Charge density wave order parameter revealed by scanning tunnelling microscopy

The charge density wave (CDW) phase in bulk and exfoliated crystals of transition metal dichalcogenides was studied by scanning tunneling microscopy (STM). In exfoliated flakes of VSe2 we found a striking non-monotonic evolution of the CDW transition temperature as function of thickness derived from the analysis of the real space charge modulation amplitude imaged by STM. This finding lifts the contradiction of previous measurements. In bulk crystals of VSe2, CuxTiSe2 and NbSe2 we examined the CDW phase by our original method to extract the local amplitude, wavelength and phase of the CDW. This high resolution spatial mapping of the full complex order parameter provided unprecedented insight into fundamental aspects of CDWs. It experimentally proved that the CDW in these materials consist of three individual charge modulations. Furthermore, it explained the different contrasts often observed in STM images and revealed a rich variety of features like phase domains and topological defects.