Coherent vibrational control of a surface structural phase transition*

The desire to exert active optical control over matter is a unifying theme across multiple scientific fields. In femtochemistry, multi-pulse optical excitation schemes address coherences in the electronic and vibrational degrees of freedom of molecules to influence the efficiencies of chemical reactions. The possible transfer of this concept to solid-state systems is, however, complicated by the high electronic and vibrational density of states, and by couplings to an external heat bath. Low-dimensional and strongly correlated systems represent a promising intermediate between molecules and solids, and often host intriguing phenomena, for example, Peierls-type metal-insulator phase transitions. The impact of coherences on the switching efficiencies and thresholds of such transitions, however, remains a largely open subject.
Here, using ultrafast low-energy electron diffraction (ULEED) in combination with sequential optical excitation, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional surface system, namely atomic indium wires on the (111) surface of silicon [1]. To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, as evidenced by oscillations in the delay-dependent switching efficiency. We identify possible control mechanisms and propose a two-dimensional potential energy model for the transition. Using multi-pulse optical excitation, we explore the selective excitation of individual modes and the repeated stimulation of the coherent phonon amplitude.

Reference:
[1] J. G. Horstmann et al., Coherent control of a surface structural phase transition. Nature 583, 232–236 (2020)