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 surrounded by two hexagonal planes of chalcogen atoms. Adjacent slabs are held together by weak van der Waals interactions to form bulk crystals 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.
Multiband charge density wave in 2H-NbSe2
Analysing the bias dependence of the phase and amplitude of the CDW contrast in STM topographic images, we expose the existence of at least two distinct charge modulations developing on different bands in bulk 2H-NbSe2 (collaboration with Jasper van Wezel and Felix Flicker).
CDW gap and contrast inversion in 1T-TiSe2
Accurately measuring the gap of the CDW ground state remains one of the major challenges, especially by vacuum tunneling spectroscopy. Analysing the CDW contrast amplitude in topographic STM images of bulk TiSe2, we found an alternative way to gain insight into this fundamental spectroscopy feature. This study provides one among very few examples of actual contrast inversion expected in topographic STM images of the CDW acquired at bias voltages above and below the CDW gap (collaboration with David Bowler).
Cu doped 1T-TiSe2
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 (collaboration with David Bowler) .