Cu nuclear spin-spin relaxation in YBa2Cu3O6.98 and YBa2Cu4O8

We have measured the temperature (T) dependence of the transverse relaxation rate (T _G ^–1 ) of the Cu(1) nuclear spin in YBa₂Cu₃O_6.98 (T _c=92 K) and YBa₂Cu₄O₈ (T _c=82 K). From the scaling ratio ofT _G ^–2 (Cu1) toT _G ^–2 (Cu2), we have estimated the strength of a covalent bonding between the CuO₂ plane and the CuO chain to be B0.38×A _zz. The experimentalT _G ^–1 (Cu1) in YBa₂Cu₄O₈ was of the same order of magnitude as the estimated one fromT _G ^–1 (Cu2). These results appear to indicate that the electrons in the CuO₂ plane fairly spread out of the plane in both compounds.     

Shape of the Cu(2) NQR spectra in YBa2Cu3O7, TmBa2Cu3O7 and TmBa2Cu4O8

We present a study of shape of the Cu(2) NQR spectra in YBa₂Cu₃O₇, TmBa₂Cum₃O₇, and TmBa₂Cu₄O₈ compounds at temperatures of 4.2–300 K. The results of the quantitative analysis lead us to conclude that the shape of the Cu(2) NQR spectra in all the samples studied can be described in the framework of the “motional narrowing” model, which implies that the Cu(2) nucleus possesses two different NQR frequencies between which it can rapidly jump. The difference in frequencies seems to be related to the charge-stripe correlations in CuO₂ planes resulting in a dynamical modulation of the electric field gradients at the Cu(2) nuclei. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 594–598 (10 November 1997)     

Silver-tracer diffusion in Cu2O

The diffusion of¹¹⁰Ag in Cu₂O has been measured by a serial-sectioning technique as a function of temperature (700–1132°C) and oxygen partial pressure (6 × 10^?6 ?8 × 10^?2 atm). The data are fit to the defect model for Cu₂O developed by the authors in the preceding paper. Silver ions have a larger impurity-vacancy binding free energy and/or a larger jump frequency for the singly charged cation vacancies relative to that for the neutral cation vacancies. The activation enthalpies for the diffusion of copper and silver ions in Cu₂O are nearly equal, but the absolute value of D¹_Ag is about three times larger than D¹_Cu even though the silver ion is 31% larger than the copper ion.     

Diffusion and point defects in Cu2O

The diffusion of⁶⁴Cu in Cu₂O has been measured by a serial-sectioning technique as a function of temperature (700–1153°C) and oxygen partial pressure (10^?6?8 × 10^2̄ atm). The oxygen-partial-pressure and temperature dependencies of D¹_Cu suggest that both neutral and singly charged copper vacancies contribute to cation self-diffusion. A defect model involving both neutral and singly charged copper vacancies, electron holes, and singly charged oxygen-interstitial ions is developed and fit to the tracer-diffusion data, the electrical-conductivity data (Maluenda et al.), and the stoichiometry data (O'Keeffe and Moore). The resulting defect data are quantitatively consistent with the chemical-diffusion data (Maluenda et al.) and with a correlation factor $\text{f}{}_{\mathrm{v}}\text{=}\text{2}\text{3}$. The defect data are also quantitatively consistent with the high-temperature oxidation studies of copper metal (Iguchi et al.) and with the copper vacancy being 10 times more mobile than the oxygen-interstitial ion at 10002C.     

Wigner crystallization in Na3Cu2O4 and Na8Cu5O10 chain compounds

We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10), which form insulating states with commensurate charge order. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by long-range Coulomb interaction and the number of holes in the d shell of Cu. Our interpretation is supported by x-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for a distinction between the (classical) Wigner lattice and the 4k(F) charge-density wave of quantum mechanical origin.     

Biexciton binding energy in Cu2O

The problem of the biexciton molecule in Cu₂O is considered from the theoretical point of view. The possible biexciton levels are classified using the symmetry of the crystal and the band structure scheme proposed by Elliott. The binding energy of the lowest state is computed on the basis of variational calculations with inclusion of the electron—hole exchange contribution. This contribution tends to make the biexciton unbound, the largest possible value of the binding energy being 0.2 meV. We conclude that no biexciton can exist in Cu₂O at any temperature, in agreement with the views of Petroff, Yu and Shen and in disagreement with those of Gross and Kreingold.     

Photocurrent characteristics in Cu2O/Pt and Cu2O/TiO2 photoelectrochemical cells in aqueous and non-aqueous electrolytes

Photocurrent characteristics of the Cu₂O/Pt and Cu₂O/TiO₂ photoelectro-chemical cells have been presented. In aqueous solution, a slow deterioration of power output occurs approximately at the rate of 50% per day. Operation in non-aqueous solutions (acetonitrile and ethanol) also produced a deterioration of power output. However in ethanol, it was found that the deterioration reversed itself and a new cycle of deterioration and rejuvenation began. These suggest that the deterioration is not due to a chemical reduction process at the electrode surface but other factors such as migration of charged defect centres in the depletion layer or chemical reaction on the electrode surface.     

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y₂BaCuO₅YBa₂Cu₃O_7-x (Y211/Y123) interfaces in melt-processed YBa₂Cu₃O_7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa₂Cu₃O_7-x and YBa₂Cu₄O₈(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.     

Band structure and chemical bonding in Cu2O and Ag2O oxides

The band structure, total and projected densities of states, and distributions of the valence and difference electron densities for copper and silver oxides are calculated in the framework of the density functional theory in the local approximation with ab initio norm-conserving pseudopotentials in the basis set of pseudoatomic orbitals. The results obtained are compared with the experimental data and calculations performed by other authors. The energy spectrum and spatial distribution of electrons in crystals are similar to each other. Metal ions are bonded to each other through charge density channels with a weakly pronounced maximum at the center of the empty tetrahedron.     

Quantum beats in excitons in Cu2O

A theoretical study is carried out of quantum beats (QB) in the time-dependent intensities of absorption of a test pulse and of spontaneous luminescence in a Cu₂O crystal under conditions of double optical resonance. It is assumed that pumping is effected by a CO₂-laser pulse which dynamically couples the exciton levels 1s(Γ ₅ ⁺ ) and 2p(Γ ₄ ⁻ ,Γ ₅ ⁻ ,Γ ₃ ⁻ Γ ₂ ⁻ ) and splits them into two or three pairs of quasi-energy levels. The frequency of the test pulse is in resonance with the frequency Γ ₅ ⁺ of the exciton. The corresponding intensities for various directions of the electric vector of the pump field E _L, the polarization vector ξ, and the wave vector q of the test pulse are obtained. The frequency of the quantum beats is twice the Rabi frequency, which for different values of E _L, ξ and q contains different sets of matrix elements of the dipole transitions between the levels 1s(Γ ₅ ⁺ ) and 2p(Γ ₄ ⁻ ,Γ ₅ ⁻ ,Γ ₃ ⁻ Γ ₂ ⁻ ). Thus, by measuring the period of the quantum beats it is possible to determine the unknown matrix elements of the indicated transitions. Fiz. Tverd. Tela (St. Petersburg) 39, 844–847 (May 1997)     

Vortex-creep activation energy in YBa2Cu3O7/PrBa2Cu3O7 superlattices

YBa₂Cu₃O₇/PrBa₂Cu₃O₇ (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U₀ was determined by analyzing the in-plane resistive transition of 200 μm wide bridges with the external magnetic field B oriented along the c axis. It was found that U₀ is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U₀ with the current I in the specimen: U₀ exhibits a plateau in the low-I region, then decreases significantly with increasing I. This behaviour is explained in terms of a crossover plastic vortex creep – elastic (collective) creep induced by the transport current.     

Comparing Y−Ba−Cu−O superconductors by Cu,O and Ba NMR/NQR

This paper discusses various NMR/NQR studies performed by the resonance group at the Physik-Institut on the structurally related compounds YBa₂Cu₃O₇ and YBa₂Cu₄O₈. Employing different isotopes and comparing similar measurements offers the possibility to gain new insights into these compounds. The discussion addresses the following topics: Electric field gradients and their temperature and pressure dependence;T _c enhancement by Ca doping; Cu and O Knight shifts; Ba relaxation. The wide range of applicability of the resonance methods in superconductivity research is demonstrated.     

Vibronic non-equivalence of cupric ions in a CuOCu cluster

EPR of copper doped CaO exhibits pair spectra with an isotropic g-factor and a tetragonally anisotropic hyperfine structure. Assignment is made to a linear CuOCu cluster having an S(total)=1 ground state and the bridging oxygen off-center. Complementary vibronic states of the two copper ions are stabilized and the octahedra surrounding them assume antiferrodistortive-type configurations.     

Pressure-Induced Cubic-to-Hexagonal Phase Transition in Cu2O

Phase transition and band structure of Cu2O are systematically investigated by using the HSE06 range-separated hybrid functional. Cubic Cu2O under pressure transforms to the metastable hexagonal phase. Our further investigations reveal that the first-order phase transition is driven by the elastic and dynamical instabilities. Furthermore, the stable band gap of cubic phase is always direct and greatly enhanced by pressure, whereas the hexagonal phase shows the semi-metallic band structure.