Superconductivity and bosonic fluid emerging from moire flat bands

With the evidence of intervalley attraction-mediated by phonon or topological fluctuations, we assume the intervalley attraction and aim at identifying universal properties of moire flat bands that shall emerge. We show that by matching the interaction strength of intervalley attraction with intravalley repulsion, the flat-band limit becomes exactly solvable. Away from the flat-band limit, the system can be simulated via quantum Monte Carlo (QMC) methods without sign problem for any fillings. Combining analytic solutions with large-scale numerical simulations, we show that upon increasing temperature, the superconducting phase melts into a bosonic fluid of Cooper pairs with large/diverging compressibility. In contrast to flat-band attractive Hubbard models, where similar effects arise only for on-site interactions, our study indicates this physics is a universal property of moire flat bands, regardless of microscopic details such as the range of interactions and/or spin-orbit couplings. At higher temperature, the boson fluid phase gives its way to a pseudogap phase, where some Cooper pairs are torn apart by thermal fluctuations, resulting in fermion density of states inside the gap. Unlike the superconducting transition temperature, which is very sensitive to doping and twisting angles, the gap and the temperature scale of the boson fluid phase and the pseudogap phase are found to be nearly independent of doping level and/or flat-band bandwidth. The relevance of these phases with experimental discoveries in the flat-band quantum moire materials is discussed.

Automated summary

By considering the evidence of intervalley attraction-mediated by phonon or topological fluctuations, this study assumes the existence of intervalley attraction in moire flat bands and aims to identify universal properties that may emerge. The results show that the flat-band limit can be solved exactly by matching the interaction strength of intervalley attraction with intravalley repulsion. The study also demonstrates the presence of a boson fluid phase and a pseudogap phase at high temperatures.