Engineering Excitonic Metal-Insulator Transitions in Ultrathin Doped Copper Sulfides.

Delicate engineering of the bands remains challenging due to complex electronic, structural, and compositional interplay. We demonstrate the formation of distinct metallic and insulating ground states in ultrathin 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 the 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 or lattice symmetry breaking. The evidence of the excitonic insulating phase is further provided by carrier density dependent transitions, a combined effect of electron screening and Coulomb interaction strength. Our findings demonstrate tunability of the band structure of copper sulfides, allowing for new opportunities to study exotic quantum phases.