Low temperature magnetic instabilities in triply charged fulleride polymers

The electronic properties of the C-60(3-) polymer in Na2Rb0.3Cs0.7C60 were studied by X-band and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On furt her cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below T-N approxima te to 15 K, as confirmed by the observation of an antiferromagnetic resonan ce mode. The Na2Rb1-xCsxC60 family of polymers offers a unique way to chemi cally control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive funct ion of the interchain separation. PACS numbers: 75.30.Fv, 61.48.+c, 76.30.- v.