Many-Body Effects on High-Harmonic Generation in Hubbard Ladders

We unveil multifaceted effects of spin-charge couplings on the high-harmonic generation (HHG) in Mott insulators by analyzing the two-leg ladder Hubbard model, where spin dynamics activated by the interchain hopping t_{y} drastically modifies the HHG features. When the two chains are decoupled (t_{y}=0), HHG originates from the dynamics of coherent doublon-holon pairs because of spin-charge separation. With increasing t_{y}, the doublon-holon pairs lose their coherence due to their interchain hopping and resultant nonlocal spin strings. Furthermore, the HHG signal from spin polarons-charges dressed by spin clouds-leads to an additional plateau in the HHG spectrum. For large t_{y}, we identify unconventional HHG processes involving three elementary excitations-two polarons and one magnon. Our study is an essential step toward a microscopic understanding of nontrivial many-body effects on HHG in correlated materials beyond conventional semiconductors.