On the search for Fermi surface nesting in quasi-2D materials

We investigate a number of isostructural, quasi-two dimensional transition metal dichalcogenides with respect to Fermi surface nesting. Using angle-resolved photoemission we find no clear evidence for Fermi surface nesting as a key scenario for charge density wave formation in these materials. However, interesting and unusual behavior has been discovered. For <i>1T</i>-TaS₂ in the charge density wave phase, instead of finding an intact Fermi surface along the rounded parts of its elliptically shaped contours and gaps along the parallel regions, the Fermi surface is essentially completely pseudogapped. For <i>1T</i>-TiSe₂ a model based on an excitonic insulator phase as described by Kohn [Phys. Rev. Lett., 19 (1967) 439] fits well with experimental observations. Bandstructure calculations give interesting insight into the electron configuration indicating the changes in d-band occupation and metal-d chalcogen-p overlap when going from one material to the other.