Observation of a topological defect lattice in the charge density wave of 1T-TaS2

Abstract

Charge density wave (CDW) phases native to the transition metal dichalcogenide 1T-TaS2 are probed through a graphene layer covering its surface by using local scanning tunneling microscopy/spectroscopy and transport measurements. We show that over a wide range of temperatures, the graphene cover, while protecting the air-sensitive 1T-TaS2 from oxidation, does not obstruct access to the electronic properties of the CDW. At room temperature, where the system is in the nearly commensurate CDW phase, we observe contiguous domains of the CDW phase separated by a network of topological defects. These topological defects, dubbed CDW vortices, form a lattice with a quasi-long range translational order, which resembles the Bragg glass phase in Abrikosov vortex lattices in type-II superconductors. While the quasi-long range order in the CDW itself is not uncommon, its existence in the CDW vortex lattice is observed here for the first time. Finally, when reducing the temperature to 78 K into the commensurate CDW phase, we find that the graphene cover layer acquires properties consistent with a proximity-induced CDW.