CuO2 Sheets Research Articles

High pressure syntheses of copper oxychlorides with K2NiF4-type and related structures

Several new copper oxychlorides with the K2NiF4-type and related structures (the 02(n-1)n where n=1, 2 and 3, and the 0222 structures) are synthesized using a high pressure technique. These are (Ca,K)2CuO2Cl2 (Tc = 24 K), Ba2CuO2Cl2 for the 0201; (Sr,Ca)3Cu2O4+δCl2−y (Tc = 80 K) for the 0212; (Sr,Ca)4Cu3O6+δCl2−y (Tc = 46 K) for the 0223; CaLnCuO3Cl (Ln=Y, Ho, Er) for the 0222. Hole carrier doping to induce superconductivity and size tuning of the CuO2 sheet are attained in these new oxychlorides.

Cumulant expansion for the p–d model: density of states and hole occupation

We propose a nonstandard perturbative solution of the p - d model, based on a generalized cumulant expansion. Using a temperature dependent Green's functions formalism, the behaviour of the density of states and the hole occupation number is investigated in the charge-transfer and in the Mott-Hubbard regime, to describe realistic systems with CuO2 sheets. A comparison with different analytic and numerical methods is also discussed.

Cation substitution and deoxygenation effect on superconductivity of Tl-Srl212 cuprate

The relation between the superconductivity and the Cu valence of TlSr 2Ca 1− x Y x Cu 2O y is investigated. The facts that the Cu valence of the optimum-doped specimen, if Tl 3+ is assumed, is rather low and that even the specimen with a Cu valence as low as 2.02 shows superconductivity suggest a small charge transfer from the TlO-layer to the CuO 2-sheets. The binding energy shift of the Tl 4f core level measured by XPS lends support to the slight decrease in Tl valence.

Thin film series arrays of intrinsic Josephson junctions

The intrinsic Josephson effect between the CuO2 sheets in Tl-2212 and Bi-2212 was used to prepare thin film mesa structures acting as series arrays of a number of junctions corresponding to the height of the mesa. We investigated the electrical properties of these arrays at various temperatures, in external magnetic fields, and under irradiation of microwaves. The results are discussed within simple theoretical models of 1-D Josephson arrays.

Structure and properties of SrCu(OH)4·H2O

The structure of SrCu(OH)4·H2O has been determined from a single-crystal X-ray refinement and shown to contain isolated square-planar [Cu(OH)4]2– groups separated by strontium ions and hydrogen bonded to water molecules. ESR, IR and UV/VIS spectra are also reported for this compound and are consistent with the presence of discrete [Cu(OH)4]2– square planes. Thermal decomposition of this material proceeded by a complex multistage process via Sr2(OH)2Cu(OH)4, through CuO and Sr(OH)2, to Sr2CuO3 and CuO. Consideration of the structure of SrCu(OH)4·H2O shows that the relative orientation of the [Cu(OH)4]2– units is such that they do not crosslink to form infinite CuO2 sheets.

Physical and chemical properties of (Y1?xCax)(Ba2?xLax)Cu3O7??

The metallic conductivity of both the CuO2 sheets and the CuOx chains of YBa2Cu3O7−δ has been established. We have used double doping in (Y1−xCax)(Ba2−xLax)Cu3O7−δ to destroy the integrity of the CuO1−δ chains while keeping constant the total oxidation state of the Cu-O array. A break-up of the chain segments allows us to establish the temperature dependence of the Seebeck coefficient, α(T), due to the CuO2 sheets and, by difference, that of the CuO1−δ chains in YBa2Cu3O7−δ. We also show that double doping enhances significantly the chemical stability of the superconductive phase. The origin of the chemical stability and of the behavior of α(T) is interpreted.

Non-adiabatic electron-lattice interactions in the copper-oxide superconductors

Transport measurements distinguish the superconductive phase from the parent antiferromagnetic and the overdoped metallic phases in La2−xSrxCuO4. For small x, holes in the CuO2 sheets form non-adiabatic polarons containing, experimentally, five Cu atoms. Within a polaron, a first-order contraction of the mean Cu-O bond length collapses the Hubbard U and transfers spectral weight from the Hubbard bands to the gap between them. In the superconductive phase above Tc, condensation into a polaron liquid produces an ɛ(k) dispersion curve, and dynamic pseudo Jahn-Teller deformations within the polaron give a Seebeck enhancement with Tmax≈140 K. The polarons order pair-wise into 〈110〉 stripes alternating with parent-phase stripes; below Tc, the stripes are perfectly ordered. In the overdoped phase, the parent phase is squeezed out; the vibronic states become homogeneous.

Critical sheet resistance and two-dimensional properties of Bi2Sr2CuOx thin films.

Electrical properties of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{\mathit{x}}$ thin films prepared by the RF magnetron sputtering method have been investigated. By varying the Bi content in the film, the resistance of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{\mathit{x}}$ changed over a wide range. Samples with low resistance showed superconductivity with a zero-resistance critical temperature of about 5 K, and the temperature dependence of the resistance above the superconducting transition temperature was metallic. On the other hand, samples with high resistance did not show superconducting transition and the temperature dependence of the resistance was semiconductorlike. The critical value of the normal resistance per Cu-O sheet for the appearance of the superconductivity was found to be very close to the quantum resistance (h/4${\mathit{e}}^{2}$). In the case of superconductive films, the power law of current-voltage characteristics (V\ensuremath{\propto}${\mathit{I}}^{\mathrm{\ensuremath{\alpha}}}$) was observed and the temperature dependence of the exponent \ensuremath{\alpha} showed the universal jump, which suggested the dissipation at the superconducting transition due to the Kosterlitz-Thouless (KT) transition. The temperature dependence of the resistance also supported the KT transition. The fluctuation effect above the mean-field BCS transition temperature was interpreted by the 2D Aslamazov-Larkin theory. In the case of insulating films, their behavior was examined by several mechanisms. The Anderson localization could be responsible for the ${\mathit{R}}_{\mathrm{\ensuremath{\square}}}$-T characteristics of the sample of which sheet resistance was just above the critical resistance in the low-temperature region. The intriguing possibility of the KT transition of charges for high-resistance samples was also discussed.

Transmission electron microscopy study on 2a×2b×c superstructure in bulk (Bi,Pb)2Sr2Ca2Cu3O10+δ cuprates

For 7,7,8,8-tetracyanoquinodimethane (TCNQ)-treated (Bi,Pb)2Sr2Ca2Cu3Oz[(Bi,Pb)-2223] super- conductors, a superstructure which can be described as 2a0×2b0×c0 has been observed by electron diffraction. Superconductivity is virtually destroyed in the TCNQ-treated (Bi,Pb)-2223 cuprate. Superconductivity (with Tc up to 110 K) may be recovered for a TCNQ-treated sample by being annealed in flowing oxygen gas. Then the 2a0×2b0×c0 superstructure is found to disappear. This suggests that the superstructure may be caused by an ordering of oxygen vacancies. High resolution transmission electron microscopic observations reveal that the oxygen vacancies reside not only in the CuO2 sheets, but also likely in the SrO sheets. A model for the superlattice in both CuO2−x and SrO1−y sheets is proposed.

Electronic structure of the T*-type superconductors (Bi 0.5 Sr 1.5 )(Y, Ce) 2 Cu 2 O y studied by photoelectron spectroscopy

The electronic structure of T*-type compounds (Bi0.5Sr1.5)(Y2-xCex)Cu2 Oy has been studied by x-ray photoelectron spectroscopy. From analysis of the results, it was found that cation deficiency in Ce4+ introduced holes to conduction planes in the T* phase as in the p-type infinite layer compounds. Some Cu ions in the sample without cation-deficiency contained two-oxygen coordination, which resulted in the incompleteness of the CuO2 sheets. Bi ions in the T* phase cuprates take exact valence of 3+, Pb ions cannot replace them due to the too large an ionic radius of Pb2+/Pb4+.

Atomic-Scale Images of the Growth Surface of Ca 1– x Sr x CuO 2 Thin Films

The surface microstructure of c-axis (Ca,Sr)CuO(2) thin films, grown by laser molecular beam epitaxy on SrTiO(3)(001) substrates, was studied by ultrahigh-vacuum scanning tunneling microscopy (STM). Images were obtained for codeposited Ca1-xSrxCuO(2) thin films, which show a layered-type growth mode. The surfaces consist of atomically flat terraces separated by steps that are one unit cell high. A pronounced dependence of the growth mechanism on the Sr/Ca ratio of the films was observed. Atomic resolution STM images of the CuO(2) sheets in the ab plane show a square lattice with an in-plane spacing of 4 angstroms; the lattice contains different concentrations of point defects, depending on the polarity of the sample-tip bias.

新たな観点から見つめ直した高温超伝導体の結晶構造

The extended electrostatic valence rule is applied to several superconductors and related compounds to explain differences in interatomic distances within coordination polyhedra. A general structural feature is indicated that bonds between apical oxygen and atoms opposite to Cu on CuO2 sheets are relatively short and covalent. A new method in conformity with this idea is proposed for describing the crystal structures of superconductors.

Effect of V, Nb, and Ta Doping on the 2223 Phase Formation and the Crystal Structure in the BiSrCaCuO System

X-ray diffraction and electrical and diamagnetic analyses revealed that the 2223 phase was significantly enhanced by high-valence cation (V5+, Nb5+, Ta5+, etc.) doping in BiSrCaCuO samples. The optimum nominal composition was Bi1.6M0.4Sr2Ca2Cu3 Oy(M = high-valence cations) for all three elements. A quasi-high-valence peak of Cu2p3/2 (∼534.5 eV) has been observed. A reversible reaction, Bi3+⇆annealedquenched Bi5+ + Bi+, was also encountered in the sample doped with Nb5+. The effects of doping high-valence cations on the high-Tc phase formation and the crystal structure are two-fold: (1) The high-valence cation substitution for the fourfold-coordinated Cu site in the CuO2 sheet strengthens the coupling among the Ca-CuO2-Ca layers of the 110 K phase, and (2) the cations incorporated into the Bi-O layers increase the oxygen bonding, which is beneficial for the formation of the Aurivillius-type structures and stabilizes the structure of the 2223 phase. We argue that the beneficial effect for promoting the formation of the 2223 phase might be a general property of doping with high-valence cations.

Low-Frequency Relaxation Spectrum of La 2 CuO 4 Oxide Compound

La_2CuO_4, the parent compound of the La system high-temperaturesuperconductors, can be rendered superconducting at～40 K by substituting A~(2+)(A=Ca, Sr, Ba) at La~(3+) sites or introducing excess oxygen to compound by annealingunder high oxygen pressures as well as other treatments. It is a hard Moot insulatorof which crystal cells contain Cu-O sheet, and 3D antiferromagnetic property has

Optical absorption in the two-dimensional two-band model

The optical properties are studied within the two-dimensional two-band model with the antiferromagnetic exchange interaction between nearest-neighbord andp spins in the Cu-O sheet. The optical-absorption spectra are computed within the mean-field theory with the periodic boundary condition. An insulating gap of about 1.7 eV appears for nondoping. A Drude peak appears for a singlep hole. The Drude peak and the in-gap state appear for twop holes. The intensity of the charge-transfer band decreases, while those of the Drude peak and the in-gap state increase with doping. It turns out that the strong Coulomb repulsion and the $$d_{x^2 - y^2 } $$ orbital are important such as thep x orp y orbital in the electronic properties of high-T c copper oxides.

Irreversibility line for a Pb-doped Hg-Ba-Ca-Cu-O superconductor

By measuring the magnetization hysteresis, we studied the critical current density, Jc, and irreversibility fields, Hirr, of a c-axis aligned Hg-Ba-Ca-Cu-O sample doped with Pb containing the Hg-1223 phase as the main phase. Anisotropy in ΔM was clearly observed in two different magnetic field directions. Even though the Hg-1223 phase contains three CuO2 sheets, the Hirr vs [1-(T/Tc)] curve was located at roughly the same positions as those reported for the Hg-1201 and Hg-1212 phases, indicating that the higher Tc phase, i.e., Hg-1223, is advantageous to the other phases in practical uses.

Thallium solubility range in Tl2 –yBa2Can– 1CunO2n+ 4 –xsuperconductors

The Tl2 –yBa2Can– 1CunO2n+ 4 –xphases corresponding to n= 1, 2 and 3 have been synthesized at 860–880 °C under sealed-tube conditions and characterized by wet chemical analysis, X-ray diffraction and magnetic susceptibility. A lower Tl solubility limit of (2 –y)= 1.25 was found for all n, but the upper limit decreased from 1.89 for n= 1 to 1.80 for n= 2 and 1.65 for n= 3. Under our controlled synthetic conditions, which allowed variation of the Tl concentration under similar temperatures and oxygen partial pressures, the equilibrium oxygen content was found to vary with the change in Tl concentration so as to keep a constant oxidation of the CuO2 sheets. Over the entire Tl solubility ranges, the superconductive onset temperatures were nearly constant: Ton= 117 ± 1 K for n= 3, Ton= 105 ± 1 K for n= 2, and Ton= 86 ± 1 K for n= 1. The superconductive properties improve sensitively with chemical homogenization, and homogenization was achieved most easily for n= 2. Processing of Tl-2212 superconductors with a controlled Tc under sealed-tube conditions appears to be feasible.

Quantitative HREM analysis of planar defects in high-pressure synthesized infinite-layer superconductor

Planar defects in the infinite-layer (Sr1-xCax)1-yCuO2 structure (planar CuO2 sheets separated by single Sr and Ca cations) have been suggested to be responsible for superconductivity with Tc up to about 110K. It is therefore important to understand the details of these defects at the atomic scale. In this work, we have used high resolution transmission electron microscopy to identify these defects and χ2 minimizations to best fit the experimental images; an optimized structure model from these studies is proposed.Thin samples of the infinite layer superconductor with the nominal chemical formula (Sr1-xCax)1-y CuO2 (x=0 and 0.3; y=0.9, 1.0 and 1.1) synthesized by high pressure treatment were prepared using a combination of mechanical polishing, dimpling and ion milling techniques. Through focal (10 nm steps) [100] HREM images of the defects (Fig. 1) were obtained on a 300kV Hitachi H-9000 microscope. HREM image simulations were carried out using NUMIS software and these were quantitatively compared against the experimental images using conventional χ2 minimizations with SEMPER software.

Crystal Structure and Magnetic Properties of BaCu 2(PO 4) 2 Phosphate

The crystal structure of BaCu 2(PO 4) 2 has been determined from X-Ray single crystal data. The crystal symmetry is triclinic, space group P1̄, with unit cell constants a = 9.226(1) Å, b = 9.271(1) Å, c = 10.516(1) Å, α = 106.76(5)°, β = 101.69(5)°, and γ = 115.70(5)°. The structure was solved using 2802 independent reflections ( R = 0.0411, R w = 0.0413). The framework corresponds to isolated sheets of mixed CuO 5 and PO 4 polyhedra which are linked by the Ba 2+ cations. The copper atoms constitute isolated magnetic chains parallel to the [110] direction. The magnetic susceptibility of BaCu 2(PO 4) 2 has been interpreted in terms of a one-dimensional magnetic system.

Mechanisms for hole doping in thallium cuprate superconductors having single copper-oxygen sheets

This article describes some recent studies on the effect of chemical substitution on the superconducting properties of thallium cuprates containing single Cu-O sheets interleaved with single and double Tl-O layers. These studies strongly suggest that, for Tl 2Ba 2CuO 6, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x 2− y 2 bands, i.e., the Tl-O layer creates holes in the CuO 2 layers. This is in contrast to TlBa 2CuO 5-type phases, where the metal chemistry forces the oxidation of CuO 2 layers. However, studies on TlSr 2− x La x CuO 5 and TlBa 1− x Sr x LaCuO 5 systems indicate that for Tl-O single rock-salt layer phases with a short in-plane Cu-O distance, such as TlSrLaCuO 5, the CuO 2 layer x 2− y 2 band is raised high enough in energy for the Tl-O layer to create holes in the CuO 2 layer