Hole concentrations, Hall number, and Tc relationships in substituted YBa2Cu3Oy.
One of the fundamental questions involving the high-temperature copper oxide containing superconductors is to understand the relationship between hole concentration and the superconducting transition temperature, T,. We help resolve this question by a study of Nd-, La-, Ga-, Ca-, and Zn-substituted YBa2Cu30~ with controlled oxygen stoichiometry. The total hole concentration is determined by a chemical method and trends in the mobile hole concentration by Hall measurements. By considering T, to be a function of the mobile sheet holes only, there is a maximum in T, (92 K) at a sheet-hole concentration of about 0.20-0.25 per Cu site. The importance of the hole concentration in Cu oxide containing superconductors is widely recognized and its relationship to the superconducting transition temperature (T, ) has been the object of numerous studies. ' '2 The hole concentration, which we call p, is an electron deficiency and can be determined chemically, as well as by Hall measurements. Although it is generally accepted that it is associated with the oxygen rather than the copper, i.e., 0, the chemistry of its determination is identical to that of Cu + or a hole in the copper-oxygen band. The Hall measurement is complimentary to the chemical measurement since it measures only the mobile holes. Previously, using both techniques we showed a rather linear relationship existed between T, and p in the La2 Sr„Cu04 system (for p &0.15) and discussed the possible T, vs p relationship in the 1:2:3-type materials, e.g. , YBa2Cu30». ' Our data suggested a possible relationship between the two systems if similar hole concentrations per sheet (CuOz), which we showed reached a maximum at about 0.15 in the one-sheet La2 „Sr,Cu04 system, could be produced in each of the two sheets of the 1:2:3 materials. Torrance et al. ' further studied the
Hall effect of epitaxial YBa2Cu3O7-x and Bi2Sr2CaCu2Oy films: Interpretation of the Hall effect on the basis of a renormalized tight-binding model.
The Hall effect of epitaxial YBa2Cu3O7−x and Bi2Sr2CaCu2O y films has been investigated experimentally and theoretically. In both compounds the normal state Hall coefficient R H (B || c-axis) is positive and exhibits a strong temperature dependence, which is more pronounced in YBa2Cu3O7−x than in Bi2Sr2CaCu2O y . The Hall coefficient R xyz H in the normal phase has been calculated on the basis of a renormalized two-dimensional tight binding model using the relaxation time approximation. Strong correlation effects are accounted for via doping dependent hopping terms leading to band narrowing. The influence of the choice of the relaxation time on the Hall coefficient is explored. The calculations have been performed for a wide range of parameters and are compared to experimental data.
Satellites and Education in Australia--Learning from International Experience
Australia's domestic satellite service, AUSSAT, began operating in August 1985 and will be fully operational with the launch of the third satellite in late 1986. An examination of international educational satellite systems relevant to primary and secondary education in Australia is presented. Information collected and analysed is intended to inform decision-making.