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.

We report the anomalous phase evolution in ferroelectric single crystals Ba1-xCaxTiO3 (0.02

We have performed the first-principles electronic structure calculation for the novel superconductor Ca 4 Al 2 O 6 Fe 2 As 2 which has the smallest a lattice parameter and the largest As height from the Fe plane among the Fe-As superconductors. We find that one of the hole-like Fermi surfaces is missing around the À point compared to the case of LaFeAsO. Analysis using the maximally-localized-Wannier-function technique indicates that the xy orbital becomes more localized as the As-Fe-As angle decreases. This induces rearrangement of bands, which results in the change of the Fermi-surface topology of Ca 4 Al 2 O 6 Fe 2 As 2 from that of LaFeAsO. The strength of electron correlation is also evaluated using the constraint RPA method, and it turns out that Ca 4 Al 2 O 6 Fe 2 As 2 is more correlated than LaFeAsO. The discovery of the new iron-pnictide superconductor LaO 1Àx F x FeAs 1) stimulated extensive work to clarify the basic features of material properties, particularly the origin and character of superconductivity and also to search for new superconductors. In fact, a variety of related superconductors have been synthesized: they have the topologically identical iron (Fe)-pnictogen (Pn) layer structure combined with various block layers.2-4) Among them, a family of materials which have block layers of the perovskite structure are unique in the sense that the distance between the neighboring Fe-Pn layers can be systematically controlled by changing the thickness of the block layer. 4)The rich variation of the materials with the perovskite structure is also attractive. Quite recently, Ogino et al. synthesized (Ca nþ2 (Al,Ti) n O 3nÀy )(Fe 2 As 2 ) (n ¼ 2; 3; 4) and observed the start of resistivity drop at about 39 K for n ¼ 4.5) For the synthesis of these materials at ambient pressure, the presence of Ti turns out to be essential not to produce strong internal strain. However, subsequent to this work, Shirage et al. have successfully synthesized Ca 4 Al 2 O 6Ày Fe 2 Pn 2 with Pn ¼ P and As without Ti using a high-pressure synthesis technique.6) The superconducting transition temperatures are 17.1 K for P and 28.3 K for As. These materials are characterized by the smallest a-lattice parameters and the largest pnictogen heights from the Fe plane among the iron-pnictide superconductors. It is pointed out that these quantities are crucial in determining the transition temperature. [7][8][9][10] In the present work, we have performed DFT calculations for (Ca 4 Al 2 O 6Ày )(Fe 2 As 2 ) in order to analyze the effects of the above unique structural features on the electronic structures paying particular attention to the energy range near the Fermi level. The absence of Ti in this material makes our analysis simpler because d states near the Fermi level comes only from Fe. We used a computational code QMAS 11) based on the projector augmented-wave method, 12) which has been applied to the study of the ground state properties of LaFeAsO 13,14) and SrFe 2 As 2 .15) The generalized gradient approximation (GGA) 16) was ad...

We have obtained the first-principles electronic structure of solid coronene, which has been recently discovered to exhibit superconductivity with potassium doping. Since coronene, along with picene, the first aromatic superconductor, now provide a class of superconductors as solids of aromatic compounds, here we compare the two cases in examining the electronic structures. In the undoped coronene crystal, where the molecules are arranged in a herringbone structure with two molecules in a unit cell, the conduction band above an insulating gap is found to comprise four bands, which basically originate from the lowest two unoccupied molecular orbitals (doubly-degenerate, reflecting the high symmetry of the molecular shape) in an isolated molecule but the bands are entangled as in solid picene. The Fermi surface for a candidate of the structure of Kxcoronene with x = 3, for which superconductivity is found, comprises multiple sheets, as in doped picene but exhibiting a larger anisotropy with different topology.

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with laser for photoconduction. A hole effective mass of 2.24 m0 and the hole mobility µ h ≥ 9.0 cm 2 /Vs (298 K) were deduced in Γ-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.

We theoretically explore the crystal structures of K x picene for which a new aromatic superconductivity has recently been discovered for x = 3, by systematically performing first-principles full structural optimization covering the concentration range x = 1-4. The crystal symmetry (space group) of the pristine picene is shown to be preserved in all the optimized structures despite significant deformations of each picene molecule and vast rearrangements of herringbone array of molecules. For K x picene (x = 1-4), optimization indicates that (i) multiple structures exist in some cases and (ii) dopants can enter in the intralayer region as well as in the interlayer region between the stack of herringbone structures. In the electronic structure obtained with the local density approximation for the optimized structures, the rigid-band approximation is invalidated for multiple reasons; the dopants affect the electronic properties not only through the rearrangement and distortion of molecules, but also through hybridizations between the molecules and metal atoms. As a consequence, the resultant Fermi surface exhibits a variety of multiband structures, which take diverse topology for K 1 and K 3 picenes.