Two-Dimensional Charge Density Waves in a Single-Layer Thick Islands of a Molecular Dirac Fermion System*

Charge density waves have been intensely studied in inorganic materials such as transition metal dichalcogenides however its counterpart in organic materials has yet to be explored in detail. Here we report the finding of a robust two dimensional charge density waves in molecular layers formed by α-(BEDT-TTF)₂-I₃ on a Ag(111) surface. Low temperature scanning tunnelling microscopy images of a multi-layer thick α-(BEDT-TTF)₂-I₃ on Ag(111) substrate reveal coexistence of 5a₀ x 5a₀ and  a₀ x  a₀ R9° charge density wave patterns commensurate with the underlying molecular lattice at 80 K. Both charge density wave patterns remain in nano-size molecular islands with just a single constituent molecular-layer thickness at 80 K and 5 K. Local tunneling spectroscopy measurements reveal the variation of the gap from 244 meV to 288 meV between the maximum and minimum charge density wave locations. Density functional theory calculations further confirm a vertical positioning of BEDT-TTF molecules in the molecular layer. While the observed charge density wave patterns are stable for the defect sites, they can be reversely switched for one molecular lattice site by means of inelastic tunnelling electron energy transfer with the electron energies exceeding 400 meV using a scanning tunneling microscope manipulation scheme.