Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the √3×√3⇔3×3 structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the √3×√3⇔3×3 transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.