University of Virginia, USA
Title: Universal and system-specific charge density wave features in 2H-transition metal dichalcogenides
Biography: Utpal Chatterjee
Recently, the studies of incommensurate charge density wave (CDW) phases in various 2H-polytypes of transition metal dichalcogenides (TMDs), e.g., 2H-NbSe2 and 2H-TaSe2, have attracted a lot of attention due to intriguing experimental observations, some of which are reminiscent of the enigmatic pseudogap phase in cuprate high temperature superconductors (HTSCs). We present a comprehensive Angle Resolved Photoemission spectroscopy. ARPES) study on 2H-TaS2, a canonical incommensurate CDW material. Comparing our ARPES data together with arguments based on a tight-binding analysis on 2H-TaS2, with those on related materials like 2H-NbSe2 and 2H-TaSe2, we identify the generic and system-specific characteristics of these systems. We find the following generic features of incommensurate CDW TMDs: (i) opening of CDW energy gap (Δcdw) along part of the underlying Fermi Surface (FS) sheets; (ii) finite Δcdw at temperatures above the CDW transition temperatures and particle-hole asymmetry in Δcdw and a lack of one-to-one correspondence between CDW wave vectors and the FS nesting vectors. We have also observed some system-specific features. For example, in contrast to 2H-NbSe2, where Δcdw is non-zero only at a few “hot spots” on a specific FS sheet, Δcdw in 2H-TaS2 is non-zero along the entirety of multiple FS sheets. Using a tight-binding model, we describe this in terms of the difference in the orbital orientations of their electronic states close to the Fermi level. In short, our strong-coupling model can describe both the generic and the material-specific features of these compounds. Therefore, we argue that the strong electron-phonon coupling, including its orbital and momentum-dependence, is key to the incommensurate CDW instability in TMDs.