin manipulation efficiency.
COVER ARTICLE
2021, 70 (6): 068702.
doi: 10.7498/aps.70.20201438
Abstract +
The fabrication of precise arrays of atoms is a key challenge at present. As a kind of biomacromolecule with strict base-pairing and programmable self-assembly ability, DNA is an idea material for directing atom positioning on predefined addresses. Here in this work, we propose the construction of iron atom arrays based on DNA origami templates and illustrate the potential applications in cryptography. First, ferrocene molecule is used as the carrier for iron atom since the cyclopentadienyl groups protect iron from being affected by the external environment. To characterize the iron atom arrays, streptavidins are labelled according to the ferrocene-modified DNA strand through biotin-streptavidin interactions. Based on atomic force microscopy scanning, ferrocene-modified single-stranded DNA sequences prove to be successfully immobilized on predefined positions on DNA origami templates with high yield. Importantly, the address information of iron atoms on origami is pre-embedded on the long scaffold, enabling the workload and cost to be lowered dramatically. In addition, the iron atom arrays can be used as the platform for constructing secure Braille-like patterns with encoded information. The origami assembly and pattern characterizations are defined as encryption process and readout process, respectively. The ciphertext can be finally decoded with the secure key. This method enables the theoretical key size of more than 700 bits to be realized. Encryption and decryption of plain text and a Chinese Tang poem prove the versatility and feasibility of this strategy.
EDITOR'S SUGGESTION
2021, 70 (6): 069801.
doi: 10.7498/aps.70.20201482
Abstract +
Hexagonal boron nitride (h-BN) is considered as an ideal substrate material for new electronic devices and nano-electromechanical (NEMS) devices, owing to its hexagonal network lattice structure and high chemical and mechanical stability. It can be used to seal gas with a long-term stability, and then has a big potential in further applications in electronics and NEMS. Recently, researchers have discovered that hydrogen atoms can penetrate multiple layers of h-BN non-destructively, forming the bubbles between layers, which can be used as NEMS devices. In this article, we investigate the effect of hydrogen plasma treatment duration on the size of h-BN bubbles. It is found that the size of bubbles becomes larger with the increase of treatment time while their distribution density decreases. It is also observed that the prepared h-BN bubbles have similar morphological characteristics, which are related to Young’s modulus of h-BN and interlayer van der Waals interaction. With the help of force-displacement curve measurement, it is obtained that the internal pressure is about 1—2 MPa for micro-sized bubbles, while the internal pressure of nano-sized bubbles can reach a value of GPa.
REVIEW
2021, 70 (6): 060501.
doi: 10.7498/aps.70.20201517
Abstract +
In lunar circumstances, lunar dust has special properties such as conductivity, which can cause lunar dust to easily adhere to the surface of detection equipment. And this behavior will cause the equipment to fail to function properly and thus affecting the lunar exploration missions. According to the researches of lunar dust protection, in this article the passive protection technology of lunar dust is mainly analyzed. Firstly, the lunar-dust caused adverse factors and effects on detection equipment are analyzed. Then the mechanism of lunar dust adhesion is studied, and the theoretical basis of the two main forces that cause adhesion is discussed. Secondly, the main methods of reducing the adhesion of lunar dust particles are systematically explained according to different adhesion mechanisms, and the latest progress of the passive protection technology of the lunar dust is introduced in detail. Combined with the different protection methods, the method of testing the adhesion of the lunar dust is summarized. These studies lay the foundation for effectively protecting the surface of detection equipment from being affected by the lunar dust.
GENERAL
2021, 70 (6): 060201.
doi: 10.7498/aps.70.20201523
Abstract +
Since the discovery of carbon nanotubes (CNTs), they have attracted extensive attention from scholars in various fields because of their excellent properties. The hollow-structured CNTs are often regarded as conduits and containers, which can act as nano-channels for various molecular substances in the membrane structure. As a source of life, water is indispensable to any living organism. In the application of carbon nanotubes as nanochannels, the most important is the ability of carbon nanotubes to store and transport liquids, especially nanoscaled aqueous solutions. Water molecular clusters in confined spaces exhibit unusual structures and properties. The study of special water structures in carbon nanotubes is of great theoretical importance in chemistry, biology and materials science. There are great difficulties in making the experiment on a nanoscale, but molecular dynamic simulation enables us to better study and analyze the structure and properties of water in confined space of CNT on a nanoscale. One has also studied the influence of temperature on the structure of water, but there are few studies focusing on the effect of temperature on the structure of water in confined space. Therefore, molecular dynamics simulation is used to investigate the effects of CNT diameter, CNT chirality and temperature on the water structure and distribution in a confined space. The simulation calculation is completed by GROMACS, the SPE/C water model is used for water molecules, and GROMOS96 54a7 force field is used. Because of the presence of carbon nanotubes, water molecules tend to line up against the walls of the tubes, both inside and outside. In addition, water molecules tend to form highly ordered multi-ring structures in the carbon nanotubes with a size of 1.018–1.253 nm at a certain temperature. It is difficult to form the ordered structure of water in the outer carbon nanotubes. In the above range, with the increase of pipe diameter, the structure of multi-element ring water changes from three-element ring to six-element ring. On the one hand, the ordered structure depends on the diameter of the carbon nanotube, but the chirality of the carbon nanotube does not have a great influence on it. On the other hand, the stability of the ordered structure is temperature-dependent, and the ordered structure of multiple ring water in the carbon nanotube with a larger diameter is more likely to disappear with the increase of temperature. The van der Waals potential distribution is calculated by Multiwfn, and it is concluded that the van der Waals potential inside the tube is extremely low, resulting in a very large dispersion effect, and molecules can spontaneously move from the outer area to the tube. The van der Waals potential can also be negative outside the tube. This explains why water molecules tend to line up against the wall of the tube.
2021, 70 (6): 060701.
doi: 10.7498/aps.70.20201530
Abstract +
The infrared detector can generate nonlinear response error when the Fourier transform infrared spectrometer is used for implementing the radiometric calibration or observing the high temperature targets. Based on the relationship between the incident radiation intensity and the electron concentration in the optical conduction band, the mechanism of the nonlinear response error caused by the high incident photon flow is analyzed. According to Planck radiation law and interference principle, the effect of nonlinear error on spectrum is studied by simulating blackbody radiation data with nonlinear error. It is found that the nonlinear response with a different order has a different influence region, and the higher-order nonlinear response has a wider influence range and generates a larger nonlinear response error. By the general nonlinear response correction method the nonlinear response coefficient is obtained through constructing the nonlinear response model of the interference data and then the spectral distortion produced by the detector is corrected. According to the convolution iteration method, the polar orbit meteorological satellite CrIS constructs the convolution equation to correct the second-order nonlinear response by taking the low-wave number band of 50－500 cm–1 as the characteristic region. The European Meteorological Agency’s Airborne Infrared Interferometer Evaluation System (ARIES) selected two feature areas, 50－500 cm–1 and 2000－2500 cm–1, and iteratively corrected the second-order and third-order nonlinear response. The gradient descent method is often used to solve the optimization problems of unconstrained multivariate functions. Based on the gradient descent algorithm, an iterative method suitable for correcting the high-order nonlinear response errors is proposed in this paper. In this method, the information about the iteration point is obtained by constructing the nonlinear response function of the high-order detector and setting the appropriate iteration initialization. According to the initial value of the iteration and the information about the known iteration point, the gradient of the iteration variable is calculated to determine the iteration value of the next unknown variable, thus quickly searching for the global minimum point and determining the nonlinear response coefficient. We use Fourier transform infrared spectrometer to carry out radiometric calibration experiment and compare the effects of three correction methods: convolution, cross iteration and gradient descent method. The results show that the three correction methods can effectively reduce the nonlinear error, and improve the fitting extent by 0.15%, 0.29% and 0.39% respectively. The spectral data corrected by gradient descent method are more accurate.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2021, 70 (6): 061301.
doi: 10.7498/aps.70.20201557
Abstract +
The jet tagging task in high-energy physics is to distinguish signals of interest from the background, which is of great importance for the discovery of new particles, or new processes, at the large hadron collider. The energy deposition generated in the calorimeter can be seen as a kind of picture. Based on this notion, tagging jets initiated by different processes becomes a classic image classification task in the computer vision field. We use jet images as the input built on high dimensional low-level information, energy-momentum four-vectors, to explore the potential of convolutional neural networks (CNNs). Four models of different depths are designed to make the best underlying useful features of jet images. Traditional multivariable method, boosted decision tree (BDT), is used as a baseline to determine the performance of networks. We introduce four observable quantities into BDTs: the mass, transverse momenta of fat jets, the distance between the leading and subleading jets, and N-subjettiness. Different tree numbers are adopted to build three kinds of BDTs, which is intended to have variable classifying abilities. After training and testing, the results show that the CNN 3 is the neatest and most efficient network under the design of stacking convolutional layers. Deepening the model could improve the performance to a certain extent but it is unable to work all the time. The performances of all BDTs are almost the same, which is possibly due to a small number of input observable types. The performance metrics show that the CNNs outperform the BDTs: the background rejection efficiency increases up to 150% at 50% signal efficiency. Besides, after inspecting the best and the worst samples, we conclude the characteristics of jets initiated by different processes: jets obtained by Z boson decays tend to concentrate in the center of jet images or have a clear differentiable substructure; the substructures of jets from general quantum chromodynamics processes have more random forms and not only just have two subjets. As the final step, the confusion matrix of the CNN 3 indicate that it comes to be kind of conservative. Exploring the way of keeping the balance between conservative and radical is our goal in the future work.
ATOMIC AND MOLECULAR PHYSICS
2021, 70 (6): 063101.
doi: 10.7498/aps.70.20201657
Abstract +
The development potential of germanene-based integrated electronics originates from its high carrier mobility and compatibility with the existing silicon-based and germanium-based semiconductor industry. However, the small band gap energy band (Dirac point) of germanene greatly impedes its application. Thus, it is necessary to open a sizeable band gap without reducing the carrier mobility for the application in logic circuits. In this study, the effects of organic molecule (benzene or hexafluorobenzene) adsorption and substrate on the atomic structures and electronic properties of germanene under an external electric field are investigated by using density functional theory calculations with van der Waals correction. For benzene/germanene and hexafluorobenzene/germanene systems, four different adsorption sites are considered, with the center of the organic molecules lying directly atop the upper or lower Ge atoms of germanene, in the Ge-Ge bridge center, and on the central hollow ring. Meanwhile, different molecular orientations at each adsorption site are also considered. Thus, there are eight high-symmetry adsorption configurations of the systems, respectively. According to the adsorption energy, we can determine the most stable atomic structures of the above systems. The results show that the organic molecule adsorption can induce the larger buckling height in germanene. Both the adsorption energy and interlayer distance indicate that there is no chemical bond between the organic molecules and germanene. Mulliken population analysis shows that a charge redistribution in the two sublattices in germanene exists since benzene is an electron donor molecule and hexafluorobenzene is an electron acceptor molecule. As a result, the benzene/germanene system exhibits a relatively large band gap (0.036 eV), while hexafluorobenzene/germanene system displays a small band gap (0.005 eV). Under external electric field, germanene with organic molecule adsorption can exhibit a wide range of linear tunable band gaps, which is merely determined by the strength of electric field regardless of its direction. The charge transfer among organic molecules and two sublattices in germanene gradually rises with the increasing the strength of electric field, resulting in the electron density around the sublattices in germanene unequally distributed. Thus, according to the tight-binding model, a larger band gap at the K-point is opened. When germanane (fully hydrogenated germanene HGeH) substrate is applied, the band gaps further widen, where the band gap of benzene/ germanene/germanane system can increase to 0.152 eV, and that of hexafluorobenzene/germanene/germanane system can reach 0.105 eV. The sizable band gap in germanene is created due to the symmetry of two sublattices in germanene destroyed by the dual effects of organic molecule adsorption and substrate. Note that both of organic molecules and substrate are found to non-covalently functionalize the germanene. As the strength of the negative electric field increases, the band gaps can be further modulated effectively. Surprisingly, the band gaps of the above systems can be closed, and reopened under a critical electric field. These features are attributed to the build-in electric field due to the interlayer charge transfer of the systems, which breaks the equivalence between the two sublattices of germanene. More importantly, the high carrier mobility in germanene is still retained to a large extent. These results provide effective and reversible routes to engineering the band gap of germanene for the applications of germanene to field-effect transistor and other nanoelectronic devices.
2021, 70 (6): 063301.
doi: 10.7498/aps.70.20201427
Abstract +
Surface magnetic resonance sounding (MRS) has generally been considered to be an efficient tool for hydrological investigations. As is well known, the effective relaxation time $ T_2^*$ which characterizes the decay rate of MRS free-decay-induction (FID) signal and is used to measure pore-scale properties, is particularly limited for several special cases (e.g. areas with magnetic rock subsurfaces). Recent years, the transverse relaxation time $ T_2$ obtained from spin-echo signal was adopted to implement the surface MRS, and showed great potentials for estimating the porosity and permeability. However, owning to the short period of development, the related modeling and inversion strategies are rarely introduced and summarized. Actually, the general practice for surface MRS $ T_2$ measurement fits the spin-echo by the exponential function and the fitting line was directly used as the FID signal for inversion. This scheme not only limits the precision of interpretation, but also loses part of valid information about original field data. Aiming at these problems, in this paper, we introduce the calculation of forward model and thus a two-stage framework with singular value decomposition (SVD) linear inversion involved is derived to quantify the $ T_2$ distributed with depth. Considering the fact that the inversion result of SVD is always strongly affected by the noise level, an improved method which combines the simultaneous iterative reconstruction technology (SIRT) with SVD is proposed. To be specific, we compare the measurement schemes with kernel functions between $ T_2$ and the original theory in MRS, and then provide the forward and inversion formulations. In order to substantiate the effectiveness of this method, we conduct the synthetic experiments for Carr-Purcell-Meiboom-Gill sequence and explain the dataset with the mentioned strategies. As expected, the combined approach possesses a better performance in shallow layer with an error of 1.5% and 0.02 s for water content and $ T_2$ for the contaminated data, respectively. With these advantages, it is expected to realize the adoption of the SVD with SIRT in field applications and further investigate the aquifer characterizations in the future.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2021, 70 (6): 064101.
doi: 10.7498/aps.70.20201393
Abstract +
The research on space transmission characteristics of terahertz wave is of great significance for the application of terahertz wave in space. In order to study the transmission characteristics of terahertz wave in sand and dust storm weather, according to the lognormal distribution of dust particle sizes, Mie scattering theory and Monte Carlo method are used to analyze the attenuation characteristics of six dry sand modes of sand and dust storm in different regions of China in a frequency band of 1–10 THz, and the relationship of the extinction parameters and attenuation rate to the frequency is given. The results show that with the increase of frequency, the attenuation rate of 1–10 THz terahertz wave first increases and then decreases. Different mode of sand and dust storm leads to different frequency range of strong attenuation of terahertz wave. In order to analyze the influence of sand dust particle moisture content on terahertz wave propagation attenuation, the relationship of three efficiency factors to water content of sand dust particles with different sizes is calculated. The results show that the influence of water content on extinction is different from that of the particle size. Monte Carlo method is used to calculate the attenuation of terahertz wave by sand and dust storm in two kinds of wet sand modes, and the relationship of the attenuation rate and water content to the frequency is given, the results are compared with those from the dry sand mode, showing that the albedo of wet sand mode is obviously lower than that of dry sand mode with the same size distribution. The absorption of wet sand particles increases with water content increasing. The extinction of wet sand and dust storm results from scattering and absorption. With the increase of water content in sand particles, the frequency band with strong attenuation of terahertz wave by wet sand and dust storm moves toward low frequency. When the water content is less than 5%, the attenuation rate of terahertz wave increases significantly with the increase of water content. Sand and dust storms with higher humidity have a greater influence on the transmission attenuation of terahertz wave.
2021, 70 (6): 064201.
doi: 10.7498/aps.70.20201573
Abstract +
Ultraviolet femtosecond laser pulse is an important tool in studying ultrafast chemical and physical processes. Realizing broadband ultraviolet laser pluses with a wide tunable range would significantly facilitate the study of ultrafast processes. As an effective and convenient method, the cascaded four-wave mixing (CFWM) has been widely adopted to generate broadband and tunable ultraviolet femtosecond laser pulses. In this work, we carry out CFWM in MgO crystal by using two 400-nm pulses to generate tunable ultraviolet femtosecond pulse. The MgO crystal is chosen due to its high third-order nonlinear susceptibility, large band gap and high transmittance in the ultraviolet region. In the experiment, nine frequency up-converted and five frequency down-converted sidebands are observed. The measured wavelength and scattering angle of each sideband are consistent with the CFWM theory predictions. The wavelength range of the sidebands covers 350–450 nm. The total conversion efficiency of the ultraviolet sidebands is 1.2%, which is higher than the reported values with visible/near infrared driven lasers. Meanwhile, the spectra of the high-order sidebands present a Gaussian profile and can support a Fourier-transform-limited pulse duration of less than 50 fs. Besides, the central wavelengths of the sidebands can be effectively tuned by adjusting the time-delay between the two pre-chirped pump pulses. Our study provides an efficient and convenient scheme to generate short ultraviolet femtosecond pulses with a wide tunable range.
2021, 70 (6): 064202.
doi: 10.7498/aps.70.20201474
Abstract +
Aiming at the phenomenon of single measurement parameters and low sensitivity of most Mach-Zehnder sensors based on fiber core mismatch, in this paper we design and build a Mach-Zehnder sensor based on single-mode-no-core-single-mode-no-core-single-mode fiber structure, which can be used to measure refractive index and temperature simultaneously. In this sensor, two no-core optical fiber serve as input and output couplers, the intermediate single-mode is used as a sensing arm. Using finite element simulation and theoretical analysis, the optimal length of the coupler and the sensing arm are determined to be 15 mm. High-order modes excited by no-core optical fiber propagate through the cladding of single-mode fiber, which is affected by the ambient refractive index and temperature because of the influence of the evanescent filed. Trough of different interference orders of transmission spectrum is selected as a research object to realize the simultaneous measurement of refractive index and temperature by using sensitivity coefficient matrix. After the further Fourier transform of the transmission spectrum, the frequency of the main mode that interferes with the fundamental mode is analyzed from the spectrogram to be 0.00098 nm–1. Because of the influence of temperature on the refractive index of water during temperature sensitivity measurement, temperature sensitivity formula and water temperature coefficient are introduced to perform temperature compensation to eliminate the cross sensitivity. In this paper, the 10 mm and 15 mm sensing arms are selected for refractive index comparison experiment, and the temperature experiment is focused on the sensing arm with an optimal length of 15 mm. The experimental results show that the transmission spectrum is blue-shifted with the increase of refractive index in a refractive index range of 1.333–1.397, and the transmission spectrum is red-shifted with the increase of temperature in a temperature range from 30 ℃ to 70 ℃. The refractive index and temperature sensitivity of the interference valley near 1545 nm are –153.89 nm/RIU and 0.166 nm/℃, respectively; the refractive index and temperature sensitivity of the interference valley near 1570 nm are –202.74 nm/RIU and 0.183 nm/℃, respectively. The experimental results are consistent with the theoretical analyses. Compared with the sensor of the same type, this sensor can still maintain high sensitivity while achieving simultaneous measurement of refractive index and temperature, and has a simple structure, which has a good application prospect in biomedical and other aspects.
2021, 70 (6): 064301.
doi: 10.7498/aps.70.20201726
Abstract +
The sound propagation problems in range-dependent waveguides are a common topic in underwater acoustics. The range-dependent factors, involving volumetric and bathymetric variations, significantly influence the propagation of sound energy and information. In this paper, a coupled-mode method based on the multimodal admittance method is presented for analyzing the sound propagation and scattering problems in range-dependent waveguides. The sound field is expanded in terms of a local basis with range-dependent modal amplitudes. The local basis corresponds to the transverse modes in a waveguide with constant physical parameters and constant cross section equal to the local cross section in the range-dependent waveguide. This local basis takes the advantage that it is easier to compute than the usual local modes which are the transverse modes in a waveguide with local physical parameters and constant cross-section equal to the local cross-section, especially for waveguides with complex environments. Projection of the Helmholtz equation that governs the sound pressure onto the local basis gives the second-order coupled mode equations for the modal amplitudes of the sound pressure. The correct boundary conditions are used in the derivation, giving rising to boundary matrices, in order to guarantee the conservation of energy among modes. The second-order coupled mode equations include coupled matrices and boundary matrices, which directly describe the effect of mode coupling due to contribution from volumetric variation (range-dependent physical parameters) and bathymetric variation (range-dependent boundaries). By introducing the admittance matrix, the second-order coupled mode equations are reduced to two sets of first-order evolution equations. The Magnus integration method is used to solve the first-order evolution equations. These first-order evolution equations allow us to obtain the numerical stable solutions and avoid the numerical divergence due to the exponential growth of evanescent modes. The numerical examples are presented for the waveguides with range-dependent physical parameters or range-dependent boundaries. The agreement between the results computed with the coupled mode method and COMSOL verifies the accuracy of the coupled mode method. Although the analysis and numerical implementation in this paper are based on two-dimensional waveguides in Cartesian coordinate system, it can be generally extended to study more complex waveguides.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2021, 70 (6): 065201.
doi: 10.7498/aps.70.20201339
Abstract +
Recently, the short-pulse intense laser has become a common tool for studying the relativistic plasma with tremendous physical parameters. And the laser-driven magnetic reconnection is one of the hot topics and has received much attention. The laser-driven magnetic reconnection experiments are usually conducted by closely focusing two laser beams on a planar coil target. However, it is always hard to distinguish the physical property of magnetic reconnection from the complex background of laser-produced plasma. In this paper, we present the particle-in-cell simulation results of magnetic reconnection driven by two short-pulse lasers as well as a single laser pulse irradiating the solid planar target, and discuss the correlation between the potential distribution behind the target and the magnetic reconnection. When a single laser is used, the potential behind the target shows a double-peak distribution, which is in agreement with recent experimental results. When two lasers irradiate the target, the potential behind the target shows a three-peak distribution. The accumulated spatial distribution of plasma ions with fixed energy (4.5 and 6 MeV) at 3 μm behind the target shows several peaks, which is in agreement with the potential distribution when either a single laser or two lasers are used. In addition, after the laser pulse terminates, in the two-laser case there is extremely strong effect on the topological structure of the electric field compared with in the singlelaser case. When the magnetic reconnection happens (which can be identified through the reconnection electric field and the electron energy spectrum), the amplitude of the x component of the electric field has different evolution characteristics from the single laser case. The line outs of the y component of the electric field in two cases also have completely different shapes. In summary, the simulation results reveal that the potential distribution behind the target can directly affect the spatial distribution of the accelerated ions. This could be possibly used to identify the short pulse laser-driven magnetic reconnection in experiment.
2021, 70 (6): 065202.
doi: 10.7498/aps.70.20201587
Abstract +
Benefiting from laser preheat and magnetization, magnetized liner lnertial fusion (MagLIF) has a promising potential because theoretically it can dramatically lower the difficulties in realizing the controlled fusion. In this paper, the end loss effect caused by laser preheat in MagLIF process is chosen as an objective to explore its influences, and a one-dimensional and heuristic model of this effect is proposed based on the jet model of ideal fluid, in which the high-dimensional influences, such as geometric parameters and sausage instability, are taken into consideration. To complete the verification progress, the calculation results of one-dimensional MIST code and two-dimensional programs TriAngels and HDYRA are compared, and the application scopes of this heuristic model are discussed and summarized. Based on this model, the key parameters and influences of the end loss effect on the MagLIF implosion process and pre-heating effect are obtained. The calculation results show that the MagLIF load maintains a similar hydrodynamic evolution process in most of the implosion processes with different laser entrance radii, and experiences the same percentage of mass (～16%) lost during stagnation stage. With the same driving current, the fuel temperature will rise higher in the model with more mass losing, so the fusion yields do not change too much. The mass loss ratio seems to play a dominant role. It is recommended to design the laser entrance hole as small as possible in the experiment to increase the yield. The predictions obtained after considering the end loss effect lower the preheating temperature and fusion yield, but no change happens to the regularity trend. As the liner height increases, the preheating temperature, peak current, fuel internal energy, and fusion yield each still show a monotonically downward trend. Therefore, under the premise of fixed driving capability and laser output capability, it is suggested that the liner height in MagLIF load design should be as short as possible. The established heuristic model and conclusions are helpful in better understanding the physical mechanism in the process of MagLIF preheat and end loss.
2021, 70 (6): 065203.
doi: 10.7498/aps.70.20201574
Abstract +
The fast Z-pinch plasma formation, exploding dynamics, and the evolution of the instability can be controlled experimentally by making special structures on metal surface layer to change the initial state of material, which is valuable for studying the Z-pinch physics. Experiments on the explosion of thin flat foils which have been etched into a periodical structure on surface are performed on the QG-1 facility (～1.4 MA peak current, ～100 ns rise time) in order to study the effects of different surface conditions on explosion and control the evolution of the instability in fast Z-pinch plasma. A kind of inverse load configuration is used in experiment in which the return current post is set at the central axial-position and two modified flat foils are strained outside symmetrically as the main load. So the corresponding J × B force directs outward from the return current post orthogonal to the foil plane, creating an acceleration and pushing the foil plasma away from the center in this configuration. Different surfaces of the foil are also investigated in different conditions because of the asymmetric magnetic field distribution which is useful to study the different evolutions of instability. The foils used in the experiment mainly are the 30-μm-thick aluminum foil. The wavelength of groove perturbations seeded on the surface is 2 mm wide and ～10 μm deep. The plasma explosion dynamic behaviors around conditioned area are diagnosed by laser shadowgraphy, laser interferometry, multiframe optical self-emission imaging and B-dot. It is found that the initially etched periodical structure on surface can control the plasma structure in exploding process which can be concluded as follows. Developing plasma structure shows a periodic character similar to the initial surface structure and the eigenwavelength of the Al is suppressed. In the meantime, the surface without etched perturbations is also influenced by the etched side, showing a similar instability structure but with a lower amplitude. The correlation between two surfaces turns stronger than the case of normal foils. A faster expanding rate occurs in the deep region of the initial periodical groove structure which causes a reverse structure to form. In the discontinuous area of the conditoned structure, a narrow stream of plasma jets perpendicularly from the metal surface which causes a half-wavelength to occur in spectrum analysis. The magneto-hydro-dynamic theory analysis shows that the change of electrothermal instabilities is caused dominantly by the modulation of current density flowing around the periodical structure.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2021, 70 (6): 066101.
doi: 10.7498/aps.70.20201697
Abstract +
There are many problems during the preparation of the scintillation crystal Gd3(Al,Ga)5O12:Ce (abbreviated as GAGG:Ce), such as inclusions and antisite-defect. In order to inhibit these defects and obtain large-size and high-quality GAGG:Ce crystal, this study uses Gd3(Al,Ga)5O12 as the matrix and Ce3+ as the doping ions to grow the GAGG:Ce crystal by the Czochralski method. The phase structure, micro-region composition, optical and scintillation properties of GAGG:Ce are tested and compared. It is found that tipical Ce3+ absorption bands are at 340 nm and 440 nm, and the linear transmittance at 550 nm is 82%. The transmittance of the crystal tail drops to about 70% due to the macroscopic defects such as inclusions. The micro-region composition analysis shows that the three types of inclusions in GAGG:Ce crystal are Gd-rich phase, Ce-rich phase, and (Al,Ga)2O3 phase. The Ce3+ ion emission wavelength of GAGG:Ce crystal is about 550 nm excited by the X-ray, and there is also an emission wavelength caused by the GdAl/Ga antisite-defect at 380 nm. The emission intensity of GdAl/Ga antisite-defect in the lack of (Al,Ga) component is higher than that in the excess (Al,Ga) component. The inclusions and GdAl/Ga antisite-defect make the luminous efficiency of GAGG:Ce crystal decrease by 12.5% and the corresponding light yield decreases from 58500 to 52000 photon/MeV. The tunneling effect between GdAl/Ga antisite-defect ions and neighboring Ce3+ ions makes the decay time of the GAGG:Ce crystal extend from 117.7 to 121.9 ns, and the ratio of slow component increases from 16% to 17.2%. The migration of energy along the Gd3+ sublattice makes the rise time of the GAGG:Ce crystal extend from 8.6 to 10.7 ns. The above conclusions further deepen the understanding of the source of inclusions and the relationship between the GdAl/Ga antisite-defect and crystal composition, and provide a theoretical basis for restraining the defects and improving the crystal properties.
2021, 70 (6): 066201.
doi: 10.7498/aps.70.20201591
Abstract +
Graphene has been thought to be an ideal reinforcement material for metal matrix composite due to its superior mechanical properties and unique two-dimensional geometry. However, the deformation mechanism of graphene/aluminum matrix composite is still unclear. In this paper, molecular dynamics simulation is used to elucidate the evolution details of the dislocation microstructure and the underlying interaction behavior between dislocation and graphene during nanoindentation of the graphene/aluminum matrix composite with various graphene orientations. To this end, four different cases, i.e. the pure aluminum and the graphene/aluminum matrix composite with the graphene orientation of 90°, 45° and 0° are examined, respectively. Based on the force-indentation depth curve, the interaction behavior between dislocation and graphene and its effect on the plastic zone are analyzed. The results indicate that the graphene can act as an effective dislocation motion barrier, and the elastic deformation of graphene can occur locally along the direction of dislocation slip. Using the visualization technique of dislocation extraction algorithm, the nucleation and propagation of dislocation are investigated. The results show that the differences in interaction behavior between dislocation and graphene with various orientations affect the spreading trend of the plastic zone and the blocking strength of graphene to dislocation. For the composite with the graphene orientations of 45° and 0°, the interaction between graphene and dislocation causes the number of dislocations to increase. Additionally, the plastic zone of the composite with the graphene orientation of 45° is tangent to two symmetrical graphene sheets. For the composite with the graphene orientation of 90°, the interaction between graphene and dislocation shortens the total length of the dislocation line, and the volume shrinkage of plastic zone is most significant after indenter retraction. Here, the hardness is also calculated to quantitatively evaluate the influence of graphene orientation on the mechanical properties of graphene/aluminum matrix composite. The hardness of the composite with the graphene orientation of 45° is highest, which is due to the decrease of the volume of the plastic zone and the increase of dislocation number. The decrease of the hardness of the composite with the graphene orientation of 90° is attributed to the reduction of dislocation number in the plastic zone. However, for the composite with the graphene orientation of 0°, the interaction between graphene and dislocation results in the softening effect, because of a wide range of elastic deformation in the graphene plane. The study can provide a certain theoretical guidance for designing and preparing the high-performance graphene/metal matrix composites.
2021, 70 (6): 066401.
doi: 10.7498/aps.70.20201748
Abstract +
During directional solidification of binary alloy mixtures, instability in the solid/liquid interface appears due to constitutional undercooling. As a result of this instability, a reactive porous medium, namely mushy layer, is formed, and it separates the liquid phase from the solid phase completely. The intrinsic structure of the mushy layer is of fine-scale dendritic crystal that shelters solute in the interstitial fluid. In a gravitational field, the rejection of lighter solute components from an advancing solidification front brings about unstable density gradient. Ensuing convective motions in the mush are driven by a density difference. The convection can change the solid matrix of the mushy layer. Hence, the dynamic response of the mushy layer is driven by interaction among heat transfer, solute transport and convection. As a contactless control tool, external magnetic field can change the heat and solute transport, which has a significant effect on the phase change process. Therefore, when magnetic field, thermal diffusion, solute transport and buoyancy convection are considered simultaneously in the phase transformation process, the mechanism of mushy region will become more complex and interesting. In this paper, the effect of external magnetic field on the stability of mushy layer during binary alloy solidification is studied. The coupling effects of magnetic field, temperature field, concentration field and convection are considered in the model. Including the direct mode and the oscillation mode, the resulting dispersion relation reveals the influence of magnetic field on the stability of mushy layer through linear stability analysis. It is found that the Lorentz force can reduce the instability effect which is caused by buoyancy convection. In the oscillation mode, an external magnetic field brings about a stabilizing effect on the mushy layer, but in the direct mode, the effect of external magnetic field on stability of the mushy layer is uncertain. In conclusion, the finding in this paper provides an important theoretical reference for improving products quality by applying an external magnetic field in the metals processing industry.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2021, 70 (6): 067101.
doi: 10.7498/aps.70.20201728
Abstract +
First principles calculations are performed to explore the electronic structure and optical properties of BlueP/X Te2 (X = Mo, W) van der Waals heterostructures after biaxial strain has been applied. The type-II band alignments with indirect band gap are obtained in the most stable BlueP/X Te2 heterostructures, in which the photon-generated carriers can be effectively separated spatially. The BlueP/MoTe2 and BlueP/WTe2 heterostructures both have appreciable absorption of infrared light, while the shielding property is enhanced. The increase of biaxial compressive strain induces indirect-direct band gap transition and semiconductor-metal transition when a certain compressive strain is imposed on the heterostructures, moreover, the band gap of the heterostructures shows approximately linear decrease with the compressive strain increasing, and they undergo a transition from indirect band gap type-II to indirect band gap type-I with the increase of biaxial tensile strain. These characteristics provide an attractive possibility of obtaining novel multifunctional devices. We also find that the optical properties of BlueP/X Te2 heterostructures can be effectively modulated by biaxial strain. With the increase of compression strain, the absorption edge is red-shifted, the response of light absorption extends to the mid-infrared light and the absorption coefficient increases to 10–5 cm–1 for the two heterostructures. The BlueP/MoTe2 shows stronger light absorption response than the BlueP/WTe2 in the mid-infrared to infrared region and the ε1(0) increases significantly. The BlueP/X Te2 heterostructures exhibit modulation of their band alignment and optical properties by applied biaxial strain. The calculation results not only pave the way for experimental research but also indicate the great potential applications of BlueP/XTe2 van der Waals heterostructures in narrow band gap mid-infrared semiconductor materials and photoelectric devices.
2021, 70 (6): 067301.
doi: 10.7498/aps.70.20201558
Abstract +
With the rapidly increasing demands pertaining to high voltage applications in modern power electronic systems, power devices have become widely used in today’s power applications. As a major carrier device without unreliable metal-semiconductor Schottky contact, super barrier rectifier (SBR) has been one of promising substitutions for traditional diodes since it was first introduced, owing to its excellent performance and reliability. The main principle behind SBR approach is to create an adjustable potential barrier in the MOS channel. The height of this barrier can be easily adjusted by the doping concentration in the channel and by the oxide thickness. Trench-gate-type SBR (TSBR) with a trench gate is so designed that the junction-type field-effect transistor effect of planar gate structure enables TSBR to be eliminated to have ultralow forward voltages and a good tradeoff between the forward voltages and reverse leakage currents. However, the charge coupling effect under reverse bias, which is usually neglected and not intensively studied, plays an important role in determining the breakdown voltage of TSBR for high voltage applications (above 200 V). In this paper, the two-dimensional electric field distribution influenced by the charge coupling effect is explained and verified by the analytical model and device simulations with TCAD software Sentaurus. Adjusting the key parameters including the trench depth, oxide thickness and mesa width can improve the tradeoff between the forward voltage drops and breakdown voltages. The optimization of key parameters can provide the significant guidance for designing the device structure. Furthermore, some considerations for designing the TSBRs are discussed in this paper. In addition, a novel TSBR with the stepped oxide structure (SO-TSBR) is proposed and demonstrated. Similar to, yet different from, the stepped oxide structure for dual trench MOSFET, the stepped oxide design equipped with this new rectifier possesses the ability to enhance the forward conduction. As indicated by the simulation results, the SO-TSBR reduces the forward voltage drop by 51.49% at a forward current density of 2.5 A/cm2 compared with the normal structure of TSBR, with almost the same breakdown voltage of 270 V. The optimized TSBRs and SO-TSBRs are very promising rectifiers that can be used in high power electronic systems, because their breakdown voltages are both greater than 250 V.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2021, 70 (6): 068502.
doi: 10.7498/aps.70.20201961
Abstract +
Sensory nervous system (SNS) can build the connections between organism and outside environment. Both of synapse and neuron are cornerstones of human biological system, which can transmit information to human brain and receive the feedback from central nervous system. Finally, the corresponding responses to the external information are performed. However, the information from outside environment should be received by SNS all the time. It is important for organism to distinguish between the stimuli that required attention and those that are irrelevant and no need to response. Habituation is one of fundamental properties of SNS to form such discrimination. It plays an important role for organism to adapt the environment and filter out irrelevantly repetitive information. In this study, an nc-Al/AlN structured based memristor with a thickness of 40 nm is produced by the sputtering method. The top and bottom electrode are of Ag and Al respectively, forming a sandwiched structure device. Habituation is found in the nc-Al/AlN thin film based memristor which has been rarely reported before. Both of current-voltage (I-V ) and pulse voltage measurement are executed on this device at room temperature. In the I-V measurement, the memristor shows unipolar switching properties which may be caused by conductive filament connecting or breaking. In the voltage pulse measurement, pulse interval is an important factor to affect memristor conduction. If the pulse interval is quite large, that is, the pulse frequency is low, the memristor will get maximized conduction very slow or in infinity time. If choosing an appropriate pulse voltage and interval value, the habituation will be observed after several stimulus pulses. The larger pulse interval needs more pulse numbers to cause memristor to be habituated, but which results in higher device conduction finally. A habituation memristor can act as synapse and connect with neuron to build the whole leaky integrate-and-fire (LIF) model which is quite often used in circuit design to mimic a real organism neuron behavior. In this model, neuron could be fired only when it gets enough stimuli from previous neuron. If the stimulus pulse frequency is low, there is observed no firing phenomenon in this case. In this study, the input signal of LIF model is a continuous voltage pulse with an amplitude of 1.2 V and interval of 5 ms. Such an input signal will be transmitted by habituation memristor to a neuron electronic element. The output signal is the pulse generated by neuron when it is fired. According to the results, the frequency of output signal is smaller than input information which complies with the basic characteristics of habituation. It is supposed that organisms should not response to this repetitive pulse any more and it will make neuron have more capabilities to handle following information.
2021, 70 (6): 068701.
doi: 10.7498/aps.70.20201659
Abstract +
Reduced activation ferritic/martensitic (RAFM) steel, as a typical body centered cubic (bcc) iron based structure material, has become a candidate material for future fusion reactor. Nano-scale prismatic interstitial dislocation loops formed in irradiated RAFM have been studied for many years because of their significant influences on the mechanical properties (e.g. irradiation embrittlement, hardening, creep, etc.). Compared with edge dislocation, screw dislocation has very important influence on plastic deformation behavior because of its low mobility. Thus, the mechanism of interaction between screw dislocation and interstitial dislocation loops has become an intense research topic of interest. In this study, the slip behavior of screw dislocation and the mechanisms of interaction between screw dislocation and ½[$11\bar 1$ ] dislocation loop in bcc-Fe at different temperatures are investigated by molecular dynamics simulation. The results show that the screw dislocation mainly slides along the ($\bar 2 11$ ) plane at a low temperature of 2 K under the increase of shear stress. With the temperature increasing to 823 K, it is prone to cross slip, and then the cross slip occurs alternately in the ($\bar 1 10$ ) plane and the ($\bar 2 11$ ) plane. Therefore, with the increase of temperature, the critical shear stress decreases gradually. When the screw dislocation slips close to the dislocation loop, the mechanism of interaction between screw dislocation and dislocation loop is different at different temperature: at low temperature of 2 K, there is repulsive force between screw dislocation and dislocation loop, when screw dislocation slip approaches to the dislocation loop, the cross slip of screw dislocation can occur, and shear stress is lower than that from the model without dislocation loop; at medium temperatures of 300 K and 600 K, the influence of repulsive force on the cross slip of screw dislocation can be weakened, and screw dislocation will slip through the dislocation loop then form the new structure named helix turn, which further hinders screw dislocation slipping and results in the increase of shear stress; at a high temperature of 823 K, the screw dislocation is more likely to cross slip due to the thermal activation, and the slip of dislocation loop is also easier to occur, but the screw dislocation and the dislocation loop do not contact each other in the whole shearing process, therefore the shear stress is lowest.
2021, 70 (6): 068801.
doi: 10.7498/aps.70.20201651
Abstract +
Space charge layer (SCL) effect induced by interfaces, e.g., grain boundaries in the polycrystals or heterointerfaces in the composites, may make the characteristics of the charge carrier transport near the interfaces significantly different from those in the bulk area. In previous studies, the Poisson-Boltzmann (PB) equation was widely used to model the SCL effect, in which all the charge carriers were assumed to be in electrochemical equilibrium. However, the assumption of the electrochemical equilibrium is no longer valid when the charge carriers exhibit macroscopic motion. In this paper, we develop a model to simulate the charge carrier transport within the oxygen-ion conductor, particularly in the SCL, in which the charge carrier mass conservation equation is coupled to the Poisson equation. Our present coupled model, in which the assumption of the electrochemical equilibrium is not employed, is therefore able to simulate charge carrier transport with macroscopic motion. Two key dimensionless parameters governing the SCL effect are deduced, i.e. the dimensionless Debye length characterizing the ratio of Debye length to the thickness of oxygen-ion conductor, and the dimensionless potential representing the relative importance of the overpotential to the thermal potential. Taking AO2-M2O3 oxide for example, the conventional model with using PB equation and our present coupled model are compared for predicting the SCL effect. Furthermore, the mechanism of the oxygen vacancy transport in the oxygen-ion conductor with considering the SCL effect is thoroughly discussed. In a brief summary, with increasing the current density at the interface, the SCL resistance shows a non-monotonical tendency, i.e., it firstly decreases and then increases. Besides, enlarging the dimensionless Debye length significantly increases the SCL resistance. The influence of increasing the dimensionless potential on the oxygen vacancy transport is obvious when the overpotential is comparable to the thermal potential, but it becomes negligible when the overpotential is far less than the thermal potential. These results may offer helpful guidance for enhancing the performance of oxygen-ion conductors by rationally designing the grain boundaries and heterointerfaces.
2021, 70 (6): 068901.
doi: 10.7498/aps.70.20201626
Abstract +
In this study, the unidirectional pedestrian flow in the corridor is taken as a research object, the generation mechanism of the pedestrian zipper phenomenon is analyzed, and a velocity correction model based on the Voronoi diagram is established for the simulation research. First, the generation mechanism of the pedestrian zipper phenomenon is analyzed from the perspective of optimal visual field and walking comfort of pedestrians. Then the visual attention and visual occlusion of pedestrians are used to describe the factors which affect the zipper deviation during pedestrian movement, the local density of pedestrians is used to describe the walking comfort of pedestrians, the zipper sensitivity coefficient is adopted to describe the willingness of pedestrians to move objectively, and the mechanism of lateral deviation of a single pedestrian is considered to obtain the optimal deviation position of pedestrians. Besides, the Voronoi diagram is introduced to effectively determine the pedestrians surrounding the target pedestrian within the visual field. And the influence of surrounding pedestrians with different distances and directions on the moving velocity of the target pedestrian based on the Voronoi diagram is considered. Then, a velocity correction model of pedestrians based on the Voronoi diagram is constructed, whether the pedestrian has a subjective willingness to deviate is considered, and the deviation rule is embedded to simulate and reproduce the zipper phenomenon of pedestrians. The simulation results truly reproduce the normal pedestrian flow through the corridor and show that our model can overcome the deficiency of the jitter and overlap phenomenon of the traditional social force model. The self-organized pedestrian flow with uniform distribution and the pedestrian zipper effect can also be observed. Furthermore, through the simulation results, we can see that the number of zipper layers for pedestrians is proportional to the width of the corridor. The comparison of simulated pedestrian data with the empirical data indicates that the fundamental diagram of velocity-density relation of our model is in good agreement with the empirical data. A comparison between with and without considering the zipper effect shows that the larger the proportion of pedestrians actively willing to laterally deviate, the more helpful it will be to improve the moving velocity, comfort and space utilization of pedestrians in the corridor.
2021, 70 (6): 068902.
doi: 10.7498/aps.70.20201486
Abstract +
In this paper, the influence of opinion dynamics on rumor propagation is studied by introducing the mechanism of stifler’s comments on rumors into the susceptible-infected-recovered (SIR) model. In this model, individuals can form a group together with their direct neighbors, through which the spreader can spread rumors, and the stiflers can express opinions and refute the rumors. The mechanism of rumor refuting can not only reduce the acceptance rate of the ignorant to rumor, but also increase the transition probability of the spreader to be a stifler. In this paper, we use the Erdös-Rényi (ER) random network, scale-free network and real social network as the underlying interaction structure to study the influence of stifler’s silence probability on the rumor spreading dynamics. First of all, we find that the process of rumor propagation can be roughly divided into two stages, i.e., the early stage of free propagation of rumors and the later stage of checks and balances between the stiflers and the spreaders, respectively. Secondly, it is found that the rumor will break out with the increase of stifler’s silence probability. Under a threshold of rumor outbreak, the increase of silence probability will not lead the number of spreaders to significantly increase, but will cause more ignorance to perceive the rumor and quickly turn into spreaders, and then change into stiflers under the guidance of other stiflers. When the silence probability reaches a threshold, the stiflers will not be able to control the spread of rumors, which will lead the number of stiflers to decrease and the rumors to break out. Finally, the early stage of rumor propagation in scale-free networks is shorter than that of random network, which makes rumor more difficult to break out. Our model comprehensively considers the influence of opinion dynamics on the spreading of rumors and more realistically simulates the rumor diffusion process, which provides a useful insight for the rumor control in real-world social networks.
EDITOR'S SUGGESTION
2021, 70 (6): 068903.
doi: 10.7498/aps.70.20202084
Abstract +
Population migration is an essential medium for the spread of epidemic, which can accelerate localized outbreaks of disease into widespread epidemic. Large-scale population movements between different areas increase the risk of cross-infection and bring great challenges to epidemic prevention and control. As COVID-19 can spread rapidly through human-to-human transmission, understanding its migration patterns is essential to modeling its spreading and evaluating the efficiency of mitigation policies applied to COVID-19. Using nationwide mobile phone data to track population flows throughout China at prefecture-level, we use the temporal network analysis to compare topological metrics of population mobility network during two consecutive months between before and after the outbreak, i.e. January 1st to February 29th. To detect the regions which are closely connected with population movements, we propose a Spatial-Louvain algorithm through adapting a gravity attenuation factor. Moreover, our proposed algorithm achieves an improvement of 14% in modularity compared with the Louvain algorithm. Additionally, we divide the period into four stages, i.e. normal time, Chunyun migration, epidemic interventions, and recovery time, to describe the patterns of mobility network’s evolution. Through the above methods, we explore the evolution pattern and spatial mechanism of the population mobility from the perspective of spatiotemporal big data and acquire some meaningful findings. Firstly, we find that after the lockdown of Wuhan and effective epidemic interventions, a substantial reduction in mobility lasted until mid-February. Secondly, based on the economic interaction and geographic location, China has formed an urban agglomeration structure with core cities centering and radiating toward the surroundings. Thirdly, in the extreme cases, the dominant factor of population mobility in remote areas is geographic location rather than economy. Fourthly, the urban agglomeration structure of cities is robust so that when the epidemic weakens or disappears, the city clusters can quickly recover into their original patterns.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2021, 70 (6): 069101.
doi: 10.7498/aps.70.20201282
Abstract +
In this paper, an efficient algorithm of three-dimensional (3D) explicit sensitivity (or called Fréchet derivatives) matrix for marine controlled source electromagnetic measurements is established by combining an electric coupled potential finite volume method with a direct solver PARDISO direct method. Firstly, on the Yee’s staggered grids, the coupled potential Helmholtz equations are discretized to form a large, sparse and complex linear system which is excited by mobile transmitters. Secondly, through the inversion of the discrete matrix and 3D linear interpolation formula, the interpolation operators and projection operators are established for each receiver at different positions. Because these interpolation operators and projection operators are unrelated to transmitters, they can be computed in advance according to the positions of all receivers. Then the multiple projection operators with discrete vector of each transmitter source can efficiently produce the electromagnetic (EM) responses. On the basis, the goal conductivity of block model and pixel model is expressed as a piece-wise constant function. By perturbing the goal conductivity, the scattered electric current density can be decomposed into a series of electric current elements distributed on Yee’s grids. Each scattered current element is equal to the product of relative perturbation of conductivity and the electric intensity on the grid. The discrete vector on the right-hand side is computed by integrating each scattered current element on the Yee’s grid and then being multiplied with the project operator. Then the linear relationship between the changes in EM field and the relative conductivity perturbation on each block or pixel can be fast produced to obtain the explicit sensitivity matrix about EM responses. Finally, numerical results demonstrate the efficiency and accuracy of this method. The characteristics of 3D sensitivity in three different cases are further investigated.
