Since the first report of superconductivity in synthetic organic conductors in 1980, chemistry and physics of molecular-based conductors have achieved remarkable progress and a number of exotic phenomena have been reported. From the viewpoint of the electronic structure, this progress has been driven along the following two major trends: (1) from “onedimensional” to “higher dimensional” and (2) from “single-component” to “multicomponent”.1 Since planar π-conjugated molecules tend to stack to form the column structure, molecular metals developed in the early stage had the one-dimensional (1D) electronic structure. The 1D metallic electron system characterized by a pair of planar Fermi surface is inherently unstable and undergoes a metal-insulator transition accompanied by the density wave formation at low temperatures.2 Much effort to increase the dimensionality of the electronic structure has been made by means of chemical modification and/or application of pressure. The first organic superconducting system, (TMTSF)2X (TMTSF ) tetramethyltetraselenafulvalene), forms a quasi1D system.3 The organic donor BEDT-TTF (ET, bis(ethylenedithio)tetrathiafulvalene) has provided various types of two-dimensional (2D) metallic systems.4 And, the DCNQI-Cu (DCNQI ) N,N′-dicyanoquinonediimine) salt is the first molecular conductor where the existence of a three-dimensional (3D) Fermi surface is confirmed.5 On the other hand, for many years, molecular conductors were assumed to be electronically single-component systems which exhibit only one electron type. Although the first organic metal TTF-TCNQ (TTF ) tetrathiafulvalene; TCNQ ) tetracyanoquinodimethane) is well-known to consist of two conducting components, the electronic structure of this system has an only 1D pπ electron character.1 Over the past decade, however, an increasing number of multicomponent systems, where there exist “two” energy bands with different characters (for example, orbital character and dimensionality) near the Fermi level or where a strong interaction between itinerant pπ electrons and localized d electron spins operates, have been reported. Reizo Kato was born in 1955 in Yamaguchi, Japan. He received his B.Sc. in 1979, M.Sc. in 1981, and D.Sc. in 1984 from the University of Tokyo. He was appointed research associate of Department of Chemistry at Toho University in 1984, and he was promoted to lecturer in 1988. He joined the Institute for Solid State Physics in The University of Tokyo as an associate professor in 1990. Since 1999 he has been a chief scientist and a director of the Condensed Molecular Materials laboratory in RIKEN (The Institute of Physical and Chemical Research). He received the Chemical Society of Japan Award for Young Chemists in 1990 and the IBM Japan Science Prize in 1995 for his works on molecular conductors. His research has been focused on development of new molecular materials, especially molecular metals and superconductors. 5319 Chem. Rev. 2004, 104, 5319−5346

Conducting metal dithiolene complexes: structural and electronic properties.

The electronic transport properties of conventional three-dimensional metals are successfully described by Fermi-liquid theory. But when the dimensionality of such a system is reduced to one, the Fermi-liquid state becomes unstable to Coulomb interactions, and the conduction electrons should instead behave according to Tomonaga-Luttinger-liquid (TLL) theory. Such a state reveals itself through interaction-dependent anomalous exponents in the correlation functions, density of states and momentum distribution of the electrons. Metallic single-walled carbon nanotubes (SWNTs) are considered to be ideal one-dimensional systems for realizing TLL states. Indeed, the results of transport measurements on metal-SWNT and SWNT-SWNT junctions have been attributed to the effects of tunnelling into or between TLLs, although there remains some ambiguity in these interpretations. Direct observations of the electronic states in SWNTs are therefore needed to resolve these uncertainties. Here we report angle-integrated photoemission measurements of SWNTs. Our results reveal an oscillation in the pi-electron density of states owing to one-dimensional van Hove singularities, confirming the one-dimensional nature of the valence band. The spectral function and intensities at the Fermi level both exhibit power-law behaviour (with almost identical exponents) in good agreement with theoretical predictions for the TLL state in SWNTs.

Direct observation of Tomonaga–Luttinger-liquid state in carbon nanotubes at low temperatures

We introduce the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, and conductive liquids, particularl...

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

Toward systematic understanding of diversity of electronic properties in low-dimensional molecular solids.

Coordination and organometallic chemistry of metal-NO complexes.

A new molecular superconductor β′-Et2Me2P[Pd(dmit)2]2 (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate)

Preparation and physical properties of an alloyed (DMe-DCNQI)2Cu with fully deuterated DMe-DCNQI (DMe-DCNQI = 2,5-dimethyl- N,N′-dicyanoquinonediimine)

Crystal and electronic structures and physical properties of the molecular conductors Me4Z [Pd(L)2]2 (Z = N, P, As, and Sb; L = dmit and dmise) are described Every salt has very similar crystal structure with “solid-crossing” columns The behavior of resistivity under pressure depends on the Se-substitution and the choice of the cation Tight-binding band calculations indicate that these salts form a unique two-band system The narrow and two-dimensional HOMO (anti-bonding) band is thought to form the half-filled conduction band Magnetic properties suggest that the insulating state under ambient pressure comes from the strong electron-electron correlation The one-dimensional LUMO (bonding) band is located below the HOMO band The application of pressure induces an overlap of these two bands and changes the band filling, which brings forth the metallic state The system under higher pressure, however, shows non-metallic behavior probably due to the one-dimensional instability associated with th

Se-Substitution and Cation Effects on the High-Pressure Molecular Superconducior, β-Me4N[Pd(dmit)2]2 -A Unique Two-Band System

We systematically study transport and optical properties of charge-transfer salts, (Me 2 -DCNQI) 2 Li 1- x Cu x (0≤ x ≤1). The temperature dependence of electrical resistivity exhibits semiconducting behavior at x =0, a metal-semiconductor transition for 0.08≤ x ≤0.29, and metallic behavior down to 4.2 K for x ≥0.39. A semiconductor-semiconductor transition is observed around 60 K for x ≤0.14. The reflectance spectra at room temperature reveal two optical excitations in the infrared region. The first excitation appears in salts with x ≠0 far below 650 cm -1 . The second excitation is the so-called mid-infrared band , which appears around 3000 cm -1 for 0≤ x ≤0.14, and shifts downward for x ≥0.17. We propose a hypothesis which provides an explanation both for the metallic conduction and for the mid-infrared band in a one-dimensional conductor having a quarter-filled band.

Reflectance spectra and electrical resistivity of (Me2-DCNQI)2Li1-xCux

We have made a high-resolution photoemission study of quasi-one-dimensional partially deuterated (DMe-DCNQI${)}_{2}$Cu, where DMe-DCNQI denotes dimethyl N,${\mathit{N}}^{\mathcal{'}}$-dicyanoquinonediimine. The spectra in the metallic phase exhibit a power-law dependence on the electron binding energy \ensuremath{\Vert}\ensuremath{\omega}\ensuremath{\Vert} on a high-energy scale (0.3\ensuremath{\gtrsim}\ensuremath{\Vert}\ensuremath{\omega}\ensuremath{\Vert}\ensuremath{\gtrsim}0.05 eV) or at high temperatures (T\ensuremath{\gtrsim} 300 K) while a weak but finite intensity is found at the Fermi level at low temperatures, consistent with the existence of a Fermi edge. These results imply how a crossover occurs from a one-dimensional Luttinger liquid to a three-dimensional Fermi liquid as a function of excitation energy, temperature, and mass anisotropy.

Shuji Aonuma

Papers

A new molecular superconductor β′-Et2Me2P[Pd(dmit)2]2 (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate)

Preparation and physical properties of an alloyed (DMe-DCNQI)2Cu with fully deuterated DMe-DCNQI (DMe-DCNQI = 2,5-dimethyl- N,N′-dicyanoquinonediimine)

Se-Substitution and Cation Effects on the High-Pressure Molecular Superconducior, β-Me4N[Pd(dmit)2]2 -A Unique Two-Band System

Reflectance spectra and electrical resistivity of (Me2-DCNQI)2Li1-xCux

Fermi-liquid versus Luttinger-liquid behavior and metal-insulator transition in N , N ’ -dicyanoquinonediimine-Cu salt studied by photoemission