Electronic states of the Ba-${\mathrm{C}}_{60}$ compounds

Synchrotron-radiation photoelectron spectroscopy has been used to investigate the phases that are formed upon doping of ${\mathrm{C}}_{60}$ with Ba and to examine the electronic properties of those phases. Core-level analysis reveals a ${\mathrm{Ba}}_1$${\mathrm{C}}_{60}$ phase and confirms the formation of ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$ at 300 K. A continuous range of solubility is indicated for 3<x<6. ${\mathrm{Ba}}_1$${\mathrm{C}}_{60}$ has states at the Fermi level but transport measurements indicate that these states are localized. The width of the ${\mathit{t}}_{1\mathit{u}}$-derived band for ${\mathrm{Ba}}_1$${\mathrm{C}}_{60}$ points to the continued importance of electron correlation. ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ is an insulator with a gap of at least ∼0.35 eV. The insulating character of ${\mathrm{Ba}}_3$${\mathrm{C}}_{60}$ reflects transfer of two electrons per Ba atom to the fullerene and complete filling of the ${\mathit{t}}_{1\mathit{u}}$-derived band. ${\mathrm{Ba}}_6$${\mathrm{C}}_{60}$ exhibits a clear Fermi cutoff, as expected from its superconducting character, and resonant photoemission shows that the states near ${\mathit{E}}_{\mathit{F}}$ are ∼25% Ba in character with hybridization in both ${\mathit{t}}_{1\mathit{u}}$- and ${\mathit{t}}_{1\mathit{g}}$-derived bands.