Vol. 67, No. 21 (2018)
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2018, 67 (21): 217501. doi: 10.7498/aps.67.20181129
The hypothetical half-Heusler alloys VLiBi and CrLiBi containing only one transition metal element are constructed. The electronic structure and magnetic properties of VLiBi and CrLiBi are investigated by using the first-principles full-potential linearized augmented plane wave method based on density functional theory. The spin-polarized calculations of electronic structure for the half-Heusler alloys VLiBi and CrLiBi are performed. The calculation results reveal that VLiBi and CrLiBi are half-metallic ferromagnets with the half-metallic gaps of 0.25 eV and 0.46 eV and the total magnetic moments of 3.00 μB and 4.00 μB per formula unit, respectively. The total magnetic moments mainly originate from the magnetic moment on V or Cr atom. Li and Bi have small atomic magnetic moments, where the atomic magnetic moment of Bi is negative. The mean field approximation method is used to estimate the Curie temperatures of the alloys. The calculated results show that the values of Curie temperature for VLiBi and CrLiBi are 1401 K and 1551 K, respectively. To study the robustness of the half-metallicity with the change of lattice constant, the electronic structures of VLiBi and CrLiBi are also calculated under their lattice constant changing from-10% to +10% relative to the equilibrium lattice constant. It is found that the VLiBi and CrLiBi can maintain their half-metallicity and retain their total magnetic moments of 3.00 μB and 4.00 μB per formula unit even when their lattice constants change from-5.6% to 10.0% and from-6.9% to 10.0%, respectively. To discuss the effect of strongly correlated interaction on the half-metallicity, the electronic structure of VLiBi and CrLiBi are calculated by the LDA+U method with U for V-3d and Cr-3d orbital. The calculation results indicate that VLiBi and CrLiBi can keep their half-metallicity and integer total magnetic moments of 3.00 μB and 4.00 μB when the value of U reaches to 5 eV. Also, the electronic structure of VLiBi and CrLiBi are recalculated by the GGA+SOC method. The calculated results show that 1) there are some spin-down bands crossing the Fermi level, 2) the spin polarizations of VLiBi and CrLiBi at the Fermi level are 98.8% and 94.3%, respectively, and 3) total magnetic moments of VLiBi and CrLiBi are 3.03 μB and 4.04 μB per formula unit, respectively. The spin-orbit coupling has a weak effect on the half-metallic of half-Heusler alloy VLiBi and the spin polarization is still high for the half-Heusler alloy CrLiBi. The half-Heusler alloys VLiBi and CrLiBi may be useful in spintronics and other applications.
2018, 67 (21): 217101. doi: 10.7498/aps.67.20181297
Scale effect and topological frustration can form magnetic order in the finite graphene structures (graphene nanoflakes (GNFs)). In this paper, the GNFs that can generate large net electron spin or electron spin antiferromagnetic coupling between local regions of net electron spins are classified reasonably. Representative special GNF configurations are proposed to be effectively used as fundamental logic gate devices for ultra-fast high density spintronics, and theoretically investigated by the first-principles electron structure calculations based on spin-polarized density functional theory. The first-principles calculations are performed by utilizing all-electron numerical-orbital scheme in the M11-L form of meta-GGA exchange-correlation functional. The energy spectrum of singly occupied states and the isodensity surface of total spin distribution indicate evidently that spin-single-state electrons are localized on two sides of a representative double-triangle GNF and the spin polarizations of two GNF segments are in opposite directions, resulting in antiferromagnetic coupling, which is consistent with the results derived from the graph theory and Lieb theorem. The energy of antiferromagnetic spin-coupled state is 55 meV lower than that of ferromagnetic spin-coupled state, which is obviously higher than the thermodynamic threshold of the minimum energy dissipation at room temperature. The spin coupling energy of the double triangle GNF increases with the scaling of GNF dimension increasing. The magnetic coupling strength of the double triangle GNF with and without mirror symmetry approach to the maximum stable values of 50 meV and 200 meV respectively, which are remarkably higher that of quantum dots and transition metal atom systems. Due to the fact that the spin coupling strength of the GNF logic gate spin device can reach 200 meV, it can operate normally at ambient temperature with an error rate of 0.001 which can be easily improved by an error correction technique. The calculation results demonstrate that the proposed GNF logic gate can finely operate at ambient temperature with significantly low and correctable error rate. Recent experimental studies show that graphene nanodevices on a scale of only a few nanometers can be successfully fabricated by etching technique of electron beam and scanning probe. Furthermore, the properties of GNF spin logic devices are not sensitive to intrinsic defects. The triangular GNF with n carbon rings has only (n+2)2-3 carbon atoms, while it can endure n-1 internal defects, thus persisting in non-bond states and local magnetic moments. It is suggested that the full spin logic gate devices based on GNF can be realized by using the current advanced nano-processing technology.
Determination of carrier bulk lifetime and surface recombination velocity in semiconductor from double-wavelength free carrier absorption
2018, 67 (21): 217201. doi: 10.7498/aps.67.20181509
In microelectronic and photovoltaic industry, semiconductors are the basic materials in which impurities or defects have a serious influence on the properties of semiconductor-based devices. The determination of the electronic transport properties, i.e., the carrier bulk lifetime (τ) and the front surface recombination velocity (S1), is important for evaluating the semiconductor material. In this paper, a method of simultaneously measuring the bulk lifetime and the front surface recombination rate of semiconductor material by using double-wavelength free carrier absorption technique is presented. The effect of the carrier bulk lifetime and the front surface recombination rate on the modulated free carrier absorption signal (Ampratio and Phadiff) are qualitatively analyzed. The process of extracting the bulk lifetime and the front surface recombination rate by the proposed double-wavelength free carrier absorption method are also given. At the same time, the uncertainties of the parameters extracted by this method are calculated and compared with those obtained by the traditional frequency-scan free carrier absorption technique. The results show that the proposed method can significantly reduce the uncertainties of the measurement parameters, especially for the samples with higher surface recombination rate. For the sample with a lower front surface recombination rate (S1=102 m/s), the uncertainty of the carrier bulk lifetime and the front surface recombination velocity obtained by the proposed method are almost in agreement with those obtained by the conventional frequency-scan method. On the contrary, for the samples with higher front surface recombination rate (S1 ≥ 103 m/s), the uncertainties of the carrier transport parameters are much smaller than those from the conventional frequency-scan method. For example, the estimated uncertainty of the carrier bulk lifetime and the front surface recombination velocity for the sample with τ=10 μs and S1=103 m/s are approximately ±5.55% and ±2.83% by the proposed method, which are more improved than ±18.50% and ±31.46% by the conventional frequency-scan method with a wavelength of 405 nm. Finally, we explain the above phenomenon by analyzing the distribution of excess carrier concentration at different pump wavelengths. As the pump wavelength decreases, the more excess carriers are excited near the surface of the sample due to the greater absorption coefficient, and the influence of the surface recombination by the impurities and defects on the signal is more obvious. Therefore, the measurement accuracy of the front surface recombination rate can be improved effectively by using double wavelength pumping.
2018, 67 (21): 217301. doi: 10.7498/aps.67.20181479
Graphene has excellent electrical, optical, thermal and mechanical properties, so it has been used in high-performance field effect transistors, sensors, optoelectronic devices, and quantized devices. It is crucial to realize a high-quality junction between metal electrode and graphene. For example, in the field of electrical measurement, due only to the contact resistance in a proper order of magnitude, the quantum Hall effect can be realized. The lower the contact resistance, the higher the measurement accuracy of Hall resistance is. In order to reveal the factors affecting the contact resistance we propose an effective method to reduce it, and a physical model is established in this paper. The carrier transport between the metal electrode and graphene is divided into two cascaded processes. Carriers first transport from the metal electrode to the graphene underneath it, then transport between the graphene underneath metal and the adjacent graphene. The transport probability of first step is considered through the effective coupling length and the mean free path. The transport probability of second step is considered through the effective length of potential step change between the graphene under the metal and the adjacent graphene. The contact resistance is analyzed by combining the distribution of carriers. In order to verify the correctness of the theoretical results, an experimental sample with gold as the metal electrode is fabricated. The transport line model is used to measure the contact resistance. The length of contact area is 4 μm. The lengths of graphene channel are set to be 2, 4, 6, 8, and 10 μm, respectively. The current values are set to be 10, 20, 40, 60, and 80 μA, respectively. The results show that the relationship between current and voltage is almost linear. The total resistance can be obtained with different lengths of graphene. According to the transmission line model, the resistance value can be estimated as (160±30) Ω when the graphene length is zero. Considering that the measured result is obtained under two metal electrodes contacting the graphene, the contact resistance of experimental result is (320±30) Ω·μm which agrees well with the theoretical result. From the analysis of theoretical process, the factors that affect the contact resistance is determined by material, drain-source voltage, gate voltage, doping concentration, distance between metal electrode and graphene atoms, distance between graphene and gate. Finally, in order to reduce the contact resistance between graphene and metal electrode, we propose some corresponding solutions for choosing the metal material whose work function is close to graphene's, reducing the thickness of the silicon dioxide layer, increasing carrier mean free path, improving the surface morphology of the metal material, and reducing the coupling length between metal and graphene.
Hydrogen adsorption mechanism on single-layer MoSe2 for hydrogen evolution reaction: First-principles study
2018, 67 (21): 217102. doi: 10.7498/aps.67.20180882
Based on the first-principles of the density functional theory, the Gibbs free energies (△GH0) of the hydrogen adsorption on the 2H-phase molybdenum diselenide monolayer (MoSe2) with different active sites and hydrogen coverage rates are calculated. The calculated results reveal that several ideal adsorbed rates and sites are very close to those at thermoneutral state (△GH0~0). To compare their catalytic ability in the hydrogen evolution reaction (HER), the exchange current density (i0) as a function of △GH0 is calculated as a volcano curve. Two sites located at the top of volcano curve present higher exchange current densities than that of Pt catalyst. The charge transfers and the bonding details of the two edge-hydrogen-adsorptions (Mo edge and Se edge) are analyzed by the charge density difference and electronegativity as the associated structures and relative △GH0 are further explained. It is found that the localized charge transfer distributed uniformly between the hydrogen atoms and the adsorption sites can facilitate the catalytic ability of HER. For this reason, the catalytic ability of HER for the Se edge is more stable than that of Mo edge with less sensitivity to the absorption sites and hydrogen coverage rates. Based on the first-principles molecular dynamics (MD) simulation, finally, the influences of the thermal motion on the two kinds of structures of hydrogen adsorption at the higher temperature are explored, with the critical temperature for the hydrogen desorption as well as the atomistic dynamics discovered. It is worth mentioning that during the structure optimization and MD simulation, the edge deformation and reconstruction are discovered, respectively, which indicates that the ideal edge of MoSe2 may not be the most stable structure, which will change with the external conditions. This theoretic study reveals the atomistic mechanisms of the hydrogen adsorption and desorption of the single-layer 2H-phase MoSe2 at different temperatures, with the edge lattice deformation and reconstruction discovered, which can deepen our insights into the HER mechanisms near the edges of the 2H-phase MoSe2 at different temperatures and provide theoretic guidelines for designing the high-efficient and low-cost catalyst in the HER through tuning the MoSe2 edges.
2018, 67 (21): 217802. doi: 10.7498/aps.67.20181191
The Pt/Au Schottky contacts to InGaN samples with different background carrier concentrations are fabricated. The crystal qualities of InGaN samples are characterized by X-ray diffraction (XRD) and atomic force microscope (AFM), and the correlation between threading dislocation density of InGaN and growth temperature is further clarified. The full width at half maximum (FWHM) values of the InGaN (0002) XRD rocking curves show that the density of threading dislocations in InGaN, which can seriously deteriorate InGaN crystal quality and surface morphology, decreases rapidly with increasing growth temperature. The Hall measurements show that the background carrier concentration of InGaN increases by two orders of magnitude as growth temperature decreases from 750 to 700℃, which is due to a reduced ammonia decomposition efficiency leading to the presence of high-density donor-type nitrogen vacancy (VN) defects at lower temperature. Therefore, combining the studies of XRD, AFM and Hall, it can be concluded that the higher growth temperature is favorable for realizing the InGaN film with low density of VN defects and threading dislocations for fabricating high-quality Schottky contacts, and then the barrier characteristics and current transport mechanism of Pt/Au/n-InGaN Schottky contact are investigated by current-voltage measurements and theory analysis based on the thermionic emission (TE) model and thermionic field emission (TFE) model. The results show that Schottky characteristics for InGaN with different carrier concentrations manifest obvious differences. It is noted that the high carrier concentration leads to the Schottky barrier height and the ideality factor obtained by TE model are quite different from that by TFE model due to the presence of high density of VN defects. This discrepancy suggests that the VN defects lead to the formation of the tunneling current and further reduced Schottky barrier height. Consequently, the presence of tunneling current results in the increasing of total transport current, which means that the defects-assisted tunneling transport and TE constitute the current transport mechanism in the Schottky. However, the fitted results obtained by TE and TFE models are almost identical for the InGaN with lower carrier concentration, indicating that TE is the dominant current transport mechanism. The above studies prove that the Pt/Au/n-InGaN Schottky contact fabricated using low background carrier concentration shows better Schottky characteristics. Thus, the properly designed growth parameters can effectively suppress defects-assisted tunneling transport, which is crucial to fabricating high-quality Schottky devices.
First-principles study of stacking effect on second harmonic generation of graphene-like two-dimensional silicon carbide
2018, 67 (21): 217803. doi: 10.7498/aps.67.20181337
Two-dimensional layered silicon carbide (2d-SiC), a semiconductor with graphene-like structure, has potential applications in nonlinear optical frequency conversion. The effect of stacking and strain on the nonlinear second harmonic generation (SHG) coefficient are studied by using the first-principles calculation of the all-electron full-potential linearized augmented-plane wave combined with the sum-over-states method. The analysis of physical origin of the SHG process shows that the single-particle transition channel formed by three bands dominates the SHG process of 2d-SiC. The interband motion of electrons is significantly tuned by the intraband motion. The angle dependence of the SHG coefficient of 2d-SiC is given as a reference for future experiments. A tunable SHG enhancement could be obtained by straining 2d-SiC.
2018, 67 (21): 217801. doi: 10.7498/aps.67.20181170
In this paper, we present a kind of broadband absorbent material. The broadband absorbent material is designed based on topology optimization and tested. The optimizing of metamaterials with a genetic algorithm has become one of the most effective methods of designing metamaterials in recent years. An integral system with interactive simulation of MATLAB and CST Microwave Studio is developed, and the main program of genetic algorithm is written in MATLAB; with simulation and computation in CST the metamaterial is optimized by a genetic algorithm with power of global optimization. Vacuum assistant resin infusion process is a new cost-effective and high-performance process. The proposed radar absorbent material possesses a sandwich structure, which consists of transparent composite skin panel, resistive metasurface, polyurethane foam and reflective composite skin panel. The transparent composite skin panel is low-dielectric-constant glass fiber reinforced composite, which has excellent physical properties and weather resistant property. The core material is composed of low density polyurethane foam and metamaterials, which can well meet the requirements for weight reduction and the invisibility. The reflective composite skin panel is a low-resistance carbon fiber reinforced composite, which prevents the electromagnetic waves from transmitting and also provides electrical boundary conditions for metamaterial. Simulation and test results indicate that the reflectivity of the radar absorbent material is less than-12 dB in a range of 2-18 GHz. Because of the symmetrical structure design of the resistance film, the radar absorbent material is polarization-independent. We preliminarily produce a batch of radar absorbent materials and test their various performances. Such a radar absorbent material has a strong absorption and other properties such as light quality, high temperature resistance, low temperature resistance, humidity resistance and corrosion resistance. The radar absorbent material which has been widely used in the engineering field is easy to achieve the compatibility of absorption, mechanical properties and environmental performance. Compared with previous design method, the topology optimization design is simple in programming operation, good in generality, and short in design periode. The radar absorbent materials owns strong application value.
2018, 67 (21): 210601. doi: 10.7498/aps.67.20181274
Optical frequency comb is a kind of new pulse source, whose repetition rate and phase are locked. Optical frequency comb plays an important role in absolute distance measurement and time-frequency metrology. Lots of laser ranging methods such as time-of-flight and multi-heterodyne interferometry based on femtosecond laser pulse have been used in distance measurement. In this paper, a high-precision distance measurement system based on optical sampling by cavity tuning is set up to realize a long absolute distance measurement. And a kind of error compensation method is proposed based on the asymmetric cross-correlation patterns. In traditional optical sampling by cavity tuning measurement system, the fiber link is inserted into the reference path to extend the non-ambiguity distance, which does not have a good performance in arbitrary distance measurement. In our system, we use a 116-meter-long fiber which is inserted into the measuring path to extend the non-ambiguity distance. Besides, dispersion compensation technique is used to control the shape of the laser pulse. An asymmetric optical pulse is used as the light source, so that we can obtain extremely asymmetric cross-correlation patterns. The cross-correlation patterns can be acquired by sweeping the repetition frequency. We use an arbitrary waveform generator to provide the scanning voltage, and the scanning voltage can adjust the repetition rate of the pulse and has a frequency of 1 Hz. There will be two peaks on the envelope of cross-correlation pattern, and both peaks can be used to obtain the distance information. When the laser propagates in vacuum and the system is stabilized, the distance between these two peaks is constant, and we can use this distance to obtain the important factor N, which is used to describe the number of the pulse. As a result, we can realize absolute distance measurement without the help of other measurement systems. However, due to the dispersion of the medium, the distance between these two peaks is not constant, which means that the asymmetry of the cross-correlation patterns in dispersion medium will influence the measurement results. And the deviation is relevant to the peak-to-peak distance. We use the difference among the peak-to-peak distances at different positions to correct the measurement results. A comparison of our results with those from a commercial He-Ne laser interferometer shows that they are in agreement within 2 μm over 50 m distance, corresponding to a relative precision of 1.9×10-7.
2018, 67 (21): 210501. doi: 10.7498/aps.67.20180789
In order to solve the problem of extracting ultrasonic signals from strong background noise, a novel method, which is termed APSO-SD algorithm and based on improved adaptive particle swarm optimization (APSO) and sparse decomposition (SD) theory, is proposed in this paper. This method can convert the ultrasonic signal denoising problem into optimizing the function on the infinite parameter set. First, based on the sparse decomposition theory and the structural characteristics of ultrasonic signal, the objective function of particle swarm optimization algorithm and the reconstruction algorithm of the denoised signal are constructed, so that particle swarm optimization and ultrasonic signal denoising can be combined. Second, in order to improve the robustness of the proposed approach, an APSO algorithm is proposed. What is more, because particle swarm optimization algorithm can be used to optimize in continuous parameter space, and according to the empirical characteristics of the ultrasonic signals used in practical engineering, a continuous super complete dictionary for matching ultrasonic signals is established. Since the super complete dictionary is continuous, there are an infinite number of atoms in the established dictionary. The redundancy of dictionaries is enhanced by the method in this paper. Based on the fact that the inner product of the optimal atom and the ultrasonic signal is one and the inner product of the noise and the optimal atom is zero in the established dictionary, the objective optimization function of APSO-SD algorithm is established. Finally, the optimal atom is determined based on the optimization result of the objective function. In this way, the denoising ultrasonic signal can be reconstructed by using the optimal atom according to the reconstruction algorithm. The processing results of simulated ultrasonic signals and measured ultrasonic signals show that the proposed method can effectively extract weak ultrasonic signals from strong background noise whose signal-to-noise ratio is lowest, as low as-4 dB. In addition, compared with the adaptive threshold based wavelet method, the proposed method in this paper shows the good denoising performance. In this paper, it is demonstrated that the problem of ultrasonic signal denoising can be transformed into the optimization of constraint functions. Furthermore, the ability of the proposed APSO-SD algorithm to accurately recover signals from noisy acoustic signals is better than that of the common wavelet method.
Mechanism of a nonlinear bistable piezoelectric cantilever beam under narrow-band random excitations and its energy harvesting
2018, 67 (21): 210502. doi: 10.7498/aps.67.20180072
While wireless sensors, data transmission devices and medical implant devices tend to miniaturization and low consumption, energy supply modes such as batteries, solar energy and wind energy are limited due to their defects. Instead, vibration energy harvesting can open up new possibilities for self-supplying the low-consumption devices. The narrow-band random vibration with center frequency is a typical vibration in the environment, and its characteristics are closely related to the environment.This paper takes the energy harvesting system with bi-stable piezoelectric cantilever beam as a research object, and the characteristics of system's equivalent linear natural frequency, linear and nonlinear stiffness under different intervals between magnets are analyzed. By using the narrow-band random excitation with a certain bandwidth output of the bandpass filter to simulate environment vibration and using Runge-Kutta method to solve the system equation numerically, the response of system and the characteristics of energy harvesting are studied.It is observed that the variation of the magnet spacings at peak output voltage, which possesses a central frequency, is related to the variation of the equivalent linear natural frequency of the system with the interval between magnets. When the variation of magnet spacing is triggered by the narrow-band random excitation with a certain bandwidth, there is always a constant interval between magnets, making the system produce a peak output, which is like a bi-stable system that produces the peak output at optimal spacing under broad-band excitation. On the other hand, there are also more than one or two different magnet spacings making the system produce peak outputs while excitation's center frequency changes in a certain range, and the peak outputs are formed by bi-stable or single-stable “resonance” of the system, induced at the equivalent linear natural frequency. And the demarcation point spacing of the single-stable and bi-stable vibration of the system are the magnet spacing when linear stiffness is zero.Therefore, for the narrow-band random excitation in the actual environment, the magnet spacing of the energy harvesting system can be reasonably arranged according to the specific working conditions to achieve better electromechanical energy conversion. The findings in this paper can provide some theoretical and technical support for the study of harvesting the vibration energy with characteristics of narrow-band random excitation.
Multispectral image enhancement based on illuminance-reflection imaging model and morphology operation
2018, 67 (21): 210701. doi: 10.7498/aps.67.20181288
In this paper we propose a multispectral image enhancement algorithm based on illuminance-reflection imaging model and morphology operation that enables us to solve the problem of improving the multispectral degraded images. Firstly, we transform the image from RGB space to HSV color space, and the hue remains unchanged. As for the saturation component, we use the adaptive nonlinear stretching to improve the image color saturation and brightness. Secondly, according to the illuminance-reflection imaging model, we adopt the guided image filtering method to decompose the brightness into illuminance component and reflection component. Usually, the illumination component mainly determines the dynamic range of the pixels in the image, corresponding to the low frequency part of the image, reflecting the global characteristics of the image and the edge detail information of the image; the reflected component represents the intrinsic essential characteristics of the image, corresponding to the high frequency part of the image, and contains most of the local detail information of the image as well as all noise. Thirdly, we present an improved adaptive gamma function, which can dynamically adjust the illuminance component by the local distribution characteristics, and use the contrast-limited adaptive histogram equalization to correct the illuminance component. Afterwards we propose a detail-feature weighted fusion strategy. The original illumination and the two corrected illuminations are fused to obtain the final illumination component. Fourthly, we propose an improved morphological operation to denoise and enhance the details of the reflection component. Finally, the corrected illumination component and the enhanced reflection component are combined to obtain the improved brightness component. In order to verify the efficiency of the algorithm proposed in the paper, we use both subjective visual effectiveness method and quantitative parameter analysis method to measure the enhancement performance in multispectral imaging scenarios, including low illumination image, underwater image, high-dynamic range image, sandstorm image, haze image and thermal infrared image. Then standard deviation, information entropy and average gradient are used as evaluation indices respectively, and qualitative and quantitative comparison with a variety of image enhancement algorithms show that the proposed algorithm can not only well suppress noise but also obviously improve local details and global contrast. Experimental results show that the proposed method proves to be better in performance.
2018, 67 (21): 210702. doi: 10.7498/aps.67.20180493
With the innovation of infrared imaging technology, the performance evaluation of the infrared imaging system plays an indispensable role in the general technology. Therefore, the establishment of a comprehensive, scientific and reasonable performance evaluation model is a prerequisite for accurately predicting the performance of the imaging system, and is also an effective technology to design and develop the high performance imaging system. According to previous studies, in this paper we analyze the key problems that need to be solved urgently in the current research and the factors limiting the development of performance evaluation techniques. The main researches of this paper are as follows. 1) Through improving the deficiencies of the inherent physical effect evaluation methods, the accuracy and reliability of the performance evaluation model are enhanced. 2) In order to meet the needs of performance optimization design, the performance parameters provided by product design and production requirements must be selected correctly to achieve an organic combination of performance evaluation model and imaging system.The NVThermIP model, a widely used performance evaluation model, is slightly inadequate for guiding the optimization of the parameters of an infrared system. A more scientific and reasonable performance evaluation model is proposed in this paper, in which the contrast-threshold function of the system in the NVThermIP model is corrected by noise equivalent temperature difference based on the theory of the human-eye noise. By quantitatively analyzing the typical physical effects on infrared imaging system, the modeling theory and process of NVThermIP model are introduced in detail. The simulation results give a visual representation of evaluating the performance of an infrared imaging system. The limitations of the NVThermIP model used to guide the design and production of the system and the deficiencies of the early theoretical basis for system optimization design are analyzed. The noise equivalent temperature difference is introduced to revise and perfect the NVThermIP model combined with the theory of human eye noise. The accuracy of the newly proposed model is verified by two experiments. Experimental results show that the corrected model is more accurate in system prediction and can be used to guide the design of a new system.
ATOMIC AND MOLECULAR PHYSICS
2018, 67 (21): 213101. doi: 10.7498/aps.67.20180974
Many investigations indicate that molecular electronics opens up possibilities for continually miniaturizing the electronic devices beyond the limits of the standard silicon-based technologies. There have been significant experimental and theoretical efforts to build molecular junctions and to study their transport properties. The electron transport in molecular device shows clearly quantum effect, and the transport property for molecular device would be strongly affected by chemical and structural details, including the contact position and method between molecule and electrodes, the angle between two electrodes connecting to the molecule. Till now, the micro-fabrication technology still does not guarantee metal electrodes contacting the molecules surfaces ideally. During molecular device fabrication, any tiny variations for the contact configuration usually exist in the molecular device, which would change the device transport property. Hence, it is necessary to investigate the effects of electrode position and electrode cross section size on the transport property.We take Au-benzene-1, 4-dithiol (BDT)-Au (Au-BDT-Au) molecular junctions as example, and systematically calculate its transport properties with various contact positions, and several electrode cross section sizes. The contact face for Au electrode is set to be the (001) face. In the calculations, the density functional theory combined with the Keldysh non-equilibrium Green's function formalism is utilized. The local density approximation is selected as an exchange correlation potential, and atomic core is determined by the standard norm conserving nonlocal pseudo-potential.Our investigations show that the relative position between the electrodes plays a crucial role in the transport behavior of Au-BDT-Au device. When both electrodes are set to be at the counter-position, the preferable transport behavior could be found. The counter-position indicates that the two electrodes are on the same line, which is beneficial to the fabrication. As the angle, which is defined as the angle of electrode deviating from the axis, is larger than five degrees, the transport behavior deteriorates. Hence, the angle for the electrode deviating from its axis should be less than five degrees. To study the effect of electrode cross section size, we calculate the transport properties for three electrode cross sections, i.e. 3×4, 4×4 and 5×4 supercell. Our calculations indicate that when electrode cross section is less than 4×4, the transmission, near the Fermi level, is discontinuous, which would deteriorate the transport performance. Hence, the section size of electrode should not be less than 4×4. This research will provide a scientific index for the electrode position and its cross section size during the fabrication.
2018, 67 (21): 213102. doi: 10.7498/aps.67.20181228
Wide bandgap semiconductor materials have received more and more attention because of their unique properties and potential applications. Single-doped tin dioxide (SnO2) has been studied extensively, however the calculation of SnO2 doped with Sb and S is less involved. Co-doping can effectively improve the solubility of the dopant, increase the activation rate by reducing the ionization energy of the acceptor level and the donor level, and increase the carrier mobility at low doping concentration. Co-doping can solve the problem that is difficult to solve with single doping. Based on the density functional theory of the first principle and the plane wave pseudopotential method, in this paper we study the electronic structure and electrical properties of SnO2 doped with Sb and S by using the generalized gradient approximation algorithm. The geometrical optimization calculation is carried out for the doped structure. The Broyden-Fletcher-Goldfarb-Shanno algorithm is used to find the stable structure with the lowest energy. The plane wave cutoff energy is set to be 360 eV, and the internal stress is less than or equal to 0.1 GPa. By analyzing the electronic structures, it is found that the material is still direct bandgap n-type semiconductor after being co-doped. The electron density is changed, and the overlap of atomic orbital is enhanced. It is conducive to the transfer of electrons. New energy levels are observed in the energy band of co-doped SnO2, and the bandgap width is narrower than that of single doping, thus making electronic transitions become easier. Fermi level is observed in the conduction-band, which leads to the metal-like properties of the material. The electronic density of states is further investigated. The results of the density of states confirm the correctness of electron transfer. In the middle of the valence-band, the hybridization is found to happen between the S atomic orbital and the Sn and Sb orbitals. The top of the valence-band is occupied by the S-3p orbit, thus providing more hole carriers to move up to the top of valence-band. With the increase of S doping concentration, the bandgap and the width of conduction-band both continue to decrease. As a result, the conductive performance turns better.
2018, 67 (21): 213201. doi: 10.7498/aps.67.20181168
The transfer mechanism from the amplitude noise of the coupling light to the phase noise of the probe light in a Rydberg electromagnetic induced transparency effect derived from a ladder-type system including 6S1/2↔6P3/2↔62D5/2 of Cs atoms is demonstrated by using Mach-Zehnder interferometer and balanced homodyne detection technology. In our experiments, the transmission signal of 852 nm probe light is measured by scanning the coupling light frequency nearby the transition from 6P3/2 to 62D5/2 Rydberg state, while the frequency of the probe light is locked at the resonance transition of the 6S1/2↔6P3/2. The relative phase stability of two arms of Mach-Zehnder interferometer, which is constructed with the first order diffraction light of probe light through an acoustic-optic modulator, is accomplished by the controlled piezoelectric ceramic with the PID feedback loop. The interferences between the probe light and the reference light of Mach-Zehnder interferometer under the different relative phases are observed. The interference spectrum of probe light is in good agreement with the theoretical simulation result of the ladder-type three-level system. Therefore, we study the transfer characteristics from the frequency noise of coupling light to the phase noise of probe light when coupling light frequency resonance happens at the transition 6P3/2↔62D5/2. We find the significant suppression of the phase noise of probe light at the higher frequency noise. Moreover, we observe the characteristics of the phase noise of the probe light varying with the power of the coupling light under the different detuning degrees of coupling light. In the red detuning side, the transferred phase noise of probe light decreases with the increase of coupling light power, which is different significantly from the scenario under the blue detuning condition. The ions produced in the ionization process of Rydberg atoms will form the local electric field that would cause the energy level of Rydberg states to shift. The investigation of the noise transfer between the coupling light and probe light in the Rydberg electromagnetically induced transparency effect is important for understanding the coherence mechanism of ladder-type system and the some potential applications, such as in Rydberg-atom-based electric field metrology.
Rovibrational spectrum calculations of four electronic states in carbon monoxide molecule: Comparison of two effect correction methods
2018, 67 (21): 213301. doi: 10.7498/aps.67.20181469
Accurate calculation of molecular energy is of great significance for studying molecular spectral properties. In this work, the potential energy curve and rovibrational spectrum (Gν) of the ground state X1∑+ and the excited states a3Π, a'3∑+ and A1Π of carbon monoxide molecule are calculated by the multi-reference configuration interaction method. In the calculation, the core-valence correlation correction (CV) effect and scalar relativistic (SR) effect are included.In order to obtain an accurate energy of molecule, two computational schemes are adopted. In the first scheme, i.e. (m MRCI+Q/CBS(TQ5)+CV+SR), the molecular orbital wavefunction is obtained from the Hartree-Fock self-consistent field method by using the basis set aug-cc-pVnZ. The wavefunction is first calculated by the state-averaged complete active space self-consistent field approach. Then the multi-reference configuration interaction method (MRCI) is adopted to calculate the dynamic correlation energy in the potential energy curve. Finally, we use the basis set cc-pCVQZ and aug-cc-pVQZ to calculate the CV effect and SR effect by the MRCI method. In the second scheme (aug-cc-pwCVnZ-DK (n=T, Q, 5)), the potential energy curves (PECs) of these four electronic states are calculated by the MRCI method whose basis set (aug-cc-pwCVnZ-DK) contains the CV effect and SR effect. Finally, in order to reduce the error caused by the basis set, we extrapolate the basis sets of the two computational schemes to the complete basis set. On the basis of the PECs plotted by the different methods, we obtain the spectroscopic parameters of the X1∑+, a3Π, a'3∑+ and A1Π states of the carbon monoxide by solving the internuclear Schrödinger equations through utilizing the numerical integration program “LEVEL”.In this paper, we calculate the SR effect and the CV effect by using different schemes, and the latter is indispensable for accurately calculating the molecular structure. For the lowest two electronic states, we consider the dependence of the two effects on the calculation of the Gaussian basis group (Method B), and find that the accuracy of the rovibrational spectrum is improved. It can be seen that these electronic states have higher requirements for electronic correlation calculation. For higher electronic states, the electron cloud distribution is relatively loose, and the electronic correlation obtained by a single Gaussian basis group can achieve the corresponding calculation accuracy. Of course, since the calculation of the rovibrational spectra is essentially only the relative energy, the offset effect of the electronic correlation effect of different electronic states is also included here in this paper.
2018, 67 (21): 213401. doi: 10.7498/aps.67.20181321
Sympathetic cooling is one of the most promising techniques for producing ultracold molecules from precooled molecules. Previous researches have shown that it is inadequate to use the ultracold alkali-metal atoms as coolant for sympathetic cooling. To explore the possibility of ultracold alkali-earth-metal atoms as coolant, in this paper a theoretical investigation is performed of the cold collision dynamics for Xe-NH(X3∑-) system in magnetic fields. The interaction potential energies of Xe-NH complex are calculated respectively by using the single and double excitation coupled-cluster theory with the noniterative treatment of triple excitations[CCSD(T)] method and complete basis set limit extrapolated method. An analytic express of potential energy surface (PES) is given for the first time. A single global minimum value occurs at R=7.14a0, θ=102.76° with an energy of-153.54 cm-1, and the PES has a weak anisotropy. Combine the ab initio PES with quantum scattering theory, then the cold collisional dynamics of Xe-NH system in a magnetic field will be studied. The elastic and inelastic transition cross sections and their ratios of NH molecules in the lowest low-field following state (n=0, mj=1) under different magnetic fields and collisional energies are calculated. The results show that the elastic cross section is independent of magnetic field, and the inelastic cross section changes with magnetic field, especially at an ultracold temperature. A common rule of thumb is that to successfully implement cooling, the ratio of elastic cross section to inelastic cross section needs to reach 100 at least. The results suggest that it is likely to be a challenging work to perform sympathetic cooling of NH molecule by ultracold Xe atom.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (21): 214202. doi: 10.7498/aps.67.20181570
The accurate and precise controlling of the attosecond time delay between the sub-pulses within a hundredth of an optical cycle is the key ingredient for the sophisticated custom-tailored coherent waveform synthesizer. The attosecond delay control technique commonly experiences the “complete” characterization of the ultrashort sub-cycle pulses, which includes the spatiotemporal pulse characterization of the synthesized waveform and the attosecond relative delay between the parent pulses. In this work, the relative time delay between spectrally separated ultrashort parent pulses is characterized in an interferometer scheme with a background-free transient-grating frequency-resolved optical grating (TG-FROG). The TG-FROG geometry accurately measures the full time-dependent intensity and phase of ultrashort laser pulses in a wide range of regime (from ultraviolet to infrared) and offers significant advantages over other nonlinear-optical processes geometries (i.e., the polarization-gate-FROG, the self-diffraction-FROG, the second-harmonic generation-FROG and the third-harmonic-generation-FROG). The attosecond measurement accuracy is achieved for the first time, to the best of our knowledge. In this experiment, the output of a carrier-envelope-phase-stable Ti:sapphire amplifier (sub-30-fs, over-1-mJ, 1 kHz) is spectrally broadened in a neon-filled hollow-core fiber with an inner diameter of 250μm. The transmission through the pressure-gradient hollow-core fiber results in an mJ-level octave-spanning whitelight supercontinuum, supporting a sub-3-fs Fourier transform-limited pulse. The supercontinuum is spectrally divided into two parent pulses by using a dichroic mirror. The sub-pulses are individually compressed by the custom-designed double-chirped mirrors and wedge pairs. The short and long wavelength pulses are separately compressed in few-cycle regime, yielding pulses with 6.7 fs and 9.8 fs, respectively. This technique overcomes the bottlenecks in the traditional delay measurement and should be applicable for many ultra-broadband pulse characterizations with extremely simple and alignment-free delay control device used. Furthermore, this new method will be easily adapted for the ultra-broadband two-dimensional electronic spectroscopy, the advanced temporal cloaking, and the field of sub-cycle arbitrary coherent waveform synthesizer for controlling strong-field interactions in atoms, molecules, solids, and nanostructures. We foresee that in the near future this novel technology will be very attractive for various applications in the next-generation light sources such as the Synergetic Extreme Condition User Facility in Beijing, China.
2018, 67 (21): 214208. doi: 10.7498/aps.67.20180611
One of the outstanding challenges in phononic crystal development is the ability to achieve bandgap tunability in a low frequency range. The introduction of piezoelectric materials into phononic crystals is an attractive technique for actively controlling the bandgaps, which is reliable, economical and light in weight. Phononic crystal possesses an artificial periodic composite structure whose elastic constant, density and sound velocity change periodically. When the elastic wave passes through a phononic crystal, special dispersion curve is formed due to the interaction among periodically arranged materials. In order to study the tunability of phononic crystal bandgap, we propose a novel two-dimensional piezoelectric phononic crystal structure possessing a wider complete bandgap, which is composed of piezoelectric materials with hard coatings periodically connected by four thin bars. The dispersion relation, transmission spectrum and displacement field are studied by using the finite element method in combination with the Bloch theorem. Numerical results show that the frequency of the first complete bandgap of the new designed phononic crystal slab is lower and the band width is enlarged by a factor of 5 compared with the band width of the traditional binary phononic crystal. Instead of changing the geometry or orientation of the phononic crystal units or inclusions, electrical boundary conditions are used to actively control the frequency bandgap. The boundary condition for electrical open circuit and short circuit are considered in this paper. With different electrical boundary conditions imposed on the surfaces of the piezoelectric inclusions, multiple complete bandgaps can be controlled actively, which means that the new designed phononic crystal structure can adapt to the vibration and noise reduction requirements under different vibration environments. The effect of piezoelectric effect on the band structure is investigated as well. The piezoelectric effect has a great influence on the band structure, with the increase of the piezoelectric constant, a part of bands move to high-frequency and the other part of the bands are kept at the original position, which means that the piezoelectric effect is of benefit to the opening of the complete bandgap. Furthermore, according to the tunability of the bandgap, the switchable piezoelectric phononic crystal slab waveguide is analyzed. Calculation shows that the electrical boundary defects can result in defect bands existing in the complete band gap, and the elastic wave energy flows can be limited by changing the applied electrical boundary conditions. This investigation is conducive to controlling the bandgaps and also reveals potential applications in designing the sensing system and different piezoelectric devices.
2018, 67 (21): 214206. doi: 10.7498/aps.67.20181112
Visible light communication (VLC) is a new type of wireless communication technology, and its applications in offshore ships and ship-shore lamp signal systems are drawing increasing attention as a supplement of communication net. In maritime environment, VLC system is affected by many factors, of which the wave fluctuation and atmospheric turbulence are the most noticeable. The turbulence will make signal intensity fluctuate randomly, and thus reducing the performance of VLC system operating in the atmosphere. To establish an effective VLC network in the actual marine environment, an effective channel transmission model needs to be established and used to study the performance of the maritime VLC link. Considering large aperture diameter receiver with the aperture averaging effect, log-normal distribution model is employed to deduce the mathematical expression of average bit error rate of maritime VLC system in atmospheric turbulence. By using time-diversity to transmit interleaved symbols with repeated coding in a maritime VLC system, it is possible to ensure that the code-word passes through multiple channels to resist the deep fade performance, and to reduce the bit error rate due to the occurrence of deep fading in a single channel. In the actual application process, in order to improve the system performance, the average signal-to-noise ratio usually increases with the transmission power increasing, but for a VLC system, there are some difficulties in making the high-power high-rate visible light transmitters. And the power will produce light pollution and even damage the naked eye. The implementation of the repetitive coding principle is simple, and in some special cases it is even better than the complex orthogonal space-time coding and other schemes, so studying the system performance of the repetitive coding scheme is of considerable value for practical application. Based on the modified Pierson-Moskowitz spectrum, the effect of wave height, transmission distance, atmospheric turbulence intensity, receiver aperture size and visibility on the average bit error rate of VLC system are analyzed. The performance of the VLC system between lighthouse and ship is affected by the fluctuations of the sea waves, and the average bit error rate changes with randomness and complexity like the sea waves in a short distance. As the wind speed increases, the marine environment becomes worse and the average bit error rate is undulate. The average bit error rate of maritime VLC increases with the increasing of transmission distance and atmospheric turbulence intensity, and with the decreasing of receiver aperture size, wavelength and average signal-to-noise ratio. Atmospheric turbulence intensity and visibility have a significant effect on the system performance, and it should be emphatically considered to take measures to reduce the influence. Increasing receiver aperture and repetitive coding are effective to a certain extent. In the present work a new model is proposed for evaluating the performance of a maritime VLC system and providing reference for practical application.
2018, 67 (21): 214201. doi: 10.7498/aps.67.20180307
In this paper, the all-optical spatial modulation of monolayer graphene-coated microfiber is proposed. Graphene is used as a saturable absorber wrapped on the microfiber produced by heating the carbon dioxide laser. When the signal light travels along the microfiber, part of the light will pass along the surface of the microfiber in the form of an evanescent field, and it will be absorbed by the graphene. Simultaneously we shoot the 808 nm pump light into the micro-nanofiber wrapped by the graphene vertically from the space. According to graphene characteristic of preferential absorption, the absorption of the signal light is controlled by the pump light, thus the broadband all-optical space modulation is realized. In a conventional graphene microfiber all-optical modulator, signal light and pump light are generally input into a microfiber via a coupler. However, the mode of operation of pump light and graphene in all-optical spatial modulation are different from those of the traditional modulation, the pump light works on the graphene outside the microfiber, which realizes the separation of the pump light and the signal light. The output signal does not need to be optically filtered for the pump light to obtain the modulated signal. The output signal light of the spatial all-optical modulator has the characteristics of “clean”. We also verify this in experiment. In addition, the pump light is vertically incident from space, the effect of the graphene length on the modulation is not considered and the modulation time is only related to the relaxation time of graphene, which is helpful in improving the response time. Modulation experiments include static spectral modulation and dynamic frequency modulation. In the static spectral modulation, the broad spectrum signal has a maximum modulation depth of 6 dB at 1095 nm when the pump power is 569 mW. The relationship among pump power, wavelength and modulation depth is also analyzed. The higher the pump power, the higher the modulation depth will be; with the same pump power, the modulation depth of long wave length is higher than that of short wave. In the dynamic modulation experiment with the modulation bandwidth~50 nm and the modulation rate~1.5 kHz, the influence of pump light and signal light on output dynamic signal are studied, the feasibility of all-optical space modulation based on graphene is verified experimentally. The composite waveguide of all-optical spatial modulator opens the door to micro-nano ultrafast signal, processing in a more flexible and efficient way.
Temperature dependence of characteristic parameters of 1550 nm vertical-cavity surface-emitting laser
2018, 67 (21): 214203. doi: 10.7498/aps.67.20180572
Compared with conventional edge-emitting semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs) exhibit many advantages such as low power consumption, low threshold current, single longitudinal-mode operation, circular output beam with narrow divergence, on-wafer testing capability, high bandwidth modulation, low cost and easy large-scale integration into two-dimensional arrays, etc. VCSELs have been widely adopted in various applications such as optical communication, optical storage, parallel optical links, etc. At the same time, the rich dynamic characteristics of VCSELs have always been one of the frontier topics in the field of laser research, and many theoretically and experimentally investigated results have been reported. For theoretically investigating the dynamical characteristics of VCSELs, the spin-flip model (SFM) is one of most commonly and effectively used methods. In order to accurately predict the nonlinear dynamical performance of a 1550 nm-VCSEL, six characteristic parameters included in the rate equations of the SFM need to be given accurately. The six characteristic parameters are the decay rate of field k, the decay rate of total carrier population N, the linear anisotropies representing dichroism a, the linear anisotropies representing birefringence p, the spin-flip rate s, and the linewidth enhancement factor . In this work, through experimentally analyzing the output performances of a 1550 nm-VCSEL under free-running and parallel polarized optical injection, such six characteristic parameters included in the SFM are extracted first in the case that the temperature of the VCSEL is set to be 20.00℃. Furthermore, through gradually increasing the temperature of the 1550 nm-VCSEL from 10.00℃ to 30.00℃, the dependence of the six characteristic parameters on the temperature of the 1550 nm-VCSEL is investigated emphatically. The results show that with the increase of temperature of the 1550 nm-VCSEL, the linear anisotropy representing birefringence p behaves as an increasing trend, and the linewidth enhancement factor shows a decreasing trend. However, the other four characteristic parameters present complex varying trends with the increase of the temperature of the 1550 nm-VCSEL. The research in this paper is helpful in accurately understanding and controlling the dynamical characteristics of the VCSEL, and we hope that it can give a guidance for practical applications.
2018, 67 (21): 214205. doi: 10.7498/aps.67.20181574
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.
2018, 67 (21): 214207. doi: 10.7498/aps.67.20180786
In this paper, we mainly simulate the characteristics of the ground state dark soliton and the multipole dark soliton in the nonlocal and cubic-quintic nonlinear medium. Firstly, the influences of the degree of nonlocality on the amplitude and the width of the dark soliton in the self-defocusing cubic-and self-focusing quantic-nonlinear medium are studied. Secondly, we find the nonlinear parameters affecting the amplitude values of solitons, but the refractive index induced by the light beam is always a fixed value. The numerical results show that the ground state dark soliton can be propagated stably alone the z axis, and the stable states of the dipole soliton and the dark tri-pole and quadru-pole solitons are stable. However, some quadru-pole dark soliton is unstable after propagating the remote distance. Furthermore, we also discuss the characteristics of the ground state dark soliton and the dark dipole soliton in the local cubic-nonlinear and nonlocal quantic nonlinear media. Both the amplitude and the beam width of the dark ground state soliton and dark dipole soliton are also affected by the degree of nonlocality and nonlinearity. Two boundary values of the induced refractive index change with the variations of the three nonlinear parameters. The dark soliton and the dipole dark soliton are more stable in the self-focusing cubic nonlinear and the nonlocal self-defocusing quantic nonlinear medium than those in the self defocusing cubic nonlinear and nonlocal self-focusing quantic nonlinear medium. The powers of single dark soliton and dark tri-pole soliton decrease monotonically with the increase of propagation constant when the cubic-quintic nonlinearities are certain values and these degrees of nonlocalities are taken different values. Furthermore, we also analyze linear stabilities of various nonlocal spatial dark solitons. And we find that the dipole dark soliton is unstable when the propagation constant is in the region[-0.9,-1.0]. These properties of linear stabilities of other multi-pole dark solitons are the same as their propagation properties.
2018, 67 (21): 214401. doi: 10.7498/aps.67.20180743
In general, when the one-dimensional heat conduction equation is solved by the method of separation of variables, we need to know the governing equations, two boundary conditions and initial condition. Because the thermophysical parameters in different layers of laminated materials are different, the heat conduction model cannot be expressed by the same governing equation. For each layer of laminated material, the boundary condition is unknown. That equation can-not be solved directly by the general separation variable method. In this work the separation of variable method is extended. The temperature field of laminated material's heat transfer is divided into many minute time intervals on the time axis. Based on differential conception, in a minimum time interval, the temperature at the junction of laminated materials can be considered to be proportional to time. Assume that the slope coefficient makes the boundary condition known, then for each layer of laminated materials, the general separation of variables method will be used to solve the temperature field. According to the same temperature and the energy continuity at the junction of laminated materials, one can solve the slope coefficient. The temperature field in the whole time domain can be obtained through cycling. Then the three-layer insulation materials are analyzed by the extended separation variable method. The correctness of the method is verified by comparing the calculated results with those from the finite element method. The influences of the type and thickness of heat insulation layer, heat transfer coefficient, air temperature on the heat insulation are studied. It is found that the thermal conductivity of the thermal insulation layer has a great influence on the insulation. The material with low heat conduction coefficient can enhance the heat insulation effect. The thicker the thickness of the insulation layer, the more slowly the surface temperature of the heat insulation material rises, and the lower the final temperature, the better the insulation effect is. The thicker the thickness of the insulation layer, the smaller the heat flux density of the heat insulation material shell is, and the better the heat insulation effect when the heat transfer reaches a stable state. All calculation results are consistent with physical phenomena. In this work, the analytical method is used to solve the heat transfer problem of laminated materials. Compared with the general numerical methods, the analytical method presents clear physical meaning and high efficiency of operation as well.
2018, 67 (21): 214701. doi: 10.7498/aps.67.20181192
In this paper, the boundary layer flow stability is investigated experimentally in a 7° half-angle straight cone under the condition of Mach number 6 and unit Reynolds number 3.1×106/m. Expanded shock wave generated by focusing laser in a limit space is used as the small artificial disturbance, and the influence of the laser-generated perturbation on the stability of the hypersonic boundary layer is analyzed. In the experiment, the wall fluctuation pressure is measured by the high-frequency pressure sensors whose response frequencies each reach a value on the order of megahertz. Through the short time Fourier transformation and power spectrum density analysis of the pressure data, the results show that when the laser-generated perturbation is added to the flow field, the position of the second mode wave advances and the amplitude of the disturbance wave greatly increases. Within the same flow range, the laser focusing on disturbance pushes the disturbance wave in the boundary layer from the linear development phase into the nonlinear development state. The laser-generated perturbation has a significant effect on the promotion of the development of disturbance waves in the boundary layer. At the same time, laser-generated perturbation that has different influences on the boundary layer when it focuses on different positions. When the laser focus disturbance focuses on the location X=100 mm, the amplitude of the disturbance wave with a frequency of 90 kHz in the boundary layer grows fastest, and the amplitude magnification at the position of X=500 mm is 3.81. When the laser perturbation is added to the free flow in front of the cone, the frequency of the disturbance wave with the fastest amplitude increase speed greatly decreases to 73 kHz. In the same range, the amplitude magnification is 4.51 times. It can be seen that when the laser focuses on the free stream upstream from the cone, its effect on the disturbance wave in the boundary layer is more significant.
Influence of leading-edge curvature on excited unsteady cross-flow vortices in three-dimensional boundary-layer
2018, 67 (21): 214702. doi: 10.7498/aps.67.20181343
Three-dimensional boundary-layer receptivity is the first stage of the laminar-turbulent transition in a three-dimensional boundary layer, and also a key issue for predicting and controlling the laminar-turbulent transition in the three-dimensional boundary layer. At a high turbulence level, the three-dimensional boundary-layer instability in the transition is caused mainly by the unsteady cross-flow vortices. And the leading-edge curvature has a significant influence on three-dimensional boundary-layer receptivity. In view of this, the direct numerical simulation is utilized in this paper to study the mechanism of receptivity to exciting unsteady cross-flow vortices in the three-dimensional (swept-plate) boundary layer with various elliptic leading edges. In order to solve the Navier-Stokes equation numerically, a modified fourth-order Runge-Kutta scheme is introduced for discretization in time; high-order compact finite difference schemes are utilized for discretization in the x-and y-direction; and Fourier transform is used in the z-direction. The pressure Helmholtz equation is solved by a fourth-order iterative scheme. Additionally, the numerical calculation is performed in the curvilinear coordinate system via Jaccobi transform. And the elliptic equation technique is used to gene-rate the body-fitted mesh. The effect of leading-edge curvature on the propagation speed and direction, distribution and receptivity coefficient of the excited unsteady cross-flow vortex wave packet, and the amplitude, dispersion relation and growth rate of the extracted unsteady cross-flow vortex are revealed. In addition, the inner link among the receptivity to unsteady cross-flow vortex, intensity, and direction of free-stream turbulence is established. Furthermore, the receptivity to anisotropic free-stream turbulence is also analyzed in detail. The numerical results indicate that the more intense receptivity to the unsteady cross-flow vortex wave packets is triggered with a smaller leading-edge curvature; whereas, the less intense receptivity is triggered with a greater leading-edge curvature. The receptivity to the unsteady cross-flow vortex wave packets in different curvatures are also found to vary with the angle of free-stream turbulence. Moreover, the anisotropic degree of free-stream turbulence can affect the excitation of the unsteady cross-flow vortex obviously. Through the above study, a further step can be taken to understand the prediction and control of laminar-turbulent transition in the three-dimensional boundary layer and also improve the theory of the hydrodynamic stability.
2018, 67 (21): 214703. doi: 10.7498/aps.67.20181020
The aim of the present paper is to investigate the gravity-driven draining process containing soluble surfactant when considering the coupling effects of surface elasticity and surfactant solubility. A nonlinear coupling evolution equation including liquid film thickness, surface velocity and surfactant concentration (both on the surface and in the bulk) is established based on the lubrication theory. Assuming that the top of liquid film is attached to the wireframe and the bottom is connected to a reservoir, the drainage evolution is simulated with the software called FreeFem. The effects of surface elasticity and solubility on liquid film draining are discussed under their coupling. The simulation results show that the surface elasticity is an indispensable factor in the process of liquid film drainage with soluble surfactant, and the surfactant solubility also has an important influence on the process. At the initial stage of liquid draining, the initial thickness of liquid film increases with increasing surface elasticity, and the surface tends to be more rigid; with the drainage proceeding, the liquid film with high and low elasticity illustrate different notable draining features:in the case of low surface elasticity, the distribution of surfactant forms a surface tension gradient from top to bottom on the film surface, leading to positive Marangoni effect that counteracts gravity. However, in the case of high elasticity, the film surface presents a surface tension gradient from bottom to top, resulting in a reverse Marangoni effect, which accelerates the draining and makes the film more susceptible to instability. The solubility of surfactant dominates the number of adsorbent molecules on the film surface, which affects the surface elasticity. When the solubility of the surfactant is great (β → 0), the film is extremely unstable, and it breaks down quickly. As the solubility decreases (namely, β increases), the stability of the film increases, and the initial surface elasticity also rises. The surface elasticity gradually approaches to the limiting dilational elasticity modulus due to the film being thinner.
2018, 67 (21): 214704. doi: 10.7498/aps.67.20172501
A thermal nonequilibrium reactive flow model is proposed to deal with the detonation dynamics of solid explosive. For the detonation in solid explosive, the solid-phase reactant and gas-phase product in the chemically reactive mixture zone do not have molecular collisions as in the case of gaseous detonation, so the solid-phase reactant and gas-phase product can arrive at a mechanical equilibrium but cannot reach a thermal equilibrium when the detonation happens. The main properties of the present detonation model are as follows. The Euler equations for chemical mixture and the mass conservation equation for solid-phase reactant are used to express the chemically reactive flows in solid explosive detonation as a traditional way, and an additional set of governing equations of the species physical variables for solidphase reactant is derived to give an expression to the thermal nonequilibrium between the solid-phase reactant and gas-phase product. The chemical mixture within a control volume is defined as a collection of species which possess distinct internal energy or temperature, and the same pressure and velocity. For the explosive detonation, the species include solid-phase reactant and gas-phase product. Based on the mixing rule that every species can preserve the conservation of its internal energy in the reactive mixture zone, the evolution equation of internal energy for solid-phase reactant may be obtained, meanwhile, based on the property of mechanical equilibrium in the reactive mixture zone, the total volume fraction is equal to one, and the equation of state of every species, the evolution equation of volume fraction for solid-phase reactant and the evolution equation of pressure for chemical mixture can be derived. Thus, the theoretical model of solid explosive detonation includes the conservation equation of mass, momentum, total energy and the evolution equation of pressure for the chemical mixture, and the conservation equation of mass and the evolution equation of internal energy and volume fraction for the solid-phase reactant. The partially differential equations of the detonation model are numerically solved by a finite volume scheme with two-order spatiotemporal precision, through using a wave propagation algorithm by means of Strang splitting operator. The validation of the proposed detonation model is checked by the propagation of planar one-dimensional detonation, the propagation of cylindrically divergent detonation and the interaction between two cylindrically divergent detonations, and the typical examples demonstrate that the proposed theoretical model of solid explosive detonation is reasonable.
2018, 67 (21): 214209. doi: 10.7498/aps.67.20180823
Due to the Abbe diffraction limit, the resolution of a traditional optical microscopy is limited to about half of the illumination wavelength. Recent studies show that super-resolution imaging through dielectric microsphere has emerged as a simple imaging technique to overcome the diffraction limit under the illumination of white light. However, for imaging through microsphere, sometimes it is needed to enhance the reflection of a sample by depositing a metallic thin film on the top of the sample. Metallic thin films with different surface roughness have different optical properties. However, the effect caused by the surface roughness of a metallic film on microsphere imaging is rarely studied. In this paper, we study the effects of silver films with different surface roughness deposited on the surfaces of samples on the imaging properties of BaTiO3 (BTG) microspheres. Silver thin films are deposited respectively at evaporation rates of 1.5-3 Å/s and 5-10 Å/s, and the surface roughness values (root mean square (RMS) values) of the obtained silver films are about 3.23 nm and 6.80 nm, respectively. Using a BTG microsphere to observe a sample with a silver film deposited on its surface, we find that the surface roughness of the silver film will affect the imaging resolution and the range of focal image position (RFIP) of the BTG microsphere. When we use a 15-μm-diameter BTG microsphere to observe a 250-nm-diameter microsphere array and 580-nm-diameter microsphere array, the RFIP of the BTG microsphere increases with the RMS of the silver film increasing from 3.23 to 6.80 nm. Moreover, a 200-nm-diameter microsphere array can also be clearly discerned. The simulation results obtained by the commercial software COMSOL show that when the surface of a microsphere array sample is deposited with a rough silver film, the electric field intensity is enhanced not only in the gaps between adjacent microspheres, but also on the silver particles due to the excitation of localized surface plasmons. We propose that the scattering effect and the local electric field intensity enhancement caused by the rough silver film allow more high-frequency information of the sample to be coupled into the BTG microsphere, and thus improving the resolution and RFIP of the microsphere. As the imaging law of microsphere imaging still needs to be explored, our research work will be helpful in further revealing the mechanism in microsphere imaging.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Radiation induced parasitic effect in silicon-on-insulator metal-oxide-semiconductor field-effect transistor
2018, 67 (21): 216102. doi: 10.7498/aps.67.20181372
In this paper, we investigate the total ionizing dose (TID) effects of silicon-on-isolator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) with different sizes by using 60Co γ-ray. The SOI MOSFET contains a shallow trench isolation (STI) edge parasitic transistor and back gate parasitic transistor, in which STI oxide and buried oxide (BOX) are used as gate oxide, respectively. Although these parasitic effects are minimized by semiconductor device process, the radiation-induced trapped-charge can lead these parasitic effects to strengthen, thereby affecting the electrical characteristics of the main transistor. Since both the STI and BOX are sensitive to the TID effect, we try to distinguish their different influences on SOI devices in this work.The experimental results show that the characteristic degradation of device originates from the radiation-enhanced parasitic effect. The turning-on of the STI parasitic transistor leads the off-state leakage current to exponentially increase with total dose increasing until the off-state leakage reaches a saturation level. The threshold voltage shift observed in the narrow channel device results from the charge sharing in the STI, while the back gate coupling is a dominant contributor to the threshold voltage shift in short channel device. These results are explained by two simple models. The experimental data are consistent with the model calculation results. We can conclude that the smaller size device is more sensitive to TID effect in the same process.Furthermore, the influence of the negative bias at back gate and body on the radiation effect are also studied. The negative bias at back gate will partially neutralize the effect of positive trapped-charge in STI and that in BOX, thus suppressing the turning-on of STI parasitic transistor and the back gate coupling. The parasitic transistors share a common body region with the main transistor. So exerting body negative bias can increase the threshold voltage of the parasitic transistor, thereby restraining the TID effect. The experimental and simulation results show that the adjustment of the threshold voltage of parasitic transistor by body negative bias is limited due to the thin body region. The modulation of body negative bias in depletion region is more obvious in back gate parasitic transistor than in STI parasitic transistor. The weakening of parasitic conduction in the back channel is more noticeable than at STI sidewall under a body negative bias.
2018, 67 (21): 216101. doi: 10.7498/aps.67.20181140
In the early 1990s, Japanese scholars unexpectedly observed that single crystal changes into polycrystal in deuterium-implanted aluminum under electron irradiation, but never found the same phenomenon in the hydrogen-implanted aluminum. However, previous study of our group has proved that the polycrystalline phenomenon can also be observed in hydrogen-implanted aluminum during electron irradiation. In this paper, the behavior of inert gas bubbles in aluminum under electron irradiation is investigated, aiming to further explore the effects of ion species, electron voltage and the pressure of bubbles on the anomalous heat-releasing reaction of bubbles induced by electron irradiation. In the experiment, the transmission electron microscope (TEM) samples of pure aluminum were implanted with He+, Ne+, Ar+ respectively by ion accelerator at room temperature. The TEM is used to in-situ observe and investigate the evolution of microstructure and the change of selected electron diffraction patterns of gas bubbles during electron irradiation. The results show that gas bubbles form in aluminum sample after ion implantation. During 200 keV electron irradiation TEM results show that the three kinds of inert gas bubbles all coalesce, grow up and bust separately. Finally, lots of nanoscale black dots appear inside them. At the same time, the electron diffraction patterns change from single crystal diffraction spots to polycrystalline diffraction rings. The dark field images indicate that the diffraction rings are induced by these black dots. Moreover, from the characterization of the diffraction rings, it is known that these black dots are pure aluminum rather than aluminum oxide. Therefore, the possibility that the diffraction rings result from aluminum oxide is eliminated. It is assumed that a certain kind of heat-releasing reaction should happen when the gas bubbles are irradiated by electrons, which leads to the poly-crystallization of aluminum after electron irradiation. However, while helium bubbles are irradiated by electrons with an energy of 80 keV, no diffraction ring is observed after electron irradiation. The same phenomenon as that in the case of helium bubbles irradiated by 80 keV electrons is observed. When helium and argon mixed bubbles with polygonal shape are irradiated by 200 keV electrons, no diffraction ring is observed after electron irradiation either. The reason might be related to the energy of the electron beam and the pressure of gas bubbles separately. There should be a threshold value of electron voltage for the heat-releasing reaction. In addition, the pressure of the gas bubbles is also a key factor for the heat-releasing reaction. The heat-releasing phenomenon of gas bubbles reminds us of the sonoluminescence phenomenon. By model calculation, it is predicted that there is a plasma core in the bubble during sonoluminescence. According to the hint from researches of sonoluminescence, an assumption is made to explain the mechanism of heat-releasing reaction of gas bubbles during electron irradiation. It is that the implanted gas in high pressure bubbles in aluminum is excited into plasma during electron irradiation. When the energy of plasma in the bubbles is accumulated to a certain degree, the plasma is extinguished suddenly. In this process, a lot of heat is released to melt the aluminum, thus leading the aluminum to recrystallize.
2018, 67 (21): 218201. doi: 10.7498/aps.67.20181636
Since the discovery of graphene, mechanical exfoliation technology has become one of the important methods of preparing high-quality two-dimensional (2D) materials. This technology shows some unique advantages in the study of the intrinsic properties of 2D materials. However, traditional mechanical exfoliation method also has some obvious deficiencies, such as low yield ratio and small size of the resulting single-or few-layer flakes, which hinders the research progress in the field of 2D materials. In recent years, we made a series of breakthroughs in mechanical exfoliation technology, and independently developed a new type of mechanical exfoliation method with universality. The core of this new method is to enhance the van der Waals interaction between the layered material and the substrate by changing multiple parameters in the exfoliation process, thereby increasing the yield ratio and area of the monolayer. Taking graphene for example, we can now increase the size of graphene from micron to millimeter, increase over 100000 times in area, and yield ratio more than 95%, in the meantime graphene still maintains very high quality. This new mechanical exfoliation method shows great universality, and high-quality monolayer flake with a size of millimeters or more has been obtained in dozens of layered material systems including MoS2, WSe2, MoTe2, and Bi2212. More importantly, some special structures can be fabricated by optimizing exfoliation parameters, such as bubble and wrinkle structures, which paves the way for the study of these special material systems. Many scientific problems are still worth exploring in the mechanical exfoliation technology, and the breakthrough of this technology will greatly promote the research progress in the field of 2D materials.
2018, 67 (21): 215202. doi: 10.7498/aps.67.20181451
Cold plasma is a kind of non-thermal plasma, and characterized by high electron temperature (1-10 eV) and low gas temperature, which can be close to room temperature. It has been proved to be a fast, facile and environmentally friendly new method for synthesizing supported metal catalysts. Enhanced synthesis of metal catalysts by cold plasma consists of complex physical and chemical reactions. On the one hand, the active environment provided by cold plasma, can not only speed up the chemical reactions, shorten the reaction time from a few hours to several minutes, but also realize the kinetically or thermodynamically infeasible chemical reactions to achieve unconventional preparation. On the other hand, the phase contact behavior on a mesoscopic scale is influenced during cold plasma enhanced preparation, thereby the metal catalysts with structure different from that synthesized by traditional method. This review summarizes the reactor structure, physical and chemical mechanism for synthesizing metal catalysts by cold plasma, as well as the structure characteristics of the obtained metal catalysts. According to the working pressure, cold plasma can be categorized into low-pressure (LP) cold plasma and atmospheric-pressure (AP) cold plasma. The LP cold plasma is often generated by radio frequency glow discharge or direct current glow discharge, while the AP cold plasma is generally generated by dielectric barrier discharge and AP cold plasma jet. Energetic electrons are deemed to be the reducing agents for LP cold plasma. However, due to the frequent collisions among the electrons and gas molecules at atmospheric pressure, the electron energy in AP cold plasma is not high enough to reduce the metal ions directly. Therefore, hydrogen-containing gases are often adopted to generate active hydrogen species to reduce the metal ions. The process of synthesizing the metal catalysts by using the cold plasma is a fast, low-temperature process, and in the preparation process there exists a strong Coulomb repulsion. Therefore, metal catalysts with small size and high dispersion of metal nanoparticles, strong metal-support interaction, as well as specific metal structures (alloying degree and crystallinity) and modified supports can be obtained. Correspondingly, metal catalysts with high catalytic activity and stability can be synthesized. In addition, the challenges of preparing the cold plasma are discussed, and the future development is also prospected.
2018, 67 (21): 212501. doi: 10.7498/aps.67.20180563
The high-accuracy nuclear datum of prompt fission neutron spectrum (PFNS) is not only an important parameter for evaluating nuclear data, but also relevant to a more fundamental understanding of the fission process. However, the PFNS experimental data of main actinides (uranium and plutonium) are very scarce and the existing experimental data are in significant discrepancy. The error of some experimental data is markedly large:the relative uncertainty value reaches above 30%. In order to clarify these discrepancies and reduce the uncertainty, more reliable and accurate fission spectrum measurements are necessary. Most of the traditional measurements of the PFNS rely on time-of-flight technique. In this technique the fission sample needs to be prepared in a fission chamber and the fission neutron is identified by fission fragment. Since the particle range of fission fragment is very short, the thickness of sample is limited. So the number of samples cannot be large and the statistic count of fission neutrons will not be great either. Now a new technique to measure PFNS has appeared based on fission gamma multiplicity, which could be used to discriminate fission neutron from other neutrons. The new technique is based on a physical fact:when a fission event happens, seven-to-eight associated gamma photons are emitted while the inelastic scattering effect emits only one or two associated gamma photons. Accordingly, the fission neutron could be discriminated from other neutrons by fission gamma multiplicity. The principles of the new method and the realizing approach are presented in detail. The experimentally measuring system is established. A spontaneous fission neutron source 252Cf is used to measure the PFNS in order to validate the measuring system. The measured spectra are compared with those of ENDF/B-VⅡ library. The PFNS of 238U induced by D-T neutron is measured by the measuring system. The results show that the new method based on fission gamma identification is be available in the measuring of PFNS. The identification efficiency, the total uncertainty analysis of the new method and the suggestion for improvements in the future are also included in the paper.
2018, 67 (21): 214204. doi: 10.7498/aps.67.20181227
In recent years, with the development of laser technology, various non-diffraction beams each with a central spot unchanged after a long distance propagation, have been generated, they being the Bessel beam, higher Bessel beam, Mathieu beam, higher Mathieu beam, cosine beam, parabolic beam, and Airy beam. Diffraction-free beams are widely used in laser drilling, laser precision alignment, optical precision control, optical micromanipulation, optical communication, plasma guidance, light bullet, synthesis of autofocusing beam, nonlinear optics, etc. In this paper, the expressions, generation methods and corresponding experimental results of the various non-diffraction beams are presented. There are many ways to generate the Bessel beam, they being circular slit, computed hologram, spherical aberration lens, resonant cavity, axicon, and metasurface. The main methods of generating the non-diffraction beams are summarized, and each method is analyzed in depth from the cost of the system, and then some suggestions for improving and perfecting are made. For the generation of non-diffraction beams, the passive methods are used most to convert other beams into corresponding non-diffraction beams by optical components. Due to the low damage threshold and high cost of optical components, the power, energy and beam quality of a non-diffracting beam will be limited. How to generate a high-power, high-beam quality non-diffracting beam will be a hot research spot. Diffractionless beams have attracted a great deal of interest due to their unique non-diffraction, transverse-accelerating (or self-bending) and self-healing property. Transverse-accelerating property refers to that non-diffraction beams propagate along a parabola trajectory. The diffractionless beams' propagation trajectory control method implemented by changing system parameters is simple and easily successful, but cannot reverse acceleration direction, and its controlling range is limited. The self-healing property means that the non-diffraction beam tends to reform during propagation in spite of severe perturbations imposed. Both the Airy beam and the Bessel beam exhibit self-healing properties during propagation. And non-diffraction beams have potential applications in many fields. In atmosphere, such as in optical communication, non-diffracting beam exhibits more resilience against perturbations. Finally, brief summary and outlook of non-diffraction beams playing important roles in future study, and their application prospects are presented. In addition to Airy beam and Bessel beam, for other non-diffraction beams due to the complexity of the beams themselves, by comparison, their applications are investigated very little, so the applications in Mathieu beam, cosine beam, and parabolic beam will be a hot research spot.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (21): 215201. doi: 10.7498/aps.67.20180932
Compton and inverse Compton scattering from relativistic Maxwellian electrons both have an important feature, i.e. calculating the radiation transport in high-temperature and full-ionized plasma. Description and evaluation of relativistic photon-Maxwellian electron scattering are numerically complex and computationally time consuming. A Monte Carlo method is proposed to simulate photon scattering with relativistic Maxwellian electron and compute the scattering cross-sections. To compute the total cross-section of a photon of energy hν interacting with electrons at temperature Te in the laboratory coordinate, the calculation steps of Monte Carlo scheme are described as follows. The first step is to sample the velocity of an electron, the directions are isotropically sampled, and the speed is sampled from the relativistic Maxwellian distribution at temperature Te. The second step is to transform the photon energy hν into the photon energy in the coordinate in which the electron is at rest. The third step is to use the exact Klein-Nishina formula to compute the cross-sections. The fourth step is go back to the first step, and cycle this many times. The last step is to summarize all computed cross-sections and averaged them, and the average value is what we need. The operation and corresponding formula for each step are described in this paper. A better method of sampling the speed of a relativistic electron is expected to be found.A Monte Carlo processor is developed to compute the scattering cross-section of a photon of any energy, interacting with electrons at any temperature. To check this method, scatterings of the photons of various energies with electrons with various temperatures are simulated, and the results are compared with those from the numerical integration method. The comparison indicates that the simulated cross-sections are in pretty good agreement with those from the multiple integration method for the cases of electron temperature less than 25 keV. But unfortunately, the difference is obvious for the case of temperature more than 25 keV, and the error increases with temperature increasing. Why so? When the temperature is more than 25 keV, the sampling of electron speed is inaccurate when using the present method, which maybe results in this difference. So, we need to find a more accurate method of sampling relativistic electron speed to solve this problem in the future.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Growth and characterization of grid-like β-Ga2O3 nanowires by electric field assisted chemical vapor deposition method
2018, 67 (21): 218101. doi: 10.7498/aps.67.20180805
Gallium oxide (Ga2O3) has five crystalline polymorphs, i.e. corundum (α-phase), monoclinic (β-phase), spinel (γ-phase), bixbite (δ-phase) and orthorhombic (ε-phase). Among these phases, the monoclinic structured β-Ga2O3 is the most stable form, and is a ultraviolet (UV) transparent semiconductor with a wide band gap of 4.9 eV. It is a promising candidate for applications in UV transparent electrodes, solar-blind photodetectors, gas sensors and optoelectronic devices. In recent years, one-dimensional (1D) nanoscale semiconductor structures, such as nanowires, nanobelts, and nanorods, have attracted considerable attention due to their interesting fundamental properties and potential applications in nanoscale opto-electronic devices.Numerous efforts have been made to fabricate such devices in 1D nanostructures such as nanowires and nanorods. Comparing with the thin film form, the device performance in the 1D form is significantly enhanced as the surface-to-volume ratio increases. In order to realize β-Ga2O3 based nano-optoelectronic devices, it is necessary to obtain controlled-synthesis and the high-quality β-Ga2O3 nanomaterials. According to the present difficulties in synthesizing β-Ga2O3 nanomaterials, in this paper, the grid-like β-Ga2O3 nanowires are prepared on sapphire substrates via electric field assisted chemical vapor deposition method.High-purity metallic Ga (99.99%) is used as Ga vapor source. High-purity Ar gas is used as carrier gas. The flow rate of high-purity Ar carrier gas is controlled at 200 sccm. Then, oxygen reactant gas with a flow rate of 2 sccm enters into the system. The temperature is kept at 900℃ for 20 min. The effect of the external electric voltage on the surface morphology, crystal structure and optical properties of β-Ga2O3 nanowires are investigated. It is found that the external electric voltage has a great influence on the surface morphology of the sample. The orientation of the β-Ga2O3 nanowires grown under the action of an applied electric field begins to improve. Only a grid composed of three different growth directions appears. And with the increase of applied voltage, the distribution of nanowires becomes denser and the length increases significantly. In addition, it is found that the chemical vapor deposition method assisted by this external electric field can significantly improve the crystallization and optical quality of the samples.
2018, 67 (21): 218701. doi: 10.7498/aps.67.20180305
A two-dimensional axisymmetric immersed boundary thermal lattice Boltzmann (IB-TLB) model is presented to study the phase transition in Czochralski silicon crystal growth for improving the morphology of the melt-crystal interface and the crystal quality. Specifically, the Euler grid and the Lagrange grid are established, respectively. The melt-crystal interface is considered as an immersed boundary, and it is described by a series of Lagrange nodes. In this paper, the melt-crystal interface is tracked by the immersed boundary method, and the melt flow and heat transfer are simulated by the lattice Boltzmann method. The D2Q9 model is adopted to solve the axial velocity, radial velocity, swirling velocity and temperature of the melt. The finite difference method is used to solve the temperature distribution of the crystal. Then the solid-liquid transition in crystal growth with moving boundary is solved by the proposed IB-TLB model. The proposed model is validated by the solid-liquid phase transition benchmark. In addition, the flatness of the melt-crystal interface is evaluated by the mean value of the absolute value of the interface deviation and the standard deviation of the interface deviation. The effects of the process parameters on the morphology of melt-crystal interface, melt flow structure and temperature distribution are analyzed. The results show that the morphology of the melt-crystal interface is relevant to the interaction of the crystal pulling rate, the crystal rotation parameter, and the crucible rotation parameter. When the crystal and crucible rotate together, the deviation and fluctuation of the melt-crystal interface can be effectively adjusted, whether they rotate in the same direction or rotate in the opposite directions. And a flat melt-crystal interface can be obtained by appropriately configurating the ratio of crystal rotation parameter to crucible rotation parameter. Finally, according to a series of computations, it is found that when the crucible and crystal rotate in the opposite directions, the crystal rotation parameter and the crucible rotation parameter satisfy a functional relation, with a flat interface maintained. The obtained relationship has a certain reference for adjusting and improving the crystal growth parameters in practice.
Young Scientists' Forum
2018, 67 (21): 218102. doi: 10.7498/aps.67.20181571
The concept of compliant substrate epitaxy was first proposed by the scientists engaged in crystal growth in the early 1990s. The core idea is to take advantage of such an ultra-thin substrate that the film and the substrate generate strain together to relieve the lattice mismatch during the epitaxy growth. The quality of the epitaxial film is improved due to the reduction of the mismatch dislocation density. However, the preparation of the artificial ultra-thin substrate with good quality requires rather complicated fabrication process. On the other hand, many transition metal dichalcogenides naturally form the compliant substrates, due to their layered structure and weak van der Waals interlayer interaction. In this paper, we introduce the transition metal dichalcogenides based compliant substrate epitaxy model and relevant applications. Through combining density functional theory, linear elasticity theory and dislocation theory, we introduce the model comprehensively by using the Au-MoS2 as a prototypical example. And we explain the experimental results of Au growing on MoS2 from the early transition electron microscopy. In addition, we introduce the experimental work related to the model, especially the Au-mediated exfoliation of large, monolayer and high-quality MoS2. Future directions and relevant important problems to be solved are also discussed.