Copper-oxygen Chains Research Articles

Schottky Anomalies in the Low-Temperature Specific Heat of YBa2Cu3Oy HTSC

The evolution of the Schottky anomalies in the temperature dependence of the specific heat (in the temperature range T = 2–10 K and the magnetic field range H = 0–9 T) of polycrystalline YBa2Cu3Oy HTSCs is studied when the oxygen content in coarse-crystalline samples changes and when the degree of structural disordering in optimally doped fine-crystalline samples of the same compound changes. The number of paramagnetic centers with uncompensated spin moments, which generate the Schottky anomalies, and the splitting of the lower energy levels are estimated. A correlation between these parameters and both the number of oxygen vacancies in chain planes and the method of oxygen ordering in chains is found. The experimental data are compared with the results of studying the magnetic susceptibility at T > Tc in the same samples. Based on this comparison, we can conclude that the nature of the Schottky centers is related to the formation of paramagnetic centers at breaks in the Cu1–O4 copper–oxygen chains in the basal planes of the crystal structure.

Paramagnetism of copper–oxygen chains in high-temperature YBa2Cu3O6 + δ superconductors

Experimental evidence for the existence of chain paramagnetic Curie-type contributions to the temperature dependence of static magnetic susceptibility χ(T) in the normal (nonsuperconducting) state is obtained for a series of pure YBa2Cu3O6 + δ high temperature superconductors with different oxygen contents (0.6 < δ ≤ 1). It is shown that the chain contribution is obvious on χ(T) curves only in the T < 150K range of temperatures, grows along with the number of oxygen vacancies in Cu1–O4 chains, and depends on the ordering of these vacancies.

Metal Oxygen Connectivities in Cu(II) Complexes of Meso-2,3-dibromosuccinic Acid (H2DBrS): [Cu3(H2O)2(μ 3–OH)2(μ 2–OAc)2(DBrS)]⋅4H2O 1 and [Cu3(H2O)4(DBrS)2](DBrSM)⋅4H2O 2

Two metal oxygen connectivities Cu(II) complexes [Cu3(H2O)2(μ3–OH)2(μ2–OAc)2(DBrS)]⋅4H2O 1 and [Cu3(H2O)4(DBrS)2](DBrSM)⋅4H2O 2 have been synthesized (H2DBrS = HOOC–(CHBr)2–COOH, DBrSM = (3aR,6aS)-3a,6a-dibromo-3,6-dioxohexahydrofuro[3,4-c]furan-1,4-bis(olate)). Within 1, two CuO5 square pyramid are edge-shared to generate di-square pyramids, each of which are corner-shared to one another CuO6 octahedron to form polyhedral copper oxygen chains. The resulted polynuclear single chains are bridged by meso-2,3-dibromossuccinate anions to generate 2D layers. Compound 2 features a trimer polyhedrons unit with two CuO5 square pyramid and CuO6 octahedron, which are further interconnected by meso-2,3-dibromossuccinate anions into 3D coordination polymer. The study of magnetic property indicates that antiferromagnetic coupling interactions exist between the adjacent Cu(II) ions.

First-principles study of magnetic interactions in cupric oxide

Cupric oxide (CuO) has been described as belonging to the quasi-one-dimensional antiferromagnetic compounds. It has also been suggested that cupric oxide possesses strong magnetic anisotropy, which is possibly related to the observed ferroelectricity in this material. In this paper, the magnetic interactions of CuO are investigated using the embedded cluster approach. Accurate wave-function-based methods have been employed to describe the interactions along all copper-oxygen chain directions. Both two-center and three-center clusters are considered in our calculations. The antisymmetric anisotropic interaction parameters are also calculated for the two-center clusters by applying an effective Hamiltonian theory. Our results show that the magnetic interactions are dominated by the antiferromagnetic coupling between copper ions along the chain of largest Cu-O-Cu angles in agreement with experiment. The results for the interplane magnetic interactions reveal competition between nearest-neighbor ferromagnetic coupling and second-nearest-neighbor antiferromagnetic interaction along the direction where two copper ions are connected via doubly bridged oxygen ligands. We also find nonnegligible Dzyaloshinskii-Moriya interactions with magnitude comparable to the weak isotropic interchain interactions.

Thermally activated transformations in stable and metastable copper(II) pyrovanadate polymorphs

The structural transformations of α- and β′-Cu2V2O7 phases over the entire temperature range of their existence and α → β′-Cu2V2O7 and β′ → β-Cu2V2O7 polymorphic transitions in α-Cu2V2O7 are described from the crystal-chemical standpoint. Variations in the parameters of the polyhedral blocks of the α-Cu2V2O7 structure implies that the greatest deformations occur with a negative and near-zero bulk thermal expansion in the range from room temperature to 400°C. The compression and rotation of vanadium-oxygen diortho groups is accompanied by unbending of zigzag copper-oxygen chains, with the distances between them unchanged, which is the reason for the anomalous volume expansion of the structure. Thermal distortion of β′-Cu2V2O7 is insignificant. The thermal expansion coefficients (TECs) of unit cell parameters are as follows: αa = −1.36 × 10−5 1/K, αb = 1.95 × 10−5 1/K, αc = 1.37 × 10−5 1/K, αβ = −0.18 × 10−5 1/K, and αV = 1.93 × 10−5 1/K. We demonstrate that the low-temperature Cu2V2O7 phase can be formed without admixtures of metastable β-Cu2V2O7 upon slow cooling (at about 1 K/min) of the high-temperature phase.

New 1D and 2D metal oxygen connectivities in Cu(II) succinato and glutarato coordination polymers: [Cu3(H2O)2(OH)2(C4H4O4)2] · 4H2O, [Cu4(H2O)2(OH)4(C4H4O4)2] · 5H2O and [Cu5(OH)6(C5H6O4)2] · 4H2O

Two Cu(II) hydroxo succinates [Cu3(H2O)2(OH)2(C4H4O4)2] · 4H2O (1) and [Cu4(H2O)2(OH)4(C4H4O4)2] · 5H2O (2) and one Cu(II) hydroxo glutarate [Cu5(OH)6(C5H6O4)2] · 4H2O (3) have been prepared and structurally characterized by single crystal X-ray diffraction methods. They feature 1D and 2D copper oxygen connectivity of elongated {CuO6} octahedra in “4 + 1 + 1” and “4 + 2” coordination geometries. Within 1, linear trimers of three edge-sharing {CuO6} octahedra are connected into copper oxygen chains, which are bridged by the anti conformational succinate anions to generate 2D layers with mono terminally coordinating gauche succinate anions on both sides. The layers are assembled into a 3D framework by interlayer hydrogen bonds with lattice H2O molecules distributed in channels. Different from 1, the principal building units in 2 are linear tetramers of four edge-sharing {CuO6} octahedra. The tetramers are condensed into copper oxygen chains and the succinate anions interlink them into a 3D framework with triangular channels filled by lattice H2O molecules. The {CuO6} octahedra in 3 are edge-shared to form unprecedented 2D inorganic layers with mono terminally coordinating glutarate anions on both sides. Interlayer hydrogen bonding interactions are responsible for supramolecular assembly of the layers into a 3D framework with lattice H2O molecules in the channels. The inorganic layers in 3 can be described as hexagonal close packing of oxygen atoms with the Cu atoms in the octahedral cavities. The title compounds were further characterized by elemental analyses, IR spectra and thermal analyses.

Determination of atom displacement distribution in YBCO superconductors induced by gamma radiation

AbstractThe calculation results of the atom displacement distribution induced by gamma radiation in the superconducting material YBa2Cu3O7–x are presented. Firstly, a direct calculation procedure for the atom displacement distribution inside a given material is introduced, starting from the application of the energy flux distribution values of secondary electrons, formerly calculated using the Monte Carlo method based code system MCNP‐4C. Finally, calculation results of the atom displacement distribution, regarding the occupied oxygen crystalline sites, are presented for different incident gamma radiation energies, where an atom displacement threshold energy value of 20 eV was assigned to O(1–3) sites, while a value of 3.45 eV was used for the copper–oxygen chain sites at the basal plane. According to the present calculations, a predominating atom displacement rate at copper‐oxygen chain sites was obtained. On the other hand, nearly linear correlations among atom displacements and energy deposition profiles were also found. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural phase transition of edge-sharing copper oxide Ca0.85CuO2 under high pressure

The perovskite-like structure compound Ca0.85-CuO2 has interesting structural properties: it has infinite one-dimensional edge-sharing copper-oxygen chains as well as partial occupancy of the Ca sites resulting in an incommensurate superstructure.In situ high-pressure energy dispersive X-ray diffraction measurements on polycrystalline powder Ca0.85CuO2 have been performed by using diamond anvil cell (DAC) instrument with synchrotron radiation. The results for the first time show that edge-sharing copper oxide Ca0.85CuO2 undergoes a structural transition at 14.5 GPa, and furthermore the structural transition is reversible.

Charge and orbital excitations inLi2CuO2

We report a resonant inelastic x-ray scattering study of electronic excitations in Li_2CuO_2, an insulating compound comprised of ribbons of edge-sharing copper-oxygen chains. Three excitations, which show little dependence on momentum transfer, are observed in our measurements. The lowest energy excitation at ~2.1 eV is dispersionless and is attributed to a localized d-d orbital excitation. We also observe two excitations at ~5.4 eV and ~7.6 eV which we assign to charge-transfer excitations. These high-energy excitations are also dispersionless along the copper-oxygen chain direction. However, in each case we observe a small energy dispersion along the direction perpendicular to the copper-oxygen ribbons, suggesting a significant interchain coupling in this system. We also discuss the possible implications of ferromagnetic nearest-neighbor intrachain coupling on the charge excitation spectra.

Raman-study of photoinduced chain-oxygen ordering in RBa/sub 2/Cu/sub 3/O//sub 7-γ/

We investigated Raman-forbidden signals in RBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// with R = Y or Pr, which lose intensity under illumination at low temperatures. These defect-induced peaks are only visible in oxygen-deficient material and for light polarized parallel to the copper-oxygen chains along the material's b-axis. In a first, two-laser based experiment we established that the bleaching effect of the light is polarization dependent as well. From this and other known properties we conclude that the signal is connected to the copper-oxygen chains, reflecting their state of disorder, and is thus closely related to aging and to the persistent photoconductivity effect in the material. In a second set of experiments we followed the temperature dependence of the Raman spectra and the intensity decrease.

Dynamic Phase Transition in Amorphous YBaCuO Films under Ar Laser Irradiation

The effect of laser irradiation (Ar laser, λ = 488 nm) on the atomic order and electronic structure of amorphous YBaCuO films is studied. It is shown that the laser irradiation of amorphous YBaCuO films leads to a sequence of processes having an oscillating character: energy accumulation (Iincoh → max), accompanied by stronger atomic disordering, and relaxation (Jcoh → max) due to atomic ordering and a change in the predominant cluster configuration, (11l) – (10l), at incident powers above 2 W. This corresponds to a local topological ordering of the solid solution with the participation of copper–oxygen chains, accompanied by an increase in conductivity by two orders of magnitude.

Heat transport by lattice and spin excitations in the spin-chain compoundsSrCuO2andSr2CuO3

We present the results of measurements of the thermal conductivity of the quasi one-dimensional spin S=1/2 chain compound SrCuO_2 in the temperature range between 0.4 and 300 K along the directions parallel and perpendicular to the chains. An anomalously enhanced thermal conductivity is observed along the chains. The analysis of the present data and a comparison with analogous recent results for Sr_2CuO_3 and other similar materials demonstrates that this behavior is generic for cuprates with copper-oxygen chains and strong intrachain interactions. The observed anomalies are attributed to the one-dimensional energy transport by spin excitations (spinons), limited by the interaction between spin and lattice excitations. The energy transport along the spin chains has a non-diffusive character, in agreement with theoretical predictions for integrable models.

Strong anisotropy of superexchange in the copper-oxygen chains of La(14)-xCaxCu24O41.

Electron spin resonance data of Cu2+ ions in La(14)-xCaxCu24O41 single crystals (x = 9,11,12) reveal a very large width of the resonance line in the paramagnetic state. This signals an unusually strong anisotropy of approximately 10% of the isotropic Heisenberg superexchange in the Cu-O chains of this compound. The strong anisotropy can be explained by the specific geometry of two symmetrical 90 degrees Cu-O-Cu bonds, which boosts the importance of orbital degrees of freedom. Our data show the apparent limitations of the applicability of an isotropic Heisenberg model to the low-dimensional cuprates.

Raman scattering study of heavily oxygenated YSr 2Cu 3O 7+ δ and AuBa 2YCu 2O 7+ δ superconductors

Raman scattering spectroscopy was used to monitor structural peculiarities and charge distribution in highly oxygenated YSr 2Cu 3O 7+ δ and AuBa 2YCu 2O 7+ δ superconductors, which do not possess linear copper-oxygen chains. The oxidation state of the chain metal ions (Cu or Au) in these materials was estimated by comparison of the experimentally determined frequency of the c-axis apex oxygen vibration and the results of model calculations which account for the nearest-neighbor interactions. The enrichment of the Raman spectra with respect to optimally doped RBa 2Cu 3O 6.95 superconductors is interpreted as being due to the lower symmetry of atomic sites in the basal plane.

Ca4Cu5O10:Copper oxide chains highly occupied by Zhang-Rice singlets

The crystal structure of ${\mathrm{Ca}}_{2+x}{\mathrm{Y}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{5}{\mathrm{O}}_{10}$ consists solely of one-dimensional copper-oxygen chains made from ${\mathrm{CuO}}_{4}$ squares sharing edges. Over the range of $x$ between 0 and 2, here synthesized using high-pressure oxygen annealing, formal copper valences between 2 and 2.4 can be obtained. All the members of the series are electrically insulating. Magnetization measurements show long-range antiferromagnetic ordering near $x=0,$ with decreasing ${T}_{N}$ on increasing $x,$ and the appearance of a new kind of short-range-ordered state for $x$ greater than 1.5. The high hole concentration regime represents the first observation of the magnetic and transport properties of a mixture of Zhang-Rice singlets and magnetic copper on one-dimensional chains without the complication of additional copper-oxygen units, and may be an excellent model system for studying their spin and charge dynamics as a key to understanding the behavior of ${\mathrm{CuO}}_{2}$ planes in copper oxide superconductors.

Optical phonons in the orthorhombic double-chainSr1−xCaxCuO2(x=0, 0.5)

We report micro-Raman and far-infrared reflectivity spectra obtained from the two members ${\mathrm{SrCuO}}_{2}$ and ${\mathrm{Sr}}_{0.5}$${\mathrm{Ca}}_{0.5}$${\mathrm{CuO}}_{2}$ of the solid solution ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{CuO}}_{2}$ with orthorhombic chain structure. The analysis of the spectra is based on the similarity between the investigated stucture and related copper oxides, which contain the same type of copper-oxygen chains. The assignment of the observed lines to atomic vibrations is supported by shell model lattice-dynamics calculations.

Formation and interaction of ferromagnetic clusters in antiferromagneticYBa2Cu3O6+xfilms

The magnetization of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ antiferromagnetic films was experimentally studied as a function of magnetic field, temperature, oxygen index, and photoillumination dose. The results are evidence of the formation of ferromagnetic clusters, promoted by oxygen holes, in a general accordance with the phase separation approach. However, field and concentration dependences of magnetization display some essential features which can be explained only with allowance for the motion of holes in long-distance potential wells, created in the ${\mathrm{CuO}}_{2}$ plane (where the clusters are formed) by charged copper-oxygen chains (allocated in the ${\mathrm{CuO}}_{\mathrm{x}}$ plane where excess oxygen ions are incorporated). A significant straggling of ferromagnetic clusters over sizes (from 100 to ${10}^{4}$ or more spins per cluster) was discovered, the properties of clusters being essentially dependent on their size.

Effect of gold doping on the superconducting critical temperature and transition width in single-crystal YBa2Cu3O? (YBCO)

Single-crystal YBa2Cu3O−δ (YBCO) has been made with some of the copper atoms in the copper-oxygen chains replaced by gold. The samples were made by a flux method. The gold concentration was determined by EDX (energy-dispersive X-ray spectroscopy) as a function of the nominal (i.e., flux) composition. Up to about 4.8% of all the copper atoms can be replaced by gold. The effect of the gold doping on the superconducting transition temperature (T c ) and the transition width (ΔT c ) were determined.

Model of oxygen diffusion in YBa2Cu3O6+x with next-nearest-neighbor hopping: A Monte Carlo study.

To the antisymmetric next-nearest-neighbor Ising (ASYNNNI) model of oxygen diffusion in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ we have added a possibility for an oxygen atom to jump not only to a nearest neighbor but also to a next-nearest-neighbor site through a jump remaining in the ab plane. The usual Monte Carlo simulation procedure has been used to calculate the tracer diffusion coefficient which turns out to be less sensitive to the oxygen content than in the standard ASYNNNI model, as found out in experiments. The Arrhenius plots show nearly linear behavior. We have also calculated the average energy per particle in the system. This when plotted as a function of oxygen concentration yielded some information about possible phase transitions in the system. At low temperatures the diffusion is probably taking place along the copper-oxygen chains whereas for higher temperatures and low concentration of oxygen, interchain diffusion as well as reorientation of short oxygen chains is also possible.

Ca induced line defects and superconductivity in Y xSr 2Cu 2GaO 7

The microstructure of Ca doped Y 1− x Ca x Sr 2Cu 2GaO 7 and Ca substituted YSr 2− x Ca x Cu 2GaO 7 have been investigated by electron microscopy. A common feature found in the Y 1− x Ca x Sr 2Cu 2GaO 7 superconductors is the existence of line defects, which are not observed in non-superconducting YSr 2− x Ca x Cu 2GaO 7. At the higher doping level, x ≈ 0.35, the line defects develop into modulated structures. The transition temperature and Meissner fraction increases with the doping level as does the defect concentration, indicating that the defects and superconductivity in these materials are interrelated. The line defects are associated with an asymmetrical distortion ( a p ≠ b p) of the gallium-oxygen plane which plays a similar role to the copper-oxygen chains in YBa 2Cu 3O 7−°.