Polarons and Kinetic Magnetism in Flat Band Models*

In strongly interacting Fermi-Hubbard systems, charged carrier dopants can couple intricately with spin degrees of freedom, dramatically affecting the low-lying excitations and the surrounding magnetic background. For example, it has been known that kinetic effects give rise to spin-polaron bound states between carriers and magnons. Here we develop a general analytic framework for such problems and furthermore explore the question of whether such polarons can arise without kinetic energy, but rather purely from interactions. In a paradigmatic model for flat band ferromagnetism on a one-dimensional chain, we use analytic methods and density matrix renormalization group numerics to study the existence and pairing of polarons arising completely from interaction effects. Our work paves the road towards placing kinetic magnetism and interaction-induced direct exchange on a more equal footing.