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 pterphenyl 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 Kdoped 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.Organic molecular materials provide an important playground in condensed matter physics, exhibiting charge density wave (CDW) phases [1], anti-ferromagnetism [1-3], Mott-insulating state [1-3] and superconductivity [2-9]. Among these, organic superconductors are of particular interest for their theoretically predicted high critical temperatures [10]. Known organic superconductors include graphite intercalation compounds [4,5], alkali-metal doped fullerenes [2, 3], organic salts [1, 6] and alkalimetal doped aromatic hydrocarbon molecular crystals [7-9]. The record for the superconducting transition temperature (Tc) in the organic superconductors has long been 38 K for doped fullerene (Cs3C60 under high pressure) [2, 3] with minimal further progress obtained thereafter. Recently, reports of low-temperature diamagnetism up to 120 K in K-doped para-terphenyl (p-terphenyl or PTP) rekindled the interest for exploring possible high temperature superconductivity in doped aromatic hydrocarbon molecular crystals [11]. p-Terphenyl consists of three benzene rings linked by single C-C bonds in the para position. By doping K into p-terphenyl molecular crystals, Wang et al. observed a weak Meissner-like effect below 7.2 K, 43 K and 120 K for different doping levels [11][12][13][14]. Shortly thereafter, a gap persisting up to at least 60 K was observed in surface K-dosed p-terphenyl crystals by photoemission spectroscopy [15]. Taken together, these indicate possible high-temperature superconductivity in electron-doped p-terphenyl. However, since the reported diamagnetic signal is very low, corresponding to less than a 0.1% superconducting shielding fraction, further experiments on structural characterization and electronic properties are essential to clarify the origin of these unique phenomena.Here, via high vacuum deposition, we have grown well-ordered (K-doped) p-terphenyl films on Au (111). A clear low energy gap of about 11-13 meV is observed in two of the ordered phases, but...
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