Non-linear transport in tetrathiofulvalene-tetracyanoquinodimethane: An electron spin resonance study

We describe an ESR study of pure and irradiated TTF-TCNQ in the low temperature range (1.2 K–4.2 K) where nonlinear transport has been observed. We find no evidence of an activated term in the susceptibility; our results yield an upper limit of about 10 ppm for nonmagnetic impurities with a spin-$\text{1}\text{2}$ excited state. Since in pure samples, the activated susceptibility term is not present whereas the transport nonlinearity is clearly seen, we conclude that the latter cannot be attributed to hot electron effects. The susceptibility and dc bias studies are consistent with a nonmagnetic soliton picture. We have monitored the spin resonance in the presence of a dc bias current. Using the Curie law as an internal thermometer we are able to set an upper limit on sample heating of 0.1 K at the highest dc levels used.