Orbital Selectivity and Particle-Hole Asymmetry of the Charge Density Wave Energy Gap in Transition Metal Dichalcogenides

Angle Resolved Photoemission Spectroscopy (ARPES) study of the incommensurate Charge Density Wave (CDW) material, 2H-TaS₂, is presented in comparison to a similar layered transition metal dichalcogenide (TMD) 2H-NbSe₂. Similarities were observed in the selective appearance of CDW energy gap about some specific symmetry points in the momentum space, particle-hole asymmetry of this gap and the persistence of a pseudogap above CDW transition temperature. As per differences, in 2H-TaS₂, the gap was significant for all momentum locations about a symmetry point, while in the case of 2H-NbSe₂ the gap opened up only in specific momentum locations. Analysis of momentum and temperature dependence of the electronic band dispersion shows many body renormalization due to a phononic origin. As the model of Fermi surface nesting was unable to explain above observations, a tight binding model with emphasis on orbital selectivity and strong electron-phonon coupling was utilized. In the light of similar behaviors exhibited by other related material, we suggest that this model can be generalized for a wide spectrum of CDW materials beyond TMDs.