Electrical resistivity at large temperatures: Saturation and lack thereof

Many transition metal compounds show saturation of the resistivity at high temperatures, T, while the alkali-doped fullerenes and the high-Tc cuprates are usually considered to show no saturation. We present a model of transition metal compounds, showing saturation, and a model of alkali-doped fullerenes, showing no saturation. To analyze the results we use the f-sum rule, which leads to an approximate upper limit for the resistivity at large T. For some systems and at low T, the resistivity increases so rapidly that this upper limit is approached for experimental T. The resistivity then saturates. For a model of transition metal compounds with weakly interacting electrons, the upper limit corresponds to a mean free path consistent with the Ioffe-Regel condition. For a model of the high Tc cuprates with strongly interacting electrons, however, the upper limit is much larger than the Ioffe-Regel condition suggests. Since this limit is not exceeded by experimental data, the data are consistent with saturation also for the cuprates. After "saturation" the resistivity usually grows slowly. For the alkali-doped fullerenes, "saturation" can be considered to have happened already for T=0, due to orientational disorder. For these systems, however, the resistivity grows so rapidly after "saturation" that this concept is meaningless. This is due to the small band width and to the coupling to the level energies of the important phonons.Comment: 22 pages, RevTeX, 19 eps figures, additional material available at http://www.mpi-stuttgart.mpg.de/andersen/fullerene

Nonresonant inelastic light scattering in the Hubbard model

Inelastic light scattering from electrons is a symmetry-selective probe of the charge dynamics within correlated materials. Many measurements have been made on correlated insulators, and recent exact solutions in large dimensions explain a number of anomalous features found in experiments. Here we focus on the correlated metal, as described by the Hubbard model away from half filling. We can determine the B1g Raman response and the inelastic X-ray scattering along the Brillouin zone diagonal exactly in the large dimensional limit. We find a number of interesting features in the light scattering response which should be able to be seen in correlated metals such as the heavy fermions.Comment: 9 pages, 7 figures, typeset with ReVTe

Radiophysics field stations and the early development of radio astronomy

During the period 1946–1961 Australia was one of the world’s leading nations in radio astronomy and played a key role in its development. Much of the research was carried out at a number of different field stations and associated remote sites situated in or near Sydney which were maintained by the Commonwealth Scientific and Industrial Research Organisation’s Division of Radiophysics. The best-known of these were Dover Heights, Dapto, Fleurs, Hornsby Valley and Potts Hill. At these and other field stations a succession of innovative radio telescopes was erected, and these were used by a band of young scientists—mainly men with engineering qualifications—to address a wide range of research issues, often with outstanding success