Taichi Kosugi

Metal-intercalated aromatic hydrocarbons: a new class of carbon-based superconductors

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.

Ab initio Derivation of Low-Energy Model for κ-ET Type Organic Conductors

We derive effective Hubbard-type Hamiltonians of κ-(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 =Cu(NCS) 2 and 0.055 eV for a Mott insulator X =Cu 2 (CN) 3 , as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended Huckel results, especially much stronger onsite interaction U ∼0.8 eV ( U / t ∼12–15) than the Huckel 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.

First-Principles Electronic Structure of Solid Picene

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 =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.

Accessing surface Brillouin zone and band structure of picene single crystals

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 a laser for photoconduction. A hole effective mass of 2.24m(0) and the hole mobility μ(h)≥9.0 cm(2)/V s (298 K) were deduced in the 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.

First-principles structural optimization and electronic structure of the superconductor picene for various potassium doping levels

We theoretically explore the crystal structures of K

Ab initioelectronic structure of solid coronene: Differences from and commonalities to picene

_x

Electronic Structure of Novel Superconductor Ca4Al2O6Fe2As2

picene, for which a new aromatic superconductivity has recently been discovered for

Slab Thickness Dependence of Rashba Splitting on Au(111) Surface: First-Principles and Model Analyses

x=3

Pauli Equation on a Curved Surface and Rashba Splitting on a Corrugated Surface

, by systematically performing first-principles full structural optimization covering the concentration range

Range-Separation Density-Fitting Band Structure Calculation with Gaussian Auxiliary Function

x=1