SIMES Seminar: Chris Nicholson

Date(s) - Apr 12 2018
10:30 AM - 11:30 AM

Room 335, McCullough Building


Ultrafast dynamics of bands and bonds during a photo-induced phase transition

Christopher Nicholson

Fritz Haber Institute of the Max Planck Society, Berlin

In the Born-Oppenheimer picture, nuclear dynamics evolve on a free energy surface determined by the transient occupation of electronic states. Access to the non-equilibrium distribution of excited states therefore allows a determination of the forces that govern the trajectory along the reaction co-ordinate during a photo-induced ultrafast structural transition. Such a description including the complete electron dynamics goes far beyond that of a “molecular movie” [1].

To address these ideas we investigate a model phase transition system – indium nanowires at the silicon(111) surface – which undergoes an order-order structural transition accompanied by an electronic insulator-to-metal transition [2]. Utilizing femtosecond time and angle resolved photoemission spectroscopy with a novel 500 kHz XUV laser source at 22 eV, we obtain direct access to the transient k-resolved electronic structure during the photo-induced phase transition (PIPT) in In/Si(111). By observing the dynamically changing band structure a detailed reaction pathway is resolved, including temporal separation of the insulator-to-metal (200 fs) and structural (700 fs) phase transitions; the latter in extremely good agreement with recent time-resolved electron diffractionmeasurements [3]. The reaction pathway is reproduced by ab initio molecular dynamics simulations, which reveals the crucial role played by localized photo-holes in shaping the potential energy landscape of the structural transition. This furthermore allows us to extend the description of ultrafast PIPTs to real space, and chart the ultrafast formation of chemical bonds during the phase transition.

1.           Dwyer, J. R. et al. Femtosecond electron diffraction: ‘Making the molecular movie’. Philos. Trans. A. 364, 741–78 (2006)

2.           Yeom, H. et al. Instability and Charge Density Wave of Metallic Quantum Chains on a Silicon Surface. Phys. Rev. Lett. 82, 4898–4901 (1999)

3.           Frigge, T. et al. Optically excited structural transition in atomic wires on surfaces at the quantum limit. Nature 544, 207 (2017)