Surface-Limited Superconducting Phase Transition on 1T-TaS₂

Controlling superconducting phase transition on a two-dimensional (2D) material is of great fundamental and technological interest from the viewpoint of making 2D resistance-free electronic circuits. Here, we demonstrate that a 1T-to-2H phase transition can be induced on the topmost monolayer of bulk (<100 nm thick) 1T-TaS2 by thermal annealing. The monolayer 2H-TaS2 on bulk 1T-TaS2 exhibits a superconducting transition temperature (Tc) of 2.1 K, which is significantly enhanced compared to that of bulk 2H-TaS2. Scanning tunneling microscopy measurements reveal a 3 × 3 charge density wave (CDW) in the phase-switched monolayer at 4.5 K. The enhanced Tc is explained by the suppressed 3 × 3 CDW and a charge-transfer doping from the 1T substrate. We further show that the monolayer 2H-TaS2 could be switched back to 1T phase by applying a voltage pulse. The observed surface-limited superconducting phase transition offers a convenient way to prepare robust 2D superconductivity on bulk 1T-TaS2 crystal, thereby bypassing the need to exfoliate monolayer samples.