Non-equilibrium lattice dynamics of one-dimensional In chains on Si(111) upon ultrafast optical excitation

Abstract

The photoinduced structural dynamics of the atomic wire system on the Si(111)-In surface has been studied by ultrafast electron diffraction in reflection geometry. Upon intense fs-laser excitation, this system can be driven in around 1 ps from the insulating false(8×2false) reconstructed low temperature phase to a metastable metallic false(4×1false) reconstructed high temperature phase. Subsequent to the structural transition, the surface heats up on a 6 times slower timescale as determined from a transient Debye-Waller analysis of the diffraction spots. From a comparison with the structural response of the high temperature false(4×1false) phase, we conclude that electron-phonon coupling is responsible for the slow energy transfer from the excited electron system to the lattice. The significant difference in timescales is evidence that the photoinduced structural transition is non-thermally driven.

Excited-state band mapping and momentum-resolved ultrafast population dynamics in In/Si(111) nanowires investigated with XUV-based time- and angle-resolved photoemission spectroscopy

“…Such a high energy content remaining in the electronic system seems to contradict the analysis of UED measurements for the same fluence range (1 -3 mJ cm −2 ). In this work, a maximum increase of the lattice temperature of no more than 40 K was extracted from a Debye-Waller analysis of diffraction spots [39]. Furthermore, the lattice temperature was found to reach its maximum after 6 ps, which is considerably longer than the reduction in electronic temperature within 1 ps that we observe.…”

“…Furthermore we show that such a verification of an electronically thermal model is important for understanding the energy flow from electrons to the lattice in the system. The fact that the highly excited electronic temperature is at odds with UED measurements, which derived a much smaller temperature increase of the surface In atoms of only ∼30 K for similar excitation conditions [39], implies a highly non-thermal distribution of optical phonons at the surface, and a bottleneck for the cooling of the electronic system as a result of electron-phonon and phonon-phonon coupling.…”

Condensation of ground state from a supercooled phase in the Si(111)-(4 × 1) → (8 × 2)-indium atomic wire system

“…On the same time scale, complementing this behavior, the (4 × 1) spot intensity increases by a factor of two. Figure 2(c) shows a transient drop of (4 × 1) spot intensity with a minimum at 6 ps time delay which is caused by the Debye-Waller effect and is due to delayed heating of the surface 22 …”

“…It is important to note that the structural transition occurs before an overall heating of the surface can be detected: the transient heating of the surface is delayed by 6 ps (Ref. 22) with an overall temperature rise between Δ T = 15–70 K for incident fluences of Φ in = 2–7 mJ/cm 2 , respectively. Considering the static sample temperature T 0 = 30 K ≪ T c , the maximum transient surface temperature T max (6 ps) = (100 ± 10) K at Φ in = 7 mJ/cm 2 remains below T c = 130 K. Thus, the structural transition must be nonthermal and is driven by electronic entropy in less than one picosecond 19 …”

Ultrafast Probing and Coherent Vibrational Control of a Surface Structural Phase Transition

“…In this thesis, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system [53]. Specifically, using ultrafast low-energy electron diffraction (ULEED) [17,18,[53][54][55], we investigate the (8×2) → (4×1) transition of atomic indium wires on the (111) surface of silicon, a prominent Peierls system which recently attracted interest for its ultrafast dynamics [9][10][11][12][56][57][58] (see artist's impression in Fig. 1.1).…”