A Mössbauer study of YBa2(Cu1−x)Fex3O7−σ

⁵⁷Fe Mössbauer spectra of a sample of YBa₂(Cu_0.985Fe_0.015)₃O_7−σ for which T_c≈59 K show that long-range magnetic order is established below ∼ K. A model in which the Fe spins are aligned with the crystallographic c-axis provides satisfactory agreement with the observed relaxation spectra. This result is discussed with reference to current theoretical models of high-T_c superconductivity which involve magnetic coupling mechanisms.     

The effect of low-temperature electron irradiation on the transition temperature of YBa2Cu3O7−δ superconductors

Measurements of the complex susceptibility =–i of electron-irradiated YBa₂Cu₃O_7– show a strong influence of the electron irradiation dose, ·t on the transition temperatureT _c . For irradiation doses of ·t=2.2·10¹⁹ e^–/cm² we find a damage rate of T _c /(·t)=–1.6·10^–19 K/(e^–/cm²). It is assumed that the decrease ofT _c is mainly a bulk effect due to the production of atomic defects like vacancies and interstitials in the Cu–O–Cu chains and in the basal planes of the unit cells.     

Superlattice structures in solid solution of high-Tc YBa2Cu3O7−δ and (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7−δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7−δ has been found with general stoichiometry (Pb_0.75xCu_1−0.75x)(Sr_2xBa_2−2x)(Ca_0.5xY_1−0.5x)Cu₂O_7−δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship with the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.     

Effect of deoxygenation on the weak-link behavior of YBa2Cu3O7−δ superconductors

A systematic study of the weak-link behavior for YBa₂Cu₃O_7−δ polycrystalline samples has been done using the electrical resistivity and AC susceptibility techniques. The experiments were performed with two samples of similar grain, a sample of well-coupled grains, and a deoxygenated sample in such a way that the oxygen mostly comes from the intergrain region. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's critical state model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor normal–metal superconductor (SNS) for well-coupled samples to superconductor insulator normal–metal–superconductor (SINS) type of junctions for the deoxygenated sample. These results are interpreted in terms of oxygen depletion from grain boundaries, which in turn decreases the intergranular Josephson coupling energy with a concomitant decrease of pinning of the intergranular vortices.     

Transition to gapless superconductivity in YBa2(Cu1−x Zn x )3O7

Point-contact spectra of YBa₂(Cu_1–x Zn _x )₃O₇/Ag are studied at various temperatures. The differential resistance dV/dI of the point contacts shows gap-related structures belowT _c which can be attributed to Andreev reflection. Evaluation of many spectra for each sample taken at 4.2 K yields a wide distribution of voltages /e at which these structures occur. The upper limit varies roughly as expected from the depression ofT _c by Zn-doping from /e=29 mV (x=0) to 9 mV (x=0.05), while the lower limit decreases much faster and disappears forx=0.05. Hence, the Zn doped samples exhibit a tendency to gapless superconductivity as suggested earlier on the basis of specific-heat measurements.     

Peculiarities of magnetic flux penetration and trapping in monocrystalline (YBa2Cu3O7−x and Bi2Sr2CaCu2O8+x ) and polycrystalline (YBa2Cu3O7−x ) samples

A comparative analysis of the magnetic flux trapping in monocrystalline and polycrystalline HTSC samples is carried out, and the possibility of employing the dependence of the trapped magnetic flux on the external magnetic field for obtaining comparative estimates of the effect of pinning centers is determined.     

Superconducting cluster in YBa2Cu3O7−x ceramic

The method of point laser heating of a sample is used to perform experimental investigations of the electrical conductivity of YBa₂Cu₃O_7−x ceramic in the vicinity of its critical state. It is found that, in the vicinity of transition to the superconducting state, the electrical conductivity of the ceramic exhibits a clusterlike behavior substantially nonuniform over the sample cross section. The topology of a superconducting cluster is investigated, as well as its spatial localization in the sample. A model of the formation of a superconducting cluster in a ceramic superconductor is suggested.     

Misfit stresses in YBa2Cu3O7−x films

Misfit and thermoelastic stresses in HTSC layers 1-2-3 grown on various substrates are, analyzed with the use of ultrasonic measurements of Young's moduli inYBa ₂ Cu ₃ O _7−x . It is shown that the misfit stress gives the main contribution to the formation of the HTSC layer strain. Military Academy of Strategic Rocket Forces. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 87–90, April, 2000.     

Magnetotransport study of in situ magnetic field textured Dy1Ba2Cu3O7−δ superconductors

The electrical resistivity, thermoelectric power and the Nernst effect have been studied as a function of temperature and magnetic field for a typical superconductor DyBa₂Cu₃O_7– magnetically textured in situ at 1035°C. The three transport coefficients show hybrid microscopic features. In particular, we show the anisotropy with respect to the field direction (H//a,H//c) in all transport coefficients. We verify that Tinkham's law is obeyed for the broadening of the resistive transition in a magnetic field. Similarly we obtainanisotropic broadening exponents for each integrated excess property. From Arrhenius plots we obtain orders of magnitude for the activation energies characterizing each property. They are markedly different from each other.     

The Hubbard model and pressure effects of YBa2Cu3O7−δ superconductors

We apply a mean field approach to the extended Hubbard model on a square lattice to the YBa₂Cu₃O_7−δ family of superconductors under pressure. The parameters of the tight-binding band are taken from experiments, and the coupling strength U and V are estimated by the zero pressure phase diagram (T_c×n_h). This scheme yields the non-trivial dependence of the superconductor critical temperature T_c as a function of the hole concentration n_h in the CuO₂ plane. With the assumption that the pressure P modifies the potential V and the on-plane hole content n_h, we can distinguish the charge transfer and the intrinsic contribution to T_c(P). We show that the changes on T_c(P) for the YBa₂Cu₃O₇ optimally doped compound at low pressures are almost entirely due to the intrinsic term.     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

Structural transition of secondary phase oxide nanorods in epitaxial YBa2Cu3O7−δ films on vicinal substrates

A theoretical study of a structural transition of secondary phase oxide nanorods in epitaxial YBa₂Cu₃O_7?δ films on vicinal SrTiO₃ substrates is presented. Two possible types of film/substrate interface are considered, with one assuming complete coherence, while the other is defective as manifested by the presence of antiphase grain boundaries. Only in the former case does the increase of the vicinal angle of the substrate lead to a substantial change of the strain field in the film, resulting in a transition of the nanorod orientation from the normal to the in-plane direction of the film. The calculated threshold vicinal angle for the onset of the transition and lattice deformation of the YBa₂Cu₃O_7?δ film due to the inclusion of the nanorods is in very good agreement with experimental observations. This result sheds lights on the understanding of the role of the film/substrate lattice mismatch in controlling self-assembly of dopant nanostructures in matrix films.     

EFG determination in magnetic field oriented powders of YBa2(Cu1−x 57Fe x )3O7−y superconductors: A test of existing models

YBa₂(Cu_1-x ⁵⁷Fe _x )₃O_7–y powder samples withx=0.02 and 0.03 were uniaxiallyc-aligned in epoxy in a magnetic field of 7.5 T. An X-ray map of the distribution of the orientation of the crystallographicc-axes of the individual crystallites was determined.⁵⁷Fe Mössbauer spectra were recorded at room temperature as a function of the angle between the texture axis and the observation direction. Different sets of EFG parameters were selected from the literature and the fit constraints were equally set for all. Using a model of the measured texture function and accounting for polarization-dependent blackness effects, a simultaneous fit of the Mössbauer spectra results in minimum ² for the EFG set determined previously from CEMS data on this films.On leave from: Institute for Low Temperature Physics and Engineering, Ukr. Acad. Sci., Kharkov, Ukraine.     

Positron annihilation anomaly in the high-temperature superconductor YBa2Cu3O7−x

We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH _b =1.2 (mG) and a coherence length =2.2T ^–1/2 (m) for the Cu, as well as a field penetration depth -34T ^1/2 (m) at the Cu/Nb interface.     

Internal magnetic fields in the high-temperature superconductor YBa2Cu3O7−δ from muon spin rotation experiments

Muon spin rotation (SR) measurements of the internal magnetic fields in sintered YBa₂Cu₃O_7– samples are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) SR signals, hysteresis effects and flux trapping are observed. The SR results are compared with the results of macroscopic experiments (susceptibility, magnetization, resistivity) and with predictions obtained from models of weakly-coupled superconducting regions.     

Magnetoresistance hysteresis of granular YBa2Cu3O7−δ high-temperature superconductor in weak magnetic fields

The variation of the transverse magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconducting ceramic with external magnetic field intensity H _ext first increasing from zero to ～500 Oe (Δρ⁺ / ρ_{273 K}) and then decreasing from about 500 Oe to zero (Δρ^? / ρ_{273 K}) is studied for transport current densities varying from j/j _c ≈ 0.01 to ≈0.99 (where j _c is the critical current density in the absence of the magnetic field) at 77.3 K. It is found that the field dependence of the magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconductor is characterized by pronounced hysteresis, the difference Δρ⁺ / ρ_{273 K} ? Δρ^? / ρ_{273 K} increasing with j/j _c. As j/j _c grows, the effective critical fields of Josephson weak links, H _c2J , and the lower critical fields of superconducting grains, H _c1A , decline. When field H _ext rises, the critical fields are lower than when H _ext diminishes: H _c2J ⁺ < H _c2J ^? and H _c1A ⁺ < H _c1A ^? .     

Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics $E(j)_{H_{treat} } = const$ of ceramic (granular) YBa₂Cu₃O_6.95 samples preliminarily magnetized in different transverse magnetic fields H _treat have been measured in a zero field (H _ext = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 ≤ H _trap ≤ H _c2J (where H _trap is the magnetic field trapped as a result of the pretreatment in H _treat and H _c2J is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance ρ_A(H _treat) _{j = const} of superconducting grains. It has been established that the magnetoresistance ρ_A of the superconducting grains is significantly lower than the magnetoresistance ρ_J for the Josephson medium. The dependence of ρ_A on H _treat and on the transport current density j has been investigated. It has been shown that the dependences ρ_A(H _treat) _{j = const} exhibit a clearly pronounced tendency to saturation, ρ_satur, and the value of ρ_satur increases with increasing j. It has been found that the lower critical field H _c1A of the superconducting grains strongly depends on the transport current density.     

Structural phase transition in YBa2Cu3O7−δ: the role of dimensionality for high temperature superconductivity

We have performed detailed high temperature X-ray diffraction to study the nature of the structural phase of YBa₂Cu₃O_7−δ. The results indicate the existence of a reversible orthorhombic to tetragonal phase transition at a temperature close to 750°C. If the high temperature tetragonal phase is quenched-in at low temperatures the critical superconducting temperature is considerably reduced from 92.5 K. This suggests that the one dimensional CuO chains present in the orthorhombic structure are necessary for high temperature superconductivity.     

Pinning potential in thick PrBa2Cu3Ox/YBa2Cu3O7−δ quasi-multilayers

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa₂Cu₃O_x (PrBCO) nano-dots and layers of YBa₂Cu₃O_7?δ (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 nm, and total thickness between 1.13 and 5.31 μm. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J_c at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J_c only in high fields. We have also investigated the frequency dependence of J_c from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nano-dots, both isotropic and c-axis correlated.     

Normal state resistivity of YBa2Cu3O7−x

A systematic study has been carried out on the normal state resistivity of YBa₂Cu₃O_7–x . Samples were cut from the same well-annealed material and were quenched from different temperatures (T _Q ). The resistivity is metallic whenT _Q 500 °C and becomes semiconductor-like whenT _Q 600 °C. The data on the latter was interpreted in terms of the three-dimensional localization model proposed by Mott.The work at USCD was supported by the California MICRO program