Optically excited structural transition in the In/Si(111) nanowire array at the quantum limit*

The Si(111)-(4x1)/(8x2)In atomic wire array is an extremely popular model system for one-dimensional electronic systems. It features a reversible temperature-induced metal-insulator transition into a charge density wave (CDW) ordered ground state. The transition is driven by a strong electron-phonon coupling. We carried out constrained density functional theory calculations and ultrafast pump-probe electron diffraction measurements. We demonstrate that an optical excitation breaks and creates specific bonds, leading to a non-thermal excitation of particular soft phonon modes. The excitation of these modes drive the structural transition in the limit of critically damped nuclear motion and allows to switch between metallic and insulating states within 350 fs. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in a regime of deterministic and directed nuclear motion [Nature 544, 207 (2017)].