Critical thickness for ferroelectricity in perovskite ultrathin films

The integration of ferroelectric oxide films into microelectronic devices, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 A (ten unit cells), ref. 3. Recent atomistic simulations have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric-electrode interface. We show that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (approximately 24 A). A depolarizing electrostatic field, caused by dipoles at the ferroelectric-metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.     

Artificial charge-modulationin atomic-scale perovskite titanate superlattices

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.     

钠米钙钛石型复合氧化物的研究与应用

介绍了纳米钙钛石犁复合氧化物的结构、制备方法以及在空气电极的电化学催化机理等方面的研究进展，并在现有研究的基础上，展望了该类型氧化物的发展前景。     

钙钛矿型复合氧化物降解染料废水的性能研究

以A,B的硝酸盐为原料,以氨水或碳酸钠为沉淀剂,利用混合沉淀的方法合成了系列钙钛矿型氧化物(ABO3)。合成中A选碱土金属元素Mg,Ca,Sr,Ba等及稀土金属元素La;B选过渡金属元素Cr,Mn,Fe,Co,Ni等。以酸性红B染料废水为目标物,考察了不同种ABO3复合氧化物光催化活性。活性测试结果表明,A的影响较小,B的影响较显著,总的变化趋势是随着B原子序数的增大光催化活性提高。在上述各催化剂中,SrNiO3的催化活性最高。TEM测试结果显示所合成SrNiO3为纯度较高的粒径为150~200 nm的近球形颗粒。考察了工艺条件对光催化降解酸性红B的影响:在废水溶液中加入H2O2后光催化活性提高,且随着加入量的增大有增长的趋势;提高溶液酸性有利于染料废水降解;酸性红B废水溶液初始质量浓度越高,光催化越难降解。     

Controlled properties of perovskite oxide films by engineering oxygen octahedral rotation

Complex perovskite oxides exhibit extremely rich physical properties in terms of magnetism, electrical transport, and electrical polarization characteristics due to the competition and coupling of many degrees of freedom. The B-site ions and O ions in perovskite form six-coordinated octahedral units, which are connected at a common vertex toward the basic framework of the perovskite oxide, providing a crucial platform to tailor physical properties. The rotation or distortion of the oxygen octahedra will tip the competing balance, leading to many emergent ground states. To further clarify the subtle relationship between emergent properties and oxide octahedral behavior, this article reviews the structure of perovskite oxides, the characterization methods of oxygen octahedral rotation and the response of transport, electrical polarization and magnetism of several typical perovskite heterostructures to oxygen octahedral rotation modes. With knowledge of how to manipulate the octahedral rotation behavior and regulate the physical properties of perovskite oxides, rationally designing the sample manufacturing process can effectively guide the development and application of novel electronic functional materials and devices.     

Room-temperature ferroelectricity in strained SrTiO3

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.     

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.     

道德教育普及的经验与误区

历代许多道德普及的经验，如尊崇孔子，奉儒家著作为经典；编写蒙书，用通俗浅近的方式教育人；修祠堂立牌坊，编方志，表彰遵从儒家道德的人，利用各种文艺形式对人民潜移默化。日本、朝鲜、越南、新加坡等国在这方面也有许多行之有效的经验。但近几十年来在这方面却出现一些误区。如盲目学外国，极左思潮干扰，理论脱离实际，不重视普及工作等。致使道德教育不力，与社会主义建设不适应。只有使道德教育普及，才能更好地建设社会主义精神文明，才能更好地继承中国革命传统和中华民族传统美德，提高国民素质，加快四化建设步伐。     

Structural diversity in binary nanoparticle superlattices

Assembly of small building blocks such as atoms, molecules and nanoparticles into macroscopic structures--that is, 'bottom up' assembly--is a theme that runs through chemistry, biology and material science. Bacteria, macromolecules and nanoparticles can self-assemble, generating ordered structures with a precision that challenges current lithographic techniques. The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice (BNSL) can provide a general and inexpensive path to a large variety of materials (metamaterials) with precisely controlled chemical composition and tight placement of the components. Maximization of the nanoparticle packing density has been proposed as the driving force for BNSL formation, and only a few BNSL structures have been predicted to be thermodynamically stable. Recently, colloidal crystals with micrometre-scale lattice spacings have been grown from oppositely charged polymethyl methacrylate spheres. Here we demonstrate formation of more than 15 different BNSL structures, using combinations of semiconducting, metallic and magnetic nanoparticle building blocks. At least ten of these colloidal crystalline structures have not been reported previously. We demonstrate that electrical charges on sterically stabilized nanoparticles determine BNSL stoichiometry; additional contributions from entropic, van der Waals, steric and dipolar forces stabilize the variety of BNSL structures.     

对我国家庭教育误区的心理学探讨

比较美国与中国的家庭教育，不难发现我国家庭教育中存在着种种误区，包括过度照顾、“完美”教育、“成龙”心结、权利侵占等，问题主要反映在家长的心态上，表现为虚荣心理、补偿心理、焦虑心理、从众心理与本位心理等。要想改变我国家庭教育的现状，克服家庭教育中的偏见，关键在于提高家长的心理素养，运用心理学知识培养孩子健全的个性。     

旅游文本汉译英中“失当信息”处理

旅游文本翻译的预期功能是传递信息和吸引游客。由于语言文化的“异质”，中英旅游文本在表达方式上有很大的不同。从英语的视角分析，汉语旅游文本在语言表达上存在失当信息即信息夸大、信息误导、信息冗余。着重从这三方面进行例析。     

师生互动中信息传递操作误差与课堂窘迫

师生互动是社会关系和人际关系在课堂上的直接体现。教师在师生互动过程中出现信息传递操作误差,影响到师生交往层面,从而引起课堂窘迫。信息传递操作误差的起因及其矫正办法需从三个方面着手,即课堂语言,课堂提问,课堂纠错。     

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Materials in which magnetic and electric order coexist--termed 'multiferroics' or 'magnetoelectrics'--have recently become the focus of much research. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments but here is found to impede long-range order of the structural degrees of freedom that drive the formation of the ferroelectric state.     

2018年钙钛矿太阳能电池研发热点回眸

 2018年是钙钛矿太阳能电池发展的第10年，国际上该领域取得了一系列重要进展。从器件效率的持续刷新，钙钛矿材料和器件稳定性问题的解决，器件到模块化制备以及柔性和半透明电池的应用等方面简要介绍了代表性研究进展。     

Strong polarization enhancement in asymmetric three-component ferroelectric superlattices

Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.     

Strain-induced metal-insulator phase coexistence in perovskite manganites

The coexistence of distinct metallic and insulating electronic phases within the same sample of a perovskite manganite, such as La(1-x-y)Pr(y)Ca(x)MnO3, presents researchers with a tool for tuning the electronic properties in materials. In particular, colossal magnetoresistance in these materials--the dramatic reduction of resistivity in a magnetic field--is closely related to the observed texture owing to nanometre- and micrometre-scale inhomogeneities. Despite accumulated data from various high-resolution probes, a theoretical understanding for the existence of such inhomogeneities has been lacking. Mechanisms invoked so far, usually based on electronic mechanisms and chemical disorder, have been inadequate to describe the multiscale, multiphase coexistence within a unified picture. Moreover, lattice distortions and long-range strains are known to be important in the manganites. Here we show that the texturing can be due to the intrinsic complexity of a system with strong coupling between the electronic and elastic degrees of freedom. This leads to local energetically favourable configurations and provides a natural mechanism for the self-organized inhomogeneities over both nanometre and micrometre scales. The framework provides a physical understanding of various experimental results and a basis for engineering nanoscale patterns of metallic and insulating phases.     

Superconductivity in the non-oxide perovskite MgCNi3

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.