Mechanism of the Band Gap Opening across the Order-Disorder Transition of Si(111)(4x1)-In

The ground state properties of indium atom chains on the Si(111)8x2-In surface and the nature of their insulator-metal (IM) transition near 120 K are under intense dispute. We compare experimental scanning tunneling microscopy (STM) images of the low temperature (LT) 8x2 phase with STM image calculations from Density Functional Theory (DFT). Our LT studies clearly indicate the existence of a frozen shear distortion between neighboring atom chains, resulting in the formation of indium hexagons. Tunneling spectra furthermore indicate that the IM transition coincides with the collapse of a similar to 0.3 eV surface-state band gap at the Gamma point of the 4x2 Brillouin zone. This implies that the IM transition is driven by a shear phonon, not by Fermi surface nesting.