β-(ET)2X organic superconductors: Linear-anion control of structure and ambient-pressure superconductivity, and the first point-contact tunneling studies of β-(ET)2AuI2 implying extremely strong coupling

Abstract Three ambient-pressure organic superconductors have been reported for linear-anion-containing 2D metallic β-(ET) 2 X systems: X = I 3 − (T c 's ⋍ 1.5 K and 8 K under > 0.6 kbar pressure), IBr 2 − (T c ⋍ 2.8 K), and AuI 2 − (T c ⋍ 5 K). These systems from the basis for structure-property correlations because the T c 's scale with unit cell volume, which increases with increasing anion length. When the linear anion length becomes approximately 14–20% shorter than that of the longest member, I 3 − , as is the case for IC1 2 − and AuCl 2 − , the electrical properties become those of a 1D metallic system in which spin-density-wave insulating ground states dominate at low temperature. In the case of the highest ambient-pressure T c material, X = AuI 2 − , we report the first unambiguous point-contact tunneling measurements of the superconducting energy gap, Δ, which reveals a BCS temperature dependence but with a gap 5 times larger than weak-coupling value, implying extremely strong coupling.