Electronic properties of two dimensional solids: The layer type transition metal dichalcogenides

Abstract

This paper gives the results of recent experiments on the electrical, magnetic, optical and structural properties of layer type crystals. It deals in particular with the layer type transition metal dichalcogenides, MX₂, and representative members are ZrS₂, NbSe₂, MoS₂. These materials have been the subject of fairly intense activity during the past few years. It is argued that in NbSe₂, there is a relatively narrow “d” band based on overlap of d_z ² orbitals on the Nb atoms lying in the plane of the sandwich, and that it is this band which is the basis of many of the interesting physical properties, Phenomena such as excitons, screening effects, conduction in narrow “d” bands, superconductivity and the metal-insulator transtion are all encountered. Interaction of the MX₂ crystals with a variety of organic and inorganic molecules and atoms can lead to quite dramatic changes in the physical properties. Long chain organic amines inserted between the MX₂ layers in the van der Waal's gap can separate the layers by large distances approaching 60 Å, and the intercalated crystals still remain superconductors. These experiments, together with those in which superconductivity is found in crystals of NbSe₂ a until cell in thickness (c 12 Å), suggests we may be concerned with essentially two dimensional superconductivity, but this is still an open and controversial point.