Séminaire - Claude Monney (Université de Fribourg - Suisse)

Angle-resolved photoemission spectroscopy as a probe of the electronic structure of materials: from static to transient properties

Claude Monney

Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland

The electronic structure of materials reflects not only the symmetry of the underlying lattice, but also the presence of electronic correlations. Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique to reveal these properties, since it gives access to the momentum-resolved electronic structure of materials. I will illustrate these different aspects with two cases.

Tantalum disulphide 1T-TaS2 is a layered material that hosts a series of charge density wave (CDW) phases at temperatures below ~350 K, and an insulating commensurate CDW (CCDW) phase below ~165 K. In 1976, already, it was recognized that the particular rearrangement of atoms in so-called Stars-of-David within the CCDW phase might lead to a Mott phase due to the presence of strong electronic correlations [1]. However, this picture has been recently challenged by the proposal that interlayer hybridization is in fact responsible for the insulating CCDW phase [2]. I will present recent ARPES measurements on 1T-TaS2 surfaces combined with advanced electronic structure calculations showing that both pictures, the Mott correlations and the interlayer hybridization, are actually leading to band gap formation in the CCDW phase in 1T-TaS2 [3]. I will then also show how strain can be employed to modify its ground state and induce a bandwidth-driven insulator-metal transition [4].

In a second part, I will explain how ARPES can measure the ultrafast dynamics of electrons using the pump-probe technique. This will be illustrated with the case of α-GeTe(111), a bulk ferroelectric semiconductor that has been recently used for spin-to-charge conversion [5]. Here, using an infrared photoexcitation, α-GeTe(111) is driven out-of-equilibrium and its low-energy electronic structure is probed with time-resolved ARPES. By comparison with density functional theory calculations, we show that the changes observed in the electronic structure are caused by a transient increase of the ferroelectric distortion [6]. We identify a surface photovoltage effect as the responsible mechanism and link it to a delayed displacive excitation of the coherent phonon of the ferroelectric distortion.

This work is supported by the Swiss National Science Foundation.

[1] Tosatti and Fazekas, Journal de Physique 37, C4-165 (1976).

[2] Ritschel et al., Phys. Rev. B 98, 195134 (2018).

[3] Petocchi et al., Phys. Rev. Lett. 129, 016402 (2022).

[4] Nicholson et al., arXiv:2204.05598 (2022).

[5] Varotto et al., Nature Electronics 4, 740 (2021).

[6] Kremer et al., arXiv:2204.11630, accepted for publication in Nature Comm..