Abstract
Molecular solids are generally highly insulating. The creation of conducting molecular solids proved to be a major scientific challenge. As in the case of Si technology, the challenge started as impurity doping in band insulators and then developed into highly doped polymers, which are not crystalline. More conducting materials in crystalline forms have been realized in charge transfer (CT) complexes with two different kinds of molecules, where electrons are transferred between them in solids. In such CT complexes, not only conducting, but also even superconducting systems were achieved in 1980 and today more than 100 different superconductors are known. The most remarkable achievement in this direction was the realization of a truly metallic state in molecular solids based on a single kind of molecule. These are called single component molecular metals (SCMM) and consist of a rich variety of electronic properties. In these conducting molecular solids, CT and SCMM, many interesting electronic properties resulting from mutual Coulomb interactions and electron-phonon interactions have been explored so far, and these will be reviewed briefly in this article from a theoretical viewpoint. Challenges to come, based on these achievements, are also discussed at the end of this review.
Highlights
As has been most beautifully phrased by Anderson [1] “More is different”, properties of solids consisting of a huge number of atoms and molecules are truly diverse and completely different from those of each isolated constituent
It is to be noted that targets in realizing metallic states will be clearly identified by understanding another limiting factor, i.e., insulating states: band insulator (BI), Mott insulator (MI), charge ordered state (CO) and Anderson localized state
In order to carry out systematic researches in this direction, coordinated activities are needed among sample preparation, measurement and theoretical analysis, through collaboration between condensed-matter physics, chemistry and biology [110,111]
Summary
As has been most beautifully phrased by Anderson [1] “More is different”, properties of solids consisting of a huge number of atoms and molecules are truly diverse and completely different from those of each isolated constituent. It is to be noted that targets in realizing metallic states will be clearly identified by understanding another limiting factor, i.e., insulating states: band insulator (BI), Mott insulator (MI), charge ordered state (CO) and Anderson localized state. The realization of conducting state here is through substitution of host by a small amount of elements with a different number of electrons in the outer shell, i.e., carrier doping into otherwise insulating host crystals (doped band insulators). This situation is similar to doped silicon. The processes toward such insulating states and their electronic properties have been studied extensively both experimentally and theoretically [9]
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