Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped 'YBCO' materials YBa2Cu3O(y) and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3O(y) and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (T(c)) superconductors.     

Probing bulk states of correlated electron systems by high-resolution resonance photoemission

Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds. Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions. High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique. Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref. 6), it should become feasible to probe the bulk electronic states in these materials. Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2. Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands. Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences.     

Small phonon contribution to the photoemission kink in the copper oxide superconductors

Despite over two decades of intense research efforts, the origin of high-temperature superconductivity in the copper oxides remains elusive. Angle-resolved photoemission spectroscopy experiments have revealed a kink in the dispersion relations (energy versus wavevector) of electronic states in the copper oxides at binding energies of 50-80 meV, raising the hope that this anomaly could be a key to understanding high-temperature superconductivity. The kink is often interpreted in terms of interactions between the electrons and a bosonic field. Although there is no consensus on the nature of the bosons (or even whether a boson model is appropriate), both phonons and spin fluctuations have been proposed as possible candidates. Here we report first-principles calculations of the role of phonons and the electron-phonon interaction in the photoemission spectra of La(2 - x)Sr(x)CuO4. Our calculations within the standard formalism demonstrate that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments. Therefore, our result rules out electron-phonon interaction in bulk La(2 - x)Sr(x)CuO4 as the exclusive origin of the measured kink. Our conclusions are consistent with those reached independently in a recent study of the related compound YBa2Cu3O7.     

Designing fast oxide-ion conductors based on La2Mo2O9

The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite, perovskites, intergrowth perovskite/Bi2O2 layers and pyrochlores. Here we report a family of solid oxides based on the parent compound La2Mo2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors, this material undergoes a structural transition around 580 degrees C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 x 10(-2) S cm(-1) at 800 degrees C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with beta-SnWO4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation that does not have a lone pair--and which has a higher oxidation state--could be used as an original way to design other oxide-ion conductors.     

Fluctuating stripes at the onset of the pseudogap in the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+x)

Doped Mott insulators have a strong propensity to form patterns of holes and spins often referred to as stripes. In copper oxides, doping also gives rise to the pseudogap state, which can be transformed into a high-temperature superconducting state with sufficient doping or by reducing the temperature. A long-standing issue has been the interplay between the pseudogap, which is generic to all hole-doped copper oxide superconductors, and stripes, whose static form occurs in only one family of copper oxides over a narrow range of the phase diagram. Here we report observations of the spatial reorganization of electronic states with the onset of the pseudogap state in the high-temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+x), using spectroscopic mapping with a scanning tunnelling microscope. We find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes. As expected, the stripe patterns are strongest when the hole concentration in the CuO(2) planes is close to 1/8 (per copper atom). Although they demonstrate that the fluctuating stripes emerge with the onset of the pseudogap state and occur over a large part of the phase diagram, our experiments indicate that the stripes are a consequence of pseudogap behaviour rather than its cause.     

脉冲激光烧蚀金属氧化物的发光光谱

研究了３５５ｎｍ脉冲激光烧蚀Ｌａ２Ｏ３,ＭｎＯ２,ＬｉＭｎ２Ｏ４,Ｌａ０．６７Ｃａ０．３３ＭｎＯ３等金属氧化物体系产物的发光光谱,观察到了激发态Ｌａ,Ｌａ＋,ＬａＯ以及Ｍｎ等粒子的发光谱线或谱带。在Ｏ２和Ａｒ气氛中,各谱线（带）的强度迅速下降。激光能量密度的增加可以改变羽状物中Ｌａ＋,Ｌａ和ＬａＯ的相对强度。对多组分金属氧化物攻蚀的结果与单组分金属氧化物类似。     

RECrO3电子结构与导电性能的关系

利用量子化学的SCF－Xα-SW电子结构计算方法,计算得到了RE4Cr2O9(RE=La,Pr,Nd,Sm,Gd,Dy,Ho,Er)等体系的电子态密度分布、费米能和能隙宽度等电子结构参数,并结合RECrO3导电陶瓷的导电特征,分析了导电性能和电子结构参数之间的关系。研究表明：随着RE原子序数的增加,其费米能附近的态密度依次增大,主要为f电子,非f电子数量逐渐减少,因此,参与导电的电子应与f电子无关。这是RECrO3导电陶瓷电导率随RE原子序数增大而下降的主要原因。     

TR唯象模型中的集体激发对比热的贡献

通过计算非费米液体重费米子体系的ＴＲ唯象模型中的集体激发对比 热的贡献,发现集体激发对比热的贡献与温度的４／３次幂成正比．这说明不能从 集体激发对比热的贡献来解释在实验上观察到的电子比热系数为温度的对数函数的非费来液体行为．     

镧—菸酸配合物的电子结构与化学键性质研究

用分子轨道法研究了镧－菸酸配合物的电子结构和化学键性质．配合物的ＨＯＭＯ（最高占据分子轨道）主要由Ｃ、Ｏ、Ｎ原子的２ｐ轨道组成；ＬＵＭＯ（最低非占据分子轨道）主要由Ｌａ３＋离子的６ｓ、５ｄ、４ｆ轨道组成．在配位过程中有配体向金属离子的电荷转移．配体共转移１．０１７个电子到金属离子上，与Ｌａ３＋离子带１．９８３个正电荷相符合．Ｌａ—Ｏ键具有一定的共价性，由Ｃ原子、两个Ｏ原子与金属离子形成的共价键有一定的流动性．其中５ｄ轨道对形成Ｌａ—Ｏ配位键贡献最大，４ｆ轨道贡献最小．     

Effect of La and Co-doping on microstructure and electrical properties of Bi FeO_3 thin films

Adding both La3＋and Co3＋was used to tune the microstructure and electrical properties of Bi Fe O3（BFO） thin films, and Bi1-xLaxFe0.90Co0.10O3 thin films were grown on the Sr Ru O3-buffered Pt-coated silicon substrates by a radio frequency sputtering. A polycrystalline structure with（110） orientation was shown in thin films, and their resistivity dramatically increases as the La3＋content increases. Their dielectric constant increases,and dielectric loss decreases with increasing La3＋content.In addition, their ferroelectric and fatigue properties were enhanced with rising La3＋content. The thin films with x = 0.03 have optimum electrical properties（e.g., remanent polarization 2Pr＊ 175.6 l C/cm2, coercive field2Ec＊ 699.5 k V/mm, dielectric constant er＊ 257 and tan d ＊ 0.038）, together with a good fatigue behavior. The impendence analysis of the films was conducted to identify the defects type during conductivity, and both hopping electrons and single-charged oxygen vacancies are mainly responsible for the conduction of grain and grain boundaries regardless of La3＋content. As a result, the doping with both La3＋and Co3＋benefits the improvement in the electrical properties of BFO thin films.     

Electronic properties and stabilities of methoxy-substituted Lindqvist polyoxometalates [Nb2W4019CH3]3- by DFT

The electronic properties and stabilities of five [Nb2W4O18OCH3]3-isomers have been investigated using a density functional theory method.The results show that the isomer with the methoxy group occupying a bridging position between two tungsten atoms(two tungsten atoms in the plane that contains two niobium atoms) in the [Nb2W4O18OCH3]3-framework is the most stable isomer in acetonitrile.The stability of the one-electron-reduced isomers changes little.The most stable one-electron-reduced isomer has the methoxy group occupying a bridging position between niobium atoms in the [Nb2W4O18OCH3]4-framework.The M-Ob(M = Nb,W;b denotes bridging) bond lengths in anions in which the metal atoms are connected by a methoxy group are longer than those in [Nb2W4O19]4-.The highest occupied molecular orbitals(HOMO) in [Nb2W4O19]4-mainly delocalize over the bridging oxygen atoms of two niobium atoms and two tungsten atoms located in the equatorial plane,and the bridging oxygen atoms on the axial surface.The lowest unoccupied molecular orbitals(LUMO) of [Nb2W4O19]4-are mainly concentrated on the tungsten atoms and antibonding oxygen atoms.Methoxy substitution modifies the electronic properties of the [Nb2W4O18OCH3]3-isomers.The HOMOs in the five isomers formally delocalize over the bridging oxygen atoms,which are distant from the surface containing the methoxy group and four metal atoms.The LUMOs delocalize over the d-shells of the four metal atoms that are close to the methoxy group,and the p-orbitals of oxygen.One-electron reduction occurred at the tungsten atoms,not the niobium atoms.     

The structure of the high-energy spin excitations in a high-transition-temperature superconductor

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-T(c)) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T(c). The most obvious feature in the magnetic excitations of high-T(c) superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)(x)CuO4 family and is not universally present in Bi2Sr2CaCu2O8+delta (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T(c), and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBa(x)CuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.     

La2—xSrxCuO4的局域电子态密度

本文用紧束缚近似计算了La_2-_xSr_xCuO_4(x=0, 0.15)的局域电子态密度.计算结果与能带论和实验符合,但比能带的结果更为细致.本文讨论了掺杂Sr以后的效果.与YBa_2Cu_3O_(7-x)材料相比较,可以认为这两种材料主要的超导机理是相同的.     

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.     

甲烷氧化偶联催化剂和部分氧化反应机理的原位及非原位谱学表征

采用原痊红外和原位显微Raman光谱技术及XPS、吡啶（Py)吸附的漫反射UV谱、Py-TPD、CO2-TPD等方法对含氟稀土基催化剂上甲烷氧化偶联（OCM）反应活性氧物种、催化剂表面酸碱性进行了考察。在O2预处理和/或工作条件下的SrF2/La2O3,SrF2/Nd2O3,LaOF,BaF2/LaOF和BaF2/CeO2等催化剂上原位观测到超氧物种（O2^-),并在其中前4种催化剂上检测到O2^-物种与CH4反应生成的气相C2H4,CO2和表面碳酸盐等OCM反应主、副产物。这些结果为O2^-是相应催化剂上OCM反应的活性氧物种首次提供了直接的光谱证据。研究结果表明,催化剂的OCM反应性能与其表面酸碱性的强弱并无简单的对应关系。采用原拉时间分辨红外光谱和原位显微Raman光谱技术对SiO2和γ-Al2O3负载的Rh、Ru催化剂上甲烷部分氧化（POM）制合成气反应的研究表明,由CH4直接氧化生成CO和H2是Rh/SiO2上POM反应的主要途径,而燃烧-重整机理是Ru/γ-Al2O3和Ru/SiO2上CO和H2生成的主要途径,反应条件下催化剂表面氧（O^2-)物种浓度的差异很可能是导致这两种催化剂体系上POM反应机理不同的主要原因,其本质可能源于Rh和Ru对氧的亲合力的不同。     

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.     

6种氮氧化物的量子化学理论研究

利用量子化学密度泛函(DFT)方法在6-311++G(d,p)水平上对N2O,NO,NO2,N2O3,N2O4及N2O5等6种氮氧化物的分子结构、布居电荷、偶极矩、极化率、热力学性质等进行了优化计算.理论研究表明:计算结果与实验测定结果基本符合,误差较小.     

Superconductivity phase diagram of Na(x)CoO2*1.3H2O

The microscopic origin of superconductivity in the high-transition-temperature (high-T(c)) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting T(c) on the degree of electronic band filling. The recent report of superconductivity near 4 K in the layered sodium cobalt oxyhydrate, Na(0.35)CoO2*1.3H2O, is of interest owing to both its triangular cobalt-oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting T(c) of this compound displays the same kind of behaviour on chemical doping that is observed in the high-T(c) copper oxides. Specifically, the optimal superconducting T(c) occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that Na(x)CoO2*1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.     

硅酸镓镧晶体的喇曼散射及密度泛函研究

根据空间群理论（SPT）计算了硅酸镓镧晶体(La3Ga5SiO14，简称LGS)的喇曼光谱，并测量了LGS的室温喇曼光谱.根据晶体结构，构造了3个团簇（La3Ga4O12, La3Ga3SiO12和La3Ga2Si2O12），利用密度泛函理论(DFT)对喇曼光谱进行了计算和模拟.结果表明理论与实验非常吻合，LGS良好的压电性起源于3个团簇大的极化率各向异性.     

Dimensional reduction at a quantum critical point

Competition between electronic ground states near a quantum critical point (QCP)--the location of a zero-temperature phase transition driven solely by quantum-mechanical fluctuations--is expected to lead to unconventional behaviour in low-dimensional systems. New electronic phases of matter have been predicted to occur in the vicinity of a QCP by two-dimensional theories, and explanations based on these ideas have been proposed for significant unsolved problems in condensed-matter physics, such as non-Fermi-liquid behaviour and high-temperature superconductivity. But the real materials to which these ideas have been applied are usually rendered three-dimensional by a finite electronic coupling between their component layers; a two-dimensional QCP has not been experimentally observed in any bulk three-dimensional system, and mechanisms for dimensional reduction have remained the subject of theoretical conjecture. Here we show evidence that the Bose-Einstein condensate of spin triplets in the three-dimensional Mott insulator BaCuSi2O6 (refs 12-16) provides an experimentally verifiable example of dimensional reduction at a QCP. The interplay of correlations on a geometrically frustrated lattice causes the individual two-dimensional layers of spin-(1/2) Cu2+ pairs (spin dimers) to become decoupled at the QCP, giving rise to a two-dimensional QCP characterized by linear power law scaling distinctly different from that of its three-dimensional counterpart. Thus the very notion of dimensionality can be said to acquire an 'emergent' nature: although the individual particles move on a three-dimensional lattice, their collective behaviour occurs in lower-dimensional space.