Scanning tunneling microscopy study of a new charge density wave phase in VSe$_{2}$ thin films

Ultra-thin two-dimensional materials of transition metal dichalcogenides have recently attracted great interest due to their diverse electronic properties and potential applications. Upon cooling to low temperature, some materials exhibit interesting phenomena of collective electronic states such as superconductivity and charge density waves. While charge density waves in bulk materials of transition metal dichalcogenides have been extensively studied in the past few decades, the understanding of this collective electronic state in materials with reduced dimensionality is still in its infancy. Here, we report \textit{in-situ} ultra-low temperature scanning tunneling microscopy and spectroscopy measurements on VSe$_{2}$ thin films synthesized by molecular beam epitaxy. We observed an unconventional charge density wave which does not follow previous reports of hexagonal symmetry of VSe$_{2}$. Spectroscopy results will be discussed in relation to other characterizations using electrical transport and transmission electron microscopy.