Vol. 67, No. 15 (2018)

SPECIAL TOPIC—Multiferroicity: Physics, materials, and devices
(1-x)Sr3Sn2O7+xCa3Mn2O7 ceramics and their photo-electric characteristics
Huang Yu-Tian, Wang Yu, Zhu Min-Min, Lü Ting, Yang Hong-Chun, Li Xiang, Wang Xiu-Zhang, Liu Mei-Feng, Li Shao-Zhen
2018, 67 (15): 154203. doi: 10.7498/aps.67.20180954
Abstract +
In order to obtain new hybrid improper ferroelectricity, the (1-x)Sr3Sn2O7+xCa3Mn2O7 (0 x 0.2) samples each with a Ruddlesden-Popper phase are prepared by the conventional solid-state reaction method. The phase purity is characterized by X-ray diffraction through using the GSAS refinement. The results indicate that (1-x)Sr3Sn2O7+xCa3Mn2O7 sample has an orthorhombic structure and its cell constants decrease with content x increasing. At room temperature, the dielectric constant decreases with frequency increasing. Sr3Sn2O7 exhibits diamagnetism. With increasing the content of Ca3Mn2O7, the sample presents diamagnetism, paramagnetism, weak ferromagnetic and paramagnetism in sequence. And the sample with x=0.1 has a weak ferromagnetic property. All these pave a way for selecting Ruddlesden-Popper multiferroic materials.
Theoretical study on magnetoelectric effect in multiferroic tetragonal BiMnO3
Yuan Ye, Tian Bo-Bo, Duan Chun-Gang
2018, 67 (15): 157511. doi: 10.7498/aps.67.20180946
Abstract +
Perovskite BiMnO3 with ferroelectric and ferromagnetic ordering simultaneously, as a kind of multiferroics, can be expected to have the coupling between the magnetic and dielectric properties as well as their control by the application of electric fields. This advantage can make BiMnO3 a good candidate for an artificial synapse material. Under the framework of the density functional theory, in this paper we adopt the generalized gradient approximation (GGA+U) plane wave pseudopotential method to calculate the ferroelectricity double-well potential curves and magnetic moments of Mn of tetragonal BiMnO3, with 0.18% and 4% strain exerted in its x-y plane. The results show that the magnetic moment of Mn monotonically increases from paraelectric state to ferroelectric state. It means that the ferromagnetic property of tetragonal BiMnO3 can be controlled by the intensity of polarization. The greater the stress, the greater the range of magnetic moment is. This would imply that the multiferroic artificial synapse device based on BiMnO3 can bring another degree of freedom into designing the complex cognitive systems of artificial intelligence in the future.
Hybrid improper ferroelectricity and multiferroic in Ruddlesden-Popper structures
Liu Xiao-Qiang, Wu Shu-Ya, Zhu Xiao-Li, Chen Xiang-Ming
2018, 67 (15): 157503. doi: 10.7498/aps.67.20180317
Abstract +
Hybrid improper ferroelectricity (HIF) is a secondary ferroelectric ordering induced by the coupling between oxygen octahedral in-plane rotation and out-of-plane tilt in a metal-oxide containing the perovskite structure units. Investigation of HIF will greatly extend the connotation and denotation of ferroelectric physics and material science, and it is expected to develop the room temperature single phase multiferroic material with large polarization and strong magnetoelectric coupling, owing to its intrinsic characteristic of the electric-field control of magnetism through HIF in magnet. In the present paper, the recent primary progress of HIFs and the multiferroics with Ruddlesden-Popper structures is reviewed, and the perspective of the future development is also presented.
Research progress of low-dimensional ferroelectric materials
Hu Ting, Kan Er-Jun
2018, 67 (15): 157701. doi: 10.7498/aps.67.20180483
Abstract +
Ferroelectricity, which exhibits a spontaneous electrical polarization under Curie temperature, is of potential value for sensors, photonics and energy-efficient memories, solar cell, and photoelectrochemical applications. With the rapid development of high-density electronic devices, miniaturized and integrated ferroelectric devices have been a development tendency for ferroelectric materials. However, the size effect and surface effect restrict the applications of traditional bulk ferroelectric materials on a nanometer scale. Therefore the ferroelectric properties of low-dimensional nanomaterials have become an extensively studying subject in the field of material science. In this article, we review the theoretical and experimental researches of low-dimensional ferroelectric materials in recent years, including two-dimensional van der Waals layered ferroelectric materials, covalent functionalized ferroelectric materials, low-dimensional perovskite materials, external regulation and two-dimensional hyperferroelectric metal. We first give a concise outline of the basic theory, which relates to the existence of ferroelectricity. And then, we introduce the intrinsic ferroelectricity into two-dimensional materials. Many samples have been predicted, and the origin of ferroelectricity can be attributed to the soft modes of phonon, which leads to the ion displacements. Further, we discuss the ferroelectricity in covalent-modified two-dimensional materials. In such structures, the modified groups produce spontaneous electric dipoles, and lead to the macroscopical ferroelectricity. Therefore, we focus on how to design such structures, and the consequent ferreoelectricity. Considering the big potential of perovskite structures in ferroelectric family, we also discuss the recently reported low-dimensional perovskite structures, indicating several competitive mechanisms in such complex compounds. Additionally, we also introduce the research progress of other aspects in this field, including charge-polar induced ferroelectricity, two-dimensional ferromagnetic ferroelectrics, and hyperferroelectric metal. The reported new physical mechanisms are also provided to explain the low-dimensional ferroelectrics. Thus, such results not only mark the research of low-dimensional materials entering into a new stage, but also provide abundant physics in this area. Finally, the development prospects for low-dimensional ferroelectrics are also discussed.
Photovoltaic effect in ferroelectrics
Cai Tian-Yi, Ju Sheng
2018, 67 (15): 157801. doi: 10.7498/aps.67.20180979
Abstract +
Ferroelectric oxides are attractive materials for constructing efficient solar cells. The mechanism includes the anomalous photovoltaic effect (APE) and the bulk photovoltaic effect (BPE). The BPE refers to the generation of a steady photocurrent and above-bandgap photovoltage in a single-phase homogeneous material lacking inversion symmetry. The mechanism of BPE is different from the typical p-n junction-based photovoltaic mechanism in heterogeneous materials. We survey the history, development and recent progress in understanding the mechanisms of BPE, with a focus on the shift current mechanism, an intrinsic BPE that is universal to all materials lacking inversion symmetry. We also review the important factors to the APE, i.e., the domain boundary, the Schottcky junction, and the depolarization field. The recent successful applications of inorganic and hybrid perovskite structured materials in solar cells emphasize that ferroelectrics can be used in conventional photovoltaic architectures. We review the development in this field, with a particular emphasis on the perovskite materials and the theoretical explanations. In addition to discussing the implication of a ferroelectric absorber layer and the solid state theory of polarization, the design principles and prospect for high-efficiency ferroelectric photovoltaics are also mentioned. Considering the coupling between the degrees of freedom, some special ferroelectrics are expected to have prominent multi-functionality. With the introduction of the additional degree of freedom, some ferroelectrics, i.e., ScFexCr1-xO3 (1/6 x 5/6), can be a promising candidate for highly efficient solar cells and spin photovoltaic devices.
Oxygen vacancies induced tuning effect on physical properties of multiferroic perovskite oxide thin films
Zhao Run, Yang Hao
2018, 67 (15): 156101. doi: 10.7498/aps.67.20181028
Abstract +
By controlling the position and concentration of oxygen vacancies, the relevant physical properties of the multiferroic ABO3 perovskite thin film can be modulated, including electric, optical and multiferroic properties. In this paper, we briefly review the various typical multiferroics. The details of oxygen vacancies are introduced, including the formation mechanism, oxygen octahedral structure, relationship between strain and oxygen vacancy, and specific tuning effect on the physical properties (multiferroic, superconductivity and electrochemical behavior). The latest research progress of the oxygen vacancies induced tuning effect, especially in the field of the multiferroic, provides valuable reference for exploring novel magnetoelectric functional materials and devices.
Recent progress of improper ferroelectricity in perovskite oxides
Zhao Guo-Dong, Yang Ya-Li, Ren Wei
2018, 67 (15): 157504. doi: 10.7498/aps.67.20180936
Abstract +
Perovskite oxides show many potential applications in the research fields of emerging materials and devices for electronics, information and communication because of their rich functionalities, e.g. magnetic, ferroelectric, multiferroic, mechanical and optical properties. Among them, ferroelectricity is currently being studied intensively due to the existence of many different mechanisms, and the coupling with magnetism and strain. In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force, the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders. In this review, we focus on the inorganic perovskite oxides to summarize the recent research progress of the improper ferroelectricity in general, but we review the magnitude of polarization, and the generation mechanism of improper ferroelectricity in perovskite superlattice, double perovskite structures and a specific SmFeO3 single crystal possessing antiferromagnetic domain walls in particular. This review will hopefully provide routes to systematically understanding the improper ferroelectricity.
High pressure synthesis and physical properties of multiferroic materials with multiply-ordered perovskite structure
Zhou Long, Wang Xiao, Zhang Hui-Min, Shen Xu-Dong, Dong Shuai, Long You-Wen
2018, 67 (15): 157505. doi: 10.7498/aps.67.20180878
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Perovskite is one of the most important material systems for magnetoelectric multiferroic study. However, multiferroic is not expected to occur in a cubic perovskite on account of the highly symmetric crystal structure. Besides, magnetoelectric multiferroics with large ferroelectric polarization and strong magnetoelectric coupling have not been found to occur simultaneously in a single-phase multiferroic material discovered so far, challenging to the potential applications of this kind of material. Here we briefly review two multiferroic materials with multiply-ordered perovskite structure synthesized under high pressure and high temperature conditions. In the cubic perovskite LaMn3Cr4O12, we observed spin-induced ferroelectric polarization, providing the first example where ferroelectric takes place in a cubic perovskite material. In another multiply-ordered provskite BiMn3Cr4O12, type-I and type-Ⅱ multiferroic phases successively developed when cooled. It provides a rare example where two different types of multiferroic phases occur subsequently so that both large polarization and strong magnetoelectric effect are achieved in a single-phase material. In addition, since double ferroelectric phases take place in BiMn3Cr4O12, one can obtain four different polarization states by adopting different poling procedures, thus opening up a new way for generating multifunctional spintronics and multistate storage devices.
Recent progress of multiferroic magnetoelectric devices
Yu Bin, Hu Zhong-Qiang, Cheng Yu-Xin, Peng Bin, Zhou Zi-Yao, Liu Ming
2018, 67 (15): 157507. doi: 10.7498/aps.67.20180857
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Multiferroic composites possess the coupling effect among mechanical, electrical, and magnetic ordering, showing potential applications in compact, fast, and low-power magnetoelectric devices. Owing to the increasing application demand, the researches of device design, micro-/nano-fabrication, and performance test of magnetoelectric devices have made continuous progress. In this review, we briefly introduce several prototype devices based on magnetoelectric coupling, analyze the noteworthy application techniques, and summarize the working mechanisms and performances of devices including tunable inductors, RF/microwave filters, magnetoelectric memories, energy harvesters, magnetoelectric sensors, magnetoelectric antennas, etc. Besides, we discuss the issues and challenges in researches of multiferroic magnetoelectric devices, and present the perspectives for improving the device performance.
Research progress of multiferroicity in Bi-layered oxide single-crystalline thin films
Zhai Xiao-Fang, Yun Yu, Meng De-Chao, Cui Zhang-Zhang, Huang Hao-Liang, Wang Jian-Lin, Lu Ya-Lin
2018, 67 (15): 157702. doi: 10.7498/aps.67.20181159
Abstract +
Room temperature multiferroics with a single phase is very rare, and magnetic elements doped Bi-layered Aurivillius oxides are an important family of room temperature single phase multiferroics. However, due to the lack of single crystalline samples, the multiferroic related researches of these materials are mostly based on polycrystalline bulk or thin film samples. And the multiferroic characterizations are performed mostly by using the bulk type of samples. Therefore the studies of the origin and mechanism of the multiferroicity of these materials are extremely difficult. Recently, multiple magnetic elements doped singlecrystalline thin films have been successfully prepared, which makes it possible to study the physics mechanism of the Bi-layered Aurivillius oxides of multiferroicity. The current study shows that most of the single-crystalline thin films exhibit in-plane orientated spontaneous ferroelectric polarization and very weak room temperature magnetism. Moreover, at low temperatures the single-crystalline films exhibit a second magnetic transition. The resonant inelastic X-ray scattering experiments indicate that the doped structure exhibits a changed crystal field split, which may enhance the weak ferromagnetism through Dzyaloshinskii-Moriya interaction. On the other hand, the polarized neutron reflectivity experiments reveal that the single-crystalline thin film possesses much weaker room temperature magnetism than the bulk sample, which indicates that the origin of the magnetism and the magnetoelectric coupling in the single-crystalline samples are different from those in the polycrystalline samples. The current study of the multiferroicity in the single-crystalline Bi-layered Aurivillius thin film opens the road to designing better multiferroic systems of the Aurivillius materials.
Magnetoelectric effect in stretch-shear mode self-biased LiNbO3 based composite with high-frequency resonant response
Xin Cheng-Zhou, Ma Jian-Nan, Ma Jing, Nan Ce-Wen
2018, 67 (15): 157502. doi: 10.7498/aps.67.20180810
Abstract +
Magnetoelectric (ME) composites have received attention abidingly due to the promising potential applications in magnetic field sensor and energy harvester. In recent years, shear mode ME composite was frequently discussed with promising applications in high-frequency magnetic field with large signal-to-noise ratio. Single-crystal LiNbO3, as a lead-free piezoelectric phase with high mechanical quality factor and small dielectric constant, is suitable for achieving a large shear ME effect with large shear piezoelectric coefficient d15 or d24, and different piezoelectric coefficients can be obtained by crystal-cut transformation. The transformation rule of shear ME coefficient with transformation of LiNbO3 crystal orientation and the MHz high-frequency magnetic detection is still lacking. Furthermore, self-biased ME composite can be obtained with SrFe12O19 ribbon, which is useful for the integration and miniaturization of ME sensor. In the present work, we use a series of X-cut LiNbO3 to obtain different d15 or d16 in a stretch-shear ME composite. Piezoelectric coefficient d15 and ME coefficient E15 of Metglas/LiNbO3 composite are obtained in experiment, respectively. The results show that LiNbO3 xzt/30 has the largest d15 and E15, and the transformation rule of E15 is consistent with the coordinate transformation of d15. The structure of stretch-shear ME composite is optimized to improve the ME coefficient. Then the stretch-shear mode self-biased SrFe12O19/Metglas/LiNbO3 composite is fabricated, and shear ME response is observed under zero external direct current magnetic bias. Moreover, E15 at electromechanical resonance frequency is gained at shear-mode high frequency (0.991 MHz and 3.51 MHz). The largest ME coefficient E15 is acquired in the stretch-shear 5-foil Metglas/LiNbO3 (xzt/30) composite of 134.16 mV/(cmOe) at 1 kHz and 9.17 V/(cm Oe) at 3.51 MHz. This work is beneficial to the confirming of the corresponding rules of shear ME coefficient and LiNbO3 piezoelectric coefficient, showing that the composite possesses the potential applications in integration, miniaturization and high-frequency resonant sensor.


Magnetoelectric heterostructure and device application
Yang Na-Na, Chen Xuan, Wang Yao-Jin
2018, 67 (15): 157508. doi: 10.7498/aps.67.20180856
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The magnetoelectric (ME) heterostructure is composed of ferromagnetic and ferroelectric materials. The heterostructural ME effect originates from piezoelectric effect in the ferroelectric component and magnetostrictive effect in the ferromagnetic component. The magnetoelectric heterostructure has higher magnetoelectric coupling coefficient and lower dielectric loss than the particulate composites, and thus leading to several promising applications such as in the magnetic field sensors, the energy harvesters, antenna and memory devices. In this paper, we review the recent research progress in ME heterostructure for device applications, and present a development course of ME heterostructure. Finally, we also summarize the challenges of developing the ME heterostructure and point out its perspectives.
Excited charge-transfer organics with multiferroicity
Yuan Guo-Liang, Li Shuang, Ren Shen-Qiang, Liu Jun-Ming
2018, 67 (15): 157509. doi: 10.7498/aps.67.20180759
Abstract +
Multiferroics, showing simultaneous electric and magnetic degree of freedom, has aroused increasing interest due to tailored multiferroic properties and magneto-electric coupling for shaping the development of energy-efficient multifunctional devices. Now, the multiferroics can be classified as two groups:1) inorganic multiferroics, which can be single-phase, multi-phases oxide multiferroic or multiferroic heterojunction and 2) organic counterpart, which is mostly determined by instinct charge-transfer behavior. But it is difficult to find the polarization and the magnetization co-exist in a single-phase oxide multiferroic material, and their coupling range in the multiferroic heterojunction is only several atomic layers, which limits the applications. As a result, more and more different types of organic multiferroics have been studied. Some organic complexes can display dual ferroelectric and ferromagnetic properties at ambient temperature, e.g. thiophene-fullerene donor-acceptor charge-transfer networks. The organic charge-transfer complex is based on electron donor (D+) and acceptor (A-) assembly. D+A- are long-range ordering, the excitons have s lifetime and 1/2 spin, which contributes to the room temperature ferroelectricity and ferromagnetism. The excitons can be excited by external magnetic field, electric field, illumination and stress, and eventually influence the polarization, magnetization and magnetoelectric coupling coefficient. However, there are still many problems to be solved, i.e., searching for new charge-transfer systems and preparing supramolecular co-crystal with ordered molecular chain, further improving magnetoelectric properties; developing the heterojunction technology and epitaxial growth of organic ferroelectric or ferromagnetic systems on excited organic films, which is expected to greatly improve their magnetoelectric coupling effects; inventing more new charge transport organic multiferroic devices to extend the application scope of new multiferroic devices in actual industrial production. Generally speaking, the organic charge-transfer complexes not only greatly enrich the room temperature multiferroics materials, but also provide the technical basis for developing the new multifunctional electronic devices.


Electric field driven magnetic switching in nanoscale multiferroic heterostructures
Song Xiao, Gao Xing-Sen, Liu Jun-Ming
2018, 67 (15): 157512. doi: 10.7498/aps.67.20181219
Abstract +
Recently, there has been a surge of research interest in the electric field control of magnetism due to its promising application in spintronic and memory devices, which has become a hot topic in the field of multiferroic research. In current spintronic technology, magnetic reversal is usually driven by a large electric current via current generated magnetic field or spin-torque effect to write/erase a magnetic bit, and thus producing large power consumption and heat dissipation. While using insulating multiferroic materials, the reversal of magnetization can be triggered by applying an electric field instead of current, hence dramatically reducing the energy consumption and heat dissipation. With the current miniature trend in microelectronic technology, it is very essential to explore the electric field driven magnetic reversal (EFMS) behaviours in a micro/nanometer scale. In this article we briefly review the new progress in the field of EFMS based on multiferroic heterostructures, including some new features arising from size reduction, as well as some recent experimental and theoretical advances towards nanoscale EFMS, e.g. strain-mediated coupling, or spin exchange coupling in BiFeO3-based heterostructures, and their associated mechanisms. Finally, some key challenges in developing future EFMS based magnetoelectric devices, and some prospects for future research are also discussed.
Progress of converse magnetoelectric coupling effect in multiferroic heterostructures
Chen Ai-Tian, Zhao Yong-Gang
2018, 67 (15): 157513. doi: 10.7498/aps.67.20181272
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Electric-field control of magnetism has recently received much attention because of low-power consumption, which has potential applications in low-power multifunction devices. Ferromagnetic/ferroelectric multiferroic heterostructure is a useful way to realize the electric-field control of magnetism. Strain-mediated magnetoelectric coupling with large magnetoelectric coupling coefficient at room temperature is one of the current research hotspot. In this paper, we give an overview of recent progress of strain-mediated magnetoelectric coupling in multiferroic heterostructures.This review paper consists of five parts:introduction of multiferroics, electric-field control of magnetism in multiferroic heterostructures, electrical control of magnetization reversal, electric-field control of magnetic tunnel junctions, and the future prospects of multiferroic heterostructures. The basic concepts of multiferroics and background of magnetoelectric coupling effect are introduced in the first part.In the second part, a brief review of the recent work on the Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) based multiferroic heterostructures is given. The PMN-PT has a FE domain structure, which plays a vital role in electric-field control of magnetism, especially the 109 domain switching. For PMN-PT (001), the importance of 109 domain switching on the nonvolatile electrical control of magnetism is discussed. For PMN-PT (011), it is shown how to obtain nonvolatile strain which induces magnetic easy axis to be rotated by 90. The work on electric-field modulation of ferromagnetic material with perpendicular magnetic anisotropy is also mentioned.Electric-field control of magnetization reversal is still a challenge and remains elusive. Combination of strain-mediated magnetoelectric coupling and exchanging bias is a promising method to reverse magnetization by electric field, and the exchange-biased system/ferroelectric structures are given in the third part. There are also some theoretical attempts and proposals to realize the electrical control of 180 magnetization reversal. Then the method to manipulate magnetic tunnel junctions by electric field is given through integrating multiferroics and spintronics. Further outlook of the multiferroic heterostructures is also presented finally.


Progresses of magnetoelectric composite films based on PbMg1/3Nb2/3O3-PbTiO3 single-crystal substrates
Xu Meng, Yan Jian-Min, Xu Zhi-Xue, Guo Lei, Zheng Ren-Kui, Li Xiao-Guang
2018, 67 (15): 157506. doi: 10.7498/aps.67.20180911
Abstract +
Owing to the rapid development of microelectronic technology,higher requirements are raised for miniaturization, intellectualization,sensitivity,reliability,low-power consumption and versatile functions of electromagnetic functional devices,but conventional block magnetic or electrical functional materials cannot meet those requirements mentioned above any longer.Layered magnetoelectric composites,by contrast,have ferroelectric,ferromagnetic and magnetoelectric properties,so they are possible to satisfy these demands above and be applied to the next-generation magnetoelectric functional devices.Layered magnetoelectric composites not only have rich physical phenomena and effects,but also possess broad application prospects in weak magnetic field detectors,multi-state memories,electric-write/magnetic-read memories,electrically tunable filters,phase shifters,antennas,etc,which have attracted extensive attention of material scientists and physicists.Among layered magnetoelectric composites,the functional thin film/ferroelectric single crystal heterostructures have aroused increasingly interest due to their simple preparation method,flexible structural design,effective electric field control and low power consumption.Currently,because of the excellent ferroelectric and piezoelectric properties of the (1 -x) PbMg1/3Nb2/3O3-xPbTiO3(PMN-PT) single crystal,the functional thin film/PMN-PT single crystal heterostructure has become one of the hot research topics in the multiferroic composite thin film material field.On this research topic,Chinese scientists have made their own significant contributions to the research of functional thin film/PMN-PT single crystal heterojunction.So far,researchers have built multiple types of thin film/PMN-PT heterostructures,such as manganese oxide/PMN-PT,ferrite/PMN-PT,ferromagnetic metal/PMN-PT,dilute magnetic semiconductor/PMN-PT,luminescent material/PMN-PT,two-dimensional material/PMN-PT,multi-layer film/PMN-PT,superconductive material/PMN-PT,etc.,and they have made great achievements in both theoretical and experimental studies.In this review,we summarize the research progress of magnetoelectric composite thin films based on PMN-PT single crystal substrates in the last decade.We first briefly describe the current status of articles related to functional film/PMN-PT heterostructures.Then we introduce the phase diagram and electric-field-induced strain properties of the PMN-PT single crystal around the morphotropic phase boundary.We also classify the heterostructures according to different categories of functional thin film materials and discuss the representative research findings of each category in the past few years.Our discussion focuses on the magnetoelectric properties of materials and the intrinsic physical mechanism.Finally,we also discuss the scientific problems to be solved and predict the possible application directions in the future.
Multiferroic properties of exotic double perovskite A2BB' O6
Wu Mei-Xia, Li Man-Rong
2018, 67 (15): 157510. doi: 10.7498/aps.67.20180817
Abstract +
Multiferroic material in which there co-exist at least two of the ferro-phases,namely ferroelectricity,(anti-) ferromagnetism,and ferroelasticity,has attracted considerable attention in recent years due to its intriguing physics and potential applications for advanced multifunctional devices.However,multiferroic materials are rare due to the contradictory requirements between electrical polarization and magnetism.So far,only several compounds have been reported to show above-room temperature multiferroics.Thus,it is essential to search for new materials.The two most significant strategies to obtain multiferroics are 1) to incorporate magnetic transition-metal ions into polar structures to obtain polar magnets,and 2) to introduce special magnetic structure to drive ferroelectricity (the so-called type-Ⅱ multiferroics).Exotic double perovskite-related oxide A2BB'O6 with small A-site cations is one of the most extensively studied multiferroic families in recent years. The small A-site cations give small perovskite tolerance factor (t),and mostly high-pressure synthesis is required to stabilize the exotic perovskite structure.The crystal structure of exotic A2BB' O6 oxides can crystallize into either the centrosymmetric alumina corundum (AL),ilmenite (IL),or distorted GdFeO3-type perovskite structure,or the polar LiNbO3(LN),Ni3TeO6(NTO),or ordered ilmenite (OIL) structure.The polar LN,NTO,and OIL structures can accommodate magnetic transition-metal ions at both the A and B/B'sites in octahedral coordination,giving enhanced magnetic interactions and thus robust magneto-electric effect and high spontaneous polarization as well (usually above 50 C/cm-2,more than twice that in the renown BaTiO3),examples include the LN-type Mn2FeNbO6,and Mn2FeTaO6,OIL-type Mn2FeMoO6,and NTO-type Mn2FeMoO6,Mn2FeWO6,and Mn2MnWO6.These polar magnets show potential multiferroic responses even above room temperature (e.g.,ferromagnetic ordering temperature up to 340 K in NTO-type Mn2FeMoO6) and magnetoelectric coupling effect as in Mn2MnWO6.Magnetoelectric coupling can also arise in centrosymmetric IL structure in the absence of helical spin structure,such as those that are observed in Mn2FeSbO6,which exhibits colinear ferrimagnetic spin arrangement but magnetostriction induced antiferroelectricity.The corundum derivatives (AL,LN,IL,OIL,and NTO) and perovskite phases are competitive,depending on the electron configuration and synthesis pressure,and usually higher pressure favors the formation of perovskite structure.Compared with polar magnets in the corundum family,the exotic double perovskite adopts distorted GdFeO3-type structure (P21/n) with eight-coordination of the A-sites.In some double perovskite materials,the electric polarization can be induced by the special magnetic order,such as the ⇈⇊ magnetic structure induced type-Ⅱ multiferroics exemplified by A2CoMnO6(A=Lu,Y,Yb,Lu).In this review paper,we first compare the structure features of conventional and exotic double perovskite A2BB'O6 derived from the simple ABO3 analog,then summarize the recent progress of multiferroics in exotic double perovskite family,such as the polar magnets with transition-metal (Mn and Ni) cations at the A sites,type-Ⅱ multiferroic Mn2FeSbO6,and A2CoMnO6(A=Lu,Y,Yb,Lu). Finally,the problems and prospection of multiferroics in exotic double perovskite A2BB'O6 are also discussed to give a reference for the future research.
Resistive switching characteristics and resistive switching mechanism of Au/TiO2/FTO memristor
Yu Zhi-Qiang, Liu Min-Li, Lang Jian-Xun, Qian Kai, Zhang Chang-Hua
2018, 67 (15): 157302. doi: 10.7498/aps.67.20180425
Abstract +
Resistance random access memory is regarded as one of the most promising candidates for the future nonvolatile memory applications due to its good endurance, high storage density, fast erase speed and low power consumption. As one of the most important transition-metal oxides, the anatase TiO2 has received intense attention due to its inexpensive cost, strong optical absorption, favorable band edge positions and superior chemical stability. In the last decade, the nanometer-sized TiO2 has been shown to exhibit a wide range of electrical and optical properties, such as nanoscale electronics and optoelectronics, which rely mainly on the unique size and shape. Recently, various anatase TiO2 based devices such as the anatase TiO2 nanotube based memristor and the anatase TiO2 nano-film based memristor have been intensively studied due to their nonvolatile resistive switching performances. Furthermore, many conduction mechanisms have been used to elucidate the resistive switching behaviors of the anatase TiO2 based devices. However, the direct growth of anatase TiO2 nanowire arrays (NWAs) on the FTO substrate is still a challenge since there exists a large lattice mismatch of about 19% between the anatase TiO2 NWAs and the FTO substrate. Moreover, the Au/TiO2/FTO based device has not been reported and the resistive switching mechanism of the anatase TiO2 NWAs based memristor is still unclear. In this work, the anatase TiO2 NWAs with (101) preferred orientation are successfully grown on the FTO substrate by a facile one-step hydrothermal process. The resistive switching characteristics and resistive switching mechanism of the as-fabricated Au/TiO2/FTO memristor are investigated systemically. The result indicates that the Au/TiO2/FTO memristor exhibits nonvolatile bipolar resistive switching behavior. Meanwhile, the resistance ratio between high resistance state and low resistance state exceeds 20 at 0.1 V, which can be maintained over 103 s without significant degradation. In addition, the conduction mechanism of the low resistance state is governed by the ohmic conduction mechanism, while the trap-controlled space charge limited current conduction mechanism dominates the high resistance state. The resistive switching model of the Au/TiO2/FTO memristor is developed, and the resistive switching mechanism could be attributed to the formation and rupture of the conductive filaments relating to the localized oxygen vacancies. It demonstrates that the Au/TiO2/FTO memristor may be a potential candidate for the future nonvolatile memory applications.
Effects of oxide isolation layer on magnetic properties of L10 FePt film grown on Si substrate
Li Dan, Li Guo-Qing
2018, 67 (15): 157501. doi: 10.7498/aps.67.20180387
Abstract +
Magnetic force microscope (MFM) is a powerful tool to subtly detect the stray field distribution of magnetic film or particles on a sub-micrometer scale. Due to its huge uniaxial magnetocrystalline anisotropy (Ku~7107 erg cm-3) and high Currie temperature (TC~500℃), FePt alloy in an L10 phase is expected to be coated on the MFM tip to display high coercive force (Hc) and to improve the magnetic stability and MFM resolution. A grain size of~3 nm will be enough to overcome the super paramagnetism. However, the growing fresh FePt films must experience a high temperature annealing (exceeding 700℃) in order to transform their structures thoroughly from a soft A1 phase into the desired hard L10 phase. This brings the risk of diffusion between FePt coating layer and the underneath Si cantilever. Several admixtures have been attempted by other researchers to obtain granular films with FePt grains separated by oxides, with the purpose to prevent the diffusion from happening between FePt and Si. But apparently, it will be very difficult to fabricate a separated FePt grain exactly on the top of MFM tip. This is a critical factor to affect the MFM resolution. And discussion about the influence of the interface diffusion is avoided in most of published papers. Alternatively, some oxide isolation layers with higher melting temperature can be useful for separating the top FePt film from the bottom Si crystal. In this paper, MgO and SiO2 are selected as isolation layers, deposited by magnetron sputtering. Subsequently, the FePt films are deposited at 400℃ and annealed at different temperatures (500℃ to 800℃) for 2 h. The experimental results indicate that the diffusion between FePt and Si substrate always occurs in the absence of any isolation layer, leading to a reluctant maximum Hc of~5 kOe for 50 nm FePt film. However, the coercive force could remarkably exceed 10 kOe if an isolation layer is used. In the case of MgO, a maximum Hc of~12.4 kOe for 50 nm FePt could be stably measured. However, the annealing temperature must be lower than 600℃ to hold back the occurrence of brittle cracks in isolation layer. Because of the smaller lattice mismatch and expansion coefficient difference between SiO2 isolation layer and Si substrate, the highest annealing temperature could exceed 800℃ when replacing MgO with SiO2. The Hc of FePt film could be adjusted in a range from~5 kOe to~15 kOe by changing the annealing temperature. These findings greatly benefit the fabrication of FePt-based MFM tips with high Hc. And it is expected to be able to effectively enhance the resolution of MFM image.
Multipactor in parallel-plate transmission line partially filled with dielectric material
Zhai Yong-Gui, Wang Rui, Wang Hong-Guang, Lin Shu, Chen Kun, Li Yong-Dong
2018, 67 (15): 157901. doi: 10.7498/aps.67.20180351
Abstract +
Due to the poor conductivity of the dielectrics, if an electron collides with the dielectric material, a charge will be deposited on the surface as a consequence of the secondary electron emission. Thus, the multipactor process in dielectric-loaded microwave devices differs from those in metallic devices. The objective of this paper is to study the self-extinguishing physical mechanism of the multipactor in parallel-plate transmission lines partially filled with dielectric layers by particle-in-cell simulation. The self-consistent field generated by the electrons in the simulation is assumed to be neglected, since there do not exist too many electrons in the self-extinguishing process. To illustrate the self-extinguishing phenomenon in a dielectric-loaded waveguide device, the strength of electric field in the vacuum area needs to be the same as that in a metallic device. When the input power is slightly higher than the multipactor threshold, the self-extinguishing phenomenon occurs after the initial electron multiplication while the number of electrons increases exponentially with the simulation duration in metallic device. Based on this fact, the physical mechanism of self-extinguishing phenomenon is investigated in detail. By analyzing the temporal evolution of the electrons and the average secondary electron yield (SEY), it can be concluded that the self-extinguishing phenomenon is caused by the electrostatic field generated by the charges deposited on the surface of the dielectric. Moreover, the average SEY of the dielectric tends to be one or greater than one when the number of electrons drops to nearly zero. Hence, it is necessary to further analyze the ability to continue accumulating charges on the dielectric surface when extra electrons are injected into the simulation region at the instant when the number of electrons is close to zero. For the former case, the charges deposited on the dielectric surface remain steady all along, while the charges reach to a stable state eventually as the number of injected electrons increases for the latter one. Both of them mean that the average SEY of the dielectric surface will be unity in the end. Since the electrostatic field generated by the charge deposited on the dielectric surface can reduce the risk of occurrence of multipactor, the electret material could be used in the design of the dielectric-loaded microwave devices to improve the multipactor threshold.
Optical property of X-two ring structure
Pan Ting-Ting, Cao Wen, Deng Cai-Song, Wang Ming, Xia Wei, Hao Hui
2018, 67 (15): 157301. doi: 10.7498/aps.67.20172582
Abstract +
In this paper, the metal periodic array structure of X-two ring based on the principle of Fano resonance is proposed, which is composed of two concentric rings around the center X. The optical properties of the structure are investigated by using the finite difference time domain method. According to the simulated transmission spectra, electric field distribution and charge distribution, when linearly polarized light is incident to the metal surface, Fano resonance can be excited and the interaction between resonance modes can be produced in the structure of X-two ring, which can make resonance valleys generated at different positions. Fano resonance is mainly formed by the coherent interference between a bright mode with the larger radiation broadening and a dark mode with the weak radiation broadening, thus the structural resonance valley of X-two ring based on Fano resonance is strongly dependent on the relative parameters of the structure (the arm length of X, the distance between the inner ring and outer ring, the width of the inner ring and outer ring, the period, the number of ring, and the angle of X). In other words, over the wavelength range of 450 nm to 3000 nm, the intensity and position of the structural resonance valley are adjustable as the change of the relative geometric parameters of the structure. In addition, due to weak radiation damping and strong local electromagnetic field enhancement of Fano resonance, the resonance frequency and line type can significantly shift with the change of the environmental refractive index. Therefore, the further analysis of the variation of the structural resonance valley under the conditions of different refractive indices can be concluded that the structure of X-two ring has a higher sensitivity to the refractive index of surrounding environment, up to 1300 nm/RIU. The above results show that the structure of X-two ring not only is simple, economical, compact and efficient, but also has great potential applications in refractive index sensors and some photonic devices.
Correlation and coherence for two-qubit system coupled to XY spin chains
Yang Yang, Wang An-Min, Cao Lian-Zhen, Zhao Jia-Qiang, Lu Huai-Xin
2018, 67 (15): 150302. doi: 10.7498/aps.67.20180812
Abstract +
Quantum coherence has played a decisive role in quantum information processing. On the other hand, quantum correlation can be considered as a powerful resource for delivering quantum information. Both quantum coherence and quantum correlation may occur in an information propagating process, which challenges us to understand the relationship between coherence and correlation. This is also an important procedure for physicists to know the features of quantum resources. Any quantum system interacting with its surrounding environment will destroy the quantum coherence and fail to fulfil any task of delivering quantum information. In this sense, studying the dynamics of quantum correlation and quantum coherence is very fascinating. In this paper, we investigate the dynamics of the quantum correlation and quantum coherence for two central qubits coupled to their own spin baths modeled by the XY spin chain with Dzyaloshinsky-Moriya interaction. We employ the quantum discord to characterize the quantum correlation, and use the relative entropy to measure quantum coherence. In this way the evolution law of the quantum discord and the relative entropy of quantum coherence of two-qubit system are derived, and the evolution law depends not only on the Dzyaloshinsky-Moriya interaction, the anisotropy parameter and the total number of spin chain sites, but also on the coupling strength between the central spin and its spin chain. Our findings are as follows. Firstly, we find that near the critical point of spin chain the quantum coherence abruptly changes, which can be used to detect the existence of quantum phase transition. Secondly, at the critical point, the relative entropy of quantum coherence is the same as that of classical correlation when time tt0, and it is the same as that of quantum discord when time tt0. At time t0, the sudden transition from quantum discord to classical correlation occurs. All in all, the relative entropy of quantum coherence reflects the behaviors of classical correlation and quantum discord for times tt0 and tt0, respectively, which is caused by the change of the optimal basis for quantum discord. Thirdly, the dynamics of quantum correlation and quantum coherence keep invariant under the scaling variation of the total number of spin chain sites and the coupling strength. Moreover, we find that all the Dzyaloshinsky-Moriya interactions and the anisotropy parameters, as well as the coupling strengths will enhance the decay of quantum coherence and quantum correlation, while they have no obvious effect on the relationship between dynamics of coherence and correlation. The above discussion reveals some new features of quantum coherence and quantum correlation, which may be useful in further developing quantum information theory.
Translation compensation and micro-motion parameter estimation of laser micro-Doppler effect
Guo Li-Ren, Hu Yi-Hua, Dong Xiao, Li Min-Le
2018, 67 (15): 150701. doi: 10.7498/aps.67.20172754
Abstract +
Precise target identification is significant for commanding and identifying enemies. The micro-Doppler effect (MDE) can reflect the subtle movement characteristics of the targets, which provides a new way of detecting and recognizing the target. However, the current research mainly focuses on the micro-motion feature extraction and classification of the targets, which is not capable of identifying the targets of the same type. In fact, by accurately estimating the micro-motion parameters and combining sufficient prior knowledge, the target can be accurately identified. Compared with the microwave radar, the laser detected MDE has high sensitivity and precision in micro-motion parameter estimation. This is more conducive to realizing the accurate classification and fine identification of the targets. In real detection, the MDE always exists in the moving targets. This will generate a mixed echo signal modeled by the polynomial phase signal and sinusoidal frequency modulation (SFM) signal. So far, there have been no effective methods of estimating the micro-motion parameters in such mixed signals. In this regard, a set of translational motion compensation and micro-motion parameter estimation methods is proposed in this paper. A bandwidth searching method based on the fractional Fourier transform (FrFT) is presented to precisely estimate the translation parameters, which will be used to realize the compensation for the translational motion. The advanced particle filtering (PF) method using the static parameter model is designed for the micro-motion parameters in the remaining SFM term. Given the lack of particle diversity in static parameter PF, the Markov chain Monte Carlo sampling is employed, which also helps to improve the algorithm efficiency. Meanwhile, a new likelihood function in calculating the particle weights is designed by using the cumulative residual. With this improvement, the correct convergence under multi-dimensional parameter condition is guaranteed. The proposed method can avoid the influence from error transfer and achieve efficient and accurate estimation. Compared with the typical method that combines the time-frequency analysis and the polynomial fitting through the simulation, the proposed FrFT method is verified to have little computation complexity and high estimation accuracy, where the relative estimation errors of the translational parameters are kept at 0.64% and 0.45%, respectively. The waveform similarity of the SFM signal phase between the compensated signal and the real one indicates that the accuracy fully meets the requirement for accurate estimation of the micro-motion parameters. Further, the simulation result also shows the high efficiency of the improved PF algorithm. The convergence time consumed by the proposed algorithm is 0.353 s, while the traditional method needs 0.844 s. In the end, the comparison with the experimental data from the traditional inverse Radon transform shows the effectiveness and necessity of the proposed method. The research results are conducive to the accurate and rapid estimation of micro-motion parameters, which lays a foundation for the fine target recognition based on the MDE.
Chaotic analysis of fractional Willis delayed aneurysm system
Gao Fei, Hu Dao-Nan, Tong Heng-Qing, Wang Chuan-Mei
2018, 67 (15): 150501. doi: 10.7498/aps.67.20180262
Abstract +
The dynamic system of Willis aneurysm (WAS) has played an important role in theoretical and clinical research of cerebral aneurysms. Fractional differential is an effective mathematical tool that can describe the cerebral aneurysm system accurately and profoundly. However, the existing fractional Willis aneurysm system (FWAS) cannot describe the delayed aneurysm rupture of unknown cause in reality. Therefore, by introducing the time-delay factors into the existing fractional Willis aneurysm system as a rational extension, a new fractional Willis aneurysm system with time-delay (FWASTD) is proposed in this paper.First, FWASTD is introduced in the context, and the comparison of time sequences map between FWAS and FWASTD proves that FWASTD is feasible in the depiction of time-delay situations. The bifurcation diagram and the largest Lyapunov exponent diagram as well as the phase diagram of fractional order also confirm the chaotic characteristics of the FWASTD.Then, the classical analysis methods in chaotic dynamics, such as time series diagram, phase diagram and Poincar section are used to analyze FWASTD in detail. When studying the diagrams of time-delay factors for the important physiological parameters of the system, we find that blood flow resistance coefficient can exert a remarkable effect on the system stability under time-delay. Besides, the experimental results show that the FWASTD becomes stable with the increase of blood flow resistance under a certain condition. Usually, promoting thrombosis is a kind of adjunctive therapy in clinic for cerebral aneurysm. The results of this part can accord with the treatment in clinic and has great significance in clinical diagnosis.Finally, as the chaotic state of the time-delay system indicates that cerebral aneurysm is in a dangerous situation, the primary task of the control for this new system is to achieve stability rather than synchronization. The stability theory of fractional time-delayed system is adopted in a strict proof of the uniqueness of solution for the FWASTD. To make FWASTD stable, a corresponding linear controller is designed based on the stability theory of fractional order delay system. The numerical simulation indicates that the linear controller can control the blood flow velocity and speed up the periodic fluctuation within a small range, which illustrates that it is not easy to rupture the cerebral aneurysm. We also make self-synchronization control between FWASTD and FWAS just in case that the coefficients of the system are not clear.The research results in this paper, to some extent, can serve as theoretical guidance for the clinical diagnosis and the treatment of aneurysm.
Entanglement properties of multi-cascaded beamsplitter and its applications
Jia Fang, Zhang Kui-Zheng, Hu Yin-Quan, Zhang Hao-Liang, Hu Li-Yun, Fan Hong-Yi
2018, 67 (15): 150301. doi: 10.7498/aps.67.20180362
Abstract +
Beam splitter,as a kind of linear optics instruments,has many applications such as in quantum optics and quantum information,including the preparation of nonclassical quantum states and entangled state representation.In Heisenberg picture,on the one hand,the relation of input-output of beam splitter can be easily obtained.Especially for the multicascaded beam-splitters,the input-output relation can also be directly obtained by the input-output relation of single beam splitter.On the other hand,we often need to calculate the probabilities of detecting photon number in many cases,thus we need to turn into Schrdinger picture for simplifying our calculation.Based on the equivalence between both pictures,the relation between transformation matrixes connecting these two pictures is derived.That is to say, the transform matrix corresponding to the Schrdinger picture can be obtained by transposing the transform matrix in Heisenberg picture.This concise relation constructs a bridge connecting two pictures and simplifies our calculation in the Schrdinger picture rather than step by step.Using the relation between transform matrixes of both pictures and combining the technique of integration within ordered product of operator,we further consider the coordination representation,normally ordering form and exponential expression of single beam-splitter.Then we further examine the coordination representation,normally ordering form and exponential expression of two-cascaded beam-splitters.As a generalization,the method is extended to the case of multi-cascaded beam-splitters.These investigations provide an effective way to prepare multi-mode entangled states and qubit states.In addition,a general method is shown of obtaining the total operator and its normally ordering form as well as Schmidt decomposition of the linear systems consisting of beam-splitters.As applications,2-cascaded beam-splitters is used to generate a new quantum mechanics representation and prepare the qubit states with the help of conditional measurement.The Schmidt decomposition of three-mode entangled state representation can be directly obtained by the coordination representation of 2-cascaded beam-splitters,which shows the property of entanglement.In addition,based on this representation we can clearly see that when the input states of first beam splitter are two coordinate states,the output states cannot be entangled.This implies that although the coordinate states are nonclassical,the entangled state can not be prepared either.The new proposed quantum mechanics representation will be further used to investigate the optical transformations,including wavelet transformation,Fourier transform,fractional Fourier transform,et al.Therelevant discussion will be our aim in the future research.
Theoretical and experimental research on influence of cavity frequency difference in birefringent laser feedback system
Niu Hai-Sha, Zhu Lian-Qing, Song Jian-Jun, Dong Ming-Li, Lou Xiao-Ping
2018, 67 (15): 154201. doi: 10.7498/aps.67.20180230
Abstract +
The internal stress of glass material directly affects the processing quality of glass components and the service life of optical components. It is an important factor that relates to the overall system performance, safety, and reliability. Aerospace, precision optical systems, precision machining and other areas generally highly value the stress measurements of glass components. For example, the internal stress in the medium-glass material of precision imaging system will lead to the degradation of optical performance and reduce the image quality; the stress in the glass material used as the gain medium of high-power solid-state lasers not only directly affects the polarization state of the output light, but also shortens the service life of the laser; the stress concentration in the load-bearing glass of aircraft windshields, building glass curtain walls, etc., will cause serious accidents such as popping due to the reduction of glass mechanical properties. Therefore, the high sensitivity and large measurement range of stress detection technology has become a current research hotspot. Stress measurement techniques based on the birefringent external cavity laser feedback effect has received widespread attention due to its advanced and novel measurement principle. It is generally accepted in the traditional theory that the output phase of the laser in a feedback system is only determined by the phase retardation of birefringent element in an external cavity, and the measurement error is induced by the non-linear movement of external mirror. In this paper, the orthogonally polarized laser principle and the three-cavity equivalent model are combined to explain the influence of cavity frequency difference on the output of laser in feedback system. The frequency difference caused by the birefringence of the laser cavity is measured by comparing the intervals between adjacent longitudinal modes, and the frequency tuning feedback experiment is carried out. Theoretical analysis and experimental results show that the output phase of the laser is determined by the phase retardation of the external cavity, the frequency difference of the internal cavity, and the length of the external cavity. This conclusion is also confirmed by the measurement of the standard quarter wave plate. For a feedback system with an internal cavity frequency difference of 5 MHz and external cavity length of 150 mm, the phase difference induced by internal cavity frequency difference is about 0.573. The laser can output a single longitudinal mode below 40 MHz of the internal cavity frequency difference, and the length of the external cavity is generally larger than 150 mm when the actual system is designed, so the phase difference introduced by these two parameters cannot be ignored and must be calibrated. This study summarizes the phase characteristics of the orthogonally polarized laser under the joint of anisotropy feedback cavity, supplements the physical content of the laser feedback, and has great significance for accurate laser measurement of stress-birefringence, displacement, and distance.
Theoretical and numerical study on narrow-linewidth nanosecond pulsed Raman fiber amplifier
Su Rong-Tao, Zhang Peng-Fei, Zhou Pu, Xiao Hu, Wang Xiao-Lin, Duan Lei, Lü Pin, Xu Xiao-Jun
2018, 67 (15): 154202. doi: 10.7498/aps.67.20172679
Abstract +
Narrow-linewidth nanosecond pulsed Raman fiber amplifiers possess many applications such as in nonlinear frequency generation, remote sensing and quantum information. By considering nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), self-phase modulation (SPM) and cross-phase modulation (XPM), we build a nonlinear dynamical model of narrow-linewidth nanosecond pulsed Raman fiber amplifier. A numerical simulation model is also built and the simulation is carried out based on the parallelizable bidirectional finite difference time-domain method. The pulse evolution processes in time and spectral domain are simulated. The influences of pump pulse width, fiber length and signal laser power are studied in detail. It is found that SRS peak power threshold is not influenced by pump pulse width, however, pump pulse width will affect SBS threshold and output linewidth. When the pump pulse width is 800 ns, tens of MHz narrow linewidth can be obtained, but the SBS occurs as the increasing of pump energy, which limits the power scaling of the narrow-linewidth laser pulses. When the pump pulse width is 80 ns, the SBS is effectively suppressed and the peak power can be further increased, but the linewidth of output laser is easily broadened to hundreds of MHz. The simulation results also show that lower SRS threshold and higher efficiency can be obtained by using longer passive fiber, however, if shorter passive fiber is used, SPM and XPM can be weakened and narrower linewidth can be obtained. We build an experimental setup to study the influence of fiber length. In our experiment, a polarization-maintained passive fiber with a core diameter of 10 m and core numerical aperture of 0.08 is used as the Raman gain fiber. The signal laser is a 1120 nm single frequency continuous wave fiber laser with an average power of 20 mW, and the pump laser is a 1064 nm pulsed laser with a pulse width of~40 ns and repetition rate of 500 kHz. When the fiber lengths are 100 m and 80 m, the efficiencies of the pulsed Raman amplifier are, respectively, 51.5% and 38.2% at a pump power of 6.8 W. It can also be found that increasing signal power can increase the efficiency of the amplifier, but it will reduce the SBS threshold at the same time. Therefore, in order to balance the different nonlinear effects in the arrow-linewidth nanosecond pulsed Raman fiber amplifier, we should take laser power, linewidth and efficiency into consideration, and choose the suitable system parameters such as pump pulse width, fiber length and signal power. These analyses can serve as design guidelines for narrow-linewidth nanosecond pulsed fiber Raman amplifiers.
Influence of background gas on two-dimensional metal evaporation
Lu Xiao-Yong, Zhang Xiao-Zhang
2018, 67 (15): 154701. doi: 10.7498/aps.67.20180066
Abstract +
The spatial distributions of macroscopic parameters such as density, bulk velocity and temperature of the metal vapor have influences on the photo ionization yield of target isotope and the utilization ratio of material, which is related to the separation efficiency and the cost of atomic vapor laser isotope separation. To study this problem more practically, a system of binary gas Bhatnagar-Gross-Krook (BGK) model equations, which describe both the metal vapor and the background gas, is established. The physical characteristics are dealt with by dimensionless method for simplifying the calculations. The model equations are discretized by one-order upwind difference and then are solved by iteration method for obtaining stable results. The computational grids are adjusted to the flow field in order to acquire modest computational cost and accurate result simultaneously. Non-uniform grids in the phase space and in the velocity space are constructed to match the macroscopic parameter gradient and the form of the velocity distribution, respectively. The macroscopic parameters in the cases of different background gas densities, temperatures of tail plate and absorptivities are obtained for studying the influence of the background gas. The results show that with the increase of density of the background gas, the density and temperature of the metal vapor increase, the bulk velocities in the x and z$ direction decrease obviously in the domain far from the evaporation source, while the macroscopic parameters that are close to the evaporation source hardly change. As a result, the evaporation rate is not affected. Meanwhile, a circulation of the background gas is driven by the metal vapor, which in turn affects the diffusion of the metal vapor. Besides, as the temperature of tailing plate rises, the influence of the background gas on the macroscopic parameters of the metal vapor weakens. However, the rise of the temperature of tail plate leads the absorptivity of metal vapor to decrease, which enlarges the influence of the background gas. Therefore, it is appropriate to adjust the temperature of the tail plate based on the relationship between the absorptivity of metal vapor and the temperature. The results of theoretical calculation can serve as a reference for designing the vacuum and laser spot of the separation device.
Poly-L-lysine induced shape change of negatively charged giant vesicles
Sheng Jie, Wang Kai-Yu, Ma Bei-Bei, Zhu Tao, Jiang Zhong-Ying
2018, 67 (15): 158701. doi: 10.7498/aps.67.20180456
Abstract +
Decoration of biomembrane with polymer may improve its physical properties, biocompatibility, and stability. In this study, we employ the inverted fluorescence microscopy to characterize the polylysine (PLL) induced shape transformation of the negatively charged giant unilamellar vesicles (GUVs) in low ionic medium. It is found that PLL may be adsorbed to the 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1, 2-dioleoyl-sn-glycero-3-phosphatidic acid (DOPA) binary mixture vesicles, resulting in the attachment between the membranes, the formation of the ropes, and rupture of the GUVs. The response of GUVs generally is enhanced with the increase of the negatively charged DOPA in the membranes. The experimental observations are concluded as follows. Firstly, for the PLL induced attachment of GUVs, the attachment area grows gradually with time. Secondly, ropes can only be found in relatively large GUVs. However, the hollow structure is not discernable from the fluorescence imaging. Thirdly, after the rupture of GUVs, some phase-separated-like highly fluorescence lipid domains form in the adjacent intact vesicles. Through careful discussion and analysis, we show that on the one hand, the positively charged PLL adheres to the negatively charged membrane surface, bridging the neighboring GUVs and drawing the originally electrical repulsive vesicles together. The contact zone between GUVs expands with the increasing adsorption of PLL in this area. And the local high fluorescence areas in the GUVs originate from the PLL induced membrane attachment as well. Some membrane segments from ruptured vesicles are adsorbed to the particular areas of GUV, forming a few lipid patch structures above the latter membrane. On the other hand, PLL is adsorbed to the membrane area enriched in the negatively charged DOPA, reversing the surface charge of the upper leaflet and deteriorating the stability of the lipid bilayer. The original equilibrium of the system is broken by the change of the electrical interaction between the neighboring lipid domains as well as the interaction between the domain and water-dispersed PLL. The lipid packing density and inter-lipid force are affected by the PLL adsorption. Lipid membranes have to bud to release the stress built in the spontaneous curvature incompatibility in the two leaflets. The system may become stable again after buds grown into rods with a certain length. All in all, this study deepens the understanding of the interaction mechanism between lipid membrane and oppositely charged polymer. The conclusions obtained will provide valuable reference for the further studies on the polymer-GUV application areas including drug delivery, control release, cell deformation, micro-volume reaction, and gene therapy.
The method of suppressing spatial filter output noise-power gain for cardiac electrical activity imaging
Zhou Da-Fang, Zhang Shu-Lin, Jiang Shi-Qin
2018, 67 (15): 158702. doi: 10.7498/aps.67.20180294
Abstract +
For non-invasive imaging of cardiac electrical activity using magnetocardiogram (MCG) data measured on human body surface, a key problem that needs to be solved is to enhance the spatial resolution of reconstructing distributed current source dipole moment strength in MCG imaging. In this paper, a beamforming method of suppressing spatial filter output noise-power gain (SONG) is proposed based on the minimum variance beamforming (MVB). The purpose is to improve the resolution of the distributed source dipole moment strength reconstruction, i.e., the ability to resolve the source for distributed current source spatial spectrum estimation, in order to enhance the resolution of the cardiac electrical activity magnetic imaging. The method offers a new spatial filter weight matrix by using a low-trace positively-semidefinite matrix that will affect the spatial filter output power, on the premise that the influence of noise spatial spectrum of spatial filter on the estimation of current source spatial spectrum has been constrained by the noise spatial spectrum intensity normalization. The positively-semidefinite matrix is specially constructed to satisfy the condition that the eigenvalue is not greater than 1 and the trace of the matrix is lower than its order, so that it can be used to constrain the spatial filter output noise-power gain for improving the robustness to noise of the source spatial spectrum estimation. In addition, a classical model of the horizontally layered conductor is used as the heart-torso model to calculate the lead-field matrix that needs to be used in source spatial spectrum estimation. The results obtained in this study are as follows. For validating the proposed method, a theoretical analysis and simulation tests of the current source reconstruction are performed, where the SONG and MVB methods are compared and a parameter of the signal-to-noise ratio is considered according to the realistic MCG data. In this paper we also give the cardiac electrical activity imaging of 36-channel cardiac magnetic field data of single-cycle from two healthy people, where a heart profile from the magnetic resonance imaging is used as a reference and adjusted to the MCG measurement system. The results show that the SONG method has ability to better resolve the current source and can observe the significant electrophysiological characteristics such as the strong electrical activity in the ventricles of the healthy people at the time of Rpeak. In summary, our proposed method can improve the visual effect of the cardiac electrical activity imaging, when the signal-to-noise ratio of the single-cycle cardiac magnetic signal is not lower than 10 dB. Therefore, this method of measuring the non-invasively imaging cardiac electrical activity is a promising one and helpful for relevant medical research and applications.
Quality management of high-efficiency planar heterojunction organic-inorganic hybrid perovskite solar cells
Li Shao-Hua, Li Hai-Tao, Jiang Ya-Xiao, Tu Li-Min, Li Wen-Biao, Pan Ling, Yang Shi-E, Chen Yong-Sheng
2018, 67 (15): 158801. doi: 10.7498/aps.67.20172600
Abstract +
The energy extracted from solar radiation is the most abundant and accessible source of renewable energy, which will become progressively more important as time goes on. Solar cells are regarded as one of the most promising candidates for generating renewable clean energy. Recently, a new class of semiconducting material called organic-inorganic halide perovskite has received great attention of academia, and the record power conversion efficiency (PCE) of perovskite solar cell (PSC) rapidly increased from 3.8% in 2009 to 22.7% in late 2017 through intensive research due to some advantages as follows. 1) Excellent optoelectronic property. Perovskite materials exhibit excellent properties, including long diffusion length, high carrier mobility, and high absorption coefficient. 2) Low cost. The ingredients of perovskite materials are cheap, and PSCs can be manufactured by a solution process. 3) Tunable bandgap. Perovskite materials have highly tunable bandgap (1.2-2.2 eV), contributing to the further improvement in PCE of single junction PSCs by realizing the ideal bandgap (1.3-1.4 eV) as demonstrated by the Shockley-Queisser detailed balanced calculation. The basic architectures of PSCs are divided mainly into mesoscopic and planar heterojunction structures. Compared with the former configuration, the later configuration combined with low-temperature processable interlayers provides a method of fabricating flexible PSCs and tandem PSCs. Furthermore, the nonuse of the mesoscopic structure simplifies the structure of PSCs and reduces the cost and time of fabrication. The key requirement to achieve an efficient and reproducible planar heterojunction PSCs is that the perovskite layer should be uniform, continuous, and pinhole free to minimize shunting pathways. So, significant research effort is being devoted to the quality management of perovskite films with the goal of achieving the controllable preparation, including the optimization of their morphology (uniformity, coverage, roughness) and microstructure (grain size/distribution, texture), and the elimination of defects (voids, pinholes, grain boundaries), which influence the PSC performance directly. Especially for the one-step solution coating method, the film quality of perovskite on different planar substrates under varied deposition conditions exhibits a large difference, due to the complex crystallization process and the heightened sensitivity to environmental conditions. In this paper, the characteristics of perovskite materials, the nucleation-growth mechanism of films in the one-step solution method, and the evolution of cell structures are described briefly. The latest quality control methods of high-quality perovskite films prepared by solution method are then discussed emphatically. Finally, to provide references for the future research, the development and existing problems of PSCs are addressed and prospected.