Coexistence of Superconductivity with Enhanced Charge Density Wave Order in the Two-Dimensional Limit of TaSe2

Bulk 2H-TaSe2 is a model charge density wave (CDW) metal with superconductivity emerging at extremely low temperature (T-c = 0.1 K). Here, by first-principles calculations including the explicit calculation of the screened Coulomb interaction, we demonstrate enhanced superconductivity in the CDW state of monolayer 1H-TaSe2 observed in recent experiments. Its ground-state 3 X 3 CDW phase features triangular clustering of Ta atoms and possesses a large electron-phonon coupling of lambda = 0.74, yielding an order of magnitude higher superconducting T-c compared to the bulk. Upon lowering the thickness from bulk to monolayer TaSe2, the CDW intensifies with slightly decreased Fermi-level density of states, while superconductivity gets boosted via a largely increased intrinsic electron-phonon coupling strength, which overcomes both the CDW effect and naturally reinforced Coulomb repulsion. These results uncover the simultaneously enhanced CDW and superconducting orders in the two-dimensional limit for the first time and have key implications for other CDW metals like 2H-TaS2.