CuO2 Sheets Research Articles

Superconductivity in the alkaline earth-copper-oxygen system with Tc=110K

Abstract Use of high pressure of a few GPa at elevated temperatures around 1200K has proved to provide us with new materials containing two-dimensional CuO2 sheets. As one of the results the “infinite-layer” parent structure was found to be stabilized for a wide composition of Ba 1 3 Sr 2 3 CuO2 to Ca 2 3 Sr 1 3 CuO2 through SrCuO2. If the atmosphere during high pressure treatment is oxidizing, superconductivity appears in a wide composition range. It has been found that the superconductor crystallizes in the infinite-layer structure which is not ideal but is modified to tolerate nonstoichiometry in a partially ordered way. Typical experimental data obtained from (Ca1−xSrx)1−yCuO22 (x=0.7,y

Local Structure and Superconductivity of CaLaBaCu3Oy

The local structure and electronic state of CaLaBaCu3Oy (CLBCO) as a function of the oxygen composition (6.66<y<6.90) are investigated by XAFS on the Cu K-edge using synchrotron radiation. The XAFS spectra show that the in-plane local structure of CaLaBaCu3Oy is tetragonal from the microscopic viewpoint due to the disordered oxygen vacancy and it is indicated that the self-doping of hole into the CuO2 sheet does not occurred for y>6.66 in contrast with YBa2Cu3Oy (y<6.70).

Superconductivity and Electrical Resistivity in the T′-Phase Pr2-yLay-xCexCuO4-δ

Effects of the La substitution for Pr on the superconductivity and the electrical resistivity have been investigated in Pr2-yLay-xCexCuO4-δ of the T′-phase. For heavily La-substituted samples of 1.0≦y≦1.2, superconductivity of the bulk appears in the range of smaller x values, 0.10≦x\\lesssim0.15. This is discussed in relation to the electrical resistivity and the carrier concentration in the CuO2 sheet.

High-Temperature Superconductors 1992: Bringing the Materials Under Control

As I write, it is about six years since Bednorz and Müller sent off their amazing paper reporting superconductivity at about 30 K for a mixed phase sample in the La-Ba-Cu-O system. After an incubation period of a few months, during which only a handful of people paid any attention, the community suddenly woke up (literally overnight) to the realization that the discovery was genuine, when Kitazawa (Tokyo University) and Chu (University of Houston) confirmed the result at the 1986 Fall Meeting of the Materials Research Society. No need to repeat the stories of the next frantic couple of years: the Nobel Prize for the discovery, the tantalizing prospect of another prize for understanding the superconducting mechanism, the almost limitless prospects of new superconducting technologies which appeared in article after article, designed not just for scientists and engineers, but for the general public at large. Now, six years later, perhaps some perspective on the high-temperature superconductivity discoveries is possible.At the fundamental science level, the discoveries have indeed been spectacular. Many layered structures based on the CuO2 sheets have been discovered. The first advance was the Tokyo group's discovery of the structure and composition of the superconducting phase La2-xBaxCuO4-δ, where x is optimally about 0.15. Then came the Alabama/Houston discovery of superconductivity at 92 K in YBa2Cu3O7-δ, followed rather quickly by the discoveries of 110 K superconductivity in the Bi-Sr-Ca-Cu-O (BSCCO) system at Tsukuba and then 125 K in the Tl-Ba-Ca-Cu-O (TBCCO) system by the Arkansas group.

On the optimum hole concentration, nanostructure and superconductivity in Bi-cuprate ceramics

Many outstanding questions about optimum hole concentrations, hole doping mechanisms and complex structural issues in Bi-Sr-Ca-Cu-O high temperature superconductor system could be addressed by their study in the undoped system, and by cationic substitution able to modify the anion stoichiometry of the system, in various reducing and oxidizing environments. Such critical studies in different environments are very limited. A number of studies exist in the system, in which the role and concentrations of charge carriers in oxide superconductivity are discussed by correlating Tc with in-plane Cu-O bond lengths, or with substitutional contents. However, most of the work relates to samples treated in air or oxygen where overdoping of holes in the CuO2 sheet can occur due to excess oxygen.

A new copper oxide superconductor containing carbon

SUPERCONDUCTIVITY in the high-transition-temperature (high-Tc) copper oxide superconductors seems to arise from layers of copper–oxygen squares, pyramids or octahedra. Recently the compound Sr2CuO2CO3 was found to contain layers of CuO6 octahedra1 (Fig. 1), suggesting that it might be made superconducting by appropriate doping. But the presence of carbonate as an impurity is known to degrade the superconducting properties of materials such as and (refs 2, 3), much effort having been made to minimize residual carbon in these compounds by optimizing processing methods4. It is thus of some interest to see whether Sr2CuO2CO3 can be made superconducting despite the carbonate ions incorporated in the structure. Here we report the synthesis, at 50 atm oxygen partial pressure, of superconducting (0.4≤x≤0.65, y≈O.1), with Tc (onset) up to ∼40 K and zero resistance at up to ∼26K. The crystal structure contains CuO2 sheets alternating with , slabs, which serve as charge reservoir layers: substitution of ∼10% copper for carbon in the slabs introduces holes into the CuO2 sheets, making the compound superconducting.

Superconductivity at 110 K in the infinite-layer compound (Sr1-xCax)1-yCuO2

THE 'infinite-layer' parent structure1 of the copper oxide superconductors (Fig. 1) is the simplest structure containing the CuO2 sheets that are apparently essential to high-transition-temperature (high-Tc) superconductivity. At ambient pressure only Ca1-x SrxCuO2 with x≈O.l can be stabilized in this structure1,2but at high pressures and temperatures compounds ranging from Ba1/3Sr2/3CuO2 to Ca2/3Sr1/3CuO2 through SrCuO2 can be synthesized3. We have previously reported superconductivity with Tc = 40–100 K in the Ba–Sr–Cu–O system4,5 but have not until now been able to isolate a superconducting phase. Here we report the isolation of an alkaline-earth-deficient infinite-layer phase, (Ca1-xSrx)1-yCuO2 (y~O.l), with Tc up to 110 K. In contrast to Sr1-xRxCuO2 (with R a rare-earth element and Tc⩽43 K), which from the composition dependence of the lattice constants is thought to be an n-type superconductor6,7 our data suggest that the present superconductor is of p-type, with the carriers arising from calcium and strontium vacancies. High-resolution electron micrographs reveal defect layers, which we suggest are where the calcium and strontium vacancies are concentrated.

Compensation effect of substitutions at Cu(1) and Y sites on superconductivity in the YBa2Cu3O6.5+z system

A series of single-phase samples with the composition of Y1-xCaxBa2Cu3-yCoyO6.5+z (0<x<0.25; 0<y<0.25) have been synthesized and studied by X-ray powder diffraction analyses, and measurements of the resistivity, AC susceptibility, oxygen content and X-ray photoelectron spectroscopy (XPS). It is found that the reduction in the superconductivity induced by the substitution of Co for Cu(1) can be counterbalanced, to an extent, by the replacement of Ca for Y. This phenomenon has been explained with the opposite effect of Co doping for Cu(1) from Ca doping for Y, on the mobile hole concentration of the system. The substitution of Ca for Y induces a delocalization effect which releases the holes in the Cu-O chain and transfers them to the CuO2 sheets, leading to an increase in the mobile hole concentration while the substitution of Co for Cu(1) reduces the mobile hole concentration because of its localization effect on the holes. The XPS measurement for the core level of Cu 2p and O 1s reveals that the oxidation state of the copper changes with the dopant content, and a peak corresponding to the Cu3+ oxidation state is observed in some co-doped samples.

Hole concentration and critical temperature in Tl2 –yBa2 –zLazCuO6 –x

Samples of the system Tl2 –y Ba2 –zLazCuO6 –x with starting values of y= 0.00 and 0.35 for 0 ⩽z⩽ 0.2 have been synthesized at 860–880 °C in sealed gold tubes and then quenched in liquid N2. The lattice parameters and critical temperature Tc, were measured, and chemical analysis was made for y and x of the single-phase products before a further anneal at 350 °C in 1 atm O2. During the O2 anneal, samples with a starting value of y= 0.00 extruded thallium to form Tl2O3 and transformed from tetragonal to orthorhombic; samples with a starting value of y= 0.35 did not. The data indicate that the superconductive samples are p-type at the high oxidation side of the range of superconductor solid solutions and that all samples contain Tl 6s electrons. Without a quantitative knowledge of the concentration of Tl 6s electrons, it was not possible to obtain a quantitative plot of Tcversus the hole concentration, p, per Cu atom in the CuO2 sheets, but for Δp= 0.10, ΔTc=–52 °C, showing that Tc drops sharply with increasing p. We also conclude that Tl extrusion to form Tl2O3proceeds by an ordered removal of Tl until the Fermi energy EF falls below a mobility edge in the Tl 6s band.

Synthesis of Bi-Sr-Ca-Cu-O High-Tc Phase from a Parent Compound of Ca(Sr)CuO2

The synthesis of bulk Bi-Sr-Ca-Cu-O superconductors has been performed from Ca(Sr)CuO2 as a parent structure reacting with Bi and Sr as other containing compounds. It is proven that the high-Tc phase with three Cu-O2 sheets, Bi2Sr2Ca2Cu3O10, is readily obtained from the above special starting materials. Using the carbonates of each metal ion as starting materials, however, the low-Tc phase is mainly obtained.

The effect of a layered structure of YBa2Cu3Ox and Bi2Sr2CaCu2O8+x oxides on their superconducting properties

Anomalous temperature dependences of the lower critical fields Hc1(T) for H||c measured in YBa2Cu3Ox by the extrapolation method, the superconducting gaps Δ|| and Δ⊥ measured in Bi2Sr2CaCu2O8+x by Raman spectroscopy in two different polarizations parallel and perpendicular to the CuO2 sheets, and the scaling behavior of the Bi1.7Pb0.3Sr1.9CaCu1.908+x magnetoresistance arising from giant fluctuations in the decoupled quasi 2D CuO2 superconducting planes are analyzed in terms of the models taking into account the layered structure of the 2212 and 123 cuprates with a very weak coupling between the CuO2 planes. The importance of the proximity induced superconductivity in explaining unusual Hc1(T) and Δ⊥(T) features is emphasized.

アルカリ土類‐銅酸化物超伝導体の高圧合成

Cupric oxide superconductors are made of 2D CuO2 sheets and 2D counter sheets stacked alternatively along a crystallographic axis. These sheets match their 2D lattices through intersheet metal-oxygen bonding. Because the Cu-O and counter cation-O bonds should have different compressibilities and different thermal expansion coefficients, it is quite reasonable to assume that preparation in various pressure-temperature regions give us new materials having new structures and/or new compositions.We found previously that the so-called infinite layer structure could be stabilized at high pressure (6GPa) for a much wider alkaline earth composition range including SrCuO2 than at ambient pressure. Fortunately we recently found a superconductor with Tc-110 K having an alkaline earth deficient composition (Sr, Ca)1-xCuO2 (x-0.1).

Hole Concentration and Tc in the TI2-x-zBa2+xCa2+xCu3O10-y System

ABSTRACTSingle phase Tl-2223 samples were prepared in the TI2-x-zBa2Ca2+xCu3O10-y (z represent Tl vacancies in the range 0.22 – 0.27 and y represent oxygen vacancies in the range 0.17 – 0.33) system with x between 0 and 0.4. A small impurity phase was found at x = 0.6. All samples were found to be tetragonal. The Tc varied between 112 and 118 K for the “as-synthesized” samples. After post-annealing in a sealed quartz tube under vacuum at 750*deg;C for 10 days, the Tc increased from 116 to 122 K for x = 0.3 and from 118 to 120 K for x = 0,4 samples. Although x = 0.3 sample had small impurity phases after post-annealing, a large amount of impurity phases were found for x = 0.4 sample after post-annealing. Wet chemical analysis showed that all samples were both Tl and oxygen deficient. The Tc was found to increase with increasing hole concentration. The origin of the holes for all these samples is due to Ca2+ substitution on the TI3+ sites and overlap of Tl-6s band with the conduction band of the CuO2 sheets.

Tuning High- T c Superconductors via Multistage Intercalation

Multistage intercalation has been used to tune the interaction between adjacent blocks of CuO(2) sheets in the bigh-T(c) (high superconducting transition temperature) superconductor Bi(2)Sr(2)CaCu(2)Ox. As revealed by atomic-resolution transmission electron microscopy images, foreign iodine atoms are intercalated into every nth BiO bilayer of the host crystal, resulting in structures of stoichiometry IBi2nSr2nCanCu2nOx with stage index n up to 4. An expansion of 3.6 angstroms for each intercalated BiO bilayer decouples the CuO(2) sheets in adjacent blocks. A comparison of the superconducting transition temperatures of the pristine host material and intercalated compounds of different stages suggests that the coupling between each pair of adjacent blocks contributes approximately 5 K to T(c) in Bi(2)Sr(2)CaCu(2)Ox.

Scaling of the mixed state magnetoresistance of high-Tc superconductors in terms of two-dimensional superconducting fluctuations

We show scaling properties of the magnetoresistance of Bi2Sr2CaCu2O8 thin films below Tc=89 K in fields up to 20 T. The functional dependence is that of two-dimensional superconducting fluctuations above a critical temperature of 30 K. We propose that the double CuO2 sheets in each unit cell behave as 2D superconducting units with giant fluctuations.

Material overview of high-T c oxides

Structures of the existing cuprate superconductors can be viewed as alternating stacks of conducting CuO 2-sheets and insulating ionic layers (block layers). With a concept of the block layer, we review the classification and material design of CuO 2-layer based superconductors and present the empirical diagram for the carrier-doping characteristics. Strong dependence of T c on the nominal metal valence (or filling parameter) in copper- and non-copper based oxide superconductors is pointed out.

SUPERCONDUCTIVITY MECHANISM IN YBa2Cu3O7−x SYSTEMS FROM DIFFERENT DYNAMICAL ORBITAL POLARIZATIONS

The electronic structure of the normal state of YBa 2 Cu 3 O 7−x for x=0 is analyzed by means of a model which includes different dynamical potentials for each localization channel (p and d). A feature of the electronic structure of YBa 2 Cu 3 O 7−x which has large influence on the appearance of the superconductivity is the different number of holes in the atoms of the CuO2 sheets with respect to the same atoms of the CuO3 chain. For increasing x-values, vacancies of neutral O4 atoms are produced and approximately an electron per O4 vacancy is added to the conduction band. The free charge added to the conduction band will tend to occupy fundamental states arising from the pz-orbital of O1 and dx2-y2/d3z2-y2 of Cu1 since these symmetries are dominant of EF. The process of O4 vacancy production produces a reorganization of the Cu sheet holes and they suffer an interatomic screening which implies coupling for the determined bandwidths and densities of unoccupied states.

Influence of the spacing of Cu-O2 sheets on the superconducting transition temperature

Relationships between Tc and crystal structures of copper-oxide superconductors are examined from the view point of the interaction between Cu-O2 sheets. The Tc correlates with the Cu-Cu distance between the different Cu-O2 sheets, and there is a critical length around 4-6 AA. For longer distances, the Tc are less than 50 K. For shorter distances, on the other hand, the Tc are distributed from 50 to 125 K. In the latter case, Tc seems to linearly decrease with the Cu-Cu distance.

特集 原子層制御から原子制御へ ９． 各種材料の層状成長 ９．１ 高温超伝導薄膜結晶の原子層成長

The mechanism and the method of atomic layer growth of copper-based oxide superconducting thin films are discussed. Taking advantage of the two dimensional layered structure of high-Tc superconductors, these layered structures have been constructed by a layer-by-layer deposition method using laser MBE method. The basic structural parameters based on the CuO2 sheets have been successfully changed to control the high-Tc superconductivity with these methods.

Oxygen content and superconductivity of Ca-to-Y substitution in YBa2Cu3Oy

The oxygen content and superconductivity of Ca-to-Y substitution in YBa2Cu3Oy was studied. By comparison with that of Pr substituted YBa2Cu3Oy system, it is found that the oxygen content as well as the formal oxidation state of Cu is not a factor directly predominating Tc. The depression of Tc is connected with the reducing of the mobile holes in CuO2 sheets.