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Keywords = tilted Weyl equation

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18 pages, 764 KB  
Review
Achievements and Challenges in Molecular Conductors
by Hidetoshi Fukuyama
Crystals 2012, 2(3), 875-892; https://doi.org/10.3390/cryst2030875 - 5 Jul 2012
Cited by 3 | Viewed by 6248
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Molecular Conductors)
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18 pages, 705 KB  
Review
Theory of Dirac Electrons in Organic Conductors
by Yoshikazu Suzumura and Akito Kobayashi
Crystals 2012, 2(2), 266-283; https://doi.org/10.3390/cryst2020266 - 20 Apr 2012
Cited by 12 | Viewed by 7772
Abstract
The dynamical property of electrons with the tilted Dirac cone was examined using the tilted Weyl equation. The polarization function exhibits cusps and nonmonotonic structures by varying both the frequency and the momentum. A pair of tilted Dirac cones exhibits a new plasmon [...] Read more.
The dynamical property of electrons with the tilted Dirac cone was examined using the tilted Weyl equation. The polarization function exhibits cusps and nonmonotonic structures by varying both the frequency and the momentum. A pair of tilted Dirac cones exhibits a new plasmon for the intermediate magnitude of momentum owing to the combined effects of two tilted cones. Dirac electrons with the zero-gap state (ZGS) in organic conductor α-(BEDT-TTF)2I3 are examined by calculating the Berry curvature, which displays the peak structure for a pair of Dirac particles between the conduction band and the valence band. The ZGS is theoretically predicted for α-(BEDT-TTF)2NH4Hg(SCN)4 under uniaxial pressure. Examining the band structure of the stripe charge ordered state of α-(BEDT-TTF)2I3 under pressure, we have found a topological transition from a conventional insulator to a new phase of a pair of Dirac electrons with a finite mass. Further, investigating the zero-energy (N = 0) Landau level under a strong magnetic field, we propose ferromagnetism breaking the SU(2) valley-pseudo-spin symmetry, and the phase fluctuations of the order parameters leading to Kosterlitz-Thouless transition at lower temperatures. Full article
(This article belongs to the Special Issue Molecular Conductors)
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