Emergent Collective Modes and Kinks in Electronic Dispersions

Recently, it was shown that strongly correlated metallic fermionic systems [Nature Phys. 3, 168 (2007)] generically display kinks in the dispersion of single fermions without the coupling to collective modes. Here we provide compelling evidence that the physical origin of these kinks are emerging internal collective modes of the fermionic systems. In the Hubbard model under study these modes are identified to be spin fluctuations, which are the precursors of the spin excitations in the insulating phase. In spite of their damping, the emergent modes give rise to signatures very similar to features of models including coupling to external modes.