On the strength of electron correlations in high-T c superconducting copper oxides

We analyze the strength of electronic correlations in the half-filled antibonding Cu–O orbitals of high-T _c superconducting copper oxides by considering a Cu₁₂O ₁₇ ^n– cluster withn=8 or 10, respectively. The correlated ground state is calculated by the method of the local approach (LA) in a version which allows the treatment of stronger correlated electrons. As mean-field basis a semiempirical Hamiltonian of the ZDO (zero differential overlap) type has been adopted. It is found that the correlations are particularly strong in the Cu 3d _x ²–y² orbitals. The nonintegral orbital occupation allows for valence fluctuations between Cu⁺ and Cu²⁺ in spite of the remarkable correlations. According to the present model excess holes are located at the oxygen sites. The theoretical findings are compared with the results of spectroscopic investigations. The present electronic-structure analysis allows for a straighforward rationalization of previous experimental measurements. The strong connection between the importance of electronic correlations and the symmetry properties of the electronic wave function is emphasized.     

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.     

Geometric and electronic structures of AlnCum (n = 5–9, m = 1–3) clusters: genetic algorithm combined with ab initio models

The geometries, stabilities and electronic structures of Al _n Cu _m (n?=?5–9, m?=?1–3) clusters were explored by using the genetic algorithm combined with ab initio methods. The geometric structures are almost spherical when the valence electrons are around the magic number 20, otherwise the structures are oblate or prolate. The stabilities of the clusters are related to both the Cu/Al ratios and the electronic configurations. The clusters with lower Cu/Al ratios have high stabilities. The molecular orbitals are in accord with the shell structures predicted by the jellium model. The 3d orbitals of the Cu atoms are localised, although their orbital energies are between the 1P and 1D jellium orbitals. The Al₆Cu₂ with 20 valence electrons forms closed 1S²1P⁶1D¹⁰2S² shells, and shows large binding energy and removal energy, large ionisation potential and small electron affinity. For the no-magic clusters, the structure deformation leads to crystal-field-like splitting of the degenerate shells and stabilises the clusters.     

CNDO studies of Y-Ba-Cu-O superconductors V. Co/Cu substitution

Electronic properties of [Co _n Cu_3–n O₁₂]^–17 and [Ba₈Co _n Cu_3–n O₁₂]^–1 model clusters (n=0–3) are investigated using CNDO molecular orbital method. Our results well reproduce the trends of experimental data on Cu-O bond lengths and supposed negative charge transfer from central Cu(1)O chains to lateral Cu(2)O₂ planes with increasing Co content.     

EPR and Magnetic Susceptibility of Na2O–CuO–P2O5 Glasses

Abstract

(50?x/2)Na₂O–xCuO–(50?x/2)P₂O₅ glasses (x=1, 5, 15, or 30 mol%) have been prepared and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The shape of the Cu²⁺ EPR spectrum depends on the Cu content, and the corresponding computer simulations suggest that the Cu²⁺ ions occupy two different sites in these glasses: one of them is preponderant at low Cu content and the other is preponderant at high content, in which the Cu²⁺–Cu²⁺ interactions are more important. From EPR parameters, it was found that for the site at low content, the covalency of copper ion bonding with the surrounding ligands is appreciable. The magnetic susceptibility data appear to follow the Curie–Weiss law (χ=C/(T?θ_p)) with negative paramagnetic Curie temperature θ_p indicating antiferromagnetic interactions between Cu²⁺ ions that are more significant in the samples with high Cu content, in agreement with EPR results.     

CNDO studies of Y-Ba-Cu-O superconductors I. Comparative study of CuO n polyhedra

Electronic structure of CuO _n ^q clusters (n=2, 4, 5) is calculated in experimental geometries of YBa₂Cu₃O₆ and YBa₂Cu₃O₇ for various formal oxidation states of Cu modelled by total cluster charges,q, both by non-relativistic and by quasi-relativistic CNDO and INDO methods. The results for standard Cu (d¹⁰s¹p⁰) as well as for nonstandard Cu (d⁹s²p⁰) reference configurations are compared from the point of view of the Cu oxidation state. Cu (III) oxidation state seems to be improbable.     

57Fe and57Co Mössbauer study of suppression of superconductivity in PrBa2Cu3O7−d

⁵⁷Fe and⁵⁷Co Mössbauer spectroscopy and X-ray diffractometry were used to study⁵⁷Fe- and⁵⁷Co-substituted PrBa₂Cu₃O_7–d samples. Our most striking result is that the isomer shifts belonging to both Co and Fe at four- and five-coordinated Cu(1) sites in the PrBa₂Cu₃O_7–d cuprate are significantly higher than those observed for the superconducting 1-2-3 compounds. This result is considered as being a consequence of suppression of superconductivity because the extra electrons from Pr fill in the holes in Cu-O planes due to the valence state of Pr higher than 3+ and due to hybridization of Pr-Cu-O orbitals, via the charge transfer mechanism between the Cu(2)-O planes and Cu(1)-O chains. Our results give evidence of the existence of the charge transfer mechanism between the planes and the chains, and of its important role in controlling the superconductivity with a charge reservoir behavior of the Cu(1)-O chains.     

Neutron scattering study of short-range correlations and ionic diffusion in copper selenide

The paper reports the results of a neutron scattering study of Cu_{2 - d}Se {\hbox{C}}{{\hbox{u}}_{{2 - \delta }}}{\hbox{Se}} superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Se↔Cu (8c,32f) and Cu (8c)↔Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.     

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).     

Nuclear Magnetic Resonance Relaxation Study of the Phase Transitions of Rb2CuCl4·2H2O and Cs2MnCl4·2H2O Single Crystals

The physical properties and phase transitions of Rb₂CuCl₄·2H₂O and Cs₂MnCl₄·2H₂O crystals were investigated by performing ⁸⁵Rb, ⁸⁷Rb, and ¹³³Cs nuclear magnetic resonance relaxation analyses. The temperature dependences of the spectra and the spin–lattice relaxation times T ₁ near T _C are related to changes in the symmetry of the octahedrons consisting of four Cl⁻ ions and two water molecules around Rb⁺ or Cs⁺; the forms of these octahedrons are disrupted by the loss of H₂O. The difference between the relaxation times of the two crystals is possibly due to the difference between the electron structures of the Cu and Mn ions. Cu²⁺ has nine valence electrons in its 3d orbital, whereas Mn²⁺ has five valence electrons in its 3d orbital.     

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT _c and the relaxation rate σ αn _s/m ^* (carrier density/effective mass) of hole-doped high-T _c cuprate superconductors (La₂, Sr₃)CuO₄, YBa₂Cu₃O₇ (Y_1−xPr_x)Ba₂Cu₃O₇, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)₂Cu(NCS)₂.     

Inhomogeneous state of the electron system in the Sr14 ? x Ca x Cu24O41 superconducting cuprate: 63Cu-17O NMR study

Charge fluctuations in quasi-one-dimensional hole-doped Sr₁₄Cu₂₄O₄₁ (Ca-0) and Sr₂Ca₁₂Cu₂₄O₄₁ (Ca-12) spin-ladder cuprates have been studied with the use of ⁶³Cu-¹⁷O NMR. Spin-echo decay rates ¹⁷(1/T ₂) and ⁶³(1/T ₂) have been measured in the temperature range of T = 10–300 K. The variation of ¹⁷(1/T ₂) and ⁶³(1/T ₂) in the Ca-0 compound is monotonic in the entire temperature range of the NMR study. In the Ca-rich compound, pronounced peaks have been observed in the decay rates ¹⁷(1/T ₂) and ⁶³(1/T ₂) at temperatures of 25 and 50 K, respectively. This result indicates the presence of collective low-frequency (τ_c ∼ T ₂) charge density fluctuations in superconducting Ca-12 with an activation energy of E _A = 100(10) K. The fluctuations gradually slow down as the temperature decreases. The amplitude of the charge density fluctuations is only 0.01–0.02 hole per site. ¹⁷O-⁶³Cu spin-echo double-resonance (SEDOR) experiments in Sr_{14 − x} Ca _x Cu₂₄O₄₁ oxides with x = 0 and 12 have been performed depending on the temperature and orientation of single crystals in a magnetic field. The constants of an indirect heteronuclear ¹⁷O-⁶³Cu interaction of nuclear spins mediated by conduction electrons have been measured. The estimates of the indirect interaction constants for nearest neighbors O1-Cu and O2-Cu, as well as SEDOR experiments with selective excitation of separate sections of ¹⁷O and ⁶³Cu NMR spectra, provide convincing evidence of the microscopically inhomogeneous spatial distribution of spin density developing in a crystal. Original Russian Text ? Yu.V. Piskunov, V.V. Ogloblichev, S.V. Verkhovsky, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 850–855.     

Bifunctional photocatalysis of TiO2/Cu2O composite under visible light: Ti in organic pollutant degradation and water splitting

Photocatalytic experiment results under visible light demonstrate that both TiO₂ and Cu₂O have low activity for brilliant red X-3B degradation and neither can produce H₂ from water splitting. In comparison, TiO₂/Cu₂O composite can do the both efficiently. Further investigation shows that the formation of Ti³⁺ under visible light has great contribution. The mechanism of photocatalytic reaction is proposed based on energy band theory and experimental results. The photogenerated electrons from Cu₂O were captured by Ti⁴⁺ ions in TiO₂ and Ti⁴⁺ ions were further reduced to Ti³⁺ ions. Thus, the photogenerated electrons were stored in Ti³⁺ ions as the form of energy. These electrons trapped in Ti³⁺ can be released if a suitable electron acceptor is present. So, the electrons can be transferred to the interface between the composite and solution to participate in photocatalytic reaction. XPS spectra of TiO₂/Cu₂O composite before and after visible light irradiation were carried out and provided evidence for the presence of Ti³⁺. The image of high-resolution transmission electron microscopy demonstrates that TiO₂ combines with Cu₂O tightly. So, the photogenerated electrons can be transferred from Cu₂O to TiO₂.     

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.     

Chemical origin of C–O blue shift and nature of Cu+…C bonding in non-classical copper carbonyl Cu+(CO)n (n = 1–8) complexes

Natural bond orbital (NBO) method and atoms-in-molecules (AIM) theory are used to study the chemical origin in the direction and ordering of C–O frequency shift, as well as Cu^+…C bond nature in copper carbonyl cations Cu⁺(CO)_n (n = 1–8). This study emphasises the role of π-back donation in explaining the ordering of C–O blue shift. NBO analyses show that the interplay of two competing factors, including π-back donation and rehybridization, is responsible for both the direction and the ordering of C–O stretching frequency shift in Cu⁺(CO)_n (n = 1–8) complexes. In addition, the Cu^+…C interaction is interesting because Cu⁺(CO)_n (n = 1–4) structure has one-sphere CO ligands but Cu⁺(CO)_n (n = 5–8) has two-sphere CO ligands. Topological analyses of electron density are applied to characterise the Cu^+…C interactions of first- and second-sphere CO with Cu⁺ and to explore the Cu^+…C interactions in the nature.     

A Mössbauer study of Bi2Sr2Ca n-1 (Cu,Fe) n O y (n=1,2,3) oxide superconductors

Mossbauer spectra of⁵⁷Fe have been measured for Bi₂Sr₂Ca_n-1(Cu, Fe)_nO_y (n=1,2,3) oxide superconductors. Each spectrum was decomposed to a single set or two sets of double peaks. The assignment of these peaks to the specific sites for the iron atoms are discussed. The 4 fold planar sites of Cu₂O plane are preferentially substituted by Fe atoms.     

Ab Initio Study on Structures and Vibrational Spectra of M+(H2O)Ar2 (M = Cu,Au)

Previous investigations have shown that it is difficult to acquire the infrared (IR) spectra of M⁺(H₂O) (M?=?Cu, Au) using a single IR photon by attaching an Ar atom to M⁺(H₂O). To explore whether the IR spectra can be obtained using the two Ar atoms tagging method, the geometrical structures, IR spectra and interaction energies are investigated in detail by ab initio electronic structure calculations for M⁺(H₂O)Ar₂ (M?=?Cu, Au) complexes. Two conceivable isomeric structures are found, which result from different binding sites for two Ar atoms. CCSD(T) calculations predict that two Ar atoms are most likely to attach to Cu⁺ for the Cu⁺(H₂O)Ar₂ complex, while the Au⁺(H₂O)Ar₂ complex prefers the isomer in which one Ar atom attaches to an H atom of the H₂O molecule and the other one is bound to Au⁺. Moreover, the calculated binding energies of the second Ar atom are smaller than the IR photon energy, and so it is possible to obtain the IR spectra for both Cu and Au species. The changes in the spectra caused by the attachment of Ar atoms to M⁺(H₂O) are discussed.     

Mössbauer study of tin-doped 1-2-3 and 2-1-4 highT c superconductors

Samples of YBa₂(Cu_0.85Sn_0.15)₃O_7–y (1-2-3), La₂(Cu_0.95Sn_0.05)O4_4–x, and La_1.85Ba_0.15(Cu_0.95Sn_0.05)O_4–x (2-1-4), with different oxygen stoichiometry, have been studied by Mössbauer spectroscopy. These measurements reveal the existence of two inequivalent sites for Sn in the (1-2-3) compounds. The (1-2-3) spectra display two quadrupole doublets which we associate with Sn in Cu(1) and Cu(2) sites, respectively. La compounds show a single quadrupole doublet.     

Electronic structure and binding mechanism of Cu2O

A self-consistent LAPW band structure calculation of Cu₂O is presented. Total and partial densities of states and electron densities were calculated and are used to give an interpretation of chemical bonding. It is found that there are significant deviations from a simple ionic picture due to a depletion of the valence band of Cu-3d electrons leading to a non-spherical charge density around Cu. A critical discussion of theoretical and experimental work on Cu₂O is given.     

Preparation,structural, photoluminescence and magnetic studies of Cu doped ZnO nanoparticles co-doped with Ni by sol–gel method

Zn_0.96−xNi_0.04Cu_xO nanoparticles have been synthesized by varying different Cu concentrations between 0% and 4% using simple sol–gel method. X-ray diffraction studies confirmed the hexagonal structure of the prepared samples. The formation of secondary phases, CuO (111) and Zn (101) at higher Cu content is due un-reacted Cu²⁺ and Zn²⁺ ions present in the solution which reduces the interaction between precursor ions and surfaces of ZnO. Well agglomerated and rod-like structure noticed at Cu=4% greatly de-generate and enhanced the particle size. The nominal elemental composition of Zn, Cu, Ni and O was confirmed by energy dispersive X-ray analysis. Even though energy gap was increased (blue-shift) from Cu=0–2% by quantum size effect, the s–d and p–d exchange interactions between the band electrons of ZnO and localized d electrons of Cu and Ni led to decrease (red-shift) the energy gap at Cu=4%. Presence of Zn–Ni–Cu–O bond was confirmed by Fourier transform infrared analysis. Ultraviolet emission by band to band electronic transition and defect related blue emission were discussed by photoluminescence spectra. The observed optical properties concluded that the doping of Cu in the present system is useful to tune the emission wavelength and hence acting as the important candidates for the optoelectronic device applications. Ferromagnetic ordering of Cu=2% sample was enhanced by charge carrier concentration where as the antiferromagnetic interaction between neighboring Cu–Cu ions suppressed the ferromagnetism at higher doping concentrations of Cu.