Strong correlation in atomically thin 1T-TaSe₂

Charge density wave (CDW) alters the electronic structure of its host material by periodically distorting the lattice. As a result, electronic states drastically different from the host may emerge after the reconstruction, sometimes exhibiting surprising properties. In 1T-phase tantalum dichalcogenides, a unique star-of-David shape CDW forms at low temperatures, where 12 tantalum atoms aggregate towards the central one. While band structure calculations have shown the reconstruction results in a flat band of 5d character around the Fermi level, subject to a Mott transition into a correlated insulator, controversial experimental evidence of metallic and insulating phases coexists, possibly due to complicated vertical stacking of the CDW pattern. Fortunately, recent advances in van der Waals materials provides the access to individual layer properties with unprecedented control and therefore make the “bottom-up” study of these complicated systems possible. With state-of-the-art two-dimensional material preparation and characterization techniques, we successfully prepared 1T-TaSe₂ devices down to bilayer thickness and discovered the emergence of a correlated insulating phase at the atomic limit with transport and tunneling measurements. Density functional theory calculation revealed the charge transfer insulator nature and demonstrated a suppression of this phase in the bulk due to the hybridization between the central tantalum 5d_z² orbital and nearby selenium 4p_z orbitals. An 1T/1H heterostructures was further prepared via thermally induced polymorph transition to examine the magnetic ground state in monolayer 1T-TaSe₂. Both transport and scanning tunneling spectroscopy measurements confirmed the existence of localized magnetic moments and established the system as a quantum spin liquid candidate. Our results have provided a promising and versatile platform for the investigation of strongly correlated physics in atomically thin 1T-TaSe₂.