Spin Polarons in Flat Band Ferromagnets

TL;DR

This paper demonstrates the formation of spin polarons in a one-dimensional flat band ferromagnet model, revealing new mechanisms of polaron binding unrelated to traditional topological or frustrated lattice settings.

Contribution

It uncovers the emergence of spin polarons in the Mielke-Tasaki chain, a flat band ferromagnet with trivial topology, using combined numerical and analytical methods.

Findings

Spin polarons form in the Mielke-Tasaki chain upon electron doping.

Polaron binding occurs even when the interaction-induced kinetic energy is zero.

Weak mixing with dispersive bands facilitates polaron formation in this flat band system.

Abstract

Spin polarons are bound states of electrons and spin-flips that form above spin polarized electronic insulators.These bound states conventionally form in one of two settings: in frustrated lattices with dispersive bands -- where the motion of an electron preferences binding a nearby spin-flip -- or in topological flat bands -- where the Chern number enforces an effective dipolar interaction between electrons and spin flips. In this work, we report the formation of a spin polaron in a context that doesn't fall cleanly into either of these paradigms. In particular, we study the one-dimensional Mielke-Tasaki chain, a paradigmatic model of flat band ferromagnetism, which has an exact ferromagnetic ground state, trivial band topology, and quenched kinetic energy in its lowest band. Despite these features, our density matrix renormalization group simulations reveal the presence of spin polarons upon electron doping this model. More surprisingly, combining these numerics with analytic calculations, we show that polaron binding occurs when the interaction-induced kinetic energy of the model is zero -- contrary to intuition from kinetic magnetism -- and the glue binding the electrons and spin-flips arises from weak mixing with the model's dispersive band -- contrary to what occurs in topological flat bands. Our results open the doors to exploring how the quantum geometry of flat bands drives the formation of exotic charge carriers.