Anomalous narrow-band correlation in a natural superconducting heterostructure

A new frontier in condensed matter physics is to stack atomically thin layered-materials with different properties and create intriguing phenomena which do not exist in any of the constituent layers. Transition metal dichalcogenide 4Hb-TaS$_2$, with an alternating stacking of a spin liquid candidate 1T-TaS$_2$ and a superconductor 1H-TaS$_2$, is a natural heterostructure for such a purpose. Recently, rare phenomena are indeed observed, including chiral superconductivity, two-component nematic superconductivity, topological surface superconductivity and enigmatic magnetic memory. A widely proposed starting point to understand such a mysterious heterostructure requires strong electronic correlation, presumably provided by 1T-TaS$_2$ layers with a narrow flat band near the Fermi level ($E_F$). Here, by using angle-resolved photoemission spectroscopy, we reveal the theoretically expected flat band near $E_F$ in the energy-momentum space for the first time. However, this flat band only exists on the 1T-TaS$_2$ terminated surface layer with broken translational symmetry, but not on the 1T-TaS$_2$ layers buried in the bulk. These results directly challenge the foundation of the current theoretical paradigm. On the 1T-TaS$_2$ terminated surface layer, we further reveal a pseudogap and an anomalous doping effect. These phenomena and the dichotomy between surface and bulk layers also shed new light on the unusual coexistence of distinct electronic orders in this mysterious heterostructure.