Evidence for endohedral muonium in ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ and consequences for electronic structure

Positive muons injected into solid ${\mathrm{C}}_{60}$, ${\mathrm{K}}_4$${\mathrm{C}}_{60}$, and ${\mathrm{K}}_6$${\mathrm{C}}_{60}$ form vacuumlike muonium (${\mathrm{\mu }}^{+}$${\mathit{e}}^{\mathrm{-}}$) with a (6–12)% probability. Observation of coherent spin precession of muonium establishes that all three materials are nonmagnetic and nonconducting at low temperatures. From the temperature dependence of the signals we estimate the electronic band gaps in ${\mathrm{K}}_4$${\mathrm{C}}_{60}$ and ${\mathrm{K}}_6$${\mathrm{C}}_{60}$ to be considerably smaller than in ${\mathrm{C}}_{60}$. The similarity of the muonium centers supports a model in which a muonium atom is caged inside the ${\mathrm{C}}_{60}$ molecule in pure ${\mathrm{C}}_{60}$ or the ${\mathrm{C}}_{60}^{\mathit{x}}$ molecular ion in ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$.