First-order quantum phase tranistion in a hybrid transition metal dichalcogenide compound

A small change in the crystal structure of a material can completely modify its electronic properties. This is the case in the transition metal dichalcogenide TaS₂ where a slight rotation of the sulphur atoms from the trigonal prismatic 1H structure to the octahedral 1T varies it from a superconductor to a Mott insulator possibly hosting a spin liquid state. We have studied 4Hb-TaS₂ that interleaves the 1T and 1H polytypes. This has allowed us to investigate using scanning tunneling spectroscopy the fate of a Mott insulator when coupled to a metal. We find a sharp first order quantum phase transition between a flat band and a Kondo cluster states. It is induced by the combination of electric field and temperature as well as by tuning the inter-layer coupling with the scanning tunneling microscope's tip. The transition line suggests it is governed by the difference in compressibility of the two states involved. The 4Hb-TaS₂ system thus offers a platform to investigate the evolution of Kondo lattices from isolated Kondo impurities via an intermediate correlated Kondo cluster state.