Surface-Limited Superconducting Phase Transition on 1 T-TaS.

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