  Recently, superconductivity in potassium (K) doped p-terphenyl (C18H14) has
been suggested by the possible observation of the Meissner effect and
subsequent photoemission spectroscopy measurements, but the detailed lattice
structure and more-direct evidence are still lacking. Here we report a low
temperature scanning tunneling microscopy/spectroscopy (STM/STS) 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 would caution against superconductivity as an interpretation in
previous reports of K-doped p-terphenyl materials. Such gapped phases are
rarely (if ever) observed in single layer hydrocarbon molecular crystals. 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.
