Doped Aromatic Thin Films with Superconducting Capabilities

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Polycyclic aromatic hydrocarbons (PAHs) recently emerged as a new class of superconducting materials upon doping by metal atom intercalation; with critical temperatures already higher than 30 K. Key factors for the superconductivity in the organic compounds are the appropriate ratio of dopant atoms per organic molecule as well as the interplay between the doping and the crystal structure formed. In this research project we aim at a deeper understanding of the influence of doping by alkali and earth alkali metals on the physical properties (i.e., structural, optical, and electronic properties) of several PAHs - with either potential or already documented superconductivity - in thin films, as opposed to powder samples used in most previous studies. By covering the thickness range from (epitaxial) monolayers up to films several nanometers thick and applying selected spectroscopic methods, namely optical differential reflectance spectroscopy (DRS) and scanning tunneling spectroscopy (STS) in a temperature range from 300 K down to 1 K, we shall obtain comprehensive and complementary data to characterize the influence of the applied doping on the electronic and optical properties. Those measurements will be accompanied by thorough structural characterization (MCP-LEED, ST[H]M, AFM), in order to obtain information on the influence of the doping on the crystallographic structure and to elucidate the docking sites of the doping atoms on the molecules in doped mono- and multilayers. In particular scanning tunneling hydrogen microscopy (STHM) shall be employed, which has already been successfully applied in our group for the precise location of individual potassium dopants within a molecular layer. The ultimate goal is to unequivocally demonstrate the superconducting states by determining the corresponding energy gap via STS and to establish general rules identifying which PAHs are successful candidates for organic superconductors. Our working hypothesis is that the presence of degenerate molecular levels may play a major role. To elucidate this, we will include different PAHs in our research, some of those possessing degenerate frontier orbitals, others not.