Michael Zuerch

Our group specializes in ultrafast spectroscopic methods, enabling in-depth studies of material chemistry in intricate environments and the control of quantum phenomena on femtosecond timescales. In the first part of this seminar, I will discuss the role of lithium in various systems from its contribution to symmetry breaking (LiNbO₃), to an exotic quantum material (polar metal LiOsO₃), to unravel the reasons behind the low hopping rate of lithium ions at the surface of a solid-state electrolyte (Li_xLa_(2-x)/3TiO₃). All these systems share the common feature that Li occupies a symmetry-broken state which we can selectively probe using extreme-ultraviolet second-harmonic generation spectroscopy (XUV-SHG), a novel spectroscopy pioneered in my group. In the second part I will discuss recent results on 1T-TiSe₂, a prototypical charge-density-wave (CDW) compound that also exhibits strong excitonic correlations in its low-temperature phase. I will discuss how we unravel the interplay of these different aspects via cryogenic attosecond transient extreme-ultraviolet absorption spectroscopy (cryo-ATAS), for which we built a unique laboratory instrument, and mega-electron-volt ultrafast electron diffraction (MeV-UED). I will show how photoexcitation leads to a 3D-to-2D dimension crossover in the CDW order parameter, a process dictated by the excitonic correlations in the system. The excitonic effect is further evidenced in the initial response of selected core-level absorption edges, and these observations help pinpoint the specific role of excitonic correlations in the CDW transition. I will also illustrate the hidden 1D nature of the CDW and its implications for the formation mechanism of domain-wall-like topological defects, which are found to emerge well under one picosecond following photoexcitation.

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