Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques1 combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds2,3,4,5,6. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer7,8,9. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In–In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic)8. This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors10,11.
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This work was supported by the Deutsche Forschungsgemeinschaft through SFB616 ‘Energy dissipation at surfaces’, FOR1700 ‘Metallic nanowires on the atomic scale: electronic and vibrational coupling in real world systems’, SFB1242 ‘Non-equilibrium dynamics of condensed matter in the time domain’, FOR1405 ‘Dynamics of electron transfer processes within transition metal sites in biological and bioinorganic systems’ and the High Performance Computing Center Stuttgart and the Paderborn Center for Parallel Computing. We acknowledge discussions with R. Ernstorfer and K. Sokolowski-Tinten.
The authors declare no competing financial interests.
Reviewer Information Nature thanks J. Freericks, J. Ortega and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Figure 1 Excitation of characteristic phonon modes through the (de-)population of certain electronic states.
Orbital character of indium surface states that are particularly susceptible to photo-induced structural changes in the Si(111)(8 × 2)–In surface; colour coding corresponds to Extended Data Fig. 3. The occupation of the blue and green interchain In–In bond states (empty in the (8 × 2) phase) excites the shear phonon mode of the indium chain. Emptying the yellow and orange intrachain In–In bond states (occupied in the (8 × 2) phase) excites the rotary phonon mode of the indium chain. The nuclear motion of the excited phonon modes is indicated by arrows.
Transient intensity of the (01) spot of the (4 × 1) phase at a substrate temperature of T0 = 142 K for two different incident fluences: Φ = 1.3 mJ cm−2 (pink) and Φ = 3.1 mJ cm−2 (orange). The decreases in the fits to the intensity (solid lines) can be converted into maximum temperature jumps of ΔT ≈ 19 K and ΔT ≈ 19 30 K, respectively.
a, b, Calculated electronic band structure for Si(111)(8 × 2)–In (a) and Si(111)(4 × 1)–In (b) (phases depicted as insets). The grey shaded areas show the projected silicon bulk bands. Excitations with a partially emptied zone-boundary valence state (a, orange and yellow) and partially occupied zone-centre conduction state (a, green and blue) are indicated with red open and filled circles.
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Frigge, T., Hafke, B., Witte, T. et al. Optically excited structural transition in atomic wires on surfaces at the quantum limit. Nature 544, 207–211 (2017). https://doi.org/10.1038/nature21432
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