Pairing symmetries in a Fermi-Hubbard ladder with band flattening

Band flattening has been identified as a key factor in correlation phenomena in moiré materials and beyond. Here, we examine strongly repulsive fermions on a ladder - a minimal platform for unconventional d-wave pairing - and demonstrate that flattening the lower band via an additional diagonal hopping term yields non-Fermi liquid behavior. This behavior is evidenced by the violation of Luttinger's theorem and axial d-wave pairing correlations. Similar features have been observed in systems with plaquette ring exchange, albeit with distinct diagonal d-wave pairing symmetries. Therefore, our findings demonstrate a competition between different unconventional pairing channels and show how band geometry can induce fermionic pairing via a simple model. These insights are broadly relevant to correlated flat-band systems, ranging from ultracold atoms to strongly interacting electrons in solids.