Studying superconducting granular aluminum with microwaves

This paper reports on the methods of electron paramagnetic (EPR) and electron spin resonance (ESR) in solids. The former describe investigations of microwave transitions between Zeeman levels of paramagnetic ions, whereas the latter indicate transitions between nearly free, but stationary spins in radicals and semiconductors as well as itinerant carriers in semimetals and metals. As early as 1962 an ESR study in intercalated graphites was undertaken in the latter. ESR was observed in C8K, C24K and C28Rb, but not in C8Rb, C8Cs and C24Cs. From the observed linewidths, which are larger for heavier alkali intercalates with larger spin-orbit coupling, it was concluded that the carrier wave function was composed not only of carbon π but also of alkali-metal s orbitals, as theories later corroborated