Conductivity Studies on High-PurityN-Methylphenazinium Tetracyanoquinodimethan

An experimental study of the electrical conductivity in high-purity single crystals of $N$-methylphenazinium tetracyanoquinodimethan (NMP)(TCNQ) is presented. The conductivity at high temperature is larger than that previously reported while the low-temperature conductivity (below 100 °K) is more than a factor of 10 smaller than that of earlier data. Both regimes are consistent with considerably improved sample purity. The data are analyzed from the point of view of a continuous metal-to-insulator transition with fluctuations playing a central role. The low-temperature results indicate that for (NMP) (TCNQ) the curvature in ${log}_{10}\sigma \mathrm{vs} T^{-1\mathrm{}}$ observed in earlier data arises from impurity contributions with the intrinsic behavior being exponential in $T^{-1\mathrm{}}$. The exponential behavior is consistent with the magnetic data which indicate that (NMP) (TCNQ) is a magnetic Mott-Hubbard insulator at low temperature.