Collective Excitations in Bose-Fermi Mixture*

Low-lying collective excitations are sensitive probes to quantum many-body systems. In particular, quasiparticle properties and the quantum gas’s equation of state are studied through these elementary excitations. We focus on exploring the Bose-Fermi mixture in the fermionic impurity limit, which exhibits rich many-body phase diagram and dynamics. The mixture is created by immersing fermionic $^{40}$K atoms into a $^{23}$Na Bose-Einstein condensate (BEC). We optically excite dipole and other low-lying collective modes and study the Bose-Fermi mixture response upon varying interspecies interaction strength and temperature. Our experiments reveal a collisionless to hydrodynamic transition of fermionic impurities.