Structure Of High-temperature Superconductors Research Articles

Determination of The Local Atomic Structure of LA2-x(SR,BA)xCUO4 Materials From Neutron Powder Diffraction Data

ABSTRACTWe describe the use of neutron powder diffraction for studying the local structure of high-temperature superconductors. This is accomplished by carrying out a pair distribution function (PDF) analysis. This approach does not presume a periodic structure and allows short range deviations from perfect crystalline order to be observed. Data from a sample of La2CuO4 collected on two different diffractometers are compared to determine the degree of reproducibility of the results. It is found that the data reproduce very well; however, a model-PDF, fit to the data assuming the average crystal structure in the low-temperature-orthorhombic (LTO) phase, differs from the data in a number of significant ways. This is related to earlier observations that the local structure of the La1.875Ba0.125CuO4 compound differs from the average crystal structure.

Role of bonding, reduced screening, and structure in the high-temperature superconductors.

In this paper we have studied the role of bonding, reduced screening, and structure in high-temperature superconductors (HTSC) using ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ as a prototype material. In addition to the ionic and metallic component of bonding recently investigated, we estimate the influence of covalence effects on the structural parameters and the phonon spectrum. Also, the effect of a reduced dispersion of the electronic band structure on the polarizability and the phonon frequencies is considered and (hypothetical) mechanisms are investigated, which would favor ferroelectricity in the HTSC. In a further topic we deal with reduced screening, introduced by nonadiabatic effects related to the quasi-two-dimensional band structure and its consequence for the electron-phonon interaction, the phonon dispersion, and the pairing mechanism. We then focus our interest on features related to the tetragonal-to-orthorhombic phase transition in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. Structural parameters are calculated and the influence of the transition on the phonon dispersion as well as on the phonon-induced charge fluctuations is discussed. Finally, we present, on the basis of nonlocal electron-phonon interaction of the charge-fluctuation type, a theoretical interpretation of a universal relationship between ${\mathit{T}}_{\mathit{c}}$ and the hole content in the HTSC.

Neutron Diffraction, Structural Inorganic Chemistry, and High-Temperature Superconductors

This paper discusses neutron diffraction which has been of fundamental importance for the determination of the structure of high-temperature superconductors and for understanding the influence of structure on the critical temperature. This is because the new superconductors are heavy metal oxides; X-rays are mainly scattered by the metal atoms, but thermal neutrons are scattered as strongly by oxygen, which is the atom of most interest in these materials. In fact, for the past 20 yr, neutron diffraction has been steadily gaining ground as an important technique in structural inorganic chemistry.

Local structure of high temperature superconductors from x-ray absorption studies

Considerable effort has been devoted to determining the mechanism responsible for the electron pairing in the perovskite-structured oxide superconductors. Different measurements have provided conflicting evidence on structural instabilities and phonon anomalies in these systems. X-ray absorption fine structure (XAFS) provides a structural probe that can accurately characterize the local structure and dynamical behavior of the component atoms. Here we summarize XAFS results on YBa2Cu3O7-δ and compare and contrast these results with those on a system that exhibits charge density wave fluctuations, namely, the perovskite-structured, oxide superconductors Ba(Pb1-xBix)O3 and (Ba1-xKx)BiO3. Emphasis is placed on structural results that may shed some light on possible mechanisms of superconductivity in these systems.

Theory of Broad Resistive Transition in High Temperature Superconductors under Magnetic Field

We further develop the theory of renormalized superconducting fluctuations in a strong field, taking account of layer structure in high temperature superconductors and renormalization of fluctuations of higher Landau levels, both of which we neglected in our previous works. Performing detailed comparisons with recent resistivity data for bulk single crystal samples of YBCO and BSCCO, we find that the resulting expressions of fluctuation conductivities quantitatively explain the broadening of resistive transition for both cases of the field perpendicular and parallel to current. In particular, the single expression for the case I ⊥ B can cover not only the broad transition region but also the flux flow region observed at lower temperature.

Electronic Structure of High‐Temperature Superconductors: Comparison of Theory and Experiment

The ab initio pseudofunction method has been used to compute electronic properties of superconductors in good agreement with experiment. La2CuO4 and CuO are computed to be antiferromagnetic and semiconducting in agreement with experiment. The 0.4‐eV peak in the optical conductivity spectrum has been shown to be an interband transition for YBa2Cu3O7. The bands for Bi2Sr2CaCu2O8 as well as YBa2Cu3O7 agree well with angle‐resolved photoemission near Ef. The shake‐up peaks near 9 and 12 eV observed in photoemissio can be understood in terms of two‐electron processes envolving the local densities of states of Cu and O. A mechanism for superconductivity based on these electronic properties will be advanced. All calculations use local density potentials with no parameters.

Flux creep and vortex potential well structure in high-temperature superconductors

The resistive transition and the V-I characteristics in the presence of a magnetic field in epitaxial films of YBa 2Cu 3O 7, Bi 2Sr 2CaCu 2O 8, YBa 2Cu 2.985Ag 0.015O 7, and GdBa 2Cu 3O 7/ YBa 2Cu 3O 7 multilayers have been investigated. The common qualitative features of all the systems are the current dependent thermally activated resistivity and the power law V-I characteristics at elevated current densities. An extended flux creep model, which incorporates the shape of the vortex potential well, is shown to account for these results.

On the theory of formation of a twin (Ferroelastic) structure in high-temperature superconductors with oxygen nonstoichiometry

A model similar to the Frenkel-Kontorove one is used to describe the HTSC twin structure. It is shown that at low temperatures such a structure may exist as an equilibrium one only in slightly nonstoichiometric samples.

Electronic structure of high-temperature superconductors I. Y-Ba-Cu-O systems

The electronic structure of high-Tc copper based superconductors has been studied using the quasirelativistic INDO molecular orbital method. Model clusters of YBa2Cu3O y systems have been investigated fory ranging between 6·0 and 7·0. Monomeric units [Ba8Cu3O x ] q and [Y4Ba8Cu4O x ] q , dimeric units [Ba12Cu6O x ] q and [Y6Ba12Cu8O x ] q as well as tetrameric units [Ba18Cu12O x ] q over 300 valence orbitals have been studied. From the set of occupied molecular orbitals the density of states (DOS) function has been calculated along with its d-orbital projection. The highest occupied molecular orbitals cover the oxygen p-band whereas the copper d-band lies at lower energies. Superconducting phase models (y=6·75 to 7·0) have a higher d-DOS function at higher energies with respect to the non-superconducting phase (y=6·0).

The Electronic Structure of High Tc Copper-Oxide Superconductors from Photoemission Spectroscopy

This article will summarize the information gained from photoemission spectroscopy (PES) studies of the electronic structure of high temperature superconductor (HTSC) materials, with emphasis on distinguishing models for the normal state. Inverse photoemission spectroscopy (IPES) is a closely related measurement, and taken together PES and IPES yield the single-particle electronic structure, meaning the the spectrum to remove and add electrons, i.e., the electronic ionization and affinity spectrum. It is assumed that the sudden approximation holds, so that the photon event can be characterized by a photoemission cross section σ. The emission from a particular kind of electronic state i can often be identified because its cross section has a particular photon energy (hv) dependence σ (hv).We will emphasize a few recent results especially pertinent to the conceptual issues posed by these materials, and we will divide the discussion between angle-integrated spectra and angle-resolved spectra. For angle-integrated spectra, electrons are collected from the sample surface in a large solid angle, while for angle-resolved spectra, electrons are collected in a well-defined direction. Angle resolution can be used to select emitted electrons of a particular value of crystal momentum k.

Elektronnaya struktura vysokotemperaturnykh sverkhprovodnikov v normal'nom sostoyanii

Elektronnaya struktura vysokotemperaturnykh sverkhprovodnikov v normal'nom sostoyanii