Mechanism of the band gap opening across the order-disorder transition of Si(111)(4 x 1)-In.

The ground state properties of indium atom chains on the Si(111) 8 x 2-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) 8 x 2 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 approximately 0.3 eV surface-state band gap at the Gamma point of the 4 x 2 Brillouin zone. This implies that the IM transition is driven by a shear phonon, not by Fermi surface nesting.