Controlling charge density order in 2H-TaSe_2 using a van Hove singularity

We report on the interplay between a van Hove singularity and a charge density wave state in 2H-TaSe_2. We use angle-resolved photoemission spectroscopy to investigate changes in the Fermi surface of this material under surface doping with potassium. At high doping, we observe modifications which imply the disappearance of the (3× 3) charge density wave and formation of a different correlated state. Using a tight-binding-based approach as well as an effective model, we explain our observations as a consequence of coupling between the single-particle Lifshitz transition during which the Fermi level passes a van Hove singularity and the charge density order. In this scenario, the high electronic density of states associated with the van Hove singularity induces a change in the periodicity of the charge density wave from the known (3× 3) to a new (2× 2) superlattice.