-terphenyl on Au (111)*

Recently superconductivity in potassium (K) doped p-terphenyl (C₁₈H₁₄) has been suggested by Meissner effect and subsequent photoemission study, sparking the research of doped phenyl materials. Here we report a low temperature STM study on K-doped single layer p-terphenyl films grown on Au (111). We observe several ordered phases with different morphologies and electronic behaviors, in two of which a sharp and symmetric low-energy gap of about 11 meV opens below 50 K. In particular, the gap shows no obvious response to a magnetic field up to 11 Tesla, which rule out the possibility of a superconducting gap. Such gapped phases are rarely (if ever) observed in single layer hydrocarbon molecular crystals, several candidates for its origin are discussed but still need further investigation. Our work also paves the way for fabricating doped two-dimensional (2D) hydrocarbon materials, which will provide a platform to search for novel emergent phenomena.