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 (ΔCDW) of 2H-TaS2 is localized along the K-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of ΔCDW even at temperatures higher than the CDW transition temperature TCDW in 2H-TaS2, reflects the similar pseudogap behavior observed previously in 2H-TaSe2 and 2H-NbSe2. However, in sharp contrast to 2H-NbSe2, where ΔCDW is nonzero only in the vicinity of a few "hot spots" on the inner K-centered Fermi surface barrels, ΔCDW 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 ΔCDW 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.