Gaps and excitations in fullerides with partially filled bands: NMR study of ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ and ${\mathrm{K}}_4{\mathrm{C}}_{60}$

We present a nuclear magnetic resonance (NMR) study of ${\mathrm{Na}}_2{\mathrm{C}}_{60}$ and ${\mathrm{K}}_4{\mathrm{C}}_{60},$ two compounds that are related by electron-hole symmetry in the ${\mathrm{C}}_{60}$ triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate ${(1/T}_1)$ measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) ${\mathrm{C}}_{60}^{2-}$ or ${\mathrm{C}}_{60}^{4-}.$ However, the extended temperature range of the measurements presented here (10 K to 700 K) allows one to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, ${1/T}_1$ deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected by the appearance of quadrupole effects on the ${}^{23}\mathrm{Na}$ spectra of ${\mathrm{Na}}_2{\mathrm{C}}_{60}.$ In ${\mathrm{K}}_4{\mathrm{C}}_{60},$ modifications of the ${}^{13}\mathrm{C}$ spectra line shapes also indicate a structural modification. We discuss this high-temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, ${1/T}_{1}T$ tends to a constant value for ${\mathrm{Na}}_2{\mathrm{C}}_{60},$ both for ${}^{13}\mathrm{C}$ and ${}^{23}\mathrm{Na}$ NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.