Spectroscopic studies of the high-Tc superconducting cuprate perovskites

We present XRD, ESR, IR, DR(UV, VIS) as well as resistivity and magnetic susceptibility data of single phase superconducting perovskites YBa₂Cu₃O_x and EuBa₂Cu₃O_x. A rich phonon structure for both oxide compounds has been observed with a peak at: 72, 108, 135, 148, 172, 570 and 620 cm^-1. The edge of the low energy side was observed at 55 cm^-1. The estimated carrier density for YBa₂Cu₃O_x is (1–2) × 10²¹ cm^-3. A weak ESR signal has also been observed for superconducting phases which is associated with the lattice defects.We also report ESR and optical measurements of constitutent materials as well as not superconducting phases Y₂Cu₂O₅ and BaCuO₂.     

The Tl-Ba-Ca-Cu-O superconducting ceramics containing Al2O3

The Tl-Ba-Ca-Cu-O superconducting ceramics containing Al₂O₃ have been prepared utilizing the reaction, Tl₂O₂+BaCuO₂+Ca₂CuO₃+A1₂O₃ or Tl₂O₃+BaCuO₂ +Ca₂CuO₃+AlCuO_2.5. It is shown that the resulting ceramics show the superconductivity at temperatures over 100 K when the Al/Cu molar ratio is kept below 0.5. EPMA analyses revealed that there is always an Al₂BaO₄ layer between the superconducting Tl-Ba-Ca-Cu-O and Al₂O₃ phases, leading to the separation between the superconducting and A1₂O₃ phases.     

Atomic-level studies of superconducting and nonsuperconducting YBa2Cu3O7−x

Atomic-resolution images from superconducting and nonsuperconducting samples of YBa₂Cu₃O_7–x have been obtained by field ion microscopy. Both phases of the material exhibit a similar layered structure in the field ion images. Transmission electron micrographs of superconducting field ion tips show vividly the twinned structure of the orthorhombic phase. Time-of-flight (atom-probe) mass spectra from the two materials differ primarily in an enhanced molecular oxygen signal from the superconducting samples indicating that oxygen in the O(4) sites (CuO chains) is field desorbed preferentially as molecular oxygen ions.     

YBa2Cu3O6+δ/BaTiO3系复合功能陶瓷特性研究

报道了超导陶瓷ＹＢａ₂Ｃｕ₃Ｏ_6+δ与铁电陶瓷ＢａＴｉＯ₃进行复合的结果。研究了该复合功能陶瓷的物相、（超）导电性和低温电阻温度特性。结果表明，采用合理的合成工艺，可得到呈现混和分布的两相复合功能陶瓷材料；该复合材料的电导特征符合三维渗流导电行为，发现在较高ＹＢａ₂Ｃｕ₃Ｏ_6+δ含量时，样品呈超导电性，并对此作了初步讨论。 关键词：     

Crystallization process of Bi(Pb)-Sr-Ca-Cu-O glass

A crystallization study has been carried out for rapidly solidified Bi₂Pb_0.5Sr₂Ca₄Cu₅O_x glass. Glass transition temperature T _g, crystallized superconducting phases and microstructural changes were measured and analysed by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The crystallization mechanism of the three superconducting phases — (2201) 20 K phase, (2212) 80 K phase, and (2223) 110 K phase — has been discussed, and a time-temperature-transformation diagram for the glass has been constructed.     

Mössbauer studies of YBa2Cu3O7−δ-type high-T c superconductors

The local magnetic, electronic, and structural properties of (RE)Ba₂Cu₃O_7?δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mössbauer spectroscopy using the resonances of¹⁵⁵Gd,¹⁶¹Dy,¹⁵¹Eu, and⁵⁷Eu. In GdBa₂Cu₃O_7?δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa₂Cu₃O_7?δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa₂Cu₃O_7?δ and EuBa₂Cu₃O_7?δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa₂ (Cu_0.995Fe_0.005)₃O_7?δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.     

Resonance enhancement of electron-phonon interaction

A superconducting state near the phase transition curve from paramagnetic phase to superconducting phase for perovskites La_{2 ? x} Sr _x CuO₄ and YBa₂Cu₃O₆ is considered. Expressions for effective parameters of electron-spin-phonon interaction are obtained. It is shown that the critical temperature of the phase transition from paramagnetic phase to superconducting phase T _c is determined by the enhancement of electron-phonon interaction by spin fluctuations of exchange type.     

Superconducting weak links in YBa2Cu3O7−σ an AC magnetic susceptibility study

AC magnetic susceptibility, χ' and χ″, and ac resistivity, ϱ, of two different samples of superconducting YBa₂Cu₃O_7-σ have been measured. The results show a strong non-linear behaviour and are very sensitive to the exciting amplitudes. From the analysis of the data and comparison with appropriate models and other superconducting compounds, evidence for the existence of weak superconducting links is inferred. Finally, the utility of ac χ measurements for detection of different superconducting phases is made evident.     

A Mössbauer study on TiBaCa3(Cu1−xFex)3O8.5 superconductors

Two main superconducting phases, 1223 and 2223 in TiBaCa₃Cu₃O_8.5 superconductor, are identified by powder x-ray analysis and⁵⁷Fe Mössbauer spectroscopy analysis. The Mössbauer fitting results shown that iron atoms in the lattice substitute two inequivalent sites of copper atoms for each main phase. The temperature dependence of Mössbauer parameters are obtained, and several problems are discussed which relate to the electric charge transfer which takes place around the superconducting transition temperature, the lattice dynamics and the effect on oxygen deficiency at different temperatures.     

Competing energy scales in high‐temperature superconductors: Ultrafast pump–probe studies

We present a review of photoexcited quasiparticle dynamics of cuprate and pnictide high‐temperature superconductors in regimes (temperature, doping) where different phases such as superconductivity, spin‐density‐wave (SDW) and pseudogap phases coexist or compete with one another. We start with the overdoped cuprate superconductor Y_1–xCa_x Ba₂Cu₃O_7–δ, where the superconducting gap and pseudogap coexist in the superconducting state. In another cuprate Tl₂Ba₂Ca₂Cu₃O_y, we ob‐ serve a competition between SDW and superconducting orders deep in the superconducting state. Finally, in the underdoped iron pnictide superconductor (Ba,K)Fe₂As₂, SDW order forms at 85 K, followed by superconductivity at 28 K. We also find the emergence of a normal‐state order that suppresses SDW at a temperature T * ～ 60 K and argue that this normal‐state order is a precursor to superconductivity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observation of the second harmonic in the phase dependence of a superconducting current in Nb/Au/YBCO heterojunctions

The second harmonic in the dependence of a superconducting current on the phase difference of superconducting wave functions of the electrodes was observed experimentally in Nb/Au/(001)YBa₂Cu₃O_x heterojunctions. Possible reasons for its appearance were discussed within the framework of a mixed (d±s) symmetry order parameter of YBa₂Cu₃O_x.     

Symmetry of superconducting compositions in the BaPb1−xBixO3 system

Reexamination of the symmetry of superconducting phases in the BaPb_1−xBi_xO₃ system was carried out by x-ray diffraction using a synchrotron source which offers exceptional resolution. These studies confirm our original finding of tetragonal symmetry. We believe that confusion on the symmetry has resulted because some compositions in this system are two phase mixtures.     

On Gossamer metals and insulating behaviour

We extend the Gossamer technique recently proposed to describe superconducting ground states to metallic ground states. The Gossamer metal in a single band model will describe a metallic phase that becomes arbitrarily hard to differentiate from an insulator as one turns the Coulomb correlations up. We were motivated by the phase diagram of V₂O₃ and f-electron systems which have phase diagrams in which a line of first-order metal–insulator transition ends at a critical point above which the two phases are indistinguishable. This means that one can go continuously from the metal to the ‘insulator’, suggesting that they might be the same phase.     

Phase formation and crystallization in Bi-Sr-Ca-Cu-O thick films

Thick films of Bi₂Sr₂CaCu₂O_x superconducting at 84 K have been prepared on polycrystalline MgO substrates. The effect of heat treatment on phases formed at various stages of the film preparation is studied. Investigations show that a thin layer at the top is highly oriented while the bulk of the 212 phase material beneath it consists of randomly oriented grains. This appears to be the main reason for the low critical current densities obtained in practice.     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

Optimization of the superconducting phase of hydrogen sulfide

The electron and phonon spectra, as well as the densities of electron and phonon states of the SH₃ phase and the stable orthorhombic structure of hydrogen sulfide SH₂, are calculated for the pressure interval 100–225 GPa. It is found that the I4/mmm phase can be responsible for the superconducting properties of metallic hydrogen sulfide along with the SH₃ phase. Sequential stages for obtaining and conservation of the SH₂ phase are proposed. The properties of two (SH₂ and SH₃) superconducting phases of hydrogen sulfide are compared.     

Structure of high-T c superconducting tape of the second generation with high current-carrying capacity

Commercial high-T _c superconducting tape produced by Superpower Inc. (the U.S.A.) is studied by scanning and transmission electron microscopy. The superconductor structure is shown to consist of a superconducting layer of average composition GdBa_1.5Cu_2.5–3.0O _y coated by a silver layer and buffer layers of LaMnO₃, MgO,Y₂O₃, and Al₂O₃ deposited on a metallic tape of nickel alloy (Ni-Cr-Mo-Fe-W-Co-Mn). The superconducting layer is formed by the superconducting oxide GdBa₂Cu₃O _y containing lamellar inclusions Gd₂O₃.     

X-ray photoelectron spectra and composition of YBa2Cu3O7 − δ films prepared by laser ablation

The Y3d, Ba3d _5/2, Cu2p _3/2, and O1s X-ray photoelectron spectra of thick (600 nm) superconducting YBa₂Cu₃O_{7 ? δ} films deposited on textured Ni-W substrates with Y₂O₃ + ZrO₂ and CeO₂ buffer layers have been studied. It has been established that, after the mechanical removal of surface layers with a diamond scraper (and as the analyzed region of the film approaches the interface), a decrease in the oxygen content leads to a decrease of the orthophase fraction and an increase of the tetraphase and Cu⁺ ion fractions. This is caused by the presence of elastic stresses in the superconducting film due to the lattice misfit between the phases making up a composite sample. These stresses prevent oxygen diffusion involved in oxidizing annealing. The spectra of the superconducting film have not revealed signals generated by elements of the substrate and buffer layers.     

Crystal chemistry of the Tl2−xBa2Ca n Cu n+1O2n+6 (n=0, 1, 2) high-T c superconductors

We report on a comparative crystal chemical study of the Tl_2–xBa₂Ca _n Cu _n+1O_2n+6(n=0, 1, 2) high-T _c superconductors. Superconducting properties were determined by DC-conductivity and-susceptibility measurements. An X-ray diffraction analysis reveals the nonstoichiometry of the compounds as a link between structural and superconducting behavior. A comparison of the different structures gives evidence that T1 deficiency in the TlO double layers increases with increasingn; this deficiency causes apparent Cu valences larger than 2+in the superconducting phases thus maintaining the charge balance. By extracting appropriate chemical formulas and performing a bond strength calculation we obtained the mixed-valent oxidation state of copper for the superconducting tetragonal phases as being 2.17+(n=0), 2.25+(n=1) and 2.31+(n=2) respectively; for the non-superconducting orthorhombic phase Tl_2–xBa₂CuO_6–y a Cu valence of about 2+is suggested.     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.