Phase transition of submonolayer Pb/Ge(111): α−3×3R30° ↔ 3 × 3at∼ 250 K

We have studied the low 3 × 3 and high (α−3×3R30°) temperature phases of Pb/Ge(111) at Pb coverage 1/3 of monolayer using synchrotron radiation angular resolved photoemission spectroscopy and low-energy-electron-diffraction measurements. The Ge 3d and Pb 5d core levels and valence bands have been recorded above and below the transition temperature (Tc ≈ 250K). We have observed that the 3 × 3 reconstructed surface is a semiconductor, while the α−3×3R30° phase is metallic. As the system undergoes this transition, a band gap opens in precise areas of the Brillouin zone, supporting the charge-density-wave model for the transition.