Transport studies near phase transitions in NbSe3

The non-Ohmic transport properties of Nb${\mathrm{Se}}_3$ are studied near the two phase transitions at $T_1$ and $T_2$. From the non-Ohmic data a parameter $\alpha$ is obtained which provides information on the fraction of Fermi surface affected by the phase transitions. By relating $\alpha$ to a BCS-type gap and modifying the calculations of Fedders and Martin on the spin-density-wave state in Cr we have obtained a theoretical curve for $\alpha$ which agrees well with the experimental data. This good agreement provides new indirect evidence for the charge-density-wave (CDW) state in Nb${\mathrm{Se}}_3$. The derivative of the low-field resistivity versus temperature has also been measured and is shown to be divergent at $T_1$ and $T_2$. The index of this divergence is presented and its significance to the dimensionality of the system is discussed. Finally, we briefly discuss the possibility of explaining the non-Ohmicity below $T_1$ in terms of nonlinear excitations in a CDW condensate.