The Nanoscale and Quantum Phenomena Institute (NQPI) Seminar Series is pleased to welcome Nohora Hernandez, Ohio University, as our featured speaker on Thursday, October 16 at 4:10 p.m. Seminars are held in Walter Hall 145 and are also available virtually via Microsoft Teams.

Nagaoka polarons and their robustness under magnetic fields in finite-size arrays.

Quantum dot (QD) platforms have enabled the first observation of Nagaoka Ferromagnetism (NFM) in a 2 x 2  array after Nagaoka’s prediction in 1966. However, the finite-size cluster characteristics that maximize ground-state spin with a single hole away from half-filling and the robustness of NFM against magnetic fields remain to be studied. Employing exact diagonalization of the Hubbard Hamiltonian, we find that NFM appears in bipartite finite-size arrays for a finite value of t/U, which can be maximized by minimizing the bulk sites (nodes with 4 links) available for the hole. In the NFM regime, ferromagnetic clouds form around the hole with a strength that increases with the number of bulk sites, but vanish out of this regime. Applying a magnetic field perpendicular to the array induces an Aharonov-Bohm phase, which prompts a critical flux at which NFM can be destroyed. This flux decreases linearly with t/U, with a maximum in the infinite interaction limit. Furthermore, we demonstrate that tuning the magnetic flux up to a critical value always generates a Nagaoka polaron; when ϕ=π, the ground state exhibits antiferromagnetic correlations. Our results illustrate how NFM and polaron formation depend on the array’s connectivity, as well as leading to the counterintuitive conclusion that ferromagnetism can be destroyed by applying a magnetic field.