Structural dynamics in atomic wire systems at surfaces studied by ultrafast-electron diffraction: Excitation, metastable states and relaxation

The (4×1) reconstructed In atomic wires on Si(111) under-go a Peierls like symmetry breaking at T_c = 130 K. During this 1^st order structural transition the system doubles the periodicity along and normal to the wires resulting in a (8×2) ground state [PRB89,121107(2014)]. The structural dynamics upon excitation by fs-IR laser pulses is probed by ultra-fast electron diffraction at a resolution of 350 fs [RSI78,013906(2007), RSI90,045119(2019)]. The excitation results in an accelerated displacive structural transition of the atomic wires to the (4×1) excited state in only 700 fs [Nature 544,207(2017)].

Transient heating of the In atoms from 30 to 60 K occurs delayed at 6 ps [Struct.Dyn.5,025101(2018)]: transition is driven by electronic entropy and not thermally. An energy barrier for the atoms collective motion from the (4×1) to the (8×2) hinders the immediate recovery of the ground state: the system remains for ns in a super cooled metastable (4×1) state inaccessible under equilibrium conditions. Relaxation into the (8×2) ground state happens through nucleation of the (8×2) at pre-existing defects like adsorbates [PRL109,186101(2012), PRL111,149602(2013)] or step edges triggering a recrystallization front propagating at a speed of ~100 m/s [Struct.Dyn.6,045101(2019)].