Lieb Lattice, Anomalous 2D Raman modes and Localized Defect States in a Novel 2D Metal

Here, we report the synthesis and characterization of a novel air stable layered van der Waals metal with tetragonal symmetry. First, the metallic character and electronic structure of this material were characterized in bulk crystals via scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES). The bulk crystals exhibit high density of state resonances corresponding to flat bands. Defects in the bulk crystals show localized defect states under STM, an as yet unreported phenomenon in metallic crystals. This can be attributed to the electronic Lieb lattice that arises from strong interactions between surface atoms arranged in a square geometry. Raman spectroscopy and STM of monolayer exfoliated flakes are evidence for the long-term stability of this metal under ambient conditions. Notably, new, strongly Raman active modes appear in few-layer samples yet are absent in bulk crystals. As such, we have a new air stable metal, which allows for a wide range of applications as a good conductor for nanoscale devices involving other 2D materials and heterostructures. Additionally, evidence for flat bands and a Lieb lattice support the use of this material as a template in which to induce exotic many-body states. Finally, the emergence of high intensity Raman modes in the few-layer limit is a previously unobserved phenomenon. This material is of fundamental interest to further our understanding of the mechanisms which yield new physics in novel 2D materials.