Pure and doped C₆₀:

Organic Conductors, Superconductors and Magnets

Mix of tabletop and large scale experiments
Measuring optical transmission, reflection as the function of frequency.
Two fundamental aspects:
- molecular vibrations (Example: linear molecule, like CO₂ or CS₂)
- free electron excitations

Fullerenes discovered in 1985 by Curl, Kroto and Smalley
The most abundant fullerene, the C₆₀ molecule has icosahedral molecular symmetry, and it can modified by endohedral and on-site doping.
Solid C₆₀ forms an approximate face centered cubic fcc structure
The fullerene molecule forms a variety of interesting bonded structures
Solid C₆₀ can be doped with alkali and alkali earth metals , other elements and molecules.

Four IR allowed modes
Spectrum of pure C₆₀ single crystal looks a bit more complex....
- temperature dependence
- pressure dependence
- comparing the natural and C¹³ enriched single crystals
Model system: Liquid CS₂
Model calculation: Perturbed linear molecule
Two sources of "extra" modes:
- Many more modes than the four "allowed" IR active modes: symmetery breaking
- Modes extend to high frequencies: anharmonicity
- Not sensitive to C¹³, except for slight line broadening
- Pressure dependence and temperature dependence suggests intramolecular coupling.
- All 46 fundamental vibrational modes can be deduced.

What is the magnitude of the gap? Optical and tunneling studies.