Superconducting state of the organic conductor (TMTSF)2ClO4

(TMTSF)(2)ClO4 is a quasi-one-dimensional organic conductor and superconductor with T-c = 1.4 K, and one of at least two Bechgaard salts observed to have upper critical fields far exceeding the paramagnetic limit. Nevertheless, the Se-77 NMR Knight shift at low fields reveals a decrease in spin susceptibility chi(s) consistent with singlet spin pairing. The field dependence of the spin-lattice relaxation rate at 100 mK exhibits a sharp crossover (or phase transition) at a field H-s similar to 15 kOe, to a regime where chi(s) is close to the normal state value, even though H-c2 >> H-s.