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We report the observation of zero resistivity (p) in a hydrocarbon superconductor, and describe the temperature dependence of p in metal-doped hydrocarbons. The resistivity of K-doped picene (Kx picene) has been recorded from pellet samples in a four-terminal measurement mode. A drop in p is observed below 7 K for K3.1 picene and below 11 K for K3.5picene, which clearly displays zero resistivity. The resistivity drop at 7 K is consistent with the superconducting critical temperature (Tc) obtained from the magnetic susceptibility of the 7 K phase of K3picene, while the drop at 11 K is inconsistent with the Tc's of both the 7 and 18 K phases of K3 picene reported previously. The temperature dependence of p for both samples exhibits granular-metal-like behavior in the normal state. [ABSTRACT FROM AUTHOR]

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 Tc in 7- and 18-K phases of K3picene. 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 K3picene by a solution process with monomethylamine CH3NH2. This method enables us to prepare selectively the K3picene sample exhibiting 18-K superconducting transition. The method for preparing K3 picene with Tc = 18 K found here may facilitate clarification of the mechanism of superconductivity. [ABSTRACT FROM AUTHOR]