Engineering metal-insulator transitions in ultra-thin doped copper sulfides

Abstract: Band structure is essential for understanding the electronic behavior of a material. The delicate engineering of the bands remains challenging because of the complex interplay of electronic, structural, and compositional factors. Here, we demonstrate the formation of distinct metallic and insulating ground states in ultra-thin copper sulfide films by effectively tuning the band structure via changing the composition of Cu and S in the system. Using angle-resolved photoemission spectroscopy (ARPES), we observed a continuous band renormalization and opening of a full gap at low temperatures over a wide range of doping. The electronic origin of this metal-insulator transition is supported by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) measurements, which show no indication of superlattice modulation and lattice symmetry breaking. The evidence of excitonic insulating phase is further provided by carrier density dependent transitions, a combined effect of electron screening and Coulomb interaction strength. Our findings demonstrate the tunability of the band structure of copper sulfides, allowing for new opportunities to study exotic quantum phases.