Physics (Internal)

Ultrafast spectroscopy of electronic dynamics in complex materials

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract: The doping of charges in Mott insulators gives rise to a wealth of new phenomena including high-TC superconductivity, colossal magnetoresistance or metal-insulator transitions. Self-organized electronically-ordered phases are a recurring feature in such materials, resulting e.g. in fluctuating stripes whose role for the high-TC mechanism is under debate. However, the cause-effect relations between low-energy excitations and correlations often remain hidden in time-averaged studies. In this talk, I will discuss experiments that employ femtosecond mid-IR and X-ray probes to study intrinsic processes in correlated solids. In particular, I will focus on recent experiments that reveal ultrafast charge localization and lattice coupling in the model stripe-phase nickelate La1.75Sr0.25NiO4, as dynamical precursors to stripe formation. The onset of electronic localization is manifested in the opening of a pseudogap at a crossover temperature T* far above long-range stripe formation. Ultrafast excitation triggers a sub-picosecond dynamics that exposes a strong and rapid dependence of Fano electron-phonon coupling on the local charge arrangement. These results clarify the role of charge localization in the nickelate pseudogap, suggesting new ways to understanding this mysterious phase in a broader class of complex oxide materials.