从对Cu的取代看YBa2Cu3O7-y中Cu-O平面和Cu—O链的作用

通过测量YBa₂Cu_3-xM_xO_7-y(M=Zn,Ni,x=0,0.025,0.05,0.075,0.1,0.15,0.2,0.3)系列样品的晶体结构、正常态电子输运性质、超导电性以及O含量,给出了Zn择优取代Cu(2)的更直接证据;同时,观察到掺Zn系统中的由正交相Ⅰ到正交相Ⅱ的结构相变以及Ni取代Cu带来的电子局域化效应。研究结果表明,具有正二价态的Zn对Cu(2)具有择优取代性,而具有正二价和正三价的Ni并不具有明显的择优取代。对Cu(2)的取代引起超导临界温度的显著变化,但对正常态电子输运性质的影响不明显;对Cu(1)的取代更显著地影响了晶体结构和正常态电子输运性质,同时对T_c产生抑制,我们认为,YBa₂Cu₃0₇中的高温超导电性以及正常态输运性质是由CU-O平面和Cu-0链共同承担,而Cu-O平面和Cu-0链之间的耦合强度决定着该系统的超导电性的强弱。 关键词：     

Zn取代Cu抑制YBa2Cu3O7-y超导电性的机理研究

通过精心制备样品,系统地研究分析YBa₂Cu₃O_7-y体系的超导转变、电阻率、热电势、晶体结构,以及氧含量等随Zn浓度变化的规律。并指出,Zn取代Cu导致CuO面上Cu的数目减少,载流子浓度下降;正是以Zn杂质为中心的正常芯及随之产生的渗流性破坏抑制了YBa₂Cu_3-xO_7-y的超导电性;以Zn杂质散射为基础的渗流超导电性模型与实验结果一致。     

STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.     

Theory of the fano resonance in the STM tunneling density of states due to a single kondo impurity

The conduction electron density of states nearby single magnetic impurities, as measured recently by scanning tunneling microscopy (STM), is calculated, taking into account tunneling into conduction electron states only. The Kondo effect induces a narrow Fano resonance in the conduction electron density of states. The line shape varies with the distance between STM tip and impurity, in qualitative agreement with experiments, but is very sensitive to details of the band structure. For a Co impurity the experimentally observed width and shift of the Kondo resonance are in accordance with those obtained from a combination of band structure and strongly correlated calculations.     

Local density of states around a magnetic impurity in high- T(c) superconductors based on the t-J model

The local density of states (LDOS) around a magnetic impurity in high- T(c) superconductors is studied using the two-dimensional t-J model with a realistic band structure. The order parameters are determined in a self-consistent way within the Gutzwiller approximation and the Bogoliubov-de Gennes theory. In sharp contrast with the nonmagnetic impurity case, the LDOS near the magnetic impurity shows two resonance peaks reflecting the presence of spin-dependent resonance states. It is also shown that these resonance states are approximately localized around the impurity. The present results have an immediate connection with the scanning tunneling spectroscopy observation of Bi2Sr2Ca(Cu1-xNi[Zn](x))(2)O(8+delta).     

Mixed layer formation of copper overlayers on Ni(1 1 0) surface

Copper overlayer formation on the Ni(1 1 0) surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Atom-resolved STM images showed that initially deposited Cu is replaced with surface Ni atoms forming atom-size depressions on the Ni(1 1 0) terraces and a Ni-rich quasi-one-dimensional island along the direction. Further Cu deposition yields a mosaic structure on the islands, indicating Cu/Ni mixed layer formation. From the quantitative measurement of the Cu/Ni ratio on the substrate and the islands, impinging Cu will be replaced with surface Ni whereas expelled Ni and directly impinging Cu to the island form the mixed island. The number of Cu atoms in the islands, however, more than the directly impinging Cu, indicate significant Cu/Ni replacement at the periphery of the island.     

Organic superconductors revisited

The surfaces of a ten years aged crystal and a freshly prepared κ-(BEDT-TTF)₂Cu(NCS)₂ crystal were compared by scanning tunneling microscopy (STM). The molecularly-resolved STM images of the bc plane of the crystals agree with each other and with the electronic contrast obtained by new density functional theory (DFT) based simulations. Even after ten years STM images of the molecular stacking of BEDT-TTF display a variation in brightness at the positions of different molecules. We attribute this symmetry breaking concerning the brightness in the STM images of the otherwise equivalent BEDT-TTF dimers to the electronic states of a relaxed surface.     

YBa2Cu1-xCox(Cu1-yZny)2Oz体系的Co,Zn共同掺杂效应

本文报道通过对YBa₂Cu_1-xCo_x(Cu_1-yZn_y)₂O_z(0≤x,y≤0.1)体系晶体结构、氧含量、正常态电阻-温度关系、Hall效应以及超导临界温度等的综合测量,发现随着Co和Zn含量的增加,体系经历了从正交结构的超导金属向四方结构的非超导半导体的转变,超导临界温度T_c和载流子浓度n_h均迅速下降,Co 关键词：     

Impurity-induced local magnetism and density of states in the superconducting state of YBa2Cu3O7

17O NMR is used to probe the local influence of nonmagnetic Zn and magnetic Ni impurities in the superconducting state of optimally doped high TC YBa2Cu3O7. Zn and Ni induce a staggered paramagnetic polarization, similar to that evidenced above TC, with a typical extension xi=3 cell units for Zn and xi>or=3 for Ni. In addition, Zn is observed to induce a local density of states near the Fermi energy in its neighborhood, which also decays over about 3 cell units. Its magnitude decreases sharply with increasing temperature. This allows direct comparison with the STM observations done in BiSCCO.     

Monolayer structure of phenoxy species on Cu(110): an STM study

The monolayer structure of phenoxy species (PhO) on a metal surface has been studied for the first time with scanning tunneling microscopy (STM). The phenoxy species was produced by exposing the Cu(110) surface to phenol at room temperature. The monolayer exhibits a c(4 × 2) LEED pattern; however, molecular resolution STM imaging of the monolayer reveals three types of overlayer structures: (1) a densely packed c(4 × 2) or 4002 structure with a coverage of 0.25 PhO per surface copper atom, (2) more densely packed phenoxy chains separating the c(4 × 2) regions with a local coverage of 0.33 PhO per Cu, and (3) a distorted c(4 × 2) or 4102 structure consisting of phenoxy chains. Structural models are proposed for each case based on the real dimensions measured with STM. These structures exhibit a constant binding site for the phenoxy species with different orientations (from nearly parallel to almost normal) with respect to the surface plane. The “15°–20° tilts” measured by other techniques on Pd(110) and Ni(110) may be affected by the sum of different structures.     

Spatial effects of Fano resonance in local tunneling conductivity in vicinity of impurity on semiconductor surface

We present the results of local tunneling conductivity spatial distribution detailed theoretical investigations in vicinity of impurity atom for a wide range of applied bias voltage. We observed Fano resonance in tunneling conductivity resulting from interference between resonant tunneling channel through impurity energy level and direct tunneling channel between the tunneling contact leads. We have found that interference between tunneling channels strongly modifies form of tunneling conductivity measured by the scanning tunneling microscopy/spectroscopy (STM/STS) depending on the distance value from the impurity.     

STM analysis of triosmium carbonyl cluster adsorption at HOPG

The study of metallic carbonyl clusters as precursors in tailoring the heterogeneous metal catalysts has been of great importance. The catalytic nature of the adsorbed clusters in thin film form depends on the chemical properties of the substrate used. The metal-support interaction will determine various properties such as the surface morphology, adsorption features and the structural orientations. We report a scanning tunneling microscopy (STM) study of an osmium carbonyl cluster (Os₃(CO)₁₁(NCCH₃)) adsorbed on highly oriented pyrolytic graphite (HOPG). STM measurements showed that the osmium carbonyl cluster interacts with HOPG in such a way that it adsorbs on the basal plane showing regular lattice structure, whereas the axial planes of the HOPG surface shows no ordered structure. The regular cluster lattice structure of the carbonyl cluster on the basal plane of the graphite has lattice parameters of a=1.4 nm and b=1.5 nm. We believe that the regular orientation of the cluster indicates a monolayer adsorption of the cluster on the graphite basal planes. Scanning tunneling spectroscopy (STS) measurements also indicated an insulating behavior for the cluster molecules on HOPG, with a significant energy gap value of ca. 300 mV. The cluster interaction at the active sites, i.e. axial planes of the graphite, was also observed by in situ STM measurements.     

Oscillatory melting temperature of the vortex smectic phase in layered superconductors

We report on transport measurements of YBa 2Cu 3O (7-delta) single crystals with different oxygen contents in the geometry B, J ||ab (J perpendicularB). Our data show that the vortices become confined between the Cu-O planes below a well-defined temperature at which the effective size 2xi of the vortex core is approximately equal to the period of the Cu-O layers. This confinement strongly increases the vortex liquid freezing temperature. A new melting line is found separating a vortex liquid and a smectic phase, which shows an oscillatory field dependence reflecting differences between commensurate and incommensurate smectic states.     

α-Sexithiophene on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O: An STM and NEXAFS study

The adsorption of α-sexithiophene (6T) on Cu(1 1 0), Cu(1 1 0)-(2 × 1)O and the mesoscopically patterned Cu-O striped surface have been studied by STM (scanning tunnelling microscopy), XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure). The molecular resolution of the STM allowed to determine the orientation and local order of the molecules in the submonolayer and monolayer regime. It is shown that the 6T molecules align with their long molecular axis along the densely packed copper rows on Cu(1 1 0) and along the Cu-O rows on the Cu(1 1 0)-(2 × 1)O surface. On the striped phase with alternating copper and Cu-O regions the molecules adsorb first on the Cu regions and after complete filling of these regions, on the Cu-O. The orientation is the same on both areas as on the respective pristine surfaces with the only exception that the molecules reorient by 90° if the width of the copper regions is smaller than the molecular length. The NEXAFS measurements allowed for a determination of the adsorption geometry of the molecules: while 6T lies flat on the surface on clean copper, the molecular planes are inclined with an angle as high as 39° with respect to the substrate on (2 × 1)O. For the latter, this inclination angle is 4° higher than in the bulk crystal structure of 6T observed for thicker films to release stress and allow commensurability with the substrate lattice, while for the former it is a result of the aromatic system bonding to the Cu(1 1 0) surface, as confirmed by XPS.     

Doping dependent tunneling conductance in SDW ordered copper oxide superconductors

It is observed that doping suppresses the long range anti-ferromagnetic order and induces superconducting phase for a suitable doping. In order to study this effect, we present a model study of the doping dependence of the tunneling conductance in high-T_c systems. The system is described by the Hamiltonian consisting of spin density wave (SDW) and s-wave type superconducting interaction in presence of varying impurity concentrations. The gap equations are calculated by using Green’s functions technique of Zubarev. The gap equations and the chemical potential are solved self-consistently. The imaginary part of the electron Green’s functions shows the quasi-particle density of states which represent the tunneling conductance observed by the scanning tunneling microscopy (STM). We investigate the effect of impurity on the gap equations as well as on the tunneling conductance. The results will be discussed based on the experimental observations.     

天然杂质对黄铁矿的电子结构及催化活性的影响

采用密度泛函理论和平面波赝势方法对含天然杂质黄铁矿的电子结构和光学性质进行了计算,并讨论了二十种天然杂质:钴、镍、砷、硒、碲、铜、金、银、钼、锌、铊、锡、钌、钯、铂、汞、镉、铋、铅和锑,对黄铁矿催化活性的影响.结果表明在过渡金属杂质中,杂质能级主要由它们的d轨道产生,而在主族金属及非金属杂质中,杂质能级主要由它们的s或p轨道产生.含铜、钼、砷、金、银或镍的黄铁矿对氧的还原的电催化能力增强.除锌、钼、钌、砷、锑、硒和碲外,其余杂质能增强黄铁矿表面俘获电子的能力,使光生电子和空穴复合的概率减小.光学性质计算表 关键词： 天然黄铁矿 杂质 电子结构 光学性质     

Cu,O共掺杂AlN晶体电子结构与光学性质研究

采用基于密度泛函理论的平面波超软赝势方法对纯Al N,Cu单掺杂以及Cu与O共掺杂Al N超胞进行了几何结构优化,计算了掺杂前后体系的晶格常数、能带结构、态密度与光学性质.结果表明:掺杂后晶格体积增大,系统能量下降;Cu掺入后Cu 3d电子与N 2p电子间有强烈的轨道杂化效应,Cu与O共掺后Cu和O之间的吸引作用克服了Cu原子之间的排斥作用,能够明显提高掺杂浓度和体系的稳定性.光学性质分析中,介电函数计算结果表明Cu与O共掺杂能改善Al N电子在低能区的光学跃迁特性,增强电子在可见光区的光学跃迁;复折射率计算结果显示Cu与O掺入后由于电磁波穿过不同的介质,导致折射率发生变化,体系对低频电磁波吸收增加.     

STM study of the Ni(110)-(2 × 1)-2CO system: structure and bonding-site determination

The structure of the Ni(110)-(2 × 1)-2CO system has been studied with scanning tunneling microscopy (STM) at room temperature. Contrary to many previous studies, it is found that atomically-resolved STM imaging of CO molecules is attainable. By comparing the apparent CO adlayer registry with coadsorbed low concentrations of O and S, it is found that the CO molecules bond in short-bridge sites along the Ni [11̄0] rows and alternately tilt away from the normal direction within the (001) plane.     

Channel selective scanning tunneling spectroscopy

In this paper we simulate STM and STS experiments for CO monomers and dimers on Cu(1 1 1) surface. We show that the contrast of STM images can be attributed to interference effects between tunneling channels, and suggest that functionalizing the microscope tip improves the channel selectivity of STM. Furthermore, we show that voltage and position dependent tunneling spectra also reflect the same interference effects, but adds the energy resolution to the channel analysis. Especially in the case of nonresonant tunneling, STS measures local density of states only indirectly. The present study suggests that STS in constant height mode can be used in investigating the phase and energy sensitivity of tunneling channels in adsorbate molecules and nanostructures.     

Atomic structure of the Cu(410)-O surface: STM visualization of oxygen and copper atoms

The structure of the Cu(410)-O surface is studied by a scanning tunneling microscope (STM) with atomic resolution. Tunneling is accomplished for various states of the tungsten STM probe, which allows oxygen and copper atoms to be visualized separately.