Electronic Structure Of High-temperature Superconductors Research Articles

Filamentary model of vibronic spectra of YBa2Cu3O6.95

Ellipsometric synchrotron spectroscopy of YBCO provides persuasive evidence supporting the Phillips-Jung (PJ) filamentary nanodomain model of the electronic structure of high-temperature superconductors (HTSCs). The PJ model is part of a universal off-lattice topological platform that explains the origin of the intermediate phase that is responsible for high-temperature superconductivity, the stiffness transition and reversibility window in network glasses, and the folding transitions of proteins. In HTSCs it is both more specific and more general than descriptions based on conventional collective modes related to charge-density waves.

Andreev Bound-State Tunneling and ESR Spectroscopy of High-Temperature Superconductors and Observations of Broken Time-Reversal Symmetry

An emphasis on reliable materials growth and development of new fabrication techniques has allowed us to investigate the electronic structure of high-temperature superconductors by planar quasiparticle tunneling and electron paramagnetic resonance (EPR) spectroscopies. The quasiparticle (QP) density of states (DoS) is investigated by tunneling into oriented thin films of Y1Ba2Cu3O7 (YBCO) and single crystals of Ba2Sr2Ca1Cu2O8 (BSCCO). Data are obtained as a function of crystallographic orientation, temperature, doping, damage, and applied magnetic field. These data demonstrate that the observed zero-bias conductance peak (ZBCP) is composed of Andreev bound states (ABS) which intrinsically form at a symmetry-breaking interface of an unconventional superconductor, for example, a (110)-surface of d-wave YBCO. Tunneling into doped or ion-damaged YBCO provides a measure of the QP scattering rate below Tc. An applied field causes Doppler shift of the ABS, arising from the scalar product between the QP velocity and the superfluid momentum, vF·Ps, observed as a splitting in the ZBCP. Magnetic hysteresis of the splitting is consistent with the effects of strong vortex pinning near the interface. The directional field dependence shows that the ABS is highly anisotropic in its transport. These results, plus in-plane crystallographic orientational dependence on single-crystal BSCCO, demonstrate the d-wave symmetry of this superconductor. Below ∼ 8 K and in zero applied field, the ZBCP splits, indicating a transition into a superconducting state with spontaneously broken time-reversal symmetry (BTRS). EPR experiments are used to detect directly the spontaneous formation of the magnetic moments in the BTRS state.

Electronic structure studies of Bi 2 Sr 2 CaCu 2- x Sn x O 8+δ system

Photoemission measurements have been carried out for Bi2Sr2CaCu2-xSnxO8+? system with conventional x-ray photoemission spectroscopy for core-level spectra and synchrotron radiation photoemission spectroscopy for valence band. With Sn doping, all core levels shift differently in binding energy, and the intensity near fermi energy becomes smaller in valence band. From the experiment, we can deduce that the shifts of all core levels and valence bands may involve some other mechanisms, such as electrostatic effects, in addition to binding energy referencing effects. We argue that the chemical environment plays a crucial role in the electronic structure of high-temperature superconductors.

Strong electron correlation effects in X-ray and photoelectron spectra of high-temperature superconductors

This review is a detailed analysis of the literature (161 references) dealing with experimental and theoretical investigations of the electronic structure of high-temperature superconductors. Special attention is paid to the sudden perturbation model, which was effectively used to describe and interpret a series of X-ray and photoelectron experiments.

Electronic structure of high-temperature superconductors II. La-Ba-Cu-O systems

Electronic structure of La2−x Ba x CuO4 superconductors has been studied. Model systems La10CuO 22 −12 , La10cuO 14 +4 and LagBaCuO 14 +3 with various Ba positions have been investigated using the quasirelativistic INDO molecular orbital method. The oxidation state of copper atoms corresponds to Cu(II). The influence of Ba/La substitution on copper atoms is much lower than on their oxygen ligands sphere.

Alloy model for high-temperature superconductors

An alloy model is proposed for the electronic structure of high-temperature superconductors. It is based on the assumption that holes and extra electrons are localized in small copper-oxygen clusters, that would be the components of such an alloy. This model, when used together with quantum chemical calculations on small clusters, can explain the structure observed in the experimental densities of states of both hole and electron superconductors close to the Fermi energy. The main point is the strong dependence of the energy level distribution and composition on the number of electrons in a cluster. The alloy model also suggests a way to correlate T c with the number of holes, or extra electrons, and the number of adequate clusters to locate them.

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.

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).

Electronic structure of high-temperature superconductors in the normal state

Electronic structure of high-temperature superconductors in the normal state

Elektronnaya struktura vysokotemperaturnykh sverkhprovodnikov v normal'nom sostoyanii

Elektronnaya struktura vysokotemperaturnykh sverkhprovodnikov v normal'nom sostoyanii

Electronic structure of high-temperature superconductors.

The electronic energy bands, local densities of states, and valences have been calculated for ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, with a tight-binding model fitted to previous calculations for ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. Strong d-state peaks, predicted about 5 eV above the Fermi energy for La, Y, and Ba, should be observable in inverse photoemission, electron energy-loss spectroscopy, and final-state photoemission experiments.

Chemical composition and electronic structure of high-temperature superconductors: Ba2EuCu3O7-x and La2-xSrxCuO4.

We have studied the electronic structure of ${\mathrm{Ba}}_{2}$${\mathrm{EuCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ and ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ (x=0--0.15) with synchrotron-radiation photoemission and electron-energy-loss spectroscopy. The results were compared to those previously obtained on ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$. The observed photoemission resonances are consistent with a prevailing ${\mathrm{Cu}}^{2+}$ valence state for copper. Partial-yield spectra of ${\mathrm{Ba}}_{2}$${\mathrm{EuCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ indicate that the valence state of Eu is ${\mathrm{Eu}}^{3+}$, and that of Ba is ${\mathrm{Ba}}^{2+}$, similar to the corresponding elements in ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$. From the energy-loss spectra, we derived plasmon energies and indications for peaks in the density of unoccupied states. The change in x for the ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ samples only causes rigid shifts of the photoemission spectra.