When a material absorbs the energy of a photon, individual electrons within itsstructure can change in a myriad of ways, resulting in macroscopic changes to the material’s properties and generating new out-of-equilibrium phases. Many emerging phenomena in quantum materials stem from competing ground states in different, often coupled, degrees of freedom in a material. In such cases, optical excitation can enable well-controlled design of material properties transiently by tipping the balance between competing ground states in a well-defined fashion. Likewise, charge transfer processes at interfaces are most essential for chemical processes such as in photocatalysis or batteries. Understanding the material chemistry and dynamics at a molecular level is of striking importance for a wide array of current challenges, such as clean water production, carbon dioxide capture, understanding the behavior of plastics in water, clean energy production by photocatalysis, designing ultra-low power electronic devices, and energy storage in next generation batteries. In our group we develop and apply novel ultrafast spectroscopic methods that allow us to study materials chemistry in complex chemical environments, and to study and control quantum phenomena and materials chemistry on femtosecond timescales. In this seminar, I will discuss a recent stream of research focused on the role of lithium in various systems from its contribution to symmetry breaking (LiNbO 3 ), to an exotic quantum material (polar metal LiOsO 3 ), to unraveling why lithium ions exhibit a lower hopping rate at the surface of a solid-state electrolyte (Li x La (2-x)/3 TiO3) as compared to bulk. In another stream of research, we solved a 40-year-old puzzle regarding the relation between the intrinsic exciton condensate and a charge-density wave (CDW) that coexist in a single material, the layered excitonic insulator (1T- TiSe 2 ). In this material, we demonstrate that by melting the exciton condensate abruptly using femtosecond laser pulses, we can collapse the CDW from a 3D coordination into 2D planes in a controlled and fully reversible fashion. This is an exquisite example of how adapted probes can guide discovery to control the state of matter with the coordinate time as new degree of freedom.
Speaker: Michael Zuerch, UC Berkeley
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