On the structure of non-superconducting YBa2Cu3O6+ɛ

The crystal structure of non-superconducting, tetragonal YBa₂Cu₃O₆₊ ( close to 0) (s.g.P4/mmm) was studied at room temperature by means of single crystal X-ray diffraction, electron microscopy and electron diffraction. Crystals may easily be obtained by heating YBa₂Cu₃O₇ samples under vacuum. For 0, the tetragonal single crystals are chemically and crystallographically homogeneous — without any twin domains. The only defects immediately apparent are stacking faults alongc. The structure is very closely related to that of the high-T _c superconductor YBa₂Cu₃O₇. The most important difference is, that the oxygen on theb-axis (O(1)), which together with Cu(1) forms Cu–O–Cu–O-chains alongb in YBa₂Cu₃O₇, is vacant in YBa₂Cu₃O₆. Some crystals show a superstructure with superstructure reflections which are sharp in thea ^*–a ^*-plane but form diffuse streaks alongc ^*. We propose a model which relates these superstructure reflections to a 2-dimensional ordering of residual oxygen atoms on the O(1) site (corresponding to >0 in the chemical formula YBa₂Cu₃O₆₊).     

Phase transformation and microtwinning in crystals of the high-T c superconductor YBa2Cu3O8−x,x≅1.0

We employed transmission electron microscopy (TEM) and single crystal X-ray diffraction techniques, supplemented by differential thermal and thermo-gravimetric analysis (DTA, TG) and high temperature powder X-ray diffraction, to study microtwinning in orthorhombic crystals of the high-T _c superconductor YBa₂Cu₃O_8–x (x1.0). This twinning is associated with a structural phase transition at 750°C from a tetragonal high temperature phase (s.g.P4/mmm) to the orthorhombic ambient temperature phase (s.g.Pmmm) and seems to be inherent to virtually all orthorhombic crystals of YBa₂Cu₃O_8–x . The domain size ranges from typically 100 Å to 1000 Å. All our observations are compatible with a twin law where the tetragonal (110)-mirror plane is the twin element.     

Stable tetragonal HTSC Fe-doped 1-2-3 phase with oxygen excess

Mössbauer study was performed on the YBa₂Cu_2.95Fe_0.05O _y compound prepared in three different states: orthorhombic phase with y7, tetragonal non-superconducting phase with oxygen deficiency, and a new tretragonal superconducting phase with oxygen excess (y>7). The new synthesis method was used for preparation of the Fe-doped YBa₂Cu₃O _y (named 1-2-3) compound. The comparison of Mössbauer spectra of three different states allowed us to identify the oxygen environments of the Fe atoms in the Cu1 positions.     

Evaluation of the low temperature Mössbauer spectra of YBa2(Cu0.9Fe0.1)3O6.9 by means of a field distribution model

57Fe Mössbauer spectra of YBa₂(Cu_0.9Fe_0.1)₃O_6.9 measured at low temperatures show spontaneous magnetic ordering combined with broad resonance lines. It is shown that these spectra can be well understood assuming an asymmetric distribution of the absolute values of the magnetic field at the nucleus. For the fitting procedure, we used the full Hamiltonian for combined electric and magnetic static hyperfine interaction. The number of free parameters for the fits was restricted as much as possible. For the iron sites A and B (Cu(1) sites), the ordering temperature is 26.5 K, for the iron site C (Cu(2) site), it is 31 K. The mean saturation hyperfine magnetic field is different for the different iron sites (26 T for the A site, 20 T for the B site and 43.5 T for the C site). Using these fit results and our results on thin films, we can give a consistent interpretation of the Mössbauer spectra of YBa₂(Cu_1-xFe_x)₃O_7–.     

Temperature dependence of the magnetic penetration depth in YBa2Cu3O6.95

We report precision transverse fieldSR measurements of the internal field distribution in the vortex state of crystalline YBa₂Cu₃O_6.95. A novel low background apparatus was used to study a mosaic sample of three high quality single crystals (T_c=93.3K). The observed frequency spectra in magnetic fields of 5kG and 15 kG applied along the c-axis have the characteristic features expected for a regular vortex lattice with some additional broadening. From a preliminary analysis we find that [(0)/(T)]² has a linear temperature coefficient forT<30 K. Such a term is inconsistent with simple s-wave pairing in the superconducting state. These results support recent microwave measurements of(T) on similar crystals in zero applied field but differ significantly from previousSR reports on sintered powders and crystals with lower T_c.This research was supported by the Natural Sciences and Engineering Research Council of Canada.     

Mössbauer and X-ray diffraction studies of the high-temperature superconductor YBa2(Cu0.99Fe0.01)3O7−δ

The high-T _c superconductor YBa₂(Cu_0.99Fe_0.01)₃O_7?δ was studied by Mössbauer spectroscopy and X-ray diffraction. Spectra taken at room temperature were fitted with four components. Annealing experiments showed that changes in the spectra at various temperatures are related to the nearest environment of Fe atoms rather than to the global lattice symmetry. Based on the results obtained, a model of lattice site assignment of Fe atoms has been proposed.     

A photoemission study (XPS and UPS) of the superconductor ErBa2Cu4O8 and comparison with YBa2Cu3O7-δ(δ≈0,1)

Core level and valence band photoemission spectra of the 124 superconductor ErBa₂Cu₄O₈ are investigated using MgK , HeI, and HeII radiation and compared with photoemission spectra of orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O_6.2 samples. It is found that the Cu 2p _3/2 core level and the UPS valence band spectra of ErBa₂Cu₄O₈ and YBa₂Cu₃O₇ are very similar but differ significantly from those of YBa₂Cu₃O_6.2. From the resemblance between the ErBa₂Cu₄O₈ and YBa₂Cu₃O₇ spectra it follows that the electronic structures of these two compounds are closely related in spite of an additional CuO chain in the 124 phase. The differences between the YBa₂Cu₃O_6.2 spectra and the other spectra are explained by the the presence of monovalent copper at the crystallographic Cu(1) sites. The Cu 2p _3/2 lineshapes of the here studied superconductors and some Cu^II compounds are discussed.     

On the problem of Fe-substitution in YBa2Cu3O7-δ: a neutron powder diffraction study

Neutron powder diffraction study of YBa₂(Cu₁-xFe _x )₃O_7- was carried out for a series of samples with 0.005x0.12. The results obtained suggest that Fe substitutes for Cu at both, Cu1 and Cu2 sites with equal probabilities. There is a significant number (9%) of vacancies at the Cu1 site. This number is decreasing with an increasing concentration of Fe in a specimen. The oxygen content does not vary forx<0.08, but increases to about 7.06 for 12% Fe. There is a very low probability that Fe substitutes for Ba.     

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 .     

μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

An X-ray diffraction and Mössbauer spectroscopic study on thec-axis texture YBa2(Cu1−x Fe x )3O7−y thick films

YBa₂(Cu_1–x Fe _x )₃O_7–y thick films (x=0, 0.01, 0.02, 0.03) on ceramic substrate were prepared. X-ray diffraction determinations show the formation of partialc-axis texture perpendicular to the surface of the ceramic substrate in the preparation process. The⁵⁷Fe Mössbauer spectra were measured at 300 K, where the angle between the incidence-ray beam and the surface of the film is 90° and 36°, respectively. The⁵⁷Fe Mössbauer spectra with=90° possess four sets of asymmetrical doublets.     

Studies of oxides related to high temperature superconductors

Samples of the series of compounds YBa₂(Cu_1?x Fe _x )₃ O_6.5+p with 0<x≤0.2 have been characterised by X-ray diffraction, oxidation titration and Mössbauer spectroscopy yielding information on the oxygen content parameterp and the charge state ratios Cu³⁺/Cu²⁺ and Fe⁴⁺/Fe³⁺.     

Localization effects in the YBa2Cu3-xCoxOy system

By measuring the X-ray diffraction, R(T) relation and Hall effect for YBa₂Cu_3-xCo_xO_y, a metal-seminconductor transition accompanied by an orthorhombic to tetragonal structural transition was found with the increase of impurity content. Simultaneously, the carrier concentration reduces sharply. The concept of electron localization is used to explain this phenomenon.     

Evidence for incipient O dimerization and absence of Cu+3 in temperature-dependent XPS of highT c superconducting oxides

From temperature-dependent X-ray photoemission (XPS) spectra between 300 and 10 K of Sr doped La₂CuO₄ and 123 compounds (YBa₂Cu₃O_7–) we deduce the absence of the Cu3d ⁸(Cu³⁺) configuration in the ground state of these Mott insulators. The metallic character of these samples and involving those Cu atoms which are relevant for the superconducting properties arises from (d ⁹ p ⁶)+(d ¹⁰ p ⁵) valence fluctuations, while the superconducting transition is indicated to be closely related to an incipient oxygen dimerization which is best detected upon cooling far belowT _c .     

X-ray superstructure determination of ordered oxygen and Cu-displacements in a single domain of YBa2Cu3O6.35

The superstructure of ordered oxygen atoms in a single domain of YBa₂Cu₃O_6.35 has been determined by X-ray diffraction. The 45^o rotated superstructure cell is orthorhombic—spacegroup Pbam, lattice constants , witha=3.8652(3) Å,c=11.815(2)Å the dimensions of the fundamental tetragonal cell. The arrangement of the oxygen atoms in the basal plane minimizes their electrostatic repulsion by avoiding near and next-near neighbors. There are displacements of copper atoms of 0.13 Å in the Cu(1) plane and 0.06 Å in the Cu(2) plane. Only 12% of the oxygen atoms in the basal plane contribute to the superstructure.     

Mössbauer study of Fe-substituted orthorhombic and tetragonal YBa2(Cu1-xFex)3O7-σ system

Mössbauer study of orthorhombic and tetragonal YBa₂(Cu_1-xFe_x)₃O_7-σ;x=0.02, 0.04, 0.08 has been done to investigate the two inequivalent Cu-sites. Fe substituting Cu having pyramidal oxygen co-ordination is in Fe³⁺ state while Fe substituting Cu having square planar co-ordination is in Fe³⁺ and Fe⁴⁺ states in dynamic equilibrium.     

Structure of grain boundaries in YBa2Cu3O7 thin-film step-edge junctions

The atomic structure of 90° [100] (or [010]) tilt grain boundaries in YBa₂Cu₃O₇ thin-film step-edge junctions and, for comparison, in the interface between a-axis and c-axis oriented YBa₂Cu₃O₇ grains is investigated by means of high-resolution transmission electron microscopy. For (100)(001)-type boundaries two different structures are found. In the first a (001) CuO₂ plane of one grain faces a (100) Y–Ba–O plane of the other grain, in the second a (001) BaO plane faces a (100) Cu–O plane. In the former structure an incomplete unit cell of YBa₂Cu₃O₇ terminates at the boundary and a smaller strain in the adjacent CuO₂ planes is detected in comparison with the latter. It is found that a combination of a partial dislocation with a 124 stacking fault is a way to accommodate the lattice mismatch between c and 3a of YBa₂Cu₃O₇ in the boundary. For a symmetric (103)(103)-type boundary a displacement of the Cu-atoms of the CuO₂ planes is found near the boundary plane. From this a redistribution of the oxygen atoms around the Y-atoms located right in the boundary plane is inferred. The possible effect of the boundary structure on the superconducting properties of YBa₂Cu₃O₇ films is discussed.     

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.     

Femtosecond relaxation dynamics of electronic processes and electron-phonon coupling in YBa2Cu3O7-δ and PrBa2Cu3O7 epitaxial films

The room temperature femtosecond relaxation dynamics of reflection spectra of YBa₂Cu₃O_7- with different oxygen contents and PrBa₂Cu₃O₇ epitaxial films have been studied. The relaxation times of R(t) in YBa₂Cu₃O_7- increase from less than 100 fs to more than 1 ps with increasing and with substitution of Pr for Y. The electron and lattice temperature profiles at these quasi-equilibrium state are calculated from the nonequilibrium energy-balance model. The process of electronphonon energy transfer was found to be dominated in our experiments. These results reveal that in 123 phase superconductors decrease of carrier concentration slows down the carrier-phonon coupling and the type of the electron-phonon interaction would be localized. The dependence of electron-phonon coupling to oxygen defects in YBa₂Cu₃O_7- was found to be exp(–).     

Superconductivity and magnetic order in RBa2Cu3Oz

Mössbauer studies of⁵⁷Fe in RBa_2?y K _y (Cu_1?x Fe _x )₃O_z, with R=Y and Pr;y=0 and 0.5;x=0.01, 0.05 and 0.1 andz between 5.9 and 7.1, have been performed. A minority of the iron ions enter the Cu(2) site and reveal its magnetic order. In nonsuperconducting YBa_1.5K_0.5(Cu_0.95Fe_0.05)₃O_6.1 two distinctly inequivalent magnetic iron sites are observed, probably corresponding to iron in the Cu(2) site with different Ba?K neighbours. In superconducting (T _c =60 K) YBa_1.5K_0.5(Cu_0.95Fe_0.05)₃O_6.5 one Cu(2) subsite remains magnetic (T _N=440 K). The implications of these findings on the valencies of the Cu ions are discussed.