High-temperature Oxide Superconductors Research Articles

Nodal quasiparticle in pseudogapped colossal magnetoresistive manganites

A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction. Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs'. This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity or by invoking orders which are natural competitors of d-wave superconductors. Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La1.2Sr1.8Mn2O7. Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state.

High Tc superconductor re-entrant cavity filter structures

Microwave cavity filters are widely used in high frequency communication systems for high quality factor signal filtering. However, their large size makes it inconvenient for them to be used at lower operating frequencies especially in the 1–4 GHz frequency range of most of the present day commercial wireless applications. In order to improve their performance and to reduce their overall size, superconducting cavity filters were designed and simulated. Cylindrical and re-entrant cavity filters were fabricated, using seeded melt grown yttrium barium copper oxide high temperature superconductors. The resonant characteristics of these filters were measured, verifying the concept that small, high Q, cavity filters can be made for wireless applications.

Pairing interactions and pairing mechanism in high-temperature copper oxide superconductors

The polaron binding energy ${E}_{p}$ in undoped parent cuprates has been determined to be about $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the unconventional oxygen-isotope effect on the antiferromagnetic ordering temperature. The deduced value of ${E}_{p}$ is in quantitative agreement with that estimated from independent optical data and that estimated theoretically from the measured dielectric constants. The substantial oxygen-isotope effect on the in-plane supercarrier mass observed in optimally doped cuprates suggests that polarons are bound into the Cooper pairs. We also identify the phonon modes that are strongly coupled to conduction electrons from the angle-resolved photoemission spectroscopy, tunneling spectra, and optical data. We consistently show that there is a very strong electron-phonon coupling feature at a phonon energy of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ along the antinodal direction and that this coupling becomes weaker towards the diagonal direction. We further show that high-temperature superconductivity in cuprates is caused by strong electron-phonon coupling, polaronic effect, and significant coupling with $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ charge transfer fluctuation.

The Breakdown of Migdal's Theorem and the Divergence of Electron Polarizations in HTSC

We assume that Fermi surface nesting is a common feature in all oxide high temperature superconductors (HTSC) of perovskite or layered-perovskite structure. We show that this feature results in the breakdown of the Migdal's theorem, and the singularity of some electronic polarizations. These singularities trigger various kinds of instabilities in these materials.

Plasticity and strength of metal oxide high-temperature superconductors (Review)

The results of research on the plasticity and strength of a wide class of metal oxide perovskite-like compounds which have the property of high-temperature superconductivity or which can be used as base compounds for making high-temperature superconductors (HTSCs) are systematized and presented from a unified point of view. The mechanical properties of materials with different morphology—single crystals, polycrystals, and composites,—measured by different methods of mechanical testing in the low-temperature, room-temperature, and high-temperature regions, are discussed. The characteristic defects of the crystal structure for these compounds are considered, the crystallography of two modes of plastic deformation—slip and twinning—is described, and the stress-induced structural rearrangement of the twin structure that appears at a high-temperature phase transformation is discussed. The features of plastic deformation and fracture of metal oxide materials due to structural microdefects (dislocations, impurities, twin and grain boundaries) and macrodefects (voids, cracks, heterophase inclusions) are noted, and the role of heavy-cation diffusion in the kinetics of high-temperature deformation is discussed. The influence of structural phase transformations and the superconducting transition on the mechanical properties of metal oxides is considered. This review is a continuation of a review of the elastic and acoustic properties of HTSCs published earlier by the authors in Fiz. Nizk. Temp. 21, 475 (1995) [Low Temp. Phys. 21, 367 (1995)].

Polaronic origin of the isotope effect on the London penetration depth in high-temperature superconducting oxides

Direct measurements of the in-plane London penetration depth λ L have recently been performed on high-temperature superconducting copper oxides by a new low-energy muon spin rotation technique. The results show that λ L is isotope dependent, evidencing unconventional electron–phonon interactions as its source. The data are interpreted here in terms of polaronic effects on the single-particle energies, which leads to level shifts and exponential band narrowing. Good agreement with the experimental data is obtained.

On the break in the single-particle energy dispersions and the ‘universal’ nodal Fermi velocity in the high-temperature copper oxide superconductors

Recent data from angle-resolved photoemission experiments published by Zhou et al. [Nature, 423, 398 (2003)] concerning a number of hole-doped copper-oxide-based high-temperature superconductors reveal that in the nodal directions of the underlying square Brillouin zones (i.e. the directions along which the d-wave superconducting gap is vanishing) the Fermi velocities for some finite range of k inside the Fermi sea and away from the nodal Fermi wave vector k F are to within an experimental uncertainty of approximately 20% the same both in all the compounds investigated and over a wide range of doping concentrations and that, in line with earlier experimental observations, at some characteristic wave vector k ☆ away from k F ( typically amounts to approximately 5% of ) the Fermi velocities undergo a sudden change, with this change (roughly speaking, an increase for ) being the greatest (smallest) in the case of underdoped (overdoped) compounds. We demonstrate that these observations establish four essential facts: firstly, the ground-state momentum distribution function must be discontinuous at k = k ☆; with and denoting the measured velocities close to and , we obtain in which is the ‘Fermi’ velocity corresponding to the case in which (for two-body interaction potentials of shorter range than the Coulomb potential, whereby ); secondly, the single-particle spectral function must at k = k ☆ possess a coherent contribution corresponding to a well-defined quasiparticle excitation at an energy of approximately 70 meV below the Fermi energy; thirdly, the amount of discontinuity in at the nodal Fermi points must be small and ideally vanishing; fourthly, the long range of the two-body Coulomb potential is of vital importance for realization of a certain aspect of the observed behaviour in the Fermi velocities (specifi- cally) in the underdoped regime. The condition conforms with the observation through an earlier angle-resolved photoemission experiment by Valla et al. [Science, 285, 2110 (1999)], on the optimally doped compound Bi2Sr2CaCu2O8 + δ, which shows that the imaginary part of the self-energy along the nodal directions of the Brillouin zone and in the vicinity of the nodal Fermi points satisfies the scaling behaviour characteristic of marginal Fermi-liquid metallic states, for which is indeed vanishing. We present arguments advocating the viewpoint that the observed ‘kink’ in the measured energy dispersions cannot be a direct consequence of electron-phonon interaction, although a finite may possibly arise from this interaction. In other words, even though possibly vital, the role played by phonons in bringing about the latter ‘kink’ must be indirect. Our approach further provides a consistent interpretation of the observed sudden decrease in the width of the so-called ‘momentum distribution curve’ on ‖k| increasing above .

ＲＥＢａ２Ｃｕ３Ｏｙ膜を用いた永久電流スイッチ材料の開発

Many applications for the superconducting magnets using high critical temperature (Tc) oxide superconductors are expected to operate in the persistent current (PC) mode. For the PC mode operation, the PC switch (PCS) is considered to be an essential device. The following properties are required for the PCS material; high critical current, high normal electric resistance in order to cut off the current flow efficiently through thermal switching, and controlled Tc of about several ten degrees higher than the operating temperature in order to attain a quick switching.The liquid phase epitaxy (LPE) method could provide REBa2Cu3Oy (RE123) thick films with a high critical current density in the superconducting state, a reasonable electric resistivity in the normal state and could control Tc with doping a third element. We studied how to fabricate Zn-doped RE123 LPE films with a meander shape to realize its Tc value of about 40-60 K and high electric resistance in the normal state as persistent current switch materials for operation of the magnet at about 20 K. We also studied controllability of the Tc and the lateral growth length of Zn-doped RE123 LPE films on the BaZrO3 buffer layer to develop the process for fabrication of persistent current switch materials.

Study on the spin-polarized electronicstructures and atomic magnetic moments ofcluster La8-xBaxCuO6

The Xα method is applied to study the spin-polarized electronic structures and atomic magnetic moments of clusters La8-xBaxCuO6 in the compound La2-yBayCuO4.This paper shows that Cu magnetic moment in La2CuO4 is 0.37μB, which is in better agreement with the experimental value(0.48±0.15μB). Ba substitution lowers the structure symmetry, removes molecular orbital degeneracy, and hence strengthens orbital hybridization and weakens quasi-two-dimensional feature, which enhance the coupling between Cu-O layer and the neighbormg layers and influence the charge transfer. These changes strongly affect the properties of carriers. Holes are introduced due to Ba substitution and they mainly enter into O sites, a few d-electronic holes enter into Cu sites, which causes important effects on the density of states, atomic magnetic moment and valence states of Cu and O in Cu-O layers. The results confirm that the holes are not always beneficial to the superconductivity. At the beginning the holes are beneficial to the superconductivity; they can decrease the superconductivity when more holes are introduced. This paper can explain that the transition temperature of high-temperature superconductive oxides changes with the variation of the doping level.

Open atmosphere laser patterning of RBCO high-temperature superconductors

We show that it is possible to use CO2 laser radiation in an open atmosphere to dope high-temperature superconductors with oxygen. We show that, without using a vacuum chamber, high-temperature oxide superconductors can be doped to fabricate devices with electronics applications. The present experimental study shows that, for a given structure of high-temperature superconductor materials, irradiating the samples using an intense laser beam in an inert gas or an oxygen gas atmosphere can change the oxygen content. The reflection infrared spectra show that the laser irradiation of the samples has had suitable results on the samples. The x-ray diffraction spectra of the samples reveal that the intensities of some of the planes change dramatically.

Tunable photonic band schemes in two-dimensional photonic crystals composed of copper oxide high-temperature superconductors

We theoretically demonstrate the tunability of two-dimensional photonic crystals composed of copper oxide high-temperature superconductors (HTSCs). The photonic crystals we deal with are composed of rods whose axes are parallel to the c axes in the copper oxide HTSC. Photonic crystals composed of copper oxide HTSCs exhibit the large tunabilities by temperature and magnetic fields. That is, a photonic band gap increases and a midgap frequency decreases with increasing temperature and the magnetic field. Their characteristics are due to the temperature dependence of London penetration depths and the magnetic field dependence of the critical temperature and the London penetration depths. The frequency range in which the photonic band gap appears are strongly dependent on materials in the copper oxide HTSCs.

The effect of superstructural ordering on the properties of high-temperature oxide superconductor systems

The effect of superstructural ordering in the oxygen sublattice (in addition to the influence of the antiferromagnetic interaction of copper ions) on the electron and phonon characteristics of oxide high-temperature superconductor (HTSC) systems has been studied. Taking into account this ordering effect, it is possible to explain a wide range of experimental data, including doping-induced changes in shape of the Fermi surface, features of the phonon spectra, the existence of stripes, the presence of a pseudogap and its coexistence with the superconducting gap, and some peculiarities in the phase diagrams of HTSCs.

Magnetic resonant mode in the single-layer high-temperature superconductor Tl2Ba2CuO(6+delta).

An unusual spin excitation mode observed by neutron scattering has inspired numerous theoretical studies of the interplay between charged quasiparticles and collective spin excitations in the copper oxide high-temperature superconductors. The mode has, thus far, only been observed in materials with crystal structures consisting of copper oxide bilayers, and it is absent in the single-layer compound La(2-x)Sr(x)CuO(4+delta). Neutron-scattering data now show that the mode is present in Tl(2)Ba(2)CuO(6+delta), a single-layer compound with a superconducting transition temperature of approximately 90 kelvin, demonstrating that it is a generic feature of the copper oxide superconductors, independent of the layer sequence. This restricts the theoretical models for the origin of the resonant mode and its role in the mechanism of high-temperature superconductivity.

814 高温酸化物超伝導体の交流磁化率測定装置の開発研究

In order to measure the superconductive phase transition temperature of the high-Tc oxide superconductor, the ac susceptibility measuring apparatus was developed. The measurement system that used together a pick-up coil and lock-in amplifier was produced for measuring the susceptibility of small sample. And the phase transition temperature of high-Tc oxide superconductor in the Y_1-Ba_2-Cu_3-O_y system was measured. Measuring apparatus detects the signal generated from the pick-up coil as a minute voltage change with lock-in amplifier. Consequently, it turns out that the phase transition temperature of a high temperature oxide superconductor can detect and measure by the resolution of 0.1K.

An unusual phase transition to a second liquid vortex phase in the superconductor YBa2Cu3O7.

A magnetic field penetrates a superconductor through an array of 'vortices', each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex 'matter'. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a 'solid', in which the vortices are pinned, but the solid can 'melt' into a 'liquid' phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition-namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory-is in accord with theoretical predictions of a transition to a second vortex-liquid phase.

Strongly linked current flow in polycrystalline forms of the superconductor MgB2.

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.

The normal state of high temperature oxide superconductor

Looking back on the experiments about the normal state of the high temperature superconductor (HTS), the authors point out nine important experimental results. On the basis of these results, the authors have argued that the two-dimension, two-subsystem Hamiltonian is the appropriate starting point for describing the normal state of HTS. By this Hamiltonian, using the decoupling approximation of Green's function method by Kaga through numerical calculations, the authors have obtained the temperature dependent pseudogap in the density of states (DOS), which is consistent qualitatively with the experimental results by angle-resolved photoemission spectroscopy (ARPES). Theoretically, this Hamiltonian has the superconducting order parameter of d+s symmetry with d-wave as the main component, which is consistent with experiments. Further, the quantum electronic liquid in HTS is a near Fermi liquid in which there is coexistence of the delocalized states and nearly localized states, and there is finite probability for the nearly localized carriers to form the nearly localized carrier pairs at any finite temperature.

Pseudo-gap phenomenon in the Sn-doped mercury-based cuprate superconductors

The Sn-doped mercury-based copper oxide high temperature superconductors, with nominal composition (Hg1 − xSn x )Ba2Ca2Cu3O8 + δ, have been successfully fabricated. These samples were used to study the pseudo-gap phenomenon in their normal phase. By means of ln[1/ρ(T) − 1/ρ N (T)] versus 1/T plots we found three characteristic temperatures T*, TS, and TF, which describe the role of pseudo-gap phenomenon. From the stated plots, we are able to estimate the magnitude of the pseudo-gap for our samples. The pseudo-gap ΔPG changes into the spin pairing dipolar superconducting gap ΔSPDS, when the Fermi kinetic energy shrinks to zero at the critical temperature Tc. It is proved that the spin pairing dipolar superconducting gap \(\Delta _{{\text{SPDS}}} = \sqrt {(2/3)} \Delta _{\text{T}}\), where, ΔT being the total superconducting gap of the cuprate superconductors, which has the value determined by \(2\Delta _{\text{T}} = 3.52\sqrt 3 k_B T\). It is also proved that the spin pairing dipolar superconducting gap is smaller than the dipolar pseudo-gap, ΔPG.

Preliminary experiments on metallisation of ceramic oxide superconductors

AbstractThe present paper briefly reports on preliminary experiments on two new methods for the metallisation of ceramic oxide high temperature superconductors )HTS compounds): sinter-permeate silver and chemically precipitated silver. By using well characterised HTS samples for the measurement of superconducting properties it is shown that successful metallisation and jointing were obtained. The two methods are considered to have potential for further development and application in this field.

Valence States of Lead and Bismuth Atoms in the High-Temperature Superconductor BaPb1-xBixO3

X-ray photoelectron spectra of valence bands and core levels of BaPb0.8Bi0.2O3, PbO, PbO2, BaPbO3, BaBiO3, NaBiO3, and Bi2O3 were studied. Comparison of the electron binding energies of the Pb 4f7/2 or Bi 4f7/2 core levels for all the oxides studied showed that the high-temperature oxide superconductor BaPb0.8Bi0.2O3 contains simultaneously two different valence forms of lead atoms (PbIV and PbII) and two different valence forms of bismuth atoms (BiV and BiIII). Parameters of the X-ray photoelectron spectra of the valence bands do not contradict the conclusion on heterovalent states of lead and bismuth atoms in BaPb0.8Bi0.2O3.