$(\mathrm{TMTSF}{)}_2{\mathrm{ClO}}_4$ is a quasi-one-dimensional organic conductor and superconductor with $T_c=1.4\mathrm{K}$, and one of at least two Bechgaard salts observed to have upper critical fields far exceeding the paramagnetic limit. Nevertheless, the ${}^{77}\mathrm{Se}$ 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 $H_s\sim 15\mathrm{kOe}$, to a regime where ${\chi }_s$ is close to the normal state value, even though $H_{c2}\gg H_s$.

• Received 8 November 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.147002