Organic Superconductors: When Correlations and Magnetism Walk in

Abstract

This survey provides a brief account for the start of organic superconductivity motivated by the quest for high T c superconductors and its development since the eighties'. Besides superconductivity found in 1D organics in 1980, progresses in this field of research have contributed to better understand the physics of low dimensional conductors highlighted by the wealth of new remarkable properties. Correlations conspire to govern the low temperature properties of the metallic phase. The contribution of antiferromagnetic fluctuations to the interchain Cooper pairing proposed by the theory is borne out by experimental investigations and supports supercondutivity emerging from a non Fermi liquid background. Quasi one dimensional organic superconductors can therefore be considered as simple prototype systems for the more complex high T c materials.

Low-temperature structural effects in the (TMTSF)2PF6and AsF6Bechgaard salts

“…2. Table I compares the 4 K lattice parameters of PF 6 -D12 and PF 6 -H12. It shows that the PF 6 -D12 lattice parameters are larger than those for PF 6 -H12.…”

“…[2][3][4] One of the main challenges of present research activities remains the determination of the microscopic origin of the Cooper pairing, 5 an issue all the more interesting because of the accumulating evidences in favor of unconventional d-wave symmetry superconductivity at least for the quasi-one-dimensional (quasi-1D) Bechgaard salts. 6 In the latter salts, it has been proposed 7 that the d-wave Cooper pairing is achieved through spin-density-wave (SDW) fluctuations, which arise because of the close proximity of a SDW ground state. In this scenario, unconventional superconductivity should emerge from a non-Fermi liquid due to the pertinence of SDW correlations.…”

“…In this scenario, unconventional superconductivity should emerge from a non-Fermi liquid due to the pertinence of SDW correlations. 6,8 This scenario, however, requires a detailed description of the metallic state of the Bechgaard salts. Such a description should consider both the various thermal crossovers, affecting the dimensionality of the coherent charge transport, as well as the nature of the incipient density-wave instabilities provoked by the reduced electronic dimensionality in the presence of electron-electron repulsions and/or electron-lattice coupling.…”

“…In all these aspects, (TMTSF) 2 PF 6 already behaves as a complex metal at ambient pressure. At high temperature the coherent charge transport is confined (1D behavior) to the stack direction a where (TMTSF) 2 PF 6 behaves as a Tomonaga-Luttinger liquid. 9,10 In this regime this salt exhibits both 2k F SDW 11 and bond-order-wave (BOW) 12,13 1D instabilities.…”

“…The Bechgaard salts crystallize in the triclinic P1 space group, where in particular each anion is located on an inversion center of the structure. 22 At ambient conditions, the anions, even centrosymmetric ones such as PF 6 , as well as the methyl groups, are subject to a considerable thermal motion. Both anion and methyl-group disorders are progressively removed upon cooling.…”