The origin and nature of the charge ordering in monolayer transition metal di-chalcogenide VSe2

Transition Metal Di-chalcogenides (TMDs) are known to exhibit a wide array of electronic properties, including superconductivity, charge ordering, and non-trivial band topology. The manifestation of these properties is strongly influenced by the dimensionality of the material.  In addition, when thin layers of TMDs are stacked together to form heterostructures, they often exhibit unique properties that are distinct from those of their individual constituent materials. This has led to a significant interest in understanding the interplay between these properties and their evolution from bulk to monolayer materials.  The group V transition metal dichalcogenide VSe2 undergoes a charge density wave (CDW) transition at 110 K with (4 x 4 x 3) charge ordering in its bulk form. Our studies using angle-resolved photoelectron spectroscopy (ARPES) and low energy electron diffraction (LEED) reveal that monolayer (ML) VSe2 displays an enhanced transition at approximately 140 K with very different charge ordering. Moreover, this transition is accompanied by a full gapping of the Fermi surface. We have used time and angle resolved photoelectron spectroscopy (TR-ARPES) to understand the electron dynamics in ML VSe2 above and below the transition temperature. We have also modelled the ARPES intensity using a modified BCS self-energy and density functional theory calculated bare bands. We find the gapped phase vanishes upon pumping in about 500 fs and takes unusually long (me to recover (more than 10 ps). This behaviour underscores the role of lattice degrees of freedom in controlling the charge ordered phase in ML VSe2.