Speaker:

Ralph Ernstorfer, Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin, Germany (ernstorfer@fhi-berlin.mpg.de)

Title:

Momentum-resolved dynamics of electrons and phonons: from electron-phonon coupling to light-induced phase transitions

We investigate the electronic and atomic structure of crystalline materials in laser-prepared non-equilibrium states with time- and angle-resolved photoelectron spectroscopy (trARPES) [1] and femtosecond electron diffraction (FED). The momentum-resolved view of the structure and dynamics of excited electrons and phonons provides information on microscopic coupling phenomena, e.g. the coupling of electronic and vibrational degrees of freedom. We aim for a quantum-state-resolved picture of coupling on the level of quasi-particle self-energies, which goes beyond established ensemble-average descriptions. I will exemplify this experimental approach by discussing electron, exciton and phonon dynamics in the semiconducting transition metal dichalcogenide WSe₂ [2,3] and discuss the extension of this approach to the study of nanoscale heterostructures. By combining momentum- and time-resolved dynamics of electronic and vibrational excitation, a microscopic picture of electron-lattice coupling and energy dissipation emerges. In addition, we investigate the ultrafast semiconductor-metal phase transition in indium nanowires on Si(111) as a model system for photoinduced chemical reactions. TrARPES reveals the transient electronic structure, which defines the potential energy surface on which nuclear dynamics evolve, and a detailed reaction pathway including temporally separated transitions of the electronic and the nuclear structure [4,5].

References:

[1]        M. Puppin, et al., arXiv: 1811.06939.

[2]        R. Bertoni, et al., Phys Rev. Lett. 117, 277201 (2016).

[3]        L. Waldecker, et al. , Phys. Rev. Lett. 119, 036803 (2017).

[4]        C.W. Nicholson et al., Science 362, 821 (2018).

[5]        C.W. Nicholson et al., arXiv: 1812.11385.