Ultrafast dynamics of electrons, excitons and phonons in momentum space

S. Dong, S. Beaulieu, M. Dendzik, T. Pincelli, P. Xian, W. Windsor, J. Maklar, D. Zahn, H. Seiler, T. Vasileiadis, Y. Qi, M. Puppin, C. Nicholson, A. Neef, Y. Deng, L. Waldecker, M. Wolf, L. Rettig, and R. Ernstorfer^*

Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany

*ernstorferprotect me ?!fhi-berlin.mpgprotect me ?!.de, https://pc.fhi-berlin.mpg.de/sesd/

The dynamics of quasi-particles in non-equilibrium states of matter reveal the underlying microscopic coupling between electronic, spin 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, and which requires ultrafast momentum-resolving techniques. The dynamics of electrons and excitons is measured with four-dimensional time- and angle-resolved photoelectron spectroscopy (trARPES), featuring a high-repetition-rate XUV laser source [1] and momentum microscope detector [2]. I will exemplify this experimental approach by discussing electron and exciton dynamics in the semiconducting transition metal dichalcogenide WSe₂ [3,4]. Our approach provides access to the transient distribution of hot carriers in the entire Brillouin zone of photo-excited semiconductors and allows the quantification of energy relaxation dynamics. I will sketch the capability of multidimensional photoemission spectroscopy of providing orbital information [5], of visualizing the change of the electronic structure during phase transitions [6,7], and of revealing interfacial energy transfer processes in nanoscale heterostructures. The complementary view of ultrafast phonon dynamics is obtained through femtosecond electron diffraction. The elastic and inelastic scattering signal reveals the temporal evolution of vibrational excitation of the lattice and momentum-resolved information of transient phonon populations [8].

1. M. Puppin et al., Rev. Sci. Inst. 90, 23104 (2019).
2. J. Maklar et al., arXiv:2008.05829 (2020).
3. R. Bertoni et al., Phys. Rev. Lett. 117, 277201 (2016).
4. D. Christiansen et al., Phys. Rev. B 100, 205401 (2019).
5. S. Beaulieu et al., Phys. Rev. Lett., accepted; arXiv:2006.01657 (2020).
6. C.W. Nicholson et al., Science 362, 821 (2018).
7. S. Beaulieu et al., arXiv:2003.04059 (2020).
8. L. Waldecker et al., Phys. Rev. Lett. 119, 036803 (2017).