Soft Phonon, Dynamical Fluctuations, and a Reversible Phase Transition: Indium Chains on Silicon

The In/Si(111)-(4×1) surface is a paradigmatic example of a quasi-one-dimensional system showing a reversible structural and electronic (metal-insulator) phase transition when the temperature is lowered. In this work, we use first-principles simulation techniques to uncover the atomic and electronic origin of this controversial transition. Our calculations show that the ground state consists of insulating (4×2) indium chains with a weak interchain coupling that induces opposite shear distortions in alternate chains. First-principles molecular dynamics simulations show that the (4×1)↔(8×2) phase transition is due to the ``dynamical fluctuations'' the system undergoes when, at high temperature, it fluctuates chaotically between degenerate ground states. The metallicity of the In/Si(111)-(4×1) surface is related to the low energy cost for the shear distortion.