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...
Possible dissociative adsorptions of organic molecules such as CH 3 OH (CH 3 NH 2 ) to form CH 3 O‚‚‚H or CH 3 ‚‚‚OH (CH 3 NH‚‚‚H or CH 3 ‚‚‚NH 2 ) on the Si(100)-2 × 1 surface are discussed by using the hybrid density functional B3LYP method. The Si 9 H 12 cluster is used as a model of the Si(100)-2 × 1 surface. First, a CH 3 -OH adsorbs molecularly on the Si(100)-2 × 1 surface with no barrier and its stabilization energy is estimated to be 14 kcal/mol. Next, the molecularly adsorbed CH 3 OH dissociates to CH 3 O‚‚‚H or CH 3 ‚‚‚OH. The activation energies to form CH 3 O‚‚‚H/Si 9 H 12 and CH 3 ‚‚‚OH/Si 9 H 12 from the molecularly adsorbed CH 3 OH/Si 9 H 12 are calculated to be 3.8 and 27.8 kcal/mol, respectively. The transition states in these reactions lie 10.5 kcal/mol below and 14.0 kcal/mol above the energy of the initial state (isolated bare Si 9 H 12 cluster + CH 3 OH molecule), respectively, and thus the former reaction has no barrier to dissociative chemisorption and will occur under milder conditions than the latter. The overall exthothermicity from initial state is calculated to be 65.3 kcal/ mol in the former reaction, while 80.9 kcal/mol in the latter reaction and thus the product in the latter reaction is more stable than in the former reaction. Therefore, the former reaction would proceed under mild conditions while the latter reaction would proceed under severe conditions. Same results are obtained in the case of the dissociative adsorptions of CH 3 NH 2 on the Si(100)-2 × 1 surface; the dissociatively adsorbed CH 3 NH‚‚‚H/ Si 9 H 12 is more likely to be produced under mild conditions while the dissociatively adsorbed CH 3 ‚‚‚NH 2 / Si 9 H 12 is more likely to be produced under severe conditions. Orbital interactions and charge transfers have been analyzed to clarify the differences of the adsorption mechanisms. Significant negative charge transfer from the silicon surface to the CH 3 OH and CH 3 NH 2 molecules is the main reason these molecules dissociatively adsorb on the silicon surface in a reductive manner.
Superconductivity was recently discovered in solid potassium-intercalated picene (K(3)22ph), in which the picene molecule becomes trianionic (22ph(3-)). In this Letter, we conduct a theory-based study of the superconductivity of 22ph(3-) within the framework of BCS theory. We estimate the density of states N(ε(F)) on the Fermi level to be 2.2 states per (eV molecule spin) by using the theoretical intramolecular electron-phonon coupling l(x) and the experimental superconducting transition temperature T(c) of 18 K. The theoretical value is consistent with the 1.2 states per (eV molecule spin) determined experimentally for K(3)22ph with T(c)=18 K, indicating the validity of our theoretical treatment and the electron-phonon mechanism for superconductivity. The predicted l(x), 0.206 eV, for 22ph(3-) is larger than any value reported for organic superconductors, so picene may have the largest l(x) among the superconductors reported so far.
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