Speaker
Description
We unveil a mechanism for ferromagnetism in quasiperiodic moiré systems hosting a narrow band of localized states. By projecting the full interacting Hamiltonian onto the narrow band, we numerically and analytically determine the stability of the fully polarized ferromagnetic state at half filling of the narrow band against charge and spin-flip excitations, converging the results to the thermodynamic limit in both 1D and 2D.
Our analytical theory shows that the geometry of single-particle eigenstate overlaps governs magnon excitations and the stability of the ferromagnet.
Under certain off-resonant conditions for the single-particle eigenstates, the critical interaction strength for ferromagnetism can become much smaller than the energy gap to remote bands, a regime where our projected theory becomes asymptotically exact.
We contrast our result with the celebrated Lieb theorem for half-filled Hubbard models on bipartite lattices - where a fully-polarized ferromagnetic ground state is forbidden for any repulsive interaction strength - and show how weakly breaking bipartiteness can stabilize ferromagnetism at small interaction strengths in our setup.