Probing Spin Pairing of Superconductiing State in (TMTSF)$_2$ClO$_4$ with $^{77}$Se Knight Shift Measurement

We report measurements of the $^{77}Se$ Knight shift at low temperatures and $\vec{B}(=1.38T)\parallel \vec{a}$ in the organic superconductor (TMTSF)$_2$ClO$_4$. Recent observations of an upper critical magnetic field H$_{c2}$ exceeding the spin paramagnetic limit by a factor of order two led to suggestions that this system may be a candidate for a triplet pairing state [1]. The Knight shift is proportional to the spin susceptibility, which does not vanish for the case of triplet pairing. We were limited by heating effects to a range of temperatures $T>.8T_c(1.38T)$, but nevertheless observed a change in the NMR frequency consistent with a 10-15 percent reduction in the spin susceptibility relative to the normal state. We discuss the implications of these results. This research is supported by NSF grant DMR-0203806. [1] J.I. Oh and M.J. Naughton, Phys.Rev.Lett. $\bf92$, 067001 (2004)