Transition metal dichalcogenides (TMDs) are layered materials with the general chemical formula MX2, where M is a transition metal (e.g. Mo, Ti, Ta, V) and X is a chalcogen (S, Se, Te). Each slab is composed of a covalently bonded sandwich of a sheet of metal atoms and two hexagonal planes of the chalcogen atoms. The adjacent layers are held together by week Van der Waals interactions to form the bulk crystal in different polytypes which vary in stacking orders and metal atom coordination.
This chemically versatile family of materials spans the entire range of electronic structures, from insulator to metal, and hosts a number of interesting properties such as charge density wave (CDW) modulations, orbital ordering and superconductivity. These materials are the subject of intense studies both in bulk and exfoliated few layer forms.
Copper intercalated TiSe2 hosts several other properties: the CDW exhibits a remarkable instability towards the formation of stripes at low Cu concentrations. We also unveiled a remarkable energy dependent patchwork of 2×2 ordered regions providing valuable insight on the nature of the CDW gap. Above x=0.4% copper content, CuxTiSe2 becomes superconducting with a maximum critical temperature of 4K at x=0.8. The interplay between charge ordered phases and superconductivity is a key question in a number of superconducting systems, including high temperature superconductors. CuxTiSe2 offers a fantastic playground to explore these topics by means of scanning tunnelling microscopy and spectroscopy.