title: Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces.
abstract: Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.
authors.affiliation: International School for Advanced Studies (SISSA) and Istituto Nazionale di Fisica della Materia (INFM/DEMOCRITOS), via Beirut 2-4, I-34014, Trieste, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Ballabio
authors.forename: G
authors.initials: G
authors.lastname: Profeta
authors.forename: S
authors.initials: S
authors.lastname: de Gironcoli
authors.forename: S
authors.initials: S
authors.lastname: Scandolo
authors.forename: G E
authors.initials: GE
authors.lastname: Santoro
authors.forename: E
authors.initials: E
authors.lastname: Tosatti