Transport properties in transition metal trichalcogenides

The structure of transition metal trichalcogenides is made up of infinite chains of atoms in the center of trigonal prisms stacked on the top of each other. Depending on the effective coupling between chains, this family of compounds is well suited to exhibit properties which extend from pseudo one-dimensionality to anisotropic three dimensionality. Nb-Nb pairs makes NbS3 a semiconductor ; TaS3 exhibits a metal-insulator transition due to a Peierls distorsion at 210 K. TaSe3 is superconductor below 2.1 K and NbSe3 undergoes two independant CDW transitions at 145 K and 59 K. The resistivity anomalies associated with the CDWs formation in NbSe3 have been shown to be strongly nonlinear with the applied current. We summarize several theoretical models to explain these striking properties. NbSe3 exhibits also two others transitions at 2.2 K and around 0.6 K. The resistivity drops below 2.2 K by 30 to 75 % of its residual value at 4.2 K and reaches a plateau. Below 0.4 K the resistivity decreases linearly down to 7 mK. Initial susceptibility measurements down to 50 mK do not show any increase in diamagnetism indicating flux expulsion. However NbSe3 is a bulk superconductor under pressure. We discuss the nature of these low temperature transitions with the eventual occurence of superconductivity.