High-field electron paramagnetic resonance study of the polymerization in Na2Rb1-xCsxC60

We report on the study of the magnetic properties of the low-temperature po lymer phases in Na2Rb1-xCsxC60 at 110 GHz microwave frequency with x varyin g between 0 and 1. The magnetic and structural properties of the polymer ph ase strongly depend on the Cs content and its electronic structure progress ively becomes quasi-one-dimensional as x is increased. While the electronic properties of the polymeric Na2RbC60 appear to be close to three-dimension al metal, Na2Rb0.3Cs0.7C60 shows characteristics of quasi-one-dimensional m etal where instability in the electronic structure was found as detected by the sudden disappearance of the ESR intensity due to the opening of the ga p at the Fermi surface. The observation of an additional resonance line bel ow 15 K, which could be attributed to antiferromagnetic resonance, suggests that the low-temperature polymeric phase in Na2Rb0.3Cs0.7C60 has a well-de fined magnetic ground state.