The transport properties of YBa 2 Cu 3 O y with various y and YBa 2 Cu 3− x M x O y (M = Fe, Co, Mn) with various χ have been studied. The results obtained have been analyzed on the basis of the phenomenological theory of electron transport in the case of a narrow conductive band. The narrow-band model permits one to account for the specific features of the transport coefficients of HTSC and to determine the band-structure parameters from comparison of the experimental and calculated temperature dependences of the thermoelectric power. The dependences of band-structure parameters and superconductive properties on y and χ have been analyzed together. The differences in the influence of different 3d metals on the band structure were revealed. Taking into account the crystallochemical-studies data we drew the conclusion that the oxygen sublattice disordering has a determinant effect on band-structure parameters and superconductive properties of YBa 2 Cu 3 O y .

The electron transport phenomena in Y based HTSC's and their analysis on the basis of phenomenological narrow-band theory the band structure transformation with oxygen content and substitution for Cu

A theory of highly correlated layered superconducting materials isapplied for the cuprates. Differently from an independent-electron approximation, their low-energy excitations are approached in terms of auxiliary particles representing combinations of atomic-like electron configurations, where the introduction of a Lagrange Bose field enables treating them as bosons or fermions. The energy spectrum of this field accounts for the tendency of hole-doped cuprates to form stripe-like inhomogeneities. Consequently, it induces a different analytical behavior for auxiliary particles corresponding to "antinodal" and "nodal" electrons, enabling the existence of different pairing temperatures at T^* and T_c. This theory correctly describes the observed phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover in the normal state, the existence of Fermi arcs below T^* and of a "marginal-FL" critical behavior above it. The qualitative anomalous behavior of numerous physical quantities is accounted for, including kink- and waterfall-like spectral features, the drop in the scattering rates below T^* and more radically below T_c, and an effective increase in the density of carriers with T and \omega, reflected in transport, optical and other properties. Also is explained the correspondence between T_c, the resonance-mode energy, and the "nodal gap".

A Theory for the High-T_c Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing

A theory of highly correlated layered superconducting materials is applied for the cuprates. Differently from an independent-electron approximation, their low-energy excitations are approached in terms of auxiliary particles representing combinations of atomic-like electron configurations, where the introduction of a Lagrange Bose field enables treating them as bosons or fermions. The energy spectrum of this field accounts for the tendency of hole-doped cuprates to form stripe-like inhomogeneities. Consequently, it induces a different analytical behavior for auxiliary particles corresponding to “antinodal” and “nodal” electrons, enabling the existence of different pairing temperatures at T
 ∗ and T
 c
 . This theory correctly describes the observed phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover in the normal state, the existence of Fermi arcs below T
 ∗ and of a “marginal-FL” critical behavior above it. The qualitative anomalous behavior of numerous physical quantities is accounted for, including kink- and waterfall-like spectral features, the drop in the scattering rates below T
 ∗ and more radically below T
 c
 , and an effective increase in the density of carriers with T and ω, reflected in transport, optical and other properties. Also is explained the correspondence between T
 c
 , the resonance-mode energy, and the “nodal gap”.

A Theory for the High-Tc Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing

Theories describing superconductivity in high-Tc oxides and in doped fullerenes can be placed in four categories: Fermi-liquid (FL), BCS-like schemes and non-FL theories with electro-phonon or electronic mechanisms of pairing. The main difficulties are analysed in the light of some experimental observations paying special attention to those in which pairs exist or are claimed to be observed above Tc.

Do pairs exist above Tc

The in-plane resistivity data, ϱ ∥ ( T ), as a function of temperature for various Cu-oxide superconductors have provided strong evidence for Fermi-liquid normal state and also important clues for understanding the mechanism responsible for high-temperature superconductivity. A dominant quadratic temperature dependence of ϱ ∥ ( T ) above T c is observed in the electron-doped NdCeCuO system and several relatively low- T c hole-doped cuprates. On the other hand, the high- T c Cu-oxides are always characterized by a linear temperature dependence of ϱ ∥ ( T ). Within the framework of the Fermi-liquid model, the correlation of T c with the temperature variation of T c can be understood as caused by effects arising from a two-dimensional Fermi surface (FS). The flatness of FS and the amount of nesting determine the temperature dependence of ϱ ∥ ( T ) and the magnitude of T c . Both the T 2 and the T dependences are due to electron-electron interaction and the linear temperature variation of ϱ ∥ ( T ) is mostly a manifestation of partial FS nesting. Even with a modest electron-phonon interaction (λ≲1), a T c of the order of 100 K can be achieved with the aid of a sharply varying density of states N ( E ) near the Fermi energy E F . Such a van Hove like singularity in N ( E ) has been proposed in the past as a T c -enhancement mechanism for A15 superconductors and more recently for the Cu oxides. It is shown that T c can be limited by various N ( E ) broadening effects on the scale of k B T c . However, these effects are not sufficient to invalidate the high- T c mechanism in Cu oxides. In addition, experimental findings such as the near-zero isotope effect, the Cu-substitution effect on T c , and the lack of correlation between λ and energy gap to T c ratio 2Δ(0)/ k B T c are consistent with this high- T c mechanism.

Origin of high-temperature superconductivity in copper oxides - clues from the normal-state resistivity

Four-probe resistivity (ρ) and thermoelectric power (TEP) measurements were carried out on samples of YBa2Cu3O7−δ up to 950°C, in air and in flowing oxygen at 1 bar. Below 700 K the TEP is small and increases rapidly above it, reaching, at 1200 K, +140μV/K in air and +120μV/K in oxygen. At the changeover temperature (700 K) the slope of logρ vs.T changes abruptly. These results are interpreted in terms of a model of transport of carriers in a narrow band, which is full forδ = 1 and half-filled forδ = 0. Possible origins for such a narrow band are discussed in detail.

Thermoelectric power measurements of YBa2Cu3O7−δ up to 950°C and their application to test the band structure near EF

We report the results of simultaneous measurements of resistivity and of thermoelectric power of YBa 2 Cu 3 O x over a range of temperatures and oxygen pressures (100 K T p (O 2 )≤1 atm), both under conditions of equilibrium and of non-equilibrium with the ambient. The results have been interpreted using a simple model of conduction in a narrow band, where the carrier concentration in the band and the dominant scattering mechanism are determined by the concentration of oxygen vacancies. There is good agreement between the experimental results and the predictions of the model.

On the normal state resistivity and thermoelectric power of YBa2Cu3Ox: Experiments and interpretation

The resistivity and thermoelectric power (S) of YBa2Cu3O7−δ (δ> 0.5), at constant oxygen pressure, pass through a maximum as a function of temperature. At still higher temperature S changes sign. We interpret this behaviour in terms of a simple model.

The relation between transport properties and oxygen stoichiometry in oxygen depleted YBa2Cu3O7−δ

Resistivity and thermoelectric power (TEP) measurements were carried out on YBa 2 Cu 3 O 7−δ up to 950°C, in atmospheres with different oxygen partial pressures P(O 2 ). The results are interpreted in terms of a narrow band model, which is full forδ = 1 and half filled forδ = 0.

I.O. Lelong

Papers

Thermoelectric power measurements of YBa2Cu3O7−δ up to 950°C and their application to test the band structure near EF

On the normal state resistivity and thermoelectric power of YBa2Cu3Ox: Experiments and interpretation

The relation between transport properties and oxygen stoichiometry in oxygen depleted YBa2Cu3O7−δ

Resistivity and thermoelectric power of YBa2Cu3O7−δ up to 950°C