Surface behaviour at the charge density wave transition in NbSe2

2H-NbSe2 is a van der Waals bonded layered structure, which undergoes a charge density wave (CDW) transition. We have investigated the CDW transition in NbSe2 using grazing incidence X-ray diffraction. The evolution of the satellite reflection 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 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'