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 derive effective Hubbard-type Hamiltonians of -(BEDT-TTF) 2 X, using an ab initio downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer t $ 0:067 eV for a metal X ¼ Cu(NCS) 2 and 0.055 eV for a Mott insulator X ¼ Cu 2 (CN) 3 , as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended Hückel results, especially much stronger onsite interaction U $ 0:8 eV (U=t $ 12 {15) than the Hückel estimates (U=t $ 7 { 8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis. Organic conductors with BEDT-TTF molecules [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene, abbreviated as ET hereafter], (ET) 2 X with a number of choices of anions X, offer a variety of prototypical behaviors of strongly correlated electron systems with two-dimensional (2D) anisotropies.1) Examples range from correlated metals with superconductivity at low temperatures to Mott insulators either with a quantum spin liquid or with antiferromagnetic, charge-density or spin-Peierls orders. Intriguing Mott transitions are also found. They are all in front of recent active research for unconventional quantum phases and quantum critical phenomena in nature, while their essences of physics are still under strong debates.In particular, an unconventional nonmagnetic Mott-insulating phase is found near the Mott transition in the -type structure of ET molecules, X ¼ Cu 2 (CN) 3 referred to as -CN, where no magnetic order is identified down to the temperature T ¼ 0:03 K, four orders of magnitude lower than the antiferromagnetic spin-exchange interaction J $ 250 K.2) The emergence of the quantum spin liquid near the Mott transition has been predicted in earlier numerical studies, [3][4][5] while the full understanding of the spin liquid needs more thorough studies. It is also crucially important to elucidate the real relevance of the theoretical findings to the real -ET compounds. Most of numerical 6) and theoretical 7) studies have also been performed for a simplified single-band 2D Hubbard model based on an empirical estimate of parameters combined with extended Hückel calculations. 8,9) A more realistic description of -ET compounds is certainly needed beyond the empirical model.Another fundamental finding achieved in this series of compound is the unconventional Mott transition found for X ¼ Cu[N(CN) 2 ]Cl under pressure.10) The novel universality class of the Mott transition is in good agreement with the marginal quantum criticality at the meeting point of the symmetry breaking and topological change. [11][12][13][14] Because of its significance to the basic understanding on the physics of quantum criticality, the relevance of theoretical concept to the experimental observation needs to be further examined on the realistic and first-principles grounds. Furthermore, an unconventional superco...

To explore the electronic structure of the first aromatic superconductor, potassium-doped solid picene which has been recently discovered by Mitsuhashi et al. with the transition temperatures T c ¼ 7 { 20 K, we have obtained a first-principles electronic structure of solid picene as a first step toward the elucidation of the mechanism of the superconductivity. The undoped crystal is found to have four conduction bands, which are characterized in terms of the maximally localized Wannier orbitals. We have revealed how the band structure reflects the stacked arrangement of molecular orbitals for both undoped and doped (K 3 picene) cases, where the bands are not rigid. The Fermi surface for K 3 picene is a curious composite of a warped two-dimensional surface and a three-dimensional one.