We report electronic-structure and physical properties of metal-doped picene as well as selective synthesis of the phase that exhibits 18-K superconducting transition. First, Raman scattering is used to characterize the number of electrons transferred from the dopants to picene molecules, where a softening of Raman scattering peaks enables us to determine the number of transferred electrons. From this, we have identified that three electrons are transferred to each picene molecule in the superconducting doped picene solids. Second, we report pressure dependence of $T_c$ in 7- and 18-K phases of K${}_3$picene. The 7-K phase shows a negative pressure dependence, while the 18-K phase exhibits a positive pressure dependence which can not be understood with a simple phonon mechanism of BCS superconductivity. Third, we report a synthesis method for superconducting K${}_3$picene by a solution process with monomethylamine CH${}_3$NH${}_2$. This method enables us to prepare selectively the K${}_3$picene sample exhibiting 18-K superconducting transition. The method for preparing K${}_3$picene with $T_c$ $=$ 18 K found here may facilitate clarification of the mechanism of superconductivity.

• Received 26 September 2012

DOI:https://doi.org/10.1103/PhysRevB.86.214507