C13Knight shift in TTF-TCNQ(C13): Determination of the local susceptibility

The temperature dependent ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ Knight shift and spin-lattice relaxation rate of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) labeled at the CN group of TCNQ are presented. The Knight shift is a direct measure of the time-averaged local susceptibility on the TCNQ chain. From these data, the local susceptibilities on the TTF and TCNQ chains are obtained. The TCNQ chain susceptibility falls to zero below 54 K, indicating the onset of long-range order. The TTF susceptibility persists to lower temperature decreasing below approximately 49 K. The temperature dependence of the TCNQ chain susceptibility is in quantitative agreement with the calculated susceptibility based on the Peierls-Fröhlich model with a complex order parameter for $T>\mathrm{}54\mathrm{}$ K which becomes pinned below 54 K. The susceptibility and thus the density of states on the TTF chain is considerably larger (a factor of 2 at 300 K) than that of the TCNQ chain suggesting a correspondingly narrower bandwidth.