Low Temperature Magnetic Instabilities in Triply Charged Fulleride Polymers

The electronic properties of the ${\mathrm{C}}_{60}^{3-}$ polymer in ${\mathrm{Na}}_2{\mathrm{Rb}}_{0.3}{\mathrm{Cs}}_{0.7}{\mathrm{C}}_{60}$ were studied by $X-\mathrm{band}$ and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On further cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below $T_N\approx 15\mathrm{K}$, as confirmed by the observation of an antiferromagnetic resonance mode. The ${\mathrm{Na}}_2{\mathrm{Rb}}_{1-x}{\mathrm{Cs}}_x{\mathrm{C}}_{60}$ family of polymers offers a unique way to chemically control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive function of the interchain separation.