Ultraflat bands in twisted moiré superlattices under shallow potential wells

In twisted moir\'e superlattices, the inherent periodic potential fields can form flat bands, causing many novel physical phenomena with strong correlation effects. In existing research findings, moir\'e flat bands with strong charge localization can be achieved by reducing the twist angle to increase the moir\'e period or selecting materials with large band edge energy differences that correspond to deep potential wells. Here, we propose a design approach for realizing flat bands in moir\'e superlattices at large twist angles even under shallow potential wells. We find that charges with greater effective mass are more susceptible to localization in the superlattice in spite of small potential well depth. We have confirmed this idea in twisted ${\mathrm{MoSe}}_{2}_\mathrm{TeMoS}$ bilayers, achieving an ultraflat energy band with a bandwidth of 0.8 meV (1.3 meV) at a twist angle of $7.34{}^{\ensuremath{\circ}}$ based on $AB$ ($AA$) stacking configuration with the band edge energy difference of only 13.5 meV (22.6 meV), due to the large hole effective mass of $11.8{m}_{0}$ ($11.6{m}_{0}$). Our discovery provides an approach for achieving ultraflat energy bands at large angles, further enriching the flat-band mechanism.