Experimental Investigations of Correlated Electron Systems: Alkali Fullerides and Sesquioxides

Abstract

The work reported in this thesis systematically investigates the structural, electronic and/or magnetic properties of fcc AxCs3-xC60 fullerides (A = K and Rb) and of sesquioxide Cs4O6 under ambient and non-ambient temperature and pressure conditions, employing X-ray and neutron powder diffraction, muon spin relaxation, and SQUID magnetometry.
In the AxCs3-xC60 alkali fullerides, unconventional superconductivity can emerge through tuning of intermolecular distances. While conventional BCS-like response is found in the underexpanded regime of the electronic phase diagram, significant deviations appear in the overexpanded region adjacent to the Mott boundary where strong electron correlations are prominent. In this work, the solid-state synthesis of fcc KxCs3−xC60 (0.12 ≤ x ≤ 2) superconducting materials, with intermolecular distances controlled via adjusting the K+/Cs+ dopant ratio, is reported. Whilst the structural, electronic and magnetic properties of fcc KxCs3−xC60 are reminiscent of those previously reported for fcc RbxCs3−xC60, i.e. the Mott boundary can be shifted to ambient pressure and the metal-insulator crossover temperature can be tuned by such chemical pressurisation of Cs3C60, cation-specific effects are of importance on the electronic properties of fcc AxCs3−xC60. This work also provides strong evidence for correlated behaviour in the overexpanded regime through examining the presence or not of the Hebel-Slichter coherence peak and extracting the superconducting gap magnitude.
Crystallographically, the alkali sesquioxides A4O6 (A = Rb, Cs), at high and low temperature, had been reported to adopt a cubic structure which generates a single crystallographic position for dioxygen, implying charge disorder: A4(O2(4/3)-)3. Conversely, spectroscopy studies at 5 K found evidence for two localized valence states of dioxygen, indicating charge ordering: (A+)4(O2−)2(O22−). This issue is addressed with the first systematic investigation of the temperature- and cooling-protocol-dependent structural evolution of Cs4O6, revealing the existence of a valence-ordered state at low temperature and also that valence disorder-order transitions can be induced by temperature, pressure and X-ray illumination.