  To explore underlying mechanism for the superconducting phase in recent
discovered aromatic hydrocarbons, we carry out the first-principles
calculations on benzene, the basic and the simplest unit of the series and
examine the structural and phase stability when doped by potassium,
K$_x$C$_6$H$_6$ ($x=1,2,3$). We find that K$_2$C$_6$H$_6$ with the space group
of $Pbca$ is the most stable phase with superconducting transition temperature
around 6.2 K. Moreover, we argue that all existing hydrocarbons should have a
unified superconducting phase in the same temperature range of 5$-$7 K, when
doped by two potassium atoms. Our results indicate that the electron-phonon
interaction is enough to account for the superconductivity of this unified
superconducting phase.
