Two-band model for NbSe3(Ohmic regime)

Abstract

The charge-density-wave (CDW) linear-chain metal Nb${\mathrm{Se}}_{3}$ shows striking non-Ohmic behavior when the applied electric field exceeds \ensuremath{\sim} 0.1 V/cm. Hall effect, transverse magnetoresistance, conductivity anisotropy, and Shubnikov-de Haas measurements using sufficiently low current densities to avoid Ohmic breakdown have been published. We propose a simple two-band model to account for the temperature dependence of these quantities as well as the (magnetic) field dependence of the Hall constant in the Ohmic regime below 58 K. The model has six unknowns (carrier concentrations and mobilities) that are fixed by six experimental numbers at each temperature $T$. The solution shows that all the mobilities obey a power-law behavior versus $T$, whereas the carrier concentrations are both $T$ independent up to 40 K. Above 40 K the hole population rises sharply, analogous to the theoretical predictions for an excitonic insulator. This implies that the CDW gap occurs on the hole surface. Using the parameters of the model, we have recomputed the resistivities, Hall constant, and magnetoresistance, and they have been shown to agree with all the available experimental data. Thus the conventional single-particle picture with the additional hypothesis of a BCS-type gap on the hole surface is adequate for understanding the transport properties of Nb${\mathrm{Se}}_{3}$ in the zero-frequency-Ohmic regime. We also interpret the SdH data in terms of the two-band model.