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arXiv · 2311.09279

Polaronic mechanism of Nagaoka ferromagnetism in Hubbard models

Rhine Samajdar·R. N. Bhatt
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Abstract

The search for elusive Nagaoka-type ferromagnetism in the Hubbard model has recently enjoyed renewed attention with the advent of a variety of experimental platforms enabling its realization, including moir\'e materials, quantum dots, and ultracold atoms in optical lattices. Here, we demonstrate a universal mechanism for Nagaoka ferromagnetism (that applies to both bipartite and nonbipartite lattices) based on the formation of ferromagnetic polarons consisting of a dopant dressed with polarized spins. Using large-scale density-matrix renormalization group calculations, we present a comprehensive study of the ferromagnetic polaron in an electron-doped Hubbard model, establishing various polaronic properties such as its size and energetics. Moreover, we systematically probe the internal structure of the magnetic state$\unicode{x2014}$through the use of pinning fields and three-point spin-charge-spin correlation functions$\unicode{x2014}$for both the single-polaron limit and the high-density regime of interacting polarons. Our results highlight the crucial role of mobile polarons in the birth of global ferromagnetic order from local ferromagnetism and provide a unified framework to understand the development and demise of the Nagaoka-type ferromagnetic state across dopings.

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Rhine Samajdar, R. N. Bhatt. 2023-11-15. Polaronic mechanism of Nagaoka ferromagnetism in Hubbard models. https://doi.org/10.1103/physrevb.109.235128

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