CAN SIMPLE HYDROCARBON MOLECULAR SOLIDS BE SUPERCONDUCTORS?
Xuhui Luoa, b, Serdar Oguta and Taner Yildirimb
aDepartment of Physics, The University of Illinois at Chicago, Chicago, Illinois
bNCNR, NIST, Gaithersburg, Maryland
Hydrocarbons are essential to everyday life — for example, the use of hydrocarbons as a combustible fuel source. Most people, however, wouldn't think of hydrocarbons as being electrically interesting. Even for physicists, the possibility of superconductivity in molecular hydrocarbon solids remains a controversial issue, unlike many well-established high-temperature (Tc) superconductors such as cuprates, M3C60, MgB2 and iron-pnictides. However, a recent study reporting superconductivity up to Tc ~ 17 K in potassium-doped Picene  challenged the preconception. At this moment, understanding the superconducting mechanism in the hydrocarbon systems is the first step to predicting what hydrocarbons are good candidates for improving the superconducting Tc. First of all, we would like to see if doped Picene is a conventional superconductor driven by electron-phonon coupling. Secondly, we wish to find a convenient way of looking for good hydrocarbon superconductors. Motivated by these goals, we present a detailed theoretical study of the electron-phonon (el-ph) coupling in doped organic molecular solids. Due to large system size, the calculation of el-ph coupling using the standard linear response theory is not feasible. Hence, we have developed a finite-displacement method where both the phonon energies and el-ph coupling can be easily calculated for large systems. Our method gives the theoretical Tc in good agreement with experimental results. Thus we conclude that the el-ph coupling is the superconducting mechanism in potassium-doped Picene. As a comparison, we have also studied the el-ph coupling in alkali-doped Pentacene, a similar well-studied hydrocarbon in which no superconductivity has been observed. The comparison of Picene and Pentacene indicates that armchair edges in hydrocarbons help to improve the superconducting Tc compared with zigzag edges. Finally, we search for better hydrocarbons among aromatic compounds. Phenanthrene is our theoretical winner in all the aromatic hydrocarbons, whose doped solid may produce a higher Tc than metal-doped Picene.
 Mitsuhashi et al. Nature, 464, p. 76 (March, 2010).