The 1983 discovery of superconductivity in (BEDT-TTF)4(ReO4)2 — the first unambiguous superconducting sulfur-donor organic system, superconducting only under pressure near 2 K with a large negative pressure derivative dTc/dP≈−0.3dT_c/dP\approx -0.3 K/kbar and a first-order metal–insulator transition attributed by the discoverers to anion rearrangement — founded the entire BEDT-TTF (ET) family of organic superconductors. Four decades and dozens of superconducting salts later, with transition temperatures pushed above 10 K, the question the original paper already posed is still open: what is the microscopic pairing mechanism and the superconducting gap symmetry in these quasi-two-dimensional strongly correlated molecular metals, and why does superconductivity in the founding salt appear only in a narrow pressure window adjoining the anion-ordering metal–insulator transition?

Background

Organic charge-transfer salts are crystalline conductors built from electron-donor molecules and inorganic or organometallic counter-anions. BEDT-TTF (also called ET, bis(ethylenedithiolo)tetrathiafulvalene) is a sulfur-based donor molecule whose radical-cation salts form conducting sheets separated by insulating anion layers, giving quasi-two-dimensional electronic transport. In January 1983, Parkin, Engler, Schumaker, Lagier, Lee, Scott, and Greene reported evidence for superconductivity in (BEDT-TTF)4(ReO4)2 near 2 K for pressures above about 4 kbar — the first unambiguous superconductivity in a sulfur-donor organic system, complementing the earlier selenium-based (TMTSF)2X superconductors. Their abstract records three striking observations: at higher pressures the transition temperature falls rapidly, dTc/dP∼−0.3dT_c/dP\sim -0.3 K/kbar, an unusually large negative value compared with conventional superconductors; at low pressures the material undergoes a first-order metal–insulator (MI) transition, which the authors postulated to result from a change in the arrangement of the ReO4 anions, which are already ordered at room temperature; and the mechanism of superconductivity was not determined. Here anion ordering denotes the ordering pattern of the tetrahedral perrhenate (ReO4) anions in the insulating layers, which can modulate the site energies of the donor stacks and open gaps in the conduction band.

The subsequent history of the ET family fixed the broader context in which these questions are now asked. Ambient-pressure superconductivity at 8 K was found in beta-(BEDT-TTF)2I3 in 1985, about 10 K in kappa-(BEDT-TTF)2Cu(SCN)2 in 1988, and about 11.6 K (resistive onset) in kappa-(BEDT-TTF)2Cu[N(CN)2]Br in 1990, so the original exploration question of whether other ET salts could exceed the roughly 2 K of the ReO4 salt is affirmatively answered. Structural systematics mapped the main polymorphs — different molecular-packing motifs conventionally labeled beta, kappa, and theta — and the superconducting kappa-phase salts are now described as dimer-Mott systems: the ET molecules form face-to-face dimers, one carrier per dimer gives an effectively half-filled Hubbard model (the standard lattice model of electrons with on-site Coulomb repulsion) on an approximately triangular lattice, and superconductivity appears near the boundary between a paramagnetic metal and an antiferromagnetic Mott insulator (a Mott insulator being a charge-localized state driven by on-site Coulomb repulsion rather than band filling).

The unresolved core is the superconducting state itself. Candidate pairing mechanisms that have coexisted since the 1980s are: antiferromagnetic spin-fluctuation-mediated pairing, which favors unconventional gap symmetry with sign changes between Fermi-surface sheets (a dd-wave state, with line nodes where the superconducting energy gap vanishes along certain directions); coupling to intramolecular vibrational modes (the electron–molecular-vibration channel, a Holstein-type coupling in the sense that conduction electrons interact with local molecular distortion modes); and valence-bond or charge-order-fluctuation-mediated pairing, i.e. pairing driven by fluctuations of singlet bonds or of the pattern of charge localization on the molecular sites. Experiments probe the question through nuclear magnetic resonance (NMR), whose Knight shift (the displacement of the resonance frequency by electron spin polarization) and spin–lattice relaxation are sensitive to spin pairing and low-energy quasiparticles, magnetic-field penetration depth (gap nodes), angle-resolved heat capacity, scanning tunneling spectroscopy, and ultrasound velocity; these probes have given partially conflicting results, and recent heat-capacity work even finds different gap symmetries in different kappa-phase salts. Reviews as late as 2019 open by stating that the nature of the superconducting state in organic charge-transfer salts is still not fully understood. For the founding salt (BEDT-TTF)4(ReO4)2 specifically, superconductivity survives only in a pressure window of a few kbar directly above the first-order, hysteretic anion-rearrangement MI transition, and the discoverers' original questions — why this compound becomes superconducting at all, why only under pressure, and why TcT_c falls so steeply as pressure increases — have no established microscopic answer.

Problem Statement

Determine the microscopic origin of superconductivity in the BEDT-TTF (ET) family of organic charge-transfer superconductors, as first posed at the family's founding by the pressure-induced superconductor (BEDT-TTF)4(ReO4)2. Concretely: (1) identify the pairing mechanism — antiferromagnetic spin fluctuations, electron–molecular-vibration (phonon) coupling, or charge-/valence-bond-fluctuation pairing — and the resulting superconducting gap symmetry (including whether the gap has line nodes and which irreducible representation of the crystal point group it belongs to) in the superconducting ET salts; (2) explain the founding salt's own phenomenology, namely why (BEDT-TTF)4(ReO4)2 becomes superconducting only above roughly 4 kbar, in a narrow pressure window that begins directly at the first-order metal–insulator transition postulated to arise from a rearrangement of the ordered ReO4 anions, and why its transition temperature falls so steeply with further pressure, dTc/dP≈−0.3dT_c/dP\approx -0.3 K/kbar; and (3) establish what role the neighboring anion-ordering or charge-order insulating phase plays in enabling, competing with, or constraining the superconducting state. The earlier exploration sub-question of whether other ET salts can reach higher transition temperatures is already answered affirmatively and is not part of the open target; the remaining open objective is the microscopic mechanism and gap structure, the founding salt's narrow pressure window and steep dTc/dPdT_c/dP, together with its interplay with the adjacent ordered insulating phases.

References

  1. S. S. P. Parkin, E. M. Engler, R. R. Schumaker, R. Lagier, V. Y. Lee, J. C. Scott, and R. L. Greene, "Superconductivity in a New Family of Organic Conductors", Physical Review Letters 50 (4), 270–273 (1983). DOI: 10.1103/PhysRevLett.50.270, https://doi.org/10.1103/PhysRevLett.50.270
  2. S. S. P. Parkin, E. M. Engler, R. R. Schumaker, R. Lagier, V. Y. Lee, J. Voiron, K. Carneiro, J. C. Scott, and R. L. Greene, "A sulphur based organic molecular superconductor: (BEDT-TTF)4(ReO4)2", Journal de Physique Colloques 44 (C3), C3-791–C3-797 (1983). DOI: 10.1051/jphyscol/1983004, https://doi.org/10.1051/jphyscol/1983004
  3. K. Carneiro, J. C. Scott, and E. M. Engler, "Comparative ESR study of three (BEDT-TTF):ReO4 salts: An organic superconductor, a Peierls metal and a semiconductor", Solid State Communications 50 (6), 477–481 (1984). DOI: 10.1016/0038-1098(84)90311-9, https://doi.org/10.1016/0038-1098(84)90311-9
  4. M. Tokumoto, K. Murata, H. Bando, H. Anzai, G. Saito, K. Kajimura, and T. Ishiguro, "Ambient-pressure superconductivity at 8 K in the organic conductor beta-(BEDT-TTF)2I3", Solid State Communications 54 (12), 1031–1034 (1985). DOI: 10.1016/0038-1098(85)90753-7, https://doi.org/10.1016/0038-1098(85)90753-7
  5. S. Gärtner, E. Gogu, I. Heinen, H. J. Keller, T. Klutz, and D. Schweitzer, "Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2", Solid State Communications 65 (12), 1531–1534 (1988). DOI: 10.1016/0038-1098(88)90645-X, https://doi.org/10.1016/0038-1098(88)90645-X
  6. J. Wosnitza, "Superconductivity of Organic Charge-Transfer Salts", Journal of Low Temperature Physics 197 (3–4), 250–271 (2019). DOI: 10.1007/s10909-019-02230-6, https://doi.org/10.1007/s10909-019-02230-6
  7. R. T. Clay and S. Mazumdar, "From charge- and spin-ordering to superconductivity in the organic charge-transfer solids", Physics Reports 788, 1–89 (2019; published online 2018). DOI: 10.1016/j.physrep.2018.10.006, https://doi.org/10.1016/j.physrep.2018.10.006
  8. S. Imajo, K. Kindo, and Y. Nakazawa, "Symmetry change of d-wave superconductivity in κ-type organic superconductors", Physical Review B 103 (6), L060508 (2021); arXiv:2102.11542. DOI: 10.1103/PhysRevB.103.L060508, https://arxiv.org/abs/2102.11542
  9. D. Jérôme and C. Bourbonnais, "Quasi one-dimensional organic conductors: from Fröhlich conductivity and Peierls insulating state to magnetically-mediated superconductivity, a retrospective", Comptes Rendus. Physique 25 (G1), 17–178 (2024); arXiv:2311.03104. DOI: 10.5802/crphys.164, https://doi.org/10.5802/crphys.164
  10. H. H. Wang, A. M. Kini, L. K. Montgomery, U. Geiser, K. D. Carlson, J. M. Williams, J. E. Thompson, D. M. Watkins, and W. K. Kwok, "Synthesis of the new highest Tc ambient-pressure organic superconductor, kappa-(BEDT-TTF)2Cu[N(CN)2]Br, by five different routes", Chemistry of Materials 2 (5), 482–484 (1990). DOI: 10.1021/cm00011a003, https://doi.org/10.1021/cm00011a003