Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.     

Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films

The evolution of Raman spectra with illumination has been studied in YBa₂Cu₃O_6.4 films at temperatures between 5–300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm^–1, O(2)-O(3) at 338 cm^–1, and O(4) at 488 cm^–1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequencies, which is in agreement with the enhancement of the static conductivity(0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.     

Microwave surface impedance of YBa2Cu3O7?x:S-wave versusD-wave pairing

The low temperature behavior of the microwave surface resistanceR _s and the penetration provides information on the pairing state. Fully oxygenated films with low de-resistivity in the normal state exhibit an exponential temperature dependence corresponding to values of the energy gap up to 10 meV. For oxygen deficient films a linearR _s(T) was observed, similar to single crystals. This can be explained qualitatively by induced superconductivity in the copper-oxygen chains.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the linear term is presented and discussed.     

Fluctuation conductivity in Li-doped YBa2Cu3O7?x

The paraconductivity of Li-doped YBa₂Cu₃O_7–x was measured. We have found that the character of the fluctuation changes as lithium content increases: instead of a crossover from 2D to 3D behavior (D is the dimensionality), as we have observed for a low doping level, a double crossover to the 2D percolative and 3D percolative regimes becomes manifest by increasing the lithium content.     

Insulators for YBa2Cu3O7 Josephson Junction Technology

Because ideal PrBa₂Cu₃O₇ superconducts at 90 K, it should be replaced by RBa_2–u Sr _u Cu₂MO₈ (where R is a light-mass rare earth and M is Nb or, preferably, Ta) as the insulator of choice for YBa₂Cu₃O₇ Josephson junction or microstructure technology. Furthermore, (110)- or (100)-oriented substrates of RSr₂Cu₂MO₈ may provide an opportunity for optimal device fabrication.     

Electronic Raman scattering in YBa2Cu4O8

We report on the electronic Raman scattering in YBa₂Cu₄O₈ and find that, In contrast to the other superconducting cuprates, the continuum in the normal state is strongly temperature pendent in all polarizations. This temperature dependence is found to follow a Bose-Einstein-like form. We conclude that the absence of the Bose-Einstein factor in the electronic continuum is not essential to high-T_c superconductivity, as is sometimes implied. Additionally, we report on the rearrangement of the continuum which occurs belowT _c.     

Electronic specific heat of YBa2Cu3O6+x from 1.8 to 300 K

We have determined for the first time the electronic specific heatγ(x, T) of YBa₂Cu₃O_6+x for 0.16≤x≤0.97 from 1.8 to 300 K. Weakly superconducting behavior betweenx=0.4 and 0.8 progresses rapidly to strong coupling BCS-like superconducting and metallic normal state behavior forx> 0.9. However, the continuous development of the entropyS(x, T) withx andT across the entire series suggests a progressive modification of the low-energy spin spectrum with hole doping rather than a simple band model. Fermi statistics andk-space pairing for allx is indicated by the magnitude andT-dependence ofS(x, T). Pseudogap behavior inS(x, T) is observed over a temperature region aboveT _c , which increases rapidly with oxygen depletion to around 200 K forx～0.7. This loss of entropy reflects normal-state correlations apparently unrelated to the superconducting pairing.     

Charge Density Variations or Stripes in YBa2Cu3O6+y

For many years it was believed that NMR on the YBa₂Cu₃O_6+y family of superconductors does not support charge density variations or stripes. We discuss the NMR data of YBa₂Cu₃O_6+y (y > 0.63) and show that large charge density variations are actually necessary in order to explain the data.     

Model simulation study of the structural stability of oxygen order in the YBa2Cu3O6+x superconductor

A stability analysis is made of a model of the high-temperature superconductor YBa₂Cu₃O_6+x by computer simulation of the oxygen ordering processes in the CuO-basal plane. The oxugen ordering is modelled within a two-dimensional lattice-gas model subject to varying external oxygen pressures and different annealing schedules. This model has previously proved successful in describing static as well as dynamic properties of the oxygen ordering processes. It is found that the domain formation and the stability of the oxygen-ordered orthorhombic phases (ortho-I and ortho-II) at low temperatures are crucially dependent on the temperature where the oxygen pressure is decreased, as well as the cooling rate employed in the annealing. For higher temperatures, the cooling rate and the initial oxygen content are less important. It is found that the size of the ortho-I domains, which is important for a high superconductivity transition temperature, is optimal when a high external oxygen pressure is maintained until low temperatures have been achieved.     

X-Ray and neutron scattering studies of YBa2Cu3O6+x compounds: Oxygen insertion and spin dynamics

In the first part of this paper the results of X-ray and neutron diffraction studies of the structural aspects related to oxygen insertion in the CuO _x planes of YBa₂Cu₃O_6+x compounds are presented. The second part is devoted to the inelastic neutron scattering study of the spin dynamics in the three regimes (weakly doped, heavily doped, and overdoped) of the metallic phases of YBa₂Cu₃O_6+x .     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Microstructures of high-Jc melt-textured YBa2Cu3O7?x/Ag superconductors

Silver has been previously added to the melt-textured YBa₂Cu₃O_7–x in order to increase the critical current density (J _c ) of these materials. However, the effect of this addition on theJ _c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7–x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ _c of the MTG YBCO/Ag samples is more than 10⁴A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ _c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.     

Investigations of the Spin Hamiltonian Parameters for the Cu2+ Sites in PrBa2Cu3O6+x and Pr0.5Er0.5Ba2Cu3O6+x

The spin Hamiltonian parameters (the anisotropic g factors g _∥ and g _⊥ and the hyperfine structure constants) for the Cu²⁺ sites in PrBa₂Cu₃O_6+x and Pr_0.5Er_0.5Ba₂Cu₃O_6+x are theoretically investigated using the high order perturbation formulas of these parameters for a 3d⁹ ion in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are determined from the superposition model and the local structures of the Cu²⁺ sites. The theoretical spin Hamiltonian parameters are in good agreement with the observed values for both systems, and the results show improvements as compared with those based on various adjustable covalency coefficients in the previous work. The larger hyperfine structure constants for PrBa₂Cu₃O_6+x than those for Pr_0.5Er_0.5Ba₂Cu₃O_6+x can be attributed to the weaker covalency due to the relatively longer Cu²⁺– O^[`2]mathrm{O}^{bar{2}} bonds in the former.     

Chemical solution deposition of YBa2Cu3O7−x coated conductors

At present, the development of superconducting YBa₂Cu₃O_7−x coated conductors attracts much attention due to their enormous application potential in electric power systems. Worldwide research is focused on the investigation and improvement of buffer materials and YBa₂Cu₃O_7−x superconducting properties as well as low-cost manufacturing processes in cooperation with industrial companies. Accordingly, chemical solution deposition has emerged as a highly competitive, versatile, and cost-effective technique for fabricating coated conductors of high performance. New chemical solution approaches are under development for buffer layer deposition. In order to achieve high critical current carrying YBa₂Cu₃O_7−x layers, the established trifluoroacetate route is favored. This paper reviews the most recent work on chemical solution deposition within the IFW Dresden while also considering achievements on this specific research topic worldwide.     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0 < x < 0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2− x} Sr _x CuO₄ and Y_{1− x} Ca _x Ba₂Cu₃O₆ cuprate systems.     

Electronic structure of the ladder-chain compound Sr14−xCaxCu24O41

The electronic structure of the ladder-chain compound Sr_14−xCa_xCu₂₄O₄₁ is studied by ab initio calculations within the local density approximation. The effects of Ca substitution and structure modulation on electronic structure are discussed. It is found that 0.05 holes per copper atom are on the ladder layers for fully substituted compound, Ca₁₄Cu₂₄O₄₁.     

Effect of PrBa2Cu3O7?x buffer layer thickness on the properties of YBa2Cu3O7?x thin films grown on sapphire by laser ablation

Superconducting YBa₂Cu₃O_7–x (YBCO) thin films were deposited onr-plane A1₂O₃ substrates with PrBa₂Cu₃O_7–x (PBCO) buffer layer by XeCl excimer laser ablation. The thickness of PBCO buffer layer was systematically changed to investigate the superconducting properties of YBCO thin films on sapphire. The structure and surface morphology of the films were characterized by X-ray diffraction and scanning electron microscopy (SEM). Superconducting transition temperatures were varied depending on the buffer layer thickness. Interdiffusion between laser-ablated YBCO thin films and A1₂O₃ substrates had been studied by Auger electron spectroscopy (AES). The results of this study show that diffusion does not occur between the YBCO thin film and the substrate even with 20 Å thick PBCO buffer layer.     

Effect of tungsten substitution in the YBa2Cu3O7−x system

A series of YBa₂Cu_3−x oxides was synthesized. The results show a cubic system with a simple perovskite structure for x < 0.2 and a face-centered doubled perovskite cell for x > 0.2. There was no indication of the existence of the superconducting 1-2-3 phase for x > 0.2. Solid solution of tungsten was exhibited over the composition region 0.2 < x as well as in the simple perovskite composition range. Analyses of a sample of nominal composition YBa₂Cu₂W oxide indicates a non-superconducting perovskite with .     

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 ? x Bulk Superconductors

We studied the YBa₂Cu₃O_{7 – x} bulk superconductor doped with BaZrO₃ up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO₃ doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO₃ does not lead to degradation of the superconducting properties of bulk YBa₂Cu₃O_{7 – x} .