The upper temperature for superconductivity in organic conductors has increased from 1 kelvin in 1980, when the phenomenon was discovered in the quasi-one-dimensional cation radical salt tetramethyltetraselenafulvalene phosphorus heptafluoride to 12 kelvin in a new series of organic salts that show nearly two-dimensional electronic properties. These superconductors are attracting interest because of the wide range of new phenomena that they exhibit, including the competition between various ground states, the influence of a magnetic field on a quasi-one-dimensional conductor, the quantization of the Hall effect in a three-dimensional material, the giant magnetoresistance effects related to the two-dimensional nature of the Fermi surface of some materials, and the coherent voltage oscillation of a spin-modulated ground state. Furthermore, there is reason to believe that organic conductors with high superconducting transition temperatures could be produced in the near future. The recent finding of superconductivity in "fullerene" doped with alkali metals supports this optimism.

https://science.sciencemag.org/content/sci/252/5012/1509.full.pdf

The physics of organic superconductors.

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.

Conventional superconductivity, as used in this review, refers to electron-phonon coupled superconducting electron-pairs described by BCS theory Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e g spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 06 K in 1979 Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994 Further progress in high temperature superconductivity would be aided by understanding the cause of such unconventional pairing This review compares the fundamental properties of 9 unconventional superconducting classes of materials - from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples, with the hope that common features will emerge to help theory explain (and predict!) these phenomena In addition, three new emerging classes of superconductors (topological, interfacial [e g FeSe on SrTiO3], and H2S under high pressure) are briefly covered, even though their conventionality is not yet fully determined

Unconventional Superconductivity

Over the past two decades, unconventional superconductivity with gap symmetry other than s wave has been found in several classes of materials, including heavy fermion, high Tc, and organic superconductors. Unconventional superconductivity is characterized by anisotropic superconducting gap functions, which may have zeros (nodes) along certain directions in the Brillouin zone. The nodal structure is closely related to the pairing interaction, and it is widely believed that the presence of nodes is a signature of magnetic or some other exotic, rather than conventional phonon mediated, pairing mechanism. Therefore experimental determination of the gap function is of fundamental importance. However, the detailed gap structure, especially the direction of the nodes, is an unresolved issue for most unconventional superconductors. Recently it has been demonstrated that thermal conductivity and specific heat measurements under a magnetic field rotated relative to the crystal axes provide a powerful method for determining the shape of the gap and the nodal directions in the bulk. Here we review the theoretical underpinnings of the method and the results for the nodal structure of several unconventional superconductors, including borocarbide YNi2B2C, heavy fermions UPd2Al3, CeCoIn5 ,a nd PrOs 4Sb12, organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, and ruthenate Sr2RuO4, determined through angular variation of the thermal conductivity and heat capacity.

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Nodal structure of unconventional superconductors probed by angle resolved thermal transport measurements

The proton spin lattice relaxation rate (1 / T 1 ) in the superconducting state of(TMTSF) 2 ClO 4 has been measured at zero magnetic field using the field cycling technique. It was found that the enhancement of 1 / T 1 just below T c , commonly observed in typical BCS superconductors, was absent and 1 / T 1 varied as T 3 . These results indicate that the superconductivity in this system is associated with the anisotropic order parameter vanishing along lines on the Fermi surface.

Proton Spin Relaxation in the Superconducting State of (TMTSF)2ClO4

Values of the lattice constants of the organic conductor β-(BEDT-TTF) 2 I 3 up to 2 GPa at room temperature are obtained by Buerger precession method and the compressibilities d x / x d p [/GPa] are determined as -0.044, -0.032 and -0.026, where x = a , b and c , respectively. The anisotropy factor of the compressibilities is less than 2, in contrast with the quasi-one-dimensional case of (TMTSF) 2 PF 6 . The difference is caused by the two-dimensional character of the electronic structure originated from the corrugated network of BEDT-TTF molecules.

Gunzi Saito

Papers

Proton Spin Relaxation in the Superconducting State of (TMTSF)2ClO4

X-ray study of compressibilities of β-(BEDT-TTF)2I3 under hydrostatic pressure