Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition

Abstract

Ultrafast non-equilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photo-induced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a non-equilibrium population of excited electronic states. Utilizing femtosecond photoemission we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photo-holes in shaping the potential energy landscape, and enables a combined momentum and real space description of PIPTs, including the ultrafast formation of chemical bonds.Artists view of the excitation and formation of chemical bonds along Indium nanowires (red balls) on a Silicon(111) surface during the ultrafast photoinduced phase transition between the 8x2 and 4x1 structures. This real space view of atoms and bonds is complemented by detailed measurememets of the electronic structure of electrons in their "momentum space" exhibiting the evolution of the band stuctrue providing a complete picture of the phase transition. 3 In/Si(111) undergoes a transition from an insulating (8x2) to a metallic (4x1) structure above 130 K (27,28), r-space schematics of which are shown in Fig. 1, C and D, respectively. The bonding motif in the insulating phase (Fig. 1C) consists of distorted hexagons, while in the conducting phase the In atoms rearrange into zig-zagging chains (Fig. 1 D).The k-space band structures of the two phases calculated within the GW approximation are given below the relevant structures in Fig. 1, E and F. In contrast to the (4x1) phase which has three metallic bands (m1 -m3) that cross EF (17), the (8x2) phase is gapped at the Γ 8x2 and X 8x2 points. Upon increasing the temperature across the (8x2) to (4x1) phase transition, the states initially lying far above EF at Γ 8x2 shift down in energy and eventually cross EF, forming the metallic m1 band of the (4x1) phase. Concurrently the energy gap in the m2 and m3 bands at the X 8x2 point closes, while at the same time the bands shift apart in momentum along the kx direction (23). We note that the three metallic bands predicted from the calculation in the (4x1) phase are clearly observed in Fig. 1B. The Fermi surface of the (4x1) phase in Fig. 1G shows the momentum cut along which our data are obtained.

Role of a higher-dimensional interaction in stabilizing charge density waves in quasi-one-dimensional NbSe3 revealed by angle-resolved photoemission spectroscopy

“…The observation of the rigid band shift during CDW formation presented above may indeed point in the direction of a redistribution of charge in the Nb states at E F , although the details remain unclear. The potential relevance of such a description can also be seen in the context of recent time-resolved experiments on femtosecond timescales where the transient occupation of bonding and anti-bonding states was shown to photo-induce a CDW transition [42] in another quasi-1D system. The band shift and gradual gapping of the Fermi surface as seen in Fig.…”

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

“…The discrepancy is even more striking as these states are expected to be symmetric to those at Γ (k x =0) in the (8x2) unit cell, suggesting a k-dependent mechanism causing this band renormalization. Since these m 1 states are directly linked to the phase transition, in particular, as they shift down into the occupied region during the photo-induced phase transition [35], it is tempting to think that the discrepancy may result from an incomplete initial transition into the (8x2) ground state which pulls the m 1 band down in energy. As described before, the measurements presented in Fig.…”

Formation of Sn‐Induced Nanowires on Si(557)

“…Nanowires grown by self‐assembly on surfaces were shown to provide a perfect template to study fundamental ground state properties of one‐dimensional systems. Moreover, they also appear to be interesting candidates for getting a deep insight into the transient of a phase transition or dynamics of solitons, as impressively demonstrated for In/Si(111) …”