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

Abstract

We investigate the excited state electronic structure of the model phase transition system In/Si(111) using femtosecond time-and angle-resolved photoemission spectroscopy (trARPES). An extreme ultraviolet (XUV) 500 kHz laser source at 21.7 eV is utilized to map the energy of excited states above the Fermi level, and follow the momentum-resolved population dynamics on a femtosecond time scale. Excited state band mapping is used to characterize the normally unoccupied electronic structure above the Fermi level in both structural phases of indium nanowires on Si (111): the metallic (4x1) and the gapped (8x2) phases. The extracted band positions are compared with the band structure calculated within density functional theory (DFT) within both the LDA and GW approximations. While good overall agreement is found between the GW calculated band structure and experiment, deviations in specific momentum regions may indicate the importance of excitonic effects not accounted for at this level of approximation. To probe the dynamics of these excited states, their momentum-resolved transient population dynamics are extracted with trARPES. The transient intensities are compared to a simulated spectral function modeled by a state population employing a transient elevated electronic temperature as determined experimentally. This allows the momentum-resolved population dynamics to be quantitatively reproduced, revealing important insights into the transfer of energy from the electronic system to the lattice. In particular, a comparison between the magnitude and relaxation time of the transient electronic temperature observed by trARPES with those of the lattice as probed in previous ultrafast electron diffraction studies implies a highly non-thermal phonon distribution at the surface following photo-excitation. This suggests that the energy from the initially excited electronic system is initially transferred to high energy optical phonon modes followed by cooling and thermalization of the photo-excited system by much slower phonon-phonon coupling. arXiv:1812.11385v3 [cond-mat.str-el]

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

“…In this case, density functional theory (DFT) predicts a strong coupling of excited electronic states to shear and rotation phonons realising the structural transition. At the same time, the weak coupling of these modes to lower-lying optical or acoustic phonons of the indium wires and the substrate found in recent trARPES [57] promises long-lived vibrational coherence in decisive degrees of freedom. Altogether, this renders indium on Si(111) and ideal system to test the applicability of coherent control schemes to low-dimensional solids.…”

“…It is a prime example of how the development and improvement of new techniques in theoretical and experimental physics can contribute to a better understanding of processes in complex materials. Even today, the system serves as a benchmark for cutting edge time-resolved techniques such as time-resolved ARPES (trARPES) [11,12,57,58] or ultrafast electron diffraction (UED) [10,56], revealing fascinating nonequilibrium dynamics (see Sec. 4.4).…”

“…In a follow-up experiment, Nicholson et al focused on the transfer of energy from the electronic system to the lattice [57], proposing an initial coupling of excited electronic states to high-energy optical modes, and a subsequent, much slower energy relaxation by means of phonon-phonon coupling. This again highlights the key role of shear and rotation modes, which are likely to represent a phonon bottleneck for the energy transfer [57].…”

“…In a follow-up experiment, Nicholson et al focused on the transfer of energy from the electronic system to the lattice [57], proposing an initial coupling of excited electronic states to high-energy optical modes, and a subsequent, much slower energy relaxation by means of phonon-phonon coupling. This again highlights the key role of shear and rotation modes, which are likely to represent a phonon bottleneck for the energy transfer [57]. Other trARPES studies by Cháves-Cervantes et al [12,58,153] demonstrated that the (8×2) → (4×1) transition can be driven by multiphoton absorption across the X (8×2) band gap and found possible signatures of shear [153] and rotation [12,66,153] modes in the excited state populations.…”

“…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).…”