Transient response of electronic collective modes in pump-probe measurements

The collective modes in an itinerant electron system are a key manifestation of its quantum correlations. Recent advances in ultrafast pump-probe technologies allow us to access subtle properties of these modes and to manipulate them for control of quantum states and phases. I will demonstrate this by describing a nontrivial topological structure associated with correlation functions of 2D Fermi liquids, which manifests itself in unconventional collective modes, for example “hidden” zero-sound modes that don’t give rise to peaks in the spectral function, but do determine the long-time transient response of the Fermi liquid. I will also present a theory of controlling the soft collective mode that exists near a nematic quantum phase transition via nonlinear phonon excitation. The theory describes both quasi-equilibrium control and quantum quenches, and I will demonstrate its applicability to the unconventional superconductor FeSe.