The State of Superconductivity in the 1D Organics

We report on the status of the superconducting state in the quasi-1D molecular organic superconductors (TMTSF)$_{2}$X, on this 25$^{th}$ anniversary of organic superconductivity. A number of experimental results over the last decade, mostly on X=ClO$_{4}$ and PF$_{6}$, address the nature of the Cooper pairing in these materials, which surprisingly is not a settled issue. Critical field (via transport), NMR (including Knight shift) and impurity studies are suggestive of unconventional pairing, converging on spin triplet as a top candidate. Muon spin rotation and thermal conductivity results are less conclusive. More recently, the large $H_{c2}$ (well beyond the Pauli limit) has been confirmed by magnetization, and tunneling on a bicrystal shows a large midgap (zero-bias) state, strengthening the case for triplet superconductivity. The talk will include a discussion of the spin triplet configurations ($p$-- and $f$ --wave) available for (TMTSF)$_{2}$X, and will propose an order parameter \textbf{d}-vector consistent with the experiments. \newline \newline Work done in collaboration with Heon-Ick Ha (present address, Harvard University) and Jeong-Il Oh