Organic Conductors and Superconductors: New Directions in the Solid State
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Conductors and Superconductors: New Directions in the Solid State J.S. Brooks
Introduction In single-crystal organic salts, we find a keen competition between superconducting, magnetic, insulating, and metallic states. The physics of these materials is further enriched by the sensitivity of these states to pressure, temperature, chemical formulation, and magnetic field. A growing international community of scientists have turned their attention to these materials, and are applying the techniques and theories of metal and semiconductor physics to probe these new systems. In this article we will explore these materials. We will discover that these materials have given us many new things: a renaissance in fermiology, new high-magnetic-field states of matter, a bulk quantum Hall effect, new challenges in the calculation of energy bands on a small energy scale, and elusive behavior which seems one step away from our present understanding of physics in low dimensions. Electron correlations probably play an important role in determining the phenomena, and should be considered in more microscopic theoretical treatments of these systems. This article presents a challenge to those who might be looking for new ways to direct their expertise in metal and semiconductor physics, both experimentally and theoretically. The subjects of this review are organic charge transfer salts, important aspects of which are described in Figure 1. They consist of large organic donor molecules (cations), which can stack in various ways, and smaller inorganic acceptor molecules (anions), which arrange themselves between the stacks. Generally, one electron is transferred from two
MRS BULLETIN/AUGUST 1993
cations to one anion. Hence, one finds the formulas (TMTSF)2X and (BEDT-TTF)2X, which represent the most studied compounds. The donors have spectacular names—tetramethyltetraselenaf ulvalene
a)
and bis(ethylenedithio)tet rathi a f u 1 va lene, respectively—but they comprise simple stacked or layered structures. In a manner similar to silicon or gallium arsenide in the world of semiconductors, or 3He and 4He in the area of quantum fluids and solids, (TMTSF)2X and (BEDT-TTF)2X have become paradigm systems for the study of highly anisotropic (low-dimensional) electronic and magnetic phenomena. There are some general observations which should make the behavior of these materials (in spite of the complicated chemical formulas) clear to the reader. The TMTSF cations stack in chains, and the most studied choices of anion are C1O4 and PF6 (see Figure la). They are called the Bechgaard salts.1 Hence these materials are highly one-dimensional in their electronic character (see Figures lb and lc), and have instabilities that give rise to magnetic phases at low temperatures and in magnetic fields. The shapes of the C1O4 (a little pyramid) and the PF6 (nearly a sphere) cause dramatic differences in the properties, which theory is far from explaining. The phase diagram of (TMTSF)2PF6 is shown in Figure 2a. The (BEDT-TTF)2X cations generally stack in planes
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