Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds, boron-doped diamond and-most prominently-iron arsenides such as LaO(1-x)F(x)FeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (T(c)) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (K(x)picene) shows T(c) values of 7 K and 18 K, depending on the metal content. The drop of magnetization in K(x)picene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite. The T(c) of 18 K is comparable to that of K-intercalated C(60) (ref. 4). This discovery of superconductivity in K(x)picene shows that organic hydrocarbons are promising candidates for improved T(c) values.

New carbon-based superconductors are synthesized by intercalating metal atoms into the solid-phase hydrocarbons picene and coronene. The highest reported superconducting transition temperature, T(c), of a hydrocarbon superconductor is 18 K for K(3)picene. The physics and chemistry of the hydrocarbon superconductors are extensively described for A(x)picene (A: alkali and alkali earth-metal atoms) for x = 0-5. The theoretical picture of their electronic structure is also reviewed. Future prospects for hydrocarbon superconductors are discussed from the viewpoint of combining electronics with condensed-matter physics: modification of the physical properties of hydrocarbon solids is explored by building them into a field-effect transistor. The features of other carbon-based superconductors are compared to clarify the nature of hydrocarbon superconductors.

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 2 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 K 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 cannot be understood with a simple phonon mechanism of BCS superconductivity. Third, we report a new 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.Corresponding author: Takashi Kambe, kambe@cc.okayama-u.ac.jp & Yoshihiro Kubozono, kubozono@cc.okayama-u.ac.jp 3 I. IntroductionRecently a new class of organic superconductors has been discovered in aromatic systems. They are solids of hydrocarbons that include picene, coronene, phenanthrene and 1,2:8,9-dibenzopentacene, 1-6 doped with metal atoms. Namely, the superconductivity was first discovered in potassium-doped picene, K 3 picene, which showed two different superconducting transition temperatures, one with T c = 7 K and the other as high as 18 K. 1 This has been followed by other studies, and the highest T c among these hydrocarbon superconductors to date attains 33 K observed in K 3.45 dibenzopentacene, 6 whose T c is much higher than the highest T c (14.2 K at 8.2 GPa 7 in β'-(BEDT-TTF) 2 ICl 2 ) in charge-transfer organic superconductors. Thus the hydrocarbon superconductors are very attractive from viewpoints of development of new high-T c superconductors as well as fundamental physics of superconductivity.Theoretical calculations for picene superconductors were also achieved, which suggests that the electron-phonon coupling is strong, 8,9 the conduction band comprises four bands arising from two LUMO orbitals, 10 and that strong hybridization between the dopants and molecules invalidates a rigid-band picture. 10The departure from the rigid-band picture was experimentally evidenced by photoemission spectroscopy. 11 This photoemission study clearly showed a metallic ground state for potassium-doped picene films. Our recent resistivity data also indicate a metallic behavior for the K 3 picene phase. 12 Further, a Pauli paramagnetic susceptibility was observed for a K 3 picene bulk sample. 1 These results support a metallic ground state for K 3 picene.The T c for the solid K 3 picene was found to be either 7 or 18 K, 1,2 while the T c of K 3 phenant...

We use photoemission spectroscopy to study electronic structures of pristine and K-doped solid picene. The valence band spectrum of pristine picene consists of three main features with no state at the Fermi level ͑E F ͒ while that of K-doped picene has three structures similar to those of pristine picene with new states near E F , consistent with the semiconductor-metal transition. The K-induced change cannot be explained with a simple rigid-band model of pristine picene but can be interpreted by molecular-orbital calculations considering electron-intramolecular-vibration interaction. Excellent agreement of the K-doped spectrum with the calculations points to importance of electron-intramolecular-vibration interaction in K-doped picene.

The electronic properties of Eu 2.75 C 60 are studied using magnetic susceptibility and electron spin resonance (ESR) from 2 to 300 K. Both the magnetic susceptibility and the ESR parameters clearly show an anomaly around the valence transition temperature, T V = 70 K. The magnetic susceptibility shows weak temperature dependence above T V , while it changes drastically to Curie-Weiss behavior below T V . The low-temperature susceptibility can be reproduced by assuming the moment of free Eu 2+ ions. This result reveals that Eu 2.75 C 60 changes from the intermediate valence state to the divalent state below T V . Although ESR signals above T V should be attributed to conduction electrons, the ESR intensity below T V follows the Curie-Weiss law with a distinct increase in the g-factor. This should be associated with a strong localization of π electrons. We also found that, below ∼17 K, the isothermal magnetization exhibits a weak hysteresis and thermoremanent magnetization appears. These results suggest that valence-ordered Eu 2.75 C 60 undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Néel temperature, T N = 17 K.