We report an in-depth angle-resolved photoemission spectroscopy study on $2H\text{−}{\mathrm{TaS}}_2$, a canonical incommensurate charge density wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, $2H\text{−}{\mathrm{TaSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$, the energy gap (${\mathrm{\Delta }}_{\text{CDW}}$) of $2H\text{−}{\mathrm{TaS}}_2$ is localized along the $K$-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of ${\mathrm{\Delta }}_{\text{CDW}}$ even at temperatures higher than the CDW transition temperature ${\mathit{T}}_{\text{CDW}}$ in $2H\text{−}{\mathrm{TaS}}_2$, reflects the similar pseudogap behavior observed previously in $2H\text{−}{\mathrm{TaSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$. However, in sharp contrast to $2H\text{−}{\mathrm{NbSe}}_2$, where ${\mathrm{\Delta }}_{\text{CDW}}$ is nonzero only in the vicinity of a few “hot spots” on the inner $K$-centered Fermi surface barrels, ${\mathrm{\Delta }}_{\text{CDW}}$ in $2H\text{−}{\mathrm{TaS}}_2$ 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 ${\mathrm{\Delta }}_{\text{CDW}}$ 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.

• Received 12 May 2017

DOI:https://doi.org/10.1103/PhysRevB.96.125103