Metal-to-insulator evolution in(NH3)xNaK2C60:An NMR study

Abstract

A singular evolution toward an insulating phase, shown by ${}^{23}\mathrm{Na}$ and ${}^{13}\mathrm{C}$ NMR, has been observed in the superconducting fullerides $({\mathrm{NH}}_{3}{)}_{x}{\mathrm{NaK}}_{2}{\mathrm{C}}_{60}$ for $x>1.$ Unlike most common cases, this insulating phase is nonmagnetic and ${}^{13}\mathrm{C}$ spin lattice relaxation shows the presence of a spin gap. These two features suggest that a charge disproportion from ${\mathrm{C}}_{60}^{3\ensuremath{-}}$ to ${\mathrm{C}}_{60}^{2\ensuremath{-}}$ and ${\mathrm{C}}_{60}^{4\ensuremath{-}}$ can drive the system from the metallic to the insulating state. The restoring of the ${\mathrm{Na}}^{+}$ cation in the center of the octahedral interstice in the insulating phase, as indicated by ${}^{23}\mathrm{Na}$ and ${}^{2}\mathrm{H}$ line shape analysis, confutes the current belief that the cation off-centering is effective in quenching the superconductivity.