Indication of Charge-Density-Wave Formation in Bi(111)

Photoemission spectroscopy of Bi(111) reveals a small hexagonal two-dimensional Fermi surface (FS) associated with an electron band centered in the surface Brillouin zone. Along the hexagon the Fermi momentum $k_F$ ranges from 0.053 to $0.061{\mathrm{\AA }}^{-1}$. Temperature dependent valence band spectra show an anisotropic energy gap $\Delta$ near the Fermi level. We find a transition temperature of about 75 K. At 11 K, the gap is $\Delta =4\mathrm{m}\mathrm{e}\mathrm{V}$ at the corner and $\Delta =7.5\mathrm{m}\mathrm{e}\mathrm{V}$ at the side of the hexagon. Arguments based on susceptibility $\chi (\overrightarrow{q})$ calculations of a hexagonal FS are used to discuss an incommensurate charge-density-wave (CDW) formation associated with a $q_{\mathrm{C}\mathrm{D}\mathrm{W}}=0.106{\mathrm{\AA }}^{-1}$.