Room-temperature electronic phase transitions in the continuous phase diagrams of perovskite manganites

Highly correlated electronic systems--such as transition-metal oxides that are doped Mott insulators--are complex systems which exhibit puzzling phenomena, including high-temperature superconductivity and colossal magnetoresistivity. Recent studies suggest that in such systems collective electronic phenomena are important, arising from long-range Coulomb interactions and magnetic effects. The qualitative behaviour of these systems is strongly dependent on charge filling (the level of doping) and the lattice constant. Here we report a time-efficient and systematic experimental approach for studying the phase diagrams of condensed-matter systems. It involves the continuous mapping of the physical properties of epitaxial thin films of perovskite manganites (a class of doped Mott insulator) as their composition is varied. We discover evidence that suggests the presence of phase boundaries of electronic origin at room temperature.     

Mn基钙钛矿样品相图修正及庞磁电阻起因

对具有不同A位平均离子尺寸的AMnO3型Mn基钙钛矿样品的阻温关系和电子顺磁共振谱等进行了实验研究．结果表明,在温度与A位平均离子尺寸的相图上,除了普遍公认的高温顺磁绝缘体相和低温长程铁磁有序的金属相外,在两相之间还存在一过渡相,其起因源于顺磁绝缘体背景上出现短程铁磁有序的金属集团所致．研究结果还表明,外加磁场加速铁磁金属集团的生成和长大是导致庞磁电阻效应的主要原因．     

Gas-liquid phase coexistence and finite-size effects in a two-dimensional Lennard-Jones system

The gas-liquid phase coexistence in a two-dimensional Lennard-Jones system is investigated using Maxwell construction method together with molecular dynamics simulations.The results of phase coexistence in different truncations of the potential are compared with data obtained from the literature,and the corresponding critical properties calculated.The crossover from Ising-like to mean field behavior is observed and confirmed as the temperature approaches the critical point from below.Performing simulations on systems with different sizes,we find that a finite size effect is more significant than those shown in most of the previous results,and a lower critical temperature is obtained when the full extent of this finite size effect is considered.     

Electronically soft phases in manganites

The phenomenon of colossal magnetoresistance in manganites is generally agreed to be a result of competition between crystal phases with different electronic, magnetic and structural order; a competition which can be strong enough to cause phase separation between metallic ferromagnetic and insulating charge-modulated states. Nevertheless, closer inspection of phase diagrams in many manganites reveals complex phases where the two order parameters of magnetism and charge modulation unexpectedly coexist. Here we show that such experiments can be naturally explained within a phenomenological Ginzburg-Landau theory. In contrast to models where phase separation originates from disorder or as a strain-induced kinetic phenomenon, we argue that magnetic and charge modulation coexist in new thermodynamic phases. This leads to a rich diagram of equilibrium phases, qualitatively similar to those seen experimentally. The success of this model argues for a fundamental reinterpretation of the nature of charge modulation in these materials, from a localized to a more extended 'charge-density wave' picture. The same symmetry considerations that favour textured coexistence of charge and magnetic order may apply to many electronic systems with competing phases. The resulting 'electronically soft' phases of matter with incommensurate, inhomogeneous and mixed order may be general phenomena in correlated systems.     

含Nb多元微合金钢的应变诱导析出行为

利用松弛法,在Gleeble-1500热模拟机上测定了多元微合金钢的应力弛豫曲线,并采用萃取复型技术跟踪了析出物的长大过程。实验结果表明：应力弛豫曲线分为三个阶段,分别对应析出的形核、长大和粗化：在850℃变形后等温弛豫60-200s时,应变诱导析出颗粒的尺寸仍然小于10nm,析出相为复杂的（Nb,Ti,Mo）（C,N）。     

Giant magneto-elastic coupling in multiferroic hexagonal manganites

The motion of atoms in a solid always responds to cooling or heating in a way that is consistent with the symmetry of the given space group of the solid to which they belong. When the atoms move, the electronic structure of the solid changes, leading to different physical properties. Therefore, the determination of where atoms are and what atoms do is a cornerstone of modern solid-state physics. However, experimental observations of atomic displacements measured as a function of temperature are very rare, because those displacements are, in almost all cases, exceedingly small. Here we show, using a combination of diffraction techniques, that the hexagonal manganites RMnO3 (where R is a rare-earth element) undergo an isostructural transition with exceptionally large atomic displacements: two orders of magnitude larger than those seen in any other magnetic material, resulting in an unusually strong magneto-elastic coupling. We follow the exact atomic displacements of all the atoms in the unit cell as a function of temperature and find consistency with theoretical predictions based on group theories. We argue that this gigantic magneto-elastic coupling in RMnO3 holds the key to the recently observed magneto-electric phenomenon in this intriguing class of materials.     

Nodal quasiparticle in pseudogapped colossal magnetoresistive manganites

A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction. Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs'. This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity or by invoking orders which are natural competitors of d-wave superconductors. Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La1.2Sr1.8Mn2O7. Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state.     

金属一绝缘颗粒复合介质逾渗阈值的研究

在金属—绝缘颗粒复合介质中 ,当金属颗粒的浓度达到渗流阈值时 ,复合介质发生金属—绝缘体转变 采用有效介质近似 (EMA)研究了颗粒系统中阈值与颗粒形状、分布的关系 ,讨论了逾渗阈值与退极化因子的关系     

Magnetization distribution in the mixed-phase state of hole-doped manganites

The effect of 'colossal magnetoresistance' (CMR) in hole-doped manganites--an abnormal decrease of resistivity when a magnetic field is applied--has attracted significant interest from researchers in the past decade. But the underlying mechanism for the CMR phenomenon is not yet fully understood. It has become clear that a phase-separated state, where magnetic and non-magnetic phases coexist, is important, but the detailed magnetic microstructure of this mixed-phase state is so far unclear. Here we use electron microscopy to study the magnetic microstructure and development of ferromagnetic domains in the mixed-phase state of La(1-x)Sr(x)MnO3 (x = 0.54, 0.56). Our measurements show that, in the absence of a magnetic field, the magnetic flux is closed within ferromagnetic regions, indicating a negligible magnetic interaction between separated ferromagnetic domains. However, we also find that the domains start to combine with only very small changes in temperature. We propose that the delicate nature of the magnetic microstructure in the mixed-phase state of hole-doped manganites is responsible for the CMR effect, in which significant conduction paths form between the ferromagnetic domains upon application of a magnetic field.     

Strain-induced modulation of magnetic anisotropy in Co/BaTiO_3 composite

A uniaxial magnetic anisotropy Co film was grown on a single-crystal Ba Ti O3（BTO） substrate. The strain yielded by the voltage-induced ferroelastic domain switching in the BTO substrate was recorded by atomic force microscope and modulated the magnetism of the Co film. The manipulation of the magnetism of the Co film is experimentally demonstrated by voltage dependence of magnetic hysteresis loops measured via magneto-optic Kerr effect.     

烧结温度对多铁Bi_(0.6)La_(0.4)MnO_3陶瓷钙钛矿相的影响

采用溶胶-凝胶法制备多铁Bi0.6La0.4MnO3(BLMO)系列陶瓷样品,研究烧结温度对BLMO钙钛矿相稳定性的影响.X射线衍射(XRD)、傅里叶红外光谱(FT-IR)、差热-热重分析(TG-DTA)及扫描电子显微镜(SEM)等结果显示:当烧结温度升到533℃时,BLMO样品的钙钛矿相减弱,同时伴有Bi2Mn4O10中间相的出现,这是由BLMO相分解所致;当烧结温度升高到770℃,Bi2Mn4O10与Bi2O3反应生成稳定的BLMO钙钛矿相;进一步升高温度到925℃,样品仅有单一的BLMO钙钛矿相.研究表明:为了得到稳定的BLMO钙钛矿相,较高的烧结温度是必需的.     

四种操作系统共存于同一台计算机中的实现

针对很多计算机应用场合 ,要求一台计算机具有多种职能 ,需要多种操作系统支持的需求 ,本文讨论了多种操作系统共存于一台计算机中的技术     

具有长程关联性的二维金属-绝缘体复合材料光学性质的数值研究

主要利用电阻-电感-电容的二维方格模型研究了具有长程关联性的二维金属-绝缘体复合材料的光学性质.我们采用了Frank和Lobb所发展的Y-Δ转换算法进行数值计算.并且利用修正的傅立叶过滤方法产生符合长程关联性〈xixi l〉~l-γ的随机数,其中γ是关联指数.我们发现如果复合材料中的颗粒具有长程关联性,则它的远红外吸收要强于颗粒随机分布的复合材料,而且关联性材料光学吸收也符合ω2的正比关系.同时还发现随着关联指数γ的增大(空间关联性减弱),材料的渗流阈值会增大,而表面等离子吸收峰也会增强.     

Improper ferroelectricity in perovskite oxide artificial superlattices

Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds. Here we show that superlattices with very short periods possess a new form of interface coupling, based on rotational distortions, which gives rise to 'improper' ferroelectricity. These observations suggest an approach, based on interface engineering, to produce artificial materials with unique properties. By considering ferroelectric/paraelectric PbTiO3/SrTiO3 multilayers, we first show from first principles that the ground-state of the system is not purely ferroelectric but also primarily involves antiferrodistortive rotations of the oxygen atoms in a way compatible with improper ferroelectricity. We then demonstrate experimentally that, in contrast to pure PbTiO3 and SrTiO3 compounds, the multilayer system indeed behaves like a prototypical improper ferroelectric and exhibits a very large dielectric constant of epsilon(r) approximately 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications, is distinct from that of normal ferroelectrics, for which the dielectric constant is typically large but strongly evolves around the phase transition temperature and also differs from that of previously known improper ferroelectrics that exhibit a temperature-independent but small dielectric constant only.     

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.     

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。     

Investigation of the thermal conductivities across metal-insulator transition in polycrystalline VO2

Previous reports about the thermal conductivities of VO2 showed various temperature dependences across metal-insulator transition (MIT) temperature. In this work, polycrystalline VO2 samples were fabricated by spark plasma sintering of VO2 powder. Temperature dependences of their thermal conductivities were investigated using laser flash technique, and the thermal conductivity showed a significant decrease trend from metal-phase to insulator phase. Electrical transport properties were investigated to confirm both carrier and lattice contribution to the thermal conductivity. It is found that the lattice thermal conductivity decreased significantly across MIT point, which may be caused by soft phonon mode in metal phase of VO2 .     

Shape coexistence and triaxiality in the superheavy nuclei

Superheavy nuclei represent the limit of nuclear mass and charge; they inhabit the remote corner of the nuclear landscape, whose extent is unknown. The discovery of new elements with atomic numbers Z > or = 110 has brought much excitement to the atomic and nuclear physics communities. The existence of such heavy nuclei hangs on a subtle balance between the attractive nuclear force and the disruptive Coulomb repulsion between protons that favours fission. Here we model the interplay between these forces using self-consistent energy density functional theory; our approach accounts for spontaneous breaking of spherical symmetry through the nuclear Jahn-Teller effect. We predict that the long-lived superheavy elements can exist in a variety of shapes, including spherical, axial and triaxial configurations. In some cases, we anticipate the existence of metastable states and shape isomers that can affect decay properties and hence nuclear half-lives.