On the defect structure of grain boundaries in Ba2YCu3O7–x

A central issue in the field of high temperature superconductivity is the nature of the normal state. Here we review normal state transport, thermodynamic, magnetic and spectroscopic data and discuss their interpretation within a Fermi liquid based framework. Comparison with other highly correlated systems, such as the heavy fermnion metals and the quantum liquid 3 He, is also presented. It is demonstrated that the cuprate data are not consistent with canonical Fermi liquid behavior. However, if the Fermi liquid is characterized by low energy scales, arising from narrow bandwidths or soft spin fluctuations, then canonical behavior is not expected at the relatively “high” temperatures of the normal state. In this way anomalies above T c may be reconciled with a Fermi liquid (ground state) picture. Here we review the two main Fermi liquid based schools and their interpretation of the data. These are the ”almost localized” and “almost magnetic” Fermi liquid descriptions. The dichotomy between these two nearinstabilities has appeared in the context of the heavy fermoins as well as liquid 3 He and seems to be generally associated with highly correlated systems. Both approaches have addressed the data with some success and it is clear that they are, in many respects, closely related. The relationship between other Fermi-liquid-like, as well as more unconventional schemes is also addressed. Here it is claimed that the strongest support for a Fermi liquid based approach to the cuprates derives from the comparison with other strongly correlated Fermi liquid systems, which exhibit similar normal state anomalies.

Normal state properties in the cuprates and their Fermi liquid based interpretation

The thermopower, $\alpha$, of many members of the $Bi_2Ca_{1-x}Y_xSr_2Cu_2O_{8+\delta}$, $Bi_{2-y}Pb_yCaSr_2Cu_2O_{8+\delta}$, $Tl_yCa_{1-x}Y_xBa_2Cu_2O_{6+\delta}$ $TlCa_{1-x}Ln_xSr_2Cu_2O_{6+\delta}$ and $TlSr_{2-x}La_xCuO_{5+\delta}$ systems is positive down to $T_c$, but is negative in some of them. The slope of the $\alpha-T$ plots above $T_c$ is generally negative independent of the sign of $\alpha$. The value of $\alpha$ extrapolated to 0 K, $\alpha(0)$, from the high-tem perature regime is finite and generally, positive. Compared to the behaviour of $YBa_2Cu_3O_7$, the data on the $Bi$ and $Tl$ cuprate systems examined here seem to suggest that electrons may be involved in the conduction process as well, at least in systems where both the $\alpha$ and the $\alpha-T$ slope are negative. Of the different systems examined here, $Tl_{1-x}Pb_xCaSr_2Cu_2O_{6+\delta}$ containing $Pb$ in the 4+state, deserves special mention since it shows negative $\alpha$ over the entire temperature range in addition to a negative slope suggesting that this is likely to be a genuine high-$T_c$ electron-superconductor. The thermopower of cuprates has been discussed in some detail in terms of entropic and quasiparticle contributions.

Systematics in the thermopower behaviour of several series of bismuth and thallium cuprate superconductors:: An interpretation of the temperature variation and the sign of the thermopower

A measurement algorithm and the required instrumentation for computer automation of temperature-dependent thermopower measurements over the range of 4.2 to 350 K are described. The performance of the measurement system is demonstrated by examining typical results obtained on bulk and thin film samples of the thallium-based ceramic superconductor. The quality of the measurement data obtained is improved by increasing the density of the collected data and by improving reproducibility of the results. >

http://ieeexplore.ieee.org/iel1/19/2129/00058621.pdf

Computer control of a slow-AC technique for thermopower measurements

The superconducting and normal state transport properties of ceramic samples with composition Bi 2 Sr 2− x K x CaCu 2 O y ( x  = 0, 0.1, 0.2, 0.3, 0.4) have been investigated by measuring thermoelectric power. The samples were prepared by the solid state route, and single phase Bi-2212 was found for all compositions. The T c (0) and thermoelectric power data were collected from 50 to 300 K. The thermoelectric power changes slightly with increasing doping level and increases linearly with decreasing temperature reaching a maximum value at T c (0). The T c (0), hole concentration and thermoelectric data were correlated. The results obtained are analysed in terms of phenomenological theoretical models that makes it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. Following models applied such as (a) Hubbard model, (b) two-band model, (c) Fermi-liquid behaviour. Applicability of these models to the present HTSC system is discussed and the parameters related to the models have been evaluated.

Thermoelectric power in Bi2Sr2−xKxCaCu2Oy

We report measurements of the thermoelectric power of the high-temperature Tl-Ca-Ba-Cu-O superconductor. The data indicate a transition which is centered at 118 +- 3 K, in agreement with magnetic measurements and published resistivity results. The thermoelectric power is positive and, aside from the high transition temperature, is remarkably similar in magnitude and temperature dependence to that of the rare-earth-based oxide superconductors which have been reported previously.

Thermoelectric power of the Tl-Ca-Ba-Cu-O superconductor.

The temperature dependence of the thermoelectric powers of the high-temperature oxide superconductors Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub x/ and (Eu-Y)/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub x/ is reported. The sign of the dominant carriers in the normal state is observed to be positive. Well above the superconducting transition, the magnitude of the thermoelectric power is a weakly decreasing function of temperature.

N. Mitra

Papers

Thermoelectric power of the Tl-Ca-Ba-Cu-O superconductor.

Thermoelectric power of Y-Ba-Cu-O and Eu-Y-Ba-Cu-O.

Manufacture and testing of high-Tc superconducting materials