INVITED REVIEW
INVITED REVIEW
2019, 68 (1): 017101.
doi: 10.7498/aps.68.20181211
Abstract +
Spin is an intrinsic nature of the angular momentum of elementary particle like electron and photon. Currently the collective spin behaviors of the multi-electrons in condensed matter, such as GMR, CMR and topological insulator which are the behaviors of ground state, have been a research focus in the condensed matter physics, due to the fact that the collective spin is related to electronic transports. Exciton is another type of bosonic quasiparticle, an excited state of electronhole pair in solid, which has a short lifetime and can recombine to emit light. Whether excitons can also exhibit the spin-polarized dominance before they recombine, has not been understood yet. It is proposed that excitons form condensate by themselves or light binding. Can coupled spins conduce to the formation of the exciton condensate in solid? Excitonic magnetic polaron (EMP) is the composite exciton of ferromagnetically coupled spins and free excitons in magnetic semiconductors, which may lead to ferromagnetic Bose-Einstein condensate (BEC) due to the binding of collective spins in a microstructure, like the photon binding excitons (exciton polaritons) in an optical cavity However, this subject has not been a research focus yet. Here in this paper, we review the progress of the EMP formation, its dynamic behaviors and spin polarized collective EMP emission and lasing in Ⅱ-VI dilute magnetic semiconductor micro-structures in our group Besides, we also present some expectations for the applications or advances in the quantum phenomena such as spin-related emission and lasing, spin induced BEC, photon induced magnetism and Hall effect, etc. Even more achievements of EMP could be expected in the future.
SPECIAL TOPIC—Critical topics in water research
2019, 68 (1): 018202.
doi: 10.7498/aps.68.20181312
Abstract +
Directly splitting water into carbon-free H2 fuel and O2 gases by sunlight is one of the most environmentally-friendly and potentially low cost approaches to solving the grand global energy challenge. Recent progress of electronic structure theory and quantum simulations allow us to directly explore the atomistic mechanism and ultrafast dynamics of water photosplitting on plasmonic nanoparticles. Here in this paper, we briefly introduce the relevant researches in our group. First we propose that the supported gold nanoparticles on oxide thin film/mental should be able to potentially serve as efficient photocatalysts for water splitting. Then, under the light illumination, we identify a strong correlation among light intensity, hot electron transfer rate, and water splitting reaction rate. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation, which can help to design nanoparticles in water photosplitting cells. Finally, we simulate the ultrafast electron-nuclear quantum dynamics of H2 generation with plasmonic gold cluster on a time scale of~100 fs in liquid water. We identify that the water splitting is dominated by field enhancement effect and associated with charge transfer from gold to antibonding orbital of water molecule. Based on all atomistic mechanism and quantum dynamics above, we present a “chain-reaction” H2 production mechanism via high-speed (much higher than their thermal velocity) collision of two hydrogen atoms from different water molecules under light illumination.
2019, 68 (1): 016803.
doi: 10.7498/aps.68.20182180
Abstract +
Catalysis of water, normally occurring at the interface, is crucial for the development of renewable energy and the environmental protection. Understanding the structures and chemical/physical properties of interfacial water during catalysis is of paramount importance for the sustainable development of human society, such as clean energy, wastewater treatment, and etc. However, owing to its complexity structure and mysterious property, the effect of water during catalysis is still an open question. The role of water during reactions, as reactant, catalyst, solvent, or both, has not been resolved. Recently, with the fast-development of in-situ experimental techniques and the computational capacity, the scientists started to investigate the behaviors of interfacial water using the real-time characterization and theoretical modeling at the atomic level, which provides the evidences and pictures to understand the effects of interfacial water. This paper will briefly introduce the current opportunities and challenges in studying the interfacial water, and the latest development and facing difficulty in experiment and theory, which will be beneficial for the future design of efficient catalysts for their applications in water.
2019, 68 (1): 015101.
doi: 10.7498/aps.68.20181742
Abstract +
Water molecules in the very proximity to the solute differ a lot from those in the far and the bulk water in both structure and property, they are usually referred to as hydration water or bound water. There is no doubt about the effect of hydration water on the property and structure of solute in solution, in particular when biological macromolecules are of concern. However, by far, there are even significant controversies over the understanding of hydration water, including the accurate definition and quantification of hydration water, the quantitative evaluation of the difference in the properties between the hydration water and free water, and how the hydration water is involved in the various biological processes, etc. For resolving the aforementioned issues, it would be of essential importance to formulate a quantification scheme for the hydration water on a sound footing. In the present article, the principles of various spectrometric techniques for determining hydration water are briefly examined, and the main deficiency in quantification of hydration water for the individual techniques is analyzed. Those techniques based on the inflection point of the concentration dependence of some physical properties of the solution are also scrutinized. Finally, we present in detail a quantification scheme for hydration water based on the concentration dependence of glass transition temperature, which leads to quite a universal categorization of an aqueous solution into three distinct zones. Also the crystallization dynamics thus revealed might be helpful for understanding the water-involved processes in other circumstances.
2019, 68 (1): 018801.
doi: 10.7498/aps.68.20182131
Abstract +
Water and mass transport in low-dimensional confined structures is of great importance in solving many challenging problems in interface chemistry and fluid mechanics,and presents versatile applications including mass transport,catalysis,chemical reaction,and nanofabrication.Recent achievements of water and mass transport in low-dimensional confined structures are summarized.Water flow confined in nanochannels with different wettability reveals the viscosity in the interface region increases as the contact angle decreases,whereas the flow capacity of confined water increases as the contact angle increases.Small difference in the nanochannel size has a big effect on the confined water flow,especially for nanochannels with a diameter smaller than 10 nm.The phenomena of ultrafast mass transport are universal in the nanochannels with smaller diameter (<10 nm),e.g.,ultrafast ionic transport across the biological and artificial ionic channel;ultrafast water flow through aligned carbon nanotube (CNT) membrane;ultrafast water permeation through GO membranes with hydrophilic end-group.From the classical hydrodynamics,the penetration barrier in such a small channel in both biological and artificial systems is huge,which is contradictory with the actual phenomena.Thus,we propose a concept of quantum-confined superfluid (QSF) to understand this ultrafast fluid transport in nanochannels.Molecular dynamic simulations of water confined in 1D nanochannel of CNTs (with diameter of 0.81 nm) and 2D nanochannel of graphene (two graphene layers distance <2 nm) demonstrate ordered chain of water molecules and pulse-like transmission of water through the channel,further provide proof for the QSF concept.Reversible switching of water wettability in the nanochannel via external stimuli (temperature and voltage) are presented,raising the temperature causes water wettability switching from hydrophilic to hydrophobic state,while increasing the voltage induces water wettability change from hydrophobic to hydrophilic state.The ultrafast liquid transport performance promotes the application of nanochannels in separation.There exist an upper limit for the surface tension of the liquid (≈ 180mN/m) below which the nanochannels of CNTs can be wetting.Then,we summarized versatile applications of low-dimensional confined structures in catalysis,chemical reaction,nanofabrication,and battery.Despite considerable advances over the last few decades,many challenging issues on water and mass transport in low-dimensional confined structures are still unresolved.The biggest obstacle is focused on understanding the physical origin of the non-classical behavior of liquid under confinement.In this situation,our proposed QSF concept will provide new ideas for the fluidic behavior in the nanochannels,and the introduction of QSF concept might create QSF-based chemistry.By imitating enzyme synthesis,the reactant molecules can be arranged in a certain order,and the reaction barrier will be greatly reduced to achieve highly efficient and selective chemical synthesis.Some previous works including organic reaction and polymeric synthesis have approached the example of QSF-like chemical reactions.On the other hand,the advances in nanomechanical techniques such as surface forces apparatus,atomic force microscope,and sum-frequency vibrational spectroscopy will provide useful experimental approaches to understand the mechanism of water and mass transport in low-dimensional confined structures,and promote wider application of nanoconfined structures.
2019, 68 (1): 013101.
doi: 10.7498/aps.68.20181273
Abstract +
The specific water molecules that are confined within the solvation shell adjacent to the surface of biological macromolecules (including protein, enzyme, DNA, RNA, cell membrane, etc.) are called biological water molecules. Such water around the biomolecule surface plays a very important role in the structure, stability, dynamics, and function of biological macromolecules. A molecular-level understanding of the structure and dynamics of biological water, as well as the nature of its influence on biological structure and function is the key to revealing the mechanism of the biological functions. However, the researches in this field are still in the initial stage. Here in this paper, we review the relevant researches and recent progress of hydration water from three aspects. The first aspect is about the influence of hydration water on biological structure and function. It is evident that water actively participates in many biological processes such as protein folding, proton donation and migration, ligand binding and drug design, and allosteric effects. For example, water mediates the collapse of the chain and the search for the native topology through a funneled energy landscape. The second aspect is about the structure of water molecules around the biomolecules investigated by nuclear magnetic resonance (NMR), dielectric relaxation, neutron scattering, X-ray diffraction and ultrafast optical spectroscopy. The third aspect is about the dynamic behaviors of biological water, including the relaxation time scale, dynamic property, dynamic coupling between biomolecules and water molecules, and sub-diffusive motion of the water molecules along the protein surfaces. Different techniques measure different timescales for the motion of proteins and their hydration environment. While NMR and dielectric relaxation methods reveal the motion of biological water on a time scale from several tens of picoseconds to nanoseconds, ultrafast optical spectroscopy such as fluorescence and vibrational spectroscopy probes the hydrogen-bonding fluctuations of water on a time scale from the femtosecond to picosecond. It is therefore highly necessary to acquire a real and complete picture of the structure and dynamics of biological water by combining several different techniques. Finally, some unsolved scientific problems are also summarized in this review.
2019, 68 (1): 019201.
doi: 10.7498/aps.68.20181357
Abstract +
Soil is the foundation of food security, water safety and wider ecosystem security. China's water resources is featured by its poverty and uneven distribution. Flood irrigation in traditional agriculture not only uses large amount of water, but also destroys soil aggregate structure, resulting in soil degradation, such as soil compaction and soil salinization. Underground drip irrigation have obvious water saving efficiency with the effective utilization rate of water larger than 95%, but it will also destroy the soil structure to a certain extent. It has been reported in many researches that using aerated water drip irrigation can not only increase crop yields, but also improve crop quality. The influence of several factors such as the burial depth of drop head, the frequency of dripping, the amount of irrigation, the growth period of plant, the mode of aerating and the equipment and so on, and the effects of the aerated drip irrigation on the water environment, the air environment, the microbial environment, the nutrient environment and the mineral environment of soil are summarized. And the regulation mechanism of soil environment by the aerated drip irrigation is put forward. The changes in water, gas, microorganism, nutrition and minerals are the result of the change of soil structure. The experimental results of in situ synchrotron radiation X-ray computed tomography confirmed that aerated drip irrigation can change the structure of soil.
2019, 68 (1): 018203.
doi: 10.7498/aps.68.20181639
Abstract +
Clathrate hydrates are energy and environmental related materials for energy storage and extraction, as well as for waste gas sequestration. The three general structures of natural clathrates, structure I, structure Ⅱ and structure H are reviewed in the aspects of stability, cage size, and preferred guest molecule encapsulation. Neutron scattering technique has its unique advantage of clathrate hydrates characterization, such as large bulk property determination, penetration of high pressure vessel and the clathrate sample inside, sensitive to light elements (clathrate hydrates mainly containing C, H, and O atoms). Neutron diffraction and inelastic neutron scattering of clathrate hydrates are covered on the abilities of H/D atoms positions and anisotropic thermal parameters, pressure-temperature-dependent guest molecule occupancy, the disordered distributions of guest molecules and the nuclear density distributions, the thermodynamic and kinetic process of formation and decomposition, the translational and rotational vibration models of guest molecules and their quantum state transitions. Using CO2 to gently replace CH4 in methane hydrate is one of the most attractive exploiting schemes for its benefits to both geologic hazard consideration and cost efficiency (energy extraction and CO2 sequestration).
2019, 68 (1): 016802.
doi: 10.7498/aps.68.20182201
Abstract +
Surface and interfacial water is ubiquitous in nature and modern technology.It plays vital roles in an extremely wide range of basic and applied fields including physics,chemistry,environmental science,material science,biology,geology, etc.Therefore,the studies of surface/interfacial water lies at the heart of water science.When water molecules are brought into contact with various materials,a variety of phenomena can show up,such as wetting,corrosion,lubrication, nanofluidics,ice nucleation,to name just a few.Due to the complexity of hydrogen-bonding interactions between water molecules and the competition between water-water interaction and water-solid interaction,surface/interfacial water is very sensitive to local environment,which makes it necessary to study the structure and dynamics of water at the molecular level.In recent years,the development of new scanning probe techniques allows detailed real-space research on surface/interfacial water at single-molecule or even submolecular scale.In Section 2,several representative scanning probe techniques and their applications in surface/interfacial water are reviewed.The first one is ultra-high vacuum scanning tunneling microscopy,which allows molecular imaging of single water molecules,water clusters,wetting layers,and even water multilayers on metal surfaces as well as ultrathin insulating films.Based on scanning tunneling microscopy,the single-molecule vibrational spectroscopy can be further developed to probe the vibration and movement of individual water molecules,which assist us in understanding water diffusion,dissociation and quantum nature of hydrogen bonds.As a versatile tool at liquid/solid interfaces,electrochemical scanning tunneling microscopy opens up the unique possibility of probing the double electric layer and identifying water dynamics during electrochemical reactions. Moreover,non-contact atomic force microscopy yields higher resolution than scanning tunneling microscopy,such that the topology of hydrogen-bonding skeleton of surface/interfacial water and even the degree of freedom of hydrogen atoms can be discerned.To conclude this review,the challenges and future directions of this field are discussed in Section 3, focusing on non-invasive imaging under ambient conditions,ultrafast molecular dynamics,and novel structures under high pressures.
REVIEW
2019, 68 (1): 018101.
doi: 10.7498/aps.68.20181645
Abstract +
Suspensions include solvent and uniformly dispersed particles. Solidification of suspensions is to freeze the solvent while numerous particles disturb the pattern formation during the growth of the solid/liquid interface. It is a new interdisciplinary subject, involving the fields of freeze-casting porous materials, frost heaving, sea ice and biological tissue engineering and so on. Especially in recent years, many advanced materials with excellent properties were developed based on the processing of suspension solidification. Experimental phenomenon in suspension solidification is different from that in alloy solidification, such as the close-packed particle layer and self assembly, the ice lamellae structure and the periodic ice lenses and so on. Up to now, the formation mechanisms of these microstructures are still unclear. In this paper, we first review the historical development of suspension solidification in theory and in experiment. Then we demonstrate some recent progress of microstructural evolution and dynamical particle packing of suspension solidification. Finally, the outlooks of the future study on solidification of suspensions are also presented.
GENERAL
2019, 68 (1): 010301.
doi: 10.7498/aps.68.20181634
Abstract +
The geometric momentum was originally introduced for defining the momentum of particle constrained on a hypersurface, but it is in fact not necessarily defined on a curved surface only. If a coordinate system contains a family of hypersurfaces and a normal vector on hypersurface used as a unit vector, the geometric momentum can be defined on the family of hypersurfaces and can be used to determine a complete set of commuting observables. For instance, the spherical polar coordinate system is such a kind of coordinate, in which for a given value of radial position, the spherical surface is a hypersurface. It is well-known that any vector in the space can be decomposed into components along each axis of the spherical polar coordinates, but the geometric momentum has a different decomposition, for it requires a projection of the momentum on the hypersurface, and then needs to decompose the projection into the Cartesian coordinates of the original space where the whole spherical coordinates are defined. Explicitly, with a relation-iħ▽= p Σ + p n where-iħ▽ can be usual momentum operator in Cartesian coordinates, and p Σ is the momentum component on the hypersurface which turns out to be the geometric momentum, and p n is the momentum component along the radial direction, we have a nontrivial definition of radial momentum as p n ≡-iħ▽- p Σ. Once-iħ▽ and p Σ are measurable, p n is then indirectly measurable. The three-dimensional isotropic harmonic oscillator can be described in both the Cartesian and the spherical polar coordinates, whose quantum states thus can be examined in terms of both momentum and geometric momentum distributions. The distributions of the radial momentum are explicitly given for some states. The radial momentum operator that was introduced by Dirac has clear physical significance, in contrast to widely spreading belief that it is not measurable due to its non-self-adjoint.
2019, 68 (1): 010501.
doi: 10.7498/aps.68.20181714
Abstract +
Piezoelectric material, which exhibits excellent electro-mechanical conversion properties, is widely used in smart sensors and structures for sonar systems, weather detection and remote sensing. Hyperbolic shell structure made of piezoelectric material is liable to break down when it is used in high temperature environment, which is caused by the unexpected chaotic dynamic motion under the coupling effect of thermal filed and force field. Therefore, the chaotic nonlinear dynamic vibration of simply-supported piezoelectric material hyperbolic shell is studied under the combined action of temperature field and simple harmonic excitation. Based on the theory of finite deformation, the non-linear vibration equation and coordination equation of the hyperbolic shell are established. The non-linear dynamic equation of the structure is obtained by the Bubnov-Galerkin principle. The corresponding undisturbed Hamilton system has a homoclinic orbit. Using Melnikov function, the chaotic motion condition of the dynamic system under the criterion of Smale-horseshoe transformation is obtained. Furthermore, the mathematical model is established by Simulink software and the numerical simulations are performed by the fourth-order Runge-Kutta method. The simulation results accord well with those from the Melnikov method. The bifurcation diagram, the Lyapunov exponent diagram, the phase diagram and Poincaré section diagram are acquired to analyze the influence of temperature field on the non-linear characteristic of piezoelectric material hyperbolic shell system. When the temperature is close to 32℃ and 41℃, the Lyapunov index is less than 0 and the corresponding movement of the system is in the periodic zone, which is the same as that for a temperature range from 36℃ to 37℃. When the Lyapunov index is greater than 0, the corresponding movement of the system is in chaos zone. Therefore, the change of temperature has an additional effect on the stiffness of the system which affects the vibration of the system. The chaos and periodic zones of the system alternate with the increase of temperature and the vibration characteristics of the system can be controlled by changing the temperature field. Therefore, adjusting the temperature field can control the motion state of the system, which helps to improve reliability of the structure.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2019, 68 (1): 014301.
doi: 10.7498/aps.68.20181761
Abstract +
There appears a convergence effect on the sound filed under the condition of sound channel in the deep sea due to the refraction effect of the sea water. For the deep water environment with an incomplete channel, sea bottom has an important influence on sound propagation. A long-range sound propagation experiment was conducted in the South China Sea in April 2018. Hyperbolic frequency modulated (HFM) signals with a frequency band of 250-350 Hz are transmitted by an acoustic source which is towed at a speed of 4 knots away from a vertical line array (VLA). The VLA consists of 20 hydrophones which are arranged from 85 m to 3400 m with an unequal depth space. Using the data collected in the experiment, the effects of bathymetry variation on sound propagation are studied. The physical causes of the seafloor reflection convergence phenomenon are explained by using the parabolic equation combined with ray theory. The observed phenomenon is different from the convergence phenomenon in the typical deep water environment, the spatial variation of bathymetry contributes to the formation of the seafloor reflection convergence zone in advance, and the sound intensity in part of shadow zone is significantly increased. Due to the reflection from the seabed, two obvious seafloor reflection convergence zones are observed near the range of 20 km and 40 km respectively, in which both gains increase up to 10 dB, and a high sound intensity area is formed in the shadow zone near the range of 11 km, where the gain is less than the gains in the two convergence zones. In addition, the grazing angle of the sound ray reaching the second convergence zone is smaller than that reaching the first convergence zone when the receiving depth is the same as the source depth, and the rays with smaller glancing angle have less reflection loss, which leads to a higher gain in the second convergence zone. As the water depth becomes gradually shallower with range increasing, the convergence zone near the range of 51 km under the SOFAR channel is destroyed, and the sound field energy in the corresponding range is much smaller since the number of arriving refracted sound rays is reduced. In the first convergence zone, the path of arriving rays is gradually increased as the receiver becomes deeper. Therefore, the arrival structure tends to be complicated, and the multi-path effect is more obvious. The study result is meaningful for the performance analysis of sonar in complex deep water environment.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2019, 68 (1): 015201.
doi: 10.7498/aps.68.20181796
Abstract +
Description of photon scattering with relativistic Maxwellian electrons is numerically complex, and computationally time consuming for the final photon energy and angle distribution. A Monte Carlo method is used to simulate photon scattering with relativistic Maxwellian electrons. The main idea of this method is to transform the interaction of photonmoving electrons in the laboratory coordinate system into that in a new coordinate system in which the electrons are at rest, then to use the exact Klein-Nishina formula to describe this interaction and obtain the outgoing photon energy and angle, finally, to transform it into the primary laboratory coordinate system. In sum, there are eight steps, i.e.two two-dimensional (2D) transforms and two three-dimensional (3D) transforms and two Lorentz transforms, and two sampling. Repeating this process, summarizing and averaging all computed energy values and angles, the distribution of scattered energy and angle can be obtained.A Monte Carlo processor is developed to simulate a photon of any energy interacting with electrons at any temperature. Some typical cases are simulated. The computed results indicate that the photon spectrum is different from that of the photon scattering with rest electrons remarkably, especially for a low energy photon scattering with the high temperature electrons. The main phenomena are Doppler broading and blue shifting. The moving electron can extend the distribution of the outgoing photon energy, and for a low energy photon scattering with the high temperature electrons, the photon maybe obtains the energy from electrons with significant probability. The angle distribution is very complicated, and it is determined by the incident photon energy, the outgoing photon energy, and the electron temperature. This processor can calculate the energy scattering differential cross-sections or energy-angle scattering double differential cross-sections, and provide the data in a tabulated form for other transport methods.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2019, 68 (1): 016101.
doi: 10.7498/aps.68.20181498
Abstract +
Low electrical resistivity and high strength are a basic requirements for copper alloys.However,it has been widely known that these two properties are contradictory to each other:high electrical resistivity means extensive electron scattering by obstacles in the alloy,which in turn blocks dislocation movement to enhance mechanical strength.That is to say,any increase in strength necessarily brings about an increase in electrical resistivity.Essentially,strength and electrical resistivity are coupled in metal alloy as both are issued from a similar microstructural mechanism. That is why it is generally difficult to evaluate these alloys comprehensively and to select the materials appropriately. The present work addresses this fundamental problem by analyzing the dependence of hardness (in relation to strength) and electrical resistivity on solute content for deliberately designed ternary[Moy/(y+ 12)Ni12/(y+12)]xCu100-x alloys (at.%),where x=0.3-15.0 is the total solute content,y=0.5-6.0 is the ratio between Mo and Ni.The Mo-centered and Ni-nearest-neighbored[Mo1-Ni12]cluster structure are used to construct a short-range-order structure model of solid solution.The cluster[Mo1-Ni12]in solution enhances the strength,without increasing the electrical resistivity much,for the solutes are organized into cluster-type local atomic aggregates that reduce the dislocation mobility more strongly than electron scattering.The short-range-order structure has an essentially identical function for strength and electrical resistivity. In this solution state,both hardness and resistivity increase linearly with solute content increasing.When the solute constituents do not meet the requirement for ideal solution,i.e.,Mo-Ni ratio exceeds 1/12,the maximum value that the cluster[Mo1-Ni12]can accommodate,the solid solution should be destabilized and precipitation should occur,such as Mo precipitation in this case.The deviation from the linear change of resistivity and strength with solute content are caused by different alloy states,that is,solid solution and precipitation,which contribute to the resistivity and strength differently.Here we define a new term,the ratio of residual tensile strength to residual electrical resistivity,i.e.the “strength/resistivity ratio” in short,which represents an essential property of the alloy system.This ratio is 7×108 MPa/Ω· m) for the Cu-Ni-Mo alloy in complete solid solution state,and it is in a range of (310-490) 108 MPa/Ω·m) for the Cu-Ni-Mo alloys in a fully precipitation state (i.e.,most of Mo solute atoms precipitate out of the Cu matrix). Finally this new parameter is applied to the classification of common copper industrial alloys for the purpose of laying the basis for material selection.It is found that the strength/resistivity ratio of 310 effectively marks the boundary between the fully precipitated state and precipitation plus solution state.Using this criterion,it is concluded that alloys based on Cu-(Cr,Zr,Mg,Ag,Cd) are suitable for high-strength and high-conductivity applications.However,alloys based on binary systems Cu-(Be,Ni,Sn,Fe,Zn,Ti,Al) cannot realize the same purpose.The finding of the line dividing the characteristic properties of alloy having a strength-resistivity-ratio of 310 provides a key quantitative basis for comprehensively evaluating the alloy performance,which can effectively guide material selection and development of high strength and high conductivity copper alloys.
2019, 68 (1): 016801.
doi: 10.7498/aps.68.20181905
Abstract +
According to the molecular dynamics simulations and the mechanism of energy dissipation of nanofriction, we construct a model system with a flake sliding in commensurate configuration on a monolayer suspended graphene anchored on a bed of springs. The system is to analyze the contributions of different regions (T1-T7) of the graphene flake to friction force, with the substrate characterized by different stiffness gradients and midpoint stiffness.The results indicate that the soft region of contact (T1) always contributes to the driving force, whereas the hard region (T7) leads to the biggest friction force on all column atoms of the flake. Moreover, as the support stiffness increases, when the stiffness gradient and the midpoint stiffness are equal to 1.34 nN/nm2 and 12 nN/nm, respectively, the contribution ratio of T7 to the total friction increases from 33% to 47%, which is approximately 4-15 times greater than those of each column atoms in T3-T6. The results also indicate that the energy barrier decreases with the increase of support stiffness along the stiffness gradient direction of the substrate, which induces the resistance forces on the relative motion to decrease. Meanwhile, the amplitude of the thermal atomic fluctuation is higher in the softer region while lower in the harder one. This difference in amplitude leads to the considerable potential gradient that ultimately causes the driving force. Finally, for a given point at the end of the flake (T1 or T7), the intensity of the van der Waals potential field is mainly determined by the nearest substrate atoms at that point. Part of these nearest atoms lie inside the contact region while the others do not. Consequently, the thermal vibration of the atoms inside the contact region is different from that of the atoms outside the confinement. The different thermal vibrations induce the greater edge barriers. In addition, T1 lies in the soft edge region and T7 in the hard one. As a result, the normal deformations of these two regions are always different, and therefore they also generate the driving force.At these points, the results reported here suggest that the friction force in each contact region is caused by the coupling of the energy barrier and the elastic deformation between the graphene surfaces. The former contribution, i.e.the energy barrier, includes the interfacial potential barrier in commensurate state which is against the sliding of the surfaces with respect to each other, and the potential gradient caused by the different vibration magnitudes of the substrate atoms against the different spring stiffness in the direction of stiffness gradient. The latter contribution, i.e. the elastic deformation, is the unbalanced edge energy barrier resulting from the asymmetrical deformation and the different degrees of freedom between the edge atoms of the slider and atoms inside and outside the contact area of the substrate. Results of this paper are expected to be able to provide theoretical guidance in considering the influence of stiffness gradient on friction between commensurate surfaces and in designing the nanodevices.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2019, 68 (1): 017201.
doi: 10.7498/aps.68.20181769
Abstract +
Two-dimensional transitional metal dichalcogenide (2D TMD) emerges as a good candidate material in optoelectronics and valleytronics due to its particular exciton effect and strong spin-valley locking. Owing to the enhancement of quantum confinement effect and the decline of dielectric shielding effect, the optical excitation of electron-hole pair is enhanced substantially, which makes large TMD exciton binding energy and makes excitons observed easily at room temperature or even higher temperature. Optical response of 2D TMD is dominated by excitons at room temperature, which provides an ideal medium for studying the generation, relaxation and interaction of excitons or trions. By employing ultrafast time resolved spectroscopy, we investigate experimentally the dynamic behaviors of A-exciton and spin relaxations for two types of TMDs, i.e. WS2 and WSe2 monolayers, respectively. By tuning the excitation wavelength of the degenerate pump and probe laser beam, the WS2 monolayer and WSe2 monolayer are excited at their A-exciton resonance transition position or near their A-exciton resonance transition position in order to compare the dynamical evolutions of band structure and exciton polarization of the two similar WS2 and WSe2 monolayer structures. Our experimental results reveal that the relaxation of A exciton in WS2 shows biexponential decay, while that of WSe2 shows triexponential decay, and the A-exciton life time in WSe2 is much longer than that of WS2 counterpart. The spin relaxation of A exciton in WS2 shows a monoexponential feature with a lifetime of 0.35 ps, which is dominated by the electron-hole exchange interaction. For the case of WSe2, the spin relaxation can be well fitted with biexponential function, the fast component has a lifetime of 0.5 ps and the slow one has a lifetime of 28 ps. The fast relaxation is dominated by the electron-hole exchange interaction, and the slow one comes from the formation of dark exciton via spin-lattice coupling. By tuning the excitation wavelength around A-exciton transition, the formation of dark exciton in WSe2 is demonstrated to be much more effective than that in WS2 monolayer. Our experimental results provide qualitative physical images for an in-depth understanding of the relationship between exciton and TMD structure, and also provide reference for further designing and regulating the TMDs based optoelectronic devices.
White organic light emitting devices based on ultrathin emitting layer and bipolar hybrid interlayer
2019, 68 (1): 017202.
doi: 10.7498/aps.68.20181803
Abstract +
In this paper, efficient phosphorescent white organic light-emitting diodes (WOLEDs) with stable spectra are fabricated based on doping-free ultrathin emissive layers and mixed bipolar interlayers. To achieve WOLEDs, at least three kinds of light-emitting layers, i.e. blue, green and red, are needed. The traditional method to fabricate emissive layers is by co-evaporation, which can improve electroluminescent efficiency. However, the co-evaporation rate and dopant concentration are difficult to control, which leads to a bad reproducibility and thus goes against commercialization. In order to simplify the structures of WOLEDs and improve repeatability, several doping-free ultrathin emissive layers are used in this paper with 3 nm mixed bipolar interlayers separating them. The optimal ratio of bipolar hybrid material is determined by hole-only device, electron-only device and blue phosphorescent OLED. In addition, green, orange and red monochromatic OLED have also been fabricated separately, which are used to prove that mixed bipolar material is also suitable for the three phosphorescent emitting material. The WOLED with TCTA interlayers is fabricated to confirm that mixed bipolar material is beneficial to the characteristics of WOLEDs. The energy transfer process between different emitting materials is verified by studying the transient photoluminescence lifetime. The maximum efficiency of three-color and four-color doping-free WOLED are 52 cd/A (53.5 lm/W) and 13.8 cd/A (13.6 lm/W), respectively, and the maximum external quantum efficiency of three-color and four-color doping-free WOLED are 17.1% and 11.2%, respectively. Due to the sequential energy transfer structure between different emitting layers, the Commission Internationale de L'Eclairage coordinates shows a very slight variation of (0.005, 0.001) from 465 cd/m2 to 15950 cd/m2 for three-color WOLED. The Commission Internationale de L'Eclairage coordinates shows a variation of (0.023, 0.012) from 5077 cd/m2 to 14390 cd/m2 for four-color WOLED. The four-color WOLED shows a maximum color rendering index of 92.7 at 884 cd/m2, and it reaches 88.5 at 14390 cd/m2. In addition, the lifetime of phosphorescent OLED is usually poor due to the trap formed by triplet-polaron annihilation. The exciton distribution can be broadened and the exciton concentration can be reduced by using ultrathin light emitting layers (< 1 nm) and mixed bipolar interlayers. Therefore, triplet-polaron annihilation will be reduced, and the lifetime of OLEDs will be improved.
2019, 68 (1): 017301.
doi: 10.7498/aps.68.20181663
Abstract +
Recent studies showed that the nominal AlN interlayers in InAlN/AlN/GaN heterostructures had high GaN mole fractions, especially those grown by metalorganic chemical vapor deposition. The Al and Ga mole fraction in the AlGaN interlayer determine the electron wave function and penetration probability, and thus affecting the scattering mechanism related to the InAlN/AlGaN potential layers. In this paper we study the effects of Al mole fraction of the AlGaN interlayer on three scattering mechanisms related to the potential layer, i.e. alloy disorder scattering, subband energy fluctuation scattering and conduction band fluctuation scattering induced by In compositionally inhomogeneous InAlN layer. The wave function and penetration probability in the InAlN/AlGaN/GaN heterostructure are determined by self-consistently calculating the Schrödinger-Poisson equations and then used to calculate the scattering mechanisms. The results show that penetration probabilities in the InAlN and AlGaN both decrease with increasing Al mole fraction. The combination of the contribution of the screening effect and the two-dimensional electron gas (2DEG) density inhomogeneity results in an initial decrease and subsequent increase in the subband energy fluctuation scattering-limited mobility with increasing Al mole fraction, and the heterostructure with a smaller InAlN thickness has a larger mobility increase. The penetration probability and non-periodic arrangement of Al and Ga in the AlGaN predict an Al mole fraction dependence of the alloy disorder scattering-limited mobility similar to the subband energy fluctuation scattering-limited mobility, and the alloy disorder scattering occurs mainly in the AlGaN because the penetration probability in the AlGaN is much higher than in the InAlN. The conduction band fluctuation scattering-limited mobility monotonically increases with increasing Al mole fraction due to the decrease of the penetration probability. The subband energy fluctuation scattering-limited mobility is less sensitive to variation in the Al mole fraction than the other two scattering mechanisms-limited mobilities. In a small Al mole fraction range around 0.1, the alloy disorder scattering is a dominant scattering mechanism, while the subband energy fluctuation scattering dominates the mobility beyond this compositional range. When Al mole fraction is above 0.52, the three scattering mechanisms-limited mobility exceeds that in the InAlN/GaN heterostructure without the AlGaN interlayer, indicating the promotion of the mobility by the AlGaN interlayer. The mobility is raised by more than 50 percent in the InAlN/AlN/GaN heterostructure with an AlN interlayer compared with that in the InAlN/GaN heterostructure without the interlayer.
2019, 68 (1): 017401.
doi: 10.7498/aps.68.20181531
Abstract +
ZnO is a wide bandgap semiconductor with the advantages of good stability, strong radiation resistance, and low cost. It has become a hot material in the field of photocatalysis, but it can only absorb purple light. Therefore, it is a valuable problem to study how to expand the response range of ZnO to visible light. Doping modification is a common method to solve this problem. In order to carry out the relevant research, the calculation in this paper are carried out by the CASTEP tool in Materials Studio software based on the first-principles of ultrasoft pseudopotential of density functional theory, the geometric structures of ZnO, Zn0.875Pr0.125O, ZnO0.875N0.125, Zn0.875Pr0.125O0.875N0.125, Zn0.75Pr0.25O0.875N0.125, Zn0.625Pr0.375O0.875N0.125 are constructed. All the models are based on the optimization of the geometry structure. By using the method of generalized gradient approximation plus U, we calculate the band structure, density of states, population, absorption spectra and dielectric functions of the models. The results show Co-doped system is easier to form than single-doped system, and the stability of the co-doped system increases first and then decreases with the increase of Pr concentration. The population ratio of the shortest Zn-O bond to the longest Zn-O bond in the same system increases first and then decreases with the impurity concentration, which shows that the doping of impurities has a great influence on the lattice distortion of the system, and the distortion is benefit for the separation of photogenerated hole-electron pairs. Therefore, the photocatalytic activity of the materials can be improved. Hybridization of N-2p and Pr-4f states destroys the integrity of crystals and forms crystal fields around impurity atoms, which results in splitting of energy levels and narrowing of bandgap. Compared with intrinsic ZnO, the static dielectric constant of all doped systems increases, especially the constant of Pr-N co-doped systems increases with the increase of doped Pr concentration, which indicates that the polarization ability of the co-doped systems increases with the increase of doped Pr atomic concentration. The main peaks of the dielectric function imaginary part of the doping systems move to the low energy region, and the absorption spectrums are red-shifted. As the concentration of impurity Pr atom increases, in the visible region, the absorption capacity of each co-doped system increases, their response range is enlarged in turn, showing the co-doping of N and Pr is benefit for improving the photocatalytic activity of ZnO.
2019, 68 (1): 017402.
doi: 10.7498/aps.68.20181996
Abstract +
Non-centrosymmetric superconductors have received considerable attention because of their possible possession of unconventional spin-triplet pairing.For this reason,the non-centrosymmetric Re3W with α -Mn structure has been widely concerned.However,almost all the previous studies support that the non-centrosymmetric phase of Re3W is a conventional weak-coupling s-wave superconductor.Later on,it is proved that Re3W has two different superconducting phases,one is the non-centrosymmetric phase and the other has a centrosymmetric hexagonal structure.Thus,a comparative study of these two superconducting phases could provide more information about the effect of non-centrosymmetric structure on the pairing symmetry of Re3W.In this paper,point-contact Andreev reflection experiments are carried out on Re3W/Au and the data can be well fitted by isotropic s-wave Blonder-Tinkham-Klapwijk (BTK) theory.In combination with our previous researches,we find that both centrosymmetric and non-centrosymmetric phases have similar temperature dependence of superconducting gap (△) with almost the same gap ratio of △/Tc.These results present strong evidence that both phases of Re3W are weak coupling Bardeen-Cooper-Schrieffer superconductors.Another interesting finding is that both phases of Re3W could easily form an ideal point-contact junction (i.e.,inelastic scatterings at the interface can be ignored) with a normal metal tip.This is manifested as an extremely small broadening factor (Γ) used in the fitting process,and indicates a clean (and possibly transparent) interface.Keeping this in mind,we can assume that the effective barrier (Z) at the interface mainly comes from the mismatch between the Fermi velocity of the superconductor and that of the normal metal,which can be estimated from the formula Z2=(1-r)2/4r,where r is the ratio between those two Fermi velocities.From this formula,we can obtain the Fermi velocity of Re3W by using the known value of Au's Fermi velocity and the fitting parameter Z for the Re3W/Au point contacts.It is interesting to find that the chemical property of Re3W is stable in the atmospheric environment.Even if the samples are exposed to the atmospheric environment for nearly six months,the inelastic scatterings are still very weak,and the superconducting properties are unchanged.Such an exceptional performance of Re3W can be utilized to study the physical properties of its counter electrode in a point contact.As an attempt,we build a point contact between Re3W and a ferromagnetic Ni tip,and measure its Andreev reflection spectra which are then fitted with a modified BTK model by considering spin polarization.The determined spin polarization of Ni is in good agreement with previously reported result. Moreover,using the Fermi velocities of Re3W and Ni,we can calculate the effective barrier to be around 0.3 in the Re3W/Ni interface,which coincides with the fitting parameter Z.These results self-consistently demonstrate the validity of the determination of Re3W's Fermi velocity and the cleanness/transparency of the studied point-contact interface.
2019, 68 (1): 017501.
doi: 10.7498/aps.68.20181621
Abstract +
Nano-magnetic logic device (NMLD) is a novel nanoelectronic device that stores, processes, and transfers information by dipole-coupled magneto-static interactions between nanomagnets. In the NMLD, long axis tilted nanomagnet attracts the attention of researchers due to its flexibility in magnetic logic design. Edge-slanted nanomagnet is wildly used, whose long axis is tilted due to its asymmetric shape. However, there are three defects in edge-slanted nanomagnets. 1) This type of nanomagnet requires a larger size, thus increasing the nano-magnetic logic (NML) space and introducing the C-shape and vortex clock errors that are often found in large-sized nanomagnets. 2) The irregular shape of nanomagnet increases the requirements for fabrication. 3) Complex calculations caused by the irregular shape are inevitable.In this paper, the tilt of the long axis of the nanomagnet is realized by placing the regular-shaped (elliptical cylinder) nanomagnet (50 nm×100 nm×20 nm) obliquely. According to the flipping preference of tilted nanomagnet, the authors design a two-input AND (OR) logic gate clocked by stress. The authors choose PMN-PT (Pb (Mg1/3Nb2/3) O3-PbTiO3) as the piezoelectric layer material to use its high piezoelectric coefficient. For magnetic materials, the authors choose Terfenol-D (Tb0.7Dy0.3Fe2), whose magnetic crystal anisotropy is smaller. The material of the subatrate is not discussed in this paper, which will be further studied in future experimental work. The mathematical model is established, and the dynamic magnetization of the gate is calculated. A stress of 90 MPa is applied to the output nanomagent for 3 ns. The nanomagnet is flipped to “NULL” at 1.8 ns and is then flipped to the final stable state after the stress has been removed for 0.9 ns. The output will become logic “0” (“1”) only if the input is logic “00” (“11”), otherwise the output will be logic “1” (“0”), thus successfully implementing OR (AND) logic. In addition, the gate is simulated by using the micromagnetic method. The results are basically consistent with our model. Unlike the designs based on edge-slanted nanomagnets, the basic logic gate based on tilted nanomagnets has three advantages. 1) This design allows high-aspect-ratio (2:1) nanomagnets to be used in logic functions. Therefore, less vortex and C-shaped error will be generated. 2) The regular shape can reduce the fabrication requirements and computational complexities. 3) Using stress as a clock, the energy consumption is greatly reduced, which can be only one-tenth of the general designs clocked by spin electronics.This model provides a greater energy efficiency and reliable basic logic unit for NML design. In the experimental preparation, there may be a large preparation error tilting the nanomagnet. As a solution, the stress electrodes can be tilted instead. So the stress will also make an angle with respect to the long axis of the nanomagnet.
2019, 68 (1): 017801.
doi: 10.7498/aps.68.20181869
Abstract +
Ge-Sb-Se chalcogenide glass is environmentally friendly, and has wide infrared transmitting window, high optical nonlinearity, as well as good mechanical property. These make it a good material for infrared transmission and nonlinear optics. In optical designs, the refractive index (n) and thermo-optic coefficient (ζ) of the glass are key technical parameters. In order to predict and tailor the n and ζ of Ge-Sb-Se glass, compositions with different chemical and topological features are prepared, their n, ζ, density (d) and volume expansion coefficient (β) are measured, and the composition dependence of the parameters is systematically investigated. The chemical feature of the glass is quantified by the percentage deviation of the composition from the stoichiometric ratio and denoted as dSe. The topological feature is represented by the mean coordination number <r> of each atom in the composition. It is shown that the n of Ge-Sb-Se glass increases with d increasing; the ζ decreases almost linearly with β increasing; and the β decreases with dSe decreasing or <r> increasing. When the Ge content is fixed, the d increases with dSe decreasing or <r> increasing; when the Sb concentration is fixed, the d has a minimum value at dSe=0. Based on the measured d and n, the molar refractivity (Ri) of Ge, Sb and Se elements in a spectral range of 2-12 μm are calculated. The obtained value of RGe is in a range of 10.16-10.50 cm3/mol, RSd in a range of 16.71-17.08 cm3/mol, and RSe in a range of 11.15-11.21 cm3/mol. When the Ri and d are used to compute n of any composition, the discrepancy between the calculated value and the measured one is less than 1%. According to the measured ζ and β, the thermal coefficients of the molar refractivity (φi) of Ge, Sb, and Se elements in a wavelength region of 2-12 μm are computed. The optimal value of φGe is in a range of 21.1-22.6 ppm/K, φSb in a range of 7.2-8.4 ppm/K, and φSe in a range of 90.2-94.2 ppm/K. When the φi and β are used to compute ζ of any composition, the discrepancy between the calculated value and the measured value is less than 6 ppm/K.
2019, 68 (1): 017802.
doi: 10.7498/aps.68.20181833
Abstract +
Dye-sensitized solar cell (DSSC) has been widely investigated due to its low cost, simple fabrication process, and excellent photoelectric conversion efficiency. Generally, the DSSC is composed of photoanode, electrolyte and counter electrode. At present, platinum (Pt) film delivers the highest photoelectric conversion efficiency in the available counter electrode materials. However, Pt film is very expensive and prepared by relatively complicated and high-cost magnetron sputtering, which seriously hinders the large-scale applications in DSSC. Therefore, it is of highly academic and engineering significance to develop novel counter electrode materials with low cost and high photoelectric conversion efficiency to replace expensive Pt counter electrode. Previous research shows that carbon-based nanomaterials such as graphene and carbon nanotubes ard promising to be used as highly efficient counter electrode materials. However, the high-cost and complicated fabrication process restrict their practical applications in DSSC. To address such issues, here in this work, we present and fabricate a highly efficient and low-cost three-dimensional porous carbon composite, which is constructed by the relatively dense and conductive graphite film as bottom layer (PC layer), and the porous carbon nanoparticle film as top layer (CC layer). Our fabricated DSSC consists of commercial TiO2 photoanode (m 4 mm×4 mm), and PC, CC, CC/PC composite, or Pt counter electrode with a size of m 8 mm×8 mm. The results show that under illumination (100 mW/cm2) provided by a solar simulator, the short circuit current densities (open circuit voltages) of DSSCs with PC, CC, CC/PC, and Pt counter electrodes are 11.45 mA/cm2 (0.72 V), 11.88 mA/cm2 (0.73 V), 12.00 mA/cm2 (0.75 V), and 13.46 mA/cm2 (0.74 V), respectively. The filling factors of DSSCs with PC, CC, and CC/PC are 56.09%, 59.80%, 65.28%, and 62.69%, respectively; the photoelectric conversion efficiencies of DSSCs with PC, CC, and CC/PC are 4.61%, 5.20%, 5.90%, and 6.26%, respectively. It is noted that compared with CC layer or PC layer counter electrode, the CC/PC counter electrode delivers better photovoltaic performance. Particularly, the filling factor of DSSC with CC/PC (65.28%) is even 4.10% higher than that of DSSC with commercial Pt (62.69%), and the photoelectric conversion efficiency of the CC/PC-based DSSC is as large as 5.90%, which reaches 94.2% of the Pt-based DSSC (6.26%). The excellent performance of DSSC with CC/PC counter electrode is attributed to the unique three-dimensional porous structure, which can not only facilitate the transfer of electrons and ions, but also provide abundant catalytic sites; these synergistic effects greatly enhance the photovoltaic conversion performance of CC/PC-based DSSC.
2019, 68 (1): 017803.
doi: 10.7498/aps.68.20181797
Abstract +
Fluorescence lifetime is an important characteristic parameter of quantum dot, which plays an important role in studying the optical properties of quantum dot. As a common method to obtain fluorescence lifetime, fluorescence decay curve fitting has been broadly accepted. The least squares fitting to the fluorescence decay curve is performed by using the exponential decay function to obtain fluorescence lifetime with taking the instrument response function into account. However, since the fluorescence decay curve inevitably involves noise photons such as dark counts and stray photons, there is a certain error in the fluorescence lifetime obtained by the method. In order to reduce the error and improve the accuracy of the results, enough photons are required. Nevertheless, too many photons will result in low efficiency of lifetime analysis and temporal resolution, and therefore this method can hardly extract dynamic information on a smaller temporal scale. In this paper, we propose a new method of obtaining the fluorescence lifetime of quantum dot, namely the single photons modulation spectrum. The basic idea is based on the relationship between the fluorescence lifetime and the signal amplitude of pulse repetition frequency in a single dynamic process. The experimental results show that the fluctuation errors and deviation errors of lifetime obtained by our method are significantly lower than those of the previous method when the same number of photons is used. Therefore, high-accuracy fluorescence lifetime can be obtained. When the fluctuation error is 5%, the accuracy is increased by more than one order of magnitude. And to obtain the fluorescence lifetime of the same error level, the number of photons required for our method is much smaller than that of the previous one, which indicates that our method can effectively suppress the disturbance of noise photons and enables the lifetime measurement with high efficiency and temporal resolution. When the fluctuation error and deviation error are both 5%, the efficiency and temporal resolution are increased by more than four times. Finally, real-time lifetime trajectory corresponding to the photoluminescence intensity time trajectory is obtained by our method, where rich dynamic information can be obtained on a sub-second temporal scale. The method of obtaining fluorescence lifetime with powerful anti-noise capability, high efficiency and temporal resolution proposed in this paper can play an important role in studying the fluorescence dynamics of single quantum systems.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2019, 68 (1): 018201.
doi: 10.7498/aps.68.20181726
Abstract +
Separator is an important component of lithium-ion battery,and the microstructure of separator has an important influence on the performance of lithium-ion battery.In the present paper,an electrochemical-thermal full coupling model is developed to accurately describe the complex physicalchemical phenomena in lithium-ion battery in charge and discharge process.The simulation results by the present model are closer to the experimental results than those by the previously published model.What is more,the present model is widely used to investigate the effects of the separator porosity and tortuosity on the performance of lithium-ion battery,respectively.The simulation results show that with separator porosity decreasing or separator tortuosity increasing,the output voltage,maximum discharge capacity and average output power of lithium-ion battery decrease,and the lithium-ion battery surface temperature and its rising rate increase.In the initial stage of discharge,when the separator porosity decreases or separator tortuosity increases to a certain degree,the output voltage of lithium-ion battery first decreases and then increases.The smaller the separator porosity or the higher the separator tortuosity,the larger the range and rate of reducing the output voltage of lithium-ion battery become and the longer the rise time needs in the initial stage of discharge.To ensure that the output voltage of lithium-ion battery is higher than the cut-off voltage,the separator tortuosity must be less than the critical tortuosity (It is equal to the separator tortuosity of the lithium-ion battery with the lowest output voltage,which is just equal to the cut-off voltage in the initial stage of discharge).Finally,a comprehensive analysis is conducted on the dynamic distribution of the electrochemical parameters and various heat productions in lithium-ion battery during charge and discharge.It can be clearly found that the electrochemical reactions in the end of discharge,the diffusion coefficients and the conduction coefficients of Li+ of electrolyte in the initial and middle stage of discharge are mainly influenced by the separator porosity and tortuosity.The research results in the present paper not only provide theoretical and technical support for the separator microstructure design and optimization,but also has important realistic meanings for improving or perfecting the preparation technology of the separator.
2019, 68 (1): 018401.
doi: 10.7498/aps.68.20181854
Abstract +
In recent years, the solution-processed organic-inorganic perovskite solar cells have attracted considerable attention because of their advantages of high energy conversion efficiency, low cost, and easily processing. Organometallic halide perovskite solar cells have gradually demonstrated particular superior properties in energy field due to their excellent photoelectric properties. This has been triggered by the unprecedented increase in its overall power conversion efficiency reaching 23% in just a few years, and it is becoming a direct competitor against the existing leading technology silicon. In this paper, 5-AVA-doped organometal halide perovskite films, (5-AVA)0.05(MA)0.95PbI3 and (5-AVA)0.05(MA)0.95PbI3/Spiro-OMeTAD, are prepared by the two-step method. The generation and recombination mechanism of charge carriers in two kinds of film samples are discussed in detail. The bivalent band structure of perovskite film material CH3NH3PbI3 is determined by ultraviolet-visible absorption spectra of perovskite film (5-AVA)0.05(MA)0.95PbI3 and (5-AVA)0.05(MA)0.95PbI3/Spiro-OMeTAD. We investigate the photocarrier dynamics and band filling effects in these two organometal halide perovskite films by using femtosecond transient absorption spectroscopy. For (5-AVA)0.05(MA)0.95PbI3, the photoinduced bleach recovery at 760 nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via the recombination of free electrons and holes. With regard to the perovskite film (5-AVA)0.05(MA)0.95PbI3 and (5-AVA)0.05(MA)0.95PbI3/Spiro-OMeTAD, the signal is photoinduced absorption from 550 nm to 700 nm. As the delay time increases, the electrons and holes are recombined, which results in a red shift of absorption spectrum in (5-AVA)0.05(MA)0.95PbI3. This can be referred to as Moss-Burstein band filling model. In contrast, the electrons and holes of (5-AVA)0.05(MA)0.95PbI3/Spiro-OMeTAD perovskite film sample are separated after photoexcitation. The holes rapidly transfer to the hole transport layer of Spiro-OMeTAD. It will lead to an increase in sample absorbance and a rapid recovery of bleaching signals. Consequently, electron-hole recombination is no longer a dominant pathway to the relaxation of photocarriers and the band filling effect is not significant in the composite film. Our findings provide a valuable insight into the understanding of the charge carrier dynamics and spectral band filling in mixed perovskites. These results conduce to the understanding of the intrinsic photo-physics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications, and provide a reference for the future research of perovskite solar cells.
2019, 68 (1): 018501.
doi: 10.7498/aps.68.20181577
Abstract +
In Atkinson-Shiffrin model, the formation of human memory includes three stages:sensory memory (SM), short-term memory (STM), and long-term memory (LTM). A similar memory formation process has been observed and reported in the experimental studies of memristors fabricated by different materials. In these reported experiments, the increase and decrease of the memristance (resistance of a memristor) would normally be regarded as the loss and formation of the memory of the device. These memristors can be divided into two types based on the memory formation process. The memory formation of some memristors consists of only STM and LTM, and these memristors in this paper are called STM → LTM memristors; the memory formation of other memristors contains all three stages like human memory, and these memristors here are named SM → STM → LTM memristors. The existing mathematical model of this kind of memristor can only describe the STM → LTM memristor. Three state variables are included in this model:w describes the memory of the device, wmin describes the long-term memory, and τw0 is the time constant of the forgetting curve of the short-term memory. In this paper, a phenomenological memristor model is proposed for SM → STM → LTM memristors. The model is designed by redefining a+, a constant in the existing STM → LTM memristor model, as a state variable, and the design of corresponding state equation is based on the reported experimentally observed behaviors of SM → STM → LTM memristors during the SM period. Simulations of the proposed model show its ability to describe the behavior of SM → STM → LTM memristors. Stimulated by repeated positive pulses starting from the high-memristance state, the memristor stays in the SM state during the stimulation of first several pulses, and no obvious memory is formed during this period; STM and LTM would be gradually formed when the following pulses are applied. A faster memory formation speed can be achieved by applying pulses with longer duration, shorter interval, or higher amplitude. The formation and annihilation of the conductive channel between two electrodes of a memristor is a commonly used explanation for the change of the memristance. In this model, w can be understood as the normalized area index of the conductive channel, wmin is the normalized area index of the stable part of the conductive channel, τw0 describes the amount of time taken by the annihilation of the unstable part, and a+ determines the variation of the conductive channel when different positive voltages are applied.
2019, 68 (1): 018901.
doi: 10.7498/aps.68.20181388
Abstract +
Structure entropy can evaluate the heterogeneity of complex networks, but traditional structure entropy has deficiencies in comprehensively reflecting the global and local network features. In this paper, we define a new structure entropy based on the number of the K-order neighbor nodes which refer to those nodes which a node can reach within K steps. It can be supposed that the more K-order neighbors a node has, the more important role the node plays in the network structure. Combining the formula of Shannon entropy, the K-order structure entropy can be defined and figured out to explain the differences among the relative importance among nodes. Meanwhile, the new structure entropy can describe the network heterogeneity from the following three aspects. The first aspect is the change tendency of structure entropy with the value of K. The second aspect is the structure entropy under a maximum influence scale K. The last aspect is the minimum value of the K-order structure entropy. The simulation compares the heterogeneities of five classic networks from the above three aspects, and the result shows that the heterogeneity strengthens in the from-weak-to -strong sequence:regular network, random network, WS (Watts-Strogatz) small-world network, BA (Barabási-Albert) scale-free network and star network. This conclusion is consistent with the previous theoretical research result, but hard to obtain from the traditional structure entropy. It is remarkable that the K-order structure entropy can better evaluate the heterogeneity of WS small-world networks and suggests that the greater small-world coefficients a network has, the stronger heterogeneity the network has. Besides, the K-order structure entropy can fully reflect the heterogeneity variation of star networks with network size, and reasonably explain the heterogeneity of regular networks with additional isolated nodes. It suggests that when i additional isolated nodes are added to a regular network with n nodes, the new network has weaker heterogeneity than the old one, but has stronger heterogeneity than the regular network with n+i nodes. Finally, the validity of the K-order structure entropy is further confirmed by simulations for the western power grid of the United States. Based on the minimum value of the K-order structure entropy, the heterogeneity of the western power grid is the closest to that of WS small-world networks.
