Transient response of electronic collective modes in ultrafast measurements

The collective modes in an itinerant electron system are a key manifestation of its quantum correlations. Recent advances in 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 zero-sound propagating collective modes: “hidden” modes that don’t give rise to peaks in the spectral function but still dominate the dynamics, and “mirage” modes that do give rise to peaks in spectroscopy but actually decay faster than the single-particle scattering time. Both modes can be readily identified via time-dependent measurements in pump-probe configurations. 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.