Nanoscale Modulation of Flat Bands in 4Hb-TaS2 via Controllable Defects

The reduced bandwidth of flat bands enhances electron correlation in condensed matter system. The 4Hb-TaS2 system offers a unique opportunity to control electron correlation in flat bands by tuning the interlayer coupling between alternating 1T and 1H layers. Atomic defects effectively influence interlayer coupling locally; however, the role of native defects in 4Hb-TaS2 on this interlayer coupling remains largely unexplored. Using scanning tunneling microscopy and spectroscopy (STM/S) combined with first-principles calculations, we investigated intrinsic charge-density-waves (CDW) defects in the 1T layer of 4Hb-TaS2 to elucidate their effects on flat bands and interlayer coupling. We identified two types of defects: Type 1 induces structural distortion, which removes the flat band feature and introduces hole doping, while Type 2 causes electron doping, increasing the filling factor and exhibiting a Mott gap with Kondo resonance. Density functional theory (DFT) calculations suggest that Type 1 corresponds to a sulfur vacancy, whereas Type 2 is associated with a structurally intact but with locally suppressed interlayer coupling. Furthermore, we demonstrated manipulation of flat bands by creating and erasing individual CDW defects through the application of bias pulse and tunneling currents. Our findings provide a pathway for tuning flat bands in strongly correlated systems with atomic precision.