| Summary: | μSR studies of crystalline C₆₀ reveal the existence of two distinct paramagnetic states. The experiments described in this thesis confirm that one is an exohedrally bonded muonium radical, while the other is endohedral muonium with a hyperfine parameter Aμ, close to that of muonium in vacuum. The signal from the muonium-C₆₀ radical -- which is characterized by a small Aμ, (10% of Avacμ) and an anisotropic hyperfine interaction -- is sensitive to the molecular dynamics and is used to study the structural phase transition of solid C₆₀ near 260K. Only endohedral muonium is observed in the alkali-metal-doped fullerites K₄C₆₀, K₆C₆₀, and Rb₆C₆₀. From its coherent spin precession, we find all three to be semiconductors with small gaps on the order of 0.5 eV. Our results conflict with the simple band structure model of doped fullerites indicating that electron-electron correlation effects may be important in determining the electronic structure of these solids.
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