PGI Kolloquium: Prof. Dr. Michael Bauer, Institute of Experimental and Applied Physics, Kiel University, Kiel, Germany

Please note: You will receive the link to the online talk in the e-mail invitation, usually sent out a few days before the lecture takes place. It is also available on request from the contact person below.

Exploring Electron-Phonon Interactions in 2D Quantum Materials using Time-Resolved ARPES

Electron-phonon interaction is one of the most fundamental quasiparticle interaction in solids governing for instance transport and thermodynamic properties of materials, but being also responsible for unconventional phenomena such as low-temperature superconductivity and the formation of charge-ordered phases.

In my talk I will present and discuss two recent time- and angle resolved photoemission studies of our group in which we addressed, how electron-phonon interactions can affect electron and spin degrees of freedom on ultrafast timescales in two different types of 2D transition metal dichalcogenides.

In a singly oriented film of single-layer WS₂ deposited on a Au(111) surface we studied the spin- and valley-selective photoexcitation and decay of free carriers at the K and K’-points [1]. Our results reveal that in the valence band maximum an ultimate valley polarization of free holes of 84% can be achieved upon excitation with circularly polarized light at room temperature. For the photoexcited free electrons in the conduction band minimum, we observe a significantly smaller valley polarization. Clear differences in the carrier dynamics between electrons and holes imply intervalley scattering processes into dark states being responsible for the efficient depolarization of the excited electron population. The observed characteristic timescale for this process match very well values predicted from theory for the formation of momentum forbidden intervalley dark excitons in W-based SL TMDCs due to electron-phonon interaction [2].

In the second example I will discuss results on the electron-phonon interaction in the Weyl-semimetal Td-WTe₂ as probed in a TRARPES experiment following the excitation of coherent phonons [3]. We can show that a phonon-frequency selective analysis of the experimental data providew high-resolution information on strengths and types of couplings of the individual modes to the electronic bands. I particularly will discuss results on the transient modulation of a Dresselhaus-type spin splitting of electronic bands driven by the selective coupling of an interlayer shear mode of the layered compound. The latter results reveal real-time insights into electron-phonon coupled processes that are of vital importance for a light-driven topological phase transition in Td-WTe₂ [4]. Furthermore, the frequency domain analysis developed for the analysis of the data may provide in the future a quantitative electronic band- and phonon mode-resolved view onto electron phonon interactions in general [3, 5, 6].

References

[1] H. Beyer, et al., “80% Valley Polarization of Free Carriers in Singly Oriented Single-Layer WS2 on Au(111)” Phys. Rev. Lett. 123, 236802 (2019).

[2] M. Selig, et al., “Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides” Nature Commun. 7, 13279 (2016)

[3] P. Hein, et al., “Mode-resolved reciprocal space mapping of electron-phonon interaction in the Weyl semimetal candidate Td-WTe₂”Nat. Commun. 11, 2613 (2020).

[4] E.J. Sie, et al., “An ultrafast symmetry switch in a Weyl semimetal” Nature 565, 61 (2019)

[5] T. Suzuki, et al., “Detecting electron-phonon coupling during photoinduced phase transition”, Phys. Rev. B 103, L121105 (2021).

[6] U.D. Giovannini, et al., “Direct Measurement of Electron-Phonon Coupling with Time-Resolved ARPES”, Phys. Rev. Lett. 125, 136401 (2020).