Hopping frustration interfaces in kinetic ferromagnetism 

Cold atom platforms have enabled observation of Nagaoka ferromagnetic (FM) and antiferromagnetic (AFM) spin clouds around a hole dopant in square and triangular lattices, respectively. It has been shown that including frustrated hoppings in square arrays suppresses FM in the ground state and gives rise to AFM (counter-Nagaoka) behavior. These observations highlight the interplay between hopping, interactions, and connectivity to yield kinetic magnetism.   

In this work, we investigate the presence of an interface where an abrupt change from non-frustrated to frustrated hoppings exists in a ladder geometry. By analyzing the hole-spin-spin correlator, we find that FM clouds survive and are localized near the frustrated hopping interface up to a finite value of t/U. The entanglement entropy between ladder halves is found to clearly identify the critical t/U value at which this occurs. We also find that frustration decreases the degree of entanglement between halves as t/U increases, so that the counter-Nagaoka response takes over in the low doping regime away from half filling. The three-point correlator not only indicates the presence of spin polarons but it appears to effectively measure the entanglement entropy of the system.