PHASE-TRANSITION OF SUBMONOLAYER PB GE(111) - ALPHA-ROOT-X-ROOT-R30-DEGREES[--]3X3 AT SIMILAR-TO-250 K/

We have studied the low 3 x 3 and high (alpha-root 3 x root 3 R30 degr ees) temperature phases of Pb/Ge(111) at Pb coverage 1/3 of monolayer using synchrotron radiation angular resolved photoemission spectroscop y and low-energy-electron-diffraction measurements. The Ge 3d and Pb 5 d core levels and valence bands have been recorded above and below the transition temperature (T-c approximate to 250 K). We have observed t hat the 3 x 3 reconstructed surface is a semiconductor, while the alph a-root 3 x root 3R30 degrees phase is metallic. As the system undergoe s this transition, a band gap opens in precise areas of the Brillouin zone, supporting the charge-density-wave model for the transition. (C) 1998 Elsevier Science B.V.