Librational modes in solid C60 and doped solid C60.

Abstract

We present a theoretical study of librational modes in solid ${\mathrm{C}}_{60}$ and doped solid ${\mathrm{C}}_{60}$. General results are derived by utilizing the space-group symmetries of the solids. At the Brillouin-zone center \ensuremath{\Gamma}, in solid ${\mathrm{C}}_{60}$ there should be in general five distinct libron frequencies in the simple-cubic phase, and ten in the so-called 2${\mathit{a}}_{0}$-fcc phase; in ${\mathit{M}}_{3}$${\mathrm{C}}_{60}$, where M=K,Rb, a single libron frequency is expected; in ${\mathit{M}}_{6}$${\mathrm{C}}_{60}$, one also expects a single libron frequency. The case of ${\mathit{M}}_{3}$${\mathrm{C}}_{60}$ is of particular interest since in the first approximation, the potential governing the libron motion contains a single undetermined parameter, and is thus completely determined by a measurement of the libron frequency.