High-pressure synthesis and special physical properties of several ordered perovskite structures
Strongly correlated electronic systems with ABO3 perovskite and/or perovskite-like structures have received much attention. High pressure is an effective method to prepare perovskites, in particular A-site and/or B-site ordered perovskites. In these ordered perovskites, both A and B sites can accommodate transition-metal ions, giving rising to multiple magnetic and electrical interactions between A-A, B-B, and A-B sites. The presence of these new interactions can induce a wide variety of interesting physical properties. In this review paper, we will introduce an A-site ordered perovskite with chemical formula AA3'B4O12 and two A- and B-site ordered perovskites with chemical formula AA3'B2B2'O12. All of these compounds can be synthesized only under high pressure. In the A-site ordered LaMn3Cr4O12 with cubic perovskite structure, magnetoelectric multiferroicity with new multiferroic mechanism is found to occur. This is the first observation of multiferroicity appearing in cubic perovskite, thereby opening the way to exploring new multiferroic materials and mechanisms. In the A- and B-site ordered perovskite CaCu3Fe2Os2O12, a high ferrimagnetic Curie temperature is observed to be around 580 K. Moreover, this compound exhibits semiconducting conductivity with an energy band gap of about 1 eV. The CaCu3Fe2Os2O12 thus provides a rare single-phase ferrimagnetic semiconductor with high spin ordering temperature well above room temperature as well as considerable energy band gap. Moreover, theoretical calculations point out that the introducing of A'-site Cu2+ magnetic ions can generate strong Cu-Fe and Cu-Os spin interactions. As a result, this A- and B-site ordered perovskite has a much higher Curie temperature than that of the B-site only ordered perovskite Ca2FeOsO6 (~320 K). In addition, we also for the first time prepare another A- and B-site ordered perovskite LaMn3Ni2Mn2O12. In the reported ordered perovskites with Mn3+ at the A' site, the A'-B intersite spin interaction is usually negligible. In our LaMn3Ni2Mn2O12, however, there exists the considerable A'-B interaction, which is responsible for the rare formation of B-site orthogonal spin structure with net ferromagnetic moment.
Equations of state and thermodynamic properties of hot plasma
The equations of state (EOS) and the thermodynamics properties of plasma under high temperature are widely applied to the fields of astrophysics, controllable fusion, weapon design and damage. In this paper we mainly review the theoretical model and computing method of the EOS of hot plasma on different density scales and temperature scales. For an ideal plasma, the interaction between ions can be ignored, the EOS is simple and the theories turn matured. Under the condition of extremely high temperature, ions are ionized completely and the EOSs of ions and electrons can be approximated by the EOS of ideal gas. When the temperature is not very high and ions are just partly ionized, the EOS can be obtained by Saha model or its modified model. When atoms are strongly compressed, the EOS can be calculated by Thomas-Fermi model or its modified model. For the non-ideal plasma, there is a strong coupling between ions. No unified theoretical model can completely describe the interaction between ions at arbitrary density and arbitrary temperature. In principle, the quantum molecular dynamics (QMD) can accurately describe the EOS of plasma in large density range and large temperature range. However, due to the enormous computation and the difficulty in converging, it is difficult to apply QMD to the plasma under high temperature. With simple computing method and small computation, classical molecular dynamics using semi-empirical potential can calculate the EOS accurately at high temperature. However, it will produce great error at lower temperature. It is a simple and effective way to obtain a global EOS by using different theoretical models in different density range and different temperature range and by interpolating in the vacant density range and vacant temperature range.
High-pressure structural and optical properties of organic-inorganic hybrid perovskite CH3NH3PbI3
Recent advance in highly efficient solar cells based on organic-inorganic hybrid perovskites has triggered intense research efforts to ascertain the fundamental properties of these materials. In this work, we utilize diamond anvil cell to investigate the pressure-induced structural and optical transformations in methylammonium lead iodide (CH3NH3PbI3) at pressures ranging from atmospheric pressure to 7 GPa at room temperature. The synchrotron X-ray diffraction experiment shows that the sample transforms from tetragonal (space group I4cm) to orthorhombic (space group Imm2) phase at 0.3 GPa and amorphizes above 4 GPa. Pressure dependence of the unit cell volume of CH3NH3PbI3 shows that the unit cell volume undergoes a sudden reduction at 0.3 GPa, which can prove the observed phase transition. We provide the high-pressure optical micrographs obtained from a diamond anvil cell. Upon compression, we can visually observe that the opaque black sample gradually transforms into a transparent red one above 4 GPa. We analyze the pressure dependence of the band gap energy based on the optical absorption and photoluminescence (PL) results. As pressure increases up to 0.25 GPa, the absorption edge and PL peak move to the longer wavelength region of 9 nm. However, abrupt blueshifts of the absorption edge and PL peak occur at 0.3 GPa, followed by a gradual blueshift up to 1 GPa, these phenomena correspond to the previously observed phase transitions. Phase transition increases the band gap energy of CH3NH3PbI3 as a result of reductions in symmetry and tilting of the[PbI6]4- octahedral. Upon further compression, the sample exhibits pressure-induced amorphization at about 4 GPa, which significantly affects its optical properties. Further high pressure Raman and infrared spectroscopy experiments illustrate the high pressure behavior of organic CH3NH3+ cations. Owing to the presence of hydrogen bonding between organic cations and the inorganic framework, all of the bending and rocking modes of CH3 and NH3 groups are gradually red-shifted with increasing pressure. The transition of N–H stretching mode from blueshift to redshift as a result of the attractive interactions between hydrogen atoms and iodine atoms is gradually strengthened. Moreover, all the observed changes are fully reversible when the pressure is completely released. In situ high pressure studies provide essential information about the intrinsic properties and stabilities of organic-inorganic hybrid perovskites, which significantly affect the performances of perovskite solar cells.
Photodissociation dynamics of organic molecules in condensed phase by femtosecond transient grating spectroscopy
In condensed phase, the dissociation mechanism of molecule is different from that of isolated molecule due to the effect of interaction between molecules. How to effectively trace the reaction process and products in condensed phase is a technical problem which needs to be solved urgently. In this paper, femtosecond transient grating spectroscopy is used to investigate dissociation dynamics in condensed phase. Transient grating spectroscopy, as a coherent spectral technique, has some advantages such as high signal-noise ratio and free background, thus it can identify trace numbers of reaction products in dissociation. The investigation about model molecules such as iodomethane and nitromethane demonstrates that the transient grating technique can observe relaxation in electronic excited state and also has ability to track reactants, products, and vibration of molecule or perssad. The dissociation dynamics in condensed phase material is significant for understanding the reaction mechanism in the fields of biochemistry and detonation. Thus the femtosecond transient grating has a wide application prospect in these fields. In addition, the transient grating technique, as a non-contact diagnostic approach, can be easily adapted to high temperature and high pressure conditions, etc. Thus, the transient grating technique also has a potential value in the fields of phase transform dynamics and high pressure synthesis, etc.
First-principle simulation of solid nitrobenzene under uniaxial compression
Energetic materials (EMs) including explosives, propellants and pyrotechnics have been widely used for the military and many other purposes. Solid nitrobenzene (an organic molecular crystal) could be considered as a prototype of energetic material. Up to now, numerous studies have been devoted to crystal structures, spectrum properties and decomposition mechanisms for solid nitrobenzene experimentally and theoretically. However there has been a lack of the comprehensive understanding of the anisotropic characteristics under different loading conditions. Thus we investigate the hydrostatic and uniaxial compressions along three different lattice directions to determine this anisotropic effect. In this work, the density functional theory calculations are performed based on Cambridge Sequential Total Energy Package (CASTEP) code using normconserving pseudo potentials and a kinetic energy cutoff of 700 eV. The generalized gradient approximation with the Perdew-Burke-Ernzerhof parameterization is used. Monkhorst-Pack k-point meshes with a density of 0.05 Å-1 are used for Brillouin-zone integration. The empirical dispersion correction by Grimme is taken to account for week intermolecular interactions. The hydrostatic compressions are applied from 0 GPa to 20 GPa. Cell volume, lattice shape and coordinates of the atoms could be fully relaxed. while uniaxial compression is applied up to 70% of the equilibrium cell volume in steps of 2% along their lattice directions respectively. At each compression step, only atomic coordinates are allowed to relax, with the lattice fixed. The equilibrium lattice structures under hydrostatic compressions are obtained by full relaxation at 0 K temperature. In ambient condition, the calculated volume and parameter of the unit cell are underestimated compared with the experimental data, and corresponding errors are -2.98%, 0.01%, -4.39%, 5.71% respectively. In contrast, the calculated lattice energy is overestimated compared with the range of experimental results with 5.71% of the error. In high pressure condition, the volume and cell parameter of the unit cell as a function of compression ratio are plotted and compared with the experimental data. The theoretical and experimental values are close with the increase of the pressure, for instant, the error decreases from -4.39% at 0 GPa to -1.93% at 4 GPa. On the other hand, the uniaxial compression is applied along the directions of three lattice vectors. The changes of stress tensor, band gap, energy per atom as a function of compression ratio are also plotted and discussed, which can characterize the anisotropic effect of solid nitrobenzene. The most noticeable effect of anisotropy in solid nitrobenzene is the metallization at V/V0=0.76 compressed along the X axis, while the solid nitrobenzene under hydrostatic pressure or other uniaxial compressions up to V/V0=0.76 remains semiconductor with band gap larger than 1.591 eV. By analyzing the local density of states and charge density distribution of nitrobenzene crystal, we confirm that the metallization is caused by the overlap of the π electron from benzene ring. Through calculating different physical parameters, we find that X axis is the most sensitive direction of nitrobenzene crystal. The studies of anisotropic effects are expected to shed light on the physical and chemical properties of solid nitrobenzene on an atomistic scale and provide several insights for experiments.
Structures and novel superconductivity of hydrogen-rich compounds under high pressures
Metallic hydrogen can be realized theoretically at high pressure, which suggests that it will be a room-temperature superconductor due to the high vibrational frequencies of hydrogen atoms. However, the metallic state of hydrogen is not observed in experiment at up to 388 GPa. Scientists have been exploring various new ways to achieve hydrogen metallization. Hydrogen-rich compounds can be metallized at much lower pressures because of chemical pre-compression. Moreover, because such materials are dominated by hydrogen atoms, some novel properties can be found after metallization, such as high Tc superconductivity. Therefore, hydrogen-rich compounds are potential high-temperature superconductors, and this method is also believed to be an effective way to metalize hydrogen, which has aroused significant interest in lots of fields, such as physics, material science, etc. In a word, hydrogen-rich compounds are expected to become a new member of superconductor family:hydrogen-based superconductor. Very recently, the theoretical prediction and the successful experimental discovery of high-temperature superconductivity at 200 K in a sulfur hydride compound at high pressure have set a record, which inspired further efforts to study the superconductivity of hydrogen-rich compounds. The present review focuses on crystal structures, stabilities, interaction between atoms, metallization, and superconductivity of several typical hydrogen-rich compounds at high pressures. Furthermore, higher Tc superconductors can be expected to be found in hydrogen-rich compounds in the future.
Structures and properties of functional transition metal borides
Transition metal borides (TMBs) are hard or potential superhard materials due to abrasion resistant, corrosion preventive, oxidation resistance and high hardness. However, few TMBs are superhard materials, so, discussing the strength of TMBs to understand hardness mechanism is necessary. Moreover, there are superconductors, magnetic materials, and catalysts in TMBs. But uncovering more functions in TMBs is important for finding a new kind of functional hard or superhard material. While, high energy is necessary to synthesize TMBs due to strong B–B covalent bonds and high melting of transition metal. Thus high temperature or extreme condition is necessary for synthesizing single crystal or bulk sample with high density, which is important for testing physical properties. Various ways of hybridizing boron atoms and high content of valence electron of transition metal are used to induce a large number of structures and potential new properties in TMBs. Boron atoms can form different substructures with different content of boron in TMBs, such as one-dimensional, two-dimensional and three-dimensional (3D) structures. These different boron atom substructures can affect the stability of structure and physical properties, especially hardness, because of the strong covalent bonds between boron atoms. Thus the structure and hardness of TMBs have always received much attention. The multiple electron transfer between transition metal and boron induces diverse chemical bonds in TMBs. All of covalent bonds, ionic bonds, and metal bonds in TMBs determine the mechanic performances, electricitic and magnetic properties, and chemical activity of TMBs. In this work, synthesis method, stability of structure, hardness, and functional properties of TMBs are discussed. The using of high pressure and high temperature is an effective method to prepare TMBs, because under high pressure and high temperature the electrons can transfer between transition-metal atoms and boron atoms in TMBs. There are not only stable TMBs which are even under very high pressure, but also many metastable structures in TMBs. Hardness values of TMBs are discussed by different content of boron, the high boron content or even 3D boron structure is not superhard material. Because insufficient electron transfer can form the distorted B–B covalent bond which is weaker than directional covalent bonds like C–C in diamond. Thus electron transfer is significant in TMBs for designing hard or even superhard materials. Besides high hardness, there are superconductor, magnetic material, and catalyzers in TMBs, but there are many potential properties of TMBs which are unknown. Further study to uncover the new properties of TMBs is significant for finding a new kind of functional hard material.
Design of ultra-hard multifunctional transition metal compounds
Transition metals have special characteristics, such as a large number of valence electrons, multi valence states, high electron density, etc. Introducing a light element, such as boron, carbon, nitrogen, oxygen, etc. into a transition metal is an important means for searching the new multifunctional hard materials. With the development of ab intio calculation, advance in computer and the more in-depth understanding of the nature of hardness, it is possible to design new multifunctional ultra-hard transitional metal with using the advanced structure searching software, which could now serve as the experimental syntheses of these materials. In the present article, we introduce the design of ultra hard multi functional transition metal materials. We first introduce some basic ideas of hardness and material design, then conduct some studies, afterwards we discuss some difficulties in this kind of research. Hopefully these results in the present study could be helpful for designing and synthesizing the ultra-hard multifunctional materials.
High pressure synthesis of nanotwinned ultrahard materials
In this review, we present our recent research progress in superhard materials, with specially focusing on two topics. One topic is to understand hardness microscopically and establish the quantitative relationship between hardness and atomic parameters of crystal, which can be used to guide the design of novel superhard crystals. The other topic is to identify the fundamental principle and technological method to enhance the comprehensive performances (i.e., hardness, fracture toughness, and thermal stability) of superhard materials, and to synthesize high-performance superhard materials. Starting from the chemical bonds associated with crystal hardness and electronic structure, we propose a microscopic understanding of the indentation hardness as the combined resistance of chemical bonds in a material to indentation. Under this assumption, we establish the microscopic hardness model of covalent single crystals and further generalize it to polycrystalline materials. According to the polycrystalline hardness model, we successfully synthesize nanotwinned cubic boron nitride and diamond bulks under high pressure and high temperature. These materials exhibit simultaneous improvements in hardness, fracture toughness, and thermal stability. We also clarify a long-standing controversy about the criterion for performing a reliable indentation hardness measurement. Our research points out a new direction for developing the high-performance superhard materials, and promises innovations in both machinery processing industry and high pressure science.
High pressure synthesis of anhydrous magnesium carbonate (MgCO3) from magnesium oxalate dihydrate (MgC2O4·2H2O) and its characterization
Stimulated by the extensive application and research value, the study of anhydrous magnesium carbonate (MgCO3) has been a subject of great concern recently, so that a basic problem in designing a method of effectively synthesizing MgCO3 is very worth considering. In previous studies, different methods were reported to synthesize MgCO3 successfully but they still have some obvious deficiencies. The micro-particle sizes are too small to satisfy the basic requirements of micro-analysis. Thus, it is needed to explore the new methods of artificially synthesizing MgCO3 with the simple process and the high efficiency.
By using magnesium oxalate dihydrate (MgC2O4·2H2O) as starting material, MgCO3 sample is successfully synthesized by a solid reaction under high temperature and high pressure for the first time in this work. The properties of as-synthesized sample are investigated by X-ray powder diffraction and Raman spectroscopy:neither of them shows any impurities existing in the sample. Significantly, the crystallinity quality is greatly improved in the terms of the maximum grain sizes up to 200 micrometers, which could provide a base for MgCO3 single crystal growth in the future. Moreover, compared with the results of previous studies, the reaction time of high pressure synthesis is controlled within 1 h so that the efficiency of the synthesis is greatly improved.
Based on thermogravimetric analyses and the results of high pressure experiment under the various pressures and temperatures, the P-T phase diagrams of MgC2O4·2H2O-MgCO3-MgO at high pressures of 0.5, 1.0 and 1.5 GPa are obtained, and in this case, it is reasonable to explain the principle of MgCO3 synthesis under high pressure strictly. From the P-T diagram, high pressure can greatly improve the thermal stability of material, and the decomposition temperature of MgCO3 obviously increases with pressure increasing. However, due to decomposition temperature of MgCO3 increasing more quickly than that of MgCO4·2H2O, the stable phase regions of MgC2O4·2H2O and MgCO3 are separated from each other, and hence, the corresponding temperature and pressure can be controlled to decompose the phase of MgC2O4·2H2O while stabilizing the phase of MgCO3 so as to obtain MgCO3 successfully. Besides, by using polarizing microscope, the morphology of MgCO3 sample as well as its crystal cleavage plane (1011) is observed clearly, and it is noted that as-synthesized MgCO3 has good optical properties and high-quality crystallinity. The electron probing analysis for MgCO3 thin section is performed to quantify the Mg content and the calculation indicates that the sample composition is Mg0.99CO3.
High pressure single-crystal synchrotron X-ray diffraction technique
A lot of great work has been done since the high pressure research carried out on synchrotron radiation facility almost 40 years ago. The history of high pressure single-crystal diffraction research on synchrotron radiation facility has also been more than 20 years. Recently, with the development of synchrotron X-ray optical techniques and high pressure technology, especially the invention and improvement of large opening diamond anvil cell (DAC), high pressure single-crystal X-ray diffraction (HPSXRD) method has become more and more popular in high pressure studies. The HPSXRD can be used to perform structure determination and refinement to obtain the information about lattice parameter, space group, atomic coordinate and site occupation. Compared with powder X-ray diffraction, the HPSXRD can not only obtain the three-dimensional diffraction information of samples, but also have much better signal-to-noise ratio. Furthermore, the HPSXRD data can be used to study the electron density distribution to obtain more information about chemical bonds and electron distribution. In this work, we introduce the HPSXRD method in synchrotron radiation facilities, including the knowledge of single-crystal X-ray diffraction experimental system, DAC for HPSXRD, sample loading, and HPSXRD data processing.
Theoretical study of the equation of state for warm dense matter
In this paper, we present in detail various theoretical models for studying the equation of state of warm dense matter, including the fluid variational theory, the chemical model, the ionization equilibrium model, the average atom model and INFERNO model. The method of calculating the equation of state of a mixture is also given. The results from the first principles molecular dynamics simulation and the quantum Monte Carlo simulation are also provided. Typical materials such as hydrogen, deuterium, helium, xenon, gold, tungsten, etc. are studied in warm dense region by using all the methods, showing the effects of dissociation and ionization in the equation of state.
Pressure-tuned magnetic quantum critical point and unconventional superconductivity
Magnetic quantum critical point (QCP) arises when a long-range magnetic order occurring at finite temperature can be suppressed to absolute zero temperature by using chemical substitutions or exerting high pressure. Exotic phenomena such as the non-Fermi-liquid behaviors or the unconventional superconductivity are frequently observed near the magnetic QCP. In comparison with chemical substitutions, the application of high pressure has some advantages in the sense that it introduces no chemical disorder and can approach the QCP in a very precise manner. In this article, our recent progress in exploring the unconventional superconductors in the vicinity of pressure-induced magnetic QCP is reviewed. By utilizing the piston-cylinder and cubic-anvil-cell apparatus that can maintain a relatively good hydrostatic pressure condition, we first investigated systematically the effect of pressure on the electrical transport properties of the helimagnetic CrAs and MnP. We discovered for the first time the emergence of superconductivity below Tc=2 K and 1 K near their pressure-induced magnetic QCPs at Pc≈0.8 GPa and 8 GPa for CrAs and MnP, respectively. They represent the first superconductor among the Cr- and Mn-based compounds, and thus open a new avenue to searching novel superconductors in the Cr- and Mn-based systems. Then, we constructed the most comprehensive temperature-pressure phase diagram of FeSe single crystal based on detailed measurements of high-pressure resistivity and alternating current magnetic susceptibility. We uncovered a dome-shaped magnetic phase superseding the nematic order, and observed the sudden enhancement of superconductivity with Tcmax=38.5 K accompanied with the suppression of magnetic order. Our results revealed explicitly the competing nature of nematic order, antiferromagnetic order, and superconductivity, and how the high-Tc superconductivity is achieved by suppressing the long-range antiferromagnetic order, suggesting the important role of antiferromagnetic spin fluctuations for the Cooper paring. These aforementioned results demonstrated that high pressure is an effective approach to exploring or investigating the anomalous phenomena of strongly correlated electronic systems by finely tuning the competing electronic orders.
Superconductivities of pressurized iron pnictide superconductors
As one of the independent control parameters, pressure plays an important role in finding new phenomena, testing related theories and guiding the explorations for new superconductors. In this review article, we will briefly review the progress achieved from high pressure studies on some main types of the iron pnictide superconductors, including 1111-type, 122-type, 111-type, 10-3-8 type and 112-type. A few typical results from high pressure studies are introduced in more detail, including the positive pressure effect on the superconducting transition temperature TC of 1111-type iron pnictide superconductors, which indicates a way to enhance the TC by using a smaller cation to replay La ion; the maximum TC of iron pnictide superconductors estimated by high-pressure studies on a series of 1111-type iron-based superconductors etc. More importantly, high pressure studies on the parent compounds of iron pnictide superconductors clearly demonstrate that pressure can suppress the transition temperatures of magnetic order and crystal structure, and then drive a superconducting transition. Furthermore, many examples are given in this review to reveal how the magnetic order competes with superconductivity under pressure, which provides new constrains for the establishment of the theory on superconductivity. These high pressure results are expected to be helpful for the studies of high-TC superconductors and for the exploring of new superconductors.
In-situ magnetic measurements of substances under extreme conditions
Temperature and pressure are the two most important thermodynamic elements, which determine the existent state of substance. Low temperature and high pressure are significant and key extreme conditions in the modern experimental science, providing new routes for many subjects such as physics, chemistry, materials and biology, and playing an important role in finding new phenomena. The magnetic research under extreme conditions is an important branch of the study of the extreme conditions, which not only presents the magnetic changes of the material under extreme conditions, but also is an important means to explore the high temperature superconductors. In this article, we elaborate the principle and method of measuring the magnetic susceptibility and superconducting transition temperature under high pressure. The in-situ magnetic measurement system under high pressure and low temperature is also briefly introduced, designed and installed by ourselves. Using the in-situ magnetic measurement system, the magnetic transition of iron and the superconducting transition temperature of the yttrium barium copper oxide sample under high pressure are measured.
Syntheses and studies of superhard composites under high pressure
Materials having Vickers hardness (HV) higher than 40 GPa are considered to be superhard. Superhard material is exclusively covalent and displays superior hardness, incompressibility, and wear resistance, which make this kind of material essential for a wide range of industrial applications, such as turning, cutting, boring, drilling, and grinding. Most of superhard materials are prepared under extreme pressure and temperature conditions, not only for scientific investigations, but also for practical applications. With the development of high pressure science and technology, the field of superhard composites is more active and more efficient, energy saving and environmental protection. Ultrahigh pressure and ultrahigh temperature method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases and the way of sintering high-density nanosuperhard materials. In this paper, the recent achievements and progress in high-pressure synthesis and research of superhard materials are introduced mainly in the nanopolycrystalline diamond, nanopolycrystalline cubic boron nitride (cBN), ultrahard nanotwinned cubic boron nitride, submicron polycrystalline cubic boron nitride, cBN-Si composites material, cubic-Si3N4-diamond nanocomposites and diamond-cubic boron nitride superhard alloy (composite) material prepared under ultrahigh pressure and high temperature, by using multi-anvil apparatus based on the hinged-type cubic press. These superhard composite materials are successfully synthesized by high temperature and high pressure, and a variety of performance tests show that their hardness values and thermal stability properties exceed those of the traditional superhard materials. At the same time, some new ideas, approaches to the study of superhard composite materials in recent years have been introduced, such as nanostructuring approaches and special treatments of the starting material for high-performance superhard materials, using the formation of alloys or solid solution to fill the performance gap between different materials for enhancing comprehensive performance (i.e., hardness, fracture toughness, and thermal stability), or changing and optimizing the assembly method to improve the uniformity of performance. Finally, the prospect of superhard composite material is also discussed. In the research field of superhard materials, on the one hand, the relationship between macrohardness and microstructure of superhard materials is studied continuously to establish hardness models with atomic parameters, which can be used to guide the design or prediction of novel superhard crystals. On the other hand, highly comprehensive performance and larger size of super-hard composite materials are synthesized for practical application.
Studies of quasi one-dimensional nanostructures at high pressures
The ultimate goals of researches of one-dimensional (1D) nanomaterials, quasi-one-dimensional atomic/molecular chains are expected to exhibit their strong quantum effects and novel optical, electrical, magnetic properties due to their unique 1D structures. At present, synthesis and manipulation of 1D atomic/molecular chains on an atomic/molecular level in a controllable way have been the frontier subject of scientific research. The 1D atomic/molecular chains, which can be stable in ambient conditions, have been prepared successfully by using a confinement template, such as carbon nanotubes (CNTs), zeolite, etc.
High pressure can effectively tune the interatomic and intermolecular interactions over a broad range of conditions and thus to change the structures of materials. High pressure techniques have been recently adopted to investigate the 1D nanomaterials. In this paper, we briefly review some recent progress in the high pressure studies of 1D nanostructures, including iodine chains (I2)n confined in the 1D nanochannels of zeolite, multiwalled carbon nanotube (MWNT) arrays, and 1D carbon chains confined in CNTs. Particularly, polarized Raman spectroscopy combined with theoretical simulations has been used in the high pressure studies of 1D nanostructures. These studies reveal many interesting phenomena, including pressure-induced population increase and growth of 1D atomic/molecular chains. The underlying driven mechanisms have also been uncovered. Induced by pressure, the I2 molecules in zeolite 1D nanochannels rotates to the channel axial direction and the compression of the channel length in turn leads to a concomitant decrease of the intermolecular distance such that the iodine molecules come sufficiently close to the formation of longer (I2)n polymers. The novel polarized photoluminescence (PL) from the iodine chains and the pressure-induced PL enhancement due to the growth of 1D iodine chains under pressure. The depolarization effect vanishing in the polarized Raman spectra of compressed MWNT arrays. These are related to the pressure-induced enhancement of intertube interactions and inter/intratube sp3 bonding. The results obtained by polarized Raman spectroscopy overcome the difficulty:MWNTs have no obvious fingerprints for identifying the structural transformation under pressure.
Above all, the 1D nanostructures exhibit interesting and fantastic behaviors under pressure, which deserve further investigations in this research field. In addition, polarized Raman spectroscopy is an effective tool to study the structural transformations of 1D nanomaterials at high pressures, which can be extended to the studies of other analogous 1D nanostructures under pressure.
Wetting behaviors of the molten silicon on graphite surface
A theory which was proposed by Scheid et al. in 2010 (Scheid B, van Nierop E A, Stone H A 2010 Appl. Phys. Lett. 97 171906) suggests that very thin ribbons of molten material can be drawn out of a melt by adequately tuning the temperature gradient along the dynamic meniscus that connects the static meniscus at the melting bath to the region of the drawn flat film. Based on this theory, one-step manufacturing ultra-thin silicon wafer by pulling out from a molten silicon bath has attracted considerable attention in recent year due to its many attractive performances such as low cost, simple process, etc. By using this method, solar cell can have intensive applications due to its low cost and stable output efficiency. The results show that the thermal capillarity effect plays a great role in preparing the ultra-thin silicon. The thickness of the silicon wafer is sensitive to the capillary length and the strength of the surface tension variation as well. In order to reveal the mechanism for the effect of thermal capillary on the fabrication of ultra-thin silicon wafer, a thermal capillary finite element model is developed for the horizontal ribbon growth system to study the wetting behaviors of molten silicon on graphite. The mathematical model is established and simulated by using the commercial software; several parameters such as mass, viscous stress and capillary force are calculated. The wetting processes are tested by changing surface roughness (Ra=0.721 μm and Ra=0.134 μm), system temperatures (1737–1744 K), and durations (10–30 s) at constant temperature on a high-temperature, high-vacuum contact angle measurement instrument. It is found that the wetting angle of silicon droplet on graphite decreases with surface roughness and temperature increasing; the wetting angle comes down with time going by (lasting 30 s) at constant temperature, which is consistent with the theoretical result of Wenzel. The influence of surface tension on wetting process is studied by analyzing the distributions of pressure and velocity field. It is shown that the differential pressure at the solid-liquid interfaces, induced by thermal capillary effect, decreases in the wetting process and reaches a balance which prevents the droplet from being wetted. At T=1700 K, the wetting angle and the shape of droplet change quickly within 0.4 ms and eventually become stable after 5 ms as shown in the simulation. The spreading length L and droplet height h at the steady-state are calculated with considering the influence of droplet radius on the wetting process. The results show that both L and h are directly related to the steady-state of wetting angle. The surface tension dominates the wetting process for droplet radius R0 <<5mm; while for R0 >>5 mm, the wetting process is dominated by gravity.
A first-principles study on magnetic properties of the intrinsic defects in rutile TiO2
The TiO2 based diluted magnetic semiconductors (DMSs) have aroused the considerable interest as one of the promising candidates for the spintronic devices accommodating both charge and spin of electrons in a single substance. Unfortunately, however, throughout most of the published papers, the question how to clearly elucidate the role of defects which may be played in the experimentally observed room temperature ferromagnetism (RTFM) remains open, especially after a new concept of d0 ferromagnetism. In such a case, to further understand this issue and also to explore the origin of the RTFM in rutile TiO2, we here first perform a first principles calculation on the magnetic properties of the intrinsic defects, namely oxygen vacancy (VO), Ti vacancy (VTi), Ti interstitial (Tiin), oxygen interstitial (Oin) and two complex defects of VO+Oin and VTi+Tiin. Combining the density functional theory and the Perdew-Burke-Ernzerhof functional of the generalized gradient approximation, we calculate various model structures of rutile TiO2 constituted by 48-atom 2×2×2 supercell. The cutoff energies in these calculations of the planewave basis are all set to be 340 eV and the Monkhorst-Pack scheme k points are set to be 3×3×4 for an irreducible Brillouin zone. The convergence threshold for self-consistent field iteration is 0.1455×10-6 eV/atom. Structural relaxation is taken into account in each of all calculations. It is found that each defect we created in the structure leads to a lattice expansion and that the positive value for spin up and the negative value for spin down of the density of states (DOS) of the structure without defect are symmetric, suggesting that the perfect rutile TiO2 lattice is nonferromagnetic. For the system with one VO, the total energy of the spin-polarized system is 200 meV lower than that of the non-spin-polarized system, which indicates ferromagnetic behavior in this system. The defect brings in an impurity state near Fermi level located at about 0.7–1.0 eV down below the conduction band, resulting in an excess of spin up over spin down for the presences of the two localized electrons left by the vacancy. At this point the supercell bears a magnetic moment of about 1.62 μB. In contrast, VTi also brings in an impurity state near Fermi level but above the valence band, which reveals a p-type characteristic semiconductor nature. Since a lower total energy requires more spin-up electrons, the asymmetric DOS induces a magnetic moment of 2.47 μB. When a neutral Ti occupies an interstitial lattice site, the system requires it to be oxidized into a Ti3+ ion to increase the stabilization of the system. The three delocalized electrons tend to occupy the 3d or 4s orbital of the neighbor Ti4+ ions and then have strong exchange interactions with the 2p electrons of the local O atom. This can distort octahedral symmetry and give rise to a ferromagnetic moment of 3.91 μB. Oin defect in the supercell is extremely unstable. It can easily be reduced and escape from the host in terms of an oxygen molecule so that the system is in a manner similar to the perfect lattice, showing no ferromagnetism. It is interesting to note that the properties of the system with the complex defect of one VO and Oin are similar to that of the structure with one VO and the magnetic moment of this system is 1.63 μB. For the Ticom complex defect, our results point out the fact that the magnetic properties of the supercell are related to the distance between VTi and Tiin. The spin up and spin down states are symmetric when they are close to each other, while, in addition to some ferromagnetic behavior, the system mainly exhibits antiferromagnetism when the distance increases.
Experimental and theoretical study on terahertz spectra of all-trans β-carotene
The β-carotene is a short chain polyene molecule containing nine π-electron conjugated double-bonds. Because of its special molecular structure, β-carotene has been used widely in many fields, including functional materials, optoelectronic devices and biological applications of light collection, light protection, anti-cancer, etc. Recently, new applications of β-carotene in generation and detection of terahertz (THz) wave have also attracted great attention. In this work, all-trans β-carotene films are prepared by spray coating, and the THz spectra in a wavenumber range of 30-400 cm-1 (a frequency range of 0.9-12 THz) of the as-prepared products are experimentally measured at room temperature by Fourier transform infrared spectroscopy. For comparison, the THz spectra in 0.5-3.0 THz are also characterized at the same temperature by THz time-domain spectroscopy. Based on these measurements, the fingerprint peaks of all-trans β-carotene in the THz region are experimentally identified to be located at 54 cm-1 (1.62 THz), 57 cm-1 (1.71 THz), 64 cm-1 (1.91 THz), 77 cm-1 (2.32 THz), 90 cm-1 (2.69 THz), 98 cm-1 (2.95 THz), 115 cm-1 (3.45 THz), 124 cm-1 (3.72 THz), 134 cm-1 (4.02 THz), 170 cm-1 (5.11 THz), 247 cm-1 (7.42 THz), and 279 cm-1 (8.38 THz), respectively. It is worth noting that the recent results about the THz spectra of palm leaves are thus verified. Particularly, the B3 LYP method of density functional theory is further utilized in this work to theoretically simulate the THz spectra of all-trans β-carotene molecule. It is revealed that the theoretical simulation results accord well with those experimentally measured data. In addition, we also find that the absorption peaks are caused by the torsion, deformation and rocking vibration of the molecules. Accordingly, the vibrational modes of the measured THz characteristic peaks at 148 cm-1 (4.44 THz), 132 cm-1 (3.96 THz), 115 cm-1 (3.45 THz), 76 cm-1 (2.28 THz) and 52 cm-1 (1.56 THz) are theoretically assigned, which provides a reference to explain the formation mechanism of the THz spectra. The valuable results presented in this work will be helpful for promoting the studies of the THz spectral features and response mechanisms of the organics.
Growth, structural and optical properties of orderly SnO2 microhemispheres on patterned sapphire substrates
One-dimensional nanoscaled materials, such as nanotubes, nanowires and nanobelts, have attracted a great deal of attention in recent years because of their unique electronic, optical, and mechanical properties. Their potential applications are found in next generation devices, functional materials, and sensors. A material of particular interest is stannic oxide (SnO2), which is a novel oxide semiconductor material for ultraviolet and blue luminescence devices due to its wide band gap of 3.6 eV at room temperature. SnO2 can also be widely used in many fields, such as gas sensors, optoelectronic devices, and transparent conductive glass, because of its high optical transparency in the visible range, low resistivity, and higher chemical and physical stability. In recent years, one-dimensional nanostructures of SnO2 materials, such as nanobelts, nanotubes, and nanowires, have been reported. However, the preparations of orderly SnO2 micro/nanostructures have been rarely reported. In this paper, orderly SnO2 microhemispheres with different sizes are grown on patterned sapphire substrates by a traditional chemical vapor deposition method without using any catalyst. The patterned sapphire substrates are cleaned by using a standard sapphire wafer cleaning procedure. High-purity metallic Sn powders (99.99%) and oxygen gas are used as Sn and oxygen sources, respectively. The flow rate of high-purity Ar carrier gas is controlled at 200 sccm, and the oxygen reactant gas with a flow rate of 100 sccm is introduced into the system. In the growth process, the whole system is kept at 1000℃ for 30 min. The surface morphologies, structural and optical properties of the SnO2 microhemispheres are investigated by the field emission scanning electron microscope (HITACHI S4800), the X-ray diffraction with a Cu Kαup radiation (0.15418 nm), the optical absorption spectroscope (UV-3600 UV-VIS-NIR, Shimadzu), and the photoluminescence spectroscope with an excitation source of He-Cd laser (λ=325 nm) to identify the As related acceptor emission, respectively. These results show that the diameters of SnO2 microhemispheres become larger, and the crystal quality is degraded with the increase of Sn powder mass. The special selective growth of SnO2 microhemisphere on a patterned sapphire substrate is found. In addition, we also find that the optical band gaps of the samples A-D are all redshifted with the increase of Sn powder mass. The shrinkage of Eg in the absorption spectrum should be partly attributed to the degradation of crystal quality because of excess Sn sources. This growth method of SnO2 microhemisphere provides a feasible and effective way of preparing the high density, orderly arrangement of SnO2 micro/nanostructures.
Analytical modeling for the plate with a flat-bottom hole based on the reflection and transmission theory in pulsed eddy current testing
Ferromagnetic structures such as pipes or vessels are widely used in petroleum, chemical and power generation industries. Periodical nondestructive testing (NDT) is vital for continued safe operation. As a NDT technology, pulsed eddy current testing (PECT) technology which is excited by a square-wave pulse rather than a sinusoidal waveform has been widely used for its advantages of non-contact and acquisition of information at various depths in one excitation process. In PECT, the analytical modeling is important because it gives a better understanding of the signal and benefits the inverse process of PECT in extracting information of structures. The foundation of theoretical model of PECT is the Dodd-Deeds model presented by Dodd and Deeds in 1968, Theodoulidis and Kriezis represented the integral solution of Dodd-Deeds model in the form of series by using the truncated region eigenfunction expansion (TREE) method. Using the Dodd-Deeds model and the TREE method, other analytical modelings have been solved. However, most modelings assume that the wall thinning of the specimen is uniform, and the analytical solution only contains the variables in the z direction (the direction perpendicular to the surface of the specimen), such as the thickness of the specimen. With the rapid development of PECT, problems such as the footprint of the probe, the quantitative analysis of local wall thinning also need to be solved. These problems are related to the variable in the r direction (the direction parallel to the surface of the specimen), so the analytical modelings mentioned above are not available any more. To solve these problems, the analytical modeling of the plate with a flat-bottom hole is proposed. Considering the fact that the boundary condition in the analytical modeling of the plate with a flat-bottom hole is complicated, the assumption that the transverse wave number and the longitudinal wave number in the layer where the flat-bottom hole located are the same is made in this paper, and the transverse wave number is set to be only related to the structure in the r direction. Firstly, the expressions of magnetic vector potential in all the layers are obtained by using the reflection and refraction theory of electromagnetic wave. Then the analytical solution is solved based on the extended Cheng's matrix method by introducing the construction coefficient Wn. Finally, the 16MnR specimen with the flat bottom holes is conducted as an example, and experiments are carried out. The good agreement between results calculated by the analytical model and the experimental results measured verifies the developed analytical model.
Analysis on the compressive sensing based narrow-band radar super resolution imaging mechanism of rapidly spinning targets
According to the characteristics of spinning targets, the narrow-band radar echoes can be directly used for imaging spinning targets. However, spurious peaks appear due to azimuth down sampling with a low pulse repetition frequency (PRF). By exploiting the sparsity of the targets, the compressed sensing (CS) theory can be adopted to obtain super resolution image under sub-sampling condition. This paper mainly focuses on analyzing the physical mechanism of the CS-based narrow-band imaging method. Firstly, the narrow-band radar's under-sampling echoes' model from rapidly spinning targets is established. The relationship between CS and the model is analyzed. Then the reasons why the CS-based narrow-band imaging method can guarantee the exact recovery of the spinning target are given from physical view. The theoretical lower limit of sub-sampling pulse numbers is provided. Finally, the simulation results verify the effectiveness of the theoretical analysis. The main results obtained in the paper are listed as follows. One is that the mechanism of the CS-based narrow-band imaging method differs from those of the conventional range Doppler imaging methods. The spurious peaks appear due to calculating the Doppler frequency directly under a low PRF. To avoid this phenomenon, the CS-based method searches the positions of the scatterers instead. The variation from calculating the Doppler frequency directly to searching the positions of the scatterers is the physical mechanism of the CS-based super resolution imaging method. The other is that the resolution and the allowable grid mismatch of the CS-based imaging method are related to the wavelength, which is 0.4λ and unrelated to the bandwidth. So the performance of the CS-based imaging method is related to the sub-sampling rate, the number of the scatters and the wavelength, and unrelated to the bandwidth of the wave. However, this paper only considers the ideal point scattering model and the grid is perfectly matched with the model. In the following, three aspects can be further studied. First, due to the spinning target distribution on a continuous scene, the off-grid problem would severely affect the performance of the CS-based imaging method. The continuous compressive sensing theory can be used for solving the off-grid problem and explaining the related physical mechanism. Second, the illumination of the radar cannot reach some scatterers on the target in some observation intervals, which results in the occlusion effect and the time-varying scattering amplitude. The dynamic CS theory can be used for reference in solving this problem. Finally, if the estimated spinning frequency has error, how to correct and compensate for the error adaptively needs to be further studied.
A method to calculate the electron sheath profile of the nonaxisymmetrical magnetic insulation
The nonaxisymmetrical magnetic insulation would occur due to the disalignment of inner electrodes in long magnetically insulated transmission lines, or the nonuniform distributions of injected currents in induction cavities of magnetically insulated induction voltage adders (MIVA). The electron sheath profile is a very important parameter to characterize the nonaxisymmetrical magnetic insulation. In the past, the three-dimensional particle in cell simulation was usually used to determine the electron sheath profile, which is extremely time-consuming and inefficient. In this paper, a fast and efficient calculation method is proposed. The classical one-dimensional Creedon theory of the magnetic insulation equilibrium is generalized to a two-dimensional plane of (r, θ) via introducing a parameter defined as the azimuthal mode number. Two-dimensional Creedon is developed to model the asymmetric magnetic insulation of the MIVA. Provided the azimuthal distributions of magnetic flux density on the cathode, which is in proportion to the cathode current, the two-dimensional Creedon model is numerically solved. A numerical solution method to calculate the electron sheath profile is proposed, and then the calculation error is also given. As the azimuthal distribution of magnetic flux density on the cathode meets a cosine function, the profile of the electron sheath is approximate to the Gauss function. As the nonuniform portion of cathode current increases, the electron sheath becomes more eccentric, and the calculation error is also much larger.
Information radiation model with across neighbor spread abilities of nodes
Information is spread as a kind of energy in the network, and it has the ability to spread to nodes that go beyond the neighbors, that is, the information has a radiation effect. However, most of the studies of information dissemination in complex networks only consider the dissemination between neighbors, ignoring that their neighborhood will also be affected by the information radiation. According to this, we propose a new information radiation model with the ability to communicate across neighbors. Firstly, the concepts of information radiation range and radiation attenuation are put forward by combining the theory of complex network and the radiation theory. Secondly, by proposing the hypotheses and analyzing the information content, the nodes in the network are divided into three states:the radiation state, the known state, and the unknown state with the information amount serving as the criterion. At the same time, the transition rules between node states are defined. Thirdly, a three-layer information radiation network model is established based on the physical layer serving as the network structure, the radiation layer as the information dissemination environment, and the state layer as the radiation state statistics. Then, on the basis of the model, the differential equations of the state changes of the nodes are constructed by using the mean field theory and defining the network statistic such as the nth degree, the average nth degree and the nth degree distribution. By analyzing the mechanism of information radiation, the expression of information radiation threshold is deduced by using the differential equation set. Afterwards, the existence of information radiation threshold is proved in each of NW network, BA network, Jazz network, Net-science network, and E-mail network. And the results of numerical simulation and theoretical analysis are well fitted, verifying the correctness of theoretical analysis and the validity of the model. Finally, considering the practical situation of the application, the influences of the state transition probability and the radiation attenuation on the information radiation are analyzed in the BA network by using computer simulation. The results show that the radiation attenuation can stabilize the radiation, and the number of nodes in the initial state of radiation can be increased, which will accelerate the demise of the unknown state nodes but will not increase the number of nodes in the steady state. The results show that increasing the attenuation of the radiation can not only increase the number of radiation nodes in steady stage of radiation, but also speed up the demise of unknown state nodes. And increasing the state transition probability α or γ will affect only the number of the radiation nodes in the initial stage of radiation, also accelerate the demise of the unknown state nodes but will not increase the number of radiation nodes in steady stage of radiation. The analyses of the state transition probability between nodes and the radiation attenuation also prove the correctness of the theoretical analysis.
Node importance measurement based on neighborhood similarity in complex network
Ranking node importance is of great significance for studying the robustness and vulnerability of complex network. Over the recent years, various centrality indices such as degree, semilocal, K-shell, betweenness and closeness centrality have been employed to measure node importance in the network. Among them, some well-known global measures such as betweenness centrality and closeness centrality can achieve generally higher accuracy in ranking nodes, while their computation complexity is relatively high, and also the global information is not readily available in a large-scaled network. In this paper, we propose a new local metric which only needs to obtain the neighborhood information within two hops of the node to rank node importance. Firstly, we calculate the similarity of node neighbors by quantifying the overlap of their topological structures with Jaccard index; secondly, the similarity between pairs of neighbor nodes is calculated synthetically, and the redundancy of the local link of nodes is obtained. Finally, by reducing the influence of densely local links on ranking node importance, a new local index named LLS that considers both neighborhood similarity and node degree is proposed. To check the effectiveness of the proposed method of ranking node importance, we carry out it on six real world networks and one artificial small-world network by static attacks and dynamic attacks. In the static attack mode, the ranking value of each node is the same as that in the original network. In the dynamic attack mode, once the nodes are removed, the centrality of each node needs recalculating. The relative size of the giant component and the network efficiency are used for network connectivity assessment during the attack. A faster decrease in the size of the giant component and a faster decay of network efficiency indicate a more effective attack strategy. By comparing the decline rates of these two indices to evaluate the connectedness of all networks, we find that the proposed method is more efficient than traditional local metrics such as degree centrality, semilocal centrality, K-shell decomposition method, no matter whether it is in the static or dynamic manner. And for a certain ranking method, the results of the dynamic attack are always better than those of the static attack. This work can shed some light on how the local densely connections affect the node centrality in maintaining network robustness.
An information diffusion dynamic model based on social influence and mean-field theory
With the development of online social networks, they rapidly become an ideal platform for information about social information diffusion, commodity marketing, shopping recommendation, opinion expression and social consensus. The social network information propagation has become a research hotspot correspondingly. Meanwhile, information diffusion contains complex dynamic genesis in online social networks. In view of the diversity of information transmission, the efficiency of propagation and the convenience of interaction, it is very important to regulate the accuracy, strengthen the public opinion monitoring and formulating the information control strategy.
The purpose of this study is to quantify the intensity of the influence, especially provides a theoretical basis for studying the state transition of different user groups in the evolution process. As existing epidemic model paid less attention to influence factors and previous research about influence calculation mainly focused on static network topology but ignored individual behavior characteristics, we propose an information diffusion dynamics model based on dynamic user behaviors and influence. Firstly, according to the multiple linear regression model, we put forward a method to analyze internal and external factors for influence formation from two aspects:personal memory and user interaction. Secondly, for a similar propagation mechanism of information diffusion and epidemics spreading, in this paper we present an improved SIR model based on mean-field theory by introducing influence factor.
The contribution of this paper can be summarized as follows. 1) For the influence quantification, different from the current research work that mainly focuses on network structure, we integrate the internal factors and external factors, and propose a user influence evaluation method based on the multiple linear regression model. The individual memory principle is analyzed by combining user attributes and individual behavior. User interaction is also studied by using the shortest path method in graph theory. 2) On modeling the information diffusion, by referring SIR model, we introduce the user influence factor as the parameter of the state change into the epidemic model. The mean-field theory is used to establish the differential equations. Subsequently, the novel information diffusion dynamics model and verification method are proposed. The method avoids the randomness of the artificial setting parameters within the model, and reveals the nature of multi-factors coupling in the information transmission.
Experimental results show that the optimized model can comprehend the principle and information diffusion mechanism of social influence from a more macroscopic level. The study can not only explain the internal and external dynamics genesis of information diffusion, but also explore the behavioral characteristics and behavior laws of human. In addition, we try to provide theoretical basis for situation awareness and control strategy of social information diffusion.
A memristor-based time-delay chaotic systems and pseudo-random sequence generator
Memristor, a controllable nonlinear element, is easy to generate a chaotic signal. More significantly, it can improve the complexity of the chaotic system and the randomness of signals. Although the memristor chaotic system is a hot spot of research currently, little attention has been paid to the memristive time-delayed chaotic system. Therefore, a new memristor-based time-delayed chaotic system is proposed in this paper. We construct the time-delayed chaotic system with single delay time by using the nonlinear relationship between the memristance and charge of memristor. The existence of time delay enhances the complexity of chaotic system, which makes the system produce richer and more complex dynamics. In order to study the complex dynamic characteristics of this memristive time-delayed system, we investigate the proposed system by theoretical derivation, numerical simulation, stabilization of equilibrium points, and power spectrum. In addition, the corresponding parameter region of the stable equilibrium point of the system is discussed in detail. Then, we discuss the effect of parameter variation on the dynamic behavior of the system, and a series of phase diagrams with different time-delayed parameters and system parameters is described by numerical simulation. We find that different combinations of parameters and slight changes of parameters can make the system a completely different phase diagram, which indicates that the proposed system has rich nonlinear characteristic. Moreover, the proposed time-delayed system is used to generate pseudo random sequences, and the experimental results show that the proposed system has good self-correlation, cross-correlation, and the significant approximate entropy. According to the theoretical analyses and experimental results, we conclude that the proposed new time-delayed chaotic system has complex dynamic behavior and good randomness, which can meet the needs of the applications in spread spectrum communication, image encryption and many other fields. This research provides a significant reference for further studying the usage of memristor.
Chaotic laser-based ultrafast multi-bit physical random number generation without post-process
Random numbers have great application value in the fields of secure communications, which are commonly used as secret keys to encrypt the information. To guarantee that the information is absolutely secure in the current high-speed communication, the applied random keys should possess a generation speed not less than the encrypted data rate, according to “one-time pad” theory found by Shannon (Shannon C E 1949 Bell.Syst.Tech.J. 28 656)
Pseudo-random numbers generated by algorithm may easily reach a fast speed, but a certain periodicity makes them difficult to meet the aforementioned demand of information security. Utilizing physical stochastic phenomena can provide reliable random numbers, called physical random number generators (RNGs). However, limited by the bandwidth of the conventional physical sources such as electronic noise, frequency jitter of oscillator and quantum randomness, the traditional physical RNG has a generation speed at a level of Mb/s typically. Therefore, real-time and ultrafast physical random number generation is urgently required from the view of absolute security for high-speed communication today.
With the advent of wideband photonic entropy sources, in recent years lots of schemes for high-speed random number generation are proposed. Among them, chaotic laser has received great attention due to its ultra-wide bandwidth and large random fluctuation of intensity. The real-time speed of physical RNG based on chaotic laser is now limited under 5 Gb/s, although the reported RNG claims that an ultrafast speed of Tb/s is possible in theory.
The main issues that restrict the real-time speed of RNG based on chaotic laser are from two aspects. The first aspect is “electrical jitter bottleneck” confronted by the electrical analog-to-digital converter (ADC). Specifically, most of the methods of extracting random numbers are first to convert the chaotic laser into an electrical signal by a photo-detector, then use an electrical ADC driven by radio frequency (RF) clock to sample and quantify the chaotic signal in electronic domain. Unfortunately, the response rate of ADC is below Gb/s restricted by the aperture jitter (several picoseconds) of RF clock in the sample and hold circuit. The second aspect comes from the complex post-processes, which are fundamental in current RNG techniques to realize a good randomness. The strict synchronization among post-processing components (e.g., XOR gates, memory buffers, high-order difference) is controlled by an RF clock. Similarly, it is also an insurmountable obstacle to achieve an accurate synchronization due to the electronic jitter of the RF clock.
In this paper, we propose a method of ultrafast multi-bit physical RNG based on chaotic laser without any post-process. In this method, a train of optical pulses generated by a GHz mode-locked laser with low temporal jitter at a level of fs is used as an optical sampling clock. The chaotic laser is sampled in the optical domain through a low switching energy and high-linearity terahertz optical asymmetric demultiplexer (TOAD) sampler, which is a fiber loop with an asymmetrical nonlinear semiconductor optical amplifier. Then, the peak amplitude of each sampled chaotic pulse is digitized by a multi-bit comparator (i.e., a multi-bit ADC without sample and hold circuit) and converted into random numbers directly.
Specifically, a proof-of-principle experiment is executed to demonstrate the aforementioned proposed method. In this experiment, an optical feedback chaotic laser is used, which has a bandwidth of 6 GHz. Through setting a sampling rate to be 5 GSa/s and selecting 4 LSBs outputs of the 8-bit comparator, 20 Gb/s (=5 GSa/s×4 LSBs) physical random number sequences are obtained. Considering the ultrafast response rate of TOAD sampler, the speed of random numbers generated by this method has the potential to reach several hundreds of Gb/s as long as the used chaotic laser has a sufficient bandwidth.
Trade-off optimization for low-dissipation heat engine with generic heat transfer process
The heat transfer process inevitably occurs in the operation of real heat engine. In this article, a low-dissipation heat engine with generic heat transfer process is proposed based on the low-dissipation Carnot model. The formulas for the power and the efficiency of heat engine with generic heat transfer law are derived, and the low-dissipation heat engine performance is also optimized by the trade-off optimization method, which offers a unified scheme to understand the behaviors of heat engines with generic heat transfer processes. Furthermore, the characteristics of the power as well as the efficiencies for thermal engines with the different heat transfer processes are discussed in detail, and it is found that the power and the efficiency without heat transfer process are independent of heat leak, but are related to contact time, heat dissipation and Carnot efficiency. The power output of heat engine monotonically increases as Carnot efficiency increases, but the large contact time ratio and the large dissipation ratio make it difficult to provide the big power output. When the heat leak is absent and α(β) is fixed, the efficiency of heat engine decreases (increases) with the increase of β(α). It is noted that the heat transfer process greatly influences the performance of heat engine, and ηΩ/ηC versus β displays the similar properties under three heat transfer laws. It is clearly shown that ηΩ/ηC versus β shows the transition from the monotonic decrease to monotonic increase with α increasing, but ηΩ/ηC versus α is opposite to the former, and the maximum value of ηΩ/ηC also shifts rightwards with the increase of β. Additionally, the corresponding efficiency of heat engine diminishes significantly as m decreases and n increases. When heat engines are dominated by different heat transfer laws, the curves of ηΩ versus ηC are consistent as ηC is relatively large or small, but it is observed that there exist the evident differences among three characteristic curves in the middle regime. The relatively large β or small α will also lead to the reduction of the working regime where heat engine can function normally. Our results are very helpful in understanding the design principle and the optimization mechanism for actual thermal engines and refrigerators.
Theoretical analysis of spectrum reconstruction imaging using single-pixel detection
As a novel imaging method, single-pixel imaging based on spectrum reconstruction is interesting. To date, however, there has not yet been a theory that can analyze the method in detail. In order to obtain a comprehensive understanding of the single-pixel imaging technique, a detailed theoretical analysis is proposed. Firstly, in the presented imaging theory, we analyze the effects of several factors involved in the imaging process on imaging reconstruction, including the direct detection, namely, the back scattering light from the imaged object is received by a detector; indirect detection, namely, the back scattering light is received after undergoing the diffuse reflections by multiple diffusing surfaces in proper sequence; multi-channel detection, namely, the scattering light is together detected after experiencing multiple diffuse reflections in proper sequence along multiple different paths; the size and location of the single-pixel detector. The theoretical results show that whether it is direct detection or indirect detection and whether it is single-channel detection or multi-channel detection, the above imaging method is valid as long as the scattering light can be received by the detector, and the obtained results are also in accord with the existing experimental results. Since the single-pixel detector is treated as a component of many point detectors due to its practical size, the effect of the single-pixel detector size on image reconstruction is equivalent to the integration of reconstructed images from multiple point detectors at different locations. Secondly, the spectrum reconstruction based on a three-step phase shift technique is also derived to increase the image reconstruction speed. Finally, the experimental results of the imaging reconstruction of an object, whose surface reflectivity is uniform, are demonstrated.
Controlling of dark or bright soliton type in a cascade-type electromagnetically induced transparency semiconductor quantum well by the coupling longitudinal optical phonons
In the past few years, with developing the technology of electromagnetically induced transparency (EIT) and improving the semiconductor technology, it has become possible to realize the application of optical soliton to communication device. Studies show the reduction of group velocity of the optical soliton in EIT medium under weak driving condition, which possibly realizes the storing of optical pulses in information storage. More importantly, semiconductor quantum wells have the inherent advantages such as large electric dipole moments of the transitions, high nonlinear optical coefficients, small size, easily operating and integrating. So it is considered to be the most potential EIT medium to realize the application of quantum devices. The optical soliton behavior in the semiconductor quantum well is studied, which can provide a certain reference value for the practical application of information transmission and processing together quantum devices.
Although there has been a series of researches on both linear and nonlinear optical properties in semiconductor quantum wells structures, few publications report the effects of the cross-coupling longitude-optical phonon (CCLOP) relaxation on its linear and nonlinear optical properties. However, to our knowledge, the electron-longitude-optical phonon scattering rate can be realized experimentally by varying the sub-picosecond range to the order of a picosecond. According to this, we in the paper study the effects of the CCLOP relaxation on its linear and nonlinear optical properties in a cascade-type three-level EIT semiconductor quantum well.
According to the current experimental conditions, we first propose a cascade-type three-level EIT semiconductor quantum well model. And in this model we consider the longitudinal optical phonons coupling between the bond state and anti-bond state. Subsequently, by using the multiple-scale method, we analytically study the dynamical properties of solitons in the cascade-type three-level EIT semiconductor quantum well with the CCRLOP. It is shown that when the CCRLOP strength is smaller, there exhibits the dark soliton in the EIT semiconductor quantum well. Only if the strength of the CCRLOP is larger, will in the system there exists bright soliton. That is to say, with increasing the strength of the CCRLOP, the soliton type of the system is converted from dark to bright soliton little by little. So, the temporal soliton type can be effectively controlled by the strength of the CCRLOP. In addition, we also find that the group velocity of the soliton can also be controlled by the strength of CCRLOP and the control light. These results may provide a theoretical basis for manipulating experimentally the dynamics of soliton in semiconductor quantum wells.
Numerical study of thermal erosion and topographical change of divertor target plates induced by type-I edge-localized modes
The high-Z material tungsten (W) is a promising candidate of the plasma facing components (PFCs) for the future tokamak reactors due to its high melting point (3683 K), low tritium retention and low sputtering yield. However, there are still many problems about W PFCs. One of them is the material melting under off-normal transient heat fluxes–it is one of the most outstanding open questions associated with the use of W divertor targets in international thermonuclear experimental reactor (ITER). This requires us urgently to understand the W melting behavior under high power flux deposition condition. In this paper, a two-dimensional (2D) fluid dynamic model is employed by solving the liquid hydrodynamic Navier-Stokes equation together with the 2D heat conduction equation for studying the erosion of the divertor tungsten targets and its resulting topographical modification during a type I-like edge-localized mode (ELM) in ITER with a Gaussian power density profile heat load. In the present model, major interaction forces, including surface tension, pressure gradient and magnetic force responsible for melt layer motion, are taken into account. The simulation results are first benchmarked with the calculated results by other code to validate the present model and code. Simulations are carried out in a wide range of fusion plasma performance parameters, and the results indicate that the lifetime of W plate is determined mainly by the evolution of the melt layer. As a consequence of the melt layer motion, melted tungsten is flushed to the periphery, a rather deep erosion dent appears, and at the dent edges two humps of tungsten form during the ELM. The humps at both edges are almost at the same height. Calculated results show the topographical modification becomes noticeable when the W plate is exposed to a heat flux of 2000 MW·m-2 for 0.8 ms (in the simulation, the parameter kα=əα/əT is taken to be -9.0×10-5 N·m-1·K-1, where α is the surface tension coefficient and T is the temperature). The values of the humps are both about 2.1 μm, and the surface roughness is about 1.1 μm. The longer the duration of the ELM, the more rapidly the humps rise. The melt flow may account for the higher surface temperature at the pool periphery, and for the larger melt thickness. It is found that when the energy flux is under 3000 MW·m-2 the surface tension is a major driving force for the motion of melt layer. Under the same heat flux, the bigger the kα used in the simulation, the more severe the surface topography of the target becomes; while at the same kα, the higher the heat flux, the more severe the surface topography of the target becomes. In addition, a modified numerical method algorithm for solving the governing equations is proposed.
Estimating optical turbulence of atmospheric surface layer at Antarctic Taishan station from meteorological data
Turbulence intensity in the near-surface layer and its decrease rate with height are closely related to the quality of potential sites. Astronomers have been pursuing a perfect astronomical site to place the large-aperture telescopes. Compared with the best mid-latitude sites, Antarctic plateau inevitably becomes an ideal site for building the next-generation large optical and infrared telescopes, which is because of its low infrared sky emission, low atmospheric precipitable water vapour content, low aerosol and dust content of the atmosphere, and light pollution. In this paper, we establish a model of the atmospheric optical turbulence in surface layer, and use it to estimate Cn2 at Antarctic Taishan station for the first time. The meteorological parameters of the model input are the data measured by a mobile atmospheric parameter measurement system at Antarctic Taishan station from 30 December 2013 to 10 February 2014. The values of Cn2, estimated by the model and measured by a micro-thermometer, are compared. Sensitivity analysis of the estimation method is also carried out. The measurement results and analyses show that Cn2 obtained at Taishan station has obvious diurnal variation characteristics, with well-behaved peaks in the daytime and nighttime, and minima near sunrise and sunset. Cn2 obtained in the nighttime is stronger than that in daytime, more specifically, it is on the order of 2×10-14 m-2/3. The comparison between model predictions and experimental data demonstrates that it is feasible to estimate Cn2 in Antarctic by using this model. The biggest differences between Cn2 values obtained from the model and measurement usually emerge at sunrise and sunset, respectively. Considering the fact that Antarctic atmosphere is in a stable state most of the time, the values of Cn2 estimated by different nondimensional structure parameter functions are nearly the same. Thus, the measurement accuracy of air temperature difference from one height to another is the main factor that affects the estimated value of Cn2.
A statistical analysis of the global distribution of plasmaspheric hiss based on Van Allen Probes wave observations
Plasmaspheric hiss plays an important role in driving the precipitation loss of radiation belt electrons via pitch angle scattering, which is also known as the major cause of the formation of the “slot” region between the inner and outer radiation belt. Therefore, it is of scientific importance to acquire a complete picture of the global distribution of plasmaspheric hiss. Using the thirty-three month high-quality wave data of the Van Allen Probes from September 2012 to May 2015, which provide excellent coverage in the entire inner magnetosphere, we investigate in detail the characteristics of the global distribution of plasmaspheric hiss bin-averaged wave amplitude and occurrence rate with respect to the geomagnetic activity level, L-shell, geomagnetic latitude, and magnetic local time. It is demonstrated that the bin-averaged hiss amplitude strongly depends on the level of geomagnetic activity and exhibits a pronounced day-night asymmetry. Dayside hiss shows a tendency intensifying with the disturbed geomagnetic condition, which is primarily confined to L=2.5-4.0. In contrast, the average hiss amplitude on the nightside tends to decrease. It should also be noted that plasmaspheric hiss at different amplitude levels varies distinctly with geomagnetic condition. As the geomagnetic disturbance increases, the occurrence rate of hiss wave at a smaller amplitude level (i.e., 5-30 pT) increases on the nightside but decreases on the dayside, while the occurrence pattern of higher amplitude (> 30 pT) hiss wave is opposite. For high amplitude hiss wave, the occurrence rate increases on the dayside during intense geomagnetic activities while decreases on the nightside. This is probably because during active times, suprathermal electron fluxes are larger on the nightside, which causes stronger Landau damping of whistler mode waves and thus limits the ability of chorus waves to propagate into the plasmasphere and evolve into plasmaspheric hiss. In addition, plasmaspheric hiss waves with the amplitudes ranging from 5 to 30 pT have the highest occurrence probability both around the geomagnetic equator and at higher latitudes. Our statistical results can provide a reasonable and accurate cognition complementary to the current knowledge of the global features of plasmaspheric hiss, especially in the inner magnetosphere of L=2-6, thereby offering essential input parameters of hiss wave distribution for future simulations of the dynamic spatiotemporal variations of radiation belt electrons at different energies and pitch angles under the influence of diverse solar wind and magentospheric circumstances. Therefore, we suggest that these new properties of hiss wave should be incorporated into the future modeling of radiation belt electron dynamics.
A jet acceleration mechanism for the black hole disk system
A jet acceleration mechanism of extracting energy from the disk-corona surrounding a rotating black hole is proposed. In this disk-corona scenario, the central object is a rotating Kerr black hole, and a geometrically thin and optically thick disk is sandwiched by a slab corona. The large-scaled magnetic field plays an important role in jet acceleration mechanism. So we obtain the value of the magnetic field in such a disk-corona system by solving the disk dynamic equations in the context of general relativity. The results show that the value of magnetic field decreases with the increase of disk radius, while increases with the increase of black hole spin parameter a*. Then the analytical expression of the jet power is derived based on the electronic circuit theory of the magnetosphere. It is found that the jet power increases obviously with increasing black hole spin parameter a* and magnetic stress parameter Δε. Furthermore, the calculation results also show that the jet power is mainly from the inner region of the disk-corona system, which is consistent with the observations of the jet. Finally, a sample composed of the 23 Fermi blazars with high jet power is used to explore our jet production mechanism. The conclusion suggests that our jet acceleration mechanism can simulate all sources with high power jet. By comparing with the observational data, we find that these high jet power sources cannot be explained by the Blandford-Znajek mechanism, even if the central object is extreme Kerr black hole.