Emergence of the Flat Band and Kondo Resonance in the Superconducting T/H-NbS₂ Heterostructure

By combining scanning tunneling microscopy and spectroscopy (STM/STS) with theoretical calculations, we investigate the interplay between the flat band and superconductivity in the T/H heterostructure of transition metal dichalcogenide (TMD). By applying electrical pulses to the STM tip, we induce the H-type to T-type structural phase transition on the surface of bulk 2H-NbS₂ to form the T/H heterostructure. The T-NbS₂ hosts a √13×√13 charge density wave (CDW) superstructure and has an isolated flat band near the Fermi level. The energy position of the flat band varies slightly in real space, originating from the disparity in the amount of charge transfer between T-NbS₂ and the underneath H-NbS₂. When the flat band touches and crosses the Fermi level, Kondo screening associated with the local moment in the √13×√13 CDW superstructure emerges. When 2H-NbS₂ is in the superconducting state, on the T-NbS₂, we observe a gap-like feature superimposed on the Kondo resonance background, which could be an electronic gap induced by the superconductivity in the underneath 2H-NbS₂. Our findings provide complete information on the interplay between the flat band and the superconductivity in the family of T/H TMD heterostructures.