Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of $2H$-TaS${}_2$

Abstract

We report an in-depth Angle Resolved Photoemission Spectroscopy (ARPES) study on $2H$-TaS${}_2$, a canonical incommensurate Charge Density Wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, $2H$-TaSe${}_2$ and $2H$-NbSe${}_2$, the energy gap ($\hat{I}{”}_{\text{cdw}}$) of $2H$-TaS${}_2$ is localized along the K-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of $\hat{I}{”}_{\text{cdw}}$ even at temperatures higher than the CDW transition temperature $T_{\text{cdw}}$ in $2H$-TaS${}_2$, reflects the similar pseudogap (PG) behavior observed previously in $2H$-TaSe${}_2$ and $2H$-NbSe${}_2$. However, in sharp contrast to $2H$-NbSe${}_2$, where $\hat{I}{”}_{\text{cdw}}$ is non-zero only in the vicinity of a few "hot spots" on the inner K-centered Fermi surface barrels, $\hat{I}{”}_{\text{cdw}}$ in $2H$-TaS${}_2$ is non-zero 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 $\hat{I}{”}_{\text{cdw}}$ between otherwise similar CDW compounds to the different orbital orientations of their electronic states that are involved in CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.

6 pages, 4 figures