Electric field gradients in PrBa2 Cu 3 O 7 : LSDA+U results and comparison with experiment

Electric-field gradient (EFG) and asymmetry parameter (η) at all oxygen and copper sites of nonsuperconducting PrBa₂Cu₃O₇ (Pr123) compound were calculated using the full-potential (linearized)-augmented-plane-wave plus local orbitals method. Exchange and correlation effects were treated by LSDA+U for Cu(3d) and Pr(4f) electrons. The effects of changing screened Coulomb parameters U_Pr, U_Cu1, and U_Cu2 on the results were individually studied. The calculated EFG of O2 site is close to the EFG of O3 site at variance with the experimental result. It was shown that by increasing superconducting holes in O2 and O3 sites the EFG at these sites increase and vice versa. The most famous theories which have been proposed to explain the suppression of superconductivity in perfect (without any mis-substitution or other defects) Pr123 compound are not consistent with the experimental EFG at O2 and O3 sites. By replacing one Pr atom at the Ba site on unit cell of Pr123 (Pr_Ba), it was shown that Pr_Ba mis-substitution reduces the superconducting holes in both O2 and O3 sites and could be responsible for the suppuration of superconductivity in Pr123 samples. It is very probable that the unusual behaviors of experimental EFG at O2 and O3 sites of Pr123 are related to oxygen defects which are produced with Pr_Ba mis-substitution.     

原子簇La8-xBaxCuO6的原子磁矩和自旋极化的电子结构研究

利用MS-Xα方法研究了化合物La_2-yBa_yCuO₄的原子磁矩和自旋极化的电子结构.理论计算得到母相氧化物La₂CuO₄的Cu原子磁矩为0.37μ_B，与实验值0.48±0.15μ_B基本一致. 研究结果显示, 由于Ba原子对部分La的替代，使构成化合物的基本原子簇La_8-xBa_xCuO₆的 关键词： 电子结构 自旋极化 磁矩 态密度 超导电性     

Copper NQR study of the La 214 system from an aspect of microscopic environment of copper

The nuclear quadrupole resonance (NQR) has been investigated for Cu in La_2–x A _x CuO _y (A=Sr and Ba). Three Cu NQR lines, A, B and C, have been observed, which correspond to three different Cu sites. From the analysis of these NQR intensities, site assignments are possibly as follows. These lines A, B and C are attributed to CuO₆ octahedral, CuO₅ pyramidal and CuO₄ planar sites, respectively. The apical oxygen O(2) is induced to transfer to the interstitial O(3) site between LaO planes when two or more Sr²⁺ ions are located in its neighboring La sites. The excess holes doped into the CuO₂ plane are provided mainly by O(3) as well as the unpaired Sr²⁺ ion.     

Electronic structure of superconducting Cu oxides

On the basis of photoemission results on La_2-xSr_xCuO_4-y and YBa₂Cu₃O_y it is suggested that carriers doped in the otherwise insulating samples are oxygen p holes rather than Cu d holes. The p holes interact with the localized Cu d states through hybridization and may give rise to an anomaly near the Fermi surface as described by a periodic Anderson model with low carrier density.     

Unified picture of the distribution of electric field gradients at Cu, O, and Tm sites in ReBa2Cu3O7−δ

An abundant set of published experimental NMR/NQR data on electric field gradients in ReBa₂Cu₃O_7−δ high temperature conductors, where Re=Y and Tm, is used as a test for the singlet-correlated band theory. Because of the unusual spectral weight (“capacity”) of this band, it has been possible to match the number of holes per lattice site to photoemission data on the energy spectrum and the location of the Fermi level. In the framework of a unified picture of the distribution of holes (charges) it has been possible for the first time to explain satisfactorily the observed electric field gradients at the Cu(1), Cu(2), O(1), O(2), O(3), and O(4) sites, as well as NMR and inelastic neutron scattering data on the crystal electric field at the Tm ions. Zh. éksp. Teor. Fiz. 111, 144–157 (January 1997)     

Charge states of atoms in HgBa2CuO4 and HgBa2CaCu2O6 lattices

Mössbauer emission spectroscopy on the ⁶⁷Cu(⁶⁷Zn) isotope has been used to determine the parameters of the electric-field gradient tensor at the copper sites of the HgBa₂Ca _n?1Cu_nO_2n+2 lattices (n=1,2), as well as to calculate these parameters in the point-charge approximation. An analysis of the results, combined with available NQR literature data for the ⁶³Cu isotope, has shown that the best fit of calculated to experimental data can be reached by assuming that the holes due to the presence of defects in the material localize primarily in the sublattice of the oxygen lying in the copper plane.     

On the lattice properties and electron-lattice interaction in highT c superconductors

The experimental results on the unusual crystalline lattice properties in high temperature superconductors are reviewed. Special attention is paid to their dependence on temperature and doping and their anomalous behaviour associated with the superconducting and metal-insulator transition. The most physically relevant features are: certain atoms fluctuating between different positions in the unit cell, unusually large oscillator strength of certain vibrational modes strongly coupled to a broad spectrum of electronic excitations and the polaronic nature of charge carriers. A synthesis of these experimental results is attempted in view of constructing a coherent picture for the metal-insulator transition and the superconducting state. We conclude that in the insulating materials the holes are localized on the CuO₂ units in the immediate vicinity of the dopant ions. In the metallic materials we expect charge-fluctuations of holes between the units containing the O(4) apex ions (Cu(1)-20(4)-20(1) for YBa₂Cu₃O_7–) and the Cu(2)-20(2)-20(3) complex in the CuO₂ layers. This charge transfer being linked to large Cu(1)-O(4) bond fluctuations ultimately leads — via a polaronic mechanism — to pairing of holes in the Cu(2)-2O(2)-2O(3) units.     

Charge states of atoms in the lattices of the high-temperature superconductors Tl2Ba2Ca n−1CunO2n+4 and Bi2Sr2Ca n−1CunO2n+4

To determine the charges of atoms in the lattices of the compounds Tl₂Ba₂Ca _n?1Cu_nO_2n+4 and Bi₂Sr₂Ca _n?1Cu_nO_2n+4 (n=1,2,3), the parameters of the electric field gradient tensor at the copper sites of the indicated lattices were found by emission Mössbauer spectroscopy on the isotopes ⁶¹Cu(⁶¹Ni) and ⁶⁷Cu(⁶⁷Zn), and a calculation of these parameters was performed in the point-charge approximation. A comparison of the resulting values and the published data on ⁶³Cu nuclear quadrupole resonance showed that agreement between the experimental and computed values of the parameters obtains for models in which the holes resulting from a reduction in the valence of some of the thallium (bismuth) atoms are localized predominantly at oxygen sites located in the same plane as the copper atoms (for the compounds Tl₂Ba₂Ca₂Cu₃O₁₀ and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀—at oxygen sites in the same plane as the Cu(2) atoms).     

Mössbauer study of the HgBa2Ca n−1CunO2n+2 ceramics

The parameters of the electric-field gradient tensor at the copper, barium, and mercury sites in the HgBa₂Ca _n?1Cu_nO_2n+2 lattices (n=1,2,3) have been determined by Mössbauer emission spectroscopy on the ⁶⁷Cu(⁶⁷Zn), ¹³³Ba(¹³³Cs), and ¹⁹⁷Hg(¹⁹⁷Au) isotopes, and calculated in the point-charge approximation. An analysis of these results combined with available ⁶³Cu NMR data showed that the experimental and theoretical data can be reconciled by assuming that the holes originating from defects in the material are localized primarily on the sublattice of the oxygen lying in the copper plane [for HgBa₂Ca₂Cu₃O₈, in the plane of the Cu(2) atoms].     

Magnetism and superconductivity in R1.5Ce0.5MSr2Cu2O10‐δ (M=Ru and Fe) compounds

It is shown here, that the superconducting (SC) R_1.5Ce_0.5RuSr₂Cu₂O_10-δ (RCeRuSCO, R= Eu and Gd) materials (T_c ～ 32 and 42 K) are also antiferromagnetically (AFM) ordered at T _N(Ru) ～ 122 and 180 K, respectively, thus, T_N ? T_c, a trend which is contrary to that obtained in “magnetic‐SC” intermetallic systems. Mössbauer spectroscopy (MS) on 0.5% ⁵⁷Fe doped samples shows that all Fe ions reside in the Ru site which is magnetically ordered, whereas SC is confined to the CuO₂ planes. On the other hand, for Y_1.5Ce_0.5FeSr₂Cu₂O₉ (YCeFSCO) no SC is found and the Cu–O planes order AFM at T _N(Cu)=31 K. MS studies reveal two in equivalent Fe sites, and that Fe resides predominantly (60%) in the Cu(1) site. In both sites, the Fe does not order magnetically, and the low T _N(Cu) obtained is due to frustration of the Cu moments by the presence of Fe. T _N is sensitive to oxygen concentration and shifts toward 260 K when oxygen is depleted.     

Molecular and atomic adsorption states of oxygen on Cu(111) at 100–300 K

Adsorption states of oxygen on Cu(111) at 100–300 K were investigated by means of HREELS. Two molecular species were characterized by different OO stretching frequencies (v(OO)) at 610 cm⁻¹ and 820–870 cm⁻¹, which are assigned to the peroxo-like species (O²⁻₂) adsorbed in a bridged form and the one in a bidentate form bound on an atop site, respectively. The bridged peroxo species is preferred at the low coverage and the atop peroxo species becomes dominant at the higher coverage. In addition to the peaks due to the molecular oxygen, a peak assigned to v(CuO) of atomic oxygen was observed at 370 cm⁻¹ at the high coverage. The frequency of this mode was higher than the frequency reported for Cu(111) exposed to oxygen above 300 K, indicating that the adsorption state of atomic oxygen formed at 100 K is different from that above 300 K. The v(OO) modes became faint after annealing to 170 K because of O₂ dissociation. The v(CuO) mode of the atomic oxygen formed at 100 K remained up to 230 K and disappeared after annealing to 300 K. No desorption of O₂ was detected on annealing to 300 K. It was also found that vibrational spectra for adsorbed NH₃ are influenced by the adsorption states of atomic oxygen on Cu(111).     

Correlated one-hole spectra of YBa2Cu3O7 in the bulk and at the surface

The one-hole valence spectrum of YBa₂Cu₃O₇ is calculated using an accurate tight-binding hamiltonian and a Hubbard model to treat correlation effects. Intra-atomic Coulomb integrals for holes sitting on Cu or O sites are considered and the one-hole self-energy is computed in the low density approximation. The calculated spectrum is compared with photoemission and X-ray emission experiments. This allows to point out a number of problems that arise in assigning the observed structures to a specific atom and to understand some aspects of the data. To evaluate the role of the surface in determining the shape of the photoemission energy distribution curves, the one-hole spectrum is calculated for the Ba-O₄ surface. Oxygen induced surface states are shown to be important in determining the intensity of the observed structures.     

Effects of copper(II) and iron(II) doping on the bound electronic states in the “one-dimensional” semiconductor Magnus' green salt

EPR and conductivity measurements on the “one-dimensional” semiconductor Magnus' Green Salt doped with Fe or Cu impurities are described. It is found that the introduction of these impurities causes significantly higher localization of the previously identified d_z2 holes.     

Charge-transfer hole-lifetime broadening of the Cu Auger-electron spectra of high Tc superconductors

The satellite intensity in the Cu L₂₃-VV Auger-electron spectrum of the high T_c superconductor (YBa₂Cu₃O_7−x, 123) is much more enhanced as compared to that of CuO. This enhancement was previously interpreted by Ramaker et al. [D.E. Ramaker, N.H., Turner, F.L. Hutson, Phys. Rev. B 38 (1988) 11368] as a result of the mixing between the ddp and dpp states. Here, d is a hole in the Cu 3d band and p is a hole in the O 2p band. However, the dramatic Auger electron spectroscopy (AES) spectral lineshape change from CuO to 123 is not only in the charge-transfer (CT) satellite but also in the main-line width. The change arises from the transit of the “pairing” of two bound d holes in the ddp state to that of two bound p holes in the dpp state. As a result, in CuO there is no CT satellite but the dd state becomes a resonant state broadened by the CT hole-lifetime broadening, whereas in 123 the dd state becomes a mixture of a resonant-like state and nonresonant band states. The present many-body theory can explain the overall AES lineshape change from CuO to 123.     

EPIR effect of Cu2O films by electrochemical deposition

Cuprous oxide (Cu₂O) films and Cu/Cu₂O/Cu/FTO sandwich structures were prepared by electrochemical deposition on conductive FTO substrates with different pH value conditions but constant deposition potential. The phase composition, crystal structure and microstructure of the Cu₂O films were characterized by XRD, SEM and EDS as well as by Electric–Pulse–Induced–Resistance (EPIR) perturbation. In particular, the switching effects of the Cu/Cu₂O/Cu/FTO device are examined in this work. The result shows that the EPIR-effect is large for the Cu/Cu₂O/Cu/FTO device at room temperature and strongly related to the pH value of the solution. In both acidic and neutral conditions, for example at pH = 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value. It disappears when the pH value goes further into the alkaline regime, i.e. pH = 8, 9 and 10. Space charge barriers at the interface of electrode and Cu₂O are used to explain the I–V characteristic of the layer structure and the EPIR-effect.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

Density functional theoretical study of Cun, Aln (n = 4-31) and copper doped aluminum clusters: Electronic properties and reactivity with atomic oxygen

A DFT study of the electronic properties of copper doped aluminum clusters and their reactivity with atomic oxygen is reported. Firstly we performed calculations for the pure Cu_n and Al_n (n = 4, 9, 10, 13, 25 and 31) clusters and we determined their atomization energy for some frozen conformations at the B3PW91 level. The calculated work functions and M-M (M = Cu, Al) bond energies of the largest clusters are comparable with experimental data. Secondly, we focused our attention on the change of the electronic properties of the systems upon the substitution of an Al atom by a Cu one. This latter stabilizes the system as the atomization energy of the 31-atoms cluster increases of 0.31 eV when the substitution is done on the surface and of 1.18 eV when it is done inside the cluster. We show that the electronic transfer from the Al cluster to the Cu atom located at the surface is large (equal to 0.7 e⁻) while it is negligible when Cu is inserted in the Al_n cluster. Moreover, the DOS of the Al₃₁ and Al₃₀Cu systems are compared. Finally, the chemisorption energies of atomic oxygen in threefold sites of the Al₃₁, Cu₃₁ and Al₃₀Cu clusters are calculated and discussed. We show that the chemisorption energy of O is decreasing on the bimetallic systems compared to the pure aluminum cluster.     

YBa2Cu3Oy中对Y位与Cu(Ⅰ)位取代导致的空穴浓度变化

本文研究Co对Cu(I)和Ca对Y作取代时,YBa₂Cu₃O_y系统中的空穴浓度、晶体结构和超导临界温度随掺杂量的变化关系及其起源。结果表明,上述取代对体系中的Cu-O平面上的空穴浓度P_sh都有重要影响,但其作用和方式是不同的。前一种取代主要是通过对空穴的复合和增强对空穴的局域化而使P_sh下降:后者则具有明显的退局域化作用,从而使得P_sh上升。当两种取代同时进行时,局域化作用不 关键词：     

Hole doping by Li substitution and antiferromagnetism in YBa Cu O studied by neutron powder diffraction measurements

The magnetic structure of tetragonal insulating YBa₂Cu_3-xLi_xO_y has been studied as a function of x and y. The Néel temperature and the mean ordered magnetic moment on the Cu2 sites were determined by neutron powder diffraction measurements. The decrease of these two parameters as compared to YBa₂Cu₃O₆ is much stronger for lithium than for zinc substitution. The difference is quantitatively explained by the presence of holes created in the CuO₂ planes. These holes arise from the substitution of plane Cu²⁺ by Li⁺. We suggest an explanation why such holes are not seen for the same substitution of plane Cu²⁺ by Li⁺ in orthorhombic superconducting YBa₂Cu_3-xLi_xO _{7 - δ} . Received 31 October 2001 and Received in final form 6 March 2002 Published online 25 June 2002