An X-ray diffraction investigation of the charge density wave transition at the NbSe2 surface
Niobium diselenide 2H-NbSe2 is a van der Waals bonded layered structure, which undergoes a charge density wave transition (CDW). We have investigated the CDW transition in NbSe2 using grazing incidence X-ray diffraction. The evolution of a satellite reflection associated with the CDW has been observed above and below the critical angle of total external reflection in order to carry out a direct comparison between the surface and bulk behaviour. We successfully isolated the surface CDW structure on a high quality single crystal. The central finding of this thesis is that the behaviour of the surface CDW satellite differs from that in the bulk: At the surface, the CDW transition occurs at a higher temperature than in the bulk; also, the transition appears to be continuous. It is likely that we observe the unusual case defined as a “surface transition“ and not the usual case of an “ordinary transition“. A novel experimental method, grazing incidence inelastic X-ray scattering, has been demonstrated. A first successful experiment on 2H-NbSe2 where surface phonons are measured is reported.