Showing papers in "Physica C-superconductivity and Its Applications in 1989"

Magnetism in La2CuO4 based compounds

Abstract: To explain recent observations of incommensurate spin modulation in doped La2CuO4 an interacting electron system is investigated from the itinerant side where a magnetic band is nearly half-filling. An analytical solution of spin density wave with infinite higher harmonics is obtained, giving rise to a soliton lattice spin structure, striped charge domain walls which run perpendicular to the modulation vector Q and associated midgap state. Temperature and doping dependences of Q are predicted.

AC susceptibility of high temperature superconductors in a critical state model

Abstract: A critical state model for a granular superconductor is employed to calculate the temperature and AC and DC magnetic field dependence of the complex susceptibility, χ = χ ′ + iχ ″, of a sintered bulk YBa 2 Cu 3 O 7- δ superconductor. Inter granular Josephson vortices are assumed to sweep in and out of the weak-link network while intragranular Abrikosov vortices move in and out of the superconducting grains, both causing bulk pinning hysteresis losses. The predictions of the model for χ′ and χ″ are consistent with experimental data and model parameters which characterize a high temperature granular superconductor can be determined. These parameters are the inter- and intragranular pinning force densities, the fraction of the superconducting grains, the grain size distribution and a London penetration depth which neglects grain anisotropy.

Correlation between the in situ growth conditions of YBCO thin films and the thermodynamic stability criteria

Abstract: Data from the literature on the successful in situ growth of YBCO thin films in terms of the growth temperature and the oxygen pressure are compared with the thermodynamic stability as determined by an electrochemical method. The correlation over a wide range of growth temperatures and oxygen pressures, and for most methods of deposition, suggests that proximity to the equilibrium decomposition line may be favorable for in situ growth.

Analysis of magnetic resonance experiments in YBa2Cu3O7

Abstract: A consistent explanation of the unusual combination of anisotropies of the Knight shifts and relaxation rates of Cu(1) and Cu(2) nuclei in YBa2Cu3O7 is given. The key features are an anisotropic on-site hyperfine coupling to local Cu2+ electronic spins and an isotropic transferred hyperfine coupling to the neighboring Cu2+-spins. We use a quantum chemical model of the Cu-O hybridization to estimate the strength of these hyperfine interactions. Good agreement is found between our estimated parameters and experiment.

Low temperature anomalies and pressure effects on the structure and Tc of the superconductor YBa2Cu4O8 (Tc=80 K)

Abstract: The structure of the 80 K superconductor YBa 2 Cu 4 O 8 has been measured as a function of temperature and pressure between 5 K and 165 K by high resolution neutron powder diffraction. The oxygen bridging CuO planes to double CuO chains moves away from the chains, toward the planes as T c increases with increasing pressure, indicating charge transfer to the planes. A similar effect occurs when YBa 2 Cu 3 O 6+ x is oxidised to YBa 2 Cu 3 O 7 , again increasing T c . The a - and c -axes of YBa 2 Cu 4 O 8 increase smoothly with temperature, but the b -axis, parallel to the direction of the CuO chains, appears to have a minimum near T c . This results in a slight inflexion in the orthorhombic distortion near T c , similar to that observed in YBa 2 Cu 4 O 7 . The oxygens within these chains appear disordered even at low temperature, again as in YBa 2 Cu 4 O 7 . However, in YBa 2 Cu 4 O 8 oxygens from the CuO-planes also have relatively large out-of-plane amplitudes. High resolution electron microscopy and diffraction show that YBa 2 Cu 4 O 8 , unlike YBa 2 Cu 3 O 7 , does not have planar structural defects; there are no twins or superstructures.

ACuO2 (A: alkaline earth) crystallizing in a layered structure

Abstract: ACuO 2 (A: Ba 1/3 Sr 2/3 ∼Sr∼Sr 1/3 Ca 2/3 ) has been found to crystallize in a 2D structure made of alternatively stacked A and [CuO 2 ] ∞ layers when treated under high pressures of 60 kbar at 1050°C. The specific volume for SrCuO 2 is contracted by 9.3% in comparison with that for the structure stabilized under ambient pressure. The tetragonal lattice constants increase linearly with increasing average A ion size below r A ≦0.121 nm but tend to be saturated above this value. The expansion is strongly anisotropic reflecting the stiffness of the Cu-O bond. For r A ≧0.134 nm the BaCuO 2 type structure is stabilized.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract: Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double PbO layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Lattice vibrations in high-Tc superconductors: Optical spectroscopy and lattice dynamics☆

Abstract: Raman and IR-measurements on the high- T c material series La 2 CuO 4 and YBa 2 Cu 3 O 7 are reviewed. This summary focuses on lattice vibrations in these superconducting systems, on impurity phases, the influence of oxygen stoichiometry and element substitution. The determination of the superconducting gap, the phonon renormalization in the superconducting state, and the isotope effect with its implications on the theory of superconductivity in these materials are discussed. In addition, the measured lattice vibrations are compared with the results of lattice dynamics calculations.

Two new electron cuprate superconductors, Pr1.85Th0.15CuO4−y and Eu1.85Ce0.15CuO4−y, and properties of Nd2−xCexO4−y

Abstract: We report the first observation of superconductivity in the Pr 2− x Th x CuO 4− y and Eu 2− x Ce x CuO 4− y systems. Superconducting resistive onset temperatures of ∼ 23 K and ∼ 13 K were observed for Pr 1.85 Th 0.15 CuO 4− y and Eu 1.85 Ce 0.15 CuO 4− y , respectively; the corresponding transition midpoints are T 0.5 c = 19.5 and 8 K . Magnetic susceptibility data indicate that the superconductivity is a bulk property. Transition temperatures higher than previously reported for Pr 1.85 Ce 0.15 CuO 4− y and Sm 1.85 Ce 0.15 CuO 4− y were also observed. Oxygen content analysis reveals that the superconducting Ce-doped specimens have quite small oxygen deficiencies. Specific heat data for Nd 2 CuO 4− y ( y ≈0.07) and Nd 1.85 Ce 0.15 CuO 4− y ( y ≈0.02) display a large peak, presumably associated with magnetic ordering of Nd 3+ ions, Which decreases in magnitude with Ce doping. Pressure-dependent electrical resistivity measurements reveal an increase of T c with applied pressure for Nd 1.85 Ce 0.15 CuO 4− y ( y ≈0.02), at a rate of approximately d T c /d P ≈0.025 K/kbar.

The crystal structure of superconducting La2CuO4.032 by neutron diffraction

Abstract: The crystal structure of superconducting (37.5 K) La2CuO4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO6octahedra). The O4 insertion causes a 0.75 A displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) A. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La2CuO4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

Bond-charge repulsion and hole superconductivity

Abstract: Is is proposed that a bond-charge repulsion term that necessarily arises in deriving a tight-binding Hamiltonian from first principles plays a fundamental role in superconductivity. This term is repulsive for bonding states and attractive for antibonding states. Together with the reduction of the local Coulomb repulsion due to atomic polarization this effect can explain the occurence of superconductivity in solids without involving the electron-phonon interaction.

Phases and their relations in the Bi-Sr-Cu-O system

Abstract: Solid phases and their relations in the Bi-Sr-Cu-O system were investigated at 840°C in air. Bi 2+ x Sr 2− x Cu 1+ y O Z (0.1 x y x /2), Bi 17 Sr 16 Cu 7 O Z , Bi 2 Sr 3 Cu 2 O Z , Bi 4 Sr 8 Cu 5 O Z and BiSr 3 O Z have newly been found. Structural data obtained by means of powder XRD and TEM for these are presented. The superconductor with a T c ∼ 10 K corresponds to the Bi-poorest end of the solid solutions mentioned above. Carrier holes are created by the presence of excess oxygen but these tend to be trapped locally for x > 0.13. The one-dimensional modulation mode showed an interesting composition dependence, from which a modified mechanisms of the modulation has been deduced.

Strong pressure dependence of Tc of the new 80 K phase YBa2Cu4O8+x

Abstract: The pressure dependence of the superconducting transition temperature Tc of the new structure YBa2Cu4O8 was measured on bulk material and single crystals as well. The differential dTc/dp was determined to be 0.55 K/kbar. Such a large change of Tc with increasing pressure was never observed before in compounds with a Tc of about 80 K. The results are discussed on the basis of Cryot's model and on polaronic based superconductivity.

Airy Pattern, Weak-Link Modeling of Critical Currents in High-Tc Superconductors

Abstract: We have measured the transport critical current density at very low magnetic fields in samples of superconducting bulk sintered Y 1 Ba 2 Cu 3 O x , Y 1 Ba 2 Cu 4 O x obtained from several sources. The results are analyzed at low fields (≤10 mT) with a statistical model which assumes that the current is limited by Josephson weak links (SNS or SIS Josephson junctions or microbridges) whose locations are to be determined. Each weak link is assumed to be described by an Airy current-field pattern rather than a Fraunhofer pattern. The former has a better theoretical foundation and is in better agreement with the data, varying as H -3 2 upon averaging. The fitting procedure yields the average cross sectional area of the weak links. By assuming the link thickness to be twice the London penetration depth at 77 K, we find that the average linear dimensions of the links are in all cases comparable to the grain dimensions. The quantitative analysis also confirms the percolation concept, in which a subset of weakest links controls the transport current.

Superconductivity up to 100 K in lead cuprates: A new superconductor Pb0.5Sr2.5Y0.5Ca0.5Cu2O7−δ

Abstract: A new superconductor Pb 0.5 Sr 2.5 Y 0.5 Ca 0.5 Cu 2 O 7−δ has been isolated whose superconducting properties depend upon the thermal treatments, i.e. are closely related to the oxygen stoichiometry. This material exhibits a T on c =100 K whereas the T off c ( R =0) ranges from 45 K to 53 K. The magnetic measurements using a SQUID magnetometer show superconducting volume fractions ranging from 14% to 72%. Anomalies appear in the magnetization curves at low field for some samples. This effect and the broad transition are interpreted in terms of an inhomogeneous distribution of oxygen in the matrix. The structure of this tetragonal phase ( a =3.81 A, c =11.908 A) belongs to the “1212” type observed for thallium cuprates, i.e. consists of an intergrowth of double rock salt-type layers involving lead and strontium and of double copper pyramidal layers interleaved with yttrium and calcium. This oxide differs also from the classical intergrowth structures by the existence of numerous oxygen vacancies in the rock salt type layers (δ≅0.7−0.5); moreoer satellites are observed on E.D. diffraction patterns. These two effects are explained by the presence of the 6s 2 lone pair of Pb(II).

Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

Abstract: A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10 -4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high- T c superconducting materials. The low-field, step-like drop in the critical current density J c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current ( V-I characteristic, represented by the transition parameter n (i.e., V ∝ I n ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J c ( B ) characteristic.

Phase diagrams of YBa2Cu4O8 and YBa2Cu3.5O7.5 in the pressure range 1 bar≤PO2≤3000 bar

Abstract: The P-T-x phase diagram of the pseudobinary system (Y-Ba-Cu-O)-O 2 has been further investigated in the oxygen pressure range between 1 and 3000 bar. The stability ranges of the phases YBa 2 Cu 4 O 8 (124), YBa 2 Cu 3.5 O 7.5− x (123.5) and YBa 2 Cu 3 O 7− x (123) have been determined. Long duration experiments showed that the 123 phase is not stable at least down to 7 bar≤ P ≤20 bar oxygen and 900°C. It is not clear whether at lower pressures and temperatures the 123 phase is thermodynamically stable or metastable due to low reaction rates. In the presence of excess CuO, the 124 is the stable phase. The melting of 124 pellets at P O 2 =2800 bar shows that even at this pressure the 124 compound melts incongruently. Using the phase diagram data we could change the T c of 123.5 from 16 to 70 K by varying systematically the nonstoichiometry. Due to a narrow homogeneity range the T c of 124 remained constant but is different for powder pellets (81 K) and for crystals (70 K), probably due to the influence of the flux. Single crystals of both 124 and 123.5 with dimensions up to 4 mm were grown from the flux under high oxygen pressure.

Stability of 124, 123, and 247 superconductors

Abstract: The 124 phase YBa2Cu4O8 is synthesized by solid state reaction at 930°C in P(O2)>62;30 bar, while the 247 phase Y2Ba4Cu7O15−x is favored in the narrow range of P(O2) between 10 and 30 bar. The 123 phase YBa2Cu3O7−δ forms at P(O2) 62;6.6) may be thermodynamically unstable to decomposition into 124 (plus non-superconducting compounds) at all temperatures, with synthesis of 124 at low temperatures and oxygen pressure limited only by kinetics. Since very high oxygen pressures are not required, synthesis of the 124 and 247 phases is relatively easy, this may have considerable practical importance.

Absence of current direction dependence of the resistive state of high temperature superconductors in magnetic fields

Abstract: Resistive transition of Bi 2 Sr 2 CaCu 2 O 8+ y film in magnetic fields is studied in the light of the currently proposed flux creep model. Measurements with precise angular alignment have clarified that the resistive transition does not depend on the relative direction between magnetic field and transport current. This calls for a significant revision of the Lorentz-force-driven flux creep picture. A fluctuation induced phase-slip phenomenon as an alternative model is also discussed.

3-D vacancy ordered superstructures in “homogeneous” YBa2Cu3O7-δ

Abstract: High resolution electron microscopy and electron diffraction techniques were applied to study structural aspects of YBa 2 Cu 3 O 7- δ for varying δ, i.e., oxygen contents. Particular attention was paid to the superstructures arising from oxygen vacancy ordering, three types of specimens were investigated: constant stoichiometry, cooled vacuum annealed and microscope heated samples. The most stable vacancy ordered structure appears to be the orthorhombic II or 2 a 0 structure occurring in a wide range of compositions around δ =0.5. Several other superstructures were found in the electron beam heated and rapidly cooled samples which reveal apart from 2 a 0 , also 3 a 0 , 4 a 0 and 5 a 0 structures as well as 2 a 0 ×2 a 0 and 2√2 a 0 ×2√2 a 0 structures. These phases are presumably only transient. Attention is also paid to the interpretation of diffuse intensity lines in the diffraction patterns of beam heated and rapidly cooled samples which could be interpreted in terms of the cluster theory as being due to a transition state which can be considered as a precursor to the orthorhombic II structure. Crystallographic models are proposed for the various superstructures observed in this study as related to the oxygen contents. The order-disorder transition is found to occur in stages with increasing temperature. One observes a loss of correlation of the stacking of the 2-D patterns in successive layers, disordering of the chain separation, followed by disordering within the chains, leading finally to a random distribution of the remaining oxygen atoms in the CuO 1- δ layers.

Effective transfer for singlets formed by hole doping in the high-Tc superconductors

Abstract: By comparing the exact results of an extended multiband Hubbard Hamiltonian for Cu 2 O 7 and Cu 2 O 8 clusters to those obtained from an effective single band Hamiltonian, we show that the low energy scale physics is very well described by a t - t ′- J model which describes the motion of singlets in an antiferromagnetic background of spins. We obtain values for t , t ′ and J for both hole and electron doping and show that these are different. We also study the dispersion relations and density of states within the quasi particle approximation both of which show rather interesting characteristics which could be relevant for high T c superconductors.

Josephson vortices and flux penetration in high temperature superconductors

Abstract: We have investigated, experimentally and theoretically, the magnetic flux which threads a YBa 2 Cu 3 O 7− x cylinder sample at 77 K when a weak low frequency ac magnetic field is applied. We explain the highly nonlinear magnetic response in great detail quantitatively, using a critical state model where during each ac cycle, intergranular (Josephson) vortices sweep in and out of the cylinder. The Josephson vortex pinning force is found to be field independent and several orders of magnitude smaller than in conventional type II superconductors. This explains the weak shielding properties and low critical current density found in this material. At magnetic fields greater than about 50 Oe, vortices start to enter the superconducting grains.

Copper NQR and NMR in the superconductor YBa2Cu4O8+x

Abstract: The frequencies νQ, the linewidth and the spin-lattice and spin-spin relaxation times T1 and T2, respectively, of the 63Cu nuclear quadrupole resonance (NQR) signals for chain and planar copper sites in YBa2Cu4O8 were measured between 6 and 750 K; in addition nuclear magnetic resonance (NMR) spectra were taken at 150 and 300 K. The results are compared with data known for the related structure YBa2Cu3O7. Above Tc the tensors of the electric field gradient and the anisotropic Knight shift at the copper sites differ in a subtle way from those of YBa2Cu3O7. At the Cu2 site, the derivative of νQ with respect to temperature changes its sign around 200 K. No indication for magnetic ordering at the Cu sites was found. Below Tc there is no evidence for BCS type relaxation and the Cu2 Tc rates vary like exp( T T 0 ) and the Cu1 rates like Tn with n≈1.35. Above Tc the Cu1 rates suggest the presence of quasiparticles (and phonon interactions) while the Cu2 rates can perhaps be explained by a model of Monien et al. postulating the coupling of Cu nuclei to itinerant interacting quasiparticles.

Quadratic temperature dependence of the in-plane resistivity in superconducting Nd1.85CuO4 — Evidence for Fermi-liquid normal state

Abstract: The in-plane normal-state resistivity, ϱ|;( T ), of the electron-doped superconductor Nd 1.85 Ce 0.15 CuO 4 has been measured and analyzed as a function of temperature in the range of T c to 300 K where T c ( R =0)=20 K. A dominant T 2 temperature dependence of ϱ|( T ) for T T 200 K, the data indicate a logarithmic correction to the quadratic temperature dependence. These findings are consistent with the Fermi-liquid model for electron-electron scattering, which is greatly enhanced in the CuO 2 -layered oxides. The experimental results of this work lend strong support to the validity of a Fermi-liquid description for the normal-state of electron-doped Cu-oxide superconductors.

Structural behavior and chemical order of Fe in YBa2(Cu1 − xFex)3O7 + δ

Abstract: Neutron and Mossbauer effect measurements have been made as a function of Fe concentration in orthorhombic ( x = 0.01, 0.02) and tetragonal ( x ⩾ 0.05), YBa 2 (Cu 1 − x Fe x ) 3 O 7 + δ . A systematic Rietveld analysis of the neutron data for all concentrations shows that Fe principally occupies a site slightly displaced ( y , y , 0) from the Cul (0, 0, 0) site in order to approach tetrahedral coordination. Neutron data for the x = 0.05 compound at 10 K show no significant difference from those obtained at 297 K. Local ordering of oxygen about an Fe substituent is conducive to placement of a second Fe atom in an adjacent site leading to local aggregation into chains of various lengths. This clustering is reflected in the character of low temperature magnetic Mossbauer spectra and in specific heat measurements. The Mossbauer spectra yield the relative changes in oxygen configurations about Fe atoms as the Fe concentration changes.

Anti-Meissner effect in the BiSrCaCuO-system

Abstract: The low field susceptibility of a sintered sample of Bi 2 Sr 2 Ca 1 Cu 2 O 8+δ has been investigated in a SQUID magnetometer. In the low field FC susceptibility a new feature of high-T c superconductors is revealed, an antimeissner effect. Vortex pair fluctuations and pinning are believed to be essential for the observed behaviour.

Structural modulation and superconducting properties in Bi2-xPbxSr2CaCu2O8+d and Bi2-yPbySr2YCu2O8+d

Abstract: Changes in the structural modulation and the superconducting properties were examined in Bi 2- x Pb x Sr 2 CaCu 2 O 8+ d and Bi 2- y Pb y Sr 2 YCu 2 O 8+ d systems. In both systems, the b -axis decreased and the periodicity of the structur al modulation along the b -axis increased with increasing Pb content, and the structural modulation disappeared in a high Pb concentration region. These changes in structural modulation were explained by the decrease in additional oxygen in the Bi-O layer. Besides this relaxation of the structural modulation, the superconducting transition became sharper, and a slight increase in T c was observed in the Bi 2- x Pb x Sr 2 CaCu 2 O 8+ d system.

Flux pinning and creep in high-Tc superconductors

Abstract: The theory of collective pinning in the presence of thermal fluctuations of vortex lines is developed. Critical current jc is shown to decrease rapidly with temperature. Flux creep under the action of weak (j

Neutron powder diffraction study of Pb2Sr2YCu3O8, the prototype of a new family of superconductors

Abstract: The structure of Pb 2 Sr 2 YCu 3 O 8 has been analyzed with powder neutron diffraction techniques and profile analysis. The results of this study confirm the general structural features determined by X-ray single-crystal methods. The material has pseudo tetragonal symmetry, but is orthorhombic, space group Cmmm with lattice parameters a =5.3933(2), b =5.4311(2), and c =15.7334(6)A. The orthorhombic distortion is caused by the disordering in the ab plane of the oxygen atoms of the PbO layers over the general position of the space group, with x =0.275(5) and y =0.402(5) A. The structure of this compound can be derived from that of YBa 2 Cu 3 O 6 by substituting the Cu layers with blocks of … PbO.Cu.PbO… . Of the two Cu atoms in the asymmetric unit, one has two fold coordination while the other has five fold pyramidal coordination with the apex elongated along the c -axis.

Correlation between Tc and hole concentration in the cation-substituted Bi2Sr2CaCu2O8+δ system

Abstract: Correlation between superconducting T c and hole concentration was investigated in the Bi 2 Sr 2 CaCu 2 O 8+δ system. The hole concentration was varied by substituting cations with different valences for Sr or Ca. In order to increase the hole concentration, some samples were annealed at 430°C in 250 bar of oxygen pressure. We find that T c increases, takes a maximum at the hole concentration corresponding to 0.2–0.3 holes per CuO 2 unit, and then decreases, with increasing hole concentration. This correlation is markedly similar to those in the La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− δ systems.