Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of 2H−TaS₂

We report an in-depth angle-resolved photoemission spectroscopy study on 2H-TaS₂, a canonical incommensurate charge density wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, 2H-TaSe₂ and 2H-NbSe₂, the energy gap (Δ_CDW) of 2H-TaS₂ is localized along the K-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of Δ_CDW even at temperatures higher than the CDW transition temperature T_CDW in 2H-TaS₂, reflects the similar pseudogap behavior observed previously in 2H-TaSe₂ and 2H-NbSe₂. However, in sharp contrast to 2H-NbSe₂, where Δ_CDW is nonzero only in the vicinity of a few "hot spots" on the inner K-centered Fermi surface barrels, Δ_CDW in 2H-TaS₂ is nonzero along the entirety of both K-centered Fermi surface barrels. Based on a tight-binding model, we attribute this dichotomy in the momentum dependence and the Fermi surface specificity of Δ_CDW between otherwise similar CDW compounds to the different orbital orientations of their electronic states that participate in the CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.