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Phys Rev Lett. 2002 Sep 16;89(12):126803. Epub 2002 Sep 3.

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces.

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International School for Advanced Studies (SISSA) and Istituto Nazionale di Fisica della Materia (INFM/DEMOCRITOS), via Beirut 2-4, I-34014, Trieste, Italy.


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.

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