Magnon-Polarons in the Fermi-Hubbard Model

The interplay between itinerant dopants and magnetic ordering is thought to be at the heart of the rich physics of unconventional superconductivity in the cuprates. An important theoretical question in this regard is understanding the renormalization of single spin excitations, or magnons, upon doping a magnetic insulator. In this talk we report the observation of a new type of quasiparticle arising from the dressing of magnons with hole dopants, i.e. a magnon-polaron, in a cold atom Fermi-Hubbard system. Using two-photon Raman spectroscopy, we probe the energy and dispersion of magnetic excitations on top of a spin-polarized gas of lattice fermions. For the simplifying case of an undoped band insulator with strong on-site interactions, photoexcitation injects single magnons which are described by spin wave theory. We measure the evolution of the photoexcitation spectra as we move away from this limit to produce magnon-polarons due to dressing of the magnons by charge excitations. We observe a shift in the polaron energy with doping that is strongly dependent on the injected momentum, accompanied by a redistribution in spectral weight. Additionally, we characterize the dispersion of the magnon-polaron with a momentum resolved measurement of the dynamical spin structure factor. We anticipate our results to be a starting point for exploring Fermi polarons in correlated many-body states, including charge density waves and Mott insulators. The technique demonstrated here may also be useful for studying the spectroscopic signatures of other types of polarons in Hubbard systems, including itinerant spin polarons in kinetically frustrated systems.