Insulating and Conducting Phases of RbC60

Optical measurements were performed on thin films of Rb$_{x}$C$_{60}$, identified by X-ray diffraction as mostly $x=1$ material. The samples were subjected to various heat treatments, including quenching and slow cooling from 400K. The dramatic increase in the transmission of the quenched samples, and the relaxation towards the transmission observed in slow cooled samples provides direct evidence for the existence of a metastable insulating phase. Slow cooling results in a phase transition between two electrically conducting phases.Comment: Minor revisions. Submitted to PRB, RevTeX 3.0 file, 2 postscript figures included, ir_dop

Electrical resistivity and stoichiometry of KxC60, RbxC60, and CsxC60 films

Electrical resistance measurements as a function of stoichiometry have been carried out on KxC60, RbxC60 and CsxC60 thin films for 0<x<6. The annealed films show global resistance minima at K3C60, ρmin=4×10-3 Ω cm (60°C), Rb3C60, ρmin=4×10-3 Ω cm, and Cs1C60, ρmin = 7×10-2 Ω cm. All of the annealed films show additional features in the vicinity of x=4, but the manifestation of the A4C60 phase (A=K, Rb, Cs), in transport studies is dependent on the metal and the annealing conditions. The A6C60 phase is apparent for all of the metals studied and shows a relatively high resistivity. The activation energies of the conduction process show well defined stationary points at K3C60 (not activated), Rb3C60 (not activated). Cs1C60 (minimum), Cs4C60 (maximum), with less distinct features between 4<x<6 in all cases.