DECREASE OF ELECTRONIC COHERENCE LENGTH BY IMPURITIES IN THE QUASI-ONE-DIMENSIONAL CHARGE-TRANSFER SALT QN(TCNQ)2

Transport properties of the quasi-one-dimensional salt Qn(TCNQ)2 with various percentages of impurity content were measured in the temperature range 4.2-330K. Nominally pure samples show a broad maximum of the conductivity at high temperatures and a large dispersion of the conductivity at 9.1 GHz. The 9.1 GHz dielectric constant has a maximum value epsilon max=3500 at 240K and epsilon 4.2=420. In samples having a 7% impurity content (disrupting the TCNQ chains) the DC conductivity is strongly decreased, no maximum of conductivity appears and the dielectric constant is also depressed, epsilon max=450 and epsilon 4.2=130. An analysis is presented in which it is assumed that electrons move coherently in highly-conducting large segments interrupted by defects. Coherence lengths (l) are estimated from the temperature dependence of the dielectric constant. A decrease from l=110 lattice constants for the pure sample to l=16 for the most impure sample corresponds well to the impurity content of 7%.