Structural, electronic and magnetic properties of fulleride materials

Abstract

This thesis outlines new research findings into the solid-state properties of selected alkali- and alkaline-earth-intercalated fullerides, focusing on their structural, electronic and magnetic properties at ambient and non-ambient temperatures and pressures, primarily employing synchrotron X-ray powder diffraction and SQUID magnetometry.

Understanding the relationship between superconducting, neighbouring insulating and normal metallic states above Tc in unconventional superconductors is fundamentally important. Highly expanded fcc Cs3C60 behaves very differently to underexpanded A3C60 alkali fullerides such as K3C60 and Rb3C60. Whilst superconductivity in the latter seems well described by conventional Bardeen-Cooper-Schrieffer (BCS) theory, Cs3C60, a Mott-Jahn-Teller insulator under ambient pressure, exhibits distinctly non-BCS type superconductivity upon pressurisation. The intermediate regime adjacent to the Mott boundary, where strong electronic correlations are prominent, was hitherto only studied through physical pressurisation of Cs3C60 to tune the intermolecular spacing. This thesis reports the solid-state synthesis of fcc-rich RbxCs3−xC60 (0.25 ≤ x ≤ 2) bulk superconducting materials, with excellent stoichiometry control, and the effects on the electronic properties in situ of tuning intermolecular separation by varying temperature, physical and chemical pressurisation via adjusting the cation dopant ratio. It is shown that the Mott boundary can be traversed at ambient pressure upon cooling, and the metal-insulator crossover temperature tuned by chemical and physical pressurisation.

A15 Cs3C60 orders antiferromagnetically below 46 K. Previous studies found no evidence of symmetry lowering or discontinuous structural changes upon magnetic ordering, despite theoretical predictions to the contrary. This issue is addressed with the first systematic ultrahigh-resolution investigation of its structural evolution with temperature, evidencing a transition to a rhombohedral phase below TN. The structural properties of A15 Cs3C60 and Ba3C60 in situ upon pressurisation are described, extending previous work on A15 Cs3C60. This first study of the effects of compression on the latter system reveals a pressure-induced structural transition to a hitherto unreported monoclinic phase.