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.     

YBa2Cu3O7-x涂层导体的外延生长和性能对CeO2缓冲层的依赖性

利用倾斜衬底沉积法在无织构的金属衬底上生长了MgO双轴织构的模板层,在这一模板层上实现了YBa₂Cu₃O_7-x薄膜的外延生长.在外延YBa₂Cu₃O_7-x薄膜前,依次沉积了钇稳定的立方氧化锆和CeO₂作为缓冲层.利用X射线衍射2θ扫描、扫描、Ω扫描和极图分析测定了这些膜的结构和双轴织 关键词： 2Cu₃O_7-x镀膜导体')" href="#">YBa₂Cu₃O_7-x镀膜导体 2缓冲层')" href="#">CeO₂缓冲层 厚度依赖性 外延生长     

Low-temperature growth of high-Tc superconducting films by plasma-enhanced metal-organic chemical vapor deposition

Plasma-enhanced MOCVD in which metal-organic compounds are sublimated directly into the growth chamber is studied for the first time as a new low-temperature process for growing superconducting YBa₂Cu₃O-_-x thin films. Y(THD)₃, Ba(THD)₂, Cu(THD)₂ and oxygen are used as metal sources and oxydizing agent. Emission spectroscopy reveals that activated metal-organic compounds and activated oxygen species are present during film growth. Superconducting YBa₂Cu₃O_7-x films whose zero-resistivity temperature are 50 K and 82 K are grown at 550°C and 600°C.     

The role of interfacial morphology in multilayers based on high T c YBa2Cu3O7-x

The interfacial microstructures in CaTiO₃/YBa₂Cu₃O_7-x /SrTiO₃/LaAlO₃/CaRuO₃ multilayers grown on (001) LaAlO₃ substrates has been characterized using transmission electron microscopy in combination with scanning electron microscopy. The morphology of films grown on both flat surfaces and on stepped surfaces patterned by ion beam milling were interpreted in terms of effects that influence epitaxial orientation relationships, nucleation of Y₂O₃ particles, and pinhole formation. The general conclusions are that a-axis oriented YBa₂Cu₃O_7-x particles should be avoided in multilayer structures. The particles give rise to cracking and pinhole formation. Local surface roughness, which may be due to ion beam milling or to the growth mechanism of YBCO, induces Y₂O₃ particle nucleation and pinhole formation. Very steep surface slopes cause the formation of a-axis YBa₂CU₃O_7-x particles. A critical size effect of the Y₂O₃ particles is suggested.     

Relationships between X-ray photoelectron spectra,structure and superconductivity in LnBa2Cu3O7-x

X-ray photoelectron spectroscopy has been carried out on HoBa₂Cu₃O_7-x (on abraded, fractured and ion beam reduced surfaces). These measurements show that there are only minor differences with similarly obtained results from YBa₂Cu₃O_7-x. The spectral features of the Ln species (Y and Ho), the O 1s, and Ba 3d envelopes are considered in some detail. The low energy contribution to the O 1s envelope has been assigned in part to the effects of Ba–O bonding, either in the LnBa₂Cu₃O_7-x material, or in BaO in minor phase regions, and in part to the effects of the electronic linkage of the Cu planes in the superconducting compound. The anomalous low binding energy contributions to the Ba 3d envelope are also specific to the electronic structure of these materials. It is likely that Ba (and Sr and Ca for the Bi-Sr-Ca-C-O system) plays an important electronic, as well as structural role, in promoting high temperature superconductivity.     

High temperature study of yttrium--barium--copper oxides near the YBa2Cu3Ox composition by 111In/Cd time differential \gamma--\gamma perturbed angular correlation spectroscopy

The local atomic order around very dilute indium impurities in c-axis-oriented YBa₂Cu₃O₇ and YBa₂Cu₃O_6.25 films at room temperature and in YBa₂Cu₃O_x bulk powders held in air or oxygen over a wide temperature range were investigated using ¹¹¹In/Cd \gamma--\gamma perturbed angular correlation (PAC) spectroscopy. The probe atoms occupy a single substitutional lattice position in YBa₂Cu₃O_x, and evidence reported here strongly supports previous claims that this is the yttrium position. In YBa₂Cu₃O_x powders, the local atomic order changes continuously with temperature. At room temperature the electric field gradient measured by PAC in bulk YBa₂Cu₃O_x powder is indistinguishable from that in the fully oxygenated YBa₂Cu₃O₇ film. Near the decomposition temperature, the bulk YBa₂Cu₃O_x powder data are semiquantitatively similar to data in the room temperature YBa₂Cu₃O_6.25 film, with quantitative differences that we attribute to temperature averaging in the former. Other sites usually found in PAC spectra of YBa₂Cu₃O_x bulk samples arise from ¹¹¹In dissolved in Y₂BaCuO₅ and Y₂Cu₂O₅ impurity phases. The latter phase apparently arises because of significant barium loss during processing; the barium deficiency is clearly demonstrated by comparison of PAC data with the alloy phase diagram above the decomposition temperature. PAC data on pure Y₂Cu₂O₅ are reported here also. This revised version was published online in July 2006 with corrections to the Cover Date.     

Antiferromagnetic order in PrBa2Cu3O x (x≈6, 7)

Zero and weak transverse fieldμ ⁺ SR measurements on PrBa₂Cu₃O _x samples withx≈6 andx≈7 show ordered magnetism in both oxygen concentration limits. As expected for equivalent doping (nominally PrBa₂Cu₃O _x ∼YBa₂Cu₃O _x −0.5), neither sample is superconducting. Two muon signals are observed in thex≈6 sample, as in YBa₂Cu₃O₆, but only one (the weaker) has the same local magnetic field as in YBa₂Cu₃O₆. In thex≈7 sample, only one site is observed; its local field is reduced with respect to that of the primary site in YBa₂Cu₃O₆ by a factor roughly consistent with the carrier-density dependence of the Cu ion moment in antiferromagnetic YBa₂Cu₃O _x .     

Non-superconducting and rendered superconducting YBa2Cu3O7−x . An in-situ photoemission study

We present X-ray photoemission spectroscopy (XPS) results obtained on polycrystalline YBa₂Cu₃O_7–x in the non-metallic (x>0.6) and metallic state (x<0.6) by an in-situ oxygen deloading/loading procedure, thus avoiding uncontrolled surface contaminations. The transition to the metallic (superconducting) state is characterized by corresponding changes in the O1s and Cu2p photoelectron lineshapes being in accordance with charge transfer from CuO₂ planes to CuO chains. In particular, a peak at 531.1 eV binding energy is related to the presence of oxygen hole states (O^–).     

X-ray photoelectron spectroscopy of the high-temperature superconductor YBa2Cu3O7−x and the copper oxides

The X-ray photoelectron (XPS) core-level spectra of Cu-metal, Cu₂O, CuO, NaCuO₂ and YBa₂Cu₃O_7–x have been determined. It has been shown, that the Cu³⁺ ions in NaCuO₂ are reduced to lower valence states by X-rays in ultrahigh vacuum (UHV). The determination of the electron binding energy of the Cu 2 _{p 3/2} level in NaCuO₂ yields a value of 934.1 eV. The shape of the Cu 2 _{p 3/2} peak as well as the appearence of a shake-up satellite in YBa₂Cu₃O_7–x which is smaller than in CuO show that YBa₂Cu₃O_7–x contains Cu³⁺ ions. The behaviour of YBa₂Cu₃O_7–x also was studied during X-ray irradiation and a reduction was noticeable.     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa₂Cu₃O_x and Nb/Au/YBa₂Cu₃O_x heterojunctions in which the c axis of the YBa₂Cu₃O_x (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO₃ substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa₂Cu₃O_x were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.     

Pressure effect onT c and resistivity: Localization in YBa2Cu3O x oxides

We have studied the effect of hydrostatic pressures up to 20 Kbar on the temperature dependence of the resistivity (T) and the effect of quasihydrostatic pressure up to 200 Kbar on the lattice parameters of YBa₂Cu₃O _x for different oxygen concentrations (x=6.95–6.2). Pressure produces a decrease of resistivity in normal state, an increase ofT _c , and a suppression of semiconducting-like resistivity (T) at lowx. The dependence of dT _c (x)/dP onx is nonmonotonic; the record values of dT _c /dP=(1.0±0.1) K/Kbar are observed forx=6.7 and 6.8. The derivative dln/dP atT=293 K differs by the factor 1.8 between superconducting and nonsuperconducting compounds. The compressibility and its pressure derivative alonga, b andc-directions in YBa₂Cu₃O _x have been determined. The most remarkable variation is alongc-direction. A nonmonotonic dependence of dk _c (x)/dP onx has been observed.The results are discussed in the context of localized effects in disordered oxygen-deficient YBa₂Cu₃O _x .     

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y₁Ba₂Cu₃O_7–x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0x0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu²⁺ and Cu³⁺ in YBa₂Cu₃O_7–x is constructed.     

Relationship between oxygen stoichiometry and crystal structure in YBa2Cu3O6.5+x precursors to high-Tc superconducting materials

Possible mechanisms for the oxygen intercalation in the ternary mixed-Cu-valence YBa₂Cu₃O_6.5+x oxide are considered, which account for the observed cell symmetry transformation driven by the oxygen concentration. These mechanisms are consistent with the formation of both single-and double-cell superstructures of ordered oxygen vacancies that have been encountered in crystallographic investigations of the high-T_c superconductor YBa₂Cu₃O_≈7.     

NdBa2Cu3O7-δ/YBa2Cu3O7-δ多层膜体系的外延结构和磁通钉扎的研究

采用脉冲激光沉积技术,在单晶SrTiO₃基底上外延生长了一系列名义结构为p×(NdBa₂Cu₃O_7-δ(m)/YBa₂Cu₃O_7-δ(n))的多层膜和准多层膜(单元层NdBa₂Cu₃O_7-δ较厚而YBa₂Cu₃O_7-δ呈非连贯的岛状分布,m,n为激光脉冲数,p为重复周期).样品的超导转变温度在87—91 K范围,具体大小取决于不同的调制结构,多层膜的重复周期越大,层状界面越多,超导转变温度就越低.磁传输测量表明,准多层的样品不仅具有较高的超导转变温度,而且具有较强的磁通钉扎性能,77K零场下的临界电流密度高达4×10⁶ A/cm²,显示出良好的应用前景. 关键词： 2Cu₃O_7-δ')" href="#">NdBa₂Cu₃O_7-δ 多层膜 磁通钉扎 临界电流密度     

Top-seeded infiltration growth of Y–Ba–Cu–O bulk superconductors

A top-seeded melt-growth (TSMG) process is widely used to fabricate single domain YBa₂Cu₃O_y (Y–Ba–Cu–O) bulk superconductors. Pores are often found in the TSMG-processed Y–Ba–Cu–O samples due to the oxygen gas evolution during the molten stage. Recently developed liquid infiltration growth (LIG) process is known to be effective in suppressing the pore evolution and in refining the size of Y₂BaCuO₅ (Y211) particles dispersed in YBa₂Cu₃O_y matrix. The LIG process utilizes the liquid (Ba₃Cu₅O₈) infiltration into a pre- sintered Y211 contact and slow cooling through a peritectic temperature. In this study, we fabricated bulk Y–Ba–Cu–O superconductors by the LIG process combined with top-seeding with SmBa₂Cu₃O_y seed and confirmed that a single-domain bulk can be produced. Trapped field measurements however showed that some distortion in the field distribution was observed in the region near the seed crystal, which was attributed to Y211 density and its relatively large size.     

ARXPS-studies ofĉ-axis textured — superconducting YBa2Cu3O x -films

YBa₂Cu₃O _x sputter deposited cold on Al₂O₃ and on MgO grows in O₂ annealing epitaxially to a transparent, superconducting film withT _c=80 K. The unscraped surfaces of these films are smooth showing XPS lines changing with photoelectron take-off angle. This enhanced data base allows to separate the different chemical shifts and to obtain their spatial distribution. This yields the identification of the chemical compounds (hydroxide, peroxide, carbonate, carboxyle, cuprate, graphite ...), their amount and their distribution making up the cinder and the cuprate. The cinder grows with O₂-anneal at internal and external surfaces. Because of the large unit cell and of the anisotropy of the cuprate the chemical composition of the surface depends on orientation and changes over distances of the order of 1 nm.The cinder stoichiometry gives insights in the chemistry going on in O₂ annealing. Below the cinder the signature of-axis textured YBa₂Cu₃O _x surfaces are identified, showing that a Ba-oxide layer forms the stable surface. This Ba-oxide-surface coats Cu-oxide and Y-oxide layers yielding so an intrinsically insulating surface layer of about 1 nm thickness. In superconducting cuprates a metallic Ba-level shift occurs as compared to the Ba in oxides or cuprate surfaces or insulating cuprates, whereas Y does not show such a metallic chemical shift in the transition to superconducting cuprates. This asks new questions about charge transfer and superconducting interaction in the grains and the coupling between them.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

Positive muon sites and possibility of anyons in the YBa2Cu3O x system

Possible positive muon sites in YBa₂Cu₃O _x were determined from the observedμ ⁺ hyperfine fields in antiferromagnetically ordered YBa₂Cu₃O _x and GdBa₂Cu₃O₇. After determining theμ ⁺ sites, the possibility of anyons or chiral spin ordering in the superconducting YBa₂Cu₃O₇ is discussed. Positive muon implanted in YBa₂Cu₃O₇ feel static magnetic fields of average 1.4G, which are explicable in terms of nuclear magnetic dipolar fields. Non observation of static local magnetic fields of electronic origin (the upper limit is the order of 0.1 G) means that anyons or chiral spin ordering might not exist in superconducting YBa₂Cu₃O₇.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.