YBa2Cu3O7超导体中空穴状态的X射线光电子能谱研究

对正交单相YBa₂Cu₃O₇样品进行了X射线光电子能谱(XPS)测量。明显观察到Cu的类高价状态。讨论了Cu_2p和O_1s的XPS中各峰对应的电子状态,认为由于过量的O而引入的额外空穴产生在O_2p轨道上。Cu的类高价状态包含Cu3d⁹L和Cu3d¹⁰L²两种状态的组合。 关键词：     

CNDO studies of Y-Ba-Cu-O superconductors IV. The oxygen content effect

The dependence of the electronic structure of superconducting YBa₂Cu₃O _y , is investigated using a quasi-relativistic CNDO/1 molecular orbital method. Electronic structures of model clusters [Ba₈Cu₃O _n ] ^q with total chargesq=23–2n forn=11 to 14 in various Cu(1) coordination geometries are compared. The increasing oxygen content causes the electron density transfer from the central Cu(1)O(1,5) _x area to the lateral Cu(2)O(3,4)₂ sheets. The isomerization effects are important for the structures withn=12.     

Development of the method for low-temperature investigations of HTSC systems on an X-ray electron magnetic spectrometer

Peculiarities of the chemical structure of bulk polycrystalline samples of the high-temperature superconductors Bi₂Sr₂CaCu₂O₈, BiSrCaCu₂O_5.5, BiSrCaCu₃O₈, and YBa₂Cu₃O_{7 ? δ} have been investigated in detail at room and superconducting temperatures on an X-ray electron magnetic spectrometer equipped with an attachment for low-temperature studies. It is shown that covalent bonding is formed at a superconducting temperature between copper and oxygen due to Cu²⁺ ions. Due to the enhancement of the d(Cu)–p(O) hybridization of copper and oxygen electrons in the superconducting state, the d-electron density increases near E _F. The occurrence of additional peaks in the O1s and Sr3d (Ba3d) spectra after transition of the system to the superconducting state indicates changes in the nearest environment of O and Sr (Ba) atoms, in particular, the transition of Sr atoms to a higher oxidation state.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.     

Non-superconducting and rendered superconducting YBa2Cu3O7−x . An in-situ photoemission study

We present X-ray photoemission spectroscopy (XPS) results obtained on polycrystalline YBa₂Cu₃O_7–x in the non-metallic (x>0.6) and metallic state (x<0.6) by an in-situ oxygen deloading/loading procedure, thus avoiding uncontrolled surface contaminations. The transition to the metallic (superconducting) state is characterized by corresponding changes in the O1s and Cu2p photoelectron lineshapes being in accordance with charge transfer from CuO₂ planes to CuO chains. In particular, a peak at 531.1 eV binding energy is related to the presence of oxygen hole states (O^–).     

A photoemission study (XPS and UPS) of the superconductor ErBa2Cu4O8 and comparison with YBa2Cu3O7-δ(δ≈0,1)

Core level and valence band photoemission spectra of the 124 superconductor ErBa₂Cu₄O₈ are investigated using MgK , HeI, and HeII radiation and compared with photoemission spectra of orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O_6.2 samples. It is found that the Cu 2p _3/2 core level and the UPS valence band spectra of ErBa₂Cu₄O₈ and YBa₂Cu₃O₇ are very similar but differ significantly from those of YBa₂Cu₃O_6.2. From the resemblance between the ErBa₂Cu₄O₈ and YBa₂Cu₃O₇ spectra it follows that the electronic structures of these two compounds are closely related in spite of an additional CuO chain in the 124 phase. The differences between the YBa₂Cu₃O_6.2 spectra and the other spectra are explained by the the presence of monovalent copper at the crystallographic Cu(1) sites. The Cu 2p _3/2 lineshapes of the here studied superconductors and some Cu^II compounds are discussed.     

Calculation of EFGs in high Tc superconductors

The electric field gradients (EFGs) at all atomic positions of YBa₂Cu₃O₇ and YBa₂Cu₃O₆ are calculated on the basis of full potential linearized augmented plane wave calculations by an ab initio method. Cood agreement with experimental EFG_s is found for all oxygen positions in YBa₂Cu₃O₇ and the Cu1 position in both compounds, while for the Cu2 position in the superconductor the symmetry of the EFG agrees with experiment but the magnitude of our theoretical EFG is only half the experimental value. A small transfer of 0.07 electrons from d(x²−y²) to d(z²) symmetry would be sufficient to achleve agreement with experiment. Thus the underlying local density approximation is not perfect, but remains a very accurate method for describing ground state properties.     

X-ray photoelectron spectroscopy of the high-temperature superconductor YBa2Cu3O7−x and the copper oxides

The X-ray photoelectron (XPS) core-level spectra of Cu-metal, Cu₂O, CuO, NaCuO₂ and YBa₂Cu₃O_7–x have been determined. It has been shown, that the Cu³⁺ ions in NaCuO₂ are reduced to lower valence states by X-rays in ultrahigh vacuum (UHV). The determination of the electron binding energy of the Cu 2 _{p 3/2} level in NaCuO₂ yields a value of 934.1 eV. The shape of the Cu 2 _{p 3/2} peak as well as the appearence of a shake-up satellite in YBa₂Cu₃O_7–x which is smaller than in CuO show that YBa₂Cu₃O_7–x contains Cu³⁺ ions. The behaviour of YBa₂Cu₃O_7–x also was studied during X-ray irradiation and a reduction was noticeable.     

Evidence for incipient O dimerization and absence of Cu+3 in temperature-dependent XPS of highT c superconducting oxides

From temperature-dependent X-ray photoemission (XPS) spectra between 300 and 10 K of Sr doped La₂CuO₄ and 123 compounds (YBa₂Cu₃O_7–) we deduce the absence of the Cu3d ⁸(Cu³⁺) configuration in the ground state of these Mott insulators. The metallic character of these samples and involving those Cu atoms which are relevant for the superconducting properties arises from (d ⁹ p ⁶)+(d ¹⁰ p ⁵) valence fluctuations, while the superconducting transition is indicated to be closely related to an incipient oxygen dimerization which is best detected upon cooling far belowT _c .     

Studies of oxides related to high temperature superconductors

Samples of the series of compounds YBa₂(Cu_1?x Fe _x )₃ O_6.5+p with 0<x≤0.2 have been characterised by X-ray diffraction, oxidation titration and Mössbauer spectroscopy yielding information on the oxygen content parameterp and the charge state ratios Cu³⁺/Cu²⁺ and Fe⁴⁺/Fe³⁺.     

X-ray absorption spectroscopy of the 90K superconductor YBa2Cu3O7−δ

X-ray absorption spectroscopy has been used to probe the Cu environment of the new highT _c YBa₂Cu₃O_7– superconductor. Evidences of other valencies than two are found for copper. Besides Cu¹⁺ which is localized on Cu₁ sites when the unit cell is oxygendeficient, one invokes a ground state which merges bivalent 3d ⁹ to 3d ⁹ L configurations as proposed in a recent paper (L stands for a ligand hole in the CuO bond) very similar to that observed in many light rare earth oxides.     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

63Cu and17O NMR studies on the magnetic and superconducting properties of YBa2Cu3O7−y

Results of⁶³Cu and¹⁷O NMR experiments in YBa₂Cu₃O₇ and YBa₂Cu₃O_6.63 are reviewed. The normal state data revealed two important aspects of the magnetic properties of these materials, namely, the temperature dependent antiferromagnetic Cu spin correlations and the spin gap behavior, the latter being observed in the reduced oxygen material. These features appear to be the general properties of many high-T _c cuprates. Anomalous temperature dependence of the anisotropy of the Cu relaxation rate was found in the superconducting state of YBa₂Cu₃O₇, which can be explained by a d-wave pairing model.     

Percolation and electrical resistivity in the YBa2Cu3O7−Ag composite system

We have measured the resistivity of the YBa₂Cu₃O₇–Ag composite system with Ag volume fraction,p _Ag ranging from 0–100%. The percolation behavior and the normal-state resistivity are studied. We find a three-dimensional threshold for the Ag matrix occurring atp _Ag 20% and a threshold for the superconducting matrix occurring at 1–p _Ag30%. The electrical resistivity is interpreted in terms of a resistivity model for granular YBa₂Cu₃O₇ proposed by Halbritter et al. [Z. Phys. B —Condensed Matter71, 411 (1988)]. Based on this model, we show that the resistivities between the YBa₂Cu₃O₇ grains and flaws in the grains are greatly diminished in the samples, indicating significantly improved grain growth behavior of the YBa₂Cu₃O₇ phase resulting from the addition of Ag. With the diminishing of the grain boundary resistivities, an intrinsic value for the temperature coefficient of resistivity, (1/gr)(d/dT), can be inferred, which is 3.3×10^–3/K at 300 K. This value is very close to that observed in the Cu–O₂ planes in single crystals and epitaxial films of YBa₂Cu₃O₇.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

NdBa2Cu3O7-δ/YBa2Cu3O7-δ多层膜体系的外延结构和磁通钉扎的研究

采用脉冲激光沉积技术,在单晶SrTiO₃基底上外延生长了一系列名义结构为p×(NdBa₂Cu₃O_7-δ(m)/YBa₂Cu₃O_7-δ(n))的多层膜和准多层膜(单元层NdBa₂Cu₃O_7-δ较厚而YBa₂Cu₃O_7-δ呈非连贯的岛状分布,m,n为激光脉冲数,p为重复周期).样品的超导转变温度在87—91 K范围,具体大小取决于不同的调制结构,多层膜的重复周期越大,层状界面越多,超导转变温度就越低.磁传输测量表明,准多层的样品不仅具有较高的超导转变温度,而且具有较强的磁通钉扎性能,77K零场下的临界电流密度高达4×10⁶ A/cm²,显示出良好的应用前景. 关键词： 2Cu₃O_7-δ')" href="#">NdBa₂Cu₃O_7-δ 多层膜 磁通钉扎 临界电流密度     

Intercalated hydrogen in yttrium barium cuprate: The state and mobility of a “Guest” and modification of the “Host” properties

The electric, magnetic, and physicochemical properties of yttrium barium hydrocuprate (H₂YBa₂Cu₃O₇) and its oxidized form (H₂YBa₂Cu₃O_7.8) are governed, to a large measure, by poorly studied behavior of hydrogen atoms in the lattice. The crystal chemical state and the mobility of a proton intercalated into yttrium barium cuprate have been investigated by the isotope exchange and inelastic neutron scattering techniques. By their behavior in the process of inelastic neutron scattering, a substantial fraction of protons can be considered mechanically free particles without chemical bonding with oxygen ions. The self-diffusion coefficient of a proton is estimated to be several orders of magnitude larger than that of oxygen ions, the latter coefficient being equal to 10^?13 cm² s^?1 at 490 K. Considerable changes in the lattice parameters and local charge distribution in the copper-oxygen subsystem are revealed by the x-ray powder diffraction analysis and nuclear quadrupole resonance (NQR). Discrete changes in the interplanar spacings along the c axis and in the Cu NQR frequencies with a gradual variation in the degrees of hydration and oxidation indicate the formation of hydrocuprate and (or) oxyhydrocuprate in the matrix of the initial material.     

Electronic structure of high-temperature superconductors I. Y-Ba-Cu-O systems

The electronic structure of high-T _c copper based superconductors has been studied using the quasirelativistic INDO molecular orbital method. Model clusters of YBa₂Cu₃O _y systems have been investigated fory ranging between 6·0 and 7·0. Monomeric units [Ba₈Cu₃O _x ] ^q and [Y₄Ba₈Cu₄O _x ] ^q , dimeric units [Ba₁₂Cu₆O _x ] ^q and [Y₆Ba₁₂Cu₈O _x ] ^q as well as tetrameric units [Ba₁₈Cu₁₂O _x ] ^q over 300 valence orbitals have been studied. From the set of occupied molecular orbitals the density of states (DOS) function has been calculated along with its d-orbital projection. The highest occupied molecular orbitals cover the oxygen p-band whereas the copper d-band lies at lower energies. Superconducting phase models (y=6·75 to 7·0) have a higher d-DOS function at higher energies with respect to the non-superconducting phase (y=6·0).     

Relationships between X-ray photoelectron spectra,structure and superconductivity in LnBa2Cu3O7-x

X-ray photoelectron spectroscopy has been carried out on HoBa₂Cu₃O_7-x (on abraded, fractured and ion beam reduced surfaces). These measurements show that there are only minor differences with similarly obtained results from YBa₂Cu₃O_7-x. The spectral features of the Ln species (Y and Ho), the O 1s, and Ba 3d envelopes are considered in some detail. The low energy contribution to the O 1s envelope has been assigned in part to the effects of Ba–O bonding, either in the LnBa₂Cu₃O_7-x material, or in BaO in minor phase regions, and in part to the effects of the electronic linkage of the Cu planes in the superconducting compound. The anomalous low binding energy contributions to the Ba 3d envelope are also specific to the electronic structure of these materials. It is likely that Ba (and Sr and Ca for the Bi-Sr-Ca-C-O system) plays an important electronic, as well as structural role, in promoting high temperature superconductivity.