Two-dimensional electron gas with universal subbands at the surface of SrTiO(3)

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ～0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.     

凝胶-浇注法制备LaxSr1-1.5xTiO3阳极材料及其性能的研究

采用凝胶-浇注法合成钙钛矿复合系的阳极材料LaxSr1-1.5xTiO3(x=0.1～0.4),探究La掺杂量对相组成、烧结性能、力学性能及高温电导率等的影响.LaxSr1-1.5xTiO3粉体在1 100℃下预烧可得到典型的钙钛矿结构.在1 400℃下煅烧得到的烧结体的电导率数据显示:当x=0.3时电导率最高,在测试温度为800℃时高达180 S.cm-1,与传统固相法相比,其烧结温度降低了约200℃.     

A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface

Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.     

Synthesis of crystalline perovskite-structured SrTiO<Subscript>3</Subscript> nanoparticles using an alkali hydrothermal process

We report an experimental route for synthesizing perovskite-structured strontium titanate (SrTiO₃) nanocubes using an alkali hydrothermal process at low temperatures without further heating. Furthermore, we studied the influence of heating time (at 180℃) on the crystallinity, morphology, and perovskite phase formation of SrTiO₃. The SrTiO₃ powder, which is formed via nanocube agglomeration, transforms into cubic particles with a particle size of 120–150 nm after 6 h of hydrothermal sintering. The crystallinity and percentage of the perovskite phase in the product increased with heating time. The cubic particles contained 31.24at% anatase TiO₂ that originated from the precursor. By varying the weight ratio of anatase TiO₂ used to react with the strontium salt precursor, we reduced the anatase-TiO₂ content to 18.8at%. However, the average particle size increased when the anatase-TiO₂ content decreased.     

The strength of Mg(0.9)Fe(0.1)SiO3 perovskite at high pressure and temperature

The Earth's lower mantle consists mainly of (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiowüstite, with the perovskite taking up at least 70 per cent of the total volume. Although the rheology of olivine, the dominant upper-mantle mineral, has been extensively studied, knowledge about the rheological behaviour of perovskite is limited. Seismological studies indicate that slabs of subducting oceanic lithosphere are often deflected horizontally at the perovskite-forming depth, and changes in the Earth's shape and gravity field during glacial rebound indicate that viscosity increases in the lower part of the mantle. The rheological properties of the perovskite may be important in governing these phenomena. But (Mg,Fe)SiO3 perovskite is not stable at high temperatures under ambient pressure, and therefore mechanical tests on (Mg,Fe)SiO3 perovskite are difficult. Most rheological studies of perovskite have been performed on analogous materials, and the experimental data on (Mg,Fe)SiO3 perovskite are limited to strength measurements at room temperature in a diamond-anvil cell and microhardness tests at ambient conditions. Here we report results of strength and stress relaxation measurements of (Mg(0.9)Fe(0.1))SiO3 perovskite at high pressure and temperature. Compared with the transition-zone mineral ringwoodite at the same pressure and temperature, we found that perovskite is weaker at room temperature, which is consistent with a previous diamond-anvil-cell experiment, but that perovskite is stronger than ringwoodite at high temperature.     

NO在Rh(100)与(111)表面上吸附理论研究

基于slab模型,采用密度泛函理论研究了NO在Rh(100)和(111)表面上吸附的几何与电子结构.结果表明,在开放的(100)面和密堆积的(111)面上,优势吸附位分别是桥位和hcp位.在两表面上的优势吸附结构中,NO的吸附主要是其1π和5σ轨道分别与Rh的5s和4d态混合的结果.NO分子吸附后,均存在两种相同的电荷迁移过程:即NO的1π和5σ电子向金属表面的转移以及金属表面电子向NO的2π轨道上的反馈.这两种电子迁移均使得N-O键被活化,N-O键被消弱,N-O键长增加.     

Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.     

二元系SrTiO3_CaTiO3固溶体的XRD研究

进行了SrTiO3_CaTiO3系统的X_射线衍射分析 ,结果表明 ,当Ca2 +置换Sr2 +而形成ABO3钙钛矿型固溶体时 ,相对衍射强度 (I/I1) 10 0 减弱 .通过测定晶体结构的晶胞参数 ,进而比较衍射强度理论值及实测值 ,可以证明 (I/I1) 10 0 的减弱与ABO3钙钛矿型结构中A位离子的原子散射因子有关 ,由于钙的原子散射因子小于锶的原子散射因子 ,导致 (Sr ,Ca)TiO3固溶体的相对衍射强度 (I/I1) 10 0 减弱 .另外 ,通过比较衍射强度I110 /I111的实测值与计算值 ,还获得了A位离子的占位情况 .研究结果为SrTiO3基陶瓷材料的微量物相鉴定及微观结构分析提供了依据     

全钙钛矿La0.5Sr0.5CoO3/PbZr0.53Ti0.47O3/La0.5Sr0.5CoO3异质结的结构与性能

应用射频磁控溅射法在SrTiO3(STO)基片上制备了全钙钛矿结构La0.5Sr0.5CoO3/PbZr0.53Ti0.47O3/ La0.5Sr0.5CoO3(LSCO/PZT/LSCO)电容器异质结,并进行了结构和性能的表征.X射线衍射(XRD)的研究表明,LSCO/PZT/LSCO异质结在SrTiO3(STO)基片上为外延生长.对该电容器铁电性能的研究发现,在5 V驱动电压下,电滞回线饱和趋势良好,矫顽场电压为1.8 V和剩余极化强度为21.5×10-6 C/cm2,漏电流为8.9×10-8 A/cm2.实验还证实该电容器具有良好的脉冲宽度依赖性及抗疲劳特性.     

(La,Sr)(Co,Fe)O_3与(La,Sr)MnO_3阴极材料的表征与性能

采用氨水沉淀法分别制备La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)与La0.8Sr0.2MnO3-δ(LSM)两种阴极材料,并用X射线衍射(XRD)、场发射扫描电子显微镜(FESEM)和比表面积(BET)对所制备材料的结构、晶粒大小、形貌及比表面积进行表征.结果表明,LSCF和LSM前驱体分别在900℃和1200℃煅烧5h后产物均形成单一正交的钙钛矿结构,但LSCF具有较小的粒径和较大的比表面积.电化学性能测试结果表明,LSCF阴极比LSM阴极的过电位低,且LSCF为阴极单电池的输出功率较高.     

Imaging the atomic arrangements on the high-temperature reconstructed alpha-Al2O3(0001) surface.

Alumina is a technologically important oxide crystal because of its use as a catalyst and as a substrate for microelectronic applications. A precise knowledge of its surface atomic structure is a prerequisite for understanding and controlling the physical processes involved in many of its applications. Here we use a dynamic scanning force microscopy technique to image directly the atomic structure of the high-temperature phase of the alpha-Al2O3(0001) surface. Evidence for a surface reconstruction appears as a grid of protrusions that represent a rhombic unit cell, and we confirm that the arrangement of atoms is in the form of surface domains with hexagonal atomic order at the centre and disorder at the periphery. We show that, on exposing the surface to water and hydrogen, this surface structure is important in the formation of hydroxide clusters. These clusters appear as a regular pattern of rings that can be explained by self-organization processes involving cluster-surface and cluster-cluster interactions. Alumina has long been regarded as the definitive test for atomic-resolution force microscopy of insulators so the whole class of insulating oxides should now open for direct atomic-scale surface investigations.     

退火气氛对SrTiO3薄膜电学性能的影响

在不同的退火气氛下,采用金属有机物分解法在(111)Pt/Ti/SiO2/Si衬底上制备了SrTiO3薄膜材料,并对其结构和电学性能进行了研究。X射线衍射结果显示SrTiO3薄膜都呈现多晶立方钙钛矿结构。对比不同气氛下制备出的SrTiO3样品电学测量结果,发现在氧气氛中退火后,样品具有较低的漏电流和较好的介电性能,这可能是由于氧气氛中退火减少了SrTiO3薄膜内氧空位的浓度引起的。     

Local switching of two-dimensional superconductivity using the ferroelectric field effect

Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity and colossal magnetoresistance. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters--in particular the doping level--so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with 'perfect' interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres.     

双掺杂钙钛矿型复合氧化物La_(0.7)Sr_(0.3)Co_(1-x)M_xO_3的电性能研究

以开发新型激光器阴极材料和固体氧化物燃料电池阴极材料为目的 ,合成一系列钙钛矿型复合氧化物 La0 .7Sr0 .3 Co1- x Mx O3 ( M:过渡族金属元素 ) ,并对 Co位掺杂过渡族元素后样品的高温电阻率和热电动势进行研究 ,发现 Co位掺杂不同元素显著影响材料的电导率和导电类型 ,并对其相应的物理机制进行探讨 .     

Ba1-xSrxTiO3(BST)/钙钛矿型氧化物超导薄膜异质结构的研究及发展

钛酸锶钡(Ba1-xSrxTiO3(BST))铁电薄膜材料在集成铁电器件领域具有很大的优势和潜力,BST与铂(Pt)等贵金属电极材料的界面特性限制了BST的实际应用,氧化物超导薄膜代替贵金属材料做底部电极取得了不少突破性的进展,综述了国内外研究人员对Ba1-xSrxTiO3(BST)/钙钛矿型氧化物超导薄膜异质结构和基本介电性能等方面的研究及成果。     

Bi3．15Nd0．85Ti3O12纳米管阵列的制备与微结构表征

采用溶胶-凝胶法,在孔径为100nm的阳极氧化铝模板中制备了Bi3．15Nd0．85Ti3O12（BN（IT）纳米管阵列．通过XRD、SEM、TEM、HRTEM、SAED和Raman光谱测试手段对纳米管阵列的物相、微结构和声子振动特性进行了表征．研究表明,所合成BNdT纳米管为钙钛矿相多晶结构,纳米管外径约为100nm,管壁厚约10nm,长度达几个微米．Raman光谱分析表明,Nd离子部分取代了类钙钛矿层中A位的Bi离子．     

硅衬底SrTiO3薄膜的热敏特性

利用氩离子束镀膜技术在SiO2/Si衬底上淀积钛酸锶（SrTiO3）膜,并制成平面型电阻器。结果表明：在实验温区（28～150℃）内,SrTiO3薄膜具有负温度系数电阻特性,且热敏牧场生比较明显,在室温30℃时,温度系数α达－2．15％℃^-1。建立热敏电阻－电容器并联模型,分析了频率对不同温度下薄膜电阻器阻拢的影响。在实验温区内,SrTiO3薄膜的介电常数具有较好的热稳定性。     

Ag掺杂类钙钛矿Ca(Mn_2Cu_1)Mn_4O_(12)的电磁性能

采用固相反应法制备了系列Ca(Mn2Cu1)Mn4O12/mAg2O多晶类钙钛矿锰氧化物。X射线衍射表明,m≤0.3的样品呈比较纯净的类钙钛矿相,而m>0.3的样品,是由类钙钛矿和金属Ag相组成的复合物。所有样品呈现半导体导电性质,电阻率低于未掺杂的Ca(Mn2Cu1)Mn4O12样品。居里温度测量表明,随着Ag掺杂量的增加,样品的居里温度逐渐升高。对于m=0.3和m=0.6的样品,120K温度和1T磁场下磁电阻比可达-10%左右。     

Impedance effect of manganite thin film-based photodetectors

We report on the photodetector structures based on perovskite manganite La0.67Ca0.33MnO3 thin films on tilted SrTiO3 （001） substrates. The photovoltaic effect has been observed in response to excitation by 308 nm ultraviolet laser pulse irradiation in duration of 20 ns at room temperature. The outputs obtained required no amplification. To reduce the deformation of the signal detected, a series of testing measurements were made to investigate the impedance effect. When the impedance at the oscilloscope end matched to the co-axis cable, the signal trace was almost triangular and symmetrical, with response time equal to the excitation laser. In addation, the response linearly depends on the irradiated area for low on-sample energy. The devices work well under unbiased conditions and so are simple to configure for practical applications.     

面向复合材料带隙预测的两段式集成学习模型构建

带隙是钙钛矿型复合氧化物材料重要的特征参数, 对材料的物理化学性质起决定性作用, 如导电性能和光电性能等. 为了寻找适合不同应用领域的钙钛矿型材料, 利用机器学习进行带隙预测是一种重要的研究手段. 构建了一个两阶段异质集成学习模型, 在第一阶段使用多种不同的基础机器学习器(回归模型)进行预测; 在第二阶段把对预测结果影响较大的描述子和基础机器学习器进行集成学习.利用该模型对 210 种钙钛矿型复合氧化物材料的带隙进行预测, 并与多种独立的机器学习算法以及不同集成策略模型的预测性能相对比, 评估了本模型的预测性能. 结果表明, 这种两段式的集成学习模型能够更好地学习到材料数据的内在关系, 并具有较好的预测效果和较强的泛化能力.