Evidence from ${}^{77}$Se Knight shifts for triplet superconductivity in (TMTSF)${}_2$PF${}_6$

Abstract

The layered quasi-one-dimensional molecular superconductor (TMTSF)${}_2$PF${}_6$ is a very exotic material with a superconducting order parameter whose ground state symmetry has remained ill-defined. Here we present a pulsed NMR Knight shift (K) study of ${}^{77}$Se measured simultaneously with transport in pressurized (TMTSF)${}_2$PF${}_6$. The Knight shift is linearly dependent on the electron spin susceptibility ${\chi }_s$, and is therefore a direct measure of the spin polarization in the superconducting state. For a singlet superconductor, the spin contribution to the Knight shift, K${}_s$, falls rapidly on cooling through the transition. The present experiments indicate no observable change in K between the metallic and superconducting states, and thus strongly support the hypothesis of triplet p-wave superconductivity in (TMTSF)${}_2$PF${}_6$.

4 pages, 4 eps figures